High aspect ratio semiconducting nanostructures have gained increasing interest as platforms for fluorescence based biosensing. Here, well-defined biofunctionalized vertical semiconducting nanowires are used to map the interaction of light with nanowires. We identified how nano-optical effects modify biosensing fluorescence detection at the nanowire interface - as it modulates the photon flux in their surroundings through interactions with the electromagnetic field of the light. We presented a novel approach to understanding the complex effects at the nanostructure-biomolecule-light interface, both experimentally and theoretically. We show how nanowires act as antennas modifying the light distribution and the emitted fluorescence This work highlights the important of nano-optical phenomenon at nanostructures to obtain highly sensitive fluorescence based biosensing platforms.