Transmission electron microscopy (TEM) images were achieved by a JEM-1400 operating at 120 kV. The optical absorption spectrum of WS₂ QDs was obtained by a UV-3101 scanning spectrophotometer (Shimadzu, Japan) at room temperature. Fluorescence intensity was recorded on the luminescence spectrometer (Fluorolog-3). The x-ray photoelectron spectroscopy (XPS) measurement was carried out on PHI Quantera (PHI, Japan), the binding energy was calibrated with C1s=284.8 eV. Microplate reader (Thermo Scientific Multiskan MK3, Shanghai, China)

WS₂ QDs were produced using a simple liquid exfoliation of bulk crystals. Briefly, 50 mg of WS₂ powder (Alfa Aesar) was added to 30 mL of distilled water in a 50-mL beaker and sonicated in an ultrasonic cell disruptor (400 W) for 4 h. Then, the precipitation with 30-mL NMP (1-methyl-2-pyrrolidinone) was sonicated in an ultrasonic bath continuously for 3 h. After that, the solution was centrifuged (9000 rpm, 30 min) and filtrated to remove large WS₂ sheets. Then the suspension was evaporated and re-dissolved in 50-mL water and stored at 4 °C.

The growth inhibition of the WS₂-treated HepG2 cells was measured using an MTT assay. The cells were seeded into 96-well cell culture clusters (Corning, NY, USA) at a density of 6×10³ cells per well. After a 12-h incubation, the cells were treated with WS₂ for 6 h to 48 h. Thereafter, the cells were rinsed twice with PBS and incubated with 100 μL of a 0.5-mg/mL MTT solution at 37 °C for 4 h. After removing the supernatant, the formazan crystals formed by the MTT were dissolved with 150 μL of DMSO. The absorbance (A) at 490 nm was measured using a microplate reader. The cell viability was calculated from the following formula:

The UV–vis absorption (blue line) and fluorescence emission spectra (red line) of WS₂ QDs are shown in Fig. 1(a). The ultra-violet–visible (UV–vis) absorption spectrum of WS₂ QDs in water shows a broad absorption band at approximately 275 nm. Moreover, when the excitation is 280 nm, the fluorescence emission spectrum has a strong peak at 457 nm. The inset image shows that the calculated band gap energy of WS₂ QDs is about 4.8 eV and blue fluorescence is observed when the WS₂ QDs are irradiated with 365-nm UV light. Photoluminescence (PL) quantum yield of WS₂ QDs is 21.91%, using quinine sulphate as a reference at the excitation wavelength of 360 nm. The quantum yield of WS₂ QDs was calculated according to:

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. (color online) (a) Absorption and PL emission spectra of WS2 QDs (inset: calculated band gap energy of WS2 QDs). (b) PL emission spectra of the WS2 QDs obtained under excitations from 280 nm to 390 nm with 10-nm increase. The high resolution XPS survey spectra of the as-prepared WS2 QDs. (c) W 4f and (d) S 2p spectra.

where Y is the quantum yield, F is the measured integrated emission intensity, and A is the optical density. The subscript “s” refers to quinine sulphate, “x” denotes WS₂ QDs.

To further investigate optical properties, PL emission spectra of WS₂ QDs are recorded under various excitations. As shown in Fig. 1(b), with the excitation wavelength changing from 280 nm to 370 nm, emission peaks are blue-shifted from 459 nm to 447 nm, while the PL intensities are enhanced remarkably. A maximum fluorescence emission intensity can be obtained at 370 nm excitation. When the excitation wavelength ranges from 370 nm to 390 nm, the emission peaks shift from 447 nm to 462 nm with intensity decreasing gradually. This interesting result indicates that WS₂ QDs fluorescence may depend on the excitation wavelength.

To further explore the chemical property and composition of the WS₂ QDs, XPS characterization is carried out. As shown in Fig. 1(c), the W 4f_7/2 and W 4f_5/2 peaks for W⁴⁺ in WS₂ QDs can be observed at 37.1 eV and 35 eV, respectively. Meanwhile, the peaks at around 168.8 eV and 167.8 eV are attributed to the coexistence of S 2p_3/2 and S 2p_1/2 (Fig. 1(d)). Detailed compositional analysis results reveal that the atomic ratio (W:S) is 1:2.03.

As shown in scheme 1 (see Fig. 2), Cu²⁺ can easily combine with WS₂ QDs through electrostatic interactions, which leads to the aggregation of QDs. The energy or electron will transfer from WS₂ QDs to Cu²⁺ and the fluorescence of QDs will be quenched. To confirm this principle, TEM and size distribution are performed. Figure 3(a) depicts the TEM images of WS₂ QDs. Figure 3(b) shows that the diameters are mainly distributed in a range of 2 nm–12 nm (average 7 nm), suggesting that the as-prepared WS₂ QDs are uniformly arranged. When 200-μM Cu²⁺ are added (Fig. 3(c)), the WS₂ QDs aggregate obviously and the average diameter increases to about 40 nm (Fig. 3(d)). These results prove that Cu²⁺ can induce the aggregation of WS₂ QDs and quench the fluorescence.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. (color online) Illustrations of the preparation scheme of WS2 QDs and the detection of copper ions.

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. (color online) (a) TEM image of WS2 QDs. (b) Size distribution histogram of pure WS2 QDs. (c) TEM image of WS2 QDs with 200-μM Cu2+. (d) Size distribution histogram of WS2 QDs with 200-μMCu2+.

To investigate sensitivity of the sensor, Cu²⁺ with concentrations ranging from 0 μM to 200 μM are analyzed by WS₂ QDs under 365-nm excitation wavelength. Figure 4(a) shows that the WS₂ QDs are sensitive to Cu²⁺ and the fluorescence intensity at 454-nm gradually decreases with the concentration of Cu²⁺ increasing. When the concentration of Cu²⁺ reaches 200 μM, the fluorescence intensity of WS₂ QD solution decreases to about 50% of its initial value. Figure 4(b) presents the relationship between the fluorescence intensity of WS₂ QDs and the concentration of Cu²⁺ (F₀ and F refer to fluorescence intensities at 365 nm in absence and presence of Cu²⁺, respectively). The inset image indicates that a good linear relationship exists, which ranges from 0.2 μM to 1 μM with a high correlation coefficient (R² = 0.986). Based on 3σ/k (σ is the standard deviation of the corrected blank signals of the WS₂ QDs and k is the slope of the fitting line), the limit of detection of Cu²⁺ ions is estimated to be 0.4 μM, which is much lower than the maximum level (∼ 20 μM) set by the U.S. Environmental Protection Agency (EPA).

Fig. 4.

Figure Option ViewDownloadNew Window

Fig. 4. (color online) (a) Emission spectra of WS2 QDs in the presence of Cu2+ with concentrations ranging from 0 μM to 200 μM. (b) Plot of (F0 − F)/F0 of WS2 QDs at 365 nm versus the concentrations of Cu2+ (inset: The calibration curve between concentration of Cu2+ and (F0 − F)/F0). (c) PL emission spectra of the WS2 QDs at a pH range from 2 to 12. (d) Fluorescence intensity change of WS2 QDs in the presence and absence of 100-μMCu2+ at different pH values.

In order to study the practical applicability, the effect of pH on the fluorescence response of WS₂ QDs to Cu²⁺ is investigated. The experiments are carried out at a pH range from 2 to 12, with a concentration of WS₂ QDs fixed at 160 μg·ml⁻¹ and 365-nm excitation wavelength (Fig. 4(c)), and Cu²⁺ at 100 μM (Fig. 4(d)). The results show that for both WS₂ QDs with and without Cu²⁺, no obvious variation of the fluorescence emission is observed in the pH range 2 to 12, suggesting that the fluorescent sensor is independent of the pH of the solution.

To assess the selectivity of Cu²⁺ detection, the PL response of the probe to potentially interfering substances (K⁺, Na⁺,Ca²⁺,Ba²⁺,Al³⁺,Fe³⁺,Ni²⁺,Ag⁺,Hg²⁺,Zn²⁺, , Cl⁻, NO³⁻) is tested. The concentrations of the interfering substances and Cu²⁺ are tested at 200 μM under 365-nm excitation wavelength. As shown in Fig. 5(a), the interfering ions exhibit little influence on the fluorescence quenching rate of WS₂ QDs. This result demonstrates that sensing Cu²⁺ is hardly affected by these interfering ions and the fluorescent sensor is of very high selectivity for Cu²⁺ detection. Figure 5(b) shows the effect of WS₂ on the viability of HepG2 cells. The cells are cultured for 12 h and then incubated with 160 μg·ml⁻¹ WS₂ for 6, 12, 24, and 48 h. Cell viability is determined by an MTT assay. The result indicates that WS₂ has no obvious cytotoxicity against HepG2 cell line in vitro.

Fig. 5.

Figure Option ViewDownloadNew Window

Fig. 5. (color online) (a) Various values of (F0 − F)/F0 of WS2 QDs solution in the absence and presence of 200-μM interfering substances (K+, Na+, Ca2+, Ba2+, Al3+, Fe3+, Ni2+, Ag+, Hg2+, Zn2+, , Cl−, NO3−) under 365-nm excitation wavelength. (b) Effect of WS2 on the viability of HepG2 cells. The cells are cultured for 12 h and then incubated with 160-μg·ml−1 WS2 for 6, 12, 24, and 48 h. Cell viability was determined by an MTT assay.