Plant-inspired quercetin thin films: universal coatings and their postfunctionalization for non-biofouling applications

Polyphenols containing catechol (1,2-dihydroxyphenyl)/pyrogallol (1,2,3-trihydroxyphenyl) groups have been employed as precursors to prepare substrate-independent (i.e., universal) coatings. However, quercetin, an interesting flavonol due to its unique biological properties, was previously found to be an inefficient compound for substrate coatings after screening polyphenolic compounds. We report that quercetin-based thin films could be formed on various substrates, including titanium dioxide, silicon wafers, gold, nylon, and glass, when diethylenetriamine (DETA) was used as an additive. DETA acted as a cross-linker to facilitate conjugate addition and imine formation with the catechol in quercetin. The film thickness was controllable by changing the quercetin concentration. Static water contact angles for all examined substrates converged to 25 degrees regardless of the vastly different static water contact angles of the bare substrates, showing the universal coating capability of quercetin and DETA. Importantly, the films formed with quercetin and DETA were chemically modifiable; thus, specific functions on the surface could be introduced. alpha-Bromoisobutyryl bromide was immobilized on the films via esterification, and then a zwitterionic sulfobetaine polymer was subsequently grafted. Compared to the controls, the polymer-grafted surface effectively suppressed the adsorption of fibrinogen and platelets, demonstrating its non-biofouling effect. Considering its universal coating and postfunctionalization capabilities, this quercetin-based film provides a route for the preparation of functional organic coatings.