Biomedical Engineering Reference
In-Depth Information
properly immobilized to the transducer. The way in which biosensing elements are
immobilized determines the properties of the biosensor. In some cases, immobiliza-
tion leads to partial or complete loss of bioactivity. In order to fully retain biologi-
cal activity, biosensing elements should be attached onto the transducer without
affecting function. Generally, the choice of a suitable immobilization strategy is de-
termined by the physicochemical and chemical properties of both the surface of the
transducer and the biosensing element. For example, the protein activity or func-
tion is critically dependent on its 3D structure, which is very sensitive to the local
physical and chemical environment. Keeping an immobilized protein molecule in a
native state and preserving its function are a major challenge. Many immobilization
techniques have been developed, which are mainly based on the three mechanisms
of physical, covalent, and bioaffinity immobilization.
9.5.2.1 Physical Entrapment
This technique involves the simple adsorption of the biocomponent on to the elec-
trode surface. Proteins adsorb on surfaces via intermolecular forces, mainly ionic
bonds and hydrophobic and polar interactions [Figure 9.14(a)]. Adsorption is clas-
sified as physical adsorption (physisorption) and chemical adsorption (chemisorp-
tion). Physisorption is usually weak and occurs via the formation of van der Waals
bonds or hydrogen bonds between the substrate and the enzyme. Chemisorption
is much stronger and involves the formation of covalent bonds. The adsorption
capacity of flat surfaces is limited by the geometric size of the immobilized proteins.
However, physical adsorption shows random orientation and weak attachment.
An alternative is to physically encapsulate the biosensing element using thin
polymer films [Figure 9.14(b)]. A suitable monomer is polymerized in the presence
of an enzyme. The resulting polymer can be conducting (such as polypyrrols) or
nonconducting (such as polyphenols), depending on the monomer employed. The
film thickness is controlled by adjusting the monomer concentration. In addition
to immobilizing the biosensing elements, they could act as membranes to improve
selectivity and to provide a barrier against electrode fouling. Thin microporous
membranes (using materials such as nylon and cellulose nitrate) are also used to
encapsulate a biocomponent. Due to very small membrane thickness, the biocom-
ponent and the transducer are very close to each other, which maximize biosensor
response. Further bonding of the biocomponent to the transducer surface may be
done using a conductive polymer (polypyrrole). The membrane is selected for its
Sensing elements
Transducer surface
Transducer surface
(a) (b) (c)
Figure 9.14
Physical immobilization of biosensing elements to the surface: (a) physical adsorption,
(b) encapsulation, and (c) entrapment in a matrix.
