Properties of nanoparticles in contact with other surfaces are crucial for various applications. Here, we show that electrical potential of individual nanoparticles or their aggregates (sizes 5–50 nm) can be locally switched between negative and positive via work functions of surrounding materials (Si, Au, and Pt). We employed novel methodology of Kelvin probe force microscopy (KPFM) to characterize surface potentials of hydrogenated, thermally oxidized, and partially graphitized detonation diamond nanoparticles (5 nm), as well as gold nanoparticles (20 nm) deposited on the substrates (Si, Au) from colloidal dispersions in water or methanol. We present a comprehensive model of the observed effect based on work functions of the involved materials and charge trapping in or on the nanoparticles. The model is further confirmed by modification of nanoparticle charges after intentional application of up to ±4 V through the KPFM tip. The observed effects may impact other physical properties such as electron emission and photoluminescence of the nanoparticles.