Eco‑Friendly One‑Pot Hydrothermal Synthesis of Water‑Soluble WS₂ Quantum Dots for Sensitive Luminescent Detection of Hydrogen Peroxide and Glucose

Abstract

Zero‑dimensional tungsten disulfide (WS₂) quantum dots (QDs) with strong, stable photoluminescence (PL) are rare, especially when produced without toxic solvents. Here we report a simple, one‑pot hydrothermal method that yields water‑soluble WS₂ QDs using sodium tungstate dihydrate and l‑cysteine as the W and S precursors. The resulting QDs (4–7 nm) exhibit high crystallinity, excellent dispersion, and robust PL that is excitation‑dependent due to quantum‑confinement effects. The PL can be quenched by hydrogen peroxide (H₂O₂) and, in the presence of glucose oxidase, by glucose itself, enabling electro‑deless optical sensors with limits of detection (LOD) of 40 µM for H₂O₂ and 60 µM for glucose. Hybridization with carbon quantum dots (CDs) improves the glucose LOD while retaining photostability and ionic tolerance. Raman and time‑resolved PL studies confirm that H₂O₂‑induced partial oxidation of the QDs is the primary quenching mechanism. This bottom‑up route is inexpensive, scalable, and compatible with other layered‑material QDs, making it a versatile platform for environmental monitoring, biomedicine, and photocatalysis.

Introduction

Two‑dimensional (2D) materials such as graphene have inspired a surge of research, yet graphene’s lack of a band gap limits many applications. Layered transition‑metal dichalcogenides (TMDs) provide tunable electronic and optical properties, making them attractive for sensors and optoelectronics. Molybdenum disulfide (MoS₂) has been extensively studied, but tungsten disulfide (WS₂) offers advantages in abundance, cost, and chemical reactivity, especially at sulfur‑rich edge sites. Bulk WS₂ is an indirect‑gap semiconductor with weak PL, whereas quantum‑confined WS₂ QDs possess a direct gap and visible‑range emission, ideal for optical sensing. Photoluminescent WS₂ QDs remain underexplored; most synthesis relies on top‑down exfoliation that involves hazardous solvents and can degrade electronic properties. Bottom‑up hydrothermal synthesis, by contrast, is environmentally benign and affords controlled particle size and surface chemistry. In this work we present a straightforward hydrothermal protocol that yields high‑quality, water‑soluble WS₂ QDs and, for the first time, CD/WS₂ QD hybrids. We demonstrate their application as electrodeless PL probes for H₂O₂ and glucose, with the hybrids offering superior glucose sensitivity.

Methods

Reagents and Chemicals

All reagents were analytical grade and used without further purification. Sodium tungstate dihydrate and l‑cysteine served as the W and S sources, respectively. Glucose oxidase (GOx) was sourced from Sigma‑Aldrich. Ultrapure water (Milli‑Q) was used throughout.

Materials Preparation

Synthesis of WS₂ QDs

The precursor solution (0.066 g Na₂WO₄·2H₂O in 12.5 mL water, sonicated 5 min) was acidified to pH 6.5 with 0.1 M HCl. Adding 0.0242 g l‑cysteine and 50 mL water, followed by 10 min sonication, yielded the reaction mixture. This was sealed in a 100 mL Teflon‑lined autoclave and heated at 180 °C for 24 h. After cooling, the supernatant was centrifuged at 10 000 rpm for 20 min to collect the QDs, which were stored at 4 °C. For CD/WS₂ hybrids, a microwave‑derived CD solution (1 M sucrose, 20 min 500 W) was sonicated, mixed with the WS₂ precursor, stirred 15 min, and hydrothermally treated under identical conditions. Centrifugation followed the same protocol. The synthesis is depicted in Scheme 1 and illustrated in Figure 1.

Characterization

Structural analysis employed XRD (Cu Kα, λ = 1.5418 Å), TEM/HRTEM (JEOL‑3010), AFM, XPS (JEOL JPS‑9010), UV–Vis (Jasco V‑630), PL/PLE (Hitachi F‑4500), TRPL (Edinburgh OB920), and Raman (Horiba iHR320). All measurements were performed at room temperature unless otherwise noted.

Results and Discussion

Structural and Morphological Studies

TEM images show monodisperse WS₂ QDs (4–7 nm) with no aggregation (Figure 1a). HRTEM confirms the 0.27 nm lattice spacing of the (101) plane, characteristic of 2H‑WS₂. CD/WS₂ hybrids display an average size of 11.5 nm (Figure 1e) and retain the WS₂ lattice (Figure 1f). XRD patterns (Figure 2a) reveal peaks at 2θ = 28.9°, 32°, 33.9°, and 38.0° corresponding to (004), (100), (101), and (103) planes, with the (002) peak absent—consistent with few‑layer QDs. AFM shows thicknesses of 6–10 nm (Figure 2b), confirming the few‑layer nature.

Surface Chemistry and Valence States

XPS indicates W 4f peaks at 33.5 and 34.1 eV (W⁴⁺) and a 35.7 eV peak (W⁶⁺). S 2p shows S²⁻ (161.9/163.1 eV) and S⁴⁺ (166.9 eV) components, revealing edge oxidation. C 1s spectra (284.7, 286.2, 288.0 eV) match graphitic CDs. Hybridization is predominantly physical adsorption, as evidenced by similar W and S peaks in both pristine and hybrid samples (Figure 3).

Optical Properties

UV–Vis spectra exhibit a dominant absorption band near 360 nm, indicative of quantum‑confinement‑induced bandgap widening. PL measurements (excitation 300–400 nm) show emission red‑shifting from 385 to 470 nm, with maximum intensity at 450 nm under 360 nm excitation (Figure 5a,b). The PL quantum yields are 3.05 % (WS₂) and 4.1 % (CD/WS₂) at 360 nm. The PL remains stable under 1 h UV irradiation and across NaCl concentrations up to 200 mM (Figures 6b,c). TRPL decay fits a single exponential with a 3.51 ns lifetime (Figure 10b), unchanged by H₂O₂. Raman spectra show A₁g (420 cm⁻¹) and E¹₂g (353 cm⁻¹) modes with a 67 cm⁻¹ separation, confirming few‑layer WS₂. After H₂O₂ treatment, a new 385 cm⁻¹ band appears, attributed to O–W–O bending, indicating partial oxidation that quenches PL (Figure 11).

Sensing Applications

For H₂O₂ detection, PL quenching of CD/WS₂ hybrids follows a linear relation (R² = 0.99) over 0.1–1 mM, yielding an LOD of 40 µM (Figure 7b). Pristine WS₂ QDs give an LOD of 60 µM (Supplementary Fig. S1). Glucose sensing exploits GOx‑mediated H₂O₂ generation: PL decreases linearly with glucose (0.1–1 mM), achieving an LOD of 60 µM for CD/WS₂ hybrids and 120 µM for pristine WS₂ (Figure 8b, Supplementary Fig. S2). Selectivity tests against fructose, lactose, maltose, and common interferents show negligible quenching, confirming GOx specificity (Figure 9).

Conclusions

We have established a green, bottom‑up hydrothermal route that produces high‑quality, water‑soluble WS₂ QDs and CD/WS₂ hybrids. The QDs exhibit robust, excitation‑dependent PL and can be used as electrodeless optical probes for H₂O₂ and glucose with LODs of 40 µM and 60 µM, respectively. Raman and TRPL analyses confirm that H₂O₂‑induced partial oxidation, rather than nonradiative recombination, is responsible for PL quenching. Compared with conventional top‑down methods, our protocol is simpler, cost‑effective, and amenable to hybridization, positioning WS₂ QDs as promising platforms for environmental, biomedical, and photonic applications.