Recent Advances in Metal-Based Antimicrobial Coatings for High-Touch Surfaces

Abstract

International interest in metal-based antimicrobial coatings to control the spread of bacteria, fungi, and viruses via high contact human touch surfaces are growing at an exponential rate. This interest recently reached an all-time high with the outbreak of the deadly COVID-19 disease, which has already claimed the lives of more than 5 million people worldwide. This global pandemic has highlighted the major role that antimicrobial coatings can play in controlling the spread of deadly viruses such as SARS-CoV-2 and scientists and engineers are now working harder than ever to develop the next generation of antimicrobial materials. This article begins with a review of three discrete microorganism-killing phenomena of contact-killing surfaces, nanoprotrusions, and superhydrophobic surfaces. The antimicrobial properties of metals such as copper (Cu), silver (Ag), and zinc (Zn) are reviewed along with the effects of combining them with titanium dioxide (TiO₂) to create a binary or ternary contact-killing surface coatings. The self-cleaning and bacterial resistance of purely structural superhydrophobic surfaces and the potential of physical surface nanoprotrusions to damage microbial cells are then considered. The article then gives a detailed discussion on recent advances in attempting to combine these individual phenomena to create super-antimicrobial metal-based coatings with binary or ternary killing potential against a broad range of microorganisms, including SARS-CoV-2, for high-touch surface applications such as hand rails, door plates, and water fittings on public transport and in healthcare, care home and leisure settings as well as personal protective equipment commonly used in hospitals and in the current COVID-19 pandemic.

Keywords: SARS-CoV-2; antimicrobial; coating; high-touch surface; nanoprotrusion; superhydrophobic.

Figure 1

Cu-based antimicrobial coated commercial products. Adopted from public internet platforms: (A–D) [13], (E,F) [14], (G,H) [15], (I) [16], (J) [17].

Figure 2

Contact killing mechanism of (A) Cu; (B) Ag; (C) ZnO and (D) TiO₂. Reproduced with permission from: (A) [26], (B) [27], Adopted from (C) [28] (D) [29].

Figure 3

Role of Cu in antimicrobial activity: (A) effect of Cu particle size against S. aureus ATCC 25923; (B) effect of Cu concentration and humidity against E. coli M-17; (C) effect of dry and (D) moist environments against P. aeruginosa ATCC 15442. Reproduced with permissions: (A) [39], Adopted from: (B) [40], (C) and (D) [41].

Figure 4

Role of Ag in antimicrobial activity: (A) effect of Ag nanoparticle size, suggesting 10 nm size gives optimum antimicrobial efficacy against V. natriegens; (B) effect of Ag concentration and morphology suggest that the concentration of Ag particles have a significant effect on the viability of mesenchymal stem cells. Whereas the morphology of Ag particles such as rod, sphere, cubes, etc., also influence cell viability but not as significantly as Ag concentration. Whereas, ( * ) presents the significance in differences when compared with control i.e., * p < 0.05; ** p < 0.005; and *** p < 0.001. Adapted from: (A) [51], (B) [52].

Figure 5

Role of ZnO in antimicrobial activity: (A) effect of ZnO concentrations of 0.1 mg/mL (sample 1), 0.3 mg/mL (sample 2), 0.5 mg/mL (Bulk) and (B) effect of ZnO nanoparticle size on antibacterial performance against S. aureus. Adapted from: (A) [76] Reproduced with permission from: (B) [80].

Figure 6

(A) performance of TiO₂ nanoparticles in combination with Cu and Ag particles and (B) influence of TiO₂ particle size and concentration on E. coli viability. Reproduced with permission from: (A) [95], Adapted from: (B) [96].

Figure 7

Examples of superhydrophobic surfaces in nature: (a) SEM image of lotus leaf at two different magnifications; (b) SEM images of springtail showing hierarchical structures and (c) Optical image of an interaction of a 10 μL water droplet on a termite wing and a topographical view of the wing membrane. Reproduced with permission from: (a) [8], Adapted from: (b) [9], (c) [10].

Figure 8

(a) Optical and SEM images of laser-fabricated graphene mask (scale bar = 10 μm) and the resulting water contact angle of 141° on the mask; (b) FE-SEM images of hydrophobic nanoporous Anodic Aluminium Oxide (AAO) transformed into nano pillared AAO with extended post etching (inset shows improvement in water contact angle) and (c) SEM image of a film cast from a block copolymer micelle solution (0.01 g/mL) (inset shows the water contact angle > 160°). Reproduced with permission from: (a) [161], (b) [162], (c) [164].

Figure 9

(a) SEM images of a bare Stainless Steel substrate, laser-etched and with the PDA@ODA modification (inset shows water contact angles), corresponding high magnification and Laser confocal microscopy images, and plots of colony-forming units per mL (CFU/mL) for E. coli and S. aureus bacteria with a concentration of PDA@ODA compound [Reproduced with permission from reference no. S.Li et al., 2019] (b) As-prepared ZnO nanorod films at low and high magnifications (inset shows the water droplet shape before (left) and after (right) UV illumination) [Reproduced with permission from reference no. X.Feng et al., 2003] (c) Low and High magnification images of TiO₂ nanorod film (inset shows contact angle of 154°). Reproduced with permission from: (a) [165], (b) [166], (c) [167].

Figure 10

(a) Structural physiology of the forewing of cicada and SEM image of a P. aeruginosa cell sinking between the nanopillars on the wing surface (scale bar = 200 nm); (b) Optical image of wings from common sanddragon dragonfly; (c). SEM images of four E. coli bacteria attached to the uncoated nanopillar surface of a dragonfly wing in progressive stages of death and a red arrow marking the darker region formed by leakage of cellular fluid flooding the nanopillars (scale bar = 200 nm) and (d) 3-D biophysical model of the interactions between cicada wing nanopillars and rod-shaped bacterial cells. Reproduced with permission from: (a) [168], (b) [178], (c) [171], (d) [179].

Figure 11

SEM images showing surface patterns of (a) black Si and (b) dragonfly forewings (measured nanopillars highlighted by red line, magnification = 35 k, scale bar = 200 nm), (c) SEM images and confocal laser scanning micrographs show P. aeruginosa cells are significantly disrupted through interaction with both the dragonfly wing and black Si (scale bars = 200 nm) and (d) Bactericidal efficiency of black Si and dragonfly wings on various bacterial strains. Reproduced with permission from [169].

Figure 12

SEM images of (a) TiO₂ nanopillar surface developed on Ti₆Al₄V substrate; (b) K. pneumoniae bacteria envelope interaction with Ti alloy control compared to deformation induced by TiO₂ nanopillar surface; (c) Brightfield TEM and a 3D reconstruction of the tomogram, showing multiple nanopillars penetrating the bacterial envelope; and (d) Colony-forming units (CFU) determined for K. pneumoniae when incubated on the Ti alloy control or TiO₂ nanopillar surfaces for up to 10 h (*** indicates p ≤ 0.001 relative to control, as determined by one-way ANOVA and Tukey-HSD post hoc test). Adapted from [7].

Figure 13

A schematic of our proposed transmission combating strategy: (a) Virus encapsulation, (b) contamination suppression, and (c) virus elimination. Reproduced with permission from [185].

Figure 14

Comparison of the E. coli killing effect over time seen for superhydrophobic and superhydrophilic Cu surfaces prepared by nanosecond laser processing. Reproduced with permission from [186].

Figure 15

Effect of the bacteria surface density on the antibacterial activity for the 20th day of the test. M_i represents the change in fluorescence intensity caused by the increase or decrease in the bacterial population. The M_i of the “hybrid” SH+Cu surface remains low even for densities up to 13.4 × 10⁸ colony-forming units per cm² (CFU/cm²). Reproduced with permission from [187].

Figure 16

Total number of S. aureus colonies on different specimens after incubating for 1 and 5 days. (Data are shown as mean ± SD, n = 3, * represents p < 0.05 compared with Ti control surface). Reproduced with permission from [192].

Figure 17

E. coli growth inhibition in presence of: 1—TiO₂/SiO₂; 2—TiO₂/SiO₂/Ag_9.7 at.%; 3—TiO₂/SiO₂/Ag_14.4 at.%; 4—TiO₂/SiO₂/Ag_19.8 at.%; control—in absence of sample. Reproduced with permission from [194].

Figure 18

(a) Zn leached from ZnO nanoparticle Zn²⁺/Cel-6 film at different times. (b) bacteria inhibition rate of different Zn²⁺ concentrations for S. aureus, E. coli, and C. albicans. (c) bacteria inhibition rate of ZnO nanoparticle Zn²⁺/Cel-6 film not treated by UV for S. aureus, E. coli, and C. albicans. SEM images of (d) E. coli (bacteria) and (e) C. albicans (fungus) cells before and after mechanical rupture (without UV treatment) and photocatalytic oxidation inactivation of ZnO nanoparticle Zn²⁺/Cel-6 film respectively. (f) Schematic of antimicrobial mechanism of ZnO nanoparticle Zn²⁺/Cel films. Reproduced with permission from [196].

Figure 18

(a) Zn leached from ZnO nanoparticle Zn²⁺/Cel-6 film at different times. (b) bacteria inhibition rate of different Zn²⁺ concentrations for S. aureus, E. coli, and C. albicans. (c) bacteria inhibition rate of ZnO nanoparticle Zn²⁺/Cel-6 film not treated by UV for S. aureus, E. coli, and C. albicans. SEM images of (d) E. coli (bacteria) and (e) C. albicans (fungus) cells before and after mechanical rupture (without UV treatment) and photocatalytic oxidation inactivation of ZnO nanoparticle Zn²⁺/Cel-6 film respectively. (f) Schematic of antimicrobial mechanism of ZnO nanoparticle Zn²⁺/Cel films. Reproduced with permission from [196].

Figure 19

Schematic diagram illustrating the synthesis procedure of ZnO nano-spines on an activated-carbon nanofiber (ZnO/ACNF) structure. Reproduced with permission from [198].