Luminescent Pyrene-Derivatives for Hg²⁺ and Explosive Detection

Abstract

Mercury and explosives are well-known hazards that affect the environment and threaten society. Mercury generally exists as inorganic mercuric (Hg²⁺) salts, and its detection via fluorometric response is highly notable. Likewise, mainstream explosives contains a nitro (−NO₂) moiety as a functional unit, and numerous reports have quantified them using fluorescence quenching. Among the available literature, there are still noticeable concerns about the environmental and biological applicability of luminescent pyrene derivaives-tunedfluorometric detection of Hg²⁺ and explosives. In the presence of Hg²⁺ ions, pyrene derivatives tend to form excimers, which can be tuned to the chelation-enhanced fluorescence (CHEF), photo-induced electron transfer (PET), or fluorescence resonance energy transfer (FRET), etc., to exhibit “Turn-On” or “Turn-Off” fluorescence responses. On the other hand, π-π stacking of emissive pyrene-derivatives may lead to J- or H-type aggregation via self-excimers (Py-Py*), which has been found to be quenched/enhanced by explosive hazards. In fact, −NO₂-containing explosives interact with pyrene derivatives, leading to exceptional fluorescence quenching or enhancement. This review details the use of pyrene derivatives toward the sensing of Hg²⁺ and explosives with demonstrated applications. Further, the design requirements, sensory mechanisms, advantages, limitations, and the future scope of using the reported pyrene derivatives in Hg²⁺ and explosives sensing are discussed.

1. Introduction

Environmental toxicity related to hazardous metal ions is a global threat that must be resolved with care [1,2,3]. Among common metal hazards, mercury detection is a serious concern requiring environmental remediation. Mercury can enter the environment in three ways: 1. volcano explosions into the air, forest fires, and the weathering of rocks and soils; 2. The burning of industrial fossil fuels, coals, and from industries that produce gold, thermometers, barometers, caustic soda, mercury lamps, etc.; and, finally, 3. ocean evaporation can re-introduce mercury into the environment [4,5,6,7]. The accumulation of mercury in the environment affects the living eco-system by entering into the daily food cycle, thus requiring more scrutiny [8,9,10]. At high concentrations, mercury can cause diseases such as brain damage, Minamata disorders, cognitive and motion disorders, central nervous syndromes, impaired infant development, cardiovascular diseases, anxiety depression, etc. [11,12,13,14,15]. Thus, the U.S. Environmental Protection Agency (EPA) has regulated the levels of mercury allowed in drinking water and fish tissue to 2 ppb (10 nM) and 3 ppm (1.5 µM), respectively [16,17]. Likewise, the allowable mercury concentration in normal soil is fixed at 625 ppb by the Agency for Toxic Substances and Disease Registry (ATSDR) of the U.S. Department of Health and Human Services [18,19]. To overcome the mercury toxicity in the eco-system, several detection and removal tactics were proposed, including organic/polymeric probes, nanoparticle-based colorimetric detection, instrumentation-based quantification, covalent/metal–organic frameworks (COFs/MOFs), and nanomaterials-based fluorescent sensors, etc. [20,21,22,23,24,25,26].

Like mercury’s environmental toxicity, explosive hazards threaten human life and society [27,28,29]. An explosion may cause damage to living systems and properties. The severity of damage caused by an explosion can be ordered as follows: (A) the death of living people, (B) diseases or loss of health, (C) damage to properties such as buildings, and (D) loss of activity, i.e., physical inactivity [30]. Even though explosives are stored in a non-exploding state, they can harm the environment and lead to numerous diseases in living species [31]. This might be due to the atomic composition present in the explosive materials. For example, most explosive materials consist of electron-deficient nitro groups, which destroy the natural pressure in the environment during an explosion [27,28,29,30,31]. However, for security purposes and to restrict terrorism, the use of explosives by the military cannot be avoided. Instead, while explosives are in the hands of terrorists, their rapid detection using suitable strategies is necessary. Therefore, numerous methods, including COFs/MOFs, luminescent nanomaterials, electrochemical detection, colorimetric assay, fluorescent organic probes, carbon dots, fluorescent semiconductor quantum dots, perovskite nanomaterials, etc., were developed for explosive detection with various applications [32,33,34,35,36,37,38,39,40,41]. Among these, the luminescent organic probe-based detection of nitro-explosives seems to be particularly efficient. In general, the strong fluorescence of organic probes is quenched (static/dynamic) by the interactive nitro groups in explosives, which alter the π-π stacking of the probe, or by the supramolecular interactions of explosive materials with the probe [42,43,44]. In the same vein, through similar mechanistic approaches, organic probes for the fluorescent “Turn-On” detection of explosives are also available; however, it should be noted that reports on these approaches are rare [45,46].

In the design of fluorescent organic probes to be applied in Hg²⁺ and explosives detection, pyrene-based derivatives were established as extraordinary innovations in terms of their fluorescent properties and real-time application [47,48]. In fact, due to the π-π-stacking and highly emissive excimer-forming abilities of pyrene moieties, their derivatives display exceptional fluorescence properties and are applied in optoelectronic devices, sensor bioimaging, etc. [49,50,51]. By tuning the π-π stacking of pyrene derivatives, its aggregation/self-assembly features can be enhanced toward in vitro/in vivo theranostic applications [52]. In this way, numerous reports on the aggregation-induced emission enhancement (AIEE) of pyrene derivatives with certain applications are available [53], allowing researchers to design new pyrene-based probes for Hg²⁺ and explosive detection. Among them, a few reports reveal π-π stacking variations and H-aggregation (head-to-head) and J-aggregation (head-to-tail) during sensing [54,55,56,57,58,59,60]. The detection of Hg²⁺ by pyrene-based probes was achieved by either “Turn-On” or “Turn-Off” fluorescence responses by optimizing the solvent and probe’s design. Likewise, several highly emissive pyrene-based probes (in certain solvent conditions) can detect explosives via a “Turn-Off” fluorescence response via π-π stacking and supramolecular interactions. In contrast, only one report on a hybrid pyrene-based probe is available with a “Turn-On” fluorescent response to explosives; thus, this should be noted as a rare case (see Figure 1). Until now, no review has described the use of pyrene-based probes in the selective detection of Hg²⁺ and explosives with mechanistic details, which allows us to deliver this review.

In this review, the use of luminescent pyrene-based probes to detect Hg²⁺ ions and nitro-explosives is described with mechanistic evidence. Figure 1 briefly outlines the use of weakly/strongly emissive pyrene derivatives in detecting Hg²⁺ ions and explosives via photoluminescence (PL) “Turn-On” and “Turn-Off” fluorescence responses directed towards applications. Finally, requirements for the probe’s design, merits, limitations, and perspective points are debated for the readers.

2. Pyrene Derivatives for Hg²⁺ Detection via PL for “Turn On” Responses

2.1. Excimer Facilitated Hg²⁺ Detection

Yang et al. developed a simple pyrene and amino acid-conjugated probe 1 (See Figure 2A) via multi-step synthesis and employed it for PL “Turn-On” detection via excimer formation [61]. Probe 1 undergoes 2:1 excimer formation to induce the PL “Turn-On” response. In the presence of Hg²⁺_, the excimer emission at 480 nm was enhanced with a simultaneous decrease at 383 nm. Linear regression was witnessed between 0 and 750 nM with an LOD of 57.2 nM (nM = nanomole (10⁻⁹ M)). The association constant (K_a) of 1 to Hg²⁺ was estimated as 7.84 × 10¹² M⁻² (M = Mole), which defines the probe’s high sensitivity to Hg²⁺ ions. Though this work looks impressive, it lacks real-world applications. Further, the design and development of such probes require careful optimization of all the intermediate synthetic steps and critical analysis. However, this work has driven researchers to invent numerous sensory probes, as detailed below.

Areti et al. synthesized the pyrene–glucose conjugate 2 (See Figure 2A) using a simple Schiff base condensation between glucosamine and 1-pyrenecarboxaldehyde and engaged it in the ratiometric PL detection of Hg²⁺ ions [62]. The emission at 398 nm was quenched with a 32-fold enhancement at 500 nm. The formation of a 2-Hg²⁺-2 excimer complex was clarified from the mass data with a proposed LOD of 45 ± 5 nM. Further, the excimer complex displayed reversibility with tetrabutylammonium fluoride (TBAF), which extended up to five cycles. This work looks impressive in terms of the mechanism and LOD. However, it lacks linear regression and real-time demonstration.

Following on from the previous report [61], Thirupathi and co-workers developed two similar pyrene and amino acid conjugates, 3 and 4 (see Figure 2A), with slight modifications and employed them in the discrimination of Hg²⁺ ions via 2:1 excimer formation [63]. Both probes have a linearity between 0 and 500 nM with LODs of 22.2 nM (3) and 44 nM (4). Further, the association constants of 3-Hg²⁺ and 4-Hg²⁺ were estimated as 5.72 × 10¹³ M⁻² and 1.15 × 10¹³ M⁻², respectively. Since this work was a follow-up work and lacks real applications, it cannot be labelled as a unique report. Shellaiah et al. described the use of the aggregation-induced emission (AIE) active pyrene-based probe 5 (See Figure 2B) for the PL “Turn-On” detection of Hg²⁺ ions via 2:1 excimer complex formation and a photoinduced electron transfer (PET) mechanism [64]. Probe 5 was synthesized by refluxing 1-pyrenecarboxaldehyde and 2-Aminothiol in ethanol and show “AIE” in the presence of water (H₂O). The linear regression of 5 to Hg²⁺ was estimated as 0–60 µM (µM = micromole (10⁻⁶ M)) with an LOD of 2.82 µM and a K_a value of 7.36 × 10⁴ M⁻¹. The sensitivity of 5 to Hg²⁺ and the 2:1 complex was defined by time-resolved photoluminescence (TRPL), fluorescence quantum yield (Φ_F; varied from 0.035 to 0.289), and mass data. HeLa cellular imaging interrogations attested to the Hg²⁺ detection. Later, the authors reported the OTFT-based detection of Hg²⁺ with a nanomolar LOD using probe 5 [47]. However, additional studies are still mandatory due to the solution’s LOD at the micromolar level. Using a Schiff base reaction, Wu et al. constructed the “AIE” active probe 6 (See Figure 2B) and demonstrated its Hg²⁺ detection via 2:1 excimer and PET-tuned PL “Turn-On” response [65]. Figure 2C schematically defines the 2:1 excimer complex formation driven by pyrene probes 1–6. With the incremental addition of water, 6 displays an “AIE” and detects Hg²⁺ in H₂O:DMF (v:v = 2:3, PBS buffer, pH 7.0; PBS = phosphate-buffered saline, and DMF = dimethylformamide). As seen in Figure 3a–c, in the presence of Hg²⁺, the emission of 6 at 450 nm was enhanced compared to that of other ions. The linear detection range of Hg²⁺ lies between 0 and 20 µM with an LOD of 0.42 µM and a K_a value of 3.62 × 10⁴ M⁻¹. Spiked real water recoveries were found to be between 89.0 and 94.5%. HeLa cellular imaging studies supported this research, hence it can be noted as a unique work. However, in-depth studies on energy band gaps, Φ_F, and π-π stacking models are still in demand.

Similar to 2:1 complex-tuned excimer formation, a few pyrene-based conjugates display sensitivity to Hg²⁺ ions via 1:1 and 1:2 (probe to metal) complex formation. Figure 4A,B represent the structures of the pyrene-based probes described in this review. The 1:1 excimer complex construction during Hg²⁺ binding is schematically depicted in Figure 3c. Zhou et al. synthesized the azadiene–pyrene probe 7 and utilized it to sense Hg²⁺ via a 1:1 excimer mechanism [66]. The one pot-synthesized Schiff base probe showed a PL “Turn-On” response at 462 nm at 365 nm excitation in HEPES-CH₃CN (80:20, v/v; HEPES = 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid and CH₃CN = Acetonitrile). The fluorescence emission enhancement was visualized between 0.1 and 10 µM. From nonlinear curve fitting, the K_a value was established as 4.32 × 10⁵ M⁻¹ with a LOD of 0.2 µM. The 1:1 excimer complex was recognized from ¹H NMR (NMR = Nuclear Magnetic Resonance) titration. This work appears to be preliminary, not in line with the current state of the art, and requires further investigation to assess the real-world utility of this strategy. Lee et al. developed a series of pyrene-based calix[4]arene compounds; among them, probe 8 displays Hg²⁺ selectivity via 1:2 (probe to Hg²⁺) complex formation, disturbance of the initial pyrene–pyrene excimer, and a fluorescence resonance energy transfer (FRET) mechanism [67]. Here, the pyrene excimer emission at 470 nm was diminished with ratiometric PL enhancement at 576 nm (at 343 nm excitation). An FRET between pyrene and rhodamine was inhibited due to the excimer formation, which became a “Turn-On” response during Hg²⁺ binding via the rhodamine ring opening. The ratiometric response (I₅₇₆/I₄₇₀) starts from 5 µM onwards, which is labelled as an LOD. This work was carried out in CHCl₃/CH₃CN (50:50, v/v; CHCl₃ = chloroform), which raises the question of its actual applicability. Wang and co-workers constructed the pyrene–thiourea conjugate probe 9 to detect Hg²⁺ ions via a 1:1 complex-tuned excimer formation [68]. The monomer emission of 9 at 381 nm diminished while binding with Hg²⁺ with an enhanced excimer peak at 490 nm (22-fold). In this study, the linear regression lies between 0 and 1 µM, with an LOD of 0.74 µM and a K_a value of 1.34 × 10⁴ M⁻¹. Cellular imaging, C. elegans fluorescence images, and Zebra fish imaging interrogations defined the in vitro and in vivo applications of 9. This is an impressive report, but it should be extended with details of material applications.

Rodr’ıguez-Lavado et al. produced the calix[4]arene 10, consisting of pyrene and thiourea conjugation for detecting Hg²⁺ and Ag⁺ ions [69]. The monomer emission at 377 and 395 nm decreased with ratiometric PL enhancement at 477 nm due to the metal complex stimulating excimer formation. The linear regression of 10 to Hg²⁺ ranged between 0 and 1 µM with an LOD of 8.11 nM. The apparent association constant “K_app” value for 10-Hg²⁺ was estimated to be 10.64. Though the application of logic gate supports this research, this probe shows more sensitivity to Ag⁺ ions. Thus, this can be considered a supplementary report, and its applicability is still questionable. Pramanik et al. used the hexaphenylbenzene–pyrene derivative 11 to detect Hg²⁺ via 1:1 a complex-tuned excimer formation [70]. Probe 11 shows an “AIE” with an increasing H₂O percentile. Next, during the Hg²⁺ detection, the emission of 11 (Φ_F = 0.076; at 456 nm, λ_ex = 342 nm) displayed a 10-fold enhancement at 468 nm (Φ_F = 0.74). The linear detection of Hg²⁺ lies between 0 and 100 µM with an LOD of 4.5 nM. Figure 5a,b show the 1:1 (11-Hg²⁺) complex-tuned excimer formation and emission enhancement. The 11-Hg²⁺ was reversible by adding ethylenediamine tetraacetic acid (EDTA) and iodide (I⁻) anions. A supramolecular assembly has been witnessed when using 11-Hg²⁺ to detect picric acid, as described in Section 4.1. This report used the strip method, but it requires further development for real-world applications.

Tu¨may et al. developed the pyrene–cyclotriphosphazene derivatives ‘12–14’ and engaged them in the quantification of Hg²⁺ via 1:1, 1:2, and 1:3 complex-induced excimers and the “CHEF” mechanism [71]. Figure 6a,b display the structure of 12–14 and their 1:1, 1:2, and 1:3 complex-promoted excimer formation.

The linear regression of 12-Hg²⁺, 13-Hg²⁺, and 14-Hg²⁺ ensembles at 450 nm (λ_ex = 365 nm) in ACN (ACN = CH₃CN) were estimated as 0–50 µM, 0–25 µM, and 0–20 µM with LODs of 0.223 µM, 0.114 µM, and 0.050 µM, correspondingly. Subsequently, the Φ_F values of 12, 13, and 14 (0.10, 0.12, and 0.15) were also enhanced for 12-Hg²⁺, 13-Hg²⁺, and 14-Hg²⁺ (0.74, 0.76, and 0.79). The Job’s plot and ¹H-NMR titrations confirm the 1:1, 1:2, and 1:3 complexes of 12-Hg²⁺, 13-Hg²⁺, and 14-Hg²⁺. HeLa cellular imaging investigations attested to the applicability of this report. However, the synthetic complications may raise questions about the cost effectiveness of this research. Using CH₃CN for sensor applications could still be evaluated using the appropriate aquatic solvents.

2.2. CHEF-Tuned Hg²⁺ Detection

A pyrene–NBD (NBD = 7-nitrobenzo-2-oxa-1,3-diazolyl) cyclam probe 15, pyrene–thiosemicarbazone derivative 16, pyrene-conjugated Schiff base derivatives 17–19, phenanthro[4,5-fgh]pyrido[2,3-b]quinoxaline 20, and twisted (pyrene)₂–pyridine Schiff base derivative 21 were reported for the “CHEF”-promoted PL “Turn-On” sensing of Hg²⁺ ions [72,73,74,75,76,77,78]. Figure 7 shows the structures of analogues 15–21.

Probe 15 displays an Off–On PL response to Hg²⁺ and an On–Off PL response to Cu²⁺ at 538 nm [72]. The estimated LOD of Hg²⁺ by 15 was 7.9 µM with a K_a value of 2.5 × 10⁴ M⁻ from nonlinear curve fitting. This preliminary report supports several sensory reports but lacks a discussion of interference, practical applications, discrimination between Cu²⁺ and Hg²⁺, and mechanisms. Pyrene–thiosemicarbazone derivative 16 witnesses a PL “Turn-On” response to Hg²⁺ via the “CHEF” mechanism [73]. The semilogarithmic plot defined its LOD as 3.98 µM. This report lacks the use of interferences, applications, and in-depth discussion. Thus, the findings are still in question. The pyrene-based Schiff base 17 shows 12-fold PL enhancement (at 472 nm; λ_ex = 365 nm) in the presence of Hg²⁺ ions via a 1:1 “CHEF” complex [74]. The 17-Hg²⁺ base has a linearity from 0.1 to 10 µM with an LOD of 22 nM and a K_a value of 1.02 × 10⁴ M⁻¹. Candida albicans cellular imaging clarified the Hg²⁺ sensor. However, this work is lacking in interference studies; thus, additional interrogations are still mandatory. A “CHEF”-promoted PL “Turn-On” detection of Hg²⁺ was described using pyrene-based Schiff base probe 18 [75]. Upon adding Hg²⁺, probe 18 exhibits a PL enhancement at 395 nm (λex = 345 nm) in water–DMSO (1:2 v/v, pH:8.0; DMSO = Dimethylsulfoxide). This might be attributed to the “CHEF” mechanism, as seen in Figure 8.

The linear range lies between 0.008 and 38 μM with an LOD of 8.32 nM and a K_a value of 1.0⁵ × 10⁵ M⁻¹ (1:1 complex). The Φ_F value of 18-Hg²⁺ was estimated to be 0.71 (70-fold enhancement) and shows reversibility in the presence of S²⁻ ions (See Figure 8). Notably, the sensing ability of 18 was well defined by its application on test papers, food, and environmental samples. Thus, this can be considered an impressive work. Bai et al. used the pyrene-based Schiff base 19 to measure Hg²⁺ ions [76]. At 470 nm excitation, 19-Hg²⁺ in HEPES/ACN (10 mM, pH = 7.4, 3:7 (v/v)) revealed a PL “Turn-On” peak at 607 nm through a 1:2 “CHEF”-complex. The linear regression ranged from 0 to 21 µM with an estimated LOD of 36 nM. The 19-Hg²⁺ shows reversibility while adding I⁻ via the formation of HgI₂. This work lacks real-world applications (except preliminary test strip studies); hence, further investigations are essential. During the sensing of Hg²⁺ ions, a ratiometric fluorescence shift was witnessed by the pyrene-based probe 20 [77]. As explored in Figure 9, the initial emission peak at 487 nm red-shifted to 570 nm in the presence of Hg²⁺, which might be assigned to excimer, “CHEF”, and Intramolecular charge transfer (ICT) mechanisms.

The LOD of Hg²⁺ by 20 was stated to be 1.31 µM, and 20-Hg²⁺ (1:1 complex) was found to be reversible with cysteine. Test strips were used to demonstrate its applicability. However, sensor studies in tetrahydrofuran (THF) may affect real applications. Thus, this could be counted as a supplementary report. Twisted (pyrene)₂–pyridine Schiff base 21 (Φ_F = 0.31; λ_ex = 368 nm and λ_{em =} 408 and 430 nm) shows selectivity to both Hg²⁺ and Pb²⁺ ions via 1:1 “CHEF” complexation [78]. In the presence of Hg²⁺, the emission at 460 nm is enhanced, with a Φ_F value of 0.75. The limit of detection (LOD) for Hg²⁺ was established at 12.0 μM, demonstrating considerable sensitivity. This work lacks information on the discrimination between Hg²⁺ and Pb²⁺, hence it can be counted as a supplementary report.

2.3. Pyrene Conjugates for Reaction-Based Detection of Hg²⁺

Pyrene-attached rhodamine conjugates (22–24), pyrene–hydrazone 25, and pyrene–bissulfanes (26–28) were described for the reaction-based detection of Hg²⁺ ions [79,80,81,82,83]. Figure 10 displays the structures of probes 22–28.

Pyrene–rhodamine 6G conjugate 22 undergoes a ring-opening reaction with Hg²⁺ ions, which induces FRET between the pyrene and rhodamine units to trigger an emission “Turn-On” at 550 nm [79]. Probe 22 at 0.5 µM in ACN displays excellent selectivity for Hg²⁺ and is reversible with EDTA. However, the emission enhancement of 22-Hg²⁺ was saturated at 34 µM. The proposed binding stoichiometry is 1:1 and 2:1, but further clarification and evidence are needed. This work emphasizes FRET but lacks linear regression analysis and real applications; hence, it is considered as a supplementary report. Two pyrene–rhodamine B probes (23 and 24) were synthesized and applied in a reaction-based Hg²⁺ sensing application (in ethanol: water (1:1)) [80]. For Hg²⁺, probe 23 exhibited fluorescence enhancement at 452 and 576 nm (λ_ex = 365 nm) with an estimated LOD of 19.1 µM and a quantification limit (LOQ) of 63.7 µM. In contrast, probe 24 shows an excellent response to Hg²⁺ ions at 582 nm (λ_ex = 365 nm) with an LOD and LOQ of 0.43 and 1.45 µM, respectively. Both probes show a greater sensitivity to Hg²⁺ due to the inhibition of initial FRET. The mass data also confirms 1:1 stoichiometry and the formation of Rhodamine B, thus clarifying the reaction-based mechanism. Figure 11a,b schematically represent the sensing mechanism of 23-Hg²⁺ and 24-Hg²⁺. This work must be updated using real sample applications.

Refluxing the 1-pyrenecarboxaldehyde and hydrazine afforded probe 25, which undergoes hydrolysis to detect Hg²⁺ ions in CH₃CN–CH₂Cl₂ (1:1, v/v; CH₂Cl₂ = dichloromethane) [81]. The Hg²⁺ induces the hydrolysis of 25 to yield 1-pyrenecarboxaldehyde at 440 nm, confirmed through NMR titration. It has been stated that 0.5 equivalent of Hg²⁺ was found to be enough to cause the hydrolysis. This work lacks details on linear regression, LOD, and real applications. Thus, further investigations are still mandatory for to support this technique.

Due to the high affinity of Hg²⁺ for S, pyrene-based derivatives with bisethylsulfane (26) and thioacetal units consisting of carboxyl and hydroxyl groups (27 and 28) were designed as chemodosimetric probes for the detection of Hg²⁺ ions [82,83]. The bisethylsulfane units of probe 26 (Φ_F = 0.01) interact with Hg²⁺ to afford the chemodosimetric response at 455 nm (Φ_F = 0.27; λ_ex = 365 nm) [82]. This might be attributed to the formation of 1-pyrenecarboxaldehyde, as confirmed by NMR titration. A stable response was seen in the pH range of 4–10. A linear PL response was witnessed from 0 to 30 µM with an LOD of 55 nM. This work was carried out using RAW 264.7 cellular and zebra imaging studies; thus, it can be regarded as innovative research. However, further investigations in environmental samples still need to be performed. By using a similar interactive reaction-based mechanism, the probes 27 and 28 in ethanol/PBS (pH 7.4; 2:1 (v/v)) revealed PL “Turn-On” responses at 457 nm (λ_ex = 395 nm) [83]. Figure 12a,b depict the chemodosimetric sensory response of 28 to Hg²⁺ ions.

The stoichiometric coordination of 27-Hg²⁺ and 28-Hg²⁺ was established at 1:1 and 1:2 ratios, respectively. The linear regression of 27-Hg²⁺ and 28-Hg²⁺ was found to be 0–3.5 µM and 0–12 µM with corresponding LODs of 1.49 nM and 1.03 nM. Probes 27 and 28 show an excellent sensitivity to Hg²⁺ in a direct buffer solution, with linearity ranging from 0–0.2 µM to 0–0.25 µM, and impressive limits of detection (LODs) of 1.74 nM and 1.53 nM. The in vitro imaging ability of 27 and 28 for Hg²⁺ was defined in A549 cell lines. In addition, real water analysis showed >90% recovery with RSD (RSD = Relative Standard Deviation) values < 1%. This work is an impressive one, and should be extended toward commercialization.

Table 1. Summary of pyrene-based derivatives illustrated for PL “Turn-On” detection of Hg²⁺ ions.

Probe Number	Detection Conditions (Solvent; pH)	Linear Regression	Detection Limit (LOD)	Assigned Mechanism	Applications	Ref
1	H2O/DMF (98:2, v/v, 10 mM HEPES; pH 7.4)	0–750 nM	57.2 nM	2:1 (1-Hg2+) excimer formation	NA	[61]
2	Ethanol; NA	NA	45 ± 5 nM	2:1 (2-Hg2+) excimer formation	NA	[62]
3 and 4	H2O–DMSO, 95:5, v/v, 10 mM HEPES; pH 7.4	0–500 nM (for both)	22.2 nM (3) and 44 nM (4)	2:1 (3-Hg2+ and 4-Hg2+) excimer formation	NA	[63]
5	DMSO/H2O (v/v = 7/3); pH 7.0	0–60 µM	2.82 µM	2:1 (5-Hg2+) excimer formation and PET	Cellular Imaging	[64]
6	H2O/DMF (v/v = 2:3, PBS buffer); pH 7.0	0–20 µM	0.42 µM	2:1 (6-Hg2+) excimer formation and PET	Real water and cellular Imaging	[65]
7	HEPES-CH3CN (80:20, v/v); pH 7.2	NA	0.2 µM	1:1 (7-Hg2+) excimer formation	NA	[66]
8	CHCl3/CH3CN (1:1, v/v); NA	NA	5 µM	FRET “On”	NA	[67]
9	DMSO/HEPES buffer (20Mm; 9:1 v/v); pH 7.4	0–1 µM	0.74 µM	1:1 (9-Hg2+) excimer formation	Cellular, C-elegans, and Zebra imaging	[68]
10	CH3CN/DMSO (99:1); NA	0–1 µM	8.11 nM	1:1 (10-Hg2+) excimer formation	Logic gate applications	[69]
11	H2O/EtOH (1:1, v/v) buffered with HEPES; pH = 7.05	0–100 µM	4.5 nM	1:1 (11-Hg2+) excimer formation	Test Strip Method	[70]
12, 13, and 14	CH3CN; NA	0–50 µM, 0–25 µM, and 0–20 µM, respectively	0.223 µM, 0.114 µM, and 0.050 µM, respectively	1:1, 1:2, and 1:3 (12-Hg2+, 13-Hg2+, and 14-Hg2+) excimer formation and “CHEF”	Cellular Imaging	[71]
15	H2O-CH3CN (10:90, v/v; pH 4.8	NA	7.9 µM	“CHEF”	NA	[72]
16	CH3CN; NA	NA	3.98 µM	“CHEF”	NA	[73]
17	H2O–CH3CN (30:70, v/v); NA	0.1–10 µM	22 nM	“CHEF”	Cellular imaging	[74]
18	H2O:DMSO (1:2 v/v); pH:8.0	0.008–38 μM	8.32 nM	“CHEF”	Test paper, food, and environmental samples	[75]
19	HEPES buffer (10 mM)/CH3CN (30:70, v/v); pH = 7.4	0–21 µM	36 nM	“CHEF”	Test strips and silica gel plate	[76]
20	THF:H2O (99:1, v/v); NA	NA	1.31 µM	Excimer, “CHEF”, and ICT	Test strips	[77]
21	THF; NA	NA	12 µM	“CHEF”	NA	[78]
22	CH3CN; NA	NA	NA	FRET and Chemodosimeter	NA	[79]
23 and 24	Ethanol/H2O (1:1, v/v); pH 7.0	0–300 µM and 0 –4 µM	19.1 µM and 0.43 µM	FRET and Chemodosimeter	Water samples	[80]
25	CH3CN–CH2Cl2 (1:1, v/v); NA	NA	NA	Chemodosimeter	NA	[81]
26	H2O-CH3CN (v/v = 50/50); NA	0–30 µM	55 nM	Chemodosimeter	Cellular and Zebra imaging	[82]
27 and 28	Ethanol/PBS buffer (2:1, v/v); pH 7.4	0–3.5 µM and 0–12 µM	1.49 nM and 1.03 nM	Chemodosimeter and ICT	Cellular imaging and real water samples	[83]

NA = not available; µM = micromole (10⁻⁶ M); nM = nanomole (10⁻⁹ M).

summarizes the pyrene-based probes utilized in the PL “Turn-On” detection of Hg²⁺, their linear ranges, LODs, and applications.

2.4. Critical Analysis of PL “Turn-On” Detection of Hg²⁺

Pyrene-based conjugates deliver PL “Turn-On” responses to Hg²⁺ via excimer formation, PET, CHEF, and the chemodosimeter mechanism [61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83], as discussed in the previous sections. Those reports still have certain limitations, as follows: (A) in the case of the excimer complex-tuned mechanism, the reports are still lacking a discussion of the PET and FRET mechanisms [61,62,63,64,65,66,67,68,69,70,71], which require critical in-depth interrogations; (B) for pyrene–rhodamine conjugate probes [67,79,80], the FRET mechanism was inconsistently recorded, which must be addressed via critical examinations; (C) reports describing the “CHEF” mechanism [72,73,74,75,76,77,78] are lacking theoretical support, which requires critical inquiries using DFT (Density Functional Theory); (D) the majority of pyrene-based probes [61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83] lack information on “AIE” and solvent effects on real-time sensory responses, and this requires critical analysis to address aquatic sensor studies; (E) the extensive stability of probes and their proposed sensory complexes under temperature, pressure, diverse solvent conditions, and distinct environmental conditions require integrative critical examinations.

3. Pyrene Derivatives for PL “Turn-Off” Detection of Hg²⁺

Pyrene derivatives with initial emissive properties undergo PL quenching while interacting with Hg²⁺ ions. Figure 13 represents the pyrene derivatives that detect Hg²⁺ via a 1:1 complex-tuned PL “Turn-Off” response [84,85,86,87,88].

Wu’s research group conjugated pyrene and ribose through triazole linkage to afford probes 29 and 30 and applied them to quantify Hg²⁺ ions [84]. These probes possess a strong emission at 478 nm (λ_ex = 343 nm), and while titrating with metal ions in CH₂Cl₂/CH₃OH (8:2, v/v; CH₃OH = Methanol), with both showing a high sensitivity to Hg²⁺ via PL “Turn-Off” response. Probes 29 and 30 form a 1:1 ligand-to-metal complex via triazole units, inhibiting pyrene excimer formation, as shown in Figure 14. From Stern–Volmer quenching plots, the LODs of Hg²⁺ by 29 and 30 were estimated as 10 µM and 15 µM with K_a values of 1.73 × 10⁵ M⁻¹ and 4.44 × 10⁵ M⁻¹. The DFT and NMR titrations support the proposed binding mechanism. This work lacks suitable solvents and applications, which require further research. Following a similar 1:1 complexation and binding mechanism, three pyrene-based probes (31–33) with triazole units were developed to detect Hg²⁺ ions [85]. In the presence of Hg²⁺, probes 31, 32, and 33 display fluorescence quenching at 474 nm (λ_ex = 343 nm) via the 1:1 binding of triazole, which inhibits the pyrene–pyrene* (Py-Py*) excimer formation. The LODs of 31-Hg²⁺, 32-Hg²⁺, and 33-Hg²⁺ were defined as 1.74, 2.42, and 3.83 µM with K_a values of 1.68 × 10³ M⁻¹, 1.57 × 10³ M⁻¹, and 1.52 × 10³ M⁻¹, respectively. Imaging of 31-Hg²⁺, 32-Hg²⁺, and 33-Hg²⁺ was performed on RAW 264.7 cell lines. This work still requires real sample analysis.

Banerjee and co-workers proposed using the methionine–pyrene hybrid probe 34 for detecting Hg²⁺ ions via a 1:1 complex-mediated PL “Turn-Off” response [86]. Due to the excimer formation, the hybrid Schiff base probe 34 holds the emission at 455 nm (λ_ex = 360 nm) in methanol/water (2:1, v/v). Upon adding Hg²⁺ to 34 in methanol/water (2:1, v/v), the emission at 455 nm was quenched linearly between 0 and 40 nM, with a LOD 0.14 nM and K_a value of 7.5630 × 10⁴ M⁻¹. The Φ_F values of 34 and 34-Hg²⁺ were calculated as 0.1206 and 0.0757. This work impresses the researchers by delivering NMR titrations, computational calculations, and industrial waste water applications. A pyrene-based Schiff base probe 35 (Φ_F = 0.07) was marked for the PL “Turn-Off” detection of Hg²⁺ through 1:1 complex formation [87]. Probe 35 shows linear quenching at 450 nm (λ_ex = 348 nm) in ACN/water (1:1, v/v; pH 7.2) from 100 nM to 2.5 µM with an estimated LOD of 0.35 nM and K_a value of 9.08 × 10⁵ M⁻¹. Cellular imaging and real water analysis define the uniqueness of this research, but the low Φ_F value of the probe’s emission restricts its further exploration. Pyrene-tied imidazole derivative 36 witnessed the PL “Turn-Off” response through 1:1 complexation [88]. The emission at 469 nm in buffered CH₃CN (2:1 v/v, pH 5.0) was linearly quenched from 0 to 100 µM with a projected LOD and LOQ of 98 nM and 0.295 µM. The NMR titrations and mass analysis clarify 1:1 complexation, and real sample analysis justifies its applicability. This work requires a detailed discussion on DFT and recovery studies. Figure 15 shows the pyrene-based probes 37–41 detecting the Hg²⁺ through PL “Turn-Off” response employing 1:2 complexation, chemodosimeter, and solvent control [89,90,91,92]. Among BINOL-pyrene derivatives (37 and 38), 37 shows selectivity to Ag⁺ and Hg²⁺ via 1:1 and 1:2 complexes [89]. For 37-Hg²⁺, the K_a value was established as 4.332×10⁸ M⁻². However, this work can be counted as a supplementary report due to the possible chances of Ag⁺ interference. Similarly, a solvent-tuned PL “Turn-Off” detection of Hg²⁺ in H₂O: CH₃OH (7:3 v/v) was proposed by probe 39 with a LOD of 2.8 nM [90]. However, the sensing of Hg²⁺ seems to be greatly affected by Cu²⁺ ions, thus noted as a supplementary report.

The pyrene and boronic acid-containing probe 40 undergoes self-assembly in the presence of Hg²⁺ to reveal a PL “Turn-Off” response at 387 nm (λ_ex = 387 nm) [91]. The HgCl₂ replaces the boronic acid in 40 to afford a different pyrene moiety with HgCl, which induces self-assembly to deliver the PL quenching. This chemodosimetric probe 40 shows linear quenching of Hg²⁺ from 0 to 3 µM with an LOD of 6.6 nM and a Stern–Volmer quenching constant (K_SV) value of K_SV = 1.8 × 10⁶ M⁻¹. Real water- and soil-based test strip interrogations revealed the exclusive features of this research. However, this work must be supported with nano-instrumental inquiries. The pyrene-based probe 41 shows selectivity for Hg²⁺ with a K_SV value of 2.9 × 10⁴ M⁻¹ through 1:1 complex formation. However, probe 41 also shows a selectivity for Cu²⁺ and Pb²⁺; thus, it should be regarded as a supplementary report and requires additional research.

Critical Analysis of PL “Turn-Off” Detection of Hg²⁺

Though pyrene derivatives 29–41 show PL “Turn-Off” responses to Hg²⁺ [84,85,86,87,88,89,90,91,92], reports showing this have critics, as noted here: (A) the majority of reports do not clarify the information regarding the “AIE” effect of the probe in the presence of water and the “AIE” effect on the sensitivity of Hg²⁺ also requires critical investigation; (B) the excimer emission of the probe stated in 35 is still in question considering its ΦF value [87], and its demonstrated Hg²⁺ sensing ability in ACN–water (1:1) requires critical evaluation. This is due to the feasible “AIE” effect induced by pyrene derivatives; (C) many reports lack theoretical support for the probe’s π-π stacking and proposed Hg²⁺ sensing capabilities, which should be critically evaluated; (D) the sensing ability of probes 37–39 and 41 [89,90,92] does not equal the detection capability of Hg²⁺ due to the effects of interference and solvents, which requires additional critical assessment.

4. Pyrene Derivatives for Explosive Detection

This section describes the detection capability of pyrene derivatives for types of explosives. There are numerous explosive materials known throughout the world. The names of common explosives and their denotations are as follows: 2,4-Dinitro Anisole (DNAN), Picric Acid (2,4,6-Trinitro Phenol) (PA (TNP)), Styphnic Acid (SA), 1,3,5-Triethoxy-2,4,6-Trinitro Benzene (TETNB), 2,4-Dinitro Toluene (DNT), 2,4,6-Dinitro Toluene (TNT), 1,3,5-Trinitro-1,3,5-triazine (RDX), Nitrobenzene (NB), 2,4-Dinitro Aniline (DNA), Pentaerythritoltetranitrate (PETN), 2,3-dimethyl-2,3-dinitrobutane (DMDNB), and Trinitroglycerin (TNG). Figure 16 shows the structures of the explosives mentioned above.

4.1. Pyrene Derivatives for Picric Acid (PA) Detection

Pyrene derivatives 42–58 were used in the detection of PA via PL “Turn-Off” and ratiometric PL “Turn-On” responses [93,94,95,96,97,98,99,100,101,102,103,104,105,106,107]. Figure 17A–C display the structures of pyrene derivatives 42–58. A few of them have a certain selectivity for other explosives [93,94,96,97,100,103,105]. However, those reports still possess a higher PL quenching percentile, which allows us to include them in this section. Apart from the above probes, the emissive pyrene–mercury complex probe 11-Hg²⁺ undergoes quenching with PA [70], as detailed below.

Due to the energy transfer between self-assembled 11-Hg²⁺ and PA, the PL emission at 550 nm (λ_ex = 342 nm in H₂O/ethanol (1:1, v/v) with HEPES, pH = 7.05) was quenched with a K_SV value of 0.55 × 10⁵ M⁻¹ and LOD of 35 nM. Paper strip and DFT studies were used to describe PA detection and its proposed sensing mechanism. This is a unique report describing the selectivity for Hg²⁺ and PA. Probe 42 showed a quenching efficiency of 70% (PA), 60% (TNT), and 46% (NB) in the solution state [93]. But when exposed to vapours, the quenching efficiency was reversed in the order of NB > TNT > PA. Due to the interference effect and lack of real-time detection, this report was regarded as a supplementary report for PA quantification. The electrospinning of pyrene over polyethersulfone afforded Py-PES, which shows a static PL quenching sensitivity in the order of PA > TNT > DNT > RDX [94]. From linear plots (0−1 μM), the K_SV values of PA, TNT, DNT, and RDX were established as 1.263 × 10⁶ M⁻¹, 1.80 × 10⁵ M⁻¹, 7.52 × 10⁴ M⁻¹, and 3.26 × 10⁴ M⁻¹, respectively, with corresponding LODs of 23, 160, 400, and 980 nM. This work demonstrates the application of this probe in real water samples but does not provide details for discrimination between PA, TNT, DNT, and RDX. This work requires additional critical investigations.

Probe 43 was reported as a PA sensor via a PL “Turn-Off” response (λ_em = 402 nm; λ_ex = 345 nm in 10 mM HEPES) through π-π interactions and a charge transfer mechanism [95]. The quenching efficiency of PA was found to be 80% with an LOD of 23.2 nM and K_SV value of 1.16 µM⁻¹. Similar to PA, Nitrophenol (NP) and Dinitrophenol (DNP) also show PL quenching with K_SV values of 0.087 µM⁻¹ and 0.31 µM⁻¹, respectively. The strip method defines the detection of PA. However, this work can be further tuned by describing the “AIE” effect of the probe followed by PA detection. Deshmukh and co-workers developed the pyrene-based probe 44 for the selective detection of Cu²⁺ and PA [96]. The probe undergoes “AIE” and becomes emissive at 60% water fraction; the emissive probe 44 was used to detect Cu²⁺ and PA via a PL “Turn-Off” response. Between 0 and 39.6 μM, the PL emission at 498 nm (λ_ex = 343 nm) was linearly quenched with an estimated LOD of 155 nM. The detection mechanism was assigned to π-π stacking and charge transfer (intercalation). This work lacks quenching (static/dynamic) and real application studies. Thus, additional research is mandatory. Push–pull pyrene compounds 45–47 were developed with a nitroaromatic sensing ability [97]. However, 45–47 show greater sensitivity to PA with defined LODs of 0.18, 528, and 0.29 µM compared to all nitroaromatics. The K_SV values of 45–47 for PA were quantified as 5.5 × 10⁶ M⁻¹, 7.6 × 10⁴ M⁻¹, and 3.5 × 10⁶ M⁻¹, correspondingly. Due to the interference effect of the remaining nitroaromatics and the lack of applicability, this work is regarded as a supplementary report.

The pyrene–imidazole conjugate probe 48 was proposed for the PL “Turn-Off” detection of PA via a 1:1 guest–host stoichiometric complex and a resonance energy transfer (RET) mechanism [98]. In the presence of PA, the fluorescence of 48 (Φ_F = 0.126) quenches due to complex-tuned charge transfer and RET. The PLQY value of 48-PA was established as 0.02 with an LOD of 2.5 µM and a K_SV of 8.47 × 10⁴ M⁻¹. The high selectivity of PA by 48 was supported by DFT and single crystal X-ray diffraction (XRD) investigations. This work must be boosted with additional research on real-time applications. At 80% water content, probe 49 (Φ_F = 0.001) drives the formation of emissive “AIE” (Φ_F = 0.79; λ_em = 476 nm; λ_ex = 390 nm), which undergoes static PL quenching with PA [99]. Between 0 and 250 µM of PA, the emission (of “AIE”) was quenched to create a nonlinear curve, and the K_SV value was recorded as 1.4 ± 0.02 × 10⁴ M⁻¹. Due to π-π stacking and charge transfer, the “AIE” emission of 49 was quenched in the presence of PA. This work was applied in test strips. However, more clarification through LOD and interference studies is required. Compound 50 undergoes static and dynamic quenching with PA and 2-nitro-p-cresol (NPC) with estimated K_SV values of 1.4 × 10⁵ M⁻¹ and 1.1 × 10³ M⁻¹ [100]. This work is classified as a supplementary report because it lacks interference studies and real-world applications.

Kathiravan and co-workers constructed pyrene-based probe 51 (Φ_F = 0.19) for the selective sensing of PA via the PL “Turn-Off” response [101]. PA and probe 51 underwent intramolecular hydrogen bonding (H-bonding) to reveal emission quenching at 530 nm with a recorded LOD of 63 nM. A smartphone-attached portable fluorimeter model and aqueous PA detection validates this report. This is innovative research. Pyrene-attached conjugates 52 and 53 (Φ_F = 0.64 and 0.95) display fluorescence quenching at 513 nm and 482 nm via electrostatic interaction-tuned charge transfer [102]. After the PA interaction, the PLQY of 52 and 53 reaches >1% and 15% with K_SV values of 1.75 × 10⁴ M⁻¹ and 6.04 × 10⁴ M⁻¹, respectively. Among all nitroaromatics, PA displays a high quenching efficiency with 52 and 53. Due to the extended conjugation, probe 53 shows greater quenching of PA than 52. Figure 18a–d demonstrate the charge transfer between 52 and 53 to PA. This work requires additional information on LOD and real-world applications.

“AIE” active nanoaggregates were developed using compound 54 by adding 90% water and were then utilized for PA detection [103]. At 0% water fraction, probe 54 in THF possesses a Φ_F value of 0.01, which is enhanced to 0.546 for a 90% water fraction (λ_em = 469 nm; λ_em = 375 nm). The PA, DNP, and Trinitrobenzene (TNB) show quenching efficiency with K_SV values of 1.37 × 10⁴ M⁻¹, 2.903 × 10³ M^−1, and 1.732 × 10³ M⁻¹. In actuality, PA shows a greater quenching efficiency than other analytes. From TRPL, cyclic voltammetry (CV), and time-correlated single photon counting (TCSPC), FRET was assigned as the mechanism. Evaluation of the LODs, real applications, and discrimination between PA and DNP requires extended interrogations. The 1:1 guest–host stoichiometric complex-induced PL “Turn-Off” quantification of PA was illustrated by probe 55 (Φ_F = 0.007) [104]. A red-shifted PL quenching of 55 from 425 nm to 462 nm was witnessed during the titration with PA. The K _SV of 55-PA was calculated to be 2.29 × 10⁷ M⁻¹ and its LOD of was 19 nM. DFT investigations revealed the band gaps of 55 and 55-PA as 3.04 eV and 2.77 eV (eV = electron volts), which suggest a feasible charge transfer mechanism. The strip method detection of PA at different temperatures (288 K, 298 K, and 308 K) attests to its applicability. This probe is also applied in the PL “Turn-On” detection of Cu²⁺, standing out as being unique. However, for PA detection, extended inquiries on interferences are still mandatory.

Pyrene-methylimidazolium chloride (56; Φ_F = 0.266 in water) was reported in the PL “Turn-Off” detection of PA via a charge transfer mechanism [105]. Due to the n-π* and π-π* transition between PA to 56, the PL emission around 374, 393, and 415 nm (λ_ex = 342 nm) was quenched with a K_a value of 1.69 × 10⁵ M⁻¹ and a LOD of 50 μM. The Φ_F value drops down to 0.003 during the sensing of PA. Though this work shows a good sensitivity of 56 in real water analysis and compares the results with anthracene derivatives, other phenolic compounds may affect the sensitivity in different pH conditions. Extended investigations on the “AIE” of 56 should address this issue. One-pot-synthesized pyrene compound 57 displays PL quenching of PA in ACN–water (1:2, v/v) with a K_a value of 2.13 × 10⁴ M⁻¹ [106]. Due to the π-π interaction, the PL emission at 470 nm (λ_ex = 310 nm) was quenched linearly between 0 and 180 µM with an estimated LOD of 0.814 µM. This work was applied to smartphone-assisted quantification and invisible ink. More data on DFT are needed to justify the mechanism.

Pyrene–rhodamine conjugate 58 (Φ_F < 0.001) was reported for the ratiometric PL “Turn-On” detection of PA via PET inhibition, ring opening, and FRET initiation [107]. As shown in Figure 19, probe 58 reveals both colorimetric and fluorometric responses for PA.

The PL emission of pyrene at 428 nm was quenched along with the emergence of rhodamine peak at 578 nm. From mass analysis, the stoichiometry of 58-PA (Φ_F = 0.44) was established as 1:2. The K_a value of 58-PA is approximately 10¹¹ M⁻¹, with a limit of detection (LOD) of 13.8 ± 0.2 nM, demonstrating both its potency and sensitivity. Probe 58 initially possesses a PET between the amino group and pyrene, which is inhibited via ring opening and PA-induced FRET between rhodamine and pyrene. None of the nitro-aromatics show an interfering response, but acetate anion reverses the PA-induced sensor signal. Spiked soil analysis demonstrates the applicability of this work. This is a unique, stand-out ratiometric sensor for PA.

Critical Analysis of Detection of PA

Pyrene conjugate-based detection of PA [93,94,95,96,97,98,99,100,101,102,103,104,105,106,107] requires the following critical points to be taken into account: (A) Many of the reports [93,94,96,97,100,103,105] do not answer ‘how to avoid the interference effect?’, and discussions on the discrimination between the interferences are not valid enough; (B) The charge transfer mechanism, FRET, and the intramolecular H-bonding proposed for the detection of PA require supportive experimental and theoretical evidence. This requires multiple critical inquiries, which are lacking in many reports; (C) None of the reports provide guidelines for developing application models (except test strips and spiked water) that directly detect the PA. This issue must be critically optimized for innovative designs; (D) So far, only one ratiometric PL “Turn-On” sensor for PA quantification is available, which is insufficient to justify such designs. Thus, researchers must critically evaluate similar designs for PA discrimination; (E) The majority of “AIE” active probes for PA are lacking nano-level morphological and crystalline analyses, which should be critically examined further.

4.2. Pyrene Derivatives for Trinitrotoluene (TNT) Detection

In this section, the TNT-sensing ability of pyrene derivatives 59–76 is discussed in detail [108,109,110]. Figure 20A,B show the structures of compounds 59–67 engaged in TNT detection [108,109,110,111,112,113,114,115,116]. A pyrene excimer formation over the mesoporous thin films was described for the sensing of TNT via the PL quenching response [108]. After 5 min exposure of the film to nitroaromatics, the quenching efficiency at 470 nm (λ_ex = 340 nm) was in the order of TNT (>80%) > DNT (>60%) > NB (>60%). The π-π stacking between pyrene and nitroaromatic analytes (TNT, DNT, and NB) and the mesoporous structure of thin films play a vital role in achieving the higher PL quenching efficacy. This work also defines the reusability of films for extended cycles of TNT detection. This is an innovative work. However, extensive research is mandatory for validation of this technique.

Venkatramaiah and co-workers reported the phosphonated pyrene probe 59 (λ_em = 515 nm; λ_ex = 330 nm) with a high sensitivity towards TNT via π-π stacking and effective intramolecular H-bonding [109]. The probe shows a 91% quenching efficiency for TNT with a K_SV value of 8.4 × 10⁴ M⁻¹. Similarly, it holds a 74% quenching efficiency for PA with a K_SV value of 3.6 × 10⁴ M⁻¹. Probe 59 shows PL quenching sensitivity in the order TNT > TNP > DNP > DNT. This study confirms the feasibility TNT sensing with a thin film and its reusability while pointing out important interference effects. Later on, Han’s research group carried out an in-depth mechanistic investigation on 59-TNT [110]. The theoretical investigation suggests the involvement of the PET mechanism along with the energy level crossing between the charge transfer state (CT) and the local excited state (LE). These findings could help upcoming researchers create novel designs for TNT detection. Probe 60 detects TNT and RDX via a fluorescence quenching response [111]. The fluorescence at 481 nm (λ_ex = 345 nm in cetyl trimethylammonium bromide (CTAB)) was quenched with TNT and RDX with LODs of 8.8 nM and 1.5 µM, respectively. The K_a values were 1.0 × 10⁶ M⁻¹ and 6.8 × 10⁴ M⁻¹. This work must be investigated further with additional interferences such as PA, DNP, NP, etc. Also, this work lacks real-world applications, thus it is regarded as an additional study.

Pyrene–carboxylic acid and polyethylene glycols were condensed to form ester probes 61 and 62, which were then engaged in sensing TNT and PETN [112]. Probe 61 is more sensitive to TNT than probe 62. The Φ_F values lie between 35 and 62%, and upon interaction with TNT and PETN, the K_SV values were appraised in the order of 10⁴ M⁻¹ with LODs at µM levels. However, this work appears to be preliminary without experimental sensor details, short interference studies, and real applications. 12-pyren-1-yldodecanoic acid (63) was employed in the selective detection of TNT via the PL “Turn-Off” response [113]. Probe 63 displays a higher sensitivity to TNT than other types of explosives, as shown in Figure 21.

The emission at 482 nm (λ_ex = 365 nm) was quenched linearly between 0.001−15 μM with an appraised LOD of 5 nM and a K_SV value of 6.87 × 10⁵ M⁻¹. The effectiveness of this work was illustrated by a soil extract analysis, which estimates the LOD at 10 nM. It has been concluded that the charge transfer between pyrene excimers and TNT afford the 90% PL quenching response. This is a standout work and can be extended for commercial device development.

The use of bispyrenylalkane 64 was proposed for the sensing of TNT [114]. The PL emission of 64 was quenched in the presence of TNT with a quantified LOD of 0.6 µM and a K_SV value of 4.67 × 10⁵ M⁻¹. This is a preliminary work without details of interferences, applications, and mechanisms. Similarly, compounds 65 and 66 were utilized to detect DNT and TNT [115]. The K_SV values were at 10³ M⁻¹, and the DFT studies supported the charge transfer. Due to a lack of interference and real applications, this work can be considered as a supplementary report on the TNT sensor. Hao and co-workers synthesized probe 67 and validated its “AIE” and TNT sensing ability [116]. When adding the TNT, the PL emission at 505 nm (λ_ex =376 nm, in water) was quenched linearly between 0 and 3 µM with a calculated LOD of 4.24 nM and K_SV value of 6.2 × 10⁵ M⁻¹. The K_SV value of TNT was higher than PA (5.24 × 10⁴ M⁻¹; LOD = 56.3 nM). Notably, the PL quenching efficiency reaches 96.18% with 67-TNT. Using the naked eye, the strip method detects the TNT even at 10 nM concentration. Based on TRPL and DFT studies, the PET mechanism was assigned as the sensor response. Apart from the unique performance, this work still needs to be extended to interference and real-time applications. Yeşilot’s research group developed a series of dendritic pyrene-functionalized cyclotriphosphazene-based dye molecules 68–74 (see Figure 22A,B) for the detection of TNT and other explosives [117,118].

The Φ_F values of 68–74 were reported to be >0.5 and underwent static PL quenching in the presence of TNT, DNT, NP, and NB. Due to the π-π interaction and charge transfer between nitroaromatics (NACs) and 68–74, the fluorescence emission was quenched rapidly. These reports focused on explaining the photophysical and thermal properties of 68–74 and lacked an in-depth discussion of NAC sensors. In addition, the unspecific selectivity and sensitivity of TNT and insufficient information on real applications make them supplementary reports.

A ring-opening metathesis polymer (ROMP) with pyrene substitution (75; see Figure 23A) was designed and used in the identification of TNT [119]. In water, the polymer 75 forms a nano-sized micelle structure with emission bands at 376 and 395 nm (λ_ex = 340 nm). The PL emission was quenched linearly for TNT in the 0.1–30 µM range with a quantified LOD and LOQ of 64 nM and 190 nM, respectively. Interestingly, TNT shows a 72% quenching efficiency compared to other interferences (quenching efficiencies at 22 -35%). The estimated K_SV value of 75-TNT was 1.5 × 10⁵ M⁻¹, which attests to the probe’s effectiveness. This work requires protracted investigations for applications. A pyrene-attached polystyrene nanofibrous copolymer 76 was developed via the electrospinning technique and applied in the sensing of TNT [120]. The fluorescent polymeric nanofibrous membrane shows a PL “Turn-On” response to TNT at 1 mM compared to other analytes, as shown in Figure 23B. The quenching efficiency was linear between 5 nM and 1 mM with a maximum of 99% at 1 mM TNT. The established LOD of 5 nM for TNT and time-dependent investigations supported this report. However, extra interrogations of the mechanism, interference, and applicability are mandatory.

Critical Scrutiny of Detection of TNT

Many reported pyrene derivatives for TNT sensing lack proper validation and need critical evaluation. (A) The sensitivity of the probes to TNT was possibly affected by the presence of other NACs, which requires perilous investigations to discriminate between them; (B) probes 65, 66, and 68–74 were able to detect the TNT along with other NACs [115,117,118], thus selectivity of them towards TNT is questionable. This requires additional critical inquiries; (C) many reports on TNT detection lack an interference effect, K_SV values, quenching efficiency, detailed mechanistic investigations, and validated real applications. This could lead to the question, “Why the need for such jumbled research?”, which could be assessed by more critical studies; (D) few reports on pyrene derivatives on nanostructure formation for TNT detection have been delivered without assessing the nano-level variation during the sensing studies. Upcoming researchers could critically evaluate these points further.

4.3. Pyrene Derivatives for RDX and Multi-Explosive Discovery

Similar to compounds 65, 66, and 68–74, pyrene derivatives 20 and 77–86 (see Figure 24) were effective in multi-NAC detection using fluorescence quenching [121,122,123].

Compound 20 (in THF; λ_{ex =} 330 nm and λ_{em =} 487 nm) displays PL quenching responses to 2,4-dinitroaniline (2,4-DNA), p-nitroaniline (p-NA), p-nitrophenol (p-NP), PA, and 2,4-DNP of 99%, 89%, 75%, 55%, and 31%, respectively, with K_SV values at the 10⁵ M⁻¹ level [121]. The LODs for 2,4-DNA, p-NA, p-NP, PA, and 2,4-DNP were estimated to be 0.27, 0.21, 0.43, 0.52, and 0.71 µM, respectively. The PL quenching was attributed to the PET and electron transfer mechanism. Further, 2,4-DNA follows static and dynamic quenching, whereas 2,4-DNP follows the static quenching phenomena seen with 20. The strip method shows the research’s relevance. We need to confirm if 2,4-DNP is present at significant levels and how other analytes affect quenching. Kovalev and co-workers developed a series of pyrene derivatives ‘77–82 (Φ_F = 0.07–0.78)’ to quantify 2,4-DNT and 2,4,6-TNT in aqueous solvent [122]. Among them, 81 and 82 (Φ_F = 0.76 and 0.78; λ_{em =} 380 nm) show exceptional quenching response to 2,4-DNT and 2,4,6-TNT with an LOD of 1 (for both) and K_SV values at 10⁴ M⁻¹ level. The PET process, π-π interaction, and false static quenching mechanism were assigned as mechanisms due to the “sphere of action”. The strip method validates applications. This work must be boosted by additional research on discriminative quantification and real sample analysis. The pyrene-based iptycenes probes 83–86 were reported for sensing RDX, TNT, and DNT via the PL “Turn-Off” response [123]. Probe 83 shows a higher response at an order of RDX>TNT>DNT with K_SV values at the 10³ M⁻¹ level. The cavity of iptycenes provides a better binding site for the RDX moiety than others. Interfering NACs can affect this work, so it is included as a supplementary report.

1-Hydroxy pyrene-based derivatives 87–90 (See Figure 25) were used for sensing RDX, PETN, DMDNB, and TNG via the PET-driven PL quenching response [124]. The Φ_F values of 87–90 were appraised between 0.23 and 0.98 and possess K_SV values at 10⁵ M⁻¹ for those explosives. For RDX, the PL emission bands at 384 and 403 nm (λ_ex = 345 nm) were quenched linearly between 0 and 1 µM, with an LOD of 67.5 nM. Dynamic Light Scattering (DLS), DFT, and TRPL investigations validated the nano size of the probes and quenching mechanism. Due to insufficient information on interferences and applications, this research is considered supplementary. Mosca and co-workers described the PET-induced PL “Turn-On” detection of RDX by pyrene-based probes 91–93 (See Figure 25) [125]. RDX undergoes base-promoted hydrolysis to afford formaldehyde (HCHO) and Dinitrogen oxide (N₂O). The interaction of HCHO with 91–93 is responsible for the PL “Turn-On” response. However, in the presence of TNT, DNT, and DMDNB, the PL “Turn-On” response was quenched with appraised K_SV values within the 10³ to 10⁴ M⁻¹ range. Intramolecular H-bonding between −NH and the explosives and FRET mechanism was proposed for the PL quenching induced by explosives other than RDX. Later on, this mechanism was excluded by DFT and time-dependent Density Functional Theory (TDDFT) investigations by Lu’s research unit [126]. In this model, PET and π-π interactions are assigned as mechanisms. This is unique research showing a PL “Turn-On” response to RDX. But, additional inquiries are mandatory to exclude the quenching effect of interfering explosives and real applications.

As seen in Figure 25, pyrene-conjugated organosilicon polymers ‘94–96’ were developed for the excimer–monomer dual emissive “Turn-Off” detection of TNT and TNP [127]. Polymer probe 94 (at 417 nm and 524 nm; λ_ex = 350 nm) shows a high response to TNT and TNP. From theoretical studies, π-π interactions and intramolecular H-bonding were assigned as mechanisms. Details of interference and real-time applications are missing from this report. A polypseudorotaxane (λ_ex = 345 and λ_em = 480 nm) was formed from a copolymer and pyrene-ended ammonium salt via a guest–host interaction and reported as a multi-stimuli responsive probe [128]. As depicted in Figure 26, the cavities prevailing in the polymer play a vital role in interacting with bases, anions, temperature, and explosives. The LODs for NACs are at the µM level. Due to the lack of interference, detailed mechanistic studies, and real applications, this work is counted as a supplementary one.

Critical Views on RDX and Multi-Explosive Encounters

Reports on pyrene-based compounds for RDX and multi-explosive detection are lacking in the literature. (A) RDX detection by pyrene conjugates was ultimately affected by the presence of other moderate explosives. This is not enough to convince society and researchers. Thus, in-depth critical analysis is mandatory for unique design and development; (B) Pyrene-derivatives for multi-NACs detection are not convincing enough to address explosive threats, which should be critically evaluated for their specific sensitivity; (C) Exposure of RDX and multi-explosive vapours to pyrene-based derivatives may lead to decomposition of probe’s structure in the solid state. Researchers should address this issue; (D) On what basis are the multistep synthetic polymers being synthesized and explored for RDX and multi-explosive sensing? None show a specific selectivity, have an interference effect, or have confirmed applications, and lack they mechanistic studies; (E) Exploration of polypseudorotaxane to NACs may lead to new research directions. However, due to the involvement of multi-synthetic steps to afford low-yield polypseudorotaxane, how could it be cost-effective research for developing countries?

5. Pyrene-Conjugated Hybrid Probes for Hg²⁺ and Explosive Detection

5.1. Hg²⁺ Sensors

This section explores the pyrene units conjugated with nanostructural materials for Hg²⁺ discriminations [129,130,131,132,133,134,135]. The structures of pyrene derivatives ‘97–102’ utilized for developing the hybrid Hg²⁺ sensors are shown in Figure 27. The pyrene Schiff base 97 was attached to hybrid magnetic silica nanoparticles (Fe₃O₄@SiO₂ NPs) to afford Py–Si–Fe₃O₄@SiO₂ NPs and applied to detect Hg²⁺ via PL “Turn-On” response [129]. The hybrid system shows linear PL enhancement (at 450 nm; λ_ex = 363 nm) from 4 to 40 µM with an LOD of 55 ± 15 nM. This hybrid probe shows >95% recoveries in real water and serum samples, hence it has been noted as a good innovation. However, additional information on the mechanism is required for validation.

A silica gel hybrid sensor was developed using compound 98 to detect Cu²⁺ and Hg²⁺ ions via a PL “Turn-On” response [130]. The hybrid sensor shows 10-fold PL enhancement to Hg²⁺ at 506 nm (λ_ex = 365 nm) than Cu²⁺ ions (1.5-fold). Linear regression of Hg²⁺ ranged from 0 to 20 µM with an appraised LOD of 3.4 pM (pM = Picomole; 10⁻¹² M) and K_a value of 2.5 × 10⁵ M⁻². The 2:1 stoichiometric complex-induced excimer formation was assigned as the mechanism. HeLa cellular imaging, the test strip method, and real water recovery studies validated this work. The recoveries were found to be >95%. This is good work, but the discrimination between Cu²⁺ and Hg²⁺ must be investigated further. Pyrene derivative 99 was conjugated with magnetic core–shell fibrous silica (CoFe₂O₄/SiO₂/KCC-1) to afford a “Py-CoFe₂O₄/SiO₂/KCC-1” hybrid model that could be applied in Hg²⁺ quantification [131]. The hybrid system has linear sensitivity to Hg²⁺ from 1 to 20 µM with an LOD of 4.1 nM. Notably, the sensor shows >99% recoveries in environmental water samples and also possesses reversibility with EDTA (up to six cycles). Based on the available results, this can be considered as a good innovation.

Cationic oligopyrene 100 was complexed with oligothymine to afford a OHPBDB/poly(dT) complex, which reveals a PL “Turn-Off” selectivity towards Hg²⁺ [132]. The PL emission at 465 nm (λ_ex = 340 nm) was quenched via thymine–Hg²⁺–thymine (T-Hg²⁺-T) base pair formation. The LOD of H g²⁺ by PL and using the naked eye are expressed as 5 nM and 5 µM. This is innovative work, but it must be explored further for extended its applications. A cryogel consisting of polyacrylamide and pyrene derivative 101 was expressed to discriminate Hg²⁺ ions [133]. This hybrid system detects Hg²⁺ with an estimated LOD of 10 nM. However, extended investigations on interference studies, mechanisms, and environmental applications are still needed. Coupling of 1-pyrenecarboxylic acid and NH₂@SiO₂ NPs leads to Pyr-NH₂@SiO₂ NPs, which detects Hg²⁺ ions via the PL “Turn-Off” response [134]. The Brunauer–Emmett–Teller (BET) surface area of the hybrid sensor was found to be 44.5 m² g⁻¹. The sensor shows PL quenching linearity from 0 to 25 µM with a LOD of 50 nM and a K_SV value of 1.14 × 10⁵ M⁻¹. This work still needs to be investigated to assess static or dynamic quenching and applicability. The reaction between 102 and mesoporous silica-functionalized epoxy-terminated organosilanes forms the hybrid sensor (MSN@Py-EOA) for PL “Turn-Off” detection of Hg²⁺ ions [135]. The BET surface area of MSN@Py-EOA was estimated to be 164.41 m² g⁻¹ with a 2.2 nm pore size. Linear regression of MSN@Py-EOA-Hg²⁺ (at 498 nm) was exhibited between 1 and 20 µM with an LOD of 0.62 µM and a K_SV value of 4.0 × 10⁴ M⁻¹. The NMR titration validates the feasible 1:1 complexation. This work falls short in real applications.

5.2. Explosive Sensors

Similar to Hg²⁺ sensing, hybrid models with pyrene units were effectively employed to detect NACs [136,137,138,139,140,141,142,143,144,145,146,147]. The pyrene derivatives 103–110 (see Figure 28) were used to construct the hybrid models for the sensing of NACs.

Pyrene-functionalized ruthenium nanoparticles [136], 104-linked 2-bromoethyl methacrylate polymer (PBEMA) [138], 105-functionalized self-assembled monolayer (SAM) film [139], 106-functionalized fluorescent film [140], 107-coupled mesoporous silica nanomaterials [141], and 109-attached mesoporous silica microspheres [143] were demonstrated for multi-NAC detection with LODs between nM to µM and their K_SV values are lies between 10³–10⁵ M⁻¹. The 1-pyreneiodide coupled polyvinylpyrrolidone-functionalized gold nanoclusters (Au-PVP) (or) CTAB-stabilized gold nanorods (Au-CTAB) also follow a similar PL quenching trend for multi-NACs with a high K_SV value of 3.88 × 10⁶ M⁻¹ and an LOD of 10 nM [145]. Next, polyacrylonitrile nanofibers coupled with pyrene units (Py-PAN NFM) were also used for the detection of TNT and DNT [146]. This work seems impressive in terms of the K_SV (10⁵–10⁶ M⁻¹), LODs (at nM), and real water analysis. However, these reports reveal similar mechanistic aspects and lack interference studies and real applications, as stated in Section 4.1 and Section 4.3; thus, additional details are not provided here.

The compound 103-assembled glass substrate detects the PA with high sensitivity via a PL “Turn-Off” response at 500 nm (λ_ex = 353 nm) [137]. The substrate follows the PL quenching trend of PA > TNT > DNT > NB. The estimated LOD of PA is 10 nM with a K_SV value of 5.1 × 10⁴ M⁻¹. The π-π interaction between 103 and PA was assigned as the mechanism. This work defines the reversible use of film by using ethanol exposure. In-depth mechanistic investigations may further validate this technique. The copper-complexed pyrene-conjugate 108 (Φ_F = 0.51) shows a high sensitivity to PA (>95% quenching efficiency at 360 µM) [142]. The PL emission at 387 nm and 407 nm (λ_ex = 342 nm) was quenched between 0 and 360 µM with an LOD of 1.2 µM. The PL quenching was assigned to electron/energy transfer and the involvement of static and dynamic quenching phenomena. Preliminary soil tests attest to the applicability of the complex. However, extended investigations for interference and mechanistic aspects are still needed.

A mesoporous silica nanomaterial generated from pyrene derivative 110 was used for the discovery of the PL “Turn-On” by TNT and Tetryl [144]. The mesoporous support was loaded with a tris(2,20-bipyridyl)ruthenium(II) chloride ([Ru(bipy)₃]Cl₂) complex and then functionalized with 3-(azido-propyl)triethoxysilane, which undergoes a click reaction with 110 to afford the hybrid sensor model. In the presence of TNT and Tetryl, the PL intensity at 625 nm (λ_ex = 453 nm) was enhanced due to the complexation between the electron donor pyrene unit and the electron-withdrawing benzene rings of NACs. This is innovative research, but additional details on sensitivity, discrimination between TNT and Tetryl, applications, and effects (pH, time, and interference) are missing. Pyrene-functionalized mesoporous Si NPs (pMSNs) were utilized to selectively identify TNT via the PL “Turn-Off” response [147]. Herein, cetyltrimethylammonium micelles, pyrene, and tetraethyl orthosilicate (TEOS) were self-assembled to form the hybrid sensor with emission located at 475 nm (λ_ex = 340 nm). The emission quenched linearly (for TNT) from 10 nM to 1 µM with an established LOD of 12 nM. Incredibly, the quenching efficiency of pMSNs reached 98.6% for TNT at a 1 µM concentration, which is higher than all other NACs. This innovative research requires additional support from detailed mechanistic and real-application studies.

5.3. Critical Opinions on Hybrid Sensors for Hg²⁺ and Explosives

Hybrid probes constructed by coupling with pyrene derivatives hold certain criticisms and require extended investigations. (A) Most of the hybrid models derived from pyrene derivatives were possibly affected by the proposed protocols, and their stability is still in question. This requires careful optimization; (B) The sensitivity of hybrid probes to Hg²⁺ and specific NAC cannot be judged before analytical interrogations. Thus, while choosing the nanomaterial and probe structure, critical thinking is necessary; (C) Most of the pyrene derivatives already show a high selectivity to Hg²⁺ and specific NACs in the solution state, so why is there a need for a hybrid design for sensing? This requires critical evaluation; (D) Many pyrene-based hybrid sensors are designed for multi-explosive detection. This begs the question: “How do these developments benefit the research community?” Future researchers must tackle this directly; (E) Hybrid sensory materials for Hg²⁺ and NAC quantification lack evidence for mechanistic analysis and proposed applications. This requires in-depth critical investigations.

6. Design Requirements, Advantages, and Limitations

6.1. Design Requirements

To sense Hg²⁺ and specific NAC, pyrene derivatives with the following requirements may be helpful.

• The probe for Hg²⁺ must hold hetero atoms or functional groups such as nitrogen/imine (N/−C=N), sulphur/thiols (S/−SH), oxygen/hydroxyl (O/−OH), carboxylic acid (−COOH), etc. This could lead to strong coordination with Hg²⁺ to deliver either PL enhancement or a quenching response.
• The pyrene-based probe for Hg²⁺ must hold exceptional fluorescent Φ_F values in two ways: (i) For the PL “Turn-On” sensor, the probe must possess a low Φ_F value with weak or no-emission; (ii) For the PL “Turn-Off” sensor, the probe must possess a higher Φ_F value with superior emission. This could be achieved by the inclusion of heteroatoms, functional units, Schiff base imine (−C=N) bonds, and extended conjugation, etc. [148,149,150,151], which initiates either PET/FRET On or Off, “CHEF”, and chemodosimetric approaches mechanisms.
• For the chemodosimetric and ratiometric detection of Hg²⁺, the probe must be designed with highly reactive groups like sulphur/thiols (S/−SH) and a suitable acceptor moiety such as rhodamine, Fluorinated Boron-Dipyrromethene (BODIPY), etc.
• To detect the NACs via the PL “Turn-Off” response, the designed pyrene-based probe must have extended conjugation, “AIE” characteristics, and exceptional π-π stacking. On the other hand, for the PL “Turn-On” response, the probe must be designed with H-bonding and electron/charge transfer units that initiate the emission enhancement.
• To design the pyrene-based hybrid sensor, suitable nanostructured materials must be selected to attach with the proposed pyrene conjugate to afford a weak/highly emissive unique structured material for Hg²⁺/NACs detection. However, before synthesis, it is important to ensure the hybrid system has a particular affinity for either Hg²⁺ or NACs.

6.2. Advantages

Reported pyrene-conjugated probes for Hg²⁺ and NACs detection have advantages, as noted below.

• Most of the probes that detect Hg²⁺ ions hold unique luminescent features to afford PL “Turn-On” or “Turn-Off” responses with exceptional changes in Φ_F values and can be applied for in vitro/in vivo imaging and real sample analysis. This is advantageous over nanomaterials such as quantum dots (QDs), carbon dots (CDs), nanoparticles, 2D-materials, etc. [152,153,154,155].
• The sensitivity of pyrene-based probes to Hg²⁺ is comparable and reliable to other known tactics such as electrochemical, colorimetric, chemiluminescence, etc. [154,156,157,158]. This is an added advantage of using pyrene derivatives in Hg²⁺ quantification.
• The distinct PL response achieved during the ratiometric Hg²⁺ sensor may be helpful to quantify the Hg²⁺ via dual/multi-channel response, which is an advantage over common probes.
• Most pyrene-based probes for NACs show incredible fluorescence quenching, and many of them are applied in real-time detection that can be extended for commercial device fabrication. This is a reliable advantage over other sensory materials [159,160].
• Hybrid pyrene–mesoporous conjugates show specific affinity to Hg²⁺/NACs by affording a large surface area to deliver greater PL responses. This could be advantageous and reliable compared to other hybrid systems.

6.3. Limitations

Though pyrene derivatives have many advantages over other methods and sensors, they also have certain limitations, as noted below.

• Pyrene is a light-sensitive moiety; hence, the stability and durability of pyrene-based probes are still in question. This limits the long-term use of the pyrene-based probes for Hg²⁺ and NACs detection.
• Most of the pyrene-based probes for Hg²⁺ and NACs are operable in organic solvent or semi-aqueous media, and few probes show “AIE” features in an aqueous environment. This could limit the easy operation and real-time performance of sensors.
• Many reports on the use of pyrene derivatives for Hg²⁺ discrimination lack precise mechanistic evidence and extended applications. This raises doubts about the reliability of the probes and limits further investigations.
• Recently reported pyrene-based probes for NAC detection still lack evidence of their interference effect, mechanistic evidence, and details of real applications [161,162,163,164]. This could restrict the emergence of novel pyrene-based designs.
• The few reports on pyrene-conjugated hybrid systems for Hg²⁺ and their multi-NAC sensing ability limit upcoming researchers investigating such a direction. In addition, the test-strip method using probes reported for NAC detection is described without proper protocols. This limits the real applications.

7. Conclusions and Perspectives

This comprehensive review detailed the use of pyrene derivatives 1–110 for Hg²⁺ and nitroaromatic explosive detection. The py-py* excimer-tuned “Turn-On” detection, using the “CHEF” chemodosimetric approaches, and the PL “Turn-Off” detection of pyrene derivatives 1–41 are discussed with mechanism and their applications. Subsequently, pyrene derivatives 42–96 applied in the detection of diverse NACs are described alongside their mechanisms and real applications. Rare examples of the pyrene-conjugated hybrid sensors 97–110 for Hg²⁺ and NACs are described in detail. At the end of each section, critical thoughts and queries are offered. Ultimately, the design requirements, advantages, and limitations of the pyrene-based probes for Hg²⁺ and nitroaromatic explosive detection are explored for future researchers. Addressing the following perspective points may help the society to remove hazardous Hg²⁺ and NACs: (A) Though numerous reports are available on pyrene-based probes for Hg²⁺, a “state-of-the-art” procedure is still missing; this could be addressed with great care and optimization; (B) To date, pyrene-based probes are rarely reported for the ratiometric sensing of Hg²⁺, which should be extended with novel acceptor dyes and molecules; (C) The conjugation of pyrene derivatives with 0–2D-materials such as Graphene, Mxenes, perovskites, QDs, MOFs, etc., may enhance the sensitivity to Hg²⁺ and NACs; (D) Numerous reports are available on the utilization of pyrene-conjugated probes for the PL “Turn-Off” detection of NACs. But, none of these methods are standardized for commercialization, and this could be a unique research direction; (E) The design and development of novel pyrene-based probes for the PL “Turn-On” detection of specific NAC is still in demand, and this could be focused on by upcoming researchers; (F) Developing pyrene probes with longer alkyl and alkoxy chains may help to detect Hg²⁺ and NACs via gelation studies [165]. This could be an exclusive direction; (G) The development of p- or n-type organic thin-film transistor (OTFT) might be advanced by using pyrene-conjugated probes for Hg²⁺ and NACs detection [47]; (H) The design of pyrene derivatives to detect specific NAC in the presence other NACs is important and ground-breaking research, which upcoming researchers could focus on.

Apart from the above open questions, pyrene derivatives are still noted as unique candidates for the discrimination of Hg²⁺ and NACs. Innovations from current and upcoming researchers might address all of the issues described and deliver ground-breaking outputs.