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Benzo[ghi]perylene and coronene as ratiometric fluorescence probes for the selective sensing of nitroaromatic explosives
Talanta 207 (2020) 120316
Contents lists available at ScienceDirect
Talanta journal homepage: www.elsevier.com/locate/talanta
Benzo[ghi]perylene and coronene as ratiometric fluorescence probes for the selective sensing of nitroaromatic explosives
T
Ejaz Hussaina,b,c,∗∗∗, Yongxin Lia,b, Cheng Chenga,b, Huipeng Zhuoa,b, Sohail Anjum Shahzadd, Sajjad Alic, Mohammad Ismailc, Hong Qie,∗∗, Cong Yua,b,∗ a
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China University of Chinese Academy of Sciences, Beijing, 100049, PR China c Department of Chemistry, Faculty of Life Sciences, Karakoram International University Gilgit, 15100, Gilgit-Baltistan, Pakistan d Department of Chemistry, COMSATS University Islamabad, Abbottabad Campus, Abbottabad, 22060, Pakistan e Tumor Hospital of Jilin Province, Changchun, 130061, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Fluorescence probes Nitroaromatic explosives Ratiometric sensing Fluorescent strips Visual detection
A simple and efficient fluorometric sensing method is developed for the rapid detection of nitroaromatic explosives, based on the quenching of monomer and excimer emission of benzo[ghi]perylene and coronene. The ratiometric method (IE/IM) offers a linear response as a function of the concentration of picric acid (PA, i.e. 2,4,6trinitrophenol), which is used as a model example of the nitroaromatic compounds (NOCs). The detection range is observed to be 0.1–120 μM of PA (22.9 ppb–27.5 ppm). The bright emission of the stable probe excimer and monomer can be easily distinguished under UV lamp from the quenched solution with nitro-aromatic molecules that enables naked-eye detection of nitro-aromatic explosives. The fluorescent paper strips prepared by embedding the probes on the surface of the paper are used for fast, portable and selective detection of NOCs. Our optimized methods can easily detect and quantify NOCs down to 0.1 μM. The sensing process is free of commonly encountered interferences such as volatile organic compounds (VOCs), acids, bases, oxygen, and salt solutions.
1. Introduction Nitroaromatic compounds (NOCs) such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), mononitrotoluene (MNT), cyclotrimethylenetrinitramine (RDX), nitroglycerin (NG), etc., are extensively used as explosives for military and civil purposes [1,2]. The explosive comprises of a mixture of reductant and oxidant that on triggering can undergo a highly exothermic reaction to yield gaseous products [2]. The detection of NOCs has, therefore, immense importance in the military operation, environmental safety and security [1,2]. In addition, anemia and abnormal liver functions can be caused upon exposure to TNT. NOCs are highly toxic and carcinogenic [1]. Although they are relatively rare in nature, however, the human activities have introduced them to the environment and cause contamination of the environment and pollute the water. Approximately 80 ppm of TNT is found in industrial wastewaters [1]. Thus the toxicity of NOCs is a threat to human life. And the quick detection methods and
safety measures are seriously required to address the environmental challenges related to NOCs. Highly sensitive and efficient methods for the detection of NOCs are inevitable to monitor the contaminations of NOCs in industries, and forensics etc. Various fluorescent probes such as quantum dots, metal complexes, fluorescent polymers, small organic molecules have been used for the sensing of explosives [1–13]. Fluorescence quenching of AIEE active iridium complexes was used for the detection of picric acid [14]. Nanofibrous membranes loaded with ZnS quantum dots were used to construct multichannel fluorescence sensor array for the discriminative detection of explosives [20]. Many NOCs sensing methods have been designed by employing organic fluorescence molecules. Such as mesoporous ormosil thin films embedded with pyrene was used for the visual detection of nitro-explosives [3]. Hydrophilic fluorescent paper sensors have earlier been developed by exploiting a PAH derivative that can detect nitro-aromatic pollutants in aqueous media [1]. Polyethersulfone thin films doped with organic fluorescent molecule
∗ Corresponding author. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China. ∗∗ Corresponding author.Tumor Hospital of Jilin Province, Changchun. 130061, PR China. ∗∗∗ Corresponding author. Department of Chemistry, Faculty of Life Sciences, Karakoram International University Gilgit, 15100, Gilgit-Baltistan, Pakistan. E-mail addresses: [email protected] (E. Hussain), [email protected] (H. Qi), [email protected] (C. Yu).
https://doi.org/10.1016/j.talanta.2019.120316 Received 27 June 2019; Received in revised form 30 August 2019; Accepted 3 September 2019 Available online 03 September 2019 0039-9140/ © 2019 Elsevier B.V. All rights reserved.
Talanta 207 (2020) 120316
E. Hussain, et al.
were tested for the detection of vapors of NOCs that showed sensitive fluorescence quenching detectable by the naked eye. In continuation, pyrene based iptycenes with extended cavity were synthesized and introduces as sensors of NOCs [6]. Calix [4]arene based fluorescent sensors and several other fabricated materials were also introduced for similar applications [7-16]. Besides the existing techniques, more sensitive, selective, efficient, reliable and easy to use methods for explosives sensing with simple fabrication are required to address the safety concerns and control over the threats of terror. The high quantum yield, long lifetime, and excellent chemical stability of bezo[ghi]perylene and coronene are advantageous for the fluorescence-based sensing applications [17–19]. These compounds display two main emission bands acting as dual emissive fluorescent probes i.e., the monomer emission between 380 and 450 nm and the excimer emission at around 520 nm. These dual emissive fluorescent probes are sensitive to the micro-environmental conditions and molecular oxygen and serve as ratiometric sensors of oxygen and microenvironment changes [17–19]. Herein simple and portable methods for the sensitive and selective detection and quantification of nitro-aromatic explosives are developed based on the fluorescence quenching of BzP and Cron by NOCs. The fluorescence quenching of such electron-rich fluorophores occurs due to the photoinduced electron transfer from the fluorophore to the electron-poor nitro-aromatic molecule. HOMO energy level of nitro-aromatic molecule holds relatively less energy as compare to LUMO of the fluorophore. The fluorophores with extended delocalized π system show good sensitivity for the detection of NOCs [21]. The ratiometric methods developed by the monomer and excimer emission quenching of probes gave linear response for quantification of NOCs via IE/IM as a function of the concentration of NOCs. The bright fluorescence of probe solutions visible under UV lamp is quenched efficiently in the presence of NOCs. Thus visual detection by the naked eye becomes possible. The fluorescent paper strips prepared by embedding the probes in the paper are used to detect the NOCs selectively. Around 0.1 μM (22.9 ppb) solution of PA can easily be detected by our proposed method. The sensitivity of our method with detection of PA down to 0.1 μM is found better or comparable to the published literature methods where detection limits were reported to be 1 μM, 0.3 μM and 5 ppm [7,14,22]. The quenching of fluorescence is tested against commonly interfering species such as volatile organic compounds (VOCs) i.e. alcohols, DMSO, benzene, toluene, molecular oxygen, acids, bases, and salt solutions. The sensing methods are found free of these interferences.
2. Experimental 2.1. Materials Benzo[ghi]perylene, coronene, picric acid (PA), 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), and nitroglycerine (NB) were purchased from commercial suppliers and used as received. Ethanol, n-hexane, toluene, benzene, xylene, ethylbenzene, DMSO, acetic acid, KOH, Whatman filter papers and all other chemicals used were available at Biosensors Lab CIAC. Ultra-pure water purified by Milli-Q water purification system was used for solution preparation. Fluorescence spectra were collected by Fuoromax-4 spectro-fluorometer. 3. Methods The stock solutions of benzo[ghi]perylene and coronene in ethanol were prepared in the concentration range of 1–2 mM. For only monomer fluorescence quenching studies by NOCs, 500 μL of 2 μM probe solutions were prepared in ethanol by dilution of the stock solutions. 500 μL of 6 μM probe solution in ethanol/water mixture (1 : 4) was optimized to obtain the monomer-excimer fluorescence emissions as dual-emissive probe. The emission spectra were collected by gradual addition of 1 μL of 3 mM PA solution to the sample solutions of the probe. The emission spectra of BzP were collected in the range of 380–650 nm at an excitation wavelength of 365 nm with a slit width of 2 nm. For the spectral acquisition of coronene monomer emission, 305 nm of excitation wavelength was selected; and for excimer emission, 365 nm of the excitation source was applied with silt width of 3 nm. The percent quenching efficiency was calculated from the gradual decrease in fluorescence emission with the addition of PA. The fluorescent paper strips were prepared by immersing the Whatman filter paper strips in the ethanol solution of benzo[ghi]perylene. The strips were dried in the oven at 50 °C over 2 h and used for direct sensing of NOCs. 4. Results and discussion 4.1. Optical sensing of NOCs based on fluorescence quenching of probe monomers The luminescence properties of benzo[ghi]perylene and coronene favor various sensing applications [17–19]. The sensing potential of
Fig. 1. Monomer fluorescence emission quenching of BzP and plot of percent quenching as a function of PA concentration (λext: 365 nm, slit: 2 nm).
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Fig. 2. Monomer emission spectra of coronene and plot of percent quenching as a function of PA concentration (λext: 305 nm, Slit: 2 nm).
these probes for electron-deficient nitro-aromatic explosives was interrogated based on the gradual quenching in fluorescence emission of the monomers. A 2 μM ethanol solution of BzP displayed monomer emission spectra with multiple sharp peaks at 380–450 nm. The emission spectra of BzP were monitored with the increasing concentrations of PA as a model example of NOCs. The emission intensity decreased rapidly with the increase in the concentration of PA at 0.1–125 μM as shown in Fig. 1. A plot of percent quenching as a function of the concentration of PA was obtained with a linear response up to 30 μM of PA. At higher concentrations of PA, changes remained constant because of quenching saturation. This method could detect 0.1 μM concentration of PA easily. Coronene displayed fluorescence emission with multiple vibronic bands in the range of 400–500 nm. Like BzP, it showed quenching in fluorescence emission on interactions with the electron-deficient nitroaromatic explosives due to the energy transfer from a conjugated system of excited probes to the nitro-group of NOCs [6,7]. The monomer emission intensity of coronene decreased gradually with the addition of PA at 0.1–90 μM concentration. The percent quenching as a function of the micromolar concentration of PA gave a linear response up to 45 μM as demonstrated in Fig. 2. Thus the monomer emissions of both probes are behaving as signaling elements for the detection of NOCs.
aggregates due to their hydrophobic nature [17]. The 6 μM solution of the probe in ethanol/water (1 : 4) mixture forms nano-aggregates [18]. At this optimized condition, the solution contained a mixture of monomer and excimer and exhibits a spectrum displaying both monomer and excimer emission, which could act as a dual emissive probe for the sensing of NOCs as illustrated in Fig. 3a. The fluorescence intensity of both the excimer and monomer emissions decreased gradually with the increase in the concentration of PA. The plot of excimer emission intensity and IE/IM values were used for the quantitative measurements of nitro-explosives to the lowest concentration of 0.1 μM. The plot of excimer emission of BzP as a function of the concentration of PA exhibited linearity from 0.1 to 30 μM of PA (Fig. 3b). However, the plot of IE/IM as a function of the concentration of PA (μM) gave a larger range of linear response expanded in the range of 0.1–120 μM concentration as illustrated in Fig. 3c. Like BzP, at the excitation wavelength of 365 nm the fluorescence emission spectrum of 6 μM solution of coronene in ethanol/water (1 : 4) mixture displayed both monomer and excimer emissions (Fig. 4a). The monomer and excimer emission were quenched efficiently with the addition of PA. The plot of percent fluorescence quenching of excimer emission gave a linear response up to 50 μM (Fig. 4b). However, the ratio of excimer and monomer emission IE/IM offered a large linear range for the PA sensing as demonstrated in Figure- 4c. The probes form nano-aggregates in the solution of ethanol/water (1 : 4) mixture [18]. Thus the nano-aggregates enhanced the luminescence intensities and increased the sensitivity of methods. Under UV lamp at 365 nm, the ethanol solution of BzP showed bright blue emission. However, in the presence of NOCs, the probe solution was nonluminescent. As demonstrated in Fig. 5a, the ethanol solution of BzP exhibited bright blue fluorescence. With the addition of PA, the fluorescence of solution was quenched. However, BzP exhibited bright green
4.2. The fluorescence of Probe's excimers based and ratiometric optical sensing of NOCs Herein, the excimer emission-based and ratiometric methods for sensing of nitro-aromatic explosive are developed employing benzo [ghi]perylene and coronene. These compounds are soluble in organic solvents such as ethanol. However, in water they tend to form
Fig. 3. Excimer emission spectra of BzP (a) and the plots of % quenching of excimer emission (b) and IE/IM (c) as a function of TNT concentration (μM) (λext: 365 nm, Slit: 2 nm). 3
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Fig. 4. The emission spectra of coronene (a) and the plot of percent quenching of excimer emission (b) and IE/IM (c) as a function of the concentration of PA (μM).
devices. The fluorescent paper strips prepared by adsorbing probes onto the filter-papers exhibited bright fluorescence under UV radiation. A tiny drop of a micro-molar solution of NOCs quenched the fluorescence of strips. This enabled the quick detection of NOCs. The paper strips were tested against common interfering chemical species i.e. volatile organic compounds (VOCs), some benzene derivatives (toluene, xylene, and ethylbenzene), common solvents, acids, and bases. The fluorescence of strips remained unchanged by common interferences that ensure the selectivity of strips for NOCs as demonstrated in Fig. 6. Similar to the paper strips, the visual detection of NOCs by probe solutions under UV lamp revealed good selectivity for NOCs against interfering species. Picric acid (PA), 2,4,6-trinitrotoluene (TNT), 2,4dinitrotoluene (DNT), and nitroglycerin (NG) were tested as model examples of NOCs and they caused efficient quenching in fluorescence of probe solutions. The volatile organic compounds such as n-hexane, toluene, DMSO, and aqueous solutions of KOH, acetic acid, NaCl and CaCl2 didn't influence the fluorescence of probes as demonstrated in Fig. 7. The oxygen (O2) would not interfere with the assay because the excimer emission of the probe is not quenched by molecular oxygen [19]. It ensured the good selectivity of probe towards sensing of NOCs.
Fig. 5. The fluorescence emission of BzP monomer in ethanol and monomer/ excimer in 1 : 4 of ethanol/water mixture under UV lamp at 365 nm in the presence and absence of PA.
5. Conclusions
fluorescence due to the formation of nano-aggregates in the ethanol/ water (1 : 4) mixture. The fluorescence of nano-aggregates was quenched efficiently with the addition of PA (Fig. 5b). The visual detection of NOCs, therefore, became possible under UV lamp with the naked eye.
Herein we presented simple, portable, and efficient methods for sensing of nitroaromatic explosives by using benzo[ghi]perylene and coronene as fluorescence probes. The process involved fluorescence quenching by photon-induced electron transfer from highly conjugated probes to the electron-deficient nitro-aromatic compounds. The probes formed nanoaggregates in ethanol/water (1 : 4) mixture at 6 μM concentration and showed monomer-excimer dual emissions. The percent quenching of monomer and excimer emissions and IE/IM as a function
4.3. Sensing of NOCs by fluorescent paper strips and selectivity studies The probes are efficient solid-state fluorescent materials for the preparation of paper sensors, thin films, and sensory materials for
Fig. 6. Sensing of PA down to 0.1 μM by fluorescent paper strips (Left) and selectivity of strips towards NOCs against common interferences.
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Fig. 7. Selectivity of sensing process towards NOCs against common interferences i.e. VOCs, water-soluble salts, benzene derivatives, acids, and bases.
of the concentration of PA were introduced for the quantitative determination of NOCs down to 0.1 μM of PA as a test model. The monomers and excimers of BzP exhibited bright fluorescence under UV radiation and the addition of NOCs to the solution caused fluorescence turn-off. Hence, a solution-based method was established for visual detection of NOCs. Portable fluorescent paper strips were prepared that showed good selectivity and sensitivity for the sensing of NOCs. These methods are helpful to address the safety concerns via monitoring of NOCs. The probes can be further employed to construct powerful sensory devices for commercial purposes.
phase, RSC Adv. 5 (2015) 33306–33311. [8] H.S. Jang, H.S. Cho, D. Uhrig, M.P. Nieh, Insight into the interactions between pyrene and polystyrene for efficient quenching nitroaromatic explosives, J. Mater. Chem. C 5 (2017) 12466–12473. [9] Y. Yang, H. Wang, K. Su, Y. Long, Z. Peng, N. Li, F. Liu, A facile and sensitive fluorescent sensor using electrospun nanofibrous film for nitroaromatic explosive detection, J. Mater. Chem. 21 (2011) 11895–11900. [10] S. Shanmugaraju, S.A. Joshi, P.S. Mukherjee, Fluorescence and visual sensing of nitroaromatic explosives using electron rich discrete fluorophores, J. Mater. Chem. 21 (2011) 9130–9138. [11] H. Turhan, E. Tukenmez, B. Karagoz, N. Bicak, Highly fluorescent sensing of nitroaromatic explosives in aqueous media using pyrene-linked PBEMA microspheres, Talanta 179 (2018) 107–114. [12] C. Zhang, Y. Che, Z. Zhang, X. Yang, L. Zang, Fluorescent nanoscale zinc(II)-carboxylate coordination polymers for explosive sensing, Chem. Commun. 47 (47) (2011) 2336–2338. [13] R.C. Stringer, S. Gangopadhyay, S.A. Grant, Detection of nitroaromatic explosives using a fluorescent-labeled imprinted polymer, Anal. Chem. 82 (2010) 4015–4019. [14] W. Che, G. Li, X. Liu, K. Shao, D. Zhu, Z. Su, M.R. Bryce, Selective sensing of 2,4,6trinitrophenol (TNP) in aqueous media with ‘‘aggregation-induced emission enhancement’’ (AIEE)-active iridium(III) complexes, Chem. Commun. 54 (2018) 1730–1733. [15] N. Singh, U.P. Singh, R.J. Butcher, Luminescent sulfonate coordination polymers: synthesis, structural analysis and selective sensing of nitroaromatic compounds, CrystEngComm 19 (2017) 7009–7020. [16] S. Zhang, F. Lu, L. Gao, L. Ding, Y. Fang, Fluorescent sensors for nitroaromatic compounds based on monolayer assembly of polycyclic aromatics, Langmuir 23 (2007) 1584–1590. [17] E. Hussain, Z. Hu, H. Zhou, C. He, S.A. Shahzad, C. Yu, Benzo[ghi]perylene and coronene: ratiometric fluorescent probes for the sensing of microenvironment changes and micelle formation in aqueous medium, New J. Chem. 42 (2018) 6949–6954. [18] E. Hussain, Z. Hu, H. Zhou, C. He, S.A. Shahzad, C. Yu, Aggregation enhanced excimer emission (AEEE) of benzo[ghi]perylene and coronene: multimode probes for facile monitoring and direct visualization of micelle transition, Analyst 143 (2018) 4283–4289. [19] E. Hussain, C. Cheng, Y. Li, N. Niu, H. Zhou, X. Jin, J. Kong, C. Yu, Benzo[ghi] perylene and coronene as ratiometric reversible optical oxygen nano-sensor, Sens. Actuators B Chem. 287 (2019) 27–34. [20] Z. Wu, H. Duan, Z. Li, J. Guo, F. Zhong, Y. Cao, D. Jia, Multichannel discriminative detection of explosive vapors with an array of nanofibrous membranes loaded with quantum dots, Sensors 17 (2017) 2676–2889. [21] N. Niamnont, N. Kimpitak, K. Wongravee, P. Rashatasakhon, K.K. Baldridge, J.S. Siegel, M. Sukwattanasinitt, Tunable star-shaped triphenylamine fluorophores for fluorescence quenching detection and identification of nitro-aromatic explosives, Chem. Commun. 49 (8) (2013) 780–782. [22] P.S. Hariharan, J. Pitchaimani, V. Madhu, S.P. Anthony, Perylene diimide based fluorescent dyes for selective sensing of nitroaromatic compounds: selective sensing in aqueous medium across wide pH range, J. Fluoresc. 26 (2016) 395–401.
Acknowledgments This work was supported by the CAS-TWAS President's Ph.D. Fellowship Program (2015CTF031), the National Natural Science Foundation of China (51761145102, 21561162004, 21874128), the Jilin Provincial Science and Technology Development Project (20190201069JC), and Higher Education Commission Pakistan (SRGP, 21-2460). References [1] W. Lu, J. Zhang, Y. Huang, P. Théato, Q. Huang, T. Chen, Self-diffusion driven ultrafast detection of ppm-level nitroaromatic pollutants in aqueous media using a hydrophilic fluorescent paper sensor, ACS Appl. Mater. Interfaces 9 (2017) 23884–23893. [2] M.E. Germain, M.J. Knapp, Optical explosives detection: from color changes to fluorescence turn-on, Chem. Soc. Rev. 38 (2009) 2543–2555. [3] P. Beyazkilic, A. Yildirim, M. Bayindir, Formation of pyrene excimers in mesoporous ormosil thin films for visual detection of nitro-explosives, ACS Appl. Mater. Interfaces 6 (2014) 4997–5004. [4] F.N. Xiao, K. Wang, F.B. Wang, X.H. Xia, Highly stable and luminescent layered hybrid materials for sensitive detection of TNT explosives, Anal. Chem. 87 (2015) 4530–4537. [5] G.B. Demirel, B. Daglar, M. Bayindir, Extremely fast and highly selective detection of nitroaromatic explosive vapors using fluorescent polymer thin films, Chem. Commun. 49 (2013) 6140–6142. [6] A.F. Khasanov, D.S. Kopchuk, I.S. Kovalev, O.S. Taniya, K. Giri, P.A. Slepukhin, S. Santra, M. Rahman, A. Majee, V.N. Charushin, O.N. Chupakhin, Extended cavity pyrene-based iptycenes for the turn-off fluorescence detection of RDX and common nitroaromatic explosives, New J. Chem. 41 (2017) 2309–2320. [7] K. Boonkitpatarakul, Y. Yodta, N. Niamnont, M. Sukwattanasinitt, Fluorescent phenylethynylene calix[4]arenes for sensing TNT in aqueous media and vapor
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Contents lists available at ScienceDirect
Talanta journal homepage: www.elsevier.com/locate/talanta
Benzo[ghi]perylene and coronene as ratiometric fluorescence probes for the selective sensing of nitroaromatic explosives
T
Ejaz Hussaina,b,c,∗∗∗, Yongxin Lia,b, Cheng Chenga,b, Huipeng Zhuoa,b, Sohail Anjum Shahzadd, Sajjad Alic, Mohammad Ismailc, Hong Qie,∗∗, Cong Yua,b,∗ a
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China University of Chinese Academy of Sciences, Beijing, 100049, PR China c Department of Chemistry, Faculty of Life Sciences, Karakoram International University Gilgit, 15100, Gilgit-Baltistan, Pakistan d Department of Chemistry, COMSATS University Islamabad, Abbottabad Campus, Abbottabad, 22060, Pakistan e Tumor Hospital of Jilin Province, Changchun, 130061, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Fluorescence probes Nitroaromatic explosives Ratiometric sensing Fluorescent strips Visual detection
A simple and efficient fluorometric sensing method is developed for the rapid detection of nitroaromatic explosives, based on the quenching of monomer and excimer emission of benzo[ghi]perylene and coronene. The ratiometric method (IE/IM) offers a linear response as a function of the concentration of picric acid (PA, i.e. 2,4,6trinitrophenol), which is used as a model example of the nitroaromatic compounds (NOCs). The detection range is observed to be 0.1–120 μM of PA (22.9 ppb–27.5 ppm). The bright emission of the stable probe excimer and monomer can be easily distinguished under UV lamp from the quenched solution with nitro-aromatic molecules that enables naked-eye detection of nitro-aromatic explosives. The fluorescent paper strips prepared by embedding the probes on the surface of the paper are used for fast, portable and selective detection of NOCs. Our optimized methods can easily detect and quantify NOCs down to 0.1 μM. The sensing process is free of commonly encountered interferences such as volatile organic compounds (VOCs), acids, bases, oxygen, and salt solutions.
1. Introduction Nitroaromatic compounds (NOCs) such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), mononitrotoluene (MNT), cyclotrimethylenetrinitramine (RDX), nitroglycerin (NG), etc., are extensively used as explosives for military and civil purposes [1,2]. The explosive comprises of a mixture of reductant and oxidant that on triggering can undergo a highly exothermic reaction to yield gaseous products [2]. The detection of NOCs has, therefore, immense importance in the military operation, environmental safety and security [1,2]. In addition, anemia and abnormal liver functions can be caused upon exposure to TNT. NOCs are highly toxic and carcinogenic [1]. Although they are relatively rare in nature, however, the human activities have introduced them to the environment and cause contamination of the environment and pollute the water. Approximately 80 ppm of TNT is found in industrial wastewaters [1]. Thus the toxicity of NOCs is a threat to human life. And the quick detection methods and
safety measures are seriously required to address the environmental challenges related to NOCs. Highly sensitive and efficient methods for the detection of NOCs are inevitable to monitor the contaminations of NOCs in industries, and forensics etc. Various fluorescent probes such as quantum dots, metal complexes, fluorescent polymers, small organic molecules have been used for the sensing of explosives [1–13]. Fluorescence quenching of AIEE active iridium complexes was used for the detection of picric acid [14]. Nanofibrous membranes loaded with ZnS quantum dots were used to construct multichannel fluorescence sensor array for the discriminative detection of explosives [20]. Many NOCs sensing methods have been designed by employing organic fluorescence molecules. Such as mesoporous ormosil thin films embedded with pyrene was used for the visual detection of nitro-explosives [3]. Hydrophilic fluorescent paper sensors have earlier been developed by exploiting a PAH derivative that can detect nitro-aromatic pollutants in aqueous media [1]. Polyethersulfone thin films doped with organic fluorescent molecule
∗ Corresponding author. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China. ∗∗ Corresponding author.Tumor Hospital of Jilin Province, Changchun. 130061, PR China. ∗∗∗ Corresponding author. Department of Chemistry, Faculty of Life Sciences, Karakoram International University Gilgit, 15100, Gilgit-Baltistan, Pakistan. E-mail addresses: [email protected] (E. Hussain), [email protected] (H. Qi), [email protected] (C. Yu).
https://doi.org/10.1016/j.talanta.2019.120316 Received 27 June 2019; Received in revised form 30 August 2019; Accepted 3 September 2019 Available online 03 September 2019 0039-9140/ © 2019 Elsevier B.V. All rights reserved.
Talanta 207 (2020) 120316
E. Hussain, et al.
were tested for the detection of vapors of NOCs that showed sensitive fluorescence quenching detectable by the naked eye. In continuation, pyrene based iptycenes with extended cavity were synthesized and introduces as sensors of NOCs [6]. Calix [4]arene based fluorescent sensors and several other fabricated materials were also introduced for similar applications [7-16]. Besides the existing techniques, more sensitive, selective, efficient, reliable and easy to use methods for explosives sensing with simple fabrication are required to address the safety concerns and control over the threats of terror. The high quantum yield, long lifetime, and excellent chemical stability of bezo[ghi]perylene and coronene are advantageous for the fluorescence-based sensing applications [17–19]. These compounds display two main emission bands acting as dual emissive fluorescent probes i.e., the monomer emission between 380 and 450 nm and the excimer emission at around 520 nm. These dual emissive fluorescent probes are sensitive to the micro-environmental conditions and molecular oxygen and serve as ratiometric sensors of oxygen and microenvironment changes [17–19]. Herein simple and portable methods for the sensitive and selective detection and quantification of nitro-aromatic explosives are developed based on the fluorescence quenching of BzP and Cron by NOCs. The fluorescence quenching of such electron-rich fluorophores occurs due to the photoinduced electron transfer from the fluorophore to the electron-poor nitro-aromatic molecule. HOMO energy level of nitro-aromatic molecule holds relatively less energy as compare to LUMO of the fluorophore. The fluorophores with extended delocalized π system show good sensitivity for the detection of NOCs [21]. The ratiometric methods developed by the monomer and excimer emission quenching of probes gave linear response for quantification of NOCs via IE/IM as a function of the concentration of NOCs. The bright fluorescence of probe solutions visible under UV lamp is quenched efficiently in the presence of NOCs. Thus visual detection by the naked eye becomes possible. The fluorescent paper strips prepared by embedding the probes in the paper are used to detect the NOCs selectively. Around 0.1 μM (22.9 ppb) solution of PA can easily be detected by our proposed method. The sensitivity of our method with detection of PA down to 0.1 μM is found better or comparable to the published literature methods where detection limits were reported to be 1 μM, 0.3 μM and 5 ppm [7,14,22]. The quenching of fluorescence is tested against commonly interfering species such as volatile organic compounds (VOCs) i.e. alcohols, DMSO, benzene, toluene, molecular oxygen, acids, bases, and salt solutions. The sensing methods are found free of these interferences.
2. Experimental 2.1. Materials Benzo[ghi]perylene, coronene, picric acid (PA), 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), and nitroglycerine (NB) were purchased from commercial suppliers and used as received. Ethanol, n-hexane, toluene, benzene, xylene, ethylbenzene, DMSO, acetic acid, KOH, Whatman filter papers and all other chemicals used were available at Biosensors Lab CIAC. Ultra-pure water purified by Milli-Q water purification system was used for solution preparation. Fluorescence spectra were collected by Fuoromax-4 spectro-fluorometer. 3. Methods The stock solutions of benzo[ghi]perylene and coronene in ethanol were prepared in the concentration range of 1–2 mM. For only monomer fluorescence quenching studies by NOCs, 500 μL of 2 μM probe solutions were prepared in ethanol by dilution of the stock solutions. 500 μL of 6 μM probe solution in ethanol/water mixture (1 : 4) was optimized to obtain the monomer-excimer fluorescence emissions as dual-emissive probe. The emission spectra were collected by gradual addition of 1 μL of 3 mM PA solution to the sample solutions of the probe. The emission spectra of BzP were collected in the range of 380–650 nm at an excitation wavelength of 365 nm with a slit width of 2 nm. For the spectral acquisition of coronene monomer emission, 305 nm of excitation wavelength was selected; and for excimer emission, 365 nm of the excitation source was applied with silt width of 3 nm. The percent quenching efficiency was calculated from the gradual decrease in fluorescence emission with the addition of PA. The fluorescent paper strips were prepared by immersing the Whatman filter paper strips in the ethanol solution of benzo[ghi]perylene. The strips were dried in the oven at 50 °C over 2 h and used for direct sensing of NOCs. 4. Results and discussion 4.1. Optical sensing of NOCs based on fluorescence quenching of probe monomers The luminescence properties of benzo[ghi]perylene and coronene favor various sensing applications [17–19]. The sensing potential of
Fig. 1. Monomer fluorescence emission quenching of BzP and plot of percent quenching as a function of PA concentration (λext: 365 nm, slit: 2 nm).
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Fig. 2. Monomer emission spectra of coronene and plot of percent quenching as a function of PA concentration (λext: 305 nm, Slit: 2 nm).
these probes for electron-deficient nitro-aromatic explosives was interrogated based on the gradual quenching in fluorescence emission of the monomers. A 2 μM ethanol solution of BzP displayed monomer emission spectra with multiple sharp peaks at 380–450 nm. The emission spectra of BzP were monitored with the increasing concentrations of PA as a model example of NOCs. The emission intensity decreased rapidly with the increase in the concentration of PA at 0.1–125 μM as shown in Fig. 1. A plot of percent quenching as a function of the concentration of PA was obtained with a linear response up to 30 μM of PA. At higher concentrations of PA, changes remained constant because of quenching saturation. This method could detect 0.1 μM concentration of PA easily. Coronene displayed fluorescence emission with multiple vibronic bands in the range of 400–500 nm. Like BzP, it showed quenching in fluorescence emission on interactions with the electron-deficient nitroaromatic explosives due to the energy transfer from a conjugated system of excited probes to the nitro-group of NOCs [6,7]. The monomer emission intensity of coronene decreased gradually with the addition of PA at 0.1–90 μM concentration. The percent quenching as a function of the micromolar concentration of PA gave a linear response up to 45 μM as demonstrated in Fig. 2. Thus the monomer emissions of both probes are behaving as signaling elements for the detection of NOCs.
aggregates due to their hydrophobic nature [17]. The 6 μM solution of the probe in ethanol/water (1 : 4) mixture forms nano-aggregates [18]. At this optimized condition, the solution contained a mixture of monomer and excimer and exhibits a spectrum displaying both monomer and excimer emission, which could act as a dual emissive probe for the sensing of NOCs as illustrated in Fig. 3a. The fluorescence intensity of both the excimer and monomer emissions decreased gradually with the increase in the concentration of PA. The plot of excimer emission intensity and IE/IM values were used for the quantitative measurements of nitro-explosives to the lowest concentration of 0.1 μM. The plot of excimer emission of BzP as a function of the concentration of PA exhibited linearity from 0.1 to 30 μM of PA (Fig. 3b). However, the plot of IE/IM as a function of the concentration of PA (μM) gave a larger range of linear response expanded in the range of 0.1–120 μM concentration as illustrated in Fig. 3c. Like BzP, at the excitation wavelength of 365 nm the fluorescence emission spectrum of 6 μM solution of coronene in ethanol/water (1 : 4) mixture displayed both monomer and excimer emissions (Fig. 4a). The monomer and excimer emission were quenched efficiently with the addition of PA. The plot of percent fluorescence quenching of excimer emission gave a linear response up to 50 μM (Fig. 4b). However, the ratio of excimer and monomer emission IE/IM offered a large linear range for the PA sensing as demonstrated in Figure- 4c. The probes form nano-aggregates in the solution of ethanol/water (1 : 4) mixture [18]. Thus the nano-aggregates enhanced the luminescence intensities and increased the sensitivity of methods. Under UV lamp at 365 nm, the ethanol solution of BzP showed bright blue emission. However, in the presence of NOCs, the probe solution was nonluminescent. As demonstrated in Fig. 5a, the ethanol solution of BzP exhibited bright blue fluorescence. With the addition of PA, the fluorescence of solution was quenched. However, BzP exhibited bright green
4.2. The fluorescence of Probe's excimers based and ratiometric optical sensing of NOCs Herein, the excimer emission-based and ratiometric methods for sensing of nitro-aromatic explosive are developed employing benzo [ghi]perylene and coronene. These compounds are soluble in organic solvents such as ethanol. However, in water they tend to form
Fig. 3. Excimer emission spectra of BzP (a) and the plots of % quenching of excimer emission (b) and IE/IM (c) as a function of TNT concentration (μM) (λext: 365 nm, Slit: 2 nm). 3
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Fig. 4. The emission spectra of coronene (a) and the plot of percent quenching of excimer emission (b) and IE/IM (c) as a function of the concentration of PA (μM).
devices. The fluorescent paper strips prepared by adsorbing probes onto the filter-papers exhibited bright fluorescence under UV radiation. A tiny drop of a micro-molar solution of NOCs quenched the fluorescence of strips. This enabled the quick detection of NOCs. The paper strips were tested against common interfering chemical species i.e. volatile organic compounds (VOCs), some benzene derivatives (toluene, xylene, and ethylbenzene), common solvents, acids, and bases. The fluorescence of strips remained unchanged by common interferences that ensure the selectivity of strips for NOCs as demonstrated in Fig. 6. Similar to the paper strips, the visual detection of NOCs by probe solutions under UV lamp revealed good selectivity for NOCs against interfering species. Picric acid (PA), 2,4,6-trinitrotoluene (TNT), 2,4dinitrotoluene (DNT), and nitroglycerin (NG) were tested as model examples of NOCs and they caused efficient quenching in fluorescence of probe solutions. The volatile organic compounds such as n-hexane, toluene, DMSO, and aqueous solutions of KOH, acetic acid, NaCl and CaCl2 didn't influence the fluorescence of probes as demonstrated in Fig. 7. The oxygen (O2) would not interfere with the assay because the excimer emission of the probe is not quenched by molecular oxygen [19]. It ensured the good selectivity of probe towards sensing of NOCs.
Fig. 5. The fluorescence emission of BzP monomer in ethanol and monomer/ excimer in 1 : 4 of ethanol/water mixture under UV lamp at 365 nm in the presence and absence of PA.
5. Conclusions
fluorescence due to the formation of nano-aggregates in the ethanol/ water (1 : 4) mixture. The fluorescence of nano-aggregates was quenched efficiently with the addition of PA (Fig. 5b). The visual detection of NOCs, therefore, became possible under UV lamp with the naked eye.
Herein we presented simple, portable, and efficient methods for sensing of nitroaromatic explosives by using benzo[ghi]perylene and coronene as fluorescence probes. The process involved fluorescence quenching by photon-induced electron transfer from highly conjugated probes to the electron-deficient nitro-aromatic compounds. The probes formed nanoaggregates in ethanol/water (1 : 4) mixture at 6 μM concentration and showed monomer-excimer dual emissions. The percent quenching of monomer and excimer emissions and IE/IM as a function
4.3. Sensing of NOCs by fluorescent paper strips and selectivity studies The probes are efficient solid-state fluorescent materials for the preparation of paper sensors, thin films, and sensory materials for
Fig. 6. Sensing of PA down to 0.1 μM by fluorescent paper strips (Left) and selectivity of strips towards NOCs against common interferences.
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Fig. 7. Selectivity of sensing process towards NOCs against common interferences i.e. VOCs, water-soluble salts, benzene derivatives, acids, and bases.
of the concentration of PA were introduced for the quantitative determination of NOCs down to 0.1 μM of PA as a test model. The monomers and excimers of BzP exhibited bright fluorescence under UV radiation and the addition of NOCs to the solution caused fluorescence turn-off. Hence, a solution-based method was established for visual detection of NOCs. Portable fluorescent paper strips were prepared that showed good selectivity and sensitivity for the sensing of NOCs. These methods are helpful to address the safety concerns via monitoring of NOCs. The probes can be further employed to construct powerful sensory devices for commercial purposes.
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