Facile Route to Effective Antimicrobial Aluminum Oxide Layer Realized by Co-Deposition with Silver Nitrate

Abstract

The emergence and spreading of the SARS-CoV-2 pandemic has forced the focus of attention on a significant issue: the realization of antimicrobial surfaces for public spaces, which do not require extensive use of disinfectants. Silver represents one of the most used elements in this context, thanks to its excellent biocidal performance. This work describes a simple method for the realization of anodized aluminum layers, whose antimicrobial features are ensured by the co-deposition with silver nitrate. The durability and the chemical resistance of the samples were evaluated by means of several accelerated degradation tests, such as the exposure in a salt spray chamber, the contact with synthetic sweat and the scrub test, highlighting the residual influence of silver in altering the protective behavior of the alumina layers. Furthermore, the ISO 22196:2011 standard was used as the reference protocol to set up an assay to measure the effective antibacterial activity of the alumina-Ag layers against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria, even at low concentrations of silver. Finally, the Ag-containing aluminum oxide layers exhibited excellent antimicrobial performances also following the chemical–physical degradation processes, ensuring good durability over time of the antimicrobial surfaces. Overall, this work introduces a simple route for the realization of anodized aluminum surfaces with excellent antibacterial properties.

Innovative Codeposition of a Ag–Al2O3 Layer: An Attractive Combination of High Durability and Lack of Cytotoxicity for Public Space Applications

“…Definitely, silver nitrate, which is easily dissolved in the sulfuric acid bath, does not influence the growth of the aluminum oxide layer. This result was already observed in a previous literature work, 37 which studied in depth the silver deposition process during the anodization, that occurs by self-reduction in proximity to interstitial elements, such as silicon and manganese, present in the aluminum alloy. Figure 2 reveals this phenomenon: silicon and manganese counteract the growth of the oxide layer but at the same time favor the reduction of silver, which is trapped in the cavities of the Al 2 O 3 film.…”

“…The addition of silver in the aluminum oxide layer was achieved by a single-step codeposition process. The samples were realized following a procedure described in a previous study, 37 introducing the silver nitrate directly into a sulfuric acid anodization bath. First, the aluminum plates (80 mm × 60 mm × 2 mm dimensions) were subjected to a pretreatment process, consisting of an etching step of 300 s in 5 wt % NaOH solution, followed by a 30 s desmutting step in 10% v/v HNO 3 .…”

“…A previous work 37 has highlighted the degradation these surfaces undergo in contact with a solution-simulating human sweat. In this study, the authors focused on evaluating the silver ion release values by the composite layer exposed to this solution to assess the level of danger of multiple or prolonged contact of the samples with human skin.…”

“… 36 The latter method seems to be the most encouraging, as the recent work has highlighted the excellent bactericidal performance of the surfaces obtained by codeposition with silver nitrate. 37 …”

“…Thus, this work focuses on the characterization of the durability behavior of silver-containing anodized aluminum surfaces, whose bactericidal performances have already been assessed. 37 In detail, the surfaces of aluminum oxide layers were exposed to aggressive environments, with particular thermal changes, UV radiation, and sulfur dioxide atmosphere, evaluating the effect introduced by silver in modifying their durability. Thus, the release of silver ions was assessed by placing the samples in contact with a lactic acid solution simulating human sweat.…”