Adsorption of cysteine on TiO2 at different pH values: Surface complexes characterization by FTIR-ATR and Langmuir isotherms analysis

Surface complexes of cysteine amino acid onto TiO2 films at pH 2.0, 5.0 and 8.0 were characterized by IR spectroscopy. The FTIR spectra at pH 2.0 show the coexistence of surface complexes with protonated and deprotonated carboxylate groups. This points to the occurrence of electrostatic interactions between the zwitterionic form of cysteine and the positively charged TiO2 film. A Langmuir isotherm analysis was performed to obtain the binding constants values, which are consistent with electrostatically adsorbed surface species. At pH 8.0, the TiO2 film is charged negatively, and the amino acid molecules approach the TiO2 surface through the amino protonated groups. This new arrangement generates a larger surface concentration at saturation coverage.