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Surface plasmon resonance-enhanced photothermal nanosensor for sensitive and selective visual detection of 2,4,6-trinitrotoluene
Sensors and Actuators B 237 (2016) 224–229
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Surface plasmon resonance-enhanced photothermal nanosensor for sensitive and selective visual detection of 2,4,6-trinitrotoluene Xilin Bai, Suying Xu, Gaofei Hu, Leyu Wang ∗ State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Environmentally Harmful Chemical Analysis, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 26 February 2016 Received in revised form 14 June 2016 Accepted 15 June 2016 Available online 17 June 2016 Keywords: Nanosensor Explosives detection Visualized sensing Surface plasmon resonance (SPR)
a b s t r a c t The exploration of feasible, reliable sensors for the selective and sensitive detection of nitroaromatic explosives is a critical issue nowadays as mandated by homeland security concerns. Our endeavor is to develop an instant on-site visual and ultra-sensitive photothermal nanosensor for 2,4,6-trinitrotoluene (TNT) detection, which could be applied on a broad scale with high selectivity and sensitivity. The fundamental principle of our work is that TNT could effectively induce the aggregation of the cysteaminestabilized gold nanoparticles (Au NPs) by forming the Meisenheimer complex, which further enhanced the surface plasmon resonance (SPR) properties and the photothermal effects of Au NPs. Other coexisting nitroaromatics have negligible influences on the photothermal responses. By means of the temperature enhancement and photothermal imaging technique, the presence of TNT could be readily recognized and visualized, indicating wide potential applications in analytical sensing. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Rapid, sensitive and selective detection of nitroaromatic explosives such as 2,4,6-trinitrotoluene (TNT) is always desirable due to their great threat to the global security and human health [1–6]. To date, various methods have been developed for selective recognition of nitroaromatics, of which the optical sensing strategy holds great potentials in fabricating ultra-high sensitive probes. Taking advantages of nanotechnology, the development of nanomaterials with unique luminescent properties, such as semiconductor quantum dots (QDs) [5,7–11], lanthanide-doped nanocrystals [12–16], noble metal and carbon-based nanomaterials [17,18], organic dye-based nanomaterials [19,20] and metal-organic frameworks (MOFs) [21], has opened up new horizon for detection of nitroaromatics [22]. Yet, it is still intriguing to exploit a facial and versatile nanoplatform for the rapid and selective detection of TNT. The surface plasmon resonance (SPR) nanostructures have been widely investigated for enhanced catalysis and tumor ablation [23–29]. SPR-enhanced luminescence has also drawn wide attention for sensitive detection of small molecules [15,30]. In addition, noble metal nanostructure based-SPR and surface-enhanced Raman scattering (SERS) [10,31] have been extensively explored for constructing
∗ Corresponding author. E-mail address: [email protected] (L. Wang). http://dx.doi.org/10.1016/j.snb.2016.06.093 0925-4005/© 2016 Elsevier B.V. All rights reserved.
optical biosensors, especially for label-free assays [32–38]. Yet, direct employment of SPR spectroscopy could be problematic since only those analytes with high molecular weight could afford observable SPR shifts. Alternatively, the SPR responses induced by small molecules need to be either amplified through some strategies or transformed into other types of signals that could be read out easily [39]. In recent decades, nanomaterials with photothermal effect have received wide attention for their applicability in photothermal therapy and photothermal imaging [29,40–45]. In particular, SPR-induced photothermal responses have gained substantial interests across various disciplines [44,46,47]. Notably, for the near-infrared light responsive photothermal agents, the consequent temperature changes are mainly originated from the photothermal effect of the photothermal agents, thus displaying low background signals from the external environment and possessing high sensing sensitivity. Despite of the intrinsic high sensitive properties, the photothermal agents have rarely been utilized for sensing purposes. To date, our group has utilized the photothermal properties of polyaniline to successfully fabricate a photothermal sensor for selectively detection of nitroaromatics, which has demonstrated its great applicability [48]. The coinage metal-based nanostructures such as gold nanoparticles (AuNPs) are frequently utilized in designing sensors [49] owing to their localized SPR properties. Since the SPR changes of AuNPs arising from recognition event with analytes could also induce a color change, which has successfully been
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Scheme 1. Schematic illustration of SPR-enhanced photothermal nanosensor based on TNT-induced aggregation of AuNPs.
utilized for constructing nitroaromatic sensors [18,50]. However, there is no platform that makes use of the SPR-enhanced photothermal effect of these nanomaterials for the highly selective and sensitive detection of TNT. Here, for the first time we explored the possibility of developing a photothermal nanosensor with superior selectivity and in situ detection of TNT under the assistance of SPR-enhanced photothermal effect. In this work, the cysteamine-stabilized AuNPs were prepared, which were used to form the Meisenheimer complex with electron-deficient TNT through charge-transfer processes [10,12,22,48]. Such complexation further induces the aggregation of gold nanoparticles, resulting in a remarkable photothermal enhancement upon irradiation at 650 nm (Scheme 1). More importantly, the temperature of the Au NPs/TNT solution increases proportionally along with the increase of TNT concentration, which were scarcely affected by other nitroaromatics such as 2,4,6-trinitrotophenol (TNP), 2,4-dinitrotoluene (DNT), nitrobenzene (NB), toluene (PhMe) and benzene (Ph). In addition, the temperature changes induced by TNT could also be visualized through photothermal imaging technique. Therefore, a novel SPR enhanced photothermal nanosensor for on-site visual and selective detection of TNT has been successfully developed. 2. Experimental 2.1. Reagents 2,4,6-Trinitrotoluene (TNT) and 2,4,6-trinitrotophenol were supplied by the National Security Department of China and were recrystallized with ethanol before use. For safety considerations, all of the explosives must be kept away from fire, striking and friction, and they should be handled carefully. 2,4-Dinitrotoluene, nitrobenzene toluene and benzene were purchased from the Aladdin Chemistry Co. Ltd. These nitroaromatics were dissolved in mixed solvents of ethanol and acetonitrile (volume ratio of 4:1) to obtain the stock solution. In brief, 3.0 mg of each of nitroaromatics was dissolved in 15.0 mL of the mixture solvent containing ethanol (12.0 mL) and acetonitrile (3.0 mL) to get the stock solution with a final concentration of 0.2 mg/mL. All other reagents were analytical grade and used as received without further purification. Deionized (DI) water was used throughout. Cysteamine was purchased from Biological Science and Technology Co. Thiolated poly (ethylene glycol) (SH-mPEG, Mw = 5000) was purchased from Beijing Kaizheng Biological Engineering Development Co. Ethanol, acetonitrile, NaOH, NaHCO3 , Na2 CO3 , NaAc, HAc, Na2 HPO4 , NaH2 PO4 , NaF and HAuCl4 ·2H2 O were received from Beijing Chemical Company (China). 2.2. Apparatus The size and morphology of AuNPs were characterized by the H-800 transmission electron microscope (TEM). Dynamic
Fig. 1. TEM images (a, b) and DLS size distribution (c, d) of AuNPs-NH2 before (a, c) and after (b, d) addition of TNT (4.0 g/ mL).
light scattering (DLS) particle size was measured by a Zetasizer Nano-ZS90 (Malvern) size analyzer. The absorption spectra were conducted on a Shimadzu UV-3600 spectrophotometer with a spectral window range of 200–3600 nm. The thermal imaging and temperature evolution plots were performed on a FLIR-A600 infrared (IR) camera. 2.3. Synthesis of amine-stabilized AuNPs In brief, 1.0 mL of cysteamine (6.5 mM) was added to 40 mL of HAuCl4 (2.0 mg) solution (100 ◦ C) under magnetic stirring. Then 1.0 mg SH-mPEG was added to improve the stability of the assynthesized Au NPs. The solution was kept boiling for 15 min till the solution turned into rose color. Finally, the NH2 moiety functionalized hydrophilic gold nanoparticles (AuNPs-NH2 ) were obtained after centrifugation and then redispersed into 3.0 mL of deionized water for later use. 2.4. Photothermal detection of TNT in solution The amine-functionalized gold nanoparticles (AuNPs-NH2 ) were obtained according to the previously published method [15]. Briefly, 50.0 L of various concentrations of TNT or other nitroaromatics was mixed with 300 mL of the as-prepared AuNPs-NH2 . And then the mixed solution was diluted to 0.5 mL with NaH2 PO4 Na2 HPO4 (pH = 8.0, 20 mM) before being exposed to the laser irradiation. The photothermal tests were carried out with an IR camera to monitor and record the temperature changes while keeping the solution exposed to irradiation light at 650 nm with a power density of 2.25 W/cm2 . 3. Results and discussion 3.1. Morphology and photothermal properties of AuNPs-NH2 NPs as photothermal nanosensors In this study, cysteamine was used both as the electron donor to interact with electron-deficient nitroaromatics and surface ligands to stabilize the as-prepared AuNPs. As shown in Fig. 1a and c, initially, the AuNPs-NH2 was well dispersed in deionized water
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Fig. 2. (a) Temperature increment (T) of AuNPs-NH2 with different concentrations under irradiation (650 nm) with a power density of 2.25 W/cm2 . Here, T = T − T0 , where T0 and T represent the temperature before and after irradiation, respectively. (b) Temperature increment of AuNPs-NH2 colloidal solution before and after adding TNT (4.0 g/mL) under different pH conditions. Buffer solutions were prepared as follows: pH (4.0 5.0, HAc-NaAc, 20 mM); pH (6.0–8.0, NaH2 PO4 -Na2 HPO4 , 20 mM); pH (9.0–11.0, NaHCO3 -Na2 CO3 , 20 mM), pH (12.0, NaH2 PO4 -NaOH, 20 mM).
with an average size of 51.4 ± 8.0 nm. Upon addition of TNT into the colloidal solution, an obvious aggregation of gold NPs appeared as a result of the formation of the Meisenheimer complex with electrondeficient TNT through charge-transfer processes [10,12,22,48], and the DLS size of particles increased to about 240.3 ± 12.4 nm (Fig. 1b, d). Additionally, the changes of UV–vis spectra also demonstrated the aggregation of gold NPs (Fig. S1 in the Supporting information). Along with the addition of TNT, the absorption at 570 nm increased accordingly. This phenomenon powerfully verifies our conception of forming the Meisenheimer complex between TNT and amine-functionalized AuNPs. Furthermore, to explore the optimal conditions for constructing photothermal nanosensor, the amount of Au NPs as well as the pH value was optimized. It is of noted that AuNPs-NH2 have maximum absorption at about 570 nm and the absorption at 650 nm increased accordingly with the addition of TNT. Actually, due to the SPR effects, if the 570 nm laser was adopted, the photothermal effects would be better than that irradiated by 650 nm laser. However, the 570 nm laser was not available, and a 650 nm diode laser was chosen instead. Upon irradiation at 650 nm (2.25 W/cm2 ), the temperature increased along with the irradiation time. In addition, it was easily found that the temperature increased more rapidly with the increment of AuNPs-NH2 concentration in the absence of TNT (Fig. 2a). When the concentration of AuNPs-NH2 was 100 mg/mL, the temperature increment increased up to 8.1 ◦ C within 3 min after irradiation. The influence of AuNPs-NH2 concentration on the temperature increment (T) in the presence of TNT was also investigated. Here, T = T − T0 , T and T0 represent the temperature before and after irradiation with laser, respectively. As shown in Fig. S2, by fixing the TNT concentration (4.0 g/mL), the largest T difference (T2 − T1 ) was obtained with 60 mg/mL of AuNPs-NH2 . Here, T2 and T1 represent the temperature increment (T) in the presence and absence of TNT, respectively. On the other hand, the temperature increment of AuNPs-NH2 solution under different pH conditions was also investigated, which indicated that the value of T follows similar trend, regardless the presence or absence of TNT. As shown in Fig. 2b, the value of (T2 − T1 ) increased slightly
Fig. 3. (a) Temperature increment of AuNPs-NH2 (60 mg/mL) in the presence of various concentrations of TNT under irradiation of 650 nm with a power density of 2.25 W/cm2 recorded by an IR camera; (b) Calibration plot of temperature increment (T) versus TNT concentrations.
in the pH range of 4.0–8.0 and decreased a little when the pH value varied from 8.0 to 12.0, suggesting that it is most suitable for charge transfer process at pH of 8.0. Therefore, the following photothermal detection of TNT was carried out at pH of 8.0.
3.2. Photothermal detection and linear range To verify the feasibility of the AuNPs-NH2 /TNT system as potential photothermal sensor, various concentrations of TNT were added to the solutions containing fixed amount of AuNPs-NH2 , respectively. Upon laser irradiation (650 nm, 2.25 W/cm2 ), the T of the resulting solution increased rapidly within 3.0 min (Fig. 3a), which demonstrated that the photothermal effect had been enhanced along with the increase of TNT from 0 to 1.0 g/mL. It is known that the electron withdrawing properties of nitro groups on the TNT molecule render it highly electron-deficient, which is prone to form Meisenheimer complex with electron-rich moieties such as amine through charge-transfer process [8,15,48]. Thus, the TNT molecules in the solution would act as bridges to pull together the cysteamine functionalized AuNPs, leading to the aggregation of the AuNPs-NH2 [7] . Accordingly, the resulting aggregation clusters of AuNPs in turn enhance the photothermal effect, leading to the enlargement of the temperature differences. In addition, by plotting the temperature increment (T) against to the concentration of TNT, a well linear fitting was obtained in the range of 0.1–1.0 g/mL with a calibration function of T = 4.383 + 1.642C (Fig. 3b). Here, T is the temperature difference (◦ C) and C is the concentration of TNT (g/mL). In addition, our nanosensor displayed a low limit of detection (LOD = 3/K) of 78 ng/mL, herein, is the standard deviation of the blank measurements (n = 6), and K is the slope of the calibration curve. Meanwhile, when under the same conditions, in the range of 1.0–10.0 g/mL, it also showed good linear relationship (R2 = 0.9947) with a calibration function of T = 5.544 + 0.410C and a LOD of 0.9 g/mL (Fig. S3). The low limit of detection and high sensitivity convincingly confirmed that our photothermal nanosensor is suitable for TNT detection.
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Fig. 5. Chemical structures of all the tested compounds and the temperature changes of the AuNPs-NH2 colloidal solution in the presence of the tested compounds (5.0 g/mL) under irradiation (650 nm, 2.25 W/cm2 ). Inset are the corresponding photothermal images acquired after irradiation for 3.0 min.
Fig. 4. (a) The photos of the AuNPs-NH2 /TNT solution captured by a digital camera under daylight. (b) The photothermal images of the AuNPs-NH2 /TNT solution under irradiation at 650 nm for 3.0 min. The values on the tubes and images are the concentration of TNT being used (g/mL).
3.3. Visual detection and selectivity test Based on the color change of AuNPs-NH2 caused by the controllable change in their dispersion/aggregation states, Au NPs have already been widely used for colorimetric sensing [18,51–53]. However, in most cases where the concentration of analytes was very low, it was hard to accurately tell the color difference for sensitive visual detection. Therefore, developing a more reliable, feasible sensing platform is of significant importance. As shown in Fig. 4a, after adding various amounts of TNT, the optical photos of all the AuNPs-NH2 colloidal solution showed similar rose color, which can hardly be used to discriminate the presence of different amounts of TNT. Nevertheless, when placed under irradiation at 650 nm for 3.0 min, an obvious difference on the photothermal images was presented (Fig. 4b), offering a facile on-site sensing platform. It is worth noting that except for the high sensitivity, the photothermal imaging technique is ready to be carried out by a portable infrared camera, which holds great advantages in practical applications.
Furthermore, to illustrate the good selectivity of this nanosensor, the interference was also investigated and the results are shown in Fig. 5. Though all the interferences have similar chemical structures, after adding the same amount of them, no changes on temperature values as well as photothermal images were observed, same to that of control case where no TNT or interferences were added. However, in the presence of TNT, the temperature increment was higher and the photothermal image was brighter than that of other cases. To verify the adaptability of our photothermal nanosensor, we carried out the experiments for detection low amount of TNT while in the presence of high concentration of other interferences. As indicated in Fig. S4, even in the presences of interferences with 10 times higher than that of TNT, still give no obvious photothermal effect as compared to the controls. In addition, the presence of little amount TNT, still induce significant photothermal effect, that is, detection of TNT would not be affected by other interferences. These results demonstrate that highly selective detection of TNT could be rationally achieved through the proposed nanoplatforms. The comparison of our method with the related techniques for TNT detection has also been carried out and summarized in Table 1. The comparison results suggest that our nanosensor truly owns some advantages for the sensitive, selective and visual detection of TNT. 4. Conclusions In summary, for the first time we have developed a SPRenhanced photothermal nanosensor for on-site visual, selective, and sensitive detection of TNT. The TNT molecule could act as a linker to pull together the amine-rich AuNPs through the formation of Meisenheimer complex, leading to aggregation of AuNPs, which further enhanced the photothermal performance upon irradiation. Taking advantage of photothermal imaging techniques, subtle temperature changes induced by TNT could be directly visu-
Table 1 Comparison of our photothermal nanosensor and reported methods. Refs.
materials
Linear range (TNT)
LOD (TNT)
Visual detection
this work [15] [20] [48] [49]
AuNPs AuNPs tetrathiafulvalene-capped hybrid material polyaniline gold nanostar
0.4–40 M 0–32 M 10−3 –10−8 M 4–800 M 10−3 –10−7 M
0.31 M N/Aa 66 M N/Aa 0.72 M
yes no no yes no
a
Not available.
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alized, which prevails over conventional colorimetric methods in terms of sensitivity. Therefore, this proposed novel detection assay paves the way for constructing more photothermal nanosensors, and will definitely offer great opportunities in chemical and biochemical sensing platform. Acknowledgements This research was supported in part by the National Natural Science Foundation of China (Grant Nos. 21475007, 21275015 and 21505003). We also thank the support from the “Innovation and Promotion Project of Beijing University of Chemical Technology”, the “Public Hatching Platform for Recruited Talents of Beijing University of Chemical Technology, the High-Level Faculty Program of Beijing University of Chemical Technology (buctrc201507, buctrc201608) and BUCT Fund for Disciplines Construction and Development (Project No. XK1526)”. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.06.093. References [1] E.R. Goldman, I.L. Medintz, J.L. Whitley, A. Hayhurst, A.R. Clapp, H.T. Uyeda, et al., A hybrid quantum dot-antibody fragment fluorescence resonance energy transfer-based TNT sensor, J. Am. Chem. Soc. 127 (2005) 6744–6751. [2] H. Sohn, R.M. Calhoun, M.J. Sailor, W.C. Trogler, Detection of TNT and picric acid on surfaces and in seawater by using photoluminescent polysiloles, Angew. Chem. Int. Ed. 40 (2001) 2104–2105. [3] H. Sohn, M.J. Sailor, D. Magde, W.C. Trogler, Detection of nitroaromatic explosives based on photoluminescent polymers containing metalloles, J. Am. Chem. Soc. 125 (2003) 3821–3830. [4] H.B. Zhou, Z.P. Zhang, C.L. Jiang, G.J. Guan, K. Zhang, Q.S. Mei, et al., Trinitrotoluene explosive lights up ultrahigh raman scattering of nonresonant molecule on a top-closed silver nanotube array, Anal. Chem. 83 (2011) 6913–6917. [5] Y.S. Xia, L. Song, C.Q. Zhu, Turn-on and near-infrared fluorescent sensing for 2,4,6-trinitrotoluene based on hybrid (gold nanorod)-(quantum dots) assembly, Anal. Chem. 83 (2011) 1401–1407. [6] F. Akhgari, H. Fattahi, Y.M. Oskoei, Recent advances in nanomaterial-based sensors for detection of trace nitroaromatic explosives, Sens. Actuators B: Chem. 221 (2015) 867–878. [7] W.C.W. Chan, D.J. Maxwell, X.H. Gao, R.E. Bailey, M.Y. Han, S.M. Nie, Luminescent quantum dots for multiplexed biological detection and imaging, Curr. Opin. Biotechnol. 13 (2002) 40–46. [8] M. Bai, S.N. Huang, S.Y. Xu, G.F. Hu, L.Y. Wang, Fluorescent nanosensors via photoinduced polymerization of hydrophobic inorganic quantum dots for the sensitive and selective detection of nitroaromatics, Anal. Chem. 87 (2015) 2383–2388. [9] R.Y. Tu, B.H. Liu, Z.Y. Wang, D.M. Gao, F. Wang, Q.L. Fang, et al., Amine-capped ZnS-Mn2+ nanocrystals for fluorescence detection of trace TNT explosive, Anal. Chem. 80 (2008) 3458–3465. [10] K. Zhang, H.B. Zhou, Q.S. Mei, S.H. Wang, G.J. Guan, R.Y. Liu, et al., Instant visual detection of trinitrotoluene particulates on various surfaces by ratiometric fluorescence of dual-emission quantum dots hybrid, J. Am. Chem. Soc. 133 (2011) 8424–8427. [11] S.H. Yang, J. Liu, Y. Chen, J.X. Guo, L. Zhao, X. Wei, et al., A facile one-step photochemical strategy for preparation of polyacrylamide functionalized CdTe(S) quantum dots and their application in sensitive determination of 2,4,6-trinitrotoluene, Sens. Actuators B: Chem. 212 (2015) 1–9. [12] Y.X. Ma, S. Huang, M.L. Deng, L.Y. Wang, White upconversion luminescence nanocrystals for the simultaneous and selective detection of 2,4,6-trinitrotoluene and 2,4,6-trinitrophenol, ACS Appl. Mater. Inter. 6 (2014) 7790–7796. [13] Y.X. Ma, L.Y. Wang, Upconversion luminescence nanosensor for TNT selective and label-free quantification in the mixture of nitroaromatic explosives, Talanta 120 (2014) 100–105. [14] Y.X. Ma, S. Huang, L.Y. Wang, Multifunctional inorganic-organic hybrid nanospheres for rapid and selective luminescence detection of TNT in mixed nitroaromatics via magnetic separation, Talanta 116 (2013) 535–540. [15] N.N. Tu, L.Y. Wang, Surface plasmon resonance enhanced upconversion luminescence in aqueous media for TNT selective detection, Chem. Commun. 49 (2013) 6319–6321. [16] S.G. Wang, L.Y. Wang, Lanthanide-doped nanomaterials for luminescence detection and imaging, Trends Anal. Chem. 62 (2014) 123–134.
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Biographies Xilin Bai is currently a Ph.D. candidate at Beijing University of Chemical Technology, Beijing, China. She is engaged in the preparation, characterization of multifunctional nanoparticles, and potential analytical application. Suying Xu received her Ph.D. degree in University of Bath. She is currently an associate professor at Beijing University of Chemical Technology. Her research areas of interests include design of fluorescent probes and fabrication of nanoscale materials for biomedical applications. Gaofei Hu received his Ph.D. degree in Beijing Institute of Technology. His research interests are focused on the nuclear magnetic resonance spectroscopy, metabonomics, food and environmental safety analysis. Leyu Wang received his Ph.D. degree in Tsinghua University. He is currently a full professor of State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology. His research interests are focused on the synthesis, characterization of nanostructured materials and their applications in sensing and bioimaging.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Surface plasmon resonance-enhanced photothermal nanosensor for sensitive and selective visual detection of 2,4,6-trinitrotoluene Xilin Bai, Suying Xu, Gaofei Hu, Leyu Wang ∗ State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Environmentally Harmful Chemical Analysis, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 26 February 2016 Received in revised form 14 June 2016 Accepted 15 June 2016 Available online 17 June 2016 Keywords: Nanosensor Explosives detection Visualized sensing Surface plasmon resonance (SPR)
a b s t r a c t The exploration of feasible, reliable sensors for the selective and sensitive detection of nitroaromatic explosives is a critical issue nowadays as mandated by homeland security concerns. Our endeavor is to develop an instant on-site visual and ultra-sensitive photothermal nanosensor for 2,4,6-trinitrotoluene (TNT) detection, which could be applied on a broad scale with high selectivity and sensitivity. The fundamental principle of our work is that TNT could effectively induce the aggregation of the cysteaminestabilized gold nanoparticles (Au NPs) by forming the Meisenheimer complex, which further enhanced the surface plasmon resonance (SPR) properties and the photothermal effects of Au NPs. Other coexisting nitroaromatics have negligible influences on the photothermal responses. By means of the temperature enhancement and photothermal imaging technique, the presence of TNT could be readily recognized and visualized, indicating wide potential applications in analytical sensing. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Rapid, sensitive and selective detection of nitroaromatic explosives such as 2,4,6-trinitrotoluene (TNT) is always desirable due to their great threat to the global security and human health [1–6]. To date, various methods have been developed for selective recognition of nitroaromatics, of which the optical sensing strategy holds great potentials in fabricating ultra-high sensitive probes. Taking advantages of nanotechnology, the development of nanomaterials with unique luminescent properties, such as semiconductor quantum dots (QDs) [5,7–11], lanthanide-doped nanocrystals [12–16], noble metal and carbon-based nanomaterials [17,18], organic dye-based nanomaterials [19,20] and metal-organic frameworks (MOFs) [21], has opened up new horizon for detection of nitroaromatics [22]. Yet, it is still intriguing to exploit a facial and versatile nanoplatform for the rapid and selective detection of TNT. The surface plasmon resonance (SPR) nanostructures have been widely investigated for enhanced catalysis and tumor ablation [23–29]. SPR-enhanced luminescence has also drawn wide attention for sensitive detection of small molecules [15,30]. In addition, noble metal nanostructure based-SPR and surface-enhanced Raman scattering (SERS) [10,31] have been extensively explored for constructing
∗ Corresponding author. E-mail address: [email protected] (L. Wang). http://dx.doi.org/10.1016/j.snb.2016.06.093 0925-4005/© 2016 Elsevier B.V. All rights reserved.
optical biosensors, especially for label-free assays [32–38]. Yet, direct employment of SPR spectroscopy could be problematic since only those analytes with high molecular weight could afford observable SPR shifts. Alternatively, the SPR responses induced by small molecules need to be either amplified through some strategies or transformed into other types of signals that could be read out easily [39]. In recent decades, nanomaterials with photothermal effect have received wide attention for their applicability in photothermal therapy and photothermal imaging [29,40–45]. In particular, SPR-induced photothermal responses have gained substantial interests across various disciplines [44,46,47]. Notably, for the near-infrared light responsive photothermal agents, the consequent temperature changes are mainly originated from the photothermal effect of the photothermal agents, thus displaying low background signals from the external environment and possessing high sensing sensitivity. Despite of the intrinsic high sensitive properties, the photothermal agents have rarely been utilized for sensing purposes. To date, our group has utilized the photothermal properties of polyaniline to successfully fabricate a photothermal sensor for selectively detection of nitroaromatics, which has demonstrated its great applicability [48]. The coinage metal-based nanostructures such as gold nanoparticles (AuNPs) are frequently utilized in designing sensors [49] owing to their localized SPR properties. Since the SPR changes of AuNPs arising from recognition event with analytes could also induce a color change, which has successfully been
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Scheme 1. Schematic illustration of SPR-enhanced photothermal nanosensor based on TNT-induced aggregation of AuNPs.
utilized for constructing nitroaromatic sensors [18,50]. However, there is no platform that makes use of the SPR-enhanced photothermal effect of these nanomaterials for the highly selective and sensitive detection of TNT. Here, for the first time we explored the possibility of developing a photothermal nanosensor with superior selectivity and in situ detection of TNT under the assistance of SPR-enhanced photothermal effect. In this work, the cysteamine-stabilized AuNPs were prepared, which were used to form the Meisenheimer complex with electron-deficient TNT through charge-transfer processes [10,12,22,48]. Such complexation further induces the aggregation of gold nanoparticles, resulting in a remarkable photothermal enhancement upon irradiation at 650 nm (Scheme 1). More importantly, the temperature of the Au NPs/TNT solution increases proportionally along with the increase of TNT concentration, which were scarcely affected by other nitroaromatics such as 2,4,6-trinitrotophenol (TNP), 2,4-dinitrotoluene (DNT), nitrobenzene (NB), toluene (PhMe) and benzene (Ph). In addition, the temperature changes induced by TNT could also be visualized through photothermal imaging technique. Therefore, a novel SPR enhanced photothermal nanosensor for on-site visual and selective detection of TNT has been successfully developed. 2. Experimental 2.1. Reagents 2,4,6-Trinitrotoluene (TNT) and 2,4,6-trinitrotophenol were supplied by the National Security Department of China and were recrystallized with ethanol before use. For safety considerations, all of the explosives must be kept away from fire, striking and friction, and they should be handled carefully. 2,4-Dinitrotoluene, nitrobenzene toluene and benzene were purchased from the Aladdin Chemistry Co. Ltd. These nitroaromatics were dissolved in mixed solvents of ethanol and acetonitrile (volume ratio of 4:1) to obtain the stock solution. In brief, 3.0 mg of each of nitroaromatics was dissolved in 15.0 mL of the mixture solvent containing ethanol (12.0 mL) and acetonitrile (3.0 mL) to get the stock solution with a final concentration of 0.2 mg/mL. All other reagents were analytical grade and used as received without further purification. Deionized (DI) water was used throughout. Cysteamine was purchased from Biological Science and Technology Co. Thiolated poly (ethylene glycol) (SH-mPEG, Mw = 5000) was purchased from Beijing Kaizheng Biological Engineering Development Co. Ethanol, acetonitrile, NaOH, NaHCO3 , Na2 CO3 , NaAc, HAc, Na2 HPO4 , NaH2 PO4 , NaF and HAuCl4 ·2H2 O were received from Beijing Chemical Company (China). 2.2. Apparatus The size and morphology of AuNPs were characterized by the H-800 transmission electron microscope (TEM). Dynamic
Fig. 1. TEM images (a, b) and DLS size distribution (c, d) of AuNPs-NH2 before (a, c) and after (b, d) addition of TNT (4.0 g/ mL).
light scattering (DLS) particle size was measured by a Zetasizer Nano-ZS90 (Malvern) size analyzer. The absorption spectra were conducted on a Shimadzu UV-3600 spectrophotometer with a spectral window range of 200–3600 nm. The thermal imaging and temperature evolution plots were performed on a FLIR-A600 infrared (IR) camera. 2.3. Synthesis of amine-stabilized AuNPs In brief, 1.0 mL of cysteamine (6.5 mM) was added to 40 mL of HAuCl4 (2.0 mg) solution (100 ◦ C) under magnetic stirring. Then 1.0 mg SH-mPEG was added to improve the stability of the assynthesized Au NPs. The solution was kept boiling for 15 min till the solution turned into rose color. Finally, the NH2 moiety functionalized hydrophilic gold nanoparticles (AuNPs-NH2 ) were obtained after centrifugation and then redispersed into 3.0 mL of deionized water for later use. 2.4. Photothermal detection of TNT in solution The amine-functionalized gold nanoparticles (AuNPs-NH2 ) were obtained according to the previously published method [15]. Briefly, 50.0 L of various concentrations of TNT or other nitroaromatics was mixed with 300 mL of the as-prepared AuNPs-NH2 . And then the mixed solution was diluted to 0.5 mL with NaH2 PO4 Na2 HPO4 (pH = 8.0, 20 mM) before being exposed to the laser irradiation. The photothermal tests were carried out with an IR camera to monitor and record the temperature changes while keeping the solution exposed to irradiation light at 650 nm with a power density of 2.25 W/cm2 . 3. Results and discussion 3.1. Morphology and photothermal properties of AuNPs-NH2 NPs as photothermal nanosensors In this study, cysteamine was used both as the electron donor to interact with electron-deficient nitroaromatics and surface ligands to stabilize the as-prepared AuNPs. As shown in Fig. 1a and c, initially, the AuNPs-NH2 was well dispersed in deionized water
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Fig. 2. (a) Temperature increment (T) of AuNPs-NH2 with different concentrations under irradiation (650 nm) with a power density of 2.25 W/cm2 . Here, T = T − T0 , where T0 and T represent the temperature before and after irradiation, respectively. (b) Temperature increment of AuNPs-NH2 colloidal solution before and after adding TNT (4.0 g/mL) under different pH conditions. Buffer solutions were prepared as follows: pH (4.0 5.0, HAc-NaAc, 20 mM); pH (6.0–8.0, NaH2 PO4 -Na2 HPO4 , 20 mM); pH (9.0–11.0, NaHCO3 -Na2 CO3 , 20 mM), pH (12.0, NaH2 PO4 -NaOH, 20 mM).
with an average size of 51.4 ± 8.0 nm. Upon addition of TNT into the colloidal solution, an obvious aggregation of gold NPs appeared as a result of the formation of the Meisenheimer complex with electrondeficient TNT through charge-transfer processes [10,12,22,48], and the DLS size of particles increased to about 240.3 ± 12.4 nm (Fig. 1b, d). Additionally, the changes of UV–vis spectra also demonstrated the aggregation of gold NPs (Fig. S1 in the Supporting information). Along with the addition of TNT, the absorption at 570 nm increased accordingly. This phenomenon powerfully verifies our conception of forming the Meisenheimer complex between TNT and amine-functionalized AuNPs. Furthermore, to explore the optimal conditions for constructing photothermal nanosensor, the amount of Au NPs as well as the pH value was optimized. It is of noted that AuNPs-NH2 have maximum absorption at about 570 nm and the absorption at 650 nm increased accordingly with the addition of TNT. Actually, due to the SPR effects, if the 570 nm laser was adopted, the photothermal effects would be better than that irradiated by 650 nm laser. However, the 570 nm laser was not available, and a 650 nm diode laser was chosen instead. Upon irradiation at 650 nm (2.25 W/cm2 ), the temperature increased along with the irradiation time. In addition, it was easily found that the temperature increased more rapidly with the increment of AuNPs-NH2 concentration in the absence of TNT (Fig. 2a). When the concentration of AuNPs-NH2 was 100 mg/mL, the temperature increment increased up to 8.1 ◦ C within 3 min after irradiation. The influence of AuNPs-NH2 concentration on the temperature increment (T) in the presence of TNT was also investigated. Here, T = T − T0 , T and T0 represent the temperature before and after irradiation with laser, respectively. As shown in Fig. S2, by fixing the TNT concentration (4.0 g/mL), the largest T difference (T2 − T1 ) was obtained with 60 mg/mL of AuNPs-NH2 . Here, T2 and T1 represent the temperature increment (T) in the presence and absence of TNT, respectively. On the other hand, the temperature increment of AuNPs-NH2 solution under different pH conditions was also investigated, which indicated that the value of T follows similar trend, regardless the presence or absence of TNT. As shown in Fig. 2b, the value of (T2 − T1 ) increased slightly
Fig. 3. (a) Temperature increment of AuNPs-NH2 (60 mg/mL) in the presence of various concentrations of TNT under irradiation of 650 nm with a power density of 2.25 W/cm2 recorded by an IR camera; (b) Calibration plot of temperature increment (T) versus TNT concentrations.
in the pH range of 4.0–8.0 and decreased a little when the pH value varied from 8.0 to 12.0, suggesting that it is most suitable for charge transfer process at pH of 8.0. Therefore, the following photothermal detection of TNT was carried out at pH of 8.0.
3.2. Photothermal detection and linear range To verify the feasibility of the AuNPs-NH2 /TNT system as potential photothermal sensor, various concentrations of TNT were added to the solutions containing fixed amount of AuNPs-NH2 , respectively. Upon laser irradiation (650 nm, 2.25 W/cm2 ), the T of the resulting solution increased rapidly within 3.0 min (Fig. 3a), which demonstrated that the photothermal effect had been enhanced along with the increase of TNT from 0 to 1.0 g/mL. It is known that the electron withdrawing properties of nitro groups on the TNT molecule render it highly electron-deficient, which is prone to form Meisenheimer complex with electron-rich moieties such as amine through charge-transfer process [8,15,48]. Thus, the TNT molecules in the solution would act as bridges to pull together the cysteamine functionalized AuNPs, leading to the aggregation of the AuNPs-NH2 [7] . Accordingly, the resulting aggregation clusters of AuNPs in turn enhance the photothermal effect, leading to the enlargement of the temperature differences. In addition, by plotting the temperature increment (T) against to the concentration of TNT, a well linear fitting was obtained in the range of 0.1–1.0 g/mL with a calibration function of T = 4.383 + 1.642C (Fig. 3b). Here, T is the temperature difference (◦ C) and C is the concentration of TNT (g/mL). In addition, our nanosensor displayed a low limit of detection (LOD = 3/K) of 78 ng/mL, herein, is the standard deviation of the blank measurements (n = 6), and K is the slope of the calibration curve. Meanwhile, when under the same conditions, in the range of 1.0–10.0 g/mL, it also showed good linear relationship (R2 = 0.9947) with a calibration function of T = 5.544 + 0.410C and a LOD of 0.9 g/mL (Fig. S3). The low limit of detection and high sensitivity convincingly confirmed that our photothermal nanosensor is suitable for TNT detection.
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Fig. 5. Chemical structures of all the tested compounds and the temperature changes of the AuNPs-NH2 colloidal solution in the presence of the tested compounds (5.0 g/mL) under irradiation (650 nm, 2.25 W/cm2 ). Inset are the corresponding photothermal images acquired after irradiation for 3.0 min.
Fig. 4. (a) The photos of the AuNPs-NH2 /TNT solution captured by a digital camera under daylight. (b) The photothermal images of the AuNPs-NH2 /TNT solution under irradiation at 650 nm for 3.0 min. The values on the tubes and images are the concentration of TNT being used (g/mL).
3.3. Visual detection and selectivity test Based on the color change of AuNPs-NH2 caused by the controllable change in their dispersion/aggregation states, Au NPs have already been widely used for colorimetric sensing [18,51–53]. However, in most cases where the concentration of analytes was very low, it was hard to accurately tell the color difference for sensitive visual detection. Therefore, developing a more reliable, feasible sensing platform is of significant importance. As shown in Fig. 4a, after adding various amounts of TNT, the optical photos of all the AuNPs-NH2 colloidal solution showed similar rose color, which can hardly be used to discriminate the presence of different amounts of TNT. Nevertheless, when placed under irradiation at 650 nm for 3.0 min, an obvious difference on the photothermal images was presented (Fig. 4b), offering a facile on-site sensing platform. It is worth noting that except for the high sensitivity, the photothermal imaging technique is ready to be carried out by a portable infrared camera, which holds great advantages in practical applications.
Furthermore, to illustrate the good selectivity of this nanosensor, the interference was also investigated and the results are shown in Fig. 5. Though all the interferences have similar chemical structures, after adding the same amount of them, no changes on temperature values as well as photothermal images were observed, same to that of control case where no TNT or interferences were added. However, in the presence of TNT, the temperature increment was higher and the photothermal image was brighter than that of other cases. To verify the adaptability of our photothermal nanosensor, we carried out the experiments for detection low amount of TNT while in the presence of high concentration of other interferences. As indicated in Fig. S4, even in the presences of interferences with 10 times higher than that of TNT, still give no obvious photothermal effect as compared to the controls. In addition, the presence of little amount TNT, still induce significant photothermal effect, that is, detection of TNT would not be affected by other interferences. These results demonstrate that highly selective detection of TNT could be rationally achieved through the proposed nanoplatforms. The comparison of our method with the related techniques for TNT detection has also been carried out and summarized in Table 1. The comparison results suggest that our nanosensor truly owns some advantages for the sensitive, selective and visual detection of TNT. 4. Conclusions In summary, for the first time we have developed a SPRenhanced photothermal nanosensor for on-site visual, selective, and sensitive detection of TNT. The TNT molecule could act as a linker to pull together the amine-rich AuNPs through the formation of Meisenheimer complex, leading to aggregation of AuNPs, which further enhanced the photothermal performance upon irradiation. Taking advantage of photothermal imaging techniques, subtle temperature changes induced by TNT could be directly visu-
Table 1 Comparison of our photothermal nanosensor and reported methods. Refs.
materials
Linear range (TNT)
LOD (TNT)
Visual detection
this work [15] [20] [48] [49]
AuNPs AuNPs tetrathiafulvalene-capped hybrid material polyaniline gold nanostar
0.4–40 M 0–32 M 10−3 –10−8 M 4–800 M 10−3 –10−7 M
0.31 M N/Aa 66 M N/Aa 0.72 M
yes no no yes no
a
Not available.
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alized, which prevails over conventional colorimetric methods in terms of sensitivity. Therefore, this proposed novel detection assay paves the way for constructing more photothermal nanosensors, and will definitely offer great opportunities in chemical and biochemical sensing platform. Acknowledgements This research was supported in part by the National Natural Science Foundation of China (Grant Nos. 21475007, 21275015 and 21505003). We also thank the support from the “Innovation and Promotion Project of Beijing University of Chemical Technology”, the “Public Hatching Platform for Recruited Talents of Beijing University of Chemical Technology, the High-Level Faculty Program of Beijing University of Chemical Technology (buctrc201507, buctrc201608) and BUCT Fund for Disciplines Construction and Development (Project No. XK1526)”. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.06.093. References [1] E.R. Goldman, I.L. Medintz, J.L. Whitley, A. Hayhurst, A.R. Clapp, H.T. Uyeda, et al., A hybrid quantum dot-antibody fragment fluorescence resonance energy transfer-based TNT sensor, J. Am. Chem. Soc. 127 (2005) 6744–6751. [2] H. Sohn, R.M. Calhoun, M.J. Sailor, W.C. Trogler, Detection of TNT and picric acid on surfaces and in seawater by using photoluminescent polysiloles, Angew. Chem. Int. Ed. 40 (2001) 2104–2105. [3] H. Sohn, M.J. Sailor, D. Magde, W.C. Trogler, Detection of nitroaromatic explosives based on photoluminescent polymers containing metalloles, J. Am. Chem. Soc. 125 (2003) 3821–3830. [4] H.B. Zhou, Z.P. Zhang, C.L. Jiang, G.J. Guan, K. Zhang, Q.S. Mei, et al., Trinitrotoluene explosive lights up ultrahigh raman scattering of nonresonant molecule on a top-closed silver nanotube array, Anal. Chem. 83 (2011) 6913–6917. [5] Y.S. Xia, L. Song, C.Q. Zhu, Turn-on and near-infrared fluorescent sensing for 2,4,6-trinitrotoluene based on hybrid (gold nanorod)-(quantum dots) assembly, Anal. Chem. 83 (2011) 1401–1407. [6] F. Akhgari, H. Fattahi, Y.M. Oskoei, Recent advances in nanomaterial-based sensors for detection of trace nitroaromatic explosives, Sens. Actuators B: Chem. 221 (2015) 867–878. [7] W.C.W. Chan, D.J. Maxwell, X.H. Gao, R.E. Bailey, M.Y. Han, S.M. Nie, Luminescent quantum dots for multiplexed biological detection and imaging, Curr. Opin. Biotechnol. 13 (2002) 40–46. [8] M. Bai, S.N. Huang, S.Y. Xu, G.F. Hu, L.Y. Wang, Fluorescent nanosensors via photoinduced polymerization of hydrophobic inorganic quantum dots for the sensitive and selective detection of nitroaromatics, Anal. Chem. 87 (2015) 2383–2388. [9] R.Y. Tu, B.H. Liu, Z.Y. Wang, D.M. Gao, F. Wang, Q.L. Fang, et al., Amine-capped ZnS-Mn2+ nanocrystals for fluorescence detection of trace TNT explosive, Anal. Chem. 80 (2008) 3458–3465. [10] K. Zhang, H.B. Zhou, Q.S. Mei, S.H. Wang, G.J. Guan, R.Y. Liu, et al., Instant visual detection of trinitrotoluene particulates on various surfaces by ratiometric fluorescence of dual-emission quantum dots hybrid, J. Am. Chem. Soc. 133 (2011) 8424–8427. [11] S.H. Yang, J. Liu, Y. Chen, J.X. Guo, L. Zhao, X. Wei, et al., A facile one-step photochemical strategy for preparation of polyacrylamide functionalized CdTe(S) quantum dots and their application in sensitive determination of 2,4,6-trinitrotoluene, Sens. Actuators B: Chem. 212 (2015) 1–9. [12] Y.X. Ma, S. Huang, M.L. Deng, L.Y. Wang, White upconversion luminescence nanocrystals for the simultaneous and selective detection of 2,4,6-trinitrotoluene and 2,4,6-trinitrophenol, ACS Appl. Mater. Inter. 6 (2014) 7790–7796. [13] Y.X. Ma, L.Y. Wang, Upconversion luminescence nanosensor for TNT selective and label-free quantification in the mixture of nitroaromatic explosives, Talanta 120 (2014) 100–105. [14] Y.X. Ma, S. Huang, L.Y. Wang, Multifunctional inorganic-organic hybrid nanospheres for rapid and selective luminescence detection of TNT in mixed nitroaromatics via magnetic separation, Talanta 116 (2013) 535–540. [15] N.N. Tu, L.Y. Wang, Surface plasmon resonance enhanced upconversion luminescence in aqueous media for TNT selective detection, Chem. Commun. 49 (2013) 6319–6321. [16] S.G. Wang, L.Y. Wang, Lanthanide-doped nanomaterials for luminescence detection and imaging, Trends Anal. Chem. 62 (2014) 123–134.
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Biographies Xilin Bai is currently a Ph.D. candidate at Beijing University of Chemical Technology, Beijing, China. She is engaged in the preparation, characterization of multifunctional nanoparticles, and potential analytical application. Suying Xu received her Ph.D. degree in University of Bath. She is currently an associate professor at Beijing University of Chemical Technology. Her research areas of interests include design of fluorescent probes and fabrication of nanoscale materials for biomedical applications. Gaofei Hu received his Ph.D. degree in Beijing Institute of Technology. His research interests are focused on the nuclear magnetic resonance spectroscopy, metabonomics, food and environmental safety analysis. Leyu Wang received his Ph.D. degree in Tsinghua University. He is currently a full professor of State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology. His research interests are focused on the synthesis, characterization of nanostructured materials and their applications in sensing and bioimaging.