Biomedical Engineering Reference
In-Depth Information
layer was vapor deposited on the gold film that inhibited the deposition of metal
on Au film. With the resulting low background achieved by SiO
2
-coated Au
films, sensitive detection of DNA hybridization with the catalytic growth of
AuNPs was demonstrated with enhanced SPR detection.
Colloidal AuNPs was assembled on the surface of SPR gold chip through
2-aminoethanethiol (forming AuNP-gold chips) to enhance sensitivity in a label-
free detection system.
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Gold-binding polypeptides were fused to protein A result-
ing in the fusion protein, GBP-ProA, that was directly self-immobilized on SPR
chips. h-IgG was bound on the ProA domain that targeted the Fc region of antibod-
ies and anti-h-IgG. It was also used to immobilize antibodies against
Salmonella
typhimurium
that achieved 10-fold increase in sensitivity in compared with the
bare electrode. The signal enhancement in the AuNPs-gold chips demonstrated
signal enhancement of biomolecular interaction. In addition, the GBP-ProA pro-
tein allowed for simple and properly oriented immobilization of antibodies onto
Au chip surfaces without additional surface chemical modification.
4.7.1.5 SERS Biosensors
Raman spectroscopy is a technique that is based on inelastic scattering of mono-
chromatic light that is usually from a laser source.
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Inelastic scattering refers
to the process where the frequency of photons in a monochromatic light changes
upon interaction with a sample that are absorbed by the sample and then reemit-
ted. The Raman effect refers to the process where the frequency of the reemitted
photons is shifted up or down in comparison with the original monochromatic
frequency. This shift in frequency provides information about vibrational, rota-
tional, and other low-frequency transitions in molecules.
Raman scattering is very weak and it is difficult to separate the weak inelasti-
cally scattered light from the intense Rayleigh scattered laser light. But, this has
been overcome through the discovery of SERS by Fleischmann et al.
122
in 1974
when pyridine was adsorbed onto a roughened silver electrode. The SERS signal
can be enhanced by a factor proportional to the fourth power of the enhancement
of the local incident near field with metals that have high optical reflectivity such
as silver, gold, and copper. Transition metals such as Fe, Ni, Pd, and Pt that have
low optical reflectivity do not serve for efficient SERS effect. Based on the elec-
tromagnetic (EM), theory, the size of the metal particles is required to be much
smaller than the wavelength of the exciting radiation (Rayleigh approximation).
SERS-based biosensors have some advantages over fluorescent, SPR, and
electrochemical biosensors including (i) label-free detection, (ii) excellent
reproducibility, (iii) more reliable multiplexing capability because of finger-
printing Raman spectra, (iv) much higher sensitivity, and (v) potentially greater
flexibility due to larger pools of available and nonoverlapping Raman reporters.
Local electromagnetic field is significantly enhanced at the gap between two
particles, the so-called “hot spot,” which allows SERS enhancement factor to
increase up to 10
12
-10
15
.
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For this reason, nanostructures with sharp or rough
junctions are preferable for SERS-based biosensors.
