Enhanced fluorescence in a nanoporous waveguide and its quantitative analysis

Fluorescence behavior was examined for fluorophore-labeled protein (BSA-AF) adsorbed on the nanopore surface of a nanoporous waveguiding film. The waveguiding film has a bilayer structure of a porous anodic alumina (PAA) layer on a metallic aluminum (Al) layer, and this structure allows efficient interaction of fluorophores entrapped in the nanoporous waveguiding film with a hotspot of the enhanced electromagnetic field of the waveguide modes. Fluorescence response of BSA-AF depends on the enhanced field within the waveguiding film and the enlarged adsorbed amount in the PAA layer where most of the light is confined. Enhancement of the field in the waveguiding film can be controlled by the refractive index of the PAA layer and enlargement of the pore size efficiently affects the enhancement of the fluorescence response. Compared to the film without a PAA layer, the PAA/Al film exhibits more than 140-fold larger fluorescence response due to the large adsorption capacity of the PAA nanopores and the enhanced field formed by the waveguide modes in the PAA layer with a low refractive index.