Microfluidic devices with selectable optical pathlength for quality control of alcoholic solutions exploiting NIR spectroscopic properties of water and ethanol

In this work, we present a smart micro-opto-fluidic platform for the analytical detection of fluids exploiting the absorption spectroscopy technique in the near infrared wavelength region, from 1.15 μm to 1.65 μm. In the experimental configuration, the broadband radiation provided by a Tungsten lamp is fiber-coupled and shone onto a rectangular glass micro-capillary containing the sample. Top and bottom external sides of the micro-device are coated with thin Aluminum layers, deposited by sputtering, to create a zig-zag guiding effect: light crosses the capillary multiple times, thus increasing the optical path-length inside the sample fluid. Then, output light is directed towards an optical spectrum analyzer. For instance, this platform allows to detect pollution of alcohols by small quantities of water. As the work was performed during the Covid-19 pandemic, we took the opportunity to exploit it for specific quality control of hand sanitizer gels based on ethanol. The effect of water absorption around 1.45 μm is enhanced thanks to the long light path and the spectral shape of the absorption profile can be detected with a higher level of detail with respect to a single crossing of the channel, since the sensitivity increases. The sensing platform was calibrated by testing ethanol-water mixture and a responsivity parameter, defined as the ratio of the output power at two wavelengths, was retrieved. To validate the results, a sophisticated theoretical model, based on geometrical optical ray tracing approximation and Lambert-Beer law for solutions, was implemented in MATLAB environment: a good agreement between experimental results and theoretical predictions was found. The proposed optical readout technique is non-invasive, contactless and remote; moreover, the use of integrated reflectors is a simple and low-cost technology that makes our micro-opto-fluidic platform a smart device for specific sensing, suitable for investigation of ultra-low fluid volumes.