Interaction fluorophore-metal

Recently, Schalkhammer et al. have discussed the relevance of surface enhanced fluorescence in immunosensing apphcations (surface-enhanced fluoroimmunoas-say, SE-FIA) [322]. The enhancement mechanism was explained by an electrodynamic model and the interaction between metal particle and fluorophore for the excitation and emission process was discussed. It was shown that the discrimination power increased with decreasing quantum efficiency of the fluorophore. This suggested that in SE-FIA a low-quantum efficiency fluorophore needs to be used, as was shown by experiments with model compounds. 

In recent years we have been reporting our observations on the favorable effects of silver nanoparticles deposited randomly on glass substrates (silver island films) for increasing the intensities and photostability of fluorophores, particularly those with low quantum yields. These reports described fluorophores with visible excitation and emission wavelengths. Since the fluorophores interact with metal through die surface plasmon resonance, for-silver the absorption maximum is near 430 nm, we did not know if silver particles would enhance the emission of ICG with absorption and emission... 

While implementation of RDE is relatively simple, understanding the principles of RDE is difficult. The concepts are widely distributed in the optics and chemical physics literature, often described in terms difficult to imderstand by biophysical scientists. In this volume we have presented chapters from the experts who have studied metal particle optics and fluorophore-metal interactions. We believe this collection describes the fundamental principles for the widespread use of radiative decay engineering in the biological sciences and nanotechnology. 

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. 

There is a fluorophore-metal interaction, which results in an increase in the intrinsic radiative decay rate of the fluorophore. 

Fig. 6 Modified Jablonski diagram for illustrating metal-fluorophore interactions, (a) the transition of dye excited by the incident light, (b) the enhanced excitation according to enlarged electromagnetic field, (c) the fluorescent emission of dye molecule, (d) the nonradiative relaxation, (e) the enhanced emission of the fluorophores and metal coupling in far field. Reproduced with permission from Ref. [77]...

Calixarene containing a dioxotetraaza unit, PET-18, is responsive to transition metal ions like Zn2+ and Ni2+. Interaction of Zn2+ with the amino groups induces a fluorescence enhancement according to the PET principle. In contrast, some fluorescence quenching is observed in the case of Ni2+. PET from the fluorophore to the metal ion is a reasonable explanation but energy transfer by electron exchange (Dexter mechanism) cannot be excluded. 

---