Chemistry Reference
In-Depth Information
(Shim et al. 2004). When the NHS ester-containing liposomes are polymerized prior
to immobilization on the glass, the blue liposomes turn red upon contact with the
slide. However, if the liposomes are immobilized prior to polymerization and sub-
sequently irradiated, the immobilized vesicles are blue and can undergo a subsequent
chromatic transition in response to heat (Fig. 12.11).
Although the bulk of PDA sensors involve vesicles and Langmuir monolayers, a
few examples of responsive PDA assemblies based on bolaamphiphiles and diyne
silica nanocomposites have been reported (Lu et al. 2001; Song et al. 2001, 2004;
Yang et al. 2003; Peng et al. 2006). Although these materials have not been
broadly utilized for analyte sensing, they do exhibit the thermochromic, solvatochro-
mic, and pH responsive behavior seen with monolayers and liposomes and hold
promise for future development.
12.3. POLYTHIOPHENES
2,5-Unsubstituted thiophenes undergo polymerization to provide polythiophenes, a
class of conjugated polymers that have absorbances in the visible region of the spec-
trum. Polymerization is carried out using iron(III) salts or initiated electrochemically.
The absorbance maxima of the polythiophenes are dependent on the aggregation
state of the polymer, mediated by p-stacking interactions, as well as the degree of
coplanarity between consecutive thiophene monomers. Numerous examples of
ionochromic polythiophenes have been reported where addition of various cations
to functionalized polythiophenes induces a color change. An early report described
the synthesis of the bridged crown ether shown in Figure 12.12, which exhibits a
91-nm shift in l
max
upon binding of sodium (Marsella and Swager 1993). The struc-
turally related oligoethylene polymer 8 was reported to be potassium selective
(Levesque and Leclerc 1995).
Polymers of thiophene carboxylic acids (9) undergo different chromatic transitions
upon the addition of a variety of cations (McCullough et al. 1997). Although the acid
form of the polymer was not water soluble, addition of various ammonium or metal
Figure 12.12 Modulation of polythiophene coplanarity using metal binding. Reprinted from
Marsella and Swager (1993). Copyright 1993 American Chemical Society.
