  Plasmonic biosensing chips were prepared by fabricating wavelength-scaled
dielectric-metal interfacial gratings on polymer film covered bimetal layers.
Lysozyme biomolecules (LYZ) and gold nanoparticle bioconjugates (AuNP-LYZ) with
1:5 mass ratio were seeded onto the biochip surfaces. Comparison of the
reflectance curves measured in a modified Kretschmann arrangement and computed
numerically proved that monitoring the narrower secondary peaks under optimal
rotated-grating coupling condition makes it possible to achieve enhanced
sensitivity in biodetection. The enlarged resonance peak shift is due to the
horizontally and vertically antisymmetric long-range plamonic modes propagating
at the edge of the valleys and hills, which originate from Bragg scattered
surface plasmon polaritons. The sensitivity is further increased in case of
bioconjugates due to the coupled localized resonances on Au NPs.
