Thermally Enhanced Paper Microfluidics for Photonic Crystal Biosensors in Amorphous Silicon

Paper-based lateral flow devices (LFDs) are the technology behind low-cost, rapid, simple and portable point of care devices capable of displaying results within minutes. Targeted lateral flow devices are widely used for medical and environmental analyte detection but typically suffer in quantifying a detection. Paper-based LFDs are volume limited due to an upperabsorption limit but present a more environmentally friendly alternative to polydimethylsiloxane-based microfluidic alternatives. Photonic crystal biosensors are a valuable tool for label-free biosensing. Combining a paper-based lateral flow device with a photonic crystal biosensor allows for quantified analyte concentration measurements using guided-mode, or leaky-mode, resonance to measure a refractive index change at the photonic crystal’s surface. The primary challenge with any form of biosensor is the limit of detection. The photonic crystal biosensors reported in this work have a limit of detection of 10µg/mL. To reduce this limit of detection, without altering the performance of the sensor, we utilise two effects. Firstly, a thermal gradient across the paper LFD to accentuate a phenomenon known as the coffee-ring effect, which causes an outward flow of solute as a body of solution dries. Secondly, the physical design of the paper LFD and the resulting flow-manipulation of the analyte. The system we propose consists of a paper-based lateral flow device acting as a power-free pumping system, in which a thermal gradient is created using a targeted heat source to increase analyte localisation to reduce the limit of detection of a photonic crystal biosensor. Through optical measurements in reflection with a temperature gradient of 30˚C, we were able to reduce the limit of detection of a 2D photonic crystal to 6ng/mL. This result is a factor 103 lower than previously reported detection limits.