Slide- and Micoarray-Based Biosensors Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 60

4.2 Localized Surface Plasmon Resonance

For the development of integrated lab-on-a-chip and array-based
biosensors, 10 the LSPR phenomenon 11-13 associated with nanoscale
structures of noble metals appears to be especially attractive. This
is due to a change in interfacial refractive index upon biomolecular
binding induces a change in the color of the sensor substrate, thus
providing an easily detectable optical signal. Moreover, since LSPR
excitations are conined to the nanostructures, miniaturization down
to the scale of single nanoparticle can be accomplished. This is a strong
contrast to conventional SPR, also call propagating surface plasmon
resonance (PSPR), in which the laterally propagating nature of the
surface plasmon polaritons 14 limits the lateral resolution to tens of
micrometers. 15 In PSPR, bulk plasmons cannot be excited by freely
propagating light owing to the longitudinal nature of the oscillating
charges in the plasmon and the transverse nature of the electric
ield of light. As such, a momentum matching scheme such as prism
coupling or Bragg scattering is required for excitation of PSPR. 16
On the contrary, LSPR is attributed to the collective oscillations of
surface electrons in a nanostructure excited directly by an incident
optical wave, resulting in peaks in the extinction (absorption and

Figure 4.3 (Left) Transmission electron micrographs of Au nanospheres
(upper left) and Au nanorods (lower left) and (Right) UV-
vis absorption spectra of a sample of Au nanorods having an
aspect ratio of 3.2 (solid line) compared to that of 16 nm Au
nanospheres (dot line).