We report the application of an all-fiber setup for spectral reflectivity measurements on rectangular glass microcapillaries, in view of their application as micro-opto-fluidic devices for detection of solution concentration. We tested two kind of capillaries, both with 50 μm × 500 μm cross section but different wall thickness, respectively equal to 35 μm and 50 μm. Optical readout was provided by a broadband light source with λ_c = 1.55 μm. We here compare the results obtained on the two different devices in terms of sensitivity and limit of detection when tested with Glucose solutions in water.