Label-Free Fluorescence Sensing of Lead(II) Ions and Sulfide Ions

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Research Article pubs.acs.org/journal/ascecg

Label-Free Fluorescence Sensing of Lead(II) Ions and Sulfide Ions Based on Luminescent Molybdenum Disulfide Nanosheets Yong Wang,*,† Jie Hu,† Qianfen Zhuang,† and Yongnian Ni*,†,‡ †

College 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: Fluorescent molybdenum disulfide (MoS2) nanosheets were synthesized hydrothermally by employing sodium molybdate and thiourea as the starting materials. Lead(II) ion was introduced as a chemical dopant into the fluorescent nanosheets for the first time, and it was found that the fluorescence of the doped MoS2 nanosheets showed a considerable enhancement compared with that of initial MoS2 nanosheets, implying that lead(II)-doping into the MoS2 nanosheets could result in an increase in the fluorescence quantum yield. In parallel, we exploited the lead(II)-induced fluorescence enhancement of MoS2 nanosheets to design a green and facile fluorescent “turn on” nanosensor for lead(II) detection. Moreover, we found that the fluorescent intensity of the doped MoS2 nanosheets was drastically quenched by the successive addition of sulfide ions. Hence, the “turn off” process was used to fabricate a green fluorescence quenching sensor for detection of sulfide ions. Finally, we elucidated the origin of the lead(II)-induced fluorescence enhancement and sulfide-induced fluorescence reduction by using various analytical techniques like scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and UV−vis spectroscopy. The work not only opens a door for the further development of new approaches for the preparation of various fluorescent layered transition metal dichalcogenides with high quantum yields but also provides a versatile and sustainable sensing platform for ion detection. KEYWORDS: Molybdenum disulfide nanoflakes, Luminescence, Two-dimensional materials, Lead, Sulfide, Sensors

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INTRODUCTION

With the rapid development of nanotechnology, inorganic graphene-like two-dimensional fluorescent nanomaterials have attracted tremendous research interest in recent years because they possess a wealth of unusual and fascinating physicochemical properties like low toxicity, excellent photostability and biocompatibility, and superior chemical stability.9 Among the fluorescent graphene-analogous materials, a surge of intense research interest over the past few years has focused on layered transition metal dichalcogenides (TMDs), notably molybdenum disulfide (MoS2).10,11 More recently, two independent groups have reported the fluorescence MoS2 nanosheets that were fabricated using microexfoliation techniques on oxidecovered Si substrates.12,13 In addition, the MoS2 nanoflakes prepared by some approaches including exfoliation,14 intercalation,15 and chemical vapor deposition16 have been also found to emit fluorescence. However, as far as we are aware, almost no literature has reported that the MoS2 nanoflakes prepared by the bottom-up wet-chemical hydrothermal synthetic route were able to exhibit fluorescence. On the other hand, until now, the quantum yield (QY) of the fluorescent MoS2 nanosheets has been significantly low. To improve the QY, the fluorescent property of MoS2 nanosheets

In the past years, the development and application of chemical sensors has become a hot topic.1−5 Among all sensors, the development of sensors for the recognition and detection of anions or heavy metal ions has attracted the most interest because these ions are closely associated with personal health or environmental problems, which undermines global sustainability.2−5 Traditional detection methods have been applied to detect anions or heavy metal ions based on liquid chromatography, mass spectrometry, or atomic spectrometry.6−8 However, these methods usually possess some shortcomings such as the usage of toxic solvents, the application of expensive instruments, the need for complex and timeconsuming operations, and the resulting waste products, which fall short of sustainability goals.6−8 Compared with these approaches, simple fluorescent sensors can overcome these drawbacks, and are commonly used for routine analysis. At present, among these fluorescent sensors, they can be roughly divided into three categories: organic dye, semiconductor quantum dots, and other fluorescent nanomaterials.1−5 Nevertheless, most organic dyes and semiconductor quantum dots have usually high toxicity.1−5 Hence, the fabrication of fluorescent sensors based on other environmentally friendly fluorescent nanomaterials to detect these ions is very important. © 2016 American Chemical Society

Received: December 5, 2015 Revised: March 17, 2016 Published: April 5, 2016 2535

DOI: 10.1021/acssuschemeng.5b01639 ACS Sustainable Chem. Eng. 2016, 4, 2535−2541

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on an AJ-III Instrument (Shanghai AJ Nano-Science Development Co., China) under tapping mode. Scanning electron microscopy (SEM) images were obtained with a JSM-6701F microscope system (JEOL Co., Tokyo, Japan). X-ray photoelectron spectra (XPS) analyses were carried out on an ESCALab 250Xi using 200 W monochromated Al Kα radiation. X-ray diffraction (XRD) patterns were collected from a Bede D1 high-resolution X-ray diffractometer. Fourier transform infrared spectra (FT-IR) were measured in a Nicolet spectrometer equipped with a DTGS KBr detector and a KBr beam splitter in the transmission mode. UV−vis absorption spectra were recorded on an Agilent 8453 UV−visible spectrometer. All fluorescence (FL) spectra were conducted on a PerkinElmer LS-55 fluorescence spectrophotometer with an excitation wavelength of 250 nm. Synthesis of Fluorescent Molybdenum Disulfide (MoS2) Nanoflakes. The fluorescent MoS2 nanoflakes were synthesized as below. In short, after 2.2 g of sodium molybdate was mixed with 2.0 g of thiourea in 70 mL water, the mixture precursors were strongly stirred for ca. 10 min. Next, the pH value of the mixture solution was adjusted to less than 1 with 12 M hydrochloric acid. Then, the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 200 °C for 24 h. After cooling to room temperature, the final solid product was obtained by centrifugation, washed with water and absolute ethanol for several times, and dried in a vacuum oven overnight at 60 °C. Fluorescent Detection of Lead(II) Ions. In a typical experiment, 20.0 μL of 1.0 mg mL−1 MoS2 solution and appropriate amounts of water were sequentially micropipetted into a 3.5 mL cuvette, followed by Pb2+ solution with different concentrations (range, cPb2+ = 0−15.0 μM; total, 31 samples). Then, the resultant solution was mixed thoroughly and incubated for 5 min. The FL spectra of the solution were measured at an excitation wavelength of 250 nm using water as a blank. All measurements were taken at room temperature. Fluorescent Detection of Sulfide Ions. In a typical experiment, 20.0 μL of 1.0 mg mL−1 MoS2 solution and appropriate amounts of water were sequentially micropipetted into a 3.5 mL cuvette, followed by 24.0 μL of 1.0 mM Pb2+ solution. Next, the resultant solution was mixed thoroughly and incubated for 5 min. Then, the solution of S2− was added to obtain a series of samples with different concentrations (range, cS2− = 0−20.0 μM; total, 32 samples). After 10 min of incubation, the FL spectra of the solution were collected at an excitation wavelength of 250 nm using water as a blank. All measurements were taken at room temperature.

has been currently tuned in various ways, including electrical gate,17 strain engineering,18 defect engineering,19 substrate engineering,20 and layer-by-layer assembly.21 These modulation routes usually require specialized and expensive systems, or need harsh conditions, or involve cumbersome and complex processes. Hence, exploring a simple, convenient, and costeffective means to enhance the FL of MoS2 nanosheets is strongly desired. The solution-based chemical doping is a wellknown method and commonly used for enhancing FL efficiency. More recently, many researchers have mixed the solution containing fluorescent MoS2 nanosheets into various dopant molecules, and have successfully achieved the QY improvement.22−24 However, to our knowledge, most of the studies focused on the use of organic compounds or DNA as molecular dopants, and few reports emerged concerning the chemical doping of inorganic compounds into the fluorescent MoS2 nanosheets. In the work, we found that MoS2 nanosheets prepared hydrothermally by employing sodium molybdate and thiourea as precursors could emit fluorescence. Then, lead(II) (Pb2+) ions were first introduced as a novel chemical dopant into the nanosheets, and could induce a considerable FL enhancement of the initial MoS2 nanosheets, indicating that lead(II)-doping into the MoS2 nanosheets could result in an increase in the QY. Hence, the observation was naturally exploited to design a label-free FL “turn on” nanosensor for the detection of heavy metal ions, Pb2+. Moreover, we found that further addition of sulfide (S2−) ions could remarkably quench the FL of the doped MoS2 nanosheets. Therefore, the “turn off” process was used to fabricate a FL quenching sensor for detection of the anions, S2−. Compared with other publications on sensing of Pb2+ or S2−,25−28 the advantages of the present work lay in the use of environmentally friendly fluorescent materials, and simultaneous detection of Pb2+ and S2− in one pot. In addition, we further provided a plausible mechanism for the Pb2+induced FL enhancement and S2−-induced FL reduction by using various analytical techniques. This brief protocol is schematically displayed in Scheme 1.

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Scheme 1. Schematic Illustration of Hydrothermal Synthesis of Fluorescent MoS2 Nanosheets and Chemical Tuning of Fluorescence via Pb2+ or S2−

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RESULTS AND DISCUSSION Physicochemical Characterization of Fluorescent MoS2 Nanosheets. The morphology of the prepared MoS2 samples was identified by SEM imaging (Figure 1a). As shown in Figure 1a, the samples have a controllable nano-sheet-like morphology. Figure 1b displays the representative TEM images of the hydrothermally synthesized MoS2 nanosheets. A graphene-analogous structural morphology with many scaled and curled nanosheets was clearly observed. The highmagnification TEM graph, as shown in Figure 1c, indicated that the MoS2 nanosheets were mainly composed of 1−5 layers stacking of the monatonmic sheets, and had an interlayer separation of ca. 0.65 nm, which was attributed to the (002) plane of MoS2.30 AFM analysis was carried out to measure the thickness of the MoS2 nanosheets (Figure 1d,e). The step height was determined to be in the range of 0.5−3.5 nm, which corresponded to 1−5 monolayers. Analysis of the FL behavior of the MoS2 nanosheets was conducted (Figure 2). The FL emission spectrum of the MoS2 nanosheets shown in Figure 2 exhibited a broad band peaked at ca. 406 nm, suggesting the successful synthesis of fluorescent MoS2 nanosheets. However, the emerging FL intensity of the MoS2 nanosheets was somewhat low. The FL QY measurements of the MoS2

EXPERIMENTAL SECTION

Reagents and Chemicals. Sodium molybdate dihydrate (Na2MoO4·2H2O) was purchased from Tianjin Chemical Reagent fourth Factory Kaida Chemical Plant (Tianjin, China). Thiourea was obtained from Tianjin Chemical Reagent fourth Factory Fengchuan Chemical Plant (Tianjin, China). Graphite powder was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Graphene oxide (GO) was prepared by an improved Hummers method, as reported previously.29 All chemicals were used as received without further purification. Deionized distilled water was used throughout. Apparatus. Transmission electron microscopy (TEM) images were recorded using a JEOL JEM-2100 transmission electron microscope. Atomic force microscopy (AFM) images were performed 2536

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results given by other doped MoS2 nanosheets.22−24 The addition of S2− to the doped MoS2 nanosheets then resulted in a drastic reduction of FL intensity (Figure 2). By comparison, it was clearly observed from Figure 2 that the direct addition of S2− to the undoped MoS2 nanosheets almost did not give any changes of FL intensity. Therefore, the FL intensity suppression caused by the addition of S2− to the doped MoS2 nanosheets presumably arose from the complex chemical reaction of S2− with the doped MoS2 nanosheets. Mechanism for Pb2+-Induced Fluorescence Enhancement and S2−-Induced Fluorescence quenching. To validate further our explanations for the above-mentioned FL changes of the MoS2 nanosheets, some characterization techniques like transmission electron microscopy (TEM), Xray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and UV−vis spectroscopy were performed to analyze the changes in the surface of the MoS2 nanosheets under the aforementioned circumstances. Figure S1 represents the TEM micrographs of the fluorescent MoS2 nanosheets with Pb2+ alone and the mixture of Pb2+ and S2−. As seen from Figure S1, after adding Pb2+ or Pb2+ and S2−, some nanoparticles were found on the edges of the nanosheets. This was likely due to adsobates at the sufaces of the nanosheets, which offered hosting materials for nanoparticles growth. The high-resolution XPS spectra of Mo and S core level regions of the fluorescent MoS2 nanosheets are shown in Figure S2. The S 2p XPS spectrum in Figure S2a was deconvoluted into six peaks. The six peaks were divided into three groups, representing the 2p doublets, 2p3/2 and 2p1/2 respectively, of three valence states of S. Taking account of the three 2p doublets, those at 162.36 and 163.62 eV derived from the 2p3/2 and 2p1/2 of MoS2 which had an S 2p3/2 binding energy of 162.20 eV.33 The other two doublets, 163.65 eV (2p3/2) and 164.91 eV (2p1/2), 168.97 eV (2p3/2) and 170.18 eV (2p1/2), were assigned to polysulfides (Sn2−) and sulfate (SO42−).34−36 The Mo 3d XPS spectrum in Figure S2b displayed a peak at 226.65 eV corresponding to the S 2s binding energy of MoS2 and two doublets composed of 3d5/2 and 3d3/2 components.34 Two intense characteristic peaks arising from 229.54 and 232.72 eV were attributed to the Mo 3d5/2 and 3d3/2 binding energies for a Mo(IV) oxidation state of MoS2,34 whereas two very weak Mo 3d5/2 and 3d3/2 components located at 231.66 and 234.94 eV were characteristic of MoO3.34 Concerning that the S 2p spectrum had three forms of sulfur, additional S 2s components with binding energies of 227.69 and 233.99 eV were assigned to Sn2− and SO42−.34,37 On the basis of the aforementioned results, we could find that MoS2, Sn2−, sulfate (SO42−), and MoO3 were present in the sample. The possibility of the presence of SO42− could be further confirmed by using barium chloride in the presence of dilute hydrochloric acid (Figure S3 in the Supporting Information). We suppose that SO42− very likely originated from the oxidation of thiourea involved in hydrothermal synthesis. The quantification of the XPS peaks revealed that the atomic ratio of Mo to S was approximately 1:2.18, which was close to the stoichiometric MoS2. Figure S4 represents the high-resolution XPS patterns of Mo, S, and Pb regions of the fluorescent MoS2 nanosheets with Pb2+. It was noticed from Figure S4a that the Pb 4f XPS spectrum possessed two peaks with binding energies of 139.42 eV (Pb 4f7/2) and 144.28 eV (Pb 4f5/2), which were assigned to PbSO4.38 The S 2p XPS spectrum in Figure S4b was fitted by

Figure 1. Characterization of the MoS2 nanosheets: (a) SEM image, (b) TEM image, (c) high-magnification TEM image, (d) AFM image, and (e) the height profile along the black line overlaid on the AFM image of panel d.

Figure 2. FL spectra of the MoS2 nanosheets alone, the mixture of the MoS2 nanosheets with Pb2+, the mixture of the MoS2 nanosheets with S2−, and the mixture containing the MoS2 nanosheets and Pb2+ after the addition of S2−.

nanosheets were calibrated by quinine sulfate in a H2SO4 aqueous solution (0.1 mol L−1, literature quantum yield is 0.54), and the result showed that the QY at 250 nm excitation was estimated to be approximate 0.28%, which was close to the previous results of single-layer MoS2 (usually less than 0.4%).31,32 FL of a mixture solution of the MoS2 nanosheets and Pb2+ was also detected. An obvious enhanced FL of the mixture solution was clearly depicted in Figure 2. On the basis of the previous findings that molecular doping of MoS2 nanosheets by some organic compounds strengthened their FL,22−24 we suppose that Pb2+ imposed chemical doping in the MoS2 nanosheets. The QY of the doped MoS2 nanosheets was calculated to be ca. 0.73%, which was comparable to those 2537

DOI: 10.1021/acssuschemeng.5b01639 ACS Sustainable Chem. Eng. 2016, 4, 2535−2541

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ACS Sustainable Chemistry & Engineering three 2p3/2−2p1/2 spin−orbit doublets. The high-energy S 2p doublet, 168.50 and 169.75 eV, corresponded to the S 2p3/2 and S 2p1/2 lines of PbSO4.38 The low-energy S 2p doublet, 161.58 and 162.70 eV, was assigned to the S 2p3/2 and S 2p1/2 lines of MoS2.33 The last doublet, 163.07 eV (S 2p3/2) and 164.39 eV (S 2p1/2), was associated with Sn2−.34 The Mo 3d XPS spectrum in Figure S4c showed three S 2s components and two doublets composed of 3d5/2 and 3d3/2 components. The three S 2s peaks at 225.93, 226.70, and 232.99 eV indicated the presence of MoS2, Sn2−, and PbSO4, respectively.34,38 The two doublets, 228.93 eV (3d5/2) and 232.01 eV (3d3/2), 232.60 eV (3d5/2) and 235.50 eV (3d3/2), were considered to originate from MoS2 and MoO3.34 Therefore, the sample contained PbSO4. MoS2, Sn2−, and MoO3. In light of the aforementioned results, it was logical to consider that PbSO4 was formed on the surface of the fluorescent MoS2 nanosheets upon the addition of Pb2+, and then it was used as a molecular dopant into the MoS2 nanosheets to cause an increase in FL intensity. Figure S5 depicts the high-resolution XPS patterns of Mo, S, and Pb regions of the fluorescent MoS2 nanosheets in the presence of Pb2+ and S2−. As observed in Figure S5a, there were a high-energy doublet, 138.33 eV (Pb 4f7/2) and 143.23 eV (Pb 4f5/2), and a low-energy doublet, 137.34 eV (Pb 4f7/2) and 142.24 eV (Pb 4f5/2), in the Pb 4f XPS spectrum. The former was assigned to PbSO4,38 whereas the latter arose from PbS.39 Four doublets, 160.47 eV (2p3/2) and 161.73 eV (2p1/2), 161.50 eV (2p3/2) and 162.76 eV (2p1/2), 163.09 eV (2p3/2) and 164.20 eV (2p1/2), 168.07 eV (2p3/2) and 169.18 eV (2p1/2), were resolved from the S 2p XPS spectrum (Figure S5b). These doublets were respectively related to PbS, MoS2, Sn2−, and PbSO4.34,38,39 After deconvolution of the Mo 3d XPS spectrum (Figure S5c), the binding energies of 228.74 and 231.79 eV corresponded to Mo 3d5/2 and Mo 3d3/2 in MoS2, and the binding energies of 232.07 and 235.17 eV arose from Mo 3d5/2 and Mo 3d3/2 in MoO3.34 The remaining peaks located at 225.27, 226.20, 226.99, and 233.08 eV corresponded to the S 2s lines of PbS, MoS2, Sn2−, and PbSO4, respectively.34,39 Hence, PbS, PbSO4, MoS2, Sn2−, and MoO3, coexisted in the sample. These results suggested conversion of PbSO4 into PbS after the addition of S2−, which led to a diminishment in FL intensity. Figure 3a displays the X-ray diffractogram of the fluorescent MoS2 nanosheets. It was observed that the XRD pattern had a weak diffraction peak at 2θ = 14.4°and two strong peaks at 2θ = 34.2° and 2θ = 57.6°, which are assigned, respectively, to the (002), (l00), and (110) lattice planes of MoS2 (ICDD card no. 77-1716). They were in good agreement with the reported literature.40 The low intensity of the (002) peak indicated that the MoS2 were comprised of a few layers of nanosheets. The widening of these peaks implied the incomplete crystallization of MoS2. Figure 3b depicts the XRD pattern of the fluorescent MoS2 nanosheets after the addition of Pb2+. From Figure 3b, it was found that in addition to the diffraction peaks of MoS2, there were also many intense and sharp peaks. Most of the peaks could be indexed according to the power diffraction card of PbSO4 (ICDD card no. 82-1854). Seven distinct intense peaks at 2θ = 21.0°, 23.5°, 26.8°, 27.7°, 29.8°, 43.9°, 44.7°, which were indexed as 011, 111, 210, 102, 211, 122, 401, respectively, could be clearly detected. All of the information suggested the presence of PbSO4 on the surface of the MoS2 nanosheets. The XRD pattern of the fluorescent MoS2 nanosheets after the addition of Pb2+ and S2− is shown in Figure 3c. Looking at Figure 3c, we found that besides the diffraction peaks of MoS2 and PbSO4, the XRD pattern

Figure 3. XRD diffractograms of (a) the fluorescent MoS2 nanosheets, (b) the fluorescent MoS2 nanosheets with Pb2+, and (c) the fluorescent MoS2 nanosheets with Pb2+ and S2−.

consisted of the peaks of PbS at 2θ = 26.0°, 30.2°, 43.2°, 51.0°, 53.8°, 62.8°, 69.0°, 71.3°, 79.2°, which were indexed as 111, 200, 220, 311, 222, 400, 331, 420, 422, respectively (see ICDD card no. 78-1897). All these illustrated that PbSO4 on the surface of the MoS2 nanosheets was very likely transformed into PbS, which was in line with the XPS observation. Figure 4a shows the corresponding FT-IR spectra. Looking at the FT-IR spectrum of the MoS2 nanosheets, a characteristic

Figure 4. (a) FT-IR spectra and (b) UV−vis absorption spectra of the fluorescent MoS2 nanosheets, the fluorescent MoS2 nanosheets with Pb2+, and the fluorescent MoS2 nanosheets with Pb2+ and S2−.

weak peak around 480 cm−1 was found, which was attributed to the Mo−S vibration. Three intense infrared bands observed at 580, 1110, and 1398 cm−1 were assigned to the sulfate vibrations. After the addition of Pb2+, a vibratonal band at ca. 773 cm−1 appeared in the IR spectra of the MoS2 nanosheets. This was possibly related to the formation of PbSO4. When S2− was added into the MoS2 nanosheets with Pb2+, we noticed the appearance of a new broad band at ca. 1022 cm−1 arising from the Pb−S vibration. Simultaneously, it was found that the band at ca. 773 cm−1 exhibited an attenuated light transmission, which gave an important indication of the conversion of PbSO4 into PbS. Correspondingly, the UV−vis spectra were also recorded for the aforementioned samples (Figure 4b). An absorption band centered at 217 nm was observed for the fluorescent MoS2 nanosheets, which corresponds with the previous reports.15 When Pb2+ were added into the MoS2 nanosheets, the absorption spectra showed a new peak at ca. 350 nm, which was attributed to the formation of PbSO4.41 2538

DOI: 10.1021/acssuschemeng.5b01639 ACS Sustainable Chem. Eng. 2016, 4, 2535−2541

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ACS Sustainable Chemistry & Engineering After S2− were added into the MoS2 nanosheets with Pb2+, there was a reduction of the peak intensity located at 350 nm, and the characteristic peak of the PbS nanocrystals colloid appeared at around 282 nm.42 All these indicated the transformation of PbSO4 to PbS during the process. On the basis of the above-mentioned results, we could further elucidate the origin of the Pb2+-induced enhancement and S2-induced quenching in the fluorescence of the prepared MoS2 nanosheets. The recent literature demonstrated that the fluorescent MoS2 nanosheet was an n-type semiconductor, and its FL modulation (enhancement or quenching) was dominated by charge transfer effect between dopant molecules and the MoS2 nanosheet, i.e., its FL intensity could either be suppressed or increased by changing its electron density.20−22 Generally, the decrease in the total electron concentration of the host MoS2 system lead to its FL enhancement, or vice versa. In the work, the positively charged Pb2+ bound to the negatively charged SO42− on the surface of the MoS2 nanosheet to form PbSO4 nanoparticles as dopants. This observation suggested that the electron concentration of the MoS2 nanosheet decreased, which resulted in its FL enhancement. Further S2− addition to the doped MoS2 nanosheet promoted the transformation of PbSO4 into PbS because of the small solubility product constant of PbS. Compared with PbSO4, PbS injected a number of the electrons more easily into the MoS2 nanosheet, resulting in a greater increase of electron concentration. The increase of electron concentration in the MoS2 nanosheet could be reflected in its FL quenching. Fabrication of Fluorescence Sensor for Detection of Pb2+. The FL spectra of the fluorescent MoS2 nanosheets with the successive addition of Pb2+ are shown in Figure 5a. Noticeably, the FL intensity gradually increased with increasing Pb2+ concentration. The FL intensity was found to be linear with the concentration of Pb2+ over the range of 0.5−12.0 μM, and the detection limit of 0.22 μM was obtained on the basis of a signal-to-noise (S/N) ratio of 3. The calibration curve for

Pb2+ sensing is depicted in Figure 5b, and the corresponding linear calibration equation was F = 53.9 cPb2+ (μM) + 241 with the correlation coefficient of 0.9991. To assess the selectivity of the sensor, the fluorescence response to the other 17 common metal ions at a concentration of 5.0 μM was further challenged (Figure 5c,d). It was observed that the FL enhancement of the sensor to Pb2+ was more pronounced than those to the other metal ions, suggesting the high selectivity of this MoS2-based sensing platform for Pb2+. In addition, under the same synthetic conditions as the fluorescent MoS2 nanosheets, we prepared the reduced graphene oxide (most common two-dimensional nanomaterials) by using graphene oxide and thiourea as precursors. It was observed from Figure S6 (see the Supporting Information) that the FL spectra of the reduced graphene oxide (RGO) nearly underwent no change after the addition of Pb2+. This observation implied that the high selectivity of the sensor very likely originated from the intrinsic nature of the fluorescent MoS2 nanosheets, not from the two-dimensional structure. As previously mentioned, the Pb2+-induced FL enhancement arouse from the formation of PbSO4. Therefore, the high selectivity toward Pb2+ was considered to originate from the low solubility product constant of PbSO4 to a greater degree. Ba2+ did not induce the noticeable increase in FL intensity, possibly because its atomic radius was greater than that of Pb2+, leading to its lower positive charge density. Fabrication of Fluorescence Sensor for Detection of S2−. Figure 6a depicts that the continuous addition of S2− into

Figure 6. (a) Fluorescence spectra of the mixture of the fluorescent MoS2 nanosheets and Pb2+ (5.0 μM) with the addition of S2− at different concentrations (0, 0.5, 1.0, ..., 11.0, 12.0, ..., 20.0 μM). (b) Calibration curve for S2− sensing. (c) Fluorescence spectra of the mixture of the fluorescent MoS2 nanosheets with Pb2+ (12.0 μM) in the presence of each individual anion. The concentration of each anion is 10.0 μM. (d) Selectivity of the fluorescent MoS2 nano-sheets-based sensor for S2−. The concentration of each anion is 10.0 μM. F′ and F0′ represent the fluorescence intensity of the mixture of the fluorescent MoS2 nanosheets with Pb2+ in the presence or absence of each anion at the maximum emission wavelength of 406 nm, respectively. Figure 5. (a) FL spectra of the fluorescent MoS2 nanosheets with different Pb2+ concentrations (0, 0.5, 1.0, ..., 15.0 μM). (b) Calibration curve for Pb2+ sensing. (c) FL spectra of the fluorescent MoS2 nanosheets with each individual metal ion. (d) Selectivity of the fluorescent MoS2 nano-sheets-based sensor for Pb2+. F and F0 denote the fluorescence intensity of the fluorescent MoS2 nanosheets with or without each metal ion at the maximum emission wavelength of 406 nm, respectively.

the mixture solution of the MoS2 nanosheets and Pb2+ resulted in a gradual decrease of FL intensity. A good linear correlation (r = 0.9983) was obtained in the range from 0.5 to 12.0 μM, and the detection limit of 0.42 μM was estimated on the basis of S/N = 3. The calibration curve for S2− sensing is given in Figure 6b, and the related quenched fluorescence intensity 2539

DOI: 10.1021/acssuschemeng.5b01639 ACS Sustainable Chem. Eng. 2016, 4, 2535−2541

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ACS Sustainable Chemistry & Engineering followed the linear calibration equation of F = −27.1 cS2− (μM) + 894. The selectivity of the sensor for S2− over other twentyone common anions was also investigated (Figure 6c,d). It was seen that all of the tested anions at a concentration of 10.0 μM could not elicit a remarkable FL diminishment similar to S2−, which showed that the sensor for S2− had an excellent selectivity relative to a wide range of anions. Meanwhile, it was found from Figure S6 (see the Supporting Information) that the addition of S2− into the mixture of RGO and Pb2+ had a negligible effect on its FL spectral change. On the basis of the result, we suppose that the high selectivity of the sensor possibly arose from the intrinsic nature of the fluorescent MoS2 nanosheets. Similarly, as previously mentioned, the S2−-induced FL quenching arouse from the formation of PbS. Hence, it is plausible that the selectivity toward S2− arouse from the lower solubility of PbS (i.e., the stronger binding affinity of S2− toward Pb2+) and the stronger reducing property. Repeatability, Reproducibility, and Stability of the Fluorescence Sensor. The repeatability, reproducibility, and stability of the developed fluorescence sensor for detection of Pb2+ or S2−, which are also critically important in the practical application, were studied, respectively. For repeatability and reproducibility, each sample containing Pb2+ (12.0 μM) or S2− (10.0 μM) was respectively analyzed in triplicate within 1 day by the same analyst and three different analysts. The results for detection of Pb2+ or S2−, which are expressed as relative standard deviation (RSD%), were found to be below 7.2%, suggesting good repeatability and acceptable reproducibility of our proposed sensing approach. For stability, each sample containing Pb2+ (12.0 μM) or S2− (10.0 μM) was analyzed intermittently for a period of 30 days. The results showed that the RSD% was less than 9.3%, denoting that the fluorescence sensor could maintain good stability for detection of Pb2+ or S2−. Application in Real Samples. To evaluate further the feasibility of the sensor for determination of Pb2+ and S2−, we used the sensor to detect of Pb2+ and S2− in water samples. The water samples were obtained from Longten Lake (Nanchang, China). The samples obtained were filtered via a 0.22 μm membrane, and then centrifuged for 30 min at 12,000 rpm. It was found that the FL sensor detected no Pb2+ and S2−, and thus the water samples were spiked with Pb2+ and S2− at different concentrations to test the feasibility. The results are listed in Tables S1 and S2. The results showed that the mean recoveries of Pb2+ and S2− reached to 96−105% and 95−110% with the relative standard deviation (RSD) of 1.3−9.0% and 7.0−9.7%, respectively, indicating that analytical performance of the sensor for the detection Pb2+ and S2− in water samples was satisfactory.

and the chemical tuning of FL via Pb2+ or S2− in the fluorescent MoS2 nanosheets could be further expanded to the area of highperformance optical modulators, solar cells, and biological applications. Moreover, the work may open a door for the further development of new and facile synthetic approaches for the preparation of various fluorescent TMDs with high quantum yields, and the construction of versatile and sustainable sensing platforms for ion detection. Much work toward this direction is under intense study.

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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/acssuschemeng.5b01639. TEM images of the fluorescent MoS2 nanosheets with Pb2+ alone and the mixture of Pb2+ and S2−; XPS spectra for the fluorescent MoS 2 nanosheets alone, the fluorescent MoS2 nanosheets with Pb2+, and the fluorescent MoS 2 nanosheets with Pb 2+ and S 2 ; identification for sulfate ions in a typical hydrothermal synthesis process of the fluorescent MoS2 nanosheets; fluorescence spectra of RGO, RGO with Pb2+, and RGO with Pb2+ and S2−, and determination of Pb2+ and S2− spiked in water samples (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*Y. Wang. Telephone: +86 791 83969500. Fax: +86 791 83969500. E-mail: [email protected]. *Y. Ni. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the National Natural Science Foundation of China (NSFC-21305061), the Natural Science Foundation of Jiangxi Province (20151BAB213014 and 20151BAB203021), 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-201612).

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REFERENCES

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CONCLUSION In brief, we found that the MoS2 nanosheets prepared by a simple hydrothermal route could emit fluorescence. And we observed that Pb2+ enhanced the FL of the fluorescent MoS2 nanosheets, and the subsequent addition of S2− quenched their FL. Various characterization techniques were used to demonstrate that the Pb2+-induced FL enhancement was attributed to the formation of PbSO4 at the surface of the fluorescent MoS2 nanosheets, and the S2−-induced FL quenching was due to the transformation of PbSO4 at the MoS2 surface into PbS. More importantly, the observations were explored for sensing of Pb2+ and S2−, respectively. It is anticipated that the as-prepared fluorescent MoS2 nanosheets 2540

DOI: 10.1021/acssuschemeng.5b01639 ACS Sustainable Chem. Eng. 2016, 4, 2535−2541

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