An alternative dopant method is to use a second polymer, forming a copolymer. The film properties can be regulated by adjusting the monomer concentrations. PANI co-deposited with poly(vinyl sulfonate) (PVS) has recorded improved electrode surface morphology and electrical conductivity of the polymer at non-acidic pH. 120 Poly(2-acrylamido-2-methyl propanesulphonic acid) (PAMPS) is used as a dopant to make PANI soluble in water. 121 Lignosulfonate (LGS) was shown to increase the conductivity of PANI by more than two folds as well as improving the thermal stability. 122 PANI polymers containing aniline derivatives such as o-ABA or m-aminobenzenesulphonic acid, have displayed electrochemical activity over a wide pH range, while the additional aniline increases the rate of polymerisation. Such polymers are often referred to as self-doped PANI. PPy can similarly be self-doped and be co-polymerised with pyrrole-2-carboxylic acid. As previously mentioned, these additional functional groups on the self-doped polymer films can serve as a linker for covalent attachment to a biomolecule. Polystyrene sulphonic acid (PSSA) has also successfully been incorporated into polymer layers such as PANI and PEDOT. These molecules have a higher molecular weight than counter ions and thus have a larger impact on the polymer's morphology. Additionally they are unlikely leach out or de-dope as they are partially entrapped in the layer. 103