Room-Temperature Phosphorescence Chemosensor and Rayleigh

used for room-temperature phosphorescence (RTP) to sense and for RS ... phosphorescence (RTP) of dopants and host semiconductors has been studied in a...
0 downloads 0 Views 5MB Size
Download PDF
Anal. Chem. 2011, 83, 30–37

Room-Temperature Phosphorescence Chemosensor and Rayleigh Scattering Chemodosimeter Dual-Recognition Probe for 2,4,6-Trinitrotoluene Based on Manganese-Doped ZnS Quantum Dots Wen-Sheng Zou, Dong Sheng, Xin Ge, Jun-Qin Qiao, and Hong-Zhen Lian* Key Laboratory of Analytical Chemistry for Life Science, Ministry of Education, School of Chemistry and Chemical Engineering and Center of Materials Analysis, Nanjing University, 22 Hankou Road, Nanjing 210093, China Rayleigh scattering (RS) as an interference factor to detection sensitivity in ordinary fluorescence spectrometry is always avoided in spite of considerable efforts toward the development of RS-based resonance Rayleigh scattering (RRS) and hyper-Rayleigh scattering (HRS) techniques. Here, combining advantages of quantum dots (QDs) including chemical modification of functional groups and the installation of recognition receptors at their surfaces with those of phosphorescence such as the avoidance of autofluorescence and scattering light, L-cyscapped Mn-doped ZnS QDs have been synthesized and used for room-temperature phosphorescence (RTP) to sense and for RS chemodosimetry to image ultratrace 2,4,6-trinitrotoluene (TNT) in water. The L-cys-capped Mn-doped ZnS QDs interdots aggregate with TNT species induced by the formation of Meisenheimer complexes (MHCs) through acid-base pairing interaction between L-cys and TNT, hydrogen bonding, and electrostatic interaction between L-cys intermolecules. Although the resultant MHCs may quench the fluorescence at 430 nm, interdots aggregation can greatly influence the light scattering property of the aqueous QDs system, and therefore, dominant RS enhancement at defect-related emission wavelength was observed under the excitation of violet light of Mn-doped ZnS QDs, which was applied in chemodosimetry to image TNT in water. Meanwhile, Mn-doped ZnS QDs also exhibited a highly selective response to the quenching of the 4T1-6A1 transition emission (RTP) and showed a very good linearity in the range of 0.0025-0.45 µM TNT with detection limit down to 0.8 nM and RSD of 2.3% (n ) 5). The proposed methods are well-suited for detecting the ultratrace TNT and distinguishing different nitro compounds. In the recent decade, because the interference from autofluorescence emission and scattering light can be easily avoided due to the long lifetime of phosphorescence,1 room-temperature phosphorescence (RTP) of dopants and host semiconductors has * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +86-25-8368-6075. Fax: +86-25-8332-5180.

30

Analytical Chemistry, Vol. 83, No. 1, January 1, 2011

been studied in a number of doped semiconductor systems.2,3 Very recently, Zhang’s group4 synthesized amine-capped ZnS-Mn2+ nanocrystals and used it in the field of fluorescence analysis for the first time. Subsequently, Yan’s group applied such nanocrystals in RTP sensing DNA,3a persistent organic pollutants (POPs) in water,3b enoxacin,3c and glucose3d in biological fluids. Since the early 1990s, at least two types of Rayleigh scattering (RS)-based techniques, resonance Rayleigh scattering (RRS)5 and hyper-Rayleigh scattering (HRS),6 have been developed extensively. RRS takes place when the incident beam is close in energy to an absorption band and has attracted more and more interests owing to its simplicity, rapidness, and high sensitivity. It has been used to investigate intermolecular interaction and aggregation,7 determine urinary adenosine and inorganic ions,8 screen homoadenine and anticancer drugs,9 and monitor growth mechanism of nanoparticles,10 etc. HRS is a convenient technique for the determination of hyperpolarizability of nonlinear optical (NLO) species in solution,11 and the study of second-order NLO surface, dimension and structure dependence of nano particles,12 and the evaluation of potential of NLO materials for second-harmonic generation (1) Sa´nchez-Barraga´n, I.; Costa-Ferna´ndez, J. M.; Valledor, M.; Campo, J. C.; Sanz-Medel, A. Trends Anal. Chem. 2006, 25, 958–967. (2) Chung, J. H.; Ah, C. S.; Jang, D. J. J. Phys. Chem. B 2001, 105, 4128–4132. (3) (a) He, Y.; Wang, H. F.; Yan, X. P. Chem.sEur. J. 2009, 15, 5436–5440. (b) Wang, H. F.; He, Y.; Ji, T. R.; Yan, X. P. Anal. Chem. 2009, 81, 1615– 1621. (c) He, Y.; Wang, H. F.; Yan, X. P. Anal. Chem. 2008, 80, 3832– 3837. (d) Wu, P.; He, Y.; Wang, H. F.; Yan, X. P. Anal. Chem. 2010, 82, 1427–1433. (4) Tu, R.; Liu, B.; Wang, Z.; Gao, D.; Wang, F.; Fang, Q.; Zhang, Z. Anal. Chem. 2008, 80, 3458–3465. (5) (a) Pasternack, R. F.; Collings, P. J. Science 1995, 269, 935–939. (b) Pasternack, R. F.; Bustamante, C.; Collings, P. J.; Giannetto, A.; Gibbs, E. J. J. Am. Chem. Soc. 1993, 115, 5393–5399. (6) (a) Clays, K.; Persoons, A. Rev. Sci. Instrum. 1992, 83, 3285–3289. (b) Clays, K.; Persoons, A. Phys. Rev. Lett. 1991, 66, 2980–2983. (7) (a) Li, Y.; Li, H.; Chen, X.; Zhu, F.; Yang, J.; Zhu, Y. J. Polym. Sci., Part B: Polym. Phys. 2010, 48, 1847–1852. (b) Zhang, W. Z.; Chen, X. D.; Yang, J.; Luo, W.-A.; Zhang, M. Q. J. Phys. Chem. B 2010, 114, 1301–1306. (8) Jiang, Z.; Fan, Y.; Chen, M.; Liang, A.; Liao, X.; Wen, G.; Shen, X.; He, X.; Pan, H.; Jiang, H. Anal. Chem. 2009, 81, 5439–5445. (9) Song, G.; Chen, C.; Qu, X.; Miyoshi, D.; Ren, J.; Sugimoto, N. Adv. Mater. 2008, 20, 706–710. (10) Wang, W.; Yang, X.; Cui, H. J. Phys. Chem. C 2008, 112, 16348–16353. 10.1021/ac1008942  2011 American Chemical Society Published on Web 11/30/2010

(SHG) imaging.13 In the past 2 years, gold nanoparticles and HRS technique-based detection methods have been developed for biomolecules and heavy metal ions.14 At the same time, HRS has been applied to investigate the relationship between second-order NLO properties and structure of organic conjugated chromophores15 and provided instruction for the design of organic optical functional materials featuring better nonlinearity and transparence.16 Moreover, HRS proved to be a powerful tool for the real-time monitoring of the conformation of conjugated polymers and nucleation and growth of nanoparticles.17 Gold nanomaterials-based two-photon Rayleigh scattering (TPRS) can be used for rapid, highly sensitive, and selective detection of E. coli bacteria,18a Alzheimer’s disease biomakers,18b breast cancer cells,18c and so on. However, RS as an interference factor to detection sensitivity in ordinary fluorescence spectrometry is always eliminated through excitation wavelength blue-shifted or a filter inserted in front of sample cell. Few works have, so far, been reported regarding the foundation of RS-based protocols to directly detect target molecules utilizing a nanocrystal-level RS device. In this paper, we report the synthesis of L-cys-capped Mn-doped ZnS QDs with unusual 2,4,6-trinitrotoluene (TNT) dual-recognition function and the study of its RTP sensing and RS chemodosimetry imaging features. The L-cys can bind TNT species to form Meisenheimer complexes (MHCs) through acid-base pairing interaction between electron-rich amino ligands and electron-deficient aromatic rings and hydrogen bonding and electrostatic interaction between 19 L-cys intermolecules. The resultant MHCs leading to interdots aggregation among L-cys-capped Mn-doped ZnS QDs likely dominate the light scattering property of the aqueous system. Also, owing to the violet excitation of Mn-doped ZnS QDs, dramatic (11) (a) Ma, X.; Ma, F.; Zhao, Z.; Song, N.; Zhang, J. J. Mater. Chem. 2010, 20, 2369–2380. (b) Coe, B. J.; Fielden, J.; Foxon, S. P.; Brunschwig, B. S.; Asselberghs, I.; Clays, K.; Samoc, A.; Samoc, M. J. Am. Chem. Soc. 2010, 132, 3496–3513. (12) (a) Duboisset, J.; Russier-Antoine, I.; Benichou, E.; Bachelier, G.; Jonin, C.; Brevet, P. F. J. Phys. Chem. C 2009, 113, 13477–13481. (b) Li, Q.; Wu, K.; Wei, Y.; Sa, R.; Cui, Y.; Lu, C.; Zhu, J.; He, J. Phys. Chem. Chem. Phys. 2009, 11, 4490–4497. (13) (a) Reeve, J. E.; Collins, H. A.; Mey, K. D.; Kohl, M. M.; Thorley, K. J.; Paulsen, O.; Clays, K.; Anderson, H. L. J. Am. Chem. Soc. 2009, 131, 2758– 2759. (b) Meulenaere, E. D.; Asselberghs, I.; de Wergifosse, M.; Botek, E.; Spaepen, S.; Champagne, B.; Vanderleyden, J.; Clays, K. J. Mater. Chem. 2009, 19, 7514–7519. (14) (a) Griffin, J.; Singh, A. K.; Senapati, D.; Lee, E.; Gaylor, K.; Jones-Boone, J.; Ray, P. C. Small 2009, 5, 839–845. (b) Darbha, G. K.; Singh, A. K.; Rai, U. S.; Yu, E.; Yu, H.; Ray, P. C. J. Am. Chem. Soc. 2008, 130, 8038–8043. (15) Coe, B. J.; Foxon, S. P.; Harper, E. C.; Helliwell, M.; Raftery, J.; Swanson, C. A.; Brunschwig, B. S.; Clays, K.; Franz, E.; Garı´n, J.; Orduna, J.; Horton, P. N.; Hursthouse, M. B. J. Am. Chem. Soc. 2010, 132, 1706–1723. (16) (a) Pe´rez-Moreno, J.; Zhao, Y.; Clays, K.; Kuzyk, M. G.; Shen, Y.; Qiu, L.; Hao, J.; Guo, K. J. Am. Chem. Soc. 2009, 131, 5084–5093. (b) Sergeyev, S.; Didier, D.; Boitsov, V.; Teshome, A.; Asselberghs, I.; Clays, K.; Velde, C. M. L. V.; Plaquet, A.; Champagne, B. Chem.sEur. J. 2010, 16, 8181– 8190. (17) (a) Vandendriessche, A.; Asselberghs, I.; Clays, K.; Smet, M.; Dehaen, W.; Verbiest, T.; Koeckelberghs, G. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 3740–3747. (b) Segets, D.; Tomalino, L. M.; Gradl, J.; Peukert, W. J. Phys. Chem. C 2009, 113, 11995–12001. (18) (a) Singh, A. K.; Senapati, D.; Wang, S.; Griffin, J.; Neely, A.; Candice, P.; Naylor, K. M.; Varisli, B.; Kalluri, J. R.; Ray, P. C. ACS Nano 2009, 3, 1906–1912. (b) Neely, A.; Perry, C.; Varisli, B.; Singh, A. K.; Arbneshi, T.; Senapati, D.; Kalluri, J. R.; Ray, P. C. ACS Nano 2009, 3, 2834–2840. (c) Lu, W.; Arumugam, S. R.; Senapati, D.; Singh, A. K.; Arbneshi, T.; Khan, S. A.; Yu, H.; Ray, P. C. ACS Nano 2010, 4, 1739–1749. (19) Dasary, S. S. R.; Singh, A. K.; Senapati, D.; Yu, H.; Ray, P. C. J. Am. Chem. Soc. 2009, 131, 13806–13812.

RS enhancement at defect-related emission wavelength was observed. Notably, Mn-doped ZnS QDs also exhibited a highly selective response toward TNT analyte through significant color change and quenching of the 4T1-6A1 transition emission. Meanwhile, RS chemodosimetry imaging experiments revealed that Mn-doped ZnS QDs are TNT-responsive and cause RS enhancement in water by TNT complexation. It is very interesting that high RS response of ZnS QDs was found, and a Mn-doped ZnS QDs based RTP chemosensor and RS chemodosimeter dual-recognition probe for TNT was constructed. In comparison with the publication pioneered the Mndoped ZnS QDs based analytical application,4 both are consistent in substance. This seems to be two faces of one and the same coin (see the section Possible Mechanism of Mn-Doped ZnS QDs for RS Enhancement at Defect-Related Emission Wavelength but Quenching of the 4T1-6A1 Transition in the Presence of TNT in detail). On the basis of individual spectroscopic phenomena, different novel sensitive and selective analytical methods for the detection of TNT have been developed, respectively. TNT, one of the most notorious highly explosive nitroaromatic compounds with significant deleterious effects on the environment and human health,20 has magnetized tremendous societal concern due to the gradually increasing needs for a secure society and green environment. Therefore, the development of selective and sensitive methods for its real-time analytical detection has attracted considerable research efforts in recent years.21 Current technologies for the detection of TNT are less sensitive and usually timeconsuming with the employment of cumbersome and expensive gas chromatography coupled to mass spectrometry, ion mobility spectrometry, and neutron activation analysis.22 In the recent decade, because subtle changes of the surface properties of photoluminescent materials can result in dramatic changes in their optical properties, the endeavor has always been aiming at developing optosensing devices or techniques featuring high sensitivity, cost-effectivity, simplicity, on-the-spot for real-time determination of TNT in environments.23-26 QDs and dye-labeled nanomaterials also provided the chemodetection selectivity and sensitivity to target species.4,27-29 Particularly, to achieve the tailored selectivity of nitroaromatic analytes, a promising way is the use of molecularly imprinted polymers (MIPs).21b,c,30 Recently, functional groups modified gold nanoparticles exhibited high surface-enhanced Raman spectroscopy (SERS) response19 and colorimetric sensitivity31 of TNT at picomolar levels. The above approaches have led to significant contributions to the TNT assay; (20) (a) Hernandez, R.; Zappi, M.; Kuo, C. Environ. Sci. Technol. 2004, 38, 5157–5163. (b) Lynch, J. C.; Myers, K. F.; Brannon, J. M.; Delfino, J. J. J. Chem. Eng. Data 2001, 46, 1549–1555. (c) Ro, K. S.; Venugopal, A.; Adrian, D. D.; Constant, D.; Qaisi, K.; Valsaraj, K. T.; Thibodeaux, L. J.; Roy, D. J. Chem. Eng. Data 1996, 41, 758–761. (21) (a) Brill, T. B.; James, K. J. Chem. Rev. 1993, 93, 2667–2692. (b) Xie, C.; Zhang, Z.; Wang, D.; Guan, G.; Gao, D.; Liu, J. Anal. Chem. 2006, 78, 8339–8346. (c) Gao, D.; Zhang, Z.; Wu, M.; Xie, C.; Guan, G.; Wang, D. J. Am. Chem. Soc. 2007, 129, 7859–7866. (22) (a) Swager, T. M. Acc. Chem. Res. 1998, 31, 201–207. (b) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537–2574. (23) (a) Sohn, H.; Calhoun, R. M.; Sailor, M. J.; Trogler, W. C. Angew. Chem., Int. Ed. 2001, 40, 2104–2105. (b) Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. J. Am. Chem. Soc. 2003, 125, 3821–3830. (24) (a) Rose, A.; Zhu, Z. G.; Madigan, C. F.; Swager, T. M.; Bulovic´, V. Nature 2005, 434, 876–879. (b) Zahn, S.; Swager, T. M. Angew. Chem., Int. Ed. 2002, 41, 4225–4230.

Analytical Chemistry, Vol. 83, No. 1, January 1, 2011

31

it is still a continuous demand to develop inexpensive, easy to operate, environmentally friendly, and reliable methods for this purpose. Such an approach requires the integration of simple and green synthetic techniques, selective recognition, and highly sensitive detectable signals, etc. The reported QDs-based chemosensors and chemodosimeter here open up a potential prospect for the sensitive and convenient sensing of TNT explosive. EXPERIMENTAL SECTION Chemicals. 2,4,6-Trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), and 4-trinitrotoluene (NT) were gifted friendly by Professor Zhao-Wu Shen (University of Science and Technology of China, Hefei, Anhui, China) and used as recrystallized from methanol-water (2:1, v/v) mixture. 2-Nitrophenol (2-NP), 4-nitrophenol (4-NP) with a purity of over 99.5%, and fulvic acid with carbon content over 40% were commercially purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). LCysteine (L-cys, purity >98%) was the product of Alfa Aesar (Tianjin, China). ZnSO4 · 7H2O, Na2S · 9H2O, and MnCl2 · 4H2O were purchased and used as received. The stock solutions of TNT, DNT, NT, 2-NP, and 4-NP were prepared by dissolving in ethanol with concentration of 2 × 10-3 M for TNT, and 2 × 10-2 M for others, as used they were diluted to desired concentration. The concentration of Mn-doped ZnS QDs was 5 mg/L. Doubly deionized water was prepared in Elix 5 pure water system (Millipore, Milwood, MA). All other reagents were of analytical grade and used without further purification. Apparatus. UV-vis absorption spectra were obtained using a UV-3600 UV-vis spectrophotometer (Shimadzu, Tokyo, Japan), the RTP, fluorescence, and RLS spectra were recorded on an F-4600 (Hitachi, Tokyo, Japan), and the measurements were performed with excitation wavelength at 316 nm equipped with a plotter unit and a quartz cell (1 cm ×1 cm) in a variety of modes. The acidity was measured with a Sartorius PB-10 pH meter (Sartorius, Dietikon, Switzerland). The X-ray diffraction (XRD) spectra were collected on a Shimadzu XRD-6000 diffractometer with Cu KR radiation. Fourier transform infrared (FT-IR) spectra (4000-400 cm-1) in KBr were recorded on a Nicolet-6700 spectrometer (Nicolet, Madison, WI). The morphology and microstructure of the QDs were characterized by high-resolution transmission electron microscopy (HRTEM) on a JEM200CX (JEOL, Tokyo, Japan) microscope operating at a 200 (25) (a) Yang, J.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864–11873. (b) Yamaguchi, S.; Swager, T. M. J. Am. Chem. Soc. 2001, 123, 12087– 12088. (26) West, R.; Sohn, H.; Bankwitz, U.; Calabrese, J.; Apeloig, Y.; Mueller, T. J. Am. Chem. Soc. 1995, 117, 11608–11609. (27) (a) Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H. Nat. Mater. 2005, 4, 435–446. (b) Han, M. Y.; Gao, X. H.; Su, J. Z.; Nie, S. M. Nat. Biotechnol. 2001, 19, 631–635. (c) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016–2018. (d) Medintz, I. L.; Clapp, A. R.; Brunel, F. M.; Tiefenbrunn, T.; Uyeda, H. T.; Chang, E. L.; Deschamps, J. R.; Dawson, P. E.; Mattoussi, H. Nat. Mater. 2006, 5, 581–589. (28) Goldman, E. R.; Medintz, I. L.; Whitley, J. L.; Hayhurst, A.; Clapp, A. R.; Uyeda, H. T.; Deschamps, J. R.; Lassman, M. E.; Mattoussi, H. J. Am. Chem. Soc. 2005, 127, 6744–6751. (29) (a) Gao, D.; Wang, Z.; Liu, B.; Ni, L.; Wu, M.; Zhang, Z. Anal. Chem. 2008, 80, 8545–8553. (b) Geng, J.; Liu, P.; Liu, B.; Guan, G.; Zhang, Z.; Han, M.Y. Chem.sEur. J. 2010, 16, 3720–3727. (30) Riskin, M.; Tel-Vered, R.; Lioubashevski, O.; Willner, I. J. Am. Chem. Soc. 2009, 131, 7368–7378. (31) Jiang, Y.; Zhao, H.; Zhu, N.; Lin, Y.; Yu, P.; Mao, L. Angew. Chem., Int. Ed. 2008, 47, 8601–8604.

32

Analytical Chemistry, Vol. 83, No. 1, January 1, 2011

kV accelerating voltage. The samples for TEM were obtained by drying sample droplets from water dispersion onto a 300mesh Cu grid coated with a lacey carbon film, which was then allowed to dry prior to imaging. The confocal images were obtained using a Leica TCS SP5 microscope equipped with a 63× oil-immersion objective (Leica, Germany). The samples were excited at the UV band. A band-pass from 405 to 435 nm was adopted for observation. The mineral ions of pond water sample were analyzed by an ELAN 9000 ICPMS (Perkin-Elmer, U.S.A.). Synthesis of the Mn-Doped ZnS QDs. Mn-Doped ZnS QDs were prepared according to the reported procedures in literatures3c,32 with slight modifications. Briefly, 50 mL of 0.02 M L-cys and 5 mL of 0.1 M ZnSO4 were added into a three-necked flask. The mixed solution was adjusted to pH 11.0 or so with 2 M NaOH and stirred under dry nitrogen at room temperature for 30 min, and subsequently, 1.5 mL of 0.01 M MnCl2 was added into the above mixture and stirred for 20 min. Five milliliters of 0.1 M deoxygenated Na2S was injected into the solution quickly. The mixture was stirred for another 20 min, and then the solution was aged at 50 °C under air for 2 h to form L-cyscapped Mn-doped ZnS QDs. At last, the QDs were precipitated with ethanol, separated by centrifuging, and washed with ethanol, then dried in a vacuum. The prepared QDs powder is highly soluble in water. Aggregation of Mn-Doped ZnS QDs with TNT. The Mndoped ZnS QDs was immersed into a 2.0 mM stock solution of TNT assisted by ultrasonic vibration for 1 h; the Mn-doped ZnS QDs were taken out by centrifuging, washed several times with ethanol, and then dried under nitrogen flow at room temperature. TNT molecules were accordingly aggregated onto the surface of Mn-doped ZnS QDs with a change from gray to blood-red. This solid sample was used for FT-IR and XRD characterization. Spectroscopic Measurement. Spectral measurements were carried out with excitation and emission slit widths both of 10 nm for fluorescence and synchronous modes and with excitation and emission slit widths of 10 and 20 nm, respectively, for phosphorescence mode. As for photomultiplier tube voltage, 700, 400, and 950 V were used for fluorescence, synchronous, and phosphorescence, respectively. Fluorescence at emission wavelengths of 430 and 580 nm and phosphorescence at an emission wavelength of 580 nm were monitored at an excitation wavelength of 316 nm, and RLS spectra were recorded by scanning simultaneously the excitation and emission monochromators (∆λ ) 0) of the F-4600 spectrophotometer. Analysis of Water Samples. The water samples were collected in a precleaned glass bottle from a pond in the south area of Anhui University of Architecture (Hefei, Anhui, China). The samples were filtered through 0.45 µm Supor filters and stored in a refrigerator, and the pH was adjusted to 7.5 by Tris-HCl before analysis. Because no TNT in the collected water samples was detectable by the proposed method, a recovery test was carried out on the samples spiked with 2.5, 5.0, and 20 nM TNT to evaluate its reliability. (32) Zhuang, J. Q.; Zhang, X. D.; Wang, G.; Li, D. M.; Yang, W. S.; Li, T. J. J. Mater. Chem. 2003, 13, 1853–1857.

Figure 1. (A) XRD patterns of Mn-doped ZnS QDs aged at 50 °C for 2 h (curve 1) and aggregation of TNT with Mn-doped ZnS QDs (curve 2). (B) HRTEM image of Mn-doped ZnS QDs.

RESULTS AND DISCUSSION Characterization of the Mn-Doped ZnS QDs. The -CH2(∼2928 cm-1) existing in the FT-IR spectra (see Figure S1 in the Supporting Information) of L-cys-capped Mn-doped ZnS QDs indicated the L-cys covalently interacted onto the surface of nanocrystals through thiols. The XRD pattern of Mn-doped ZnS QDs exhibited a cubic structure with peaks for (311), (220), and (111) planes and showed more distinguishable planes than those of the Mn-doped ZnS QDs with TNT aggregating on the surface, which testified the latter is more instable zinc blende (see Figure 1A). The HRTEM image revealed Mn-doped ZnS QDs with spherical shape and almost uniform size in diameter about 3.6 nm (see Figure 1B). Spectral Characteristics of the Mn-Doped ZnS QDs. Mndoped ZnS QDs are one kind of the most interesting nanometer luminescent materials because of their distinctive emission behaviors; that is, Mn-doped ZnS QDs can emit fluorescence and RTP while the spectrofluorometer setting as fluorescence and phosphorescence modes, respectively. The fluorescence and RTP emission curves are shown in Figure S2, part A (see the Supporting Information). It can be seen that when excited at 316 nm the peaks of fluorescence and RTP of the Mn-doped ZnS QDs lay in 430 and 580, and 580 nm, respectively. The phosphorescence lifetime of 2 ms for the synthesized Mn-doped ZnS QDs were evaluated from the decay curve of its phosphorescence emission (see Figure S2, part B). The fluorescence emission at 430 nm is ascribed to a defect-related emission and disappears in the phosphorescence mode. The L-cys-capped ZnS QDs cannot emit RTP (see curve 1 in Figure S2, part A). The observed orange emission at 580 nm (both RTP and fluorescence modes) is known to result from the 4T1-6A1 transition of Mn2+ impurities incorporated into the ZnS host lattice excited via energy transfer from the ZnS host,2 and the emission of Mn2+ on the surface of QDs was quenched by dissolved oxygen.3c Mn-doped ZnS QDs display bright orange fluorescence in a UV lamp box (see the inset in Figure S2, part A), suggesting a relatively high quantum yield of Mn2+ transition emission (see Quantum Yield Measurement in the Supporting Information). Fluorescence, RTP, and RLS Spectra of TNT Titrating MnDoped ZnS QDs. The effect of pH on the emission intensity of the 4T1-6A1 transition of Mn2+ impurities is very significant. The intensity of the 4T1-6A1 transition emission both increased

gradually with the increase of pH from 6.0 to 11.0 while spectroscopic measurement setting as fluorescence and phosphorescence modes, but there is a relatively stable range between 7.0 and 9.0. In addition, weaker alkalinity is propitious to the interaction between electron-rich and electron-deficient groups. Therefore, all experiments were performed in aqueous solution buffered with Tris-HCl at pH 7.5. To explore the potential applications of the Mn-doped ZnS QDs, the effects of TNT on the fluorescence, RTP, and RLS spectra of the Mndoped ZnS QDs were investigated at the same time. The results shown in Figure 2, parts A and B, indicate that the 4T1-6A1 transition gradually reduced with the TNT being injected into the QDs aqueous solution no matter what emission mode was set as fluorescence or phosphorescence mode. The blue emission at 430 nm became greater and greater. Such enhancement was eventually testified to mainly result from RS but include the contribution of gradually decreased defectrelated emission of ZnS QDs (see the section Possible Mechanism of Mn-Doped ZnS QDs for RS Enhancement at DefectRelated Emission Wavelength but Quenching of the 4T1-6A1 Transition in the Presence of TNT). The high RS response of QDs should be valuable in analytical applications, and then the RTP- and RS-based dual-recognition probe is established utilizing Mn-doped ZnS QDs and an ordinary fluorescence spectrometer. In comparison with reported emission quenching,4,29a our Mndoped ZnS QDs probe held equivalent signal-amplified output which made the developed methods also ultrasensitive in monitoring TNT analyte down to subnanomolar levels (see section Application of the Mn-Doped ZnS QDs for RTP to Sense TNT and for Fluorescent Chemodosimetry to Image TNT in Water). Seen from the quenching ratio of the 4T1-6A1 transition and enhancement ratio of RS at the same TNT concentration, the efficiency of the latter was much higher than the former, suggesting that RS holds comparably much better sensing sensitivity. The signal change of RLS shown in Figure 2C also demonstrated the existence of aggregation between Mn-doped ZnS QDs and TNT. Selectivity of the Mn-Doped ZnS QDs Based RS and RTP Methods. To learn the high selectivity of the present probe for TNT, the UV-vis spectra and colorimetric visualization were performed in order to investigate the interaction of L-cys and MnAnalytical Chemistry, Vol. 83, No. 1, January 1, 2011

33

the same experiments were also performed between Mn-doped ZnS QDs and these aromatic compounds. The change of system color happened at a TNT concentration of 0.3 mM, with very strong absorption at 465 and 510 nm measured. Whereas, even while the concentration of nitro aromatic compounds was high to 100 mM, there were no new absorption bands between 450 and 800 nm, and no change of system color was observed. Therefore, Mn-doped ZnS QDs probe clearly exhibited excellent selectivity for TNT over DNT, NT, and nitro phenols. Similarly, the excellent selectivity of our Mn-doped ZnS QDs probe for TNT was tested simultaneously under fluorescence and phosphorescence modes, and the following eqs 1 and 2 were used to characterize RS selectivity: SN ) (Ii - I0)Ci

(1)

SN' ) SN/STNT

(2)

where I0 and Ii are the instant and original blue RS (or fluorescence) intensity in the absence and presence of TNT or other nitro aromatic compounds, respectively, Ci is the instant concentration of above compounds, SN denotes the selectivity of every analyte, and SN′ represents the selectivity relative to TNT. The RTP quenching followed the Stern-Volmer equation shown in eq 3:

Figure 2. TNT concentration-dependent fluorescence (A), RTP (B), and RLS spectra (C) of the Mn-doped ZnS QDs. The inset in panel C shows the change in RLS signal of Mn-doped ZnS QDs with the increase of TNT (10 µL 4 × 10-5 M of TNT stock solution was injected into ZnS QDs solution every time).

doped ZnS QDs with TNT. As shown in Figure 3, while TNT was added into L-cys and Mn-doped ZnS QDs aqueous solutions, new absorption bands were observed for both two solutions; the peak absorption lay in 510 nm for L-cys solution, 465 and 510 nm for Mn-doped ZnS QDs solution, and the absorption peak gradually increased with the persistent addition of TNT, which implied the generation of a third phase in solution, namely, the formation of MHCs (see the section Possible Mechanism of Mn-Doped ZnS QDs for RS Enhancement at Defect-Related Emission Wavelength but Quenching of the 4T1-6A1 Transition in the Presence of TNT). Meanwhile, the photographs of TNT titrating the above two solutions were taken and displayed the change of system color before and after the addition of TNT (see the inset in Figure 3). To clarify the selectivity of Mn-doped ZnS QDs for TNT over other nitro aromatic compounds, such as DNT, NT, 2-NP, and 4-NP, 34

Analytical Chemistry, Vol. 83, No. 1, January 1, 2011

I0 /I ) 1 + KSVCq

(3)

S ) KSV,TNT /KSV,OA

(4)

where I0 and I are the phosphorescent intensities in the absence and presence of quenchers, respectively, Cq is the concentration of the quencher, OA represents other nitro aromatic compounds except for TNT, and KSV is the quenching constant of the quenchers. The ratio (S) of KSV of TNT and other analytes was adopted to evaluate the selectivity of the materials. The same measurement process replicated three times. The results are illustrated in Figure 4. It can be found that different from the 4T1-6A1 transition quenching and RS enhancement of TNT, the addition of other nitro aromatic compounds all much more laggardly quenched the 4T1-6A1 transition and defect-related emission (see Figure S3 in the Supporting Information). Detailed data (see Table S1 in the Supporting Information) indicates that QDs possess excellent selectivity for TNT and therefore can sense TNT in environmental and biological samples ultraselectively without the interferences from other nitro aromatic compounds. The Mn-doped ZnS QDs gave excellent selectivity for RTP detecting TNT in the presence of the main relevant metal ions and organic compounds in pond water sample (see the Matrix Effect section and Table S2 in the Supporting Information). Quenching of the RTP emission due to the addition of TNT at 0.1 µM was unaffected by 10 000-fold excesses of Li+, Na+, K+, Mg2+, Ca2+, Sr2+, and Al3+, 10-fold excesses of Mn2+, Fe2+, Co2+, and Ni2+, and 5-fold excesses of Zn2+, Cu2+, Pb2+, and Cd2+. Fulvic acid as the representative of organic compounds was permitted at saturated solution of pH 7.5, without interference with the detection of TNT.

Figure 3. UV-vis spectra of the MHCs between TNT and L-cys (2.0 mM) (A) and between TNT and Mn-doped ZnS QDs (B). The concentration of TNT is 0, 0.3, 0.5, 0.7, and 1.0 mM, respectively. The insets show the corresponding colors at different concentrations of TNT. The inset in panel A shows the corresponding SN′ magnification of DNT, NT, 2-NP, and 4-NP.

Figure 4. Demonstration of selectivity of Mn-doped ZnS QDs probe for TNT over other nitro aromatic compounds. Log SN and KSV (µM-1) for fluorescence (A) and phosphorescence (B), respectively.

Possible Mechanism of Mn-Doped ZnS QDs for RS Enhancement at Defect-Related Emission Wavelength but Quenching of the 4T1-6A1 Transition in the Presence of TNT. TNT molecules may quench the emission of Mn-doped ZnS QDs with the band gap approaching the absorption band edge of TNT through a charge-transfer process.33 The UV absorption band of the MHCs is close to the band gap of the Mn-doped ZnS QDs from the absorption spectra of the QDs (see Figure 3).4 The charge at the conductive band of the QDs can directly transfer to the lowest unoccupied molecular orbital (LUMO) of the ultraviolet band of the MHCs. Moreover, because the visible absorption wavelengths of MHCs (at 465 and 510 nm) are much shorter than the emission wavelength of the 4T1-6A1 transition of Mn2+ (at 580 nm) but longer than the emission wavelength of Mn-doped ZnS QDs (at 430 nm), the electrons may also transfer from the conductive band and defect band to the LUMO of the visible band of the MHCs. Such a mechanism has also been reported by Zhang’s and Yan’s groups, respectively. However, how to explain the enhancement at 430 nm in our present work? Enhancement at 430 nm but quenching at 580 nm seems to be a new kind of photoinduced electron-transfer (PET) phenomenon. TNT is inclined to form MHCs with L-cys, which has been reported recently using 13C NMR spectroscopy by Fant et al.34 MHCs are σ-complexes formed by covalent addition of nucleophiles to a ring carbon atom of electron-deficient aromatic substances. Since TNT is typically electron-deficient due to the (33) Nieto, S.; Santana, A.; Herna´ndez, S. P.; Lareau, R.; Chamberlain, R. T.; Castro, M. E. Proc. SPIEsInt. Soc. Opt. Eng. 2004, 5403, 256–259.

strong electron-withdrawing effect of the nitro group, a chargetransfer complexing interaction takes place between the electron-deficient aromatic ring of TNT and electron-rich amino group to form TNT anion in solution.21b,c,24a,35 The generation of TNT anion can strongly absorb the visible light, leading to the change of solution color; this phenomenon was first observed by Janovsky and Erb in 188636 and later interpreted by other research groups.37 TNT is also able to form MHCs even in the gas phase.34b The interdots aggregation among capped gold nanoparticles by means of the formation of MHCs can result in the enhancement of the Raman signal or the change of colorimetric visualization for ultrasensitive and selective detection of TNT in solution.19,31 In this work, the interactions between Mn-doped ZnS QDs and TNT were also investigated by monitoring the phosphorescence emission decay. It is found that in the presence of TNT the phosphorescence emission lifetime of Mn-doped ZnS QDs did not undergo any evident variation (see Figure S4 in the Supporting Information), indicating that TNT led to the interdots aggregation without affecting the decay kinetics of radiative and nonradiative processes.38 Our TEM image confirmed the aggregation (see Figure 5), and the RLS spectra gave the same result (see Figure (34) (a) Fant, F.; de Sloovere, A.; Matthijsen, K.; Marle, C.; Fantroussi El, S.; Verstraete, W. Environ. Pollut. 2001, 503, 507–512. (b) Jehuda, Y.; Johonson, V. J.; Bernier, R. U.; Yost, A. R.; Mayfield, T. H.; Mahone, W. C.; Vorbeck, C. J. Mass Spectrom. 1995, 30, 715–722. (35) (a) Kang, S.; Green, J. P. Proc. Natl. Acad. Sci. U.S.A. 1970, 67, 62–67. (b) Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321–5322. (36) Janovsky, J. V.; Erb, L. Ber. Dtsch. Chem. Ges. 1886, 18, 2155–2158. (37) Caldin, E. F.; Long, G. Proc. R. Soc. London, Ser. A 1955, 228, 263–285.

Analytical Chemistry, Vol. 83, No. 1, January 1, 2011

35

Figure 5. TEM image demonstrating the interdots aggregation induced by the formation of MHCs of Mn-doped ZnS QDs in the presence of TNT. Scale bar: 10 nm.

2C and RLS in the Supporting Information). As for other nitro aromatic compounds, the lack of a nitro group results in the lack of negative charge in the benzene ring, which confined the formation of MHCs.19 The Rayleigh scattering equation can be written as I ) KυV2 /λ4

(5)

where K is constant, υ is the amount of QDs, V is the volume of aggregation, and λ is the excitation wavelength. In the presence of TNT, given half an amount of QDs interdots aggregated with TNT through the formation of MHCs, then both V and I increased 2-fold. Moreover, the excitation light of Mn-doped ZnS QDs is violet light, and the edge of the defect-related emission of Mndoped ZnS QDs is close to the excitation wavelength; thus, significant RS enhancement at this emission wavelength was observed, which implied L-cys-capped Mn-doped ZnS QDs have great potential as an RS probe for TNT featuring high sensitivity and selectivity. The TNT-dependent fluorescence experiments were repeated on a Perkin-Elmer LS-45 spectrometer while a 400 nm filter being inserted in front of the sample cell to avoid the interference from scattering light and double frequency according to Zhang’s group’s method.4 The results indicated the intensity at 430 and 580 nm both decreased (see Figure S5 in the Supporting Information), which manifested the nonoccurrence of the PET process in TNT titrating Mn-doped ZnS QDs experiments. So did the lifetime measurement experiments (see Lifetime Measurement and Figure S6 in the Supporting Information). Finally, ZnS QDs were testified to hold analogous RS response (see Figure S7 in the Supporting Information). Abundant exhibition of RS response was obtained when the stabilizer of Mn-doped ZnS QDs aggregated with TNT through acid-base pairing interaction (stabilizer is cysteamine)4 or acid-base pairing

interaction assisted by hydrogen bonding and electrostatic interaction (stabilizer is L-cys, possible analogues are GSH and homocysteine) (experimental data not shown).19 Besides, due to the close proximity between QDs and MHCs, and partial overlap of the absorption band of MHCs with the emission of Mn-doped ZnS QDs, the fluorescence resonance energy transfer (FRET) also possibly contributed to the quenching of emission of Mn-doped ZnS QDs. Application of the Mn-Doped ZnS QDs for RTP to Sense TNT and for Fluorescent Chemodosimetry to Image TNT in Water. Contamination of soil and groundwater with TNT is the major concern because of its biological persistence, toxicity, and mutagenicity.39 TNT can be absorbed through skin, and people who are exposed to TNT over a prolonged period tend to experience aplastic anemia, skin hemorrhages, and abnormal liver functions.40 Other effects include leukocytosis, peripheral neuritis, cardiac irregularities, renal irritation, and bladder tumors.41 The development of specific and sensitive methods for TNT determination in environmental and biological samples is of significant importance. The most unique and important advantage of RTP over other luminescence analyses was the minimum interference from shortlife autofluorescence and scattering light, which is a remarkable merit of the RTP for the analysis of complex biological and environmental samples since tedious sample pretreatment procedures can be avoided.3 The MHCs quenching phosphorescence had a distinct linearly reduce toward TNT in the concentration range of 0.0025-0.45 µM with a correlation coefficient of 0.9961 and a linear regression equation of ∆I ) 696.8C + 22.5 (where C is the concentration of TNT in µM, I represents phosphorescence intensity); the precision for five replicate detections of 0.1 µM TNT was 2.3% (RSD). The detection limit (DL), calculated as the concentration of TNT which produced a phosphorescence reduce three times the standard deviation of the blank signal, was 0.8 nM, close to the reported fluorescence methods.4,28 Figure S8 (see the Supporting Information) shows the difference in the background of a pond water matrix between the two detection modes of fluorescence and phosphorescence. No RTP background (curve 2) was observed for the real water sample, whereas the fluorescent background (curve 1) was significant. The quantitative recoveries were 107%, 103%, and 98.4%, respectively, for the pond water spiked with 2.5, 5.0, and 20.0 nM TNT, demonstrating the potential applicability of Mn-doped ZnS QDs for the determination of TNT in real samples. The developed RTP method based on Mn-doped ZnS QDs was also applied to analyze pond water samples spiked TNT without the interference of autofluorescence and scattering light. The analytical results are given in Table 1. It can be seen that the autofluorescence interference was

Table 1. Analytical Results for the Detection of TNT in Pond Water Samples added (µM)

36

samples

TNT

DNT

NT

2-NP

4-NP

TNT determined (µM, mean ± sd, n ) 3)

1 2 3 4 5 5 6

0 0.1 0.2 0.2 0.2 0.2 0.2

0 0 2.0 0 0 0 2.0

0 0 0 2.0 0 0 2.0

0 0 0 0 5.0 0 5.0

0 0 0 0 0 5.0 5.0

not detectable 0.10 ± 0.01 0.19 ± 0.01 0.20 ± 0.02 0.20 ± 0.01 0.20 ± 0.01 0.19 ± 0.03

Analytical Chemistry, Vol. 83, No. 1, January 1, 2011

Figure 6. RS images of the mixture of Mn-doped ZnS QDs with different concentrations of TNT: 0 µM (left), 5.0 µM (middle), and 10.0 µM (right).

completely avoided under the phosphorescence detection mode and the concentration of TNT in the spiked pond water samples determined by the above method coincided with those of added TNT. As a result, the experiments clearly showed the potential applicability of the RTP optosensor based on Mn-doped ZnS QDs for the quantification of TNT in real samples. Fluorescence-based “turn-on” chemodosimetry for target analytes is a perfect analytical method;42 such a method was employed to investigate the RS-based sensing possibility and ability for TNT in water. The solutions with the same Mn-doped ZnS QDs concentration but different TNT concentrations were prepared in water, and 0.1 mL of the above solutions was dropped onto glass slides and dried with argon; then their RS images were obtained using confocal laserscanning microscopy at UV excitation wavelength. Results in Figure 6 display that the RS chemodosimetry image became clearer and clearer with the gradual increase of TNT into Mn-doped ZnS QDs aqueous solution, which indicated the interdots aggregation enhanced RS at 430 nm. RS chemodosimetry image has advantages over solution-based methods: the effect of solvation on the interaction between analyte and the receptor is inclined to weaken.43 Figure 6 shows that the brightness of the image was so strong that we could facilely distinguish the color variation before and after the addition of TNT when the concentration of TNT was only 5.0 µM. However, these changes in RS intensity were not observed in other nitro compounds with the concentration even high to 100 mM. Moreover, we found that RS images on the glass slide with TNT were reproducible owing to the strong RS enhancement effect (replicated three times). These results implied that RS images could be utilized as an RS chemodosimeter for the detection of TNT. In addition, at a high concentration of TNT (0.3 mM), the colorimetric response (colorless to red, see the UV spectra in Figure 3B) can be observed by the naked eye. CONCLUSIONS In summary, the interdots aggregation caused by the formation of MHCs through L-cys stabilizer anchored at the surface of Mn(38) Sharma, S. N.; Pillai, Z. S.; Kamat, P. V. J. Phys. Chem. B 2003, 107, 10088– 10093. (39) (a) Dillewijn, P. V.; Couselo, J. L.; Corredoira, E.; Delgado, A.; Wittich, R. M.; Ballester, A.; Ramos, J. L. Environ. Sci. Technol. 2008, 42, 7405– 7410. (b) Hawari, J.; Beaudet, S.; Halasz, A.; Thiboutot, S.; Ampleman, G. Appl. Microbiol. Biotechnol. 2000, 54, 605–618. (40) (a) Nipper, M.; Carr, R. S.; Biedenbach, J. M.; Hooten, R. C.; Miller, K.; Saepoff, S. Arch. Environ. Contam. Toxicol. 2001, 41, 308–318. (b) Nipper, M.; Carr, R. S.; Biedenbach, J. M.; Hooten, R. C.; Miller, K. Mar. Pollut. Bull. 2002, 44, 789–806. (41) Djerassi, L. S.; Vitany, L. Br. J. Ind. Med. 1975, 32, 54–58. (42) Kim, Y.-R.; Kim, H. J.; Kim, J. S.; Kim, H. Adv. Mater. 2008, 20, 4428– 4432. (43) Xiang, Y.; Tong, A.; Jin, P.; Ju, Y. Org. Lett. 2006, 8, 2863–2866.

doped ZnS QDs interacting with TNT resulted in emission quenching of the 4T1-6A1 transition of Mn2+ impurities and RS enhancement at defect-related emission wavelength. Accordingly, we have developed novel Mn-doped ZnS QDs based RS enhancement and RTP quenching methods for simply, sensitively, and selectively sensing and imaging TNT in water. Although the research on the nanocrystal-based RTP chemosensor and RS chemodosimeter dual-recognition probe is in its infancy, the obtained results confirmed the possibility of modulating the RS, implying a great potential for the design and synthesis of nanometer devices and machines aiming at a turbidity sensor. Endeavors to explore the application of this RS device are currently in progress. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (90913012, 20575027), National Basic ResearchProgramofChina(973program,2009CB421601,2011CB911003), National Science Funds for Creative Research Groups (20821063), and Analysis & Test Fund of Nanjing University. The authors thank Professor Dr. Zhongping Zhang, Key Laboratory of Biomimetic Sensing and Advanced Robot Technology, Institute of Intelligent Machines, Chinese Academy of Sciences, Professor Dr. Weijun Jin, College of Chemistry, Beijing Normal University, and Dr. Hefang Wang, College of Chemistry, Naikai University, for their instructive comments. The authors also thank Miss Ling Wang, School of Life Sciences, Nanjing University, for her help in the fluorescent chemodosimetry image experiment. SUPPORTING INFORMATION AVAILABLE FT-IR spectra of Mn-doped ZnS QDs and the aggregation of Mn-doped ZnS QDs with TNT; fluorescence and RTP spectra of Mn-doped ZnS QDs; RLS; data of fluorescence selectivity (SN), relative selectivity (SN′), and slope of the Stern-Volmer curve of phosphorescence; 4-NP concentration-dependent fluorescence (A) and RTP (B) spectra of the Mn-doped ZnS QDs; TNT concentration-dependent fluorescence spectra of the ZnS QDs; TNT concentration-dependent fluorescence spectra of the ZnS QDs recorded on a Perkin-Elmer LS-45 spectrometer; lifetime measurement; fluorescence (curve 1) and RTP (curve 2) spectra of pond water matrix, matrix effect. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review April 6, 2010. Accepted November 4, 2010. AC1008942

Analytical Chemistry, Vol. 83, No. 1, January 1, 2011

37