Microfluidics for Biosensor Chips - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 148

spacing distance between the two dipoles, this interaction is highly
sensitive to distance. This energy transfer process must also proceed
faster than the spontaneous emission of excited electrons (~10 −9 s).
As a result, resonance energy transfer luorescence or quenching
typically only occurs between two static luorophores close to each
other. The mean free paths of freely moving dyes in dilute solutions
are usually too long to maintain close contact throughout the lifetime
of spontaneous emission. In principle, when a luorophore-tagged
analyte docks with the recognition probe planted on noble metal
nanoparticles, this excited luorophore is able to dissipate its energy
to the nanoparticles via FRET to quench its luorescence.
When a tagged donor luorophore on a single-strand DNA probe
is initially absorbed on the noble metal surface, the hybridization
of this probe with the complementary strand allows the absorbed
luorophore to jut away from the surface. Therefore, although the
donor luorescence is initially quenched, this dissipated emission
is able to resume when DNA hybridization is complete. Chen and
colleagues implemented this idea in developing a microluidic
biosensing device to detect the polymerase chain reaction (PCR)
product of the dengue virus. 36 In their study, gold nanoparticles
forming a self-assembled monolayer were deposited on the
microchannel wall. Targeting dengue DNA probes tagged with Cy3
luorescence dyes were immobilized in the monolayer via a thiol
group at the end of probe. The DNA probe, whose dye-tagged end
was bound to the nanoparticle surface, formed an arch conformation.
The luorescence of tagged dye was quenched to disappear at this
conformation. When the PCR products of the dengue virus were
added to hybridize DNA probes, the conformation of targeting
DNA probes changed. The probes stretch outward and restore
luorescence. Prior to this hybridization step, the enterovirus PCR
products were mixed in sample as a negative control.

7.5.2 Using Silver Nanoparticles in Surface Plasmon

Coupled Fluorescence Detection

When the luorophores spatially immobilize away from the surface
of metallic nanoparticles, non-radiative quenching due to resonance
energy transfer decreases dramatically. At a distance of hundreds of
nanometers, luorophore emission can couple with surface plasmon
of metallic nanoparticles to occasionally, sometimes to enhance
emission intensity. This mechanism is called SPCE. 37