Biodetection Based on Fluorescence Quenching and Surface-Enhanced Fluorescence Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 234

be suitable for the detection systems and/or labeling involved
in speciic experiments. When using metal surfaces, it may be
desirable to use luorophores with low quantum yields. Consider the
schematic immunoassay displayed in Fig. 10.9. Assume that a capture
antibody is covalently bound to the surface near the metal particles.
The presence of the analyte (An) results in surface binding of a
second antibody that is labeled with a nonluorescent chromophore.
Upon binding to the antigen the previously nonluorescent species
emits due to increases in the radiative rate. The unbound species
that are located at longer distances from the metal site will not
interfere because they do not luoresce. The nonluorescent species
becomes a molecular switch, emitting only when close to the metal
particles. Similarly, a sandwich-type immunoassay has been used for
the determination of the cardiac marker myoglobin. 10 Myoglobin is
irst captured on silver-island ilms and glass surfaces coated with
anti-myoglobin antibodies; the surfaces are then incubated with
rhodamine Red-X-labeled anti-myoglobin antibodies. The luorescence
emission from the rhodamine Red-X-labeled antibody on the silver-
island ilms is about 10-15-fold that on the glass substrate alone.
This approach provides an LOD of 50 ng/mL for myoglobin, a value
much lower than the clinical cut-off concentrations for myoglobin in
healthy patients (100 ng/mL).
Indocyanine green is widely used in medical imaging and testing
because of its safety in use and has been approved by the U.S. Food
and Drug Administration. Indocyanine green-labeled human serum
albumin that is bound to a noncontinuous coating of silver particles
on a glass substrate increases its intensity approximately 20-fold
and decreases the concomitant decay time. 87 Since the rate of
photobleaching does not increase, indocyanine green-silver particle
complexes can yield at least 20-fold more photons per indocyanine
green molecule for improved medical imaging. The signal of a
luorophore can be further improved on Ag colloid coated surfaces
that have elongated colloid aggregates. When a monolayer of Texas
Red-labeled bovine serum albumin is separately deposited on the
surfaces of noncontinuous coating of Ag particles and Ag colloid
coated surfaces, the last provides a luorescence signal approximately
twice as strong under the same experimental conditions. 88 The
last surface is also more eficient for the intensity decays of the
luorophore and does not provide the residual long-component