Tapering to Enhance the Performance of Nanowire-Based Fluorescent Biosensors

High-refractive-index nanowires are of interest for signal enhancement in fluorescence-based biosensing. Here, through electromagnetic modelling, we demonstrate substantial fluorescence signal enhancement by incorporating a tapered section into a cylindrical nanowire and investigate two complementary tapering configurations to address practical implementation challenges. Under near-normal excitation, the excitation enhancement increases approximately linearly with taper length, reaching more than three times that of the corresponding non-tapered nanowire at a taper length of 8000 nm. In a non-imaging signal detection scheme using a low numerical aperture (NA), the proposed tapered nanowire achieves more than three orders of magnitude enhancement in fluorescence signal compared with a planar biosensor. Thus, the tapered nanowire should enable efficient fluorescence detection using compact, low-NA optical configuration. In imaging mode, similarly as for non-tapered nanowires, imaging sharpness beyond conventional microscopy limitations is obtained when imaging from the non-tapered nanowire end, while strongly increased excitation enhancement is simultaneously obtained when exciting from the opposite, tapered end.