R ESEARCH ARTICLE doi: 10.2306/scienceasia1513-1874.2020.076 Cyclic voltammetric determination of capsaicin by using electrochemically deposited tin and reduced graphene oxide on screen-printed carbon electrodes Wasukamol Numphud, Orapin Chienthavorn, Wilai Siriwatcharapiboon∗ Department of Chemistry and the Center of Excellence for Innovation in Chemistry, Faculty of Science, Kasetsart University, Bangkok 10903 Thailand ∗Corresponding author, e-mail: [email protected] Received 28 May 2020 Accepted 19 Aug 2020 ABSTRACT: Tin and reduced graphene oxide (Sn/rGO) was prepared by an electrochemical deposition and modified on a screen-printed carbon electrode (SPCE) in order to improve the electrode selectivity and sensitivity for determination of capsaicin in real samples. The modified electrode was characterized for its surface morphology by scanning electron microscopy (SEM). An elemental analysis of prepared catalysts was confirmed by energy dispersive spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS). The experimental conditions influencing determination of capsaicin were optimized. The experiment was carried out in a sodium acetate buffer solution pH 3.0 at a scan rate of 50 mV/s. The Sn/rGO/SPCE showed a linear working range of 0.2–22 µM of capsaicin concentrations. The limit of detection and limit of quantification were 0.005 µM (S/N=3) and 0.02 µM (S/N=10), respectively. The prepared electrode was successfully applied to determine capsaicin in real chili samples and chili sauces. KEYWORDS: capsaicin detection, modified electrode, electrocatalyst, electrochemical sensor INTRODUCTION chromatography (HPLC) [6], gas chromatography- mass spectrometry (GC-MS) 15 , and liquid Chili is one of the most popular ingredients in food [ ] chromatography-mass spectrometry (LC-MS) 16 , owing to its unique characteristics for desirable [ ] have been reported. However, these methods re- taste and improving appetite 1 . The spiciness [ ] quire the use of expensive instruments with compli- and pungency of chili pepper is attributed to a cated procedure and long response time. Alternate compound group known as “capsaicinoids”, which approach for capsaicin determination could be done are found in members of the capsicum family 2, 3 . [ ] via electrochemistry due to its advantages, such as Capsaicin (C H NO ) is a main compound in the 18 27 3 high efficiency, good reproducibility, greater linear capsaicinoid group that includes dihydrocapsaicin, response range, and easy operation. Among electro- homocapsaicin and homodihydrocapsaicin 4–6 . [ ] chemical techniques, cyclic voltammetry is the most Capsaicin has been widely used in pharmaceuti- widely used to study the electrochemical behavior cal and medical applications due to its remark- of electroactive species by using biosensor 17, 18 able properties, such as boosting metabolism to [ ] or non-biosensor 19 . increase amount of heat in body and burn more [ ] Electrochemical determination of capsaicin was calories 7 , reducing blood pressure 8 , helping [ ] [ ] firstly reported by Kachoosangi et al 20 by us- dilation of blood vessel 9 , and improving blood cir- [ ] [ ] ing adsorptive stripping voltammetry at a carbon culation 10 . Capsaicin also has antioxidative 11 , [ ] [ ] nanotube (CNT) based electrochemical sensors. antibacterial 12 and anticarcinogenic 13 proper- [ ] [ ] Other approaches were reported on different types ties. of electrodes for detection of capsaicin, such as Due to the importance of capsaicin in culi- boron-doped diamond electrode 21 , glassy car- nary and biomedical uses, a simple, rapid and [ ] bon electrode 22 , carbon nanotube (CNT) 19 , accurate determination of capsaicin is necessary, [ ] [ ] graphene oxide (GO) 23 , reduced graphene oxide particularly in food and pharmaceutical industries. [ ] (rGO) 24 , precious metal 25, 26 , non-precious Several analytical techniques for determination of [ ] [ ] metals 23 , carbon paste electrode modified with capsaicin, such as spectrophotometry, gas chro- [ ] amino-functionalized mesoporous silica [27], and matography (GC) [14], high performance liquid www.scienceasia.org 2 ScienceAsia 46 (2020) screen printed single-walled CNT [19]. Further- MATERIALS AND METHODS more, several metallic nanoparticles modified elec- Reagents trode surface and their enhanced sensitivity and selectivity were also investigated, for example, Capsaicin (C18H27NO3) with ¾99% purity, tin (II) chloride dihydrate (SnCl 2 H O) with 99.995% Ag/Ag2O functionalized reduced graphene oxide 2 2 ¾ purity and graphene oxide· (2 mg ml dispersion (rGO) on screen-printed electrode (SPE) [25] and / in H O) were obtained from Sigma-Aldrich (St. ruthenium nanoparticles decorated CNTs [28]. The 2 precious metals always show high catalytic activ- Louis, USA). Standard tested interfering substances, ity; however, the obtained electrodes have a high such as D-(+)-glucose, D-(-)-fructose with ¾99% cost of production. The complicated and expensive purity, zinc (II) chloride anhydrous with ¾97% pu- electrode fabrication process may restrict the im- rity, copper (II) chloride were also obtained from proving of capsaicin sensor in the future. Hence, Sigma-Aldrich (St. Louis, USA). Potassium chlo- a disposable, cheap, sensitive, and selective electro- ride and sodium hydroxide anhydrous pellets were analytical sensor for determination of capsaicin is purchased from Carlo Erba Reagents (Barcelona, necessary. Spain). The electrolyte solution was a mixture of The screen-printed carbon electrode (SPCE) is a glacial acetic acid and anhydrous sodium acetate ob- low cost sensor. It can be modified and tailored to tained from Carlo Erba reagents (Barcelona, Spain) appropriately serve the demand of the end user and and Sigma-Aldrich (St. Louis, USA), respectively. offer high potential for commercial applications. As Carbon paste and Ag/AgCl paste were purchased the sensitivity and selectivity of the SPCE towards from SunChemical (Torfaen, UK). the target molecules are imperative, the reduced graphene oxide (rGO), a derivative of graphene, Preparation of tin and reduced graphene oxide has attracted tremendous attentions due to its prop- screen-printed carbon electrode (Sn/rGO/SPCE) erties, such as high surface area and excellent A screen-printed technique was applied for making electrochemical properties. However, the limited electrodes on a chip for electrochemical measure- conductivity of the rGO and its poor dispersibility ment. Carbon paste was screen-printed onto a are still challenging factors for the rGO modified 10 mm 25 mm of polyvinyl chloride (PVC) sheet electrode. In order to solve these problems, rGO has (Paper Thai× Co., Ltd.) and dried at 60 °C for 30 min been functionalized with precious metals [25, 26] to form a working electrode area of 3 mm diameter and non-precious metals [22]. Tin (Sn) is a p- and a counter electrode. After that, Ag/AgCl paste block metal that has been widely used for promoting was applied onto the pre-screened PVC to form a adsorption of several molecules on an electrode reference electrode and dried at 60 °C overnight. surface [29, 30]. Tin can supply highly oxophilic An insulating ink was finally screen-printed onto sites where one oxygen atom in the target molecule three electrodes to exclude the detection area. The can be adsorbed on the electrode surface. It is printed carbon electrode was dried at room temper- possible for tin to form a key part of active site, ature and kept in a desiccator. either purely as a coordination site or indeed as A mixture of 0.5 ml GO, 103 µl of 0.05 mM tin part of the redox system. Furthermore, tin has been (II) chloride solution and 1.4 ml deionized water used as a catalyst due to its high selectivity and was sonicated for 1 h to prepare tin and graphene performance to facilitate electron transfer between oxide (Sn/GO) catalyst ink. A 6 µl Sn/GO catalyst electroactive species and electrode surface [31, 32]. ink was drop-casted on the working electrode of the Based on the facts mentioned above, we aimed bare SPCE and dried at room temperature. A 100 µl to develop disposable electrochemical sensors by sodium acetate buffer solution was dropped to cover using non precious metal, rGO and SPCE. There- all modified SPCE surface, and the electrode was fore, Sn and rGO were employed to enhance elec- subjected to chronoamperometry with an applied trocatalytic activity of SPCE for determination of potential of 0.44 V for 5 min to obtain a Sn/r- capsaicin in our present work. The Sn/rGO/SPCE GO/SPCE. To− obtain optimum potential and time was prepared by using electrochemical deposition. for Sn electrodeposition, the potentials and time Investigation of capsaicin adsorption on the newly were varied, as shown in Fig. S1 in supplementary modified electrode was then achieved by cyclic data. The potential of 0.44 V and the deposition voltammetry. The electrode was subsequently used time for 300 s showed the− maximum peak current. for determination of capsaicin in Bird’s eye chili, The potential of electrode was controlled by the Karen chili and chilli sauces. Autolab PGstat20 potentiostat (Metrohm-Autolab, www.scienceasia.org ScienceAsia 46 (2020) 3 Netherlands). (a) (b) Electrochemical measurement A 0.00764 g of capsaicin powder was dissolved in 250 ml sodium acetate buffer solution (pH 3.0) to obtain 100 µM capsaicin stock solution. The stock solution was then diluted with sodium acetate buffer solution (pH 3.0) to obtain capsaicin standard solutions in a concentration range of 0.2–22 µM. (c) (d) (e) For the the study of capsaicin determination, the cyclic voltammetric (CV) measurement was con- ducted over the range from 0 to 1.0 V at a scan rate of 50 mV/s in 100 µl of 0.01 M sodium acetate buffer solution (pH 3.0) to check the performance of the electrode. The sodium acetate buffer solution was removed. A 100 µl of the capsaicin standard (f) solution was then dropped on the electrode surface for the determination of capsaicin at a scan rate of 50 mV/s. Preparation of real samples The as-prepared Sn/rGO/SPEC electrode was used to test the electrochemical determination of cap- saicin in real samples. Bird’s eye chilli, Karen chili and chili sauces were obtained from a local market Fig.