Chemistry Reference
In-Depth Information
The color change is attributed to the formation of a complex between the cationic
polythiophene and the polyanionic DNA. Complexation arises through increased
linearization and unfolding of both the polythiophene and the nucleic acid, followed
by aggregation-induced color changes. The color change is attenuated in the presence
of potassium that promotes folding of the nucleic acid into a G-quartet structure, pre-
venting linearization. A related strategy, employing a G-quartet that binds to the
enzyme thrombin, has been used for thrombin detection. When a complementary
DNA sequence is added to the solution, the color reverts to yellow (Ho et al. 2005).
Cationic polythiophenes have been used to detect cleavage of single-stranded DNA by
nucleases or hydroxyl radicals. The cleaved strands of DNA are not as effective in aggre-
gating polythiophene as the uncleaved DNA, resulting in the restoration of the yellow
color of uncomplexed polythiophene upon cleavage (Tang et al. 2006; Fig. 12.17).
The imidazolium salt functionalized polymer 12 also functions as an iodide
sensor, rapidly changing color in the presence of sodium iodide (Ho and Leclerc
2003). Iodide-mediated aggregation was highly sensitive to the length of the side
chain connecting the imidazolium salt to the polythiophene backbone. A variety of
Figure 12.18 (a) UV visible and (b) circular dichroism spectra of 13 (0.10 mM) in the
absence and presence of various amounts of adenosine triphosphate (ATP) in water at 20 8C.
ATP concentrations (from front to back): 0, 0.001, 0.0125, 0.025, 0.10, 0.15, 0.175, 0.20,
0.25, 0.375, and 0.50 mM. Reprinted from Li et al. (2006). Copyright 2006 Wiley-VCH
Verlag GmbH and Co. KGaA.
