## 2 Results and Discussion
### Recovery of Polyphenols from mPEG-Grafted Silica Particles  
3.1 Screening Solvent Systems Coupled with Reagents That Can Break Hydrogen Bonding for the Recovery of Polyphenols from mPEG-Grafted Silica Particles  
Effect of Sorbitol and Salt in Aqueous Ethanol on Polyphenol Recovery  
Figure 4 shows the desorption ratios of various solvent systems. When sorbitol or NaCl was individually added to 30% aqueous ethanol, the desorption ratios increased by \(\sim\)8-fold in comparison to 30% aqueous ethanol, yet the desorption ratios were at \(\sim\)13% level, which is likely considered as a noneffective recovery process for polyphenols. The stronger elution power, seen with the addition of \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **C (wt \%)** & **H (wt \%)** & **N (wt \%)** & **Surface Area** & **Total Pore Volume** \\  & & & **(m\({}^{2}\)/g)** & **(cm\({}^{3}\)/g)** \\ \hline
**Bare Silica** & 1.02 \(\pm\) 0.01 & 0.50 \(\pm\) 0.01 & 0.07 \(\pm\) 0.05 & 270.74 \(\pm\) 1.45 & 0.869 \\
**mPEG-2000** & 2.28 \(\pm\) 0.02 & 0.62 \(\pm\) 0.00 & 0.08 \(\pm\) 0.01 & 248.29 \(\pm\) 1.35 & 0.806 \\
**mPEG-5000** & 5.02 \(\pm\) 0.04 & 1.24 \(\pm\) 0.02 & 0.11 \(\pm\) 0.00 & 234.83 \(\pm\) 1.35 & 0.768 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Elemental composition and physical characteristics of bare and mPEG-grafted silica particles. NaCl to the aqueous ethanol, can be partially explained by the salt effect seen in the reverse phase chromatography [43]. When NaCl and sorbitol were added to 70% aqueous ethanol, desorption ratios increased by \(\sim\)4- and 7-fold in comparison to 70% aqueous ethanol, whereas combined sorbitol/NaCl increased the desorption ratios by \(\sim\)11-fold, resulting in a desorption ratio of 36.1% (Figure 4). We attempted to use hydrogen bond breaker reagents to release the polyphenols strongly bonded to mPEG-grafted silica particles. Sorbitol is a monosaccharide with six hydrogen donors and acceptors and is commonly used as a biomolecule stabilizer. Sorbitol was previously used to elute large molecule biologics presumably by breaking the hydrogen bonds between the resin and the biomolecule of interest [44]. Moreover, in combination with salts, the elution power was strengthened [44]. In analogy to elution of biologics in hydrogen bonding chromatography [44], sorbitol and NaCl was used in an aqueous ethanol binary solvent system. Although the desorption ratio of 100 mM NaCl + 200 mM sorbitol in 70% aqueous ethanol was promising, further optimization of sorbitol concentration was not performed due to the viscosity constraints. Effect of Acid and Base on Polyphenol Recovery  
Acidic and basic conditions were screened to test the ability of these conditions to release the polyphenols from mPEG-grafted silica particles. Both 1% HCl and 1% NaOH in 70% aqueous ethanol showed poor desorption ratios (\(<\)14%) (Figure 4), whereas 1% HCl showed \(\sim\)2-fold higher desorption ratios compared to 1% NaOH, suggesting that acidic conditions are more favorable than basic for the release of polyphenols. However, further optimization was not performed with the strong acids due to the concerns about leaching mPEG ligands from the surface of silica particles [41]. In contrast to strong acids, previous studies using a weak acid--citric acid--showed promising elution performance for the recovery of a model polyphenol--quercetin--from titania-modified silica particles [41]. Citric acid was also supplemented to an extraction solvent used to isolate rutin from _Sophora japonica_[45]. This work, therefore, aimed to increase the desorption ratio by supplementing anhydrous ethanol with varying concentrations of citric acid (Figure 4). Unlike quercetin recovery from titania-modified silica particles using 10 and 20% citric acid in ethanol [41], only \(\sim\)12 to 15% desorption ratios were observed when polyphenols were eluted from mPEG-grafted silica particles (Figure 4).