Laser‐Scanning Microscopy for Electrophoretic Mobility Characterization of Single Nanoparticles

Abstract To enable detailed studies of interactions between nanoparticles and their environment and the correlations between various nanoparticle properties, one must go beyond ensemble averages and toward single‐particle measurements. However, current methodologies for the single‐nanoparticle analysis of charge and size either lack the flexibility to study dynamic processes on the single‐particle level or are highly specific and require complex microfluidic devices. In addition, accurate measurements of the electrophoretic mobility (or zeta‐potential) based on the optical detection of single nanoparticles remain challenging due to the low photon budget, the required sampling frequency, and the fact that electroosmosis in typical microfluidic devices must be analyzed carefully. In this study, a method is investigated to accurately characterize the electrophoretic mobility of individual nanoparticles and estimate their size by simultaneously analyzing the electrokinetic‐ and Brownian motion in a simple microfluidic channel. Fast laser scanning excitation and sensitive detection of fluorescent photons enable single‐nanoparticle velocimetry experiments in an oscillating electric field at high frame rates.