2 2 plasmonic phenomena

The analysis of the expression based on the Brillouin theorem provides a very illustrative description of how the plasmon phenomenon arises. For this purpose we introduce the following assumptions ... 

In this section, we describe theoretical methods that describe the macroscopic optical properties of metal nanoparticles (a.k.a quantum dots). Recently, silver and gold nanoparticles have foimd tremendous use in biological assays, detection, labelling and sensing because of their sensitive optical spectra. While some works in the literature refer to these as quantum dots , in optical absorption experiments their quantized energy structure is not probed. The spectrum is a probe of the localized surface plasmon phenomenon, a collective electronic excitation that is localized in spatial extent owing to the small size of the nanoparticle compared with the wavelength. 

Remarkably the position of the final plasmon peak of the alloy particles is dependent on the molar ratio of gold to silver nanoparticles. When the ratio is shifted favoring either metal, an alloy of any desired composition can be formed. This alloying phenomenon indicates that it is possible for true tuneability of the properties of a set of nanoparticles. 

The affinity (interaction strength), multiple interactions, and the changes in concentration can be also monitored from those studies. To deliver data in real time, the natural phenomenon of surface plasmon resonance (SPR) is employed. Since the refractive index (r ) at the interface changes as molecules are immobilized on the sensor surface, instant measure of r provides real-time assessment. The Tlcxchip platform exploits grating-coupled SPR (GC-SPR) for this purpose. 

The term surface plasmon resonance (SPR) can refer to the phenomenon itself or to the use of this phenomenon to measure biomolecules binding to surfaces. This method is now widely used in the biosciences and provides a generic approach to measurement of bio molecule interactions on surfaces. 

Immunosensors have been developed commercially mostly for medical purposes but would appear to have considerable potential for food analysis. The Pharmacia company has developed an optical biosensor, which is a fully automated continuous-flow system which exploits the phenomenon of surface plasmon resonance (SPR) to detect and measure biomolecular interactions. The technique has been validated for determination of folic acid and biotin in fortified foods (Indyk, 2000 Bostrom and Lindeberg, 2000), and more recently for vitamin Bi2. This type of technique has great potential for application to a wide range of food additives but its advance will be linked to the availability of specific antibodies or other receptors for the various additives. It should be possible to analyse a whole range of additives by multi-channel continuous flow systems with further miniaturisation. 

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. 

The detection of sharp plasmon absorption signifies the onset of metallic character. This phenomenon occurs in the presence of a conduction band intersected by the Fermi level, which enables electron-hole pairs of all energies, no matter how small, to be excited. A metal, of course, conducts current electrically and its resistivity has a positive temperature coefficient. On the basis of these definitions, aqueous 5-10 nm colloidal silver particles, in the millimolar concentration range, can be considered to be metallic. Smaller particles in the 100-A > D > 20-A size domain, which exhibit absorption spectra blue-shifted from the plasmon band (Fig. 80), have been suggested to be quasi-metallic [513] these particles are size-quantized [8-11]. Still smaller particles, having distinct absorption bands in the ultraviolet region, are non-metallic silver clusters. 

Thus the value of the dielectric constant at the sample/metal interface determines the shift of the resonance. When adsorption of molecules at the metal surface results in the change of the refractive index or of the local value of the dielectric constant, the change of reflectivity is observed. This phenomenon has been used as the mechanism for detection of gases (Fig. 9.18a) and of adsorbed biomolecules (Fig. 9.18b). The depth of penetration of the surface plasmon is comparable to that of the evanescent field, that is, 100-500 nm for the visible-near infrared range. 

Alternatively, various analytical methods based on SPR phenomenon have been developed, including surface plasmon field-enhanced Raman scattering (SERS) [7], surface plasmon field-enhanced fluorescence spectroscopy (SPFS) [8-11], surface enhanced second harmonic generation (SHG) [12], surface enhanced infrared absorption (SEIRA) [13], surface plasmon field-enhanced diffraction spectroscopy (SPDS) [14-18], Most of these methods take advantage of the greatly enhanced electromagnetic field of surface plasmon waves, in order to excite a chromophoric molecule, e.g., a Raman molecule or a fluorescent dye. Therefore, a better sensitivity is expected. 

Based on their experimental results concerning ethanol and methanol photooxidation by Au/Ti02 composites under visible-light illumination, Tatsuma et al. (Tian and Tatsuma 2005) proposed a plasmon-induced charge separation scheme. They observed a surprising phenomenon, in which the photoelectrons were excited from Au nanoparticles and transferred to the CB of Ti02 (Wood et al. 2001 Subramanian et al. 2004). Meanwhile, the oxidized Au species accepted electrons from the donor molecules present in the solution to recover the charge balance. The process is illustrated in Fig. 16.23. 

Surface plasmons (SPs) are surface electromagnetic waves that propagate parallel along a metal/dielectric interface. For this phenomenon to occur, the real part of the dielectric constant of the metal must be negative, and its magnitude must be greater than that of the dielectric. Thus, only certain metals such as gold, silver, and aluminum are usually used for SPR measurements. The dispersion relation for surface plasmons on a metal surface is ... 

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