Microfluidics for Biosensor Chips - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 140

in recent research have utilized this label-free sensing scheme. 14
Commercial SPR detection equipment such as Biacore systems (GE
Healthcare) have become popular tools for studying biomolecular
interactions.
Although the thin ilm of bulk noble metals such as gold (Au)
or silver (Ag) is mirror-like and shines at all wavelength ranges,
the nanoparticles of these metals are colored to scatter or absorb
light at speciic wavelength ranges. This phenomenon is known as
PPR, referring to the plasmon excitation phenomenon occurring
on nanometer-sized metallic structures. The absorbed incident
light induces the oscillation of conduction electrons conined in
the metallic nano-scale substrates to scatter. When the electrons
collectively oscillate at the resonance wavelength of incoming light,
the scattering intensiies to the degree that the nanoparticles appear
colored. The scattering wavelength is dependent on the permittivity
of nanoparticles and their surrounding medium. Similar to the
principles of developing SPR biosensing methods, the dielectric
properties of nanoparticles are tailored when immobilized probes
on the surface recognize targets to dock. The shifting of PPR
scattering or absorbance bands can be observed to conirm the
binding event. Chapters 3 and 4, respectively, illustrate colorimetric
and spectroscopic biosensing assays and slide-based biosensors
with PPR detection methods using functionalized noble metal
nanoparticles.

7.3.1 Microfluidic Devices to Facilitate PPR Detection

These functionalized nanoparticles can be illed into microchannels
or coated on channel walls to develop miniaturized assays. Dark-
ield microscopy is suitable for observing the color changes of
these nanoparticles due to binding events in the micro-channel. 15
Gold nanoparticles of 80 nm diameter implanted with biotinylated
antibodies are purged into PDMS (poly(dimethylsiloxane)) or PMMA
(poly(methyl methacrylate)) microchannels to monitor the bindings
of antigens such as Helicobacter pylori and Escherichia coli at the
detection limit of 10 ng, which is comparable to the limit provided
by dot blot assays.
Colorimetric assays can also be performed by measuring the
light transmission through the coating layer of functionalized
nanoparticles on substrate slides of transparent materials such
as quartz. The slides can be integrated with microluidic chips for