When biomolecules bind to gold and silver nanoparticle surface, it causes a shift in localised surface plasmon resonance wavelength towards higher wavelength.
IMHO, and from a practical view, you can think about a overall change in the surrounding media refractive index, i.e. this sort of molecules are usually dielectric and its refractive index is quite upper than the RI of the medium where they are embedded (either aqueous or air), so that the LSPR is red-shifted. If you simulate -by means of a simple Mie calculus- both the effect of a thin dielectric film on the metallic particle and the effect of the surrounding medium (aqueous, n=1.33, vs air, n=1.0), you will obtain a similar red shift.
If you talk about SERS, there is a red shift in the frequency of the antenna contribution (see Resolving the electromagnetic mechanism of surface-enhanced light scattering at single hot spots: http://www.nature.com/ncomms/journal/v3/n2/full/ncomms1674.html?WT.ec_id=NCOMMS-20120221, for further details).
Yes, it changes the dielectric function of the surrounding media at the interface. This also influences the non-localized mode, the SPP. As an example the impact of C60 on Silver has been measured here: http://link.springer.com/article/10.1007/s11468-011-9298-0
http://link.springer.com/article/10.1007/s11468-011-9298-0
Just in agreement with all the previous answers. Other two very complete and interesting publicationso of our group here in leiden
DOI: 10.1038/nnano.2012.51
DOI: 10.1088/0034-4885/74/10/106401
I hope these refs help you in your research and also to all the people who has shown interest in your question.
I also agree with the previous answers. If you do this biosensor research, you might find that some researchers add Au nanoparticles to better see the red-shift 'coz these additional nanoparticles can induce a strong coupling from the Au surface and nanoparticles themselves will change the effective RI of the medium.
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