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Detailed Program
Paper Number : NA-O02
Time Frame : 11:47~11:59
Presentation Date : Friday, 28, November
Session Name : Nano-particles & Nano-Structured Materials
Session Chair 1# : Chang-Yeoul Kim
Session Chair 2# : Yamamoto Hayashi
Enzyme-mimetic Activity of Ce-doped Titanate Nanosheets
Kai KAMADA
Nagasaki University
Cerium-doped titanate nanosheets were fabricated through a hydrolysis reaction of titanium tetraisopropoxide liquid mixed with Ce(NO3)3, and their reactive oxygen species (ROS) scavenging activity was investigated. The hydrolysis reaction was carried out by an addition of aqueous tetrabutylammonium hydroxide solution to the mixture, followed by aging at 60oC for 2 h. The resultant transparent colloidal solution contained nanoparticles with a tiny size less than 10 nm. XRD and Raman spectroscopy revealed that crystal structure of the nanoparticles is akin to layered tetratitanate (Ti4O92-) and the nanoparticles (called below as nanosheets) consist of several tetratitanate layers interstratifying Ce ions.
ROS scavenging activity of the nanosheets was evaluated for superoxide anion radical (O2-) as a target. The evaluation was performed by a well-known cytochrome c reduction technique [1]. The dependence of nanosheet concentration on the O2- quenching is shown in Figure 1(a), where the 1 mol% Ce-doped titanate nanosheets were employed. The figure clearly indicates dose-dependent O2- scavenging behavior, implying that the nanosheets possess superoxide dismutase (SOD) mimetic activity. Figure 1(b) plots 50% inhibitory concentrations (IC50) as a function of Ce-doping amounts. The presence of Ce in the nanosheet drastically reduced the IC50 even at a low concentration below 1 mol%.
Typically, an enzyme SOD catalyzes dismutation of O2- to O2 (oxidation) and H2O2 (reduction) accompanied by valence fluctuation of transition metal ions (Cu, Mn, etc.) in active centers. Judging from the favorable effect of Ce-doping on IC50, the Ce ions appear to bring about the redox reaction of O2-. To clarify an annihilation route of O2- in the presence of nanosheets, H2O2 produced by the quenching of O2- was quantified by means of peroxidase assay [2]. The H2O2 concentration after scavenging by the nanosheets was lower than that after spontaneous O2- dismutation without the scavengers, suggesting that the oxidation (O2-/O2) is a preferential route. This fact is not conflicted with thermodynamic consideration of redox potentials of Ce3+/4+ (Eo = +1.15 V vs. NHE at pH 7), O2-/H2O2 (+0.89 V), and O2-/O2 (-0.33 V). That is, Ce4+ ions can oxidize O2- to O2, whereas Ce3+ cannot reduce it to H2O2. In conclusion, the present study demonstrated that the Ce-doped titanate nanosheets have the scavenging ability of O2-, i.e., SOD-mimetic activity, and the O2- annihilation is due to the oxidation activity of Ce4+ ions.

Figure 1 (a) Dose-dependent O2- scavenging activity of 1 mol% Ce-doped titanate nanosheets and (b) influence of doping amounts of Ce on IC50.

References:
[1] C. Korsvik, S. Patil, S. Seal and W.T. Self, Chem. Commun., (2007) 1056.
[2] N. Fujita and K. Kamada, J. Ceram. Soc. Jpn., 122 (2014) 141.
Acknowledgements :