Page 135 - Analytical Electrochemistry 2d Ed - Jospeh Wang

120 PRACTICAL CONSIDERATIONS

Liquid

Substrate

End group

Organic chain

Head group
FIGURE 4-14 Formation of a self-assembled monolayer at a gold sustrate. (Reproduced
with permission from reference 38.)

and topographical variations in the ®lm architecture. The novelty of using SAMs
stems from their ability to be further modi®ed into chemically or biologically
reactive surface layers (via covalent coupling of different materials to the functional
end group, X). This use of SAMs for anchoring various functionalities can impart
speci®c interactions essential for various sensing applications (see Chapter 6). These
applications can bene®t from the use of mixed SAMs that minimize steric hindrance
effects. The high degree of order has also allowed detailed structure±function
relationships to be developed for various electron-transfer processes. Other highly
ordered ®lms, such as of alkyl siloxane, can be formed on metal oxide surfaces
(particularly SiO 2 ).

4-5.3.2 Sol-Gel Encapsulation of Reactive Species Another new and
attractive route for tailoring electrode surfaces involves the low-temperature encap-
sulation of recognition species within sol-gel ®lms (41,42). Such ceramic ®lms are
prepared by the hydrolysis of an alkoxide precursor such as, Si(OCH 3 ) 4 under acidic
or basic condensation, followed by polycondensation of the hydroxylated monomer
to form a three-dimensional interconnected porous network. The resulting porous
glass-like material can physically retain the desired modi®er but permits its
interaction with the analyte that diffuses into the matrix. Besides their ability to
entrap the modi®er, sol-gel processes offer tunability of the physical characteristics