Nanoparticle-fluorophore distance, metal

At very short metal nanoparticle-fluorophore distances ( 1 to 3 nm), a large decrease in fluorescence, known as quenching, is expected [8,19,20]. At greater distances however, the fluorescence can undergo enhancement or continue to experience a degree of quenching. The examples outlined below will illustrate that whether enhancement or quenching is observed depends on nanoparticle size and shape, the distance between the fluorophore and the metal nanoparticle surface, and on the overlap between the SPR and the excitation and/or emission transitions in the fluorophore. 

Many experiments have been carried out where the distance between the fluorophore and the metal nanoparticle surface is varied yet only quenching is observed. In these cases the nanoparticles are usually small spherical metal nanoparticles. The following examples demonstrate distance-dependent quenching in a couple of nanoparticle-fluorophore systems. 

The PAH polymeric layer played an important role in our fluorescence sensor design. First, its positive charges enabled the deposition of anionic dextran that was labeled with the pH indicator fluorescein on the surface of the nanoparticles. More importantly, the PAH polymeric layer separated between the fluorescein molecules and the metal particle. In fact, the thickness of the polymeric layer was over 10 nm, which is larger than the Forster distance required for efficient energy transfer between the fluorophore and the metallic gold particles. 

An increase in fluorescence quantum yield is possible due to the suppression of the nonradiative decay of fluorophores upon binding to metallic nanoparticles. However, radiative decay is dramatically increased near the surface of metal nanoparticles, whereas changes in knr are less sensitive to distance. 

Quenching At shorter distances, ranging from few nanometers to the physical contact with the metallic structure, a mechanism tends to increase the total decay rate. This effect, which is responsible for fluorescence quenching, is due to the absorption of fluorescence photons in the metallic structure itself (99). Another effect is based on interactions of the fluorophore with free electrons in the metal, wherein the plasmon absorption leads to lower fluorescent emission efficiency (100). Theoretical study asserts that the optimized distance between the excitation source and the fluorophore is around 2-5 nm (99, 101,102). Nanoparticles coated with a thin shell (e.g. silica, 5nm in thickness) and the dye attached to the dielectric shell could overcome quenching effects (84, 103). The quenching effect can also be found in the quantum dot / GNP system (104). It is noted that as the concentration of fluorophore is high, the self-quenching effect should also be considered. (100)... 

Both Geddes and the Lakowicz group s have investigated the metal-enhanced fluorescence of fluorophores on silver island films (SIFs) [11,26,27] and variously aggregated silver nanoparticles in solution [28,29]. One example of enhancement on SIFs is discussed below [26]. In this work the distance-dependent MEF of a monolayer of sulforfiodamine B (SRB) on SIFs was studied. A SRB monolayer was electrostatically incorporated into the Langmuir-Blodgett (LB) layers of octadecylamine (ODA) deposited... 

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]. 

There are essentially two models that describe the interaction between an excited fluorophore and the SPR of the metal to account for quenching and enhancement of the fluorescence. They both depend on coupling of the fluorophore excited state to the SPR and this is dependent of the spectral overlap of the emission of the fluorophore and the SPR, and the distance between the fluorophore and the metal nanoparticle surface. 

As discussed above biochips based on metal particles layers are among the best in enhancing fluorescence. The REF -chip is a slide with a coating of designed metal nanoparticles and an overcoat of a transparent inert non-metal distance layer and topmost an array of biorecognitive components, which do bind the fluorophores in the proper distance during an assay. 

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