Fluorophore-metallic surface

Two different nanomaterials, namely colloidal core / shell Quantum Dots (QDs) and Quantum Rods (QRs) were synthesized as described in [51]. In the case of CdSe / ZnS QDs, the synthesis yielded samples emitting at Inux = 580 nm widi a spectral width of the fluorescence emission of 40 nm. CdSe quantum rods showed an emission peak centered at l x = 567 nm with similar linewidth. The NCs were subsequently dispersed in PMMA and deposited onto the substrate by spin-coating. In order to study tiie influence of the average fluorophore-metallic surface distance on the MEF efi t, several thicknesses of the active layer were investigated, finding an o(Aimum value of 35 nm, as measured from the surface of the metallic nanostructures. 

Gersten, J. (2005] Theory of fluorophore-metallic surface interaction, in Topics in Fiuorescence Spectroscopy, Volume 8 Radiative Decay Engineering (ed. by Geddes, C. D., and Lakowicz, J. R., Springer), New-York. 

The presence of metallic surfaces or particles in the vicinity of a fluorophore can dramatically alter the fluorescence emission and absorption properties of the fluorophore. The effect, which is associated with the surface plasmon resonance of the metallic surface, depends on parameters such as metal type, particle size, fluorophore type and fluorophore-particle separation. 

Figure 1.1 A schematic diagram of the unified plasmon/fluorophore description. Fluorophores induce surface plasmons in metals and energy is effectively transferred in a non-radiative fashion. This interaction of excited states with surface plasmons leads to a wealth of new fluorescence, chemiluminescence and phosphorescence phenomena and technologies we describe as a Unified Description.

When a fluorophore is located in close proximity to a metal surface, both its absorption and emission properties may be affected dramatically. This in turn affects its fluorescence properties and may result in either a quenching or an enhancement of the fluorescence signal. This latter situation is obviously of interest for many applications using fluorophores. Let us discuss these steps in more detail. 

In conclusion, the fluorescence signal is not necessarily completely quenched for molecules directly adsorbed on the metal surface, but it is rather much less enhanced than the Raman signal. As a consequence, if SERS peaks can be observed for a fluorophore, they should in most cases be accompanied by a MEF signal. 

Size and the spatial geometry of nanostructured arrays or aggregates which also allow for plasmonic manipulation of the substrate optimization of the fluorophore-nanostructure distance orientation of the fluorophore on the metallic surface. 

Although it has been difficult to separate the effects of excitation and emission enhancement, both of these effects should be extremely sensitive functions of the shape of the metal particle, the orientation of the fluorophore, and the distance between the fluorophore and the metal, because the local-field effects depend strongly on these parameters. Many groups have studied variations in fluorescence intensity as a function of the distance between a layer of fluorophores and a number of nanostructured metal surfaces, adsorbed colloidal particles or suspended colloidal particles. Single-molecule experiments have even provided strong evidence for the existence of a local maximum in the fluorescence intensity versus distance curve. ... 

The fluorescence amplification provided by the plasmonic nanostructures has been shown to be applicable to many fluorophores. Hence fluorophores currently employed in assays would still be suitable. However, the use of low quantum yield fluorophores would lead to much larger fluorescence enhancements (i.e. 1 / Qo) and could significantly reduce unwanted background emission fi om fluorophores distal fi om the metallic surface. 

Localized surface plasmon resonance (LSPR) at the metal surface has been exploited to enhance the signal obtained from optical biochips and thereby lower the limits of detection. There are two main enhancement factors (i) an increase in the excitation of the fluorophore by localizing the optical field on the nanoparticles near the fluorophore and (ii) an increase in quantum efficiency of the fluorophore. The plasmon resonance wavelength should coincide with the fluorophore absorption band to obtain the maximum emission efficiency. Several parameters concerning the signal detection enhancement are as follows (84)... 

Several other studies (150-153) reported that metal surfaces were able to either enhance or suppress the radiative decay rates of fluorophores. Furthermore, an increase in the extent of resonance energy transfer was also observed. These effects might be due to the interactions of excited-state fluorophores with SPs, which in turn produce constructive effects on the fluorophore. The effects of metallic surfaces include fluorophore quenching at short distances ( 0-5 nm), spatial variation of the incident light field (-0-15 nm), and changes in the radiative decay rates (-0-20 nm) (64). The term of metal-enhanced fluorescence could be referred to the appplication of fluorophore and metal interactions in biomedical diagnosis (64). 

Variation in fluorescence intensity as a function of the distance between a layer of fluorophores and a number of nanostructured metal surfaces (154,155), suspended colloidal particles (156,157), and adsorbed colloidal particles (158) have been intensively studied. 

The use of fluorophores and nano-structured metal surfaces or adsorbed colloidal particles led to the formation of metal-enhanced planar immunoassay, such as sandwich assay or DNA hybridization assay as illustrated in Scheme 8.9. 

Table 8.3 Studies on fluorescent intensity between fluorophores and nanostructured metal surfaces or adsorbed colloidal particles...

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]. 

It is possible that surface enhancement effects, similar to the observations made earlier in metal-fluorophore systems [11, 83-85] may occur. Metal surfaces are known to have effects on fluorophores such as increasing or decreasing rates of radiative decay or resonance energy transfer. A similar effect may take place in ZnO nanomaterial platforms. However, decay lengths of fluorescence enhancement observed in the semiconducting ZnO NRs are not commensurate with the length scale seen on metals such as Au or Ag. For effective metal enhanced fluorescence, fluorophores should be placed approximately between 5-20 nm away from the metal surface. However, fluorescence enhancement effect on ZnO NRs is observed even when fluorophores are located well beyond 20 nm away from the NR surface. At the same time, no quenching effec en when they are placed directly onto ZnO NR surfaces. In addition, there overlap between the absorption and emission... 

Over the last several years, many groups have described the use of metallic surfaces and sub-wavelength sized metallic nanoparticles to modify both the far and near-field emissive prr rties of optically excited fluorophores, a technology named... 

It is well established that fluorophores located near a metal surface can directly transfer energy from their excited state to surface plasmon modes (63). When the metal has a surface texture, these sur ce plasmon modes can directionally scatter to the far field (64). This effect has been most widely studied in reflection, but it is known that it can occur in transmissicm as well (65). SPCE in transmission... 

Electron-transfer process has been observed between fluorophore and metal surface by photocurrent measurement [9, 29, 30]. Transient absorption studies have shown the charge separation between the dye molecule and gold nanoparticies upon pulse laser excitation [31]. In the collision quenching process, the nonradiative rate is proportional to the quencher concentration [19]. 

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