Other electrode modifications require the bioreceptor to be immobilised after the electrode has been modified. The bioreceptor may be physiosorbed onto the modified surface, 174, 177 though this is not common due to dissociation from the surface. If the modification layer is charged it can electrostatically interact with the bioreceptor. The pH must be suitable for the bioreceptor to be charged as well. This method has drawbacks as the conformation of the protein can be changed causing denaturation; there is no control of the packing density; and there is a possibility of the biomolecules leaching. Instead, as mentioned in Section 4.1, DSP, ProLinker™ B, GPMS and GPMES can directly covalently bind to the bioreceptor. Other surfaces not equipped to directly bind to the bioreceptor opt for an additional step to ensure a stable immunosensor surface.