3.1.4. DR-UV–Vis Characterization

The DR-UV–Vis spectra of CR-containing samples along with the spectrum of CR are displayed in Figure 7a,b.

**Figure 7.** Diffuse reflectance (DR)-UV–Vis spectra: (**a**) neat curcumin (CR) and precipitated samples, PZn3Al-CR(Aq), PZn3Al-CR(Et); (**b**) reconstructed samples RZn3Al-CR(Aq), RZn3Al-CR(Et).

The spectrum of the neat curcumin powder has the highest intensity absorption band in the visible region at 425 nm. This band is shifted to 455 nm and is clearly evidenced only in the spectrum of the sample RZn3Al-CR(Et) which shows also the second absorption band specific to CR at 364 nm. Meanwhile, in the spectra of the precipitated samples PZn3Al-CR(Aq) and PZn3Al-CR(Et) the maximum absorption in the visible domain appears at 361.5 and 349 nm, respectively and only an inflection of the absorption curve is noticed at 423 nm. The absorption maxima observed in these two spectra indicate that during the preparation of the samples curcumin was partially decomposed to a mixture of feruloyl methane and ferulic acid whose most intense absorption peaks are at 340 nm, and 305 nm respectively [43]. In the spectrum of RZn3Al-CR(Aq), only the characteristic absorption bands for feruloyl methane at 340 nm and 241 nm were noticed.

## *3.2. Curcumin Release Studies*

The results of the CR-release studies performed by contacting the synthesized solids with different buffer solutions (pH 1, pH 2, pH 5, pH 7 and pH 8) at 25 ◦C during 24 h under mild stirring are presented in Figure 8a–d. The release of curcumin from pure curcumin powder into the same buffer solutions was determined using an amount of curcumin powder equal to the average amount of curcumin incorporated in 0.5 g of the LDH samples (e.g., 56.25 mg) which was immersed in 50 mL of buffer solution. The results obtained under operating conditions (temperature, stirring and duration) similar to those employed for CR-containing LDH samples are presented in Figure 9.

The results displayed in Figure 8 indicate that for all samples the release of curcumin was favored at lower pH values and was almost insignificant at pH values higher than 5. It is also noticeable that CR-loaded samples prepared with ethanolic solutions were able to release higher amounts of CR than the samples prepared with aqueous solutions. At each pH value, the amount of curcumin released in the buffer solutions varied in the order: RZn3Al-CR(Et)>PZn3Al-CR(Et)>PZn3Al-CR(Aq)>RZn3Al-CR(Aq). At pH 1, CR was released faster from RZn3Al-CR(Et) since half of the total amount of released curcumin was reached after 2 h, while for the other solids only about one third of the total amount was reached in that period. Under similar conditions, the results plotted in Figure 9 show that the pH decrease led also to an increased release of curcumin from pure curcumin powder into the buffer solutions, but the highest value obtained was lower than 0.8%. When the release tests with pure curcumin powder were performed during 4 h at 37 ◦C (the physiological temperature) the amount of released CR varied in the order: 1.0 ± 0.1% at pH 1, 0.8 ± 0.1% at pH 2, 0.5 ± 0.1% at pH 5, 0.4 ± 0.1% at pH 7 and 0.3 ± 0.1% at pH 8. The effect of the

temperature on the CR-release after 4 h from the sample RZn3Al-CR(Et) was also not significant as it may be seen from the results plotted in Figure 10.

**Figure 8.** Curcumin release from the solid samples during 24 h in different pH buffers: (**a**) PZn3Al-CR(Aq); (**b**) RZn3Al-CR(Aq); (**c**) PZn3Al-CR(Et); (**d**) RZn3Al-CR(Et).

**Figure 9.** Curcumin release from pure curcumin powder.

**Figure 10.** Curcumin release from RZn3Al-CR(Et) in different buffer solutions at 25 and 37 ◦C.

The solid samples recovered after each CR-release test were analyzed by DR-UV–Vis spectroscopy and the spectra are presented in Figure 11a–d.

The alteration of the spectra of the samples after they were contacted with different buffered solutions indicates how the chemical composition of each buffer affected the CR-functionalized solids. After the tests performed at pH 7 and pH 8 in buffered solutions containing potassium dihydrogen phosphate/di-sodium hydrogen phosphate, and potassium dihydrogen phosphate/sodium hydroxide, respectively, none of the specific absorption bands observed in the spectra of the fresh samples could be noticed. This fact suggests the occurrence of chemical reactions between the components of the buffer and the solid leading to the decomposition of the chemical species responsible for the absorption bands noticed in the DR-UV–Vis spectra of the fresh samples. After the tests performed in acid buffers at pH 1 and 2, the main bands specific to the fresh solids were preserved but their relative intensity compared to the spectra of the fresh solids decreased, suggesting that a partial dissolution of the inorganic host took place. In addition to that, for the samples PZn3Al-CR(Aq), PZn3Al-CR(Et) and RZn3Al-CR(Aq) their position was shifted to lower wavelengths. The decrease was more intense at pH 1 when the highest amount of curcumin was released for each sample. The contact of the CR-loaded solids with the pH 5 buffer solution containing citric acid and sodium hydroxide affected all the samples. In the spectra of the samples prepared with CR-ethanolic solution (Figure 11b,d), after the contact with pH 5 buffer, the absorption maximum characteristic to bicicyclopentadione at 232 nm was noticed [43]. This maximum was also noticeable as a shoulder in the spectrum of PZn3Al-CR(Aq) (Figure 11a), but it was absent in the spectrum of RZn3Al-CR(Aq) (Figure 11c).

The interactions between the CR-containing samples and different buffer solutions were also evidenced in the ATR-FTIR spectra of the solids recovered after the CR-release tests performed with each buffer solution which are presented in Figure 12.

The spectra of all the samples recovered from the alkaline buffer solutions (pH 7 and 8) have the main absorption bands around 1000 cm<sup>−</sup><sup>1</sup> (marked with \* in Figure 12) and they were attributed to the phosphate anion which was intensely adsorbed on the solids. In this case, the spectra of the two samples prepared by co-precipitation (Figure 12a,b) presented the same absorption maxima, whereas in the spectra of the samples RZn3Al-CR(Aq) and RZn3Al-CR(Et) recovered from pH 8 buffer solution, the band corresponding to water bending at 1620 cm<sup>−</sup><sup>1</sup> was absent. The spectra of the samples recovered from acid buffer solutions (pH 1, pH 2 and pH 5, respectively) were mostly altered in the mid-infrared region 1600–1300 cm<sup>−</sup><sup>1</sup> (delimited by a dashed rectangle in Figure 10) where the bands specific to citrate occur (<sup>υ</sup>as(COO−) at 1612 cm<sup>−</sup>1; <sup>υ</sup>s(COO−) at 1396 cm<sup>−</sup>1, <sup>δ</sup>(CH2) at 1365 cm<sup>−</sup><sup>1</sup> [44]). For the precipitated samples (Figure 12a,b) the relative intensity of the band around 1600 cm<sup>−</sup><sup>1</sup> was sensibly higher than in the spectra of the fresh samples, while for the reconstructed ones, the relative intensity of all the bands in the domain 1600–1300 cm<sup>−</sup><sup>1</sup> was definitely increased compared to the spectra of the fresh samples (Figure 12c,d) indicating that citrate was better adsorbed on the surface of the reconstructed solids.

**Figure 11.** DR-UV–Vis spectra of the CR-containing samples after the CR-release tests at different pH values: (**a**) PZn3Al-CR(Aq); (**b**) RZn3Al-CR(Aq); (**c**) PZn3Al-CR(Et); (**d**) RZn3Al-CR(Et).

**Figure 12.** ATR-FTIR spectra of the CR-containing samples after the CR-release tests at different pH values: (**a**) PZn3Al-CR(Aq); (**b**) RZn3Al-CR(Aq); (**c**) PZn3Al-CR(Et); (**d**) RZn3Al-CR(Et).
