Optical Waveguide-Based Biosensors Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 91

diffusion limited, the sensor's DL and response time are limited by
analyte mass transport. As compared to the DL (9.8 × 10 −11 M) for
streptavidin using the FO-PPR sensor in a conventional liquid cell, 23
we have found that the DL (1.1 × 10 −11 M) under similar conditions
has been improved by about ninefold when the FO-PPR sensor was
integrated with a microluidic cell. Since the invention of the FO-PPR
sensor, we have been working on a number of immunoassays with
various binding afinities. So far, the DL found is limited to the pico-
molar level. It has been suggested that nanoscale sensors without
active mass transport will be subject to picomolar detection limit for
practical time scales. 62 As an example, the inset of Fig. 5.4 shows a
calibration graph of the sensor response ( I S / I R ) versus log anti-DNP
concentration. From this linear plot ( R = 0.9910), a DL of 6.2 × 10 −13 M
for anti-DNP is estimated.
As the refractive index increment induced by an adlayer as
shown in Eqs. (18−21) is related to the analyte concentration to be
measured, the detection of absolute number quantities of adsorbed
analyte becomes more sensitive with increasing mass of adsorbed
analyte (with thickness d ). On the other hand, the detection of
absolute number quantities of adsorbed analyte becomes less
sensitive with increasing mass of immobilized receptor (with
thickness h ). With this in mind, we have carried out of a series of
biomolecular interaction studies using various sizes of immobilized
receptors and analytes. The results are summarized in Table 5.1. It
can be seen that the DL is mainly dependent on K a as well as the size
the analyte.

Table 5.1 Detection limits and equilibrium association constants of
different receptor-analyte pairs obtained by the FO-PPR
sensor under similar immobilization and detection conditions

Immobilized receptor Analyte Detection limit (M) K a (M)

2.7 × 10 −10
1.6 × 10 −10

4.8 × 10 8
3.6 × 10 8

DNP
HSA

Anti-DNP
Anti-HSA

Biotin Anti-biotin 8.5 × 10 −8 1.7 × 10 6

4.6 × 10 −7 1.3 × 10 6

Anti-HSA HSA

DNP: dinitrophenyl, MW = 297 Da
Anti-DNP: MW = 220 kDa 63
Biotin: MW = 244 Da
Anti-biotin: MW = 150 kDa 18
HSA: human serum albumin, MW = 67 kDa 64
Anti-HSA: MW = 160 kDa 64