Electrochemical detection of sulfite using gold nanoparticles decorated poly[2-(methacryloyloxy) ethyl] trimethylammonium chloride: kinetic and mechanistic studies

Sulfites (SO₃²⁻) are widely used in various industries as a preservative in beverages, pharmaceutical products, wines, foods, and cosmetics. As a preservative, it prevents foods from spoiling and is also used as a bleaching agent due to its antioxidant, anti-browning, and antibacterial activity. Despite its widespread use, inhalation of SO₃²⁻ can lead to health issues such as headaches, nausea, asthma, dizziness, and reduction of red blood cells. Thus, accurate and efficient detection of sulfite is crucial. The deployment of positively charged poly [2-(methacryloyloxy) ethyl] trimethylammonium chloride (PMTC) decorated with gold nanoparticles (AuNPs) offers a novel approach, enhancing sensitivity and specificity in the electrochemical detection of negatively charged sulfite ions (SO₃²⁻). A sensor for detecting SO₃²⁻ was developed using PMTC decorated with AuNPs on a glassy carbon electrode (GCE). Transmission electron microscopy (TEM) was employed to examine the structural morphology of the composite material, and the formation of AuNPs was confirmed through ultraviolet-visible spectroscopy. Zeta potential analysis affirmed the positive charge of the PMTC composite, highlighting its effective coordination with the negatively charged SO₃²⁻. The surface conductivity of the modified GCE was studied using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Kinetic analyses, focusing on scan rate and pH dependencies, elucidated the SO₃²⁻ oxidation dynamics and the interaction between the positively charged PMTC and negatively charged SO₃²⁻. Quantitative evaluation was performed using the current-time (I-t) technique, achieving a limit of detection of 0.41 ± 0.003 μM (S/N = 3) within a linear range of 6.66 μM to 1020 μM. The modified electrode demonstrated remarkable stability, repeatability, and resistance to common interferents. Real sample analysis using laboratory tap water with a fixed SO₃²⁻ concentration exhibited excellent recovery. The oxidation of SO₃²⁻ on the AuNPs-PMTC-GCE proceeded via first-order kinetics and followed a stepwise pathway facilitated by the charge interactions.