Titania nanotubes grown on titanium through the electrochemical anodization are an attractive nanostructure due to their applicability in areas such as photocatalysis, dye-sensitized solar cells, sensors, etc. Nanotubes functionalization with nanoparticles or different molecules is becoming a common practice that can either aid or enhance their performance in a variety of applications. Dyes have been used in solar cells and photocatalysis to sensitize the semiconductor and, biological dyes such as anthocyanins have been tested for these purposes due to their low cost and environmentally friendly nature (1).

In this work, titania nanotubes electrodes were functionalized with anthocyanins in order to test their performance as sensitizers for photoelectrocatalytic purposes. Anthocyanins alcoholic extract has been obtained from hibiscus flower (Hibiscus sabdariffa L) and immobilized on titania nanotubes (TiO₂ Nts ) grown from 2 cm² titania plates (TiO₂ Nts/Ti) by anodization at 30 V in a glycerol:water (2:1) solution containing sodium fluoride and sodium sulfate. Anthocyanins immobilization was achieved through immersion for 48 h of the TiO₂/Ti plates in the alcoholic extract at O°C. The antioxidant activity from both, the as-extracted and the immobilized, anthocyanins was tested by the ABTS assay. This essay showed that a 1:100 dilution from the extract had an antioxidant activity of 550 μTrolox while the immobilized anthocyanins had a higher antioxidant activity of 699 μTrolox (2).

Electrochemical techniques have gained importance in the study of nanostructured materials especially in the field of surface characterization. Within this work, cyclic voltammetry was used to determine surface characteristics from the bare TiO₂ nanotubes and from the nanotubes functionalized with anthocyanins. Through cyclic voltammetry in a pH 7.00 1.0 M buffer phosphate it was possible to confirm TiO₂ Nts functionalization with anthocyanins as it can be seen from the change in the voltamperogram´s shape for the bare nanotubes and after functionalization with anthocyanins (Figure 1). Cyclic voltammetry on anthocyanins-TiO₂-Nts electrode showed two oxidation peaks at -0.390 V and -0.096 V (vs. mercurous sulfate electrode) which can be attributed to the delphinidin-3-O-glucoside (3), the main anthocyanin found in Hibiscus sabdariffa L (4). Complementary results concerning these electrodes performance will be presented at the conference.

References

1. Zyoud, A., Zaatar, N., Saadeddin, I., Helal, M.H., Campet, G., Hakim, M., Park, D., Hilal, H. 2011. Alternative natural dyes in water purification: Anthocyanin as TiO₂-sensitizer in methyl orange photo-degradation. Solid State Sciences, 13: 1268-1275.
2. Pérez‐Jiménez, J., & Saura‐Calixto, F. (2008). Anti‐oxidant capacity of dietary polyphenols determined by ABTS assay: a kinetic expression of the results. International journal of food science & technology, 43(1), 185-191.

3. Janeiro, P., Oliveira-Brett, A. M. 2007. Redox behavior of anthocyanins present in Vitis vinifera L. Electroanalysis. 19(17): 1779-1786.
4. Galicia-Flores, L. A., Salinas-Moreno, Y., Espinoza- García, B. M. y Sánchez-Feria, C. 2008. Caracterización fisicoquímica y actividad antioxidante de extractos de jamaica (Hibiscus sabdariffa L) nacional e importada. Revista Chapingo Serie Horticultura. 14(2):121-129.