BODIPY based fluorescent chemodosimeter for explosive picric acid in aqueous media and rapid detection in the solid state

Abstract

A novel fluorescent chemodosimeter, for specific recognition of explosive picric acid over other nitroaromatic compounds, was developed. It was also demonstrated for the first time that the reported chemodosimeter can selectively detect picric acid, both in solution and solid state.

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Picric acid (2,4,6-trinitrophenol), bearing phenolic and nitro functionalities, is a very useful reagent in organic synthesis, drug analysis, manufacture of rocket fuel and dye industries.^1,2 In addition to its importance for various applications, picric acid is also a powerful explosive ingredient like many polynitrated organic compounds such as TNT, DNT and RDX.^3,4 Because of its wide use, this compound pollutes the environment and causes severe health problems such as skin irritation and damage to respiratory organs.⁵ Thus, sensitive detection of picric acid at low concentrations by suitable sensors is an actively pursued research area.⁶ Among various techniques employed for the detection of highly explosive nitroaromatic compounds (NACs),⁷ the fluorescence technique is widely used because of its high selectivity, quick response and easy sample preparation.⁸ The fluorescence detection of an analyte by a sensor can be either fluorescence quenching or fluorescence turn-on of the sensor upon binding with the analyte.⁹ It is a well known fact that sensing by enhancement of fluorescence is superior to quenching of fluorescence due to two distinct reasons. First, the appearance of a bright signal on a completely dark background is qualitatively easier to detect than the dimming of an already bright signal. Second, turn-on signals result from a stoichiometric binding event, rather than from a collisional encounter.¹⁰ Thus, the recent focus of research is on turn-on fluorescence sensors rather than fluorescence quenching sensors. A perusal of the literature revealed that there are very few reports on fluorescence sensors for picric acid.¹¹ Furthermore, the fluorescence sensors reported for picric acid were mainly based on the fluorescence quenching methodology¹² and very recently Pang and coworkers¹³ reported the first “off–on” ratiometric fluorescence sensor for picric acid. Therefore, it is challenging to construct probes that can exhibit an enhanced fluorescence signal in response to picric acid. In this paper, we report a boron–dipyrromethene (BODIPY) based fluorescence turn-on sensor for picric acid. BODIPYs are interesting fluorescent dyes with valuable properties such as high chemical and photostability and relatively high absorption coefficients and fluorescence quantum yields.¹⁴ These BODIPY dyes can be optically excited by visible light and their absorption and emission properties can be fine tuned by introducing appropriate substituents onto the BODIPY framework.^15–17 Here, we prepared 3,5-bis(acetal) BODIPY 2 from 3,5-diformyl BODIPY 1 and showed it as a specific fluorescence turn-on chemodosimeter for picric acid.

The desired 3,5-bis(acetal) BODIPY 2 was synthesized by treating BODIPY 1 in CH₃OH with 0.1 equivalent of TFA (Scheme 1) and was characterized using various spectroscopic techniques (Fig. S1–S6, ESI†). The ground and excited state properties of the designed chemodosimeter 2 were studied using UV-vis absorption, electrochemistry and steady-state and time-resolved fluorescence techniques.

Scheme 1 Synthesis of 3,5-bis(acetal) BODIPY 2.

In a chloroform solution, BODIPY 2 showed a strong absorption band at 508 nm with a distinct shoulder at 478 nm. The emission spectra peaks were found to be reasonably narrow (line width ∼1100 cm⁻¹), with a maxima located at 514 nm and a quantum yield of 0.08 (Table S1, ESI†). The fluorescence lifetime measurements obtained using time-correlated single photon counting (TCSPC) revealed that the singlet state of BODIPY 2 decayed by following single exponential kinetics with a life time of ∼0.42 ns. The absorption and fluorescence studies of BODIPY 2, in different solvents of varying polarity, showed blue-shifts in the absorption and emission bands, a decrease in the quantum yield and a reduction in the singlet state lifetime upon increasing the polarity of the solvent (Fig. S7–S8, ESI†). Similar behaviour of BODIPY 2 in terms of the emission quantum yield and radiative lifetime were observed in a semi-aqueous (water–acetonitrile) environment as well, indicating its potential to detect picric acid in aqueous solution. The cyclic voltammetric studies of BODIPY 2 showed one reversible reduction at −0.75 V and one irreversible reduction at −1.75 V but it did not exhibit any oxidation, which indicated the electron deficient nature of the BODIPY core (Fig. S9, ESI†).

To explore the potential application of BODIPY 2 as a sensory material for the detection of NACs such as picric acid (PA), 2,4,6-trinitrotoluene (TNT), 2,6-dinitrotoluene (DNT), 1,4-dinitrobenzene (DNB), 1,4-dinitrobenzoic acid (DNBA), 2,4-dinitrophenol (DNP), 1,4-benzoquinone (BQ), and 4-nitrophenol (NP), we tested BODIPY 2 with the above mentioned NACs in a CH₃CN–H₂O (9 : 1 v/v) solution. First, using visual inspection, we checked for a color change of BODIPY 2 under a UV lamp, with the addition of the afore mentioned various NACs. It was found that a discernible colour change (from pale green to bright yellow-orange) of the BODIPY 2 solution occurred only in the presence of PA (Fig. 1). However, we did not observe any noticeable colour change in the solutions of the other NACs, which indicated that BODIPY 2 has the potential to act as a turn-on chemodosimeter for PA. This prompted us to further investigate the selectivity of BODIPY 2 for PA using various spectrophotometric techniques. Fig. 1a shows the absorption spectra of BODIPY 2 in the presence of various NACs, which showed about a 15 nm red-shift of the entire absorption envelope only in the presence of PA, consistent with the colorimetric assay. In addition, the emission spectra (Fig. 1b) of BODIPY 2 were found to be considerably altered in the presence of PA, with a prominent red-shift of the transition energies (Δλ_Em ∼20 nm) and a simultaneous increase of the emission intensity (6-fold). It is noted that the emission maxima remained unaltered in the presence of the various other NACs (Fig. 2). This shows remarkable changes in both the absorption and emission peak position only in the presence of PA, which indicates that BODIPY 2 acts as a highly selective sensor for the detection of PA. Moreover, the competitive binding experiments performed by the addition of excess amounts (50 equivalents) of other NACs did not revert the spectral change induced by the PA (Fig. S10, ESI†). This demonstrates that the sensor can detect PA even in the presence of other explosive NACs.

Fig. 1 Changes in (a) absorption, (b) emission intensity of BODIPY 2 (5 μM) upon addition of various nitro aromatics (50 equiv.) in a CH₃CN–H₂O (9 : 1; v/v) solution. The color change induced after addition of various nitroaromatic compounds to BODIPY 2 under a UV lamp.

Fig. 2 Plots of relative fluorescence intensity changes (I/I₀) of BODIPY 2 (5 μM) to PA and various NACs.

We systematically studied the specific detection of PA by BODIPY 2 by following the changes in the absorption and fluorescence spectra. Fig. 3a and b show the changes in the absorption and fluorescence spectra of the sensor in a CH₃CN–H₂O solution after the addition of increasing amounts of picric acid. It is clear from Fig. 3a that with increasing amounts of PA, the absorption band centered at 504 nm undergoes a continuous red-shift with three clear isosbestic points at 512 nm, 487 nm, and 467 nm respectively. The ratio of the absorbance at 504 nm and 516 nm at different equivalents of PA is plotted as an inset in Fig. 3a, this showed the sigmoidal nature which serves as an indicator of analyte binding. With increasing equivalents of PA, the fluorescence emission spectra (Fig. 3b) of the sensor also displayed a continuous red-shift (from 518 to 535 nm). This was accompanied by a significant enhancement in the fluorescence intensity (6-fold) in conjunction with an increase in the emission quantum yield (Φ_f increased from 0.08 to 0.48) and singlet state lifetime (τ increased from 0.42 to 5.8 ns). This can be rationalized as follows: the BODIPY 2 is weakly fluorescent because of the photoinduced electron transfer that occurs from the acetal oxygen to the BODIPY core. However, on addition of the picric acid, the fluorescence intensity enhances significantly due to the arrest of the photo-induced electron transfer process. This occurs through the interaction of the picric acid with the acetal oxygen of the BODIPY core and converts it to the corresponding aldehyde. The conversion of the acetal to the aldehyde was also confirmed by NMR and IR spectral techniques (Fig. S11–S14, ESI†). We have also tested the response of the BODIPY 2 to other analogous acids such as trifluoroacetic acid (TFA) (Fig. S15, ESI†). We noted that upon addition of TFA to the solution of BODIPY 2, the emission maxima was slightly red-shifted accompanied by a slight increase in the fluorescence intensity, but the changes were minimal compared to that of PA. Based on the Benesi–Hildebrand equation, the association constant for the binding of BODIPY 2 to PA was established as K_a being equal to (9.2 ± 0.1) × 10⁶ M⁻¹. The sensitivity of BODIPY 2 to PA has been further evaluated by measuring the lowest concentration, using the linear dynamic response. The detection limit (LOD) was found to be 162 (±13) ppb (3σ/slope), suggesting its ability to detect PA, even in the ppb concentration level (Fig. S16, ESI†).

Fig. 3 Changes in (a) absorption and (b) fluorescence spectra of BODIPY 2 (5 μM) upon titration with picric acid (0 to 30 equiv.) in CH₃CN–H₂O (9 : 1; v/v) solution (λ_ex = 488 nm). Insets show: (a) the plot of absorbance vs. [PA]/[BODIPY 2] molar ratio for absorbance at 504 and 516 nm respectively; (b) the plot of fluorescence emission maxima at 534 nm as a function of [PA]/[BODIPY 2] molar ratio and the color change of BODIPY 2 solution before and after addition of picric acid.

To gain further insight into the structural aspects of BODIPY 2 upon binding with PA, we performed a ¹H NMR titration in CD₃CN–D₂O (Fig. 4). The BODIPY 2 showed two doublets for pyrrole (type a,b) protons at 6.65 and 6.95 ppm respectively whereas one singlet for the acetal protons at 3.41 ppm and one singlet for the acetal CH proton at 5.74 ppm. We noted that, upon addition of 0.5 equiv. of picric acid, the pyrrole protons experienced a downfield shift and appeared as four sets of signals due to the asymmetry caused by the conversion of one acetal to an aldehyde. More importantly, the signal corresponding the to acetal –CH proton at 5.78 ppm gradually decreased with the concomitant appearance of a new signal at 10.38 ppm corresponding to the formation of the aldehyde. These observations in the ¹H NMR study are consistent with the photophysical studies.

Fig. 4 Partial ¹H NMR spectra of BODIPY 2 (2.2 × 10⁻² M) in the presence of different concentrations of PA in 0.4 mL of CD₃CN–D₂O (97.5/2.5; v/v). Concentration of PA was varied from 0 to 2.2 equiv.

BODIPY 2 is brightly fluorescent in its solid state, which is due to the absence of the π–π interactions between the neighboring BODIPY units because of the tangling acetal groups on either side of the BODIPY core. To demonstrate the potential application of the sensor towards the detection of explosive NACs, a thin film of BODIPY 2 was fabricated by a spin coating technique on a quartz substrate with a rate of 2000 rpm and at a concentration of 1 mg per 0.4 mL in a chloroform solution. The film was annealed at 50 °C for 1 hour. The comparison of the fluorescence spectra of BODIPY 2 in the solution as well as in the thin-film is shown in Fig. 5a. The fluorescence spectra of the thin film showed a red-shifted emission band at 612 nm compared to its emission band at 534 nm in solution. The fluorescence spectra recorded immediately after exposing the film to saturated vapors of various NACs as a function of time is shown in Fig. 5b. Upon continuous exposure of the thin film to saturated vapors of PA, we observed that the initial emission intensity was reduced significantly by 26% for 120 s exposure and then reduced further by 52% after 300 s. Upon further continuous exposure, 83% quenching of the fluorescence intensity was observed after 500 s of exposure time and finally reached saturation. More importantly, we did not observe any significant change in the fluorescence intensity upon exposure of the thin film of BODIPY 2 to the saturated vapors of various other explosive NACs, indicating that BODIPY 2 can specifically detect PA in preference to the other NACs in the solid state.

Fig. 5 (a) Emission spectra of BODIPY 2 recorded in chloroform and in thin film. (b) The time dependent emission spectra of BODIPY 2 thin film upon exposure to the saturated vapor of PA at 60 s time intervals. The insets show the fluorescence quenching as a function of time and the color change of the solid BODIPY 2 before and after addition of PA.

In summary, we developed a new BODIPY based fluorescent chemodosimeter, which shows remarkable selectivity and specificity towards picric acid over other explosive nitroaromatics. BODIPY 2 shows very weak fluorescence in solution due to the occurrence of photoinduced electron-transfer from the electron rich acetal moiety to the BODIPY core. The addition of PA induced a red-shift in the emission maxima accompanied by a significant enhancement in the fluorescence intensity, which is attributed to the conversion of the acetal to an aldehyde. To the best of our knowledge, BODIPY 2 is the first example of a chemodosimeter which shows a fluorescence turn-on response for the detection of trace amounts of the powerful explosive picric acid in solution. BODIPY 2 is brightly fluorescent in the solid state and upon exposure to vapors of picric acid, BODIPY 2 becomes weakly fluorescent. Thus, BODIPY 2 can serve efficiently for the detection of picric acid in solution as well as in the solid state.

Acknowledgements

M.R. acknowledges financial support from the Department of Science and Technology, Government of India. S.M. acknowledges the IIT-Bombay for fellowship and A.B. acknowledges CSIR for SRF.

Notes and references

1. (a) Safety Data Sheet for Picric Acid, Resource of National Institutes of Health; (b) H. Sohn, R. M. Calhoun, M. J. Sailor and W. C. Trogler, Angew. Chem., Int. Ed., 2001, 40, 2104.
2. (a) M. E. Germain and M. J. Knapp, J. Am. Chem. Soc., 2008, 130, 5422; (b) J. S. Park, F. Le Derf, C. M. Bejger, V. M. Lynch, J. L. Sessler, K. A. Nielsen, C. Johnsen and J. O. Jeppesen, Chem.–Eur. J., 2010, 16, 848.
3. (a) J. Akhavan, Chemistry of Explosives, Royal Society of Chemistry, London, 2nd edn, 2004; (b) J. F. Wyman, M. P. Serve, D. W. Hobson, L. H. Lee and D. E. Uddin, J. Toxicol. Environ. Health, 1992, 37, 313.
4. (a) P. Cooper, Explosive Engineering, Wiley-VCH, New York, 1996, p. 33; (b) H. Muthurajan, R. Sivabalan, M. B. Talawar and S. N. Asthana, J. Hazard. Mater., 2004, A112, 1.
5. (a) P. C. Ashbrook and T. A. Houts, ACS Div. Chem. Health Safety, 2003, 10, 27; (b) M. Cameron, Picric Acid Hazards, American Industrial Hygiene Association, Fairfax, VA, 1995.
6. (a) Y. Peng, A. J. Zhang, M. Dong and Y. W. Wang, Chem. Commun., 2011, 47, 4505; (b) B. Gole, S. Shanmugaraju, A. K. Bar and P. S. Mukherjee, Chem. Commun., 2011, 47, 10046; (c) D. S. Kim, M. Lynch, K. A. Nielson, C. Johnsen, J. O. Jeppesen and J. L. Sessler, Anal. Bioanal. Chem., 2009, 395, 393.
7. (a) D. S. Moore, Rev. Sci. Instrum., 2004, 75, 2499; (b) Y. Zimmermann and J. A. C. Broekaert, Anal. Bioanal. Chem., 2005, 383, 998.
8. (a) M. E. Germain and M. J. Knapp, Chem. Soc. Rev., 2009, 38, 2543; (b) S. Shanmugaraju, S. A. Joshi and P. S. Mukherjee, J. Mater. Chem., 2011, 21, 9130; (c) Y. H. Lee, H. Liu, J. Y. Lee, S. H. Kim, S. K. Kim, J. L. Sessler, Y. Kim and J. S. Kim, Chem.–Eur. J., 2010, 16, 5895.
9. (a) R. Martínez-Máñez and F. Sancenón, Chem. Rev., 2003, 103, 4419; (b) D. T. Quang and J. S. Kim, Chem. Rev., 2010, 110, 6280.
10. (a) J. S. Yang and T. M. Swager, J. Am. Chem. Soc., 1998, 120, 11864; (b) S. J. Toal and W. C. Trogler, J. Mater. Chem., 2006, 16, 2871.
11. (a) Y. Salinas, R. Martínez-Máñez, M. D. Macros, F. Sancenon, A. M. Costero, M. Parra and S. Salvador, Chem. Soc. Rev., 2012, 41, 1261; (b) N. Venkatramaiah, S. Kumar and S. Patil, Chem. Commun., 2012, 48, 5007; (c) N. Venkatramaiah, S. Kumar and S. Patil, Chem.–Eur. J., 2012, 18, 14745.
12. (a) V. Bhalla, A. Gupta and M. Kumar, Org. Lett., 2012, 14, 3112; (b) B. Roy, A. K. Bar, B. Gole and P. S. Mukherjee, J. Org. Chem., 2013, 78, 1306; (c) M. Kumar, S. I. Reja and V. Bhalla, Org. Lett., 2012, 14, 6084.
13. Y. Xu, B. Li, W. Li, J. Zhao, S. Sun and Y. Pang, Chem. Commun., 2013, 49, 4764.
14. (a) A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891; (b) G. Ulrich, R. Ziessel and A. Harriman, Angew. Chem., Int. Ed., 2008, 47, 1184; (c) N. Boens, V. Leen and W. Dehaen, Chem. Soc. Rev., 2012, 41, 1130.
15. (a) J.-H. Olivier, A. Haefele, P. Retailleau and R. Ziessel, Org. Lett., 2010, 12, 408; (b) A. B. Nepomnyashchii, S. Cho, P. J. Rossky and A. J. Bard, J. Am. Chem. Soc., 2010, 132, 17550.
16. (a) S. Madhu, M. R. Rao, M. S. Shaikh and M. Ravikanth, Inorg. Chem., 2011, 50, 4392; (b) V. Lakshmi and M. Ravikanth, J. Org. Chem., 2011, 76, 8466.
17. (a) O. Buyukcakir, O. A. Bozdemir, S. Kolemen, S. Erbas and E. U. Akkaya, Org. Lett., 2009, 11, 4644; (b) H. He, P.-C. Lo, S. Yeung, W.-P. Fong and D. K. P. Ng, Chem. Commun., 2011, 47, 4748.

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Footnote

† Electronic supplementary information (ESI) available: Spectral data including absorption, fluorescence and experimental details. See DOI: 10.1039/c3ra46565a

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