A CubeSat-Compatible Optothermal Sensing Platform for Biofilm Fluorescence Monitoring

This paper presents a compact Optothermal Sensing Platform (OSP) developed as the core transducer of the BOREALIS CubeSat payload. The platform operates in reflection mode beneath a movable PMMA microfluidic chip and integrates patterned optical filtering, thin-film photodetection, local thermal actuation, temperature sensing, and optical shielding, while being interfaced with custom frontend electronics and payload-level data acquisition. The OSP includes a patterned SiO2/TiO2 interference filter that rejects the 470–490 nm excitation band while preserving transmission above 500 nm. The platform integrates two optical sensing sites, each based on a lithographically defined excitation aperture in the filter. Each aperture is surrounded by sixteen hydrogenated amorphous silicon (a-Si:H) p-i-n photodiodes. The same device incorporates a-Si:H temperature sensors and Cr/Al/Cr lateral thin-film heaters. Optical characterization showed 90–95% transmission through the excitation apertures, low dark-current densities of the photosensors, and spectral selectivity compatible with Green Fluorescent Protein (GFP) fluorescence detection. Thermal characterization demonstrated localized heating of the active region, agreement between simulations and thermography, and a temperature sensor sensitivity of approximately –3.75 mV °C−1. Integrated breadboard tests using a 470 nm LED, custom low-noise readout electronics, syringe-pump-driven microfluidics, fluorescein solutions, and preliminary measurements on a GFP-expressing Escherichia coli biofilm sample validated the OSP as the sensing core of a CubeSat-compatible fluorescence platform.