Optical Readout Techniques for Smart Detection of Fluids in the Near Infrared Wavelength Region by Means of Rectangular Glass Micro-capillaries

Three different approaches for the analytical detection of fluids by means of rectangular glass micro-capillaries working in the near infrared wavelength region are presented. At first, a non-specific refractometric measurement for the detection of glucose concentration in solutions is reported, exploiting the micro-capillaries as optical resonators: by monitoring the spectral shift of the ratio between the transmitted and reflected optical spectra (T/R) from the capillary, it is possible to extract the wavelength positions of the cavity resonances (maxima of T/R) for fluids with different refractive index. When the refractive index of the sample fluid filling the channel increases, a shift towards longer wavelengths is observed. Then, a spectral phase shift interferometric technique for the detection of the wavelength position of the resonances is proposed. When the capillary is inserted in an interferometric setup, it is possible to distinguish fluids by knowing the dependence of the wavelength positions of the steep jumps in the cosine signal on the refractive index of the filling fluid. Finally, the potentiality of micro-capillaries is investigated for specific sensing, exploiting absorption spectroscopy. All the proposed optical readout approaches are remote, contactless and non-invasive. In addition, the glass-micro-capillaries are very suitable for analytical detection of fluids: they are low-cost devices, available in several formats. Thanks to their micrometric size, they can be incorporated in micro-fluidic circuits. Borosilicate glass is a bio-compatible material, allowing the use of the micro-fluidic platforms in a wide range of applications for label-free optical sensing.