Metal-organic frameworks with photocatalytic bactericidal activity for integrated air cleaning

Abstract

Air filtration has become an essential need for passive pollution control. However, most of the commercial air purifiers rely on dense fibrous filters, which have good particulate matter (PM) removal capability but poor biocidal effect. Here we present the photocatalytic bactericidal properties of a series of metal-organic frameworks (MOFs) and their potentials in air pollution control and personal protection. Specifically, a zinc-imidazolate MOF (ZIF-8) exhibits almost complete inactivation of Escherichia coli (E. coli) (>99.9999% inactivation efficiency) in saline within 2 h of simulated solar irradiation. Mechanistic studies indicate that photoelectrons trapped at Zn⁺ centers within ZIF-8 via ligand to metal charge transfer (LMCT) are responsible for oxygen-reduction related reactive oxygen species (ROS) production, which is the dominant disinfection mechanism. Air filters fabricated from ZIF-8 show remarkable performance for integrated pollution control, with >99.99% photocatalytic killing efficiency against airborne bacteria in 30 min and 97% PM removal. This work may shed light on designing new porous solids with photocatalytic antibiotic capability for public health protection.

Fig. 1

Schematic of metal-organic framework (MOF)-based filter. Schematic representation of MOF-based filter (MOFilter) for integrated air cleaning

Fig. 2

Photocatalytic disinfection performance of ZIF-8 (zinc-imidazolate MOF). a Disinfection performance comparison among five metal-organic frameworks (MOFs). b Inactivation kinetics of E. coli in the presence of ZIF-8. c Inactivation efficiency against E. coli in the presence of Zn²⁺ (3 mg L⁻¹), 2-methylimidazole (H-MeIM) (7 mg L⁻¹), and ZIF-8 (500 mg L⁻¹), respectively. d Disinfection performance comparison among ZIF-8, TiO₂, and ZnO under-simulated solar irradiation. In the disinfection performance, the error bars are calculated via repeating the measurements for three times and the black circle represents no measurable levels of bacteria in the culture medium

Fig. 3

Band-structure characterization and photocatalytic disinfection mechanism of ZIF-8 (zinc-imidazolate MOF). a The band positions of ZIF-8 with respect to the reactive oxygen species (ROS) formation potential. Conduction band (CB) and valence band (VB) represent conduction band and valence band respectively. b Electron paramagnetic resonance (EPR) spectra of ZIF-8 at 77 K in dark and under light irradiation (300 nm < λ < 1100 nm) in different atmosphere. c EPR spectra of DMPO−•O₂⁻ for ZIF-8 under light irradiation and in dark. d Steady-state concentration of •O₂⁻ calculated from the decay of nitroblue tetrazolium (NBT) and hydrogen peroxide (H₂O₂) accumulation over time, respectively. e The first-order disinfection rate on ZIF-8 with different scavengers (IPA  → •OH, l-His → ¹O₂, Cr(VI) → e⁻, Oxalate → h⁺, SOD → •O₂⁻, CAT → H₂O₂). f Dependence of the amount of released H₂O₂ by ZIF-8 on the wavelength of incident light and the ultraviolet–visible (UV–vis) spectra of ZIF-8. The error bars are calculated via repeating the measurements for three times. SOD superoxide dismutase, IPA isopropanol, DMPO 5,5-diemthyl-1-pyrroline N-oxide

Fig. 4

Characterization of metal-organic framework (MOF)-based filter (MOFilter) and its air cleaning performance. a X-ray diffraction (XRD) patterns of non-woven fabric (NWF) and MOFilter. b Optical photo and scanning electron microscopy (SEM) images of MOFilter (scale bar, 5 μm (top); 1 μm (bottom)). c Schematic representation of the air cleaning system. d Comparison of the particulate matter (PM) filtration efficiency between MOFilter and NWF. e Comparison of the air disinfection performance between MOFilter and NWF under light and dark conditions, respectively. f Air disinfection performance of MOFiter continuously used for five cycles. The error bars are calculated via repeating the measurements for three times

Fig. 5

Antibacterial performance comparison between metal-organic framework (MOF)-based filter (MOFilter) mask (MM) and commercial mask (CM). a Bioaerosol generation apparatus and optical images of trilaminar MOFilter mask. b, c Escherichia coli levels residual on top, inner and bottom layers of MM and CM, respectively, after 30 min of light irradiation. The number of viable cells in b is determined from saline eluent used for collecting living bacteria on each layer of mask after reaction. The bacterial colonies residual on eluent-treated mask are shown in c. The top, inner, and bottom layers are denoted as T, I and B, respectively. The error bars are calculated via repeating the measurements for three times