Potential use of an anthocyanin-rich extract from berries of Vaccinium meridionale Swartz as sensitizer for TiO2 thin films – An experimental and theoretical study

Abstract

An anthocyanin-rich extract from Andean species Vaccinium meridionale Swartz (Ericaceae, common names: Andean blueberry, wild agraz) was chemically characterized and evaluated as natural dye for potential application as sensitizer for TiO2 thin films. The anthocyanin-rich extract (dye) was obtained from V. meridionale berries through percolation method using an ethanol:water mixture (1:3), in a 2:1 ratio of solvent:sample. Furthermore, the pH effect on stability of V. meridionale extract was studied. Chemical characterizations were carried out by high-performance liquid chromatography with diode array detection (HPLC-DAD), UV–vis spectrophotometry and electrochemistry impedance spectroscopy (EIS). After the anthocyanin identification, sensitization process was performed on TiO2 thin films. Finally, the electronic structure of each anthocyanin found in the extract (cyanidin-3-galactoside and cyanidin-3-arabinoside) and their interaction with a slab model of TiO2 and the UV–vis absorption spectra simulations were investigated by means of the density functional theory (DFT). Results showed that (i) cyanidin-3-galactoside and cyanidin-3-arabinoside were the main anthocyanins that constituted the hydro-ethanol extract (40% and 38%, respectively) and the total anthocyanin content equivalent to delphinidin chloride (TAEDC) per mL of extract was 48 ± 5 μg TAEDC/mL; (ii) the measurements by EIS indicated that sensitization process reduced the resistance for charge transfer from 933 Ω for TiO2 until 308 Ω for TiO2/sensitizer and (iii) the computational studies showed that delphinidin-3-pentoside had largest value of Eads (-53.62 kcal mol−1) for the bidentate mononuclear anchorage mode, besides the anchorage to TiO2 induced a red shift of the low-lying absorption band and an increase in their intensities respect to the free dyes, but the compositions of the transitions in terms of its nature and the involved molecular orbitals remained identical without involving of the glycoside motifs. All results suggested that the natural dye (anthocyanin-rich extract) from V. meridionale berries has a high potential to be used as sensitizer for TiO2.