X-ray photoelectron spectroscopy investigation of adenine thin film on titanium oxide surfaces


Purpose. Recent studies have demonstrated that nano-TiO2 induces DNA damage and increase the risk of cancer and the mechanism might relate to oxidative stress. To our knowledge, no detailed spectroscopic study is reported on adenine/TiOx system. Therefore the aim of this work is study of the adenine biomolecules interaction with the polycrystalline TiOx surface by X-Ray photoelectron spectroscopy (XPS). 
Methods. TiOx surface was obtained by thermal oxidation the cleaned Ti foil at the T= 650 K, oxygen pressure 1×10-7 torr during (30+30+60) min. The thermal evaporation of adenine at 110˚C took place in the preparation chamber (pressure 1×10−7 torr). Al Kα radiation (1486.6 eV) was used to measure the XPS core levels spectra of O1s, C1s, N1s and Ti 2p3/2 with total resolution of 1 eV. Curve fitting was performed after a Shirley background subtraction by a Lorenzian–Gaussian method. 
Results. TiOx layer (d=30 Å) on the titanium foil surface were obtained. The adenine thin film (d=9 Å) on the surface of titanium oxide were obtained by thermal deposition in vacuum. Analysis of the measured XPS spectra shows that the Ti 2p core level spectra after oxidation consist many components that can be associated with oxides TiO2, TiO, Ti2O3 and titanium nitride and/or titanium carbide. N 1s core level spectra of adenine film consist the two components (399.4 eV and 400.8 eV), which can be attributed to nitrogen with unsaturated chemical bonds (-N =) and (NH + NH2), respectively. C 1s core level spectra of adenine film contains two components with 285.3eV and 287 eV binding energies, which can be attributed to carbon in the CC/CH groups and a CN-carbon bonds of the adenine rings respectively. After heating in vacuum at T=50, 75, 100, 125, 150 and 200°C thickness of the adenine film decreases. As seen from the XPS spectra and AFM images after annealing at T = 250 C on the surface TiOx remain only single molecules of adenine. 
Conclusions. The method of obtaining titanium oxide on the polycrystalline Ti foil been developed. An adenine thin film on the titanium oxide surface has been obtained by thermal evaporation in vacuum. XPS investigation of influence of thermal treatment on structural characteristic of adenine thin film showed that thickness of adenine thin film decreases at annealing. At the same time after annealing at high temperatures (250°C), we see the presence of small amount of adenine, as evidenced the C 1s and N 1s core level spectra and AFM images of studied surface structure

Keywords: Adenine, titanium oxide, adsorption, surface, XPS