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. At the Conference on Lasers and Electro-Optics (CLEO) in May, the group revealed a lateral-field version (LOET) with the electrodes interleaved on the bottom of the cell. That configuration allowed the researchers to orient and move nanowires parallel to the plane of the electrodes, as shown in the images. In the first panel, the circled nanowire is close to the spot of light prior to the AC field being switched on. Subsequently, the wire is drawn into the spot and moved 200 microns in about 10 seconds before the light is turned off. According to Berkeley’s Aaron Ohta, the OET uses much less power than optical tweezers, and now the LOET can be used to build circuitry. (CLEO paper CThGG5, abstract available at http://www.cleoconference.org .)