A method for measuring the optical absorbance of a sample medium with a microfluidic chip that includes a substrate defining a microfluidic channel, a microresonator positioned within the microfluidic channel, and at least one waveguide or coupling surface evanescently coupled to the microresonator such that only supported Whispering Gallery Mode (WGM) resonance frequencies are transmitted from the waveguide or coupling surface into the microresonator. A sample medium is disposed into the microfluidic channel to substantially envelop the microresonator and a reader is provided to measure at least a portion of the WGM frequencies transmitted out of the microresonator.