Inner Filter Effect and Resonance Energy Transfer Based Attogram

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Inner Filter Effect and Resonance Energy Transfer Based Attogram Level Detection of Nitroexplosive-Picric acid using Dual Emitting Cationic Conjugated Polyfluorene Arvin Sain Tanwar, Laxmi Raman Adil, Mohammad Adil Afroz, and Parameswar Krishnan Iyer ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00093 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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Inner Filter Effect and Resonance Energy Transfer Based Attogram Level Detection of Nitroexplosive-Picric acid using Dual Emitting Cationic Conjugated Polyfluorene Arvin Sain Tanwar,a Laxmi Raman Adil,a Mohammad Adil Afroza and Parameswar Krishnan Iyer*a,b a

Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati-781039. India.

b

Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati, 781039. India.

KEYWORDS. inner filter effect (IFE); resonance energy transfer (RET); picric acid; sensing; conjugated polymer; nitroexplosives. ABSTRACT: A novel conjugated cationic polyfluorene (polyelectrolyte) derivative, PFBT was developed by means of simple and cost effective oxidative coupling polymerization method. PFBT displayed dual state emission in DMSO as well as in water, a characteristic phenomenon of polyfluorene homopolymers, and tested for nitroexplosive analytes detection to observe a remarkable fluorescence quenching response for picric acid (PA) in the both solvents. The polymer PFBT demonstrated substantial selectivity and ultra-sensitivity towards nitroexplosive PA in both the solvents (DMSO and H2O) with exceptional quenching constant values of 2.69 × 104 M-1 and 2.18 × 105 M-1 and a ultra-low limit of detection of 92.7 nM (21.23 ppb) and 0.19 nM (43.53 ppt) in respective solvents. Furthermore, economical portable test strip devices were prepared for easy and fast on-site PA sensing, which can detect upto 0.22 attogram level of PA. PA sensing in vapor phase was also established, that could detect up to 42.6 ppb level of PA vapors. Interestingly, the mechanism of sensing in DMSO solvent was attributed to substantial inner filter effect and photo induced electron transfer, while in H2O the sensing occurs via possible resonance energy transfer and photo induced electron transfer, which is exceptional and not reported earlier for a single probe.

The development of high energy nitroexplosive sensing probes are in huge demand.1-3 Picric acid (PA) is a wellknown nitroaromatic explosive (Scheme S1) since world war I and possesses high energy content even higher than that of TNT.4 Due to extraordinary high solubility of PA in aqueous medium and its extensive application in the dye industries, pharmaceuticals, fireworks, chemical laboratories, and match box industries, it contaminates soil and natural water and therefore causes several serious health hazards like sycosis, cancer, damage to respiratory organs, abnormal liver functions and kidney failure.5-7 Moreover, PA is converted to picramic acid during metabolism, which possess very high mutagenic activity in higher proportions than PA itself.8 Additionally, owing to the comparable electron deficient nature of other nitroexplosives, especially TNT, which often interferes in the selective sensing of PA in presence of other competing nitroexplosive. Therefore, it turns out to be very vital and a challenging task to develop a system in order to detect PA very selectively and in trace amount with respect to homeland security and environment pollution. There are many reports available on nitroexplosive detection based on various analytical techniques viz. high performance liquid chromatography,9 surface enhance raman spectroscopy,10 dynamic light scattering,11 solid

phase microextraction-ion mobility spectroscopy,12 enzyme linked immunosorbent assay,13 and electrochemical methods etc.14 but these techniques have some disadvantages such as most of them are expensive, time consuming and less sensitive, complex to handle for on-site detection and thus unsuitable for hands-on use. In contrast, fluorescent probes based nitroexplosive sensing has attracted remarkable attention due to its simplicity, swift response time, noteworthy sensitivity, and capability of sensing in multiple state (solution and solid state).2,15 Therefore, researchers have created various fluorescence based probes such as quantum dots, organic molecule dyes, metal complexes, metal organic frameworks, nanoaggregates and conjugated polymers (CPs) for the nitroexplosive detection.16-33 Nevertheless, these fluorescent probes have some limitations like poor sensitivity and selectivity, difficult to use for on-site detection purposes, vapor phase and utilization of organic medium for sensing. In addition, mechanism of fluorescence quenching for nitroexplosive detection reported previously are mainly based on either PET processes, resonance energy transfer (RET), ground state complex formation, inner filter effect (IFE) and indicator displacement assay (IDA).32,34-37 Hitherto, not much attention has been given for developing a sensory sytem with likely multiple sens-

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ing mechanisms and their elucidation. Hence, development of an efficient sensor for detection of PA in multiple platforms like in aqueous, organic and solid state as well as in different PA state (solution and vapor phase) by maintaining the high selectivity and ultra-sensitivity is extremely desirable and challenging. CPs are forefront among various materials used in the area of sensing.38-42 They possess excellent light harvesting properties and due to the ‘molecular wire effect’ an amplified fluorescence signal response is achieved making them highly sensitive toward analytes.40 Recently, few reports appeared for nitroexplosive detection based on CP probes, yet sensitivity, selectivity and applicability in water can be improved by choosing an efficient receptor.21,43-52 Hence, signal amplifying nature of CP sensors for nitroexplosives sensing with desirable receptor units, which offer high selectivity and extraordinary sensitivity to the CPs remains challenging. Previously, two cationic CPs for sensing of PA based on RET and charge transfer complexation were reported.34,35 To further advance the overall sensing parameters it is necessary to explore new and alternate possible fluorescence mechanisms by designing better sensor probes. To realize this, methylbenzotriazolium appended cationic CP PFBT for very particular and fluorescence amplifying PA detection at ultra-low level was synthesized. Interestingly, the detection of PA in DMSO solvent is based on substantial IFE assisted by PET while in water (pH-7.0, 10 mMHEPES buffer), it is based on RET assisted by PET. As per the literature, there exist no reports where a single probe detects PA based on both RET and IFE mechanisms, acting independently, in different solvents with high sensitivity and on multiple platforms, making PBFT a very unique probe for PA detection at such low levels. Moreover, detection of PA in very low levels in vapor phase was also possible with this probe.

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width at 298 K. TRPL studies were done by using Edinburg Life Spec II instrument. Cyclic voltammogram was obtained by means of CH instruments Electrocchemical workstation. The sensor devices were fabricated using Whatman qualitative filter paper grade 1 test strips. Synthesis of 1-methyl-1H-benzo[d][1,2,3]triazole. In a round bottom flask containing benzotriazole (3 mmol, 357 mg) in 10 mL of DMF, fine powder of NaOH (480mg, 12mmol) and methyl iodide (187 µL, 3 mmol) were added and mixed for 1 h. After that, it was poured into aqueous media and extracted by using organic solvent (DCM). The crude triazole derivative was separated by evaporating DCM and later purified on column chromatography at 1:9 EtOAc:hexane solvent polarity.(Yield = 60%). 1

H NMR (CDCl3, 400 MHz, δ): 8.07 (d, 1H), 7.52 (q, 2H), 7.40 (m, 1H), 4.31 (s, 3H). 13

C NMR (CDCl3, 100 MHz, δ): 145.83, 133.42, 127.24, 123.80, 119.70, 109.20, 34.16. MS (ESI): C7H7N3 [M+H]+: 134.0718 (calculated); 134.0737 (found). Synthesis of M2: M2 was synthesized by previously established method.53 In a 50 mL RBF, fluorene (1 g, 6.016 mmol), aqueous NaOH (50%) and tertabutyl ammonium iodide (TBAI) (0.476 g, 1.203 mmol) were taken. The flask was purged with argon gas and degassed using free-thaw cycles followed by addition of addition of 1,6dibromohexane (6.47 mL, 42.112 mmol). It was stirred at 70 °C for 4 h followed by lowering of temperature to 25 °C, extracted with chloroform followed by washing with water thrice. Then, it was dried via Na2SO4 (anh.) and crude product was obtained upon solvent evaporation. Later it was purified via column chromatography technique (eluent-hexane). (Yield = 82 %, 2.4 g) 1

H NMR (CDCl3, 400 MHz, δ): 7.71 (d, 2H), 7.32 (m, 6H), 3.27 (t, 4H), 1.97 (t, 4H), 1.06 (m, 4H), 1.63 (m, 4H), 0.60 (m, 4H), 1.16 (m, 4H). 13

EXPERIMENTAL SECTION Attention! The nitroaromatic compounds (especially TNT, RDX and PA) possess extremely explosives nature, therefore extra caution should be maintained while handling them using appropriate safety measurements. Materials and Methods. PA was used as received and procured from Loba Chemie,. Nitroaromatic compounds 4-NT, 2,4-DNT, 2,6-DNT, 1,3-DNB, HEPES buffer were obtained from Sigma-Aldrich Chemicals. From AccuStandard, Research department explosive (RDX) and TNT were obtained and used as received. Other compounds were obtained from Merck or Alfa-Aesar. All the sensing experiments and aqueous stock solution were prepared using Milli-Q water. 1H and 13C NMR spectra were obtained through Varian-AS400 NMR spectrometer and Bruker Ascend 600 spectrometer respectively. Absorption spectra was recorded on Perkin Elmer Lambda25 spectrophotometer where the Photoluminescence spectra was recorded on a Horiba Fluoromax-4 spectrofluorometer in 1 cm path length cuvettes keeping 3 nm slit

C NMR (CDCl3, 100 MHz, δ): 141.28, 150.45, 127.01, 127.27, 119.89, 122.92, 40.40, 55.07, 32.81, 34.12, 27.93, 29.23, 23.68. Synthesis of PF: To a three necked round bottom flask, containing dissolved FeCl3 (anh.) (1.4 g, 8.124 mmol) in nitrobenzene (15 mL) an argon inlet was fitted. M2 (1 g, 2.031 mmol) in 10 mL nitrobenzene was slowly introduced to the above round bottom flask via a syringe. After stirring for 36 h at rt it was precipitated in methanol solution. These precipitates were dissolved in chloroform, reprecipitated from methanol, centrifuged and filtered (repeated thrice). Finally, it was dried in vacuum to get brown colored polymer. (Yield = 50 %, 49 mg) 1

H NMR (CDCl3, 400 MHz, δ): 7.86 (br), 7.70 (br), 7.33 (br), 3.30 (br), 2.08 (br), 1.69 (br), 1.27 (br), 1.18 (br), 0.73 (br). GPC (standard: polystyrene/THF): Mw =2.41 × 104, PDI=3.6. Synthesis of PFBT: Polymer PF (40 mg, 8.124 mmol) and 1-methyl-1H-benzo[d][1,2,3]triazole (81.24 mmol, 108 mg)

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was solubilized in dry DMF (5 mL) and dry THF (2 mL), maintaining inert conditions, allowing to stir for 24 h at 70 °C and thereafter decanted into excess diethyl ether and stirred further to obtain precipitates. The precipitates were further washed thrice with chloroform filtered out and desiccated at reduced pressure to get brown polymer (PFBT). (Yield = 75%, 46 mg) 1

H NMR (DMSO-d6, 600MHz, δ: 8.32 (br), 8.14 (br), 7.97 (br), 7.87 (br), 7.79 (br), 7.48 (br), 7.36 (br), 4.54 (br), 3.95 (br), 2.89 (br), 2.08 (br), 1.37 (br), 1.10 (br), 0.66 (br). Stock Solutions Preparation. Polymer PFBT stock solution of 1 × 10-3 M concentration was prepared in DMSO solvent. Stock solution of various nitroaromatic and phenolic compounds, namely, phenol, NM, 4-NP, NB, 2,4DNP were prepared at concentrations (1 × 10-3 M) in MilliQ water. Other nitroaromatic compounds viz. 4-NT, 1,3DNB, 2,4-DNT, 2,6-DNT solutions were carefully prepared in HPLC grade THF at concentrations of 1 × 10-2 M. Solutions of TNT and RDX of 1 × 10-2 M concentration were prepared in 1:1 MeOH:CH3CN. UV-vis and emission spectra of polymer PFBT with different nitroaromatic compounds were executed in 3 mL of DMSO and water (pH-7.0, 10 mM- HEPES buffer) having 3.3 × 10-6 M of PFBT in a 1 cm path length cuvette. Absorption spectrum of PFBT in DMSO:H2O solvents. Absorption spectra of PFBT were studied under two different solvent ratios i.e. DMSO:water. The absoption band of isolated PFBT at 370 nm in DMSO:water (100:0) undergoes red shift to 378 nm in DMSO:water (0:100) ratio (Figure S1). This red shift in the UV-vis absorption band signifies J-type aggregation among the polymer chain because of increased non-covalent interactions among them.54-55 TRPL studies. Excited state lifetime decay profile of PFBT (3.3 × 10-6 M) was obtained in both solvents before and after PA (93.3 × 10-5 M in DMSO and 26.6 × 10-5 M in aqueous medium) addition via 375 nm pulse excitation and emission at 419 nm in DMSO and 529 nm in aqueous medium. The decay profiles were bi-exponentially fitted and for uniformity in results average lifetime were considered (Table S1-S2). Portable Test Strips. Whatman filter paper was taken and dipped into the solution of PFBT (3.3 × 10-6 M) in DMSO and dried to get fluorescent paper strip. The PFBT coated filter paper was thereafter shaped to portable test strips of size (1 cm × 1 cm) by easily cutting them in desired amount and then used for on-site sensing. Photoluminescence quantum yield. Quantum yields were measured using absolute fluorescent quantum yield via the integrating sphere method using an Edinburgh FLS980 fluorescence spectrometer. Limit of Detection (LOD) Evaluation. For calculating LOD in DMSO solvent, various samples of polymer PFBT (3.3 × 10-6 M) each having PA (3.3 × 10-7 M, 6.6 × 10-7 M, 10.0 × 10-7 M, 13.3 × 10-7 M, 16.6 × 10-7 M, 20.0 × 10-7 M) were taken separately and later photoluminescence spectrum

was obtained for respective solution keeping the same excitation wavelength i.e 370 nm. A regression equation was obtained from the calibration curve between emission maxima and concentration of PA. Afterward, LOD was calculated from 3σ/k, here k represents slope of calibration curve and σ denotes standard deviation in the emission of PFBT without PA. In a similar manner, LOD was determined in water medium, where different samples of PFBT (3.3 × 10-6 M) having different amount of PA (0.0 M, 0.3 × 10-9 M, 0.6 × 10-9 M, 1.0 × 10-9 M, 1.3 × 10-9 M) were utilized to get the calibration curve between emission intensity and PA concentration. Similarly using the equation 3σ/k, LOD was calculated for aqueous medium. Cyclic Voltammetry (CV) Studies. Three-electrode cell was used for CV measurements of PFBT. Where saturated Ag/AgNO3, platinum wire and glassy carbon eletrode serve as reference, counter and working electrode respectively. CV studies were done at room temperature and in argon environment. Tertabutylammoniumhexafluorophosphate (0.1 M) (TBAPF6) was utilized as a supporting electrolyte in CH3CN and Fc+/Fc couple as the reference internally. Using onset method for PFBT, calculated HOMO level is -5.56 eV; [EHOMO = -(E(onset,ox vs Fc+/Fc) + 4.8) (eV)]. Using the onset of absorption spectrum of PFBT, optical band gap was obtained, from which LUMO level of PFBT was found to be -2.65 eV. Calculations for RET Parameters. Overlap integral value i.e. J determines the extent of energy transfer between the PFBT and PA. It was calculated by the using the below equation35 ∞

‫ = )ߣ(ܬ‬න ‫ߣ)ߣ( ܣߝ )ߣ( ܦܨ‬4 ݀ߣ 0

Where, FD (λ) denotes corrected photoluminescence intensity from λ to Δλ with the total intensity for PFBT normalized to unity, εA denotes molar extinction coefficient of acceptor (PA) in M-1cm-1 . Förster distance (R0) was calculated by using the equation given below: 1/6

ܴ0 = 0.211ሾ(‫ߟ(ܳ)ܬ‬−4 )(݇ 2 )]

Where, Q represents photoluminescence quantum yield of PFBT, η denotes refractive index of the medium and k2 denotes dipole orientation factor (0.667). Calculations of inner filter effect correction. The role of IFE in the suppression of the fluorescence of PFBT can be calculated by using the below equation for a 1 cm size of cuvette.56 Icorr/Iobs= 10(Aex+Aem)/2 Where, Icorr and Iobs are the fluorescence intensity before and after the IFE correction. Aem and Aex are the UV-vis absorption value of the fluorophore (PFBT) having analytes (PA) at emission maxima and excitation wavelengths. Vapor phase sensing. 100 mg of the anhydrous PA was placed for two days at room temperature in a sealed flask, which kept for 30 minutes at 30 °C before each titration in


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order to make sure complete saturation of air within flask with the vapors of PA. Using the (log10 P = A-B/T) equation, which is an integrated form of the ClausiusClayperon equation, PA vapor pressure (P) was determined. Here, P denotes PA vapor pressure, aforementioned equation involves two conventionally used fitting parameters i.e. A and B, and T corresponds to temperature. Furthermore, the PA vapors volume in ppm level was calculated by- saturation concentration (ppm) = P (mmHg) / 760 mmHg × 106, where P denotes the vapor pressure of PA. The amount of PA vapors was maintained constant (101.64 ppb, 2.5 mL) for purging through fluorescence cuvette using an air-tight syringe.

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from 419 to 529 nm in 100% DMSO to 100% H2O, the fluorescence spectra in various DMSO and water fraction ratios were recorded keeping the fixed concentration of PFBT (3.3 × 10-6 M). When water ratios were increased to 30%, the emission intensity at 419 nm decreases and with further increment in water ratio the peak completely shifts to 529 nm, (Figure S9). The emission of new peak at 529 nm in water is due to the spontaneously formed polymer nanoaggregates via fluorene-fluorene noncovalent interaction and leading to aggregation caused quenching (ACQ) red-shifted effect, a unique phenomenon associated with polyfluorene homopolymers.54,55 This new 110 nm orange emission has been ascribed to the intermolecular self-assembling nature or due to the J-type aggregation as visualized from the UV-Visible spectrum of PFBT and as well as due to the likely nano objects formation in aqueous environment.54,55 Fluorescence quenching experiments of PFBT were investigated in both mediums i.e. DMSO and water for various nitroexplosives and phenols viz. TNT, PA, 4-NT, 1,3-DNB, 2,4-DNT, 2,6-DNT, NM, Phenol, NB and RDX. Interestingly only PA showed significant fluorescence quenching in both these solvents. The concentration of PFBT (3.3 × 10-6 M) was kept constant in both solvents (DMSO and water) and then the quenching experiments were initiated by addition of various nitroexplosives.

Scheme 1. Synthetic scheme of PFBT. (a) 50% NaOH(aq.), TBAI, 1,6-dibromohexane, 70°C, 4h. (b) Anh. FeCl3 in nitrobenzene, rt, 36 hr, inert (c) 1-methyl-1Hbenzo[d][1,2,3]triazole, DMF, THF, 70 °C, 24 h.

RESULTS AND DISCUSSION PFBT-Synthesis and characterizations. Synthetic pathway to precursor polymer PF and target polymer PFBT are shown in Scheme 1. The monomer and polymers were characterized completely before their further usage (Scheme S2, Figures S2-S8). Molecular weight (Mw) of PF was 2.41 × 104, Polydispersity (PDI) = 3.6 using polystyrene as standard in THF. N-methyl benzotriazolium groups linked to aliphatic side chains of CP PFBT provide specific recognition sites for PA through electrostatic interactions resulting enhanced sensing efficiency. PFBT showed high solubility in DMSO with PL quantum yield of 0.81 in DMSO and 0.19 in H2O. The absorption maxima were observed at 370 nm in DMSO and at 378 nm in H2O (pH= 7.0, 10 mM-HEPES buffer) (Figure S1). The emission spectra were observed at 419 nm in DMSO and 529 nm in H2O (pH= 7.0, 10 mM-HEPES buffer), when excited at 370 nm (excitation wavelength). To investigate this unique shift of 110 nm in fluorescence spectrum

Sensing Studies in DMSO. The polymer PFBT showed blue luminescence (419 nm) in dilute solution (3.3 × 10-6 M) underneath a UV lamp (365 nm) which can be seen clearly by naked eyes (Figure S10). Addition of minor amount of PA (3.3 × 10-6 M) caused immediate fluorescence quenching ~10 % that becomes ~94 % on total addition of PA solution (93.3 × 10-6 M) (Figure 1a). This fading of fluorescence can also be clearly seen underneath UV lamp (Figure S11). To calculate efficiency of quenching of PFBT in response to PA concentration in solution, SternVolmer (S-V) plot was obtained using the equation (I0/I = 1 + Ksv[Q]), here Io represents emission intensity of PFBT devoid of any quencher; I represents the emission intensity after addition of quencher of concentration [Q]; and Ksv denotes the S-V constant (M-1). From S-V plot (Figure 1a (inset)), Ksv was obtained i.e. 2.69 × 104 M-1 signifying higher sensitivity for PFBT towards PA. LOD value for PA was calculated using standard method reported,34 and found to be 92.7 nM (Figure S12). Selectivity studies in DMSO. To investigate the selectivity of PFBT towards PA, various interfering nitroexplosives namely TNT, RDX, 1,3-DNB, 2,4-DNT, 2,6-DNT, NM, 4-NT, phenol and NB were tested in same experimental setup. As shown in Figure 1b, no substantial changes were witnessed in the fluorescence quenching of PFBT in presence of these analytes. The Stern-Volmer plots for all these nitroexplosives were also obtained (Figure 1c). At lower concentration of PA, the S-V plot was linear in nature, however, at higher concentration the curve rises exponentially. This non-linear nature of S-V curve signifies the amplified quenching in case of CP or involvement of mix-


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ture of other mechanism (static and dynamic) or an additional energy transfer process, or may be liable for selfabsorption. On the other hand, S-V plot remains linear for all other analytes confirming the absence of above phenomenon. Significantly higher value of Ksv was found for PA as compared to rest nitroexplosives, suggesting remarkable slectivity and amplified quenching. Furthermore, other inorganic ions (Cr3+ ,Cu2+, Cd2+, Hg2+, Fe3+, Co3+, Zn2+, Ag+, Pb2+, Mn2+, Al3+, Ni2+, HCO3-, SO42-, NO3-, S2-, BF4-, CN-, CO32- ,I-, F-, Cl- , PO43-, H2PO4-) that may be naturally present in contaminated water samples didn’t show interference with the emission intensity of PFBT (Figure S13), suggesting exclusive selectivity of PFBT towards PA and feasibility of PFBT system for sensing in contaminated natural water samples. Competitive Sensing Studies. The specific recognition of PA in the occurrence with different nitroaromatic compounds is highly desirable for realistic application of this platform. All other nitroaromatic compounds possess similar electron deficient nature like PA and their presence may affect the sensitivity of PFBT and compete with PA, hence detection of PA in presence of different nitroexplosives remains challenging. To validate the selectivity of PFBT system, various photoluminescence titration experiments were conducted in the presence of these interfering nitroaromatic compounds. Initially, fluorescence spectrum of PFBT (3.3 × 10-6 M) was recorded and to the above, a solution of TNT (93.3 × 10-6 M) was added in order to have maximum interaction among recognition sites of PFBT with TNT molecules, however addition of TNT caused no substantial change in emission spectrum (Figure S14). After that, PA (93.3 × 10-6 M) was added to the above mixture i.e. (PFBT+TNT), that caused substantial quenching of fluorescence. Similar experiments were repeated with other analytes and the results obtained were quite similar to above case with nearly insignificant modification in the PA quenching efficiency (Figure S15-S22 and Figure 1d). Sensing Studies in Aqueous Medium. The polymer PFBT emits orange fluorescence (529 nm) in dilute solution (3.3 × 10-6 M) underneath UV lamp (365 nm) (Figure S10). It was also noticed that cationic CP PFBT is very sensitive to the PA in aqueous medium as compared in DMSO solvent. Initial addition of minor amount of PA (3.3 × 10-6 M) caused immediate photoluminescence quenching ~48 % that becomes 86 % on total addition of 26.6 × 10-6 M PA concentration (Figure 2a). This fading of orange emission can also be clearly seen underneath UV lamp (Figure S11). In the same manner as observed with DMSO solvent, quenching efficiency of PFBT in water (pH-7.0, 10 mM-HEPES buffer) showed identical respons-

es to the PA concentration as seen from the slope of S-V plot. From S-V plot (Figure 2a (inset)), Ksv was obtained i.e. 2.18 × 105 M-1 indicating significantly high sensitivity of PFBT towards PA in water. LOD value for PA in aqueous medium was calculated using standard method reported, and found to be 0.19 nM (43.53 ppt) (Figure S23). Selectivity studies in Aqueous Medium. To examine the selectivity of PFBT towards PA in aqueous medium, various interfering nitroexplosives were tested under the same experimental setup. As shown in Figure 2b, no substantial changes were witnessed in PL quenching of PFBT by these analytes. S-V plots for all other nitroexplosive were also obtained in water (pH-7.0, 10 mM-HEPES buffer) and showed linear nature (Figure 2c). This linear nature of S-V curve in aqueous medium indicated the possibility of either static or dynamic type of quenching is happening. Significantly high value of Ksv was obtained for PA as compared to other nitroexplosives, indicating remarkable selectivity towards PA. Further, other inorganic ions (Cr3+ ,Cu2+, Cd2+, Hg2+, Fe3+, Co3+, Zn2+, Ag+, Pb2+, Mn2+, Al3+, Ni2+, HCO3-, SO42-, NO3-, S2-, BF4-, CN-, CO32- ,I-, F-, Cl- , PO43-, H2PO4-) that may be naturally present in contaminated water samples didn’t show interference with the emission intensity of PFBT in water (pH-7.0, 10 mM-HEPES buffer) (Figure S24), suggesting exclusive selectivity of PFBT toward PA and motivated us to test the feasibility of PFBT system for sensing in contaminated natural water samples. Competitive Sensing Studies. As mentioned in case of DMSO solvent, selectivity of PFBT system was also tested in water (pH-7.0, 10 mM, HEPES buffer) in a competitive environment with other nitroaromatic compounds. In this case, first PL spectrum of PFBT (3.3 × 10-6 M) was recorded and to this a solution of TNT (26.6 × 10-6 M) was added in order to have maximum interaction among the binding sites of PFBT with TNT molecules, Nevertheless, no substantial alteration in PL spectrum was witnessed after addition of TNT (Figure S25). To the same mixture i.e. (PFBT+ TNT), PA (26.6 × 10-6 M) was added which resulted in significant fluorescence quenching indicating superior recognition of PA by PFBT in presence of interfering analyte like TNT. Similar types of experiments were repeated with other analytes and no variation in results were obtained (Figure 2d and Figure S26-S33) as compared to most other chemosensor systems developed previously for the PA detection that suffer from huge interference. In sight of these studies, PFBT system provides a rapid, reliable and effective platform for PA detection in a competitive environment and irrespective of the solvent medium, which is exceptional.


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Figure 1. (a) PL spectra of PFBT (3.3 × 10 M) in presence of PA in DMSO (inset: S-V plot). (b) Effect of other nitroexplosives -6 -6 (93.3 × 10 M) on PL intensity of PFBT (3.3 × 10 M) (error bars = ±5%). (c) Comparison of S-V plots drawn for various analytes -6 -6 in DMSO. (d) PL quenching (%) by various interfering analytes (93.3 × 10 M) prior to and after PA is added (93.3 × 10 M).


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-6

Figure 2. (a) PL spectra of PFBT in water (3.3 × 10 M) with PA (pH 7.0, 10 mM of HEPES buffer) (Inset: S-V plot). (b) Effect of -6 -6 various nitroexplosives (26.6 × 10 M) on PL intensity of PFBT (3.3 × 10 M) (error bars = ±5%). (c) Comparison of S-V plots drawn for various analytes in water (pH 7.0, 10 mM of HEPES buffer). (d) PL quenching (%) with interfering analyte solution -6 -6 (26.6 × 10 M) before and after addition of 26.6 × 10 M PA.

Mechanism of Sensing. There can be many reasons for quenching of fluorescence as reported in literature. However, based on the above observations, PA detection via PFBT could be possible by the involvement of majorly three mechanism of sensing. These are (a) IFE or/and RET, (b) ground state charge transfer complexation and (c) a favored PET, between PFBT and PA. PA has a wide UV-vis absorbance spectrum from 280-480 nm and in the present PFBT system involving both the solvents i.e. (DMSO and water), PA absorption spectrum shows a huge overlapping region with both excitation-emission spectra of PFBT (Figure 3a and 4a) which is the prerequisite for IFE and RET to take place. Ideally IFE occurs whenever there is a substantial overlapping of UV-vis spectrum of quencher (PA) with excitation and/or emission spectra of fluorophore (PFBT) while in case of RET, the spectral overlap happens exclusively between emission spectrum of fluorophore (PFBT) and UV-vis spectrum of quencher. Therefore, to determine key cause of quenching, fluorescence decay time of PFBT was examined in absence and presence of PA for both the solvents. Interestingly, there was no significant change in the fluorescence decay time of PFBT (0.464 ns) after the addition of PA (0.455 ns) in DMSO (Figure 3b). This consistency in the fluorescence decay time after the addition of PA con-

firms that the quenching is of static type in DMSO solvent and also rules out the possibility of any RET, which falls under the category of dynamic fluorescence quenching. Therefore, this suggested that IFE is the key cause for PL quenching of PFBT rather than RET in DMSO. While in the case of aqueous medium, there was a significant change in the fluorescence decay time of PFBT (2.011 ns) after the addition of PA (0.692 ns) (Figure 4b). This significant change in decay time of PFBT in water medium confirms the dynamic fluorescence quenching mechanism and indicates RET is happening in water medium. These observations could be clarified by the insufficient overlap of the PFBT excitation-emission spectra with the nitroaromatic compounds absorption spectra in DMSO and water which consequently resulted in poor IFE and RET in respective solvent (Figure 3c and 4c). This was further confirmed by calculation of IFE corrections and RET parameters in both the solvents. In case of water solvent, RET plays a major role in quenching with ~52% of RET efficiency (Table S3), while IFE contribution was very insignificant when quenching in water i.e. 11%, whereas the rest of the quenching efficiency came from PET mechanism (Figure S34 and Table S5). In case of DMSO solvent, RET contributed less than 2% of the quenching efficiency while the major quenching occurred via strong inner filter


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effect (IFE) i.e. ~69% and the rest came from PET mechanism (Figure S34 and Table S4) as discussed below. Furthermore, to check the possibility of ground state charge transfer complexation, UV-vis absorbance spectra of PFBT was obtained in presence of different PA concentrations. In both solvent cases, there was just an increase in the absorbance value but no new peak appeared (Figure 3d and 4d). These results eliminate the possibility of creation of any ground state complexation between PFBT and PA in DMSO as well as in water. Hence, on the basis of above observation it can be stated that IFE is the most probable mechanism in case of DMSO solvent and eventually accountable for remarkable sensitivity and selectivity of PFBT towards PA. On the other hand, RET plays a key role in quenching mechanism in case of aqueous medium and resulting in extraordinary sensitivity and selectivity of PFBT towards PA. Further, to investigate the interaction between PFBT and PA, we performed control competitive experiments using model compounds i.e. 4-NP and 2,4-DNP which are similar to PA. The only difference in the chemical structure of the above compounds is the quantity of –NO2 groups linked to benzene ring, which makes the phenols acidic in a particular order of PA>2,4-DNP>4-NP. Moreover, PL quenching response obtained for these analytes

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followed the same order (Figure S35). Since, number of electron deficient –NO2 groups attached to phenolic ring is directly proportional to its acidity and so is the dissociation of phenol. This facilitates a favorable electrostatic interaction between cationic PFBT and the dissociated phenol. PA can undergo more dissociation than 2,4-DNP and 4-NP therefore resulting in more electrostatic interaction between PFBT and PA than 4-NP and 2,4-DNP subsequently highest fluorescence quenching. Other analytes that are devoid of –OH functional groups can seldom interact effectively with PFBT and therefore showed negligible fluorescence quenching response. Moreover, addition of an acid like trifluoroacetic acid (TFA), which is stronger acid than PA, did not influence fluorescence of PFBT (Figure S36). This justifies involvement of PET and its not sole acidity. The electrostatic interaction promotes the PET or energy transfer mechanism for the highly selective response of PBFT toward PA. To further confirm the role of electrostatic interactions, sensing experiment was carried out in high ionic strengths solution by using NaCl solution (1M).57 It was observed that the quenching efficiency of the polymer-PFBT decreases (13%) in high ionic strength solution, this shows weakening of electrostatic interactions between PFBT and PA in high

Figure 3. (a) Normalized UV-vis spectrum of PA overlap with normalized emission/excitation spectra of PFBT in DMSO. (b) -6 -6 Lifetime decay of PFBT (3.3 × 10 M) before and after adding PA (93.3 × 10 M) in DMSO. (c) Normalized UV-vis spectrum of various nitroaromatic compounds overlapping with normalized emission/excitation spectra of PFBT in DMSO (d) UV-visible -6 spectra of PFBT (3.3 × 10 M) in presence of PA.


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Figure 4. (a) Normalized UV-vis spectrum of PA overlap with normalized emission/excitation spectra of PFBT in water (pH 7.0, -6 -6 10 mM of HEPES buffer). (b) The lifetime decay readings of PFBT (3.3 × 10 M) before and after adding PA (26.6 × 10 M) in water (pH 7.0, 10 mM-HEPES buffer). (c) Normalized UV-vis spectrum of various nitroaromatics compounds overlapping with normalized emission/excitation spectra of PFBT in water (pH=7.0, 10 mM of HEPES buffer). (d) Changes in the UV-visible spec-6 trum of PFBT (3.3 × 10 M) in presence of PA.

quenching. The PA LUMO energy level is the lowest compared to nitroexplosives such as TNT (-3.7 eV), DNT (-3.5 eV), NT (-3.2 eV) etc., but the efficiency of quenching was quite insignificant for them. This is in agreement with the electrostatic interaction enhanced PET and is valid in both solvents. Therefore, PFBT possessed remarkable selectivity and ultra-sensitivity for PA due to the dominant IFE and likely PET in DMSO and RET and favorable PET in aqueous medium. Figure 5. (a) CV of PFBT film on glassy carbon electrode (50 mV/s scan rate). (b) The PET demonstration from PFBT LUMO to the PA LUMO.

ionic strength solutions leading to lessening in RET/PET mechanism (Figure S37).57 To validate this, HOMO level and LUMO level of PFBT were obtained through cyclic voltammetry studies (Figure 5a). It was observed that there is a possibility of PET from PFBT LUMO (-2.65 eV) to the PA LUMO (-3.89 eV) causing superior quenching (Figure 5b). Note that, LUMO levels of 4-NP (-2.22 eV) and 2,4-DNP (- 2.82 eV) are usually higher than LUMO of PA, resulting in reduced efficiency of fluorescence

Size and Morphological studies. To further examine the morphological and size distribution of the PFBT in presence of PA, FESEM and DLS experiments were performed in both solvents i.e. DMSO and water. FESEM morphological analysis of PFBT with various concentration of PA showed the formation of unsymmetrical nanoaggregates in both solvents, where there was a slight increase in the nanoaggreagates size without any significant change in the shape (Figure S38). This morphological transformation was also supported by the DLS studies, which could be due to the formation of PFBT-PA coaggregates (Figure S39-S46).


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PA Analysis in Natural Water Samples. To verify the feasibility of the PFBT based method, it is highly important to recognize the presence of PA in natural water samples. For this purpose, different samples of water were collected from the Brahmaputra river (near IITG campus) and sea water sample from Bay of Bengal. These samples were centrifuged and filtered using 0.2 µm membrane filter. The recoveries were carried out by using samples spiked with known concentrations of PA (Table 1). As shown in Table 1, recoveries for PA in the Brahmaputra river water samples and sea water varied from 90% to 99% and RSD was found to be lower than 10 %. Therefore, PFBT showed excellent feasibility for detection of PA in real samples. Table 1. Analysis of natural water samples for PA Sample River

Sea

Added (µM) 1.0 2.5 3.5

Found (µM) 0.92 2.34 3.49

Recovered (%) 92.0 93.6 99.7

RSD (%, n=3) 2.12 6.06 1.07

4.0 6.0 10.0

3.98 5.99 9.76

99.5 99.8 97.6

9.31 1.71 6.82

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that the paper strips were selective only for PA (Figure S47). Vapor-phase detection. As an advanced ability of the PFBT probe, PA was also detected in the vapor phase by using the PFBT in water medium. Fixed volumes of PA vapors (101.64 ppb, 2.5 mL) were passed through the PFBT solution in water via an air-tight syringe, thereafter the PL spectra were recorded and quenching in the fluorescence maxima was detected with each addition (Figure 7 and S48). Furthermore, LOD of the PFBT polymer was calculated to be 42.6 ppb based on the change in the fluorescence maxima of PFBT in water at different amounts of PA vapors (84.7, 169.4, 254.1, 338.8, 423.5, 508.2 ppb) using the standard equation 3σ/K (Figure S49). Hence, the present system demonstrated a fast and selective sensing of PA in the solution as well as vapor phase, which remains an exciting and highly challenging area of research.

Figure 7. PL spectrum of PFBT (3.3 × 10-6 M) with increasing PA vapors concentration in water (pH=7.0, 10 mMHEPES buffer).

CONCLUSION Figure 6. Photographs of fluorescent test strips after addition of 10 µL of PA solution of various concentration [under UV-light (365 nm)]. A dark spot can be seen after the addition of PA.

Sensing on Solid Support. For rapid and low cost onsite detection of PA, portable device based test strips were developed by dip coating the test strips in PFBT solution (10-4 M) before drying them at rt in air. 10 µL of various concentration of PA in water was applied on each paper strip based devices. A dark spot was observed under UV lamp (365 nm), and this darkness appeared light at lower concentration of PA indicating quenching of PFBT (Figure 6). It can be seen from the naked eyes that the paper strips can detect minimum amount of 10 µL PA (10-16 M) solution that is equivalent to 0.22 attogram level, which is among the best reported literature values (table S6 ESI). Furthermore, we checked the selectivity studies of PFBT on portable and readily usable paper strips. For this purpose, we applied 10 µL of other nitroaromatic compounds (10-3 M) and compared them with the PA spot to confirm

In summary, a new cationic conjugated polymer PFBT was synthesized using simple and economic method of oxidative polymerization for PA sensing in solution and solid state. The polymer PFBT displayed dual emission in DMSO and aqueous medium. It was found that polymer PFBT can detect PA in both solution (DMSO and water) as well as on portable device based solid platform like normal filter paper strips for onsite portability purposes and an extremely low detection limits 92.7 nm and 0.19 nm were obtained in DMSO and water respectively while minimum of 0.22 attogram of PA can be detected using portable paper strips. This system can also detect PA in vapor phase with a very low limit of detection of 42.6 ppb. The fluorescence of the PFBT gets quenched by strong inner filter effect (IFE) and PET mechanisms between polymer and PA in DMSO. Whereas in case of aqueous medium, resonance energy transfer (RET) and PET mechanism are responsible of fluorescence quenching. This method is very simple and provides excellent selectivity and high sensitivity even in competent environment, therefore can be used for analyzing natural water sam10 ples.


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ASSOCIATED CONTENT Supporting Information This material is available free of charge. SI.doc. Synthetic scheme, nitroexplosives structures, characterization data, LOD plots, photograph of PFBT solutions, competitive studies and selectivity studies, table of comparison, etc.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Funding from the Department of Electronics & Information Technology, (DeitY) No. 5(9)/2012-NANO (Vol. II), Department of Science and Technology (DST), New Delhi, (No. DST/SERB/EMR/2014/000034), DST-Max Planck Society, Germany (IGSTC/MPG/PG(PKI)/2011A/48), are gratefully acknowledged. For instrument facilities used in this work, CIF, IIT Guwahati is acknowledged. AT thanks Banpreet Kaur for GPC analysis and Ankita Saini for the quantum yield measurements.

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Inner Filter Effect and Resonance Energy Transfer Based Attogram

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