Superhydrophilicity and antibacterial property of a Cu-dotted oxide coating surface

Abstract

Background: Aluminum-made settings are widely used in healthcare, schools, public facilities and transit systems. Frequently-touched surfaces of those settings are likely to harbour bacteria and be a potential source of infection. One method to utilize the effectiveness of copper (Cu) in eliminating pathogens for these surfaces would be to coat the aluminum (Al) items with a Cu coating. However, such a combination of Cu and Al metals is susceptible to galvanic corrosion because of their different electrochemical potentials.

Methods: In this work, a new approach was proposed in which electrolytic plasma oxidation (EPO) of Al was used to form an oxide surface layer followed by electroplating of Cu metal on the top of the oxide layer. The oxide was designed to function as a corrosion protective and biocompatible layer, and the Cu in the form of dots was utilized as an antibacterial material. The antibacterial property enhanced by superhydrophilicity of the Cu-dotted oxide coating was evaluated.

Results: A superhydrophilic surface was successfully prepared using electrolytic plasma oxidation of aluminum (Al) followed by electroplating of copper (Cu) in a Cu-dotted form. Both Cu plate and Cu-dotted oxide surfaces had excellent antimicrobial activities against E. coli ATCC 25922, methicillin-resistant Staphylococcus aureus (MRSA) ATCC 43300 and vancomycin-resistant Enterococcus faecium (VRE) ATCC 51299. However, its Cu-dotted surface morphology allowed the Cu-dotted oxide surface to be more antibacterial than the smooth Cu plate surface. The enhanced antibacterial property was attributed to the superhydrophilic behaviour of the Cu-dotted oxide surface that allowed the bacteria to have a more effective killing contact with Cu due to spreading of the bacterial suspension media.

Conclusion: The superhydrophilic Cu-dotted oxide coating surface provided an effective method of controlling bacterial growth and survival on contact surfaces and thus reduces the risk of infection and spread of bacteria-related diseases particularly in moist or wet environments.

Figure 1

Red wines on test coupons: Cu (middle), Al (right) and Cu/oxide coating (left). (a, b) drops of red wines at the beginning, (c) after 12 hours, and (d) after being run through water.

Figure 2

SEM micrographs and EDX analysis of the Cu-dotted oxide coating. (a) Cu-dotted oxide coating, (b) cauliflower-like Cu dots, (c) single Cu dot, (d) EDX obtained from the oxide layer, and (e) EDX obtained from a Cu dot.

Figure 3

SEM micrograph and EDX analysis of the Cu-dotted oxide coating. (a) A Cu dot with bacteria and (b) EDX obtained from the dark area (as indicated by the arrow in (a)).

Figure 4

Bacterial reduction (BR) vs. time of the Cu plate, Al plate and Cu-dotted oxide coating tested with (a) E. coli ATCC 25922, (b) MRSA ATCC 43300 and (c) E. faecium ATCC 51299. The oxide coatings without Cu were also included in (b) and (c).