Microfluidic optical sensor comprising: an optical waveguide capable of propagating light from an optical input port to an optical output port, the optical waveguide comprising an optical waveguide interaction region; a fluidic channel capable of conducting a fluid from a fluid input port to a fluid output port, the fluidic channel comprising a fluidic channel region; the fluidic channel region being separated from the optical waveguide interaction region by an interposed spacing material configured to transmit an evanescent field of the light through the spacing material between the optical waveguide interaction region and the fluidic channel region. Microfluidic optical sensor comprising an optical resonator. Methods for making microfluidic optical sensors.