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DOI: 10.1039/C5CC02194D

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Sameer Hussain,a Akhtar Hussain Malik,a Mohammad Adil Afroz,a and Parameswar Krishnan Iyer*a,b 5

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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Picric acid (PA) detection at parts per trillion (ppt) levels is achieved by conjugated polyelectrolyte (PMI) in 100% aqueous media and on solid platform using paper strips and chitosan (CS) films. The unprecedented selectivity is accomplished via combination of ground state charge transfer and Resonance Energy Transfer (RET) facilitated by favorable electrostatic interactions. The rapid and selective detection of explosives is an imperative challenge with respect to national security and environmental protection.[1] Among various nitro aromatics (Scheme S1) 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4DNT), 2,6-dinitrotoluene (2,6-DNT) and 2,4,6trinitrophenol/picric acid (TNP/PA) are the primary constituents of industrial explosives, found at several unexploded land mines worldwide.[2] Picric acid (PA) was an extensively used explosive up to World war-I due to its superior explosive power compared to its well-known counterpart TNT even at picomolar concentrations. Additionally, it is also a serious environmental pollutant generated from leather, pharmaceutical, chemical and dye industries.[3] Owing to the high solubility of PA in water, it easily contaminates soil and groundwater and is liable for acute health effects like strong irritation to the skin/eye and potential damage to respiratory organs.[4] During metabolic processes, PA transforms into picramic acid (2-amino-4,6-dinitrophenol), which has ten times more mutagenic activity than PA.[5] Hence, development of reliable, selective and sensitive methods to detect PA to prevent terrorist threats and environmental contamination is highly significant in the present context. Although detection of explosives has been performed with sophisticated instruments,[2] they suffer primarily from low selectivity, portability and on-field use issues making them unviable and expensive. Fluorescence-based detection of nitro explosives is gaining attention due to their simplicity, high sensitivity, short response time and viability both in solution and solid phase.[6] In this regard, numerous sensors for explosives based on polymers,[7] metal-organic frameworks,[8] selfassemblies,[9] nanoaggregates,[10] etc. have been designed. However, due to similar electron affinities of nitro-aromatics, the development of a reliable and effective chemo sensor for PA with high selectivity and sensitivity remains a challenging task. Conjugated polymers (CPs), have proven to be excellent candidates for the detection of nitro explosives due to their excellent molar absorptivity, good film forming ability, high quantum efficiency and amplified signal responses.[1,7b,11] In This journal is © The Royal Society of Chemistry [year]

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recent years, few approaches to design CP based sensors for detection of PA have been reported.[7d,12] However, most of them work in organic media with lesser detection limit, low selectivity and moderate quenching efficiencies. Interference by other nitroexplosives is another key problem associated in the existing systems due to the absence of specific receptors for PA7b,d. This motivated us to develop a sensitive platform based on CP with appropriate receptor for the amplified and selective detection of PA both in aqueous phase as well as solid support. Herein, we report the highly selective detection of PA at parts per trillion (ppt) levels using conjugated polyelectrolyte (PMI) in aqueous media as well as by contact mode / instant spot testing via paper strips and efficient fluorescent film for the first time. The cationic CP PMI was developed using post-functionalization polymerization method (Scheme S2, Fig. S1-S2).[13] Imidazolium groups strapped along the side chain of PMI provide good solubility in polar solvents like DMSO, methanol and water and also act as a specific recognition site for PA due to favorable electrostatic attraction. Fluorescent signal amplification property of CP encouraged us to utilize PMI for specific recognition of PA at very low concentrations.[14]

Fig. 1 (a) Structure of PMI (b) Photoluminescence spectra of PMI (2 × 105 M) with increasing concentration of PA in aqueous media. ~95% quenching was observed at 6.67 × 10-7 M of PA. Inset: Stern-Volmer plot of PMI upon the addition of PA. (c) Color of PMI solution in water before and after adding PA (irradiation under UV lamp at 365 nm).

The cationic polymer PMI (Fig. 1a) shows absorption and emission maximum at 325 nm and 406 nm (325 nm excitation) in aqueous media. Sensing studies were performed in aqueous solution and HEPES buffer (pH = 7, 10mM), and the results were comparable. To investigate the interaction of nitro explosives with PMI, fluorescence quenching titration was performed by [journal], [year], [vol], 00–00 | 1

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Ultrasensitive detection of nitro explosive-picric acid via conjugated polyelectrolyte in aqueous media and solid support

ChemComm

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DOI: 10.1039/C5CC02194D

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PMI.6e The complex thus formed may facilitate the ground state charge transfer between PMI and picrate. UV-Vis spectrum of PMI displayed a new band at ~425 nm with a significant red shift of ~15 nm upon gradual addition of PA (Fig. S14), suggesting the formation of ground state charge transfer complex6d,e between PMI and PA which is also in agreement with the lifetime studies. To comprehend the mode of interaction between PMI and PA, both experimental and theoretical studies were performed (ESI). Cyclic Voltammetry studies (Fig. S15) ruled out the possibility of excited state electron transfer, since lower unoccupied molecular orbital (LUMO) of PMI (-4.364) is lower than the LUMO of PA (-3.89).7b,10a Since, picrate and PA are supposed to have variable HOMO/LUMO levels6d,e as illustrated by DFT studies, there is a strong possibility of ground state electronic charge transfer from HOMO of the picrate to LUMO of positively charge PMI (Fig. 3), resulting in high quenching efficiency. However, HOMO of other nitro-explosives were found to have low energy levels than LUMO of the sensor molecule, thereby discarding the possibility of electron transfer, resulting in low quenching efficiencies (Fig. S16).

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Fig. 3 Schematic representation showing ground state charge transfer from HOMO of the picrate to the LUMO of PMI.

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Fig. 2 (a) Stern-Volmer plots for different analytes in water. (b) Quenching percentage of PMI (2 × 10-5 M) with several analytes (6.67 × 10-7 M) in aqueous media before and after the addition of PA (6.67 × 10-7 M). 35

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For practical applications, the selective recognition of PA in an aquatic system is in great demand. In this regard, we have performed titration experiments in the presence of various nitro-aromatics and electron deficient analytes to monitor the selectivity of PMI for PA (Fig. 2b, Fig. S6-S13). These plots demonstrate that PMI can detect PA even in the presence of other interfering analytes. Since, most of the PA chemosensors suffer largely from the interference of other electron-deficient compounds; the higher selectivity of PMI provides a very rare and unique platform for the highly selective detection of PA in aqueous media. Encouraged by these results, the origin of remarkable selectivity was also investigated. High quenching efficiency by PA compared to other nitro-compounds is explained via deprotonation of the strongly acidic phenolic -OH group in aqueous media, resulting in anion exchange with the polymer 2 | Journal Name, [year], [vol], 00–00

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Fig. 4 Spectral overlap between the normalized emission spectrum of PMI and normalized absorption spectra of various electron deficient compounds.

High sensitivity of PMI towards PA and the non-linear nature of S-V plot for PA suggests that an energy transfer process may also be involved in quenching process.[6d] There is a possibility of RET from fluorophore (donor) to non-emissive analytes (acceptor) if emission band of the donor overlaps efficiently with the absorption band of the acceptor. Since RET has moderately higher efficiency compared to electron transfer process,[11b] it can significantly enhance the quenching efficiency viz-a-viz This journal is © The Royal Society of Chemistry [year]

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adding aliquots of various analytes to the aqueous solution of PMI. Immediate fluorescence quenching of ~25% was observed on adding just 3.3 × 10-8 M solution of PA that is further quenched ~95% on adding 6.67 × 10-7 M PA solution (Fig. 1b). The disappearance of the blue luminescence of PMI in the presence of PA was clearly visible under UV light (Fig. 1c). The efficiency of quenching studied by linear fitting of Stern-Volmer plot (Ksv) was found to be 0.1×108 M-1 (Inset-Fig. 1b), confirming very high sensitivity of PMI towards PA. The limit of detection (LOD) calculated (Fig. S3) for PA was 56.11 × 10-11 M (128 ppt), which is the best value reported in literature (Table S1). Other interfering analytes viz. 2,6-DNT, 2,4-DNT, 4nitrotoluene (4-NT), 1,3-dinitrobenzene (1,3-DNB), nitrobenzene (NB), nitromethane (NM), benzoic acid (BA) and phenol did not influence the fluorescence emission of PMI even in large quantities (Fig. S4). It is evident (Fig. 2a) that at lower PA concentration, the S-V plot is close to linear, whereas, at higher concentration it deviates from linearity and increases nearly exponentially demonstrating an amplified quenching process. The non-linear nature of S-V plot suggests self-absorption or an additional effective energy transfer mechanism.[1,15] However, all other analytes showed only linear increase in S-V plots (Fig. 2a). The significantly higher Ksv value obtained for PA compared to other analytes confirms predominant selectivity and exceptional quenching ability of PA towards PMI. Time resolved fluorescence studies validated the mechanism of quenching as static and further confirms the ground state complex formation since the fluorescence lifetime of PMI remains unchanged in the presence of PA6d,f (Figure S5).

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sensitivity. Fig. 4 confirms a large overlap between the absorption spectrum of PA and the emission spectrum of PMI, resulting in dramatic florescence quenching unlike other analytes that have almost no overlap. To identify the extent of energy transfer, overlap integral values for all analytes were calculated. The integral value (Jλ) observed for PA was highest (2.63 × 1014 M−1 cm−1 nm4) compared to other analytes by ~100 times (Table S2). The Förster distance R0 value obtained for PMI-PA interaction was 35.30 Å, which falls well in the range of 10 to 100 Å applicable for RET. Thus, unprecedented sensitivity and remarkable selectivity of PMI towards PA in 100% aqueous media is a unique feature accountable to the well-known “molecular-wire effect”, formation of ground state charge transfer complex as well as excited state energy transfer process. However, both theoretical and experimental studies suggest that charge transfer is a predominant process. Finally, a control experiment was performed to elucidate the unprecedented selectivity of PMI towards PA using nitro compounds comprising only one hydroxyl group viz. (PA), 4nitrophenol (NP) and 2,4-dinitrophenol (2,4-DNP). The fluorescence quenching efficiencies by these analytes follows the order PA>DNP>NP (Fig. S17), which are in well agreement with the acidity of these nitro compounds (PA>DNP>NP). Since, PA is a strong acid with pKa value of ~0.38, it can easily dissociate in aqueous medium, which can accelerate the electrostatic interactions between cationic PMI and PA. TFA, a stronger acid than PA, had no effect on the emission of PMI, thereby, negating the possibility of quenching by acidity effect (Fig. S18). Consequently, it can be concluded that homogeneity of both PMI and PA in aqueous media is another key factor involved which brings PMI and PA in close proximity via electrostatic attraction and facilitates efficient charge transfer and/or energy transfer mechanisms thereby providing excellent and unprecedented fluorescence quenching towards PA. However, other analytes do not favor either of these mechanisms and subsequently showed no response towards polymer PMI.

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Fig. 5 (A) Color of fluorescent test strips under UV light (a) before and after (b, c, d, e, f and g) adding 10µL of various concentrations of PA solution. (B) PMI-doped transparent CS film in day light. (C) Visualization of PMI-doped CS fluorescent film under UV light (a) Dark spot of PA residual left thumb impression (b) Right thumb impression as control (lamp excited at 365 nm).

To realize practical applications with PMI, the sensing of explosives at actual site is of immense significance. In this context, the instant surface sensing of explosives by preparing PMI coated fluorescent test strips using Whatman filter paper was performed. To check the sensitivity of PMI, the PA solutions with This journal is © The Royal Society of Chemistry [year]

varying concentrations (10-3-10-11 M) were spotted onto the different test strips. A solvent blank was also spotted as control. For better reliability, all depositions were prepared from a 10 µL volume, thereby producing a spot 0.5 cm in diameter. After drying, the filter paper was visualized under UV light (lamp excited at 365 nm). Dark spots of different strengths were observed that varies with the concentration of PA (Fig. 5A). The minimum amount of PA detectable by the naked eye was 10 µL (1 × 10-9 M) providing a very low detection limit of ∼2.29 pg which is among the best reported values.6f,9a,16 Other interfering analytes were found ineffective towards paper strip test confirming high selectivity of PMI towards PA (Fig S19). These results validate the unprecedented efficacy of the test strips for on-site instant and accurate detection of PA. PA can contaminate the human body, apparel and other materials in the environment during manufacturing of rocket fuel, preparation and packaging of fireworks. Hence, contact mode detection of PA has huge demand in the field of analytical and forensic sciences, yet no efficient methods exist. To achieve this objective, we prepared a transparent fluorescent film by doping 5% PMI with commercial chitosan (CS) (Fig. 5B) and describe its fluorescence response towards PA via contact mode. CS is an ideal substrate due to its excellent film-forming ability, ease of modification, economy, ready availability and environmental biocompatibility.[17] Since, PA is a strong acid while CS film is rich in amino groups; the affinity of PA towards CS film is enhanced due to electrostatic interaction thereby increasing its detection ability. The image (Fig. 5C) under ultraviolet light confirms that PMI-doped CS film can detect even trace residues of PA very clearly, providing an exceptional, cheap and reliable platform for PA detection on solid support. Other common interfering analytes did not cause any significant change in the fluorescence of film even in higher quantities (Fig. S20). In conclusion, the fluorescence-amplified detection of PA in 100% aqueous media in parts per trillion levels as well as on solid based platforms comprising cationic conjugated polyelectrolyte PMI is developed. PMI exhibits remarkable selectivity even in the presence of most commonly interfering nitroaromatics. Formation of ground state charge transfer complex, RET as well as favorable electrostatic interaction between PMI and PA are the key aspects for unprecedented selectivity and remarkable sensitivity of PMI towards PA. Paper strips test and contact mode sensing platform using PMI-doped chitosan film confirms this method as simple, portable and cost-effective in order to prevent environmental contamination and terrorist threats rapidly under very realistic conditions.

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Financial assistance from Department of Science and Technology (DST), New Delhi, (No. SB/S1/PC-020/2014), (No. DST/TSG/PT/2009/23), DST–Max Planck Society, Germany (IGSTC/MPG/PG(PKI)/2011A/48) Department of Electronics & Information Technology, (DeitY) No. 5(9)/2012-NANO (Vol. II), is gratefully acknowledged. The Central Instruments Facility, IIT Guwahati is acknowledged for instrument facilities.

Notes and references Journal Name, [year], [vol], 00–00 | 3

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DOI: 10.1039/C5CC02194D

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Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati-781039. India. Fax: +913612582349; Tel: +913612582314; *E-mail: [email protected] b Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati-781039. India. †Electronic Supplementary Information (ESI) available: It includes experimental details, method of preparing fluorescent paper strips and film, cyclic voltammogram, DFT studies, detection limit plot, lifetime plot and UV-Vis spectra of PMI with PA. See DOI: 10.1039/b000000x/ 1. Y. Salinas, R. Martínez-Máñez M. D. Marcos, F. Sancenón, A. M. Costero, M. Parra and S. Gil, Chem. Soc. Rev., 2012, 41, 1261–1296. 2. M. E. Germain and M. J. Knapp, Chem. Soc. Rev., 2009, 38, 2543– 2555. 3. J. Akhavan, Chemistry of Explosives, Royal Society of Chemistry, 2nd ed., 2004. 4. (a) P. C. Ashbrook and T. A. Houts, Chem. Health Safety, 2003, 10, 27; (b) Innovative Treatment Technologies: Annual Status Report, U.S. Environmental Protection Agency: Washington, D.C., 8th ed., 1996. 5. K.-M. Wollin and H. H. Dieter, Arch. Environ. Contam. Toxicol., 2005, 49, 18–26. 6. (a) Y. Peng, A.-J. Zhang, M. Dong and Y.-W. Wang, Chem. Commun., 2011, 47, 4505–4507; (b) N. Dey, S. K. Samanta and S. Bhattacharya, ACS Appl. Mater. Interfaces, 2013, 5, 8394–8340; (c) D. Dinda, A. Gupta, B. K. Shaw, S. Sadhu and S. K. Saha, ACS Appl. Mater. Interfaces, 2014, 6, 10722–10728; (d) P. S. Mukherjee and K. Acharyya, Chem. Commun., 2014, 50, 15788–15791; (e) B. Roy, A. K. Bar, B. Gole and P. S. Mukherjee, J. Org. Chem., 2013, 78, 1306– 1310; (f) V. Bhalla, A. Gupta, M. Kumar, D. S. S. Rao and S. K. Prasad, ACS Appl. Mater. Interfaces, 2013, 5, 672–679. 7. (a) W. Dong, T. Fei, A. Palma-Cando and U. Scherf, Polym. Chem., 2014, 5, 4048–4053; (b) H. Sohn, M. J. Sailor, D. Magde and W. C. Trogler, J. Am. Chem. Soc., 2003, 3821–3830; (c) K. K. Kartha, S. S. Babu, S. Srinivasan and A. Ajayaghosh, J. Am. Chem. Soc., 2012, 134, 4834–4841; (d) B. Xu, X. Wu, H. Li, H. Tong and L. Wang, Macromolecules, 2011, 44, 5089–5092. 8. (a) S. S. Nagarkar, B. Joarder, A. K. Chaudhari, S. Mukherjee and S. K. Ghosh, Angew. Chem. Int. Ed. Engl., 2013, 52, 2881–2885; (b) A. Lan, K. Li, H. Wu, D. H. Olson, T. J. Emge, W. Ki, M. Hong and J. Li, Angew. Chem. Int. Ed. Engl., 2009, 48, 2334–2338; (c) V. Bhalla, S. Pramanik and M. Kumar, Chem. Commun., 2013, 49, 895–897; (d) Y.-N. Gong, L. Jiang, and T.-B. Lu, Chem. Commun., 2013, 49, 11113–11115. 9. (a) V. Bhalla, S. Kaur, V. Vij and M. Kumar, Inorg. Chem., 2013, 52, 4860−4865; (b) J. Ma, T. Lin, X. Pan and W. Wang, Chem. Mater., 2014, 26, 4221–4229. 10. (a) V. Vij, V. Bhalla and M. Kumar, ACS Appl. Mater. Interfaces, 2013, 5, 5373−5380; (b) M. P. Sk and A. Chattopadhyay, RSC Adv., 2014, 4, 31994–31999; (c) X.-G. Hou, Y. Wu, H.-T. Cao, H.-Z. Sun, H.-B. Li, G.-G. Shan and Z.-M. Su, Chem. Commun., 2014, 50, 6031–6034; (d) H.-T. Feng and Y.-S. Zheng, Chem. Eur. J. 2014, 20, 195–201. 11. (a) S. J. Toal and W. C. Trogler, J. Mater. Chem., 2006, 16, 2871– 2883; (b)Y. Wang, A. La, C. Brückner and Y. Lei, Chem. Commun., 2012, 48, 9903–9905; (c) K. R. Ghosh, S. K. Saha and Z. Y. Wang, Polym. Chem., 2014, 5, 5638–5643; (d) D. Gopalakrishnan and W. R. Dichtel, J. Am. Chem. Soc., 2013, 135, 8357–8362; (e) H. Sohn, R. M. Calhoun, M. J. Sailor and W. C. Trogler, Angew. Chem. Int. Ed. Engl., 2001, 40, 2104–2105. 12. (a) S. Shaligram, P. P. Wadgaonkar and U. K. Kharul, J. Mater. Chem. A, 2014, 2, 13983–13989; (b) N. Sang, C. Zhan and D. Cao, J. Mater. Chem. A, 2015, 3, 92–96; (c) W. Wei, R. Lu, S. Tang and X. Liu, J. Mater. Chem. A, 2015, 3, 4604–4611. 13. S. Hussain, A. H. Malik and P. K. Iyer, ACS Appl. Mater. Interfaces, 2015, 7, 3189–3198. 14. D. T. McQuade, A. E. Pullen and T. M. Swager, Chem. Rev. 2000, 100, 2537–2574. 15. D. Zhao and T. M. Swager, Macromolecules, 2005, 38, 9377–9384. 16. (a) J.-F. Xiong, J.-X. Li, G.-Z. Mo, J.-P. Huo, J.-Y. Liu, X.-Y. Chen, and Z.-Y. Wang, J. Org. Chem. 2014, 79, 11619–11630; (b) M. Rong, L. Lin, X. Song, T. Zhao, Y. Zhong, J. Yan, Y. Wang, and X.

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Table of contents entry Ultrasensitive detection of nitro explosive-picric acid via conjugated polyelectrolyte in aqueous media and solid support

Cationic polymer PMI detects picric acid at ppt levels via combination of ground state charge transfer, RET and electrostatic interactions. Paper strips test and contact mode sensing platform using chitosan film confirms the method as simple, portable and cost-effective.

ChemComm Accepted Manuscript

Published on 19 March 2015. Downloaded by University of Saskatchewan on 19/03/2015 11:54:03.

Sameer Hussain, Akhtar H. Malik, M. Adil Afroz and Parameswar K. Iyer*

Ultrasensitive detection of nitroexplosive - picric acid via a conjugated polyelectrolyte in aqueous media and solid support.

Picric acid (PA) detection at parts per trillion (ppt) levels is achieved by a conjugated polyelectrolyte (PMI) in 100% aqueous media and on a solid p...
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