Developing a Portable Sensor for the Detection of Silver Nanoparticles in Freshwater

Abstract

Silver nanoparticles (AgNPs) are used in medicine, and industrial applications, such as textiles, because of their bactericidal, catalytic, and optical properties. However, AgNPs raise environmental and health concerns because of their release into freshwater. Detecting AgNPs in freshwater remains challenging. The present research involves steps toward developing a portable and cost-efficient (under $1000 CAD) electrochemical sensor to detect AgNPs, silver nitrate (AgNO3), and silver acetate (AgC2H3O2) in freshwater. Firstly, the redox behaviour of AgNPs, AgNO3, and AgC2H3O2 was studied using cyclic voltammetry (CV) through their oxidation and reduction reactions on glassy carbon electrodes (GCE). Then, to enhance the detection sensitivity, square wave voltammetry (SWV) was used, because it enhances the signal-to-noise ratio by measuring Faradaic (redox) currents, whereas minimizing non-Faradaic (capacitive) background through differential pulse measurements. Using SWV, the limit of detection (LOD) for AgNPs was lowered by 1.9 times (0.088 mg/L), for AgNO3 by 3.5 times (0.020 mg/L), and for AgC2H3O2 by 1.6 times (0.080 mg/L), compared to CV values of 0.16, 0.070, and 0.13 mg/L, respectively. Secondly, aqueous media parameters were assessed to identify optimal detection conditions. The highest oxidation peak current (0.083 µA) for AgNPs occurred at pH 6.9, indicating optimal oxidation conditions for AgNPs. AgNPs selectivity tests in the presence of freshwater ions reported that no significant interference was observed from sodium ion (Na⁺) and potassium ion (K⁺). However, magnesium ion (Mg2+) and calcium ion (Ca2+) reduced the oxidation peak current of AgNPs by 50.0% and 63.3%, respectively. To further enhance sensitivity for AgNPs, this sensor was fabricated by integrating graphene and Aquivion® onto a GCE by drop-casting. Graphene was selected because it facilitates electron transfer rate due to its surface area (2630 m²/g), which is twice that of carbon nanotubes (CNTs) (1315 m2/g). Aquivion® enhances the graphene dispersion because of its 10 times higher proton conductivity compared to NafionTM. A LOD of 0.015 mg/L for AgNPs was reported using SWV with the graphene/Aquivion®/GCE (G-A-GCE), with a linear range of 0.020 to 0.10 mg/L. These results suggest that G-A-GCE may serve as a suitable sensing platform for further optimization toward detecting AgNPs in simulated freshwater.