Fluorophore signal detection and imaging enhancement in high refractive index nanowire biosensors

Abstract

High refractive index semiconductor nanowires have recently been demonstrated experimentally as an efficient platform for enhancing the signal in fluorescence-based biosensors. Here, we study through modelling how a vertical GaP nanowire (i) enhances the excitation intensity at the position of the fluorophore attached to the nanowire sidewall, (ii) enhances the probability to collect photons emitted from the fluorophore by directing them preferentially into the numerical aperture of the collection objective, and (iii) through the Purcell effect increases the quantum yield of the fluorophore. With appropriate choice for the geometry of the nanowire, we can reach a larger than $10^2$ enhancement in signal compared to a corresponding conventional planar biosensor platform. We model also imaging-based detection. There, we find that thanks to waveguiding in the nanowire, we can beat the limitations set by the depth of view in conventional microscopy, enabling the use of a long nanowire to enhance the binding-area for fluorophores. As an example, we can focus to the top of a 4000 nm long nanowire and reach a 25 times sharper image from a fluorophore at the bottom of the nanowire, as compared to such a 4000 nm defocusing in a conventional planar biosensor platform.