Environmental analysis of nitrobenzene using newly synthesized anisotropic gold nanostructures: Reaction kinetics and electrocatalytic activity

Abstract

In this article, we established to produce anisotropic, colloidally stable gold nanostructures (AuNS) by rapidly mixing aqueous solutions of gold(III) chloride trihydrate (HAuCl4) and citric acid (CA) as dual reducing and capping agents at room temperature without the aid of a templating agent. Furthermore, the impact of various anions and shape-directing agents on the morphology and reaction kinetics of citric acid-capped gold nanostructures (CA-AuNS) was investigated. Additionally, the electrocatalytic activity of AuNS towards nitrobenzene (NB) was examined using a CA-AuNS modified glassy carbon (GC) electrode. The rate of reaction for the formation of CA-AuNS increased rapidly with higher concentrations of CA, indicating a strong dependence on the reductant’s concentration. This process followed first-order kinetics, suggesting that the reaction rate is directly proportional to the citric acid concentration. Consequently, optimizing CA levels can effectively control the synthesis rate and yield of CA-AuNS. The presence of nitrite ions led to rapid aggregation of gold nanostructures (AuNS) and a quick reaction time. Chloride ions increased the size of the AuNS. As bromide ion concentration increased, the reaction time slowed significantly, resulting in spherical gold nanoparticles. Both acetate ions and polyvinylpyrrolidone (PVP) inhibited the growth of AuNS. High-resolution transmission electron microscopy (HR-TEM) images showed the formation of various shapes, including spherical, hexagonal, pentagonal, and boat-like structures. In the presence of cetyltrimethylammonium bromide (CTAB), both complex formation between CTAB and HAuCl4 and the formation of spherical gold nanoparticles occurred. Upon addition of silver nitrate (AgNO3) to the colloidal solution, the color shifted to pale violet, and the morphology transitioned from anisotropic to irregular. However, when ascorbic acid (AA) was utilized, it proved ideal for maintaining the morphology of gold nanostructures (AuNS). The analysis of X-ray diffraction spectroscopy (XRD), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Fourier transform infrared spectroscopy (FT-IR) further confirmed the synthesized CA-AuNS. The FT-IR spectra of CA before and after the formation of AuNS indicated the participation of carboxylic acid (–COOH) and hydroxyl (–OH) groups in the reduction process of Au3+ metal ions. The GC/CA-AuNS electrode exhibits a larger electroactive surface area due to the coating of AuNS onto the electrode surface, enhancing the electrode’s electrical conductivity by providing additional active sites for electron transfer. In addition, the electrocatalytic activity of NB was investigated in this study. The GC/CA-AuNS electrode had better electrocatalytic activity than the unmodified GC electrode. As a result, with a LOD of 28.2 nM (S/N = 3), this electrode was chosen for sensitive NB determination.