Page 458 - Book Hosokawa Nanoparticle Technology Handbook

APPLICATIONS 3 SENSING BASED ON LOCALIZED SURFACE PLASMON RESONANCE
APPLICATION 3

3 SENSING BASED ON LOCALIZED SURFACE PLASMON RESONANCE IN METALLIC
NANOPARTICLES

1. Localized surface plasmon where r stands for radius of the nanosphere. The
polarizability ( ) is maximum with the minimum
Gold nanospheres dispersed in a solution are ruby red, absolute value of the denominator. When the nanos-
since green light interacts with electrons in the nanos- phere is covered with a dielectric thin film with a
pheres and is adsorbed. This phenomenon is called thickness d and dielectric constant ( ), equation
d
localized surface plasmon resonance (LPR). Localized (3.1) is rewritten as
surface plasmon resonance occurs not only in metallic
nanoparticles but also in a sharp metallic tip and at a () () ()

()
m
B

rough metallic surface. These LPR phenomena are () 4 ()(r ) d 3 d A () (() (3.2)
m

()
applied to enhancement of near-field scanning () 2 m B
A
d
microscopy, Raman scattering and fluorescence spec-
troscopy. LPR has the following features: where the parameters are described as
(1) Confinement of light within a nanometer region.
() ()(3 2P ) 2 ()P (3.3)
(2) The sensitive resonance condition to the dielec- A a d
tric constant of the ambient medium.
() () ()(3 P ) (3.4)
P
d
a
B
(3) A large electric field produced at the LPR
condition. ⎛ r ⎞ 3
P 1 ⎜ ⎟ (3.5)
d
Recently, nano- and micro-photonic devices and sen- ⎝ r ⎠
sors have been developed, based on LPR. This article
deals with small and potential sensing devices by use Fig. 3.1c shows scattering efficiency of gold nanos-
of metallic nanoparticles. pheres of 40 nm in diameter covered with a dielectric
The optical properties of nanoparticles are described thin film ( ( ) 2.25), in which the thicknesses of
d
in textbooks in detail [1, 2]. Here we consider the the dielectric film are 2, 5 and 10 nm. The thickness
optical response of a nanosphere described in of the thin film and its optical properties can be
Fig. 3.1a. Suppose the dielectric constant of the nanos- probed by measuring scattering spectra or absorption
phere ( ) and that of the ambient medium ( ) at a spectra, which is determined by ( ).
m
a
wavelength . If the size of the nanosphere is much
smaller than the wavelength of light, the polarizability 2. Two sensing method using plasmon
of the nanosphere can be described as follows [3].
There are two kinds of bio and chemical sensors that

() ()

() 4 ()r 3 a m (3.1) utilize LPR in metallic nanoparticles. One is the
m

() 2 () sensor that probes affinity between molecules
a
m
(a) (b) d (c) 8
e m (l) 6 10nm
e m (l) scattering Efficiency (a.u.) 4 5nm
2nm
e a (l) e a (l) 2 0nm
r
r
e d (l) 500 600 700 800
wavelength (nm)
Figure 3.1
(a) Optical geometry of a nanoparticle. (b) Optical geometry of nanoparticle covered with a dielectric film. (c) Calculated
scattering efficiency of gold nanoparticles with various thickness of dielectric films.
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