A device for monitoring the adsorption of target molecules on a substrate, the device comprising a substrate (1), at least a portion of which is formed as a thin dielectric layer. A first, modified surface (2) of the layer has ionizable groups of the substrate material thereon. A chamber contains a sample (5) including target molecules (6) to be in contact with the first surface (2). At least two electrodes (3) each have an electrode surface arranged on a second surface of said dielectric layer (1) opposite said first surface (2). A method of making the device is also disclosed.