Laying nanowires down with optoelectronic tweezers

Standard optical tweezers are used to manipulate one particle at a time by trapping it in a focused laser’s strong field gradients. OETs also use field gradients, but the gradients are created between electrodes by shining light (which needn’t be from a laser) onto a photoconductive material whose conductivity can change by three orders of magnitude. As a spot or pattern of light is moved across the material, the resulting electric field, its gradient, and any trapped particles also move. The University of California, Berkeley, group that developed OETs loaded a liquid buffer containing the particles of interest into a cell with the photomaterial and an electrode on the cell’s bottom. In the original OET, a second electrode was at the top; when an AC voltage was applied to the device, a vertical electric field was generated wherever the illumination fell, and nanorods could be stood upright and moved around....