Two-photon vibrational spectroscopy for biosciences based on surface-enhanced hyper-Raman scattering

*Wellman Center for Photomedicine, Harvard Medical School, Boston, MA 02114; and
^†Federal Institute for Materials Research and Testing, D-12489 Berlin, Germany

Communicated by Robert J. Silbey, Massachusetts Institute of Technology, Cambridge, MA, September 20, 2006 (received for review May 16, 2006)

Abstract

Two-photon excitation is gaining rapidly in interest and significance in spectroscopy and microscopy. Here we introduce a new approach that suggests versatile optical labels suitable for both one- and two-photon excitation and also two-photon-excited ultrasensitive, nondestructive chemical probing. The underlying spectroscopic effect is the incoherent inelastic scattering of two photons on the vibrational quantum states called hyper-Raman scattering (HRS). The rather weak effect can be strengthened greatly if HRS takes place in the local optical fields of gold and silver nanostructures. This so-called surface-enhanced HRS (SEHRS) is the two-photon analogue to surface-enhanced Raman scattering (SERS). SEHRS provides structurally sensitive vibrational information complementary to those obtained by SERS. SEHRS combines the advantages of two-photon spectroscopy with the structural information of vibrational spectroscopy and the high-sensitivity and nanometer-scale local confinement of plasmonics-based spectroscopy. We infer effective two-photon cross-sections for SEHRS on the order of 10⁻⁴⁶ to 10⁻⁴⁵ cm⁴·s, similar to or higher than the best “action” cross-sections (product of the two-photon absorption cross-section and fluorescence quantum yield) for two-photon fluorescence, and we demonstrate HRS on biological structures such as single cells after incubation with gold nanoparticles.

Footnotes

^‡To whom correspondence may be addressed at:
Harvard Medical School, 40 Blossom Street, Boston, MA 02114.
E-mail: kneipp{at}usa.net or jkneipp{at}janina-kneipp.de
Author contributions: J.K., H.K., and K.K. designed research; J.K., H.K., and K.K. performed research; J.K. and H.K. analyzed data; and J.K. and K.K. wrote the paper.
The authors declare no conflict of interest.
Abbreviations:

HRS,

hyper-Raman scattering;

SEHRS,

surface-enhanced HRS;

SERS,

surface-enhanced Raman scattering;

cw,

continuous wave;

GM,

Goeppert-Mayer
Freely available online through the PNAS open access option.