Im developed a SPR biosensor with gold thin-film deposited on optical fiber. After chemical groups functionalization on the gold layer, I incubate the sensor with probe-DNA and target-DNA, each in 60 minutes time. During the first 10 minutes of DNA hybridization (forming double standed DNA) I see a blue shift in resonance wavelength. After 10 minutes, the SPR resonance will be red-shifted until the end of immobilization time of 60 minutes. Can somebody explain this result? I see few papers with blue-shifted peak in localized-SPR (metal nanoparticles), but not in gold thin-film.

BTW the gold thickness is 50 nm.