Biomedical Engineering Reference
In-Depth Information
using glow discharge or plasma of organic vapors, are extremely thin, homogeneous,
mechanically and chemically stable, with strong adhesion to the substrates. Karube's
group fi rst reported the application of PPF to QCM immunosensors [55]. They veri-
fi ed that the resultant sensors were more reproducible from batch to batch, and might
have lower noise and higher sensitivity than sensors using some conventional organic
coatings (e.g. polyethylenimine). This kind of functionalized fi lm may offer promising
alternatives in interfacial design of immunosensors of various transducers.
In recent years, various nanomaterials are found to be skillfully applied in com-
bination with the covalent interaction-based immobilization procedures for immu-
nosensors. Carbon nanotubes (CNTs), for example, have been recognized as the
quintessential nano-sized materials since their discovery in 1991 [58]. These nano-
tubes are now chemically functionalized for the immobilization of biological entities
for different biosensors, i.e. electrochemical devices [59-61]. Pantarotto et al. success-
fully bound a model pentapeptide and a virus epitope of foot-and-mouth disease onto
single-walled CNTs [61]. They found that the CNTs-loaded peptide might retain the
structural integrity to be well recognized by monoclonal or polyclonal antibodies, indi-
cating the potential applications for diagnostic purposes and vaccine delivery. A sil-
ica nanoparticles-based immobilization strategy was also proposed by Wang et al. for
direct immunosensing determination of Toxoplasma gondii -specifi c IgG [62]. Herein,
the preparation strategy could allow for antigens covalently bound with higher loading
amount and better retained immunoactivity compared to the commonly applied cross-
linking methods.
The aforementioned covalent interaction-based procedures may usually allow for
the immunoactive proteins immobilized with high stability and repeatability, and the
robust covalent bonds may favor the low noise of detection. Nevertheless, problems
associated with these covalent bond immobilizations are the decrease of binding capac-
ity of antibodies (antigens) in the immobilization process. Such a phenomenon may
be presumably contributed to the partial loss of the immunoactive sites and the ran-
dom orientation of antibody molecules bound on the transducer surfaces. In addition,
cross-linking can produce a three-dimensional multilayer matrix that creates diffusion
barriers and transport limitations, resulting in long immunoreaction time and low sen-
sitivity [63]. It is established that the oriented immobilization of antibodies has low
infl uence on their immunological activities to a certain degree [64-68], which antigen
binding capacity was demonstrated with a factor of 2-8 higher than that of antibodies
randomly immobilized [68]. Therefore, special interest has been given to the develop-
ment of the orientation-controlled immobilization techniques for antibodies, i.e. mostly
through proteins A or G to specifi cally bind the antibody Fc fragment, or by directly
binding the chemical groups at antibody Fc region [64, 69-71].
Lee et al. utilized the self-assembled layer of thiol group-modifi ed protein A for
the oriented immobilization of antibodies [64]. An increased binding capacity was
further observed. As another illustrative instance, a protein A-based orientation-
controlled immobilization strategy for antibodies was proposed for the fabrication
of a QCM immunosensor using nanometer-sized gold particles and amine-terminated
PPF [65]. Moreover, in recent years, there has emerged another oriented immobilization
Search WWH ::




Custom Search