Plasmonic nanoparticle-enhanced optoelectrowetting (OEW) for effective light-driven droplet manipulation

Abstract

With the benefits of device simplicity and functionality, optoelectrowetting (OEW) has been studied as a liquid-handling approach for lab-on-a-chip applications. Recent studies have further demonstrated three-dimensional (3D) OEW technology by using a polymer-based photoconductive material, titanium oxide phthalocyanine (TiOPc), which allows device fabrication on flexible substrates via a low-temperature spin-coating method. However, one critical drawback of this TiOPc material is its low-quality photoconductive property and thus corresponds to very poor OEW modulation. Our study herein presents light absorption of the TiOPc largely enhanced by using plasmonic nanoparticles to significantly improve OEW performance for effective light-driven droplet manipulation. Metallic nanoparticles dispersed on the TiOPc layer enable to induce plasmonic light scattering with an increased optical path length. Consequently, more light rays can be absorbed onto the TiOPc and dramatically increase its photoconductivity to enhance OEW performance. Our measurement studies have verified a 2-order improvement in the TiOPc’s light absorption performance by using aluminum (Al) nanoparticles. We have experimentally demonstrated that a 4 μm thick layer of Al nanoparticles fabricated with a 2.0 wt% solution enables 60.4° more contact angle modulation than the one used with no nanoparticles. The droplet dynamics study has also presented the light-actuation speed of a droplet as 12.5 mm/s enhanced by plasmonic light scattering, which is a 39-fold faster speed than that without nanoparticles. Our plasmonic nanoparticle-enhanced OEW technology can offer device simplicity, flexibility and functionality, while providing much enhanced OEW performance useful for various digital microfluidic (DMF) applications by allowing effective light-driven droplet manipulations.