3D microelectrode arrays, pushing the bounds of sensitivity toward a generic platform for point-of-care diagnostics.

The increased sensitivity of microelectrode arrays (MEAs) over macroelectrodes for biosensing is well established, and results from reducing the diffusion gradient of the target species to and from the electrode surfaces. The current study describes the fabrication and characterisation of a polymer-based MEA, which exploits the advantages of three dimensionality (3D). Firstly, the unique 3D formfactor promotes release of the gold tips from an inert layer in a controlled fashion, to form a highly reproducible array of microelectrodes in a single step. The 3D topography of the fabricated MEAs significantly enhances the diffusion profile of the target species to the electrode which results in higher sensitivity. Furthermore, the "sharpness" of the 3D structure induces differential current distribution that is focused at the apices of the individual electrodes, reducing the active area, and thereby overcoming the requirement for the electrodes to be sub-micron in size before true MEA behaviour can be achieved. The electrochemical characteristics of the 3D MEAs shows ideal micro-electrode behaviour, with a level of sensitivity of three orders of magnitude greater than that of enzyme-linked immunosorbent assays (ELISA), as the optical based gold standard. The application of the 3D MEAs uses the combination of enzyme-label and substrate approach employed in ELISAs as a generic basis for biosensing and can hence be applied to the plethora of targets that utilise the ELISA approach. As an example, the 3D MEAs are applied to the detection of RNA and demonstrate a level of detection down to single digit picomolar concentrations.