One-Step Hydrothermal Synthesis of Highly Fluorescent MoS2 Quantum Dots for Lead Ion Detection in Aqueous Solutions

Lead ions in water are harmful to human health and ecosystems because of their high toxicity and nondegradability. It is important to explore effective fluorescence probes for Pb2+ detection. In this work, surface-functionalized molybdenum disulfide quantum dots (MoS2 QDs) were prepared using a hydrothermal method, and ammonium tetrathiomolybdate and glutathione were used as precursors. The photoluminescence quantum yield of MoS2 QDs can be improved to 20.4%, which is higher than that for MoS2 QDs reported in current research. The as-prepared MoS2 QDs demonstrate high selectivity and sensitivity for Pb2+ ions, and the limit of detection is 0.056 μM. The photoluminescence decay dynamics for MoS2 QDs in the presence of Pb2+ ions in different concentrations indicate that the fluorescence quenching originated from nonradiative electron transfer from excited MoS2 QDs to the Pb2+ ion. The prepared MoS2 QDs have great prospect and are expected to become a good method for lead ion detection.


Introduction
Heavy metal contamination in the environment has become an urgent problem to solve because of its threat to human health and ecosystems [1,2]. In industry and agricultural fields, heavy metal ions (Pb 2+ , Hg 2+ , Cd 2+ , etc.) are widely used. If not handled properly, these ions can leak into water circulation systems and further contaminate our drinking water and food, ultimately enriching in the human body and changing protein structure to cause a series of diseases. Lead pollutants are considered to be one of the most dangerous contaminants, which exhibit high toxicity and nondegradability [3,4]. So far, atomic absorption spectrometry, electrochemical technique, inductively coupled plasma mass spectrometry, among others, have been developed to detect heavy metal ions [5][6][7]. However, their application is often limited due to complicated sample pretreatment, long analysis time, and expensive equipment. To overcome these shortcomings, an approach of optical detection based on fluorescence analysis has emerged for its high sensitivity and selectivity. Hence, to detect Pb 2+ ions, it is urgent to develop an ecofriendly fluorescent material with high sensitivity and selectivity.
Owing to quantum confinement effect and edge effect, semiconductor quantum dots (QDs) possess unique photophysical properties and can be used as a fluorescence probe for ion detection. Fluorescence probes for Pb 2+ based on QDs have gained much interest. Mn-doped ZnS QDs, ZnSeS/Cu:ZnS/ZnS core/shell/shell QDs, and carbon dots have been developed for Pb 2+ detection [8][9][10]. However, the inorganic quantum dots are imperfect for their toxicity and multistep synthetic approach. Although carbon dots have created great focus for their facile synthesis and low toxicity, low quantum yield limits their application. In recent years, two-dimensional transition metal dichalcogenides such as MoS 2 and WSe 2 , and especially their quantum dots, have received much attention in sensing applications and optoelectronic devices [11][12][13][14][15][16][17]. MoS 2 QDs with large surface-to-volume ratio and abundant active edge sites can be used as a photoluminescence-sensing platform [18][19][20][21]. Different from traditional quantum dots (CdSe, CdTe, etc.) with harmful elements, MoS 2 QDs are water-soluble and nontoxic. Hence, MoS 2 QDs have received extensive attention in bioimaging, sensing and photodynamic therapy [22,23]. MoS 2 QDs have been used as a sensor to detect nitro explosives, hyaluronidase and hydrogen peroxide, along with glucose and other biomolecules [18][19][20][24][25][26]. Additionally, there are some reports on the metal ion sensor using fluorescent MoS 2 QDs. Cysteine-functionalized MoS 2 QDs have been used for sensing Al 3+ and Fe 3+ metal ions [27]. Based on the fluorescence turn-off effect, MoS 2 QDs can be used as a sensor for Fe 3+ detection [28]. However, the fluorescence quantum yield of these MoS 2 QDs reported by previous research is less than 10%, and the interaction of metal ions with MoS 2 QDs is not well studied. To obtain a high fluorescence MoS 2 QDs, a widely used bottom-up method of hydrothermal synthesis has been improved with different molybdenum and sulfur sources, which is simple, ecofriendly and easy to operate. In the hydrothermal method, MoS 2 QDs were obtained by the Xian group; they successfully adopted ammonium tetrathiomolybdate [(NH 4 ) 2 MoS 4 ] and hydrazine hydrate as precursor and reducing agent, respectively [24]. Zhang et al. have also synthesized MoS 2 QDs using this method, with sodium molybdate (Na 2 MoO 4 ·2H 2 O) and glutathione (GSH) serving as molybdenum and sulfur sources [27]. Although the photoluminescence quantum yield of MoS 2 QDs can be increased to 6% in this method, it is challenging to produce large quantities of bright MoS 2 QDs by using more appropriate Mo and S sources.
Herein, we explore high fluorescent MoS 2 QDs in a one-step hydrothermal method using (NH 4 ) 2 MoS 4 and GSH as Mo and S sources. On the one hand, GSH was used as a reductant to reduce (NH 4 ) 2 MoS 4 ; on the other hand, GSH, as a passivation agent, could eliminate the surface defects of MoS 2 QDs to enhance the fluorescence. If we introduced the Pb 2+ ions into the resultant MoS 2 QDs aqueous solution, quenched fluorescence was observed. We further acquired the linear dependence of fluorescence intensity on Pb 2+ ion concentration in a certain range. The limit of detection (LOD) is 0.056 µM, which is below the acceptable limit given by United States Environmental Protection Agency. We further study the photoluminescence decay dynamics of MoS 2 QDs with increasing the concentration of Pb 2+ ion. With a detailed exciton dynamics study, we found that the fluorescence quenching originated from the electron transfer from MoS 2 QDs to Pb 2+ ions. As a result, MoS 2 QDs prepared in this work can be used as a sensing probe to detect Pb 2+ ions in water with high sensitivity and selectivity. These results are important to understanding the sensing mechanism of MoS 2 QDs to metal ions.

Preparation of MoS 2 QDs
(NH 4 ) 2 MoS 4 and GSH were chosen as Mo and S sources to synthesis MoS 2 QDs through a pot hydrothermal process. Briefly, (NH 4 ) 2 MoS 4 (65 mg) was dispersed in ultrapure water (25 mL), sonicated for 10 min until the (NH 4 ) 2 MoS 4 was fully dissolved. Afterward, 0.1 M HCl was used to adjust the pH of solution to 6.5. In addition, GSH (308 mg) was dissolved in ultrapure water (50 mL), and subsequently added into the (NH 4 ) 2 MoS 4 aqueous solution prepared in the previous step. The mixture was further sonicated for 10 min and heated at 200 • C in a Teflon-lined autoclave (100 mL) for 24 h. The solution was then naturally cooled to room temperature. The mixture was centrifuged for 10 min at 10,000 rpm to collect the suspension, subsequently purified by a 0.22 µm microporous membrane. The target MoS 2 QDs aqueous solution was stored at 4 • C for further characterization.

Characterization of MoS 2 QDs
Commercial experimental instruments including a UV-Vis-NIR spectrophotometer (Cary 5000, Agilent, Santa Clara, CA, USA) and fluorescence spectrometer (PerkinElmer LS 55, Waltham, MA, USA) were used to measure the absorption and photoluminescence spectra, respectively. Time-resolved photoluminescence (TRPL) measurements were conducted with a home-built fluorescence lifetime setup (Picoharp300, Picoquant, Berlin, Germany) based on a time-correlated single photon counting (TCSPC) module. A pulsed excitation source (350 nm with repetition frequency of 13 MHz) was used to excite the samples.
The particle diameters and monodispersity of MoS 2 QDs were characterized by high resolution transmission electron microscopy (HRTEM) (JEM-2010, JEOL, Tokyo, Japan). The aqueous solution of appropriate concentration was dripped onto a carbon-coated copper grid, and the aqueous solvent was dried by nitrogen flow at ambient temperature.
The monolayer nature of MoS 2 QDs was checked by atomic force microscopy (AFM, Solver-P47H, NT-MDT, Moscow, Russia). The sample for AFM measurement was acquired by spin coating diluted MoS 2 aqueous solution on a mica wafer.
Elemental analysis was conducted using X-ray photoelectron spectroscopy (XPS) (PHI 5000 VersaProbeIII, Japan). The binding energy calibration was performed using C 1s of 284.6 eV as standard peak energy.
The surface functional groups of MoS 2 QDs were identified by Fourier transform infrared spectroscopy (FTIR) spectra (Perkin-Elmer spectrometer, Spectrum One B, Waltham, MA, USA), and the MoS 2 QDs powder was pressed into a tablet with KBr.

Results
The MoS 2 QDs were first characterized by TEM. The MoS 2 QDs are monodispersed and homogeneous with an average diameter of 4.4 ± 0.2 nm (Figure 1a,b). The inset in Figure 1a is the HRTEM image of a single MoS 2 QD with ordered lattice fringes. The lattice spacing is~2.3 Å, which can be attributed to the (103) plane of crystalline MoS 2 . AFM was further used to check the thickness of the MoS 2 QDs. Figure 1c also shows the monodispersity and size uniformity of the MoS 2 QDs. Additionally, the profile shown in Figure 1d indicates that the height of MoS 2 QDs is~0.7 nm, demonstrating a monolayer nature [29]. The optical properties of absorption and photoluminescence spectra are presented in Figure 1e,f. A typical excitonic peak of MoS 2 QDs at 310 nm with a remarkable shoulder is shown in Figure 1e [22]. The fluorescence emission peak is 430 nm under 350 nm excitation ( Figure 1f). The photoluminescence quantum yield of MoS 2 QDs was measured choosing quinine sulfate (54%, 350 nm excitation) as a standard sample. According to the data in Figure 1e,f, the photoluminescence quantum yield of MoS 2 QDs is calculated at 20.4%, which is relatively higher than those reported previously for MoS 2 QDs fabricated with a similar hydrothermal method [24,27]. In this study, the GSH was used as a reductant and passivation agent, which can provide enough surface functional groups to eliminate the edge defects, resulting in a high fluorescence emission.
According to the data in Figure 1e,f, the photoluminescence quantum yield of MoS is calculated at 20.4%, which is relatively higher than those reported previously for QDs fabricated with a similar hydrothermal method [24,27]. In this study, the GS used as a reductant and passivation agent, which can provide enough surface func groups to eliminate the edge defects, resulting in a high fluorescence emission. The high-resolution XPS was conducted for further elementary analysis. Th response in the Mo 3d, S 2p is shown in Figure 2a  The high-resolution XPS was conducted for further elementary analysis. The XPS response in the Mo 3d, S 2p is shown in Figure 2a,b, respectively. Figure 2a exhibits two peaks at 232.8 and 229.5 eV, which correspond to Mo 4+ 3d 3/2 and Mo 4+ 3d 5/2 . Additionally, the two characteristic peaks of S 2p 1/2 and S 2p 3/2 located at 163.4 and 162.3 eV indicate a 2H phase for the crystal structure of MoS 2 QDs [30,31]. Moreover, the atomic ratio of Mo/S is about 1:2, indicating the formation of MoS 2 QDs. The formula of GSH is drawn in Figure 2c. During synthesis of MoS 2 QDs, GSH was not only used as the reducing agent, but also as surface passivation agent to provide carboxyl, amino, and thiol groups for MoS 2 QDs. The surface functional groups of MoS 2 QDs were further verified by FTIR (Figure 2d   The high fluorescence and environment-friendly characteristics of surface-functionalized MoS 2 QDs make them a potential candidate for sensing metal ions. To explore the sensing capability of these MoS 2 QDs, selectivity toward various metal ions was performed. Various metal ions with the same concentration of 25 µM were added into the MoS 2 QDs aqueous solution to study the influence on respective MoS 2 QDs fluorescence. The (F 0 − F)/F 0 value was used to determine the fluorescence enhancement or quenching of MoS 2 QDs, where the fluorescence intensities of MoS 2 QDs without/with metal ions were represented by F 0 and F, respectively. Figure 3a indicates that only Pb 2+ ions caused obvious fluorescence quenching, while the other metal ions have a slight impact on the fluorescence intensity. These results demonstrate that the surface-functionalized MoS 2 QDs have high selectivity for Pb 2+ ion. The high fluorescence and environment-friendly characteristics of surface-functionalized MoS2 QDs make them a potential candidate for sensing metal ions. To explore the sensing capability of these MoS2 QDs, selectivity toward various metal ions was performed. Various metal ions with the same concentration of 25 μM were added into the MoS2 QDs aqueous solution to study the influence on respective MoS2 QDs fluorescence. The (F0 − F)/F0 value was used to determine the fluorescence enhancement or quenching of MoS2 QDs, where the fluorescence intensities of MoS2 QDs without/with metal ions were represented by F0 and F, respectively. Figure 3a indicates that only Pb 2+ ions caused obvious fluorescence quenching, while the other metal ions have a slight impact on the fluorescence intensity. These results demonstrate that the surface-functionalized MoS2 QDs have high selectivity for Pb 2+ ion.   To evaluate the stability of MoS 2 QDs under different conditions, the effect of pH on the fluorescence of MoS 2 QDs was also studied by adjusting the acidity to alkalinity. As shown in Figure 3b, the fluorescence intensity of MoS 2 QDs shows a slight variation with increasing the pH in a wide range from 3.7 to 10.4, indicating negligible influence of pH on the fluorescence of MoS 2 QDs. When the sample was kept for 50 days, the fluorescence intensity of MoS 2 QDs was almost unchanged while the position of maximum emission peak was red shifted slightly (Figure 3c). These observations suggest that MoS 2 QDs can be used as a stable fluorescence probe in complex underwater environments.
The sensitivity for Pb 2+ ion detection was carried out by fluorescence titration of MoS 2 QDs with varying concentration of Pb 2+ ion from 0 to 120 µM. By increasing the concentration of Pb 2+ ions from 0 to 120 µM, the fluorescence intensity of the MoS 2 QDs decreases gradually (Figure 4a). Additionally, we plotted the degree of quenching (F 0 − F)/F 0 versus Pb 2+ ion concentration in the range 0 to 60 µM (Figure 4b). We obtained a good linear relationship between (F 0 − F)/F 0 with Pb 2+ ion concentration (R 2 = 0.984), and the LOD was measured at 0.056 µM (3σ per slope, where σ is the standard deviation of blank signals, N = 10), which is less than the 15 µg/L safety value set by United States Environmental Protection Agency [32]. Moreover, compared with carbon dots and other inorganic probes used for the detection of Pb 2+ by using fluorescence method (Table 1), the LOD obtained using MoS 2 QDs as the sensor was slightly larger while the linear detection range was wider. However, compared with 1,4-diaminobutane (DAB) capped MoS 2 QDs for monitoring the Pb 2+ ions [21], the GSH-functionalized MoS 2 QDs in this work possess higher quantum yield and lower LOD, which is suitable to use as a fluorescent probe for lead ions. These results suggest that the surface-functionalized MoS 2 QDs can be used as a sensor for Pb 2+ ion detection with high selectivity and sensitivity. To evaluate the stability of MoS2 QDs under different conditions, the effect of pH on the fluorescence of MoS2 QDs was also studied by adjusting the acidity to alkalinity. As shown in Figure 3b, the fluorescence intensity of MoS2 QDs shows a slight variation with increasing the pH in a wide range from 3.7 to 10.4, indicating negligible influence of pH on the fluorescence of MoS2 QDs. When the sample was kept for 50 days, the fluorescence intensity of MoS2 QDs was almost unchanged while the position of maximum emission peak was red shifted slightly (Figure 3c). These observations suggest that MoS2 QDs can be used as a stable fluorescence probe in complex underwater environments.
The sensitivity for Pb 2+ ion detection was carried out by fluorescence titration of MoS2 QDs with varying concentration of Pb 2+ ion from 0 to 120 μM. By increasing the concentration of Pb 2+ ions from 0 to 120 μM, the fluorescence intensity of the MoS2 QDs decreases gradually (Figure 4a). Additionally, we plotted the degree of quenching (F0 − F)/F0 versus Pb 2+ ion concentration in the range 0 to 60 μM (Figure 4b). We obtained a good linear relationship between (F0 − F)/F0 with Pb 2+ ion concentration (R 2 = 0.984), and the LOD was measured at 0.056 μM (3σ per slope, where σ is the standard deviation of blank signals, N = 10), which is less than the 15 μg/L safety value set by United States Environmental Protection Agency [32]. Moreover, compared with carbon dots and other inorganic probes used for the detection of Pb 2+ by using fluorescence method (Table 1), the LOD obtained using MoS2 QDs as the sensor was slightly larger while the linear detection range was wider. However, compared with 1,4-diaminobutane (DAB) capped MoS2 QDs for monitoring the Pb 2+ ions [21], the GSH-functionalized MoS2 QDs in this work possess higher quantum yield and lower LOD, which is suitable to use as a fluorescent probe for lead ions. These results suggest that the surface-functionalized MoS2 QDs can be used as a sensor for Pb 2+ ion detection with high selectivity and sensitivity.  To further study the fluorescence quenching mechanism of MoS2 QDs for Pb 2+ ion detection, we measured the photoluminescence decay dynamics of MoS2 QDs in the  To further study the fluorescence quenching mechanism of MoS 2 QDs for Pb 2+ ion detection, we measured the photoluminescence decay dynamics of MoS 2 QDs in the presence of Pb 2+ ion (0, 5, 10, 15, 20 µM). The photoluminescence lifetime of MoS 2 QDs decreases gradually with increasing concentration of Pb 2+ ion (Figure 5a), indicating that Pb 2+ ion plays an important role in the exciton recombination of MoS 2 QDs. The photoluminescence decay curves in Figure 5a can be fitted with a bi-exponential model; the fitting parameters are shown in Table 2. There are two decay processes (short lifetime τ 1 and long lifetime τ 2 ) for the exciton deactivation. The fast component τ 1 shows a mild variation while slow component τ 2 becomes shorter with increasing concentration of Pb 2+ ions. These two lifetime components could be assigned to band-edge emission and surface-state-assisted emission, respectively [21,36]. The shorter of slow component τ 2 from surface-state-assisted emission may be ascribed to the observation that Pb 2+ ion can chelate with surface functional groups of MoS 2 QDs [33], resulting in electron transfer from the excited-state MoS 2 QDs to the Pb 2+ ion. The electron transfer rate κ ET can be calculated by the following equation [37]: where τ av and τ 0 are the average lifetime of MoS 2 QDs with and without Pb 2+ ions, respectively. When the Pb 2+ ion concentration is 5 µM, the κ ET is 3.26 × 10 7 s −1 . We further observed the linear dependence of electron transfer rate κ ET on the Pb 2+ ion concentration ( Figure 5b). This evolution of κ ET further indicates that the fluorescence quenching originates from nonradiative electron transfer from the excited MoS 2 QDs to the Pb 2+ ions. presence of Pb 2+ ion (0, 5, 10, 15, 20 μM). The photoluminescence lifetime of MoS2 QDs decreases gradually with increasing concentration of Pb 2+ ion (Figure 5a), indicating that Pb 2+ ion plays an important role in the exciton recombination of MoS2 QDs. The photoluminescence decay curves in Figure 5a can be fitted with a bi-exponential model; the fitting parameters are shown in Table 2. There are two decay processes (short lifetime τ1 and long lifetime τ2) for the exciton deactivation. The fast component τ1 shows a mild variation while slow component τ2 becomes shorter with increasing concentration of Pb 2+ ions. These two lifetime components could be assigned to band-edge emission and surface-state-assisted emission, respectively [21,36]. The shorter of slow component τ2 from surface-state-assisted emission may be ascribed to the observation that Pb 2+ ion can chelate with surface functional groups of MoS2 QDs [33], resulting in electron transfer from the excited-state MoS2 QDs to the Pb 2+ ion. The electron transfer rate κET can be calculated by the following equation [37]: where τav and τ0 are the average lifetime of MoS2 QDs with and without Pb 2+ ions, respectively. When the Pb 2+ ion concentration is 5 μM, the κET is 3.26 × 10 7 s −1 . We further observed the linear dependence of electron transfer rate κET on the Pb 2+ ion concentration ( Figure 5b). This evolution of κET further indicates that the fluorescence quenching originates from nonradiative electron transfer from the excited MoS2 QDs to the Pb 2+ ions.

Conclusions
We prepared high fluorescence MoS2 QDs by using an easily manipulated hydrothermal method, in which (NH4)2MoS4 and GSH were used as Mo and S sources. In this method, GSH was used as a reductant and capping agent to obtain high fluorescent MoS2 QDs. The morphological and structural characterization from TEM and AFM demonstrated that the ultrasmall MoS2 QDs (~4.4 nm) were monodispersed and single-layered.  Table 2. Bi-exponential fitting for photoluminescence decay curves of the MoS 2 QDs. Fitting equation: I(t) = I 0 + A 1 exp(−t/τ 1 ) + A 2 exp(−t/τ 2 ), and the average lifetime τ av = (A 1 τ 1 2 + A 2 τ 2 2 )/(A 1 τ 1 + A 2 τ 2 ), where A 1 , A 2 and τ 1 , τ 2 are the amplitudes and lifetimes, respectively.

Conclusions
We prepared high fluorescence MoS 2 QDs by using an easily manipulated hydrothermal method, in which (NH 4 ) 2 MoS 4 and GSH were used as Mo and S sources. In this method, GSH was used as a reductant and capping agent to obtain high fluorescent MoS 2 QDs. The morphological and structural characterization from TEM and AFM demonstrated that the ultrasmall MoS 2 QDs (~4.4 nm) were monodispersed and single-layered. FTIR analysis further manifested that the functional groups provided by GSH can passivate the surface of MoS 2 QDs. The photoluminescence of MoS 2 QDs was quenched in the presence of Pb 2+ ions. Based on the quenching effect, the surface-functionalized MoS 2 QDs were used as a fluorescence probe with high sensitivity and selectivity to detect Pb 2+ ions. From the fluorescence titration of MoS 2 QDs, we determined the sensitivity of this sensor at the detection limit of 0.056 µM. Moreover, the photoluminescence decay dynamics of MoS 2 QDs help us to understand the electron transfer behavior between MoS 2 QDs and Pb 2+ ions. A linear relationship between electron transfer rate κ ET and Pb 2+ ion concentration was observed. Our findings indicate that this water-soluble nanomaterial holds great promise and would be a good sensor for the detection of lead ions.