Introduction

In last years, the swift emergence of viral diseases has been recognized as a serious threat to both human and veterinary health [1]. More recently, the viral diseases surpassed other infectious diseases to become the leading cause of death worldwide. This evolution has made viral diseases a paramount concern for public health. Acyclovir (ACV) is known as a synthetic nucleoside analog of purine derived from guanine. However, it differs from guanine in that it lacks a 3'-hydroxyl on its side chain. It has been extensively utilized in the clinical treatment of various viral diseases, including Epstein-Barr virus, varicella-zoster virus (VZV), hepatitis B virus (HBV), and herpes simplex virus (HSV). This medication has demonstrated remarkable therapeutic benefits in treating viral diseases such as cold sores, encephalitis, infections of central nervous system, keratitis, and corneal blindness [2,3]. When present in high concentrations in the body, ACV can cause adverse effects such as nausea and diarrhea. In addition, high doses of this medication can potentially cause serious side effects related to the kidneys and low platelet counts [4,5]. Therefore, the necessity of using a powerful analytical tool with high selectivity and sensitivity, and quick response is clearly evident for ACV determination in pharmaceutical compounds and biological samples. So far, several analytical techniques have been reported for the ACV detection, including spectrophotometry [6], radioimmunoassay [7], capillary electrophoresis [8], and chromatography [9,10], and. Despite being well-established and widely accepted for detecting ACV, these methods often encounter limitations such as expensive equipment, extensive sample preparation, the need for specialized expertise, and time-consuming procedures. These factors can render these methods unsuitable for routine analysis in clinical settings and can restrict their availability to healthcare professionals and patients. Due to their sensitivity, simplicity, rapid response and inexpensive instrumentation, electrochemical methods based on chemically modified electrodes (CMEs) present a promising alternative [11-21].

Research indicates that the screen-printing technology commonly employed in microelectronics holds significant value in producing electrodes for disposable electrochemical (bio) sensors. Screen-printed electrodes (SPEs) offer a range of benefits such as simplified operation, adaptability, cost-effectiveness, portability, reliability, reduced dimensions, and scalability for mass production [22,23]. Consequently, it finds extensive utility within the realm of electroanalytical chemistry. Furthermore, the implementation of an SPE eliminates the need for meticulous cleaning procedures that traditional electrodes, such as glassy carbon electrodes, require. This innovation addresses the limitations inherent in conventional electrode systems, which necessitate frequent recalibration, lack stability, and prove unsuitable for on-site analyses due to their lengthy completion times spanning several hours. Furthermore, traditional electrode systems mandate the expertise of skilled professionals due to their complex isolation and washing protocols. Consequently, the shortcomings associated with traditional electrode systems have rendered them less effective compared to the advantages offered by SPEs. Nanotechnology, as one of the top advances of recent decades, has rapidly become one of the most important research and application fields in various domains [24-29]. More importantly, nanotechnology has a huge potential in the design, fabrication, and development of novel and practical sensors [30-37]. With the development of the nanostructures application in electrochemical sensors, it is known that the use of nanostructures to modify the surface of the electrode leads to the improvement of the speed of the electron transfer process, reduction of overvoltage and increase of the efficiency of the electrode [38-43].

Recently, there has been significant attention in research towards molybdenum-based two-dimensional transition metal dichalcogenides (2D nanomaterials) owing to their amazing physical and chemical attributes such as substantial surface area, heightened electronic conductivity, remarkable specific capacitance, their layered structure, inexpensive price, outstanding electrocatalytic activity, and good chemical stability [44,45].

Typically, chalcogenides like MoX₂ (where, X = S and Se) are synthesized to exhibit a two-dimensional layered configuration similar to graphite. The individual MoX₂ layer comprises a covalently bonded arrangement of Mo elements with chalcogenide elements. These layers are held together through gentle interactions (Van der Waals). MoSe₂ surpasses MoS₂ in terms of electrical conductivity and electrocatalytic performance due to its elevated metallic attributes and the presence of actively catalytic unsaturated selenium edges [46-48]. Nonetheless, the inert nature of the exposed planes on MoSe₂ nanosheets limits their reactivity, as the active sites crucial for catalysis are primarily concentrated along the unsaturated selenium edges. Therefore, due to the sparse distribution of these active edge sites, the overall electrocatalytic activity of MoSe₂ remains unsatisfactory. To tackle these limitations, MoSe₂ has been employed in conjunction with highly conductive carbonaceous materials, resulting in an enhancement of both MoSe₂'s conductivity and electrocatalytic activity. Because of its exceptional capacity to enhance electron transfer and its extensive surface area, reduced graphene oxide (rGO) can be readily combined with two-dimensional nanosheets like MoSe₂ to create heterostructures [49,50].

Concerning the aforementioned factors, the present investigation introduces the utilization of a novel and disposable sensor for the detection of ACV. This sensor capitalizes on the enhancement of the SPGE surface through the incorporation of a MoSe₂/rGO nanocomposite. Notably, the MoSe₂/rGO nanocomposite exhibited good electrocatalytic performance towards the ACV determination. This was evidenced by its notably narrow LOD, impressive sensitivity, and minimal over-voltage requirements. The resultant sensor was effectively employed for the quantification of ACV in urine samples and ACV tablets.

Experimental

Equipments and Reagents

The PGSTAT 302N Autolab (Metrohm, the Netherlands) operated by GPES software and connected to personal PC was applied to perform all electrochemical experiments. The pH control of phosphate buffer solution (PBSs) was measured by using a Metrohm 713 pH meter. The analytical grade of reagents with high purity were used in the present work as provided from Merck and Sigma-Aldrich companies without any further processing. The synthesis and characterization of MoSe₂/rGO nanocomposite has been given in our previously reported work [51]. Figure 1 displays its FE-SEM image.

Figure 1: FE-SEM image of MoSe₂/rGO nanocomposite

Preparation of the MoSe₂/rGO Nanocomposite Modified SPGE

The procedure for SPGE modification is as follow: 1.0 mg of MoSe₂/rGO was initially subjected to ultrasonication for at least 20 min in 1 mL of solvent to prepare MoSe₂/rGO suspension, and then MoSe₂/rGO suspension (4.0 µL) was dropped carefully on the SPGE surface. After drying at room temperature, the MoSe₂/rGO/SPGE sensor was prepared.

The surface areas of unmodified SPGE and MoSe₂/rGO/SPGE were calculated to determine the effect of modification process. Accordingly, the CVs at various scan rates were recorded for 0.1 M KCl solution containing 1.0 mM K₃[Fe(CN)₆]. The Randles-Sevcik equation was used to calculate the surface areas. The MoSe₂/rGO/SPGE demonstrated an effective surface area (0.113 cm²), which was 3.6 times greater than the surface area on unmodified electrode.

Results and DiscussionTop of Form

Investigating the Performance of the MoSe₂/rGO on the ACV Determination

The pH of buffer solution is a key parameter in electroanalysis of compounds. Therefore, the effect of pH in the present work was investigated in PBS (0.1 M) with various values of pH from 4.0 to 9.0 containing 75.0 µM ACV at MoSe₂/rGO/SPGE. From the voltammograms, the current responses of ACV enhanced with the increasing in pH value from 4.0 to 7.0, and then decreased at pH values higher than 7.0. Therefore, the selected optimal buffer solution is PBS 0.1 M at pH 7.0 due to the highest peak current of ACV in this pH. The oxidation mechanism of ACV is presented in Scheme 1.

Scheme 1: The oxidation mechanism of ACV

To confirm the electrocatalytic response of prepared sensor (MoSe₂/rGO/SPGE) towards the electrochemical reaction of ACV, the CV responses of 100.0 µM ACV in PBS (0.1 M-pH 7.0) were recorded at various electrodes (unmodified SPGE (Figure 2 (voltammogram a)) and MoSe₂/rGO/SPGE (Figure 2 (voltammogram b)) at a scan rate of 50 mV.s^-1. From the obtained voltammograms, it is obvious that the observed electrochemical process is an irreversible process, because only one well-defined oxidation peak was obtained due to the ACV presence. Also, a weak oxidation peak with low Ipa was observed for ACV at bare SPGE (voltammogram a). In contrast, the SPGE surface modified with a nanocomposite of MoSe₂/rGO (voltammogram b) exhibited a notably amplified Ipa = 9.35 µA) and a shifted Epa toward negative values (Epa = 990 mV) in the presence of ACV, compared to the bare SPGE. This prominent enhancement in the oxidation peak characteristics can be ascribed to the remarkable impacts of the MoSe₂ and rGO sheets and their synergistic effects in the ACV oxidation.

Figure 2: CVs of bare SPGE (a) and MoSe₂/rGO/SPGE (b) in 0.1 M PBS at pH 7.0 containing 100.0 μM ACV at 50 mV s^-1 scan rate

Effect of Scan Rate

In addition, the effect of scan rate (υ) on ACV oxidation was evaluated using LSV on the MoSe₂/rGO/SPGE in PBS (0.1 M at pH 7.0) containing 90.0 µM ACV at various scan rates (10 to 300 mV.s^-1) (Figure 3). Figure 3 illustrates that the Ipa of ACV was increased with increasing of scan rates. Moreover, the data presented in Inset of Figure 3 shows a strong linear dependence between the Ipa and the υ^1/2 within the range of 10-300 mV.s^-1 (Ipa (μA) = 1.6473υ^1/2 (mV s^-1)^1/2 - 2.2396 (R² = 0.9991)), which suggests a diffusion-controlled process on the MoSe₂/rGO/SPGE surface.

Figure 3: LSVs of MoSe₂/rGO/SPGE in 0.1 M PBS at pH 7.0 containing 90.0 μМ ACV at different scan rates ((a) 10, (b) 50, (c) 100, (d) 200, and (e) 300 mV s^-1. Inset: the corresponding plot of Ipa (µA) vs. ν^1/2 (mV s^-1)^1/2

Chronoamperometric Measurements of ACV

A chronoamperometric investigation was conducted to determine the diffusion coefficient (D) of ACV at the MoSe₂/rGO/SPGE. The results of this investigation are depicted in Figure 4, which illustrates the obtained chronoamperograms for ACV at various concentrations in a pH 7.0 PBS. The reaction of an electroactive substance with a D is described by Cottrell's equation when the process is constrained by mass transport. In Figure 4A, a linear correlation is obtained between the current (I) and the square root of time (t^1/2) for the oxidation of varying ACV concentrations. The slopes obtained from the linear fits were then correlated with the different ACV concentrations, as demonstrated in Figure 4B. By utilizing the plotted slope in conjunction with the Cottrell equation, the D of ACV was calculated to be 2.1×10^-5 cm²/s.

Figure 4: Chronoamperometric responses of MoSe₂/rGO/SPGE in 0.1 M PBS at pH 7.0 containing ACV at different concentrations ((a 0.1), (b 0.3), (c 0.6), (d 0.8), and (e 1.0 mM) of ACV). Insets: corresponding plots of I (µA)-t^-1/2 (s^-1/2) curves from the chronoamperograms (A) and corresponding plots of obtained slopes-[ACV]

Quantitative Analysis of ACV by DPV

Figure 5 demonstrates the DPV responses for ACV at different concentrations in PBS (0.1 M-pH 7.0) at the MoSe₂/rGO/SPGE in the following conditions: step potential (0.01 V) and pulse amplitude (0.025 V). It was found that the Ipa of ACV increased proportionally along with increasing in ACV concentration over a range of 0.03 µM to 190.0 µM. Likewise, by plotting the Ipa of ACV vs. its concentrations a good linearity was obtained (Ipa (μA) = 0.0853C_ACV (μM) + 0.777 (R² = 0.9995)) (Figure 5-Inset). The LOD value was obtained to be 0.01 µM. In addition, the performance of MoSe₂/rGO/SPGE sensor was compared with some of reported electrochemical sensors for ACV determination (Table 1).

Figure 5: DPVs of MoSe₂/rGO/SPGE in 0.1 M PBS at pH 7.0 containing various concentrations of ACV ((a) 0.03, (b) 0.3, (c) 2.5, (d) 5.0, (e) 15.0, (f) 45.0, (g) 75.0, (h) 100.0, (i) 150.0, and (j) 190.0 µM). Inset: The linear plot of Ipa (µA) vs. C_ACV (µM)

Table 1: The performance of MoSe₂/rGO/SPGE sensor in comparison with some of previous reported electrochemical sensors for ACV determination

Stability Studies of MoSe₂/rGO/SPGE for ACV Determination

The stability studies of MoSe₂/rGO/SPGE sensor were done by recording the voltammetric response of this sensor towards 60.0 µM ACV over 12 days. The obtained results demonstrated that the voltammetric response retained 97.1% of its initial response after 12 days, indicating the good stability of the developed sensor.

Analytical Application of MoSe₂/rGO/SPGE for Determination of ACV in Real Samples

The urine samples and ACV tablets were used to investigate the practical applicability of the MoSe₂/rGO/SPGE for ACV determination by DPV. The recovery studies were done by standard addition method to confirm the accuracy by spiking the ACV tablet and urine samples with ACV in various concentrations (Table 2). The recovery values varied from 96.7% to 104.2%. Also, the results showed a good precision as can be inferred from the low values of RSD (%) obtained (n = 5).          

Table 2: The determination results of ACV in urine samples and ACV tablets

Conclusion

In the presented study, a facile and novel MoSe₂/rGO-modified SPGE was designed and applied for sensitive and accurate determination of ACV. With the synergistic effect from MoSe₂ and rGO sheets, the designed sensor exhibited good performance for oxidation of ACV by reducing the over-potential and enhancing the current response. In addition, the MoSe₂/rGO/SPGE sensor exhibited analytical performances for determining ACV, including wide response range (0.03 µM to 190.0 µM), low LOD (0.01µM), and high sensitivity (0.0853 µA.µM^-1). Also, MoSe₂/rGO/SPGE sensor demonstrated good stability for ACV determination. Finally, the designed sensor was successfully utilized for ACV determination in ACV tablet and urine samples, with high reliability and accuracy.

ORCID

Hadi Beitollahi

https://orcid.org/0000-0002-0669-5216  

HOW TO CITE THIS ARTICLE

Tajik, F. Garkani Nejad, R. Zaimbashi, H. Beitollahi. Voltammetric Sensor for Acyclovir Determination Based on MoSe₂/rGO Nanocomposite Modified Electrode. Chem. Methodol., 2024, 8(5) 316-328

DOI: https://doi.org/10.48309/CHEMM.2024.444547.1771

URL: https://www.chemmethod.com/article_194063.html