Holographic Micro-Coulometry for Measurement of Charge Density on Individual Nanobubble Surface

Abstract Nanobubbles are promising in environmental, agricultural, biomedical, and manufacturing settings, but quantitative measurement of the interfacial charge thought to underlie their unusual stability remains difficult. Here, we introduce holographic micro-coulometry (Holo-σ), which combines in-line digital holographic microscopy with microfluidic electrophoresis for in situ analysis of individual nanobubbles. Hologram reconstruction is used to verify that tracked objects are gaseous nanobubbles rather than solid particles and to follow the same object in three dimensions under field-free and applied-field conditions. These trajectories yield hydrodynamic diameter and electrophoretic mobility, from which zeta potential and surface charge density are determined. Benchmarking with Au, polystyrene, and silica standards shows good agreement with commercial electrophoretic light scattering. Applied to confirmed nanobubbles, Holo-σ resolves bubble-to-bubble charge heterogeneity, reveals an inverse relationship between surface charge density and bubble size, and quantifies electrolyte-dependent changes in the surface charge density and zeta potential. Holo-σ thus provides a practical route for single-nanobubble electrokinetic measurements.