Multimodal Ion Sensing by Structurally Simple Pyridine-end oligo p

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Cite This: ACS Sustainable Chem. Eng. 2019, 7, 12304−12314

Multimodal Ion Sensing by Structurally Simple Pyridine-End Oligo p‑Phenylenevinylenes for Sustainable Detection of Toxic Industrial Waste Suman Kalyan Samanta,†,§ Nilanjan Dey,† Namita Kumari,† Dipen Biswakarma,† and Santanu Bhattacharya*,†,‡ †

Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India School of Applied and Interdisciplinary Sciences, Indian Association for the Cultivation of Sciences, Jadavpur 700032, India § Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, India

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ABSTRACT: Environmental pollution induced by toxic metal ions and harmful chemicals mainly from industrial waste poses a significant threat which urges for their rapid detection before release into the ecosystem above the permissible level. Optical sensors are inexpensive, simple, yet efficient in sensing such toxicants. Herein, we show that structurally simple π-conjugated pyridine-end p-phenylenevinylene oligomers can selectively sense toxic metal ions and anions in solution, supramolecular gels, as well as in solid support. Interaction of Hg2+ at nanomolar concentrations with the linear pyridine-ends via two-site coordination was clearly seen from “naked eye” color changes and fluorometric investigations. The sensitivity as well as selectivity of the oligomers toward Hg2+ was found to be greatly affected by the extent of aromatic conjugation and pKa of the end-pyridine functionalities. Interestingly, one of the oligomers (3) containing an isoniazid moiety renders visual color changes with both Hg2+ and CN− ions through two different binding sites involving two nonidentical sensing pathways which enable this probe for the detection of multiple analytes at the same time. Moreover, these ions (Hg2+/CN−) showed remarkable tuning of the supramolecular assembly (molecular gels) of 3, depicting reversible sol−gel transformation on complementary addition of Hg2+/CN− in proper stoichiometry which could be useful in scavenging toxic ions from industrial wastes. In addition, the low-cost, reusable paper discs coated with the probe molecules show rapid, onsite detection of toxic ions even from the contaminated water samples. Therefore, this highly efficient multimodal sensing of toxic ions by easy-to-synthesize molecular probes could inspire the design of new sensors with varying chromophores for the color-tunable sensing of toxic ions. KEYWORDS: Naked-eye sensing, Toxic ions, Dual-mode detection, Mutually exclusive interactions, Supramolecular gels

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INTRODUCTION

analytes in industrial wastes enhances the possibility of contamination of drinking water resources, particularly when they leach through the water-miscible organic solvents. Among various methods for the detection of ionic analytes, optical sensors are attractive since they are simple and inexpensive and can detect ions rapidly by color-changing responses.4−8 A large number of reports on Hg2+ detection based on highly conjugated boron-dipyrromethenes, cyanine dyes, and other chromophores are present in the literature.9−12 However, none of these reports offer correlation between the conjugation length of the chromophoric unit with the extent of visual changes for optical sensing. Variation in the conjugation length is known to alter the optical properties of the organic

Inexpensive techniques for the rapid detection of toxic metal ions and anions are of high contemporary significance due to the increasing extent of harmful industrial waste, since these ions pose a tremendous hazard both to the environment and human health.1 Among the toxic metal ions, Hg2+ drew special attention for the past several decades due to its severe harmful effects even at below micromolar concentrations. It induces serious damage to the digestive tracts and kidneys in the human body.2 On the other hand, CN− is the most notorious one among the biologically relevant anions. The higher toxicity of cyanide is attributed to its ability to bind with the active site of cytochrome c oxidase, which disrupts the electron transport chain and severely affects the central nervous system.3 In spite of these, both Hg2+ and cyanide are widely employed as reagents in large-scale organic syntheses and extractions in various chemical industries. Thus, the presence of these © 2019 American Chemical Society

Received: March 23, 2019 Revised: May 7, 2019 Published: May 29, 2019 12304

DOI: 10.1021/acssuschemeng.9b01644 ACS Sustainable Chem. Eng. 2019, 7, 12304−12314

Research Article

ACS Sustainable Chemistry & Engineering

phenomenon which is rarely explored.32−34 In addition, reusable, low-cost paper strips were also developed as an alternate strategy for rapid, onsite detection of these analytes.

dyes, thus it is expected also to modulate the electronics of the binding sites and alter their ion-recognition efficiencies along with the significant optical changes due to the binding event. Similarly, CN− sensing strategies including hydrogen bonding, Michael addition, or metal ion displacement gained popularity due to their easily distinguishable “output” signal.13,14 However, simultaneous detection of both of these ions involving two independent interaction sites is rare in the literature, which could provide an exclusive way to screen multiple analytes at the same time.15−19 Most of the reported methods depicted anion sensing by metal ion-responsive probes through metal-ion displacement techniques (Table S1).20−22 Notably, preaddition of toxic metal ions is essential for the detection of anions and also for an optimum performance, several parameters (such as metal ion-to-ligand ratio, complexation mode with a metal ion) needed to be standardized. Therefore, developing a single molecular probe for the simultaneous detection of Hg2+ and CN− via two different sensing pathways is desirable particularly for rapid detection purposes.23,24 Herein, we report selective ion sensing of three π-conjugated pyridine-end p-phenylenevinylene oligomers 1−3, particularly their inferences in the detection of Hg2+ and CN− ions at nanomolar concentrations (Chart 1).

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RESULTS AND DISCUSSION Design and Synthesis of Compounds. The conjugated chromophoric pyridine-end oligo p-phenylenevinylene analogues 1 and 2 with three and five aromatic rings, respectively, were synthesized by successive Witting reactions following our earlier report.35 Compound 3 having a modified conjugation length was prepared through a carbonyl-nucleophile addition protocol using isoniazid and the corresponding dialdehyde as described previously.36 The presence of metal ion coordinating pyridine ends along with anion-sensitive hydrazone units (for 3) prompted us to investigate their ion-responsive properties. Interaction of the Compounds with Toxic Metal Ions. The absorption spectrum of 1 owing three aromatic rings in conjugation showed the presence of two bands at ∼328 nm (due to a π−π* transition) and ∼394 nm (due to intramolecular charge transfer) in THF (Figure 1a). Addition of Hg2+ caused a distinct change in color of 1 from colorless to bright yellow with a prominent red-shift (∼63 nm) in the absorption maximum (394 nm) (Figure 1a and Figure S1a). This major red-shift in the intramolecular charge transfer (ICT) band might be due to the electron deficiency at the acceptor site (pyridyl ends) upon coordination with Hg2+ ions. Saturation in the optical response was achieved only upon addition of 1.2 equiv of Hg2+, which ensured 1:1 complexation of Hg2+ with 1 (also evidenced from Job’s plot) (Figure S2a).37 The 1:1 stoichiometry ensured a two-coordinated linear polymeric complex formation38 by Hg2+ with the two pyridyl ends of 1 in an intermolecular fashion (Scheme S1). The higher binding affinity (log K = 4.01 ± 0.01) and lower detectability (∼3.2 ppb) of 1 toward Hg2+ ions allowed its prompt recognition even in the presence of other competitive metal ions (Figure S2b).39 A variation in the absorbance values with the concentration of added metal ion indicates a ratiometric response with Hg2+ ion (Figure S3). It is important to note that the ratiometric sensors are often considered superior to others as they can effectively eliminate the interferences from the surrounding environment by considering the optical changes at two spectral bands. However, other commonly encountered transition metal ions did not induce any significant alteration in the absorption spectra of 1 (except Cu2+ which induced ∼30 nm red-shift) (Figure 2a). Surprisingly, addition of Hg2+ selectively transformed the bright sky-blue emission of 1 (ϕ = 0.67) into intense yellow color (ϕ = 0.23) when observed under a 365 nm UV lamp (Figure 2b). This was further evidenced by the Hg2+-driven ∼120 nm red-shift in the emission maximum of 1 (λmax = 465 nm, λex = 400 nm) (Figure 1b and Figure S1b). A titration study at these conditions revealed that ∼2 equiv of Hg2+ was sufficient for optimal spectral response (Figure S3). It is known that Hg2+ ion, due to its d10 electronic configuration, forms two-coordination complexes with nitrogen-containing monodentate ligands, such as thymine, pyridine, and imidazole.40 Formation of such complexes facilitate the intramolecular charge transfer process, resulting in the red-shifts in both absorption and emission bands.41 On the other hand, owing to the closed-shell d10-electronic configuration, Hg2+ prefers to form sp-hybridized linear complexes with pyridine ligands.42,43 Though both Zn2+ and Cd2+ also possess a similar d10 electronic configuration, they

Chart 1. Molecular Structures of the Pyridine-End pPhenylenevinylene-Based Chemosensors

Although a wide-range of π-conjugated polymers and oligomers are known in the literature for the optical sensing of metal ions,25−27 there is no report so far pertaining to one based on the structurally simple pyridine-end oligo pphenylenevinylenes. This class of molecules are of great interest due to their high quantum yield, tunable photophysical properties, and widely functionalizable backbones. Hence, the oligomers 1 and 2 were designed with different conjugation lengths in the chromophore unit. While both of them showed pronounced optical changes with Hg2+, the extent of selectivity as well as sensitivity depended mostly on the aggregation behavior of the probes and pKa of the end-pyridine moieties.28−31 Notably, compound 3 with slightly modified linker units, exhibited selective detection of both Hg2+ and CN− ions using two mutually independent sensing pathways, thus enabling the screening of multiple analytes at the same time. A physical gelation was also observed for 3, and interestingly, it exhibited a reversible sol−gel transformation upon alternating addition of Hg2+ and CN− ions in a sequence with the right stoichiometry. Such stimuli responsive ion-tuned gels could be useful in scavenging toxic ions from industrial wastes since they showed both disruption as well as restoration of gels for the dual detection of Hg2+ and CN− ions, a 12305

DOI: 10.1021/acssuschemeng.9b01644 ACS Sustainable Chem. Eng. 2019, 7, 12304−12314

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Figure 1. UV−visible titration of (a) 1 and (c) 2 with Hg2+ (0−2.5 equiv) in THF. Fluorescence titration of (b) 1 (λex = 400 nm) and (d) 2 (λex = 440 nm) with Hg2+ (0−4.5 equiv) in THF. Insets showing corresponding solutions before and after the addition of Hg2+ ions. [1], [2] = 10 μM in each case.

compared to its cadmium analogue.47 Similarly, Cu2+-induced emission quenching can be explained by the photoinduced electron transfer (PET) effect owing to its open-shell d9electronic configuration.48−50 Influence of Chromophoric Unit on Sensing. To study the effect of conjugation length of the chromophoric unit (donor site) on the extent of charge transfer and in tuning ionresponsive properties, we incorporated another probe (2) with five aromatic rings arranged in a linear fashion. It showed an absorption maximum at ∼450 nm owing to the longer conjugation length (Figure 3a). Addition of Hg2+ ions produced ∼54 nm red-shift in the absorption maximum (Figure 1c). Except Cu2+ (∼23 nm red-shift), no other metal ions showed any response in THF (Figure 2a and Figure S4a). Though in this case, also the Job’s plot showed a 1:1 interaction with Hg2+, the extent of the change in absorbance was found to be quite less as compared to 1 (Figure S5). A low binding affinity (3.90 ± 0.03 in the log scale) along with higher detection limit (∼12.3 ppb) further supported a relatively weak complexation ability of 2 with Hg2+. Similarly, the emission spectrum showed Hg2+ induced change from light-green (ϕ = 0.52) to a deep-red (ϕ = 0.11) (Figure 1d) with ∼110 nm redshift in the emission maximum (λmax = 520 nm) (Figure 2b and Figure S4b). In order to explain the poor sensitivity of compound 2 toward Hg2+ in comparison to 1, we correlated their structural aspects with metal ion binding properties. The presence of a higher number of flexible aliphatic chains in the case of 2 might restrict the motion of the targeting metal ions toward the binding sites. Second, the pKa of pyridyl nitrogens for 2 (3.2) was found to be significantly lower as compared to 1 (5.8). This low basicity of pyridyl nitrogens of 2 could possibly reduce its affinity toward metal ions. Third, the critical aggregation concentrations (CAC) of 1 and 2 were also largely different due to the difference in their conjugated aromatic lengths and number of aliphatic chains. The higher coagulation ability of 2 (CAC = ∼25 μM) in comparison to 1 (CAC = ∼61 μM) led to the formation of random aggregates in THF (Figure 3b). This could make some of the binding sites inaccessible for incoming metal ions. Formation of larger

Figure 2. Change in (a) absorbance and (b) fluorescence of 1 and 2 (10 μM) with different metal ions (2 equiv) in THF. Insets showing the resulting solutions in day-light and under 365 nm UV-light, respectively.

seldom form linear complexes like Hg2+. The arrangements of ligands around the metal ion center depend not only on its electronic configuration but also on a delicate balance between attractive electrostatic metal−ligand forces and repulsion of the bound residues.44 Consequently, Zn2+ can be found mostly in tetrahedral arrangements, while for Cd2+ preference changes between tetrahedral and octahedral.45,46 Moreover, Zn2+ being a borderline metal ion according to the hard and soft acids and bases (HSAB) principle does not prefer soft ligands such as pyridyl nitrogens, unlike Cd2+ or Hg2+. On the other hand, between Cd2+ and Hg2+, optical changes were more prominent with Hg2+ due to the higher Hg−N (pyridine) bond energy 12306

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Figure 3. (a) UV−visible spectra of 1 and 2 (10 μM) in THF. (b) Determination of critical aggregation concentration of compounds 1 and 2 in THF. Insets showing AFM images of 1 and 2 at 100 μM concentration.

Figure 4. (a) UV−visible titration of 3 (10 μM) with Hg2+ (0−2 equiv) and (b) with CN− (0−2 equiv) in THF. Insets showing the mode of interactions of these ions. Partial1H NMR spectra of 3 upon addition of (c) Hg2+ and (d) CN− as well as (e) the assigned concerned protons of probe 3.

medium (Figure 4b and Figure S7). A clear ∼43 nm red shift in the ICT band was observed upon addition of ∼5 equiv of CN− ions. Interestingly, unlike complexation with Hg2+, in the case of CN−, a 1:2 binding interaction was observed. This indicated abstraction of two acidic hydrazone protons upon interaction with CN−. On the other hand, in emission mode the compound (ϕ = 0.42) showed prominent quenching with CN− ions (ϕ = 0.09), while Hg2+ induced a considerable redshift (ϕ = 0.17) in emission maximum (∼75 nm) along with partial quenching (Figure S8). Here also, addition of analytes (Hg2+ and CN−) showed a concentration-dependent linear change in optical signal, which is beneficial for quantitative estimation (Figure S9). These contrasting emission signals allow easy differentiation between these two ions (Hg2+ and CN−), which was not possible solely by UV−visible spectroscopy. In order to check whether the presence of other ions can affect the sensing efficiency or not, we have studied the interaction of compound 3 with Hg2+ and CN− in the presence of other ions. However, no detectable variation in emission response was noticed even in the presence of excess amounts

aggregates in the case of 2 was also verified by atomic force microscopy (AFM) images (inset, Figure 3b). Simultaneous Sensing through Mutually Independent Pathways. It was evident that compound 1 showed a better response with Hg2+ compared to 2. However, significant interference from Cu2+ may limit its practical utility. To overcome this, we have introduced another probe (compound 3) with a modified backbone which could also address the effect of conjugation length. Compound 3 possesses an anion responsive acyl hydrazone moiety coupled with pyridine ends. This could be advantageous for dual mode sensing of both metal ions and anions. Indeed, compound 3 showed similar spectral changes with Hg2+ ions as observed in the case of 1 and 2 (Figure S6). Though the extent of selectivity for Hg2+ was found to be higher, the extent of spectral changes was comparatively lower (Figure 4a). This might be due to the low basicity of the pyridyl nitrogens owing to the presence of hydrazone units as well as because of the perturbed πconjugated chromophoric backbone.35 In addition, compound 3 also showed selective interaction with cyanide ions in THF 12307

DOI: 10.1021/acssuschemeng.9b01644 ACS Sustainable Chem. Eng. 2019, 7, 12304−12314

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Figure 5. (a) UV−visible titration of 3 + Hg2+ (10 μM, [Hg2+] = 20 μM) with CN− (0−20 μM) in THF. Inset showing cyanide mediated displacement of Hg2+. (b) UV−visible titration of 3 + Hg2+ (10 μM, [Hg2+] = 20 μM) with CN− (20−60 μM) in THF. Insets showing cyanide mediated deprotonation. (c) HOMO−LUMO energy gap calculation for Hg2+ and CN− interaction.

Figure 6. Schematic diagram for interaction of compound 3 with Hg2+ and CN−.

Hg2+. We suspected that distinguishable optical signals, observed in these cases, may be caused by two different modes of interactions. This experimentally found trend was further supported by their unequal highest occupied molecular orbital−lowest unoccupied molecular orbital, (HOMO− LUMO) gap obtained from the density functional theory (DFT) calculations (Figure 5c). Frontier molecular orbital (FMO) analysis showed that the HOMO of the complex was mainly concentrated on the central aromatic core, while the LUMO was located at the Hg2+-linked pyridine ends (ΔE = 3.73 eV). Metal ion coordination to the pyridyl nitrogens (acceptor site) of compound 3 decreased the electron density at the acceptor ends, which increased the extent of charge transfer from donor to acceptor compared to the native probe (ΔE = 4.2 eV). Moreover, the unique 2-coordinated linear arrangement of the probes in the presence of the Hg2+ ions might also boost the intermolecular charge transfer interaction across the molecules.40 Conversely, cyanide-mediated deprotonation of the isonicotinyl hydrazone protons resulted in charge delocalization over the central aromatic core and increased the effective conjugation length. This again narrowed

of the competitive analytes (Figure S10). Job’s plot analysis also showed contrasting results, where 1:1 complexation was observed with Hg2+ and 1:2 with CN−. The corresponding binding constants were calculated as 3.12 ± 0.08 and 8.96 ± 0.02, respectively (Figures S11 and S12). The interaction of 3 with either Hg2+ or CN− ions was probed using 1H NMR spectroscopy. Progressive addition of Hg2+ ions into 3 caused gradual downfield shifts of the protons (Ha, Hb, and Hc) situated close to the metal ion binding pyridine moiety (Figure 4c,d), which ensured that the metal ion coordination occurred through the pyridyl nitrogens. However, addition of CN− ions led to the deprotonation of the isonicotinyl hydrazone moieties of 3, which led to the accumulation of negative charges, causing the upfield shifts in the NMR spectra. The detection limits were calculated to be as low as 7.8 and 11.2 ppb for Hg2+ and CN−, respectively. Interaction of 3 with both Hg2+ and CN− led to the redshifts in absorption maxima; however, the extent of shift was found to be different. Incorporation of Hg2+ induced an ∼34 nm bathochromic shift, whereas it was ∼43 nm for CN−. Also, in fluorescence mode, a red-shift was specifically observed with 12308

DOI: 10.1021/acssuschemeng.9b01644 ACS Sustainable Chem. Eng. 2019, 7, 12304−12314

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Figure 7. (a, b) Thermoreversible gel-to-sol conversion of 3 at 70 °C and gel-to-sol transformations in the presence of (c) Hg2+ and (d) CN− ions at 25 °C. Insets showing corresponding gel and sol under 365 nm UV-light. (e) Restoration of the “gel” upon addition of complementary ions, 4 equiv of CN− or 1.5 equiv of Hg2+. (f) AFM images of 3 in the presence (bottom) and absence (top) of Hg2+ ions.

effective in creating multiresponsive gel systems because it shows reversible binding and unbinding of the responding sites.62−68 Further, rheological studies have been performed to evaluate the mechanical strength of the dioxane gel of compound 3. Both amplitude sweep and frequency sweep experiment indicates larger values of G′ (storage modulus) than that of G″ (loss modulus), which confirms the viscoelastic nature of gel sample. The frequency sweep further revealed that the viscoelasticity of the gel was maintained throughout the frequency range of 1−100 Hz at a fixed applied strain of 0.1% (Figure S14). In order to address the interesting stimuli responsive properties,69−72 dioxane gel of 3 was tested, since the probe showed selective interactions with both Hg2+ (1:1 binding) and CN− (1:2 binding) ions. Interestingly, a gel-to-sol transition was observed immediately after the addition of either of the two ions (Figure 7c,d). A significant color change was noticeable with the “naked eye”. The resulting “sol” became dark orange with ∼3 equiv of CN− ions and a light orange in the presence of ∼2 equiv of Hg2+ ions from a lightyellow gel of 3 alone. This again suggested the different interaction modes with Hg2+ and CN− ion. However, the “sol” could again be reverted back into the gel by adding complementary ions i.e., 1.5 equiv of Hg2+ on the “sol” containing 3 equiv of CN− or 4 equiv of CN− on the “sol” containing 2 equiv of Hg2+ (Figure 7e). This stoichiometry dependent reversible transformation from the “sol” to the gel could help in scavenging toxic ions from industrial wastes. The gel-to-sol transformation was further evidenced from the demolition of the fibrillar network of 3 upon addition of Hg2+ in AFM (Figure 7f). However, “sol” formation was also observed on heating the “gel” sample at ∼70 °C. However, in this case the sky-blue emission of the gel did not quench even in the “sol” state. In order to compare the mechanical strength of the recovered gel, rheological parameters were determined. Although, a little decrease in the G′ (from 510 to 430 Pa) and strain (13.5% to 10.4%) required for the gel-to-sol transition was observed, the viscoelastic nature of the gel was fully recovered upon consecutive addition of Hg2+ and CN−. The frequency sweep data of the recovered gel also indicates that

down the HOMO−LUMO energy gap (ΔE = 3.54 eV) and led to the red-shift in absorption maxima. Thus, the red-shifts in absorption maxima observed in the presence of both Hg2+ and CN− primarily originated from two different modes of interactions (Figure 6). Interaction of 3 with consecutive addition of Hg2+/CN− was examined by UV−vis spectroscopy. In the presence of the preadded Hg2+, CN− showed a concentration-dependent change in the optical signal. Addition of a stoichiometric amount (1:2) of CN− ion could only displace the Hg2+ from the vicinity of the probe molecules and regenerate the optical signal of native probe (Figure 5a). However, upon increasing the concentration further, it initiated the deprotonation process as usual (Figure 5b and Scheme S2). Similarly, addition of the stoichiometric amount of Hg2+ ions to the solution of 1 + CN− showed a color change from yellowish orange to colorless due to the formation of the free probe. However, here also further increase in Hg2+-concentration induced the formation of linear complexes and turned the solution color to yellow (Figure S13). Thus, simultaneous detection of both Hg2+ and CN− ions is achieved using two mutually independent sensing pathways, which is particularly useful in the screening of multiple analytes at the same time. Stimuli-Responsive Sol−Gel Transition in the Presence of Ionic Analytes. The structural features of 3 prompted us to check its physical gelation abilities as it contains n-hexadecyl chains and amide-like linkages which are known to impart intermolecular van der Waals and hydrogen bonding interactions, respectively. Also, weak π-stacking interactions by the aromatic moieties may further assist in gel formation.51−53 Indeed, physical gelation of 3 was obtained in dioxane having a minimum gelator concentration of 10 mg/ mL. The self-supporting gel was thermoreversible in nature, i.e., the gelation remained intact even after several consecutive heating and cooling cycles (Figure 7a). In contrast, compound 1 and 2 failed to form a self-supported gel due to the lack of any hydrogen bonding moiety. Few gelator molecules are known to bind selective ions through specific binding sites which induce either order or disorder in their self-assembly leading to the ion-tuned sol−gel transitions.54−61 Among various noncovalent interactions, coordination interaction is 12309

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Figure 8. (a) Detection of metal ions (10 μM) using paper discs coated with compound 1 and 2. (b) Simultaneous sensing of Hg2+ and CN− (10 μM) using compound 3-coated paper discs. (c) Detection of Hg2+ and CN− in the presence of other metal ions and anions by compound 3.

showed an improved solubility in the water-rich environment and can be employed for the detection of Hg2+/CN− ions even in a 6:4 THF−water mixture medium (Figure S17). These observations clearly indicate that the present method can detect Hg2+ and CN− ions in a semiaqueous environment in addition to a wide range of organic solvents. Thus, in the analysis of water samples contaminated with industrial waste, it is also possible to use the present sensor system. “Sustainable Sensing” Using Reusable Paper Discs. Laboratory-designed detection methods mostly involve sophisticated instruments, which require high-cost maintenance and trained technicians for handling. Considering this, here we have developed an alternative strategy of detecting toxic metal ions and/or anions using precoated paper discs. The probe-coated paper discs showed a distinct visual color as well as fluorescence signal for each of compounds 1−3. For example, paper discs dipped into the solution of compound 1 showed a bright cyan-colored fluorescence, whereas a prominent green emission was observed from the discs coated with compound 2. When these precoated discs were exposed to a wide varieties of metal ions, a selective color change (both in normal daylight as well as under UV light) was observed not only with Hg2+ but also in the presence of Cu2+. In the case of compound 1, addition of Hg2+ showed an appearance of yellow color, while fluorescence changed from cyan to yellow. Contrary to this, Cu2+ induced a turn-off emission response with no detectable change in visible color (Figure 8a). On the other hand, the color of the compound 2-coated paper strips changed from yellow to red when exposed to Hg2+ solution. At the same time, these Hg2+-treated paper discs can also be detected under UV lamp, where it could selectively turn green fluorescence into a red color. However, no detectable fluorescence was noticed when the same strips were spiked with Cu2+ instead of Hg2+. Similarly, the paper discs, coated with compound 3, could simultaneously detect both CN− (quenching) and Hg2+ (greenish-blue) ions by fluorescence response (Figure 8b). Since industrial waste may contain multiple ions simultaneously, the compound 3-coated paper strips were spiked with mixtures of different toxic metal ions and anions both with and without incorporating Hg2+ and CN− ions. Only the mixture of ions containing either Hg2+ or CN− showed changes in emission color (blue to cyan for Hg2+

the viscoelasticity of the recovered gel was retained over a wide frequency range under the applied strain of 0.1% (Figure S15). Application to Industrial Waste-Management. “Chemical waste” is a broad term, which includes a wide range of acids, alkalis, metals, cyanides, oxidizers, and flammable gases produced by large factories during the manufacturing process. In the laboratory, chemical wastes are usually segregated and disposed in specialized containers in order to meet public health and safety. Unlike this, in many places, industries are built without having proper disposal machinery or sanitation systems. Since most of the production sites are commonly built near natural water-sources, the untreated and partially treated chemicals are commonly fed back into a near lying body of water. Thus, they can easily get transmitted into the natural ecosystem, which is dangerous not only to the marine life but also for the organisms who use those as drinking water sources. Considering this, we checked the mercury sensing ability of our probe (1) in a wide range of organic solvents. On systematic investigations, it revealed that the probe could produce identical spectral response both in water-miscible (like THF, acetonitrile) as well as immiscible (like DCM, hexane) organic solvents. This indicates that the present system can act as a general marker for Hg2+ for a wide range of industrial waste samples. In addition, the π-conjugated backbone would allow these probes to be integrated with the electronic devices, which will be useful not only for detecting toxic analytes in waste materials but also for quantifying them even at below nanogram concentration. Contamination of nearby water resources by improper deposition of waste materials is one of the main reasons for growing health complications in industrial areas. Therefore, we intended to check whether the present system can detect the presence of toxic ions in the aqueous environment or not. However, the compounds in aqueous medium (THF content