The performance of polyaniline electropolymerization at various acidic electrolytes on gold electrodes for electrochemical biosensor application

Abstract

An electrochemical biosensor is an efficient and reliable analytical tool for analytical measurement due to providing simplicity, rapid response, high sensitivity, high selectivity, cost-effectiveness, no requirement for sample preparation and considerable ease to develop point-of-care tests (POCT)(1). To design and fabricate electrochemical biosensors, many strategies for electrode modification have been proposed to improve their biosensor performance, including, high sensitivity, wide linearity of detection, selectivity, rapid detection and long-term stability. Therefore, the properties of the supporting materials for electrode modification is an important factor for biosensor fabrication. The modification of an electrode surface can be conducted with many strategies; for example, increasing its surface area as a supporting matrix for enzyme immobilization by using nanomaterials, using different types of polymers and so on. The material modification also can lead to an improvement of the electron transfer and analytical characteristics (2).

Polyaniline (PANI) is one of the attractive conducting polymers which have been extensively used for several applications including chemical sensors, biosensors, supercapacitors, batteries and energy storage devices (3). Particularly, it has obtained high attention in electrochemical biosensors because of its unique properties such as high conductivity, great stability, high surface area as supporting materials for biomolecule immobilization, as well as ease of synthesis by monomer via chemical oxidative polymerization or the electro-polymerization (4). Nowadays, PANI synthesis using electrochemical methods has become more popular because of its ease to fabricate, and controllable thickness. Aniline monomer was electropolymerized via various methods, i.e., cyclic voltammetry, amperometry, and galvanostatic method (5).

Regarding the condition of PANI preparation, the structural forms and the electrochemical properties of PANI can be different, and the ability to electrically transform between its conductive and resistive states also involves the types of acidic electrolytes. The electropolymerization process is mostly performed in the aqueous media of strong organic acids including HCl or H₂SO₄. This is due to these strong acids can be doping and de-doping changing on ¶-conjugated system leading to the improvement the electrical signals. Moreover, PANI morphologies can be generated in different forms such as nanobeads, nanofiber which is dependent on several factors of electropolymerization, i.e., the concentration of PANI, doping agent, and also the electrochemical techniques performed for the process of PANI synthesis.

Herein, our aim is to study the electrochemical properties of PANI in the different acidic electrolytes. The behaviors of electron transfer and the changing of the current signal were investigated via cyclic voltammetry. In this study, the electropolymerization of PANI was conducted in two types of acid dopants including 1.0 M HCl and 0.50 M H₂SO₄ which was modified on a gold electrode using chronopotentiometry via applying the constant current density to the electrode surface. A gold electrode was polished using alumina slurries (0.30 and 0.05 mm, respectively) and sonicated in de-ionized water for 5 minutes. After that, the electrode was cleaned electrochemically in 0.50 M sulfuric acid using cyclic voltammetry in the range of 0 - 1.5 V at a scan rate of 100 mV s^-1 for 25 cycles (6). PANI electropolymerization was carried out in 1.0 M HCl, and 0.50 M H₂SO₄ using chronopotentiometry via applying a constant current density at 0.1 mA cm^-2. The time deposition of PANI coating at 120, 300, and 600 s was also evaluated and optimized by considering the electrochemical behaviors.

The electrochemical measurement was operated using a three-electrode system Ag/AgCl (saturated KCl) as a reference electrode, a platinum wire as a counter electrode, a gold electrode (3.0 mm diameter) as a working electrode. To investigate the electrochemical behaviors of the PANI modified electrodes, cyclic voltammetry was performed in 5.0 mmol L^-1 K₃Fe(CN)₆. The electrochemical characteristics in terms of the peak-to-peak potential separations (ΔEp), the current of the oxidation (I_pa) and redox reversible properties after PANI electropolymerized on gold electrode were observed. The concept of methodology was shown in Fig.1.

Fig.1 Schematic illustration showing the electropolymerization process of the PANI on the gold electrode surface and the electrochemical characterization via cyclic voltammetry.

                 The different conditions of both acid doping agents and the time of deposition for PANI electropolymerization on a gold electrode would affect the electrochemical properties. Hopefully, the optimal condition of PANI electropolymerization could be a promising conducting polymer for biosensing applications that provides the reversible characteristic redox peaks, improved the electron transfer process, and also enhanced the current response.