Similarly, antibodies ISB4 have been found to detect Cadmium [15]. Melton
et al., in another study carried out to detect the presence of uranium in contaminated
groundwater, developed two immunosensors. One was a portable field sensor,
while the second was an inline sensor. Both of the sensors detected the complex
formed by a 12F6 monoclonal antibody with U(VI)-dicarboxyphenanthroline
complex [16].
The sensitivity of immunosensors could be enhanced by the use of enzyme
labels, which are either coupled to the antibodies or the antigen. The enzyme
coupled immunosensors operate under equilibrium condition. Kanso et al. developed an enzyme coupled immunosensor for the detection of estradiol and
ethinylestradiol in wastewater. These endocrine disrupting chemicals are found in
wastewater. The methodology adopted for the development of the biosensor was
based on the competition between the free antigen and derivative of the antigen.
The amine derivatives of estradiol and ethinylestradiol were immobilized on the
surface of magnetic beads coated with a carboxylic acid. These immobilized
magnetic beads were in turn fixed on the surface of the electrode. The fixation of the
antigen derivative was followed by the addition of the free estrogen and
anti-estrogen antibody. This step is followed by washing and addition of alkaline
phosphatase labelled IgG antibody. The reaction involves the conversion of
p-nitrophenyl phosphate by the enzyme into p-nitrophenol, which gives a yellow
colour. This reaction gives the measurement of the endocrine disrupting chemicals
present in the wastewater [17]. Another study was carried out by Merola et al., for
the detection of b-lactam antibiotics using enzyme-conjugated immunosensor in
water. The detection was carried out by using anti-penicillin G antibody and the
response was measured by conjugated biotin extravidin peroxidase. Immobilization
was carried out on Immobilon membrane, which was regenerated after the use. The
antibody-antigen (b-lactam antibiotics) affinity was also determined and showed
that this method of detection was sensitive and reproducible [18].
Receptor-ligand reaction: Many biosensors utilization channels, binding proteins
and membrane receptors as molecular identification systems. An example of such a
system is lactose permease as the sensing element. The transducer is covered by the
planar lipid bilayer in which the protein is incorporated. The principle of detection
is based on the co-transport property of the protein due to which when the concentration of lactose increases, the concentration of proton between the quartz
surface and membrane also rises. This rise in proton concentration was detected by
a fluorescence dye, which is pH-sensitive [19].
Another approach in this category is the use of aptamer-based biosensor for the
detection of pollutants in water. Aptamers are single-stranded ribonucleic acids
(RNA) or deoxyribonucleic acids (DNA). They can bind to different types of targets
such as from ions, small molecules, cells and tissues. Their binding action is
specific and based on electrostatic, van der Waals, molecular shape complementarities and hydrogen bonding. Aptamers are selected by a process known as
Systematic Evolution of Ligands by EXponential enrichment (SELEX) including
selection, separation and amplification. The selection is done by using
192
R. Sinha et al.
et al., in another study carried out to detect the presence of uranium in contaminated
groundwater, developed two immunosensors. One was a portable field sensor,
while the second was an inline sensor. Both of the sensors detected the complex
formed by a 12F6 monoclonal antibody with U(VI)-dicarboxyphenanthroline
complex [16].
The sensitivity of immunosensors could be enhanced by the use of enzyme
labels, which are either coupled to the antibodies or the antigen. The enzyme
coupled immunosensors operate under equilibrium condition. Kanso et al. developed an enzyme coupled immunosensor for the detection of estradiol and
ethinylestradiol in wastewater. These endocrine disrupting chemicals are found in
wastewater. The methodology adopted for the development of the biosensor was
based on the competition between the free antigen and derivative of the antigen.
The amine derivatives of estradiol and ethinylestradiol were immobilized on the
surface of magnetic beads coated with a carboxylic acid. These immobilized
magnetic beads were in turn fixed on the surface of the electrode. The fixation of the
antigen derivative was followed by the addition of the free estrogen and
anti-estrogen antibody. This step is followed by washing and addition of alkaline
phosphatase labelled IgG antibody. The reaction involves the conversion of
p-nitrophenyl phosphate by the enzyme into p-nitrophenol, which gives a yellow
colour. This reaction gives the measurement of the endocrine disrupting chemicals
present in the wastewater [17]. Another study was carried out by Merola et al., for
the detection of b-lactam antibiotics using enzyme-conjugated immunosensor in
water. The detection was carried out by using anti-penicillin G antibody and the
response was measured by conjugated biotin extravidin peroxidase. Immobilization
was carried out on Immobilon membrane, which was regenerated after the use. The
antibody-antigen (b-lactam antibiotics) affinity was also determined and showed
that this method of detection was sensitive and reproducible [18].
Receptor-ligand reaction: Many biosensors utilization channels, binding proteins
and membrane receptors as molecular identification systems. An example of such a
system is lactose permease as the sensing element. The transducer is covered by the
planar lipid bilayer in which the protein is incorporated. The principle of detection
is based on the co-transport property of the protein due to which when the concentration of lactose increases, the concentration of proton between the quartz
surface and membrane also rises. This rise in proton concentration was detected by
a fluorescence dye, which is pH-sensitive [19].
Another approach in this category is the use of aptamer-based biosensor for the
detection of pollutants in water. Aptamers are single-stranded ribonucleic acids
(RNA) or deoxyribonucleic acids (DNA). They can bind to different types of targets
such as from ions, small molecules, cells and tissues. Their binding action is
specific and based on electrostatic, van der Waals, molecular shape complementarities and hydrogen bonding. Aptamers are selected by a process known as
Systematic Evolution of Ligands by EXponential enrichment (SELEX) including
selection, separation and amplification. The selection is done by using
192
R. Sinha et al.