Brain Function: Novel Technologies Driving Novel Understanding - Bioinspired Approaches for Human-Centric Technologies - page 322

surface. The nanostructures can be in the form of surface roughness or nano-
patterned metal thin layers; these metallic elements on the surface respond to the
incident light with the generation of localized surface plasmons that can enhance
Raman signals by factors up to 10 11 , greatly enhancing the sensitivity compared to
standard Raman biosensors. In comparison to roughened surfaces, nano-patterned
metal layers offer a more controlled, tunable, and narrow frequency response.
However, due to the enormous field enhancement factors, Raman signal intensities
are very sensitive to small geometric variations of the nanostructures across the
sensor's surface, which can lead to inaccurate measurements of bio-element
concentration.
Surface plasmon resonance (SPR) biosensors are based on the detection of the
variation of the resonance frequency of surface plasmons at the interface between a
noble metal and the wet environment in which the analyte is immersed. Since the
plasmons are confined at the metal surface, any small change at the interface, like
the presence of ligands or active biomolecules, results in a large variation of the
plasmon resonance frequency. This effect leads to extremely high sensitivity at
molecular level, which is the workhorse of SPR biosensors. The typical structure of
an SPR biosensor includes a glass substrate with a gold film deposited on the
surface in contact with the liquid buffer containing the target molecules under
investigation; the gold surface is functionalized with specific molecules, like pro-
teins or DNA, which will bind to or interact with the target molecules, thus
changing the gold surface's response in terms of plasmon resonance. From the
other side of the glass substrate, light of tunable wavelength can be directed to the
gold/electrolyte interface at various incident angles with the aid of a prism. Light
reflected from the gold film goes again through the glass substrate and through the
prism to a detector.
For optical transducers, the morphology and geometry of the nanostructures
plays a crucial role. While two 2D nanostructures were initially proposed (e.g.,
metallic optical nanoantennas and quantum dots), recent advances in
nanostructuring technologies are enabling a wide range of submicron three-
dimensional morphologies that include nanowires, cones, and cylinders. Cone-
like or nose-cone-like shapes are particularly interesting for optical/plasmonic
biosensing applications in which spectroscopy techniques are employed for detec-
tion. If such nanostructures are made of noble metals and have the correct size in the
nanoscale, they act as nanoantennas or nano-waveguides that can confine optical
electromagnetic fields well below their diffraction limit, augmenting field intensi-
ties up to a factor of 10 3 . The confinement effect is generated by the surface
plasmon polaritons (SPPs) that are produced by light excitement and that travel
along the metal/dielectric interface of the cone surface. In fact theoretical calcula-
tions predict an effect of adiabatic compression of SPPs at the tip of perfectly
conical shapes with tip radius of few nanometers (Ropers et al. 2007 ; Stockman
2004 ). At IIT, De Angelis et al. ( 2008 ) reported the fabrication of such
nanostructures and their use for Raman detection of very few molecules with
sub-wavelength spatial confinement. The reported 3D structure was a gold-coated
nanoantenna with a pointed tip fabricated with electron beam-induced deposition