# Sustainable Organic Dyes from Winemaking Lees for Photoelectrochemical Dye-Sensitized Solar Cells
## 3 Results and Discussion
### Winemaking Lees Dye Extraction  
Dye extraction process was carried out valorizing the lees deriving from red and white wines, characterized by a pH value of \(3.7\pm 0.5\). After centrifugation of winemaking lees, the colored solid was used to extract the dyes that were used for the realization of a DSSC photovoltaic cell. Firstly UV-Vis spectra of water diluted dyes sample, extracted with ethanol in 1:1 w/w lees-solvent ratio, acidified with concentrated HCl were registered. A peak at 280 nm and a large absorption band centered at 520 nm for red wine lees and at 280 nm for white wine lees can be noticed (Figure 6). It is evident that the red extracts absorb in the visible region of light spectrum and, hence, fulfil the primary criterion for their use as sensitizers in DSSCs. Considering that the most efficient organic DSSCs are usually based on organic dyes that consist of anthocyanins obtained from red or purple fruits [9], the red extract was further investigated. Using the same conditions reported before, several extractions were conducted with different solvents (methanol, ethanol, water, and mixture thereof). The maximum absorbance of red wine extract was used to monitor the concentration of dye solution during the extraction process, as a function of the extraction time. The kinetics of the dye extraction process, as shown in Figure 7, prove that the dye extraction from bentonite clay can occur in some hours. For all of the solvents used, absorbance increases very  
Figure 5: Diffuse Reflectance Spectroscopy spectra of as-prepared and calcined (400 \({}^{\circ}\)C) titanium dioxide samples. Figure 6: UV-Vis absorption spectra of ethanol/water solution of natural dyes extracted from red and white wine lees. rapidly in the first hour, reaching an asymptotic behavior just after 2 h. These experimental findings suggest that, at the beginning of the extraction process, solvent desorbs dye molecules from the external structure of bentonite layers, and then extracting organic molecules located in the middle of the clay layers. After optimizing the extraction process, the organic molecules were further characterized, and the concentration of the different organic molecules that were involved in color appearance was evaluated. Figure 8 reports anthocyanins, tannins, catechins, and total polyphenols concentrations. In particular, the anthocyanins responsible for red color resulted in being very concentrated (1 g/L), as compared to the typical values of anthocyanins content in red wines [33]. The presence of water is responsible for the decreasing of dye concentration in the extracted solutions, which was probably due to the different solubility of these molecules in organic and aqueous solutions. Figure 8: Characterization of extracted dyes from red wine lees. (**a**) Anthocyanins; (**b**) catechins; (**c**) tannins; and, (**d**) total polyphenols. Figure 7: Absorbance vs extraction time for red wine lees extract. Another important characterization is the color intensity (related to the dye concentration): it was determined as the sum of the absorbance values at 420 nm, 520 nm, and 620 nm. The values of color intensity were 41.3 for methanol extraction, 43.2 by using ethanol, and 32.0 with a mixture ethanol-water. Soxhlet extraction can be another possible way to extract organic dye from solid lees. In this case, the lees are continuously washed with fresh ethanol, allowing for recycling the solvent and perhaps increased the polyphenols content in dye extract. Soxhlet extraction was performed, by using 1:3 w/w lees-solvent ratio. For comparison, solid-liquid extraction was performed with 1:1 w/w or 1:3 w/w lees-solvent ratio. As reported in Figure 9, the Soxhlet extraction process led to a quite poor absorbance in comparison with both 1:3 w/w lees-solvent ratio and 1:1 w/w extraction.