All cells generate an electrical potential across their plasma membrane driven by a concentration gradient of charged ions. A typical resting membrane potential ranges from -40 to -70 mV, with a net negative charge on the cytosolic side of the membrane. Maintenance of the resting membrane potential depends on the presence of two-pore-domain potassium "leak" channels, which allow for outward diffusion of potassium ions along their concentration gradient. Disruption of the ion gradient causes the membrane potential to become more positive or more negative relative to the resting state, referred to as "depolarization" or "hyperpolarization," respectively. Changes in membrane potential have proven to be pivotal, not only in normal cell cycle progression but also in malignant transformation and tissue regeneration. Using polystyrene nanoparticles as a model system, we use flow cytometry and fluorescence microscopy to measure changes in membrane potential in response to nanoparticle binding to the plasma membrane. We find that nanoparticles with amine-modified surfaces lead to significant depolarization of both CHO and HeLa cells. In comparison, carboxylate-modified nanoparticles do not cause depolarization. Mechanistic studies suggest that this nanoparticle-induced depolarization is the result of a physical blockage of the ion channels. These experiments show that nanoparticles can alter the biological system of interest in subtle, yet important, ways.