A Si/Ge based, single mode, mid-infrared optical waveguide, is used as an evanescent sensor in liquid matrix. The selectivity is achieved by the use of quantum cascade lasers (QCLs) tuned to the maximum absorption of a given analyte. The cocaine detection in human saliva at 5.81 microns is chosen as a pilot application to demonstrate the technology. Light emitted from the QCL is butt coupled into the Si/Ge waveguide. The waveguide is passing through the detection part of the micro-fluidic sample preparatory/handler, and interacting with the analyte, it is collected and guided into the mid infrared quantum cascade detector QCD.