Chitosan films have been successfully functionalized with several
different dyes (Table 1). Dyes that act as chromogenic agents
undergo an optical absorbance change due to certain chemical or
physical conditions and are useful for optical sensing. For
example, chitosan modified with 2-(4-Pyridylazo)resorcinol is
capable of detecting cobalt ions.117 Chitosan modified with thionine
undergoes a color change with pH variation. Chitosan
modified with tungsten oxide changes its color due to redox
reactions.40 Other types of dyes are added to chitosan to exploit
their chemical properties and not necessarily their color. For
example Prussian blue has catalytic capabilities for hydrogen
peroxide, and it has been used with chitosan in amperometric
glucose sensors.121,122 Procion brown and procion green dyes are
ligands for certain proteins and have been combined with chitosan
to make affinity membranes for lysozyme purification.118–12