A label-free fluorescent probe for Hg²⁺ based on boron- and nitrogen-doped photoluminescent WS₂

Abstract

The air-stable doping of transition-metal dichalcogenides is important in enabling a wide range of optoelectronic and electronic devices while exploring basic material properties. In this work, we develop for the first time a simple, low-cost synthesis route to preparing boron-and-nitrogen-doped WS₂ (B, N-WS₂) nanosheets. Bulk WS₂, a prototypical transition-metal chalcogenide material, is an indirect band-gap semiconductor with negligible photoluminescence. The obtained B, N-WS₂ nanosheets exhibited enhanced fluorescence. The B, N-WS₂ nanosheets can be used as a green and easy sensing platform for the label-free detection of Hg²⁺ because of the high sensitivity and selectivity toward Hg²⁺. Detection can also be easily accomplished through a rapid, one-step (within 1 min) operation, with a limit as low as 23 nM (S/N = 3).

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1. Introduction

Heavy metal ion pollution has become a critical worldwide issue for years because of severe risks to human health and the environment.¹ Mercury ion has been recognized as one of the most toxic transition-metal ions in the world by the United States Environment Protection Agency.² Hg²⁺ has been demonstrated to be able to easily pass through skin, respiratory, and gastrointestinal tissues into the human body and damage the central nervous and endocrine systems,³ thereby raising serious environmental and health concerns. These health and environmental problems of the mercury(II) ion have prompted the development of efficient, rapid, facile, and applicable methods for the sensitive and selective assay of mercury(II) ions. Therefore, a convenient and rapid method for the analysis of Hg²⁺ ions is highly desirable. Fluorescence-based mercury(II)-ion detection has attracted considerable interest because of its simple operation, high sensitivity, and adaptability for in-field measurement. Recently, numerous studies have been reported for the detection of these ions.^4–8 So far, a wide variety of fluorescent probes have been developed, including fluorescent dye organic molecules, noble metal nanoclusters and metal-based quantum dots (MQDs) etc. Zhang et al.⁹ described a novel rhodamine-based fluorescent chemosensor as an “off–on” fluorescent probe for the detection of Hg²⁺ in aqueous samples. Lin et al.¹⁰ demonstrated the use of lysozyme type VI-stabilized gold nanoclusters (LysVI-AuNCs) for the detection of Hg²⁺ based on fluorescence quenching. Zhou et al.¹¹ developed carbon nanodots as fluorescence probes for rapid, sensitive, and label-free detection of Hg²⁺. Such fluorescent probes, however, suffer from photobleaching of fluorescent dye, expensive cost of noble metals, or toxicity of MQDs, limiting their widespread applications.

Over the past two decades, two-dimensional (2D) transition-metal dichalcogenides (TMDs) have garnered considerable attention from scientists, leading to wide-ranging and diversified technological applications because of their unusual properties.^12–15 TMDs are MX₂ type compounds, where M is a transition metal belonging to groups IV, V, and VI, and X is a chalcogen, such as S, Se, and Te.^16–18 For bulk-layered TMDs, strong intralayer covalent M–X bonds exist, while weak van der Waals interactions connect the adjacent layers, which could be cleaved to few-layer nanosheets. The properties of 2D TMDs are found to be advantageous in other branches of science, such as biosensors and nanomedicine, taking advantage of large surface area, good biocompatibility, fluorescence, electrical conductivity, and rapid heterogeneous electron transfer.^19,20 TMDs, typically MoS₂ and WS_2, are the most important grapheme-like two-dimensional layered materials. WS₂ has been the subject of substantial interest because the band gap is well matched to the solar spectrum.²¹ WS₂ can act as an efficient photoconductive layer in photovoltaic devices and photoelectrochemical solar cells.²² The main advantage of WS₂ is corrosion prevention, as phototransitions involve nonbonding d–d orbitals of W atoms.²³ WS₂ is a quasi-two-dimensional compound, with covalently bonded S–metal–S sheets held by van der Waals forces. In addition, the unique combinations of structural, electronic, catalytic, mechanical, and related properties of the material can offer superiority in applications of biosensors and nanomedicine for achieving detection, diagnosis, and therapy with high sensitivity, specificity, and accuracy. Heteroatom doping can drastically alter the electronic characteristics of TMDs, thus resulting in unusual properties and related applications. Ultra-thin, ultra-small, defect, and doped WS₂ prepared by different researchers have shown excellent applications in semiconductors and fluorescence materials.^24,25

For the first time in this study, we report a facile and low-cost synthetic strategy to prepare fluorescent nanosheets of B, N-WS₂ by doping WS₂ with boron and nitride using the doping method to change the band gap of WS₂. Boron and nitrogen atoms were incorporated in WS₂ by co-annealing with boric acid and melamine. The as-prepared samples were demonstrated as a highly effective fluorescent-sensing platform for label-free, sensitive, and effective Hg²⁺ detection with a detection limit of as low as 23 nM. The entire detection process can be completed within 1 min. The proposed design is simple to prepare and exhibits low background interference, high sensitivity, and ultrafast response. This work successfully demonstrates that the introduction of boron and nitrogen elements into ultrathin WS₂ nanosheets for enhanced fluorescence properties is feasible through a facile and general preparation strategy. This study may open up a potential approach toward designing more efficient TMD-based biosensors and fluorescence material.

2. Experimental section

2.1 Materials

Hg(Ac)₂, Fe(NO₃)₂, Fe(NO₃)_3, Cu(NO₃)₂, CaCl₂, LiNO_3, NaNO₃, KNO₃, Zn(Ac)₂, Mg(NO₃)₂, Ni(NO₃)₂, Pb(NO₃)₂, Mn(NO₃)₂, Ba(NO₃)₂, and CoCl₂ were purchased from Aladdin Ltd. (Shanghai, China). All reagents are of analytical grade and were used as received without further purification. Double-distilled water was used throughout the experiments.

2.2 Characterizations

Fluorescent emission spectra were recorded on a F-7000 spectrofluorometer (Hitachi, Japan). Ultraviolet-visible (UV-vis) spectra were recorded with a UV-1800 spectrophotometer (Shimadzu, Japan). The morphology of products was investigated by scanning electron microscopy (SEM, Hitachi S-4800, Japan) at 5.0 kV accelerating voltage. X-ray diffraction (XRD) analysis was carried out on a BRUKER D8-ADVANCE X-ray diffractometer by using Cu (40 kV, 40 mA) radiation. Raman spectra were obtained using a laser confocal Raman spectrometer (LABRAM-010) in the range of 300 cm⁻¹ to 2000 cm⁻¹. Transmission electron microscopy (TEM) images were obtained using a JEM-3010 (JEOL-3010, Japan). Metrology X-ray photoelectron spectroscopy (XPS) was carried out using Kα 1063 type with focused monochromatized Al Kα radiation (1486.6 eV).

2.3 Synthesis of B, N-WS₂

In a typical synthesis, bulk WS₂ (0.5 g) was thoroughly mixed with a solution of boric acid (1 g) and melamine (1 g) in anhydrous ethanol (100 mL) by ultrasonic dispersion for 1 h. The mixture was dried at 80 °C, subsequently heated to 400 °C in N₂ atmosphere at a heating rate of 5 °C min⁻¹, and maintained for 2 h. The sample was cooled to room temperature to obtain the B, N-WS₂ powder. In a typical run, 10 mg of B, N-WS₂ powder was added to 100 mL of deionized water. After ultrasonic dispersion for 2 h, the precipitate was removed by centrifugation at 2000 rpm for 3 min, and the clear supernatant extract was prepared for analysis.

2.4 Detection of Hg ions

Hg ions were detected at room temperature in aqueous solution. Hg aqueous solutions at different concentrations, together with other metal ion solutions, were freshly prepared before use. To evaluate the sensitivity towards Hg²⁺, we added different concentrations of Hg²⁺ into the as-prepared supernatant of B, N-WS₂. The mixed solutions were equilibrated for 2 min before spectral measurements. Photoluminescence (PL) spectra were recorded by operating a fluorescence spectrophotometer at 280 nm excitation wavelength.

3. Results and discussion

3.1 Characterizations of B, N-WS₂

The morphologies of the as-prepared B, N-WS₂ were characterized by SEM and TEM. Fig. 1(a) and (b) display the SEM image of bulk WS₂. The SEM image indicates that bulk WS₂ is solidly agglomerated at approximately several micrometers in size and exhibit the accumulation of large structures. Meanwhile B, N-WS₂ did not retain the original morphology of bulk WS₂ after the doping process. In contrast, a thin-layer structure is shown in Fig. 1(c) and (d), demonstrating that boron- and nitrogen-doped WS₂ two-dimensional layered ultrathin nanosheets have been successfully prepared after doping and ultrasonication.

Fig. 1 SEM images of bulk WS₂ (a) and (b) and B, N-WS₂ (c) and (d).

TEM was carried out to further analyze the layer structure of B, N-WS₂. Fig. 2(a) shows the image of B, N-WS₂, which possesses a layer structure at the micrometer scale. Ultrathin structure can be seen clearly in Fig. 2(b) and (c). The inset of Fig. 2(d) shows the high resolution transmission electron microscopy (HRTEM) image of B, N-MoS₂, with the (002) crystal plane spacing of 0.62 nm of the lattice. The result fits well with the XRD result. Fig. 2 indicates that the layered nanosheets are successfully prepared.

Fig. 2 TEM (a), (b) and (c) and HRTEM (d) analysis of B, N-WS₂.

In addition, the crystalline structure of bulk WS₂ and B, N-WS₂ were determined by powder XRD (Fig. 3). The peaks at 14.3°, 28.9°, 32.7°, 33.5°, 39.5°, 44.0°, 58.4°, 59.9°, and 60.4° correspond to the (002), (004), (100), (101), (103), (006), (105) (110), and (112) reflections of bulk WS₂ (indexed by Chem. Mater., vol. 12, no. 5, 2000), respectively. For B, N-WS₂, the peaks at 14.4°, 28.9°, 32.8°, 33.5°, 39.7°, 44.0°, 58.3°, 59.8°, and 60.5°correspond to the (002), (004), (100), (101), (103), (006), (105), (110), and (112) reflections, respectively. Compared with bulk WS₂, the d spacing of B, N-WS₂ decreases from 0.61372 nm to 0.61213 nm, indicating that B and N were doped into the WS₂ structure.

Fig. 3 XRD pattern of bulk WS₂ and B, N-WS₂.

XPS was conducted to analyze the chemical states of W and S in the bulk WS₂ and B, N-WS₂ samples (Fig. 4). According to the full survey spectra in Fig. 4(a), the elements W, S, C, and O are found in all samples. However, B and N are found on the B, N-WS₂ sample, indicating that B and N were successfully doped into WS₂. The W 4f doublet peak energies 32.75 eV (W 4f_7/2) and 34.9 eV (W 4f_5/2), and S 2p double peak energies 162.55 (S 2p_3/2) and 163.8 eV (S 2p_1/2) are identified in the spectra. The binding energies are in good agreement with the reported values for WS₂ samples.¹⁹ The peaks corresponding to the S 2p_3/2 and S 2p_1/2 similar to the bulk WS₂ sample. However, compared with the bulk WS₂ sample, the XPS spectrum of B, N-WS₂ shows W 4f peaks are shifting toward higher binding energies at 33.35 and 35.8 eV, indicating that boron and nitrogen elements are successfully doped into WS₂. These results agree with the EDS result of the B, N-WS₂ sample (ESI, Fig. S1†).

Fig. 4 XPS spectra of bulk WS₂ and B, N-WS₂. The full formation elements XPS survey spectra (a), high-resolution XPS analysis for W 4f (b) and S 2p peaks (c).

Fig. 5(a) shows the PL emission spectra of B, N-WS₂ and bulk WS₂ at the same concentration and excitation at 280 nm. Notably, the fluorescence of the bulk WS₂ dispersion can almost be ignored. However, the B, N-WS₂ dispersion shows a strong PL emission peak centered at 343 nm, indicating that the nanosheets are fluorescent. Fig. 5(b) shows the UV-vis absorption and PL emission spectra of B, N-WS₂ dispersion in water. The UV-vis spectrum shows an absorption peak at 220 nm. The dispersion exhibits a strong PL emission peak centered at 343 nm when excited at 280 nm, indicating that the B, N-WS₂ nanosheet exhibits strong fluorescence.

Fig. 5 PL spectra of B, N-WS₂ and bulk WS₂ (λ_ex = 280 nm) (a); UV, and PL spectra of B, N-WS₂ (b).

The luminescence quantum yield and emission lifetime of B, N-WS₂ have been estimated. Fluorescence quantum yield of the peptide sensor was measured using quinine sulfate (QY = 0.427 in water) as the standard. The quantum yield was calculated as the follow equation:

where Φ_s and Φ_r are the quantum yields of sample and the standard respectively; A_s and A_r are the respective absorbance of the sample and the standard; F_s and F_r are the areas of emission for sample and standard, respectively. The quantum yield was calculated to be 8.6%. The data of the lifetime were provided in ESI Fig. S2 and Table S1.†

3.2 Label-free and highly selective detection of Hg²⁺ ions with B, N-WS₂

To evaluate the selectivity of this sensing system, we examined the PL intensity changes in the presence of representative metal ions under identical conditions. Fig. 6(a) shows the PL quenching results of B, N-WS₂ in the presence of Na⁺, Al³⁺, Ba²⁺, Ca²⁺, Cd²⁺, Fe³⁺, Fe²⁺, Hg²⁺, K⁺, Li⁺, Ag⁺, Mg²⁺, Cu²⁺, Ni²⁺, Pb²⁺ and Zn²⁺ (50 μM). As shown in Fig. 6(a), evident PL quenching was observed upon the addition of Hg²⁺. We found that among the tested ions only Ag⁺ can quench the fluorescence of B, N-WS₂ nanosheets and may interfere with the Ag⁺ detection. Fortunately, this issue can be circumvented by using Cl⁻ as chelating agents for Ag⁺ ions and the results was shown in ESI Fig. S3.† The addition of Ag⁺ ions into the B, N-WS₂ mixture in the presence of chelating agents has no effect on the detection of Hg²⁺. Moreover, no tremendous decrease was observed by addition of these metal ions into the system.

Fig. 6 Selective PL response of aqueous B, N-WS₂ solution toward different metal ions (excitation at 280 nm). F₀ and F are the fluorescence intensities of B, N-WS₂ at 343 nm in the absence and presence of metal ions (a). Fluorescence quenching of B, N-WS₂ by Hg²⁺ as a function of time (λ_ex = 280 nm) (b).

Fluorescence quenching kinetics was also studied. Fig. 6(b) presents the time-dependent PL spectra of B, N-WS₂ dispersion after the addition of Hg²⁺. Only 1 min was required to complete the reaction between B, N-WS₂ and Hg²⁺, and no fluorescence change was observed within the following 12 h. All these observations indicate that the fluorescence of B, N-WS₂ was completely quenched within seconds upon adding Hg²⁺ and that detection is ultrafast.

Aside from selectivity, sensitivity is another important parameter in evaluating the performance of a sensing system. Therefore, the PL intensity changes with different concentrations of Hg²⁺ were studied to evaluate the sensitivity of the sensing system in this study. Fig. 7 shows the PL spectra of the B, N-WS₂ solution with different concentrations of Hg²⁺. The result indicates that the PL intensity of the mixture is sensitive to Hg²⁺ concentration and decreases with the increase in Hg²⁺ concentration. The introduction of Hg²⁺ into the dispersion leads to significant decrease in fluorescence intensity, indicating that Hg²⁺ can effectively quench the fluorescence of B, N-WS₂ nanosheets. The interaction between Hg (soft donor) and sulfur atom (soft acceptor) is extremely strong.^26,27 Interaction is believed to be able to bring the atoms into close proximity. Given that the redox potential of Hg²⁺/Hg⁺ lies between the conduction band (CB) and valence band (VB) of B, N-WS₂, photoinduced electron transfer from the CB to the complex Hg²⁺ occurs, leading to fluorescence quenching (Scheme 1).

Fig. 7 PL spectra of B, N-WS₂ dispersion with different Hg²⁺ concentrations (from top to bottom: 0, 0.0005, 0.001, 0.0025, 0.002, 0.01, 0.02, 0.05, 0.1, 0.5, 1.0, 2.0, 4.0, 5.0, 8.0, and 10 μM of Hg²⁺); inset: dependence of F₀/F on the concentration of Hg²⁺ ions within the range of 0.0005–10 μM (F₀ and F are the B, N-WS₂ fluorescence intensities at 343 nm in the absence and presence of Hg²⁺ ions, respectively).

Scheme 1 Schematic of the sensing principle of the B, N-WS₂-based fluorosensor for Hg ²⁺.

The inset in Fig. 7 shows the dependence of F/F₀ on the concentration of Hg²⁺. F₀ and F represent the fluorescence intensities of B, N-WS₂ at 343 nm in the absence and presence of Hg²⁺, respectively. A good linear correlation was obtained over the concentration range of 1 μM to 10 μM. The detection limit was estimated to be 23 nM at a signal-to-noise ratio (S/N) of 3. The detection limit has been compared to the other methods. Table S2† compares the sensing performance of different fluorescent probes for Hg²⁺, showing that our sensing system exhibits superior sensitivity over previously reported sensing systems.

To demonstrate the practical use of the proposed, we performed a study to determine Hg²⁺ concentrations in tap water samples. As shown in Fig. 8, PL intensity decreases with increased concentration of Hg²⁺ from 0 μM to 0.4 μM. The calibration curve for determining Hg²⁺ in tap water was obtained by plotting the values of F₀/F − 1 versus the concentrations of Hg²⁺ (Fig. 8, inset). The sensing system can distinguish between tap water spiked with 0.1 μM. In addition, the samples were also analyzed by cold vapor atomic absorption spectrometry (CV-AAS) as shown in Table S3.† The results suggest that the sensor is likely to be capable of practical Hg²⁺ detection upon further development. The proposed design is simple to prepare and exhibits low background interference, high sensitivity, and ultrafast response.

Fig. 8 PL spectra of B, N-WS₂ dispersion with different Hg²⁺ concentrations (from top to bottom: 0, 0.1, 0.2, 0.3, and 0.4 μM of Hg²⁺); inset: dependence of F₀/F − 1 on the concentration of Hg²⁺ ions within the range of 0–0.4 μM (F₀ and F are the B, N-WS₂ fluorescence intensities at 343 nm in the absence and presence of Hg²⁺ ions, respectively).

4. Conclusions

We successfully developed a simple method for the preparation of B, N-doped WS₂ with boric acid and melamine as boron and nitrogen sources, respectively. The as-synthesized B, N-WS₂ exhibited enhanced fluorescence in comparison with undoped WS₂ and utilized as a highly efficient fluorescence sensor for selective detection of Hg²⁺. The detection is rapid and ultrasensitive, with an extremely low detection limit of 23 nM (S/N = 3). The present study is important because it demonstrates an economic, stable, and environment-friendly route toward the production of B, N-WS₂ for rapid, highly selective, and sensitive optical detection of Hg²⁺ for the first time. This method may open up a potential approach for designing more efficient TMD-based biosensors and fluorescence materials.

Acknowledgements

This work is supported by National Natural Science Foundation of China (31270988, 31401577), National Undergraduate Innovational Experimentation Program (201310530007), Scientific Research Fund of Hunan Provincial Education Department (13B120), China Postdoctoral Science Foundation (2014M562118) and Hunan Provincial Natural Science Foundation of China (2015JJ2133, 13JJ9004).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27327g

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