The ability to detect and quantify specific biomarkers can lead to the development of very powerful tools for the diagnosis and prognosis of disease. For example, electrochemical nucleic acid biosensors provide an easy, rapid, portable and economic tool for the diagnosis of diseases from cancer to cardiovascular disease. This thesis reports on highly sensitive DNA sandwich assays where the probe strand is labelled with an electrocatalytic nanoparticle to sensitively signal the binding event. Specifically, an electrode is functionalised with capture strands that are complementary to approximately 50% of the target. Once the target is hybridised to the capture strand, a probe strand, that is labelled with an electrocatalytic nanoparticle and is complementary to the unbound section of the target, is allowed to hybridise, i.e., the immobilisation of the nanoparticles is mediated by the presence of the target. The magnitude of the electrocatalytic current depends on the concentration of the target and wide dynamic ranges and low LODs can be achieved. The dynamic range achieved was between 1 aM and 100 nM and limit of detection of 1.5 x 10-13 M was reached. The electrocatalytic nanoparticles comprise a hemispherical platinum core with a silver shell. These particles are highly electrocatalytic and show some promise as labels in surface enhanced Raman spectroscopy. The rate of heterogeneous electron transfer from the underlying electrode to the bound nanoparticle can influence the observed electrocatalytic current especially if the target sequence contains many bases. To investigate the possibility of improving the conductivity of the DNA strands, a polyaniline (PANI) based conducting nucleic acid analogue was designed and modelled in silico. Then it was electrochemically synthesized using a novel step-wise addition of individual bases and its binding to natural DNA was assessed. Finally, the current generated from an electrocatalytic nanoparticle bound through hybridisation was examined. The PANI-NA layer was characterized using electrochemical techniques, imaging of the attached nanoparticle and single molecule scanning tunnelling microscopy. The measurement of current densities of the electrodes modified with a PANI-NA capture strand immobilizing electrocatalytic nanoparticles was found to be 16 times the current densities obtained with conventional DNA capture strand at similar concentrations.