A NIR-spectroscopy-based approach for detection of fluids in rectangular glass micro-capillaries

Rectangular glass micro-capillaries are very interesting devices that can be inserted in a micro-fluidic path and exploited for label-free optical sensing of ultra-low volumes of fluids. In the past, such devices have been used for the detection of the refractive index of fluids. In this work, we developed a smart micro-opto-fluidic platform that can distinguish water and alcohol samples flowing in the micro-channel thanks to the profile of their absorption spectrum in the near infrared (NIR) region from 1.15 to 1.65 µm. The readout technique is non-contact, remote and non-invasive. The micro-capillary, with wall thickness of 280 µm and channel depth equal to 400 µm, is laid flat onto an Aluminum bulk mirror and the light from a Tungsten lamp is shone on its upper flat side with an angle of incidence of 14°. The readout beam crosses the glass walls and the channel depth twice, since it is reflected by the mirror, and it is then coupled to the monochromator input of an optical spectrum analyzer. The theoretical transmission spectra T(λ) of the capillary filled just with air as well as with distilled water, isopropanol, ethylene glycol, and 95% ethanol (with 5% water content) are obtained using analytical equations including the wavelength-dependent attenuation due to fluid absorption. Then experimental measurements are carried out and the experimental spectral response, defined as SR(λ) = T_sample(λ)/T_air(λ), is compared with the theoretical one, revealing a very good level of agreement.