Facile Bottom-up Preparation of WS2-Based Water-Soluble Quantum Dots as Luminescent Probes for Hydrogen Peroxide and Glucose
Photoluminescent zero-dimensional (0D) quantum dots (QDs) derived from transition metal dichalcogenides, particularly molybdenum disulfide, are presently in the spotlight for their advantageous characteristics for optoelectronics, imaging, and sensors. Nevertheless, up to now, little work has been done to synthesize and explore photoluminescent 0D WS2 QDs, especially by a bottom-up strategy without using usual toxic organic solvents. In this work, we report a facile bottom-up strategy to synthesize high-quality water-soluble tungsten disulfide (WS2) QDs through hydrothermal reaction by using sodium tungstate dihydrate and l-cysteine as W and S sources. Besides, hybrid carbon quantum dots/WS2 QDs were further prepared based on this method. Physicochemical and structural analysis of QD hybrid indicated that the graphitic carbon quantum dots with diameters about 5 nm were held onto WS2 QDs via electrostatic attraction forces. The resultant QDs show good water solubility and stable photoluminescence (PL). The excitation-dependent PL can be attributed to the polydispersity of the synthesized QDs. We found that the PL was stable under continuous irradiation of UV light but can be quenched in the presence of hydrogen peroxide (H2O2). The obtained WS2-based QDs were thus adopted as an electrodeless luminescent probe for H2O2 and for enzymatic sensing of glucose. The hybrid QDs were shown to have a more sensitive LOD in the case of glucose sensing. The Raman study implied that H2O2 causes the partial oxidation of QDs, which may lead to oxidation-induced quenching. Overall, the presented strategy provides a general guideline for facile and low-cost synthesis of other water-soluble layered material QDs and relevant hybrids in large quantity. These WS2-based high-quality water-soluble QDs should be promising for a wide range of applications in optoelectronics, environmental monitoring, medical imaging, and photocatalysis.
KeywordsSemiconductors Quantum localization Chemical synthesis Luminescence Optical properties
Atomic force microscopy
Carbon quantum dot
High-resolution transmission electron microscopy
Quantum confinement effect
Transmission electron microscopy
Transition metal dichalcogenide
X-ray photoelectron spectroscopy
In the past decade, graphene has opened a new horizon of two-dimensional (2D) materials for chemists and physicists [1, 2, 3]. Due to the inherent shortcomings of graphene, such as absence of band gap, research for other kinds of 2D materials is currently in the spotlight. Notable 2D material groups include layered transition metal dichalcogenides (TMDs), layered transition metal oxides, and carbide-based materials [4, 5, 6, 7, 8]. The characteristic 2D structure of TMD results in anisotropic physical properties, ranging from electron mobility to catalytic and optical properties. In comparison with their bulk counterpart, the general advantages of ultra-thin TMDs are the tunable physical properties and the enriched active sites for chemical reactions. As the most popular 2D TMD material, single-layer or multilayer molybdenum disulfide (MoS2) has shown great potential in a wide range of applications, such as electronics, sensors, and photocatalysis [9, 10, 11]. Especially, ultrathin atomic-layered MoS2 holds great promise for constructing biosensors because high specific surface area and ample active surface states make 2D MoS2 very sensitive to exposure to target analytes. In the field of biosensing, 2D MoS2 has a relatively low toxicity in comparison to many other nanomaterials, in particular, graphene and graphene oxides . For instance, 2D MoS2 has been employed for the detection of hydrogen peroxide (H2O2) and glucose in the last couple of years [13, 14, 15].
The detection of hydrogen peroxide, a vital reactive oxygen species, is of practical importance in chemical, pharmaceutical, clinical, and environmental fields. For example, an abnormal high level of H2O2 could mean the generation of acid rain and could indicate the risk of a few diseases like Alzheimer’s disease and Parkinson’s disease . On the other hand, glucose plays an important role in biochemical pathway and human health evaluation. Convenient and cheap detection of glucose is of considerable significance in the diabetes mellitus diagnosis, food, and biofuel cell analysis. Besides, it is known that over 80% of biosensor industry research is related to glucose sensors. Therefore, the development of a facile, low-priced, and accurate sensor for H2O2 and glucose continue to receive tremendous research effort [17, 18].
Zero-dimensional (0D) quantum dots (QDs) derived from ultrathin 2D materials are emerging as a novel category of nanoscale 0D materials [19, 20]. Compared with TMD nanosheets, TMD QDs show distinct and exceptional physical properties due to pronounced quantum confinement and edge effects. By reducing the dimensions of QDs close to the excitonic Bohr radius, it was found that the quantum confinement effect (QCE) enhanced the photoluminescence (PL) quantum efficiency of MoS2 QDs [21, 22]. Moreover, the ultrathin sizes of MoS2 QDs lead to larger surface-to-volume ratio and abundant active edge states, making them chemically sensitive to the surroundings. Thus TMD QDs can be promising for use in sensing, luminescence, bioimaging, and catalysis. In this regard, MoS2 QDs were lately employed for PL sensor to detect chemical and bioanalyte [23, 24].
Following the successful development of MoS2 in various applications, tungsten disulfide (WS2) begins to receive increasing amount of attention . The layer structure consists of 2D monolayer building blocks held by weak van der Waals interaction. Each WS2 single layer possesses a hexagonal crystal structure formed by covalently bonded S-W-S monolayers, where a tungsten atom sheet is sandwiched by two layers of S atoms. Compared with molybdenum, tungsten has several benefits such as copious natural resources, cheaper prices, and less toxicity, which is favorable for industrial applications. Additionally, the larger size of W provides more spacious interlayer channels in the 2D structure and facilitates physical property modulation via substitutional doping. WS2 is also preferential in tungsten dichalcogenides when a high chemical reactivity is in need at the unsaturated sulfur edges. 2D WS2 nanosheets have recently found a number of applications, such as FETs , photodetectors [27, 28], and photocatalysis [29, 30]. WS2 in its bulk form has an indirect bandgap and a photoluminescence (PL) band in infrared with low quantum efficiency . In QD configuration, 0D WS2 has a direct bandgap and hence shows highly efficient PL, facilitating the construction of electrodeless optical sensing templates. The resultant PL that appears in the visible range is compatible with most low-cost commercial optical platforms. Advantageously, the noncontact nature of optical sensing supports the future realization of advanced integrated multifunctional microchips.
To date, considerable efforts have been dedicated to achieve the synthesis of photoluminescent MoS2 QD materials [22, 31]. In contrast, the progress in the synthesis and application of photoluminescent WS2 QDs is still rather limited. In general, synthetic strategies can be divided into “top-down” and “bottom-up” approaches. As for the “top-down” methods, liquid exfoliation methods are usually regarded as an efficient methodology to prepare single or few-layered 2D material suspensions in large quantities. Successful preparations of WS2 QDs by intercalation techniques adopting lithium and K ions have been reported [32, 33]. In such cases, hazardous and time-consuming processes were involved. Besides, further purification was required to remove ionic residues and semiconducting properties could be weakened because of ion intercalation. On the other hand, sonication-assisted liquid-phase exfoliation technique is based on high ultrasonic powers and the match of surface tension between the solvents and the targeted stratified bulk materials [34, 35, 36]. Several recent reports on the preparation of WS2 QDs have employed this rather universal route [37, 38, 39, 40]. However, this technique is usually associated with hazardous organic solvents and laborious pretreatment, and is quite sensitive to the environmental conditions. In addition, the derived product is typically plagued with residue solvents. The high-temperature post-treatment process is thus required to get rid of excessive solvents with high boiling points. Nevertheless, it may lead to the aggregation of WS2 QDs and the formation of harmful side products in certain cases.
While most of these synthetic routes belong to “top-down” synthesis, the advancement in the “bottom-up” synthesis of photoluminescent WS2 QDs is fairly restricted [41, 42]. Among the “bottom-up” chemical synthetic approaches, the hydrothermal method has become a well-received and cost-effective technique for preparing semiconducting nanocrystals. The dimension and morphology of the synthesized nanostructures can be easily controlled by the chemical reaction parameters and precursor selection. In comparison with most “top-down” synthesis, the hydrothermal process is simple, environmentally benign, and well-suited to the facile formation of nanohybrid materials. Moreover, a recent investigation on hydrothermally prepared MoS2 QDs suggested that the solubility and stability of MoS2 QDs were improved due to some accompanying surface functional groups . Due to these favorable attributes, the exploration of facile hydrothermal synthesis of water-dispersible WS2 QDs with stable photoluminescence is significant and urgent at this stage. In this paper, we herein present a facile bottom-up hydrothermal route for the synthesis of photoluminescent WS2 QDs. Furthermore, motivated by recent progress in carbon quantum dots (CDs)/2D MoS2 composites and to show the viable hybrid formation by hydrothermal protocol, we proceeded to prepare CD/WS2 QDs for the first time [43, 44, 45]. CDs are 0D quasi-spherical nanoparticles, with diameter in the order of 10 nm or less, showing superb solubility, biocompatibility, photochemical stability, and rapid electron transfer properties . Next, the prepared WS2 QDs were characterized in detail. The intense blue emission from synthesized QDs was then used as luminescent probes to construct electrodeless PL sensors for detection of hydrogen peroxide and glucose. Likewise, the sensors displayed a good selectivity toward glucose over other probable interfering species. In the case of glucose sensing, it was found that the hybrid CD/WS2 QDs have a more sensitive LOD than that of pristine WS2 QDs. The obtained results indicated that the synthesized WS2 QDs and novel CD/WS2 hybrid QDs possess small sizes, stable and intense PL, high dispersibility, and non-toxicity. We believe that these optical active WS2 QDs are promising to serve as new platforms for chemical and biological molecules sensors and other functional devices. Extended studies toward this direction are currently ongoing.
Reagents and Chemicals
Sodium tungstate dihydrate (Na2WO4·2H2O) was obtained from Nihon Shiyaku Reagent (Tokyo, Japan). l-cysteine was purchased from Alfa Aesar. They served as starting materials for the hydrothermal synthesis of WS2 QDs. Here, l-cysteine acts as sulfur source as well as reducing agent. Glucose, fructose, maltose, and sucrose were obtained from Honeywell Fluka (Shanghai, China). Lactose, histidine, glycine, potassium chloride, and magnesium chloride were obtained from Sigma-Aldrich. All the reagents were of analytical purity and were used as received without further purification. Throughout the synthesis, ultrapure water from Milli-Q Plus water purification system (Millipore Co., Bedford, MA, USA) was adopted for solution preparation.
Synthesis of 0D WS2 QDs
Synthesis of Carbon Quantum Dots
Carbon quantum dots were prepared by an eco-friendly microwave-assisted method, which is analogous to the CD synthesis in previous reports [47, 48]. In a typical production, 17.1 g of sucrose was dissolved in deionized water to prepare 1 M sucrose solution. Next, the solution was subjected to microwave heating at 500 W for 20 min. The CD can be collected and filtered through a filter. After that, the CD solution was stored at 4 °C for further experiments.
Synthesis of CD/WS2 QDs
For synthesis of hybrid CD/WS2 QDs, certain amounts of CD solutions were sonicated for 20 min to achieve uniform dispersion. The CD solution was added to the preceding WS2 precursor solution with vigorous stirring for 15 min. Next, the homogeneous mixture was transferred into a 100-mL Teflon-lined autoclave and kept at 180 °C for 24 h. After the suspension was cooled to room temperature, the CD/WS2 QDs were collected by using centrifugation for 20 min at 10,000 rpm.
The phase structure was characterized by a Siemens D5000 powder diffractometer utilizing CuKα radiation (λ = 1.5418 Å). Further microstructural information of the samples was provided by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) by using a JEOL-3010 transmission electron microscope. X-ray photoelectron spectroscopy (XPS) measurements were carried out with an ultrahigh vacuum JEOL JPS-9010 electron spectrometer equipped with a multi-channel detector. The collected binding energies were referenced to the C1s peaks at 284.6 eV of the surface adventitious carbon. The UV–Vis spectra were recorded with a Jasco V-630 spectrophotometer (USA) with a standard 10-mm path length quartz cuvette. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the as-prepared samples were measured using a Hitachi F-4500 florescence spectrophotometer linked to a 150 W Xenon lamp as the excitation source. The PL decay time of the QDs was recorded on an Edinburgh Instruments OB920 Fluorescence Lifetime Spectrometer (Edinburgh Instruments Ltd., Livingston, UK). The Raman measurements were taken in ambient conditions with a red light laser. The scattered light was collected by the same objective lens and dispersed with a Horiba iHR320 spectrometer .
Results and Discussion
Structural and Morphological Studies
Surface Elemental and Valence State Analysis
Optical Property Studies
Under irradiation of UV light, a strong blue luminescence can be easily observed by the naked eye, as depicted in the inset of Fig. 6b. It is known that WS2 in its bulk form has very limited luminescent intensity. The strong blue emission again supports the successful fabrication of nanostructures in quantum confinement regime. The stability of luminescence is essential in the optical sensing application. The photo stability of CD/WS2 QDs was checked by the time-dependent PL measurement under an excitation of 360 nm. Figure 6b shows that the luminescent intensity is almost unchanged after UV irradiation for 1 h. Next, we study the effect of salt solution on the fluorescence intensity of QDs. As presented in Fig. 6c, the CD/WS2 QDs possess good ionic stability under different concentrations of NaCl solution, revealing the potential for sensing in a physiological environment. These results suggest that the PL properties of our synthetic QDs can be employed for luminescence sensing purpose. Parallel PL properties were found for pristine WS2 QDs except the luminescent intensity is weaker than that of hybrid QDs. Excitation wavelength-dependent PL spectra of pristine WS2 QDs are shown in Fig. 5c. Figure 5d displays the 2D PL contour map derived from the PL spectra of WS2 QDs, which shows a prominent red-shift with an increase in the excitation wavelength. Good ionic and temporal stability in luminescent intensity was also found for pristine WS2 QDs, which is shown in Fig. 6c, d, respectively. The PL quantum yields of WS2 QDs and CD/WS2 QDs are 3.05% and 4.1% using quinine sulfate as a reference at the excitation wavelength of 360 nm (theoretical quantum yield 54%).
Application to H2O2 and Glucose Detection
Time-Resolved PL and Raman Studies
One other interesting characteristic was noted in the Raman scattering results of CD/WS2 QDs after H2O2 treatment. As designated by an asterisk in Fig. 11, there exists an identifiable signal at 385 cm−1, which is attributable to neither first-order nor second-order WS2 Raman scattering modes . This peak can be ascribed to the bending (δ) mode O–W–O in WS2 QDs [71, 72], whose presence indicates the formation of W–O bonds upon H2O2 treatment. This mode became obviously pronounced because of the oxidation induced by hydrogen peroxide. As edge states are abundant in ultrathin 2D QDs, partial oxidation or doping of oxygen is facilitated in the reactions with hydrogen peroxide. It is in sharp contrast with 2D nanosheets because sheet surfaces are not very sensitive to oxidation. Recently, a first-principles calculation showed that the band structure of partially oxidized MoS2 QDs can be modified, leading to the suppression of photoluminescence by hydrogen peroxide treatment . It was shown that with certain degree of oxidation, the high efficient direct bandgap structure of MoS2 QDs can become inefficient indirect bandgap structure with certain bandgap narrowing. In this case, the photoluminescence of oxidized MoS2 QDs can be quenched and additional longer wavelength absorption could be found. These effects predicted by the above-mentioned calculations are consistent with our experimental outcome in partially oxidized WS2 QDs. Analogous mechanism is very likely to occur in our case since general features of the WS2 band structure are similar to those of MoS2. Furthermore, we found the corresponding absorption band of two types of WS2 QDs appeared red-shift after H2O2 was added to the solution, as shown by the dashed lines in Fig. 4. As a comparison, the absorption data of sole hydrogen peroxide was included as the brown dashed dot line, which indicates that the change is not due to the presence of H2O2 alone. Same behavior was recently reported for oxidation-induced luminescence quenching of MoS2 QDs . Consequently, oxidation induced by hydrogen peroxide is accounted for the sensing mechanism of our WS2 QDs by using PL quenching.
In summary, for the first time, photoluminescent WS2 QDs and CD/WS2 QDs were prepared under “bottom-up” hydrothermal conditions by using sodium tungstate dihydrate and l-cysteine. From the TEM analysis, it can be observed that the synthesized WS2 QDs had high crystallinity and featured good dispersibility. On the basis of the strong PL with high stability from as-prepared QDs, they were subsequently applied for the construction of an electrodeless PL quenching sensor for detection of H2O2 and glucose. Both types of QDs show similar capability in H2O2 sensing and hybrid CD/WS2 QDs provide a more sensitive LOD for glucose detection. The stability test showed that the produced WS2-based QDs are robust against photo-degradation and is stable during the sensing period. The Raman study implied that H2O2 causes the partial oxidation of QDs, which may lead to oxidation-induced quenching. Compared with most reported works with “top-down” approaches, the proposed “bottom-up” protocol for WS2-based QDs has the advantages of simple preparation, low cost, eco-friendliness, and ease for hybrid construction. Furthermore, these water-soluble WS2-based QDs with abundant active sites can be a promising candidate for potential applications in environmental monitoring, biochemistry, and clinical diagnostics. For instance, as there exist numerous kinds of O2-dependent oxidases which generates hydrogen peroxide, the presented facile 0D QDs may also be employed to detect other target molecules by taking the corresponding enzymes. Overall, our results provide an alternative and cost-efficient platform to exploit the diverse functionalities of 0D WS2-based nanomaterials. Further structural layout and extended applications are underway.
This work was supported by the Ministry of Science and Technology, Taiwan under grant No: MOST 106-2112-M-110-012. We acknowledge financial support from Center of Crystal Research, National Sun Yat-sen University, Kaohsiung, 80424, Taiwan, Taiwan.
DRH directed the project and finalized this manuscript. DYS and DRH conceived and designed the experiments. DYS carried out the main part of the experiments. CHC and HFW provided equipment support in the synthesis work. MMCC helped to polish the manuscript. SEI and KHS gave suggestions on the experimental design. All authors read and approved the final manuscript.
The authors declare that they have no conflict of interests.
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