Colloidal Nanoparticles Hold Elementary Charge in Nonpolar Solvent

The net charge of individual nanoparticles in nonpolar solvents plays a critical role in their intrinsic properties like charge carrier lifetime, electron transport, and interparticle interactions. However, there is a long-standing belief that the oil-dispersed nanoparticles inherently possess no net charge. This work presents an approach for directly quantifying the net charge of individual nanoparticles. We employ an alternating electric field coupled with in situ observation under an optical microscope to analyze the force and motion of sub-50 nm silver nanoparticles. This technique directly reveals that around 1-10% of these colloidal nanoparticles carry a single elementary charge in nonpolar solvents, while the majority remain essentially neutral. This represents the first quantitative measurement of a single nanoparticle's charge in a nonpolar solvent.