Nanocrystals for Fluorescence Detection of Trace TNT Explosive

Mar 13, 2008 - Renyong Tu,†,‡ Bianhua Liu,† Zhenyang Wang,† Daming Gao,† Feng Wang,† Qunling Fang,† and. Zhongping Zhang*,†. Key Labor...
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Anal. Chem. 2008, 80, 3458-3465

Amine-Capped ZnS-Mn2+ Nanocrystals for Fluorescence Detection of Trace TNT Explosive Renyong Tu,†,‡ Bianhua Liu,† Zhenyang Wang,† Daming Gao,† Feng Wang,† Qunling Fang,† and Zhongping Zhang*,†

Key Laboratory of Biomimetic Sensing and Advanced Robot Technology, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031, China, and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China

Mn2+-doped ZnS nanocrystals with an amine-capping layer have been synthesized and used for the fluorescence detection of ultratrace 2,4,6-trinitrotoluene (TNT) by quenching the strong orange Mn2+ photoluminescence. The organic amine-capped nanocrystals can bind TNT species from solution and atmosphere by the acid-base pairing interaction between electron-rich amino ligands and electron-deficient aromatic rings. The resultant TNT anions bound onto the amino monolayer can efficiently quench the Mn2+ photoluminescence through the electron transfer from the conductive band of ZnS to the lowest unoccupied molecular orbital (LUMO) of TNT anions. The amino ligands provide an amplified response to the binding events of nitroaromatic compounds by the 2- to ∼5-fold increase in quenching constants. Moreover, a large difference in quenching efficiency was observed for different types of nitroaromatic analytes, dependent on the affinity of nitro analytes to the amino monolayer and their electron-accepting abilities. The amine-capped nanocrystals can sensitively detect down to 1 nM TNT in solution or several parts-per-billion of TNT vapor in atmosphere. The ion-doped nanocrystal sensors reported here show a remarkable air/solution stability, high quantum yield, and strong analyte affinity and, therefore, are well-suited for detecting the ultratrace TNT and distinguishing different nitro compounds. Nitroaromatic compounds are highly explosive and environmentally deleterious substances that have been of pressing societal concern, and therefore the development of robust and sensitive platforms for their real-time analytical detection has attracted considerable research efforts in recent years.1-6 Fluorescence* To whom correspondence should be addressed. E-mail: zpzhang@iim. ac.cn. † Chinese Academy of Sciences. ‡ University of Science and Technology of China. (1) (a) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537. (b) Steinfeld, J. I.; Wormhoudt, J. Annu. Rev. Phys. Chem. 1998, 49, 203. (c) Moore, D. S. Rev. Sci. Instrum. 2004, 75, 2499. (d) Toal, S. J.; Trogler, W. C. J. Mater. Chem. 2006, 16, 2871. (2) (a) Rose, A.; Zhu, Z. G.; Madigan, C. F.; Swager, T. M.; Bulovic´, V. Nature 2005, 434, 876. (b) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 7017. (c) Zahn, S.; Swager, T. M. Angew. Chem., Int. Ed. 2002, 41, 4225. (3) Pushkarsky, M. B.; Dunayevskiy, I. G.; Prasanna, M.; Tsekoun, A. G.; Go, R.; Patel, C. K. N. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 19630.

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based chemosensors, where analyte binding produces an attenuation in the light emission, are highly desirable for the detection of small molecular analytes in many challenging environments, due to the high signal output and detection simplicity. In particular, the electron-deficient nitroaromatics are strong quenchers of the fluorescence of electron-rich chromophores via an electrontransfer mechanism. Of various photoluminescence materials, conjugated polymers are most extensively explored as chemosensory materials for the fluorescence detection of electron-deficient analytes such as nitroaromatic compounds.2,7 Swager and coworkers reported the amplified response to the analyte binding events in the aggregated systems and solid films of conjugated polymers by intermolecular exciton migration.2 Particularly, the multiphoton fluorescence quenching has been observed with obvious advantages for the real-time detection of TNT.7 Just recently, the molecular imprinting in the matrix of conjugated polymers has greatly improved the chemosensory selectivity to nitroaromatics.8 Other photoluminescent materials such as polytetraphenylsilole, polytetraphenylgermole, photoluminescent silica films,9 and silica microspheres with physisorbed dyes10 also exhibit the high fluorescence response to the solution and vapor of nitroaromatic explosives at low-level concentrations. Meanwhile, it would be naturally conceivable to develop the fluorescence-based sensors with a high chemoselectivity by assembling recognition receptors (binding target species) and (4) (a) 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. (b) Medintz, I. L.; Goldman, E. R.; Lassman, M. E.; Hayhurst, A.; Kusterbeck, A. W.; Deschamps, J. R. Anal. Chem. 2005, 77, 365. (5) (a) Gao, D. M.; Zhang, Z. P.; Wu, M. H.; Xie, C. G.; Guan, G. J.; Wang, D. P. J. Am. Chem. Soc. 2007, 129, 7859. (b) Xie, C. G.; Zhang, Z. P.; Wang, D. P.; Guan, G. J.; Gao, D. M.; Liu, J. H. Anal. Chem. 2006, 78, 8339. (c) Xie, C. G.; Liu, B. H.; Wang, Z. Y.; Gao, D. M.; Guan, G. J.; Zhang, Z. P. Anal. Chem., in press. (6) Guan, G. J.; Zhang, Z. P.; Wang, Z. Y.; Liu, B. H.; Gao, D. M.; Xie, C. G. Adv. Mater. 2007, 19, 2370. (7) (a) Yang, J-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864. (b) Yamaguchi, S.; Swager, T. M. J. Am. Chem. Soc. 2001, 123, 12087. (c) Narayanan, A.; Varnavski, O. P.; Swager, T. M.; Goodson, T., III. J. Phys. Chem. C 2008, 112, 881. (8) Li, J. H.; Kendig, C. E.; Nesterov, E. E. J. Am. Chem. Soc. 2007, 129, 15911. (9) (a) Toal, S. J.; Magde, D.; Trogler, W. C. Chem. Commun. 2005, 5465. (b) Sohn, H.; Calhoun, R. M.; Sailor, M. J.; Trogler, W. C. Angew. Chem., Int. Ed. 2001, 40, 2104. (c) Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. J. Am. Chem. Soc. 2003, 125, 3821. (d) Content, S.; Trogler, W. C.; Sailor, M. J. Chem. Eur. J. 2000, 6, 2205. (10) Albert, K. J.; Walt, D. R. Anal. Chem. 2000, 72, 1947. 10.1021/ac800060f CCC: $40.75

© 2008 American Chemical Society Published on Web 03/13/2008

transducers (signaling the binding events). Recently, semiconductive quantum dots (QDs) and nanocrystals with high quantum yields have found a wider range of applications as a foundation of fluorescence sensors. 4,11-13 The photoluminescence of the semiconductive nanomaterials is readily tunable within a large range of spectroscopy through the change of size or the introduction of dopant ions, which can potentially be utilized for obtaining a spectral response toward a particular target analyte.12c More importantly, the semiconductive nanomaterials allow the chemical modification of functional groups and the installation of recognition receptors at their surfaces, providing the chemodetection selectivity to target species.11-13 Therefore, the fluorescence chemosensors based on the “lab-on-QDs” concept have a remarkable advantage over other detection schemes in chemodetection sensitivity and selectivity. For example, Goldman and co-workers recently proposed a typical scheme of QDs-based chemosensors through the hybrid CdSe QDs of antibody segments and dye molecules.4 The specific detections toward 2,4,6-trinitrotoluene (TNT), maltose, DNA, and enzymic activities have been achieved through the fluorescence resonance energy transfer (FRET) between QDs and dyes.4,12 These above understandings open up the tremendous prospect for the novel scheme of chemo/ biosensors. Although the QDs-based chemosensors have been extensively demonstrated, the ion doping in semiconductive nanocrystals for obtaining the spectral sensitivity to the analytes of interest has rarely been explored.14 It is well-known that zinc sulfide (ZnS) is particularly suitable for use as a host material for a large variety of dopants because of its wide band gap (3.7 eV). A frequently cited research about doped semiconductors appeared in 1994. Bhargava and Gallagher reported that ZnS nanocrystals doped with Mn2+ exhibited a high luminescence quantum efficiency.15 The orange emission around 600 nm originates from the 4T1-6A1 transition of the Mn2+ ions on Zn2+ sites, where the Mn2+ is tetrahedrally coordinated by S2-.16,17 Usually, the fluorescence emission of doping ions has a higher photostability than the defectrelated luminescence of semiconductive nanomaterials, because the defects are greatly affected by synthesis conditions and environments. The strong Mn2+ photoluminescence at long wavelength should be better suited for the sensing application in the environments of solvent and atmosphere. Here we study the possibility of preparing the TNT chemosensors based on the spectral feature of ZnS-Mn2+ nanocrystals through the modification of amino ligands at the surface of nanocrystals. It has been (11) (a) Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H. Nat. Mater. 2005, 4, 435. (b) Han, M. Y.; Gao, X. H.; Su, J. Z.; Nie, S. M. Nat. Biotechnol. 2001, 19, 631. (c) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016. (12) (a) 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. (b) Shi, L. F.; Paoli, V. D.; Rosenzweig, N.; Rosenzweig, Z. J. Am. Chem. Soc. 2006, 128, 10378. (c) Peng, H.; Zhang, L. J.; Kja¨llman, T. H. M.; Soeller, C.; Travas-sejdic, J. J. Am. Chem. Soc. 2007, 129, 3048. (13) (a) Dayal, S.; Burda, C. J. Am. Chem. Soc. 2007, 129, 7977. (b) Zhelev, Z.; Ohba, H.; Bakalova, R. J. Am. Chem. Soc. 2006, 128, 6324. (14) Santra, S.; Yang, H.; Holloy, P. H.; Stanley, J. T.; Mericle, R. A. J. Am. Chem. Soc. 2005, 127, 1656. (15) Bhargava, R. N.; Gallagher, D. Phys. Rev. Lett. 1994, 72, 416. (16) (a) Sapra, S.; Prakash, A.; Ghangrekar, A.; Periasamy, N.; Sarma, D. D. J. Phys. Chem. B 2005, 109, 1663. (b) Biswas, S.; Kar, S.; Chaudhuri, S. J. Phys. Chem. B 2005, 109, 17526. (17) (a) Yu, I.; Isobe, T.; Senna, M. J. Phys. Chem. Solids 1996, 57, 373. (b) Bol, A. A.; Meijerink, A. J. Phys. Chem. B 2001, 105, 10197. (c) Suyver, J. F.; Wuister, S. F.; Kelly, J. J.; Meijerink, A. Nano Lett. 2001, 1, 429.

demonstrated that the strong Mn2+ photoluminescence can sensitively respond to the presence of TNT analyte in solution and atmosphere through fluorescence quenching. The chemosensory efficiency is greatly enhanced by the organic amino monolayer at the surface of nanocrystals. TNT molecules can strongly bind onto the surface of nanocrystals through the acid-base pairing interaction between TNT and amino ligands. Because the absorption band edge of TNT anions is lower to the conduction band of ZnS and higher than the 4T1 band of Mn2+ ions, the orange Mn2+ emission is efficiently quenched through the direct electron transfer from the conductive band of ZnS to the lowest unoccupied molecular orbital (LUMO) of TNT anions. The nanosensors can sensitively detect down to 1 nM TNT in solution or several partsper-billion of TNT vapor in air and selectively distinguish different types of nitro compounds. EXPERIMENTAL SECTION Materials. Analytically pure sodium sulfide (Na2S‚9H2O), zinc nitrate hexahydrate (Zn(NO3)2‚6H2O), manganese acetate tetrahydrate (Mn(CH3COO)2‚4H2O), picric acid (PA), and nitrobenzene (NB) were used as received from Shanghai Chemicals Ltd. 2,4,6-Trinitrotoluene (TNT) and 1,3,5-trinitro-1,3,5-triazinane (RDX) were supplied by the National Security Department of China and were recrystallized with ethanol before use. Cysteamine (HSCH2CH2NH2) and 2,4-dinitrotoluene (DNT) were purchased from Aldrich and Merch-Schuchardt (Hohenbrunn, Germany), respectively. Caution: The highly explosive TNT and RDX should be used with extreme caution and handled only in small quantities. Synthesis and Surface Modification of Manganese-Doped ZnS Nanocrystals. The procedure used to prepare the ZnSMn2+ nanocrystals is similar to that described in the literature17 but was carried out in water instead of methanol. Sodium sulfide was used at the mole amount equal to that of zinc nitrate. Typically, 50 mmol of zinc nitrate was dissolved in 40 mL of deionized water. An amount of 0.5 to ∼7.5 mmol of manganese acetate was added into the above solution, and the mixture was ultrasonicated for 10 min at room temperature. After the mixture solution was refluxed in a flask under nitrogen, 10 mL of aqueous solution containing 50 mmol of sodium sulfide was added dropwise into the reaction system, and the mixture was vigorously stirred for 5 h. The resultant ZnS-Mn2+ nanocrystals were centrifuged and washed with deionized water and absolute ethanol several times. For the further surface modification, 0.1 g of ZnS-Mn2+ nanocrystals was redispersed in 50 mL of dehydrated ethanol and then purged with dry nitrogen for 30 min to exclude the oxygen in the ethanol. An amount of 2.5 mmol of cysteamine was dissolved into the above solution, and the mixture was stirred for 24 h in a sealed vessel. Mercapto groups of cysteamine tightly attached onto the surface of the nanocrystals due to the excess of metal ions with respect to sulfide ions at the surface of the nanocrystals. The resultant amine-capped nanocrystals were centrifuged and washed with ethanol several times to remove the residue of cysteamine and were redispersed in ethanol for use. Fluorescence Quenching Effects of Amine-Capped Nanocrystals with Nitroaromatics. We used the amine-capped ZnS nanocrystals with 0.33 wt % of Mn2+ ions for the fluorescence detections of TNT and other nitroaromatic compounds, because the Mn2+ concentration can result in a strong Mn2+ photolumiAnalytical Chemistry, Vol. 80, No. 9, May 1, 2008

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nescence with a symmetric emission peak at 600 nm. In a typical procedure, 2 mL of stock solution containing 50 µg of aminecapped ZnS-Mn2+ nanocrystals was added to a quartz cuvette. A volume of 20 µL of the known concentration of analyte (solvent: ethanol/acetonitrile, 8:2, v/v) was then injected into the cuvette. Fluorescence spectra were recorded immediately after mixing the analyte with the solution of amine-capped nanocrystals. The tests of the fluorescence response to the ultralow concentration of analytes were carried out in ∼5 µg/mL nanocrystal solution according to the same procedure. Before the measurement of sensitivity to explosive vapors, the suspension of amine-capped nanocrystals was first coated onto a piece of filter film (1 × 2 cm2) and dried at ambient temperature. Then, the filter film was placed under a UV lamp for 24 h to make the fluorescence intensity stable.18 Small granules or liquid drops of nitro analytes were placed on the bottom of a sealed testing box with two quartz windows and a slot. Meanwhile, a small cotton gauze was tucked into the testing box to help maintain a constant vapor pressure of analyte.7,19 After the slot was sealed, the vapor of nitro compound was up to saturation in the testing box for 24 h at room temperature (25 °C). The filter film with nanocrystals was fixed onto the sample shelf tightly matched with the slot of the testing box and promptly inserted into the testing box filled with saturated vapor through the slot. The evolution of fluorescence spectra was recorded for specific time intervals after exposing the nanocrystal films to the vapor of analytes. Characterization. The phase of nanocrystals was determined by X-ray powder diffraction (XRD) with Cu KR radiation on an MXP18AHF X-ray diffractometer. The size and morphology of nanocrystals were examined by a JEOL 2010 transmission electron microscope. The concentration of Mn2+ ions was measured with inductively coupled plasma analysis (ICP, Perkin-Elmer Optima 3000). Steady-state luminescence spectra were acquired under excitation at 300 nm on a Perkin-Elmer LS-45 luminescence spectrometer. The UV-vis absorbance spectra and infrared spectra were recorded with a Shimadzu UV-2550 spectrometer and a Nicolet Nexus-670 Fourier transform infrared (FT-IR) spectrometer, respectively. RESULTS AND DISCUSSION In this research, we used the ZnS nanocrystals containing about 0.3 wt % of Mn2+ ions as chemosensory materials of nitroaromatics, because this concentration can usually lead to a high luminescence quantum yield.16,17 When 8% mol ratio of manganese acetate to zinc nitrate was used, and the mole amount of sodium sulfide (Na2S) was equal to that of zinc nitrate, the ICP revealed that the concentration of Mn2+ into ZnS crystalline lattice was about 0.33 wt %. Mercapto groups of cysteamine can tightly attach onto the surface of ZnS-Mn2+ nanocrystals due to the excess of metal ions with respect to sulfide ions at the surface of nanocrystals, leading to the formation of amine-capped nanocrystals. The surface-stabilized nanocrystals can be kept in ethanol for several months without noticeable precipitation and spectral change. The structure and composition of as-synthesized ZnSMn2+ nanocrystals confirmed by XRD are cubic zinc sulfide, and the obvious broadening of XRD lines was observed (data not (18) Bol, A. A.; Meijerink, A. J. Phys. Chem. B 2001, 105, 10203. (19) Zhang, S. J.; Lu ¨ , F. T.; Gao, L. N.; Ding, L. P.; Fang, Y. Langmuir 2007, 23, 1584.

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Figure 1. Fluorescence emission and UV absorption spectra of amine-capped ZnS-Mn2+ nanocrystals in ethanol. Inset is the optical photo under a 300 nm UV lamp.

shown). The particle size determined from the XRD data with the Debye-Scherrer formula is ∼3-5 nm, which was further examined by transmission electron microscopy. The infrared spectroscopy clearly confirmed that the ZnS-Mn2+ nanocrystals were covered with the monolayer of organic amino ligands. Figure 1 shows that the amine-capped nanocrystals display two PL emission peaks when excited at 300 nm (orange line). The weak blue emission around 420 nm is attributed to the defect-related emission of the ZnS. The strong orange emission with a symmetric peak around 600 nm can be attributed to the 4T1-6A1 transition of the Mn2+ impurity.15,16 The well-known green emission of Zn vacancies (at ∼480 nm) was not nearly observed in the ZnSMn2+ nanocrystals, indicating that this emission was quenched by the energy transfer to the Mn2+. Because of the relatively long lifetime of the ∼480 nm luminescence, the radiative decay cannot compete with the energy transfer to Mn2+.16 The fluorescence emission of dopant ions is more stable and controllable than the defect/vacancy-related emission, because the defects or vacancies in semiconductive nanomaterials are greatly affected by many factors such as synthesis procedure and environments. Meanwhile, we can see that the solution of amine-capped nanocrystals displays bright orange fluorescence under a UV lamp, as shown in the inset image of Figure 1, suggesting a high quantum yield of Mn2+ luminescence. The organic amino ligands at the surface of the nanocrystals play a role of the receptors of nitroaromatic compounds as well as the surface stabilizer of nanocrystals. As shown in our previous work,5,6 a strong molecular interaction occurs between the electron-deficient aromatic ring of TNT and the electron-rich amino group of primary amines in a solution system. A similar interaction occurred between TNT and cysteamine and was demonstrated by the measurements of UV-vis spectroscopy. Figure 2 shows the UV-vis absorption spectra before and after adding cysteamine into TNT solution. Two new visible absorptions at ∼465 and 515 nm were observed. Meanwhile, we can clearly see that the TNT solution changes from colorless into bright red with addition of cysteamine, as shown in the inset image of Figure 2. The TNT molecule is a Bronsted-Lowry acid that is apt to be deprotonated at the methyl group by basic cysteamine. The negative charge on the methyl anion is distributed throughout

Figure 2. Scheme for the acid-base reaction between TNT and cysteamine and the evolution of UV-vis spectra with addition of 50 mg of cysteamine into 20 mL of 1 mM TNT solution. Color inset shows the corresponding colors of the solutions before and after adding cysteamine (solvent: ethanol/acetonitrile, 8:2).

the TNT molecule through resonance stabilization by three electron-withdrawing nitro groups.20 The anion-cation pair of TNT- and RNH3+ was formed in the solution system, as illustrated in the scheme of Figure 2. The anionic form of TNT can strongly absorb the visible light, leading to the change of solution color, which was first reported by Janovsky and Erb in 188621 and interpreted later by others.22 A theoretical description about analogous systems has recently been given by Arnaud et al.23 These above observations suggest that the amine-capped ZnSMn2+ nanocrystals can efficiently enrich TNT species from solution by the acid-base pairing interaction between amino ligands and TNT. The resultant TNT anions strongly bind onto the surface of amine-capped nanocrystals. Figure 3A shows the schematic illustration of the amine-capped nanocrystal sensor for the fluorescence detection of TNT. The amino ligands at the surface of nanocrystals play a role of recognition receptor to bind TNT species through the strong acid-base pairing interaction. Moreover, the huge surface-tovolume ratio of the nanocrystals greatly enhances the adsorption affinity to TNT species. The resultant TNT anions bound onto the surface of nanocrystals can quench the fluorescence emission of the nanocrystals. The quenching efficiency is dependent on the binding affinity of nitro analytes to the nanocrystal surface and the electron-accepting ability of the analytes themselves. Figure 3B illustrates the quenching mechanism of fluorescence through the electron transfer from ZnS-Mn2+ nanocrystals to (20) Walker, N. R.; Linman, M. J.; Timmers, M. M.; Dean, S. L.; Burkett, C. M.; Lloyd, J. A.; Keelor, J. D.; Baughman, B. M.; Edmiston, P. L. Anal. Chim. Acta 2007, 593, 82. (21) Janovsky. J. V.; Erb, L. Ber. Dtsch. Chem. Ges. 1886, 18, 2155. (22) (a) Caldin, E. F.; Long, G. Proc. R.. Soc. London, Ser. A 1955, 226, 263. (b) Blake, J. A.; Evans, M. J. B.; Russell, K. E. Can. J. Chem. 1966, 44, 119. (c) Shipp, K. G.; Kaplan, L. A. J. Org. Chem. 1966, 31, 857. (d) Bernasconi, C. F. J. Org. Chem. 1971, 36, 1671. (23) Arnaud, V.; Berthelot, M.; Evain, M.; Graton, J.; Questel, J. Y. L. Chem. Eur. J. 2007, 13, 1499.

Figure 3. Schematic illustrations for (A) the amine-capped ZnSMn2+ nanocrystal sensors for TNT detection and (B) the quenching mechanism of fluorescence by the charge transfer from nanocrystals to TNT analytes.

TNT species. Generally, TNT molecules may quench the fluorescence of the QDs with the band gap close to the absorption band edge of TNT through a charge-transfer process.24 In the case of amine-capped nanocrystals, however, the fluorescence quenching is mainly achieved by the three pathways of charge transfer, due to the formation of TNT anions. From the absorption spectra of ZnS-Mn2+ nanocrystals (Figure 1, black line) and TNT (Figure 2), it can be seen that the UV absorption band of the TNT or TNT anions is close to the band gap of the ZnS nanocrystals. The charges at the conductive band of the nanocrystals can directly transfer to the LUMO of the UV band of the TNT molecules or TNT anions (pathway 1 in Figure 3B). On the other hand, as shown in Figures 1 and 2, the wavelengths of visible absorption of TNT anions (at 465 and 515 nm) are much shorter than the emission wavelength of orange Mn2+ emission (at 600 nm) but longer than the emission wavelength of ZnS (at 420 nm). Thus, the charges may also transfer from the conductive band of nanocrystals or Zn-defect-related band to the LUMO of the visible band of the TNT anions, as drawn in pathways 2 and 3 of Figure 3B, respectively. These processes of charge transfer lead to the extensive quenching of the ZnS and Mn2+ emissions. Particularly, the form of TNT anions may further enhance the quenching of Mn2+ fluorescence from pathways 2 and 3. Figure 4A shows the evolution of fluorescence intensity of amine-capped nanocrystals with increasing TNT concentration in the ethanol solution containing 25 µg/mL nanocrystals. Both the blue ZnS emission and the orange Mn2+ emission exhibit obviously the fluorescence quenching with the addition of TNT analytes through the charge transfer from nanocrystals to TNT anions, as illustrated in Figure 3B. The Mn2+ emission has a larger (24) Nieto, S.; Santana, A.; Herna´ndez, S. P.; Lareau, R.; Chamberlain, R. T.; Castro, M. E. Proc. SPIEsInt. Soc. Opt. Eng. 2004, 5403, 256.

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Figure 4. (A) Fluorescence spectra of amine-capped ZnS-Mn2+ nanocrystals with addition of the indicated concentrations of TNT in the ethanol solution containing 25 µg/mL nanocrystals. (B) The quenching efficiency for the different concentrations of amine-capped nanocrystals in 1 × 10-5 M TNT solution (1) and the intensity evolution of Mn2+ photoluminescence with the concentration of amine-capped nanocrystals in solution (2).

decrease in fluorescence intensity and relatively higher percentage of fluorescence quenching. On the other hand, the Mn2+ photoluminescence has a high fluorescence brightness and symmetric emission when excited at 300 nm. Thus, the strong Mn2+ photoluminescence is expected to provide a more sensitive response to nitroaromatic analytes than the weak ZnS photoluminescence in optical measurements. The quenching signal of orange Mn2+ emission as an indicator is particularly advantageous to the ultratrace detection of TNT. Furthermore, the quenching percentage of Mn2+ photoluminescence is dependent on the amount of amine-capped nanocrystals as well as the concentration of TNT in solution. Curve 1 of Figure 4B shows that the quenching percentage decreases rapidly with increasing the amount of the nanocrystals in 1 × 10-5 mol/L TNT solution. When the amount of nanocrystals exceeds the 50 µg/mL, a relatively small quenching percentage is detected. This is because the emission intensity significantly increases with the nanocrystal amount in solution, as shown in curve 2 of Figure 4B. The nanocrystal amount larger than 50 µg/mL exhibits a very high emission intensity. From these measurements, the nanocrystal concentration of 25 µg/mL is suitable for the evaluations of the fluorescence response of nanocrystals to TNT analytes, where the quenching percentage is about 10% for 1 × 10-5 mol/L TNT and the emission intensity is strong enough for optical detection. Meanwhile, a less dilute concentration of amine-capped nanocrystals should be used for the trace detection of nitroaromatic analytes. Parts A and B of Figure 5 compare the spectral response of amine-capped nanocrystals with that of bare nanocrystals to the three typical nitroaromatics: TNT (1), PA (2), and DNT (3). The fluorescence intensity of Mn2+ emission decreases with increasing successive aliquots of analyte concentrations in 25 µg/mL of the amine-capped and bare nanocrystals, respectively. However, it was clearly detected that the decrease of fluorescence intensity of amine-capped nanocrystals was much larger than that of bare nanocrystals for any of three analytes at the same concentration. The measurements confirm that the amine monolayer can greatly enhance the quenching efficiency of fluorescence, enlarging the spectral sensitivity of nanocrystals to the electron-deficient nitroaromatics. Generally, the efficiency of fluorescence quenching depends on the adsorptive affinity of nanocrystals to nitroaromatic molecules and the probability of charge transfer between nano3462 Analytical Chemistry, Vol. 80, No. 9, May 1, 2008

crystals and nitroaromatics. In the case of amine-capped nanocrystals, the electron-rich amine ligands can absorb the electrondeficient nitroaromatic species onto the surface of the nanocrystals. Therefore, the amine-capped nanocrystals have a higher affinity to nitroaromatic species than bare nanocrystals, leading to a higher quenching percentage of Mn2+ fluorescence. Moreover, the resultant anion species may increase the charge-transfer pathways from the nanocrystals to nitroaromatic analytes (Figure 3B) and, thus, further enhance the quenching efficiency of fluorescence. On the other hand, Figure 5A also shows that PA is the strongest quencher of Mn2+ fluorescence and DNT is the weakest quencher. This also suggests that the stronger the acidity of nitroaromatic analytes is, the higher the quenching efficiency is for the amine-capped nanocrystals. These above results are consistent with our expectation that the amine ligands can remarkably enhance the fluorescence response of the ZnS-Mn2+ nanocrystals to nitroaromatic analytes by the acid-base pairing interaction. Meanwhile, the Stern-Volmer equation was used to quantify the differences in quenching efficiency for different kinds of analytes as follows: (I0/I) - 1 ) KSV[A], where I0 is the initial fluorescence intensity in the absence of analyte, I is the fluorescence intensity in the presence of [A], and KSV is the quenching constant of the analyte. Parts A and B of Figure 6 show the SternVolmer plots from the amine-capped and bare ZnS-Mn2+ nanocrystals for TNT, DNT, NB, and RDX, respectively. The SternVolmer plot of PA was not together shown due to the extremely large quenching constant. A linear Stern-Volmer relationship was observed in the concentration range lower than 10-4 M. For the same analyte, the amine-capped nanocrystals exhibit a much higher quenching efficiency than bare nanocrystals, further suggesting that amine ligands can obviously enhance the fluorescence response of nanocrystals to the nitro analytes. Table 1 summarizes the Ksv values of amine-capped and bare nanocrystals for the five nitroaromatic analytes, respectively. The quenching constants for the amine-capped nanocrystals with PA, TNT, DNT, NB, and RDX are 4.8, 2.5, 2.1, 1.9, and 1.9-fold those for bare nanocrystals, respectively. On the other hand, Table 1 also shows that the amine-capped nanocrystals have very large quenching constants with PA and TNT, which are comparable to the conjugated polymer reported in the literature.9 Moreover, the large

Figure 5. Evolutions of Mn2+ fluorescence spectra of (A) amine-capped nanocrystals and (B) bare nanocrystals with increasing (1) TNT, (2) PA, and (3) DNT concentrations in the ethanol solution containing 25 µg/mL nanocrystals. The analyte concentrations from top to bottom are 0, 2 × 10-5, 4 × 10-5, 6 × 10-5, 8 × 10-5, and 1 × 10-4 mol/L, respectively.

Figure 6. Stern-Volmer plots from (A) amine-capped ZnS-Mn2+ and (B) bare ZnS-Mn2+ nanocrystals with different kinds of nitro analytes. The concentration of nanocrystals is 25 µg/mL in ethanol.

Table 1. Summary of Quenching Constants of the Amine-Capped and Bare Nanocrystals for PA, TNT, DNT, NB, and RDX Analytes quenching constants Ksv (M-1)

A-ZnS-Mn2+ ZnS-Mn2+ ratio of Ksv

PA

TNT

DNT

NB

RDX

34870 7340 4.8

5515 2258 2.5

3664 1713 2.1

2020 1050 1.9

1530 806 1.9

difference in the quenching constants was observed for different kinds of nitroaromatic analytes. The order of quenching constants for these five analytes is PA . TNT > DNT > NB > RDX. Picric acid exhibits a surprising superquenching to the photoluminescence of amine-capped nanocrystals and has the highest Ksv value

(34 870 M-1). PA is a stronger acid than TNT, and a stronger acid-base pairing interaction thus occurs between PA and amino ligands, resulting in the formation of PA anions at the surface of amine-capped nanocrystals. However, it is well-known that the DNT molecules with two nitro groups and the NB molecules with only one nitro group are much weaker Lewis acids and electron acceptors than TNT molecules. Although the weak interactions may lie between amine ligands and DNT or NB molecules, the UV-vis spectra do not detect any visible absorption peak when adding DNT or NB into the solution of primary amines. This suggests that DNT and NB are less deprotonated to form the corresponding anions by the relatively weak basic amine groups.5 The fluorescence quenching of the amine-capped nanocrystals with DNT and NB mainly results from pathway 1 of charge Analytical Chemistry, Vol. 80, No. 9, May 1, 2008

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Figure 7. Variation of the quenching percentage of amine-capped nanocrystals as a function of logarithm of the analyte concentration in solution. The concentration of amine-capped nanocrystals is 5 µg/ mL for these tests.

transfer, as drawn in Figure 3B. Therefore, the low affinity to the amino monolayer and weak electron-accepting ability lead to the low quenching efficiencies of DNT and NB. As a nonaromatic analyte, RDX is the lowest electron acceptor and does not have any specific acid-base interaction with the amino ligands at the surface of nanocrystals. Thus, the amine-capped ZnS-Mn2+ nanocrystals can obviously distinguish these nitro compounds by the large difference in quenching efficiency. It is desirable for fluorescence-based sensors to exhibit a large difference in quenching efficiency for structure-like analytes. The Mn2+-doped ZnS nanocrystals display a strong fluorescence brightness, and the amino ligands at the surface of nanocrystals have a high affinity to TNT molecules. It can thus be expected that the nanocrystal sensors are applicable to detect the ultratrace amount of TNT analyte in solution. When the concentration of the amine-capped nanocrystals is about 5 µg/ mL, the addition of the ultratrace TNT into this solution can cause the detectable decrease in fluorescence intensity. Figure 7 shows the quenching percentage of fluorescence with different nitroaromatic analytes in 5 µg/mL of amine-capped nanocrystals. At an ultralow level, 1.0 nM (∼0.2 ppb) of TNT in solution was detected as ∼6% decrease in the fluorescence intensity. TNT and PA almost exhibit an equal quenching percentage at the ultralow concentration. If about 5% decrease in fluorescence intensity is used as a detectable standard as in other fluorescence analyses,9 0.2 ppb of TNT or PA can clearly be detected by the fluorescence quenching signal using a luminescence spectrometer. Meanwhile, the DNT and NB at the ultralow concentration exhibit a much smaller quenching percentage to the fluorescence emission of the aminecapped nanocrystals. Figure 7 reveals that the detection limits of ∼20 and 150 ppb were observed for DNT and NB, respectively. The low detection limit of the TNT analyte confirms that the amine-capped nanocrystals have a extremely strong enriching effect to the ultratrace TNT in solution, and the resultant TNT anions have a high quenching ability to the Mn2+ photoluminescence. Meanwhile, there has been a considerable interest in the detection of nitroaromatic vapors. The Mn2+-doped nanocrystals with 3 to ∼5 nm size have an extremely high surface-to-volume ratio, and the amino ligands at the surface of the nanocrystals can further enhance the adsorption to the vapors of nitroaromatics. 3464 Analytical Chemistry, Vol. 80, No. 9, May 1, 2008

Figure 8. Fluorescence spectra of amine-capped nanocrystals on exposure to TNT-saturated air (∼4 ppb) for the indicated time periods.

Figure 9. Variation of the quenching percentage for amine-capped nanocrystals as a function of the time of exposure to the saturated air of TNT, DNT, and NB.

We thus used the amine-capped ZnS-Mn2+ nanocrystals to fabricate the thin-film-based fluorescence sensor for the detection of TNT vapor. The amine-capped nanocrystal suspension was coated onto a piece of filter film and dried at ambient temperature. Before measurements, the filter film was placed under a UV lamp for 24 h to make the fluorescence intensity stable in air. A control experiment using the film sensor with air produced no significant decrease in photoluminescence intensity. The dynamic quenching was measured under the saturated vapor of TNT in air. Figure 8 shows the fluorescence spectra of the film sensor exposed to 4 ppb of TNT vapor in air. The fluorescence intensity decreases with increasing the time of exposure to TNT vapor. Figure 9 shows the variation of the quenching percentage as a function of the time of exposure to TNT, DNT, and NB. After about 10 min, the quenching percentage of fluorescence was ∼7% for TNT-saturated air (vapor pressure ) 4 ppb at 25 °C), ∼6% for DNT-saturated air (vapor pressure ) 200 ppb at 25 °C), and ∼14% for NB-saturated air (vapor pressure ) 400 ppm at 25 °C). Because the vapor pressures of DNT and NB are about 50- and 1 × 105-fold that of TNT, respectively, the quenching percentage for TNT is thus surprisingly larger than that expected from the relative vapor pressure of these analytes. The enhanced sensitivity toward TNT vapor originates from the extremely strong adsorption of TNT species at the amino monolayer of nanocrystals and the larger quenching efficiency due to the high electron-accepting ability. Moreover, the high surface-to-volume ratio of nanocrystals is

further advantageous to the enhancement of the interaction between TNT species and the amino ligands, helping maximize the quenching efficiency of fluorescence. CONCLUSIONS We have demonstrated that Mn2+-doped ZnS nanocrystals can be used as highly sensitive materials for the ultratrace detection of TNT at soluble and atmospheric environments. The strong Mn2+ photoluminescence exhibits obviously the fluorescence quenching by the charge transfer from nanocrystals to the electron-deficient nitroaromatic compounds. The amine-capping monolayer at the surface of the nanocrystals can remarkably enhance the fluorescence response of the nanocrystals to nitroaromatic analytes by the acid-base pairing interaction between acidic nitroaromatic compounds and basic amino ligands. The extremely high quenching efficiency to Mn2+ photoluminescence is thus achieved for nitroaromatic analytes such as PA and TNT. Moreover, a large difference in quenching efficiency was observed for different types of nitroaromatic analytes, which is dependent on the affinity of nitro analytes to the amino ligands and their electron-accepting ability. The amine-capped nanocrystals can

sensitively detect down to 1 nM TNT in solution or several partsper-billion of TNT vapor in atmosphere. The ion-doped nanocrystal sensors described here show a remarkable air/solution stability, strong analyte affinity, and high signal output and, therefore, are well-suited for chemosensory applications in the trace detection of small-molecular analytes in many challenging environments. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 60571038, 60771036), the National 863 High Technology Project of China (2007AA10Z434), the Innovation Project of the Chinese Academy of Sciences (KJCX2-SW-W31), and the Basic Research Program of Anhui (07041420). We also thank the Hundreds Talent Program of the Chinese Academy of Sciences for financial support.

Received for review January 10, 2008. Accepted February 6, 2008. AC800060F

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