Plasma resonance

In this Section we want to present one of the fingerprints of noble-metal cluster formation, that is the development of a well-defined absorption band in the visible or near UV spectrum which is called the surface plasma resonance (SPR) absorption. SPR is typical of s-type metals like noble and alkali metals and it is due to a collective excitation of the delocalized conduction electrons confined within the cluster volume [15]. The theory developed by G. Mie in 1908 [22], for spherical non-interacting nanoparticles of radius R embedded in a non-absorbing medium with dielectric constant s i (i.e. with a refractive index n = Sm ) gives the extinction cross-section a(o),R) in the dipolar approximation as ... 

Figure 6. Absorption spectra of spherical non-interacting nanoclusters embedded in no absorbing matrices (a) effect of the size for Ag nanoclusters in silica (b) effect of the matrix for R = 2.5 nm Au clusters (the refractive index n = and the position of the plasma resonance are reported for each considered matrix) (c) effect of the cluster composition for i = 5 nm noble-metal clusters (Ag, Au, Cu) in silica. (Reprinted from Ref [1], 2005, with permission from Italian Physical Society.)...

Nuclear component of the stopping power SPR Surface plasma resonance... 

The formation of Au nanoparticles can be easily monitored by following the appearance of a surface plasma resonance band around 520-540 nm (Fig. 6.1). Yeung et al. [33] observed that the efficiency of gold particle formation was different in different alcohols (n-pentanol > propan-2-ol > methanol). This is due to the air/water surface activity of the alcohols and the ability of the solute to scavenge the primary OH radicals at the bubble/liquid interface. 

Creighton J.A., Blatchford C.B., Albrecht M.C., Plasma resonance enhancement of Raman-scattering by pyridine adsorbed on silver or gold sol particles of size comparable to the excitation wavelength, J. Chem. Soc. Faraday Trans. 1979 2 790-798. 

Kreibig et al. [68] have recently proposed three alternatives for understanding the lack of a well developed plasma resonance in the UV-visible absorption spectra of AU55 [40, 57,17, 59, 60] which one would normally expect with small metallic particles of gold [69, 70, 71, 72, 73]. These are examined here, together with a fourth possible explanation. 

Marcus et al. [40] argue that this, taken together with a very weak bump around 520 nm in the optical absorption of Au 5 5 in a tetrahydrofuran solution, when compared to similar measurements on colloids, is indicative of collective metallic behavior. 520 nm is the well characterized absorption wavelength for the plasma resonance in colloidal gold for particles of average diameters down to 1 nm [69, 59], showing delocalized electrons with correlated behaviour. 

This behavior differs completely from the discrete one-electron absorptions of low-nuclearity metal cluster molecules [17]. Instead, it resembles the 5d - 6s,6p interband transition of colloidal gold. This demonstrates clearly that the AU55 cluster has electronic energy levels which are closely spaced in a developing band structure, quite similar to colloidal gold. On the other hand, these electrons do not seem to show a collective behavior which would give rise to the plasma resonance. 

Kreibig et al. [68] have recently proposed three possible explanations for the lack of the plasma resonance in AU55. The first is that collective behavior doesn t occur, due to localization of the 6s electrons. This doesn t appear to be in agreement with the bulk of the experimental indications mentioned above, nor with that which will appear below in Sect. 5.4. 

As a further possibility, we can mention that the quantitative intensity of a plasma resonance absorption depends on the third power of the partiele diameter a [151, 72] ... 

The plasma resonance absorption in a cluster as small as Auj might simply be too weak to detect above the background interband absorption, which is quite intense at 520 nm, without the need to invoke any special broadening or damping mechanisms. 

We have already referred, in Sect. 4.1, to the development of a plasma resonance in mercury clusters [124]. 

The lack of a clearly developed peak in the UV-visible absorption spectrum due to plasma resonance places a limit on the collective metallic behavior of the electrons in the cluster. On the other hand, the 5d -y 6s,6p interband absorption is well developed toward that of large colloidal gold particles. 

Steinmann, W., 1968. Optical plasma resonances in solids, Phys. Status Solidi, 28, 437-462. 

One simple explanation for these results was as follows The electric field at a metal vacuum interface can be >10 times larger than in free space when the conditions required for a surface plasma resonance are met (47). Since the Raman cross-section is proportional to the square of the field, surface plasmons could produce enhancements of >10. This enhancement is probably not large enough to explain the tunneling junction results by itself, but an enhancement in signal of a factor of 100 by the excitation of surface plasmons would increase the Raman intensity from near the limits of detectibility. 

TRAP recruitment requires Hgand-induced changes in receptors that allow simultaneous interactions of each TRAP trimer with three receptor intracellular domains. This observation implicates that monomeric TRAF-receptor interactions are of low affinity so that the interactions do not occur in the absence of receptor activation. A number of quantitative biophysical characterizations with isothermal titration calorimetry (ITC) and surface plasma resonance (SPR) have provided solid support to this view (Table III). 

Holland WR, Hall DG (1983) Surface-plasmon dispersion relation shifts induced by the interaction with localized plasma resonances. Phys Rev B 27 7765-7768... 

One further development of this approach has been to link chelating iminodiacetic acid groups to PEG, which in turn can be bound by Cu ions and so provide a highly specific binding site for Hisj-tagged proteins (Cha et al., 2004). Another variation is gold-coated microarrays, which have the advantage that they can be combined with surface plasma resonance (SPR) and MS for further detection and analysis of the captured molecules (Bieri et al., 1999 Houseman et... 

Figure 6.1-13 To illustrate the electromagnetic (EM) enhancement of SERS a simple model of a small metal sphere (radius is much less than the wavelength) experienced by an electromagnetic oscillating field is considered. If the sphere is illuminated at the plasma resonance frequency of the metal electrons a high electric field on the metal surface is generated, whose efectric field lines are shown (Creighton, 1988).

Blatchford CG, Campbell JR, Creighton JA (1982) Plasma resonance - enhanced Raman scattering by absorbates on gold colloids the effects of aggregation. Surf Sci 120(2) 435 55... 

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