Sensitive label-free biosensors by using gap plasmons in gold nanoslits

The detection sensitivities of gap plasmons in gold nanoslit arrays are studied and compared with surface plasmons on outside surface. The nanoslit arrays were fabricated in a 130 nm-thick gold film with various slit widths. For transverse-magnetic (TM) incident wave, the 600 nm-period nanoslit array shows two distinguishable transmission peaks corresponding to the resonances of gap plasmons and surface plasmons, respectively. The surface sensitivities for both modes were compared by coating thin SiO2 film and different biomolecules on the nanoslit arrays. Our experimental results verify gap plasmons are more sensitive than conventional surface plasmons. Its detection sensitivity increases with the decrease of slit width. The gap plasmon is one order of magnitude sensitive than the surface plasmon for slit widths smaller than 30 nm. We attribute this high sensitivity to the large overlap between biomolecules and nanometer-sized gap plasmons.