Aspects of recent development of immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 236

monolayer of mercaptopropionic acid, providing a charged polyelectrolyte layer on the
transducer surface [19]. The biomolecules of avidin and anti-immunoglobulin (IgG)
antibodies were then well immobilized through electrostatic interaction. A novel bio-
sensing interfacial design strategy has been developed for immobilizing the antibodies
onto the positively charged surfaces of plasma-polymerized fi lm (PPF) via electrostatic
interaction through a polyelectrolyte-mediated layer [20]. The immunosensors so pre-
pared exhibited excellent response sensitivity due to the low disturbance of the elec-
trostatic adsorption immobilization to the activity of antibody. The PPF surfaces can
be regenerated repetitively by changing the pH of the buffer solutions to remove the
polyelectrolyte-mediated layer.
Moreover, antibodies or antigens may be physically entrapped into the fi lms of
organic high polymers or inorganic materials (e.g. sol-gel, graphite powder) with stereo
meshy structures. Of these entrapment immobilizations, the sol-gel-based immobili-
zations have recently attracted much attention due to their ability to encapsulate bio-
molecules at low temperature, as well as the physical tenability, optical transparency,
mechanical rigidity, and low chemical reactivity [22-23]. Most applications of the
sol-gel-based immobilizations have been primarily directed to the optical immunosensors
[14, 22, 24] and the electrochemical immunosensors [23, 25-28]. Martínez-Fàbregas
et al. proposed a polishable entrapment immobilization based on rigid biocomposite
materials consisting of graphite powder, rabbit IgG, and methacrylate (or epoxy resins)
[28]. The surface of the immunosensor can be regenerated by simply polishing to obtain
a fresh layer of immunocomposite ready for next immunoassay. The aforementioned
physical interaction-based immobilization procedures are demonstrated to be operated
simply and rapidly. However, their immobilization stability might be infl uenced by the
bulk metal surfaces and environmental factors such as temperature, pH, and ion strength
of solution, resulting in a loss of bioactivity or denaturation of the proteins. Moreover,
the gradual elution of proteins physically adsorbed may occur during the analytical per-
formances, which may in turn bring about some problems associated with loss of detec-
tion sensitivity and low reproducibility of the sensors.
In recent years, nanomaterials (e.g. noble metals, magnetic oxides, and carbon
nanoparticles or nanotubes) with unique physical and chemical properties have been
successfully applied to modify immunosensing interfaces to achieve greatly improved
immobilization of antibodies or antigens [29-31]. Some pioneering works have shown
that the assembly of the gold nanoparticle layer on an electrode would lead to sub-
stantially increased electrode surface areas available for direct adsorption of biological
entities, thus offering the possibility of the great enhancement of analytical sensitivity
[11, 32-36]. For example, a new immobilization procedure of antibodies for capacitive
immunosensor has been recently proposed using thiol compound and gold nanoparti-
cles [36]. It was here demonstrated that the proposed immobilization procedure could
retain the high biological activity of immobilized entities and provide favorable sens-
ing performances. Moreover, magnetic nanoparticles as special carriers for immobiliz-
ing biomolecules have also been the current hot subject of a series of investigations
for the construction of different immunosensors [37-39]. The easy localization of
magnetic beads was used to generate a sensing layer at the surface of a piezoelectric