DNA-templated nanowires for sensing volatile organic compounds

Abstract

The fabrication of gas sensors with semi-conducting nanowires has attracted considerable interest in recent times because of their potential of selective and fast detection of low quantities of gaseous analyte when incorporated into miniature and low-power consumer electronics. DNA templating is a relatively new process for fabrication of nanowires at room temperature without the requirement for vacuum technology. This thesis describes the synthesis, characterization and gas sensing application of DNA templated metal sulfides and carbon nanotube nanowires. DNA templated CdS, CdZnS2 and ZnS were synthesized in solution to form smooth and continuous nanowires.The reaction involves initial coordination of the metal ion(s) with DNA and subsequent co-precipitation with sulfide ions upon addition of Na2S.The nanowires were deposited on the substrate via molecular combing to form a wellaligned network for electrical characterisation and gas sensing experiments. The structure, chemical composition and morphology of the nanowires were characterised by atomic force microscopy (AFM), powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, UV-vis spectroscopy, photoluminescence (PL), fluorescence microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. These techniques showed that the metal sulfides interact with the DNA template to form microcrystalline nanowires of typical diameter < 10 nm and controllable Cd:Zn ratio. The current-voltage (I-V) properties as a function of temperature were measured using micro-band electrodes on a probe station, Impedance spectroscopy and current transients were used to estimate contact resistances. The nanowires showed weak conductivity with I-V curves typical of metalsemiconductor-metal systems and described by the space charge limited conduction model. The temperature dependent properties of the nanowires showed simple Arrhenius behaviour. The room temperature sensing properties of the nanowires to volatile organic compounds (VOCs) such as ethanol, acetone, chloroform and hexane were also determined. They demonstrated good and reversible sensing response to the VOCs but with a higher sensitivity towards ethanol. The result also suggests that the room temperature sensing mechanism of the VOCs molecules on CdS/DNA, ZnS/DNA and CdZnS2/DNA nanowire sensor is essentially driven by their direct adsorption on the surface and interaction with charges in the nanowires.