## 2 Discussion
The complexation of these compounds can be measured and quantified using FTIR and UV-vis spectroscopy, determining changes in the absorbance profiles of the nanotubes with or without complexed anthocyanins [110]. The changes registered in the UV-vis spectra of the nanotubes against those complexed with anthocyanins can be followed using roselle extracts as the reference, which contain mainly D3S. Additionally, the infrared spectroscopy revealed the anthocyanin immobilization through the presence of characteristic peaks from 750 to 1700 cm\({}^{-1}\). The presence of a band at 1700 cm\({}^{-1}\) indicates the presence of organic acids, such as hibiscus acid. The peaks between 1630 and 1440 cm\({}^{-1}\) were associated with the alkene and aromatic groups present in the polyphenols. The C-O bonds present in the phenolic group and the sugar moiety of the anthocyanin skeleton appeared from 1040 to 1260 cm\({}^{-1}\). Finally, the C-H bonds of the aromatic ring showed bands near 750 cm\({}^{-1}\). In another work, we evaluated the anthocyanin stability from the roselle calyxes when exposed to microwave energies. Examining the thermal stability of the anthocyanins to determine their possibility to react at microwave energies without being degraded, the anthocyanin extracts were heated at different temperatures (30-80 \({}^{\circ}\)C) for 20 min. Afterward, the phenolic compound and chroma properties were spectrophotometrically evaluated. The results showed losses in the total phenolic compounds from 9.28 at 60 \({}^{\circ}\)C to 11.34% at 80 \({}^{\circ}\)C. Meanwhile, color parameters a* and b* decreased, but L* was augmented (lighter colors). The results indicated that the extracts lose their red color by migrating to green (a*) and lose their blue color by turning into yellow (b*), indicating anthocyanin degradation. With the support of spectrophotometric applications, we can conclude from these results that the temperature increase negatively affects the anthocyanin stability, thereby generating negative repercussions on the nutraceutical and colorant features associated with phenolic compounds [111]. More recently, there are reports concerning the chemical stability of anthocyanins from roselle. Due to D3S being an anthocyanin that has been demonstrated to induce apoptosis in human promyelocytic leukemia cells (HL-60), there has been an increasing interest in the study of the extracts of this plant [112]. The process of copigmentation, highly visible during winemaking, occurs because of the chemical association of two or more anthocyanin molecules, but also with proteins, sugars, organic acids, and other molecules. This process allows the formation of new anthocyanins that exhibit new colors and chemical characteristics. For the above, this study was focused on the aqueous extraction of anthocyanins at different temperatures (40, 60, and 80 \({}^{\circ}\)C) and at different times of extraction (10, 20, and 30 min) to determine the effect of copigmentation with organic acidssuch as phytic and chlorogenic, and pH on their stability. Anthocyanin (535 nm), flavonoid (374 nm), and total phenolic (761 nm) concentrations and the antioxidant activity were spectrophotometrically analyzed by the DPPH methodology (517 nm). The results from this study show that anthocyanins from roselle exhibit high thermal stability at 60 \({}^{\circ}\)C and also reversible stability in terms of pH changes, measured by spectrophotometric titration, thus preserving the antioxidant activity of the extracts evaluated. Another bioactive compound that has been studied very recently by our research group is piecetanmol, a stilbenoid (a nonflavonoid polyphenol) with antibody properties that can be characterized, measured, and quantified using spectrophotometric tools. This nutraceutical was extracted from passion fruit seeds (_Passiflora edulis_) and quantified through the Folin-Ciocalteu methodology at 765 nm, as described earlier (data unpublished).