  Optical and optoelectronic techniques for micro- and nano-object manipulation
are becoming essential tools in nano- and bio-technology. A remarkable
optoelectronic technique that has experimented a strong development in the last
few years is the so called photovoltaic optoelectronic tweezers. It is based on
the light-induced electric fields generated by the bulk photovoltaic effect in
certain ferroelectrics such as LiNbO3. The technique is simple and versatile,
enabling a successful manipulation of a large variety of micro- and
nano-objects with only optical control, without the need of electrodes or power
supplies. However, it is still a challenge for this tool, to handle objects
immersed in aqueous solution due to the electric screening effects of polar
liquids. This has hindered their application in biotechnology and biomedicine
where most processes develop in aqueous solution. In this work, a new efficient
route to overcome this problem has been proposed and demonstrated. It uses
photovoltaic optoelectronic tweezers to manipulate aqueous droplets, immersed
in a non-polar oil liquid, but hanging at the interface air-oil. In this
singular configuration, the high electric fields generated in the photovoltaic
substrate allow a simple and flexible manipulation of aqueous droplets
controlled by the light. Droplet guiding, trapping, merging and splitting have
been achieved and efficient operation with water and a variety of bio-droplets
(DNA, sperm, and PBS solutions) have been demonstrated. The reported results
overcome a main limitation of these tweezers to handle bio-materials and
promises a high potential for biotechnological and biochemistry applications
including their implementation in optofluidic devices.
