Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 38

Figure 3.3 A list of silver and gold nanomaterials having various
morphologies, compositions, and structures together with
the typical locations of their LSPR bands in the visible regime.
Reprinted with permission from ref. 62. See also Color Insert.
Figure 3.3 displays that the LSPR absorption bands of Au and
Ag NPs can be tuned in the spectral range from 400 to 750 nm. 64
For instance, small Au NPs (e.g., 13 nm in diameter) absorb green
light, corresponding to a strong LSPR absorption band at ~520 nm
in the visible light spectrum. For small Au NPs, incoming light in a
dipole mode oscillates the surface electrons. As the size of the Au
NP increases, light can no longer polarize the NPs homogeneously,
and then higher order modes at lower energy dominate, causing
a red shift and broadening of the SPR absorption band upon
increasing the size of the Au NPs. 64 This phenomenon also explains
the corresponding LSPR absorption band shifts (redshift) and color
changes (red to purple) when the aggregation of small Au NPs occur.
When Au NPs aggregate, their surface plasmons combine. Thus, Au
NP aggregates can be considered as a single large particle although
the detailed interparticle plasmon coupling is rather complex and
dependent on many factors, such as aggregate morphology and
NP density.
Also displayed in Fig. 3.3 is the fact that different shapes of Au
and Ag materials have different colors. The LSPR peaks of Ag/Au
alloy nanomaterials are between 405 nm and 516 nm and can be
tuned from 405 to 516 nm by simply changing the content of Au
with the color changing from yellow to red. The linear relationship
between the peak position ( λ max ) and the molar fraction of Au
( x Au ) is itted with λ max = 412 + 1.13 x Au . The color of Ag triangular
nanoplates with edge lengths varied from ~30 to ~90 nm is greenish
blue. These Ag nanoplates exhibit three distinctive peaks located
at 690, 485, and 335 nm. These peaks are assigned to the in-plane
dipole, in-plane quadrupole, and out-of-plane quadrupole plasmon