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Enhanced optical absorption in droplet-based microfluidics for on-chip polymerase chain reactions

Microfluidic technologies and on-chip optical components have advanced such that on-chip sensing of minute chemical and molecular compounds is possible, e.g., detection of gases, pathogens, and DNA. Such DNA analyses require purification and amplification to maximize sensitivity. A common method for amplification of a DNA segment is polymerase chain reaction (PCR), which amplifies DNA segments through temperature changes in an assay process. As such, there is great interest in optofluidic lab-on-a-chip PCR methods. However, developments are limited due to challenges in optically driven temperature fluctuations. These challenges arise when the microfluidic samples are smaller than the optical penetration depth of the incident light and only minimal absorption is achieved. To overcome these challenges, this work presents a bio-photonic approach to the PCR method which utilizes infrared (IR) radiation with whispering gallery mode (WGM) waves. The WGM waves greatly increase the interaction length in the microdroplet, allowing smaller (and scalable) dimensions. This improved interaction length occurs because the applied IR radiation is confined along the perimeter of the microdroplet and its surrounding medium. The operation is modelled with finite-different time-domain electromagnetic simulations, comparing current optical heating with the presented technique. These simulations are validated through an experimental analysis with a thermal camera measuring temperature fluctuations. Ultimately, the presented approach is shown to greatly increase scalability in PCR lab-on-a-chip systems.