Photo-degradation of curcumin in the presence of TiO2 nanoparticles: Fundamentals and application
Graphical abstract
Photo-degradation of curcumin in the presence of TiO2 nanoparticles in aqueous-organic mixed solvents was studied upon excitation with UV–vis and only visible light from a 450 W xenon lamp. The mechanism of photo-degradation was investigated by using external hole and electron quenchers. This study was successfully applied in removing the yellow turmeric stain from cotton fabrics.
Introduction
Curcumin is an important molecule for its medicinal and several other usages [1], [2], [3], [4], [5], [6]. Curcumin is a yellow coloured compound present in turmeric and accounts for only about 3–5% of its total constituents [7]. The photo-physical studies of curcumin have earlier been reported by our group [8]. Curcumin has two phenolic hydroxyl groups and two centrally located carbonyl groups, which can exist in keto-enol tautomers in solution. It has been now concluded by several experimental studies that these two groups predominantly exist in enolic form at room temperature [9], [10]. The enolic form can also exist in two different forms, i.e. cis and trans, depending on the surroundings. The cis form has been suggested to be more stable as compared to the trans form [8], [11], [12].
The effect of light exposure towards the stability of curcumin is very important for exploring its medicinal applications for skin protection from sun-burns. The photo-degradation of curcumin has been reported by various groups in the recent years [13], [14]. Canamares et al. have studied the photo-degradation of curcumin in aqueous solution in the presence of silver nanoparticles upon UV–vis light excitation [13]. They have identified the photo-degradation products mainly as phenolic compounds which results from the breakdown of the chain connecting the two aromatic rings. Price et al. has reported the photo-decomposition of curcuminoid compounds present in turmeric. They have studied the effect of light, solvent and oxygen on the decomposition processes [14]. Thermal degradation of curcumin in methanol has been reported by Ahn and Obendorf [15]. They have analyzed the degradation products by gas chromatography coupled with mass spectroscopy. The major products include vanillin and its derivatives.
TiO2 nanoparticles are well known to be very good catalysis for the decomposition of various organic molecules and dyes in aqueous as well as organic solvents [16], [17]. TiO2 nanoparticles are also recently being used as an effective UB-B radiation skin protective compound in sunscreens [18]. So it is important to know the effect of the presence of TiO2 nanoparticles on the stability of curcumin molecules. There is no report available in the literature regarding the interaction between two such materials. In this study, we report the interaction between TiO2 nanoparticles and curcumin molecules that leads to the latter's photo-degradation. A systematic investigation has been made on the photo-degradation of curcumin molecules in the presence of TiO2 nanoparticles in two different solvent mixtures and two different types of exposures, UV–vis and only visible light. This study has been applied in removing the yellow turmeric stain from cotton fabrics, which was otherwise difficult to be removed by conventional methods using soap or detergents. The yellow coloured stains due to turmeric in the clothes are due to the binding of the curcuminoid compounds present in turmeric with cellulose fibres in cotton fabrics. When bound to fibres it becomes difficult to remove them by using common soaps or detergents. However, our method is very effective and simple to use.
Section snippets
Chemicals
Curcumin was obtained from Sigma–Aldrich and its purity was verified by HPLC analysis. Locally available high purity grade potassium iodide (KI) and methyl violegen (MV2+) were used as obtained. Commercially available turmeric powder was used for making the yellow stain on cotton fabrics. TiO2 nanoparticles have been synthesized by acid hydrolysis of titanium isopropoxide [19], [20]. Due to very poor solubility of curcumin in water, water–methanol (10–50%, v/v) and water–acetonitrile (10–50%,
Ground state interaction of curcumin with TiO2 nanoparticles
The ground state optical absorption spectra of curcumin in the absence and presence of TiO2 nanoparticles are shown in Fig. 1. The absorption spectrum of curcumin in the presence of TiO2 nanoparticles was relatively broad and slightly red-shifted associated with an increased absorbance. This confirms the formation of a charge transfer (CT) complex between curcumin molecules and TiO2 nanoparticles. The yellowish colour of curcumin solution was changed to reddish-yellow upon the addition of TiO2
Conclusion
Curcumin forms CT complexes with TiO2 nanoparticles in water-organic mixed solvents and gets decomposed upon exposure to either UV–vis or visible light. Upon visible light excitation, the CT complexes are photo-excited and the subsequent photo-degradation of curcumin takes place through electron transfer from the photo-excited CT complexes to the CB of TiO2 nanoparticles. In the case of UV–vis excitation, the photo-excited holes in the VB of TiO2 nanoparticles found to play an important role in
Acknowledgement
The author U. Singh is grateful to the Department of Atomic Energy for awarding a research fellowship.
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