Biomedical Engineering Reference
In-Depth Information
Chapter 1 described the principles of LSPR in detail. Briely,
this phenomenon refers 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 becomes so intense that the nanoparticles appear
colored. Similarly, the emission of an excited luorophore tagged on
a dielectric ilm can also excite surface plasmon to create a radiative
beam. Since the coupling of surface plasmon with incident light
is angle dependent, emission caused by this plasmon excitation
should be directional too. In addition, since evanescent wave ield
typically propagates 200-500 nm forward through a dielectric ilm,
SPCE is limited to closely-tagged luorophores on the surface of
nanoparticles.
For instance, in a model immunoassay using anti-rabbit antibody
probes tagged with luorescence labels, the probes bind to rabbit
immunoglobin on a surface coated with silver nanoparticles. The
surface plasmon on the silver signiicantly enhances luorescence. 38
Large particles with intense scattering, such as plastic beads or
red blood cells, only slightly attenuate the intensity of coupled
luorescence. However, hemoglobin and smaller molecules strongly
absorbed on the surface diminish the intensity of luorescence. In a
microchannel immunoassay, the lateral-low coniguration required
to separate blood cells prior to detection is no longer necessary
when SPCE detection is employed.
Since the sizes of nanoparticles are much larger than usual
solutes, the slow diffusion makes rapid detections employing SPCE
methods in microchannel, where luid mixing is problematic due
to small Reynolds number limitation (Section 7.3.2). Liquid-liquid
mixing in discrete droplets can be accomplished using microluidic
networks. 39,40 Reagent and sample solutions are illed into a funnel
microchannel via different inlets, in which each stream migrates
along laminar low lines without any mixing. The outlet of funnel
channel connects through a tiny hole with the other channel illed
with oil lows immiscible with the solutions. When the solution
streams come out to merge into the oil low, the streams wrap with
each other to become discrete droplets. The downstream of oil low
channel is serpent-like, in which the droplets go through the cycles of
stretching, folding, and re-orientation. Such baker's transformation
 
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