Molybdenum Disulfide Quantum Dots as a Photoluminescence

Jul 7, 2014 - MoS2 QDs. High-resolution peak-fitting XPS spectra of the S 2p (D) and Mo 3d (E) regions for MoS2 QDs. XRD pattern (F) and FT−IR spect...
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Molybdenum Disulfide Quantum Dots as a Photoluminescence Sensing Platform for 2,4,6-Trinitrophenol Detection Yong Wang† and Yongnian Ni*,†,‡ †

Department of Chemistry, Nanchang University, Nanchang, Jiangxi 330031, China State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang, Jiangxi 330047, China

‡

S Supporting Information *

ABSTRACT: Transition metal chalcogenides, especially molybdenum disulfide (MoS2), have recently attracted wide attention from researchers as graphene-analogous materials. However, until now, little literature has reported the synthesis of photoluminescent MoS2 materials and their applications in analytical chemistry. We herein presented a facile bottom-up hydrothermal route for the synthesis of photoluminescent MoS2 quantum dots (QDs) by using sodium molybdate and cysteine as precursors. The prepared MoS2 QDs were characterized by transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, photoluminescence spectroscopy, and UV−vis spectroscopy. The MoS2 QDs were then used as photoluminescent probes to construct a photoluminescence (PL) quenching sensor for detection of 2,4,6-trinitrophenol (TNP). The TNP sensor presented a wide linear range from 0.099 to 36.5 μM with a high detection limit of 95 nM. Furthermore, the sensor displayed a high sensitivity toward TNP over other structurally similar compounds like 2,4,6-trinitrotoluene, pchlorophenol, phenol, and 2,6-di-tert-butyl-4-methylphenol. To understand the origin of the high sensitivity, we assessed the emission wavelength-dependent PL quenching behavior of MoS2 QDs by the above five compounds using Stem−Volmer equation in detail. The results showed that the novel approach we put forward can satisfactorily explain the interaction mechanisms between MoS2 QDs and the five compounds, and the high sensitivity for TNP very likely originated from a combination of the PL resonance energy transfer, electronic energy transfer, and electrostatic interactions between MoS2 QDs and TNP. Finally, the sensor was successfully applied for detection of TNP in water samples and test papers.

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photoluminescence. That may be because MoS2 is an indirect bandgap material in its bulk form and has excellent chemical stability and high thermal stability, which increase the difficulty of synthesis of photoluminescent MoS2 materials. Therefore, it is of critical need to establish synthesis methods for photoluminescent MoS2 materials. To date, some researchers have dedicated considerable efforts to achieve the synthesis of photoluminescent MoS2 materials.23−30 In 2010, Splendiani et al. fabricated MoS2 material on both quartz and Si/SiO2 wafers through microexfoliation techniques and reported photoluminescent emergency in this material for the first time.23 In 2011, Coleman et al. proposed liquid-phase exfoliation of commercial MoS2 powers in a suitable organic solvent with the aid of ultrasonication as a route to prepare photoluminescent MoS2.24 Subsequently, Eda et al. prepared photoluminescent MoS2 from its bulk powers through Li intercalation and exfoliation under ultrasonication.25 Very recently, Stengl et al. have prepared photoluminescent MoS2 quantum dots (QDs) from natural molybdenite via sonication assisted liquid-phase

ince its discovery in 2004, one-atom-thick graphene has caused a great sensation in the broad interdisciplinary communities including physics, chemistry, materials science, nanoscience, engineering, and biology.1−5 Graphene possesses unique and unusual mechanical, electronic, optical, thermal, and chemical properties and provides wide-ranging uses including supercapacitors, batteries, biosensors, biodevices, and so forth.5−8 The outstanding physical and chemical properties depend to a large extent on its inherently two-dimensional nature,9 and therefore many researchers have recently started to pay attention to various inorganic graphene-analogous twodimensional materials, especially those ultrathin nanosheets of transition metal chalcogenide (TMC) with single or few atomic layers.10−15 Among those TMC materials, molybdenum disulfide (MoS2) has drawn particular attention because it can be easily exfoliated from a naturally occurring molybdenite compound, built up from layers consisting of sulfur atoms in two hexagonal planes separated by an atomic plane of molybdenum atoms.16−19 Some potential applications of MoS2, like batteries, catalysts, and transistors, have been explored by researchers over the past few years.18−22 These studies mainly focus on the electrical and catalytic properties of MoS2 materials, and there is little literature available regarding their optical properties, especially © XXXX American Chemical Society

Received: April 3, 2014 Accepted: July 7, 2014

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Figure 1. (A) TEM image of MoS2 QDs. The inset displays the HRTEM image (top) and the corresponding particle size distribution histogram (middle). (B) AFM image of MoS2 QDs. The inset represents the corresponding particle height distribution histogram. (C) XPS survey spectrum of MoS2 QDs. High-resolution peak-fitting XPS spectra of the S 2p (D) and Mo 3d (E) regions for MoS2 QDs. XRD pattern (F) and FT−IR spectrum (G) of MoS2 QDs.

exfoliation and refluxing. Yu et al.27 and Ji et al.28 have synthesized photoluminescent MoS2 films on various substrates using the chemical vapor deposition method. Liu et al. have presented an Ar+ plasma irradiation method to prepare photoluminescent MoS2 films.29 Late’s group has applied micromechanical exfoliation techniques to prepare photoluminescent MoS2 films on SiO2/Si substrates.30 Most of the approaches belong to “top-down” synthesis. They are usually sensitive to the environment, or use expensive and hazardous organic solvents, or require harsh pretreatment procedures. Hydrothermal synthesis is generally well-known, has significant economic and environmental advantages, and is the most common “bottom-up” method. Thus, the development of

hydrothermal synthesis of photoluminescent MoS2 is always attractive. Moreover, to the best of our knowledge, almost all of the experimental work about photoluminescent MoS2 material has only emphasized its preparation at present, and almost no literature has reported the study of its application in analytical chemistry. In the present work, for the first time, we prepared photoluminescent MoS2 QDs under hydrothermal conditions by using sodium molybdate and cysteine as precursors and gave the first example of application of MoS2 QDs for the fabrication of a photoluminescence (PL) sensor. To demonstrate the proof-of-concept, we selected 2,4,6-trinitrophenol (TNP) as the target analyte. TNP is a kind of important nitroaromatic B

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explosive and possesses far more violent explosive power than its well-known counterpart 2,4,6-trinitrotoluene (TNT).31−33 At present, it has been widely used in forensic research, land mine detection, manufacture of rocket fuels, and so on.31−33 It is recognized as an environmentally deleterious substance and produces lasting adverse effect on both human and wildlife health.34,35 Hence, the development of methods for detection of TNP has captured great research efforts in recent years.36−41 However, due to the extremely similar structure and properties, many methods for detection of TNP usually suffer interference from TNT.38−41 Therefore, the selective detection of TNP without a chemical separation procedure is in high demand. In the work presented below, we constructed a label-free sensor for sensitive and selective detection of TNP based on PL quenching of MoS2 QDs. Simultaneously, we investigated the emission wavelength-dependent PL quenching behavior of MoS2 QDs by TNP using Stem−Volmer (SV) analysis in detail. The computed Stem−Volmer constants for TNP were plotted versus wavelength and were compared with UV−vis spectral data of TNP to interpret the possible PL quenching mechanism involved in the system.

cysteine. In brief, 0.25 g of Na2MoO4·2H2O was dissolved in 25 mL of water. After ultrasonication for 5 min, the solution was adjusted to pH 6.5 with 0.1 M HCl. Then, 0.5 g of L-cysteine and 50 mL of water were added to the solution followed by ultrasonication for 10 min. The mixture was then transferred into a 100 mL Teflon-lined stainless steel autoclave and reacted at 200 °C for 36 h. After the solution cooled naturally, the supernatant containing MoS2 QDs was collected after being centrifuged for 30 min at the speed of 12 000 rpm. General Procedure of Photoluminescence Detection of TNP. For the typical TNP sensing experiments, the solutions of the Tris-HCl buffer (1.5 mL, 20 mM, pH 7.4) and MoS2 QDs (15 μL) were sequentially micropipetted into a 3.5 mL cuvette. The solution was mixed thoroughly and incubated for 3 min. Then, TNP solution was added to give a series of samples with different concentrations (range, cTNP = 0−323 μM; total, 36 samples). The resultant solution was mixed thoroughly. After incubation for 2 min, the PL spectral data of the resultant solution were collected over the wavelength range of 330−570 nm (excitation wavelength: 308 nm). All measurements were taken at room temperature.

EXPERIMENTAL SECTION Reagents and Chemicals. Sodium molybdate dihydrate (Na2MoO4·2H2O) was obtained from Tianjin Chemical Reagent 4th Factory Kaida Chemical Plant (Tianjin, China). 2,4,6-Trinitrophenol was obtained from Taishan Chemical Reagent Factory (Guangdong, China). L-Cysteine and phenol (PHE) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 2,4,6-Trinitrotoluene solution (1.00 mg mL−1 in methanol), p-chlorophenol (PCP), and 2,6di-tert-butyl-4-methylphenol (BHT) were obtained from Aladdin Industrial Co. Ltd. (Shanghai, China). Tris(hydroxymethyl)aminomethane (Tris) was obtained from Lanji Science and Technology Development Co., Ltd. (Shanghai, China). The pH of 20 mM Tris buffer was adjusted to 7.4 with concentrated hydrochloric acid (HCl). Unless specified, all chemicals were of analytical reagent grade. Deionized distilled water was used throughout. Apparatus. PL spectra were collected with a PerkinElmer LS-55 photoluminescence spectrometer (PerkinElmer Co., USA) with a standard 10-mm path length quartz cuvette. UV−vis spectra were recorded with an Agilent 8453 UV− visible spectrometer (Agilent Technologies, USA) with a standard 10-mm path length quartz cuvette. Transmission electron microscopy (TEM) images were obtained by using a JEM-2010 microscope (JEOL Co., Japan) operated at an accelerating voltage of 200 kV. Atomic force microscope (AFM) measurements were made on an AJ-III Instrument (Shanghai Aijian Nanotechnology, China) in the tapping mode. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALab 250Xi (Thermo Scientific, USA) using 200 W monochromated Al Kα radiation. X-ray diffraction (XRD) spectra were recorded with a Bede D1 high-resolution X-ray diffractometer system (Bede Co., UK). Fourier transform infrared spectra (FT−IR) were measured on a Nicolet 380 FT−IR spectrometer (Thermo Nicolet Corporation, USA) equipped with a DTGS KBr detector and a KBr beam splitter in the transmission mode. The measurements of dynamic light scattering (DLS) were performed using a PSA NANO2590 apparatus (Malvern Instruments Ltd., England). Preparation of MoS2 QDs. MoS2 QDs were synthesized through a hydrothermal method using sodium molybdate and

RESULTS AND DISCUSSION Physicochemical Characterization of MoS2 QDs. The MoS2 QDs were characterized by means of TEM, AFM, XPS, XRD, FT−IR, PL, and UV−vis spectroscopy. A typical TEM image of the MoS2 QDs (Figure 1A) showed highly uniform and monodisperse nanocrystals in narrow distribution of 2.15 ± 0.34 nm diameter (the size is statistically calculated from more than 100 QDs in the TEM images). From the representative AFM images of the MoS2 QDs (Figure 1B), it has a thickness of 1.79 ± 0.71 nm (the height is calculated based on more than 100 QDs in the AFM images). An XPS survey spectrum for powders formed by drying of MoS2 QDs solution is displayed in Figure 1C. In the XPS spectrum, Mo and S can be detected, together with N, C, O, and Na, likely from the chemicals involved in hydrothermal synthesis. Mo and S that occurred in the powder were further confirmed with the high-resolution XPS S 2p and Mo 3d spectrum, respectively (Figure 1D and E). The S 2p spectrum was fitted by three 2p3/2−2p1/2 spin− orbit doublets (Figure 1D). Concerning the three S 2p doublets, those at around 163.44 and 164.55 eV can be assigned to the 2p3/2 and 2p1/2 lines of MoS2.42 The S 2p doublet at around 163.52 eV (2p3/2) and 164.68 eV (2p1/2) was very likely derived from disulfides S22− or polysulfides Sn2−.43 The high-energy S 2p doublets, 168.02 eV (2p3/2) and 169.17 eV (2p1/2), were very possibly attributed to the S2O32− group.44 The Mo 3d spectrum was deconvoluted into five single peaks, as shown in Figure 1E. The three peaks at around 226.22 eV, 228.69, and 231.89 eV corresponded to the S 2s, Mo 3d5/2, and Mo 3d3/2 lines of MoS2, respectively.43 The quantitative analysis of the S 2s and Mo 3d5/2 peak intensity indicated that the atomic ratio between Mo and S was close to 1:2, confirming the formation of MoS2 QDs. Considering that the S 2p spectrum implied three forms of sulfur, two additional S 2s components with binding energies of 225.72 and 227.80 eV can be assigned to S2O32− and disulfides S22− or polysulfides Sn2−, respectively.43,45 Moreover, as can be seen from Figure 1D and E, the S 2p doublet and S 2s peaks, which were assigned to disulfides S22− or polysulfides Sn2−, had strong intensity. This was probably because the MoS2 QDs solution was not purified. The XRD diffractogram of MoS2 QDs is shown in Figure 1F. Four major diffraction peaks can be clearly observed. These

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graphene quantum dots.52,53 Furthermore, UV−vis spectra of MoS2 QDs exhibited an absorption band at ca. 275 nm (Figure 2D), which was similar to that reported previously.26 The absorbance maximum corresponded with a particle size of less than 2.5 nm,54 which agreed well with that given by TEM. PL Quenching Characteristic of MoS2 QDs toward TNP. As discussed above, the prepared MoS2 QDs are a new kind of nanoscale materials with good PL properties. Figure 3

peaks were attributed to the (002), (100), (102), and (103) planes of MoS2, respectively.26,46 The peak intensity was weak, implying the formation of a poorly crystalline form of MoS2 QDs.26,46 Figure 1G showed the FT−IR spectra of the MoS2 QDs. The weak peak around 465 cm−1 was ascribed to the Mo−S vibration.47 The peak at 3310 cm−1 was assigned to the N−H stretching vibration.48 In the lower frequency region, the peaks at 1637 and 1574 cm−1 were assigned to the N−H bending vibration and the N−H in-plane stretching vibration, respectively.48,49 All the results illustrated that the amino groups were very likely located on the surface of MoS2. The two-dimensional “total PL” contour map acquired from a typical excitation−emission matrices (EEMs) of MoS2 QDs (note the spectra are not corrected for Rayleigh scattering peaks) was depicted in Figure 2A. It was clearly observed that

Figure 3. PL spectra of MoS2 QDs in the absence or presence of TNP. The inset represents the photographs of MoS2 QDs in the absence (a) or presence (b) of TNP under irradiation of 365 nm light.

clearly showed that MoS2 QDs had PL excitation spectra peaked at 308 nm and emission spectra peaked at 402 nm. The addition of TNP to the MoS2 QDs solution led to a drastic diminishment of both the excitation band and emission band of MoS2 QDs (Figure 3). Correspondingly, as observed from the digital photo (the inset of Figure 3), the MoS2 QDs solution emitted strong blue photoluminescence on exposure to a 365 nm UV lamp, and upon addition of TNP, the naked eye visible photoluminescence turned black, confirming the PL quenching of the MoS2 QDs solution. To figure out the possible PL quenching mechanism, the quenching behaviors of TNP and some structurally similar compounds like TNT, PCP, PHE, and BHT (all chemical structures are listed in Table S2, Supporting Information) were characterized by their respective quenching ratio (F0(λ)/F(λ)) and the quenching constant (KSV(λ)) as determined by the Stern−Volmer model as shown in eq 1.55

Figure 2. (A) Contour map of the three-dimensional PL spectra of MoS2 QDs. The arrow represents the position of the PL peak (λexcitation = 308 nm, λemission = 402 nm). (B) Emission spectra of MoS2 QDs at excitation wavelengths progressively increasing from 270 to 450 nm. (C) Effect of pH on the PL intensity of MoS2 QDs. (D) UV−vis spectra of MoS2 QDs and TNP, as well as the theoretical and experimental profiles based on the sum of the MoS2 QDs and TNP spectra.

the most intense peak of MoS2 QDs appeared at an emission wavelength around 402 nm with an excitation wavelength around 308 nm. Moreover, the FL emission spectra of MoS2 QDs were plotted at excitation wavelengths progressively increasing from 270 to 450 nm (Figure 2B). It was clear that the PL emission peak of MoS2 QDs exhibited a large red-shift (from ∼400 nm to ∼480 nm) with an increase in the excitation wavelength, and the PL intensity increased until λexcitation = 310 nm, and then decreased remarkably. The strong excitation wavelength dependent PL feature of MoS2 QDs was consistent with those previously reported in the literature.26,50,51 This may be attributed to the hot PL from the K point of the Brillouin zone and the polydispersity of MoS2 QDs.26,50,51 The pHdependent PL behavior of MoS2 QDs was also studied. It was clear in Figure 2C that the PL intensity of MoS2 QDs had almost no change over the whole pH range from 2.0 to 12.0, denoting the superior resistance of MoS2 QDs to the pH of the solution. By using quinine sulfate as a fluorescent standard, the PL quantum yield at 308 nm excitation was estimated to be 2.6% (Table S1 in the Supporting Information),52 which was comparable to those reported previously for carbon dots and

F0(λ)/F(λ) = KSV(λ)[Q] + 1

(1)

where F0(λ) is the PL intensity at a specified emission wavelength λ in the absence of a quencher, F(λ) denotes the PL intensity at emission wavelength λ and quencher concentration [Q], and KSV(λ) represents the Stern−Volmer quenching constant at emission wavelength λ. Five PL quenching experiments were performed by titration of the MoS2 QDs solution with different concentrations of TNP, TNT, PCP, PHE, and BHT, respectively (Figure 4A, Figure S1 in the Supporting Information). The emission spectra were collected at each concentration of the five quenchers, and the above Stern−Volmer equation was fitted for each set of data obtained at 402 nm. The parameters for each quencher related to the fitted Stern−Volmer model were established in Table 1. As observed in Table 1, the correlation coefficients and the intercept for all Stern−Volmer models except BHT (see Figure S1 in the Supporting Information, the concentration-dependent quenching behavior cannot be found in the presence of BHT, indicating that the fitting results were D

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Figure 4. (A) PL spectra of MoS2 QDs under excitation at 308 nm in the presence of different TNP concentrations (0, 0.033, 0.099, 0.17, 0.33, 0.66, 0.99, 2.0, 3.6, 6.9, 10.2, 13.5, 16.8, 20.1, 23.4, 26.7, 30.0, 33.2, 36.5, 43.1, 49.6, 56.1, 62.7, 69.2, 82.2, 95.2, 108.1, 121.0, 133.9, 146.7, 165.9, 197.7, 229.4, 260.8, 292.0, and 323.0 μM) in a 20 mM Tris−HCl solution (pH 7.4). (B) Plot of Stern−Volmer constants for TNP quenching of MoS2 QDs against emission wavelengths, and UV−vis spectra of TNP in the 350−500 nm wavelength region. (C) Selectivity of the MoS2 QDs-based PL sensor for TNP. The concentration of TNP and other substances is 10 μM. The error bars denote the standard deviation of three measurements.

Table 1. Summary of Stern−Volmer Equations at 402 nm for the PL Quenching of MoS2 QDs by TNP, TNT, PCP, PHE, and BHT TNP TNT PCP PHE BHT a

number of samples (n)

KSV (× 104 M−1)

intercept (μM)

linear range coefficient

correlation

SFb

17 28 31 22 NDa

4.3 0.31 0.018 0.019 ND

1.01 1.01 1.01 1.01 ND

0.099−36.5 2.0−292.0 33.2−3729.0 43.1−1169.4 ND

0.999 0.999 0.999 0.990 ND

1 0.072 0.0042 0.0044 ∼0

ND means not present. bSF represents the ratio of KSV of TNP and four other quenchers.

not obtained for BHT and its KSV value was be approximate to zero) were equal to 0.999 and close to 1, suggesting that perfect fitted results were acquired. The order of the Stern−Volmer quenching constants for the five quenchers was TNP ≫ TNT ≫ PCP ≈ PHE ≫ BHT. TNP displayed a surprising superquenching to PL of MoS2 QDs and possessed the highest KSV value (4.3 × 104 M−1). The ratio of KSV of TNP and four other quenchers was herein defined as the selectivity factor (SF), and the SF value was adopted to assess the selectivity of MoS2 QDs toward TNP. From Table 1, we noted that the SF values for TNT, PCP, PHE, and BHT were 0.072, 0.0042, 0.0044, and 0, respectively, suggesting that MoS2 QDs have a superior selectivity for TNP compared to four other structurally similar compounds. Figure S2 in the Supporting Information further presented the high selectivity of MoS2 QDs for TNP at low concentrations below 36.5 μM, verifying the validity of SF.56 To understand the origin of the selectivity, we applied the above Stem−Volmer equation to further investigate the relationship between the wavelength dependence of the PL quenching and each quencher. The KSV constant for each quencher at each wavelength was estimated and plotted against

wavelength (Figure 4B and Figure S3 in the Supporting Information). Noticeably, the KSV constants for TNP and TNT were strongly wavelength dependent, while those for PCP and PHE were almost independent of emission wavelength. In addition, the UV−vis absorption spectrum of each quencher was recorded and displayed in Figure 4B and Figure S3 in the Supporting Information. Looking at the plot of KSV versus wavelength and the UV−vis spectrum, we can find that both plots for each quencher were very similar, especially for TNP and TNT. The corresponding analysis between KSV values and UV−vis spectral data gave a similarity coefficient of 0.999 for TNP and 0.910 for TNT, respectively, further confirming that there was perfect agreement between the emission wavelengthdependent KSV plot and UV−vis spectrum. On the basis of these results, we suppose that the PL quenching behaviors of MoS2 QDs by the two quenchers, especially by TNP, were very likely to predominantly follow the PL resonance energy transfer (RET) mechanism because the probability of RET largely depends on the extent of overlap of the fluorophore emission and quencher absorption spectrum. UV−vis spectra in the presence of MoS2 QDs or/and TNP were further studied. It was clear in Figure 2D that the experimental spectrum of TNP E

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analyzing the PL data collected at different concentrations of TNP, we find that the quenching photoluminescence at 402 nm had a distinct linear reduction toward TNP in the concentration range of 0.099−36.5 μM, with a linear regression equation of F0/F = 4.3 × 104 cTNP (μM) + 1.01 (correlation coefficient r = 0.999; see Table 1). The detection limit with a signal-to-noise ratio (S/N) of 3 for TNP calibration was 95 nM. The value was very comparable with or superior to the LODs obtained from other sensitive fluorimetric methods.36−41 Another important aspect of the TNP sensor was to assess its selectivity. Figure 4C clearly showed that after the addition of TNP or structurally similar compound (TNT, PCP, PHE, or BHT) or environmentally relevant metal ion (Fe3+, Al3+, Cr3+, Zn2+, Cd2+, Pb2+, Hg2+, Mn2+, Ni2+, Co2+, Ca2+, Mg2+, Ba2+, Na+, or K+) at a concentration of 10 μM, the PL quenching ratios (F0/F) of all substances except TNP were close to 1, and TNP had a high quenching ratio (∼1.42). Moreover, the PL spectra for the mixtures containing TNP (10 μM) and TNT (10 μM or 20 μM) were recorded (Figure S5A in the Supporting Information). It was found that the mixtures had only a slight or negligible effect on the PL spectral change compared with that of TNP alone, and the PL quenching ratios of the mixtures were approximate to that of TNP (see Figure S5A and B in the Supporting Information). All the results signified that the sensor had a highly selective response to TNP. The high selectivity of the TNP sensor very possibly originated from a combination of the RET, EET, and electrostatic interactions between MoS2 QDs and TNP. Application of the TNP Sensor in Water Samples and Test Papers. To further check the applicability of the TNP sensor, we employed the PL sensor for detection of TNP in both water samples and test papers. The water samples were collected from Qinshan Lake (Nanchang, Jiangxi, China). The samples collected were filtered through a 0.22 μm membrane and then centrifuged for 15 min at 10 000 rpm. Because no TNP in the water samples was detected by the PL sensor, a recovery test was done on the samples spiked with TNP at different concentration levels to assess its feasibility. The results were summarized in Table 2. It was observed that there was

upon the addition of MoS2 QDs was very similar to the theoretical one, and the tiny difference between the experimental and theoretical spectrum lay in the intensity variations of the absorption band. All of these implied that the interaction of MoS2 QDs with TNP was weak, and the Meisenheimer complex between MoS2 QDs and TNP was not formed. Therefore, we excluded the possibility of a chargetransfer mechanism between MoS2 QDs and TNP. Data obtained from DLS measurements (see Figure S4A and B in the Supporting Information) showed that the hydrodynamic diameter value (3.1 nm) for MoS2 QDs in the presence of TNP was close to that value (2.7 nm) for MoS2 QDs alone, indicating that MoS2 QDs did not aggregate after the addition of TNP. On the basis of the results, we can exclude the possibility of an aggregation-induced PL quenching mechanism. Simultaneously, it was found that the diameter of MoS2 QDs given by this technique was larger than the diameter measured from TEM. This was mainly because DLS measured the hydrodynamic size while TEM provided a more precise measurement of the hard MoS2 QDs core. On the other hand, it was observed from Figure 4B that TNP still possessed a constant KSV value at emission wavelength channel above 480 nm, although no UV−vis absorption spectral band of TNP appeared in the 480−500 nm wavelength region. The KSV value was close to 1.2 × 104 M−1, which was far less than that of 4.3 × 104 M−1 at 402 nm. These results suggested that the PL quenching by TNP may also follow an electronic energy transfer (EET) mechanism, and the RET mechanism was predominant over the EET mechanism. Similarly, TNT exhibited no absorption band in the 420−500 nm wavelength range, but it had a relatively large KSV value (∼2.6 × 103 M−1) in the wavelength range studied (Figure S3 in the Supporting Information), suggesting that both RET and EET mechanisms were present. In addition, as presented in Figure S3 in the Supporting Information, both PCP and PHE had no absorption spectrum over the whole wavelength range studied, and the KSV values given by two quenchers were very little (∼3.0 × 102 M−1), denoting that the quenching occurred only through a weak EET process. On the basis of the KSV values given by the EET mechanism, we can evaluate the strength of the EET process between MoS2 QDs and quenchers, i.e., TNP ≫ TNT ≫ PCP ≈ PHE ≫ BHT. The result suggested that the EET quenching quality strongly depended on the relative electron deficiency of each compound which was mainly determined by the electron withdrawal power of substituent groups attached to the benzene ring (−NO2 > −Cl > −H > −C(CH3)3). On the other hand, the quenching caused by the EET process was partly affected by the acidity of the compounds. As TNP had a hydroxy substituent on the benzene ring, it was a stronger acid than TNT and so led to a relatively low quenching effect. On the basis of the results, we suppose that there was an electrostatic interaction between the hydroxy group of TNP and the free basic sites (−NH2 on the surface) of MoS2 QDs. Therefore, the unprecedented selectivity of MoS2 QDs toward TNP can be explained by the favorable RET and EET mechanisms, as well as electrostatic interactions. Fabrication of PL Sensor for Detection of TNP. TNP is a common and powerful nitroaromatic explosive, and the feasibility of a MoS2 QDs-based PL sensor to detect TNP is herein explored for the first time. As displayed in Figure 4A, the PL intensity of MoS2 QDs decreased monotonically with increasing TNT concentration from 0 μM to 323.0 μM, verifying the validity of the PL assay for detection of TNP. By

Table 2. Detection of TNP Spiked in Water Samples (n = 3) samples

added (μM)

mean found (μM)

1 2 3

0.750 1.50 5.50

0.813 1.57 5.45

a

mean recovery (%)a RSD (%) 108 105 99

9.8 6.7 5.3

Recovery (%) = 100 × (cmean‑found/cadded).

good agreement with the added and found values, and the obtained recoveries varied from 95% to 110%, suggesting no severe interferences in such samples. Moreover, the relative standard deviations (RSD) of three replication determinations for each sample were below 10%, suggesting a relatively high reproducibility and precision. Consequently, it was believed that the MoS2 QDs-based PL sensor can be successfully applied for detection of TNP in water samples. For test paper assays, a piece of chromatography paper (70 mm in diameter) was inserted into a sealed glass bottle containing photoluminescent MoS2 QDs solution for 30 min. After that, the paper was removed from the solution and placed in an oven under a stable temperature of 60 °C for 20 min. Under a 365 nm UV lamp, a normal paper without any treatment showed black, while this F

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door for the applications of photoluminescent transition metal chalcogenides in the field of analytical chemistry.

MoS2 QDs-treated paper displayed strong blue photoluminescence (Figure 5), demonstrating the successful preparation of

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ASSOCIATED CONTENT

S Supporting Information *

Quantum yield of MoS2 QDs; chemical structures of TNP, TNT, PCP, PHE, and BHT; PL spectra of MoS2 QDs solution with different concentrations of TNT, PCP, PHE, and BHT; plot of the quenching ratio for TNP, TNT, PCP, PHE, and BHT at 402 nm versus their respective concentration; plot of Stern−Volmer constants for TNT, PCP, or PHE quenching of MoS2 QDs against emission wavelengths and UV−vis spectra of the three compounds; hydrodynamic diameter of MoS2 QDs with and without TNP measured by DLS; PL spectra of MoS2 QDs under different conditions and selectivity of the MoS2 QD-based PL sensor for TNP over TNT. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 5. Photoimages of the chromatography paper without (A) or with (B) MoS2 QDs treatment. (C) Visual detection of TNP by handwriting on the MoS2 QDs-treated chromatography paper with 1 mM of TNP solution as ink. (D) Test paper for detection of TNP. The concentration of TNP is 0.5 μM, 5 μM, 35 μM, and 1 mM from pattern a to pattern d, respectively. All the photos are taken under a 365 nm UV lamp.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 791 83969500. Fax: +86 791 83969500. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC-21065007 and NSFC-21305061), the Natural Science Foundation of Jiangxi Province (20132BAB213011), the Jiangxi Provincial Department of Education (GJJ13026), the State Key Laboratory of Chemo/ Biosensing and Chemometrics of Hunan University (SKLCBC2013010), and the State Key Laboratory of Food Science and Technology of Nanchang University (SKLF-ZZA- 201302 and SKLF-ZZB-201303).

the photoluminescent paper. The word “TNP” was written with TNP solution as ink on the photoluminescent paper, and the PL on the trail of handwriting was quenched immediately. In addition, four different concentrations of TNP solution were dripped on different zones in the photoluminescent test paper. Under 365 nm UV light irradiation, the PL intensity of the four different zones was remarkably different, and the intensity became darker and darker as the TNP concentration increased. The above findings implied that a paper-based PL sensor for detection of TNP can be successfully fabricated. The paper sensor may be combined with fingerprint lifting or imaging techniques for achieving the visual detection of TNP in the fields of homeland security and public safety.

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REFERENCES

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CONCLUSION In this work, for the first time, photoluminescent MoS2 QDs were prepared under hydrothermal conditions by using sodium molybdate and cysteine and were applied for the construction of a PL quenching sensor for detection of TNP. The TNP sensor possessed a wide linear range and a high sensitivity toward TNP over other structurally similar compounds such as TNT, PCP, PHE, and BHT. The PL quenching behaviors of MoS2 QDs by the five compounds were then evaluated by Stem−Volmer equations at different emission wavelengths, and the analysis of the Stern−Volmer quenching constant obtained at each wavelength suggested that the high sensitivity of the sensor toward TNP was possibly due to the favorable RET and EET between MoS2 QDs and TNP, as well as electrostatic interactions. Finally, the sensor was successfully used for detection of TNP in water samples and test papers. It is expected that the new method for the study of the PL mechanism using a multiwavelength mode Stern−Volmer equation can be extended for the analysis of other PL quenching systems. Many studies toward this direction are currently underway. In addition, the work may also open the G

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