Nanoparticles for in-vitro and in-vivo biosensing and imaging

26 Metal nanoparticles
1.8 Shape-dependent properties : the Gans theory
If the size and the environment effects are clearly important, shape effects on the optical
absorption spectrum of gold nanoparticles seem to be even more pronounced [50]
[51] [52]. The plasmon resonance absorption band splits into two bands as the particles
aspect ratio (i.e. the ratio between the long axis, lenght, and the minor axis, width)
increases; moreover, the energy separation between the resonance frequencies of the two
plasmon bands increases [38] [39] [30]. The high-energy absorption band at around 520
nm corresponds to the oscillation of the electrons perpendicular to the minor rod axis
and is referred to as the transverse plasmon absorption. This absorption band is relatively
insensitive to the nanorod aspect ratio [50] [51] [52] and coincides spectrally with
the surface plasmon oscillation of the spherically symmetric nanocrystals. The other
absorption band at lower energies is caused by the oscillation of the free electrons along
the major rod axis and is known as the longitudinal surface plasmon absorption.
Figure 1.10 (inset) shows the absorption spectra of two nanorod samples having aspect
ratios of 2.7 and 3.3; the longitudinal plasmon band maximum (open circles) red
shifts with increasing aspect ratio while the transverse absorption band maximum (open
squares) does not change [50] [51] [52] [53].
Figure 1.10: Size-dependent surface plasmon absorption of gold nanorods. Optical absorption spectra
of gold nanorods with mean aspect ratios of 2.7 and 3.3 are shown. The short-wavelength absorption band
is due to the oscillation of the electrons perpendicular to the major axis of the nanorod while the longwavelength
band is caused by the oscillation along the major axis. The absorption bands are referred to as
the transverse and longitudinal surface plasmon resonances respectively.The former is rather insensitive
towards the nanorod aspect ratio in contrast with the longitudinal surface plasmon band which red shifts
with increasing the aspect ratio. This is illustrated in the inset where the two band maxima (square for
the, transverse mode; circle for the longitudinal mode) are plotted vs. the nanorod aspect ratio R. [35]
The optical absorption spectrum of a collection of randomly oriented gold nanorods
with aspect ratio R can be predicted using an extension of the Mie theory. Within the