“9.61x6.69” b2815 Tissue Engineering and Nanotheranostics

Plasmonic Nanoparticles Application in Biosensor and Bioimaging 153

nanoparticle­based energy transfer and, surface­enhanced Raman

scattering (SERs) and therapy briefly.

2. Theory of Localized Surface Plasmon Resonance

Surface plasmons (SPs) are coherent oscillation of conduction elec­

trons on a metal surface excited by electromagnetic radiation at a

metal­dielectric interface. When it confined itself to a particle of a size

comparable to the wavelength of light, that is, the particle’s free elec­

trons participate in the collective oscillation, surface plasmon takes

the form of LSPR.^2

LSPR significantly enhances the local electric field at the metal

nanoparticle surface, which is the basis for the SERs, and increases

light absorption and scattering by the nanoparticle at the SPR fre­

quency. For noble metal nanoparticles, with a negative real and small

positive imaginary dielectric constant over a range of wavelengths, the

SPR frequency occurs at visible wavelengths. As the scattering wave­

length of nanoparticle depends on its size and shape, the refractive

index of the surrounding medium and internanoparticle coupling

interactions, addition of biomolecule, that would change the electric

field of nanoparticle, results in scattering shift. Due to this effect,

plasmonic nanoparticle has been engineered to be a sensor with high

specificity and sensitivity. To fully understand LSPR, would like to

briefly introduce the scattering theory.

2.1. Mie Theory

Gustav Mie proposed a theory to calculate the scattering and absorp­

tion cross­section of a small spherical particle in 1908.^3 Assuming the

small nanoparticles (d<<λ) interact with a plane­wave incident, the

total scattering, extinction, and absorption cross­sections can be

expressed as follows:

sca 2 ()()^22

1

2

(^21) LL,
L

L ab

k

p ∞

σ

=

=++∑

(1)