Laser driven exfoliation and in situ engineering of MoS₂/WS₂–Ag nanocomposites for high-performance electrochemical sensing and photonic applications

Abstract

We reveal a new laser-mediated strategy for the simultaneous exfoliation and functionalization of molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂), producing few-layered TMD nanosheets directly decorated with silver nanoparticles (Ag NPs) in a one-step, reductant-free process. Employing nanosecond pulsed laser ablation in liquid, the process utilizes laser-induced localized defect generation as active nucleation sites for Ag, allowing for controlled morphologies without chemical stabilizers. Subsequent laser ablated MoS₂/WS₂–Ag nanohybrids exhibit ultrahigh electrochemical sensing sensitivity for dopamine and ascorbic acid with an unprecedented low limit of detection of 0.1 nM, better than the majority of the highest-ranked nanocomposite sensors. In-depth spectroscopic and microscopic characterization confirms that laser parameters play a pivotal role in determining nanosheet thickness and Ag NP size/distribution, permitting tunability of electrochemical output. In addition, the composites display significant nonlinear optical limiting behavior, confirming their multitasking ability. This research presents an environmentally friendly, scalable approach for engineering defect-rich 2D TMD platforms with built-in plasmonic functionality and establishes a new standard for next-generation sensor and optical nanodevice design.

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1. Introduction

Two-dimensional (2D) layered materials have attracted extensive research interest in the last decade, symbolized by the emergence of graphene in sensing applications and biosensing.^1,2 Although graphene has received a lot of attention as an ideal two-dimensional (2D) material, it represents just the beginning of a rapidly growing class of layered materials with a variety of tunable physicochemical properties.³ Recent years have witnessed substantial growth in research related to 2D transition-metal dichalcogenides (TMDs).⁴ Generally, these materials are denoted as MX2, where M represents transition metals (e.g., molybdenum (Mo), tungsten (W), hafnium (Hf), zirconium (Zr) or titanium (Ti)) and X stands for chalcogens (e.g., sulfur (S), tellurium (Te) or selenium (Se)) and they exhibit fascinating structural properties. TMDs are two-dimensional layered materials similar in structure to graphene, and they can be isolated by the mechanism of intercalation or mechanical cleavage.^5,6 They are layered materials with a structure composed of stacked layers, where a layer of transition metal atoms is sandwiched between two layers of chalcogen atoms; these layers are held together by covalent bonds between the transition metal and chalcogen atoms.⁷ However, there exists a van der Waals sort of interaction that persists among the monolayers of TMDs. In general, TMDs can display a wide range of properties. These materials have tunable band gaps along with a high surface area; the band gap can be adjusted by changing the composition and structure of the material, i.e., by altering the orientation of the M and X atoms in the 2D layer. From that, a series of particularly interesting features could be obtained, such as the tunability of the conductivity from metal to semiconductors, which enables us to control their electrochemical (EC) performance and fluorescence.⁴ TMDs are multi-functional compounds that find a widespread application in storage and energy production, such as supercapacitors, electro-catalytic hydrogen evolution, and lithium-ion batteries.^8–15

In recent times, leveraging their strengths like electrical conductivity, fluorescence, high surface area, heterogeneous electron transfer, etc., monolayer TMD structures have been employed as potential candidates for developing electrochemical sensing and bio-sensing devices. Moreover, like graphene, a single-bilayer MoS₂ sheet can produce the characteristic cyclic voltammetric peaks innately for a mixture of uric acid (UA), ascorbic acid (AA), and dopamine (DA), which is unlikely for a glassy-carbon electrode (GCE).¹⁶ It is also reported that the nanomolar scale (nM) EC detection of hydrogen peroxide (H₂O₂) secreted by living cells has been achieved by employing very small MoS₂ platelets.¹⁷ The main EC activity of MoS₂ occurs on its edge planes.¹⁸ Thus, creating defects in MoS₂ sheets will help in the introduction of electroactive sites. Considering this possibility, MoS₂ nanosheets decorated with gold nanoparticles (Au NPs) have been used to facilitate the detection of DA in the presence of AA.¹⁹ Moreover, in another report, to detect DA, an Au NP decorated polyaniline–MoS₂ composite was used.²⁰ In another report, an Au NP decorated polymer/graphene–MoS₂ composite was employed to analyze eugenol.²¹ Similarly, catalytic copper nanoparticle (Cu NP)-decorated MoS₂ nanosheets were used for non-enzymatic EC glucose detection in an alkaline environment, and an Ag NP decorated chitosan–MoS₂ composite was utilized for the electro-catalytic oxidation of tryptophan (Trp).²² The decoration of MoS₂ with metallic NPs is often justified as a technique to enhance the conductivity of the composite. It is important to highlight that the metallic NPs do not serve this purpose because, even if the metallic NPs were closely spaced, the electron-transfer process would still have to happen through electron tunneling, which is highly ineffective in carrying long-range conductivity across many particles. Hence, it is more evident that the observed effects arise from the inherent features of the metallic NPs.²³

Apart from the TMDs decorated with metallic nanoparticles, graphene–TMD composites are also widely used in sensing technology, because of the capability of graphene to provide a highly conductive and porous platform, which can eventually undergo functionalization with TMDs that can serve as an electro-catalyst. The applications of such materials have been reported, with the graphene–MoS₂ composite used for sensing acetaminophen and the graphene–WS₂ composite utilized for the simultaneous detection of resorcinol, catechol, and hydroquinone.^24,25 However, a similar performance was reported several years ago on graphene surfaces and it has not been revealed that MoS₂/WS₂ can offer dramatic advantages.²⁶ Composite materials with greater complexity have also been utilized for bio-sensing applications. Such systems generally involve the TMD transducer layer being functionalized with an enzymatic bio-recognition layer. For example, to detect H₂O₂, hemoglobin was immobilized onto MoS₂ microspheres, while glucose oxidase was immobilized onto reduced MoS₂ sheets, with the help of chitosan, for EC detection of glucose.^16,27 In addition, the horse radish peroxidase (HRP) enzyme was immobilized onto graphene–MoS₂ for direct electrochemistry and sensing of H₂O₂.²⁸ In another study, it was shown that the bio-recognition layer in the bio-sensing system can also be non-enzymatic, such as single-stranded DNA (ssDNA). To validate that, Huang and co-workers fabricated an Au NPs/chitosan/graphene–WS₂ composite for the covalent immobilization of the ssDNA probe. The constructed platform was then employed for voltammetric detection of DNA hybridization using ferro/ferricyanide as the electroactive redox marker.²⁹ More comparative research is indeed needed to elucidate clear advantages of TMDs in EC and biosensing, i.e., over the well-established graphene. Besides, TMDs can offer well-defined structures compared to graphene oxide, which has a non-stoichiometric structure that greatly varies depending on the synthesis route.³⁰ To broaden the potential applications and improve the application potential of TMDs, MoS₂/WS₂-based nanocomposites have been explored, specifically a few-layered MoS₂/WS₂ composite with Ag nanoparticles. Further, the application of TMDs in the field of nonlinear photonics began around 2009 and has become a prominent research problem in terms of nonlinear optical features. It is believed that the development of 2D layered materials in the field of photonics will continue to intensify, thus laying a fine foundation for its empirical applications.

This study explores the EC sensing potential of laser-exfoliated MoS₂/WS₂ nanosheets and the nonlinear optical behavior of laser-synthesized MoS₂/WS₂–Ag nanocomposites. The synthesized MoS₂/WS₂ nanosheets exhibit EC sensing properties towards analytes such as DA and AA with a limit of detection (LOD) value of 0.1 nM. To our knowledge, there have been no previous reports on the laser-assisted synthesis of few-layered MoS₂/WS₂ nanosheets specifically for EC of DA and AA. Moreover, we employed an in situ preparation strategy to fabricate Ag nanostructures on the surfaces of MoS₂ and WS₂ sheets, excluding hazardous chemicals or reagents. Further, we propose a highly sensitive platform utilizing Ag nanoparticles embedded within the spacer layers of 2D-MoS₂/WS₂ synthesized via pulsed laser ablation in pure water.

The originality of this work is based on the laser-induced liquid-phase exfoliation process, which exfoliates MoS₂/WS₂ and induces surface defects that facilitate in situ anchoring of Ag nanoparticles, a different mechanism compared to conventional chemical or physical processes. This combined laser process provides fine control of nanoparticle size and density through adjusting irradiation parameters, and laser parameters like irradiation time and fluence enable fine control of nanosheet exfoliation and Ag nanoparticle size/density, providing direct tunability of functional properties, followed by the operational innovation in nanoparticle anchoring.^31–34 The use of defect creation under laser-induced conditions allows for controlled nucleation of Ag NPs without the necessity of extra surface functionalization agents or chemical reduction processes. This process differs from the commonly employed chemical techniques or mere physical mixing that tends to produce less homogeneous nanoparticle distributions or reduced interfacial interactions. More recent reports, cited in Table 1, have demonstrated that the surface reactivity of TMDs is controlled by defect sites, but the present work incorporates the idea into the synthesis process through the use of real-time laser modulation, resulting in a scalable and controllable process to form nanocomposites. The synthesized LA–MoS₂/WS₂ exhibits a low limit of detection for ascorbic acid and dopamine compared to typical TMD-based sensors documented in the literature. Although extensive work has been conducted on the different approaches to exfoliating transition metal dichalcogenides (TMDs) and synthesizing nanocomposites as materials for electrochemical sensing, using laser-induced methods for both exfoliation and anchoring of nanoparticles in one step is novel. For example, Sun et al. (2014)³⁵ fabricated an electrochemical sensor based on gold nanoparticle-decorated MoS₂ nanosheets for the concurrent detection of DA, AA, and uric acid. In their case, though, their approach was to follow chemical synthesis pathways instead of laser-enabled techniques. In another example, Castellanos-Gomez et al. (2012)³⁶ showed the laser-thinning of MoS₂ to create monolayer semiconductors, but their work did not proceed to metal nanoparticle functionalization or electrochemical sensing applications. The choice of laser fluence of 26.3 J cm⁻² for the synthesis was not random but optimized from our previous research by Nancy, Parvathy, et al.³⁷ In that work, we systematically linked laser fluence with the size and distribution of silver nanoparticles produced in ambient liquid and identified important correlations between laser parameters and nanoparticle morphology. The fluence value in the current study was therefore informed by those results, achieving efficient exfoliation of layer-structured TMDs and concurrent anchoring of evenly distributed Ag NPs. A comprehensive parameter-dependent study (such as fluence, spot size, pulse repetition rate, and irradiation time) will be performed in future research.

Table 1 Performance improvement of the present system over existing systems

Sample name	Synthesis route	LOD value	Analyte(s)	Ref.
This work	Laser assisted	0.1 nM	DA/AA	This work
h-BNNS/GCE	Drop-casting of hexagonal boron nitride nanosheets on GCE	0.006 ppb (∼0.1 nM)	Sodium azide	45
MoS2–graphene hybrid	Hydrothermal synthesis of MoS2 nanoflowers on graphene nanosheets	Sub-ppb	Pb^2+/Cd^2+	46
MoS2–polymer composite	Drop-casting MoS2–polymer nanocomposite on GCE	∼0.18 nM	DA	47
MoS2-based carbon paste electrode	Modified with MoS2 for simultaneous phenol detection	Picomolar levels	HQ/CC/RC	48
PVP-Graphene/GCE	Electrodeposition	0.2 nM (DA)	DA/AA	50
GPE/rGO/Pt/GCE	Electrodeposition	9.0 nM (DA)	DA/AA	51
Fe3O4/rGO/GCE	Solvothermal synthesis	0.12 μM (DA)	DA/AA	52
N-RGO/Au/GCE	Hydrothermal treatment	385 nM (DA)	DA/AA	53
AuNPs@MoS2	Electrodeposition	μM level	AA, DA and UA	54

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Herein, the strategy adopted in the present study of employing laser-induced liquid-phase exfoliation of MoS₂/WS₂ nanosheets along with in situ anchoring of silver nanoparticles for electrochemical sensing purposes is truly novel. Not only does this new approach ease the synthesis process, but it also improves functional integration of TMDs in electrochemical sensors, leading to a substantial breakthrough in the field by optimising the various parameters. This synthesis approach provides strong motivation for developing diverse functional materials through laser ablation for the design of EC sensing substrates and novel optical limiters.

2. Materials and methods

2.1. Raw materials

The bulk MoS₂ and WS₂ powders and the Ag target were purchased from Sigma Aldrich India Co. Ltd. DA was procured from Sigma Aldrich India Co. Ltd, and AA from Merck, India Ltd. All chemicals and reagents were used without additional purification. Distilled water was used in all the procedures unless specified otherwise.

2.2. Characterization

The X-ray diffraction (XRD) patterns of the samples were obtained by using a Rigaku Smart Lab SE II X-ray diffractometer with Cu Kα radiation of wavelength λ (1.5418 Å). The high-resolution transmission electron microscopy (HRTEM) analysis was performed using a JEOL JEM 2100 electron microscope at an operating voltage of 200 kV and field emission scanning electron microscopy (FESEM) images were taken on a Zeiss EVO 18 scanning electron microscope with an acceleration voltage of 15 kV. The Raman spectroscopy measurements were carried out using a WITec alpha 300RA confocal Raman spectroscope with an excitation laser wavelength of 532 nm, and the X-ray photoelectron spectroscopy analysis was carried out using XPS, PHI 5000 Versa Probe II, ULVAC-PHI Inc., USA, equipped with a micro-focused (200 μm, 15 kV) monochromatic Al-Kα X-ray source (hν = 1486.6 eV).

2.3. Synthesis of MoS₂/WS₂ nanosheets and MoS₂/WS₂–Ag nanocomposites

The experiment was conducted in two different steps.

•The liquid phase exfoliation of the bulk MoS₂/WS₂ powder using a nanosecond laser with an irradiation time of 20 minutes (Fig. 1(a)).

Fig. 1 Experimental layout for the (a) exfoliation of bulk MoS₂/WS₂ and (b) generation of MoS₂/WS₂–Ag nanocomposites.

•The synthesis of MoS₂/WS₂–Ag nanocomposites by ablating the Ag target in the aqueous suspension of as-prepared few-layered MoS₂/WS₂ nanosheets (Fig. 1(b)).

In order to achieve the objectives presented before, the bulk MoS₂/WS₂ powder was dispersed in deionized water (1 mg mL⁻¹) and stirred for 30 minutes. A Q-switched Nd-YAG laser (Litron LPY 674G-10) beam having 8 ns pulse width and 10 Hz repetition rate was focused on the aqueous suspension of the bulk MoS₂/WS₂ powder using a plano-convex lens of focal length 15 cm at room temperature, and the colloidal solution was ablated for another 20 minutes. After the ablation, the aggregated layers of the bulk material were transformed into few-layered MoS₂/WS₂ nanosheets. After that, the Ag target was properly immersed in the few-layered MoS₂/WS₂ colloidal solution and the Ag target ablation was performed for 15 minutes, which immediately led to the in situ generation of Ag nanoparticles (NPs) on the surface of the MoS₂/WS₂ nanosheets, and finally, the colloidal solution turned into a composite material. In both cases, the solution was stirred continuously during ablation with the help of a magnetic stirrer.

In this work, a novel method was developed by using nanosecond laser pulses to exfoliate bulk MoS₂/WS₂ (denoted as B–MoS₂ and B–WS₂) to make the corresponding thin nanosheets (denoted as LA–MoS₂ and LA–WS₂). Also, the in situ deposition of photon-generated Ag NPs on the MoS₂/WS₂ nanosheets, leading to the formation of MoS₂/WS₂–Ag nanocomposites, was performed via laser ablation of the metal (Ag target) in the aqueous suspension of bulk MoS₂/WS₂ powder. The resulting materials were denoted as LA–Ag–MoS₂ and LA–Ag–WS₂.

2.4. Preparation of electrodes and EC methods

Prior to the modification of the glassy carbon electrode (GCE) with the samples, it was mechanically polished with a wetted micro cloth containing alumina powder, and then carefully cleaned in distilled water by ultrasonication (2 min). After each analysis, GCEs were cleaned by ultrasonication in distilled water. The samples (MoS₂/WS₂ nanosheets dispersed in deionized water) were drop-cast over the pre-treated GCE carefully and then allowed to dry for 24 h at room temperature. The EC measurements were carried out with potentiostat/galvanostat PG 302N, AUT 83909 (Metrohm, Autolab, Netherlands) with an electrode system using a drop cast GCE (3 mm diameter) as the working electrode, platinum wire as the counter electrode, and Ag(s)/AgCl(s)/Cl⁻¹ (aq.) (saturated KCl) as the reference electrode. The EC behaviors of DA and AA (in 0.1 M phosphate buffer solutions, PBS) were investigated using cyclic voltammetry (CV) at room temperature.

3. Results and discussion

The structure and morphology of the obtained materials were further confirmed using various characterization techniques.

3.1. Morphological analysis

The morphologies of B–MoS₂, B–WS₂, LA–MoS₂, LA–WS₂, LA–Ag–MoS₂, and LA–Ag–WS₂ synthesized at 26.3 J cm⁻² laser fluence were characterized by TEM and HRTEM to elucidate the structural properties. Fig. 2(a) and 3(a) depict the typical TEM images of B–MoS₂ and B–WS₂, indicating that these nanosheets are actually in the form of a few layers aggregated together. After the laser ablation, the aggregated B–MoS₂ and B–WS₂ transformed into few-layered MoS₂ and WS₂ nanosheets (LA–MoS₂, LA–WS₂), as shown in Fig. 2(b) and 3(b). The HRTEM images in Fig. 2(c)–(e) and 3(c)–(e) clearly show that the LA–MoS₂/WS₂ layers are well stacked with an interlayer distance of 0.62 nm, corresponding to the (002) plane and the lattice d spacing is estimated to be 0.27 nm, which corresponds to the (100) lattice plane of the hexagonal MoS₂/WS₂ phase.³⁸Fig. 2(f) and 3(f) represent the selected area electron diffraction (SAED) pattern of LA–MoS₂ and LA–WS₂, respectively. From TEM analysis, it was inferred that the MoS₂ and WS₂ bulk powder is uniformly exfoliated into thin and isolated sheets of MoS₂/WS₂ nanosheets after laser ablation.

Fig. 2 TEM images of (a) B–MoS₂ and (b) LA–MoS₂; (c) HRTEM image of a few layers of LA–MoS₂; (d) and (e) HRTEM image of LA–MoS₂ to show the lattice fringes with defect sites and (f) selected area electron diffraction (SAED) pattern of LA–MoS₂.

Fig. 3 TEM images of (a) B–WS₂ and (b) LA–WS₂; (c) HRTEM image of a few layers of LA–WS₂; (d) and (e) HRTEM image of LA–WS₂ to show the lattice fringes with defect sites and (f) selected area electron diffraction (SAED) pattern of LA–WS₂.

Further, the elemental composition of the LA–MoS₂ and LA–WS₂ layers was examined through energy dispersive X-ray analysis (EDX), which is shown in Fig. S1(A) (ESI†). The analysis revealed that the most evident intensity peaks confirm the presence of Mo, W, and S without any other impurities from the source ingredients.

Although the present research primarily focuses on the experimental demonstration of laser-induced surface defect-assisted anchoring of Ag nanoparticles on MoS₂/WS₂ nanosheets, we acknowledge that theoretical confirmation, such as density functional theory (DFT) simulation or binding energy calculation, would provide further mechanistic insights into the interaction between the defect and the nanoparticle. The morphological proof by HRTEM reveals defect structures such as edge roughness and lattice strain, in support of the presence of laser-induced active sites for nanoparticle nucleation. It is provided as Fig. S1(B) in the ESI.†

The average size of the Ag NPs on MoS₂/WS₂ was approximately 15 ± 5 nm (the particle size distribution is included in the inset of Fig. 4(a) for LA–Ag–MoS₂, and in the inset of Fig. 4(e) for LA–Ag–WS₂, which has an average size of 17 ± 4 nm). Fig. 4(b) and (c) show the HRTEM image of Ag NPs on MoS₂, revealing the cognizable lattice fringes, a lattice spacing of 0.24 nm corresponding to the Ag (111) plane, and a lattice spacing of 0.27 nm assigned to the (100) lattice plane of MoS₂.^39–41 The TEM analysis demonstrated the simultaneous presence of Ag and MoS₂ in LA–Ag–MoS₂ nanocomposites. Fig. 4(d) shows the selected area electron diffraction (SAED) pattern of LA–Ag–MoS₂ nanocomposites, which was obtained by keeping the basal plane of MoS₂ sheets perpendicular to the electron beam; the three diffraction rings identified in the SAED pattern were assigned to (100)_MoS2, (111)_Ag, and (220)_Ag with a lattice spacing of 0.27, 0.24, and 0.14 nm, respectively. (111)_Ag and (220)_Ag suggested a face-centered cubic (FCC) lattice of Ag NPs, in which the stronger diffraction ring of (111)_Ag suggested that (111) was the primary orientation. Fig. 4(e) depicts the TEM micrograph of the exfoliated nanosheets of Ag-decorated LA–WS₂. The Ag NPs are randomly distributed throughout the surface of WS₂ nanosheets. During the ablation, the Ag deposition process takes place on the WS₂ nanosheets, which might result in a slight structural change (small grooves and wrinkles) in the WS₂ surface. However, the resultant formation of the LA–Ag–WS₂ nanocomposite does not significantly decrease the WS₂ active edges as shown in HRTEM images (Fig. 4(f)–(h)).

Fig. 4 The TEM images of (a) LA–Ag–MoS₂; (b) and (c) HRTEM image of a few layers of LA–Ag–MoS₂; (d) SAED patterns of LA–Ag–MoS₂; (e) TEM images of LA–Ag–WS₂; (f) and (g) HRTEM image of a few layers of LA–Ag–WS₂; and (h) SAED patterns of LA–Ag–WS₂. The Ag NP particle size distribution diagram is included in the inset of (a) and (e) for LA–Ag–MoS₂ and LA–Ag–WS₂.

Fig. S2 (ESI†) displays the FESEM image and the elemental mapping of LA–Ag–MoS₂ in which the elements S, Mo, and Ag are mapped in yellow, pink, and orange colors, respectively. The mapping suggests the co-existence of Ag and MoS₂ in the nanocomposite. Fig. S3 (ESI†) represents the FESEM image and the elemental mapping of LA–Ag–WS₂ in which the elements W, S, and Ag are mapped in pink, yellow, and orange colors, respectively. The mapping indicates the clear conjunction of Ag and WS₂ in the laser-synthesized nanocomposite. FESEM images depict the homogeneous distribution of the Ag NPs over the LA–MoS₂ and LA–WS₂ nanosheets. The immobilization and concentration of the Ag NPs are very high in the regions of folded edges/grooves in the LA–MoS₂ and LA–WS₂ nanosheets. The wrinkles present in the nanocomposite represent the defect sites and they facilitate a high rate of adsorption of the Ag NPs in such areas. These defect sites likely create localized charge variations and act as nucleation centers, promoting stronger interactions between the metal nanoparticles and the nanosheet surfaces. The high-density immobilization of Ag NPs in these regions not only improves the stability of the nanocomposite but also suggests potential enhancements in its electronic, catalytic, and plasmonic properties, making it highly suitable for applications in sensing, catalysis, and optoelectronics.

3.2. X-ray diffraction (XRD) analyses

X-ray diffraction (XRD) analysis was carried out to clarify the structural evolution of the materials B–MoS₂, B–WS₂, LA–MoS₂, LA–WS₂, LA–Ag–MoS₂, and LA–Ag–WS₂, which is depicted in Fig. 5. The diffraction patterns were acquired with a Rigaku Smart Lab SE II diffractometer using Cu Kα radiation (λ = 1.5418 Å). Bulk MoS₂ and WS₂ showed sharp reflections, indicating the crystallinity of the hexagonal 2H-phase of (002), (100), (103), and (110) planes, consistent with JCPDS card numbers 37-1492 (B–MoS₂) and 08-0237 (B–WS₂). Upon nanosecond laser ablation in solution, the exfoliated LA–MoS₂ and LA–WS₂ samples preserved the main (002) reflection while displaying extensive suppression or loss of higher-order peaks. This is an indication of a transition to few-layered structures with much disorder/spacing between the layers due to partial phase transformation or defect introduction. Of note, a slight shift in the (002) peaks was observed, suggesting strain effects and partial phase modification due to the rapid, non-equilibrium nature of laser exfoliation. After in situ decoration with Ag NPs, the appearance of further reflections at 38.1°, 44.4°, 64.5°, and 82.1° correspond to the (111), (200), (220), and (311) faces of face-centered cubic silver (JCPDS card no. 04-0783), thus indicating successful anchoring of the Ag NPs on the TMD surface. The lack of impurity phases and the presence of Ag as well as TMD reflections further indicate the structural integrity of the hybrid nano-system. Thus, the XRD data demonstrate the effective exfoliation and composite generation achieved by pulsed laser ablation, resulting in a scalable technique for developing smart 2D nanocomposites with tunable electrochemical and optoelectronic characteristics.

Fig. 5 X-ray diffraction (XRD) patterns of B–MoS₂, B–WS₂, LA–MoS₂, LA–WS₂, LA–Ag–MoS₂, and LA–Ag–WS₂.

3.3. XPS and Raman spectroscopy analyses

To reveal the interaction between Ag NPs and MoS₂ layers and the modification of MoS₂, Raman and XPS characterization of MoS₂ layers decorated with Ag NPs was performed. The XPS spectra of Ag 3d indicated its chemical composition (Fig. 6(a)). The binding energies of the two peaks at 373.4 and 367.4 eV were attributed to Ag 3d_3/2 and Ag 3d_5/2 orbitals, respectively.^42,43 Raman spectra revealed two active modes, namely E_g1² and A_1g, of MoS₂ (Fig. 6(d)), where the in-plane E_g1² mode arises from the opposite vibration of two S atoms concerning the Mo atom between them, whereas the out-of-plane A_1g mode is consistent with the opposite vibration of only two S atoms. Compared with Raman modes of LA–MoS₂, the E_g1² mode of LA–Ag–MoS₂ was downshifted, revealing that the interaction between Ag NPs and MoS₂ softened the lateral vibration between S and Mo atoms, demonstrating the adsorption effect of Ag NPs on MoS₂.^44–46Fig. 6(b) shows the XPS Mo 3d spectra of the obtained LA–Ag–MoS₂. The detailed peak assignments of Mo 3d are attributed to the S 2s orbital of divalent sulfur, the Mo⁴⁺3d_5/2 and Mo⁴⁺3d_3/2 orbitals of tetravalent molybdenum, and the Mo⁶⁺3d_3/2 orbital of hexavalent Mo, respectively. Among them, peaks around 228.2 eV and 231.5 eV are attributed to Mo⁴⁺3d_5/2 and Mo⁴⁺3d_3/2 components in the 1T phase of MoS₂, and peaks around 229.2 eV and 232.6 eV are attributed to Mo⁴⁺3d_5/2 and Mo⁴⁺3d_3/2 components in the 2T phase of MoS₂.^47–49Fig. 6(c) presents the detailed spectral assignments of the S 2p orbitals in the LA–Ag–MoS₂ samples, revealing two distinct peaks corresponding to the S 2p_3/2 and S 2p_1/2 orbitals of divalent sulfur. The analysis further indicates that the proportion of S atoms in the S 2p_3/2 orbital is lower than that in the S 2p_1/2 orbital, suggesting a decrease in the binding energy (valence state) of sulfur atoms in LA–Ag–MoS₂. This reduction in binding energy is likely associated with a phase transition of MoS₂ from the semiconducting 2H phase to the metallic 1T phase, as corroborated by the Mo 3d spectra.⁵⁰

Fig. 6 The XPS analysis of the LA–Ag–MoS₂ nanocomposite: (a) Ag 3d spectra; (b) Mo 3d spectra; and (c) S 2p spectra; and (d) Raman spectra of LA–MoS₂ and LA–Ag–MoS₂ nanocomposites.

The Raman spectrum of a few layers of WS₂ exhibits an in-plane active mode at 356 cm⁻¹ and an out-of-plane mode at 417 cm⁻¹. The increase in the intensity of Raman active modes for LA–Ag–WS₂ is due to the van der Waals interactions between the layers and the laser deposition of Ag NPs on the top of the WS₂ matrix, as shown in Fig. 7(d). From Fig. 7(a–c), the peak positions for W⁴⁺ 4f_7/2, W⁴⁺ 4f_5/2, S 2p_3/2, and S 2p_1/2 are 32.7 eV, 34.7 eV, 162.3 eV, and 163.5 eV, respectively. Here, W 4f is in its semiconducting prismatic 2H form.^51,52Fig. 7(a) indicates the two sharp peaks at 368 and 374 eV corresponding to Ag 3d_5/2 and Ag 3d_3/2, respectively.

Fig. 7 The XPS analysis of the LA–Ag–WS₂ nanocomposite: (a) Ag 3d spectra; (b) W 4f spectra; and (c) S 2p spectra; and (d) Raman spectra of LA–WS₂ and LA–Ag–WS₂ nanocomposites.

3.3.1. EC sensing of AA and DA at LA–MoS₂/GCE and LA–WS₂/GCE. The selectivity of LA–MoS₂/GCE and LA–WS₂/GCE was tested for the simultaneous detection of AA and DA. Generally, the detection of DA in the presence of AA, UA, etc. is challenging due to the overlapping voltammetric responses corresponding to their structural similarities. The EC sensing of DA by LA–WS₂/GCE was conducted in 0.1 M PBS containing 100 μM AA (higher than the biological concentration of AA) by CV and is presented in Fig. 8(a). There is no oxidation peak of AA, so the material (LA–WS₂) is selective towards DA. The lowest limit of detection (LOD) of DA was achieved in the range of 0.1 M to 0.1 nM in 0.1 M PBS as the electrolyte (Fig. 8b) and the LOD was found to be 0.1 nM for DA detection. LA-WS₂ has the superior sensing capability of dopamine (DA), which showed strong electrocatalytic (EC) characteristics like, high sensitivity and specificity to DA. It has a very low limit of detection (LOD) which is much below that generally existing in the human body. The results also indicate the absence of interference of AA in the DA sensing by LA–WS₂/GCE. Similar sensing experiments were repeated for AA in the presence of 100 μM DA, and the CV responses are given in Fig. 8(c). From the results, it is clear that there is no interference of DA. It is interesting to note that, the peak potential value and the current response of AA were unaffected by the presence of DA. This suggests the selectivity of LA–MoS₂ towards AA, possibly due to the stronger and preferential interaction between LA–MoS₂ and AA. Besides, the LOD of AA was found to be 0.1 nM (Fig. 8d) and was unperturbed by the presence of DA (100 μM). Thus, it is evident that LA–MoS₂ has a preference towards AA over DA and confirms the selectivity of LA–WS₂ towards DA. The results thus clearly manifest the merit of LA–MoS₂ and LA–WS₂ as EC sensor electrodes for AA and DA and compounds of similar structures. The solution processability and the superior performance of LA–MoS₂ and LA–WS₂ suggest that they are promising materials for the EC sensing of AA, DA, and similar compounds.

Fig. 8 (a) CV scan of DA at LA–WS₂/GCE in the presence of 100 μM AA; (b) LOD-CV scans of DA at LA–WS₂/GCE for the concentration range of 0.1 M to 0.1 nM, (c) CV scan of AA at LA–MoS₂/GCE in the presence of 1 μM DA; and (d) LOD-CV scans of AA at LA–MoS₂/GCE for the concentration range of 0.1 M to 0.1 nM.

To understand the improved sensing property of AA and DA on LA–MoS₂/GCE and LA–WS₂/GCE, the electrocatalytic ability of the electrodes was studied by EC impedance spectroscopy (EIS) using 5 mM [Fe (CN)₆]³⁻ as the redox probe and 0.1 M KCl as the supporting electrolyte. The results (Fig. S4, ESI†) revealed that the charge-transfer resistance (R_ct) value was in the order of 4.1 and 4.8 kΩ for LA–WS₂/GCE and LA–MoS₂/GCE, respectively. The lowest R_ct values indicate that the modified electrode has a superior electroactive surface, which can act as an active electron transport site that provides a smooth pathway for the exchange of redox species at the electrode–electrolyte interface.

Further, the influence of pH was studied in the range from 3 to 11 in the solution containing 1 mM AA and DA. As shown in Fig. S5 (ESI†), the peak current increased and then decreased when the pH shifted from acidic to basic conditions in both AA and DA sensing. The peak current for both AA and DA reached a maximum at neutral pH. Since both the analytes are polyprotic molecules, under acidic conditions (lower pH), both AA and DA are more likely to be protonated and positively charged, while under alkaline conditions (higher pH), they can be deprotonated and negatively charged. Hence the negatively charged surface of both WS₂ and MoS₂ nanosheets will interact with the positively charged AA and DA more under acidic conditions and slightly repel under basic conditions.

We point out that the LOD reported in our paper of 0.1 nM for DA and AA is competitive compared to the state-of-the-art in electrochemical sensing. Recent publications have shown that MoS₂-based and 2D nanostructured composites can be used to obtain ultra-low limits of detection for a variety of toxic and biologically important analytes. For example, some of the works from our group⁵³ constructed a hexagonal boron nitride nanosheet-modified glassy carbon electrode (h-BNNS/GCE) that provided an exceptionally low LOD of 0.006 ppb for sodium azide. Similarly, Arya Nair et al. employed a MoS₂ nanoflower–graphene nanosheet hybrid towards the electrochemical determination of Pb(II) and Cd(II) with sub-ppb sensitivity.⁵⁴ In neurotransmitter detection, real-time monitoring of dopamine with superior sensitivity and selectivity employing MoS₂-based nanocomposites was systematically studied.⁵⁵ In addition, simultaneous detection of various phenolic species like hydroquinone, catechol, and resorcinol with LODs at the picomolar level has been documented earlier.⁵⁶ Moreover, effective heavy metal ion removal by MoS₂ hollow nanoroses has also been realized,⁵⁷ highlighting the material's promise towards high-performance sensing and remediation, and many existing graphene-based electrochemical detectors of dopamine and ascorbic acid were compared.^35,58–61 The performance improvement of the present system over other reported systems is shown in Table 1.

3.3.2. Interference studies and real sample analysis. To analyze the selectivity of the LA–MoS₂/GCE and LA–WS₂/GCE modified electrodes towards the detection of AA and DA, the interference studies were conducted in the presence of biomolecules of similar structures such as uric acid (UA), folic acid (FA), glucose (GC), acetylcholine (AC), and certain metal ions (at 10³× concentration). The chronoamperometric results (Fig. S6A and B, ESI†) reveal that while 1 μM concentration of AA and DA shows well-defined amperometric current responses on LA–MoS₂/GCE and LA–WS₂/GCE, no current responses were observed for any of the other molecules/metal ions studied (1 mM concentration each), indicating the superior selectivity of LA–MoS₂/GCE and LA–WS₂/GCE in various environments towards AA and DA detection.

The real-time monitoring performance of the fabricated sensor was studied by detecting both AA and DA in real samples (blood serum and urine) spiked with AA and DA. The measurements are summarized in Table 2. The recoveries are in the range of 100.4–101.04 and 100.3–101.05% for AA and DA, respectively, suggesting good practicability and consistency of the proposed electrode for the simultaneous detection of AA and DA in real samples.

Table 2 The recovery test results of AA and DA in both blood and urine samples

Sample No.	Conc. of analyte spiked (μM)	Analytes	Real samples	Found (μM)	Recovery (%)
1	10	AA	Serum	10.10	101.04
			Urine	10.10	101.03
		DA	Serum	10.11	101.05
			Urine	10.10	101.01
2	20	AA	Serum	20.01	100.05
			Urine	20.00	100.04
		DA	Serum	20.01	100.07
			Urine	20.00	100.03

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3.3.3. Operational stability, shelf-life and continuous cycling studies of LA–MoS₂/GCE and LA–WS₂/GCE. The stability of both LA–MoS₂/GCE and LA–WS₂/GCE after 50 and 100 days (Fig. 9a and b) showed retention of >98% of the initial current response suggesting good storage stability. The reproducibility of LA–MoS₂/GCE and LA–WS₂/GCE was evaluated using 5 separately modified electrodes (Fig. 9c and d) and the relative standard deviation (RSD) was 1.21%, indicating an acceptable reproducibility of the biosensor. The reusability of each sensor was studied by comparing the peak currents for 1 mM DA and AA before and after multiple washing and ∼99.3 and 99.6% retention was observed after 3 consecutive washing (Fig. 9e and f).

Fig. 9 (a) and (b) The CV scans obtained for 1 mM AA and DA on LA–MoS₂/GCE and LA–WS₂/GCE after 100 days of storage showing the stability; (c) and (d) the bar diagram showing the reproducibility of LA–MoS₂/GCE and LA–WS₂/GCE by using 5 different electrodes; and (e) and (f) the CV scans after immediate washing three times showing the reusability of both LA–MoS₂/GCE and LA–WS₂/GCE.

3.3.4. Nonlinear optical studies. The Z-scan method was employed to evaluate the nonlinear optical (NLO) properties of the samples using a Q-switched second harmonic Nd: YAG laser (Litron LPY 674G-10) of the Gaussian beam profile of wavelength 532 nm with varying peak intensity and a pulse duration of 8 ns and 10 Hz repetition rate. The open-aperture Z-scan technique was employed to evaluate the NLO features of the few-layer TMDs and nanocomposite samples at room temperature. Additional information about the Z-scan experiment is outlined below.

A spatial filter was employed to improve the collimation of the excitation radiation from the Nd: YAG laser's Gaussian beam before the converging lens with a 15 cm focal length, which is utilized to concentrate the beam at focus. Carbon disulfide (CS₂) solution contained in a cuvette of 1 mm thickness was utilized as a reference standard sample to accurately calibrate the Z-scan experimental setup. The CS₂ reference sample was substituted with few-layer and composite samples of transition metal dichalcogenides (TMDs) to investigate their nonlinear optical properties through open-aperture and Z-scan measurements. In this setup, the reference and transmitted beam intensities were continuously monitored by using two pyroelectric detectors, D₁ and D₂, positioned before and after the sample, respectively, to capture variations in optical transmittance. As the sample was systematically translated through the focal region, located 15 cm from the lens along the laser propagation axis, the focused laser beam reached intensities sufficient to induce nonlinear optical effects. These effects manifested as dynamic changes in the sample's transmittance, directly influenced by the localized light intensity, revealing the material's nonlinear absorption characteristics as it interacted with the high-intensity laser field.

The samples were dispersed uniformly using continuous ultrasonication and placed in a 1 mm quartz cuvette so that the path length L < z₀ for achieving the thin sample approximation condition. The concentration was adjusted to ensure that every sample had a linear transmission of 60% at an exciting wavelength. The sample cuvette was secured in the translation stage to make moving along the z-direction through the beam focus easier with precision and accuracy. The focal point is regarded as the point at z = 0. A LabVIEW program automated various experimental stages, including shutter control, sample translation, and data acquisition from the oscilloscope to the computer. We can get the Z-scan curve from the obtained data sets by plotting the sample transmission concerning the position in the z-direction.

The optical limiting properties of LA–MoS₂, La–WS₂, LA–Ag–MoS₂, and LA–Ag–WS₂ were studied by an open-aperture Z-scan technique using a 5 ns laser pulse at 532 nm. The optical limiting performance is shown in Fig. 10(a–d). The open aperture Z-scan curves and the subsequent fluence-dependent nonlinear transmissions were acquired in LA–MoS₂, LA–Ag–MoS₂, and LA–Ag–WS₂ for 532 nm 5 ns laser excitation at an input laser pulse energy of 60 μJ. The optical limiting measurements are quantified using the nonlinear parameters which are given in Table 3.

Fig. 10 Optical limiting curves of (a) LA–MoS₂, (b) LA–WS₂, (c) LA–Ag–MoS₂, (d) LA–Ag–WS₂ laser pulse excitation at a laser pulse energy of 60 μJ. Open circles: data points, solid lines: numerical fits to the data points.

Table 3 Nonlinear absorption coefficient (β_eff) and saturation intensity (I_Sat) for ns excitations

Sample	Energy (μJ)	β eff (×10^−10 mW^−1)	I sat (×10^10 Wm^−2)
LA–MoS2	60	0.4	499
LA–WS2	60	0.48	139
LA–Ag–MoS2	60	1.08	72
LA–Ag–WS2	60	3.9	20

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It has been found that the nonlinear optical (NLO) response of the LA–Ag–WS₂ material demonstrates a significantly lower optical limiting threshold compared to the other investigated samples, underscoring its superior nonlinear absorption characteristics under high-intensity laser excitation. In TMD systems, nonlinear absorption is predominantly governed by saturable absorption (SA), reverse saturable absorption (RSA), and two-photon absorption (TPA), each exhibiting distinct intensity-dependent optical behaviours. While RSA and TPA contribute to an increase in absorption with rising irradiance, SA leads to a reduction in absorption due to the depletion of available electronic states at higher excitation levels.⁶²

The pronounced NLO effects observed in the TMD multilayer samples can be attributed to a combination of strong excitonic interactions, quantum confinement effects characteristic of two-dimensional (2D) layered materials, and band-edge resonance associated with TPA.^63,64 Notably, across all four studied samples, RSA emerges as the dominant nonlinear absorption mechanism, consistent with observations reported in monolayer TMDs, where similar intensity-dependent transitions are prevalent.⁶⁵ The transition from SA to RSA is typically ascribed to excited-state absorption (ESA), wherein charge carriers promoted to higher energy states undergo additional absorption processes, leading to a net increase in optical attenuation at elevated excitation intensities.^66,67 This intricate synergy of nonlinear absorption phenomena not only elucidates the fundamental optical processes in TMD-based materials but also highlights their potential for applications in optical limiting, ultrafast photonics and nonlinear optoelectronic devices.

4. Conclusion

Herein, a green, laser-driven approach for the simultaneous exfoliation of MoS₂/WS₂ and in situ immobilization of Ag NPs is reported, using nanosecond laser pulses to trigger photon-assisted defect creation in a one-step, chemical-free process. Structural and surface analysis using HRTEM, XPS, and Raman spectroscopy established the formation of few-layered TMDs with monodisperse Ag NPs and indicated a partial 2H-to-1T phase transition in MoS₂ that had a pivotal role in superior electrochemical performance. The as-prepared LA–MoS₂/WS₂–Ag nanocomposites exhibited ultralow detection thresholds (0.1 nM) for DA and AA, outperforming many of today's TMDs or graphene-based counterparts, coupled with intriguing nonlinear optical limiting action. This dual functionality makes the materials potential candidates for multifunctional sensing and photonic devices. In the future, systematic investigation of laser parameters like fluence, repetition rate, and spot size will be carried out to optimize morphology and phase transitions for function optimization. Additionally, comprehensive research linking phase development with electrochemical kinetics, real-time in situ diagnostics, and long-term device stability will be investigated. In terms of application, the scalable and green aspect of this synthesis process underlines its potential extension to large-scale sensor production, wearable diagnostics, and photonic protection systems, fitting into grand challenges in circular economy, biomedical monitoring, and sustainable nanomanufacturing.

Author contributions

N. K., SKY & S. T. supervised the work; P. N. and A. N. performed the experiments; P. N. and A. N. analyzed the data and wrote the article; all authors have read and agreed to the published version of the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The datasets supporting this study's findings are available from the corresponding author upon reasonable request. Additionally, supplementary materials, including raw and processed datasets, experimental protocols, and analytical details, are accessible from the corresponding author via email on request. For any proprietary or sensitive data, access may be provided in accordance with institutional guidelines and relevant confidentiality agreements.

Acknowledgements

The authors acknowledge the financial support from the DAE-Board of Research in Nuclear Sciences (BRNS Grant No. 39/29/2015-BRNS/39009), Rashtriya Uchchatar Shiksha Abhiyan (RUSA) 2.0 Scheme-Ministry of Education-Govt. of India for funding. The author NK also would like to acknowledge the financial support from the CRS program of UGC-DAE CSR, Kolkata Centre (UGC-DAE-CSR-KC/CRS/19/RC08/0485), India, SERB: CRG (Grant No. CRG/2021/001506), UGC-Govt. of India for funding through the Innovative Program and Special Assistance Program (SAP Grant No. F.530/12/DRS/2009; F.530/13/DRS II/2016), Scheme for Promotion of Academic and Research Collaboration (SPARC Grant No. P930, P1400, P1429), DST: Nano Mission (Grant No. SR/NM/NS-1420-2014(C), SR/NM/NS-54/2009), DST: Fund for Improvement of S&T Infrastructure (FIST Grant No. SR/FST/P SI-143/2009) and DST Promotion of University Research and Scientific Excellence (PURSE Grant No. SR/S9/Z-23/2010/22(C, G))-Government of India programs for providing the facilities for research and development. The author NK also acknowledges the Indo-French Centre for the Promotion of Advanced Research (IFCPAR/CEFIPRA) Project grant (6408-1), the CNRS International Research Partnership India “APONAMA” and the French PIA project « Lorraine University of excellence » reference ANR-15-IDEX-04-LUE. We acknowledge IIST, Trivandrum, SAIF-MGU, and TEM-IIUCNN-MGU for providing characterization facilities. We acknowledge Prof. Kuruvilla Joseph, Dean, IIST for all the administrative support for the smooth functioning of this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5nj01475a

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