The structural basis for giant enhancement enabling single-molecule Raman scattering

Abstract

We find that giant surface-enhanced Raman scattering for adsorbates on silver surfaces is present only on surfaces that exhibit self-similar fractal topology as inferred from atomic force microscopy. The fractal character results in localizing the energy of incident photons to volumes of a few nanometers on a side, millions of times smaller than the diffraction limit. Consistent with this finding, we have found an enhancement in spontaneous Raman cross section of >13 orders of magnitude for adsorbates on silver surfaces demonstrated to be fractal. The location of ''hot spots'' on the fractal surfaces is found to be hypersensitive to incident wavelength and polarization even though the observed Raman scattering is strictly linear in incident intensity. These observations are consistent with localization of the photon energy facilitated by the disordered nature of fractal organization through interference between the incident wave and scattered radiation from silver nanoparticle surface plasmons. We also present a surface preparation method that consistently produces fractal topologies that support single-molecule Raman scattering. U ltra-sensitive spectroscopy to detect single molecules is of great interest in chemistry, biochemistry, and biophysics (1-3). The advent of single-molecule fluorescence studies was an important development, but requires that the molecules under investigation must be intrinsically fluorescent or chemically modified. The discovery that Raman spectroscopy could also be extended to single molecules (4) is an enormous advance because vibrational spectra contain significantly more structural information and can be used to specifically identify molecules. It is widely acknowledged that electromagnetic enhancement near metallic surfaces is a prerequisite to acquire single-molecule Raman spectra, but the detailed circumstances under which surfaces exhibit adequate enhancement remain poorly understood, and methods to prepare surfaces that will reproducibly exhibit the enhancement are cumbersome (5). Additional uncertainty exists over whether molecular resonance or charge transfer interactions with the metal are necessary to make single-molecule Raman detection viable (6).Cross sections for Raman scattering depend on the square of the incident and scattered fields and, if these are both enhanced, Raman intensities scale as the fourth power of the field (7). Surface plasmon resonance where the local field at metal nanoparticle surfaces experiences large enhancement is thought to be part of the mechanism by which Raman cross sections are increased. However, experimental studies (8) showed only increases of 10 6 in surface-enhanced Raman scattering intensities using isolated silver nanoparticles at the plasmon resonance, far too low for single-molecule detection. Experiment and theory appear to confirm that single-molecule Raman scattering is observed only in metal nanoparticle aggregates (9, 10) but there is not universal agreement (4). Experimental ProceduresPreparation of Raman Substrate Silver ...

Towardsin vivointradermal surface enhanced Raman scattering (SERS) measurements: silver coated microneedle based SERS probe

“…The first is that a portion of excitation and Raman light had to go through the 760 mm top layer thus subject to light absorption and scattering in this layer. The second is that Ag on the microneedle, synthesized by Tollen's method, has a larger surface roughness, as shown by the Ag morphologies in the FESEM image [ Figure 4(a) and (b)] that could lead to higher Raman intensity variations as compared to Ag film fabricated by the vaporization method [23]. The Tollen's method is chosen in this study, since this technique is cost-effective [24] in scaling up for potential mass production in the future, and flexible in the coating of three-dimensional geometries [24] with film morphologies that adhere to the substrates even after insertion and withdrawal [ Figure 4(b)].…”

Surface-Enhanced Raman Scattering – Physics and Applications

“…Thus one can expect the single-molecule SERS enhancement up to g R ∼ 10 8 . In the single-molecule SERS experiments [11,16,17], the estimated enhancement is g R ∼ 10 12 to 10 14 , i.e., significantly greater. This certainly indicates the presence of other enhancement mechanisms.…”

“…The fact is that the fractals are the most efficient enhancers of the SERS [1,11,16]. This is likely to be related to two reasons: (i) self-similarity of fractals on average and (ii) giant fluctuations of local optical fields.…”

“…One of the most remarkable and important developments in the field of SERS has been the discovery of the single-molecule SERS [16,17] that has later been confirmed and studied in a body of experiments, see [11] in particular. In the case of the single-molecule SERS, the enhancement is truly giant, g R 10 12 to 10 14 .…”

“…We have proposed that fractal clusters, which are experimentally the most efficient enhancers of SERS [1,11], have specific properties conducive to the giant SERS enhancement [12,13,14]. One of these properties is the giant fluctuations of the local fields in fractals [15].…”