Templating Iron(III) Oxides on DNA Molecules

Fe(III) oxides were prepared as free nanoparticles and on DNA templates via the precipitation of Fe(III) salts with NaOH in the presence/absence of DNA. Through control of the pH and temperature, FeOOH and Fe₂O₃ were synthesised. The formation of templated materials FeOOH/DNA and Fe₂O₃/DNA was confirmed using UV-Vis absorption and FTIR spectra. The direct optical gap of Fe₂O₃/DNA was estimated as 3.2 eV; the absorption by FeOOH/DNA and Fe₂O₃/DNA at longer wavelengths is weaker, but consistent with indirect gaps near 2 eV. X-ray photoelectron spectra confirmed the presence of Fe(III) and DNA in the templated samples. Analysis of the X-ray diffraction patterns of both templated and non-templated FeOOH and Fe₂O₃ demonstrated that the materials were the α-FeOOH and α-Fe₂O₃ polymorphs with crystallite diameters of the DNA-templated materials estimated as 7.6 nm and 6.8 nm. Transmission electron microscopy showed needle-like crystals of both FeOOH and Fe₂O₃, but the Fe₂O₃ contains some globular structures. In contrast, the morphology of FeOOH/DNA and Fe₂O₃/DNA consists of needle-like crystallites of the respective oxides organised into complex dendritic structures with a length on the 10 μm scale formed by the DNA molecules. Finally, scanned conductance microscopy provided evidence for the conductivity of the FeOOH/DNA after alignment via molecular combing on an Si/SiO₂ substrate. Fe₂O₃/DNA did not exhibit any detectable conductivity.