Page 133 - Analytical Electrochemistry 2d Ed - Jospeh Wang

118 PRACTICAL CONSIDERATIONS

alkaline medium (copper and nickel) and of cyanide or sulfur compounds (silver).
Unlike platinum or gold electrodes, the copper electrode offers a stable response for
carbohydrates at constant potential.

4-5.3 Chemically Modi®ed Electrodes

Chemically modi®ed electrodes (CMEs) represent a modem approach to electrode
systems. These rely on the placement of a reagent onto the surface, to impart the
behavior of that reagent to the modi®ed surface. Such deliberate alteration of
electrode surfaces can thus meet the needs of many electroanalytical problems,
and may form the basis for new analytical applications and different sensing devices.
There are various ways in which CMEs can bene®t analytical applications. These
include acceleration of electron-transfer reactions, preferential accumulation, or
selective membrane permeation. Such steps can impart higher selectivity, sensitivity,
or stability to electrochemical devices. These analytical applications and improve-
ments have been extensively reviewed (35±37). Many other important applications,
including electrochromic display devices, controlled release of drugs, electrosynth-
esis, and corrosion protection, should also bene®t from the rational design of
electrode surfaces.
One of the most common approaches for incorporating a modi®er onto the
surface has been coverage with an appropriate polymer ®lm. Polymer modi®ed
electrodes are often prepared by casting a solution containing the dissolved polymer
onto the surface and allowing the solvent to evaporate, or via electropolymerization
in the presence of the dissolved monomer. The latter offers precise control of the ®lm
thickness (and often the morphology) and is particularly attractive in connection
with miniaturized sensor surfaces. The structures of some common polymeric
coatings are shown in Figure 4-13. For example, the Dupont Na®on per¯uorinated
sulfonated cation exchanger (a) has been widely used as an electrode modi®er due to
its attractive permselective, ion-exchange, and antifouling properties (see examples
below). Additional advantages can be obtained by coupling two (or more) polymers
in a mixed or multilayer con®guration. Other useful modi®cation schemes include
bulk modi®cation of composite carbon materials, covalent (chemical) attachment,
sol-gel encapsulation, physical adsorption, and spontaneous chemisorption.

4-5.3.1 Self-Assembled Monolayers Spontaneously adsorbed monolayers
of n-alkanethiols (X CH † SH with n > 10) on gold surfaces, based on the strong
2 n
interaction between gold and sulfur, are particularly well-suited for controlling and
manipulating the reactivity at the interface. Such monolayers are commonly formed
by immersing the gold electrode overnight in ethanolic solutions containing
millimolar concentrations of the alkanethiol. The formation of self-assembled
organosulfur monolayers (SAMs) has attracted considerable attention due to its
many potential scienti®c and technological applications (38±40). In addition to
fundamental studies on the structure of such monolayers and long-range electron
transfer, such applications include chemical sensors and biosensors, information
storage devices, or lithography.