Reflectance spectra illustrating resonance shifts of the BSW/BSSW modes caused by small linker molecules and the M13KO7 bacteriophage. (a) Angular reflectance spectra of an oxidized gradient index BSW/BSSW sensor measured before (black) and after the attachment of APTES (blue) and GA (red). The spectra are offset for clarity. The lowest angle resonance on each plot corresponds to the BSW mode. Three BSSW resonances appear at higher angles. (b) Resonance shifts of the BSW and 1st BSSW mode after the attachment of M13KO7 bacteriophage to the GA functionalized gradient index BSW/BSSW sensor shown in (a). Quantification of the angular shifts is reported in the text.