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Electrowetting-on-dielectric (EWOD) has been extensively explored as an active-type technology for small-scale liquid handling due to its several unique advantages, including no requirement of mechanical components, low power consumption, and rapid response time. However, conventional EWOD devices are often accompanied with complex fabrication processes for patterning and wiring of 2D arrayed electrodes. Furthermore, their sandwich device configuration makes integration with other microfluidic components difficult. More recently, optoelectrowetting (OEW), a light-driven mechanism for effective droplet manipulation, has been proposed as an alternative approach to overcome these issues. By utilizing optical addressing on a photoconductive surface, OEW can dynamically control an electrowetting phenomenon without the need for complex control circuitry on a chip, while providing higher functionality and flexibility. Using commercially available spatial light modulators such as LCD displays and smartphones, millions of optical pixels are readily generated to modulate virtual electrodes for large-scale droplet manipulations in parallel on low-cost OEW devices. The benefits of the OEW mechanism have seen it being variously explored in its potential biological and biochemical applications. This review article presents the fundamentals of OEW, discusses its research progress and limitations, highlights various technological advances and innovations, and finally introduces the emergence of the OEW technology as portable smartphone-integrated environmental sensors. 

We present a plasmonic-enhanced dielectrophoretic (DEP) phenomenon to improve optical DEP performance of a floating electrode optoelectronic tweezers (FEOET) device, where aqueous droplets can be effectively manipulated on a light-patterned photoconductive surface immersed in an oil medium. To offer device simplicity and cost-effectiveness, recent studies have utilized a polymer-based photoconductive material such as titanium oxide phthalocyanine (TiOPc). However, the TiOPc has much poorer photoconductivity than that of semiconductors like amorphous silicon (a-Si), significantly limiting optical DEP applications. The study herein focuses on the FEOET device for which optical DEP performance can be greatly enhanced by utilizing plasmonic nanoparticles as light scattering elements to improve light absorption of the low-quality TiOPc. Numerical simulation studies of both plasmonic light scattering and electric field enhancement were conducted to verify wide-angle scattering light rays and an approximately twofold increase in electric field gradient with the presence of nanoparticles. Similarly, a spectrophotometric study conducted on the absorption spectrum of the TiOPc has shown light absorption improvement (nearly twofold) of the TiOPc layer. Additionally, droplet dynamics study experimentally demonstrated a light-actuated droplet speed of 1.90 mm/s, a more than 11-fold improvement due to plasmonic light scattering. This plasmonic-enhanced FEOET technology can considerably improve optical DEP capability even with poor-quality photoconductive materials, thus providing low-cost, easy-fabrication solutions for various droplet-based microfluidic applications. 

A lab-on-a-smartphone (LOS) presents a portable environmental sensing tool that enables the monitoring of water quality by performing various detection techniques such as smartphone-integrated fluorescence microscopy and portable loop-mediated amplification (LAMP) assays. The LOS can conduct multiple laboratory functions and has experimentally demonstrated (1) automated on-chip water sample processing, (2) on-site fluorescent detection of harmful algae cells, and (3) fecal contamination of water through LAMP assays. The LOS can overcome conventional labor-intensive and time-consuming techniques for the monitoring of microbiological contaminants in environment waters. 

A smartphone-integrated dielectrophoretic (SiDEP) platform is presented for on-site and real-time monitoring of fecal indicator bacteria (FIB) to examine the presence and concentration levels of fecal contamination in environmental water via loop-mediated isothermal amplification (LAMP) assays on a smartphone. Experimental demonstrations have verified the SiDEP’s capabilities for (1) on-chip water sample processing, (2) portable LAMP assays, and (3) colorimetric analysis of fecal water quality. The SiDEP truly offers a low-cost, portable, and fully-integrated system enabling rapid on-site detection of the presence of FIB and their associated pathogens in environmental water without the need for sophisticated laboratory equipment or skilled personnel.