Nanoparticle local-field

Nanostructure-based LSPR biosensing has been classified into three schemes surface-relief nanostructures, surface-relief nanostructures coupled to nanoparticles, and nanoparticles. These schemes share many aspects of plasmon characteristics in common, such as shape and concentration dependence. Local field enhancement as a result of plasmon excitation can be used for highly sensitive biosensing. Given particular bio-sensing applications, one of these schemes can be selected to meet... 

To understand the importance of spectral overlap to metal-enhanced fluorescence, it is useful to review the basics of metal-enhanced fluorescence. Metal nanostructures can alter the apparent fluorescence from nearby fluorophores in two ways. First, metal nanoparticles can enhance the excitation rate of the nearby fluorophore, as the excitation rate is proportional to the electric field intensity that is increased by the local-field enhancement. Fluorophores in such "hot spots" absorb more light than in the absence of the metal nanoparticle. Second, metal nanoparticles can alter the radiative decay rate and nonradiative decay rate of the nearby fluorophore, thus changing both quantum yield and the lifetime of the emitting species. We can summarize the various effects of a nanoparticle on the apparent fluorescence intensity, Y p, of a nearby fluorophore as ... 

Highly enhanced local fields can be generated in the vicinity of a metallic spherical nanoparticle due to localized surface plasmon excitation. Using the result in Eq. (10b), the electric field components at the sur ce of the small sphere can be... 

Fluorescence and Enhanced Local Field of Metallic Nanoparticles... 

One may ask why these experiments only showed quenching and not enhancement. The first thing to take note is the fact that the metal nanoparticles here are spherical, and therefore the SPR does not produce a very large enhancement of the local field. Also, the nanoparticles are small, which, as will be explained in Section 11.3,... 

An increase in excitation of the fluorophore depends on the spectral overlap between the SPR and the excitation spectrum of the molecule and on the enhancement of the local field which, as can be seen below, depends on the position of the fluorophore and its distance from the metal surface. The distribution of the local (enhanced) fields for a nanoprism and nanorod are illustrated in Figure 11.11 [S]. The largest field intensities occur at the tips of the nanoprism and at the ends of the nanorods. The field intensities are calculated to be approximately 4000 times the applied field. These field enhancements are much larger than can be obtained with spheres. Even larger field oihancements can be obtained at the interface of nanoparticles in very close proximity to one another, as shown in Figure 11.12 [5]. 

Figure 19.1 (A) 2D projection of the calculated local field intensity distribution around a pair of 15 nm diameter silver nanoparticles excited with Xi = 400 nm light polarized along the interpaiticle axis. The edge-to-edge particle separation is 2 nm and the free space incident light intensity Ej,x P taken to be unity. The local field intensity near the pair is shown in false color. The calculation was done using dipole-dipole approximation (DDA) method with each dipole unit being a square with sides of 0.2 nm. (B) Model of the photophysics of a molecule represented by a three level system and how the excitation and decay dynamics are affected by plasmon enhancement of radiative rates and the introducticm of a rate for quenching Icq of the excited state due to proximity to the metal surface. E (X ) and E (X2) are the field enhancements at the position of the molecule for the excitation and emission wavelengths respectively, kn and kMR represent the radiative and non-radiative decay rates of the molecule in the absence of plasmon enhancement.

Although the connection between SERS and SPPs is established in (1.2), how to effectively produce desirable SERS on a given metallic structure remains challenging. To address this issue, different plasmonic systems at different dimensionalities have been proposed, fabricated, and characterized with the same intention to maximize the local field strength at cOexc and cors [13-16]. A variety of structures have been found to support SERS well, including roughened metal electrodes, metal nanoparticles colloids, metal island films, metal nanorods, etc., or more specifically, substrates that contain nanoscale gaps or hotspots are particularly... 

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