Slow doping rate in DNA-poly(o-methoxyaniline) hybrid: uncoiling of poly(o-methoxyaniline) chain on DNA template

Abstract

The doping rate of biomolecular hybrids of the deoxyribonucleic acid-poly(o-methoxyaniline) (DNA-POMA) system has been measured by UV-vis spectroscopy and pH study for the three different compositions (W_DNA = 0.21, 0.45, and 0.71, W_DNA = weight fraction of DNA) of the blend. Here the doping rate is very slow. The pH of the medium after an initial hike from 3.76 increases slowly with time and then levels up at around 5.75 in about 6 h. The UV-vis spectra of the hybrid solutions also exhibit a slow red shift of the p band-polaron band transition with time, and it levels up after 48 h of aging at 30 °C. Three stages of doping, e.g., (i) primary doping by phosporic acid group of DNA, (ii) uncoiling of POMA followed by doping, and (iii) slow uncoiling of POMA on DNA surface, are suggested for explaining the above behavior. The doping behavior has been studied at different isothermal temperatures, and Arrhenius analysis of the doping rate measured from the red shift of the π band-polaron band transition peak results in activation energy values ~14 kcal/mol. This value is close to that of conformational transition of other polymers, supporting that the uncoiling process is the cause for the slow red shift of the π band-polaron band transition with time. The confinement of the POMA chain on the DNA surface catalyzes the uncoiling of POMA chain due to the cooperative repulsive interaction of the neighboring radical cations generated from the primary doping process. The conductivity values of the blend vary from 10^-6 to 10^-7 S/cm, and the I-V curves characterize the hybrids as semiconductors.