Carboxymethylaniline (4-CMA) allows to immobilise coupled biomolecules to appropriate prepared surfaces. In this way, the binding of oligonucleotides to conductive surfaces (e.g. graphite) enables the detection of currents. By coupling, either to the 5´- or 3´-end of oligonucleotides, the molecules are covalently coupled in the desired orientation to the surface.