Thermo-responsive water purification: a thermo-switchable molecular brush for precision engineering of antibacterial ZnO on agro-waste filters

Abstract

Waterborne pathogens remain a major threat to public health, highlighting the need for sustainable antibacterial filtration materials. Herein, we report a thermo-responsive biomass-based filter constructed by grafting poly(N-acryloyl glycinamide) (PNAGA) molecular brushes onto delignified maize stalk pith (DMSP), followed by in situ growth of ZnO nanostructures. The grafted PNAGA layer exhibits upper critical solution temperature (UCST)-type behavior, producing temperature-dependent swelling and interfacial hydration changes that influence ZnO nucleation and growth on the porous scaffold. Compared with ungrafted DMSP, DMSP-g-PNAGA promotes more distinct ZnO morphologies under different synthesis temperatures, including needle-like structures at 25 °C and hierarchical flower-like assemblies at 40 °C. HRTEM analysis revealed representative (001)-related lattice fringes in ZDP-T25-t490 and (100)-related lattice fringes in ZDP-T40-t490, while DFT/ESP calculations suggest that temperature-dependent PNAGA conformations alter their preferential interactions with ZnO surfaces. The optimized ZDP-T40-t490 filter achieved high ZnO loading of 659.7 mg g⁻¹ and strong initial antibacterial performance, with LRV values of 5.96 ± 0.12 against Escherichia coli (E. coli) and 5.87 ± 0.15 against Staphylococcus aureus (S. aureus). The antibacterial activity is attributed to the combined effects of bacterial retention within the labyrinthine pores, Zn²⁺ release, membrane damage, intracellular oxidative stress, and morphology-assisted ZnO-bacteria contact. Repeated-filtration and Zn leaching tests further indicate that long-term stability and initial Zn release remain important limitations. This work provides a proof-of-concept strategy for integrating agro-waste-derived porous scaffolds, UCST-responsive polymer brushes, and antibacterial ZnO nanostructures, while further optimization is required before practical water-treatment application.