The present invention features use of evanescent field sensing to detect or evaluate unpurified, unlabeled analytes in raw samples. Capture molecules capable of binding to an analyte of interest can be immobilized to a sensing surface of an evanescent field sensor. The interaction between the analyte of interest in a raw sample and the immobilized capture molecules alters the refractive index at the sensing surface, thereby producing a detectable signal in the reflected light from the surface. Raw samples amenable to the present invention include, but are not limited to, cell lysates, tissue extracts, bodily fluid, or biologically-derived samples. Evanescent field sensors suitable for the present invention include, but are not limited to, grating-coupled waveguides.