Optical Waveguide-Based Biosensors Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 80

sensors cannot beneit from increasing the interaction length of the
sensor 27 while OW-PPR sensors can take advantage of multiple total
internal relections (TIR) along the optical waveguide to increase
sensitivity. 24 Fourth, the simple wet chemical approach typically
used to construct a NMNP layer on an OW element for OW-PPR
sensors has a cost advantage and allows a wider choice of the shape
of the sensor substrate, while the vacuum-based sputtering method
typically used to make a noble metal ilm for SPR sensors is expensive
and dificult to coat substrates with a complicated shape. Fifth,
chemically tailorable physically properties such as size, shape, and
composition lead to tunable PPR band, allowing the spectroscopic
sensing window to be tuned without re-alignment of the optical
setup. 28,29 Sixth, because of the lower refractive index sensitivity,
the PPR sensor requires little temperature control whereas the SPR
sensor (with large refractive index sensitivity) requires stringent
temperature control. 30 Seventh, the rate of analyte mass transport
to a nanoparticle sensor will be governed by radial diffusion and
consequently will be faster than that to the planar format SPR
sensor operating under semi-ininite linear diffusion. 18,30,31 Eighth,
the smaller electromagnetic ield decay length in PPR sensor 32 as
compared to that of SPR sensor yields better sensitivity in detecting
small molecules, 30,33 and also eliminates the need to trap the
interacting molecules of interest in a polymer matrix to enhance
the signal. 34 Finally, the most dramatic difference between the OW-
PPR and SPR sensors is cost. Commercialized SPR instruments can
vary between USD150,000~300,000, 30,35 whereas the prototype
FO-PPR system costs less than USD5,000. Thus, the unique optical
properties exhibited by NMNPs and the combined beneits with
the OW technology have paved the way for the construction of new
biosensors. We believe this novel biosensing technology has a high
potential of being commercialized.

5.2 Principle of the Biosensors

In this section, we take the FO-PPR biosensor as an example to
illustrate the principle of the biosensing platforms. A schematic
illustrating the principle and the setup of the FO-PPR biosensor
is shown in Fig. 5.1. Optical waveguide formats other than optical
iber generally follow the same principle but some modiications to
accommodate the speciic optical waveguide mode are needed.