Optical Waveguide-Based Biosensors Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 90

signiicantly reduced response time from several thousand seconds
in a conventional liquid cell 25 to several hundred seconds in this
microluidic cell. This reduction in response time can be accounted
for by an expression derived from the Fick's Law of Diffusion: 57

Figure 5.4 (A) Temporal response of a DNP-functionalized FO-PPR sensor
with serial injection of (a) a PBS buffer and samples of an anti-
DNP solution with concentration of (b) 1 × 10 −10 g/mL, (c) 1 ×
10 −9 g/mL, (d) 1 × 10 −8 g/mL, (e) 1 × 10 −7 g/mL, (f) 1 × 10 −6 g/
mL, and (g) 1 × 10 −5 g/mL. (B) Calibration graph of I S / I R versus
log anti-DNP concentration.

x

2

=

(5.25)

t

where t , x , and D are the molecular transportation time, diffusion
distance, and diffusion coeficient, respectively. This means that a
down-scale to 1/10 of the original size of the sensing cell reduces
the molecular transport time to 1/100. 58 Based on this relationship,
a decrease in the response time by reducing the width of the
microluidic channel is expected. Further reduction in response
time can be achieved by active mass transport, for example, by ac
electro-osmosis driven by ield-induced polarization. 59 Using this
micro-mixer (see Chapter 7 for details), the sensing response time of
detecting an orchid Odontoglossum ringspot virus (ORSV) has been
reduced from 1000 s to 330 s when the external ield is applied to
mix for 60 s while detection limit is maintained. 60
Although in principle a refractive index sensor is almost
totally insensitive to volume change of the sensing cell, 55 the
performance of a biosensor has been suggested to be limited by
the rate at which an analyte molecule is transported to a surface-
immobilized recognition molecule. 61,62 When this binding process is