Optical sensors are fabricated by depositing liquid sol–gel sensor material on a polycarbonate surface, which has been decorated with arrays of periodic micropillars. Using the principle of hemiwicking, the liquid material is spread, guided by the surface structures, to homogeneously fill the volume between the surface structures and form a liquid film with a thickness determined by the height of the micropillars. After evaporation of solvents, a uniform layer of sensor material resides on the surface. This fabrication method enables easy and reproducible deposits of isolated spots of different sensor materials of precise thickness to be made on plastic surfaces, and it provides an improved method for fabricating cheap optical sensors integrated in disposable lab containers.