Interfacial Characterization of Polypyrrole/AuNP Composites towards Electrocatalysis of Ascorbic Acid Oxidation

Abstract

Polypyrrole (PPy) is an interesting conducting polymer due to its good environmental stability, high conductivity, and biocompatibility. The association between PPy and metallic nanoparticles has been widely studied since it enhances electrochemical properties. In this context, gold ions are reduced to gold nanoparticles (AuNPs) directly on the polymer surface as PPy can be oxidized to an overoxidized state. This work proposes the PPy electrochemical synthesis followed by the direct reduction of gold on its surface in a fast reaction. The modified electrodes were characterized by electronic microscopic and infrared spectroscopy. The effect of reduction time on the electrochemical properties was evaluated by the electrocatalytic properties of the obtained material from the oxidation of ascorbic acid (AA) and electrochemical impedance spectroscopy studies. The presence of AuNPs improved the AA electrocatalysis by reducing oxidation potential and lowering charge transfer resistance. EIS data were fitted using a transmission line model. The results indicated an increase in the electronic transport of the polymeric film in the presence of AuNPs. However, PPy overoxidation occurs when the AuNPs' deposition is higher than 30 s. In PPy/AuNPs 15 s, smaller and less agglomerated particles were formed with fewer PPy overoxidized, confirming the observed electrocatalytic behavior.

Keywords: conducting polymers; electrocatalysis; electrochemical impedance spectroscopy; gold nanoparticles; overoxidation.

Figure 1

Representative SEM and dark-field TEM images, respectively, of bare graphite (a), modified graphite electrodes with PPy (b), PPy/AuNPs 15 s (c,d), PPy/AuNPs 30 s (e,f) and PPy/AuNPs 45 s (g,h).

Figure 2

Representative bright-field TEM image of PPy/AuNPs 15 s.

Figure 3

(a) FTIR spectra of PPy (black line), PPy/AuNPs 5 s (pink line), PPy/AuNPs 15 s (red line), PPy/AuNPs 30 s (blue line) and PPy/AuNPs 45 s (green line) modified graphite electrodes. (b) Structure of overoxidized PPy [32,33].

Figure 4

Cyclic voltammetry of PPy, PPy/AuNPs 15 s, PPy/AuNPs 30 s, and PPy/AuNPs 45 s in (a) PBS electrolyte and (b) PBS electrolyte containing 5 mmol L⁻¹ of AA. Scan rate = 20 mV s⁻¹.

Figure 5

(a) Cyclic voltammetry of pencil graphite electrode in PBS electrolyte (orange line), bare pencil graphite (purple line), and PPy (black line). (b) Cyclic voltammetry of PPy (black line), AuNPs (yellow line), and PPy/AuNPs 15 s (red line) in PBS electrolyte containing 5 mmol L⁻¹ of AA. Scan rate = 20 mV s⁻¹.

Figure 6

(a) Nyquist diagrams of PPy, PPy/AuNPs 15 s, PPy/AuNPs 30 s, and PPy/AuNPs 45 s in PBS containing 5 mmol L⁻¹ of AA at a dc potential of 0.4 V. (b) Equivalent circuit used to fit EIS data, and (c) the transmission line (TL) model is presented in the equivalent circuit.