Highly Photoluminescent Molybdenum Oxide Quantum Dots: One-Pot

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Highly Photoluminescent Molybdenum Oxide Quantum Dots: OnePot Synthesis and Application in 2,4,6-Trinitrotoluene Determination Sai Jin Xiao,†,‡ Xiao Jing Zhao,† Ping Ping Hu,§ Zhao Jun Chu,† Cheng Zhi Huang,‡,∥ and Li Zhang*,⊥ †

School of Chemistry, Biology and Material Science and ‡Jiangxi Key Laboratory of Mass Spectrometry and Instrumentation, East China University of Technology (ECUT), Nanchang 330013, China § Innovative Drug Research Centre, Chongqing University, Chongqing 401331, China ∥ College of Pharmaceutical Sciences, Southwest University, Chongqing 400715, China ⊥ College of Chemistry, Nanchang University, Nanchang 330031, China S Supporting Information *

ABSTRACT: As a well-studied transition-metal semiconductor material, MoOx has a wider band gap than molybdenum disulfide (MoS2), and its property varies dramatically for the existence of several different allotropes and suboxide phases of molybdenum oxides (MoOx, x < 3). In this manuscript, a one-pot method possessing the advantages of one pot, easily prepared, rapid, and environmentally friendly, has been developed for facile synthesis of highly photoluminescent MoOx quantum dots (MoOx QDs), in which commercial molybdenum disulfide (MoS2) powder and hydrogen peroxide (H2O2) are employed as the precursor and oxidant, respectively. The obtained MoOx QDs can be further utilized as an efficient photoluminescent probe, and a new turn-off sensor is developed for 2,4,6-trinitrotoluene (TNT) determination based on the fact that the photoluminescence of MoOx QDs can be quenched by the Meisenheimer complexes formed in the strong alkali solution through the inner filter effect (IFE). Under the optimal conditions, the decreased photoluminescence of MoOx QDs shows a good linear relationship to the concentration of TNT ranging from 0.5 to 240.0 μM, and the limit of detection was 0.12 μM (3σ/k). With the present turn-off sensor, TNT in river water samples can be rapidly and selectively detected without tedious sample pretreatment processes. KEYWORDS: MoOx quantum dots, 2,4,6-trinitrotoluene, inner filter effect, photoluminescent sensor, turn-off, Meisenheimer complexes

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INTRODUCTION In recent years, transition metal oxide (TMO) nanostructures have attracted great attention in basic scientific research due to their unique electrical, optical, and mechanical properties. Among the TMOs, molybdenum oxide (MoOx) nanomaterial occupies a vital position for the outstanding gaso-, photo-, and electrochromic coloration effects and dielectric property,1 which demonstrate great promise in a wide variety of applications including erasable optical storage media,2,3 electrochemical energy storage devices,4 excellent field emitters,5 high density memory devices,6 gas sensors,7,8 reversible lithium-ion batteries,9,10 etc. MoOx is an n-type semiconductor with a wider band gap than molybdenum disulfide (MoS2),11,12 and its property varied for the existence of several different allotropes and suboxide phases of molybdenum oxides (MoOx, x < 3) including MoO3 (α-MoO3, β-MoO3, and h-MoO3), MoO2, Mo 4 O 11, etc.13,14 Recently, increasing effort has been concentrated to the synthesis of MoOx nanomaterials with different morphology due to the high stability, special quantum size effect, surface effect, and high reactivity characteristic of nanomaterials.4,9,8,15−20 For example, Ou et al. synthesized twodimensional molybdenum oxide flakes by a grinding-assisted © 2016 American Chemical Society

liquid exfoliation method, while Hu et al. adopted a hydrothermal route for the fabrication of molybdenum oxide nanospheres and nanoribbons using a molybdenum precursor and poly(ethylene glycol).15,19 However, almost all of the reported methods were time-consuming, demanding tedious procedures or requiring external stimuli such as lasing, evaporation, decomposition, grinding, heating, sonication, or shear. On this line, the development of a rapid, one-pot, and environmentally friendly method for large-scale synthesis of MoOx nanomaterials is still challenging. Moreover, thus far, only a few MoOx nanomaterials have been reported to involve photoluminescence,13,16,21,22 not to mention the applications as photoluminescent probes in chemical and biological sensings. In this work, a one-pot method was developed for facile synthesis of highly photoluminescent MoOx quantum dots (MoOx QDs) by using commercial molybdenum disulfide (MoS2) powder and hydrogen peroxide (H2O2) as the precursor and oxidant, respectively, in which H2O2 provides Received: November 23, 2015 Accepted: March 8, 2016 Published: March 8, 2016 8184

DOI: 10.1021/acsami.5b11316 ACS Appl. Mater. Interfaces 2016, 8, 8184−8191

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ACS Applied Materials & Interfaces

were of analytical grade and were used without further purification. Deionized water was used throughout. Instrumentation. The size and height of MoOx QDs were observed on a JEOL Ltd. JEM-2010 transmission electron microscope (TEM, Japan) and a Bruker MultiMode 8 atomic force microscope with the ScanAsyst mode, respectively. The XRD patterns of the obtained MoOx QDs were recorded on a Bruker AXS D8Focus diffractometer operating at 40 kV and 40 mA, with Cu target Kα-ray irradiation. Scans were collected over a 2θ range from 10° to 90° with a step of 2°/min. The elemental composition and bonding configuration characterization were performed by X-ray photoelectron spectroscopy (XPS) (Thermo, USA). Fourier transform infrared spectra (FTIR) were measured on a Nicolet 5700 FTIR spectrometer (Nicolet), and the absolute fluorescence quantum yield was recorded by a Quantaurus-QY absolute quantum yield spectrophotometer (Hamamatsu, Japan). The absorption and fluorescence spectra were obtained by a Shimadzu UV-2450 spectrophotometer (Tokyo, Japan) and a Hitachi F-7000 fluorescence spectrophotometer (Tokyo, Japan), respectively. The fluorescence lifetimes were recorded by a FL-TCSPC fluorescence spectrophotometer (Horiba Jobin Yvon Inc., France). Synthesis of MoOx QDs. The MoOx QDs were synthesized by using commercial MoS2 powder and H2O2 as the precursor and oxidant, respectively. In brief, 100.0 mg of MoS2 powder was added into the mixture of 8.0 mL of 30% H2O2 and 2.0 mL of H2O, standing at room temperature for 30 min, and the pH was then adjusted to 7.0 with 375 μL of 425 g/L of sodium hydroxide (NaOH). Finally, the MoOx QDs were obtained by centrifugation at 8000g for 10 min to remove large particles and then dialysis for 48 h to remove the byproducts with a 1000 Da MWCO dialysis tube. Photoluminescence Quenching of MoOx QDs by TNT. The quenching of MoOx QDs by TNT was performed as follows: MoOx QDs (20 μL, 0.5 mg/mL), different concentrations of TNT, and deionized water that adjusted pH to 13.0 by NaOH were added into a centrifuge tube and immediately mixed thoroughly with a vortex mixer. After incubation for 10 min, the photoluminescence emission spectra of the resulting solution were recorded on the F-7000 spectrophotometer by excitation at 325 nm.

excess oxygen to promote and enhance the oxidization state of molybdenum (Scheme 1).23 Compared with other synthesis Scheme 1. Facile Synthesis of MoOx QDs and the Novel Turn-Off Sensor for TNT Determination Based on IFE

methods, the present method displays great advantages. First, the MoOx QDs are obtained in 1 h or less at room temperature without an external stimulus. The second and also the most important one, the obtained MoOx QDs, have strong photoluminescence that is close to that of graphene quantum dots, carbon dots, and MoS2 QDs,24−26 and the absolute fluorescence quantum yield reaches to 2.55%. The obtained MoOx QDs were further utilized as a new photoluminescent probe in 2,4,6-trinitrotoluene (TNT) determination. TNT is one of the most frequently used nitroaromatic explosives in both military and terrorist activities27 and also one of the priority pollutants in the Environmental Protection Agency list, which is significantly toxic to both human beings and the environment. Therefore, the accurate, rapid, sensitive, and selective detection of TNT is very important. Until now, many methods have been developed for TNT detection,27−32 and the methods based on new fluorescent nanomaterials are preferable.33−38 Normally, the reported fluorescence sensors for TNT determination are based on the fact that TNT is able to interact with the −OH/−NH2 groups of the nanomaterials through charge transfer, leading to the formation of Meisenheimer complex.27,28,39 In this manuscript, however, a novel turn-off sensor was constructed for TNT determination based on the inner filter effect (IFE) of the Meisenheimer complexes on MoOx QDs in the strong alkali solution. With the present turn-off sensor, TNT in water samples can be analyzed rapidly and selectively without tedious pretreatment processes, and the recoveries ranging from 98.3% to 106.2% are acceptable for TNT detection in water samples.

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RESULTS AND DISCUSSION Characterization of the Obtained MoOx QDs. Before employing MoOx QDs as a new photoluminescent probe, the morphology, surface chemical properties, and photoluminescence property of the obtained MoOx QDs were carefully investigated. Figure 1a showed the presentative TEM image of the obtained MoOx QDs whose average diameter was calculated to be about 2.02 nm from more than 100 dots in different TEM images, while the height from the atomic force microscopic (AFM) image was around 1.5 nm (Figure 1b). According to the XRD pattern (Figure S1), the MoOx QDs are of high crystallinity with relatively sharp and strong peaks, and the (020) peaks at 2θ of 12.9° were clearly detected, indicating the presence of the orthorhombic phase.40 However, the XRD pattern of MoOx QDs cannot be perfectly indexed to orthorhombic MoO3 (JCPDS card No. 35-0609), which probably belong to some oxygen-deficient byproduct due to the presence of Mo5+ during the exfoliation of the powders to single-layered nanosheets, causing some structural defects and lattice distortions.41 X-ray photoelectron spectroscopy (XPS) was also employed to characterize the elemental composition and bonding configuration of the obtained MoOx QDs, and Mo 3d, S 2p, O 1s, and Na 1s peaks were observed in the survey spectrum (Figure 1c). The high-resolution XPS demonstrated that both Mo 3d and S 2p doublets of the obtained MoOx QDs shifted to the higher binding energy pair compared with those of MoS2 powders (Figure 1d,e and Figure S2).23 The S 2p3/2 and S 2p1/2 components at 168.4 and 169.6 eV, which are

EXPERIMENTAL METHODS

Materials and Reagents. MoS2 powder, arginine (Arg), cysteine (Cys), histidine (His), Glycine (Gly), tyrosine (Tyr), bovine serum albumin (BSA), and amylase (Amy) were purchased from SigmaAldrach (USA). TNT (AR, 98%) and its analogues including 2,4dinitrotoluene (DNT) (AR, 98%), 2,4,6-trinitrophenol (TNP) (AR, 98%), 4-nitrotoluene (4-NT) (AR, 98%), nitrobenzene (NB) (AR, 98%), and toluene (T) (AR, 98%) were supplied by the National Security Department of China. Metal ions (Na+, K+, Cd2+, Ca2+, Cr3+, Hg2+, Mg2+, Pb2+) were purchased from Sinopharm Chemical Reagents Co., Ltd., (Shanghai, China). All chemicals and solvents 8185


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Figure 1. (a) TEM images of the as-prepared MoOx QDs. (b) AFM image of the as-prepared MoOx QDs, and the inset is the corresponding height distributions. (c) XPS survey of the as-prepared MoOx QDs, while (d) and (e) are the high-resolution XPS spectra of Mo 3d and S 2p, respectively. (f) FTIR spectrum of MoOx QDs.

adjacent to S2− 2p, can be identified as the S 2p peaks of oxidized sulfur (SO42−) free in the solution which can be confirmed by the phenomenon that the oxidized sulfur peaks of MoOx QDs disappeared after dialysis for 48 h in ddH2O (Figure S3).26,42,43 Meanwhile, the Mo 3d3/2 and Mo 3d5/2 doublets at 231.0 and 233.3 eV were identified as the oxidation states of Mo5+ doublets, while those at 232.0 and 235.1 eV were well matched to the oxidation states of Mo6+ doublets.41−44 The proportion of Mo5+ and Mo6+ calculated from the XPS peak area of Mo 3d was 13.5% and 86.5%, respectively, and thus the average oxidation state of Mo was calculated to be 5.87, suggesting the mixed-valence state can account for the oxygen vacancies and the as-prepared MoOx QDs are a combination of MoO3 and Mo2O5, which might be formed according to the reaction in eq 1.

Mo5+/Mo6+ accompanied by the existence of high concentration of oxygen vacancies. H2O2 as an excess oxygen supplier can prompt the spontaneous exfoliation and oxidation of MoS2, which is identical to the previous report.45 During the oxidation process, more and more S atoms are released from the lattice which can be refilled by excess oxygen atom due to the lower bonding affinity of Mo−S than Mo−O.40,44 The formation of Mo−O bonds was further confirmed by high-resolution spectra of O 1s and FTIR spectra (Figure S4 and Figure 1f), and the three characteristic peaks of 986, 835, and 553 cm−1 in FTIR spectra are attributed to the stretching vibration of Mo−O, the doubly coordinated oxygen (Mo2−O) stretching mode, and the triply coordinated oxygen (Mo 3 −O) stretching mode, respectively.46,47 The absorption and photoluminescence properties of the obtained MoOx QDs were further studied. The strong absorption between 200 and 400 nm is ascribed to the charge transfer of the Mo−O band in the MoO66− octahedron (Figure 2).48 When excited with a 300 nm beam, the photoluminescence of MoOx QDs showed a strong peak at 440 nm with a Stokes shift of 140 nm. Similar to other

3MoS2 + 26H 2O2 → MoO3 + Mo2O5 + 6SO4 2 − + 12H+ + 20H 2O

(1)

In a word, it can be clearly concluded that the S2− and Mo4+ of MoS2 were oxidized to the higher oxidation states S6+ and 8186


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luminescence of MoOx QDs, while TNB, DNP, and PNP induced slight decreases and other TNT analogues including DNT, NT, TNP, DNB, NB, and TB had no obvious changes in the photoluminescence intensities. It should be noted that the peak around 350 nm is ascribed from the Raman signal of water since it will shift with the excitation wavelength (Figure S7). The quenching efficiencies of TNT, DNT, NT, TNP, DNP, PNP, TNB, DNB, NB, and TB were 57.9, 2.5, 0.2, 0.2, 7.8, 4.3, 12.5, −0.001, 0.001, and −0.001%, respectively. The selectivity factor (SF) value, the ratio of quenching efficiency of TNT and its analogues, was adopted to further assess the selectivity of MoOx QDs toward TNT, which was 1.0, 0.04, 0.039, 0.039, 0.14, 0.07, 0.22, −0.001, 0.001, and −0.001 for TNT, DNT, NT, TNP, DNP, PNP, TNB, DNB, NB, and TB, respectively, suggesting an excellent quenching behavior of TNT to the photoluminescence of MoOx QDs. The selectivity of the MoOx QD-based photoluminescence sensor for TNT vs other chemicals (include 8 metal ions, 2 proteins, and 5 amino acids) was also investigated, and only TNT caused a dramatic decrease in photoluminescence intensity (Figure 3b). All the above results indicated that the MoOx QD-based photoluminescence sensor has a highly selective response to TNT. Quenching Mechanism of TNT. TNT is a strong electron-deficient aromatic chemical that usually serves as an electron acceptor, which is able to interact with electron donors (such as −OH, −NH2) through charge transfer, leading to the formation of the Meisenheimer complex.53,54 Normally, TNT interacted with the −OH and/or −NH2 groups of nanomaterials to form the Meisenheimer complex with the result of quenching the fluorescence of nanomaterials.27,28,39 Herein, the absorption spectra were utilized to confirm whether the Meisenheimer complex formed or not during the quenching process. As shown in Figure 4a, the absorption band around 445 nm gradually enhanced when increasing amounts of TNT were added into the buffer solution, and the color changed from colorless to orange, which is consistent with the absorption characteristic of the Meisenheimer complex,27,54 suggesting that the Meisenheimer complex was formed during the quenching process. To further confirm the formation of the Meisenheimer complex, FTIR spectra of TNT at pH 7.0 and 13.0 were measured (Figure S8). Compared with spectra of TNT at pH 7.0, a new peak around 1400 cm−1 attributed to the in-plane bending vibration of −OH was observed at the strong alkaline condition, 55 suggesting the formation of the Meisenheimer complex between TNT and the −OH group. Then, the photoluminescence decays were examined to verify whether the Meisenheimer complex formed between TNT and

Figure 2. Absorption and the excitation-dependent photoluminescence emission of the obtained MoOx QDs. Inset photographs are MoOx QDs under visible (left) and UV light at 365 nm (right).

photoluminescent nanoparticles, MoOx QDs exhibited the excitation-dependent photoluminescence behavior and the photoluminescence emission peak red-shifts from 400 to 560 nm when the excitation wavelength changes from 275 to 525 nm. The emission around 440 nm is chalked up to the d−d band transition of the MoOx QDs,13 and that around 530 nm is due to the deep-level emissions caused by oxygen vacancies, Mo interstitials, surface defects, and Mo5+ ion associated with an oxygen vacancy as neighbor.16,49,50 As shown in the Figure 2 inset, the brown aqueous solution of MoOx QDs similar to that of the reduced MoO3 samples51 emitted intense yellow fluorescence under UV light (365 nm), and the absolute fluorescence quantum yield is 2.55%. Moreover, the comparable fluorescence intensity of various batches of MoOx QDs indicated the good repeatability of the synthesis process (Figure S5). The as-prepared MoOx QDs maintained good stability in one year, and no obvious changes of the photoluminescence intensity were observed with the pH ranging from 2.0 to 13.0, suggesting the excellent stability of MoOx QDs to pH (Figure S6). Photoluminescence Quenching of MoOx QDs by TNT and Its Analogues. Recently, fluorescent nanomaterials have been applied in chemically active species detection, such as nitroaromatic explosives and enzymes.27,33−38,52 In this manuscript, the obtained MoOx QDs with excellent photoluminescence property were used as an effective photoluminescent probe for TNT determination. Figure 3a depicted the spectrofluorimetric responses of MoOx QDs with the addition of TNT and its analogues. It could be clearly seen that only TNT causes a dramatic decrease of the photo-

Figure 3. (a) Photoluminescence responses of MoOx QDs toward TNT and its analogues. TNT and its analogues: 150.0 μM; MoOx QDs: 0.1 mg/ mL; pH 13.0. (b) The selectivity of the MoOx QD-based turn-off sensor. TNT: 150.0 μM; Other chemicals: 500.0 μM; MoOx QDs: 0.1 mg/mL; pH 13.0. Excitation wavelength: 325 nm; width: 10 nm; voltage: 700 V. Error bar is calculated from three parallel samples. 8187


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Figure 4. (a) Absorbance responses of the MoOx QD-based sensor with the addition of TNT. TNT concentrations from bottom to top (and from left to right in the inset photography) were 0.0, 8.0, 30.0, 60.0, 100.0, 120.0 μM; MoOx QDs: 0.1 mg/mL; pH 13.0. (b) The photoluminescence lifetimes of MoOx QDs in the absence and presence of TNT. MoOx QDs: 0.1 mg/mL; TNT: 150.0 μM; pH 13.0. (c) pH-dependent photoluminescence quenching of TNT toward MoOx QDs. MoOx QDs: 0.1 mg/mL; TNT: 150.0 μM. (d) The absorption spectra of TNT and its analogues and the photoluminescence emission spectrum of MoOx QDs. TNT and its analogues: 120.0 μM; MoOx QDs: 0.1 mg/mL; pH 13.0.

Figure 5. (a) Parameters used for the correction of the inner filter effect. I0 represents the excitation beam; F is the observed fluorescence beam; s is the thickness of the excitation beam (0.10 cm); g is the distance between the edge of the excitation beam and the edge of the cuvette (0.40 cm); and d is the width of the cuvette (1.00 cm). (b) Observed (black curve) and corrected (red curve) quenching efficiency of TNT toward MoOx QDs (0.1 mg/mL). QF = (F0 − F)/F0, F0 and F are the fluorescence intensities of MoOx QDs in the absence and presence of TNT, respectively.

MoOx QDs or not. As shown in Figure 4b and Table S1, global analysis of the resulting data as a two exponential decay gave an average lifetime of 4.14 ns for MoOx QDs, while it was about 4.76 ns when TNT was added into the MoOx QD solution. The little change in photoluminescence lifetimes of the MoOx QDs indicated that there was no significant charge transfer between MoOx QDs and TNT, further demonstrating that the Meisenheimer complex was not formed between TNT and MoOx QDs. To further elucidate the quenching mechanism, the pH-dependent quenching of TNT was investigated, as shown in Figure 4c. The photoluminescence intensity of MoOx QDs had no obvious change in the pH range, while the quenching efficiency of TNT increased dramatically with the pH ranging from 8.0 to 13.0. It means that the −OH groups of the strongly alkaline solution acted as the electron donors, and the −OH is bound to the aromatic ring of TNT to form an “intramolecular” charge transfer complex of the TNT−

Meisenheimer anion which quenched the photoluminescence of MoOx QDs consequently. As no energy and charge transfer occurred between the Meisenheimer complex and MoOx QDs which can be supposed by the unchanged fluorescence lifetime, the inner filter effect (IFE) of the Meisenheimer complex on the MoOx QDs might be the main reason for the substantial photoluminescence quenching of the MoOx QDs by TNT. The IFE ascribes from the absorption of the excitation and/or emission light by absorbers in the system, and the IFE-based method is more sensitive than the absorption method since the absorbance changes of the absorber induces exponential changes in the fluorescence of the fluorophore.56,57 The integral overlapped spectra of the MoOx QDs and Meisenheimer complex are presented in Figure 4d. These spectra indicated a complementary overlap between the emission spectrum of MoOx QDs and the absorption band of the Meisenheimer complex in the visible range. Therefore, the 8188


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Figure 6. (a) Photoluminescence emission spectra of MoOx QDs incubated with increasing amount of TNT. TNT concentrations are 0, 0.5, 4.0, 8.0, 15.0, 30.0, 60.0, 100.0, 120.0, 150.0, 180.0, 210.0, and 240.0 μM from top to the bottom, respectively. MoOx QDs: 0.1 mg/mL. Inset is the photography of MoOx QDs incubated with TNT. The concentrations of TNT from left to right are 0.0, 30.0, 120.0, and 240.0 μM. (b) Linear plot of MoOx QD-based turn-off sensor. Linear equation: IF = 1206.04−4.69 cTNT. MoOx QDs: 0.1 mg/mL; pH 13.0.

Table 1. Detection of TNT in River Water Samples by the New MoOx QD-Based Sensor and the HPLC Method proposed method sample no.

value added (μM)

1 2 3

5.0 15.0 40.0

HPLC method

value founded (mean ±SD, n = 5, μM)

recovery (%)

value added (μM)

value founded (mean ± SD, n = 5, μM)

recovery (%)

5.31 ± 0.014 14.74 ± 0.02 42.23 ± 0.22

106.2 98.3 105.6

5.0 15.0 40.0

5.22 ± 0.01 15.21 ± 0.013 41.86 ± 0.18

104.4 101.4 104.7

S3).31,33−37 It is also worth noting that the quenching phenomena could be observed by the naked-eye under UVillumination (inset photograph in Figure 6a), in which the visible yellow photoluminescence of MoOx QDs became fainter as more and more TNTs were added, confirming the quenching of MoOx QDs by TNT. As one of the most frequently used nitroaromatic explosives, TNT, which is significantly toxic to both human beings and the environment, is also a priority pollutant from the Environmental Protection Agency list. Therefore, the feasibility of the new MoOx QD-based turn-off sensor for TNT detection in real samples was further validated. The water samples collected from Ganjiang River were filtered through a 0.22 μm membrane and centrifuged for 10 min at 6000g, and no TNT was detected by the new MoOx QD-based sensor. A recovery test was further performed by adding different amounts of TNT into the water samples. As shown in Table 1, the measured recoveries range from 98.3% to 106.2%, and the results obtained by the proposed MoOx QD-based sensor were well in accordance with those obtained by the high performance liquid chromatography (HPLC) method, suggesting the high accuracy, reproducibility, and precision of the new MoO3 QD-based sensor in water sample analysis.

absorbance enhancement of the Meisenheimer complex could be successfully converted to photoluminescence quenching of MoOx QDs, which ensured that the IFE occurred in a highly efficient way. The IFE on the TNT-induced fluorescence quenching was further corrected based on the cuvette geometry and the absorption characteristics of the aqueous solution of TNT and MoOx QDs (Figure 5a and Table S2), and the results illustrated that most of the quenching effect came from the IFE of TNT on MoOx QDs (Figure 5b). After removing the IFE, the small remaining quenching effect might come from other interactions between TNT and MoOx QDs, such as hydrogen bonds that might be formed between the O atom of MoOx QDs and the −OH groups of the Meisenheimer complex. The IFE could also explain the high selectivity of the new MoOx QD-based turn-off sensor toward TNT and its analogues, as shown in Figure 4d. Compared with the absorbance of TNT, those of TNT analogues were relatively weak, and thus the IFE could not occur in a highly efficient way between analogues and the MoOx QDs accompanied by the low quenching efficiencies of TNT analogues toward the MoOx QDs. Determination of TNT in Buffer and Water Samples with MoOx QD-Based Turn-Off Sensor. Figure 6 showed the photoluminescence responses of MoOx QDs when different concentrations of TNT were added into the system, and sequential decreases of the photoluminescence emission at 440 nm were observed with increasing amount of TNT. In the range of 0.5−240.0 μM, the decreased photoluminescence of MoOx QDs showed a good linear relationship to the concentration of TNT with a correlation coefficient of 0.991 (n = 12) (Figure 6b). Meanwhile, TNT could be measured only when its concentration is higher than 8.0 μM based on the absorption of the Meisenheimer complex (Figure 4a), illustrating that the IFE method is more sensitive than the absorption method since the absorbance changes of the absorber induce exponential changes in the fluorescence of the fluorophore.56,57 The detection limit calculated as 0.12 μM (3σ/k) is comparable to other fluorescence sensors (Table

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CONCLUSION In summary, a one-pot method was developed for facile synthesis of highly photoluminescent MoOx QDs. Compared with other synthesis methods, the present method displays great advantages. First of all, the reaction is performed at room temperature without any external stimuli. Second, the synthesis is rapid and one-pot, and the MoOx QD product is obtained in 1 h or less. The third and also the most important one, the photoluminescence intensity of the as-prepared MoOx QDs, is high enough to be used as an effective photoluminescent probe in chemical sensing. Then, a new MoOx QD-based turn-off sensor was further constructed for TNT determination based on the inner filter effect (IFE) of the Meisenheimer complex on 8189


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the MoOx QDs, which results in substantial photoluminescence quenching of the MoOx QDs. The present MoOx QD-based sensor shows high selectivity for TNT compared with its analogues, and TNT in river water samples could be rapidly and selectively detected without tedious sample pretreatment processes, which shows a promising application of MoOx nanomaterials as an easily prepared, environmentally friendly, biocompatible, and high photoluminescent probe in a variety of fields, such as chemi- and biosensors, environmental monitoring, cell imaging, biomedical, and so on.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11316. The XRD pattern of MoOx QDs, the XPS spectra of MoS2 powder, the high-resolution XPS spectra of O 1s, the photoluminescence stability of MoOx QDs, pH influence, fluorescence lifetimes obtained with twoexponential fit of the fluorescence decay curves, the inner filter effect of TNT on MoOx QDs, the analytical performance of fluorescence sensors for TNT (Figures S1−7, Tables 1−3) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the National Natural Science Foundation of China (Nos. 21205011 and 21465003), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13054), and Engineering Excellence Program of Jiangxi Poyang Area (No. 2013.58).

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REFERENCES

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Highly Photoluminescent Molybdenum Oxide Quantum Dots: One-Pot

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