An evanescent wave sensor for use in analyzing a medium, the sensor including a shell having a radiation port at a first end and a base at a second end. The base has a dimension greater than that of the radiation port, and inner and outer wall surfaces of the shell extend between the radiation port and the base. The shell is formed of a material having an index of refraction greater than that of the medium. An apparatus and method for analyzing the medium using the sensor may include a receptacle which defines a chamber about a portion of the shell for contacting the medium to the shell for analysis, optical elements for guiding radiation from a radiation source to the radiation port, and optical elements for guiding fluorescent radiation from the shell to a detector of fluorescent radiation.