Surface plasmon resonance shape dependence

The detected fluorescence can be significantly enhanced, however, by exploiting the plasmonic enhancement which can occur when a metal nanoparticle (NP) is placed in the vicinity of a fluorescent label or dye [1-3]. This effect is due to the localised surface plasmon resonance (LSPR) associated with the metal NP, which modifies the intensity of the electromagnetic (EM) field around the dye and which, under certain conditions, increases the emitted fluorescence signal. The effect is dependent on a number of parameters such as metal type, NP size and shape, NP-fluorophore separation and fluorophore quantum efficiency. There are two principal enhancement mechanisms an increase in the excitation rate of the fluorophore and an increase in the fluorophore quantum efficiency. The first effect occurs because the excitation rate is directly proportional to the square of the electric field amplitude, and the maximum enhancement occurs when the LSPR wavelength, coincides with the peak of the fluorophore absorption band [4, 5]. The second effect involves an increase in the quantum efficiency and is maximised when the coincides with the peak of the fluorophore emission band [6]. 

Figure 20.2 Size and shape dependence of the surface plasmon resonance absorption spectra of gold nanoparticles. (A) Spheres in different sizes (B) Rods in different aspect ratios.

Agarwal et showed, using a point-dipole, classical electrodynamic calculation, that the image can be considerable near a metal sphere (simulating a roughness feature) of about 2-3 nm and with similar molecule-sphere distances. At such distances the approximations are tolerable, so that their results are dependable. The reason for the increase in the interaction is a coupling to the surface plasmon (or shape resonance), which in the sphere can occur at lower frequencies than for a flat surface. 

The optical spectroscopy is commonly applied to such colloidal suspensions that obey the Rayleigh limit or the Rayleigh-Debye-Gans limit of scattering (cf. Appendix B.2). In this case the spectra usually have a smooth and monotone shape, from which only a few details of the size distribution can be deduced. Yet, for metals with a surface plasmon resonance in the optical domain (e.g. Ag or An), one observes a distinct, size dependent maximum in the turbidity spectra of nanoparticles (Fig. 2.17 cf. Njoki et al. 2007). The presence of such a maximum can clearly enhance the information content of the spectrum. 

SERS activity, which mainly arises from localized surface plasmon resonance, depends critically on the optical properties, size, shape, and inter-particle space of nanostructures. 

The localized surface plasmon resonance of individual plasmonic nanoparticles depends heavily on the size and shape of each nanoparticle. For instance, the wavelength of the dipolar surface plasmon red shifts with the increase of particle size. However, for much larger nanoparticles new bands for some multipolar modes will appear in the short-wavelength range, while the dipolar band at long-wavelength will be damped. Typically, the size of Au or Ag nanoparticles synthesized for SERS should be less than 150 nm, and larger than 20 nm. 

The first indication for the formation of silver nanoparticles is the change in its color from colorless to brown (Fig. 3a) that is due to the size and shape dependent surface plasmon resonance (SPR). Metallic nanoparticles in particular possess this... 

Size and Shape Dependence of Localized Surface Plasmon Resonances... 

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