On-Demand Droplet Spotter for Preparing Pico- to Femtoliter Droplets on Surfaces

A droplet spotter for on-demand generation of pico- to femtoliter droplets was developed to meet the requirement for high-density spotting of chemicals on a surface. Our droplet spotter involves applying a ∼1000-V and ∼10-ms pulse voltage to the tip of a capillary tube (o.d. ∼18 μm; i.d. ∼11 μm) supplied with water or a dye solution. The capability of the spotter was demonstrated by preparing a microarray of dye molecules. The microarray was prepared by spotting ∼30-fL droplets of a dye solution on a surface at the density of one spot per 20 × 20 μm2.