Visible-Light Active Flexible and Durable Photocatalytic Antibacterial Ethylene-co-vinyl Acetate-Ag/AgCl/α-Fe2O3 Composite Coating

Abstract

When particles are mixed in polymer, particle surfaces become passivated by polymer matrix, leading to significantly reduced photocatalytic and, thus, also reduced antibacterial activity, as the catalytic particles become isolated from the outer environment and microorganisms reaching the surface. Herein, we demonstrate a facile and rapid approach for coating preparation at room temperature, yielding good adhesion of particles in combination with the particles' interface location. Flexible ethylene-co-vinyl acetate Ag/AgCl/α-Fe₂O₃ composite coatings were prepared by the spin-coating method. The synthesized photocatalytically active coating surface exhibited a distinct and rapid inhibition of bacterial growth, with at least a 7-log reduction of gram-positive bacteria Staphylococcus aureus viability after 30 min of visible-light illumination. We also analyzed the shedding of the Ag-ions and reactive oxygen species production from the composite coating and showed that reactive oxygen species played the main role in the photocatalytic bacterial inactivation, destroying the bacteria cell as proven by the Confocal Laser Scanning Microscopy.

Keywords: antibacterial; coating; hematite; nanoparticles; photocatalysis; silver; silver chloride.

Figure 1

EVA-Ag/AgCl/α-Fe₂O₃ composite coating preparation scheme: (A) EVA polymer spin-coated onto the glass surface and heated for 2 h at 40 °C, (B) composite particles in hexane spin-coated onto the EVA coated surface and heated for 2 h at 40 °C, (C,D) flexible EVA-coating with interface location of composite particles.

Figure 2

XRD pattern of the Ag/AgCl/α-Fe₂O₃ composite. α-Fe₂O₃ nanowires XRD pattern can be found in the Supplementary Material (Figure S1).

Figure 3

FESEM images low resolution (A) and high resolution (B) of EVA- Ag/AgCl/α-Fe₂O₃ coating. A representative EDS spectrum and table with the atomic and weight percentage of elements (C), and elemental mapping of the elements Ag (D), Cl (E), Fe (F), and O (G). FESEM images of hematite nanowires can be found in the Supplementary Material (Figure S1).

Figure 4

Antibacterial efficiency of EVA-, EVA-α-Fe₂O₃, and EVA-Ag/AgCl/α-Fe₂O₃ coated surfaces towards S. aureus in the dark [D], and upon visible-light illumination [L] for 30 min. Image (B) graph shows log reduction values. The mean value of 2 repetitions with 3 samples in each ± standard deviation (SD) is reported.

Figure 5

Reusability of EVA-Ag/AgCl/α-Fe₂O₃ coating. S. aureus > 7-log reduction (full bacteria reduction) was obtained after 30 min of visible-light illumination in all 3 cycles (A). Additionally, S. aureus > 7-log reduction (full bacteria reduction) remained unchanged after coatings touching 500, 1000, and 5000 times (B), and 30 min ultrasound washing (C) after 30 min of visible-light illumination. The mean value of 2 repetitions with 3 samples in each (cycles) and 3 repetitions ± standard deviation (SD) is reported.

Figure 6

Generation of reactive oxygen species (ROS) measured with fluorescent dye DCF-DA in abiotic conditions (i.e., without the bacterial cells). Relative fluorescence units (RFUs) due to ROS production in the liquid in contact with EVA-coating, EVA-hematite coating, and EVA-Ag/AgCl/α-Fe₂O₃ composite coating in the dark [D], and upon visible-light illumination [L].

Figure 7

Representative Confocal Laser Scanning Microscopy (CLSM) images of Staphylococcus aureus suspension after 30 min exposure to the different surfaces in the dark (A–D) and under the visible-light illumination (E–H): viable cells—green (Syto9); dead or membrane damaged cells—red (PI). Scale bars represent 20 µmImages of bacteria growth test in tubes can be found in the Supplementary Material (Figure S2).

Figure 8

Representative Confocal Laser Scanning Microscopy (CLSM) images of live/dead staining of S. aureus on the coating surfaces of EVA-composite in the dark (A) and after the visible-light illumination (B) for 30 min. Viable cells—green (Syto9); dead or membrane damaged cells—red (PI). Scale bars represent 20 µm.