Biomedical Engineering Reference
In-Depth Information
scatter. This scattering, known as Rayleigh scattering, is elastic and
emits light of the same frequency as that of the incident light. The
molecular polarizability is changeable when the molecule bonds
vibrate. The scattering is no longer elastic. This inelastic scattering can
occur to contain shifted peaks different from the Rayleigh scattering
wavelength, including Stokes shift and anti-Stokes shift, of which the
wavelengths are longer or shorter than the Rayleigh scattering peak,
respectively. Raman scattering, of which the spectrum peaks are
determined by the vibration frequencies of probed molecular bonds,
is a highly speciic tool for elucidating molecular structures.
When luorophore molecules are absorbed on plasmonic
nanoparticles of noble metals such as Au or Ag or within the
proximity of a few nano-meters near the particle surface, Raman
scattering light intensity can be enhanced by a factor of 10
6
to 10
8
for an assembly of molecules. This scattering enhancement effect
known as surface-enhanced Raman scattering (SERS) is due to the
LSPR occurring on the particle surface that focuses the incident
light energy and ampliies the density of states at shifted scattering
frequencies.
Bead-based assays to complementarily hybrid with
oligonucleotide sequences of PCR products in a microluidic device
are bound with Raman active probe dye Rhodamine to label. The
microsphere beads are retained with a micro-ilter and heated to wash
out Raman dyes. The released dyes are mixed with one stream of Ag
nanoparticle solution in the same chip to enhance Raman scattering
detections. Similarly, adenine-absorbed Au nanoparticles can be
pinched by the step junction of the microchannel-nanochannel that
captures the particles with target molecules into optically enhanced
SERS active aggregates by employing capillary force.
43
The adenine
detection limit of 10 pM has been achieved. Deuterated isotopomers
have been used as an internal standard to correct quantitative
results when SERS detections have been used to detect nicotine and
pyridine in microluidic mixers.
44
7.6.2
Microfluidic Devices to Facilitate Thermal
Lens Detections
Gold nanoparticles have strong light absorption properties to create
a temperature gradient under intense light irradiation in solution.
This temperature gradient results in the refraction index change of
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