However, Fab' fragments can chemisorb directly onto gold electrodes as they contain a free sulfhydryl group that forms a thiolate bond with the gold substrate, favourably orientating the antigen recognition site. To produce the Fab' fragment, the Fc region must first be cleaved with an enzyme, leaving a F(ab') 2 fragment. The disulfide bond can then be reduced to separate the two Fab' fragments, though this may also cause damage to the recognition structure as other disulfide bridges connect the heavy and light chains within the immunoglobin structure. Alternatively, for the use of whole antibodies electrode modifications are undertaken to immobilise the biorecognition element to the electrode, with improved the antibody orientation and antigen accessibility as well as increased the surface binding strength, compared to physisorption. Surface modifications must have available functional groups for linking to the biorecognition element and be stable in storage.