Chirality is induced in polyaniline by doping with (R)-camphorsulfonic acid ((R)-CSA). Once de-doped, polyaniline films cast from solution remain chiral and can discriminate between the D- and L-enantiomers of phenylalanine, as the en Figure shows: chiral polyaniline appears green when doped with R-CSA (a) and blue when dedoped (b). De-doped films exposed to D-phenylalanine remain blue (c), but turn green in the presence of L-phenylalanine (d).