A Photoelectrochemical Sensor for the Sensitive Detection of Cysteine Based on Cadmium Sulfide/Tungsten Disulfide Nanocomposites

In this work, a CdS-nanoparticle-decorated WS2 nanosheet heterojunction was successfully prepared and first used to modify ITO electrodes for the construction of a novel photoelectrochemical sensor (CdS/WS2/ITO). The thin-film electrode was fabricated by combining electrophoretic deposition with successive ion layer adsorption and reaction techniques. The results indicated that the synthesized heterojunction nanomaterials displayed excellent photoelectrochemical performance which was much better than that of pristine CdS nanoparticles and 2D WS2 nanosheets. Owing to the formation of the surface heterojunction and the effective interfacial electric field, the enhanced separation of photogenerated electron–hole pairs led to a remarkable improvement in the photoelectrochemical activity of CdS/WS2/ITO. This heterojunction architecture can protect CdS against photocorrosion, resulting in a stable photocurrent. Based on the specific recognition between cysteine and CdS/WS2/ITO, through the specificity of Cd-S bonds, a visible-light-driven photoelectrochemical sensor was fabricated for cysteine detection. The novel photoelectrochemical biosensor exhibited outstanding analytical capabilities in detecting cysteine, with an extremely low detection limit of 5.29 nM and excellent selectivity. Hence, CdS-WS2 heterostructure nanocomposites are promising candidates as novel advanced photosensitive materials in the field of photoelectrochemical biosensing.


Introduction
Cysteine, a crucial amino acid rich in thiol, serves significant functions in numerous biological processes.Cysteine participates in tissue protein synthesis, post-translational modifications and the construction of active sites in certain enzymes [1].It was found that diseases such as acquired immune deficiency syndrome, liver injury and Alzheimer's disease are accompanied by cysteine deficiency [2].Cysteine levels in the body have been acknowledged as a significant marker for diagnosing diseases [3].Hence, it is crucial to investigate the functions of cysteine in cells and diagnose diseases through its sensitive and selective detection.
Numerous techniques have been reported to monitor cysteine, including colorimetric sensor [4], HPLC methods [5], capillary electrophoresis analysis [6], spectrofluorimetry [7], electrochemical methods [8,9] and photoelectrochemical (PEC) detection [10,11].Among the different methodologies used, photoelectrochemical measurements have attracted considerable interest because of their excellent analytical properties.In contrast to electrochemical analysis, photoelectrochemical detection exhibits a significantly reduced background as a result of the separation between the excitation light source and the detection photocurrent signal.Therefore, this method demonstrates encouraging analytical uses in the fields of bioanalysis, environmental monitoring and medicine detection [12,13].Despite the mentioned advantages, the development and application of high-performance photoactive materials are still urgent issues to be solved in photoelectrochemical sensing research.
Transition-metal dichalcogenides (TMDs) are a family of graphene-like, two-dimensional layered materials that has attracted widespread attention in the research community [14].Recent studies have shown that TMDs have a large specific surface area and abundant active sites, which can significantly enhance the catalytic activity of these materials [15].As members of the TMD compound group, tungsten disulfide (WS 2 ) nanosheets exhibit extraordinary properties, including less toxicity, high intrinsic conductivity and extraordinary electrical and optical performance, and thus they have been widely used in the field of photocatalysis and electrochemical detection in the past few years [16][17][18][19].Nevertheless, the inadequate water dispersion of WS 2 nanosheets, limited light absorption capability and rapid recombination of photoinduced electron-hole pairs hinder their further application in photoelectrochemical detection.Hence, researchers have attempted to build hybrid nanostructures using WS 2 to enhance dispersion characteristics and photoelectrochemical performance [20][21][22][23].Cadmium sulfide (CdS) is recognized as an attractive active material among PEC materials due to its exceptional ability to absorb visible light and ideal band gap.However, CdS alone exhibits photocorrosion and low photoelectric conversion efficiency under visible light irradiation [24].Researchers have reported that nanocomposites exhibit a significant enhancement in photocatalytic performance when they have a large area comprising a few layers of WS 2 combined with CdS particles [25].However, the reported preparation method needs a high temperature, long preparation time and complex experimental conditions.Furthermore, CdS-WS 2 hybrid materials prepared through one-pot synthesis do not easily form uniform thin films on indium tin oxide (ITO) electrodes due to their poor dispersion property.Therefore, the fabrication and application of high-performance CdS-WS 2 heterojunction thin film remain challenging tasks in the research of photoelectrochemical sensors.
In the past few decades, numerous methods have been reported for preparing multimetal nanomaterial films, including the use of layer-by-layer films and self-assembled monolayers, as well as various electrosynthesis techniques, which have been applied in different fields [26,27].Among the different methodologies, electrophoretic deposition (EPD) has been widely used for depositing various materials, such as composites, polymers and inorganic nanomaterials, to form multifunctional thin films suitable for different applications due to its advantages of simplicity, good uniformity and fast deposition speed [28].The research on zeta potential showed that WS 2 flakes can carry charges in water/ethanol solution [29].Therefore, WS 2 thin films can be prepared with the EPD technique.
In this work, a novel PEC sensor (CdS/WS 2 /ITO) was developed using CdS-nanoparticledecorated WS 2 nanosheet heterojunction thin-film-modified ITO electrodes.A simple and secure room-temperature deposition method was used to prepare the two-dimensional WS 2 nanosheet and CdS nanoparticle composites on ITO electrodes.Electrophoretic deposition was employed for the fabrication of WS 2 /ITO electrodes consisting of a few layers of WS 2 nanosheets.Next, the successive ion layer adsorption and reaction (SILAR) technique was utilized to load CdS nanoparticles onto WS 2 /ITO.The CdS/WS 2 /ITO nanocomposites obtained by this simple and well-controlled technique exhibited significant adhesion to ITO substrates and stable PEC performance.In the heterostructures, CdS served as a substance for light absorption and photocurrent generation, whereas WS 2 nanosheets served as a superb conductive matrix owing to their exceptional electron mobility.Given the advantageous presence of the heterointerface and the synergistic effect, enhanced PEC performance was ultimately achieved due to the significant facilitation of photogenerated electron migration to the electrode and the efficient strengthening of charge separation.In addition, layered WS 2 with a large specific surface area can load more CdS nanoparticles and form a large contact area, resulting in a strong, effective interfacial electric field, which promotes the charge separation to improve the optoelectronic performance.Therefore, we developed an innovative PEC biosensing platform using CdS/WS 2 nanocomposites which exhibited enhanced photocurrent response to cysteine oxidation.Based on the specific recognition between cysteine and CdS/WS 2 /ITO, through the specificity of Cd-S bonds, a light-driven photoelectrochemical sensor was fabricated for cysteine detection.The prepared biosensor exhibited excellent sensitivity, extensive linearity, exceptional selectivity and notable reproducibility in detecting cysteine.The photoelectrochemical sensing method was successfully employed to quantify cysteine levels in both human urine and blood serum.

Apparatus
A D8-Advance X-ray diffractometer (Bruker AXS, Karlsruhe, Germany), UV-2600 UV-visible spectrophotometer (Shimadzu, Kyoto, Japan), SU-8010 field emission scanning electron microscopy (Hitachi, Tokyo, Japan) and an ALPHA FT-IR infrared spectrometer (Bruker, Karlsruhe, Germany) were used to characterize the prepared nanomaterials.PEC experiments were performed on a homemade PEC system which included a 350 W xenon lamp as the irradiation source and an LK3200A electrochemical workstation (LANLIKE, Tianjin, China) for photocurrent detection.

Construction of Photoelectrochemical Sensor
Before its modification, the ITO slices (0.5 × 4 cm 2 ) were sequentially ultrasonically cleaned in acetone and ethanol for 20 min.The photoelectrochemical sensor (CdS/WS 2 /ITO) was fabricated using two well-controlled methods after some modification based on the previous literature [30,31].As the first step, WS 2 /ITO thin film was prepared via electrophoretic deposition.WS 2 suspension (1 mg/mL) was prepared by dispersing WS 2 powder in the mixed water/ethanol solution (65:35) under sonication for 30 min.Two ITO sheets arranged in parallel with a distance of 15 mm from each other were selected as anode and cathode to construct a two-electrode cell.Thus, using the prepared WS 2 suspension as the electrolyte, deposition of WS 2 thin film on the ITO cathode was realized by electrophoretic deposition at 8 V for 30 s.The obtained electrode is marked as WS 2 /ITO.As the second step, CdS nanoparticles were loaded on the prepared WS 2 /ITO via SILAR method.Firstly, the WS 2 /ITO electrode was immersed in a 0.05 M Cd (NO 3 ) 2 solution for 20 s, then it was immersed in a 0.05 M Na 2 S solution for 20 s.This process was repeated for 20 cycles to gain CdS/WS 2 /ITO.As a control, the same SILAR technology was employed to prepare a CdS/ITO electrode by depositing CdS nanoparticles on ITO.The preparation process of the CdS/WS 2 /ITO electrode is shown in Scheme 1.

Photoelectrochemical Detection of Cysteine
As shown in Scheme 2, PEC experiments were performed on a homemade PEC system which included an irradiation source as the light excitation system and an electrochemical workstation for photocurrent detection.The detection system consists of a photoelectrochemical cell composed of a traditional electrochemical three-electrode system and a data-processing system.A three-electrode cell with a saturated calomel electrode, a platinum sheet electrode and a modified ITO electrode (0.25 cm 2 ) was employed for PEC measurements.Visible light was generated by a xenon lamp with a power output of 350 W along with a UV-cut filter that blocked wavelengths below 400 nm.The distance from the working electrode to the light source remained constant at 20 cm.The current time curve method was applied to measure the photocurrent in pH 7.0 PB containing different concentrations of cysteine with an illumination interval of 20 s.During operation, the working voltage was set to 0.0 V.The experiment was conducted at ambient temperature.

Morphological and Spectroscopic Characterization of the Prepared Nanomaterials
Figure 1A,B display the surface morphology of WS2/ITO and CdS/WS2/ITO at the same magnification, respectively.From the scanning electron microscopy (SEM) image in Figure 1A, it can be observed that a dense nanosheet thin film is formed on the ITO conductive glass, which is electrodeposited from a few layers of the WS2 nanosheets.The ITO substrate is completely covered by the WS2 sheets, which have a diameter ranging from 20 to 50 nm and are distributed evenly across the surface.It can also be observed that, in certain areas of Figure 1A, the bright, white protrusion structure of the WS2 nanosheets is formed due to sufficient WS2 deposition on the surface.The sufficient deposition amount of WS2 nanosheets increases the surface roughness of the modified electrode.The SEM image of CdS/WS2/ITO in Figure 1B exhibits the uniform deposition of CdS nanoparticles on the entire thin film of the WS2 nanosheets via the SILAR method.The CdS nanoparticles are attached and loaded on the surface of WS2/ITO to form a well-distributed coating layer, suggesting the blending of CdS particles into WS2 sheets to form a stable CdS-WS2

Photoelectrochemical Detection of Cysteine
As shown in Scheme 2, PEC experiments were performed on a homemade PEC system which included an irradiation source as the light excitation system and an electrochemical workstation for photocurrent detection.The detection system consists of a photoelectrochemical cell composed of a traditional electrochemical three-electrode system and a data-processing system.A three-electrode cell with a saturated calomel electrode, a platinum sheet electrode and a modified ITO electrode (0.25 cm 2 ) was employed for PEC measurements.Visible light was generated by a xenon lamp with a power output of 350 W along with a UV-cut filter that blocked wavelengths below 400 nm.The distance from the working electrode to the light source remained constant at 20 cm.The current time curve method was applied to measure the photocurrent in pH 7.0 PB containing different concentrations of cysteine with an illumination interval of 20 s.During operation, the working voltage was set to 0.0 V.The experiment was conducted at ambient temperature.

Photoelectrochemical Detection of Cysteine
As shown in Scheme 2, PEC experiments were performed on a homemade PEC sy tem which included an irradiation source as the light excitation system and an electr chemical workstation for photocurrent detection.The detection system consists of a ph toelectrochemical cell composed of a traditional electrochemical three-electrode syste and a data-processing system.A three-electrode cell with a saturated calomel electrode platinum sheet electrode and a modified ITO electrode (0.25 cm 2 ) was employed for PE measurements.Visible light was generated by a xenon lamp with a power output of 3 W along with a UV-cut filter that blocked wavelengths below 400 nm.The distance fro the working electrode to the light source remained constant at 20 cm.The current tim curve method was applied to measure the photocurrent in pH 7.0 PB containing differe concentrations of cysteine with an illumination interval of 20 s.During operation, t working voltage was set to 0.0 V.The experiment was conducted at ambient temperatu Scheme 2. Schematic diagram of the detection process of the photoelectrochemical sensor.

Morphological and Spectroscopic Characterization of the Prepared Nanomaterials
Figure 1A,B display the surface morphology of WS2/ITO and CdS/WS2/ITO at t same magnification, respectively.From the scanning electron microscopy (SEM) image Figure 1A, it can be observed that a dense nanosheet thin film is formed on the ITO co ductive glass, which is electrodeposited from a few layers of the WS2 nanosheets.The IT substrate is completely covered by the WS2 sheets, which have a diameter ranging fro 20 to 50 nm and are distributed evenly across the surface.It can also be observed that, certain areas of Figure 1A, the bright, white protrusion structure of the WS2 nanosheets formed due to sufficient WS2 deposition on the surface.The sufficient deposition amou of WS2 nanosheets increases the surface roughness of the modified electrode.The SE image of CdS/WS2/ITO in Figure 1B exhibits the uniform deposition of CdS nanoparticl on the entire thin film of the WS2 nanosheets via the SILAR method.The CdS nanopar cles are attached and loaded on the surface of WS2/ITO to form a well-distributed coati layer, suggesting the blending of CdS particles into WS2 sheets to form a stable CdS-W Scheme 2. Schematic diagram of the detection process of the photoelectrochemical sensor.

Morphological and Spectroscopic Characterization of the Prepared Nanomaterials
Figure 1A,B display the surface morphology of WS 2 /ITO and CdS/WS 2 /ITO at the same magnification, respectively.From the scanning electron microscopy (SEM) image in Figure 1A, it can be observed that a dense nanosheet thin film is formed on the ITO conductive glass, which is electrodeposited from a few layers of the WS 2 nanosheets.The ITO substrate is completely covered by the WS 2 sheets, which have a diameter ranging from 20 to 50 nm and are distributed evenly across the surface.It can also be observed that, in certain areas of Figure 1A, the bright, white protrusion structure of the WS 2 nanosheets is formed due to sufficient WS 2 deposition on the surface.The sufficient deposition amount of WS 2 nanosheets increases the surface roughness of the modified electrode.The SEM image of CdS/WS 2 /ITO in Figure 1B exhibits the uniform deposition of CdS nanoparticles on the entire thin film of the WS 2 nanosheets via the SILAR method.The CdS nanoparticles are attached and loaded on the surface of WS 2 /ITO to form a well-distributed coating layer, suggesting the blending of CdS particles into WS 2 sheets to form a stable CdS-WS 2 heterostructure.In addition, SEM-EDS was performed to confirm the composition of the obtained nanohybrids.The EDS spectra and atomic percentages of the elements (Figure S1) reveal the presence of Cd, S and W elements, indicating that the CdS/WS 2 /ITO film contains stoichiometric WS 2 sheets and CdS nanoparticles.
Nanomaterials 2024, 14, x FOR PEER REVIEW 5 of 14 heterostructure.In addition, SEM-EDS was performed to confirm the composition of the obtained nanohybrids.The EDS spectra and atomic percentages of the elements (Figure S1) reveal the presence of Cd, S and W elements, indicating that the CdS/WS2/ITO film contains stoichiometric WS2 sheets and CdS nanoparticles.106), (110), (008), ( 112) and (114) planes (JCPDS no.87-2417) [32].Among them, the diffraction peak corresponding to the (002) crystal plane represents the vertical stacking of multilayer materials along the caxis.The two peaks corresponding to (100) and (110) indicate that WS2 has a two-dimensional nanosheet structure [33], which is consistent with the morphology of WS2 material.The XRD pattern of CdS (Figure 2A curve b) shows three diffraction peaks at 26.5°, 44.2° and 51.8°, indicating that the CdS nanoparticles have a cubic phase structure.These peaks correspond to the (111), ( 220) and (311) crystal planes of cubic CdS (JCPDS no.10-0454) [34].It can also be observed that the peaks related to cubic CdS are relatively broad, indicating that the size of CdS nanoparticles is very small.In contrast, the XRD spectrum of CdS/WS2 composite material exhibits all the diffraction peaks of pure CdS and an additional peak at 14.4° (Figure 2A, curve c).This peak corresponds to the (002) crystal plane of WS2 [32], suggesting the successful loading of CdS nanoparticles on WS2 nanosheets.However, no diffraction peaks at other positions of WS2 are observed in curve c, indicating that these small diffraction peaks of WS2 are masked when combined with CdS.The above result further proves that the WS2 nanosheets were covered with CdS nanoparticles to form CdS/WS2 nanocomposites.
Figure 2B shows the FT-IR spectra of the WS2 nanosheets, CdS nanoparticles and CdS-WS2 heterojunction.In curve a, pristine WS2 displays its characteristic absorption peaks at 438, 624, 929, 981 and 1120 cm −1 , corresponding to the stretching vibrations of W-S bonds and S-S bonds [35].For CdS, the infrared absorption peak at 693 cm −1 is attributed to the stretching vibration of CdS (curve b) [34].Both WS2 and CdS have strong absorption peaks around the range of 3200-3550 cm −1 , which is the stretching vibration of -OH in water molecules [36].By comparison, both the infrared characteristic absorption peaks of WS2 and CdS appear in curve c, which confirms the successful attachment of CdS to the surface of WS2.114) planes (JCPDS no.87-2417) [32].Among them, the diffraction peak corresponding to the (002) crystal plane represents the vertical stacking of multilayer materials along the c-axis.The two peaks corresponding to (100) and (110) indicate that WS 2 has a two-dimensional nanosheet structure [33], which is consistent with the morphology of WS 2 material.The XRD pattern of CdS (Figure 2A 220) and (311) crystal planes of cubic CdS (JCPDS no.10-0454) [34].It can also be observed that the peaks related to cubic CdS are relatively broad, indicating that the size of CdS nanoparticles is very small.In contrast, the XRD spectrum of CdS/WS 2 composite material exhibits all the diffraction peaks of pure CdS and an additional peak at 14.4 • (Figure 2A, curve c).This peak corresponds to the (002) crystal plane of WS 2 [32], suggesting the successful loading of CdS nanoparticles on WS 2 nanosheets.However, no diffraction peaks at other positions of WS 2 are observed in curve c, indicating that these small diffraction peaks of WS 2 are masked when combined with CdS.The above result further proves that the WS 2 nanosheets were covered with CdS nanoparticles to form CdS/WS 2 nanocomposites.UV-Vis diffuse reflectance spectroscopy (DRS) was applied for optical performance analysis.The DRS results in Figure 2C demonstrate that all samples had light absorption in the visible light range.The bandgap absorption edge of CdS was at 610 nm (curve b), while WS2 exhibited almost complete absorption in the visible region (curve a) [37].The UV-Vis absorption spectrum of WS2 nanosheets in a mixture of water and ethanol is shown in Figure S2.The spectra curve further proves that WS2 exhibited a significant absorption in the visible light region with an absorption maximum at 640 nm [38], which is consistent with the results obtained by the UV-Vis diffuse reflectance spectroscopy (curve a).After modification of the CdS nanoparticles, the optical absorption edge of CdS-WS2 nanocomposites was red-shifted to 650 nm.Moreover, in the whole visible region, the heterojunction showed stronger absorption intensity compared to pure CdS (curve c).The energy bandgap (Eg) was calculated from Tauc plot.The Eg values were 1.83 eV for bare WS2, 2.42 eV for bare CdS and 2.26 eV for CdS-WS2 nanocomposites, which are consistent with the results reported in the literature [21,30].The above results reveal that the composite material possesses a higher utilization rate of visible light and outstanding optical properties, which improves its photocatalytic ability.
The elemental composition of CdS-WS2 nanocomposite was evaluated using X-ray photoelectron spectroscopy (XPS).The XPS spectrum of CdS-WS2 in Figure 2D confirms the presence of W, S and Cd elements in the CdS-WS2 hybrid, which is completely consistent with the results of the FT-IR spectroscopy, indicating the successful preparation of CdS-WS2 nanocomposites.

Photoelectrochemical Performance of the Sensor
The PEC properties of the prepared sensor were investigated by measuring the photocurrent on different modified electrodes at a working voltage of 0.0 V under visible light illumination.Figure 3 shows that WS2/ITO and CdS/ITO had a weak photocurrent of 0.06 μA (curve a) and 0.16 μA (curve b), respectively, which can be attributed to the fact that the single photosensitive material exhibits insufficient separation of electrons and holes and the high recombination rate of photogenerated charges under visible light irradiation.In a great comparison, the CdS/WS2/ITO electrode produced an obvious ascended photocurrent of 0.35 μA (curve c), which was an approximately 6-fold increase compared to that of WS2/ITO and a 2-fold increase compared to that of CdS/ITO.This demonstrates that the CdS-WS2 heterostructure can effectively inhibit the recombination of charge carriers, Figure 2B shows the FT-IR spectra of the WS 2 nanosheets, CdS nanoparticles and CdS-WS 2 heterojunction.In curve a, pristine WS 2 displays its characteristic absorption peaks at 438, 624, 929, 981 and 1120 cm −1 , corresponding to the stretching vibrations of W-S bonds and S-S bonds [35].For CdS, the infrared absorption peak at 693 cm −1 is attributed to the stretching vibration of CdS (curve b) [34].Both WS 2 and CdS have strong absorption peaks around the range of 3200-3550 cm −1 , which is the stretching vibration of -OH in water molecules [36].By comparison, both the infrared characteristic absorption peaks of WS 2 and CdS appear in curve c, which confirms the successful attachment of CdS to the surface of WS 2 .
UV-Vis diffuse reflectance spectroscopy (DRS) was applied for optical performance analysis.The DRS results in Figure 2C demonstrate that all samples had light absorption in the visible light range.The bandgap absorption edge of CdS was at 610 nm (curve b), while WS 2 exhibited almost complete absorption in the visible region (curve a) [37].The UV-Vis absorption spectrum of WS 2 nanosheets in a mixture of water and ethanol is shown in Figure S2.The spectra curve further proves that WS 2 exhibited a significant absorption in the visible light region with an absorption maximum at 640 nm [38], which is consistent with the results obtained by the UV-Vis diffuse reflectance spectroscopy (curve a).After modification of the CdS nanoparticles, the optical absorption edge of CdS-WS 2 nanocomposites was red-shifted to 650 nm.Moreover, in the whole visible region, the heterojunction showed stronger absorption intensity compared to pure CdS (curve c).The energy bandgap (Eg) was calculated from Tauc plot.The Eg values were 1.83 eV for bare WS 2 , 2.42 eV for bare CdS and 2.26 eV for CdS-WS 2 nanocomposites, which are consistent with the results reported in the literature [21,30].The above results reveal that the composite material possesses a higher utilization rate of visible light and outstanding optical properties, which improves its photocatalytic ability.
The elemental composition of CdS-WS 2 nanocomposite was evaluated using X-ray photoelectron spectroscopy (XPS).The XPS spectrum of CdS-WS 2 in Figure 2D confirms the presence of W, S and Cd elements in the CdS-WS 2 hybrid, which is completely consistent with the results of the FT-IR spectroscopy, indicating the successful preparation of CdS-WS 2 nanocomposites.

Photoelectrochemical Performance of the Sensor
The PEC properties of the prepared sensor were investigated by measuring the photocurrent on different modified electrodes at a working voltage of 0.0 V under visible light illumination.Figure 3 shows that WS 2 /ITO and CdS/ITO had a weak photocurrent of 0.06 µA (curve a) and 0.16 µA (curve b), respectively, which can be attributed to the fact that the single photosensitive material exhibits insufficient separation of electrons and holes and the high recombination rate of photogenerated charges under visible light irradiation.In a great comparison, the CdS/WS 2 /ITO electrode produced an obvious ascended photocurrent of 0.35 µA (curve c), which was an approximately 6-fold increase compared to that of WS 2 /ITO and a 2-fold increase compared to that of CdS/ITO.This demonstrates that the CdS-WS 2 heterostructure can effectively inhibit the recombination of charge carriers, resulting in significantly improved photoelectric conversion efficiency and enhanced photoelectrochemical activity.As seen in curve d, the CdS/WS 2 /ITO electrode showed an enhanced photocurrent of 0.93 µA when 10 µM cysteine was present, indicating that cysteine as an electron donor can effectively promote the separation of electron-hole pairs, which leads to further ascent of the photocurrent response.
Nanomaterials 2024, 14, x FOR PEER REVIEW 8 of 14 resulting in significantly improved photoelectric conversion efficiency and enhanced photoelectrochemical activity.As seen in curve d, the CdS/WS2/ITO electrode showed an enhanced photocurrent of 0.93 μA when 10 μM cysteine was present, indicating that cysteine as an electron donor can effectively promote the separation of electron-hole pairs, which leads to further ascent of the photocurrent response.The PEC activities of the nanostructures are greatly influenced by their electrical characteristics.Electrochemical impedance spectroscopy (EIS) was conducted to investigate the interface properties of electrodes.The corresponding data and figures are provided in the Supplementary Materials.All of the EIS data (Figure S3) are in good agreement with the photocurrent change tendency in the current-time curves described above, further demonstrating that the sensing interface had been successfully constructed.
A schematic diagram of CdS/WS2/ITO for cysteine detection is presented in Scheme 3.Under visible light illumination, both CdS and WS2 can produce photogenerated charges.During the conversion of photocurrents, the electrons on the valence band (VB) of CdS are firstly excited to the conduction band (CB) of CdS to generate electron-hole pairs after absorbing visible light.Afterwards, the excitation electrons on the CB of CdS swiftly transfer to the CB of WS2 due to the presence of a heterointerface and well-matched energy levels.Together with the photogenerated electrons generated inside WS2, electrons on the CB of WS2 subsequently inject into the ITO electrode for the formation of a current in the external circuit.The separated holes on the VB of WS2 can transfer to the VB of CdS nanoparticles owing to energy level matching.Then, electron donor cysteine in PB captures the separated holes on the VB of CdS, resulting in the oxidization of cysteine.The consumption of the photogenerated holes by cysteine effectively suppresses charge recombination, leading to an enhanced photocurrent response in the existence of cysteine.The improvement of the photoelectrochemical properties of the PEC sensor is attributed to several factors as follows: (1) The formation of the surface heterojunctions and the effective interfacial electric field in CdS-WS2 nanocomposites can promote the charge separation and slow down the electron-hole recombination.This heterojunction architecture can protect CdS against photocorrosion, leading to a stable photocurrent.(2) The ITO elec- The PEC activities of the nanostructures are greatly influenced by their electrical characteristics.Electrochemical impedance spectroscopy (EIS) was conducted to investigate the interface properties of electrodes.The corresponding data and figures are provided in the Supplementary Materials.All of the EIS data (Figure S3) are in good agreement with the photocurrent change tendency in the current-time curves described above, further demonstrating that the sensing interface had been successfully constructed.
A schematic diagram of CdS/WS 2 /ITO for cysteine detection is presented in Scheme 3.Under visible light illumination, both CdS and WS 2 can produce photogenerated charges.During the conversion of photocurrents, the electrons on the valence band (VB) of CdS are firstly excited to the conduction band (CB) of CdS to generate electron-hole pairs after absorbing visible light.Afterwards, the excitation electrons on the CB of CdS swiftly transfer to the CB of WS 2 due to the presence of a heterointerface and well-matched energy levels.Together with the photogenerated electrons generated inside WS 2 , electrons on the CB of WS 2 subsequently inject into the ITO electrode for the formation of a current in the external circuit.The separated holes on the VB of WS 2 can transfer to the VB of CdS nanoparticles owing to energy level matching.Then, electron donor cysteine in PB captures the separated holes on the VB of CdS, resulting in the oxidization of cysteine.The consumption of the photogenerated holes by cysteine effectively suppresses charge recombination, leading to an enhanced photocurrent response in the existence of cysteine.The improvement of the photoelectrochemical properties of the PEC sensor is attributed to several factors as follows: (1) The formation of the surface heterojunctions and the effective interfacial electric field in CdS-WS 2 nanocomposites can promote the charge separation and slow down the electron-hole recombination.This heterojunction architecture can protect CdS against photocorrosion, leading to a stable photocurrent.(2) The ITO electrode modified with the thin WS 2 layer enhances the surface roughness, allowing for increased loading of CdS particles and more uniform coverage.Compared to CdS without a WS 2 layer, this results in an improved light absorption capacity and more oxidation sites.(3) The WS 2 nanosheet has excellent conductivity which can bring about rapid charge transport.The mechanism of charge transfer further confirms that the CdS-WS 2 heterostructure exhibits distinctly enhanced PEC activity, contributing to a higher separation efficiency of photoexcited charges, faster migration rate of electrons and an excellent synergistic effect.

Optimization of Experimental Conditions
Various experimental parameters were optimized to enhance PEC response and improve detection sensitivity, including electrophoretic deposition time of WS2, SILAR deposition cycles of CdS nanoparticles and applied potential.The corresponding data and figures are provided in the Supplementary Materials (Figure S4).The best results were obtained under the following experimental conditions: (a) electrophoretic deposition time: 30 s; (b) SILAR deposition cycles: 20 cycles; (c) applied potential: 0.0 V.

Photoelectrochemical Sensing of Cysteine
Figure 4A illustrates the photocurrent responses of the PEC sensor in different concentrations of cysteine under the optimum conditions, in which the photocurrent gradually increased along with the concentration.The sensor had a linear response in the 0.07 to 300 μM cysteine concentration range with a detection limit of 5.29 nM based on 3σ/S.As shown in Figure 4B, the regression equation was I (μA) = 0.3954 + 0.05294 c (μM) (R = 0.9973) in the concentration range of 0.07-20 μM and I (μA) = 1.2659 + 0.0081c (μM) (R = 0.9983) in the concentration range of 20-300 μM.In comparison to previously reported PEC methods, the PEC sensor described in the present work possesses a wider linear response range and higher sensitivity, demonstrating its exceptional analytical performance in detecting cysteine (Table 1).Furthermore, the proposed PEC sensor showed the outstanding advantages of simple structure, convenient operation, economy and fast detection.

Optimization of Experimental Conditions
Various experimental parameters were optimized to enhance PEC response and improve detection sensitivity, including electrophoretic deposition time of WS 2 , SILAR deposition cycles of CdS nanoparticles and applied potential.The corresponding data and figures are provided in the Supplementary Materials (Figure S4).The best results were obtained under the following experimental conditions: (a) electrophoretic deposition time: 30 s; (b) SILAR deposition cycles: 20 cycles; (c) applied potential: 0.0 V.

Photoelectrochemical Sensing of Cysteine
Figure 4A illustrates the photocurrent responses of the PEC sensor in different concentrations of cysteine under the optimum conditions, in which the photocurrent gradually increased along with the concentration.The sensor had a linear response in the 0.07 to 300 µM cysteine concentration range with a detection limit of 5.29 nM based on 3σ/S.As shown in Figure 4B, the regression equation was I (µA) = 0.3954 + 0.05294 c (µM) (R = 0.9973) in the concentration range of 0.07-20 µM and I (µA) = 1.2659 + 0.0081c (µM) (R = 0.9983) in the concentration range of 20-300 µM.In comparison to previously reported PEC methods, the PEC sensor described in the present work possesses a wider linear response range and higher sensitivity, demonstrating its exceptional analytical performance in detecting cysteine (Table 1).Furthermore, the proposed PEC sensor showed the outstanding advantages of simple structure, convenient operation, economy and fast detection.

Reproducibility, Stability and Selectivity
To assess the reproducibility, six sensors were prepared independently and applied for detecting cysteine at a concentration of 10 μM.The present evaluation yielded an RSD of 4.34%, giving favorable repeatability.To examine the stability of the sensor, the photocurrent responses of the same concentration of cysteine on the same sensor were measured 10 times (Figure 5A).The RSD of 2.89% suggests that the current response has good

Reproducibility, Stability and Selectivity
To assess the reproducibility, six sensors were prepared independently and applied for detecting cysteine at a concentration of 10 µM.The present evaluation yielded an RSD of 4.34%, giving favorable repeatability.To examine the stability of the sensor, the photocurrent responses of the same concentration of cysteine on the same sensor were measured 10 times (Figure 5A).The RSD of 2.89% suggests that the current response has good stability.This was attributed to the fact that the CdS-WS 2 heterostructure can protect CdS against photocorrosion to achieve a stable photocurrent.To test the reusability of the sensor, 10 µM cysteine was measured by the same sensor ten times.The RSD of the photocurrent was 3.31%, indicating that the PEC sensor has desirable reusability.
An interference experiment was conducted to evaluate the specificity of the sensor towards its target analyte.As displayed in Figure 5B, eleven common interferences, lysine (Lys), histidine (His), phenylalanine (Phe), tyrosine (Tyr), glutamic acid (Glu), ascorbic acid (AA), uric acid (UA), glucose, dopamine (DA), glutathione (GSH) and nicotinamide adenine dinucleotide (NADH), had no significant influence on the photocurrent response of cysteine.The mixed solution of all interfering substances and cysteine brough out a negligible photocurrent change compared to the detection signal generated by cysteine alone.The above results suggest that the proposed method has excellent selectivity for detecting cysteine owing to the specific recognition between cysteine and CdS/WS2/ITO through the specificity of Cd-S bonds.

Application in Real Sample Analysis
To assess the feasibility of the PEC sensor in detecting actual samples, the amount of cysteine in human urine and serum was measured.Human serum samples collected from a community hospital were diluted by 30 fold for detection of the photocurrent response of the PEC sensor.Human urine samples obtained from two healthy volunteers were immediately detected after a 20-fold sample dilution.The standard addition technique was used for quantitative analysis.The results in Table 2 display that the average recoveries varied from 98.3 to 102.1%, demonstrating that the developed strategy can be successfully used for complex real sample analysis.The cysteine concentrations in urine and serum detected via this method were both reasonably consistent with the results reported in the literature [46], definitely indicating the method's practicability in clinical fluids detection.An interference experiment was conducted to evaluate the specificity of the sensor towards its target analyte.As displayed in Figure 5B, eleven common interferences, lysine (Lys), histidine (His), phenylalanine (Phe), tyrosine (Tyr), glutamic acid (Glu), ascorbic acid (AA), uric acid (UA), glucose, dopamine (DA), glutathione (GSH) and nicotinamide adenine dinucleotide (NADH), had no significant influence on the photocurrent response of cysteine.The mixed solution of all interfering substances and cysteine brough out a negligible photocurrent change compared to the detection signal generated by cysteine alone.The above results suggest that the proposed method has excellent selectivity for detecting cysteine owing to the specific recognition between cysteine and CdS/WS 2 /ITO through the specificity of Cd-S bonds.

Application in Real Sample Analysis
To assess the feasibility of the PEC sensor in detecting actual samples, the amount of cysteine in human urine and serum was measured.Human serum samples collected from a community hospital were diluted by 30 fold for detection of the photocurrent response of the PEC sensor.Human urine samples obtained from two healthy volunteers were immediately detected after a 20-fold sample dilution.The standard addition technique was used for quantitative analysis.The results in Table 2 display that the average recoveries varied from 98.3 to 102.1%, demonstrating that the developed strategy can be successfully used for complex real sample analysis.The cysteine concentrations in urine and serum detected via this method were both reasonably consistent with the results reported in the literature [46], definitely indicating the method's practicability in clinical fluids detection.

Scheme 1 .
Scheme 1. Schematic diagram of CdS/WS2/ITO fabricated via a coupling technique of EPD and SI-LAR methods.

Scheme 2 .
Scheme 2. Schematic diagram of the detection process of the photoelectrochemical sensor.

Scheme 1 .
Scheme 1. Schematic diagram of CdS/WS 2 /ITO fabricated via a coupling technique of EPD and SILAR methods.

Scheme 1 .
Scheme 1. Schematic diagram of CdS/WS2/ITO fabricated via a coupling technique of EPD and LAR methods.

Figure 1 .
Figure 1.SEM image of (A) WS2/ITO and (B) CdS/WS2/ITO.The X-ray diffraction technique was employed to investigate the composition and structure.The XRD results in Figure2Awere collected on CdS/WS2 nanocomposites scraped from CdS/WS2/ITO using a sharp blade.The crystal structure of the WS2 nanosheets (Figure2A, curve a) shows the characteristic diffraction peaks of WS2 at 14.4°, 28.9°, 32.5°, 33.7°, 35.4°, 39.2°, 43.4°, 49.6°, 53.2°, 57.2°, 59.2°, 61.0° and 65.1°, which are related to the (002), (004), (100), (101), (102), (103), (006), (105), (106), (110), (008), (112) and (114) planes (JCPDS no.87-2417)[32].Among them, the diffraction peak corresponding to the (002) crystal plane represents the vertical stacking of multilayer materials along the caxis.The two peaks corresponding to (100) and (110) indicate that WS2 has a two-dimensional nanosheet structure[33], which is consistent with the morphology of WS2 material.The XRD pattern of CdS (Figure2Acurve b) shows three diffraction peaks at 26.5°, 44.2° and 51.8°, indicating that the CdS nanoparticles have a cubic phase structure.These peaks correspond to the (111), (220) and (311) crystal planes of cubic CdS (JCPDS no.10-0454)[34].It can also be observed that the peaks related to cubic CdS are relatively broad, indicating that the size of CdS nanoparticles is very small.In contrast, the XRD spectrum of CdS/WS2 composite material exhibits all the diffraction peaks of pure CdS and an additional peak at 14.4° (Figure2A, curve c).This peak corresponds to the (002) crystal plane of WS2[32], suggesting the successful loading of CdS nanoparticles on WS2 nanosheets.However, no diffraction peaks at other positions of WS2 are observed in curve c, indicating that these small diffraction peaks of WS2 are masked when combined with CdS.The above result further proves that the WS2 nanosheets were covered with CdS nanoparticles to form CdS/WS2 nanocomposites.Figure2Bshows the FT-IR spectra of the WS2 nanosheets, CdS nanoparticles and CdS-WS2 heterojunction.In curve a, pristine WS2 displays its characteristic absorption peaks at 438, 624, 929, 981 and 1120 cm −1 , corresponding to the stretching vibrations of W-S bonds and S-S bonds[35].For CdS, the infrared absorption peak at 693 cm −1 is attributed to the stretching vibration of CdS (curve b)[34].Both WS2 and CdS have strong absorption peaks around the range of 3200-3550 cm −1 , which is the stretching vibration of -OH in water molecules[36].By comparison, both the infrared characteristic absorption peaks of WS2 and CdS appear in curve c, which confirms the successful attachment of CdS to the surface of WS2.
curve b) shows three diffraction peaks at 26.5 • , 44.2 • and 51.8 • , indicating that the CdS nanoparticles have a cubic phase structure.These peaks correspond to the (111), (

Nanomaterials 2024 ,Scheme 3 .
Scheme 3. The charge transport pathway and detection principle of PEC cysteine sensor.

Scheme 3 .
Scheme 3. The charge transport pathway and detection principle of PEC cysteine sensor.

Figure 5 .
Figure 5. (A) Time-based photocurrent response of the sensor under ten on/off irradiation cycles.(B) Photocurrent responses of the sensor towards 10 μM of cysteine and 50 μM of other interference substances.

Figure 5 .
Figure 5. (A) Time-based photocurrent response of the sensor under ten on/off irradiation cycles.(B) Photocurrent responses of the sensor towards 10 µM of cysteine and 50 µM of other interference substances.

Table 1 .
Comparison of cysteine assay by various reported PEC sensors.

Table 1 .
Comparison of cysteine assay by various reported PEC sensors.

Table 2 .
Real sample analysis and recovery rate test in human serum and urine samples (n = 6).

Table 2 .
Real sample analysis and recovery rate test in human serum and urine samples (n = 6).