Sensing of Biomolecules - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 95

analyte/antigen binds to the antibody, a change of the refractive index of the surface
is taking place, and thus, the SPP resonance is shifted. As shown in Fig. 2.35 ,the
SPP is integrated with a microfluidic channel because the majority of antigens are in
solution. The net advantage of this sensor is a label-free sensing mechanism. Using
such SPP sensors, very low quantities of hemoglobin (50 ppb), BSA (100 pM), and
Tropotin I (2.5 ppb), which is a cardiac marker protein, were detected and reported
in Shankaran et al. ( 2007 ) and the references therein. Very recently, the change in
cell volumes directly grown on a gold layer was detected with SPP technique via the
SPP resonance shift ( Robelek and Wegener 2010 ), demonstrating again that SPP is
a very powerful technique for label-free sensing.
There are two categories of SPP immunoassays. In the heterogeneous immunoas-
say, an antibody or antigen is first immobilized on the interface, and a further
binding with the analyte occurs at the interface. This category of SPP immunosen-
sor, which is by far the most widespread, is depicted in Fig. 2.35 . In the other,
homogenous assay, the biochemical reaction takes place in solution. There are also
other SPP assays, such as direct assays, sandwich assay, displacement assays, all
able to detect biomolecules or biochemical reaction in real time.
There is another type of SPP resonance, termed as localized surface plasmon
(LSP), which refers to coherent charge density oscillations on metallic nanoparticles
and other metallic nanostructures. LSP manifests at a certain wavelength as the
result of light excitation of nanoparticles, and the resonance produces increased
light scattering, intense absorption bands, and local field enhancement (Hutter and
Fendler 2004).
The LSP resonance and the associated effects (scattering, absorption, and field
enhancement) are influenced by the shape and size of the metallic nanostructures.
The interaction with light of spherical particles is described by the Mie theory, which
is based on the assumption that the nanoparticles and the surrounding medium are
homogeneous and have associated permittivity function " m D " 0 m C i" 0 m and " d ,
respectively. Assuming that the excitation wavelength exc D D 2R,whereR is
the radius of the spherical nanoparticles and D their diameter, and that the induced
polarizability is P D ˛E where

˛ D 4" 0 R 3 ." m " d /=." m C 2" d /;

(2.85)

is the polarizability factor, the LSP resonance occurs when

" 0 m D 2" d :

(2.86)

This resonance condition, termed Frohlich condition, is analogous to that of SPP
and implies that the real parts of the permittivities of nanoparticles and the dielectric
medium must have opposite signs. The LSP resonance is illustrated in Fig. 2.36 .
The extinction coefficient (encompassing both absorption and scattering) of a
thin dielectric film containing spherical metallic particles with a radius R much
smaller than the excitation wavelength exc and density per unit area N is

ext D 24 NR 3 " 3= d Œ" 0 m =." 0 m C 2" d / 2 C " 00 m = exc ln.10/:

(2.87)