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Abstract
The interaction between DNA-functionalized surfaces/nanoparticles and free oligonucleotides in solution is the foundation of many devices such as DNA gene chips, therapeutic devices, and DNA biosensors. Electrochemical transduction provides a convenient means for studying DNA hybridization due to its simplicity and compatibility with point-of-care diagnostic devices. One challenge in electrochemical biosensors is how to detect ultra-low levels of analyte in large sample volumes within a reasonable time frame. This challenge arises because in any analytical device the sample volume for analysis needs to be representative of the bulk. An alternative to overcome this challenge is using the ‘dispersible electrodes’ approach. In this approach, conducting gold coated magnetic nanoparticles (Au@MNPs) are used as active elements in the capture and quantification of analytes. In this regard, Au@MNPs has been studied as dispersible electrodes in the first results chapter. Four types of Au@MNPs with different sizes, shapes, and method of synthesis have been characterized. The Cubic-Au@MNPs, which presents the simplest synthetic route, showed the best electrochemical stability and performance, responding quickly to a magnet and had a well defined shape. In the second results chapter, a dual-signalling methodology to investigate surface DNA hybridization was proposed. Ferrocene and methylene blue were used as the redox markers. When the redox molecules were immobilized on the DNA probe and located at the distal end of the DNA monolayer, higher currents were obtained in comparison to the response arising from the redox molecules on the DNA target, which were located in close proximity to the electrode surface. The limitation of ions accessibility showed to be the reason why low currents were obtained for the redox species located at the bottom of the DNA monolayer. In the third results chapter how DNA hybridization efficiency is affected by the position of the redox marker on the surface-bound DNA probe was investigated. It was showed that closer the methylene blue position to the surface, lower is the hybridization efficiency. In the last results chapter, the dual signalling DNA system was reproduced using the Au@MNPs. This chapter showed that the methylene blue signal is switched off and the ferrocene signal is switched on due to an increase in charge resistance.