## 3 Results and Discussion  
After 1 h of exposure to UV light of standard solution of tRES, partial transformation of _trans_- to _cis_-resveratrol was observed i.e., 89.87% of _trans_ isomer in solution was converted to _cis_ form. It is expected that loading tRES into SLN would contribute to enhance the stability of the _trans_-isoform, limiting the risk of conversion into the _cis_-isoform. The mean recovery of _trans_-isoform was 96.84 \(\pm\) 0.32%, whereas for _cis_-isoform it was 3.18 \(\pm\) 0.47%. The decrease in the intensity of the chromatographic peak of _trans_-resveratrol was proportional to the area of new chromatographic peak, corresponding to _cis_-resveratrol. The _cis_-resveratrol may also show health benefits, but has lower biological activity [50]. To overcome these shortcomings, our main purpose was to validate a suitable analytical procedure for the quantification of resveratrol that could be used for both isomers. Since conversion occurs from _trans_- to _cis_-, the _trans_-isomeric form has been selected as the active ingredient to be loaded in SLN. Due to the instability of _cis_-resveratrol, this isomer is not commercially available. The identification by high-performance liquid chromatography-diode array detection (HPLC-DAD) of analytes in pharmaceutical formulations was realized by [PERSON] et al. [51], and it is based on a combination of retention time and spectral matching in the UV region obtained at diode array. UV spectra generated for _trans_-resveratrol and _cis_-resveratrol could be easily differentiated due to the high resolution, accuracy, and sensitivity in the wavelength (200 to 400 nm) [52]. For a spectrum to be identified, a mathematical comparison with a spectra library is usually carried out, requiring the calculation of a similarity index within the entire spectra range and wavelengths measured allowing the recognition of very small differences (i.e., 181 points of comparison in the range of 200 to 380 nm) [51]. The distinct retention times of _trans_- and _cis_-resveratrol in the RP-HPLC chromatogram are shown in Figure 2a, and the respective UV absorption spectra recorded in the range of 280-350 nm are displayed in Figure 2b,c. The UV absorbance spectrum of _trans_-resveratrol (Figure 2b) shows a lambda max (\(\lambda\) max) at 306.6 nm, while the UV absorbance spectrum of _cis_-resveratrol (Figure 2c) exhibits \(\lambda\) max at 285.2 nm. Similar results have been described in the scientific literature, in which 285 nm and 306 nm of UV absorbance spectra have been reported of _cis_- and _trans_-resveratrol, respectively [53; 54]. After UV light exposure, a tRES solution of 50 \(\upmu\)g/mL exhibited a proportional decrease of the peak area of _trans_-resveratrol and an increase of the peak area of _cis_-resveratrol. The resolution of chromatographic peak (Rs) is a quantitative measure of the peak width, i.e., it measures the separation of two peaks of different retention times t in the chromatogram. The valley between the peaks should touch the baseline, and for complete resolution of the peak, the ideal value should be Rs \(\geq\)1.5 [55]. However, values between 1.2 \(\geq\) Rs \(\geq\) 1.5 are often enough. Our results exhibited an Rs value of 1.2; for validation of the analytical method for the quantification of tRES, the UV detection was fixed at 306.6 nm. Figure 2: Retention times of _trans_- (7.194 min) and _cis_-resveratrol (7.847 min) in reverse-phase (RP)-HPLC chromatogram of standard solution of (50 \(\upmu\)g/mL) after 1 h of exposition to UV light **(a)**; UV absorption spectrum of _trans_-resveratrol **(b)** and _cis_-resveratrol **(c)** recorded in the range of 280–350 nm from the diode array. The analytical curves were obtained at 12 concentration levels, each recorded in triplicate. According to RDC 899/2003 [44], the acceptable linearity is when the linear correlation coefficients (R\({}^{2}\)) are greater than 0.99.