Mass measurement of single nanoparticle by trapping in water droplet

We present a method of determining the mass of a single picogram particle by observing its Brownian motion near the bottom of a droplet of water. The motion of the particle caused by thermal fluctuation is restricted within a narrow region because of gravity, and the vertical displacement of the particle from the bottom of the droplet depends on its mass. Since the particle is trapped inside the droplet, the mean horizontal displacement decreases as the mass of the particle increases. Hence, the mass can be determined by observing displacement. Although a modeling error arises from neglecting the electrical double-layer interaction between a particle and water surface, we show that its influence on the mass is very small.