Synthesis and Optical Properties of Noble Metal Nanoparticles for Biodetection - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 5

were found to have a luorescence peak at 465 nm. 38 Embedded
silver nanodots were also observed to luoresce. 39,40 Most of the
observed luorescence was attributed to the radiative recombination
of an electron-hole pair between d band and sp -conduction band
above the Fermi level. Due to the plasmon resonance excitation, the
local ield created around the nanoparticles is found to modify the
observed luorescence. 41
When the size regime falls into less than ca. 2 nm, gold
nanoparticles exhibit the luorescence property. This unique
behavior is believed to be attributed to their sizes becoming
comparable to the Fermi wavelength of the electron (ca. 0.7 nm). The
quantum size effect occurs and results in molecule-like properties of
discrete electronic states and size-dependent luorescence. 42,43 The
photoluminescent properties are attributed to the recombination
involving d -band excitation. 44 While gold nanoparticles having
larger sizes show a size-dependent plasmon absorption band with
their free electrons in both the ground and excited states conined to
dimensions smaller than their mean free path, ca. 20 nm.

1 . 3 . 2 Absorption Spectral Characteristics of Noble

Metal Nanorods

Under the limit of classical electrostatic model, the absorption spectra
of spherical metal nanoparticles for several tens metal elements
have been described previously. 45 The material-dependent optical
properties in the range of 200-900 nm can be clearly described.
Based on the same classical formalism, we present, in the following
two sections, both the absorption and scattering spectra for 16
chosen metal nanorods dispersed in the surround medium of water
by the use of classical electrostatic simulation model described
previously. 13,22,46-48 Elements of Na, Ti, Mn, Re, Ni, Pd, Pt, Fe, Cu,
Ag, Au, Os, Al, Ga, Co, and Rh were selected to portrait the relative
individual and group spectral behaviors. The spectra are presented
in the wavelength range of 150-1200 nm. Metal nanorods exhibit
two surface plasmon components in their absorption and scattering
spectra: longitudinal and transverse modes. Depending on the type
of material, their relative intensities vary. But, quite similarly, the
peak positions of surface plasmon transverse band shift to shorter
wavelength, while those of the longitudinal band red-shift as the
particle aspect ratios increase. 13,14,49