Gold colloids spectra

PL spectra fi-om the samples with different thickness of polyelectrolyte spacer have been measured with the excitation wavelength, corresponding to the maximum of plasmon resonance in the absorption spectrum of the gold colloidal film (T = 550 nm). The enhancement of PL is expected to be most pronounced when excited at the frequency of surface plasmon resonance in gold colloids. 

A detailed study of gold colloid formation from inverse micelles and inverse microemulsions (the former showing much better potentiality) was reported by Wilcoxon etal. [235]. As a variety of starting chemicals was examined, it may be useful to list them up, as this would provide the reader with the wide spectrum of possibilities in gold colloid synthetic protocols. 

Adsorbed molecules or ions formed during the various syntheses of colloidal metals have been observed by SERS. A silver hydrosol prepared by ferrous reduction of Ag in the presence of citrate showed enhanced Raman intensity corresponding to adsorbed citrate, [213] and gold colloids prepared from [Au(CN)2]" by reduction with borohydride showed SER bands due to adsorbed CN". [214] The Raman spectrum of [Pt(CN)4] adsorbed on a 1.6 nm platinum colloid has been measured, [215] but it was concluded that the slight intensity increase observed (a factor of 7) for the CN mode at 2215 cm might be due to other enhancement mechanisms. It has also been shown that ligands such as (N-4-dimethylaminoazobenzene-4 -sulfonyl)aspartate [216] adsorbed on colloidal silver are highly Raman active as a result of this phenomenon. Polymer films con-... 

Because of the uncertainty surrounding TEM images of these smallest nanoclusters, other methods for obtaining particle size information were applied to the sol. The optical extinction in the ultraviolet-visible region of the spectrum (UV-vis) shows a trace of the plasmon feature typical of gold colloids, strongly suppressed due to finite-size effects [3] (Fig. 2, curve A). The spectrum resembles that of other gold cluster systems estimated at about 2 nm mean diameter [4], 1-2 nm [5] and about 1.2 run [6]. 

Figure 7. Kubelka-Munk transformed diffuse reflectance spectrum of (A) untreated and (B) calcined catalyst 10-wt% Au on aluminium oxide "C" calcination conditions 400°C, air, 4 h. The change in the plasmon absorbance of the supported gold colloid due to change in particle size can be clearly seen c.f. Figure 2 curves B and C. (the Kubelka-Mumk function F(R) is not the simple absorbance spectrum but is divided by the scattering spectrum of the white alumina support, which is normally assumed to be monotonic F(R) = K/S)...

Figure 6.5. (a) TEM of crystalline gold colloid particles (the halo of organic molecules is invisible), (b) computer simulation of a dendrimer nanocomposite, (c) HRTEM of a gold nanocomposite particle containing 14 Au atoms. (D) Dendrimer template. CNDs are in the middle of the structural "spectrum", to the far left lie dendrimer compatibilized "hard" gold nanoparticles, and "soft" dendrimers are on the right. 

As described above the procedures according to Frens is a suitable technique to produce colloid solutions. However the aimed red color is obtained after color changes from yellow to black, dark blue, violet, and finally shifts suddenly to red. These earlier stages of colloid synthesis absorb light substantially above 600 nm and can be isolated by a rapid cool-down or addition of metal ions blocking crystallization of the gold colloid. With such glass-type non-crystalline colloids also resonance enhancement at A, > 600 nm is achieved, which enables to shift the desired absorption peak all over the visible spectrum to IR. 

Figure 5. SEES spectra measured inside living cells incubated with ICG on colloidal gold SEES spectrum of ICG (A), SEES spectrum ofICG and cell environment (B), difference spectrum C displaying SEES features of cell components. For the assignment of the bands see text,...

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. 

In the remainder of this section we see how the theoretical calculations of Mie account for the observed spectrum of colloidal gold. In the next section we consider the inverse problem for a simpler system how to interpret the experimental spectrum of sulfur sols in terms of the size and concentration of the particles. Both of these example systems consist of relatively monodisperse particles. Polydispersity complicates the spectrum of a colloid since the same x value will occur at different X values for spheres of different radii according to Equations (99M101). 

B) Extinction spectrum of colloidal gold NPs with a diameter of ca. 13 nm and emission spectrum of LaP04 Ce,Tb NPs. (C) Evolution of emission intensity of biotinylated LaP04 Ce,Tb NPs with the addition of avidin coated gold NPs. Reprinted with permission from Gu et al. (2008a). Copyright 2008 American Chemical Society. 

The electronic absorption spectrum of metal nanocrystals in the visible region is dominated by the plasmon band. This absorption is due to the collective excitation of the itinerant electron gas on the particle surface and is characteristic of a nanocrystal of a given size. In metal colloids, surface plasmon excitations impart characteristic colors to the metal sols, the beautiful wine-red color of gold sols being well-known [6-8]. The dependence of the plasmon peak on the dielectric constant of the surrounding medium and the diameter of the nanocrystal was predicted theoretically by Mie and others at the turn of the last century [9-12]. The dependence of the absorption band of thiol-capped Au nanocrystals on solvent refractive index was recently verified by Templeton et al. [13]. Link et al. found that the absorption band splits into longitudinal and transverse bands in Au nanorods [6, 7]. 

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