DNA-templated poly(N-substituted pyrrole)bipyridinium nanowires

Abstract

Conductive polymers nanowires have been prepared using DNA-templating methods from monomer units designed in modular form. The monomer units comprise a polymerisable, pyrrolyl group, and a flexible alkyl linker attached to bipyridine groups in order to provide a metal-binding functionality within the polymers. The possibility for these DNA/polymer nanowires to act as templates for deposition of metal with enhanced electrical conductivity was also explored. Pyrrole with a flexible alkyl linker was combined with; pyridine (mono-I) as a control experiment, 2,2` bipyridyl (mono-II) and 4,4` bipyridyl (mono-III) with a metal ion binding site (nitrogen atom). This was in order to provide the metal-binding functionality for metal deposition to improve the conductivity as well as the morphology of the aimed hybrid templated nanowires. This series of pyrrole-pyridine derivatives were characterised using a range of techniques such as Proton Nuclear Magnetic Resonance (1H NMR) spectroscopy, Fourier Transform Infrared (FTIR) spectroscopy, Mass Spectroscopy (MS) and Elemental Analysis. Prior to the nanowires fabrication, pyrrole, as a control, and the prepared monomer units (mono-I, mono-II and mono-III) were chemically polymerised in a bulk scale using FeCl3 as an oxidant, then spectroscopic data and electrical conductivity of the resulting polymers were measured. A significant decrease in conductivity of poly-I, poly-II and poly-III compared to PPy, but was especially observed for the bipyridinium derivatives. This was suggested to be due to the steric hindrance of the alkyl side chain in the polymer backbone, in addition, the involvement of the non-quaternised pyridyl nitrogen in poly-II and poly-III by the nucleophilic attack on pyrrolyl groups in the polymerisation reaction. DNA-templated poly(N-substituted pyrrole)bipyridinium nanowires were synthesised at room temperature using the chemical oxidation method. The resulting CPs/DNA hybrids have been characterised using electronic and vibrational spectroscopic methods especially Ultraviolet-Visible (UV-Vis) spectroscopy and FTIR spectroscpy. The nanowires morphology was characterised using Atomic Force Microscopy (AFM). The electrical properties of the prepared nanowires were characterised using Electrostatic Force Microscopy (EFM), and measured using conductive AFM (c-AFM) and two iii terminal I/V technique, where the temperature dependence of the conductivity was probed. The conductivities of the prepared CPs/DNA nanowires are generally lower than PPy/DNA nanowires showing the large effect on N-alkylation in decreasing the conductivity of the polymer, but these are higher than the conductivity of their corresponding bulk films. This enhancement in conductivity could be attributed to the ordering of the polymer chains on DNA during the templating process. Finally, the prepared CPs/DNA nanowires were used as templates for the growth of copper nanowires at room temperature using aqueous solution of Cu(NO3)2 as a source of Cu2+ and ascorbic acid as reducing agent. AFM images showed that these nanowires were uniform and continuous compared to copper nanowires prepared using the templating method directly onto DNA. Electrical characterization of the nanowires by c-AFM revealed slight improvement in conductivity of these nanowires (Cu-CPs/DNA) compared to CPs/DNA nanowires before metallisation. Using similar preparation method, Poly-II/DNA nanostructures were also used as templates to direct the formation of Pd nanowires. An aqueous solution of PdCl2 was used as a source of Pd2+ ions and NaBH4 solution was used as reducing agent. AFM studies show that the resulting Pd-poly-II/DNA nanowires exhibit continuous and smooth morphology. Electrostatic Force Microscopy showed that these nanowires are electrically conductive.