Mechano-Bactericidal Surfaces Achieved by Epitaxial Growth of Metal-Organic Frameworks

Abstract

Mechano-bactericidal (MB) surfaces have been proposed as an emerging strategy for preventing biofilm formation. Unlike antibiotics and metal ions that chemically interfere with cellular processes, MB nanostructures cause physical damage to the bacteria. The antibacterial performance of artificial MB surfaces relies on rational control of surface features, which is difficult to achieve for large surfaces in real-life applications. Herein, a facile and scalable method is reported for fabricating MB surfaces based on metal-organic frameworks (MOFs) using epitaxial MOF-on-MOF hybrids as building blocks with nanopillars of less than 5 nm tip diameter, 200 nm base diameter, and 300 nm length. Two methods of MOF surface assembly, in situ growth and ex situ dropcasting, result in surfaces with nanopillars in different orientations, both presenting MB actions (bactericidal efficiency of 83% for E. coli). Distinct MB mechanisms, including stretching, impaling, and mechanical injury, are discussed with the observed bacterial morphology on the obtained MOF surfaces.

Keywords: MOF‐on‐MOF; antibacterials; biofilm; mechano‐bactericidal surface; metal–organic framework.

Figure 1

MOF mechano‐bactericidal (MB) surfaces. Caltrop‐like MIL‐88B‐on‐UiO‐66 (MoU) hybrids are used as the building blocks for MB surfaces through two assembly methods. a) Schematic showing MoU surfaces through in situ growth: 1. substrate, 2. in situ growth of UiO‐66 on the substrate, 3. epitaxial growth of MIL‐88B‐on‐UiO‐66 (MoU); b) Schematic illustration of the one‐pin up orientation in in situ MoU surfaces; SEM images of c) in situ UiO‐66 surface and d) in situ MoU surface after epitaxial growth of MIL‐88B; e) TEM image of MoU hybrid with one‐pin up orientation; f) Schematic showing MoU surfaces through ex situ dropcasting, 1. MoU hybrids through epitaxial growth, 2. Dropcasting MoU to the substrate; g) Schematic illustration of the four‐pin up orientation in dropcast MoU surfaces; SEM images of dropcast h) UiO‐66, i) MIL‐88B, and j) MoU, zoomed‐in SEM image false‐colored with UiO‐66 in blue and MIL‐88B in pink; k) TEM image of MoU hybrid with a four‐pin up orientation. Scale bar: 200 nm.

Figure 2

Material characterization of MOF MB surfaces. a) XRD patterns of in situ and dropcast MoU surfaces, UiO‐66, MIL‐88B, and their simulated patterns. The characteristic peaks of UiO‐66 (blue‐octahedron) and MIL‐88B (pink‐rod) are marked at corresponding positions; b) XPS survey spectra of the obtained MOF surfaces with C, O, Fe, and Zr characteristic peaks marked; c) The water contact angle of the silicon substrate and the in situ MoU surfaces showing the enhancement of hydrophobicity after MOF coating. The AFM scanning and cross‐section profile of d) in situ UiO‐66, providing a horizontal plane for MIL‐88B epitaxial growth e) in situ MoU, showing one‐pin up orientation with a near perpendicular MIL‐88B nanopillar, and f) dropcast MoU surfaces, showing four‐pin up orientation with random angles of the MIL‐88B nanopillars, scale bar: 200 nm.

Figure 3

Antibacterial performance of MOF MB surfaces. a) CFU counting results of attached E. coli with 24h growth on in situ MoU surfaces and dropcasting surfaces, including MoU, UiO‐66, MIL‐88B, and UiO‐66 + MIL‐88B (U+M). Data represent the mean ± standard deviation of three biological replicates (^*** p < 0.001). b) The live/dead fluorescent staining images of attached E. coli with 24h growth on in situ and dropcasting MoU surfaces, green indicating live bacteria and red indicating dead bacteria, scale bar: 10 µm. c) SEM images of 1) in situ MoU and 2) dropcast MoU surfaces with uncovered areas marked in red, scale bar: 200 nm. 3) The bar chart of the uncovered area percentage of in situ and dropcast MoU surfaces based on three different regions.

Figure 4

Mechanism study of the MOF MB surfaces. SEM images of attached bacteria on control, in situ MoU, and dropcast MoU surfaces with 24h growth: a) E. coli; b) S. epidermidis. Stress contour of bacteria on in situ MoU surfaces: c) E. coli; d) S. aureus. e) Illustration of bacterial rupture by MoU surfaces. f) SEM images of MoU impaling E. coli and S. epidermidis, false‐colored with bacteria in yellow, UiO‐66 in blue, and MIL‐88B in pink. Scale bar: 200 nm.