Based on a mid-infrared waveguide fabricated from Germanium on Silicon substrates, we present a concept of chemical sensors in aqueous solutions. The sensing scheme is utilizing the evanescent wave in a single-mode waveguide, and the absorption spectrum of chemicals dissolved in liquids. The waveguide is coupled to a singlemode mid-infrared quantum cascade laser [1], and an infrared detector. The wavelength of 5.81 microns is chosen to assure the specifity and sensitivity of the pilot application [2] for the analysis of cocaine analyte, in a water based matrix (saliva). A micro-fluidic system will be used for sample preparations.