We describe a method of selectively detecting the presence of an analyte, the method comprising: providing at least one waveguide, the waveguide having a core comprising porous material; absorbing an analyte sample into said porous material of said core such that said analyte sample is held within pores of said core; waveguiding radiation along said at least one waveguide to an output to provide output radiation; measuring one or more spectral features of said output radiation due to absorption or scattering of said waveguided radiation by said absorbed analyte sample; selectively identifying the presence of a target analyte in said analyte sample from said one or more spectral features. In embodiments spectral features are measured for multiple different waveguide core regions having different physical/chemical properties modified to provide additional selectivity to the target analyte(s), and these measurements combined to identify the target analyte.