Biomedical Engineering Reference
In-Depth Information
Figure 9.1
Components of a biosensor.
gene and gene products, have revolutionized the discovery of therapeutic agents.
In this chapter, the fundamentals of various biosensing elements and their integra-
tion with different transducers are discussed. Furthermore, microfabrication tech-
nologies used in miniaturization of biosensing tools are discussed along with high-
throughput microarray technology.
9.2 Bioelectrodes
The key issue in the development of a biosensor is converting target recognition
into a measurable signal. The transducer part of the sensor serves to transfer the
signal from the output domain of the recognition system. The transducer part of a
sensor is also called a detector, s sensor, or an electrode, but the term transducer is
preferred to avoid confusion. Among the various sensing devices developed thus far,
the electrochemical method is, in general, superior to optical methods because of its
rapid response, simple and easy handling, and low cost.
Bioelectrodes function as an interface between biological structures and elec-
tronic systems to measure biopotentials (discussed in Chapter 3) generated in the
body due to ionic current flow. Bioelectrodes carry out a transduction function
and convert ionic current flow in the body to electronic current in the biosensor.
To understand how bioelectrodes work, the basic mechanisms of the transduction
process and the effect on the bioelectrode characteristics are discussed next.
9.2.1 Electrode-Electrolyte Interface
When an electrode (material capable of transporting charge such as metals) is
placed into an electrolyte
(an ionically conducting solution where the charge is
carried by the movement of ions), an electrified interface immediately develops.
Chemical reactions occur at the surface whereby electrons are transferred between
the electrode and the electrolyte. Reaction in which loss of an electron (e
−
) occurs is
called the oxidation reaction (Figure 9.2), that is,
MM me
↔+
m
+
−
where
M
is the metal atom (e.g., silver),
M
m
+
represents the cation of the metal, and
m
is the valence of the cation. If the metal has the same material as cation in the
electrolyte, then this material gets oxidized and enters the electrolyte as a cation
