Demonstration of surface sensing using long-range surface plasmon waveguides on silica

Surface sensing experiments conducted using long-range surface plasmon-polaritons propagating along thin Au stripes in a symmetric dielectric environment are reported. The Au stripes are deposited on SiO2 and covered with an optical fluid index-matched to SiO2. The experimentation vehicles are integrated Mach-Zehnder interferometers (MZIs) implemented using these stripes and operated in the main long-range surface plasmon-polariton mode. Bovine serum albumin (BSA) was deposited on MZIs via microspotting, covering in some cases the entire MZI with BSA, and in other cases only a portion of one arm. The wavelength response of the MZIs was then measured and a wavelength shift of 10 nm was observed for an MZI having a partially covered arm relative to uncovered and fully covered MZIs. A 3.8 nm thick adlayer of BSA on the arm accounts for the observed wavelength shift as deduced via theoretical modelling. This thickness is consistent with the view that BSA adsorbs in a close-packed monomolecular layer on the surface. The 10 nm wavelength shift observed is comparable with the response expected from conventional SPR under wavelength interrogation. However, the MZIs used for the experimentation were selected based on availability and were not originally intended for surface sensing, larger shifts are therefore expected in appropriately optimised structures.