Page 168 - Vibrational Spectroscopic Imaging for Biomedical Applications

144 Cha pte r F i v e

been manufactured, like a secondary chromophore in fluorescent
staining. Only a very few relate to the area map imaging of biotissues
mounted onto a regularly patterned SERS active substrate. 5,7,76 We
have adopted the latter approach in our study of fungi in order to
gain spectroscopic information about the biochemical changes accom-
panying fungal development from a spore to a mature hypha capable
of producing new spores.
As with ordinary Raman spectroscopy, the core of the phenom-
enon rests on the inelastic scatter from a photon-molecule interaction
that produces a second photon, which differs in energy from the
incident photon by an amount (either more or less) equal to the
vibrational energy of some vibrational mode. The enhancement
factor arises from the special interaction between the incident pho-
ton and the roughened surface or nanoparticle with which the mol-
ecule is in contact.
Electrons in metals can exist in a conduction band, that is, in
energy levels that are virtually continuous and high in energy relative
to the orbitals of isolated metal atoms. The orbitals of these electrons
are delocalized across the metal particle and account for the ease of
electrical and thermal conductivity. In the same way that the elec-
trons on a molecule may be polarized by the action of an external
field [Eq. (5.1)], the electron distribution in metal nanoparticles may
also be polarized. A restoring force acts upon these electrons, due to
the strong attraction of the positively charged nuclei which are
essentially locked into the metal lattice. When the wavelength of
the external electric field is far greater than the dimension of the
nanoparticle, the entire particle experiences an effectively homoge-
neous field, and responds in a uniform manner. For a perfectly
uniform metallic sphere, there will be some natural frequency of
oscillation of the electrons, acted upon by the nuclear restoring force.
If the frequency of incident light matches this resonant frequency, the
particle will absorb photons. For gold, silver, and several other
coinage metals, the absorption bands of these electrons fall into the
visible portion of the spectrum. The electrons in the nanoparticle will
now be in a higher energy state in which they oscillate back and forth
across the particle, at this resonant frequency: this is the surface plas-
mon resonance (SPR). The existence of the SPR means that a molecule
bound to, or in intimate contact with, the surface will experience a
vastly enhanced external field, ξ in Eq. (5.1). The induced dipole
moment will be similarly enhanced, as will any derivative of the polar-
izability, thus the Raman scattering will be enormously enhanced.
Not only nanodots, but also hollow gold nanoparticles, silver island
films, roughened surfaces, and nanopatterned surfaces have been
found to promote the SERS effect. The review articles noted above pro-
vide extensive descriptions of the present state of understanding of
theory including the various shapes and designs of nanoparticles and
nanoparticle aggregates that facilitate the phenomenon. Convincing