Instant Notes: Analytical Chemistry

H1 – Chemical sensors and biosensors 325

Redox electrodesconsisting of crystalline zirconium dioxide, doped with

calcium or yttrium oxides to render it ionically conducting, are used to monitor

oxygen in combustion gases and molten metallurgical samples. The redox

equilibrium

O 2 +4e−↔2O^2 −

creates an electrode potential with a Nernstian response to concentration

through the movement of O^2 −ions through the crystal lattice.

Field effect transistors can be made sensitive to a range of ionic and gaseous

analytes. Biosensors can be fabricated by incorporating gel or polymer layers

containing immobilized enzymes, antigens or antibodies.

Amperometric and conductimetric gas sensors have been devised for the

detection of oxygen, hydrogen, ammonia, sulfur dioxide and methane. Enzyme-

based biosensors such as cholesterol or glucose electrodes for blood analysis

depend on the oxidation of cholesterol or glucose oxidase, respectively, to

produce hydrogen peroxide that is detected amperometrically (Fig. 3).

Cholesterol Cholesterone

–e–

+e–

+e–

–e–

Cholesterol

oxidaseox Cholesteroloxidase

red

H 2 O 2 O 2

Fig. 3. Example of an oxidase-based sensor reaction: enzyme catalyzed oxidation of

cholesterol. Reproduced from R. Kellner et al., Analytical Chemistry, 1998, with permission

from Wiley-VCH.

Optical sensors These utilize glass, quartz or plastic optical fibersto transmit incident and
attenuated or fluorescent radiation between a spectrophotometer, orlight-
emitting diode(LED) and detector, and the sample or sample stream. The ends
of two separate fiber cables are positioned together in a y-shaped configuration
a short distance above a reflector plate to form a sample cell (Fig. 4). Radiation
emerging from one cable passes through the sample into which the cell is
immersed and is reflected back through the sample into the end of the other
cable. The path length of this optical cell is twice the distance between the ends
of the cables and the plate. Fluorescent emission or reflected radiation from a
solid surface can also be detected. Chromogenic reagents (e.g. pH or complexo-
metric indicators) can be immobilized on the ends of the fibers in thin layers of
supporting media such as cellulose and polyacrylamide. Sensors modified in
this way are known as optrodes.
Home test kits, such as those for blood sugar or pregnancy, depend on visual
observation of a developed color, but test strips can also be assessed by instru-
mental reflectance measurements. Enzyme-based redox reactions monitored by
electrochemical biosensors may alter pH or generate products such as hydrogen
peroxide that can react with a chromogenic reagent to form the basis of an
optrode.
Optical sensors can be used to monitor pH, metal ions, dissolved gases and
organic compounds down to ppm and ppb concentrations using radiation in the