Fabrication of Biosensors - Biosensors and Biosensor Kinetics - page 41

entrapment in the polymer matrix ( Sadik and Emon, 1996; Adeloju and Wallace, 1996 ).
Tahir et al. (2007) indicate that this entrapment feature may be used for the direct measure-
ment of antibody-antigen binding ( Sergeyeva et al., 1996; Gerard et al., 2002 ). Besides, dur-
ing the entrapment procedure, the biological activity is maintained.
Tahir et al. (2007) report that the electrical, optical, chemical, and electrochemical properties
have been extensively investigated, including a wide range of applications ( Winokur, 1998 ).
Tahir et al. (2007) emphasize that the electrical property of Pani is pH dependent, with most
analyses being performed at pH less than 4.0 ( Shaolin and Jincui, 1999 ). Since most
biological and immunological reactions occur optimally around a neutral pH of 7.0, Tahir
et al. (2007) indicate that this poses a challenge to incorporate biological elements in pH-
dependent Pani. The authors indicate that there have, however, been improvements to
enhance the electrical and physiochemical properties of Pani ( Cao et al., 1993; Stejskal
et al., 1998; Lukachova et al., 2003 ).
Tahir et al. (2007) have developed a biosensor platform that uses ITO glass, spin-coating
Pani, and antibodies on it. These authors initially synthesized the self-doped Pani, and
evaluated its feasibility for incorporation in the biosensor design. They then evaluated the
self-doped Pani and antibodies specific to bovine viral diarrhea virus (BVDV). The BVDV
was selected as a model pathogen.
Initially, Tahir et al. (2007) attempted to characterize the Pani used. These authors state that
the TEM (transmission electron microscope) showed that the higher the molecular weight of
the self doped Pani the larger the polymer structure. Also, as the molecular weight increases,
the length of the polymer backbone per unit area increases ( Carbrey et al., 1971 ). This
enhances the flow of electrons and thereby the conductivity. The Pani weight is also temper-
ature dependent in the 22-75 C range ( Tahir et al., 2007 ). Thus, these authors indicate that a
temperature-controlled mechanism is required in the biosensor design to minimize the tem-
perature-dependent variations of the polymer properties.
Tahir et al. (2007) report that their ITO-Pani biosensor utilizes the antigen-antibody binding
format with self-doped Pani as the transducer. Their biosensor design concept is based on the
difference between the signal before ( I 0 ), and the signal after ( I s ) the antibody-antigen bind-
ing. The current drop is given by I 0 - I s , and the higher the current drop, the greater the amount
of antibody-antigen binding on the biosensor surface. This binding of the surface blocks the
transfer of electrons. In essence, Tahir et al. (2007) state that as the antibodies (with a molec-
ular weight around 15-kDa) are immobilized within the polymer backbone, electron flows are
reduced. As the antigen-antibody complex is formed (molecular weight of BVDV of at least
4 MDa), the electron flow is restricted even more. According to Tahir et al. (2007) , the larger
the antigen-antibody complex formed in the Pani backbone, the higher the restriction for the
flow of electrons.