Theoretical and electrochemical analysis of L-serine modified graphite paste electrode for dopamine sensing applications in real samples: https://dx.doi.org/10.5599/jese.1390 1243 J. Electrochem. Sci. Eng. 12(6) (2022) 1243-1250; https://dx.doi.org/10.5599/jese.1390 Open Access : : ISSN 1847-9286 www.jESE-online.org Original scientific paper Theoretical and electrochemical analysis of L-serine modified graphite paste electrode for dopamine sensing applications in real samples Revanappa Santhosh Kumar1, Gururaj Kudur Jayaprakash2,3,, Siddalinganahalli Manjappa1, Mohan Kumar4 and Avvaru Praveen Kumar5 1Department of Chemistry, University B.D.T. College of Engineering, Visvesvaraya Technological University, Davangere - 577004, Karnataka, India 2Laboratory of Quantum Electrochemistry, School of Advanced Chemical Sciences, Shoolini University, Bajhol, Himachal Pradesh, 173229, India 3Department of Chemistry, Nitte Meenakshi Institute of Technology, Bangalore, Karnataka, 560064, India 4Department of Chemistry, PES Institute of Technology and Management, Sagar Road, Guddada Arakere, Kotegangoor, 577204, Shivamogga, India 5Department of Applied Chemistry, School of Applied Natural Science, Adama Science and Technology University, P O Box 1888, Adama, Ethiopia Corresponding author:  rajguru97@gmail.com; Tel.: +91-953-876-2343 Received: May 27, 2022; Accepted: June 8, 2022; Published: July 4, 2022 Abstract In this study, the carbon paste electrode (CPE) was modified by grinding L-serine in a pestle and mortar. L-serine (L-s) was shown to be an effective electrocatalyst at the modified CPE (MCPE) interface for detecting dopamine (DA). L-sMCPE showed excellent activity to detect DA in commercial injection samples with a recovery range of 98.9 to 100.5 %. Theoretical studies were used to understand the electrocatalysis of L-serine at the atomic level using frontier molecular orbitals (FMO) and analytical Fukui assay. According to theoretical findings, the amine group of L- serine works as an extra oxidation site (reason for enhanced reduction peak DA) and the carboxylic acid group acts as an additional reduction site (reason for enhanced oxidation peak DA) at the L- sMCPE interface. Keywords Amino acid; redox reaction; quantum chemical modelling; voltammetry; sensor; dopamine Introduction Dopamine (DA) is a monoamine neurotransmitter that plays a variety of physiological activities in humans and animals. It is a basic organic molecule from the catecholamine family. The detection of DA in the human body is crucial. DA is also involved in the control of heart rate and blood pres- sure [1,2]. DA is involved in a person's movement, mood, and conduct [3]. An inadequate DA level https://dx.doi.org/10.5599/jese.1390 https://dx.doi.org/10.5599/jese.1390 http://www.jese-online.org/ mailto:rajguru97@gmail.com J. Electrochem. Sci. Eng. 12(6) (2022) 1243-1250 L-SERINE MODIFIED GRAPHITE PASTE ELECTRODE 1244 in the human body can lead to serious health issues such as restless leg syndrome, Huntington's disease, schizophrenia, senile dementia, and Parkinson's disease [4]. DA level in urine samples of humans is utilized as a biomarker to study renal and cardiovascular illnesses. Therefore, proper maintenance of the DA level in the human body is necessary. Because of the relevance of DA, the creation of novel dopamine detecting sensors is highly valued for therapeutic applications [5,6]. For DA detection, a variety of methods have been developed such as capillary electrophoresis [7], fluorescence-based sensing [8,9], colorimetry [10], and fluorescence [11,12]. On the other hand, their protocols are complex, costly, time-consuming, and frequently need specialist equipment. Electrochemical methods, however, provide several benefits, including quick and highly sensitive reactions, the convenience of usage, and low cost. Because of the electroactive nature of DA, its determination using electrochemical techniques is a key scientific issue. Over the last few decades, the sensitivity of DA determination has increased significantly. Carbon paste electrodes (CPEs) offer few beneficial properties, including repeatability, stability, and surface renewability, which make them one of the most appealing working electrodes [13,14]. Due to their low cost relative to other materials, CPEs are becoming more widely used in fields such as pharmaceutical, biological, and environmental investigations. Physical or chemical treatments can improve electrochemical characteristics of CPE such as adsorption capacity, selectivity, and sensitivity [15]. To create a novel sensor with appropriate electrochemical characteristics, a CPE matrix must be modified. Earlier, a lot of research has been published on carbon-based electrodes for sensing applications [16-24]. On the atomic scale, the quantum chemistry approach, such as density functional theory (DFT) can be utilized to quantify the electron transfer (ET) of electrode catalysts. As a result, electro- analytical data can be supported and explained using basic principles and DFT. In catalytical research, a combination of pre-ET (frontier molecular orbital theory (FMO)) and post-ET (Fukui functions) would be more useful [13,14,25,26]. In this study, we have modified the CPE by grinding L-serine in a pestle and mortar. The L-serine molecule redox reactive sites and mediating mechanism were predicted using conceptual DFT- based quantum chemical modelling. The nucleophilic and electrophilic regions of L-serine are identified using the frontier molecular orbitals (FMO), highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO) densities. The results of the donor-acceptor interactions were further evaluated utilizing analytical Fukui functions to corroborate the findings. Experimental Chemicals and reagents The analytical grade chemicals were used without further purifications. The sodium dihydrogen orthophosphate monohydrate, silicone oil, dopamine, L-serine, disodium hydrogen orthophos- phate, and K4Fe(CN)6 were purchased from Sigma-Aldrich Himedia. Graphite powder was purchased from Loba chemicals. Preparation of bare carbon paste electrode (BCPE) The bare graphite paste was made by extensively hand mixing the carbon (graphite) powder and the binder (silicon oil), in an agate mortar with a pestle in a 75:25 (w/w) proportion for 25 minutes until the paste was uniform and homogeneous. Then, the graphite paste was filled into a 3 mm hole Teflon tube. The electrode surface was wiped smoothly on the butter paper for a clean, consistent, and even surface [13-15]. R. S. Kumar et al. J. Electrochem. Sci. Eng. 12(6) (2022) 1243-1250 http://dx.doi.org/10.5599/jese.1390 1245 Preparation of L-serine modified carbon paste electrode (L-sMCPE) L-sMCPE was prepared by hand mixing/grinding 2 to 10 mg of L-serine with carbon paste in an agate mortar. Then, the graphite paste containing L-serine was filled into a 3 mm hole Teflon tube and smoothen on the butter paper. Electrochemical cell The potentiostat model CHI-660c was used in the tests (CH Instrument-660 electrochemical workstation). Three-electrode cell was applied, where working electrodes were bare CPE (BCPE) and L-serine modified CPE (L-sMCPE), the reference electrode was saturated calomel electrode (SCE), and the counter electrode was a platinum rod. Computational methods To build model geometries, we used the Sinapsis program [27] and density functional theory (DFT) [Auxiliary density perturbation theory] as implemented in the deMon2k [28-30] software, together with PBE [31,32] correlation functions and the TZVP [33] basis set. The FMO [highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)] and analytical Fukui functions were visually shown using Sinapsis [27]. Results and discussion Cyclic voltammetric response of dopamine at L-serine modified carbon paste electrode Figure 1 depicts cyclic voltammograms (CVs) for 10 μM DA in 0.1M phosphate buffer solution (PBS) of pH 7.4 at BCPE (red line) and L-sMCPE with different quantities of L-serine, at a scan rate of 100 mV s-1. At the BCPE, redox peak currents are low with higher separation of peak potentials (∆Ep) when compared to L-serine MCPE. E / V vs. SCE Figure 1. CVs of 10 μM DA in 0.1 M PBS, pH 7.4 at BCPE (red line) and MCPE with different quantities of L-serine (2-10 mg) In Table 1, ∆Ep and ipa values for DA are compared for BCPE and MCPE with different quantities of L-serine. From Figure 1 and Table 1, it can be seen that MCPE with 6 mg of L-serine has displayed http://dx.doi.org/10.5599/jese.1390 J. Electrochem. Sci. Eng. 12(6) (2022) 1243-1250 L-SERINE MODIFIED GRAPHITE PASTE ELECTRODE 1246 the highest redox current value and lower ∆Ep. Therefore we have considered 6 mg L-sMCPE for further analysis. Table 1. ∆Ep and ipa values of BCPE and MCPE with different quantities of L-sMCPE (data taken from Fig. 1) El. No. Electrode ∆Ep / mV ipa / A 1 BCPE 98 7.157 2 2 mg L-sMCPE 70 10.84 3 4 mg L-sMCPE 67 15.24 4 6 mg L-sMCPE 62 17.46 5 8 mg L-sMCPE 61 16.83 6 10 mg L-sMCPE 61 16.87 Computational studies of L-serine When L-serine is bound by physisorption to the surface of graphite electrode, the electrocatalytic activities of L-sMCPE towards DA are boosted. For mathematical modelling purposes, a monomer of L-serine is considered. To determine which atoms of L-serine are involved in the redox electron transfer processes, the frontier molecular orbitals were estimated and utilized the Fukui concept to predict the redox electron transfer sites. L-serine HOMO is present in the amine group (Figure 2a), while the LUMO is in the carboxylic acid group (Figure 2b). Therefore, according to FMO findings, the amine group in L-serine is engaged in oxidation, whereas the carboxylic acid group is engaged in reduction. The Fukui function is frequently used in electrochemistry to understand redox reaction pathways [13,14,25,26]. Simulations based on the Fukui function can be utilized in chemical and electrochemical applications to locate electron transfer sites [13,14]. The Fukui function [34] can be defined using Equation (1):      → +  −      0 ( ) ( ) ( ) lim N N N N f N r r r (1) Here ρ(r) is the electron density, N denotes the number of electrons in the system, and the + and - signs denote electron addition and removal, respectively. a b HOMO [ISO=0.05; Grid=0.2] LUMO [ISO=0.05; Grid=0.2] Figure 2. Average reactive orbital space of frontier molecular orbital of L-serine R. S. Kumar et al. J. Electrochem. Sci. Eng. 12(6) (2022) 1243-1250 http://dx.doi.org/10.5599/jese.1390 1247 The analytical Fukui findings for L-serine are displayed in Figure 3. Figure 3a displays f-(r) and f+(r) (Figure 3b) plots of the L-serine surface, respectively. The amine group of L-serine functions as an oxidation site and the carboxylic group of L-serine functions as a reduction site. a b f-(r) [ISO=0.0; Grid=0.2] f+(r) [ISO=0.03; Grid=0.2] Figure 3. Analytical Fukui isosurface results [H = white, C = grey, N = blue, and O = red] FMO analysis provides information on reactive redox sites without accounting for electronic relaxa- tion. Analytical Fukui's findings, however, provide details on the places that would suffer the most redox reaction changes while accounting for relaxation effects. Hence, FMO and Fukui researches should be correlated to more precisely forecast redox reactivity locations. In the current studies, both FMO and Fukui, studies are in agreement with each other. As a result, we get more consistent results that amine groups are oxidation centers and carboxylic acid groups are reduction centers. Effect of DA concentration Using the CV approach, the influence of DA concentration was investigated at the surface L-sMCPE electrochemical sensor in 0.1 M PBS, pH 7.4. The rise in anodic peak current with increasing DA con- centration (10 to 70 μM) is shown in Figure 4. A linearity was observed between anodic peak current (ipa) and DA concentration, with a corresponding linear regression equation: Ipa = 0.448896cDA + + 5.1907, with r2 =0.99532. E / V vs. SCE Figure 4. CVs of different DA concentrations (10 -70 M) at L-sMCPE in 0.1M PBS, pH 7.4 http://dx.doi.org/10.5599/jese.1390 J. Electrochem. Sci. Eng. 12(6) (2022) 1243-1250 L-SERINE MODIFIED GRAPHITE PASTE ELECTRODE 1248 Effect of scan rate The impact of changing the scan rate (150 to 350 mV s-1) on the oxidation peak current of DA in 0.1 M PBS as the supporting electrolyte is displayed in Figure 5. It can be seen that over the range of 150 to 350 mV s-1, the oxidation peak current rises linearly with the scan rate. E / V vs. SCE Figure 5. CVs of 10 μM DA at L-sMCPE in 0.1M PBS, pH 7.4 at different scan rates The graph of anodic peak current vs. scan rate shown in Figure 6 reveals a linear relation with a correlation coefficient of 0.9974. This suggests that DA oxidation at L-sMCPE is adsorption controlled process. v / mV s-1 Figure 6. Anodic peak current vs. scan rate (data from Fig. 5) Determination of DA in real samples The analysis of DA in the commercial injection sample was carried out at L-sMCPE in order to evaluate the reliability of the proposed approach by using the standard addition method. The results summarized in Table 2 reveal that the found values are close to the labeled content with a recovery range of 98.9 to 100.5 %. Therefore L-serine mediated ET can detect DA in injection samples effectively. R. S. Kumar et al. J. Electrochem. Sci. Eng. 12(6) (2022) 1243-1250 http://dx.doi.org/10.5599/jese.1390 1249 Table 2. Determination of DA in injection samples cDA / M Recovery, % Number Spiked Found 1 10 10.05 100.5 2 20 19.78 98.90 3 30 29.98 99.93 Conclusion In the current work, the CPE was modified by grinding L-serine (eco-friendly modifier) in a pestle and mortar. L-serine showed excellent electrocatalytic at the L-sMCPE interface for sensing DA in real injection samples with a recovery range of 98.9 to 100.5 %. The catalytical activity of L-serine was theoretically examined using FMO and analytical Fukui analysis. Theoretical observation proved that the amine group of L-serine acts like an additional oxidation site (reason for enhanced reduction peak of DA) and a carboxylic acid group acts as an additional reduction site (reason for enhanced oxidation peak of DA) of L-serine at the L-sMCPE interface. In the current study, both FMO and Fukui studies are in agreement with each other. 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This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.electacta.2017.11.154 https://doi.org/10.1016/j.molliq.2021.116348 https://doi.org/10.1080/14328917.2021.1945795 https://doi.org/10.1007/s41664-019-00116-w https://doi.org/10.1016/j.jsamd.2020.08.005 https://doi.org/10.1002/slct.201900794 https://doi.org/10.1080/14328917.2019.1684657 https://doi.org/10.1016/j.cdc.2019.100331 http://abechem.ir/index.php?option=com_content&view=article&id=9&Itemid=8 https://doi.org/10.1016/j.matchemphys.2019.122597 https://dx.doi.org/10.22036/pcr.2019.205211.1688 https://doi.org/10.1007/s42452-020-2785-1 https://doi.org/10.1016/j.cplett.2021.139295 https://doi.org/10.1039/C8NJ03679A http://sinapsis.sourceforge.net/ http://demon-software.com/public_html/index.html http://demon-software.com/public_html/index.html https://doi.org/10.1002/wcms.98 https://doi.org/10.1063/1.3036926 https://doi.org/10.1103/PhysRevLett.77.3865 https://doi.org/10.1103/PhysRevLett.80.891 https://doi.org/10.1139/v92-079 https://doi.org/10.1021/ja00326a036 https://creativecommons.org/licenses/by/4.0/) @Article{Kumar2022, author = {Kumar, Revanappa Santhosh and Jayaprakash, Gururaj Kudur and Manjappa, Siddalinganahalli and Kumar, Mohan and Kumar, Avvaru Praveen}, journal = {Journal of Electrochemical Science and Engineering}, title = {{Theoretical and electrochemical analysis of L-serine modified graphite paste electrode for dopamine sensing applications in real samples:}}, year = {2022}, issn = {1847-9286}, month = {jul}, number = {6}, pages = {1243--1250}, volume = {12}, abstract = {In this study, the carbon paste electrode (CPE) was modified by grinding L-serine in a pestle and mortar. L-serine (L-s) was shown to be an effective electrocatalyst at the modified CPE (MCPE) interface for detecting dopamine (DA). L-sMCPE showed excellent activity to detect DA in commercial injection samples with a recovery range of 98.9 to 100.5 %. Theoretical studies were used to understand the electrocatalysis of L-serine at the atomic level using frontier molecular orbitals (FMO) and analytical Fukui assay. According to theoretical findings, the amine group of L-serine works as an extra oxidation site (reason for enhanced reduction peak DA) and the carboxylic acid group acts as an additional reduction site (reason for enhanced oxidation peak DA) at the L-sMCPE interface.}, doi = {10.5599/JESE.1390}, file = {:D\:/OneDrive/Mendeley Desktop/Kumar et al. - 2022 - Theoretical and electrochemical analysis of L-serine modified graphite paste electrode for dopamine sensing applic.pdf:pdf;:www/jESE_V12_No6_1243-1250.pdf:PDF}, keywords = {Amino acid, dopamine, quantum chemical modelling, redox reaction, sensor, voltammetry}, publisher = {International Association of Physical Chemists (IAPC)}, url = {https://pub.iapchem.org/ojs/index.php/JESE/article/view/1390}, }