We demonstrate a robust and highly responsive optical microsensor, which probes the refractive index of liquids flowing along a ~ 100 mum radius channel formed in a polymer matrix. Sensing is based on measurement of the transmission spectrum of the whispering gallery modes, which are excited across the liquid channel by an optical microfiber imbedded into the polymer. The achieved sensitivity is 800 nm/RIU. Potentially, it is straightforward to assemble the sensing elements of this type into a lab-on-the-chip imbedded in a solidified optical material.