Recent developments in electrochemical immunoassays and immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 134

label on the immunocomplex at the same electrode. More recently, interdigitated array
(IDA) microelectrodes have gained popularity as an alternative transducer in electro-
chemical immunoassays. In general, a simple design of an IDA consists of a pair of
interdigitated microelectrode “fi ngers”. When an IDA is used as a sensing electrode
in a voltammetric experiment, the two interdigitated electrodes are usually held at dif-
ferent potentials to achieve “redox” cycling of the electroactive species to be detected.
This is illustrated in Fig. 5.9 using the detection of 4-AP discussed in section 5.5.2 as
an example. According to Scheme 1, an oxidizing potential is applied to one of the
two IDA electrodes to promote a two-electron oxidation of 4-AP to 4-QI. Then, 4-QI
diffuses to an adjacent electrode held at a more negative potential, where it is reduced
to 4-AP, which can then undergo another oxidation at the next adjacent electrode. A
major advantage of this redox cycling is that it improves the signal-to-noise ratio by
enhancing the Faradaic current relative to the background current, resulting in lower
detection limits and improved sensitivity. These features opened up many opportuni-
ties in which IDA electrodes were applied as electrochemical detectors in analytical
chemistry and biosensor systems [60]. In a recent application, Thomas et al. used an
IDA consisting of 25 pairs of platinum microelectrodes with 1.6 m

widths as a detector in immunoassays for mouse IgG [61]. A fourfold amplifi cation of
signal was obtained compared to single-electrode detection, with redox cycling of 87%
and a detection limit of 3.5 fmol mouse IgG.

µ m gaps and 2.4 µ

5.5.4 Impedimetric immunoassays and immunosensors
Various strategies were described in section 5.4 for immobilizing antibody on an elec-
trode surface, which is often the fi rst step in building the desired immunocomplex on the
surface of an electrochemical immunosensor. Upon the specifi c molecular recognition of
the antigen by the immobilized antibody, there will be changes in the interfacial charge,
capacitance, resistance, mass, and thickness at the immunosensor surface. There is thus
an emerging interest in exploiting electrochemical techniques that follow such interfa-
cial changes, which may yield quantitative determination of the targeted analyte. In this
section, we shall focus on electrochemical impedance spectroscopy (EIS) as an effective
method for probing the features of an electrode surface modifi ed by immunocomplexes.
In an EIS experiment, a low amplitude (5 to 10 mV peak-to-peak) sine wave potential
signal is superimposed on a fi xed DC potential applied to an electrochemical system.
Based on Ohm's law, the impedance can be computed from the applied sinusoidal poten-
tial and the measured sinusoidal current. As the sinusoidal potential and current will

4-AP 4-Q1 4-AP 4-AP 4-Q1 4-AP 4-Q1 4-Q1 4-AP

2e 2e 2e 2e 2e 2e

Cathode Anode Cathode Anode

FIGURE 5.9 A schematic showing redox cycling between 4-AP and 4-QI (see Scheme 1) at adjacent
“fi ngers” in an IDA electrode system.