A method of producing a surface enhanced Raman scattering spectrum which is useful for certain types of assays, in particular proximity assays. The method includes providing two SERS-active nanoparticles. The first SERS-active nanoparticle will absorb a photon at a first wavelength and emit a Raman-shifted photon at a second wavelength. The second SERS-active nanoparticle will absorb a photon at the second wavelength and emit a Raman-shifted photon at a third wavelength. Accordingly, when the first and second SERS-active nanoparticles are proximate to one another and the first SERS-active nanoparticle is illuminated at the first wavelength a Raman-shifted photon at the second wavelength may be emitted. This photon can be absorbed by the second SERS-active nanoparticle causing detectable emission of a second Raman-shifted photon at the third wavelength. Various assays may be designed based upon the above. Proximity assays using two SERS-active nanoparticles will have advantageous background signal characteristics.
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