Electrically modulated optical transmittance of silver nanoparticles: effects of concentration, pH, and applied...

The tunable localized surface plasmon resonance (LSPR) of silver nanoparticles (AgNPs) offers a sensitive optical response to changes in their surrounding environment, agglomeration state, and electron density. This study investigates the combined effects of nanoparticle concentration, pH, and externally applied electrical potential on the light transmittance of AgNP solutions, with a view to developing a simple electro-optical sensing platform. Using a blue LED (peak emission within the 400-450 nm within LSPR absorption bands of AgNPs) and an illuminance sensor, we systematically measure changes in transmitted light intensity under varying conditions. Increasing AgNP concentration leads to a monotonic decrease in illuminance due to enhanced absorption and scattering. Application of an external potential induces nanoparticle alignment, electro-migration, or aggregation depending on the surface charge and pH of the medium, thereby altering the LSPR conditions and further reducing transmittance. Notably, the magnitude and rate of this potential-dependent transmittance change are strongly influenced by pH, which modulates the nanoparticle surface charge and colloidal stability. These findings demonstrate that voltage-controlled optical transmittance in AgNP solutions can serve as a label-free, real-time transduction mechanism for detecting changes in ionic strength, local dielectric environment, or analyte-induced aggregation. The method holds promise for low-cost, miniaturized electrochemical-optical sensors, where nanoparticle concentration and solution conditions are key analytical parameters.