Developing a Triad of DNA Sensors to Analyse Forensically Important Samples - Polypyrrole, Metal-ion Implantation and Resistive Pulse Sensing

Abstract

Many medical, forensic and environmental scientific needs may be satisfied by the existence of suitable DNA sensors. For forensic applications an important DNA sensor is one that is able to selectively and sensitively detect the presence of a specific body fluid, for example blood. A variety of approaches have been adopted to develop DNA sensors including those involving polypyrrole and electrical impedance spectroscopy. Combined, these are able to sense as well as aptly transduce signal. A sensor was subsequently developed. Investigations demonstrated that improvements in stability and sensitivity, as well as reduction in non-specific DNA binding for fabricated sensors, could be achieved through the use of a specific dopant, redox couple and post-growth treatment. Much DNA sensor research-to-date has focused on short target DNA strands as model genes; therefore, following DNA sensor development, the effect of the length of oligonucleotide probe and target strands was studied as a significant step towards real world applications for DNA detection. Longer target (and probe) DNA strands produced increased response, presumably a feature of associated negative charges. Furthermore, real blood sample interactions with sensor surfaces were examined. Although there was initially non-specific binding, this was reduced through the use of a dialysis membrane. Furthermore, metal-ion implantation was investigated for its effect on polypyrrole conductivity, stability and other characteristics. Low-energy implantation of platinum and lead ions into synthesised polypyrrole films was performed. Characterisation experiments were undertaken before and following implantation. Results displayed optimal fluences, where polypyrrole films implanted with 2 x 10¹⁶ Pt at. cm⁻² and (2 and 20) x 10¹⁴ Pb at. cm⁻² display and retain enhanced conductivity compared with non-implanted samples. A simple DNA sensor was constructed with Pt-implanted film that displayed successful detection of complementary DNA. In a third DNA sensing system, probe oligonucleotide-grafted particles were used to detect target DNA through resistive pulse nanopore detection. Particle-by-particle analysis together with a statistical intercept model and a variable pressure method were used to distinguish particles before and following target hybridisation. Hybridisation was selectively detected at micromolar concentrations and confirmed by complementary techniques. In conclusion, three sensor systems were developed and were successfully shown to detect target DNA. The strengths and weaknesses of the different sensor systems provide vital information for the development of future DNA sensors.