Page 202 - Analytical Electrochemistry 2d Ed - Jospeh Wang

6-1 ELECTROCHEMICAL BIOSENSORS 187

FIGURE 6-15 Schematic representation of the ion permeability modulation for cation-
responsive voltammetric sensors based on negatively charged lipid membranes. Complexation
of the guest cation to the phospholipid receptors causes an increase of the permeability for the
anionic marker ion. (Reproduced with permission from reference 49.)

receptors tend to bind to classes of substances (possessing common chemical
properties that dictate the binding af®nity). Accordingly, receptor-based biosensors
are usually class-speci®c devices.
Instead of isolating, stabilizing, and immobilizing chemoreceptors onto electro-
des, it is possible to use intact biological sensing structures for determining relevant
chemical stimulants (52,53). This novel concept was illustrated with antennule
structures of the blue crab. Such structures are part of the crab's food-locating
system, and thus can be exploited for the determination of amino acids. Similarly,
various drugs can be monitored based on their stimulation of nerve ®bers in the
cray®sh walking leg. A ¯ow cell based on such a neuronal sensor is shown in Figure
6-16. Such a sensor responds to stimulant compounds at extremely low levels (down
to 10 15 M!), with very short response times. The relationship between the response
frequency R† and the stimulant concentration C† is given by

R max
R ˆ n 6-14†
1 ‡ K=C†

where R is the maximum response frequency, n is a cooperativity factor between
max
receptors, and K is a constant.
In addition to the use of bioreceptors, it is possible to design arti®cial molecules
that mimic bioreceptor functions (51). Such arti®cial receptors (hosts) can be
tailored for a wide range of guest stimulants. For example, cyclodextrin derivatives
have been used to provide a shape-discrimination effect in connection with ion-
channel sensors (54). The receptors are incorporated within arti®cial lipid
membranes, prepared by the Langmuir±Blodgett (LB) deposition method on the
transducer surface. The LB method (which involves a transfer of a monomolecular
®lm from an air±water interface onto the electrode surface) results in thin organic
®lms that can be organized into multilayer molecular assemblies one monolayer at a
time. Arti®cial sensor arrays (emulating biological sensory systems, e.g., the human
nose) are also being explored in various laboratories (55) (see Section 6-4). High
sensitivity and selectivity can be achieved also by using the receptor recognition
process as an in-situ preconcentration step (56).