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TOPICS IN (NANO) BIOTECHNOLOGY
Biosensors – fundamentals & applications
June 30th 2003
Substrate consumption/product liberation is measured and converted into quantifiable signal.
Liquid ion exchanger
Porous membrane containing ionophore
Urea + H2O + H+
2NH4+ + HCO3-
Creatinine + H2O
N-methylhydantoin + NH4+
In contact with pH electrode.
Id = nFADsC/d
(e.g. Pt wire)
(e.g. Pt, Au, C)
(e.g. Ag/AgCl, SCE)
(e.g. Tris, DPBS, Citrate)
(e.g. KCl, NaCl)
Glucose + O2
Gluconic Acid + H2O2
The product, H2O2, is oxidised at +650mV vs a
Ag/AgCl reference electrode.
Thus, a potential of +650mV is applied and the
oxidation of H2O2 measured. This current is directly
proportional to the concentration of glucose.
Various permselective membranes have been
developed which controlled species reaching the
electrode on the basis of charge and size.
Examples include cellulose acetate (charge and size),
Nafion (charge) and polycarbonate (size).
The disadvantage of using membranes is, however,
their effect on diffusion.
Many oxidase enzymes can utilise artificial electron
acceptor molecules, called mediators.
A mediator is a low molecular weight redox couple
which can transfer electrons from the active site of
the enzyme to the surface of the electrode, thereby
establishing electrical contact between the two.
These mediators have a wide range of structures and
hence properties, including a range of redox potentials.
3. Metallised electrodes
The purpose of using metallised electrodes is to create
conditions in which the oxidation of enzymatically
generated H2O2 can be achieved at a lower applied
potential, by creating a highly catalytic surface.
In addition to reducing the effect of interferents, due
to the lower applied potential, the signal-to-noise
ratio is increased due to an increased electrochemically
Metallisation is achieved by electrodepositing the
relevant noble metal onto a glassy carbon electrode
using cyclic voltammetry.
Successful results have been obtained from a few noble metals - platinum, palladium, rhodium and ruthenium being the most promising.
Glassy carbon electrode
Glassy carbon electrodes do not catalyse the oxidation of
GCEs metallised with ruthenium, rhodium, palladium or
As with membranes, polymers are used to prevent
interfering species from reaching the electrode surface. Polymers differentiate on the basis of size and charge.
An example is that of polypyrrole. A polypyrrole film has to be in the reduced state to become permeable for anions. If the film is oxidised, no anion can permeate.
Conductimetric methods use non-Faradaic currents. In conductimetric transducers the two electrodes (working and reference) are separated from the measuring solution by a gas-permeable membrane.
The measured signal reflects the migration of all ions in the solution. It is therefore non-specific and may only be used for samples of identical conductivity.
Total Internal Reflection
DT = nDH/cp
Catalase Hydrogen peroxide 100.4
Cholesterol oxidase Cholesterol 52.9
Glucose oxidase Glucose 80.0
Hexokinase Glucose 27.6
Lactic dehydrogenase Pyruvate 62.1
b - Lactamase Penicillin G 67.0
Urease Urea 6.6
Uricase Uric acid 49.1