## 3 Results and Discussion
### Optimization of NADES4 Aqueous Dilution for Enhanced Polyphenol Recovery from L. japonica Flowers  
Using _L. japonica_ flowers as a model system for polyphenol extraction optimization, this study evaluated NADES4 (choline chloride-urea, 1:2 molar ratio) as an environmentally friendly alternative to 80% ethanol, a conventional solvent for phenolic recovery. Consequently, the polyphenol-rich material and the eutectic mixture composed of choline chloride and urea were utilized in the next phase of this research. Research indicates that the high viscosity of eutectic mixtures can significantly hinder the extraction of bioactive compounds [32; 33]. Therefore, additional experiments were conducted to compare the effects of adding water to NADES4 for increasing the yield of phenolic compounds (Figure 5). The optimization of NADES4 dilution significantly influenced TPC recovery from _L. japonica_ flowers. The 60% (_w/w_) NADES4 achieved the highest TPC (57.46 mg/g), surpassing both 80% ethanol by 23% and water by 113%. This peak performance at 60% dilution suggests a critical balance between solvent polarity and viscosity, where moderate water content enhances phenolic solubility while mitigating NADES4's inherent viscosity limitations. Additionally, the TPC of NADES4 extracts consistently increased at high dilutions (10-40%), reaching 53.30 mg/g at 40%, which surpassed the performance of ethanol. Conversely, at lower dilutions (70-100%), TPC sharply decreased (from 48.18 to 44.42 mg/g), suggesting that higher concentrations of NADES4 lead to reduced solvent efficiency, likely due to increased viscosity that hinders mass transfer. This study phase was further enhanced by measuring antioxidant activity using the DPPH radical scavenging assay (Figure 6). At concentrations of 50% and 60% (_w/w_), NADES4 demonstrated the most significant antioxidant activities, measuring 153.69 and 151.37 umol of Trolox/g, respectively. This performance surpasses that of 80% ethanol by almost 20% and water by about 150%. The optimal antioxidant activity observed at 50-60% dilution coincides with the highest TPC at 60% NADES4 (Figure 5), indicating a strong correlation between antioxidant activity and phenolic content. Figure 5: Effect of aqueous NADES4 dilution (10–100% \(w/w\)) on total phenolic content (TPC) in \(L\). _japonic_ flower extracts. The violet dash-dotted line indicates TPC for ethanol extracts, whereas the light blue dashed line represents TPC for water extracts. Error bars represent the standard deviation (SD) of three independent measurements. The values with the same lowercase letters (a–d) do not differ significantly (\(\alpha=0.05\)). Figure 6: Effect of aqueous NADES4 dilution (10–100% \(w/w\)) on DPPH radical scavenging activity in \(L\). _japonic_ flower extracts. The violet dash-dotted line indicates antioxidant activity for ethanol extracts, while the light blue dashed line represents antioxidant activity for water extracts. Error bars represent the standard deviation (SD) of three independent measurements. The values with the same lowercase letters (a–d) do not differ significantly (\(\alpha=0.05\)). Other authors have also improved the extraction of antioxidants from \(L\). _japonica_ using DESs. [PERSON] et al. [33] optimized the composition of deep eutectic solvents (DESs), specifically by using tetramethylammonium chloride-ethylene glycol in a 1:3 molar ratio, along with the extraction conditions. The extracts obtained from our proposed method demonstrated enhanced antioxidant activity, yielding between 229.1 and 249.1 umol of Trolox/g d.m. Additionally, they contained higher antioxidant levels, with total phenolic content ranging from 34.2 to 36.5 mg of GAE/g d.m., all achieved in just 5 min without using organic solvents. Moreover, the authors recognized that these results greatly exceed those achieved using the extraction method specified in the Chinese Pharmacopeia with methanol. Furthermore, [PERSON] et al. [34] assessed the extraction of phenylpropanoids from \(L\).