Instant Notes: Analytical Chemistry

binding, methods used including physical adsorption onto or chemical bonding

to the surface of the transducer, or entrapment in a cross-linked polymeric gel.

Sensors should be robust, have a rapid and reproducible response to the

analyte(s), and have an appropriate selectivity/specificity and working range.

They should be stable in operation, with minimal drift, and ideally be

unaffected by changes in temperature, pressure and other adverse conditions.

Sensors are used for industrial process stream analysis, monitoring air quality in

urban and workplace environments, and hospital patients’ body fluids for

clinical purposes.

Potentiometric sensors are based on ion selective electrodes, solid state redox

electrodesand field effect transistors(FETs). Glass electrodes used for pH

measurements can be incorporated into gas sensors for measuring levels of

carbon monoxide, carbon dioxide, ammonia, oxides of nitrogen and sulfur and

other gaseous analytes (Topic C3). Biosensors can be made by coating a glass

electrode with a layer of an enzyme immobilized on the surface and which

catalyzes a biochemical reaction (Fig. 2). For example, urease coated onto a gas-

sensitive ammonia electrode (Topic C3) can be used to monitor urea which is

hydrolyzed by the enzyme, the electrode responding to the ammonia produced

at pH 7–8, i.e.

CO(NH 2 ) 2 +2H 2 O +H+ 2NH 4 ++HCO 3 −

NH 4 ++OH−→NH 3 +H 2 O

urease

→

Electrochemical
sensors

324 Section H – Sensors, automation and computing

Transducer Analyte

Output signal

Recognition element Matrix

Fig. 1. Schematic diagram of a chemical or biosensor.

Ammonium ISE

Protective membrane Immobilized enzyme

Fig. 2. Biosensor based on an enzyme-coated ammonium ion-selective electrode.

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

Wiley-VCH.