Biodetection Based on Fluorescence Quenching and Surface-Enhanced Fluorescence Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 242

this approach is highly dependent upon the surface DNA density.
Under optimum conditions, this approach provides an LOD at the
nanomolar level for Hg 2+ ions.

10.4 Conclusions and Outlook

Fluorescence quenching and SEF using novel metals have proved
themselves sensitive techniques for the detection of proteins,
DNA, and small solutes. Whether metal surfaces and NPs possess
quenching effects or SEF is highly dependent on the distance
between a luorophore and the metal object. When the distance is
less than 50 Å, luorescence quenching occurs. On the other hand,
when the distance is between 70-100 Å, SEF most likely takes place.
Fluorescence quenching by metal NPs shows fourth order distance
dependence, which differs from FRET. SEF eficiency can be controlled
by tailoring the EM ield of the luorophore by carefully varying the
surface morphology of the metal surfaces. Metallic nanostructured
surfaces (corrugated or rough surfaces) provide much greater SEF
eficiencies when compared to those obtained on lat metal surfaces.
When metal NPs are used, SEM effects can be controlled by varying
their sizes and shapes as well as the distance between them. SEF
does not only amplify the molecular luorescence emission but also
modiies the luorescence lifetime, photobleaching, intermolecular
coupling, and emission patterns.
Although SEF techniques are not as popular as luorescence
quenching and surface enhanced Raman spectroscopy in the present
analytical environment, its use has been recognized in the detection
of DNA and proteins. In addition, speciic illumination conigurations
(e.g., Kretschmann's coniguration) allow for the eficient excitation
of luorophores in certain local areas through the excitation of
plasmon modes, which are of particular interest for local probing
such as cell imaging. The processes described in SEF are readily
applicable to other molecular luminescence techniques such as
chemiluminescence, electroluminescence, and phosphorescence.
The advanced nanotechnology research has spurred the
preparation of many nanomaterials, including Au-Ag nanrods, Au
nanowires, Ag nanolowers, 106-111 and many others, which possess
great potential for developing highly sensitive SEF techniques.
Nanoparticles can be advantageously coupled to luorophores to