Optical characterization of nanoparticle deposition and field-driven assembly

Recent developments in interferometric scattering (iSCAT) microscopy have enabled label-free nanoscopic imaging with the sensitivity to detect single proteins and nanoparticles. Coupled with computational approaches, iSCAT provides highly quantitative measurements and has found growing use in the biosciences as a commercial analytical tool. However, its contribution to other fields, such as interfacial chemistry, reaction dynamics, and single-particle electrochemistry, remains underexplored. Here, I present a methodology to detect single nanoparticle deposition events in solution using iSCAT microscopy. Arobust data-processing pipeline, incorporating a custom-trained neural network, outputs detailed particle statistics. By integrating a transparent conducting substrate, I introduce a modular, electric field-driven deposition scheme termed electrophoretic deposition (EPD-iSCAT). Furthermore, combining a shaped incident beam with the applied potential generates a patternable pseudo-electrode on the substrate, directing localized single particle deposition in a technique we call light-directed (LD) EPD-iSCAT. This platform provides a versatile tool for studying spontaneous and fielddriven deposition kinetics, self-assembly, and nanofabrication. In addition, singleparticle counting in solution offers a quantitative measure of analyte polydispersity and concentration. Together, the methods extend the scope of iSCAT beyond biology, establishing a powerful framework for probing nanoparticle surface interactions across diverse chemical and physical systems.