Synthesis and characterization of conductive nanowires using DNA as a template

Abstract

The preparation and characterisation of conductive nanowires and nanoropes on DNA templates is the focus of this work. This work is motivated by the search for alternative bottom-up approaches to nanoscale electronics. The structures and composition of prepared nanowires were determined by Fourier transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV-Vis) absorption spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron (XPS) spectroscopy. The morphology and physical properties were examined by atomic force microscopy (AFM), electrostatic force microscopy (EFM), conductive atomic force microscopy (C-AFM) and two-terminal current-voltage (I-V) measurements using microelectrodes fabricated by photolithography techniques. Polypyrrole (PPy) nanoropes were formed by chemical polymerization of pyrrole on a DNA template. The diameter of these 'nanoropes' was between 5–30 nm. At room temperature, the conductivity of a PPy-DNA nanorope is confirmed by C-AFM, while the temperature dependence of the conductivity was observed to follow a simple Arrhenius behaviour with a characteristic temperature of T0 = 4000K. Silver (Ag) nanoparticle chains were grown along λ-DNA templates using Tollens' reagent under mild conditions (50 ºC, 10 min). UV–Vis spectroscopy of these nanowires exhibited an absorption band at 400-440 nm due to the Ag plasmon. The DNA-templated Ag wires were found to be coated with an oxidized shell. Unlike DNA-templated polypyrrole, these nanowires were often rough. Using metal-binding functionality (alkynyl) introduced into DNA-templated polymer nanowires by chemical modification of the monomer, the morphology of the wire was improved on this hybrid template. Finally, DNA was used as a template for the growth of cuprous oxide (Cu2O) nanowires using Benedict's reagent and ascorbic acid as reducing agent at room temperature. AFM showed that these nanowires are uniform and continuous (diameters of 5-30 nm and lengths of 5-16 μm). C-AFM revealed that the average resistance of Cu2O–DNA nanowires was in the range from 0.13-0.18 MΩ depending on deflection setpoints, which suggests the material is doped. In summary, a range of inorganic and organic materials can be templated on DNA in the form of nanowires and various methods for measuring the conductivity of these nanowires have been established.