Functionalized and Biomimicked Carbon-Based Materials and Their Impact for Improving Surface Coatings for Protection and Functionality: Insights and Technological Trends

Abstract

Interest in carbon materials has soared immensely, not only as a fundamental building block of life, but because its importance has been critical to the advancement of many diverse fields, from medicine to electrochemistry, which has provided much deeper appreciation of carbon functionality in forming unprecedented structures. Since functional group chemistry is intrinsic to the molecular properties, understanding the underlying chemistry of carbon is crucial to broadening its applicability. An area of economic importance associated with carbon materials has been directed towards engineering protective surface coatings that have utility as anticorrosive materials that insulate and provide defense against chemical attack and microbial colonization of surfaces. The chemical organization of nanoscale properties can be tuned to provide reliance of materials in carbon-based coating formulations with tunable features to enhance structural and physical properties. The transition of carbon orbitals across different levels of hybridization characterized by sp¹, sp², and sp³ orientations lead to key properties embodied by high chemical resistance to microbes, gas impermeability, enhanced mechanical properties, and hydrophobicity, among other chemical and physical attributes. The surface chemistry of epoxy, hydroxyl, and carboxyl group functionalities can form networks that aid the dispersibility of coatings, which serves as an important factor to its protective nature. A review of the current state of carbon-based materials as protective coating materials are presented in the face of the main challenges affecting its potential as a future protective coating material. The review aims to explore and discuss the developmental importance to numerous areas that connects their chemical functionality to the broader range of applications

1. Introduction

The elemental importance of carbon is signified by its position as the seventeenth most resourceful element that exists in the earth's outer layer, the fifth most abundant in the solar system, and the sixth most plentiful element in space [1]. According to the assessment, carbon’s relative abundance ranges between 180 and 270 parts per million (ppm) [2,3,4]. Carbon has intriguing and unique properties, and the properties of some carbon allotropes take the form of graphite [5], graphene oxide (GO) [6,7], graphene(G) [8], reduced graphene oxide(rGO) [9], fullerenes (C₆₀) [10], carbon nanotubes (CNTs) [11], noncrystalline carbon [12], diamond morphology [13], and lonsdaleite [14]. As a result of its peculiar electronic structure, carbon can transition between different mixed orbital hybridizations, including sp¹, sp², and sp³ forms. Variation in orbital hybridization has led to different structures with several other distinctive properties [15]. The carbon allotropes possess distinctive characteristics, including impenetrability to gases [16], chemical (acid, base, and salt) endurance [17,18], antibacterial potential [19,20], thermal stability [21,22], environmental friendliness, and, more critically, high specific surface area [23]. It has been found that carbon allotropes can also provide a fertile research ground for enhancing protective surface coatings because of their flexible surface chemistry and useful property diversity, making them excellent research targets for advanced material development.

In general, coatings are materials or multilayers that are applied to the surface of bulk materials to preserve the primary or undercoat material. Coatings that are synthetic or engineered can be used either individually or in combination as composites that result in altered functional properties. Individual or property combinations are represented by hierarchical patterns that are morphologically and functionally different and are tunable to deliver material properties with desirable outcomes [24]. Biological damage, corrosion, fouling, and environmental damage are some of the main reasons why functional coatings are used [25]. Besides medical instruments, electronics, and everyday household appliances, a wide range of military and marine applications are available [26]. It should be noted that, in addition to the applications previously mentioned, functional coatings have also been developed to help remove contaminants such as heavy metals and dyes from different natural and fresh waters [27,28]. Due to this, protective coatings go beyond their usual functions of adsorbing toxic substances, killing bacteria, and protecting against frost, fire, and irradiation to also address corrosion, fouling, and mechanical wear as well.

It is well known that coatings are widely used for protecting materials. Nevertheless, some metal coating technologies can negatively impact the environment they are connected with. Since they possess both carcinogenic and biocidal properties, hexavalent chromium (Cr (VI)) and tributyltin (TBT) have been completely banned from use in protective coatings [29,30]. For example, Cr (VI) has been limited to a maximum of 0.1 wt% in corrosion-protective coatings [31]. TBT has also been banned from use in antifouling coatings since 2003 to safeguard against damage to marine environments [32]. Metal coating ingredients used in the form of cadmium (Cd), cobalt (Co), and copper (Cu) have also been determined to be detrimental to the environment and have shown to be carcinogenic to humans [33,34,35]. Cadmium has been identified to cause hazardous effects on the environment and is associated with toxicity and its use as a material has been prohibited in electronic devices and equipment since 2006 [36]. The search for ecofriendly alternatives to replace these materials and fulfill the key industrial and regulatory requirements applied to the coating industry has been challenging. However, diamond-like carbon (DLC) is an interesting material for protective coatings, with useful applications for material toughness and wear resistance, bearing low friction coefficients and thermal conductivity. Further, DLCs offer considerable chemical stability, gas-barrier capability, and high infrared-permeable properties. What is most important is that DLC coatings are biocompatible in nature [37]. Therefore, their applications are diverse, including electronics (e.g., hard disks, video tapes, and integrated circuits), cutting tools (e.g., drills, end mills, and razors), molds (e.g., optical parts and injection molding), automotive parts (e.g., piston rings, cams, clutch plates, pumps, and injectors), optical components (e.g., lenses), plastic bottle oxygen-barrier films, sanitary equipment, windows, bathtub mirrors, and decorative uses [38]. The material properties associated with DLCs have gained much recognition in recent years, increasing their potential use as protective coatings. An extensive account of DLC-based materials as protective coatings is out of the scope of this review due to its rather broad nature; a separate review dedicated to DLCs is anticipated in greater detail.

A carbon-based technology that relies on polymer-based composites is currently the forefront for driving functionalized nanomaterials with carbon as one of its core materials.

The economic impact of combing costly materials with polymers is considerable, as polymers can reduce the cost burden by lowering the requirement of expensive materials in forming composites while maintaining material performance. This strategy has paved the way for fabricating novel protective coating products, particularly graphene-based composites, comprising paints and other useful formulations. With the availability of property-based materials, a broad range of applications is possible by tuning the compositional nature at the microstructure level of the final coating to enhance the level of protection provided by functionalized carbon. Such composites provide the freedom to modify and regulate the characteristics of the final product by altering the components in isolation. Hence, a functionalized carbon coating can deliver superior protective attributes with few shortcomings, depending on the application using (1) pure carbon coating, (2) a carbon-like coating, or (3) a functionalized carbon-enhanced composite-type coating. In this regard, remarkable progress in the advancement of innovative protective coatings has been achieved, aided by such technologies through the customization of functionalized carbon.

This review aims to consider recent developments related to the application of multipurpose carbon-based coatings. With reference to preventing corrosion, fouling, thermal or radiation damage, and contamination, the distinctive physical and chemical assets of carbon allotropes exhibited by different types of coatings will be discussed. The intrinsic protective mechanisms utilized by graphene, graphene oxide (GO), and/or reduced graphene oxide (rGO) are discussed as a prelude to their applicability, which further warrants an in-depth discussion of the merits of their technological importance and potential as engineered materials in design and function. The review also brings into view an analytical perspective on key developmental challenges and highlights future prospects of carbon-based materials in the evolution of protective-coating technologies.

2. Functionalized Carbon-Based Systems

All nanometric forms of carbon discovered to date, including graphite, diamonds, graphene, monohorns of carbon, fullerene, and carbon nanotubes (CNTs), are known as carbon nanoallotropes. A pictorial representation of carbon-based nanoallotropes is shown in Figure 1. Solubility limits the use of carbon allotropes in industrial and biological applications [39]. The solubility of carbon allotrope molecules may be enhanced through functionalization, achieved by introducing polar substituents into the molecular framework as an effective method of improving the solubility of carbon allotropes. The aromatic nature of carbon allotropes in their oxidized form (rGO) is more suppressed while GO exhibits nonaromatic composition, with carbon atoms hybridized with a sp³ configuration. Several deviations to the planarity of the structure are attributed to the presence of sp³ carbon atoms that are bonded to oxygen moieties. A variety of covalent and noncovalent techniques can be used for functionalizing carbon allotropes [15,40]. In the case of covalent functionalization, carbon allotropes and polar substituents are covalently coupled, while weak intermolecular forces (van der Waals forces) occur during noncovalent functionalization.

2.1. Non-covalent Approaches for Functionalization

There are several non-covalent functionalization methods using several approaches. Hydrophobic and electrostatic interactions, van der Waals forces of attraction between molecules, and a bundled assemblage conforming to a condensed state feature strongly in the commonly used non-covalent functionalization approaches that are applied to carbon allotropes [41,42]. Non-covalent functionalization provides useful routes to alter surfaces, thus permitting surface functionalization without chemical disruption of the surface structural framework of the carbon allotrope and resulting in changes to the physical and electrical properties following functionalization. Graphene analogues, such as single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) have, in recent years, undergone non-covalent functionalization. It is generally observed that in this mode of interaction, the πe⁻ density of the benzene ring(s) (e.g., pyrene and perylene) to some degree overlaps with the πe^- region of the carbon allotrope. Several non-covalent functionalization routes of GO can be seen in Figure 2. The aromatic regions of carbon allotropes are often directed to undergo π-π stacking associations. Interfaces between carbon allotropes (with supermolecules, surface-active agents, ionic liquids, etc.) can be categorized as hydrophobic. The π–π cooperativity has been reported to occur in polymers such as Kevlar [43], polyvinyl chloride, sulfonated polyaniline [44], and polyethylene glycol [45]. To investigate the electron–donor–acceptor interactions in aromatic compounds with graphene nanosheets, pyrene and perylene diimide derivatives were used. Carbon allotropes (G, GO, CNTs, etc.) work together hydrophobically with supermolecules, ionic liquids, and surfactant molecules to form stacked configurations on carbon allotropes and differ from the stacked orientation used by aromatic compounds on carbon allotropes. Both aqueous and organic solvents disperse carbon allotropes by virtue of their dispersion strength arising from chemical interactions between the carbon allotrope and the solvent environment. Electrostatic interactions play a large role in the interaction between ionic liquids and GO (conceivably with different allotrope types), in part because GO contains a variety of ionic groups, such as hydroxylates and carboxylates, which can engage in electrostatic interactions. Hydrogen bonding is not only capable of stacking and hydrophobic and electrostatic interactions but can also occur between allotropes of carbon in addition to stacking.

2.2. Covalent Approaches for Functionalization

Two kinds of target sites are available on the surface of carbon-based materials such as graphene oxide (GO) and its analogues, which are amenable to undergoing covalent functionalization and can be described by the following events: (i) oxygen-containing groups such as −OH as well as −C-O-C on the basal plane, as well as −COOH at the edges, where new species might be introduced via condensation and esterification; and (ii) −C=C functionality, which requires the attacks by free radicals or dienophile species [46,47]. Commonly used strategies for functionalization of GO include: (i) acylation; (ii) amidation; (iii) silylation; (iv) esterification; (v) cycloaddition; (vi) etherification; and (vii) azidation. The −COOH groups at the edges are possibly modified by amidation or esterification. There have been several amines used in the past in order to covalently functionalize graphene oxide (GO) and, more specifically, C₆₀ fullerenes, porphyrins, polythiophenes, and phthalocyanines. It is possible to activate hydroxyl groups in several ways, namely through salinization, silylation, and etherification.

There are numerous chemical processes that can be used to modify the GO, including ring-opening actions with epoxides, amine (R-NH₂), and alcohol (R-OH). A wide range of poly(allyl amine) and poly(vinyl alcohol) moieties can be attached to ringed structures using these reactions. Functionalization of > C=C < may be performed by free-radical-catalyzed cycloaddition reaction (Diels–Alder type reaction). Carbons undergoing sp² hybridization can transition to sp³ during cycloaddition, which reduces GO’s ability to conduct electrons. This is shown in Figure 3a.

A carbon nanotube (CNT) is a cylinder of carbon with a diameter measured in nanometers (nm) and a length also measured in nanometers (nm). Carbon nanotubes are divided into single-walled carbon nanotubes (SWCNs) and multiwall carbon nanotubes (MWCNs) [41]. Van der Waals interactions can provide an avenue for multiple CNT ring connections to form MWCNTs. In material science, CNTs and their derivatives (functionalized CNTs) are used for a variety of applications due to their exceptional mechanical properties and physical properties due to their diminished size. It is important to note that CNTs exhibit exceptional electrical conductivity and are prone to corrosion in aqueous media, as they enhance electron transfer. Despite their various advantages, CNTs are thus limited in their application as corrosion inhibitors, especially in aqueous phases. The desirable properties of anticorrosion can, however, be achieved by covalently functionalizing them. Surface modification of SWCNTs and MWCNTs covalent means can be accomplished using chemical strategies (Figure 3b). Such changes allow the adaptation of CNTs (SWCNTs and MWCNTs), allowing them to undergo a number of physiochemical changes as a result of covalent functionalization, which can be described by: (i) An enhancement in overall dispersion properties; (ii) interfacial compatibility assessed by adhesion and diminished hydrophobicity; (iii) a reduction in aggregation; (iv) a reduction in mechanical properties; and (vi) a reduction in electrical conductance. The functionalized state of GO and CNTs (SWCNTs and MWCNTs) retains the capacity to undergo further chemical refinement by targeting peripheral moieties, which remain exposed for further functionalization [43,44]. The biological and industrial applications of other allotropes of carbon can also be enhanced through covalent functionalization using different chemical species, as can GO and CNTs.

The chemistry of fullerene has been thoroughly studied and investigated to date. The functionalization chemistry of fullerene is more or less similar to the approaches used to functionalize carbon nanotubes and graphene oxide, as shown in Figure 3c.

3. Adaptation of Carbon as a Protective Coating via Surface Functionalization

3.1. Functionalized Coated Carbon for Anticorrosion Applications

Recent investigations have established that corrosion barrier coatings that have been fabricated from functionalized carbon, graphene, and graphene oxide and their composites, are effective anticorrosion layers, which also take the form of alloys used in the range of electrolytes. The reported use of chemically modified GO, however, has been rather limited, with relatively fewer examples of anticorrosive properties compared to its metallized counterparts. This may partly be explained by lower levels of polar electrolytes than in nonpolar electrolytes. Only a few reports have been published about the use of carbon support that has either been functionalized or modified with suitable organic compounds as effective corrosion inhibitors in an aqueous solution. Anticorrosion behavior of different organic functional groups, which can be used to functionalize carbon supports for designing an effective corrosion inhibitor system in an aqueous solution, are summarized in

Table 1. Different chemically functionalized carbon-based anticorrosion coatings in aqueous phases.

S.N.	Functional Group	Medium	Metal	Concentration of Inhibitor	Inhibition Efficiency (%)
1 [49]	1-tetradecylpyridazin-1-ium iodide	1.0 HCl	Carbon Steel	10−3 M	97.6
2 [50]	Schiff base	1mM HCl	Steel	1 mM	92.0
3 [51]	Benzotriazole	0.5 M Na2SO4	Galvanized steel	2.9 × 10−3 M	90
4 [52]	L-cysteine	1 wt% NaCl	Alloy Steel	400 ppm	65.7
5 [53]	Polylactic acid/2-mercaptobenzothiazole	1 M HCl	Alloy aluminum	0.5 mass ration	33.83
6 [54]	Imidazoline	1.65 wt% NaCl con. 1 g/L acetic acid	Carbon steel	0.1 g/L	99.3
7 [55]	N-(quinolin-8-yl) quinoline-2-carboxamide	1 M HCl	Mild steel	0.3 mmol L−1	94
8 [56]	(6-methyl-3-oxopyridazine-2-yl) acetate	1 mM HCl	Mild steel	10−1 mM	66.7
9 [57]	Benzotriazole	3.5 wt% NaCl	Copper	0.45 mM	92.31
10 [58]	2-(4 -nitrophenyl) benzimidazole	1 mM HCl	Carbon steel	1 mM	93.7
11 [59]	Adenosine	0.5 M HCl	Tin	10−3 M	73.0
12 [60]	1,8-Naphthyridine	1 M HCl	Mild steel	0.00004 M	98.7
13 [61]	Octadecyl amine	10−3 M Na2SO4	Carbon Steel	100 mgs kg−1	90
14 [62]	Potassium 2,4-di(2H-benzo[d][1,2,3] triazol-2-yl)benzene1,3-bis(olate)	3.5 wt% NaCl	Copper	0.30 mM	96.7
15 [63]	4-Chloro-1H-indazole	3.0 wt% wt% NaCl	Copper	0.4 mM	98.9
16 [49]	Triazole–theophylline	1 M HCl	Steel	50 ppm	87
17 [64]	4-(3-Mercapto-5,6,7,8-tetrahydro-[1,2,4]triazolo[4,3- b][1,2,4,5]tetrazin-6-yl)phenol	1 M HCl	Mild Steel	0.5mM	67
18 [65]	1,2,4-Triazole	3 wt% NaCl	Copper	10 mM	98
19 [66]	N2,N4,N6-tris(3-(dimethylamino)propyl)-1,3,5-triazine-2,4,6-triamine	0.5 M	Mild Steel	1.0 mM	95.8
20 [67]	N-ethyl-N,N,Ntrioctylammonium ethyl sulfate	1 M H2SO4	steel	100 ppm	83
21 [68]	1-benzyl-4-phenyl-1H-1,2,3-triazole	1.0 M HCl	Mild Steel	2.13 mM	81.7
22 [69]	2-amino-4,6-bis-(3-hydroxy-4-methoxy-phenyl)-5-nitrocyclohex-2-ene-1,3,3-tricarbonitrile	1 M HCl	Mild Steel	25 ppm	98.96
23 [70]	6-(Dodecyloxy)-1H-benzo[d]imidazole	1 M HCl	Mild Steel	10−4 M	95.0
24 [71]	(1-(pyridin-4-ylmethyl)-1H-1,2,3- triazole-4-yl) methanol	1.0 M HCl	Mild Steel	1.0 mM	90.2

. Due to its inherent binding tendency and high surface area, chemically modified GO exhibits a relatively high protection capacity.

Recently, covalent modifications and self-polymerization have been used to functionalize graphene oxide (GO) with polydopamine (PDA), which was then tested as a corrosion inhibitor in HCl solutions against carbon steel. At 100 mg/L, GO-PDA achieved almost 90% inhibition efficiency, which is a high level of corrosion protection for carbon steel. An investigation of the interaction between immersion time and concentration was conducted. A hydrophobic film was formed in micro–nano structures by lamellar functionalized GO adsorbed onto the surface of carbon steel [48]. The composite carbon material of this dense protective film was extremely hydrophobic, inhibiting the penetration of corrosive agents, as shown in Figure 4a. In this study, a transition from a GO-surface that had a negatively charged surface to a positively charged GO-PDA that significantly enhanced the adsorption at the material intersection. The contact angle measurement study, as shown in Figure 4b–e, evidently showed that the GO-PDA cover layer was hydrophobic in nature, which was corroborated by contact angle measurements at the interface.

It has been recently demonstrated that 1,3-diphenylprop-2-en-1-one oxime–graphene oxide (CO-GO) can be produced from chalcone, hydroxylamine, and a Cl-modified graphene. For 12 h, the mixture was continuously stirred at 80 °C at a constant speed [72]. The process was completed by subjecting CO-GO to a spinning cycle, followed by a washing process in water and ethanol, and finally drying at 60 °C, as shown in Figure 5. Further, potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) were utilized to investigate corrosion inhibition effectiveness. In addition to its outstanding corrosion inhibition performance, CO-GO exhibits a primarily anodic mechanism with a mixed-type inhibitor function of up to 94%.

A variety of techniques have been employed to confirm inhibitor adsorption onto the carbon steel surface, including scanning electron microscopy and ultraviolet-visible spectroscopy.

The presence of polar substituents on functionalized carbon not only enhance the solubility of carbon-based materials in aqueous electrolytes, but also act as an adsorption center during metal–inhibitor interaction. Surface corrosion protection is obtained by the transfer of nonbonding electrons from the functionalized polar group and/or from the electrons on the aromatic or heteroaromatic rings to the metal center. In general, functionalized carbon-based systems adsorb onto metallic surfaces in two steps. The functionalized carbon-based system is transferred from the bulk solution onto the metal surface in the initial step. Adsorption occurs through coordination bonding in the second step. Electrons transferred from a functionalized carbon-based system to a metal surface without forming bonds are known as donations. Due to the interelectronic repulsion, even metals that are already electron-rich may become thermodynamically unstable as reactions result in electron transfer. Consequently, electrons may be transferred back into heteroatoms' empty p-orbitals during the interaction of carbon-based materials. The process is known as retro (back)-donation. As a result of a phenomenon known as synergism, donations and retro (back)-donations strengthen one another.

Electrostatic force of attraction is another means of interfacing functionalized nanomaterials with metallic surfaces besides chemical (coordination) bonding by means of physisorption. The adsorption of counter ions on positively charged metallic surfaces in aqueous electrolytes incite a negative charge to the metallic surface. In a separate event, protonation of a functionalized nanomaterial inhibitor group induces a positive charge and drives an electrostatic force of attraction to the negatively charged metallic site, resulting in the induction of physisorption. Figure 6 depicts chemisorption and physisorption in a pictorial manner. The most common mechanism for adsorption of organic corrosion inhibitors is the mixed mode, also known as physiochemisorption. GO and its derivatives are described in this article for their corrosion-inhibition properties.

3.2. Functionalized Carbon for Flame-Retardant Coatings

The boundary region and basal planes of GO forming the graphene precursor constitute an abundance of epoxy, hydroxyl, carboxyl, and -C=C- groups which are freely available for functionalization [73]. Surface conversion of hydrophilic GO to its congruent hydrophobic state facilitates the dispersion of graphene sheets in the polymer matrix, as shown in Figure 7 [74]. The functionalization-based approach has been successfully applied in order to prepare organic flame retardants with functionalized graphene. With the help of this strategy, it is generally possible to covalently link organic flame retardants to graphene or GO in two steps: (i) Providing organic flame retardants with functionalities to enable their reaction with graphene or GO and (ii) reducing functionalized graphene to functionalized graphene using reducing agents. Modifiers that also serve as reducing agents can occasionally functionalize and reduce GO at the same time. A few examples of organic flame retardants functionalized with graphene, as well as the polymer composites that contain these materials, are summarized in

Table 2. Summary of flammability of polymer nanocomposites based on functionalized graphene oxide.

S.N.	Polymer	Loading	Main Results
1 [74]	Epoxy	10 (Functionalized graphene)	Compared with pure epoxy, the char yield was increased to 30.2% from 16.7% and the GO/epoxy had a char yield of 21.1%; the limiting oxygen index (LOI) increased to 26% from 20% and the GO/epoxy had a char yield of 23%, respectively, when compared with pure epoxy.
2 [75]	Epoxy	10 (Functionalized graphene)	According to the UL-94 V-0 test (3.2 mm), the pristine graphene/epoxy composite showed a limiting oxygen index (LOI) value of 36%; however, the pristine graphene/epoxy composite had a LOI value of 26%, which was not high enough to pass the test.
3 [76]	XLPE	3 (Functionalized graphene)	Cone calorimetry observed a 29% reduction in PHRR, whereas GO/XLPE observed a 9% reduction.
4 [77]	PP	20 (Functionalized graphene)	PPA/PP composites showed a 67% and 24% reduction in PHRR and THR, respectively, whereas the PPA/PP composites showed 48% and 20% reductions, respectively.
5 [78]	Epoxy	8 (Functionalized graphene)	PPA/epoxy composites reduced both the PHRRs and THRs by 41% and 50%, respectively, whereas composites based on PPA-g-GNS/EP composites reduced them by 35% and 46%, respectively.
6 [79]	EVA	1 (Functionalized graphene)	Pure GO/EVA exhibited a reduction of 46% in PHRR and TTI of 75 s, whereas EVA/CRG–PPSPB achieved reductions of 31% and 61 s.
7 [80]	Epoxy	1 (Functionalized graphene)	A 45% reduction in PHRR was observed from pyrolysis–combustion-flow calorimetry, while that of GO/epoxy was only 18%.
8 [81]	EVA	2 (Functionalized CNTs)	24.4% reduction in PHRR observed
9 [82]	UPR	2- (Functionalized CNTs)	72% reduction in PHRR observed
10 [83]	PVDF	3 (Functionalized CNTs)	57.7% reduction in PHRR observed

.

burn-out limits of GO have been overcome in the past few decades by grafting several synthetic compounds onto it, including 9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) [84,85], 2-(diphenylphosphino) ethyltriethoxy silane (DPPES) [78], hyper-branched flame retardants [86], polyphosphamide (PPA) [80], and poly(piperazine spirocyclic pentaerythritol bisphosphate) (PPSPB) [82]. Data on the limiting oxygen index (LOI), the UL-94 flame retardancy rating, and the peak heat-release rate (PHRR) for functionalized graphene sheets with organic flame retardants were compared; pristine graphene sheets and functionalized graphene sheets were found to exhibit better flame-retardant properties. The test results indicated that epoxy composites containing a mixture of DPPES-graphene at a loading of 10 wt% were capable of passing the UL-94 V-0 rating—in contrast to their counterparts containing equal amounts of either graphene or DPPES.

It has also been found that graphene can be functionalized with flame retardants in addition to sol–gel chemistry. Flame retardants modified with siloxane can facilitate reactions with silane-modified GO or silane-modified graphene, permitting a chemically modified interface between the retardant and graphene/GO surface formed via the silane-assisted hydrolysis of siloxane, resulting in a condensed structural composite. Figure 8 describes the step-by-step process for the functionalization of GO/PMMA composites. Several polymers, such as epoxy resins, polyuria (PU), polyvinyl alcohol (PVA), and polymethyl methacrylate (PMMA), can be made more flame-resistant by adding graphene or graphene oxide.

Flame retardants made from functionalized carbon can be effective for polymer materials because of its excellent thermal conductivity, thermal stability, and physical barrier properties. As a result of intensive research on graphene and its derivatives in epoxy resins [78], investigations have concluded that functionalized carbon is flame-retardant in three important areas. Layered structures of sp²-hybridized carbon backbones that prevail in graphene and its derivatives can serve as physical barriers, slowing down heat transfer and preventing the initial stages of matrix decomposition. An investigation was conducted by Qu et al. to understand the mechanisms of flame retardancy of epoxy resin with the addition of POSS (octa glycidyl ether) groups to GO, as shown in Figure 9a [87]. The decomposition of the matrix is inhibited by a tumultuous path effect during the dispersion of the FGO in the epoxy resin matrix as a result of a tortuous path effect that inhibits the diffusion of heat and the degradation of combustible gases. A further benefit is that, when the OGPOSS groups contain Si, the char layer can be strengthened in such a way that heat transfer and oxygen mixing with pyrolysis gas can be prevented, thereby enhancing the flame-retardant properties of epoxy resin as well as its capability to suppress smoke emission. There is also evidence that graphene can form harmless gases (e.g., NH₃) when functionalized with organic elements (e.g., compounds containing nitrogen). This gas dilution hinders the combustion of the epoxy resin matrix material. Phytic acid (PA) and piperazine (PiP) were added to graphene oxide by Fang et al. in order to produce PiP-functionalized graphene oxide (PPGO), as shown in Figure 9b [88]. Finally, the benefit of graphene that has been functionalized by inorganic functional groups is that it leads to the formation of a dense, stable, and heat-resistant char layer and reduces the heat loss and smoke emissions from combustion processes. The combination of graphene compound (Cu^2+-GO) and ammonium polyphosphate (APP) can enhance flame retardation in epoxy resins, as described by Ye et al. (Figure 9c) [89].

3.3. Functionalized Carbon for Enhanced Antifouling Coatings

There are many problems associated with fouling on marine vessels and structures. Fouling is caused by the deposition of organic and inorganic materials, which can cause a wide variety of problems, including fouling of cargo ships, naval vessels, recreational vessels, heat exchangers, and sensors for oceanographic investigations, as well as offshore platforms such as oil rigs, desalination processes, and aquaculture systems. For the U.S. navy fleet alone, there are estimated to be maintenance costs from USD 180 million$/year to 260 million$/year due to fouling and surface setting and hardening of hard-to-remove deposits that attach to ships. This includes fuel costs, powering penalties, and maintenance expenses due to particulate fouling. TBT is currently banned in antifouling coatings, which introduces new challenges for developing antifouling technologies. Cost reduction is a major objective in this sector and the development of cost-effective materials with antifouling properties is predicted to reduce the economic burden substantially.

A variety of strategies have been developed as a means of preventing successive fouling of surfaces as well as fighting both inorganic as well as organic foulants. There are several strategies to maintain cleanliness on surfaces, which include surfaces with self-cleaning properties, biomimetic structural design, and the use of biocidal chemical mediators to deter fouling. Amphiphilic modified surfaces are also a recent development [90,91]. An antistick coating agent based on carbon is a promising option. Graphene surfaces can be tailored to achieve the surface polarity and wettability desired for antifouling coatings. In the presence of superhydrophilic and superhydrophobic graphene coatings, particle fouling can be prevented. Coated membranes composed of graphene can eliminate salt more effectively because of its impermeable structure. The use of carbon materials in antifouling coatings is not a novel finding since their properties are known to be associated with antifouling behavior discouraging the settlement of marine organisms and have been extensively studied in the field. A recent study of allotropic carbons as ecofriendly coatings has inspired work to harmonize the use of graphene into antifouling applications as a result of its foul-releasing properties, surface flexibility, selectivity towards protein adhesion, and covalent polymer attachment which is tunable by the degree and nature of surface functionalization.

Common organic matter that act as foulants in membrane-fouling-resistance studies are bovine serum albumin (BSA) and humic acid (HA), as shown in Figure 10a. The hydrophobic properties of these organic materials, in addition to their negatively charged surfaces, are their distinguishing characteristics. Cysteine-functionalized graphene oxide (CGO) composites can impose surface densities that are negatively charged in PES nanocomposite membranes. Increase in the CGO concentration from 0 to 0.4 wt% in the PES membrane led to improvements from −17.7 mV and −28.7 mV for surface zeta potential. Negative-charge density is attributed cysteine’s high ionic charge density that resides in CGO-modified membranes, irrespective of the change in pH [1].

It has also been established that the roughness of GO-modified membranes may also play a role in the antifouling capabilities of the membrane and its ability to be hydrophilic. Contrary to the hydrophobic nature of the surface of the unmodified PES membrane and its rough surface, the smooth surface of the composite PES membrane allows foulants to be easily discarded. In Figure 10b, the schematic shows how surface cleaning is mechanistically independent of membrane hydrophilicity and surface roughness. Owing to the enhancement of surface hydrophilicity allied to the 1 wt% Cs-graphene-oxide-modified PES membrane and low surface roughness of the membrane, protein foulants on the membrane can be effortlessly removed via surface scrubbing.

The antifouling properties of zwitter ionic surfaces are effective in protecting surfaces against foulants. The modification of graphene oxide with zwitter ionic groups (GO-g-PMSA) and its incorporation into polyvinylidene fluoride (PVDF), as shown in Figure 11, generates two advantages: it increases the graphene oxide dispersibility in the membrane matrix and supplements GO with an increased antifouling capability. Water empathy and surface hydrophilicity of PVDF hybrid membranes were significantly improved by incorporating the zwitterionic additive. GO-g-PMSA/polyvinylidene fluoride membranes showed improved antifouling properties in comparison to pure polyvinylidene fluoride and graphene oxide–polyvinylidene fluoride membranes as it made use of softer and more hydrophilic properties. In the experiment, 1 wt% wt% of GO-g-PMSA/PVDF membrane showed superior antifouling performance. The study employed buffer solutions composed of phosphates in 1 g/L BSA and FRR was 95.3%, Rr was 32.3%, and R_ir was 4.7% at 0.7 MPa. Furthermore, high salt-ion refusal was noted in the salt-rejection experiments. The findings confirmed that zwitter-ionic-polymer-modified GO can potentially be used as an antifouling additive in nanocomposite membrane fabrication [93]. CNT coatings have been shown to prevent and control marine biofouling in several studies so far. These coatings consist primarily of MWCNTs, which have shown efficacy in marine environments, according to

Table 3. A summary of the fouling resistance of polymeric membranes using GO-based composites, is provided below.

S.N.	Coating	Material/Matrix	Organism	Experimental Setup	Main Conclusions
1 [95]	Graphene Oxide	Marine micro- and macrofoulers	Escherichia coli Staphylococcus aureus Pseudomonas aeruginosa	In vitro study Nutrient medium Ambient temperature After 24 and 48 h	Bacteria grew much slower on graphene-oxide-coated surfaces in vitro and were less likely to foul the surfaces in situ (up to 94% fewer viable cells).
2 [96]	Graphene Oxide	Alkyd resin	Marine micro- and macrofoulers	In situ study South Korea’s seawater (Jeju Sea) A period of three weeks
3 [97]	Graphene Oxide	Silicone rubber	Triceratium sp.	In vitro study A medium containing algal broth at a temperature of 25 °C Static assay (8 days) Assays in the dynamic mode In the vicinity of specimens, the linear velocity was 3.4 m/s for 10 days.	A single graphene oxide loading (0.16 wt%) responded slightly to diatom antiadhesion conditions (approximately 12% reduction in OD under static conditions). An approximate 67% OD reduction was observed for only one graphene oxide loading (0.36 wt%) in the dynamic assay.
4 [96]	Graphene Oxide	Calcium ion selective membrane electrode	Marine bacteria	In vitro study Luria–Bertani medium Ambient temperature 1 hr and 5 hr	In comparison to the noncoated sensor, the graphene oxide-coated sensor significantly inhibited the formation of biofilms (roughly 45% decrease in CFUs) by significantly improving antiadhesion and bacterial inactivation properties.
5 [98]	Graphene	Silicone rubber	Paracoccus pantotrophus	Study conducted in vitro Seawater artificially created Assay conducted in a quasi-static environment (7 days) Assay conducted in a dynamic environment. In seven days, speeds varied within a range of 0.2 to 0.5 m/s.	As compared to the rigid polystyrene sheet control surface, graphene–silicone membranes demonstrated similar antifouling performance under quasistatic conditions. As compared with control surfaces, graphene-based membranes reduced colony-forming units (CFU) by around 40% under dynamic conditions.
6 [99]	Graphene	Silica	Halomonas spp.	Study conducted in vitroSolution of sodium chloride(0.5 wt%)20 °C, 72 h	The graphene coatings decreased biofilm-producing bacteria, Halomonas spp., adhesion and adhesion gene expression.
7 [100]	Guanidine- functionalized graphene	Boron acrylate polymer	Escherichia coli Staphylococcus aureus	Study conducted in vitro Luria–Bertani medium 37 °C, 12 h	A 95% reduction in bacterial growth was observed in the coatings, along with an antiadhesion rate of 99% for diatoms. Neither fouling adhesion nor surface degradation were observed during the field trial.
8 [100]	Guanidine- functionalized graphene	Boron acrylatepolymer	Phaeodactylum tricornutum Nitzschia closterium f. minutíssima Halamphora sp.	Study conducted in vitro F/2 medium 21 °C, 14 days
9 [100]	Guanidine- functionalized graphene	Boron acrylate polymer	Marine micro- and macrofoulers	2 months of in situ study of natural seawater (Yellow Sea, China)
10 [93]	Acrylic acid-modified graphene oxide	Acrylic resin	Marine micro- and macrofoulers	Study conducted in situ Natural seawater (Zhoushan Sea, China) 6 months	The antifouling performance of composite-based paints was demonstrated during the testing process using natural seawater as the testing medium.
11 [101]	Polyaniline/p-phenylenediamine-functionalized graphene oxide	Epoxy resin	Simulated marine micro- and macrofoulers	Study conducted in vitro Simulated marine environment 25–27 °C, 3 months	A functionalized graphene oxide composite was added to commercialized epoxy coatings to improve their anticorrosion properties.
12 [102]	CNTs	Silicone oil-infused epoxy resin	Chlorella sp. Phaeodactylum tricornutum	Study conducted in vitro Artificial seawater 22 °C, 21 days	On surfaces coated with CNTs and epoxy resin, algae settlement rates were significantly lower than on surfaces coated with epoxy resin alone. Silicone oil infused into CNTs and epoxy resin coatings was even more effective in reducing the growth of algae biofilms (up to 90% reduction in cells) than silicone oil in CNTs alone.
13 [103]	MWCNTs	Chlorinated rubber	Pioneer eukaryotic biofilm communities	An in situ study was conducted (1.5 m below the surface) Chinese seawater (Xiaoshi Island) A temperature of 10 °C and a duration of 312 days	A significant reduction in mean species richness was observed by incorporating MWCNTs into antifouling materials (p 0.01) due to the reduced diversity and abundance of pioneer eukaryotic microbes.
14 [104]	MWCNTs	PDMS	Mytilus galloprovincialis-based foulers	An in vitro study was conducted An assay based on static data Seawater that has been filtered Temperature: 18 °C Plantigrades took six hours; pediveligers took 48 h.	Mussels’ adhesion and settlement were not affected by MWCNT incorporation compared to PDMS or PDMS alone.
15 [105]	MWCNTs	PDMS	Ulva linza	Dynamic assay conducted in vitro Seawater made from artificial sources The experiment was conducted for six days at 18 °C.	It appeared that adding MWCNTs to PDMS alone did not improve sporeling release properties.
16 [106]	MWCNTs	PDMS	marine micro- and macrofoulers	An in vitro study was conductedAssays that are dynamic The hull of a ship traveling at 15 knots (such as a ship's hull) Seawater made from artificial sources A temperature of 28 °C It takes 45 min for zoospores to settle; 7 days for sporelings to grow It takes 24 h for the adhesive to settle; it takes 3 months to determine its adhesion strength	By adding MWCNTs, sporeling release was enhanced (approximately 60%). A significant reduction in adhesion strength of adult barnacles growing on MWCNTs/PDMS was observed.
17 [107]	MWCNTs	PDMS	Bacteria and diatoms	A study conducted in situ (0.5–1.0 m below ground level) Water from the sea (Zhoushan, China) A 28-day period	Compared to bare PDMS, MWCNTs decreased mussel settlement (up to a 20% reduction) by altering the biomass and community composition of biofilms.
18 [107]	MWCNTs	PDMS	Mytilus coruscus (mussels and plantigrades)	An in vitro study was conducted Seawater that has been autoclaved and filtered The experiment was conducted at 18 °C for 12 h
19 [108]	Hydroxyl-modified MWCNTs	Silicone-oil-infused PDMS	Marine bacteria	An in vitro study was conducted Seawater fresh from the sea The temperature was maintained at 28 °C for ten days	Antiadhesion (up to 35% higher removal rate) and AF properties were enhanced, particularly when higher volume ratios of hydroxylated MWCNTs were used.
20 [108]	Hydroxyl-modified MWCNTs	Silicone-oil-infused PDMS	Marine micro- and macrofouler	The study was conducted in situ (1–2 m below the surface) A natural seawater sample (Yellow Sea, China) A period of eight months
21 [109]	Carboxyl and hydroxyl-modified MWCNTs	PDMS	Pioneer eukaryotic biofilm communities	A study conducted in situ (0.8-1.5 m below ground level) Water from the sea (Xiaoshi Island, China) A period of two months	The incorporation of MWCNTs showed excellent AF performance and effectively reduced colonization by pioneer eukaryotes, in comparison to plain PDMS (Shannon diversity index, p < 0.05).
22 [110]	Carboxyl and hydroxyl-modified MWCNTs	PDMS	Marine micro and macrofoulers	The study was conducted in situ (1.5 m below the surface) Chinese seawater (Weihai Western Port) A temperature of 11 °C, a duration of 56 days	The incorporation of a low amount of MWCNTs greatly improved the AF properties of PDMS coatings. Most modified coatings did not display any significant modulating effects on pioneer biofilm communities, as compared to plain PDMS.
23 [111]	Fluorinated MWCNTs	Silicon	Escherichia coli	An in vitro study was conducted A phosphate-buffered saline solution Temperature 37 °C, 6 h	In vitro study Phosphate-buffered saline 37 °C, 6 h
24 [112]	Fluorinated MWCNTs	PDMS	pseudobarnacles	Method for pseudobarnacle adhesion testing	Incorporating fluorinated MWCNTs into the PDMS matrix increased the adhesive properties of the FR layer by reducing the pseudobarnacle adhesion strength by 67% compared to bare PDMS, and by 47% compared to pristine MCWNT/PDMS matrix.
25 [113]	-Carboxyl and hydroxyl-modified MWCNTs -Graphitized MWCNTs -Carboxyl-modified SWCNTs	PDMS	Pioneer biofilm bacteria	An in situ study was conducted (1.5 meters below the surface) A natural seawater spring on the Chinese island of Xiaoshi Temperatures between 10 °C and 17 °C, 24 h	All CNT/PDMS composites decreased proteobacteria biofilm formation, but increased cyanobacteria biofilm development.
26 [114]	1. Tetraaniline(TANI) -CNT		S. epidermidis	-	Results revealed that the surface coverage percentage of S. epidermidis drops more than 50% from the unmodified to the modified film.
27 [115]	2. CNTs	poly(methyl methacrylate) (PMMA)	S. aureus S. mutans C. albicans	-	Significant antiadhesive effects (35–95%) against all tested bacteria were verified for the 1% CNT/PMMA compared to the PMMA control group.
28 [116]	3. CNT	Polyethylene (PE)	P. fluorescens M. smegmatis	-	Biofilm growth on PE-CNTs composites surface compared to PE decreased by 89.3% and 29% for P. fluorescens and Mycobacterium smegmatis, respectively.

.

Marine coatings made from pristine MWCNTs (p-MWCNTs) produce differing results. Some studies have shown that carbon nanomaterials, when incorporated into PDMS, can improve antifouling performance by lowering the number of eukaryotic microorganisms [110] and barnacle adhesion strengths [106,107]. This contrasts with other studies which showed that MWCNT-centered coatings have no impact on the settlement of microfoulers and macrofoulers [104].

Furthermore, carboxylate- and hydroxyl-functionalized MWCNTs have been evaluated to determine their contribution to marine coating AF performance. Sun and Zhang performed an extensive two-month field trial, in which MWCNTs were added to PDMS coatings with variations in the hydroxyl and carboxyl content (w/w), the diameter, and the length to examine the effects of introducing the changes [110]. The type of MWCNT applied to the coating, as well as the pioneer eukaryotic communities, had a significant impact on the AF behavior of the coating. The findings of Sun and Zhang demonstrate that AF behavior varies as a function of the bacteria strain type associated with the pioneer biofilm community and also shows dependency on the carboxyl- or hydroxyl-modified coating used. In a different investigation, however, Ji et al. observed that the majority of carboxyl- and hydroxyl-modified carbon coatings had weak modulatory impacts on the biofilm community [109]. It has also been demonstrated that fluorinated MWCNT-based coatings can reduce E. coli [109] to 98% and can prevent pseudobarnacles from adhering [105].

Additionally, results from a combination of silicone oil and lubricants in polymeric matrices showed that MWCNT-based coatings could be improved by the addition of lubricants [108]. Using this method, superhydrophobic fouling-release surfaces were developed by lubricant leaching from the surface over time. This resulted in the formation of a layer of oil serving as a shield against corrosion, originating from frictional forces progressing over time, as well as an enhancement in the antifouling capabilities [117].

The findings of this study suggest that when developing new antifouling coatings for marine environments, CNT-based coatings can be considered as an option.

3.4. Functionalized Carbon Coatings for Pollutant Adsorption

There are many reasons why carbon-based materials are ideal platforms for developing new adsorbents with enhanced functionality, including their noncorrosive nature, the ability to tune the surface chemistry, large surface areas, and the occurrence of oxygen-containing functional groups. In order to manufacture carbon-based materials for environmental applications, most of the processes that are used are chemical processes, such as chemical oxidation and deposition (chemical oxidation), as well as extended chemical processes (for example electrochemical, sol–gel, microemulsion, hydrothermal, and electrochemical).

Water treatment could greatly benefit from the use of these materials if it is designed to obtain suitable properties and structures. Therefore, carbon-based materials are generally functionalized deliberately with a view to obtain synergistic multifunctionalities or for solving specific problems in order to achieve synergistic advantages over conventional materials. Whenever functional groups are present on the surface of a carbon backbone, adsorption is always made easier. Several functional groups have been grafted onto the top of the surface of carbon nanomaterials (such as -NH₂, -SH, and -COOH), as well as to enhanced functional groups such as -COOH and -OH, which are present on the surface. It is important to coat the carbon backbone with other nanoparticles (NPs) in order to improve the performance of these materials [118,119]. A significant advantage of the carbon backbone stems from its strong adsorption behavior and large specific surface area [120,121].

Core–shell structure chemistry has received considerable attention in recent years due to the wide range of applications in areas such as biomedicine, catalysis, electronics, pharmaceuticals, and environmental pollution control. In carbon nanomaterials, the core and shell are usually composed of different materials. Material combinations are few, with a handful of examples that include Fe₃O₄@C, RGO@Al₂O₃, C@Si, and CNT@C. Nanoscale carbon-based core–shell materials have largely been utilized as high-capacity electrodes to isolate and remove solvated metals; their application in aqueous removal of heavy metals has yet to be established. However, complex nanocomposites have been used to remove heavy metals from aqueous solutions. In this direction, the properties of CNTs and graphene nanomaterials have been drastically altered to induce changes at the metal site, with magnetization facilitating their isolation and removal towards the adsorbing material. A heavy metal is considered toxic if it is nonbiodegradable, bioaccumulative, or extremely hazardous. There are several heavy metals, including Hg, Pb, Cr, and Cd, which are more commonly found and problematic. Plants, animals, and people are at risk for serious diseases due to the presence of heavy metals in the environment and trace quantities are often associated with heightened risk (Table 4). A variety of functionalized CNTs as well as graphene nanomaterials have been successfully applied for the purpose of aqueous heavy metal removal by adsorption.

Magnetized carbon materials have also been developed for the capture of other functionalized nanomaterials, particularly heavy metals. Contact among metal ions and oxygen-rich materials is a major factor in the processing of heavy metal adsorption on CNTs and graphene-based nanomaterials. Pretreatments are usually required to enhance the functional-group abundance of oxygen-associated elements on the surface of raw CNTs. In contrast, the oxygen-rich surface chemistry of graphene requires no acid or oxidation treatment. In the case of Cr(VI), a reduction process must be carried out before either adsorption or precipitation can occur. For iron oxide, carbonization of cellulose to aid the reduction of Fe3O4 nanoparticles can provide a route to the fabrication of magnetic carbon–iron nanoadsorbents. The functionalized nanoadsorbent surface can facilitate the removal of Cr(VI) through the reduction of cellulose and Fe(NO₃)₃ precipitation. Furthermore, easy separation of MC-N and MC-O was accomplished using a permanent magnet after the reaction. There have been a number of studies on magnetic nanocomposites in recent years. This is largely due to their low cost, environmentally friendly composition, and large surface area of mesoporous nanostructure that permits rapid adsorption and ease of retrieval using an externally applied magnetic field. Magnetic separation of heavy metals allows environmental risks to be minimized as shown in Figure 12a that heavy metal nanoparticles adsorb efficiently by magnetic nanoparticles. Further, magnetically functionalized nanomaterials, while recycling magnetic particles, offer an ecofriendly method to reapply the separation process with minimal to no damage to the environment compared to other methods (Figure 12b). Figure 13b illustrates a typical example of heavy metal removal. Here, surfactant (tween-20)-functionalized Au NPs have been applied for the removal of aqueous Hg (II) in a high-salt matrix (artificial seawater) by reducing Hg²⁺ to Hg⁰ through the deposition of Hg⁰ on the NP surface. The separation process was completed by using magnetic and reduced graphene oxide-Fe₃O₄ NPs.

Figure 12. A schematic diagram showing heavy metal absorption and separation using (a) magnetic nanoparticles (b) magnetic carbon-based nanomaterials used in heavy metal separation and nanoparticle retrieval. Reprinted with permission from [122]. 2018, Chemosphere.

Figure 12. A schematic diagram showing heavy metal absorption and separation using (a) magnetic nanoparticles (b) magnetic carbon-based nanomaterials used in heavy metal separation and nanoparticle retrieval. Reprinted with permission from [122]. 2018, Chemosphere.

Table 4. Aqueous toxic heavy metal adsorption by functionalized carbon-based nanomaterials.

S.N.	Carbon-Based nanomaterials	Surface Area (m2 g−1)	pH	Adsorption Capacity (mg g−1)
				Exp.	Cal.
1.	Pb (II)
[123]	Multiwalled Carbon Nanotube	-	6.0	-	15.9
[124]	Acidified MWCNTs	237.3	5.0	85.0	49.7
[123]	Multiwalled Carbon Nanotube/VP	-	6.0	-	37.0
[125]	Multiwalled Carbon Nanotube/TiO2	-	6.0	4.6	137.0
[126]	Carbon Nanotube/MnO2	275.0	5.0	-	78.7
[127]	Single-walled Carbon Nanotube/COOH	400.0	5.0	-	94.7
[128]	Carbon Nanotube/Fe3O4-NH2	90.7	5.3	37.6	75.0
[129]	Carbon Nanotube/Fe3O4-MPTS	97.2	6.5	42.1	65.4
[130]	Graphene Oxide	430.0	6.8	328.0	367.0
[130]	EDTA/Graphene Oxide	623.0	6.8	479.0	525.0
[130]	EDTA/Reduced graphene Oxide	730.0	6.8	204.0	228.0
[131]	Graphene Oxide/MnFe2O4	196.0	5.0	-	673.0
[132]	Reduced graphene oxide/nZVI	-	5.0	550.0	585.5
[133]	CoFe2O4/Graphene Oxide	212.7	-	81.3	82.3
[134]	M-CHAP/Graphene Oxide	106.0	4.5	244.5	246.1
[135]	HMO/Graphene Oxide	383.9	5.0	553.6	512.6
[136]	Smart Magnetic Graphene	165.0	6.5	-	6.0
2.	As (III)
[137]	Iron oxide/Multiwalled Carbon Nanotube	-	8.0	1.8	4.0
[138]	Fe3O4/Multiwalled Carbon Nanotube	70.1	-	-	53.2
[139]	Fe/Multiwalled Carbon Nanotube	-	7.0	210.0	200.0
[140]	EDA/Multiwalled Carbon Nanotube/Fe2+	198.5	8.0	0.7	N/A
[141]	MIO/Multiwalled Carbon Nanotube	209.8	7.0	17.2	20.2
[131]	Graphene Oxide/MnFe2O4	196.0	6.5	-	146.0
[142]	Fe3O4/Reduced Graphene Oxide	148	7.0	-	13.1
[143]	HA/Reduced Graphene Oxide/Fe3O4	0.9	7.0	7.5	8.7
[144]	HEG/electrodes	442.9	6.1	-	138.8
[145]	GAC/ZrO2	903.0	7.6	-	12.2
[146]	Multiwalled Carbon Nanotube/ZrO2	152.0	6.0	9.8 × 10−2	2.0
[147]	Fe3O4/Multiwalled Carbon Nanotube	153.0	4.0	9.7 × 10−2	1.7
3.	As (V)
[148]	Fe3O4/Multiwalled Carbon Nanotube	70.1	-	-	39.1
[149]	MIO/Multiwalled Carbon Nanotube	209.8	7.0	36.3	40.8
[139]	Fe/Multiwalled Carbon Nanotube	-	7.0	220.0	200.0
[131]	Graphene Oxide/MnFe2O4	196.0	4.0	-	207.0
[147]	Graphene oxide/Fe(OH)3	-	4.0	23.8	N/A
[142]	Fe3O4/Reduced graphene oxide	148	7.0	-	5.83
[143]	HA- Reduced graphene oxide/Fe3O4	0.9	7.0	16.0	61.7
[144]	HEG/electrodes	442.9	6.9	-	141.9
[148]	Mg-Al LDHs/Graphene Oxide	35.4	5.0	183.1	180.3
[146]	Reduced graphene oxide/ZrO2	152.0	6.0	0.1	5.0
[140]	EDA/Reduced graphene oxide/Fe2+	198.5	4.0	1.0	18.1
[149]	Fe3O4/Reduced graphene oxide	153.0	4.0	0.1	0.2
[136]	Smart Magnetic Graphene	165.0	6.5	-	3.3
4.	Hg (II)
[150]	Oxidized Multiwalled Carbon Nanotube	-	7.0	-	3.8
[151]	Multiwalled Carbon Nanotube/COOH	-	4.3	81.6	127.6
[151]	Multiwalled Carbon Nanotube/OH	-	4.3	89.4	120.1
[152]	Multiwalled Carbon Nanotube/SH	-	5.0	74.2	131.6
[153]	Multiwalled Carbon Nanotube/iodide	153.0	6.0	100.0	123.5
[153]	Multiwalled Carbon Nanotube/S	155.3	6.0	100.0	151.5
[154]	Carbon Nanotube/MnO2	110.4	6.0	14.3	58.8
[155]	KMnO4-DES/Carbon Nanotube	199.4	5.5	187.0	250.5
[156]	CS/Multiwalled Carbon Nanotube/COOH	-	4.0	183.2	181.8
[156]	CS/Multiwalled Carbon Nanotube	-	4.0	167.5	169.4
[129]	Multiwalled Carbon Nanotube/Fe3O4-SH	97.2	6.5	63.7	65.5
[157]	Tween 20-Au/graphite-Fe3O4	-	-	-	47.6
5.	Cr (VI)
[158]	IL-oxi-Multiwalled Carbon Nanotube	87.4	3.0	2.6	85.8
[159]	Carbon Nanotube/CeO2	-	7.0	30.2	31.6
[160]	DBSA-PANI/Multiwalled Carbon Nanotube	-	2.0	49.5	55.6
[161]	AC/Carbon Nanotube	755.8	2.0	9.0	8.6
[162]	AA/Carbon Nanotube	203.0	2.0	142.8	264.5
[163]	NH2-Fe3O4/Graphene Oxide	43.6	4.5	27.3	32.3
[164]	MC-N	136.3	1.0	327.5	N/A
[136]	Smart Magnetic Graphene	165.0	6.5	-	4.9
6.	Cd (II)
[165]	Raw Multiwalled Carbon Nanotube	187.6	8.0	1.29	1.3
[165]	Oxidized Multiwalled Carbon Nanotube	78.5	8.0	22.32	22.4
[165]	EDA/Multiwalled Carbon Nanotube	101.2	8.0	21.23	21.7
[166]	Al2O3 Multiwalled Carbon Nanotube	109.8	7.0	0.948	27.2
[167]	Single-walled Carbon Nanotube	400.0	5.0	-	21.2
[167]	COOH/Single-walled Carbon Nanotube	400.0	5.0	-	55.4
[168]	Oxidized Carbon Nanotube	128.0	5.5	-	11.0
[162]	AA/Carbon Nanotube	203.0	7.5	200.0	229.9
6.	Cu (II)
[169]	Oxidized Carbon Nanotube	-	7.0	50.4	64.9
[170]	Purified Multiwalled Carbon Nanotube	169.7	5.0	37.5	36.8
[168]	Oxidized Multiwalled Carbon Nanotube	-	5.0	29	28.5
[170]	Sulfonated Multiwalled Carbon Nanotube	28.7	5.0	59.6	43.2
[171]	OH/Multiwalled Carbon Nanotube	111.4	4.9	7.0	10.1
[171]	COOH/Multiwalled Carbon Nanotube	135.2	4.9	5.5	8.1
[167]	Single-walled Carbon Nanotube	400.0	5.0	-	22.9
[167]	COOH/Single-walled Carbon Nanotube	400.0	5.0	-	72.3
[172]	OH/Single-walled Carbon Nanotube/RGO	-	6.8	-	256.0
[172]	COOH/Single-walled Carbon Nanotube/RGO	-	6.8	-	63.0
[173]	Multiwalled Carbon Nanotube/Fe3O4	138.7	5.5	19.0	38.9
7.	Zn (II)
[169]	Oxidized Carbon Nanotube	-	7.0	58.0	74.6
[174]	Purified Single-walled Carbon Nanotube	423.0	-	15.4	41.8
[175]	Oxidized Multiwalled Carbon Nanotube	250.0	6.5	2.0	1.1
8.	Ni (II)
[176]	Multiwalled Carbon Nanotube	197.0	5.4	2.9	3.7
[176]	PAA/Multiwalled Carbon Nanotube	-	5.4	-	3.9
[177]	Oxidized Multiwalled Carbon Nanotube	102.0	6.5	12.5	17.9
9.	Co (II)
[169]	Oxidized Carbon Nanotube	-	7.0	69.6	85.7
[178]	Multiwalled Carbon Nanotube/iron oxide	-	6.4	2.9	10.6
10	U (VI)
[179]	CB/Graphene Oxide/Fe3O4	-	5.0	-	122.5
[180]	Fe3O4/Graphene Oxide	-	5.5	-	69.5
[181]	Amidoxime Fe3O4/Graphene Oxide	-	5.0	76.88	92.4
[182]	CMPEI/CMK	1350.0	4.0	-	151.5

Table 4. Aqueous toxic heavy metal adsorption by functionalized carbon-based nanomaterials.

S.N.	Carbon-Based nanomaterials	Surface Area (m2 g−1)	pH	Adsorption Capacity (mg g−1)
				Exp.	Cal.
1.	Pb (II)
[123]	Multiwalled Carbon Nanotube	-	6.0	-	15.9
[124]	Acidified MWCNTs	237.3	5.0	85.0	49.7
[123]	Multiwalled Carbon Nanotube/VP	-	6.0	-	37.0
[125]	Multiwalled Carbon Nanotube/TiO2	-	6.0	4.6	137.0
[126]	Carbon Nanotube/MnO2	275.0	5.0	-	78.7
[127]	Single-walled Carbon Nanotube/COOH	400.0	5.0	-	94.7
[128]	Carbon Nanotube/Fe3O4-NH2	90.7	5.3	37.6	75.0
[129]	Carbon Nanotube/Fe3O4-MPTS	97.2	6.5	42.1	65.4
[130]	Graphene Oxide	430.0	6.8	328.0	367.0
[130]	EDTA/Graphene Oxide	623.0	6.8	479.0	525.0
[130]	EDTA/Reduced graphene Oxide	730.0	6.8	204.0	228.0
[131]	Graphene Oxide/MnFe2O4	196.0	5.0	-	673.0
[132]	Reduced graphene oxide/nZVI	-	5.0	550.0	585.5
[133]	CoFe2O4/Graphene Oxide	212.7	-	81.3	82.3
[134]	M-CHAP/Graphene Oxide	106.0	4.5	244.5	246.1
[135]	HMO/Graphene Oxide	383.9	5.0	553.6	512.6
[136]	Smart Magnetic Graphene	165.0	6.5	-	6.0
2.	As (III)
[137]	Iron oxide/Multiwalled Carbon Nanotube	-	8.0	1.8	4.0
[138]	Fe3O4/Multiwalled Carbon Nanotube	70.1	-	-	53.2
[139]	Fe/Multiwalled Carbon Nanotube	-	7.0	210.0	200.0
[140]	EDA/Multiwalled Carbon Nanotube/Fe2+	198.5	8.0	0.7	N/A
[141]	MIO/Multiwalled Carbon Nanotube	209.8	7.0	17.2	20.2
[131]	Graphene Oxide/MnFe2O4	196.0	6.5	-	146.0
[142]	Fe3O4/Reduced Graphene Oxide	148	7.0	-	13.1
[143]	HA/Reduced Graphene Oxide/Fe3O4	0.9	7.0	7.5	8.7
[144]	HEG/electrodes	442.9	6.1	-	138.8
[145]	GAC/ZrO2	903.0	7.6	-	12.2
[146]	Multiwalled Carbon Nanotube/ZrO2	152.0	6.0	9.8 × 10−2	2.0
[147]	Fe3O4/Multiwalled Carbon Nanotube	153.0	4.0	9.7 × 10−2	1.7
3.	As (V)
[148]	Fe3O4/Multiwalled Carbon Nanotube	70.1	-	-	39.1
[149]	MIO/Multiwalled Carbon Nanotube	209.8	7.0	36.3	40.8
[139]	Fe/Multiwalled Carbon Nanotube	-	7.0	220.0	200.0
[131]	Graphene Oxide/MnFe2O4	196.0	4.0	-	207.0
[147]	Graphene oxide/Fe(OH)3	-	4.0	23.8	N/A
[142]	Fe3O4/Reduced graphene oxide	148	7.0	-	5.83
[143]	HA- Reduced graphene oxide/Fe3O4	0.9	7.0	16.0	61.7
[144]	HEG/electrodes	442.9	6.9	-	141.9
[148]	Mg-Al LDHs/Graphene Oxide	35.4	5.0	183.1	180.3
[146]	Reduced graphene oxide/ZrO2	152.0	6.0	0.1	5.0
[140]	EDA/Reduced graphene oxide/Fe2+	198.5	4.0	1.0	18.1
[149]	Fe3O4/Reduced graphene oxide	153.0	4.0	0.1	0.2
[136]	Smart Magnetic Graphene	165.0	6.5	-	3.3
4.	Hg (II)
[150]	Oxidized Multiwalled Carbon Nanotube	-	7.0	-	3.8
[151]	Multiwalled Carbon Nanotube/COOH	-	4.3	81.6	127.6
[151]	Multiwalled Carbon Nanotube/OH	-	4.3	89.4	120.1
[152]	Multiwalled Carbon Nanotube/SH	-	5.0	74.2	131.6
[153]	Multiwalled Carbon Nanotube/iodide	153.0	6.0	100.0	123.5
[153]	Multiwalled Carbon Nanotube/S	155.3	6.0	100.0	151.5
[154]	Carbon Nanotube/MnO2	110.4	6.0	14.3	58.8
[155]	KMnO4-DES/Carbon Nanotube	199.4	5.5	187.0	250.5
[156]	CS/Multiwalled Carbon Nanotube/COOH	-	4.0	183.2	181.8
[156]	CS/Multiwalled Carbon Nanotube	-	4.0	167.5	169.4
[129]	Multiwalled Carbon Nanotube/Fe3O4-SH	97.2	6.5	63.7	65.5
[157]	Tween 20-Au/graphite-Fe3O4	-	-	-	47.6
5.	Cr (VI)
[158]	IL-oxi-Multiwalled Carbon Nanotube	87.4	3.0	2.6	85.8
[159]	Carbon Nanotube/CeO2	-	7.0	30.2	31.6
[160]	DBSA-PANI/Multiwalled Carbon Nanotube	-	2.0	49.5	55.6
[161]	AC/Carbon Nanotube	755.8	2.0	9.0	8.6
[162]	AA/Carbon Nanotube	203.0	2.0	142.8	264.5
[163]	NH2-Fe3O4/Graphene Oxide	43.6	4.5	27.3	32.3
[164]	MC-N	136.3	1.0	327.5	N/A
[136]	Smart Magnetic Graphene	165.0	6.5	-	4.9
6.	Cd (II)
[165]	Raw Multiwalled Carbon Nanotube	187.6	8.0	1.29	1.3
[165]	Oxidized Multiwalled Carbon Nanotube	78.5	8.0	22.32	22.4
[165]	EDA/Multiwalled Carbon Nanotube	101.2	8.0	21.23	21.7
[166]	Al2O3 Multiwalled Carbon Nanotube	109.8	7.0	0.948	27.2
[167]	Single-walled Carbon Nanotube	400.0	5.0	-	21.2
[167]	COOH/Single-walled Carbon Nanotube	400.0	5.0	-	55.4
[168]	Oxidized Carbon Nanotube	128.0	5.5	-	11.0
[162]	AA/Carbon Nanotube	203.0	7.5	200.0	229.9
6.	Cu (II)
[169]	Oxidized Carbon Nanotube	-	7.0	50.4	64.9
[170]	Purified Multiwalled Carbon Nanotube	169.7	5.0	37.5	36.8
[168]	Oxidized Multiwalled Carbon Nanotube	-	5.0	29	28.5
[170]	Sulfonated Multiwalled Carbon Nanotube	28.7	5.0	59.6	43.2
[171]	OH/Multiwalled Carbon Nanotube	111.4	4.9	7.0	10.1
[171]	COOH/Multiwalled Carbon Nanotube	135.2	4.9	5.5	8.1
[167]	Single-walled Carbon Nanotube	400.0	5.0	-	22.9
[167]	COOH/Single-walled Carbon Nanotube	400.0	5.0	-	72.3
[172]	OH/Single-walled Carbon Nanotube/RGO	-	6.8	-	256.0
[172]	COOH/Single-walled Carbon Nanotube/RGO	-	6.8	-	63.0
[173]	Multiwalled Carbon Nanotube/Fe3O4	138.7	5.5	19.0	38.9
7.	Zn (II)
[169]	Oxidized Carbon Nanotube	-	7.0	58.0	74.6
[174]	Purified Single-walled Carbon Nanotube	423.0	-	15.4	41.8
[175]	Oxidized Multiwalled Carbon Nanotube	250.0	6.5	2.0	1.1
8.	Ni (II)
[176]	Multiwalled Carbon Nanotube	197.0	5.4	2.9	3.7
[176]	PAA/Multiwalled Carbon Nanotube	-	5.4	-	3.9
[177]	Oxidized Multiwalled Carbon Nanotube	102.0	6.5	12.5	17.9
9.	Co (II)
[169]	Oxidized Carbon Nanotube	-	7.0	69.6	85.7
[178]	Multiwalled Carbon Nanotube/iron oxide	-	6.4	2.9	10.6
10	U (VI)
[179]	CB/Graphene Oxide/Fe3O4	-	5.0	-	122.5
[180]	Fe3O4/Graphene Oxide	-	5.5	-	69.5
[181]	Amidoxime Fe3O4/Graphene Oxide	-	5.0	76.88	92.4
[182]	CMPEI/CMK	1350.0	4.0	-	151.5

Electrostatic attraction and surface complexation are major strategies by which heavy metals adsorb onto functionalized carbon-based systems [28]. Figure 13a demonstrates the utility of surface complexation and electrostatic attraction for the removal of heavy metals [183]. As an example, Madadrang et al. used modified graphene oxide complexed with hexadentate ligands using ethylenediaminetetraacetic acid (EDTA). The method has been utilized to treat mercury (II)-, lead(II)-, nickel(II)-, cadmium(II)-, and copper(II)-contaminated samples by two prospective mechanisms [184]. The electrostatic interaction of heavy metal ions (M^x+) with oxygenic functional groups complexed on GO after functionalization via hydroxyl and carboxyl moieties provided easy access to metal uptake. In addition, the nature of EDTA as a hexadentate chelating agent with the ability to form coordination complexes with heavy metal ions (M^x+) enhances the coordination interactions of functionalized GO (Figure 13b).

Figure 13. (a) Probable toxic heavy metal removal mechanism on graphene oxide as an adsorbent from water. Reprinted with permission from [183]. 2020, Chemosphere. (b) Scheme shows the chemical structure of ethylenediaminetetraacetic-acid-functionalized graphene oxide before (left) and after it has been exposed to heavy metal cations (right) Reprinted with permission from [130]. 2012, ACS Appl. Mater. Interfaces.

Figure 13. (a) Probable toxic heavy metal removal mechanism on graphene oxide as an adsorbent from water. Reprinted with permission from [183]. 2020, Chemosphere. (b) Scheme shows the chemical structure of ethylenediaminetetraacetic-acid-functionalized graphene oxide before (left) and after it has been exposed to heavy metal cations (right) Reprinted with permission from [130]. 2012, ACS Appl. Mater. Interfaces.

3.5. Bioinspired Functional Carbon Coatings

Carbon-based functionalized coatings play a pivotal role in achieving various functions of living organisms. Exoskeleton architectures are one of the better examples of functionalized carbon coatings that exist in nature. Animal exoskeletons, commonly known as cuticles, are covered with various coatings that may comprise a network of hydrocarbons, including phenolamine, which are coproduced with molecules that originate from oenocytes. These are specialized cells that offer protection from water loss, damage, and bacterial degradation. An interesting example of the role of functionalized coatings can be observed in mosquitoes that employ hydrocarbon coatings as defense barriers against insecticides and changes due to climate [184,185,186]. Another interesting study has suggested that the wings of particular fly species contain structural materials; therefore, instead of repelling contaminants, they are capable of killing contaminants and preventing bacterial colonization [187] on the wing surface. The cave barklice, Psyllipsocus Yucatan, from dolomit cave “Toca da Tiquara” in Brazil, illustrates an example of the unique coating of their wing membranes. This species has typically transparent light-brown wings, but some females have been found to have opaque black wings, with their surface covered with a coating. Crystallographic and material analysis of the wing surface has revealed an ultrathin layered-type morphology (1.5 µm) of catalyzed minerals containing iron, oxygen, and sulfur. It appears that the supersaturated climatic conditions and the presence of an iron-excreting bat species residing in caves could be the origin of the film formation. However, the confinement of deposits to only the wing surface, and their exclusion from other parts and hairs, indicate that the high-frequency vibration of the wings could aid the condensation of the thin layer. Detailed knowledge of this type has encouraged multidisciplinary efforts to mimic the natural composition and to advance material design with specific functional properties.

Moreover, in recent investigations, concepts embedded in biomimetics have been increasingly adopted and applied in coating technologies. Several applications aligned to biomimetics have been demonstrated to achieve superhydrophobic coatings by making use of petroleum pipelines, corrosion protection, and biomedical engineering [188,189,190]. These approaches have been largely inspired by features found in natural lotus leaves and have inspired a number of research directions [191,192]. These coatings offer excellent performance in the above-mentioned applications because of their antifouling, self-cleaning, and anticorrosion properties. In a recent study, Zhu et al. developed polysulfone–carbon nanotube–fluorinated ethylene propylene nanocomposite (PCFn) coatings and electrostatic powder spraying was employed to fabricate nanocomposite coatings with biomimetic golden spherical cactus surface structures (with microspheres and nanothorns). PCFn coatings combine the advantages of PSU, CNTs, and FEP to produce coatings that resemble the golden cactus structure and mimic the hydrophobic characteristics, enabling its water-repelling nature to present a durable, self-cleaning, and corrosion-resistant surface (SEM image in Figure 14a–c). Some other examples of applications of the biomimetic approach to achieve functional coatings are summarized in

Table 5. Biomimicry in functionalized carbon-based coating technology.

S.N.	Model System	Biomimicked Features	Biomimicked Properties	Biomimicked Coating Material	Coating Method	References
1	Lotus and Xanthosoma Sagittifolium leaves	Superhydrophobic surface	Corrosion resistance	Polymer (polybenzoxazine)	Templating by leaves	[193]
2	Dragonfly wings	Nanostructures	Mechanobactericidal behavior	Palmitic and stearic acid films onto HOPG	Surface recrystallization by self-assembly	[194]
3	Cicada wing surfaces	Nanopillar/Nanodagger-shaped microstructure	Bacterial capture	ZIF MOFs	Dip coating	[195]
4	Endothelium	Endothelium functions	Controllable heparin anchoring and NO release properties	Metal–catechol–amine	Surgace Engineering	[196]
5	Natural superhydrophobic surfaces	Superhydrophobic surface	Superhydrophilic/superhydrophobic surfaces	Branched poly(ethylenimine) (BPEI) and dipentaerythritol pentaacrylate (5Acl)	“Chemically reactive” dip-coating	[197]
6	Bristle worm (Calliteara pudibunda)	Superhydrophobic surface	Anticorrosion and antiscaling, self-cleaning, and antifouling properties	Composites of polyphenylene sulfide (PPS), expandable graphite (EAG), and elastic fluororubber	-	[198]
7	Earthworm	Secretion behavior under extra stimulation	Friction-reduction and antifouling	PDMS	Solution casting	[199]
8	Wood	Plant cell wall tissue	Controllable architecture viscoelasticity	Nanocellulose, lignin	Layer-by-layer (LbL) assembly	[200]
9	Owls	Velvety coating on feather surface	Flight sound reduction and Aerodynamics	Nylon fibers	Electrostatic flocking	[201]
10	Willow-leaf-like or wheat-haulm-leaf-like	Superhydrophobic surface	Corrosion resistance	PTFE/PPS_nanofibers/spheres	Coating-curing process	[202]
11	Human skin	Wrinkles	Mechanics of wrinkling at micro nanoscale	TiN on polyurethane, polycarbonate, polyamide, and polyimide	Pulsed laser deposition	[203]
12	Human bones	Orthopedic and dental binding	Bone-bonding ability	Yttria-stabilized tetragonal zirconia and CaP	Apatite coating technique by femtosecond laser processing	[204]
13	Psyllipsocus yucatan	Wing membranes surface coating	Surface coating	Iron, oxygen, and carbon	Natural coating	[205]
14	Polypedilum vanderplanki larvae	Cuticle (exoskeleton)	Protective layer	Polyphenol at air/solid interface catalyzed by gaseous oxygen	Aerobic oxidation	[206]

.

4. Conclusions and Future Perspective

It is clear from the scope and the quality of the research discussed in this review article that functionalized carbon-based materials have the potential to be useful candidates for protective coating applications. In the literature, there are some discrepancies that relate to functionalized carbons and their ability to prevent corrosion. Wet corrosion has been found to be more extensive on CVD-grown carbon-based material coatings than uncoated metal surfaces, despite showing a good corrosion barrier for a short period of time. The literature search supports O₂ and H₂O diffusion through graphene-based material defects and the subsequent oxidization of encapsulated metals. The vulnerability of the surface is also compromised by coating discontinuities in graphene layers. The literature indicates that graphene-like material can protect against corrosion; however, it should also be noted that there is also a need for improved coating technologies and methodologies to enhance the effectiveness of the carbon-family-based system. A few surprising discoveries have been made regarding hybridized or functional groups composed of sp² hybridized carbon coatings systems that are anticorrosive by nature. The use of sp² hybridized carbon coatings systems as an anticorrosion barrier is counterintuitive since graphite promotes the corrosion of metals when it is in contact with them.

A new level of fire resistance has been achieved using functionalized carbon-based materials as fire-retardant additives. It is possible to enhance the thermal and barrier properties of thermoset polymers by dispersing functionalized carbon-based materials into them. Functionalized carbon-based material composites are also able to withstand the transfer of noxious and flammable gases during a fire, thanks to their intrinsic impermeability to gases, with high surface area in combination with tumescent flame-retardant behavior. A functionalized carbon surface can reduce melt dripping and delay structural collapse at high temperatures by reinforcing and maintaining its integrity. The development of flame-retardant materials and coatings with optimized formulations is expected in the future.

Depending on the surface chemistry and wettability of functionalized carbon-based materials, the interface can alter the physisorption of different molecules, allowing materials such as antifouling coatings, carriers, and adsorbents to be passively controlled during adsorption and desorption. In addition to being pertinent to both dry and marine atmospheres, functionalized carbon-based materials are also very effective as an antifouling mediator in surface coatings, which can be applied to surfaces such as domestic and construction surfaces, salt removal systems, bioreactors, and aquatic structures. There is a serious problem with the price and the large-scale applicability of coating technology in the antifouling field, since most applications require large quantities of functionalized carbon-based materials. The application of bioinspired or biomimetic coatings to functionalized carbon-based materials can potentially be valuable for antifouling applications if an economical coating approach can be established. It is possible to reduce surface colonization by microorganisms by exploiting graphene surfaces with amphiphilic behaviour. In order to optimize coatings to address the specific and unique antifouling problems associated with specific industries and overcome them, more research is needed. In terms of surface chemistry, functionalized carbon-based materials possess a tunable surface chemistry, which allows it to be used as a highly surface-active material, capable of selectively adsorbing a large quantity of pollutants. It is possible to target a wide range of ingredients, involving hydrocarbons, heavy metal ions, and dyes, among others. Material made from Gr can adsorb and expel 35,000 times its own weight.

Applied research is increasingly reaching the point of being applied to a commercial scale and research efforts around the world are intensifying; however, there remain significant challenges to overcome, particularly in terms of low-cost and scalable Gr manufacturing and handling. The safety of Gr materials must be proven through in-depth studies in medical and environmental applications. Graphene will bring a technological revolution when it is realized, according to the research community. This versatile material will be widely adopted by the coatings industry as an additive and as a single material to improve coating performance in the present era as a genuine versatile tool of the 21st century for a multitude of coating applications.

Finally, the biomimetic significance of carbon-based coatings offers a considerably viable option to incorporate features that better align with the environment, particularly if there is structural and functional resemblance to naturally evolved scaffolds. Such composites are representative of 3D structures, exhibiting physical characteristics such as porosity, polymer morphology, adsorption, and mechanical properties, which can collectively impact both the structural and functional workability of the materials, which may be further tuned as a composite. The desirability of mimicking nature will likely be an important imperative to the design of future materials as synthetics or biosynthetic hybrids, improving the multidimensionality of the coating. The growth of such materials in this direction will likely be important. Therefore, in the future, understanding the functional behavior of coating living organisms and applying them for synthetic coating would be an effective pathway to produce improved carbon-based functional coatings.