Plasmon response

There are very many papers in the literature that address some aspect of gold nanospheres. In particular, their plasmon response (see Section 7.3.1.1) has been well studied, as has their agglomeration [50-52] and the manner in which they can be assembled into highly ordered colloidal crystals [50, 53, 54]. The latter are interesting and will be further discussed in Section 7.3.8.2. Conjugation of gold nanospheres with proteins and antibodies, for use as a stain in microscopy [55] or possibly, in medical applications [23], is another rich field. 

Such large enhancement factors for localized and isolated hot spots from few atom Ag clusters arising from only the chemical enhancement under certain conditions are supported by calculations. Zhao working with Jensen and Schatz used time-dependent density functional theory (TDDFT) to investigate the adsorption and Raman response of pyrazine molecules [21]. Figure 10.6 shows the Raman response of (a) isolated pyrazine compared to that of pyrazine complexed to the vertex of a (b) one and (c) two 20 Ag atom clusters with enhancements of 10 and 10 predicted, respectively. Small clusters of Ag atoms have little or no plasmon response, suggesting that the chemical enhancement can be quite significant and certainly may allow for enhancement hot spots. 

F. Goettmann, A. Moores, C. Boissiere, P. Le Floch, and C. Sanchez, A Selective Chemical Sensor Based on the Plasmonic Response of Phosphinine-stabilized Gold Nanoparticles Hosted on Periodically Organized Mesoporous Silica Thin Layers. Small, 2005,1, 636-639. 

Fig. 3 a SEM image of a gold/sUica composite opal b absorption spectra showing a red shift of the plasmonic response with the increase of the refractive index of the surrounding medium [35]... 

A plasmonic response of the gold/silica composite inverse opals was observed (Fig. 3b), which showed a pronounced spectral change upon the variation of the surrounding dielectric medium by addition of glycerol to the water phase. This property suggests an application of the hierarchically structured replica in the field of optical sensors. 

Lee, K.-S., El-Sayed, M.A., 2006. Gold and silver nanoparticles in sensing and imaging sensitivity of plasmon response to size, shape, and metal composition. J. Phys. Chem. B no, 19220-19225. 

P.A. Kossyrev, A.J. Yin, S.G. Cloutier, D.A. Cardimona, D.H. Huang, P.M. Alsing, J.M. Xu, Electric field mning of plasmonic response of nanodot array in liquid crystal matrix. Nano Lett. 5, 1978-1981 (2005)... 

Prodan, E. Radloff, C. Halas, N. J. Nordlander, P. A hybridization model for the plasmon response of complex nanostructures. Science 2003, 302, 419-422. 

Rodriguez-Fernandez,)., Funston, A. M., Perez-Juste,)., Alvarez-Puebla, R. A., Liz-Marzan, L. M., and Mulvaney, P. (2009) The effect of surface roughness on the plasmonic response of individual sub-micron gold spheres, Phys. Chem. Chem. Phys., 11, 5909-5914. 

As noted, plasmonic response not only depends on polarization and shape but also on the local refractive index of the surrounding medium [145]. A change in local refractive index can be induced by varying the bulk medium in which the nanorod or nanoshell is dispersed, or by an increase or displacanent of adsorbates at the interface of the nanoparticle and bulk medium. Variations in the local refractive index have a pronounced effect on the position of the plasmon resonance peaks of gold nanorods and nanoshells. This effect can be exploited to construct trace chemical or biosensors, where binding events in proximity to the nanoparticles trigger a shift in local refractive index and thus optical response. 

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