Recent developments in electrochemical immunoassays and immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 138

lowering detection limits. In most microbead-based immunoassays, after immobilizing
an enzyme-conjugated immunocomplex on the beads, a sample of the bead solution is
added to the enzyme substrate solution already making contact with an electrode (e.g.
a rotating disk electrode, a microelectrode or an IDA) where detection is carried out.
By separating the immunoreaction from the electrochemical detection steps, the work-
ing electrode surface is more accessible to the enzyme product as it diffuses to the bare
electrode surface. In some cases, the separation of steps also reduces electrode foul-
ing by biological species during analysis. For example, following the immobilization
of a -galactosidase-conjugated mouse IgG immunocomplex on microbeads, Thomas

et al. obtained a detectable oxidation current when a 10

β

µ L bead sample solution was

added to 10 -D-galactosidase)

already placed on an IDA [70]. In this way, a detection limit of 26 ng mL 1 of mouse
IgG was achieved. Similar applications of microbead-based immunoassays were more
recently demonstrated for bacteriophage MS2 [71] and E. coli [72] with detection lim-
its of 90 ng mL 1 and 20 cfu mL 1 , respectively. A disposable immunomagnetic elec-
trochemical sensor was also developed for detecting polychlorinated biphenyls [73].
In addition to polystyrene beads, Zhang and Meyerhoff have also recently extended
the type of magnetic beads by introducing a gold layer on polystyrene beads using
an electroless gold plating procedure [44]. In this way, stable immunocomplexes can
also be immobilized on gold coated magnetic beads via the thiol-gold pseudo covalent
bond. In a different system, colloidal gold nanoparticles were used to anchor carci-
noma antigen-125 in a cellulose acetate membrane on a glassy carbon electrode [74].
A competitive immunoassay was then established to detect carcinoma 125 down to
approximately 1.8 U mL 1 .

µ L of the enzyme substrate solution (4-aminophenyl β

5.7 CONCLUDING REMARKS

Although immunoassay techniques emerged over two decades ago, there are still vig-
orous research efforts and tremendous progress in the development of electrochemical
immunoassays and immunosensors. An extraordinary feature of these immunosystems
is their specifi city. There are continuing studies examining various strategies that will
aid in aligning antibodies on a solid phase in an optimal direction with minimal steric
hindrance. Developments in this area will undoubtedly further enhance the degree of
sensitivity achievable in analyses involving immunoassays and immunosensors. In con-
junction with electrochemical detection, these systems will offer sensitive and selective
analyses that are faster, simpler, and more economical. There is also continuing interest
in developing and applying suitable labels for electrochemical immunoassays such that
a more direct signal-generation scheme can be used. The application of electrochemical
impedance spectroscopy has started to facilitate a label-free scheme, and this is defi -
nitely an attractive, simpler alternative to others involving a label. Nonetheless, much
research effort is needed in this area to achieve the required sensitivity and dynamic
range obtainable in amperometric detection. At the same time, there are also encour-
aging promises in the development of microfl uidic electrochemical immunoassay