Development of Integrated Optofluidic Biosensor to Image Protein-Protein Interactions

Protein-protein interaction plays a pivotal role in most cellular activities, including cell division, proliferation, and signaling. Therefore, studying protein interactions is essential for understanding the fundamental mechanism of cellular activities, disease diagnosis, and drug development. Conventional approaches to study protein interactions include the enzyme-linked immunosorbent assay (ELISA) and the Forster resonance energy transfer (FRET), but the drawback is that these technologies require optical labeling. Currently, label-free photonic biosensors are getting attention because of their high sensitivity, small footprint, and multiplex assay. However, applications for photonic biosensors are mostly limited to the detection of biomolecules either by trapping the analyte of interest or by immobilizing interacting ligands. Therefore, our goal of this study is to develop label- and immobilization-free optofluidic biosensors that can image freely interacting proteins. The technological gaps are that (<i>i</i>) the current approaches to integrate microfluidic channels are not confined to the sensing area which requires larger sample volume than needed and (<i>ii</i>) to our best knowledge, there have been no studies reported with surface modification or functionalization which enables proteins to interact freely.<br/> Here, we demonstrate ring-assisted Mach-Zehnder Interferometer (RA-MZI)-based biosensor with microfluidics confined on the sensing area. RA-MZI photonics component measures the change of local refractive index upon protein-protein interaction. Due to the innate characteristic of the resonator, such as high quality factor or large extinction ratio, ring resonator-based sensors show low limit of detection and high sensitivity. To overcome the fabrication variations and further maximize the sensitivity of sensor, we have incorporated microheater on both reference and sensing arms. By manipulating the temperature, we have increased the extinction ratio, and in return, increased the sensitivity. Our next steps will be to coat the microfluidics surface with supported lipid bilayer so that we can image protein interaction while both interacting proteins are relatively freely moving on the lipid membrane. This label- and immobilization-free optofluidic biosensor will allow us to discover novel approaches to early-stage detection by studying the mechanisms and fundamentals of disease-associated protein interactions.