The detection and quantitation of specific nucleic acid (NA) sequences continue to grow in importance. This Thesis reports on novel, ultrasensitive methods to detect specific NA sequences without the need for amplification of the target, e.g., using PCR. An electrochemical biosensor has been developed which amplifies the current response associated with a single NA binding event. This sensor involves a three step procedure; initially the capture NA strand is immobilised onto the electrode surface, this is then hybridised to the complementary target sequence, which is then followed by target hybridisation to a complementary probe strand which is labelled with a nanoparticle. By controlling the size and shape of nanoparticles, their properties can be finely tuned to suit the application required. A double potential step electrodeposition technique was used to control the deposition of platinum nanoparticles (PtNPs) at electrodes modified with self-assembled monolayers that contain defects. By using the SAM template, hemispherical nanoparticles are formed which can then be functionalised on one side with NA prior to desorption, allowing for the underside of the nanoparticle to be left clean. Semi-log plots of the DNA concentration vs. current were linear from 1 aM to 1 µM. A microfluidic disc has been produced. The disc is pre-loaded with miRNA strands and each hybridization step is completed inside the disc by triggering the tabs of chambers to burst to release the contents. When the hybridization is complete, the target miRNA which is specific to epilepsy, is detected based on the electrocatalytic reduction of hydrogen peroxide in phosphate buffer saline (PBS) at the PtNPs which are functionalised to the probe miRNA strand. Electrochemical Impedance Spectroscopy (EIS) was also performed on the modified electrode as another method of NA detection.