Harald Kneipp

Surface-enhanced Raman scattering and biophysics

Surface-enhanced Raman scattering (SERS) is a spectroscopic technique which combines modern laser spectroscopy with the exciting optical properties of metallic nanostructures, resulting in strongly increased Raman signals when molecules are attached to nanometre-sized gold and silver structures. The effect provides the structural information content of Raman spectroscopy together with ultrasensitive detection limits, allowing Raman spectroscopy of single molecules. Since SERS takes place in the local fields of metallic nanostructures, the lateral resolution of the technique is determined by the confinement of the local fields, which can be two orders of magnitude better than the diffraction limit. Moreover, SERS is an analytical technique, which can give information on surface and interface processes. SERS opens up exciting opportunities in the field of biophysical and biomedical spectroscopy, where it provides ultrasensitive detection and characterization of biophysically/biomedically relevant molecules and processes as well as a vibrational spectroscopy with extremely high spatial resolution. The article briefly introduces the SERS effect and reviews contemporary SERS studies in biophysics/biochemistry and in life sciences. Potential and limitations of the technique are briefly discussed.

Extremely Large Enhancement Factors in Surface-Enhanced Raman Scattering for Molecules on Colloidal Gold Clusters

In agreement with previous results reported for colloidal silver clusters, effective surface-enhanced Raman cross sections of about 10-16 cm2 per molecule, corresponding to enhancement factors on the order of 1014, have also been obtained for molecules attached to colloidal gold clusters. Spatially isolated nearly spherical colloidal gold particles of about 60 nm size show maximum enhancement factors on the order of 103 at 514 nm excitation, close to the single plasmon resonance. The enhancement factor increases by eleven orders of magnitude when colloidal gold clusters are formed by aggregation of the gold colloids and when near-infrared excitation is applied. The large effective surface-enhanced Raman cross section has been estimated by a straightforward method based on steady-state population redistribution due to the pumping of molecules to the first excited vibrational state via the strongly enhanced Raman process. Our experimental finding confirms the important role of colloidal clusters for extremely large surface-enhanced Raman scattering (SERS) enhancement factors. Simultaneously, it suggests colloidal gold clusters as a substrate for high-sensitivity surface-enhanced Raman scattering, which can provide an enhancement level sufficient for Raman single molecule detection. Due to its chemical inactivity, gold might have some advantages compared to silver, particularly in biomedical spectroscopy.

Surface-enhanced non-linear Raman scattering at the single-molecule level

Abstract Surface-enhanced hyper-Raman scattering and surface-enhanced anti-Stokes Raman scattering were studied as potential tools for non-linear single-molecule Raman spectroscopy. Experiments were performed using near-infrared excitation on crystal violet adsorbed on colloidal silver or gold clusters. Strong enhancement factors on the order of 10 20 were inferred from hyper-Raman scattering experiments on colloidal silver. Such extremely high enhancement factors overcome the inherently weak nature of the effect, and surface-enhanced hyper-Raman scattering appears on comparable intensity levels as surface-enhanced Raman scattering. Surface-enhanced anti-Stokes Raman scattering starts from vibrational levels, that are populated by the very strong surface-enhanced Raman process. Thus, the anti-Stokes Raman scattering signal depends quadratically on the excitation laser intensity. For the first time, surface-enhanced anti-Stokes and Stokes Raman scattering was detected from single molecules on colloidal gold clusters.

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

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.

Surface-enhanced Raman scattering (SERS)—a new tool for single molecule detection and identification†

This report describes surface-enhanced Stokes and anti-Stokes Raman scattering of molecules in aqueous colloidal silver solution using non-resonant near-infrared excitation. We demonstrate that extremely large surface-enhanced Raman cross sections of the order of 10−16 cm2 per molecule can be combined with favorable conditions for excitation and collection of Raman scattered light provided by a Raman microscope to achieve single molecule sensitivity. Surface-enhanced Raman spectroscopy will be compared with fluorescence spectroscopy as a tool for single molecule detection.

NEAR-INFRARED SURFACE-ENHANCED RAMAN SCATTERING CAN DETECT SINGLE MOLECULES AND OBSERVE 'HOT' VIBRATIONAL TRANSITIONS

Surface-enhanced Raman scattering (SERS) at an extremely high enhancement level opens up interesting and new spectroscopic possibilities. The effect combines the sensitivity of fluorescence spectroscopy with the high structural information content of Raman spectroscopy, and can be used for single molecule detection and identification. This paper reports single molecule detection and identification of ‘non-absorbing’ molecules in colloidal silver solutions using near-infrared excited surface-enhanced Stokes and anti-Stokes Raman scattering. SERS enhancement factors of the order of 1014–1015 or, in other words, effective Raman cross-sections between 10-16 and 10-15 cm2/molecule result in a significant transfer of ground state population to the first excited vibrational state due to the strong Raman process. This allows the observation of v=1 to v=2 (‘hot’) vibrational transitions in SERS additionally to v=0 to v=1 transitions ‘normally’ probed in a Raman experiment.

Surface–enhanced Raman scattering on single–wall carbon nanotubes

Exploiting the effect of surface–enhanced Raman scattering (SERS), the Raman signal of single–wall carbon nanotubes (SWNTs) can be enhanced by up to 14 orders of magnitude when the tubes are in contact with silver or gold nanostructures and Raman scattering takes place predominantly in the enhanced local optical fields of the nanostructures. Such a level of enhancement offers exciting opportunities for ultrasensitive Raman studies on SWNTs and allows resonant and non–resonant Raman experiments to be done on single SWNTs at relatively high signal levels. Since the optical fields are highly localized within so–called ‘hot spots’ on fractal silver colloidal clusters, lateral confinement of the Raman scattering can be as small as 5 nm, allowing spectroscopic selection of a single nanotube from a larger population. Moreover, since SWNTs are very stable ‘artificial molecules’ with a high aspect ratio and a strong electron–phonon coupling, they are unique ‘test molecules’ for investigating the SERS effect itself and for probing the ‘electromagnetic field contribution’ and ‘charge transfer contribution’ to the effect. SERS is also a powerful tool for monitoring the ‘chemical’ interaction between the nanotube and the metal nanostructure.

Near-infrared excitation profile study of surface-enhanced hyper-Raman scattering and surface-enhanced Raman scattering by means of tunable mode-locked Ti: sapphire laser excitation

Abstract Excitation profiles of surface-enhanced hyper-Raman scattering (SEHRS) and surface-enhanced Raman scattering (SERS) of crystal violet on colloidal silver were measured using 1 ps Ti:sapphire laser pulses at wavelengths between 750 and 830 nm. Our measurements allowed the direct determination of the ratio between SEHRS and SERS intensities which exhibited a strong dependence on excitation wavelength. The measured variation of this ratio can be explained by different electromagnetic contributions to the enhancement factors of SEHRS and SERS at different excitation wavelengths. Total surface enhancement factors of hyper-Raman scattering can be estimated to be the order of 10 14 .

Coexistence of classical and quantum plasmonics in large plasmonic structures with subnanometer gaps

Large metal nanostructures with subnanometer interparticle separations (gaps) can provide extremely high local fields and are of particular interest in surface enhanced spectroscopy, as well as for basic understanding of plasmonics. In this experimental electron energy loss study, we monitor the transition of plasmonic dimers from a classical to a quantum system by decreasing gaps to dimensions when tunneling occurs and a conductive nanobridge evolves. Our studies show that silver dimers with atomic scale gaps can exhibit a regime, in which charge transfer plasmon modes, as a hallmark of a quantum nature, and “classical” bright and dark dipolar plasmon modes can be seen simultaneously.

Nonlinear Raman Probe of Single Molecules Attached to Colloidal Silver and Gold Clusters

We review surface-enhanced linear and nonlinear Raman scattering experiments on molecules and single wall carbon nanotubes attached to colloidal silver and gold clusters. Surface-enhanced hyper-Raman scattering and surface-enhanced anti-Stokes Raman scattering from pumped vibrational levels are studied as two-photon excited Raman processes where the scattering signal depends quadratically on the excitation laser intensity. The experimental results are discussed in the framework of strongly enhanced electromagnetic fields predicted for such cluster structures in so-called “hot spots.” The electromagnetic enhancement factors for Stokes, pumped anti-Stokes, and hyper-Raman scattering scale as theoretically predicted, and the field strengths in the hot spots, it is inferred, are enhanced of the order of 103. From our experiments we claim a very small density of hot spots (0.01 % of the cluster surface) and lateral confinement of the strong field enhancement within domains that can be as small as 10 nm.