In this work, all the chemicals were purchased from commercial institutions and applied directly without additional purification. Aqueous solutions of Fe(III), Ni(II), Cl(I), Na(I), Zn(II), Ba(II), Al(III), and Ca(II) were prepared from their compound solutions. Deionized distilled water was used throughout experiment. Fluorescence spectra were characterized using a luminescence spectrometer (Fluorolog-3) with a standard 10 mm path length quartz cuvette. UV−vis spectra of MoS₂ QDs were characterized by a UV-3101 scanning spectrophotometer (Shimadzu, Japan) using a standard 10 mm path length quartz cuvette at room temperature. Transmission electron microscopy (TEM) image was obtained from a JEM-1400 operating at 120 kV. The x-ray photoelectron spectroscopy (XPS) measurement was carried on PHI Quantera (PHI, Japan), the binding energy was calibrated with C 1s = 284.8 eV.

MoS₂ QDs were produced using a hydrothermal synthesis method. Briefly, 0.3 g sodium molybdate and 0.6 g L-cysteine were added to 50 mL deionized water in a 100 mL beaker and sonicated in an ultrasonic cell disruptor (400 W) for 20 min. Then, the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave and reacted at 180 °C for 30 h. After the solution cooled naturally, the supernatant containing MoS₂ QDs was transferred to another test tube after being centrifuged for 10 min at the speed of 8000 rpm. After that, the supernatant was filtered with 220 nm diameter filter. Finally, the as-prepared MoS₂ QDs solution was preserved in the matte, low temperature sealed environment.

The cooper ions solution was prepared by dissolving 0.1705 g in 1000 mL deionized water at a 1000-mL beaker. After that, the solution was further diluted to , , , , , , , , , , , , and , and then, different concentrations of copper ions were introduced to the MoS₂ QDs solution. Lastly, the fluorescent intensity of the mixture was obtained by the fluorescence spectrometer.

Figure 1(a) is the TEM image of MoS₂ QDs. A typical particle diameter distribution of the MoS₂ QDs (Fig. 1(b)) shows highly uniform and monodisperse nanocrystals about 2.25 nm diameter (the size is statistically calculated from more than 100 QDs in the TEM images). The x-ray photoelectron spectroscopy (XPS) measurements were carried out to survey the surface element composition and valence states of the MoS₂ QDs. As shown in Fig. 1(c), two characteristic peaks at 229.3 eV and 230.8 eV are the Mo 3d_3/2 and Mo 3d_5/2 peaks from Mo⁴⁺ in MoS₂ QDs. Meanwhile the peaks at around 162.2 eV and 163.4 eV are corresponding to S 2p_3/2 and S 2p_1/2. The atomic ratio between Mo and S is close to 1:2, confirming the formation of MoS₂ QDs.

Fig. 1.

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	Fig. 1. (color online) (a) TEM image of MoS2 QDs. (b) Size distribution histogram of MoS2 QDs. (c) XPS Mo 3d state. (d) XPS S 2p state.

The absorption (blue line) and fluorescence emission spectra (red line) of the MoS₂ QDs are presented in Fig. 2(a). The UV–vis spectrum shows a remarkable absorption peak at approximately 280 nm, which is similar to that reported previously,^[42] and the typical band is attributed to the electronic transitions of the MoS₂ QDs. Moreover, when the excitation wavelength is 280 nm, the fluorescence emission spectrum has a conspicuous peak at 415 nm. As observed from the digital photo (the inset of fig. 2(a)), the MoS₂ QDs solution has strong blue photoluminescence on exposure to a 365 nm UV lamp, and appears yellow under natural light. The normalized fluorescence spectra of the prepared MoS₂ QD dispersions were measured at room temperature under various excitation wavelengths from 320 nm to 400 nm. It shows that with the increasing excitation wavelength, the emission peak shifts to red, that is to say, the emission spectrum shows excitation wavelength dependence, which is the same as observed in MoS₂QDs from other synthesis methods.^[13]

Fig. 2.

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Fig. 2. (color online) (a) PL emission (red line) and absorption spectra (blue line) of MoS2 QDs (inset: MoS2 QDs are exposed under natural light and 365 nm UV light, respectively). (b) Normalized PL emission spectra of the MoS2 QDs obtained under excitations of 320400 nm. (c) PL emission spectra of the MoS2 QDs with different hydro-thermal synthesis time under 360 nm excitation. (d) Absorption spectra of the MoS2 QDs with different hydro-thermal synthesis time under 360 nm excitation.

In order to study the optimal reaction conditions, hydrothermal reactions with different time were carried out prepare MoS₂ QDs. As shown in Figs. 2(c) and 2(d), the MoS₂ QDs’ absorption and fluorescence emission spectra were acquired under 360 nm excition. Then, the fluorescence quantum efficiency can be measured according to

As shown in Fig. 3, MoS₂ QDs were prepared via a facile hydrothermal process using sodium molybdate and cysteine as molybdenum and sulfur sources, respectively. The prepared MoS₂ QDs have photoluminescence of blue light under 360 nm excitation. With the introduction of copper ions to the MoS₂ QDs solution, the fluorescence intensity is decreasing. It may attribute to the transfer of excited electrons from MoS₂ QDs to copper ions.^[18]

Fig. 3.

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	Fig. 3. (color online) Illustration of the preparation scheme of MoS2 QDs and the detection of copper ions.

In order to develop applications of MoS₂ QDs in the field of sensor, we studied the detection of copper ions based on the on-off switching process. As described in Fig. 4(a), the emission intensity of the MoS₂ QDs significantly reduces in the presence of different concentrations of copper ions. The relationship between the copper ions concentration (from to ) and (the degree of quenching) is plotted in Fig. 4(b). The inset image (Fig. 4(b)) indicates that there is a linear relationship between the copper ions concentration and in a range from 0 to and the correlation coefficient is 0.986. The detection limit is , which is much lower than the maximum level ( ) set by the US Environmental Protection Agency (EPA).^[9]

Fig. 4.

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	Fig. 4. (color online) (a) Emission spectra of MoS2 QDs in the presence of Cu2+ with different concentrations in a range from to . (b) The plot of MoS2 QDs with different concentrations of Cu2+. (c) PL emission spectra of MoS2 QDs with Cu2+ under different PH values. (d) The relationship between the PH value and the fluorescence intensity.

In order to investigate the effect of pH on this process, the experiment was carried out by adjusting the pH value of the reagent. As shown in Fig. 4(c), the fluorescence intensity of mixture solutions (MoS₂ QDs with copper ions) is not noticeable when the pH value increases from 2 to 11. Meanwhile, the MoS₂ QDs with no copper ions still have no obvious change (Fig. 4(d)). It suggests that the fluorescence intensity of the MoS₂ QDs and the mixture solution is insensitive to the pH, making the MoS₂ QDs a stable sensing platform in more complicated environment.

In order to assess the selectivity of the fluorescent probe established in this study, we investigated the influences of possible foreign substances (Cu²⁺, Fe³⁺, Ni²⁺, Cl, Na⁺, Zn²⁺, Ba²⁺, Al³⁺, Ca²⁺). As shown in Fig. 5, under the same conditions, the interfering ions show much less quenching effect on the fluorescence of the quantum dots, indicating high sensing selectivity of MoS₂ QDs towards Cu²⁺.

Fig. 5.

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	Fig. 5. (color online) Quenching efficiency of MoS2 QDs toward different ions after addition of 1 mM of the ions solution under 365 nm excitation.