Near-infrared surface-enhanced Raman spectroscopy of biomedically relevant single molecules on colloidal silver and gold clusters

Surface-enhanced Raman scattering (SERS) is a phenomenon resulting in strongly increased Raman signals from molecules which have been attached to nanometer sized metallic structures. The technique combines fingerprint capabilities of vibrational spectroscopy and ultrasensitive detection limits. Silver or gold colloidal clusters can provide total enhancement factors of about 14 orders of magnitude for non-resonant Raman scattering at near infrared excitation. Since non-resonant near infrared photons are used, photodecomposition of the probed molecule is avoided or, at least strongly reduced, and relatively high excitation intensities can be applied. In addition to the Stokes Raman signal, that linearly depends on excitation laser intensity, at excitation intensities higher than approximately 10(5)-10(6) W/cm(2) and 10(7) W/cm(2), "pumped" anti-Stokes Raman scattering and Surface enhanced hyper Raman scattering respectively, can be observed. Both effects can provide a non-linear or two-photon Raman probe where the Raman scattering signal depends quadratically on the excitation laser intensity.