Schematic of size-selective sensing experiment and reflectance spectra of BSW/BSSW sensors after APTES and large latex sphere attachment. (a) Schematic illustration of size-selective sensing experiment in which both small and large species are exposed to the PSi BSW/BSSW sensor. Reflectance spectra of (b) grating-coupled step index and (c) prism-coupled gradient index BSW/BSSW sensors after exposure to small (APTES) and large (spheres) species. The BSW resonance and BSSW resonances are labeled in the figure. The BSW resonance in each sensor shifts to higher angle when APTES and spheres are attached to the respective sensor but the BSSW resonances only shift with APTES attachment because the BSSW modes do not extend above the sensor surface where the spheres are bound. Table 1 summarizes the resonance shifts shown in this figure.