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conjugated polymers fluorescent micelles for trace detection of nitroaromatic explosives
Dyes and Pigments 125 (2016) 367e374
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
F127/conjugated polymers fluorescent micelles for trace detection of nitroaromatic explosives Zicheng Liang a, Hui Chen a, Xiaohui Wang a, *, Runcang Sun a, b a b
State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou, 510640, PR China Institute of Biomass Chemistry and Technology, Beijing Forestry University, Beijing, 100083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 July 2015 Accepted 26 October 2015 Available online 14 November 2015
The cost-effective and rapid detection of nitroaromatic explosives is highly desired for growing con cern about the environmental pollution and terror threats. Herein, a facile sensing system for fast and trace detection of nitroaromatic explosives (e.g. picric acid (PA) and 2,4-dinitrotoluene (DNT)) in aqueous solution is developed. In this system, the self-assembled nanomicelles of commercial available Pluronic F127 are selected as vesicles and fluorescent conjugated polymers poly(9,90 -dioctylfluorene) (PFO) and poly(2,7-(9,9-hexylfluorene)-alt-4,40 -phenylether) (PFPE) are used as sensing materials. Through the microextraction effect of F127 nanomicelles, the hydrophobic conjugated polymers can be well dispersed in aqueous media and effectively quenched by trace amount of PA or DNT based on electron transfer mechanism. In compare with the conjugated polymers dissolved in organic solvents, the conjugated polymers encapsulated in F127 nanomicelles show dramatically enhanced sensing selectivity and sensitivity. The current nitroaromatic explosives sensing system is versatile. These results suggest that the F127/conjugated polymers system is a feasible method for detecting nitroaromatic explosives in water. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Conjugated polymers F127 self-assembled nanomicelles Fluorescence quenching Explosives detection Electron transfer Microextraction effect
1. Introduction Nitroaromatic explosives, such as picric acid (PA), trinitrotoluene (TNT) and 2,4-ditrotoluene (DNT), are high-toxic pollution in soil and water and the important ingredient for military issues [1,2]. In recent years, the detection of nitroaromatic explosives has attracted widespread concerns as a consequence of increasing terrorist events and environmental pollution [3]. Currently, available detection strategies of nitroaromatic explosives include ion mobility spectrometry (IMS) [4], surface enhanced Raman scattering (SERS) [5], chromatographic analysis [6], mass spectroscopy [7] and so on. However, most of them are restricted by sophisticated instruments and time-consuming sample pretreatment process. Fluorescence sensing method is perceived as a promising alternative method due to their high sensitivity, short response time and portability since Swager and coworkers firstly reported the application of fluorescent probes in explosives detection in 1998 [8].
* Corresponding author. Tel.: þ86 20 8711 1745; fax: þ86 20 87111861. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.dyepig.2015.10.045 0143-7208/© 2015 Elsevier Ltd. All rights reserved.
Conjugated polymers (CPs) are commonly used sensing materials because of their strong fluorescence and abundant active sites [9,10]. However, the hydrophobicity of them badly limits their practical applications [8,11]. In recent years, numerous waterdispersible fluorescence sensing systems have been designed to overcome the drawback. Wu et al. developed water-dispersible conjugated polymer nanoparticles by mini-emulsion or reprecipitation methods [12,13]. Huang and coworkers constructed a water-soluble conjugated polymer for DNA sensing by a colorimetric strategy [14]. Nevertheless, stable, versatile and sensitive aqueous sensing methods for nitroaromatic explosives still remains a challenge. In recent years, amphiphilic polymers have been regarded as promising carriers in hydrophobic drug delivery and genetic therapy [15,16]. As hydrophobic fluorescent conjugated polymer molecules have similar properties to some hydrophobic drugs, they might be encapsulated in the hydrophobic cores of amphiphilic polymeric micelles. It has been reported that polymer nanomicelles were used to construct fluorescence probes for application in medicine, biology, material chemistry and sensing [17]. In our previous study, we had successfully utilized self-assembled amphiphilic cellulose micelles to encapsulate hydrophobic
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conjugated polymers for sensing nitroaromatic explosives in water phase [18]. Zhang et al. followed this work and realized the trace detection of explosives using amphiphilic cellulose micelles encapsulating hydrophobic dye [19]. However, these cellulose derivatives are not commercialized and require tedious preparation process. Pluronic F-127, a triblock copolymers composed of poly(oxyethylene)-block-poly(oxypropylene)-blockpoly(oxyethylene), is a non-toxic, non-irritant, low-cost and well commercialized macromolecular surfactant. It can self-assemble into nanomicelles in aqueous solution at relatively high solubility due to the long PPO block and high molecular weight. In this work, we chose the self-assembled micelles of commercial F127 as carriers and commercial available conjugated polymers as sensing materials to construct an aqueous sensing system for nitroaromatic explosives. This work would provide an alternative approach for ultrasensitive detection of nitroaromatic explosives in aqueous media by employing widely available materials and facilitate the commercialization of fluorescent sensing in explosives detection. 2. Experimental section 2.1. Materials Triblock copolymers PEO100-PPO65-PEO100 (F127, Mw ¼ 12,600) were purchased from Sigma Co. Ltd. Fluorescent conjugated polymers poly(9,9’-dioctylfluorene) (PFO, Mw ¼ 38,131, PDI ¼ 2.9) and poly(2,7-(9,9-hexylfluorene)-alt-4,40 -phenylether) (PFPE, Mw ¼ 66,317, PDI ¼ 1.8) were synthesized according to literature methods (structures shown in Fig. 1) [20,21]. Nitroaromatic explosives picric acid (PA) and 2,4-dinitrotoluene (DNT) were purchased from local commercial suppliers. Analogue methyl benzene (MB) was used as contrast. Other reagents in this work like tetrahydrofuran (THF) and ethyl alcohol (EtOH) were analytically pure and used without any purification. Deionized water was used throughout the whole experiment.
Here, ME was the mass of the encapsulated CPs in micelles and M0 was the mass of the total adding amounts. A Fluorolog-3 spectrometer (Horiba Jobin Yvon, France) was used to examine the fluorescence properties of the conjugated polymers in THF or in F127 micelles, with 335 nm excitation wavelength and 365e600 nm emission range. The slit widths for excitation and emission were 5 nm for F127/PFO, and 3 nm for F127/PFPE. The quantum yields of PFO and PFPE were calculated by equation (2) [22], with rhodamine 6G (QY ¼ 92% in ethyl alcohol) as standard [23].
F ¼ n2 Aref FFref
. n2ref AFref
(2)
Here, n is the refractive index of solvent, A is the absorbance at corresponding excitation wavelength, F is the integrated intensity of emission, and F was the value of quantum yield. 2.3. Preparation of F127/conjugated polymers fluorescent micelles F127 micelle solution was prepared by dissolving F127 in deionized water in desired concentrations. Then PFO/THF solution or PFPE/THF solutions were carefully dropped into F127 micelle solution under magnetic stirring. The mixtures were further treated with ultrasonic for 1 h to promote the encapsulation process. THF in the micelle solution was removed by vacuum distillation. The resultant F127/PFO and F127/PFPE micelle solution were further filtrated by 0.45 mm Millipore filters to remove insoluble. 2.4. Detection of nitroaromatic explosives with F127/CPs micelles Initially, 2.5 mL F127/PFO or F127/PFPE micelle solution was added in quartz colorimetric utensils (1 cm length). Picric acid (PA) or 2,4-dinitrophenol (DNT) ethanol solution was added to the micelle solution in desired concentrations. Then the fluorescence spectra were scanned after equilibrium for 10 s. 3. Results and discussion
2.2. Characterizations 3.1. Self-assembly of F127 nanomicelles Transmission electron microscopy (TEM) images were taken on a JEM-2100F (JEOL, Japan) electron microscope at 200 kV accelerating voltage. Scanning electron microscopy (SEM) images were taken on an S-3700N (Hitachi, Japan) electron microscope. Dynamic light scattering (DLS) measurements were measured on a Malvern 3000 HSA instrument (Malvern, UK). A UV-3600 (Shimadzu, Japan) spectrometer was used to determine the encapsulation efficiencies (EE) for the micelles, following equation (1).
EEð%Þ ¼ ðME =M0 Þ 100%
(1)
Driven by the hydrophobic interactions between the PPO segments, Pluronic F127 can self-assemble into core-shell-like nanomicelles in aqueous solution. As the TEM image shown in Fig. 2a, the self-assembled F127 nanomicelles were well-dispersed and show uniform spherical shape with the diameters varying in the range of 27e50 nm. DLS was further used to determine the hydrodynamic size and size distribution of the self-assembled F127 nanomicelles in aqueous media as shown in Fig. 2b. According to which the average particle size of F127 was 54.8 nm with a polydispersity indexes (PDI) of 0.357. The particle size determined by DLS was slightly higher than that determined by TEM, probably due to the drying shrinkage of the hydrophilic micelle shells during the sample treatment process for TEM analysis. 3.2. F127/conjugated polymers fluorescent nanomicelles
Fig. 1. Molecular structures of the sensing materials PFO (a), PFPE (b), and the quenchers PA (c), DNT (d) and MB (e).
Hydrophobic conjugated polymers (PFO or PFPE) can be encapsulated in F127 nanomicelles through the microextraction effect. The absorption and emission spectra of PFO and PFPE are illustrated in Fig. 3. PFO presented a maximum absorption peak at 385 nm in THF. In the emission spectra of PFO, there are a primary emission peak at 419 nm and two shoulder peaks at 442 nm and 476 nm. After being encapsulated in F127 micelles, obvious spectral changes were observed in both of the absorption and emission spectra of PFO. The absorption peak of the PFO in F127
Z. Liang et al. / Dyes and Pigments 125 (2016) 367e374
Fig. 2. A TEM image of F127 nanomicelles (a); Size distribution from DLS analysis (b).
nanomicelles blue-shifted to 380 nm and a weak absorption peak at 432 nm arose. The weak absorption at 432 nm may ascribe to the partially aggregation or formation of b-crystallization of PFO in micelles [24,25]. Meanwhile, the emission peaks of PFO in F127 nanomicelles red-shifted to 439 nm, 467 nm and 496 nm, respectively. The red shifts should be attributed to the spatial confinement effect of the nanomicelles. Such spatial confinement effect can enhance the intra-molecular and inter-molecular interactions between the entrapped conjugated polymers. As a consequence, the aggregative states and energy levels of the fluorophores are altered and finally form partial crystalline states [26]. PFPE had an absorption peak located at 340 nm and two emission peaks at 375 nm and 393 nm in THF. After being encapsulated by F127 nanomicelles, the absorption peak of PFPE slightly red-shifted to 345 nm and no shoulder peak was observed indicating aggregation was formed. Similar to PFO, the emission peaks of PFPE in F127 nanomicelles
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red-shifted to 400 nm and 419 nm, respectively. These red shifts should be ascribed to the intra-chain energy transfer in F127 nanomicelles. To investigate the encapsulating capacity of F127 micelles towards PFO and PFPE, different amounts of conjugated polymers were encapsulated in different concentrations of F127 nanomicelles (0.01, 0.1 and 1 mg/mL). Fig. 4 displayed the encapsulation efficiencies (EEs) and the loading amounts of PFO and PFPE in the F127 nanomicelles solutions with different concentrations. Initially, the EEs of F127 nanomicelles in a certain concentration increased with the concentration of PFO or PFPE till their concentration increased to 10 mg/mL. Then the EEs began to decrease along with continuous addition of PFO or PFPE. However, the loading amounts were increasing with the total amounts of PFO or PFPE in feed. These results suggested that in low concentration conjugated polymers solution, the adding amount of PFO or PFPE is the predominant factor determining the encapsulating capacity of F127 nanomicelles. While in higher concentration conjugated polymers solution, the solubilization effect of the nanomicelles' hydrophobic cores gradually reaches a saturation. Then excess conjugated polymers molecules began to precipitate in aqueous solution because they were unable to access into the nanomicelles. As a result, the EEs of F127 nanomicelles decreased at high concentration of PFO or PFPE. At the same concentration of conjugated polymers, F127 nanomicelles had relatively higher EEs for PFO because PFO has smaller molecular size. The quantum yields (QYs) of PFO and PFPE in F127 nanomicelles were exhibited in Fig. 5. At a certain concentration of F127 nanomicelles, the highest QYs of PFO or PFPE were obtained at the concentration of 10 mg/mL, which is corresponding to the highest encapsulation efficiencies. When the conjugated polymers concentration was over 10 mg/mL, the QYs drastically decayed with their increasing concentration. This trend became more pronounced in the solutions with higher micelles concentrations. A possible explanation is the self-aggregation of the conjugate polymers in a relative high concentration, which could result in fluorescence self-quenching. The sizes and corresponding PDI of F127 nanomicelles after encapsulating PFO or PFPE were showed in Table 1. The sizes of the fluorescent nanomicelles were much larger than the blank one and they became even larger with increasing loading amount of the conjugated polymers. After encapsulating PFO or PFPE, the enlargement of the hydrophobic cores would weaken the intermolecular interactions of the hydrophilic shells. Therefore, the more molecules being encapsulated, the larger sizes of the nanomicelles were obtained. The nanomicelles encapsulating PFPE had relatively larger sizes than the ones encapsulating PFO because
Fig. 3. UVevis and emission spectra of PFO (a) and PFPE (b) in THF (black line) and in F127 nanomicelles (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Encapsulation efficiencies and loading amounts of PFO (a, b) and PFPE (c, d) in F127 nanomicelles.
PFPE has a larger molecular size. Moreover, the PDIs of the nanomicelles were quite low (<0.2), implying relatively uniform size distribution. The SEM images of the nanomicelles encapsulating PFO and PFPE were given in Fig. 6. After encapsulating conjugated polymers, F127 nanomicelles maintained uniform spherical morphology and were still well-dispersed. The diameter distribution of the nanomicelles determined by SEM were in the range of 100e150 nm substantially in agreement with the DLS results.
Fig. 7 showed the size stability and photostability of the conjugated polymers encapsulated in F127 nanomicelles within 15 days' storage. No obvious aggregation of fluorescent nanomicelles was observed. The average sizes of the fluorescent nanomicelles became slightly larger with the prolonging of storage time and increased about 7% after two weeks. The QYs of the fluorescent nanomicelles had negligible change within 8 days' storage at room temperature, while obvious decays were observed after 8 days.
Fig. 5. Quantum yields of PFO (a) and PFPE (b) in F127 nanomicelles.
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Table 1 Sizes (nm) with PDI values (in parenthesis) of F127 micelles loaded with different concentration of PFO and PFPE. F127 (mg/mL)
0.01 0.1 1
PFO (mg/mL)
PFPE (mg/mL)
5
10
50
100
5
10
50
100
118.6 (0.087) 108.6 (0.060) 105.6 (0.069)
127.8 (0.095) 119.7 (0.167) 114.9 (0.209)
135.4 (0.181) 129.8 (0.074) 127.2 (0.091)
141.6 (0.079) 133.0 (0.006) 131.2 (0.109)
102.9 (0.133) 101.0 (0.129) 99.6 (0.105)
118.3 (0.101) 109.9 (0.111) 105.8 (0.103)
129.7 (0.070) 117.0 (0.074) 111.0 (0.159)
137.4 (0.057) 127.6 (0.083) 125.8 (0.085)
These results indicated that the fluorescent micelles can endure short-term storage for commercialized use. 3.3. Detection of nitroaromatic explosives with F127/conjugated polymers fluorescent nanomicelles A schematic diagram for nitroaromatic explosives detection in aqueous media with F127/PFO or F127/PFPE nanomicelles was illustrated in Scheme 1. Our aqueous sensing system for nitroaromatic explosive is based on the electron transfer mechanism. As nitroaromatic explosives are well known electron-deficient molecules, they are able to serve as electron acceptors. The nitro moieties on the benzene ring can capture the photo-induced electrons from the conjugated polymers, resulting in fluorescence quenching of the electron-rich fluorophores [1,2]. Therefore, F127/PFO or
Fig. 6. SEM images of PFO (a) and PFPE (b) after being encapsulated in F127 nanomicelles.
F127/PFPE nanomicelles were used to detect nitroaromatic explosives through monitoring their fluorescence quenching. Due to their hydrophobicity, nitroaromatic explosives can easily access into the hydrophobic cores through microextraction effect. In the nanomicelles, the nitroaromatic explosives (acceptors) and the conjugated polymers (donors) are close to each other to generate effective electron transfer. The spatial confinement effect of the nanomicelles endowed the high sensitivity and selectivity in our sensing system. The fluorescence quenching by nitroaromatic explosives were evaluated by continuously adding different concentrations of picric acid (PA) or 2,4-dinitrotoluene (DNT) into the PFO or PFPE -containing F127 nanomicelles aqueous solutions. To examine the sensitivity and selectivity, the fluorescence quenching of PFO and PFPE in THF were used as contrast. Fig. 8 and Fig. 9 displayed the quenching spectra of PFO and PFPE in F127 nanomicelles or in THF. PFO and PFPE in THF were significantly quenched by PA at 0.49 mM and 0.65 mM after 10 s' equilibrium. However, comparable quenching of PFO and PFPE in F127 nanomicelles can be observed when respectively adding only 0.025 mM and 0.015 mM PA after the same equilibrium time. These results indicated that the fluorescent nanomicelles have much higher sensitivity than those conjugated polymers in THF. Similar quenching can be observed in DNT detection. In THF, PFO and PFPE were significantly quenched by DNT in 4.17 mM and 2.76 mM. While in F127 nanomicelles, only 0.17 mM and 0.56 mM DNT could reach the comparable quenching effect. Both F127/PFO and F127/PFPE nanomicelles show much higher selectivity for PA. The high selectivity should be attributed to the lower unoccupied orbital energy of PA molecules thus endow them higher electron efficiency [11,27]. Methyl benzene (MB) was selected as contrast because it has similar benzene ring structure with PA and DNT but no electron-deficient nitro moiety. As expected, no obvious fluorescence quenching could be observed after adding large amount of MB. Fig. 10 illustrated the SterneVolmer quenching plots of the conjugated polymers in THF and in F127 nanomicelles. The SeV plot of PFO for PA in F127 nanomicelles was exponentially growing in the range of 0e0.0253 mM, which is attributed to the combined quenching process involving both static and dynamic quenching. While the SeV plot in THF was linear, same as for DNT in THF and in F127 nanomicelles. These linear SeV plots indicated only static or dynamic quenching existing in their quenching process. However, significant differences could be seen in the SeV plots of PFPE. The SeV plots of PFPE for PA and DNT in F127 nanomicelles was linear in the range of 0e0.0147 mM, while presented exponentially growing plots in THF. The SeV quenching constants of PFO and PFPE for nitroaromatic explosives were calculated by Stern-Volmer equation [28]:
I0 =I ¼ 1 þ Ksv ½Q ðLinearÞ
(3)
I0 =I ¼ eKsv ½Q ðExponentialÞ
(4)
where, I0 and I are the fluorescence intensity in absence/presence of the quencher, [Q] is the molar concentration of nitroaromatic explosives, and Ksv is the SeV quenching constant in M1. The Ksv of
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Fig. 7. Size stability (a) and photostability (b) of F127/PFO and F127/PFPE nanomicelles.
Scheme 1. Formation of fluorescent PFO or PFPE -containing F127 nanomicelles and their detection application of nitroaromatic explosives in aqueous media.
Fig. 8. Quenching spectra of PFO in F127 nanomicelles (aec) and in THF (def) with different concentrations of PA, DNT and MB (lex ¼ 335 nm).
PFO and PFPE for nitroaromatic explosives were given in Table 2. The Ksv of PFO and PFPE were quite low in THF, in which the Ksv for DNT were close to Swager's report (230 M1) [29]. After being encapsulated in F127 nanomicelles, the Ksv of PFO and PFPE dramatically increased. These results demonstrated that F127 nanomicelles effectively enhanced the detection selectivity and sensitivity of the conjugated polymers by the microextraction and spatial confinement effect.
4. Conclusions In summary, a sensing system for nitroaromatic explosive in aqueous solution is developed using commercial available polymeric surfactant Pluronic F127 as vesicles and light-emitting conjugated polymers as sensing materials. The microextraction and spatial confinement effect of F127 nanomicelles could maximize the electron transfer between PFO/PFPE and nitroaromatic
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Fig. 9. Quenching spectra of PFPE in F127 nanomicelles (aec) and in THF (def) with different concentrations of PA, DNT and MB (lex ¼ 335 nm).
Fig. 10. SterneVolmer plots of PFO (aec) and PFPE (def) for PA, DNT and MB in F127 nanomicelles and in THF.
Table 2 SterneVolmer constants for fluorescence quenching of PFO and PFPE by PA, DNT and MB. Sample
PA (M1)
DNT (M1)
MB (M1)
PFO in THF PFPE in THF PFO in F127 PFPE in F127
1422 1751 75,048 59,984
213 257 8305 2845
31 32 55 61
explosives, thereby achieving higher sensitivity and selectivity for explosives detection compared to those in organic media. By micelle encapsulation, the application of the hydrophobic conjugated polymers is extended to rapid, sensitive and selective detection for explosives in aqueous media. As our sensing system is constructed with commercial products, it is potentially expected to large-scaled utilization.
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Acknowledgements This work was supported by the grants from the Science and Technology Program of Guangzhou, China (2014J4100039), the New Century Excellent Talents in University (NCET-13-0215), the Fundamental Research Funds for the Central Universities, SCUT (201522036) and the Opening Project of the Key Laboratory of Polymer Processing Engineering, Ministry of Education, China (KFKT-201401). References [1] Germain ME, Knapp MJ. Optical explosives detection: from color changes to fluorescence turn-on. Chem Soc Rev 2009;38:2543e55. n ~ ez R, Marcos MD, Sanceno n F, Costero AM, Parra M, [2] Salinas Y, Martínez-Ma et al. Optical chemosensors and reagents to detect explosives. Chem Soc Rev 2012;41:1261e96. [3] Ma Y, Wang S, Wang L. Nanomaterials for luminescence detection of nitroaromatic explosives. TrAC-Trends Anal Chem 2015;65:13e21. [4] Eiceman G, Stone J. Peer reviewed: ion mobility spectrometers in national defense. Anal Chem 2004;76:390Ae7A. [5] Sylvia JM, Janni JA, Klein J, Spencer KM. Surface-enhanced Raman detection of 2, 4-dinitrotoluene impurity vapor as a marker to locate landmines. Anal Chem 2000;72:5834e40. [6] Walsh ME. Determination of nitroaromatic, nitramine, and nitrate ester explosives in soil by gas chromatography and an electron capture detector. Talanta 2001;54:427e38. [7] Mathurin J, Faye T, Brunot A, Tabet J, Wells G, Fuche C. High-pressure ion source combined with an in-axis ion trap mass spectrometer. 1. Instrumentation and applications. Anal Chem 2000;72:5055e62. [8] Yang J-S, Swager TM. Fluorescent porous polymer films as TNT chemosensors: electronic and structural effects. J Am Chem Soc 1998;120:11864e73. [9] Thomas SW, Joly GD, Swager TM. Chemical sensors based on amplifying fluorescent conjugated polymers. Chem Rev 2007;107:1339e86. [10] Fernando LP, Kandel PK, Yu J, McNeill J, Ackroyd PC, Christensen KA. Mechanism of cellular uptake of highly fluorescent conjugated polymer nanoparticles. Biomacromolecules 2010;11:2675e82. [11] Xu B, Wu X, Li H, Tong H, Wang L. Selective detection of TNT and picric acid by conjugated polymer film sensors with donoreacceptor architecture. Macromolecules 2011;44:5089e92. [12] Wu C, Szymanski C, McNeill J. Preparation and encapsulation of highly fluorescent conjugated polymer nanoparticles. Langmuir 2006;22:2956e60. [13] Wu C, Szymanski C, Cain Z, McNeill J. Conjugated polymer dots for multiphoton fluorescence imaging. J Am Chem Soc 2007;129:12904e5.
[14] Wang L, Liu X, Yang Q, Fan Q, Song S, Fan C, et al. A colorimetric strategy based on a water-soluble conjugated polymer for sensing pH-driven conformational conversion of DNA i-motif structure. Biosens Bioelectron 2010;25:1838e42. [15] Batrakova EV, Kabanov AV. Pluronic block copolymers: evolution of drug delivery concept from inert nanocarriers to biological response modifiers. J Control Release 2008;130:98e106. [16] Kabanov AV, Lemieux P, Vinogradov S, Alakhov V. Pluronic® block copolymers: novel functional molecules for gene therapy. Adv drug Deliv Rev 2002;54:223e33. [17] Tian J, Chen H, Zhuo L, Xie Y, Li N, Tang B. A highly selective, cell-permeable fluorescent nanoprobe for ratiometric detection and imaging of peroxynitrite in living cells. Chem-Eur J 2011;17:6626e34. [18] Wang X, Guo Y, Li D, Chen H, Sun R-c. Fluorescent amphiphilic cellulose nanoaggregates for sensing trace explosives in aqueous solution. Chem Commun 2012;48:5569e71. [19] Zhang L, Zhao C, Zhou J, Kondo T. Fluorescent micelles based on hydrophobically modified cationic cellulose for sensing trace explosives in aqueous solutions. J Mater Chem C 2013;1:5756e64. [20] Hou Q, Xu Y, Yang W, Yuan M, Peng J, Cao Y. Novel red-emitting fluorenebased copolymers. J Mater Chem 2002;12:2887e92. [21] Huang F, Niu Y-H, Liu MS, Zhou X-H, Tian Y-Q, Jen AK-Y. Efficient ultravioletblue polymer light-emitting diodes based on a fluorene-based non-conjugated polymer. Appl Phys Lett 2006;89:173113e6. [22] Shen C, Sun Y, Wang J, Lu Y. Facile route to highly photoluminescent carbon nanodots for ion detection, pH sensors and bioimaging. Nanoscale 2014;6: 9139e47. lez MG, Niessner R, Panne U, Haisch C, Genger UR. Determi[23] Würth C, Gonza nation of the absolute fluorescence quantum yield of rhodamine 6G with optical and photoacoustic methodseProviding the basis for fluorescence quantum yield standards. Talanta 2012;90:30e7. [24] Pecher J, Mecking S. Nanoparticles of conjugated polymers. Chem Rev 2010;110:6260e79. [25] Wu C, McNeill J. Swelling-controlled polymer phase and fluorescence properties of polyfluorene nanoparticles. Langmuir 2008;24:5855e61. [26] Dias FB, Morgado J, Macanita AL, da Costa FP, Burrows HD, et al. Kinetics and thermodynamics of poly (9, 9-dioctylfluorene) b-phase formation in dilute solution. Macromolecules 2006;39:5854e64. [27] Sohn H, Sailor MJ, Magde D, Trogler WC. Detection of nitroaromatic explosives based on photoluminescent polymers containing metalloles. J Am Chem Soc 2003;125:3821e30. [28] Feng HT, Zheng YS. Highly sensitive and selective detection of nitrophenolic explosives by using nanospheres of a tetraphenylethylene macrocycle displaying aggregatio-induced emission. Chem-Eur J 2014;20:195e201. [29] Zhao D, Swager TM. Sensory responses in solution vs solid state: a fluorescence quenching study of poly(iptycenebutadiynylene)s. Macromolecules 2005;38:9377e84.
Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
F127/conjugated polymers fluorescent micelles for trace detection of nitroaromatic explosives Zicheng Liang a, Hui Chen a, Xiaohui Wang a, *, Runcang Sun a, b a b
State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou, 510640, PR China Institute of Biomass Chemistry and Technology, Beijing Forestry University, Beijing, 100083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 July 2015 Accepted 26 October 2015 Available online 14 November 2015
The cost-effective and rapid detection of nitroaromatic explosives is highly desired for growing con cern about the environmental pollution and terror threats. Herein, a facile sensing system for fast and trace detection of nitroaromatic explosives (e.g. picric acid (PA) and 2,4-dinitrotoluene (DNT)) in aqueous solution is developed. In this system, the self-assembled nanomicelles of commercial available Pluronic F127 are selected as vesicles and fluorescent conjugated polymers poly(9,90 -dioctylfluorene) (PFO) and poly(2,7-(9,9-hexylfluorene)-alt-4,40 -phenylether) (PFPE) are used as sensing materials. Through the microextraction effect of F127 nanomicelles, the hydrophobic conjugated polymers can be well dispersed in aqueous media and effectively quenched by trace amount of PA or DNT based on electron transfer mechanism. In compare with the conjugated polymers dissolved in organic solvents, the conjugated polymers encapsulated in F127 nanomicelles show dramatically enhanced sensing selectivity and sensitivity. The current nitroaromatic explosives sensing system is versatile. These results suggest that the F127/conjugated polymers system is a feasible method for detecting nitroaromatic explosives in water. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Conjugated polymers F127 self-assembled nanomicelles Fluorescence quenching Explosives detection Electron transfer Microextraction effect
1. Introduction Nitroaromatic explosives, such as picric acid (PA), trinitrotoluene (TNT) and 2,4-ditrotoluene (DNT), are high-toxic pollution in soil and water and the important ingredient for military issues [1,2]. In recent years, the detection of nitroaromatic explosives has attracted widespread concerns as a consequence of increasing terrorist events and environmental pollution [3]. Currently, available detection strategies of nitroaromatic explosives include ion mobility spectrometry (IMS) [4], surface enhanced Raman scattering (SERS) [5], chromatographic analysis [6], mass spectroscopy [7] and so on. However, most of them are restricted by sophisticated instruments and time-consuming sample pretreatment process. Fluorescence sensing method is perceived as a promising alternative method due to their high sensitivity, short response time and portability since Swager and coworkers firstly reported the application of fluorescent probes in explosives detection in 1998 [8].
* Corresponding author. Tel.: þ86 20 8711 1745; fax: þ86 20 87111861. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.dyepig.2015.10.045 0143-7208/© 2015 Elsevier Ltd. All rights reserved.
Conjugated polymers (CPs) are commonly used sensing materials because of their strong fluorescence and abundant active sites [9,10]. However, the hydrophobicity of them badly limits their practical applications [8,11]. In recent years, numerous waterdispersible fluorescence sensing systems have been designed to overcome the drawback. Wu et al. developed water-dispersible conjugated polymer nanoparticles by mini-emulsion or reprecipitation methods [12,13]. Huang and coworkers constructed a water-soluble conjugated polymer for DNA sensing by a colorimetric strategy [14]. Nevertheless, stable, versatile and sensitive aqueous sensing methods for nitroaromatic explosives still remains a challenge. In recent years, amphiphilic polymers have been regarded as promising carriers in hydrophobic drug delivery and genetic therapy [15,16]. As hydrophobic fluorescent conjugated polymer molecules have similar properties to some hydrophobic drugs, they might be encapsulated in the hydrophobic cores of amphiphilic polymeric micelles. It has been reported that polymer nanomicelles were used to construct fluorescence probes for application in medicine, biology, material chemistry and sensing [17]. In our previous study, we had successfully utilized self-assembled amphiphilic cellulose micelles to encapsulate hydrophobic
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conjugated polymers for sensing nitroaromatic explosives in water phase [18]. Zhang et al. followed this work and realized the trace detection of explosives using amphiphilic cellulose micelles encapsulating hydrophobic dye [19]. However, these cellulose derivatives are not commercialized and require tedious preparation process. Pluronic F-127, a triblock copolymers composed of poly(oxyethylene)-block-poly(oxypropylene)-blockpoly(oxyethylene), is a non-toxic, non-irritant, low-cost and well commercialized macromolecular surfactant. It can self-assemble into nanomicelles in aqueous solution at relatively high solubility due to the long PPO block and high molecular weight. In this work, we chose the self-assembled micelles of commercial F127 as carriers and commercial available conjugated polymers as sensing materials to construct an aqueous sensing system for nitroaromatic explosives. This work would provide an alternative approach for ultrasensitive detection of nitroaromatic explosives in aqueous media by employing widely available materials and facilitate the commercialization of fluorescent sensing in explosives detection. 2. Experimental section 2.1. Materials Triblock copolymers PEO100-PPO65-PEO100 (F127, Mw ¼ 12,600) were purchased from Sigma Co. Ltd. Fluorescent conjugated polymers poly(9,9’-dioctylfluorene) (PFO, Mw ¼ 38,131, PDI ¼ 2.9) and poly(2,7-(9,9-hexylfluorene)-alt-4,40 -phenylether) (PFPE, Mw ¼ 66,317, PDI ¼ 1.8) were synthesized according to literature methods (structures shown in Fig. 1) [20,21]. Nitroaromatic explosives picric acid (PA) and 2,4-dinitrotoluene (DNT) were purchased from local commercial suppliers. Analogue methyl benzene (MB) was used as contrast. Other reagents in this work like tetrahydrofuran (THF) and ethyl alcohol (EtOH) were analytically pure and used without any purification. Deionized water was used throughout the whole experiment.
Here, ME was the mass of the encapsulated CPs in micelles and M0 was the mass of the total adding amounts. A Fluorolog-3 spectrometer (Horiba Jobin Yvon, France) was used to examine the fluorescence properties of the conjugated polymers in THF or in F127 micelles, with 335 nm excitation wavelength and 365e600 nm emission range. The slit widths for excitation and emission were 5 nm for F127/PFO, and 3 nm for F127/PFPE. The quantum yields of PFO and PFPE were calculated by equation (2) [22], with rhodamine 6G (QY ¼ 92% in ethyl alcohol) as standard [23].
F ¼ n2 Aref FFref
. n2ref AFref
(2)
Here, n is the refractive index of solvent, A is the absorbance at corresponding excitation wavelength, F is the integrated intensity of emission, and F was the value of quantum yield. 2.3. Preparation of F127/conjugated polymers fluorescent micelles F127 micelle solution was prepared by dissolving F127 in deionized water in desired concentrations. Then PFO/THF solution or PFPE/THF solutions were carefully dropped into F127 micelle solution under magnetic stirring. The mixtures were further treated with ultrasonic for 1 h to promote the encapsulation process. THF in the micelle solution was removed by vacuum distillation. The resultant F127/PFO and F127/PFPE micelle solution were further filtrated by 0.45 mm Millipore filters to remove insoluble. 2.4. Detection of nitroaromatic explosives with F127/CPs micelles Initially, 2.5 mL F127/PFO or F127/PFPE micelle solution was added in quartz colorimetric utensils (1 cm length). Picric acid (PA) or 2,4-dinitrophenol (DNT) ethanol solution was added to the micelle solution in desired concentrations. Then the fluorescence spectra were scanned after equilibrium for 10 s. 3. Results and discussion
2.2. Characterizations 3.1. Self-assembly of F127 nanomicelles Transmission electron microscopy (TEM) images were taken on a JEM-2100F (JEOL, Japan) electron microscope at 200 kV accelerating voltage. Scanning electron microscopy (SEM) images were taken on an S-3700N (Hitachi, Japan) electron microscope. Dynamic light scattering (DLS) measurements were measured on a Malvern 3000 HSA instrument (Malvern, UK). A UV-3600 (Shimadzu, Japan) spectrometer was used to determine the encapsulation efficiencies (EE) for the micelles, following equation (1).
EEð%Þ ¼ ðME =M0 Þ 100%
(1)
Driven by the hydrophobic interactions between the PPO segments, Pluronic F127 can self-assemble into core-shell-like nanomicelles in aqueous solution. As the TEM image shown in Fig. 2a, the self-assembled F127 nanomicelles were well-dispersed and show uniform spherical shape with the diameters varying in the range of 27e50 nm. DLS was further used to determine the hydrodynamic size and size distribution of the self-assembled F127 nanomicelles in aqueous media as shown in Fig. 2b. According to which the average particle size of F127 was 54.8 nm with a polydispersity indexes (PDI) of 0.357. The particle size determined by DLS was slightly higher than that determined by TEM, probably due to the drying shrinkage of the hydrophilic micelle shells during the sample treatment process for TEM analysis. 3.2. F127/conjugated polymers fluorescent nanomicelles
Fig. 1. Molecular structures of the sensing materials PFO (a), PFPE (b), and the quenchers PA (c), DNT (d) and MB (e).
Hydrophobic conjugated polymers (PFO or PFPE) can be encapsulated in F127 nanomicelles through the microextraction effect. The absorption and emission spectra of PFO and PFPE are illustrated in Fig. 3. PFO presented a maximum absorption peak at 385 nm in THF. In the emission spectra of PFO, there are a primary emission peak at 419 nm and two shoulder peaks at 442 nm and 476 nm. After being encapsulated in F127 micelles, obvious spectral changes were observed in both of the absorption and emission spectra of PFO. The absorption peak of the PFO in F127
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Fig. 2. A TEM image of F127 nanomicelles (a); Size distribution from DLS analysis (b).
nanomicelles blue-shifted to 380 nm and a weak absorption peak at 432 nm arose. The weak absorption at 432 nm may ascribe to the partially aggregation or formation of b-crystallization of PFO in micelles [24,25]. Meanwhile, the emission peaks of PFO in F127 nanomicelles red-shifted to 439 nm, 467 nm and 496 nm, respectively. The red shifts should be attributed to the spatial confinement effect of the nanomicelles. Such spatial confinement effect can enhance the intra-molecular and inter-molecular interactions between the entrapped conjugated polymers. As a consequence, the aggregative states and energy levels of the fluorophores are altered and finally form partial crystalline states [26]. PFPE had an absorption peak located at 340 nm and two emission peaks at 375 nm and 393 nm in THF. After being encapsulated by F127 nanomicelles, the absorption peak of PFPE slightly red-shifted to 345 nm and no shoulder peak was observed indicating aggregation was formed. Similar to PFO, the emission peaks of PFPE in F127 nanomicelles
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red-shifted to 400 nm and 419 nm, respectively. These red shifts should be ascribed to the intra-chain energy transfer in F127 nanomicelles. To investigate the encapsulating capacity of F127 micelles towards PFO and PFPE, different amounts of conjugated polymers were encapsulated in different concentrations of F127 nanomicelles (0.01, 0.1 and 1 mg/mL). Fig. 4 displayed the encapsulation efficiencies (EEs) and the loading amounts of PFO and PFPE in the F127 nanomicelles solutions with different concentrations. Initially, the EEs of F127 nanomicelles in a certain concentration increased with the concentration of PFO or PFPE till their concentration increased to 10 mg/mL. Then the EEs began to decrease along with continuous addition of PFO or PFPE. However, the loading amounts were increasing with the total amounts of PFO or PFPE in feed. These results suggested that in low concentration conjugated polymers solution, the adding amount of PFO or PFPE is the predominant factor determining the encapsulating capacity of F127 nanomicelles. While in higher concentration conjugated polymers solution, the solubilization effect of the nanomicelles' hydrophobic cores gradually reaches a saturation. Then excess conjugated polymers molecules began to precipitate in aqueous solution because they were unable to access into the nanomicelles. As a result, the EEs of F127 nanomicelles decreased at high concentration of PFO or PFPE. At the same concentration of conjugated polymers, F127 nanomicelles had relatively higher EEs for PFO because PFO has smaller molecular size. The quantum yields (QYs) of PFO and PFPE in F127 nanomicelles were exhibited in Fig. 5. At a certain concentration of F127 nanomicelles, the highest QYs of PFO or PFPE were obtained at the concentration of 10 mg/mL, which is corresponding to the highest encapsulation efficiencies. When the conjugated polymers concentration was over 10 mg/mL, the QYs drastically decayed with their increasing concentration. This trend became more pronounced in the solutions with higher micelles concentrations. A possible explanation is the self-aggregation of the conjugate polymers in a relative high concentration, which could result in fluorescence self-quenching. The sizes and corresponding PDI of F127 nanomicelles after encapsulating PFO or PFPE were showed in Table 1. The sizes of the fluorescent nanomicelles were much larger than the blank one and they became even larger with increasing loading amount of the conjugated polymers. After encapsulating PFO or PFPE, the enlargement of the hydrophobic cores would weaken the intermolecular interactions of the hydrophilic shells. Therefore, the more molecules being encapsulated, the larger sizes of the nanomicelles were obtained. The nanomicelles encapsulating PFPE had relatively larger sizes than the ones encapsulating PFO because
Fig. 3. UVevis and emission spectra of PFO (a) and PFPE (b) in THF (black line) and in F127 nanomicelles (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Encapsulation efficiencies and loading amounts of PFO (a, b) and PFPE (c, d) in F127 nanomicelles.
PFPE has a larger molecular size. Moreover, the PDIs of the nanomicelles were quite low (<0.2), implying relatively uniform size distribution. The SEM images of the nanomicelles encapsulating PFO and PFPE were given in Fig. 6. After encapsulating conjugated polymers, F127 nanomicelles maintained uniform spherical morphology and were still well-dispersed. The diameter distribution of the nanomicelles determined by SEM were in the range of 100e150 nm substantially in agreement with the DLS results.
Fig. 7 showed the size stability and photostability of the conjugated polymers encapsulated in F127 nanomicelles within 15 days' storage. No obvious aggregation of fluorescent nanomicelles was observed. The average sizes of the fluorescent nanomicelles became slightly larger with the prolonging of storage time and increased about 7% after two weeks. The QYs of the fluorescent nanomicelles had negligible change within 8 days' storage at room temperature, while obvious decays were observed after 8 days.
Fig. 5. Quantum yields of PFO (a) and PFPE (b) in F127 nanomicelles.
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Table 1 Sizes (nm) with PDI values (in parenthesis) of F127 micelles loaded with different concentration of PFO and PFPE. F127 (mg/mL)
0.01 0.1 1
PFO (mg/mL)
PFPE (mg/mL)
5
10
50
100
5
10
50
100
118.6 (0.087) 108.6 (0.060) 105.6 (0.069)
127.8 (0.095) 119.7 (0.167) 114.9 (0.209)
135.4 (0.181) 129.8 (0.074) 127.2 (0.091)
141.6 (0.079) 133.0 (0.006) 131.2 (0.109)
102.9 (0.133) 101.0 (0.129) 99.6 (0.105)
118.3 (0.101) 109.9 (0.111) 105.8 (0.103)
129.7 (0.070) 117.0 (0.074) 111.0 (0.159)
137.4 (0.057) 127.6 (0.083) 125.8 (0.085)
These results indicated that the fluorescent micelles can endure short-term storage for commercialized use. 3.3. Detection of nitroaromatic explosives with F127/conjugated polymers fluorescent nanomicelles A schematic diagram for nitroaromatic explosives detection in aqueous media with F127/PFO or F127/PFPE nanomicelles was illustrated in Scheme 1. Our aqueous sensing system for nitroaromatic explosive is based on the electron transfer mechanism. As nitroaromatic explosives are well known electron-deficient molecules, they are able to serve as electron acceptors. The nitro moieties on the benzene ring can capture the photo-induced electrons from the conjugated polymers, resulting in fluorescence quenching of the electron-rich fluorophores [1,2]. Therefore, F127/PFO or
Fig. 6. SEM images of PFO (a) and PFPE (b) after being encapsulated in F127 nanomicelles.
F127/PFPE nanomicelles were used to detect nitroaromatic explosives through monitoring their fluorescence quenching. Due to their hydrophobicity, nitroaromatic explosives can easily access into the hydrophobic cores through microextraction effect. In the nanomicelles, the nitroaromatic explosives (acceptors) and the conjugated polymers (donors) are close to each other to generate effective electron transfer. The spatial confinement effect of the nanomicelles endowed the high sensitivity and selectivity in our sensing system. The fluorescence quenching by nitroaromatic explosives were evaluated by continuously adding different concentrations of picric acid (PA) or 2,4-dinitrotoluene (DNT) into the PFO or PFPE -containing F127 nanomicelles aqueous solutions. To examine the sensitivity and selectivity, the fluorescence quenching of PFO and PFPE in THF were used as contrast. Fig. 8 and Fig. 9 displayed the quenching spectra of PFO and PFPE in F127 nanomicelles or in THF. PFO and PFPE in THF were significantly quenched by PA at 0.49 mM and 0.65 mM after 10 s' equilibrium. However, comparable quenching of PFO and PFPE in F127 nanomicelles can be observed when respectively adding only 0.025 mM and 0.015 mM PA after the same equilibrium time. These results indicated that the fluorescent nanomicelles have much higher sensitivity than those conjugated polymers in THF. Similar quenching can be observed in DNT detection. In THF, PFO and PFPE were significantly quenched by DNT in 4.17 mM and 2.76 mM. While in F127 nanomicelles, only 0.17 mM and 0.56 mM DNT could reach the comparable quenching effect. Both F127/PFO and F127/PFPE nanomicelles show much higher selectivity for PA. The high selectivity should be attributed to the lower unoccupied orbital energy of PA molecules thus endow them higher electron efficiency [11,27]. Methyl benzene (MB) was selected as contrast because it has similar benzene ring structure with PA and DNT but no electron-deficient nitro moiety. As expected, no obvious fluorescence quenching could be observed after adding large amount of MB. Fig. 10 illustrated the SterneVolmer quenching plots of the conjugated polymers in THF and in F127 nanomicelles. The SeV plot of PFO for PA in F127 nanomicelles was exponentially growing in the range of 0e0.0253 mM, which is attributed to the combined quenching process involving both static and dynamic quenching. While the SeV plot in THF was linear, same as for DNT in THF and in F127 nanomicelles. These linear SeV plots indicated only static or dynamic quenching existing in their quenching process. However, significant differences could be seen in the SeV plots of PFPE. The SeV plots of PFPE for PA and DNT in F127 nanomicelles was linear in the range of 0e0.0147 mM, while presented exponentially growing plots in THF. The SeV quenching constants of PFO and PFPE for nitroaromatic explosives were calculated by Stern-Volmer equation [28]:
I0 =I ¼ 1 þ Ksv ½Q ðLinearÞ
(3)
I0 =I ¼ eKsv ½Q ðExponentialÞ
(4)
where, I0 and I are the fluorescence intensity in absence/presence of the quencher, [Q] is the molar concentration of nitroaromatic explosives, and Ksv is the SeV quenching constant in M1. The Ksv of
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Fig. 7. Size stability (a) and photostability (b) of F127/PFO and F127/PFPE nanomicelles.
Scheme 1. Formation of fluorescent PFO or PFPE -containing F127 nanomicelles and their detection application of nitroaromatic explosives in aqueous media.
Fig. 8. Quenching spectra of PFO in F127 nanomicelles (aec) and in THF (def) with different concentrations of PA, DNT and MB (lex ¼ 335 nm).
PFO and PFPE for nitroaromatic explosives were given in Table 2. The Ksv of PFO and PFPE were quite low in THF, in which the Ksv for DNT were close to Swager's report (230 M1) [29]. After being encapsulated in F127 nanomicelles, the Ksv of PFO and PFPE dramatically increased. These results demonstrated that F127 nanomicelles effectively enhanced the detection selectivity and sensitivity of the conjugated polymers by the microextraction and spatial confinement effect.
4. Conclusions In summary, a sensing system for nitroaromatic explosive in aqueous solution is developed using commercial available polymeric surfactant Pluronic F127 as vesicles and light-emitting conjugated polymers as sensing materials. The microextraction and spatial confinement effect of F127 nanomicelles could maximize the electron transfer between PFO/PFPE and nitroaromatic
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Fig. 9. Quenching spectra of PFPE in F127 nanomicelles (aec) and in THF (def) with different concentrations of PA, DNT and MB (lex ¼ 335 nm).
Fig. 10. SterneVolmer plots of PFO (aec) and PFPE (def) for PA, DNT and MB in F127 nanomicelles and in THF.
Table 2 SterneVolmer constants for fluorescence quenching of PFO and PFPE by PA, DNT and MB. Sample
PA (M1)
DNT (M1)
MB (M1)
PFO in THF PFPE in THF PFO in F127 PFPE in F127
1422 1751 75,048 59,984
213 257 8305 2845
31 32 55 61
explosives, thereby achieving higher sensitivity and selectivity for explosives detection compared to those in organic media. By micelle encapsulation, the application of the hydrophobic conjugated polymers is extended to rapid, sensitive and selective detection for explosives in aqueous media. As our sensing system is constructed with commercial products, it is potentially expected to large-scaled utilization.
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Acknowledgements This work was supported by the grants from the Science and Technology Program of Guangzhou, China (2014J4100039), the New Century Excellent Talents in University (NCET-13-0215), the Fundamental Research Funds for the Central Universities, SCUT (201522036) and the Opening Project of the Key Laboratory of Polymer Processing Engineering, Ministry of Education, China (KFKT-201401). References [1] Germain ME, Knapp MJ. Optical explosives detection: from color changes to fluorescence turn-on. Chem Soc Rev 2009;38:2543e55. n ~ ez R, Marcos MD, Sanceno n F, Costero AM, Parra M, [2] Salinas Y, Martínez-Ma et al. Optical chemosensors and reagents to detect explosives. Chem Soc Rev 2012;41:1261e96. [3] Ma Y, Wang S, Wang L. Nanomaterials for luminescence detection of nitroaromatic explosives. TrAC-Trends Anal Chem 2015;65:13e21. [4] Eiceman G, Stone J. Peer reviewed: ion mobility spectrometers in national defense. Anal Chem 2004;76:390Ae7A. [5] Sylvia JM, Janni JA, Klein J, Spencer KM. Surface-enhanced Raman detection of 2, 4-dinitrotoluene impurity vapor as a marker to locate landmines. Anal Chem 2000;72:5834e40. [6] Walsh ME. Determination of nitroaromatic, nitramine, and nitrate ester explosives in soil by gas chromatography and an electron capture detector. Talanta 2001;54:427e38. [7] Mathurin J, Faye T, Brunot A, Tabet J, Wells G, Fuche C. High-pressure ion source combined with an in-axis ion trap mass spectrometer. 1. Instrumentation and applications. Anal Chem 2000;72:5055e62. [8] Yang J-S, Swager TM. Fluorescent porous polymer films as TNT chemosensors: electronic and structural effects. J Am Chem Soc 1998;120:11864e73. [9] Thomas SW, Joly GD, Swager TM. Chemical sensors based on amplifying fluorescent conjugated polymers. Chem Rev 2007;107:1339e86. [10] Fernando LP, Kandel PK, Yu J, McNeill J, Ackroyd PC, Christensen KA. Mechanism of cellular uptake of highly fluorescent conjugated polymer nanoparticles. Biomacromolecules 2010;11:2675e82. [11] Xu B, Wu X, Li H, Tong H, Wang L. Selective detection of TNT and picric acid by conjugated polymer film sensors with donoreacceptor architecture. Macromolecules 2011;44:5089e92. [12] Wu C, Szymanski C, McNeill J. Preparation and encapsulation of highly fluorescent conjugated polymer nanoparticles. Langmuir 2006;22:2956e60. [13] Wu C, Szymanski C, Cain Z, McNeill J. Conjugated polymer dots for multiphoton fluorescence imaging. J Am Chem Soc 2007;129:12904e5.
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