Biology Reference
In-Depth Information
CO 2 diffuses across the membrane where it reacts in the inner solution to
produce H 3 O + .
Potentiometric electrodes can also respond to a biochemically impor-
tant species. The most common class of potentiometric biosensors are
enzyme electrodes, in which an enzyme is immobilized at the surface of a
potentiometric electrode. The analyte reacts with the enzyme and produces
a product whose concentration is monitored by the potentiometric elec-
trode. Potentiometric biosensors have also been designed around other bio-
logically active species, including antibodies, bacterial particles, tissues, and
hormone receptors. One example of an enzyme electrode is the glucose
electrode, which is based on the catalytic hydrolysis of glucose by glucose
oxydase.
6.3. VOLTAMMETRY
In modern voltammetry, a time-dependent potential is applied to a
working electrode, changing its potential relative to a fixed potential of
a reference electrode. The resulting current, flowing between the work-
ing electrode and an auxiliary electrode, is measured as a function of the
potential. This technique was first developed by Jaroslav Heyrovsky in 1922,
for which he was awarded the Nobel Prize in Chemistry in 1959. The
auxiliary electrode is generally made of platinum, the reference electrode
can be a silver/silver chloride, calomel or, more rarely, hydrogen electrode.
For the working electrode, different materials are available including gold,
silver, platinum, mercury, and carbon. The first studies on voltammetry (e.g.
polarography) used a mercury working electrode. Mercury electrodes have
evolved from the classical dropping mercury electrode 9 through the hang-
ing mercury drop electrode, the static mercury drop electrode, the mercury
film electrode, and the mercury amalgam electrode. 10 Routine applications
of mercury electrodes are not too frequent nowadays. This is caused by fast
developments of modern spectrometric and separation techniques, by con-
cerns about mercury toxicity, and by the lack of properly validated methods.
However, liquid mercury electrodes remain perfect sensors for voltammet-
ric measurements because their renewable surface eliminates or reduces
problems with surface fouling, and their broad potential window enables
reaching negative potentials up to −2.5. 11
A typical setup for a voltammetric sensor is given in Fig. 6.3 . It mea-
sures the electrical current between the working electrode and the auxiliary
electrode induced by oxidation-reduction reactions. More precisely, if the
 
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