Defects within a self-assembled monolayer (SAM) of dodecanethiol on gold have been used as nucleation sites for the electrodeposition of mushroom shaped platinum nanoparticles (PtNPs). The top surfaces of these PtNPs were then decorated with a layer of silver creating a hemispherical - platinum : silver core : shell nanoparticle (Pt-AgNP). Thiolated probe strand miRNA was then immobilised onto the upper silver surface. These regioselectively modified particles were desorbed by applying a current jump to yield nanoparticles capable of hybridising to a complementary miRNA target with electrocatalysis occurring on the non-functionalized lower surface. A second electrode was functionalized with single stranded capture miRNA that has a sequence that is complementary to an miRNA, miR-132, associated with the childhood cancer, Neuroblastoma but leaves a section of the target available to bind the nucleic acid sequence on the core : shell Pt-AgNPs. Following hybridization of the target and capture strands the surface was exposed to the miRNA labelled electrocatalytic Pt-AgNPs. The concentration of the target was then determined by monitoring the current associated with the reduction of hydrogen peroxide in a solution of H2SO4. Calibration plots of the log[miRNA] vs. faradaic current were linear from 1 aM to 1 mu M and aM concentrations could be detected without the need for chemical amplification of the target, e.g., using PCR or NASBA. The regioselectively modified particles were also immobilised within the interior of gold microcavity arrays via miRNA hybridisation and their Raman properties investigated.