Exploring the Impact of Structural and Physical Properties of Layered Molybdenum Disulfide on Electrochemical Characteristics and The Sensitive Response to Chloramphenicol Detection

All chemicals used in this study, such as K₃[Fe(CN)₆], K₄[Fe(CN)₆]•3H₂O (98%), chloramphenicol (CAP, >99%), and molybdenum (IV) sulfide (MoS₂) (98%, <2 μm) were obtained from Sigma-Aldrich (USA). NaCl (99%), KCl (>98%), Na₂HPO₄, and KH₂PO₄ (99%), were of reagent grade and purchased from Sigma-Aldrich, Merck KgaA (USA), and Xilong Scientific Co., Ltd. (Shantou, China), were also used to prepare phosphate buffer saline (PBS-pH 7.4) as a potential supporting electrolyte. The pH value of this buffer solution was adjusted by using 0.1 M of H₃PO₄ and NaOH (>98% was obtained from Xilong Scientific Co., Ltd. China) solutions by IC-PH60 pH tester kit. Furthermore, the commercial screen-printed electrodes (SPEs) utilized, herein, were provided by DS Dropsens, Spain.

Preparation of MoS₂ nanosheets

Preparation of MoS₂/SPEs

Shrimp sample preparation

Physical measurements

Evaluation of electrochemical characterizations

Figure 1a displays the crystallinity and phase purity of the synthesized MoS₂ and bulk MoS₂ samples. As can be seen, the recorded XRD patterns clearly showed the formation of MoS₂ nanosheets resulting from the reduction in the (002) diffraction plane located at 14.4^o in all prepared MoS₂ samples, compared to the bulk MoS₂ sample. Both MoS₂-1, MoS₂-2, and MoS₂-3 samples exhibited characteristic peaks of (002), (100), (103), (104), (105), and (008), reflecting the pure hexagonal phase of the MoS₂ sample. ^5,11 These obtained results matched well with the previous reports. It is worth mentioning that the relative 2θ position of the characteristic diffraction peaks remained virtually unchanged with the changes in the prepared experimental condition. Furthermore, there were no extra peaks corresponding to impurity phases, confirming all proposed MoS₂ samples owned good crystallinity and high purity. ^12,18,19 Besides, the decrease in intensity of high-indexed diffraction peaks showed the short-range structural distribution of prepared MoS₂ samples, particularly for MoS₂-2 and MoS₂-3 samples.

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Raman spectroscopy is also considered as a powerful tool to study crystalline structure and degree of defects. Herein, under ambient pressure conditions, all Raman patterns of crystalline MoS₂ clearly showed the transformation of two internal lattice vibrations around 387 cm⁻¹ and 413 cm⁻¹, respectively. This result further confirmed the high purity and good crystalline structure of proposed MoS₂ samples. In which, the in-plane E¹ _2g mode results from opposite motions of two S atoms with respect to the Mo atoms. Also, the A_1g mode is associated with the out-of-plane relative vibration of only S atoms in opposite directions. ^20–23 As can be seen, both E¹ _2g and A_1g modes red shifted with the change in synthetic conditions. Furthermore, another Raman peak was observed in MoS₂ bulk at 460 cm⁻¹ owing to strong electron-phonon couplings, which arise from a second-order process relating to the longitudinal acoustic phonons at M point (LA(M)). ²² It is worth noting that the position of characteristic peaks directly depended on the number of layers as reported by the literature. ²⁴ According to that, the increase in the layer number led to increase the remarkable difference in frequency (Δω) between two distinct characteristic peaks (E¹ _2g and A_1g modes). Many reports demonstrated that when the frequency difference is approximately 20 cm⁻¹, the MoS₂ structure is monolayer. ^25,26 As seen in Fig. 1b, the decreasing trend in frequency difference (Δω) at all MoS₂ samples was observed, namely from 25 cm⁻¹ in MoS₂ bulk to 22 cm⁻¹ in MoS₂-2 and MoS₂-3 samples. The trend further confirms the change in the number of structural layers closely depended on synthetic conditions. Based on the estimation, the MoS₂ bulk and MoS₂-1 samples should have more than 4 monolayers of MoS₂ due to the Δω of 25 and 26 cm⁻¹, while for MoS₂-2 and MoS₂-3 samples, Δω is 22 cm^–1 for bilayer structure. Furthermore, some reports illustrated that the layer thickness was dependent on the intensity and intensity ratio of E¹ _2g and A_1g modes. ^22,27 The upshift of A_1g and downshift of E¹ _2g with the increasing thickness was considered due to the increase in interlayer interactions and dielectric screening of the long-range Coulomb forces. ²⁴ According to that, the peak intensities of E¹ _2g and A_1g modes increased significantly from 1 layer to 2 layers and reached the lowest value at MoS₂ bulk. Also, the highest peak intensity ratio between E¹ _2g and A_1g was recorded at 1 layer, and the intensity ratio dropped at 2 layers and MoS₂ bulk. Indeed, in this case, the intensity ratio was determined as follows: MoS₂-3 (1.74) > MoS₂-2 (1.45) > MoS₂-1 (1.08) > MoS₂ bulk (1.08), indicating the change of synthetic conditions resulting in the difference in intrinsic thickness within MoS₂ structure. More interestingly, the shift of characteristic peaks at prepared MoS₂ samples also exhibited changes in local electron density, strain, and interfacial interaction. Namely, the E¹ _2g peaks of all samples red shifted from 387 cm⁻¹ to 391 cm⁻¹ whereas the position of the A_1g peak was almost unchanged as shown in Fig. 1b, demonstrating the formation of defect configuration such as S-vacancies via breaking the S–Mo–S bonds as well the equivalent status of local electron densities at basal plane and edge. ²⁸

More evidence of the interaction between prepared MoS₂ samples was observed from the obtained results of FT-IR (Fig. 1c). For all the spectra of MoS₂ samples, the typical characteristic peak at ∼1622 cm⁻¹ was assigned S–S bonds. ⁵ Particularly, for MoS₂ prepared by various synthetic conditions (MoS₂-1, MoS₂-2, and MoS₂-3 samples), the presence of a new emerging peak around 3400 cm⁻¹ was clearly recorded, suggesting strong stretching vibration of the OH-adsorbed functional groups via interaction between MoS₂ samples and water. ⁵ It is well-known that the hydrophilic group can support to promote the hydrophilicity of materials and enhance contactability with different solvents.

Field-emission scanning electron microscopy (FESEM) image (Fig. 2) clearly revealed the sheet morphology. The average size of the sheets at all as-prepared samples was about 2–4 μm, and obvious distribution can be observed. For bulk MoS₂ and MoS₂-1 samples, their sheets overlapped or coalesced together to create exfoliated and wrinkled layers. However, for MoS₂-2 and MoS₂-3 samples, their distribution became more homogeneous and less aggregate, and many sheets were exfoliated effectively. The obtained results further confirmed that the change in the synthetic conditions of MoS₂ created remarkable differences in morphology, size, intrinsic thickness, and the formation of defect configuration.

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The electrochemical properties of the modified electrodes were investigated via CV and EIS measurements using a standard redox probe, [Fe(CN) ₆ ] ^3−/4− . Figure 3a presents the CV curves of bare SPE and electrodes modified with MoS₂ base, MoS₂-1, MoS₂-2, and MoS₂-3 in 0.1 M KCl containing 5 mM [Fe(CN) ₆ ] ^3−/4− (at a scan rate of 50 mV s⁻¹). All CV curves exhibited a characteristic pair of oxidation/reduction peaks, suggesting to the electron transfer reactions of Fe²⁺ reversibly into Fe³⁺ cations. In addition, the current response intensity at each modified electrode was remarkably increased compared with bare SPE. Indeed, the reduction peak currents achieved about 115.7, 134.33, 160.43, 170.39, and 178.16 μA for bare SPE, MoS₂ base/SPE, MoS₂-1/SPE, MoS₂-2/SPE, and MoS₂-3/SPE, corresponding to increase of approximately 16.1, 75.9, 86.9, and 95.39%, respectively. Particularly among them, MoS₂-3/SPE possessed the highest response current intensity.

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EIS measurements were performed to investigate the charge transfer capacity of the modified electrodes. Figure 3b shows the Nyquist plots of SPE, MoS₂ base/SPE, MoS₂-1/SPE, MoS₂-2/SPE, and MoS₂-3/SPE in the electrolyte solution of 0.1 M KCl at the frequency range from 50 kHz to 0.01 Hz. For the bare SPE, it possessed the largest semicircle diameter, corresponding to a R_ct of 11.25 Ω by fitting well to the Randles circuit as Fig. 3c, by contrast, it reduced to 8.67, 6.63, and 5.05 Ω upon modification with MoS₂ base, MoS₂-1, MoS₂-2, and achieved the lowest value of 4.34 Ω at MoS₂-3/SPE, respectively. Obviously, the conductivity of the proposed electrodes was higher remarkably than that of the bare SPE electrode.

In the more detailed study on the change of CV curves at all modified electrodes under various scan rates from 20 to 70 mV s⁻¹ in 0.1 M KCl containing 5 mM [Fe(CN) ₆ ] ^3−/4− (Figs. 4a–4c') was carried out. Their corresponding linear plots were recorded between peak current (I_p-μA) and sqrt of the scan rate (ν^1/2), suggesting the redox reactions onto the electrode surface was a diffusion-controlled process. To study in detail the reason for the enhancement of the current intensity, the electroactive surface area (EASA) value was estimated based on the Randles–Sevick equation:

$$\begin{eqnarray*}&&{{\rm{I}}}_{{\rm{p}}}=2.69\times {10}^{5}{\rm{A}}\,{{\rm{n}}}^{3/2}{{\rm{D}}}^{1/2}{{\rm{C}}{\rm{\nu }}}^{1/2}\end{eqnarray*}$$

where I_p is the peak current intensity, n is the number of electrons transferred (n = 1), D is the diffusion coefficient of [Fe(CN)₆]^3−/4− (D = 6.5 × 10⁻⁶ cm s⁻¹), A is EASA value, ν is the scan rate, and C refers the bulk concentration of redox probe. According to the calculated results, the A value of the proposed electrodes can be improved by the modification of MoS₂ as listed in Table I. Among these, the A value of the MoS₂-3/SPE was the highest and was 1.6-fold larger than that of the bare SPE. Along with that, the EASA values of MoS₂ base/SPE, MoS₂-1/SPE, and MoS₂-2/SPE also were 1.2-fold, 1.4-fold, and 1.3-fold higher than that of the bare SPE, respectively. Besides, the surface coverage area (θ) value, a measure of how much of the electrode surface is covered by electroactive species, was determined. It is calculated using the formula: θ = (nFAs)/Q, where, θ is a ratio without units, representing the coverage percentage, n is the number of electrons involved in the electrochemical reaction, F is the Faraday constant, As is the electrode's surface area (4 mm diameter for commercial SPEs), Q = ∫IdE is the charge obtained from the cyclic voltammetry curve in Coulombs (C). ^29–31 According to that, the θ value was calculated around 0.981, 0.999, and 0.987 for MoS₂-1/SPE, MoS₂-2/SPE, and MoS₂-3/SPE. Clearly, all modified electrodes exhibited high surface coverage area (θ) values (>98% coverage) by modifying with the proposed MoS₂ samples. More interestingly, the electron transfer rate constant (k_et) was calculated from the R_ct and A values obtained as described above:

$$\begin{eqnarray*}&&{{\rm{k}}}_{{\rm{et}}}={\rm{RT}}/{{\rm{n}}}^{2}{{\rm{F}}}^{2}{{\rm{AC}}}_{0}{{\rm{R}}}_{{\rm{ct}}}\end{eqnarray*}$$

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The k_et values of SPE, MoS₂ base/SPE, MoS₂-1/SPE, MoS₂-2/SPE, and MoS₂-3/SPE were calculated as 0.031, 0.035, 0.038, 0.047, and 0.053 cm s⁻¹, respectively. In comparison to the bare SPE electrode, the modified electrodes had an enhanced k_et value. This result demonstrated an increase in the reactive electron movement of the redox probe at the electrode/electrolyte interface. Thus, the modification of the electrode surface with MoS₂ offered impressive improvements in electrochemical behaviors. As for MoS₂-3/SPE, it possessed a considerably higher peak value at a lower potential, along with the highest EASA and k_et values in comparison with other modified electrodes. Thus it is expected to be one of the most promising modified electrodes for electrochemical sensing application. To explain these results, some reasons were proposed. Firstly, the enhancement of electrochemical response at the electrode modified with prepared MoS₂ samples was considered due to the presence of MoS₂ sheets, which provided positive impacts on electronic conductivity, reversible electron transfer, and specific surface area owing to its unique features. Second, the difference in kinetic parameters of redox transfer among modified electrodes arose from the difference in structural characteristics of as-prepared MoS₂ samples. Benefiting from a thin layer structure and uniform distribution as well as the formation of defect configuration within layer structure helped the MoS₂-3 sample possessing many promising characteristics, which facilitated more effectively the charge transferability at redox reactions, shortened the pathways for electron transfer, increased electroactive surface area and electron conductivity, and enhanced contact with electrolyte solvent.

The electrochemical behaviors of CAP at the electrodes modified with MoS₂-1, MoS₂-2, and MoS₂-3 in PBS buffer solution (pH 7) were analyzed and evaluated by typical CV curves at 50 mV s⁻¹ (Fig. 5a). As observed, there was no redox peak in the absence of CAP, meanwhile with the presence of CAP molecules in 0.1 M PBS solution, some characteristic cathodic/anodic peak were observed in the potential range from 0 to 1.1 V. CV curves appeared a sharp irreversible reduction peak at about −0.67 V to −0.83 V during cathode scanning, suggesting the irreversible reduction process of −NO₂ to hydroxylamine group (−NHOH) occurring toward CAP molecules (as described at Eq. 1).

$$\begin{eqnarray*}&&{\rm{R}}-{{\rm{NO}}}_{2}+4{{\rm{e}}}^{-}+4{{\rm{H}}}^{+}\to {\rm{R}}-{\rm{NHOH}}+{{\rm{H}}}_{2}{\rm{O}}\end{eqnarray*}$$

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Furthermore, a pair of reversible redox peaks were observed in the potential range of −0.24– −0.18 V, corresponding to the reversible reaction of the −NHOH to nitroso group derivative and vice versa as pointed out in some previous reports (as mentioned in Eqs. 1 and 2), ^6,10–12

$$\begin{eqnarray*}&&{\rm{R}}-{\rm{NHOH}}\leftrightarrow {\rm{R}}-{\rm{NO}}+2{{\rm{e}}}^{-}+2{{\rm{H}}}^{+}\end{eqnarray*}$$

Clearly, the presence of these peaks was only recorded when the adding CAP molecules were in PBS buffer, the current response of Red_NO2 at −0.8 V seemed to be higher and more sensitive than the oxidation current response at −0.12 V and the reduction current response at −0.22 V. Therefore the reduction peak at Red_NO2 was employed for the sensitive determination of the existence of CAP molecules in buffer solution. More importantly, the peak potential shift at all proposed electrodes was observed compared to bare SPE. Herein, it is considered due to the difference in conductivity and electron transfer among modified electrodes. In this case, the changes in thin layer structure and uniform distribution, as well as the formation of defect configuration within MoS₂-layered structure, also impacted considerably on the charge transferability at redox reactions of CAP molecules, electroactive surface area, and contact ability between electrode and electrolyte. As expected, Fig. 5b shows a bar plot of the irreversible reduction peak response of CAP for all proposed electrodes. The sensitivity of CAP on these electrodes had large differences. Namely, the reduction current response of CAP at MoS₂-3/SPE (17.41 μM) was the highest and was 2.4-fold larger than that of the bare SPE (7.39 μM). Also, the reduction peak current at MoS₂-2/SPE (15.48 μM) and MoS₂-1/SPE (12.92 μM) were 2.1-fold and 1.7-fold higher than that of the bare SPE, respectively. This result matched well the obtained results from the electrochemical studies under the presence of probes, further confirming the strong impact of structural and physical properties on the selectivity and sensitivity of modified electrodes.

To further analyze the kinetic nature of the redox reaction on the electrode surface, the effect of scanning speed on the electrochemical behaviors of CAP at different electrodes was also studied via CV measurements in 0.1 M PBS containing 50 μM CAP. Figure 6 presents the impact of varying scan rates (20–70 mV s⁻¹) on the electrochemical behaviors of CAP. By analyzing the CV obtained in Fig. 6, the reduction current response was increased along with an increase in the scanning speed at all three investigated electrodes. In addition, when the scan rate increased, the reduction peak potentials displayed a slight shift in the more negative direction. This result was explained due to the considerable formation of a diffusion layer on the electrode/electrolyte interface, resulting in the transfer limitation of electron/charge at high scan rates. ^6,13 Calibration graphs between peak current response and sweep rate were described with linear regression equations:

$$\begin{eqnarray*}&&\begin{array}{c}{{\rm{I}}}_{{\rm{p}}}\left({\mu }{\rm{A}}\right)=0.083{\rm{\nu }}\left({\rm{mV}}\,{{\rm{s}}}^{-1}\right)\\ \,+\,8.83\left({{\rm{R}}}^{2}=0.99\right)\,{\rm{for}}\,{{\rm{MoS}}}_{2}-1/{\rm{SPE}}\end{array}\end{eqnarray*}$$

$$\begin{eqnarray*}&&\begin{array}{c}{{\rm{I}}}_{{\rm{p}}}\left({\mu }{\rm{A}}\right)=0.092{\rm{\nu }}\left({\rm{mV}}\,{{\rm{s}}}^{-1}\right)\\ \,+\,10.73\left({{\rm{R}}}^{2}=0.996\right)\,{\rm{for}}\,{{\rm{MoS}}}_{2}-2/{\rm{SPE}}\end{array}\end{eqnarray*}$$

$$\begin{eqnarray*}&&\begin{array}{c}{{\rm{I}}}_{{\rm{p}}}\left({\mu }{\rm{A}}\right)=0.1{\rm{\nu }}\left({\rm{mV}}\,{{\rm{s}}}^{-1}\right)\\ \,+\,12.2\left({{\rm{R}}}^{2}=0.997\right)\,{\rm{for}}\,{{\rm{MoS}}}_{2}-3/{\rm{SPE}}\end{array}\end{eqnarray*}$$

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Based on these linear relationships, the adsorption capacity (Г) of CAP on the modified electrode surfaces was also determined through the equation: ⁴ I_p = n²F²AГν/4RT. According to that, the Г values of CAP were estimated at around 8.02 × 10⁻¹¹, 9.11 × 10⁻¹¹, and 10.04 × 10⁻¹¹ mol cm⁻² for MoS₂-1/SPE, MoS₂-2/SPE, and MoS₂-3/SPE, respectively. Clearly, the MoS₂-3-modified electrode owned the largest Г value. Besides, the redox peak potential shift of CAP was positive according to a linear relationship with ln(ν), corresponding to the linear regression equations:

$$\begin{eqnarray*}&&\begin{array}{c}{{\rm{E}}}_{{\rm{p}}}\left({\rm{V}}\right)=-0.0378\,\mathrm{ln}\left({\rm{\nu }}\right)\left({\rm{mV}}\,{{\rm{s}}}^{-1}\right)\\ \,-\,0.641\left({{\rm{R}}}^{2}=0.996\right)\,{\rm{for}}\,{{\rm{MoS}}}_{2}-1/{\rm{SPE}}\end{array}\end{eqnarray*}$$

$$\begin{eqnarray*}&&\begin{array}{c}{{\rm{E}}}_{{\rm{p}}}\left({\rm{V}}\right)=-0.0383\,\mathrm{ln}\left({\rm{\nu }}\right)\left({\rm{mV}}\,{{\rm{s}}}^{-1}\right)\\ \,-\,0.673\left({{\rm{R}}}^{2}=0.995\right)\,{\rm{for}}\,{{\rm{MoS}}}_{2}-2/{\rm{SPE}}\end{array}\end{eqnarray*}$$

$$\begin{eqnarray*}&&\begin{array}{c}{{\rm{E}}}_{{\rm{p}}}\left({\rm{V}}\right)=-0.0397\,\mathrm{ln}\left({\rm{\nu }}\right)\left({\rm{mV}}\,{{\rm{s}}}^{-1}\right)\\ \,-\,0.518\,\left({{\rm{R}}}^{2}=0.995\right)\,{\rm{for}}\,{{\rm{MoS}}}_{2}-3/{\rm{SPE}}\end{array}\end{eqnarray*}$$

According to the Laviron equation: ^6,13,14

$$\begin{eqnarray*}&&\begin{array}{c}{{\rm{E}}}_{{\rm{p}}}={{\rm{E}}}_{0}+\left({\rm{RT}}/{\alpha }{\rm{nF}}\right)\times \,\mathrm{ln}\left({{\rm{RTk}}}^{0}/{\alpha }{\rm{nF}}\right)\\ \,-\,\left({\rm{RT}}/{\alpha }{\rm{nF}}\right)\mathrm{ln}\left({\rm{\nu }}\right)\end{array}\end{eqnarray*}$$

The kinetic parameters such as the number of electron transfers during the reaction (n) and the charge transfer coefficient (α) were determined from the obtained slope values. Namely, the n value for all modified electrodes was equal to 2, corresponding to α values of 0.368, 0.373, and 0.387 for MoS₂-1/SPE, MoS₂-2/SPE, and MoS₂-3/SPE, respectively. Besides, the electron transfer rate constant (k_s) of CAP molecules can be calculated via the following equation: ^4,15,32

$$\begin{eqnarray*}&&\begin{array}{c}\mathrm{ln}\left({{\rm{k}}}_{{\rm{s}}}\right)={\alpha }\,\mathrm{ln}\left(1-{\alpha }\right)+\left(1-{\alpha }\right)\mathrm{ln}\,{\alpha }\\ \,-\,\mathrm{ln}\left({\rm{RT}}/{\rm{nF}}{\rm{\nu }}\right)-\,{\alpha }\left(1-{\alpha }\right)\times \left({\rm{nF}}\left({{\rm{E}}}_{{\rm{p}}}-{{\rm{E}}}_{{\rm{p}}/2}\right)/{\rm{RT}}\right)\end{array}\end{eqnarray*}$$

The k_s value was also calculated around 0.983, 1.138, and 1.718 s⁻¹ for MoS₂-1/SPE, MoS₂-2/SPE, and MoS₂-3/SPE, respectively. Herein, MoS₂-3/SPE exhibited the largest k_s value, demonstrating high electrical conduction and faster electron/charge transfer. More interestingly, CA measurements were performed on the proposed electrodes to observe the difference in diffusibility and electrocatalytic ability at each electrode. In terms of diffusibility, using Cottrell's law, the diffusion coefficient (D) of CAP molecules was calculated via the recorded current.

$$\begin{eqnarray*}&&{{\rm{I}}}_{{\rm{p}}}={\rm{nFA}}{{\pi }}^{-1/2}{{\rm{D}}}^{1/2}{{\rm{t}}}^{-1/2}{\rm{C}}\end{eqnarray*}$$

As described in Fig. 7, the linear relationships between the current intensity (I) and t^−1/2 at various CAP concentrations (5–50 μM) were observed. From the obtained slopes and the Cottrell equation, the D value was determined around 2.15 × 10⁻⁸, 5.11 × 10⁻⁸, and 6.15 × 10⁻⁸ cm² s⁻¹ for MoS₂-1/SPE, MoS₂-2/SPE, and MoS₂-3/SPE, respectively. Furthermore, the Galus method was also used to estimate the catalytic rate constant (k_cat) involving the catalytic reduction reactions of CAP at modified electrodes: I_C/I_L = π^1/2(k_catC_t)^1/2 where I_C/I_L refers to the catalytic current/limited current in the presence/absence of CAP. From the resulted slopes (Fig. 7), the k_cat values were about 1.43 × 10³, 5.49 × 10³, and 8.69 × 10³ M⁻¹ s⁻¹ for MoS₂-1/SPE, MoS₂-2/SPE, and MoS₂-3/SPE, respectively.

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Overall, similar to the observed results from the electrochemical characteristic investigation, the positive influence of electrode modification with as-prepared MoS₂ samples on the electrochemical behaviors of CAP was undeniable, furthermore, the difference in electrochemical behaviors of CAP among these modified electrodes was very clear. As expected, using MoS₂-3 sample prepared via ultrasonication method for 3 h to modify the electrode surface, it exhibited many positive enhancements in the electrochemical current response, adsorption capacity (Γ), electron transfer rate constant (k_s), diffusion coefficient (D), and catalytic rate constant (k_cat) for the electrochemical reaction of CAP. Namely, in this case, all parameters at MoS₂-3/SPE were higher remarkably compared with other modified electrodes. As seen, in the presence of K₃/K₄ probes, the increase was recorded mostly in electrical conductivity, charge transfer kinetics, and EASA value at all modified electrodes, however, the increase difference between these electrodes seems to be not clear. While, for adding CAP, the difference among kinetic parameters was very considerable, particularly, for diffusion coefficient (D) and catalytic rate constant (k_cat) (Table II). Namely, the electrocatalytic ability and diffusibility for redox reaction of CAP at MoS₂-2/SPE were about 3.8 and 2.3-fold higher than that of MoS-1/SPE, while it achieved approximately 6 and 2.9-fold for MoS₂-3/SPE. On the basis of the observed results, it can be concluded that the change in the synthetic condition created the MoS₂ samples owning different structural characteristics and physicochemical properties, which impacted remarkably the absorption-controlled process for the electron transfer reaction of CAP at modified electrodes. Herein, to justify that the possible impact hypothesis of MoS₂ samples could be explained as:

The first reason might be proposed due to the increasing electrical conductivity and faster charge transfer kinetic owing to the unique features of MoS₂ materials at modified electrodes similar to the diffusion-controlled process for the redox reactions of K₃/K₄ probes. The second reason should be noted that the change in the number of structural layers of MoS₂ samples led to the positive changes in mass transfer rate, intrinsic thickness, and wettability of the electrode-electrolyte interfaces, as well as intractability and exposure ability between the electrode surface and targeted molecules at modified electrodes. Indeed, thanks to the lower layer number, the electrode modified with the MoS₂-3 sample exhibited a higher adsorption capacity and diffusion coefficient compared with that of MoS₂-1/SPE and MoS₂-2/SPE. On the other hand, the presence of OH-functional groups on the electrode surface seemed to have better reactivity and interaction with CAP molecules, leading to higher adsorption capacity as mentioned in some of our reports. Notably, the outstanding electrocatalytic ability of MoS₂-3/SPE was considered due to the abundant formation of defect configuration (S-vacancies) arising from breaking the S–Mo–S bonds as well the equivalent status of local electron densities at basal plane and edge at MoS₂-3 sample.

Clearly, for the MoS₂ sample, the change in structural characteristics such as layer number, morphology, size, thickness, and distribution just slightly impacted electrochemical behaviors, namely herein, electron transferability, electroactive surface area, and electrical conductivity. In contrast, the formation of defect configuration within the MoS₂ structure and its physicochemical properties (wettability, interaction, and exposure ability) could be considered to be one of the most important characteristics, deciding the impressive enhancements of the electrochemical performance of CAP. ³³

Electrochemical sensor performance was affected considerably by experimental parameters (pH value and accumulation time), so these parameters have been investigated and optimized. ³⁴ The DPV curves of 50 μM CAP at MoS₂-3/SPE with different pH values (3–11) were shown in Fig. S1. As can be observed when the pH changed, there were noticeable changes in both the potential position and peak response. As the pH value of PBS increased, the peak potential of CAP turned negative. More importantly, the peak reduction current of CAP peaked at pH 7.0. Therefore, pH 7 was chosen as the best experimental condition for CAP detection. On the other hand, the DPV measurements at MoS₂-3/SPE were measured from 30 to 160 s to determine the optimal accumulation time. The recorded results were illustrated in Fig. S1c. The current intensity increased significantly with an increase in accumulation time and peaked at 120 s, however, it seems to not change after that. Therefore, the optimized accumulation time was 120 s for the subsequent experiments.

In order to evaluate the efficiency of CAP analysis on different MoS₂-modified electrochemical sensors, DPV measurements were carried out. The DPV curves at different CAP concentrations on proposed electrodes, MoS₂-1/SPE (a), MoS₂-2/SPE (b), and MoS₂-3/SPE (c) in 0.1 M PBS (pH 7) under optimal conditions were shown in Fig. 8. The obtained results show that the amperage increases as the CAP concentration increases for all proposed electrodes. The regression equations were as follows: I_p (μA) = 0.207 C(μM) + 0.027 (R² = 0.99) and I_p (μA) = 0.305 C(μM) + 0.044 (R² = 0.99) for MoS₂-1/SPE, MoS₂-2/SPE, in the CAP concentration range of 1–50 μM, respectively. Particularly, for sample MoS₂-3/SPE, the linear concentration range was widened from 0.5 to 50 μM, corresponding with a linear regression equation of I_p (μA) = 0.368 C(μM) + 0.422 (R² = 0.99). According to that, the limit of detection (LOD) was evaluated to be around 0.21, 0.12, and 0.1 μM from the recorded slope values of the above linear regression equations. Furthermore, the electrochemical sensitivity value of the proposed electrodes was determined respectively around 0.986, 1.362, and 1.586 μA. μM⁻¹. cm⁻² for MoS₂-1/SPE, MoS₂-2/SPE, and MoS₂-3/SPE. So compared to other electrodes, the MoS₂-3/SPE exhibited the highest electrochemical sensitivity.

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The proposed electrodes were also observed by recording the change of current intensity for 10 successive times on the same electrode/experimental condition to evaluate the repeatability. Figures S2a–c show the current intensity of CAP of three modified electrodes. The obtained results demonstrated that all proposed electrodes had promising repeatability with low relative standard deviation (RSD) values of 0.667%, 0.659%, and 0.663% for MoS₂-1/SPE, MoS₂-2/SPE, and MoS₂-3/SPE, respectively. Besides, the selectivity of the proposed electrodes in the determination of CAP was evaluated with the addition of several interfering metal ions (such as Fe³⁺, Ni²⁺, Cu²⁺, NO₃ ⁻, and SO₄ ²⁻) and even organic compounds (urea, D-glucose, and 4-nitrophenol) on the detection of 50 μM CAP. The tested results were shown in Fig. S3. The electrochemical sensors designed in the presence of other natural interferences at 10-fold concentrations can reliably determine CAP. For the determination of CAP in the real sample, the electrochemical performance of MoS2–3/SPE towards the sensitive detection of CAP was carried out on shrimp samples from a local shop. Proceeding to the experiment, firstly, different concentrations of CAP (10, 20, and 40 μM) were added to the prepared shrimp samples and then were analyzed using the DPV technique. The concentration of CAP was calculated according to the regression equation formula of the calibration curves (Table III). The results showed the average recoveries for MoS2–3/SPE were in the range from 93% to 94% corresponding to the relative standard (RSD) within 1.05%–1.35% (n = 3). The calculated results demonstrated the promising practical applicability potential of MoS2–3/SPE for the measurement of CAP in real samples.