An Electrochemical Sensor Based on Polypyrrole and Gold Nanoparticles for Sensitive Detection of 4-Nitrophenol: Experimental and DFT Insights

This study presents the development of a voltammetric sensor based on screen-printed carbon electrodes modified with polypyrrole (PPy) and electrochemically synthesized gold nanoparticles (AuNPs) for the sensitive, enzyme-free detection of 4-nitrophenol (4-NPh). The electrodeposition method allowed precise control over the nanoparticle distribution and surface morphology, improving electrocatalytic activity and electron transfer kinetics. The modified electrodes were thoroughly characterized by using cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR). The sensor exhibited a low detection limit of 16.5 μmol L–1 and a linear response up to 120 μmol L–1, along with excellent selectivity toward 4-NPh in the presence of common interfering substances. To support and interpret the experimental findings, density functional theory (DFT) simulations were employed to evaluate adsorption energies and electron transfer interactions of 4-NPh and its redox intermediates (Ph-NO and Ph-NHOH) with PPy and Au surfaces. The results showed that Au surfaces offer optimal intermediate stabilization and moderate adsorption strength, which are essential for catalytic efficiency and reaction progression. In contrast, PPy films exhibited stronger adsorption and direct hydrogen donation, which may hinder desorption and promote side reactions. The integration of experimental and theoretical approaches provided a comprehensive understanding of the sensor’s performance. The developed PPy/AuNPs-based electrode is simple, portable, cost-effective, and environmentally friendly, demonstrating strong potential for real-time monitoring of 4-NPh in environmental water samples, with high sensitivity, reproducibility, and operational stability. Also, a set of ab initio calculations based on DFT was employed to elucidate molecule–electrode interactions, explaining the factors behind the enhanced detection of the PPy-film electrode functionalized with AuNPs. These features make the sensor highly promising for rapid and in situ monitoring of environmental pollutants.