Synthesis and Optical Properties of Noble Metal Nanoparticles for Biodetection - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 2

1 . 1 Gold Nanoparticles and Their

Biomedical Applications

Noble metal, especially gold, nanoparticles have been used for
various biomedical applications, 1-8 including as contrast agents
for photoacoustic imaging. 9-11 One intriguing property of gold
nanoparticles is that their absorption spectra are geometry
dependent. The size effect on the surface plasmon absorption of
spherical gold nanoparticles was reported to result in the red-shift
of the spectral peak red shifting with increasing particle size, with
gold nanospheres with diameters of tens of nanometers exhibiting
an absorption peak at around 550 nm. 12 However, this misleading
report was later realized that the size effects on such nanoparticles
relect in the absorption cross-sections only. The spectral shift in the
experimental observation was mis-interpreted with the existence
of the shape effect. In contrast, the resonance of peak absorption
of gold nanorods depends only weakly on their diameter and
overall dimension, with it instead shifting strongly to the longer
wavelength with the aspect ratio (deined as the ratio of the length
of the major axis to that of the minor axis). 13,14 Laser irradiation at
the wavelength corresponding to the peak absorption results in the
maximal absorption as well as the maximal photoacoustic signal.
Targeting gold nanoparticles to cancer cells helps in the early
detection using photoacoustic imaging due to the large difference in
the optical absorption between gold nanoparticles and the tissue. 5
In addition, targeting different cancer cells with bioconjugated gold
nanorods having different aspect ratios can also be achieved. 11 The
bioconjugated gold nanorods interact with cells that have antigens
speciic to the conjugated antibody, and hence the cells that bind the
bioconjugated gold nanorods can be detected using photoacoustic
techniques. Multiple targeting using gold nanorods with different
aspect ratios was tested experimentally to demonstrate the feasibility
of measuring the expression levels of different oncogenes of cancer
cells simultaneously.
In this chapter, we discuss the synthetic schemes developed for
various types of dispersed gold nanostructures in aqueous solutions
and three different optical properties of noble metal nanoparticles,
which are widely adopted in the area of biodetection. In the content
for such optical behaviors, we emphasize on the simulated absorption
and scattering spectra based on the classical Mie/Drude formalism