Optoelectronic Manipulation, Trapping, Splitting, and Merging of Water Droplets and Aqueous Biodroplets Based on the Bulk Photovoltaic Effect

Optical and optoelectronic techniques for micro- and nano-object manipulation are becoming essential tools in nano- and biotechnology. The use of photovoltaic optoelectronic tweezers is a remarkable optoelectronic technique that has undergone rapid development in recent years, with excellent results and widespread potential applications. It is based on light-induced electric fields generated by the bulk photovoltaic effect in certain ferroelectrics, such as ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_{3}$. The technique is simple and versatile, enabling the successful manipulation of a large variety of micro- and nano-objects with only optical control, without the need for electrodes or power supplies. However, the handling of objects in aqueous solutions remains a challenge for this tool, due to the electric-screening effects of polar liquids. This has hindered their application in biotechnology and biomedicine, where most processes occur in aqueous solution. Here, an efficient route to overcome this problem is proposed and demonstrated. It uses photovoltaic optoelectronic tweezers to manipulate aqueous droplets, immersed in a nonpolar oil liquid, but hanging at the air-oil interface. In this singular configuration, the high electric fields generated by the bulk photovoltaic effect in the ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_{3}$ substrate allow simple and flexible manipulation of aqueous droplets controlled by the light. Droplet guiding, trapping, merging, and splitting are achieved and efficient operation with water and a variety of biodroplets (DNA, sperm, and phosphate-buffered saline solutions) is demonstrated. A discussion on the physical mechanisms responsible for the manipulation processes is also provided. The reported results overcome the main limitation of these tweezers to handle biomaterials and promise high potential for biotechnological and biochemistry applications, including their implementation in optofluidic devices.