Aspects of recent development of immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 237

sensor, where the magnetic beads bearing antibodies were immobilized with the help
of a permanent magnet at the surface of the crystal [37]. More recently, an ampero-
metric immunosensor has been developed by employing a kind of core-shell magnetic
nanoparticle of (CdFe 2 O 4 ø SiO 2 ) to immobilize antibody onto the electrode surface
with a magnet fi eld [38]. Additionally, magnetic beads may be applied to label or
attach antibodies (antigens) for the magneto-detection of the immune complex based
on the perturbation of a magnetic fi eld, which could be quantifi ed using a suitable
electronic device [39]. Compared with the conventional immobilization methods, these
magnetism-driven immobilization procedures may have some merits such as simple
manipulation, easy biomolecule modifi cation, low cost, and repeatable regeneration.

9.2.2 Covalent interaction-based immobilization procedures
The covalent interaction procedures, typically the cross-linking methods, are the most
popular immobilization manipulation for fabricating various immunosensors. Due to
the lack of an amount of active covalently binding sites at some transducer substrates
(e.g. metals, semiconductor, or optical fi bers), the precoatings of the base transducers
with thin fi lms are generally necessary for covalently binding the antibodies or anti-
gens by using the functional reagents such as glutaraldehyde, carbodiimide succin-
imide ester, maleinimide, and periodate. Many traditional coating materials, such as
polyethyleneimine [40-41], (

-aminopropyl) trimethoxysilane [42-43], and copolymer
of hydroxyethyl- and methyl-methacrylate [44], are often used as the mediate layers for
immunoactive molecule immobilization. In recent decades, however, some new coating
or functionalized fi lm techniques (materials) have been introduced into this fi eld.
Self-assembled monolayers (SAMs) offer promising functionalized fi lms for the
immobilization of antibodies or antigens [45-46]. Since sulfur donor atoms strongly
coordinate on noble metal substrates (e.g. Au, Ag, and Pt), various sulfur-containing
molecules such as disulfi des (R-SS-R), sulfi des (R-S-R), and thiols can form various
functionalized SAMs of highly organized and compact construction. The applica-
tions of the SAM technique in the immobilization of biomolecules have been widely
documented [47-49]. Knoll and coworkers presented a versatile biotin-functionalized
SAM, on which the biotinylated antibodies can be readily immobilized through an avi-
din mediator [48]. Mixed SAMs composed of long-chain thiols with carboxylic and
hydroxyl groups are also used to attain a specifi c and stable affi nity interface of immu-
nosensors [50-51]. Langmuir-Blodgett (LB) fi lms are other useful alternatives to tra-
ditional mediate layers [52-54]. LB fi lms, which are usually prepared by transferring
a monolayer on a solid substrate, have great potential in helping to control the orienta-
tion and surface density of the antibodies. Hirata et al. [52] successfully prepared the
lipid-tagged antibody/phospholipid monolayers with high immobilization properties
using the LB technique. Vikholm et al. demonstrated the incorporation of lipid-tagged
single-chain antibodies into lipid monolayers obtaining desirable retention of antibody
activities [53-54]. Moreover, recent years witness a newly emerged ultra-thin polymer
fi lm, plasma-polymerized fi lm (PPF), which is reported with successful applications
in various immunosensor designs [19, 55-57]. PPFs, which are generally prepared by

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