Diversity of Potential (Bio)Technological Applications of Amino Acid-Based Ionic Liquids

Abstract

This review explores the emerging potential of amino acid-based ionic liquids (AA ILs) in various (bio)applications, emphasizing their unique properties and versatility. It provides a comprehensive analysis of recent advancements, covering applications in drug delivery, catalysis, environmental remediation, and biotechnology. The review also offers an overview of the synthetic methods for preparing AA ILs, highlighting both traditional and innovative approaches, and examines key physicochemical properties—such as biocompatibility, stability, and tunability—that make AA ILs highly attractive for diverse applications. Additionally, challenges hindering their widespread adoption, including high production costs, toxicity concerns, scalability issues, and environmental impact, are discussed. This review concludes with perspectives on future research directions and strategies to overcome these challenges, unlocking the full potential of AA ILs in both scientific and industrial contexts.

1. Introduction

Ionic liquids (ILs) are salts composed of bulky asymmetric organic cations and organic or inorganic anions [1,2,3]. In general, they are characterized by melting temperatures below 100 °C, low vapor pressures, and, in some cases, favorable thermal and chemical stability. The first report on the synthesis of an organic salt, ethylammonium nitrate, with a melting point below room temperature was published in 1914 [2]. For several decades, there were only a few reports on low-melting organic salts. Later, in the 1960s and 1970s, research interest was focused on chloroaluminates, identified as the first generation of ILs [1,2,3]. Various ILs, mainly based on dialkylimidazolium cations, were investigated as electrolytes for thermal batteries. However, their instability in water and/or air posed a serious drawback to industrial applications.

The second generation of ILs includes a diverse range of compounds, with the most extensively studied being those containing cations such as imidazolium, pyridinium, pyrrolidinium, morpholinium, ammonium, and anions such as hexafluorophosphate (PF₆), tetrafluoroborate (BF₄), bis(trifluoromethanesulfonyl)imide (NTf₂), alkyl sulfate (ROSO₃), halide (X), carboxylate (RCOO), and others [1,2,3]. These ILs have widespread applications, including CO₂ storage, solvents for extractions or chemical reactions, catalysts, and more [4,5,6].

In the last two decades, the emergence of biomolecule-based ILs, including amino acid-based ionic liquids (AA ILs), has revolutionized the field. Unlike traditional ILs, these compounds are biocompatible, environmentally benign, and non-toxic, which broadens their application scope to industries such as biotechnology, food, cosmetics, and pharmaceuticals, as well as in green chemistry and ecological remediation [7,8,9,10]. This innovation signifies a paradigm shift in the design and utility of ILs, paving the way for their use in sustainable and bio-oriented technologies. Biomolecule-based ILs, including those incorporating active pharmaceutical ingredients as either the cation or anion, belong to the third generation of ILs [11,12].

This review highlights the novel and diverse (bio)technological applications of amino acid-based ionic liquids (AA ILs), a unique subclass of biomolecule-based ILs. AA ILs distinguish themselves with their unparalleled potential in fostering sustainable and green solutions. Additionally, this paper provides an overview of the synthesis methods and the key physicochemical properties of the most studied AA-based ILs, aiming to underscore their transformative role in modern science and industry.

1.1. General Structures, Synthesis, and Key Properties

Since the AAs have/possess both amino and carboxylic acid functional groups, they can be converted into either a cation or an anion. Some examples from the literature of ILs containing an AA anion and various cations are shown in Scheme 1A [13,14]. They are usually obtained in two steps: first, the organic halide is converted to organic hydroxide by ion exchange, and then the AA is neutralized by the organic hydroxide [13]. The ILs based on the AA cation are obtained in a single step by acidifying the neutral AA with a strong acid [15]. To minimize the H-bonding possibilities of AA and to reduce the melting point of the resulting salt, AA is sometimes first converted to AA ester chloride, which is then mixed with a metal salt to give the target product by metathesis [15,16,17]. Some examples of these types of AA ILs are shown in Scheme 1B.

1.2. General Information on Selected Physicochemical Characteristics of the Main Studied Groups of AA ILs

• Viscosity:

Viscosity is a key factor influencing the applications of ILs, especially when used as solvents in catalysis, extraction, and related processes. As a result, considerable effort has been devoted to the design and synthesis of ILs with reduced viscosity.

ILs containing the same anion and either imidazolium or amino acid ester cations have comparable viscosities [14]. They are less viscous than ILs based on quaternary ammonium cations [14]. Overall, ILs with asymmetric cations tend to be less viscous than those with symmetric cations [14].

AA ILs with strong hydrogen bonding through the carboxyl group generally exhibit high viscosity. This high viscosity is attributed to hydrogen bonding between the AA anions and the hydrogen atom at the carbon atom at the second position the imidazolium cation ring. For example, the minimum viscosity reported for 1-ethyl-3-methylimidazolium (Emim)-based AA ILs was 486 mPa·s at 25 °C [13,18]. On the other hand, tetrabutylphosphonium (P₄₄₄₄) is a relatively large cation, which leads to weakened interactions among the ILs. As a result, it does not exhibit effective hydrogen bonding with anions and is therefore less viscous than Emim-based AA ILs [19]. In addition, Kagimoto et al. reported that P₄₄₄₄-based ILs are less viscous than ammonium-based ILs when comparing ILs with the same anions [20]. However, their viscosity is relatively high; for example, [P₄₄₄₄][AAs] had a viscosity greater than 344 mPa·s at 25 °C [18]. On the other hand, mixtures of [P₄₄₄₄][Lys] and [P₄₄₄₄][Asp] in a ratio of 1:1 to 4:1 are characterized by a lower viscosity compared to the individual salts, due to hydrogen bonding between the amine group(s) of Lys and the carboxylic group of Asp. However, as suggested by Kagimoto et al., it is possible that these bonds do not form anion layers [21]. Among the series of tetraethylammonium (N₂₂₂₂)-based ILs, those with hydrophobic AA anions such as Pro, Met, Ile, and Ala are less viscous than those with Thr or Ser anions [22]. Among them, [N₂₂₂₂][L-Ala] is found to be the IL with the lowest known viscosity of 81 mPa·s at 25 °C, which is 6 times lower than [Emim][Gly] [18]. Regarding the cation, low-molecular-weight cations, as well as asymmetric tetraalkylammonium-based AA ILs with short and flexible alkyl chain substituents, are less viscous due to a decrease in van der Waals interactions [22].

Choline (Ch)-based AA ILs exhibit high viscosity, with reported values ranging from 102 to 107 mPa·s. In addition, for ILs based on AAs capable of participating in additional hydrogen bonding, such as Asp⁻, Arg⁻, Glu⁻, and His⁻, the viscosities reported by different research groups are not consistent. The discrepancies in these data in the literature have been attributed to insufficient drying during synthesis; therefore, the ILs may be hydrated to varying extents [23]. Typically, an increase in the size of the anion correlates with an increase in viscosity, which is attributed to stronger intermolecular forces such as anion stacking, tail aggregation, van der Waals interactions, hydrogen bonding, and π-stacking interactions. Consistent with this trend, Liu et al. reported the lowest viscosity for [Ch][Gly] (121 mPa·s) and the highest for [Ch][Trp] (5640 mPa·s). Additionally, ILs containing negatively charged AA anions exhibited viscosities above 2000 mPa·s among a large series of [Ch][AA] [24].

It should be noted that the ILs that were prepared using AAs as cations or AA derivatives were either solids or liquids with extremely high viscosities, up to 4180 mPa·s at room temperature [18]. The incorporation of alkyl substituents at the nitrogen atom of the AA cation-based ILs results in a decrease in their viscosity. N,N,N-trialkyl-substituted AA ILs have been reported to exhibit lower viscosities (200 to 1000 mPa·s) at room temperature (r.t.) compared to other known AA ILs [25].

• Solubility:

The miscibility of room-temperature ILs with organic solvents is dependent on the side chain structure of the corresponding AA anion. For example, [Emim][Glu] and [Emimm][Asp], ILs with an AA anion with two carboxyl groups, are insoluble in chloroform [19].

AA ester saccharinates, nitrates, and halogenates are miscible with water, ethanol, and acetone [17].

• Thermal properties and thermal stability:

The thermal stability of AA anion-based ionic liquids decreases in the following order, depending on the cation:

For the anion Ala, the decomposition temperatures (Td) for the cations above are as follows: 286 °C, 212 °C, 181 °C, 176 °C, 162 °C, and 150 °C [19]. The results are consistent with the data reported by Rahman et al. for [N₂₂₂₂]-based AAs [22].

On the other hand, for a series of triethanolammonium [TEA] salts, the thermal stability varies with the type of AA anion and decreases in the following order: His > Arg > Glu₂ > Pro > Trp > Ser > Gly > Leu > Ile > Thr > Met > Ala > Asp₂ > Gln > Phe > Glu > Val > Lys > Asp > Asn [26].

The phosphonium-based AA ILs demonstrate lower melting points and viscosities than the ammonium-based AA ILs [19]. However, AA ILs containing phosphonium cation with the same alkyl substituents exhibit a higher melting temperature due to increased crystallinity.

Most [Ch][AA] ionic liquids are stable up to 200 °C, with the exception of [Ch][Gly], which decomposes at 121 °C [23,24]. In general, elongation of the AA anion side chain, as well as the introduction of hydroxyl or carboxyl groups, leads to an increase in the decomposition temperature, with consistent results across the reported studies. An exception is [Ch][His], for which the Td varies between 128 and 171 °C [23,24].

The glass transition temperatures (Tg) of most tetraalkylammonium-based ILs are reported to be about 10 °C lower than those of phosphonium-based ILs with the same AA anion [18].

Interestingly, for a series of Gly-based [NTf₂], He et al. showed that the salts with N,N-dialkyl-substituted Gly or N,N-dialkyl-substituted Gly ethyl ester [GlyOEt] cations are more thermostable than [Gly][NTf₂]. Esterification and alkyl substitution of the Gly cation lead to a decrease in the melting temperature of the salt, proportional to the number of alkyl substituents. In addition, they exhibit higher thermal stability than [Gly][NO₃] but lower than [P₄₄₄₄][Gly] [25].

• Ionic conductivity

According to the results, [N4444][AAs] showed conductivity values in the range of 0.16–0.54 ms cm⁻¹, which were much higher than those of similar ILs, such as [Emim][AAs] (κ = 9.1 × 10⁻⁶ to 0.65 ms cm⁻¹) [22]. For the series tested, the authors found that the ionic conductivity is inversely proportional to the viscosity and is governed by the van der Waals interactions and the H-bonding ability of the ILs [22].

• Chirality

[AAOEt][NO₃] (except [ProOEt][NO₃]) have nearly the same specific rotation value as their precursors, while [AAE][Sac] undergo partial racemization. Chirality is preserved and remains the same for 3 months similar to the chirality of AAOEt-ILs, which are used as catalysts in the Diels–Alder cycloaddition of cyclopentadiene to methyl acrylate. AAOEt shows the same selectivity as [Bmim][BF₄]. [AAOEt][SA] show better and more stereoselectivity than [AAOEt][NO₃] [18].

The optical properties of [Ch][AAs] are consistent within the family and slightly higher than other common ILs. The refractive indices of all measured [Ch][AAs] lie in the range of 1.49–1.54 and are similar to many common organic solvents. There is no apparent correlation with other measured physical properties or the identity of the anion. The optical rotation of some enantiomerically pure [Ch][AAs] has also been reported. These are quite different from those of pure amino acids, as has been seen with other chiral ILs [23].

1.3. Toxicity and Biodegradability

The high stability of ILs is a desirable characteristic; however, degradation, disposal, or accidental release into the environment must be considered. Studies of their ecotoxicity generally indicate that the environmental impact of AA ILs depends on several factors, including their structure, concentration, and interactions with ecosystems. Overall, AAs with hydrophobic or bulky side chains tend to be more toxic.

For their use in topical ionic liquid formulations, the cytotoxicity of AA esters on skin cells has been extensively evaluated. In general, AAOEt is less toxic than methyl esters (OMe), except when cytotoxicity increases with longer alcohol chain lengths. For various AAOEt-based ILs, the half-maximal inhibitory concentration (IC₅₀) values range from 10 to 60 mM for keratinocytes and 1 to 20 mM for fibroblasts, with their salicylic acid salts further reducing toxicity [27].

Cations such as imidazolium, pyridinium, or quaternary ammonium are typically more toxic than naturally derived or simpler cations, such as choline and betaine. For example, our previous results showed that [Ch][AAs] had minimal effects on the growth of mouse fibroblasts (3T3 cells) even at 5.0 mmol/L [28]. Notably, significant reductions in cell growth (up to 50%) were observed only with [Ch][Arg], whereas [Ch][Glu] showed moderate antiproliferative activity (up to 25%) at the highest concentrations tested.

In contrast, the IC₅₀ values for [Bmim][Gly], [Bmim][Ala], and [Bmim][Val] against 3T3 and CaCo-2 cells are in the millimolar range and comparable to those of second-generation ILs, such as [Bmim][BF₄], [Bmim][PF₆], and [Bmim][HSO₄]. In these cases, the cation contributes significantly to the cytotoxicity [29]. Among ILs with the same BF₄ anion, those with AA cations such as [ValOMe], [GlyOMe], and [ValOMe] were found to be two to four times less toxic to 3T3 cells than those with [Emim] or [Bmim] cations [29].

Toxicity tests in aquatic organisms such as Daphnia magna, Vibrio fischeri, and Pseudokirchneriella subcapitata show that some AAILs can be moderately to highly toxic, depending on their chemical structure and concentration. Longer alkyl chains in the cation increase hydrophobicity, which often correlates with higher toxicity. An ecotoxicity analysis using the marine bacterial model Vibrio fischeri revealed that the choline-based ILs have a different mechanism of toxicity compared to imidazolium-based ILs [30]. While choline-based ionic liquids are generally considered harmless, the ecotoxicity of some is indeed higher than that of common organic solvents such as chloroform or dichloromethane. In fact, certain choline-based ILs have toxicities similar to reference pesticides such as atrazine [30]. Additionally, when tested on three aquatic organisms, the half-maximal effective concentration (EC₅₀) values for [Ch][AAs] were reported in the following ranges: brine shrimp (9000–15,000 mg/L), zebrafish (155–275 mg/L), and green algae (100–1000 mg/L), which can be considered non-toxic according to the toxicity ranking of Passino and Smith [31,32]. In general, the cation is considered to be the major contributor to IL toxicity. In a toxicity comparison of three phenylalanine-based ILs containing [N₄₄₄₄], [P₄₄₄₄], or [Ch] cations against zebrafish, all were found to be non-toxic with EC₅₀ values greater than 100 mg/L [33]. However, an increase in toxicity was observed when the cation was paired with the hydrophobic NTf₂ anion. Furthermore, ILs composed of the 1-(2-hydroxyethyl-3-methylimidazolium) cation paired with glycinate, serinate, alaninate, or prolinate counteranions were found to be more environmentally friendly than [Emim][AAs] in antimicrobial tests against the green alga Scenedesmus quadricauda and the bioluminescent marine bacterium Vibrio fischeri [34]. This reduced toxicity has been attributed to the presence of a short hydroxyl functional group in the cation [34]. A comparison of the toxic effects of a series of [P₄₄₄₄][AAs] and [N₄₄₄₄][AAs] on Poecilia reticulata fish revealed that, among ILs with the same amino acid anion, those from the phosphonium series were more toxic [35]. Interestingly, the one-half lethal dose (LD₅₀) values of [N₄₄₄₄][Phe] and [P₄₄₄₄][Phe], estimated in the same test, were found to be comparable [35]. The biodegradability of the [Ch][AA] ILs was evaluated in vitro using wastewater microorganisms. All the ILs were classified as readily biodegradable based on their high mineralization rates (62–87%) for 28 days [36]. ILs based on charged AAs (Asp, Gln, Lys, and Arg) are significantly more susceptible to microbial decomposition. In addition, for most [Ch][AA] ILs, low toxicity correlated with good biodegradability [36]. When comparing ILs with the same amino acid anion but different cations, the biodegradability rate decreased in the following order: Ch > Emim > Bmim > N, N-ethyl-methylpiperidinium > N₂₂₂₂ > N₄₄₄₄ > N,N-ethyl-methylpyrrolidinium [37].

2. Bioapplications of AA ILs: An Overview

2.1. Drug Formulations and Delivery

The conversion of active pharmaceutical ingredients (APIs) into ILs is a widely studied strategy to improve water solubility and long-term stability, prevent polymorphism, modify pharmacokinetic profiles, or suggest new routes of administration for orally administered drugs. AAs are biocompatible, and their ILs, in which the API serves as either a cation or anion, have been the focus of recent studies [38,39,40,41].

The most studied ILs contain AA ester cations paired with non-steroidal anti-inflammatory drug (NSAID) anions such as ibuprofen (Ibu), naproxen (Nap), ketoprofen (Keto), and salicylic acid (SA). Novel topical and transdermal formulations are being developed to minimize the gastrointestinal mucosal damage caused by orally administered NSAIDs, especially at high doses or during prolonged use [42].

Numerous in vitro studies using porcine skin models have demonstrated the penetration-enhancing properties of AA ILs. For example, [ProOEt][Ibu] showed a 10-fold increase in the cumulative drug amount at 96 h compared to the acid form of Ibu [43]. Notably, ProOEt is a compatible cation, and at a concentration of 50 mM, it reduced murine fibroblast viability by only 20% [43]. In addition, a large series of ILs containing isopropyl (i-Pr) esters of AAs and Ibu showed 6- to 120-fold increases in solubility in water and PBS, as well as more than a 16-fold increase in cumulative permeation through a model skin system [44]. As expected, an increase in lipophilicity and skin permeability was observed for a series of L-Glu alkyl ester Ibu salts, with permeability correlating with the increase in alkyl chain length from ethyl to pentyl [45]. These compounds show good potential for transdermal delivery systems of Ibu. Conversely, within a short series of Pro short alkyl AA ester naproxenates, the isopropyl ester exhibited the highest permeability in a porcine skin model [46]. Moshikur et al. demonstrated that [AAOEt][SA] penetrated more than nine times faster than the sodium salt. The best transport behavior was observed for [Asp][SA], followed by [Ala][SA], and then [ProOEt][SA] [27].

ILs composed of Et esters of hydrophobic amino acids (L-Ala, L-Pro, and L-Leu) as cations and free fatty acids (oleic or linoleic) as anions have been proposed as drug delivery systems for Ibu or the model peptide drug ovalbumin epitope [47]. These ILs demonstrated good miscibility with solvents such as isopropyl myristate and DMSO, as well as with emulsifiers such as Span-20, which are commonly used in formulations. Additionally, they showed no toxic effects in a reconstructed human epidermal model assay. In vitro, a formulation containing 10% [ProOEt][linoleate] in isopropyl myristate was found to enhance the skin permeation of Ibu or ovalbumin more effectively than sodium lauryl sulfate, a conventional chemical permeation enhancer [47].

Extensive research has shown that AA alkyl ester hydrochlorides are excellent chemical skin penetration enhancers. For a large series of AAOMe hydrochlorides, with the exception of [ValOMe][Cl], Zheng et al. reported a similar or improved ability to facilitate the transport of 5-fluorouracil or hydrocortisone across a skin model compared to Tween 20, Azone^®, or Transcutol^® [48]. Increasing the alkyl chain length of the ester substituent to octanoyl (Oct) or lauryl (Lau) resulted in up to a 6-fold and 2-fold improvement in skin permeation parameters (flux rate for 5-fluorouracil and hydrocortisone, respectively). A clear dependence on the chain length of the ester was observed. Among the two drugs, [LeuOLau][Cl] showed the highest potential as a skin permeation enhancer and exhibited low toxicity to fibroblasts (IC₅₀ = 387 µM) and keratinocytes (IC₅₀ = 177 µM) [48].

In addition to developing new topical drug formulations, the IL strategy can also be used to improve the pharmacokinetic properties of oral drug formulations. In a clinical trial, Martin et al. demonstrated a shorter distribution phase and better gastrointestinal tolerability with orally administered [Lys][Ibu] compared to the same dose of the parent Ibu in its acidic form administered by injection [49]. Similarly, [Lys][Keto] exhibited improved water solubility, resulting in a 4-fold faster increase in plasma concentration compared to the parent molecule [50]. Moreover, [Lys][Keto] caused no gastric side effects and was able to repair ethanol-injured gastric mucosa in an animal model [50]. On the other hand, cationic lipidoamino acids such as decyl alanine and decyl phenylalanine are excellent counterions for tolfenamic acid, an NSAID used for the treatment of acute migraines, rheumatoid arthritis, osteoarthritis, and other inflammatory diseases [51]. In vivo oral bioavailability studies with this API-IL showed that the drug is delivered effectively, dispersed, and solubilized. Subsequently, the cationic lipidoamino acid ion is digested into non-toxic components. Another remarkable result was observed with the API-IL formulations of methotrexate containing ProOEt, PheOEt, and AspOEt cations, which demonstrated excellent solubility enhancement in simulated body fluids—500–1000 times higher than the pure drug [52]. However, their solubility levels remain below those achieved by ILs based on choline, tetraalkylammonium, or tetraalkylphosphonium salts, which exhibit over 5000-fold enhanced solubility.

Some reports have shown that [Ch][AAs] have great potential as functional excipients capable of enhancing the solubility of poorly soluble drugs, nutraceuticals, and active cosmetic ingredients. For example, the solubility of ferulic acid and rutin is increased up to 6- and 45-fold, respectively, when 1% of [Ch][Glu] or [Ch][Phe] is added to w/o emulsions [53]. The solubility of these compounds depends on the concentration of ILs and temperature, while their radical scavenging properties and cytotoxicity remain unaffected. In line with these findings, Caparica et al. demonstrated enhanced solubility and maintained antitumor activity of rutin when loaded into a [Ch][Phe]- or [Ch][Gly]-based water-in-oil nanoparticle hybrid system [54]. Furthermore, in vitro studies have shown that [Ch][AAs] facilitate the permeation of ferulic acid and puerarin through an artificial model polyethersulfone membrane [55].

In addition, Julio et al. reported that, compared to polymer capsules alone, hybrid polymer materials incorporating [Ch][Phe] or [Ch][Glu] exhibited a 35–50% enhanced capacity for loading rutin [56]. However, the two ILs have almost no effect on the solubility and permeation of caffeine and SA, two hydrophilic drugs [57]. Furthermore, another study showed that the solubility of biotin (vitamin B8) in water is increased in the presence of [Ch][Gly] or [Ch][Ala] due to strong polar–polar interactions and hydrogen bonding. These interactions are stronger for [Ch][Ala], which could be used as an excipient in biotin formulations [58]. Additionally, both ILs were shown to increase the solubility of ibuprofen in PBS buffer by fivefold at low concentrations (<1 wt%) and to almost double its permeability across artificial model membranes [59]. A twofold increase in the solubility of glibenclamide has been reported with the addition of 6% [Ch][Trp], which was attributed to strong π-π interactions between the AA anion and the drug molecule [60]. All tested AA ester NSAIDs, including Ibu, Keto, Nap, and SA, showed enhanced solubility in water and PBS compared to the acid form of the drug [61].

Notably, the increased solubility and storage stability of paclitaxel in media containing [Ch][AA] is worth highlighting [62]. This formulation offers a promising alternative to the widely used co-solvent Cremophor EL, which is known to cause precipitation in aqueous solutions and hypersensitivity side effects upon injection.

2.2. Biomass Production and Potency in Crop Protection

Haematococcus pluvialis is the major source of astaxanthin, a carotenoid used in cosmetics and as a food and health supplement for its potent antioxidant, immunostimulatory, anti-inflammatory, and anti-cancer properties [63]. Although these microalgae can be cultivated in a relatively simple process, they face several challenges, including low biomass production and high sensitivity to stress, resulting in increased microalgal mortality and reduced astaxanthin yield [64]. In a pilot study, Gong et al. demonstrated that the biomass production and cell number of Haematococcus pluvialis increased by 38% and 34%, respectively, in the presence of [N₂₂₂₂][Arg] [64]. The study also showed an increase in carbohydrate, protein, and chlorophyll content. A concentration dependence of the increase in biomass production with increasing IL concentration was observed during plant development.

On the contrary, AA-based ILs have a negative effect on the growth of cereal seedlings. For example, [N₂Py][Ala], [P₂₂₂₂][Ala], [Emim][Ala], and [N₂₂₂₂][Ala], tested at concentrations ranging from 200 to 1200 mg/L, significantly inhibited the growth, chlorophyll content, and nutrient uptake of wheat seedlings, with the effect decreasing in the following order: [N₂Py][Ala] > [N₂₂₂₂][Ala] > [P₂₂₂₂][Ala] > [Emim][Ala] [65]. A similar negative effect on rice growth was observed by Habibul et al. in the presence of ILs containing 1-methyl-3-alkyl imidazolium cations and bromide anions. The inhibitory effect increased with the increasing alkyl chain length of the cation [66].

Interestingly, a stimulatory effect on phytohormone production in lettuce (Lactuca sativa L.) has been reported for a wide range of tetraalkylammonium tryptophanates and dialkyldimethylammonium tryptophanates [67]. Szymaniak et al. found a significant increase in auxin activity in the presence of all ILs tested, regardless of the alkyl chain length or the presence of aromatic substituents.

A wide range of tested dicationic ILs containing bis-ammonium or bis-phosphonium cations with short (butyl) to medium (octyl, dodecyl) alkyl chains, or an ester linker, and His or Pro anions stimulate the germination of white mustard (Sinapis alba) seeds [68]. Interestingly, those with an alkyl linker in the cation promote root length, whereas all ILs with an ester linker negatively affect root length. On the other hand, all bis-ammonium and bis-phosphonium Pro- and His-based ILs inhibit shoot length during white mustard development [68]. Additionally, all ILs with short-chain substituents, both alkyl and ester, exhibited very good to excellent protection and repellent activity against two common storage pests: granary weevil beetles (Sitophilus granarius) and khapra beetle larvae (Trogoderma granarium) [68].

Similarly, [Ch][AA] ILs have a stimulatory effect on maize germination and root and shoot growth [69]. The highest increase in the germination index, compared to controls, was observed in [Ch][Trp]-treated seeds. These ILs promote the production of growth-promoting phytohormones, such as gibberellic acid and zeatin, in maize shoot tips, and stimulate the production of AAs essential for plant development, including Glu, Gly, and Arg [69].

Wheat germination is highly sensitive to environmental conditions and is often used as an assay to evaluate the cytotoxicity of ILs. For a series of 1-ethyl-3-methylpyrrolidinium (EmPy) ILs containing AA anions, it was found that [EmPy][Gly] had no effect on wheat germination at the concentrations tested, while those containing anions such as Pro, Ala, Val, and Leu suppressed wheat growth less compared to methanol (control) [70].

2.3. Antimicrobial Activity, Antibacterial Coatings, and Antibiofilm Formation

[Ch][AAs] have been extensively studied for their antimicrobial activity. In a previous study, we demonstrated that [Ch][Arg] and [Ch][Lys] effectively inhibited the growth of Gram-positive bacteria (Staphylococcus aureus 29213, Staphylococcus saprophyticus 15305, and Bacillus subtilis 168) and Gram-negative bacteria (Escherichia coli 420, Escherichia coli 25922, and Pseudomonas aeruginosa PAO1), with estimated IC50 values in the millimolar range [71]. In contrast, [Ch][Glu] and [Ch][Asp] showed no cytotoxic effect nor did they even promote the growth of all tested bacterial strains except Staphylococcus aureus 29213 [71]. Polyionic membranes based on bacterial cellulose and [Ch][AA] exhibited good biocompatibility and strong antimicrobial activity against Staphylococcus aureus, Escherichia coli, and Candida albicans [72]. Within this series, membranes containing aromatic and hydrophobic AAs, such as Phe, Trp, and Ile, were more active than those containing Gly or polar AAs such as Ser and Pro [72]. In addition, [Ch][AA]–polyionic liquid hydrogels containing hydrophobic (Phe) or charged AA anions (Asp, Arg) were found to be hemocompatible and demonstrated good antimicrobial activity [73]. The hydrogels with Asp showed excellent antibacterial activity against Escherichia coli and Staphylococcus aureus, while those with Phe exhibited higher cytotoxicity against the fungus Cryptococcus neoformans [73]. In another experiment, it was demonstrated that a tri-component antibacterial coating composed of loofah fiber, epoxy resin, and positively charged choline AA ILs (Arg or Lys) was shown to have strong antibacterial activity against the Gram-positive bacteria Staphylococcus aureus [74].

Kaczmarek et al. found that bis-ammonium or bis-phosphonium-based ILs with Pro or His anions and short- to medium-chain linkers exhibited moderate antimicrobial activity against numerous strains, including Gram-positive bacteria such as Staphylococcus aureus ATCC 33862, Staphylococcus epidermidis ATCC 12228, Enterococcus faecalis ATCC 19433, Bacillus subtilis ATCC 11774, Micrococcus luteus ATCC 4698, Clostridium perfringens ATCC 13124, and Lactiplantibacillus plantarum DKK 003, as well as Gram-negative bacteria, including Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 9027, Serratia marcescens ATCC 8100, Proteus vulgaris ATCC 49132, Moraxella catarrhalis ATCC 25238, and Salmonella enteritidis ATCC 13076, and fungi such as Candida albicans ATCC 10231, Rhodotorula mucilaginosa DKK 040, Botrytis cinerea BPR 187, and Fusarium graminearum KZF 1 [68]. It should be noted that changing the linker in the cation from an alkyl to a two-ester linker significantly reduces the antimicrobial activity of the ILs.

The antibacterial activity of ILs containing cations such as ValOiPr, IleOiPr, ThrOiPr, or MetOiPr in combination with NSAID anions was tested against three bacteria commonly associated with skin infections: Staphylococcus epidermidis, Escherichia coli, and Micrococcus luteus [61]. The ILs demonstrated a significant inhibitory effect on the growth of Gram-negative bacteria, outperforming the acid forms of the NSAIDs. Among all the salts, the MetOiPr-based ILs exhibited the highest antibacterial activity [61].

Interestingly, the [L-ThrOiPr] salts of Keto and Nap did not inhibit the growth of Staphylococcus epidermidis, while their Ibu and SA salts effectively suppressed its growth. However, conversion of the NSAIDs to AAOiPr salts had no effect on their activity against Micrococcus luteus [61].

AA ILs containing EmPyr cations exhibit antimicrobial activity against human pathogenic bacteria, including Aeromonas hydrophila, Escherichia coli, Listeria monocytogenes, and Staphylococcus aureus [75]. The EC₅₀ values are comparable among the four strains, ranging from 18 to 66 mM, and are strongly influenced by the structure of the anion [75]. The antimicrobial activity of the investigated AA ILs increases in the order of [EmPyr][Ser] < [EmPyr][Pro] < [EmPyr][Ala] < [EmPyr][Gly]. Additionally, the activity can be enhanced by extending the chain length of the alkyl substituents on the cation.

2.4. Application in Agriculture

In addition to promoting the growth of seedlings and agricultural crops, as discussed above, ILs may have been used as co-solvents in the formulation of chemicals for plant protection. Although promising, studies in this area are still quite limited. For example, Wang et al. reported that AA ILs significantly increased the water solubility of two commonly used fungicides, fludioxonil and cyprodinil, in the presence of ILs containing 1-ethyl-1-methylpyrrolidinium, tetraethylammonium, 1-methyl-1-propylpyrrolidinium, or 4-ethyl-4-methylmorpholinium glycinates [70]. The solubility of the two fungicides increased with IL concentration, with the highest solubility observed in solutions containing 50% IL. In addition, the inclusion of rhamnolipid in the IL solution was shown to further enhance its solubilizing properties for the two fungicides [70].

Converting herbicides to ILs has several advantages. First, they are less volatile and therefore safer for agricultural workers. On the other hand, depending on the structure of the counterion, their water solubility can be reduced, thus minimizing leaching and contamination of soil and irrigation water due to unintended transport to the target. Few AA-based herbicidal ILs have been reported in the literature [76]. For example, ILs containing the cations L-Pro, L-Arg, and L-His, along with the anions 2,4-dichlorophenoxyacetic acid (2,4-D) or 4-chloro-2-methylphenoxyacetic acid (MCPA), were more effective than the parent herbicide on two agricultural weeds: cornflower (a sensitive species) and winter oilseed rape (a moderately sensitive species) [77].

Another potential application of AA ILs is in the degradation of pesticides. For example, Paraoxon, an insecticide that is 70% as effective as sarin, acts as a cholinesterase inhibitor [78]. It is a neurotoxin for both humans and animals, and its use is limited due to the risk of organophosphate poisoning. Studies have shown that conventional ILs based on the Bmim cation are able to degrade Paraoxon, producing different products depending on the anion involved [79]. Morales et al. showed that AA ILs with the Bmim cation are as effective as classical ILs, and depending on the cation, the reaction can follow three different pathways: nucleophilic attack on the phosphorus atom (1), the aromatic carbon atom (2), or the aliphatic carbon atom (3). Interestingly, in contrast to ILs containing Bmim cations and classical anions (such as PF₆, NTf₂, or BF₄), those with AA anions show greater selectivity [80]. For example, those with Gly or Met anions prefer pathway 2, while those with Cys and His anions favor pathway 3. Furthermore, the half-life of Paraoxon degradation varies from 2 to 41 min depending on the AA anion. The highest degradation rate was observed with [Bmim][Gly] and [Bmim][Met], whereas the process was slower in the presence of [Bmim][Ser] [80].

Notably, AA ILs have been used in the development of an ultrasensitive biosensor for the detection of organophosphate pesticides. The sensor is based on mineralized organophosphate hydrolase-fused cells supported on AA IL-stabilized carbon nanotubes and is capable of detecting trace amounts (3 fmol/L) of target pesticides (Paraoxon, Acetamiprid, Aldicarb, Carbaryl, Dibrom, Imidacloprid, and Nitenpyram) in tap water and spinach juice [81]. Han et al. demonstrated that AA ILs improved the conductivity and electrochemical activity of the hybrid catalysts.

2.5. Stability and Activity of Enzymes and Proteins

2.5.1. AA-ILs as Media for Biocatalytic Reactions

A comprehensive study of eighteen [Ch][AA] ILs showed that they all improved the water solubility of phytosterols (β-sitosterol, stigmasterol, campesterol, and brassicasterol) [82]. With the exception of [Ch][Trp], little to no effect on cell membrane integrity and the activity of Mycobacterium sp. MB 3683 was observed at concentrations up to 4%. Furthermore, in the presence of 1% [Ch][Asp], [Ch][Glu], or [Ch][Gln], the yield of androst-4-ene-3,17-dione, catalytically converted from phytosterols by whole cells of Mycobacterium sp., was higher compared to IL-free media [82].

On the other hand, Bisht et al. observed that the addition of [Ch][Gly] slightly increased the molecular volume of bromelain, causing partial unfolding compared to the PBS medium (control), leading to a loss of its compact structure and increased flexibility [83]. This is attributed to the formation of effective hydrogen bonds between the glycine anions and the peptide chains of the protein. However, the addition of [Ch][Gly] at concentrations up to 0.1 M had no significant effect on the hydrolytic activity of bromelain [83]. Similarly, a slight increase in hydrolytic activity of bromelain was observed in the presence of [Bmim][Gly] [84]. In the case of bromelain, interactions involving the AA anion appear to be predominant and have a stronger effect on its conformational stability and catalytic activity than those involving the cation [83,84].

In contrast, Kumar et al. reported a 30% increase in the hydrolytic activity of papain in the presence of 1 mM [Ch][Trp], whereas the addition of [N₂₂₂₂][Trp] at the same concentration resulted in a 15% decrease in the hydrolytic activity of papain [85]. However, when the concentration of either IL was increased even slightly, papain was almost completely inactivated due to the disruption of disulfide bonds in the papain molecule and subsequent unfolding of the protein. The IL-induced unfolding of papain was shown to be concentration-dependent [85].

Interestingly, the hydrolytic activity of papain was enhanced by 2-fold and 1.4-fold, respectively, when immobilized on gold nanoparticles (AuNPs) via [Ch][Trp] or [N₂₂₂₂][Trp] [86]. In addition, thermal stability was maintained, and secondary structure rearrangements without unfolding were observed for the supported papain molecules. Furthermore, AuNP/[Ch][Trp] showed a higher loading capacity for papain compared to AuNP/[N₂₂₂₂][Trp] [86].

In contrast, all 20 amino acid ILs, including mono- and bis-salts containing TEA cation and various anions, inhibited the enzymatic activities of proteolytic enzymes such as papain, bromelain, or subtilisin when added as co-solvents in a casein hydrolysis reaction [26]. The inhibition constants were estimated to be in the micromolar to millimolar range, depending on the specificity and type of IL anion.

Lipases are the second largest group of industrially used enzymes after proteases. Therefore, efforts are being made to improve their stability and catalytic activity in order to increase the yield of the target product and minimize production costs. We tested the effect of two large series of amino acid ILs containing either the [Emim] or [Ch] cation on the hydrolytic activity of a lipase from Candida rugosa (CRL) [87]. At concentrations above 0.025 mM, both series showed a clear trend of decreasing activity, with the influence of anions ranked as follows: Leu > Trp ≥ Thr > Val ≥ Met > Ile > Gly, with the effect being stronger for the [Emim] series. Additionally, all ILs induced an increase in random coils, disordered structures, and antiparallel β-sheets [87].

On the other hand, AA-based ILs with a symmetrical tetraalkylammonium cation seem to have a positive effect on the catalytic activity of lipases. Rahman et al. reported that CRL pretreated with [N₂₂₂₂][His] or [N₂₂₂₂][Asn] showed enhanced catalytic activity in the esterification of fatty acids of different chain lengths with oleyl alcohol in hexane [88]. Both IL-coated lipase preparations showed up to 50% higher yields of the target esters compared to control reactions with the native enzyme. In addition, Xu et al. used four types of chiral [N-Ac-Pro] ILs, either as cations or anions, to chemically modify Candida antarctica lipase B (CALB) [89]. The modifications resulted in up to a 4-fold increase in hydrolytic activity, improved tolerance to organic solvents such as methanol and DMSO, and no significant effect on thermal stability. CALB modified with [Bmim] cations showed preserved catalytic activity over a wider pH range compared to the enzyme modified with [N-Ac-Pro] cations [89]. Furthermore, different binding sites and conformational changes were observed between the [Bmim][N-Ac-Pro]- and [N-Ac-Pro][Cl]-modified enzymes. In the case of [Bmim][N-Ac-Pro]-modified CALB, an increase in β-structures was observed, whereas the [N-Ac-Pro][Cl]-modified enzyme became more compact and showed an increase in α-helices [89].

2.5.2. Effect of AA-ILs on the Stability and Structure of Proteins and Enzymes

Although they show great potential, the effects of AA-based ILs on the stability and structure of proteins and enzymes have been little studied compared to those of second-generation ILs (containing classical cations and anions). So far, we have found that ILs based on charged AA anions and a choline cation, when added to an insulin solution at a 1.5:1 molar ratio, cause a rearrangement of the protein molecule [28]. However, the overall structure remains very close to that of native insulin in solution. Additionally, the thermal stability of the protein was slightly increased in the presence of [Ch]₂[Asp] and [Ch][Glu] [28]. On the other hand, in a 1% solution of [Ch][Ser], [Ch][Thr], or [Ch][Lys], type I collagen undergoes rearrangement in the polyproline type II helix region, resulting in a more uncoiled and disordered structure [90]. In contrast, 1% [Ch][Phe] causes deformation due to changes in hydrogen bonding, increased order in the protein, or environmental changes around chromophores, resulting in subtle rearrangements in protein conformation. In the presence of any of the four ILs, a slight decrease (1 to 4 °C) in the thermal stability of collagen is observed [90]. Similarly, Ch-ILs containing charged AA anions such as Arg, Lys, Asp, and Glu induce conformational changes in the Rapana thomasiana hemocyanin (RtH), resulting mainly in an increase in β-structures at the expense of disordered structures [71]. However, the content of α-helices is preserved, and the molecules appear more ordered. We also found that this modification improves the thermal stability of RtH [71]. The strong effect of the AA anion is also evident from the observed significant changes in the conformation of RtH induced by Emim-AAs [91]. Interestingly, in a medium containing [Emim][Gly], the structure of RtH remains close to that of the native protein; however, this IL significantly stabilizes the protein aggregates that are present in the solution. In contrast, AA anions with longer or branched side chains, such as Leu, Ile, Val, Met, and His, induce the unfolding of RtH, as evidenced by an increase in β-structures, resulting in a slightly more open conformation. Conversely, in a medium containing either [Emim][Trp] or [Emim][Thr], the RtH molecule becomes more compact, the structure is more ordered, and an increase in α-helices is observed at the expense of β-structures and aggregated structures. All Emim-AAs have a negative effect on the thermal stability of RtH [91].

Interestingly, molecular dynamics (MD) simulations showed that pure [Emim][Trp], [Emim][Ala], and [Emim][Met], as well as their 5% aqueous solutions, efficiently solvated the Trp-cage miniprotein [92]. Compared to protein solutions in water, no structural changes and smaller thermal fluctuations are observed in these six systems, suggesting better thermal stability. These three ILs seem to be promising candidates as co-solvents for the preservation and storage of Trp-cage proteins [92]. At the same time, [Emim][Phe] induces completely reversible conformational changes in horse heart myoglobin, transitioning from β-sheet to helix and back to β-sheet, in up to 200 μM of the IL [93]. It has been shown that the ions bind directly to the protein and change the local viscosity. This not only changes the conformation of myoglobin but also induces self-assembly of the protein molecules, ultimately leading to fibril formation [93].

Regardless of the type of AA anion, all tetramethylguanidine (TMG)-based ILs induce the thermal unfolding of azurin and red fluorescent mCherry protein [94,95]. The effect is concentration-dependent and stronger for ILs containing hydrophobic AA anions. For example, in the presence of [TMG][Ala], the thermal denaturation temperature of red fluorescent mCherry protein decreased by 23 °C [95]. In addition, MD simulations revealed strong interactions between the IL and the protein, as evidenced by the significant Stokes shifts observed in the spectra of IL–protein complexes. The strong hydrophobic interactions are responsible for the [TMG][Ala]-induced unfolding of red fluorescent mCherry, even at room temperature [95].

The effect of IL anions on the structure and stability of bromelain has been clearly demonstrated in several publications [83,84]. For example, when [Bmim][Gly] is added to an aqueous solution of bromelain, its thermal unfolding temperature decreases in a concentration-dependent manner. In addition, conformational changes are observed with increasing IL content, as evidenced by a marked decrease in fluorescence intensity and a significant red shift of the maximum at 345 nm in the fluorescence spectra. Notably, [Bmim][Br], when tested in the same concentration range, has a lesser effect on the thermal stability of bromelain and almost no effect on its secondary structure [84]. Molecular docking experiments showed that the Gly anion can dissociate existing hydrogen bonds in the protein and form up to 9–10 new strong hydrogen bonds with several regions of the polypeptide backbone of the enzyme, thus destabilizing the protein. In contrast, for [Bmim][Br], the Br anion is expelled from the protein surface and interacts with water molecules, preventing protein–water hydrogen bonding and stabilizing the protein.

It is noteworthy to mention that choline-based ILs containing Gly, Ala, Leu, and Pro anions, when added to a solution at concentrations up to 100 mM, increased the thermal stability of calf thymus DNA by up to 3–4 °C [96]. Additionally, it was shown that Bmim-ILs with the same anions enter the solvation shell of the DNA and displace water molecules from the hydration shell [97]. These ILs stabilize the double helix structure through strong electrostatic and van der Waals interactions and help to maintain the DNA conformation over the long term.

2.6. AA-ILs with Application in the Processing of Biomaterials

2.6.1. Cellulose Treatment and Processing

Structural polysaccharides, which have a complex composition and include cellulose, hemicellulose, lignin, and pectin, comprise the cell walls of crops, woods, and various plant materials [87]. Lignocellulose, the most abundant natural material, is classified as a second-generation renewable resource. For example, lignocellulose from wood residues, straws, and other plant wastes is a feedstock for bioethanol production [98]. Lignin, a highly cross-linked aromatic polymer, consists of three main phenylpropane units: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol [99]. Due to its rigid and complex structure, lignin is highly resistant to enzymatic degradation. Therefore, an essential step in the processing of lignocellulosic biomass is delignification, which facilitates the subsequent extraction of valuable chemicals. Numerous studies have shown that some ILs are excellent solvents for biomass or media for biocatalysts for cellulosic processing.

The [Ch][AAs] have been extensively studied for cellulose pretreatment with the goal of increasing biomass porosity, partially removing lignin, and disrupting crystalline cellulose. Mixtures of glycerol with [Ch][Phe], [Ch][Ser], or [Ch][Ala] can partially remove lignin, inhibit xylan loss, and increase polysaccharide conversion from pretreated wheat straw [100]. Using 50% [Ch][Ala]–glycerol, 67.6% lignin removal, 95.1% cellulose retention, 89.7% cellulose conversion, and 70.9% xylan conversion were achieved, demonstrating a pretreatment efficiency comparable to that of other solvents [100]. Interestingly, pretreatment of rice straw or sugar bagasse with 50% [Ch][AA]–water mixtures resulted in approximately 60% lignin removal for ILs containing Lys, Gly, Ala, Ser, Phe, and Pro anions [101,102]. However, for the IL with the Glu anion, the efficiency was only 13%. After the IL–water pretreatment, the enzymatic digestion of the lignocellulosic biomass showed significantly improved efficiency in the samples with the most effective lignin removal. Except for the [Ch][Glu]-pretreated samples, the sugar yields were greater than 80% for glucose and about 50% for xylose [101,102]. Similarly, Liu et al. observed a strong dependence of the solubility of rice straw lignin on the type of AA anion in a series of choline-based ILs [24]. A lower concentration of [Ch][Lys] (20%) and a short pretreatment period (90 °C for 1 h) of rice straw also resulted in high sugar yields, reaching 81% for glucose and 48% for xylose during subsequent enzymatic hydrolysis [103]. It was also shown that even in the presence of 5 mM [Ch][AA] under the conditions tested, the lignin yield remained high, ranging between 140 and 220 mg/g IL in most media [24]. At the same time, xylan yield varied between 1 and 85 mg/g IL, while glucose was less than 5 mg/g IL in all ILs, indicating selectivity. In agreement with other reports, ILs containing negatively charged AA anions such as Glu, Asp, Gln, and Asn were the poorest solvents for lignin (yielding 10–26 mg/g IL), which is attributed to the lower pH and higher viscosity of these solutions [24]. In addition, facilitated enzymatic hydrolysis was observed, with up to a 7-fold increase in glucose yield from all [Ch][AA]-pretreated rice straw and microcrystalline cellulose samples [24]. The efficiency of delignification also depends on the source of lignocellulose. For example, when [Ch][Arg] is used as the pretreatment solvent, the lignin extraction capacity decreases in the following order: rice straw (80%) ≈ wheat straw (78%) > sugarcane bagasse (73%) > corncob (68%) > eucalyptus hardwood (55%) > pine (33%) [104]. Notably, IL was recovered after eight cycles of reuse, with the efficiency of the last cycle reported to be 75% [104]. Interestingly, when using a 100% concentration of [Ch][AA], the delignification time for rice straw (with a biomass-to-ionic liquid ratio of 50:1) was longer compared to reactions in a medium containing the same diluted IL [105]. However, the glucose and xylose yields from the subsequent enzymatic hydrolysis of the residue were not improved [94]. Pakdeedachakiat et al. demonstrated a 65% increase in lignin extractability from mulberry stems pretreated with [Ch][Gly] or [Ch][Ala] compared to non-IL-pretreated controls [106]. Furthermore, lignin was readily recovered from the IL-containing media, with [Ch][Gly] yielding lignin with over 92% purity. In addition, the pretreatment increased glucose yield by 52–60% and xylose yield by 22–30% [106]. Notably, the [Ch][AAs] were not toxic to the bacterial strain Actinobacillus succinogenes, which was used as a whole-cell catalyst for subsequent succinic acid production via biomass fermentation [106]. Interestingly, the tensile strength of artificially aged paper pretreated with an aqueous solution of [Ch][Gly] was shown to be maintained [107]. This effect is attributed to the hydroxyl cross-linking of cellulose polymers mediated by the hydroxyl groups of choline and the amino groups of the AA anion of ILs [107]. In addition, [Ch][Arg] has the highest solubilizing capacity for pine, dissolving up to 19% of its mass. In contrast, [Ch][Gly] achieves only 9% solubilization but facilitates decomposition at significantly lower temperatures [108]. Although not as extensively studied as Ch-based ILs, AA-ILs with cations other than Ch have also been tested as media for the pretreatment of lignocellulosic biomass. For example, a previous study demonstrated that among various [Emim][AAs], [Emim][Gly] serves as a highly effective solvent for the delignification of bamboo biomass, resulting in a significant reduction of about 85.3% in total lignin content [109]. Similarly, Ohira et al. reported that [Bmim][Trp], [Emim][Ala], and other ILs containing ammonium, phosphonium, or pyridinium cations with most amino acid anions do not significantly dissolve or do not dissolve microcrystalline cellulose [99]. However, N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium tryptophan ([N_221ME][Trp]) was able to dissolve cellulose (5 wt%) at 100 °C [110]. A 23% dissolution of cellulose was achieved by stirring for 2 h at room temperature in a mixture of dry dimethylsulfoxide (DMSO) and [N_221ME][Ala], while mixtures with DMF or CH₃CN were less effective [111]. In contrast, mixtures of the IL with other co-solvents such as water, methanol, acetone, ethanol, CHCl₃, THF, dioxane, toluene, dichloromethane, or ethyl acetate were unable to dissolve cellulose [112]. In addition, Hamaga et al. reported significant progress in the separation of cellulose, hemicellulose, and lignin from wood biomass using a simple procedure with N-methyl-N-(2-methoxyethyl)-pyrrolidinium-2,6-diaminohexanoate lysine ([P_1ME][Lys]) [112]. This method can be used to obtain 0.131 g lignin, 0.665 g cellulose (type I), and 0.093 g hemicellulose from 1 g Japanese cedar (Cryptomeria japonica) [112]. Similarly, Dong et al. reported an 18% (w/w) yield and simplified extraction of cellulose from moso bamboo (Phyllostachys heterocycla) powder using a [P_1ME][Lys]:DMF (1:1) mixture [113]. Notably, ILs are used not only in the pretreatment of raw biomass but also in various cellulosic processing steps. For example, pretreatment of wastepaper with [Cys][NO₃] effectively removed hemicellulose and CaCO₃ [114]. The resulting material was then used to produce char characterized by a unique morphology and high specific surface area (approximately 1011.21 m²/g). Compared to charcoal from non-IL-pretreated wastepaper, it exhibited more than 40% higher capacity to extract Cd²⁺, antibiotics, and bisphenol A from wastewater [114]. Meanwhile, ILs containing 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with Asp or Pro anions are able to dissolve cellulose and efficiently form refined cellulose films by hydrogen bonding between ILs and the material [115]. The thin cellulose film based on [DBN][Pro] exhibited a favorable combination of properties, including high tensile strength, high Young’s modulus, good transparency (≈70% at 550 nm), and smooth surface morphology. Furthermore, the two-step transglycosylation of microcrystalline cellulose to produce methyl β-D-glucoside was significantly enhanced by an initial pretreatment with a [P₄₄₄₄][Gly]-DMSO mixture, yielding a 38% higher amount of target product compared to reactions without pretreatment [116]. In addition, [N_1112-OH][Gly] has been used for the pretreatment of brewer’s spent grains prior to enzymatic hydrolysis to facilitate saccharification and achieve higher yields of sugar components [117]. Chen et al. found that homogeneous solutions could be obtained by using a [Ch][Gly]–water solution to dissolve starch [118]. In addition, starch molecules remain disaggregated upon heating due to strong interactions between ions and polysaccharide molecules.

2.6.2. Biodiesel Production

Biodiesel consists of long-chain fatty alkyl esters and is produced from renewable bio-resources such as vegetable oils, animal fats, and waste cooking oils [119]. Production typically involves the process of transesterification and the subsequent separation of the products into biodiesel and glycerol (a by-product). The transesterification reaction requires a catalyst, which can be a classical catalyst such as an alkali catalyst (e.g., KOH or CaO), an acid (e.g., H₂SO₄, HCl, or HNO₃), or a lipolytic enzyme [120]. However, each of these catalysts has drawbacks related to stability, efficiency, environmental impact, or cost. In the last two decades, ILs, especially those based on AAs, have shown excellent potential in biodiesel production.

Table 1. Examples of AA-ILs with excellent transesterification activity.

Catalyst	Reaction Conditions	Biodiesel Yield	Reference
[TBA][Arg] [TMA][Arg] [Ch][Arg]	Sunflower oil: methanol (1:9 mole ratio); 6% (w/w on oil base) catalyst; 60 min; 80 °C	Fatty acid esters 98.8–99.8%	[121]
[N2222][Arg]	Soybean oil: methanol (1:10 molar ratio); 20% (w/w on oil base) catalyst; 60 min; 100 °C	Fatty acid esters 98.4%	[122]
[Ch][Arg]	Soybean oil: methanol (1:9 molar ratio); 15 % (w/w on oil base) catalyst; 60 min; 80 °C	Fatty acid esters 98.6%	[123]
[Ch][Arg]	Soybean oil: glycerol (1:2 molar ratio), 10% catalyst (based on soybean oil weight); 12 h; 100 °C	Monoacylglycerols (biosurfactant) 60%	[124]
[TMA][Arg]	Soybean oil: glycerol (1:2 molar ratio), 10% catalyst (based on soybean oil weight); 30 min; 105 °C	Monoacylglycerols (biosurfactant) 65%	[124]
[Ch][Arg] [TBA][Arg] [TMA][Arg]	Castor oil: glycerol (1:6 molar ratio); 12 % catalyst (based on castor oil); 120 min; 100 °C	Monoacylglycerols (biosurfactant) 65–83%	[125]
1-n-heptyl-3-methylimidazolium alanine [C7mim][Ala]	Soybean oil: methanol (1:23 molar ratio); 8% catalyst (based on soybean oil); 60 min; 60 °C	Fatty acid esters 40 %	[126]

summarizes the main results obtained in biodiesel production using AA-based ILs as catalysts.

The type of AA anion seems to be crucial for the catalytic activity of ILs in the transesterification reaction. In three series of ILs containing [Ch], [TBA], or [TMA] cations with the same AA anion, the tested ILs show comparable activity under identical reaction conditions [121]. In contrast, within a series of ILs with the same cation but different AA anions, the activity varies considerably. For example, while transesterification of sunflower oil in the presence of 6% [Ch][Arg] yields 99% biodiesel, replacement of the anion with Ala, Ser, Leu, Phe, His, Met, or Gly significantly reduces the yield to 0.9–6% [121]. The same trend was observed in the transesterification of soybean oil with [Ch][AA] [123]. Similarly, Wang et al. observed the highest catalytic activity for [N₂₂₂₂][Arg] within a series of [N₂₂₂₂][AA] tested in the methanolysis reaction of soybean oil [122]. The transesterification of triglycerides with methanol is a typical nucleophilic substitution reaction [121]. A correlation between the basicity of the ILs and their catalytic activity was observed, with more basic ILs acting as more effective catalysts and stabilizing the nucleophile (in this case, methoxyl anion) [121]. In addition, similar to what was observed for the transesterification reaction, the glycerolysis reaction of soybean oil is at least 10 times more effective when using a catalyst based on the Arg anion compared to ILs containing other amino acid anions [125]. The rate of transesterification and the resulting biodiesel yield also depend on the composition of the fat. For example, biodiesel production using [N₂₂₂₂][Arg] varied slightly (by up to 10%) in the following order: soybean oil > olive oil > rapeseed oil > outdated lard oil > Jatropha oil, which is consistent with the order of increasing acid values [122].

2.7. AA ILs as Media for Extraction

2.7.1. AA ILs as Media for Extraction of Plant Secondary Metabolites

Compared to classical extraction methods, many studies highlight the numerous advantages of using AA ILs as extraction media, including higher yields of target products, reduced environmental impact in terms of lower CO₂ emissions, and a 3- to 5-fold reduction in energy consumption per unit of product obtained, as well as shortened extraction times and the avoidance or minimization of the use of organic solvents [127,128]. In addition, pretreatment of plant materials with ILs can help break down the rigid cell walls, thereby increasing the efficiency of the extraction process.

For example, essential oils, scutellarin, and rosmarinic acid were extracted directly from Perilla frutescens leaves by vacuum microwave-assisted rotary hydrodistillation with up to 3-fold higher efficiency in the presence of 10% [Ch][Glu] compared to the heating jacket method using pure water [129]. Similar efficiencies for essential oil extraction were observed with 10% [Ch][Trp], [Ch][Phe], [Ch][Ala], [Ch][Pro], [Ch][His], and [Ch][Tyr]. However, these ILs were 25% to 95% less effective as solvents for rosmarinic acid.

On the other hand, [Ch][Leu] was used as a component in a K₃HPO₄-based aqueous two-phase extraction (ATPE) system for the simultaneous extraction and separation of flavonoids and pectin from mandarin peels [130]. Under optimal conditions, the yield of crude flavonoids was 20%, while the yield of purified pectin was 12.8%.

Compared to classical extraction methods such as Soxhlet extraction, solvent extraction with methanol or chloroform, and microwave extraction, the yield of compounds with antioxidant properties obtained from plants can be significantly increased by ultrasound-assisted extraction in the presence of ILs [131,132,133]. For example, Roman et al. demonstrated the use of TEA salts in the preparation of aqueous extracts from Lycopodium clavatum, Cetraria islandica, and Dipsacus fullonum using an ultrasound-assisted method [134]. Specifically, the addition of 2.5% (w/v) [TEA][Thr] increased the amount of isolated polyphenols by 5.5-fold compared to the pure water medium, and the extract exhibited a 2.4-fold higher antioxidant capacity compared to the control [134]. In addition, this method reduced the extraction time, making it more effective for thermosensitive natural compounds. Another significant improvement in the AA IL-assisted isolation of polyphenols was reported by Luo et al. [135]. They developed a new polymeric material with a porous structure based on imidazole-type bicationic (ViIm)2Cn amino acid-based ILs, which selectively adsorbs tea polyphenols. The [(ViIm)₂Cn][(L-Pro)₂)] membrane extracts 521 g of tea polyphenols per gram of material, significantly outperforming other adsorption media [135]. In addition, the polyphenols can be easily desorbed with an acid–methanol solution. The membrane was used four times without significant loss of capacity.

Finally, Ni et al. reported that, compared to Emim-based ILs with classical anions such as Br, BF₄, PF₆, or pure dimethylformamide, those with AA anions such as Ala, Gly, Lys, Phe, and Ser enhanced the distribution of α-tocopherol in the water–DMF phase [136]. A suitable partition coefficient and adequate extraction capacity for α-tocopherol mixed with methyl linoleate were achieved in the [Emim][Ala]–DMF–hexane biphasic system. High separation selectivity of α-tocopherol over methyl linoleate (up to 29) was reported under the optimized extraction conditions [136].

2.7.2. AA ILs as Media for Extraction of Amino Acids and Proteins

Chiral separation is not only crucial for ensuring the safety and efficacy of pharmaceutical drugs, but it also plays an important role in the sensory properties (taste, aroma, etc.) of foods and fragrances, as well as in the study and application of biologically active compounds in biotechnology, such as gaining a deeper understanding of the interaction between chiral compounds and enzymes, receptors, and other biological targets.

AAILs have been shown to be highly enantioselective in the extraction of amino acids [137]. Tang et al. demonstrated the use of Cu²⁺-modified [Bmim][L-Pro] or [Emim][L-Pro] for the chiral separation of amino acids via chiral ligand exchange. These AA ILs act as a solvent, an efficient and stable acceptor phase for amino acids, and a selector which, with the participation of Cu²⁺, shows a distinct enantioselectivity in the separation of racemic amino acids. For liquid–liquid extraction with [Cu(L-Pro)₂][Emim]₂, enantiomeric excess (e.e.) values were reported for four racemic amino acids in the order: Phe > Tyr > Trp > His [137].

Similarly, silica-supported [Emim][L-Pro], [Bmim][L-Pro], [Hmim][L-Pro], and [Omim][L-Pro] coordinated to Cu²⁺ effectively separated underivatized D,L-Phe, D,L-His, D,L-Trp, and D,L-Tyr by HPLC or capillary electrophoresis [138]. The enantioselectivity of these AA ILs was significantly higher than that of conventional AA ligands. Among the ILs tested, those with longer alkyl chain substituents in the cation showed superior efficacy [138]. Qing et al. demonstrated that ILs based on AA cations, such as [L-Pro][CF₃COO], [L-Pro][NO₃], [L-Pro]₂[SO₄], and [L-Phe][CF₃COO], coordinated with Cu²⁺, can be effectively used in ligand-exchange chromatography for the separation of Trp enantiomers [139]. They reported a relatively good resolution (Rs) of 1.89 between the peaks of L-Trp and D-Trp.

Similarly, ILs containing [Bmim], [Hmim], or [Omim] cations, amino acid anions, such as L-Leu, L-Val, or L-Lys, and inorganic salt show good potential for the aqueous biphasic extraction of L-Trp. Hydrogen bonding with AAILs is likely to play an important role in the phase distribution of the solute and the most effective IL for this system was [Omim][Lys] [140].

Generally, the ability of [Ch][AA]-ILs to form aqueous biphasic solutions follows the order: [Ch][L-Lys] > [Ch][L-Arg] > [Ch][L-Ser] > [Ch][Gly] > [Ch][L-His] > [Ch][L-Ala] > [Ch][β-Ala] > [Ch][L-Pro] > [Ch][L-Val] > [Ch][L-Phe] [141,142,143].

On the other hand, [Ch][AAs] are commonly mixed with a low-viscosity molecular solvent such as water to form hybrid solvents, which preserve the favorable properties of ILs while enhancing their physical properties. ATPEs based on ([Ch][AAs])₂Cu complexes have been evaluated for the chiral resolution of D, L-Val [144]. The efficiency of chiral resolution depended on the specific ILs used and was ranked as follows: [Ch][Pro] > [Ch][Ala] > [Ch][Cys] > [Ch][Met] > [Ch][His]. Under optimal conditions, D-Val was preferentially distributed in the [Ch][AA]-rich phase, reaching an enantiomeric excess (e.e.) of approximately 55.6% [144]. Consistent with this result, Sun et al. observed that among the various [Ch][AA]/K₃PO₄ ATPE systems, the [Ch][L-Pro]-K₃PO₄ system exhibited the highest separation factor for Phe enantiomers, resulting in a greater extraction of L-Phe into the top phase [145].

The ATPE system containing [Ch][Ala]–K₃PO₄–water and [Ch][Ala]–KH₂PO₄–water were evaluated for the partitioning of four N-(2,4-dinitrophenyl) (DNP) amino acids: DNP-L-Leu, DNP-L-Val, DNP-L-Ala, and DNP-Gly. A clear dependence on the type and concentration of the inorganic salt was observed. The highest partition coefficients were obtained for DNP-L-Leu in the [Ch][Ala]–K₃PO₄–water system, with the partition coefficient values following the order: Leu > Val > Ala > Gly [146].

In addition to their use in amino acid separation, AA ILs are also used in protein isolation. For instance, cytochrome C is easily extracted into the IL phase by phase separation at room temperature within 10 min using a tetrabutylphosphonium-N-trifluoromethanesulphonyl–leucine ([P₄₄₄₄][Tf-Leu])–water (1:1) system [147]. Other proteins such as lysozyme, chymotrypsin, hemoglobin, albumin, and myoglobin are also readily solubilized in hydrated [P₄₄₄₄][Tf-Leu] [147]. However, proteins such as laccase, horseradish peroxidase, and triethylene glycol-modified cytochrome C show a preference for the aqueous phase.

Interestingly, N-butyl-N-methyl-piperidinium-L-ornithine ([P_1,4][L-Orn]) showed exceptional performance as a chiral ligand in a chiral ligand-exchange capillary electrophoresis system, allowing effective chiral resolution and quantification of methionine [148].

An ATPE system consisting of polypropylene glycol 400 (PPG400) and [Ch][AA] was used to partition bovine serum albumin (BSA), with the phase formation ability of the ILs decreasing in the following order: [Ch][Arg] > [Ch][His] > [Ch][Pro] > [Ch][Val] [149]. The study showed that the salting-out effect and π-π interactions, as well as the hydrophilic/hydrophobic interactions between BSA and [Ch][AA]-ILs, are crucial for BSA partitioning [150]. Similarly, for aqueous biphasic [Ch][AA]–PPG–water systems, an increased affinity of BSA or trypsin towards the IL phase was observed with decreasing hydrophobicity of the AA [142]. The pH-adjustable salting-out effect caused by the IL-rich phase at pH < pI [AA] can also enhance the hydrophobic interaction between the protein and PPG polymer, thus promoting the partitioning of protein into the PPG-rich layer [142]. BSA is also preferentially transferred to the IL phase in the [Ch][AA]–polyethylene glycol di-methyl ether 250 (PEGDME250)–water system [149,150].

2.8. AA IL Application in Wastewater Treatment and Separation Processes

In recent years, ILs, including those based on AAs, have shown significant potential for the decontamination of industrial wastewater and sludges [151,152,153]. They can be used for the adsorption and extraction of heavy metals and organic pollutants, as well as for desalination [154]. IL-functionalized membranes further increase purification efficiency. In addition, some ILs possess inherent antimicrobial properties, making them valuable for disinfection, preventing biofouling in water systems, and inhibiting bacterial growth by disrupting microbial cell membranes.

Examples of the excellent performance of AA ILs in the removal of water contaminants are summarized in

Table 2. Applications of AA ILs in contaminant isolation, wastewater purification, and separation of target materials.

Method/AA IL Used	Pollutant/Target	Achieved *	References
ATPE (25 °C; 0.1 MPa) [Ch][L-Gly]–K2HPO4–water (*) [Ch][L-Ala]–K2HPO4–water [Ch][L-Ser]–K2HPO4–water	extraction of amoxicillin	extraction efficiency (E%) = 97 ± 2; partition coefficient (K) > (16 ± 6)101	[155]
ATPE (25 °C; 0.1MPa) [Ch][L-Gly]–K2HPO4–water [Ch][L-Ala]–K2HPO4–water (*) [Ch][L-Ser]–K2HPO4–water [Ch][L-Ala]–K3HPO4–water [Ch][L-Leu]–K3HPO4–water (**)	extraction of acetaminophen	(*) E (%) =97.5 (**) E (%) =97.5	[156]
IL-assisted ligand-exchange HPLC C18 column, eluent IL, and 3.0 mM Cu(II) were dissolved in methanol/water (20/80, v/v) at a flow rate of 0.5 mL/min, where IL is [Emim][Leu][Bmim][Ala], [Bmim][Val], [Bmim][Leu] (*), [Bmim][Ser], [Bmim][Ph-Ala], and [Omim][Leu]	enantioselective resolution of (S, R) ofloxacin	(*) separation within 14 min; enantioseparation factor (α) = 1.34; resolution (Rs) = 1.63	[157]
extraction from model oil [C2mpyr][Gly] (*) [C2mpyr][Ala] (*) [C2mpyr][Ser] [C2mpyr][Pro]	extraction of naphthenic acid	(*) E (%) = 98; recovery of the naphthenic acid: 70–80%; 4 cycles without any loss of extraction capacity	[75]
solid-phase extraction (SPE) magnetic nanoparticles (Fe3O4@SiO2) grafted with-1-octyl-3-octenyl-benzimidazolium glycinate, [C8obim][Gly]	extraction of benzimidazoles	trace detection in plasma and juice	[158]
SPE and HPLC poly-{3-(3-(7-(diethylamino)-2-carbonyl-2 h-amino-3-propyl)-1-ethyl bromo imidazole} P[Phe][EMI]	extraction of Allura red	limit of detection (LOD) = 0.004 μg/mL (in food); recovery (%) 92–102.	[159]
SPE and HPLC poly-{1-ethyl-3-benzyltriethylammonium alanine P[ETA][Ala]}	extraction of sunset yellow	LOD = 0.01 μg/mL (in beverages); recovery (%) 92–110	[160]
ATPE and subsequent HPLC-UV detection [Ch][Gly]-K3PO4 [Ch][Ala]-K3PO4 (*) [Ch][Lys]-K3PO4 [Ch][Arg]-K3PO4	extraction of sunset yellow	(*) LOD = 0.021 μg/mL (in beverages); recovery 87.04–101.21% (in beverages)	[161]
extraction from model oil [N2222][L-Ala] (*), [N2222][Gly], [N2222][L-Pro], [N2222][Sarcosine], [N2222][L-Lys], [N2222][L-Arg], [N2222][L-Val]	extraction of indole	(*) E (%) = 98; separation within 5 min	[162]
oxidation–extraction from model oil [Omim][Phe] (*) [Omim][His], [Omim][Gly], [Omim][Trp]	extraction of dibenzothiophene	(*) desulfurization: 97.4% first cycle; 93.4% eighth cycle	[163]
ATPE (25 °C; 0.1MPa) [Ch][Ala]-K3PO4 [Ch][Gly]-K3PO4 [Ch][Lys]-K3PO4 [Ch][Leu]-K3PO4 (*)	extraction of naphthalene and pyrene	(*) recovery (%) naphthalene: 98.9 (lake water) and 95.2 (tap water); pyridine: 103.1% (lake water) and 107.8 (tap water)	[164]
SPE and HPLC magnetic graphene oxide grafted with [Bmim][Trp]	extraction of Sudan I–IV	high-affinity extraction on the IL-modified adsorbent; 10-fold enrichment factor; LOD=1.0 ng/mL	[165]
chiral liquid–liquid extraction system CH2Cl2-[Bmim][Trp]	enantioselective extraction of metoprolol	(S)-metoprolol, enantioselectivity (pH 8.5, 25 °C) = 1.29	[166]
chiral liquid–liquid extraction system CH2Cl2-[Bmim][L-Trp] (*) CH2Cl2-[Bmim][L-Glu] CH2Cl2-[Bmim][L-Ser] CH2Cl2-[Bmim][L-Phe]	enantioselective resolution of flurbiprofen	(S)-flurbiprofen maximum α = 1.20; 5 cycles of reuse	[167]
nanofiltration nanofiltration membrane grafted with 1,1,1-trimethylhydrazinium glycinate [αN111][Gly]	pigment adsorption	higher selectivity for Na2SO4 and NaCl than commercial DK and DL nanomembranes	[168]
extraction totuene/AA ILs (1:2) [Ch][Gly] [Ch][Pro] [Ch][His] (*) [Ch][Ser] [Ch][Phe]	extraction of asphalt from carbonate rocks	(*) 91% recovery of asphalt in a single step	[169]
extraction [N2222][L-Ala] (*) [N2222][L-Phe] N2222][Gly] [N2222][L-Pro] [N2222][L-Sar]	extraction of phenol-contaminated oil	(*) E (%) = 99 for phenol (model system toluene–phenol); E (%) from 97.9 to 98.8%; the rest of the AA ILs is between (*) 98.6% phenol extraction efficiency (real coal tar oil mixture)	[170]
microemulsion extraction [Trp][BF4]–urea	extra-heavy oil extraction from carbonate asphalt rocks	11% enhanced oil recovery in comparison to control	[171]
chiral ligand-exchange capillary electrophoresis (CE) and ligand-exchange micellar electrokinetic capillary chromatography Cu (II)-tetramethylammonium L-hydroxyproline [TMA][L-OH-Pro]	enantioselective resolution of AAs	Rs = 3.03 for Trp; Rs = 4.35 for 3,4-dihydroxy- phenylalanine	[172]
chiral ligand-exchange HPLC [Bmim][L-Ile]	enantioselective resolution of mandelic acid and its derivatives	Rs between 1.15 and 2.16 depends on the substituents	[173]
ligand-exchange chiral separations (CE and HPLC) Cu (II)-[1-alkyl-mim][L-Pro]	enantioselective resolution of D, L-Phe, D, L-His, D, L-Trp, and D,L-Tyr	Rs=3.26–10.81 for HPLC; Rs=1.34–4.27 for CE	[138]
reactive extraction [N4441][Ser] [N4441][Thr] [N4441][Cys] [N4441][Pro] [N4441][Lys] (*) [N4441][Val] [P4444][Ser] [P4444][Lys] (**) [P4444][Thr] [P4444][Pro] [P4444][Cys]	removal of naphthenic acid from crude oil	for [N4441][AA] series, % naphthenic acid removal is in the range 21–46% (*) 46%; for [P4444][AA] series, % naphthenic acid removal is in the range 27–41% (**) 41%.	[174]
ATPE [Ch][L-Met] [Ch][Gly] [Ch][L-Ala] [Ch][L-Pro]	gold(I) recovery from aurocyanide wastewater	E (%) = 99 for Au (I)	[175]
magnetic dispersive SPE Fe3O4@SiO2@MAPs@AAIL-POSS based on: [C8obim][Gly]	benzimidazole residue analysis in fruit juice and human serum	E (%)> 97 10 recovery cycles of the adsorbent	[158]
membrane adsorption Nylon 6,6/ChOH-Gly (1:5) Nylon 6,6/ ChOH:Gly (1:2) (*)	Cu (II) removal	(*) highest permeability of the membrane; 80% Cu(I) removal until steady state reached	[176]
mobile phase for ion exchange chromatography [Emim][Gly] [Bmim][Gly] [Hmim][Gly] (*) [Hmim][Ala] [Hmim][Asp]	analysis of Mg2+, Ca2+, Zn2+, and Fe3+ in oral solution and water	(*) good resolution and sensitive quantitative determination; (*) linear range: 0.01–50 mg/L, except for Ca2+, which is 0.1–100 mg/L; (*) LOD in the range of 0.57–3.58 µg/L	[177]
synchronous fluorescence spectrometry [Omim][Ala]	simultaneous determination of Magnolol and Honokiol	LOD =1.46 ng/mL (Magnolol), LOD = 0.92 ng/mL (Honokiol); the recovery rates: 98.6–100.7% (Magnolol) and 99.7–100.6% (Honokiol)	[178]
capillary electrochromatography poly(glycidyl methacrylate-co-ethylene dimethacrylate)monolithic column modified with [N1111][Arg] and graphene oxide	separation ability for amino acids, β-blockers, and nucleotides	Rs: Trp, Phe, 5-OH-Trp: 2.231 and 2.036; β-blockers: 2.779 and 2.470; and nucleotides: 8.345 and 3.321	[179]
chiral ligand-exchange capillary electrophoresis Zn(II)-[Bmim][L-Lys] Zn(II)-[Hmim][L-Lys] (*) Zn(II)-[Omim][L-Lys]	resolution of mixtures of D,L-Dns-Ile, D,L-Dns-Met, and D,L-Dns-Ser; resolution of D-Dns-Ile, D-Dns-Met, and D-Dns-Ser	(*) excellent Rs at the optimum conditions	[180]
chiral ligand-exchange capillary electrophoresis Cu(II)-[L-Pro][CF3COO] (*) Cu(II)-[L-Pro][NO3] Cu(II)-[L-Pro][BF] Cu(II)-[L-Pro2][SO4]	resolution of mixtures of Dns-D,L-AAs	nine pairs of labeled D,L-AAs were successfully separated with Rs ranging from 0.93 to 6.72	[181]
chiral ligand-exchange capillary electrophoresis Zn(II)-[Bmim][Orn] (L-ornithine)	Resolution of Dns-D,L-AAs enantiomers; screening of D-AA oxidase inhibitors	Rs values range from 0.53 to 3.53	[182]
Capillary electrophoresis Glycogen–[N1111][Arg] (*) Glycogen–[N1111][Asp]	Resolution of racemic mixtures of either drug: Nefopam hydrochloride, citalopram hydrobromide, or duloxetine hydrochloride	Rs = 2.13 for Nefopam; Rs = 3.12 for citalopram; Rs = 5.73 for duloxetine	[183]
chiral ligand-exchange capillary electrophoresis Zn(II)-[L-Epy][L-Lys] (*) Zn(II)-[L-Bpy][L-Lys] Zn(II)-[L-Hpy][L-Lys] Zn(II)-[L-Opy][L-Lys]	enantioseparation of Dns-D, L-AA (such as Ala, Asp, Asn, Ile, Met, Phe, Ser, Thr, Tyr, and Trp)	the Rs values are in the range of 0.8–3.9	[184]
non-aqueous capillary electrophoresis β-cyclodextrin-[N1111][L-Arg] β-cyclodextrin-[N2222][L-Arg] β-cyclodextrin-[N3333][L-Arg]	enantioseparation of Dns-D,L-AAs	the Rs values are in the range of 1.31–3.2	[185]
capillary electrophoresis Cu(II)-[N1111][L-Arg] (*) Cu(II)-[N1111][L-Pro] Cu(II)-[N1111][L-Glu]	enantioseparation of Dns-D,L-AAs	the Rs values are in the range of 0.86–5.51	[186]
magnetic solid-phase extraction PEG4000-nanoFe3O4-[Glu][TMG]	trypsin isolation	extraction capacity 718 mg/g; at least 5 cycles without loss of capacity	[187]

(*) Results obtained under the optimal conditions for the study. (**) Selected results obtained under the optimal conditions for the study.

.

2.9. AA ILs as Catalysts

AA ILs have remarkable catalytic properties. The amino group (-NH₂) or the carboxylate group (-COO-) in these ILs can act as Lewis bases because the carboxylate anion is electron-rich and can coordinate with metal ions or other electron-deficient species. This property allows AA ILs to coordinate with metal catalysts in catalytic cycles and to activate substrates such as CO₂, where the lone pairs on nitrogen or oxygen facilitate such interactions. The carboxylate group (-COO-) in the anion of these ILs is also a strong Brønsted base capable of deprotonating acidic substrates. Zwitterionic forms of amino acids, when paired with suitable cations, retain basic properties and act as proton acceptors. Such ILs are effective catalysts for reactions requiring proton abstraction, such as aldol condensations and Knoevenagel reactions. In addition, they can neutralize acidic by-products in organic synthesis. AA ILs containing a protonated amine group (-NH₃⁺) in the amino acid-derived cation can function as Brønsted acids. Functionalized ILs with acidic groups, such as sulfonic acid (-SO₃H), exhibit enhanced Brønsted acidity. These ILs are particularly useful for acid-catalyzed reactions, including esterification, hydrolysis, and cyclization. They also provide acidic media suitable for biocatalysis and polymerization reactions. Numerous studies have demonstrated the excellent catalytic activities of AA ILs in a wide range of reactions, including esterification, hydrolysis, carbon–carbon bond formation, cyclization, oxidation, asymmetric synthesis, the cycloaddition of CO₂, and reduction (see

Table 3. Catalytic application of AA ILs.

AA IL Catalyst	Reaction/Mechanism	Advantages of the AA IL Catalyst	References
[pyridine-N-sulfonyl glycine][Cl]	Biginelli reaction; Knoevenagel condensation; media: water, at reflux temperature	Target product(s): xanthenediones and dihydropyrimidinones; remarkable catalytic activity, water as a solvent, high yields, short reaction time	[188]
AA IL solvent	intermolecular Knoevenagel reaction; media: toluene, reflux	Target product(s): caulerpin (61 % yield) 50% higher yield than that obtained with conventional catalysts	[189]
[Pro][H2PO4]	Biginelli reaction (aminoalkylation)	Target product(s): aminoalkyl naphthols the yield of the derivatives is within the range of 84–97%	[190]
[ValOLauryl]2[CuCl4] [PheOLauryl]2[CuCl4]	cross-dehydrogenative coupling reaction (C-C bond formation); media: water	Target product(s): propargyl amines (up to 94 % yield)	[191]
[L-Pro][NO3]	cyclocondenzation reaction	Target product(s): 5-aryl-1,2,4-thiazolidine-3-thiones the yield of the derivatives is within the range of 80–95%; 5 cycles of catalyst reuse without significant loss of activity	[192]
[Emim][Boc-AAs]	solid-phase peptide synthesis; carbonate–pyridine–acetate-mediated peptide synthesis	Target product(s): dipeptides the yield of the dipeptides is within the range of 89–95% with all catalysts, except for [Emim][Boc-Asn], for which it is only 10%; 5 cycles of catalyst reuse without significant loss of activity	[193]
[N1111][Arg] (*), [N1111][Ala], [N1111][Phe], [N1111][Trp], [N1111][Ser], [N1111][Asn], [N1111][Cys]	reaction of polycondensation	Target product(s): poly(isosorbite carbonate) (*) carboxymethyl intermediates greater than 99%	[194]
[Ch][Pro]	self-condensation reaction	Target product(s): deoxyfructosazine or fructosazine 33.8% yield of deoxyfructosazine for 30 min/100 °C 16.7% fructosazine for 30 min/100 °C	[195]
[L-Pro][NO3]	Knoevenagel condensation, Michael-type addition, intramolecular cyclization, and imine–enamine tautomerism	Target product(s): 5-benzylidene-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione or pyrano[2,3-d] pyrimidine diones the yield of the derivatives is within the range of 86–94%	[196]
[ProH][CH3COO], [ProH][H2PO4], [ProH][CF3COO], [ProH][NO3], [ProH][HSO4], [ProH][Cl], [ProH][CF3SO3], [GluH][CF3SO3], [AspH][CF3SO3], [TyrH][CF3SO3], [TrpH][CF3SO3], [PheH][CF3SO3], [AlaH][CF3SO3], [ProH][CF3SO3] (*)	Baeyer–Villiger oxidation	Target product(s): β-valerolactone (*) cyclopentanone conversion 98.55%, β-valerolactone yield 64.51%; selectivity 65.46% (*) 4 cycles of catalyst reuse without affecting conversion % and selectivity %	[197]
[Pro][HSO4] [Gly][HSO4]	isobutane C4 alkylation	worse quality of the products compared to conventional catalysts like H2SO4	[198]
[Emim][Lys], [Emim][Thr], [Emim][Val], [Emim][Ala], [Emim][Ser], [Emim][His], [Emim][Asp]	catalytic copolymerization	Target product(s): poly(isosorbide carbonate) the yield of the polymers is within the range of 91–99%	[199]
[Gly][Cl] (*), [Leu][Cl], [Thr][Cl], [Phe][Cl], [Asp][Cl], [Ala][Cl], [Val][Cl], [Lys][Cl]2, [Gly][HSO4]	dehydration reaction	Target product(s): 3-acetamido-5-acetylfuran (*) 52.61% yield; fast 10 min reaction	[200]
[Mor1,4][AcGly] [Mor1,6][AcGly] [Mor1,8][AcGly] [Mor1,10][AcGly] [Mor1,12][AcGly] (*)	N-alkylation- and C-alkylation reactions of dibenzoazepine; C-alkylation reaction of fluorene derivatives	Target product(s): (*) for the synthesis of N-butyl- dibenzazepine, it has the same efficiency as [N4444][Br] (*) a 15% higher yield of 9,9-di-n-butyl-2,7-dibromofluorene was obtained from fluorene compared to using a phase-transfer catalyst	[201]
[Bmim][His] (*), [Bmim[Ala], [Bmim][Phe], Bmim][Asn], [Bmim][Ser], Bmim][Met], [Bmim[Gly], [Bmim][Pro], [Bmim][Br]	thiol-Michael addition	Target product(s): S-(2-nitro-1-phenylethyl)-L-Cysteine (*) the highest kcat (1695 M−1 s−1)	[202]
3-(Tri-ethylammonium)propan-1,2-diol [[N222,PDO]-based [N222,PDO][L-Val] [N222,PDO][L-Leu] [N222,PDO][L-Pro] (*) [N222,PDO][L-His] [N222,PDO][L-Tyr] N-((1,3-Dioxolan-4-yl) methyl)-N,N,N-tri-ethylammonium [N222,1,3-Dioxane-4yl-met]-based [N222,1,3-Dioxane-4yl-met][L-Val] [N222,1,3-Dioxane-4yl-met][L-Leu] [N222,1,3-Dioxane-4yl-met][L-Pro] [N222,1,3-Dioxane-4yl-met][L-His] [N222,1,3-Dioxane-4yl-met][L-Tyr	Morita–Baylis–Hillman reaction	Target product(s): 3-[Hydroxy(4-nitrophenyl)methyl]but-3-en-2-one and analogs (*) 1st cycle—68% yield for 4 h; 5th cycle—60% yield for 96 h	[203]
[N4444][Asp] supported on metal–organic framework	one-pot three-component reaction: deprotonation, addition reaction, dehydration, nucleophilic addition, automerization, and intermolecular cyclization	Target product(s): 1,4-dihydropyrano[2,3-c]pyrazoles the yield of the derivatives is within the range of 88–96%	[204]
[L-Pro][NO3]	thia-Michael addition for C-S bond formation; conjugate addition of the thiol to sulfonamide chalcones for the synthesis of b-sulfidocarbonyl compounds	Target product(s): sulfonamide chalcones the yield of the derivatives is within the range of 78–92%	[205]
[L-Pro][NO3]	Biginelli reaction	Target product(s): 3,4-dihydropyrimidin-2(1H)-thiones the yield of the derivatives is within the range of 86–92%	[206]
[N4444][L-Leu] [N4444][L-Val] [N4444][L-Ile] [N4444][L-Thr] [N4444][L-His] [N4444][L-L-Met] [N4444][L-Tyr [N4444][L-Trp]	Knoevenagel condensation	Target product(s): 2-benzylidenemalononitrile the yield of the derivatives obtained for 30 min is within the range of 74–89% 6 cycles of reuse of catalysts without significant loss of activity	[207]
[N1111][Pro] (*) [N1111][Gly] [N1111][Ala] [N1111][Val]	hydrolysis	Target product(s): benzaldehyde (*) 94% yield	[208]
1-(2-aminoethyl)-3-methyl-imidazolium [Aemim][Asp]	condensation reaction	Target product(s): acetophenoximes the yield of the derivatives is within the range of 85–96%	[209]
[N4444][D, L][Thr]-salicylaldehyde-Cu(II) complex	A3 coupling reaction	Target product(s): propargylamines the yield of the derivatives is within the range of 39–95%	[210]
[N2222][Pro]	one-pot synthesis: transesterification and elimination reaction	Target product(s): 68% yield of glycidol	[211]
[N2222][Pro] (*), [N2222][Gly], [N2222][Ala], [N2222][Val], [N2222][Ser]	transesterification reaction	Target product(s): (*) 72% yield of dibutyl carbonate	[212]
[P4444][Arg]	cyclocondensation	Target product(s): 2,4(1H, 3H)-diones the yield of the derivatives is within the range of 87–96% 5 times of reuse of catalyst without significant loss of activity	[213]
[P4444][Pro], [P4444][Val], [P4444][Ala], [P4444][Gly], [P4444][Ser]	Knoevenagel condensation reaction	Target product(s): α,β-unsaturated cyano derivatives the yield of ethyl-2-cyano-3- phenylacrylate is within the range of 81–96% [P4444][Pro] is the most active 6 cycles of reuse without significant loss of activity	[214]
[Pro][HSO4], [Gly][HSO4], [Ala][HSO4]	esterification	Target product(s): 99.9% yield of ethyl valerate	[215]
[P4444][Z-Asp]	enzymatic synthesis of a model peptide, Z-APM	the IL is both media and substrate much higher productivity compared with conventional IL reaction media for proteases	[216]
[Gly][NO3] (*), [Gly][SO4] [Gly][Cl]	Biginelli condensation	Target product(s): 3,4-dihydropyrimidin-2(1H)-ones (*) 92% yield of dihydropyrimidinone	[217]
[ProN4,4][Br]	cycloaddition reaction	Target product(s): cyclic carbonate 83% yield of styrene carbonate; 98% selectivity	[218]
[N2222][Pro] [N2222][ OH-Pro] [Emim][Pro] [Emim][OH-Pro]	click reaction	Target product(s): 1,4-disubstituted 1,2,3-triazoles the yield of the derivatives is from traces to 99%	[219]
[Emim][Pro]	Michael additions of cyclohexanones	Target product(s): chalcones the yield of the derivatives is within the range of 85–98%; moderate to good enantioselectivity (16–94%)	[220]
[Bmim][Pro] supported on polystyrene	N-arylation/ Buchwald–Hartwig amination	Target product(s): N-(4-methoxyphenyl)imidazole derivatives the yield of the derivatives is from traces to 97%	[221]
p-toluenesulfonic acid ionic liquid ([Lys][p-TSA])–lipase system	degradation of poly(lactide)/poly(butylene adipate-co-terephthalate	weight loss of the blend at optimal conditions reached 31.59% after 49 days, which was 3.79 times higher than that of the PLA/PBAT blend without [Lys][p-TSA]	[222]
[Bmim][Pro]	degradation of polyethylene terephthalate	Target product: bis(2-hydroxyethyl terephthalate – 75.3% yield; 100.0% conversion of the starting material	[223]
[Ch][Met] [Ch][Ala] [Ch][Gly] [Ch][Pro] (*)	hydrolysis of paraoxon (organophosphorus pesticide)	(*) τ1/2 = 19.8 min	[224]
[Ch][Gly]	glycolysis of poly(ethylene terephthalate)	Target product: bis-hydroxyethyl terephthalate – 51% yield; 85% conversion of the starting material	[225]

* The highest yields, obtained at optimal reactions are shown.

). These ILs are environmentally benign compared to conventional catalysts, with many showing good stability and activity over several consecutive catalytic cycles.

2.10. Other Applications

2.10.1. Utilization and Capture of Toxic Gases

The main strategies for eliminating CO₂ SO₂, NO₂, and other toxic gases using ILs include physical absorption and chemically reactive absorption [226,227]. In the latter, the IL interacts with CO₂ to form reversible compounds such as carbamates or bicarbonates. Additionally, ILs can act as catalysts to incorporate CO₂ into valuable compounds via carboxylation and cycloaddition reactions, as demonstrated in the examples mentioned above. ILs can also be used as electrolytes in the electrochemical reduction of CO₂ to produce fuels such as methane, ethylene, or alcohols. Some examples from the literature are given below.

Pure AA ILs based on tetraalkylphosphonium cations and AA ILs, such as [P₁₁₁₁][Gly], [P₄₄₄₄][Gly], [P₄₄₄₂]₂[Ser], [P₄₄₄₄][Ala], [P₄₄₄₄][Met], [P₄₄₄₄][Ile], [P₆₆₆₁₄][Gly], [P₆₆₆₁₄][Met], and [P₆₆₆₁₄][Pro], exhibit excellent CO₂ absorption capacity [228,229,230]. Cations with longer alkyl chain substituents exhibit an 11% increase in absorption capacity. Interestingly, [P₆₆₆₁₄][Met] and [P₆₆₆₁₄][Pro] absorb CO₂ in an almost 1:1 stoichiometry [231]. However, due to the very high viscosity of these AA ILs, SO₂ absorption is hindered, resulting in some selectivity for CO₂ over SO₂ [227]. Among the series of pure [Ch][AAs], glycinate appears to be the most effective sorbent for CO₂ [232].

High CO₂ absorption capacity and stable regeneration are achieved with aqueous biphasic systems containing AA ILs based on tetraalkylammonium and 1,3-disubstituted imidazolium cations, such as Emim, 1-propyl-3-methylimidazolium, and 1-vinyl-3-methylimidazolium [233,234,235,236,237,238,239,240,241,242,243]. Numerous studies have demonstrated enhanced absorption capacity and over 90% regeneration efficiency for [N₁₁₁₁][Gly] and [Bmim][Lys] when blended with aqueous solutions of amines or amino alcohols, including N-methyldiethanolamine and 2-amino-2-methyl-1-propanol [244,245,246,247]. Furthermore, mixed solutions of monoethanolamine (MEA) with [C_2-OHmim][Gly] or [MEA][Gly], as well as more complex systems such as diethylenetriamine-serine ionic liquid/polyethylene glycol dimethyl ether/water, also exhibit high CO₂ absorption capacity [248,249,250].

Good cyclic adsorption–desorption performance has been reported for various [Emim][AAs], [P₄₄₄₄][AA], [Bmim][Gly], and [N₂₂₂₂][Gly], supported on meso-, micro-, and nanoporous silica materials such as SBA-15, MCM-41, MCM-48, KIT-6, and zeolites, including silica particles grafted with polymethylmethacrylate, polydivinylbenzene, and other materials [251,252,253,254,255,256,257,258,259].

It is noteworthy that [P₄₄₄₄][AAs] and [Ch][AAs], when grafted onto thin film composites or membranes based on cellulose acetate, organometallic frameworks, Teflon, polyacrylamides, poly(ethylene oxide), and other materials, exhibit high permeability and exceptional selectivity for CO₂ over N₂ [260,261,262,263,264,265,266]. On the other hand, a system containing low-molecular-weight (200–400 Da) polyethylene glycol and [P₄₄₄₄][Ala] [P₄₄₄₄][Gly] or [P₄₄₄₄][Pro] is highly selective for CO₂ over H₂ [267].

AA ILs, which operate via a chemically reactive adsorption mechanism, have shown intriguing potential for CO₂ utilization. Ultrathin metal–porphyrinic metal–organic framework nanosheets, functionalized with [Bmim][Glu] or [Bmim][His], have been shown to facilitate substrate-specific cycloaddition of CO₂, yielding cyclic carbonates in 84–99% under mild conditions (solvent-free, 80 °C, 1 atm CO₂) [268].

Paired mesoporous poly(ionic liquids), based on 3-aminoethyl-1-vinylimidazolium cations copolymerized with divinylbenzene and Glu, Asp, Ala, or Gly anions, exhibited high catalytic efficiency in the cycloaddition of CO₂ to aziridines, resulting in high yields (>95%) and regioselectivity (>99%) for 5-substituted oxazolidinones [269].

High yields (96–99%) of cyclic carbonates obtained under solvent-free and halogen-free conditions have also been reported for [Bmim][Asp], [Bmim][Glu], [Mor_1,8][Glu], [HiQuin][Asp], [N₄₄₄₄][His], [N₄₄₄₄][Asp], [N₄₄₄₄][Glu], methoxylbutyl-3-methylimidazolium (MOBIm) [Arg], [MOBIm][Gly], and others [270,271,272].

On the other hand, [Ch][Pro] and [Ch][Gly] have been shown to support the fixation and formation of ammonium carbamate and ammonium carbonate [273]. Additionally, [Ch][Arg], [Ch][His], [Ch][Tyr], [Ch][Glu], [Ch][Gln], and [Ch][Pro] demonstrated the ability to undergo at least five cycles of CO₂ chemical sorption without significant loss of capacity [274].

2.10.2. Electrochemistry

ILs play a crucial and versatile role in electrochemistry due to their unique physicochemical properties. Among them, AA ILs are particularly attractive due to their safety profile. With high conductivities, wide electrochemical stability windows, low volatility, and non-flammability, AA ILs such as [Ch][AAs], [Emim][AAs], and 1-butyl-3-vinylimidazolium (VilimC₄)-based ILs such as [VilimC₄][His], [VilimC₄][Lys], [VilimC₄][Ser], and [VilimC₄][Gly]—used either in pure form or mixed with organic solvents—have found applications as electrolytes in energy storage devices such as supercapacitors and batteries [275,276,277,278,279].

AA ILs also show great promise in electrocatalysis due to their ability to stabilize reaction intermediates. For example, in CO₂ reduction reactions, they act as both a reaction medium and a co-catalyst. Furthermore, their ability to dissolve various substances and form stable electrode interfaces enhances the performance of electrochemical sensors.

In bioelectrosensing, AA ILs are particularly effective because they can specifically interact with target molecules (e.g., glucose, CO₂, etc.), thereby improving sensitivity and selectivity. For example, an electrochemical biosensing platform combining [Bmim][Pro] with a carbon nanotube composite provides excellent stabilization for glucose oxidase, maintaining high enzyme activity toward glucose with a linear range of 0.05–0.8 mM and a detection limit of 5.5 µM [280]. On the other hand, a sensitive detection of ammonia (lower limit 1 ppm) at room temperature has been reported for a device based on a cellulose membrane with incorporated [Emim][Pro] [281]. In addition, N-doped graphene quantum dots assembled on [Hmim][Trp] micelles construct a novel fluorescence “turn on/off” probe for the detection of Cu(II) and levodopa [282]. Under optimized conditions, Cu(II) and levodopa exhibited strong linear relationships within the concentration ranges of 0.051–16.00 μg/mL and 0.55–5.00 μg/mL, respectively, with LODs of 0.017 μg/mL and 0.16 μg/mL. Interestingly, poly-1-vinyl-3-butylimidazolium glutamate acid serves as a stable up-conversion fluorescence probe for H₂O₂-regulated Fe(II)/Fe(III) speciation. The LODs for Fe(II) and Fe(III) were reported to be 6.16 ng/mL and 4.48 ng/mL, respectively [283].

The suitability of AA ILs for bioelectrochemical applications, such as biosensors and biofuel cells, is further enhanced by their ability to improve enzyme stability and facilitate electron transfer. In addition, AA ILs allow precise control of the deposition of metals and alloys, enabling the production of nanostructured or smooth films. Their complexing abilities play a key role in optimizing deposition processes.

Certain AA ILs also exhibit significant anticorrosion properties. For example, 1-cyanomethyl-3-vinylimidazolium derivatives such as [CyVIm][Trp], [CyVIm][His], and [CyVIm][Pro] effectively inhibit the corrosion of mild steel in neutral media [284], while [Ch][Phe] prevent the corrosion of mild steel in 1 M HCl [285]. Similarly, [Omim][Pro] acts as an effective anticorrosive, protecting mild steel in 0.5 M H₂SO₄ and inhibiting copper corrosion in 3.5% NaCl solution by over 90% [286,287]. Similarly, [Bmim][Lys] has been shown to inhibit the corrosion of carbon steel in 5% H₂SO₄ [288].

2.10.3. Polymeric Materials for Electronic Devices and Materials for Nonlinear Optics

AA ILs hold great promise in nonlinear optics (NLO) due to their customizable molecular properties, excellent stability, and role as sustainable alternatives for advanced photonic applications [289]. Their potential lies in bridging the gap between functional materials and green technologies.

The advantages of AA ILs as NLO materials include their asymmetric structures, strong polarizability, high dipole moments, exceptional thermal stability, and their ability to stabilize organic and inorganic chromophores. These properties prevent chromophore degradation and enhance optical performance. Additionally, their ionic lattice structures amplify the local electric field, thereby boosting nonlinear optical interactions.

Furthermore, AA ILs can form molecular aggregates or self-assemble, which further enhances cooperative optical effects, making them highly valuable for NLO applications.

2.10.4. AA ILs as Heat Transfer Fluids

Heat transfer ILs are gaining attention as alternatives to conventional heat transfer fluids such as water, oils, and molten salts due to their unique properties [290,291,292]. These include exceptional thermal stability (often exceeding 300 °C), a wide liquid range with extremely low melting points (some below −50 °C), and high boiling points that allow them to remain liquid over a wide temperature range. In addition, heat transfer ILs have low vapor pressure, high thermal conductivity, and low flammability, making them safer and more efficient for various applications. They are used in solar systems, cooling systems, thermal energy storage, and other advanced heat transfer technologies.

The first AA-based ILs for future application as heat transfer media are 1-ethyl-4-butyl-1,2,4-triazolium acetylglycine [Taz(2,4)][Acgly], 1-ethyl-4-butyl-1,2,4-triazolium acetylalanine [Taz(2,4)][Acala], and 1-ethyl-4-butyl-1,2,4-triazolium acetylcysteine [Taz(2,4)][Accys] [293]. Compared to several common commercial heat transfer fluids, these ionic liquids exhibit superior heat storage density (∼2.38 MJ cm⁻³-K⁻¹) and thermal conductivity (∼0.197 W-m⁻¹K⁻¹). Interestingly, the addition of a small amount of water reduces their viscosity without affecting other thermophysical properties [293]. It has also been reported that [Ch][Trp] and [N₂₂₂₂][Trp] significantly influence the phase transition behavior of biocompatible polymers [294]. For example, the critical solution temperature of poly(N-vinyl caprolactam) in water is about 32 °C. This temperature can be precisely adjusted by the addition of ILs; for example, in a medium containing a certain amount of [N₂₂₂₂][Trp], the critical solution temperature increases to 34 °C, while in the presence of [Ch][Trp], it increases to 36 °C [294].

2.10.5. AA ILs as Surfactants

AA IL surfactants are particularly valuable in green chemistry and advanced materials due to their customizability and environmental benefits. They find applications as emulsifiers in oil–water systems for cosmetics, pharmaceuticals, and enhanced oil recovery; as templating agents for the preparation of nanomaterials and mesoporous structures; as advanced detergents; and as stabilizers for dispersing nanoparticles or stabilizing colloidal systems.

For example, choline N-lauroyl-AAs, self-assembling fatty AA-based surfactants, are significantly superior to conventional ionic surfactants such as sodium dodecyl sulfate and sodium dodecyl benzene sulfate, while being less toxic to human cells in vitro [295]. Triverdi et al. proposed task-specific surfactants based on AA propyl or butyl ester cations and a lauryl sulfate anion [296]. These ILs form closed bilayer structures, with self-assembly controlled by a fine balance of solvophobic effects and ionic arrangements via hydrogen bonding and electrostatic interactions [297]. These surfactants show excellent potential for mitigating harmful algal blooms in seawater and serving as templates for the shape- and size-specific synthesis of CeO₂ nanomaterials. Another example is dodecyl betaine N-acetyl glycinate, an IL capable of forming stable micelles in water. In this case, strong interactions between the anion and water through hydrogen bonding contribute to the micellar solubility, allowing the dissolution of hydrophobic drugs in water [298].

Interestingly, Hu et al. synthesized mesoporous silica with uniform hollow triangular prism morphology and high specific surface area using [N₁₁₁₁][Gly] and a supramolecular organogel N-lauroyl-L-glutamic acid di-n-butylamide as co-templates [299]. Using the same co-templates, Lei et al. fabricated cactus rod-like mesoporous alumina [300]. Additionally, ILs based on cations such as 1-(2-methoxyethyl)-3-methylimidazolium or choline and anions like Asp or Glu, mixed with monoolein, form liquid–crystalline matrices with three-dimensional continuity and periodicity. The cubic lattice constant of these matrices can be tuned from 88 to 100 Å by designing specific AAILs [301].

Mesoporous γ-Al₂O₃ impregnated with [Emim][Gly] demonstrated optimal textural properties as a CO₂ capture material [302]. On the other hand, silica-coated magnetic nanoparticles modified with [Emim][Pro] displayed strong chiral recognition abilities, making them suitable for the direct separation of chiral AAs using centrifugal chiral chromatography [303]. Zou et al. reported that SBA-15 modified with 1-methyl-3-(3-(trimethoxysilyl)propyl) imidazolium lysinate achieved 99% immobilization efficiency for porcine pancreatic lipase. Strong carrier–enzyme interactions prevented leaching, preserving 96% of the enzyme’s catalytic efficiency even after five cycles [304]. Similarly, a nanocomposite based on an ordered graphitized mesoporous carbon surface modified with [Emim][Ala] was found to be an excellent support for tyrosinase biosensors. These biosensors showed a linear response for phenol concentrations from 0.1 to 10 µmol·L⁻¹ with a low detection limit of 20 nmol L⁻¹ and retained 90% of their initial activity after 21 days of storage [305]. Additionally, magnetic iron oxide nanoparticles stabilized with silane ligands containing [Ch][Gly] or [Ch][Lys] were shown to be efficient catalysts for Knoevenagel condensation reactions [306].

Safavi et al. proposed a novel two-step reduction procedure for the synthesis of gold nanoparticles (AuNPs) using 1-dodecyl-3-methyl imidazolium tryptophan ([C₁₂mim][Trp]) or [Emim][Trp] [307]. Both the cation and anion of these ILs contribute to AuNP stabilization, with the longer alkyl chain of [C₁₂mim][Trp] providing superior stability. This result is consistent with another study showing that [Bmim][Gly], [Bmim][Leu], [Bmim][Pro], [Bmim][Val], and [Bmim][Ala] effectively prevent AuNP aggregation and improve their long-term storage, addressing a key challenge in drug delivery systems [308].

2.10.6. AA ILs as Lubricants

AA ILs offer excellent thermal stability, low volatility, and superior lubrication performance under extreme conditions, making them suitable for a variety of applications in automotive, aerospace, and industrial machinery. Specific applications include greases, additives in engine oils, and lubricants for high-performance gears and bearings. For example, the lubricating performance of [Ch][AA] is comparable to that of polyalphaolefin oil [309]. When added to the PEG₂₀₀ base oil, they showed excellent anti-wear- and anti-friction-reducing properties [310]. On the other hand, [Ch][AA] added to lignin is an effective additive that improves thermal stability, reduces wear rates, and stabilizes the friction coefficients of these new lubricants. It has anticorrosive properties against commercial aluminum and iron sheets [311].

Interestingly, [N₄₄₄₄][AAs] tend to readily adsorb onto the positively charged metal surfaces to form a protective boundary layer, which is a major contributor to their excellent tribological properties. Tested as pure lubricants for steel/steel, copper/steel, and aluminum/steel contacts, they show promise as an alternative to conventional polyalphaolefin oil [312,313]. In addition, [P₄₄₄₄][Trp] exhibits excellent tribological properties due to the synergistic effect of P and N [314]. It significantly improves the anti-friction and anti-wear properties of water.

3. Conclusions

AA ILs are emerging as versatile and sustainable materials with immense potential across diverse applications, including drug delivery, catalysis, adsorption, and separation processes. Their customizable properties, biocompatibility, and alignment with green chemistry principles make them particularly attractive for addressing modern scientific and industrial challenges. However, key obstacles, such as high production costs, complex synthesis, and concerns about environmental impact, must be resolved to unlock their full potential.

Future innovations in AA IL design will likely focus on the development of novel biocompatible counterions and scalable, cost-effective production methods. Counterions derived from natural compounds, such as sugars (e.g., sorbitol and gluconate), polyphenols (e.g., gallic acid and caffeic acid), carboxylates (e.g., citrate and succinate), and natural alkaloids (e.g., berberine and theobromine), hold significant promise for creating environmentally friendly and application-specific ILs. Furthermore, addressing the challenges of stability and reusability is critical for advancing AA IL applications, particularly in catalytic and adsorption processes. Innovations such as supported AA IL systems and hybrid materials offer exciting pathways to enhance operational durability and enable efficient regeneration.

Looking ahead, interdisciplinary research will be pivotal in overcoming these challenges, combining insights from chemistry, materials science, and engineering to drive transformative advancements. By addressing these hurdles, AA ILs could become indispensable tools for sustainable development, revolutionizing industries ranging from pharmaceuticals to environmental remediation. Their potential to balance functionality, eco-friendliness, and economic feasibility positions AA ILs as key players in the future of green and biotechnological innovation.