The Plasmon Resonance

Under i-polarization light, the optical spectra of 5-nm nanoparticles (Fig. 7A), recorded at various incident angles 0 do not change with increasing 0. They are characterized by a maximum centered at 2.9 cV, which is similar to that observed for isolated particles (Fig. 5B). Flowever, the plasmon resonance peak remains asymmetrical, as observed under nonpolarized light (Fig. 6). 

In order to investigate this effect, ordered arrays of metallic nano-islands were fabricated on glass substrates by a process of natural lithography using monodisperse polystyrene nanospheres. The metal particle dimensions were tailored in order to tune the plasmon resonance wavelength to match the spectral absorption of the fluorophore. The fluorophore, Cy5 dye, which is widely used in optical immunoassays and has a medium quantum efficiency ( 0.3), was used in this preliminary study of the plasmonic enhancement effect. 

Nanometals have interesting optical properties [37,73,74]. For example, suspensions of nanoscopic Au particles can be pink, purple, or blue depending on the diameter of the particles [74]. These colors arise from the plasmon resonance absorption of the nanometal particle, a phenomenon we have explored in some detail [37,73]. We have shown that membranes containing Au nanowires like those described here also show this plasmon resonance band, and as a result such membranes can show a wide variety of colors. This absorption in the visible region provides an interesting optical approach for characterizing the Au nanowire-containing membranes. 

It is important to realize that for the plasmon resonance to occur the condition of two matching plasmons at the opposite interfaces of the thin metal must be met. In other words there must be a dielectric/metal interface at which an evanescent field is created. In the Kretschmann geometry that interface is created by having the metal coated on the glass prism. Likewise, the SPR condition can also be realized in a fiberoptic format with a thin metal layer deposited on a flattened single-mode optical... 

Related to the plasmon resonance physics is the micromirror optical sensor for hydrogen (Butler, 1991). Like gold and silver, palladium is a free-electron gas metal in which charge groupings such as phonons or plasmons are likely to occur. As we have seen already, palladium has a natural selectivity due to its sorption of monoatomic hydrogen. In that sensor, the reflectivity of the thin Pd film mirror mounted at the end of cladded optical fiber (Fig. 9.19) is modulated by absorption of hydrogen. 

A relatively recent development is the exploitation of w/c microemulsions for the synthesis of metallic and semiconductor nanoparticles. By reducing silver nitrate, Ji et al. (1999) were able to harvest silver nanoparticles from a w/c microemulsion. Analysis of the plasmon resonance peak at 400 nm indicated that samples collected at intervals of 20 and 10 min were 4 nm in diameter. A subsequent decrease in the intensity of the plasmon band, over a period of 1 h, was attributed to the slow flocculation of nanoparticles. 

As previously mentioned, this section describes two examples of analytical applications involving the use of the unique spectroscopic properties of gold nanoparticles. The use of changes in the plasmon resonance absorption of gold nanoparticles as a basis for the development of improved aggregation-based immunoassays is then discussed. 

Although aggregation of metallic nanoparticles perturbs their electronic structure and especially the plasmon resonance frequency, the semiconductor quantum dots are much more resistant to such effects. 

The variations with size in the colours exhibited by colloidal particles and supported particles of similar dimensions have been discussed in Section 3.2.3 the plasmon resonance that is responsible is however influenced by a number of extraneous factors, and it cannot be used quantitatively to estimate size or degree of metallic character. 

There is also an enhancement of o off the plasmon resonance, albeit weaker, which we interpret as due to confinement of excitations, here of hot electrons in the Ag NP. The photochemical mechanism of all the processes seen is believed to involve TNI states [87, 94], which is consistent with detailed characterizations... 

Figure 4.4.8 (A) Radiation emitted from individual Ag NPs of various sizes, observed with the photon STM. Inset STM micrograph of the NP ensemble investigated. After [90]. (B) Variation of desorption cross sections, a, of NO from (NO)2 monolayers on Ag NPs as a function of mean particle size (top abscissa), for the three photon energies given. The plasmon is excited at 3.5 eV. The bottom abscissa gives the inverse radius and emphasizes the approximate scaling off the plasmon resonance. After [14],...

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