Optical Waveguide-Based Biosensors Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 87

by the sensor) of 7.0 × 10 −5 RIU was achieved. Such a bulk RIR is
inferior to that achieved by many SPR sensors. 48 Nevertheless, a
surprising low detection limit (DL) of 9.8 × 10 −11 M for detection of
streptavidin was achieved. 23 There must be a reason.
Both PPR and SPR biosensors measure changes in the RI of the
surface adlayer. A consequence of the evanescent character of both
sensing techniques is that the sensing ield strength decays quite
markedly with distance from the sensor surface. 30 The SPR response
with respect to the dissipating iled strength can be depicted by
exponent dependency on distance normal from the surface: 49

⎡ ⎤

. . 1- dI

⎦

(5.20) Δλ= Δ ⎣

-2 /

mn e

d

where Δλ is the wavelength shift response, m is the refractive index
sensitivity, Δ n is the change in RI induced by an adsorbate, d is the
effective adsorbate layer thickness, and l d is the characteristic ield
decay length. Equation 5.18 is also used in a review article to account
for the wavelength shift response in PPR biosensors, but the authors
caution that the electromagnetic ields associated with NMNPs is
oversimpliied. 50
In biosensor applications, it is common to attach a selective
receptor on the sensor surface, and an analyte subsequently binds
with the selective receptor from solution. Assuming the thickness
of the immobilized receptor is h , a simple extension of Eq. (5.20)
yields 49

. .

⎡ ⎤

.

Δλ= Δ ⎣

1- dI

-2 /

−

2 / hI

mn e

⎦

(5.21)

d d

For the PPR biosensor or FO-PPR biosensor which interrogates
the binding event via absorbance change, Δ A , similar relationships
are proposed as following:

. . 1- dI

⎡ ⎤

Δ = Δ ⎣

-2 /

Amn e

⎦

(5.22)

d

. .

⎡ ⎤

. Δ = Δ ⎣

1- dI

-2 / -2 / hI

Amn e

⎦

e (5.23)

d d

In a review article by Willets and Van Duyne, 51 the authors give
the following summary: although SPR spectroscopy provides much
higher sensitivity to changes in bulk RI (~3100 to 8800 nm/RIU) 49
than PPR spectroscopy (~70 to 880 nm/RIU), 16,19,52-54 the responses
of the two techniques become comparable when measuring short-
range changes in RI owing to a molecular adsorption layer. This is a