8-Hydroxyquinoline functionalized ZnS nanoparticles capped with amine groups: A fluorescent nanosensor for the facile and sensitive detection of TNT through fluorescence resonance energy transfer

Dyes and Pigments 97 (2013) 84e91

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8-Hydroxyquinoline functionalized ZnS nanoparticles capped with amine groups: A fluorescent nanosensor for the facile and sensitive detection of TNT through fluorescence resonance energy transfer Lijuan Feng a, b, Chunyu Wang a, Zhonglin Ma a, Changli Lü a, * a b

Institute of Chemistry, Northeast Normal University, Changchun 130024, PR China Centre of Analytical and Test, Beihua University, Jilin 132013, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 September 2012 Received in revised form 22 November 2012 Accepted 24 November 2012 Available online 14 December 2012

8-Hydroxyquinolines (HQs) functionalized ZnS nanoparticles (NPs) with an amine-capping layer (ZnSeNH2eQ NPs) were prepared by a ligand-exchange process. FT-IR, XRD, NMR, TEM and fluorescence spectrometer were used to characterize the novel ZnSeNH2eQ NPs. Upon the addition of 2,4,6trinitrotoluene (TNT), the amino groups on the surface of ZnSeNH2eQ NPs can bind TNT molecule from solution by forming Meisenheimer complex. This complex absorbs the green part of visible light, and strongly suppresses the fluorescence emission of the ZnSeNH2eQ NPs through Fluorescence Resonance Energy Transfer (FRET). The observed linear fluorescence intensity response for TNT in the range of 0e1.89 mM allows the quantitative detection TNT, with a detection limit down to 10 nM. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: ZnS nanoparticles Surface functionalization 8-Hydroxyquinoline Fluorescence TNT detection FRET

1. Introduction 2,4,6-Trinitrotoluene (TNT), as a commonly used nitro aromatic explosive, is released into the environment mainly because of military and industrial applications. TNT residues in the environment will cause severe health problems in both animals and humans, including anemia, abnormal liver function and cataract development [1e3]. Therefore, developing optosensing materials or techniques featuring high sensitivity, simplicity, on-the-spot for realtime determination of TNT in environments have been further heightened by today’s security concerns [4e6]. The fluorescence technique has been widely used to detect TNT vapor or solution owing to its strongpoint of high signal output, sensitivity and simplicity [7,8]. So far many fluorescence materials, including conjugated polymers [9e13], fluorescent dyes [14e16], semiconductor nanoparticles (NPs) [17e19], anti-TNT antibodies [20e22] and hybrid-assembly formed between these materials [23e27],

* Corresponding author. Fax: þ86 43185098768. E-mail address: [email protected] (C. Lü). 0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2012.11.023

were used to detect the traces of TNT. Among these materials, semiconductor NPs or Quantum Dots (QDs) have gradually been used to detect TNT because of their great photostability, sizedependent emission wavelengths and their readily manipulated optical properties by the metal cores or the introduction of dopant ions [28]. Zhang’s group reported amine-capped manganese (Mn2þ)-doped ZnS NPs for TNT detection through an electron transfer (ET) mechanism, with the detection limit down to 1 nM [29]. After that, Co2þ or Mn2þ-doped ZnS NPs and L-cysteinecapped CdTe QDs have also been used to detect TNT in solution [17,18,30e32]. Fe3O4 magnetic nanoparticles and Mn2þ-doped ZnS nanocomposites have been used for the magnetic separation of captured ultratrace TNT in water [33]. Chen et al. has manipulated the optical properties of Ruthenium NPs by incorporating functional molecules (such as pyrene, 4-vinylbenzo-18-crown-6, histidine, etc.) onto the surface of NPs, and applied them to detect TNT and Pb2þ ions [34,35]. It is well known that the semiconductor NPs such as ZnS and CdS have a great number of surface metal atoms that can coordinate with organic ligands to form stable fluorescent complexes on the surface of NPs [36]. However, among the literatures about the TNT detection, we found that the semiconductor NPs with regulated optical properties by the functional ligands was rarely reported for fluorescent

L. Feng et al. / Dyes and Pigments 97 (2013) 84e91

chemosensor. Our previous research revealed that ZnS NPs were easily modified by the phenanthroline and 8-hydroxyquinoline derivatives through metaleligand interfacial bonding interactions [37,38]. In addition, the white light emitting semiconductor nanocrystals have been successfully synthesized from 8-hydroxyquinoline-5-sulfonic acid decorated manganese doped ZnS NPs [39]. It has also been found that the metaleligand interfacial bonding interactions can play a significant role in regulating the optical and electronic properties of NPs [40]. These novel surfacefunctionalized NPs exhibited steady fluorescent properties, high photoluminescence quantum yields (QYs) as compared with the conventional zinc complexes. In this paper, we, for the first time, used the 2-mercaptoethylamine (MEA) and 8-hydroxyquinoline (HQ) co-functionalized ZnS NPs (ZnSeNH2eQ NPs) as effective chemosensor for the TNT detection in solution through Fluorescence Resonance Energy Transfer (FRET). The HQ molecules were attached to the surface of ZnS NPs in a single coordination fashion due to the steric hindrance effect of the spherical surface of NPs, which made the ZnSeNH2eQ NPs possess unique and high fluorescent properties in organic solvents as compared with the bis(8-hydroxyquinoline) zinc(II)(ZnQ2) complex. Within the present experimental context, the fluorescence response of ZnSeNH2eQ NPs was much drastic with the content variation of TNT. This is ascribed to that the organic amine ligands on the surface of ZnS NPs can bind TNT species by forming compact Meisenheimer complex (TNT-amine complex) between the electron-rich amino groups of MEA and electrondeficient TNT [41]. The TNT-amine complexes on the surface of ZnS NPs can absorb the green part of visible light (absorption with lmax at 519 nm) and strongly suppress the fluorescence emission of ZnSeNH2eQ NPs (emission peak at 503 nm) due to resonance energy transfer. We wish that the reported ZnSeNH2eQ NPs sensor for TNT here opens up a potential design-thought for using organic ligands functionalized semiconductor NPs as the convenient, highly efficient and inexpensive sensor materials. 2. Experimental 2.1. Materials 8-Hydroxyquinoline (HQ), 2-mercaptoethanol (ME), 2mercaptoethylamine hydrochloride (MEA$HCl), zinc acetate dihydrate [Zn(Ac)2$2H2O], thiourea, dimethylformamide (DMF), chloroform (CHCl3) and dimethyl sulfoxide (DMSO) were all of analytical-grade reagents and were used without further purification. 2,4,6-Trinitrotoluene (TNT) in methanol (1 mg,mL1) were obtained from Aladdin. 2,4-Dinitrotolene (DNT) was purchased from TCI.

85

2.3. Synthesis of ZnQ2 complex ZnQ2 complex was synthesized according to the literature [42]. The typical synthetic process is as follow: an aqueous solution (40 mL) of zinc acetate dihydrate (0.01 mol) was added dropwise to the acetone solution (80 mL) of 8-hydroxyquinoline (0.02 mol) and triethylamine (0.02 mol) under stirring. After refluxing the mixture at 50e60  C for 2 h, the precipitates were collected by centrifugation (6000 rpm, 10 min) and washed with acetone and deionized water for several times, respectively. At last, the product was dried in vacuum, and 0.32 g of ZnQ2 as a yellow powder was obtained. 2.4. Synthesis of amine-capped ZnS nanoparticles (ZnSeNH2 NPs) Zn(Ac)2$2H2O (5.5 g, 0.025 mol), ME (2.0 mL, 0.029 mol), MEA$HCl (0.86 g, 0.08 mol), 5 mL sodium hydroxide aqueous solution (16 mol/L) and thiourea (1.7 g, 0.045 mol) were dissolved in 40 mL DMF. The mixture was stirred at 150e160  C for 10 h under nitrogen and then concentrated to 10 mL at a reduced pressure. The resulting solution was poured into a large amount of ethanol and the white precipitate was collected and thoroughly washed several times with ethanol, and then dried in vacuum. 2.5. Preparation of HQ functionalized ZnSeNH2 nanoparticles (ZnSeNH2eQ NPs) HQ functionalized ZnSeNH2 NPs were synthesized according to the literature [38]. Typically, 1.0 g of ZnSeNH2 NPs was dispersed in 2.5 mL DMF, and 0.01 g of HQ was added. The reaction solution was allowed to stir for 30 h at room temperature, and then the resulting mixture was poured into a large amount of ethanol. The precipitation was isolated by centrifugation (8000 rpm, 10 min), and purified by repeated washing sonication/centrifugation cycles with, sequentially, ethanol, and methanol, and then dried under vacuum overnight at room temperature. Finally, 0.9 g of ZnSeNH2eQ NPs of yellowish-green powder was obtained and these particles have very good dispersibility in polar solvents such DMF and DMSO. 2.6. Preparation of HQ functionalized pure ZnS nanoparticles (ZnSeQ NPs) In order to compare the effect of amino groups on the TNT detection, we have synthesized pure ZnSeQ NPs without aminecapping layer. The synthesis of ZnS NPs without amine-capping layer is consistent with that of ZnSeNH2 NPs except that not adding MEA$HCl, and the synthesis of HQ functionalized ZnS NPs is also consistent with that of ZnSeNH2eQ NPs. 3. Results and discussion

2.2. Measurements The morphology of NPs were characterized by high-resolution transmission electron microscopy (HRTEM) on a Philips EM-420 at a 120 kV accelerating voltage. XRD diffraction data were collected on a Siemens D-5005 X-ray diffractometer using Cu Ka radiation. FT-IR spectra (4000e400 cm1) were determined with KBr disks on a Magna 560 FT-IR spectrometer. The 1H NMR spectra were recorded on a Bruker Avance 500 MHz spectrometer in d6-DMSO. UVevis absorption spectra were obtained using a Shimadzu UVe2550 spectrophotometer. The fluorescence spectra were recorded at room temperature on a Varian Cary Eclipse spectrofluorometer. The photoluminescence quantum yields (QYs) and excited-state decay times were measured on a transient spectrofluo-rimeter (Edinburgh FLS920) with time-correlated single-photo counting technique.

3.1. Preparation and characterization of ZnSeNH2eQ NPs sensor materials Scheme 1 illustrates the synthetic process of ME/MEA cocapped ZnS NPs (ZnSeNH2 NPs) and HQ functionalized ZnSeNH2 NPs (ZnSeNH2eQ NPs). MEA and ME co-capped ZnS NPs were prepared from zinc acetate dihydrate and thiourea in dimethylformamide (DMF) in the first step. The mercapto groups of MEA and ME can tightly bind at the surface of bare ZnS NPs due to the excess of metal ions with respect to sulfide ions at the surface of NPs. HQ was later incorporated on the ZnSeNH2 NPs surface through a facile ligand exchange route. Unlike the traditional bis(8hydroxyquinoline) zinc(II) complex, HQ molecules were attached to the surface of ZnSeNH2 NPs in a single coordination fashion due to the steric hindrance effect of the spherical surface of ZnSeNH2 NPs.

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Scheme 1. Schematic illustration for the preparation of HQ and MEA co-capped ZnS NPs and their selective detection for TNT through FRET.

The morphology of ZnSeNH2 and ZnSeNH2eQ NPs can be observed on TEM images in Fig. 1. The approximately spherical ZnSeNH2 NPs with a diameter of about 3e4 nm can be seen in Fig. 1a. We can clearly observe the internal lattice fringes of ZnSeNH2 NPs in HRTEM (see inset in Fig. 1a). The d spacing of nearly 3.1 A and 1.6  A are attributed to the (111) and (311) planes of cubic ZnS phase, respectively [38]. The selected area electron diffraction (SAED) pattern (Fig. 1b) shows the outer diffraction rings with (311), (220) and (111) of the cubic structure of ZnSe NH2eQ NPs. For ZnSeNH2eQ NPs, we can obtain the same conclusion from the Fig. 1c and d, indicating that the surface ligand exchange of ZnS NPs does not change the morphology and crystal structure of NPs. These results are in well agreement with that obtained from the XRD pattern (see Fig. 2). Both of the XRD patterns of ZnSeNH2 and ZnSeNH2eQ NPs exhibit a cubic structure with the diffraction peaks for (311), (220) and (111) planes, which is in accordance with the cubic structure of sphalerite ZnS. This shows that the crystal structure of ZnS NPs is not destroyed after functionalization with HQ. The size of the NPs can be calculated by Scherrer’s diffraction formula [43]. The average diameters of ZnSeNH2 and ZnSeNH2eQ NPs were calculated as about 3.0 nm, and this result is in well agreement with that obtained from the HRTEM characterization. Fig. 3a shows the FT-IR spectra of ZnS NPs, ZnSeNH2 NPs and ZnSeNH2eQ NPs. From all the spectra, it is clearly found that the bands at 2920 and 1429 cm1 are assigned to the methylene group of ME or MEA molecules on NPs. The characteristic absorption peaks of eOH in ME are observed at 3418 and 1255 cm1. The peak of the SeH vibration at 2550e2565 cm1 is not observed, indicating that the

Fig. 1. HRTEM image (a) and SAED pattern (b) of ZnSeNH2 NPs; HRTEM image (c) and SAED pattern (d) of ZnSeNH2eQ NPs.

L. Feng et al. / Dyes and Pigments 97 (2013) 84e91

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(a)

ZnS-NH2-Q

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ZnS

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2Theta / degree

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Fig. 2. XRD patterns of (a) ZnSeNH2 NPs and (b) ZnSeNH2eQ NPs.

mercapto groups of ME (or MEA) are bound to the surface of ZnS NPs [44]. Compared with the ZnS curve, we can clearly observe the characteristic NeH asymmetric bending peaks of eNH2 at the region of 1560e1650 cm1 and 1390e1470 cm1 [33,45] in ZnSeNH2 and ZnSeNH2eQ curves, indicating that the MEA molecules with amino groups have been successfully bonded on the surface of ZnS NPs. Fig. 3b is the partial magnification FT-IR spectra of ZnSeNH2 and ZnSeNH2eQ from 500 to 1580 cm1. The peaks at 1500, 1463, 1388 (these three peaks overlap with the bands of the NeH groups of the amino groups) and 1323 cm1 of ZnSeNH2eQ belong to the CC/CN stretching þ CH bending vibration associated with both pyridyl and phenyl group of HQ molecule [46]. The CeH out-of-plane wagging vibrations of the quinoline groups of HQ are observed at 736, 802 and 824 cm1 [47]. These results indicate that the HQ molecules have successfully attached on the surface of ZnSeNH2 NPs. Fig. 4 shows the 1H NMR spectra of ZnSeNH2 NPs (a), ZnSe NH2eQ NPs (b), and HQ (c) in DMSO-d6. The peak assignments have been marked in Fig. 4. The signal at 8.02 ppm is assigned to the acidamide protons of DMF that adsorbed at the surface of NPs [48]. The signals at 4.75 ppm for the NPs (curves a and b) are assigned to the hydroxyl and amido protons of capping agents ME and MEA, and the resonances in the region of 3.32e3.54 ppm are assigned to the methylene protons of ME and MEA [38]. The NMR response signals of HQs on the ZnSeNH2eQ NPs are very small due to the low feed ratio of HQ to ZnS NPs (1:100, w/w). To more clearly observe the characteristic peaks of HQ on ZnSeNH2eQ NPs, the partial amplificatory 1H NMR spectrum of ZnSeNH2 and ZnSe NH2eQ NPs is presented (see inset of Fig. 4). In curve (b) of inset, the corresponding NMR response signals of HQ can be seen in 5e10 ppm. Due to the change of chemical circumstance as compared with that of free HQ molecules, the chemical shifts of some protons for HQ anchored on the surface of ZnS NPs shift and their peak shapes also change. In addition, the H atomic chemical signal of eOH in quinoline also disappears in 9.5e10 ppm due to the coordination reaction between HQ and ZnS NPs, further indicating that the HQs have successfully anchored to the surface of ZnSeNH2 NPs [38]. NMR and FT-IR results show that the amine groups and HQ molecules have coordinated to the surface of ZnS NPs and the functionalized NPs have been successfully prepared by the ligand exchange procedure.

(b)

ZnS-NH2-Q 802 824 736

1323 ZnS-NH2

1463 1500 1388

1500

1350

1200 1050 900 -1 Wavenumber(cm )

750

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Fig. 3. FT-IR spectra of (a) ZnS NPs, ZnSeNH2 NPs and ZnSeNH2eQ NPs. (b) The partial magnification IR spectra of ZnSeNH2 and ZnSeNH2eQ NPs.

properties, we also present the UVevis spectra of HQ and ZnQ2 complex. HQ shows a UV absorption peak at 327 nm (curve a) [49]. For ZnQ2 complex (curve b), its characteristic absorption band is at about 400 nm [50]. As the HQ coordinates to the surface of ZnSe

3.2. UVevis absorption and fluorescent properties of ZnSeNH2eQ NPs sensor materials Fig. 5 shows the UVevis absorption spectra of ZnSeNH2 and ZnSeNH2eQ NPs in the solid state. In order to compare the optical

Fig. 4. 1H NMR spectra of ZnSeNH2 NPs (a), ZnSeNH2eQ NPs (b) and HQ (c) in DMSO-d6. Inset: partially magnified aromatic proton resonance signals of HQ for ZnSe NH2eQ NPs.

L. Feng et al. / Dyes and Pigments 97 (2013) 84e91

Absorbance(a.u.)

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b

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Fig. 5. UVevis absorption spectra of HQ (a), ZnQ2 (b), ZnSeNH2eQ NPs (c) and ZnSe NH2 NPs (d) in the solid state.

Fig. 6. (A): PL spectra of ZnSeNH2 NPs (a), ZnSeNH2eQ NPs (b) and ZnQ2 complex (c) in the solid state excited by 365 nm (ZnSeNH2 NPs excited by 310 nm). (B): PL spectra of ZnQ2 complex in CHCl3 (a) and DMSO (b), and ZnSeNH2eQ NPs in DMSO (c).

not simply derived from the metal-quinolate complexes formed on the surface of ZnS NPs. Because there is a steric hindrance effect provided by the special spherical surface of NPs and the existence of ZneS bond, the molar ratio of HQ molecules to the zinc atom in the metal-quinolates formed on the surface of ZnSeNH2eQ NPs should be 1:1 (see Scheme 1) as compared with the conventional small coordination compounds such as ZnQ2 complex.

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ZnS-NH2 in DMSO ZnS-NH2-Q in DMSO

intensity ( counts)

NH2 NPs, ZnSeNH2eQ NPs (curve c) also shows the new absorption peak at about 383 nm besides the intrinsic absorption of ZnSeNH2 NPs (curve d). This absorption peak should be assigned to the charge transfer transition from oxide-containing phenolato moiety of the quinolate ligand to its nitrogen-containing pyridyl moiety [51]. The absorption band at about 335e480 nm of ZnSeNH2eQ NPs is similar to that of ZnQ2 complex, but blue-shifted as compared with that of ZnQ2 complex (Similar shift is observed in the fluorescence spectra described below). This blue-shifted should be attributed to only one electron-donating ligand (8Hydroxyquinoline) to coordinate zinc ion of the surface of ZnS NPs, and the hindrance effect provided by the special spherical surface of NPs may affect the stacking of HQ molecules on the surface of ZnS NPs. Fig. 6A shows the PL spectra of ZnSeNH2 NPs, ZnSeNH2eQ NPs and ZnQ2 complex in the solid state excited by 365 nm (ZnSeNH2 NPs excited by 310 nm). The inset is the powder photos of these samples under a natural light (top) and UV light (l ¼ 360 nm, down). The weak emission peak at 420 nm of ZnSeNH2 NPs (curve a) corresponds to the defect state recombination on the surface of ZnS NPs [52]. Compared with the ZnSeNH2 NPs, the ZnQ2 complex (curve c) and ZnSeNH2eQ NPs (curve b) exhibited strong red-shift emission at about 500 nm. This result is in agreement with that of UVevis absorption spectra, indicating HQ molecules have anchored to the ZnSeNH2 NPs to form composite fluorescent NPs. Fig. 6B shows the PL spectra of ZnQ2 complex in CHCl3 and DMSO solution, and ZnSeNH2eQ NPs in DMSO solution. The inset is the corresponding fluorescence photos. The ZnQ2 complex in CHCl3 solution has the strong luminescent emission at about 505 nm (curve a), which is consistent with their solid state. However, ZnQ2 complex in DMSO solution exhibits a red-shifted luminescent emission at 584 nm (curve b). The PL spectra of ZnQ2 complex in DMSO are redshifted due to the interaction of polar DMSO molecules with the metal-quinolate complexes. As ZnSeNH2eQ NPs have the same fluorescence emission peak in DMSO with that in the solid state (curve c), indicating that the novel surface-functionalized NPs exhibit excellent photoluminescence properties with the reduced attack of solvent molecules. The fluorescence decay curves of synthetic NPs in DMSO solution are given in Fig. 7 and the corresponding decay time and fluorescence quantum yields (PL QYs) are shown in Table 1. The ZnSeNH2eQ NPs obviously exhibits different optical properties as compared with that of both ZnSeNH2 NPs and ZnQ2 complex. In particular, the ZnSeNH2eQ NPs in DMSO possess higher PL QYs (about 36%) and decay time (23 ns) than that of ZnQ2 complex. These results indicate that the PL properties of ZnSeNH2eQ NPs are

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Fig. 7. Fluorescence decay curves of ZnSeNH2 and ZnSeNH2eQ NPs in DMSO, and ZnQ2 complex in CHCl3 and DMSO.

L. Feng et al. / Dyes and Pigments 97 (2013) 84e91 Table 1 Emission peak positions, relative PL QYs and decay data of ZnQ2 complex, ZnSeNH2 and ZnSeNH2eQ NPs. a

ZnSeNH2 (DMSO) ZnSeNH2eQ (DMSO) ZnQ2 (DMSO) ZnQ2 (CHCl3)

303 365 421 350

lem [nm]

b

c

415 503 569 507

e 36 3 4

QYs [%]

d Decay time [ns]

75 23 5 15

a

Maximum excitation peak. Maximum emission peak. Fluorescence quantum yields (QYs). d The decay time of the corresponding sample was obtained from the decay curves. b

5000 TNT: 0 M TNT: 1.76×

4000 Intensity (counts)

lex [nm]

Samples (solvent)

89

M

3000 2000 1000

c

3.3. Mechanism for the fluorescence quenching of ZnSeNH2eQ NPs to TNT TNT is colorless in solution and does not absorb any visible light. But, with addition of organic amines such as MEA, the electron transfer from amino groups to aromatic rings leads to the formation of Meisenheimer complex (TNT-amine) [41] which can strongly absorb the green part of visible light, at last, the solution changes from colorless to red (see the inset in Fig. 8). As shown in Fig. 8, without TNT, ZnSeNH2eQ NPs in DMSO show no absorption at visible absorption region (Fig. 8a). However, the absorption spectrum of MEA-TNT solution shows a visible absorption with lmax at 519 nm in DMSO (Fig. 8b). This shows that only the interaction is formed between amine and TNT, the absorption peak at 519 nm will appear. The photoluminescence (PL) spectra of ZnSeNH2eQ NPs are shown in Fig. 8c and d. The excitation of the sample at 365 nm leads to the broad emission peak at about 503 nm. So ZnSeNH2eQ NPs as an energy donor has a spectral overlapping with the absorption peak of TNT-MEA complexes at about 519 nm. This suggests that the FRET from ZnSeNH2eQ NPs to the TNT-MEA complexes may occur on the surface of ZnS NPs. To further prove the occurrence of energy transfer from the excited state of the ZnSe NH2eQ NPs to the TNT-amine complex, we recorded the decay curves for ZnSeNH2eQ NPs in the DMSO before and after adding TNT, as shown in Fig. 9. It is known that FRET quenching process belongs to dynamic quenching, and the decays of fluorescence donor become faster as analyte was added [53]. In Fig. 9, the average lifetimes of the donor ZnSeNH2eQ NPs decrease from 23.0 ns to 21.7 ns after adding 1.76 mM TNT into the system. The decrease of fluorescence lifetime also provides a convincing evidence that the FRET process is turned on by TNT in this system [32,54].

Fig. 8. Absorption spectra of ZnSeNH2eQ NPs (a) and MEA-TNT (b); excitation (c), and emission (d) spectra for ZnSeNH2eQ NPs in DMSO. Inset: Optical image of MEA-TNT complex in DMSO solution under the natural light.

0 40

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Fig. 9. Fluorescence decay curves for ZnSeNH2eQ NPs in DMSO with and without TNT (monitored at 503 nm). Straight curve: in the absence of TNT; Dash curve: in the presence of TNT (1.76  106 M).

3.4. Fluorescence response of ZnSeNH2eQ NPs towards TNT The fluorescence quenching properties of the ZnSeNH2eQ NPs were studied by systematically examining the fluorescence intensity changes in response to TNT in DMSO solutions. Fig. 10 shows the fluorescence ratiometric responses of ZnSeQ and ZnSeNH2eQ NPs in DMSO solution upon successively adding the same concentration of TNT or DNT (2 mL 4.4  104 M TNT or DNT was injected into 5 mL of NPs solution every time. The amount of NPs is 45 mg,mL1 in DMSO) with an excitation wavelength of 365 nm. The fluorescence intensities at 503 nm decrease with the increasing concentrations of TNT in DMSO solution of ZnSeNH2eQ NPs (Fig. 10a). Herein, it can be clearly detected that the fluorescence quenching is much larger than that of ZnSeQ NPs (Fig. 10b) at the same concentration of TNT. This indicates that the amine ligands of MEA at the surface of ZnS NPs play an important role for the recognition of TNT molecules. The TNT binding will lead to the fluorescence quenching of ZnSeNH2eQ NPs by FRET. To further confirm that the ZnSeNH2eQ NPs can enhance the sensitivity to TNT, we chose 2,4-dinitrotoluene (DNT) as a contrast, which is the structural analogs of TNT. The fluorescence intensities decrease with increasing DNT concentration (Fig. 10c). But, the quenching percentage is much lower than TNT in the same concentration of ZnSeNH2eQ NPs. It is well known that the electron accepting ability of DNT with two nitro groups is much weaker than TNT molecules with three nitro groups. So the low affinity to the amino groups and weak electron-accepting ability lead to the low quenching efficiencies of DNT. The quenching response was analyzed by fitting the data to the SterneVolmer equation (I0/I) 1 ¼ KSV CA [55,56] (Fig. 10d). Where I0 and I are the fluorescence intensity in the absence and presence of analyte (quencher, A), respectively, CA is the molar concentration of the analyte, and KSV is the corresponding SterneVolmer quenching constant. The higher the quenching constant (KSV) the more sensitive is the fluorophore to the quencher. At lower concentration of TNT (0e1.23  106 M), a linear SterneVolmer plot of ZnSeNH2eQ NPs is obtained with a SterneVolmer constant (KSV) of 4.42  105 M1. The KSV value of ZnSeNH2eQ NPs to TNT is about 5.6-fold to DNT and 18-fold of ZnSQ NPs. This result shows that the ZnSeNH2eQ NPs greatly amplifies the quenching response to TNT through the highly efficient FRET. The quenching fluorescence of ZnSeNH2eQ NPs had a distinct linearly decrease toward TNT in the concentration range of 0.176e 1.89 mM with a correlation coefficient of 0.9993 and a linear regression equation of DI ¼ 292.2C7.64 (where C is the concentration of TNT in mM, I represents fluorescence intensity). The detection limit is calculated by the formula [33,57]: CL ¼ k$SB/m,

L. Feng et al. / Dyes and Pigments 97 (2013) 84e91

Intensity (a.u.)

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Fig. 10. Evolution of fluorescence spectra of ZnSeNH2eQ NPs (a) and ZnS-Q NPs (b) with the concentration increase of TNT, and DNT in ZnSeNH2eQ NPs (c). SterneVolmer plots corresponding to the above graphs (d).

where “k” is a constant, usually k ¼ 3, ‘‘SB’’ is the standard deviation of the blank signal (F0), and ‘‘m’’ is the calibration sensitivity of the fluorescence intensity change (DF ¼ F0  F) vs [TNT]. The calculated detection limit is 10 nM, which is close to or lower than the most sensitive fluorescent quenching method reported before. 4. Conclusion In summary, we have demonstrated that 8-hydroxyquinoline (HQ) and organic amine can effectively modify the surface of ZnS nanoparticles to form a hybrid monolayer of fluorescent sensor for TNT. This novel ZnSeNH2eQ NPs present good fluorescent emission properties. It shows the fluorescence quenching sensitivity and selectivity towards TNT through FRET because of the energy matching between TNT-amine complexes and ZnSeNH2eQ NPs. The FRET-based surface-functionalized nanoparticle sensors have several remarkable advantages as follows: (1) facile preparation without use of complicated chemical procedure; (2) material properties can be readily manipulated by the chemical nature of the metal cores as well as the structures of the surface organic ligands; (3) high fluorescence brightness suited well for the detection of ultratrace analytes. Briefly, we can endow the semiconductor NPs with versatile functionalization by using functional ligands molecules and the resulting nanomaterials can open up a new perspective in the design of sensors for various analytes based on FRET technology. Acknowledgment This work was supported by the National Natural Science Foundation of China (21074019) and Natural Science Foundation of Jilin Province (20101539). References [1] Hernandez R, Zappi M, Kuo CH. Chloride effect on TNT degradation by zerovalent iron or zinc during water treatment. Environ Sci Technol 2004;38: 5157e67.

[2] Singh S. Sensorsdan effective approach for the detection of explosives. J Hazard Mater 2007;144:15e28. [3] Dillewijn PV, Couselo JL, Corredoira E, Delgado A, Wittich RM, Ballester A, et al. Bioremediation of 2,4,6-trinitrotoluene by bacterial nitroreductase expressing transgenic aspen. Environ Sci Technol 2008;42:7405e6. [4] Carona T, Guillemota M, Montméata P, Veignala F, Perraut F, Prenéa P, et al. Ultra trace detection of explosives in air: development of a portable fluorescent detector. Talanta 2010;81:543e6. [5] Doussineau T, Schulz A, Fernandez AL, Moro A, Korsten S, Trupp S, et al. On the design of fluorescent ratiometric nanosensors. Chem Eur J 2010;16:10290e310. [6] Salinas Y, Manez RM, Marcos MD, Sancenon F, Costero AM, Parra M, et al. Optical chemosensors and reagents to detect explosives. Chem Soc Rev 2012; 41:1261e336. [7] Germain ME, Knapp MJ. Optical explosives detection: from color changes to fluorescence turn-on. Chem Soc Rev 2009;38:2543e613. [8] Bau L, Tecilla P, Mancin F. Sensing with fluorescent nanoparticles. Nanoscale 2011;3:121e3. [9] Yang JS, Swager TM. Fluorescent porous polymer pilms as TNT chemosensors: electronic and structural effects. J Am Chem Soc 1998;120:11864e910. [10] Toal SJ, Trogler WC. Polymer sensors for nitroaromatic explosives detection. J Mater Chem 2006;16:2871e913. [11] Rose A, Zhu ZG, Madigan CF, Swager TM, Bulovic V. Sensitivity gains in chemosensing by lasing action in organic polymers. Nature 2005;434:876e84. [12] Chen SJ, Zhang QY, Zhang JP, Gu JW, Zhang L. Synthesis of two conjugated polymers as TNT chemosensor materials. Sens Actuators 2010;149:155e6. [13] Cho J, Anandakathir R, Kumar A, Kumar J, Kurup PU. Sensitive and fast recognition of explosives using fluorescent polymer sensors and pattern recognition analysis. Sens Actuators B 2011;160:1237e47. [14] Gao DM, Wang ZY, Liu BH, Ni L, Wu MH, Zhang ZP. Resonance energy transferamplifying fluorescence quenching at the surface of silica nanoparticles toward ultrasensitive detection of TNT. Anal Chem 2008;80:8545e9. [15] Fang QL, Geng JL, Liu BH, Gao DM, Li F, Wang ZY, et al. Inverted opal fluorescent film chemosensor for the detection of explosive nitroaromatic vapors through fluorescence resonance energy transfer. Chem Eur J 2009;15:11507e8. [16] Geng JL, Liu P, Liu BH, Guan GJ, Zhang ZP, Han MY. A reversible dual-response fluorescence switch for the detection of multiple analytes. Chem Eur J 2010; 16:3720e8. [17] Chen YF, Chen Z, He YJ, Lin HL, Sheng PT, Liu CB, et al. L-cysteine-capped CdTe QD-based sensor for simple and selective detection of trinitrotoluene. Nanotechnology 2010;21:125502e6. [18] Zou WS, Qiao JQ, Hu X, Ge X, Lian HZ. Synthesis in aqueous solution and characterisation of a new cobalt-doped ZnS quantum dot as a hybrid ratiometric chemosensor. Anal Chim Acta 2011;708:134e7. [19] Pandya A, Goswami H, Lodha A, Menon SK. A novel nanoaggregation detection technique of TNT using selective and ultrasensitive nanocurcumin as a probe. Analyst 2012;137:1771e4. [20] Goldman ER, Medintz IL, Whitley JL, Hayhurst A, Clapp AR, Uyeda HT, et al. A hybrid quantum dot-Antibody fragment fluorescence resonance energy transfer-based TNT sensor. J Am Chem Soc 2005;127:6744e8.

L. Feng et al. / Dyes and Pigments 97 (2013) 84e91 [21] Medintz IL, Goldman ER, Lassman ME, Hayhurst A, Kusterbeck AW, Deschamps JR. Self-assembled TNT biosensor based on modular multifunctional surface-tethered components. Anal Chem 2005;77:365e8. [22] Parka M, Cellab LN, Chena W, Myunga NV, Mulchandani A. Carbon nanotubesbased chemiresistive immunosensor for small molecules: detection of nitroaromatic explosives. Biosens Bioelectron 2010;26:1297e305. [23] Aguilar AD, Forzani ES, Leright M, Tsow F, Cagan A, Iglesias RA, et al. A hybrid nanosensor for TNT vapor detection. Nano Lett 2010;10:380e5. [24] Feng JC, Li Y, Yang MJ. Conjugated polymer-grafted silica nanoparticles for the sensitive detection of TNT. Sens Actuators B 2010;145:438e46. [25] Xia YS, Song L, Zhu CQ. Turn-on and near-infrared fluorescent sensing for 2,4,6-trinitrotoluene based on hybrid (gold nanorod)-(quantum dots) assembly. Anal Chem 2011;83:1401e7. [26] Wei W, Huang XB, Chen KY, Tao YM, Tang XZ. Fluorescent organiceinorganic hybrid polyphosphazene microspheres for the trace detection of nitroaromatic explosives. RSC Adv 2012;2:3765e7. [27] Feng LJ, Li H, Qu Y, Lu CL. Detection of TNT based on conjugated polymer encapsulated in mesoporous silica nanoparticles through FRET. Chem Commun 2012;48:4633e43. [28] Shenhar R, Rotello VM. Nanoparticles: scaffolds and building blocks. Acc Chem Res 2003;36:549e613. [29] Tu RY, Liu BH, Wang ZY, Gao DM, Wang F, Fang QL, et al. Amine-capped ZnSe Mn2þ nanocrystals for fluorescence detection of trace TNT explosive. Anal Chem 2008;80:3458e68. [30] Zhang K, Zhou HB, Mei QS, Wang SH, Guan GJ, Liu RY, et al. Instant visual detection of trinitrotoluene particulates on various surfaces by ratiometric fluorescence of dual-emission quantum dots hybrid. J Am Chem Soc 2011; 133:8424e34. [31] Wang YQ, Zou WS. 3-Aminopropyltriethoxysilane-functionalized manganese doped ZnS quantum dots for room-temperature phosphorescence sensing ultratrace 2,4,6-trinitrotoluene in aqueous solution. Talanta 2011;85:469e77. [32] Zou WS, Sheng D, Xi Ge, Qiao JQ, Lian HZ. Room-temperature phosphorescence chemosensor and Rayleigh scattering chemodosimeter dualrecognition probe for 2,4,6-rrinitrotoluene based on manganese-doped ZnS quantum dots. Anal Chem 2011;83:30e8. [33] Zou WS, Yang J, Yang TT, Hu X, Lian HZ. Magnetic-room temperature phosphorescent multifunctional nanocomposites as chemosensor for detection and photo-driven enzyme mimetics for degradation of 2,4,6-trinitrotoluene. J Mater Chem 2012;22:4720e8. [34] Chen W, Zuckerman NB, Konopelski JP, Chen SW. Pyrene-functionalized ruthenium nanoparticles as effective chemosensors for nitroaromatic derivatives. Anal Chem 2010;82:461e5. [35] Kang XW, Li X, Hewitt WM, Zuckerman NB, Konopelski JP, Chen SW. Manipulation of intraparticle charge delocalization by selective complexation of transition-metal ions with histidine moieties. Anal Chem 2012;84:2025e6. [36] Shi YL, Fu YQ, Lü CL, Hui L, Su ZM. High luminescence, organiceinorganic nanocomposite films with covalently linked 8-hydroxyquinoline anchored to ZnS nanoparticles. Dyes Pigments 2010;85:66e7. [37] Gao JF, Lü CL, Lü XD, Du YY. APhen-functionalized nanoparticlesepolymer fluorescent nanocomposites via ligand exchange and in situ bulk polymerization. J Mater Chem 2007;17:4591e7. [38] Lü CL, Gao JF, Fu YQ, Du YY, Shi YL, Su ZM. A ligand exchange route to highly luminescent surface-functionalized ZnS nanoparticles and their transparent polymer nanocomposites. Adv Funct Mater 2008;18:3070e110.

91

[39] Lü XD, Yang J, Fu YQ, Liu QQ, Qi B, Lü CL, et al. White light emission from Mn2þ doped ZnS nanocrystals through the surface chelating of 8-hydroxyquinoline5-sulfonic acid. Nanotechnology 2010;21:115702e11. [40] Kang XW, Chen W, Zuckerman NB, Konopelski JP, Chen SW. Intraparticle charge delocalization of carbene-functionalized ruthenium nanoparticles manipulated by selective ion binding. Langmuir 2011;27:12636e46. [41] Claude F, Bernasconi J. Kinetic and spectral study of some reactions of 2,4,6trinitrotoluene in basic solution. I. Deprotonation and Janovsky complex formation. J Org Chem 1971;36:1671e9. [42] Du YY, Fu YQ, Guo XH, Li H, Lü CL, Su ZM. Fabrication of fluorescent mesoporous silica nanoparticles with confined 8-hydroxyquinoline functionalized ZnS nanoparticles and their transparent polymer nanocomposites. Micropor Mesopor Mater 2010;130:122e8. [43] Nanda J, Sapra S, Sarma DD. Size-selected zinc sulfide nanocrystallites: synthesis, structure, and optical studies. Chem Mater 2000;12:1018e27. [44] Lü CL, Cui ZC, Wang Y, Li Z, Guan C, Yang B, et al. Preparation and characterization of ZnSepolymer nanocomposite films with high refractive index. J Mater Chem 2003;13:2189e97. [45] Cooper JK, Franco AM, Gul S, Corrado C, Zhang JZ. Characterization of primary amine capped CdSe, ZnSe, and ZnS quantum dots by FT-IR: determination of surface bonding interaction and identification of selective desorption. Langmuir 2011;27:8486e8. [46] Gavrilko T, Fedorovich R, Dovbeshko G, Marchenko A, Naumovets A, Nechytaylo V, et al. FTIR spectroscopic and STM studies of vacuum deposited aluminium (III) 8-hydroxyquinoline thin films. J Mol Struct 2004;704:163e8. [47] Khaorapapong N, Ogawa M. In situ formation of bis(8-hydroxyquinoline) zinc(II) complex in the interlayer spaces of smectites by solidesolid reactions. J Phys Chem Solids 2008;69:941e8. [48] Fujiwara H, Murakoshi K, Wada Y, Yanagida S. Observation of adsorbed N, Ndimethylformamide molecules on colloidal ZnS nanocrystallites. Effect of coexistent counteranion on surface structure. Langmuir 1998;14:4070e4. [49] Khaorapapong N, Ogawa M. Solid-state intercalation of 8-hydroxyquinoline into Li(I)-, Zn(II)- and Mn(II)-montmorillonites. Appl Clay Sci 2007;35:31e8. [50] Hopkins TA, Meerholz K, Shaheen S, Anderson ML, Schmidt A, Kippelen B, et al. Substituted aluminum and zinc quinolates with blue-shifted absorbance/ luminescence bands: synthesis and spectroscopic, photoluminescence, and electroluminescence characterization. Chem Mater 1996;8:344e8. [51] Burrows PE, Shen Z, Bulovic V, McCarty DM, Forrest SR. Relationship between electroluminescence and current transport in organic heterojunction lightemitting devices. J Appl Phys 1996;79:7991e8016. [52] Becker WG, Bard AJ. Photoluminescence and photoinduced oxygen adsorption of colloidal zinc sulfide dispersions. J Phys Chem 1983;87:4888e96. [53] Lakowicz JR. Principles of fluorescence spectroscopy. New York: Plenum; 1999. [54] Liu BY, Zeng F, Wu GF, Wu SZ. A FRET-based ratiometric sensor for mercury ions in water with multi-layered silica nanoparticles as the scaffold. Chem Commun 2011;47:8913e23. [55] Lakowicz JR. Principles of fluorescence spectroscopy. New York: Plenum Press; 1986. [56] Zhao ZJ, Liu JZ, Lam JWY, Chan CYK, Qiu HY, Tang BZ. Luminescent aggregates of a starburst silole-triphenylamine adduct for sensitive explosive detection. Dyes Pigments 2011;91:258e66. [57] A closer look at the IUPAC definition, limit of detection. Anal Chem 1983;55: 712e3.