*3.1. Synthesis of the POSS–EGCG Conjugate*

To prepare the POSS–EGCG conjugate, EGCG was conjugated to API-POSS via a one-step reaction using HRP as a catalyst (Figure 1). The structure of the resultant POSS–EGCG conjugate was characterized by 1H NMR spectroscopy (Figure S1). The characteristic signals belonging to EGCG units were observed at 6.81, 6.41, 5.95, 5.84, 5.36, 4.98, and 2.77 ppm. Moreover, the peaks due to API-POSS units in the POSS–EGCG conjugate were shown at 2.69, 1.82, 1.58, 0.95, and 0.54 ppm [20,32]. The semiquinone radicals or active oxygen species can easily oxidize either the gallyl moiety (B-ring) or

the gallate moiety (D-ring) of EGCG, resulting to transform EGCG into reactive species with a quinone structure [33]. However, gallyl structures are more susceptible to oxidation than gallate structures and they only form catechol quinones through intermediate semiquinones [34]. In addition, the presence of a reactive amino group on API-POSS can provide a site for enzymatic conjugation of catechol through electrophilic addition [35]. The conjugation ratio of EGCG that was introduced to API-POSS was calculated from the result of elemental analysis to be 0.96.

**Figure 1.** Schematic diagram of enzymatic synthesis of the polyhedral oligomeric silsesquioxane– epigallocatechin gallate (POSS–EGCG) conjugate.
