Sodium chloride assists copper release, enhances antibacterial efficiency, and introduces atmospheric corrosion on copper surface
Graphical abstract
Introduction
Antibacterial surfaces are drawing attentions in the recent years [1], [2], [3]. They can be applied as touched surfaces, having enormous potential demands in those heavily microbial burdened healthcare environments [4], [5], [6] as well as in our daily lives [7], [8]. In order to verify and assess the antibacterial ability of newly fabricated surfaces, antibacterial efficiency tests were designed and carried out.
Nowadays, there are different types of evaluation methods such as dry plating [9], wet plating [10], agar disk diffusion [11], etc. Limited by the incubation of bacteria and transfer process of bacterial suspension, a certain medium must be introduced during these evaluations. This medium is usually called buffer, which provides a suitable aqueous environment for bacterial survival without further growth. Despite the wide selection of buffers, in most of the cases, two main requirements should be satisfied: (1) a manageable pH range and (2) a suitable osmotic pressure for the microbes. Take phosphate-buffer saline (PBS) as an example [12], it contains a pair of acid and its conjugate base so as to compensate the pH fluctuation around 7.4. For the second purpose, it is sodium chloride (NaCl) that plays the part.
As can be seen, the design of buffers mainly considers the needs of microbes. But when they are to be applied in antibacterial surface research, a question needs to be raised: should the potential interactions between buffers and surfaces also be considered? The answer may be “certainly”. For example, copper-containing surfaces have been recently developed for antibacterial applications in various occasions [13], [14], [15]. Their antibacterial capability originates from the multifunctional antibacterial effects of copper ions [16]. To strongly interfere the survival of harmful microbes, copper ions should be efficiently released from the surfaces. As metallic surfaces, this release process is, most of the time, achieved by corrosion. The composition and the properties of buffer could become decisive, as they could facilitate or retard the corrosion process. Therefore, the solution and the copper surfaces together display how effective the antibacterial activity would be shown in antibacterial efficiency test.
Back to the example of PBS or other saline solutions, without a doubt, NaCl is expected to play an extremely important factor in revealing the antibacterial efficiency of copper surface. Electrochemical study has investigated the roles of chloride ions (Cl−) in formation and transformation of corrosion product (CuCl, Cu2O, Cu(OH)2, etc.) [17], which could further affect the diffusion of copper ions [18]. In general, the presence of Cl− also leads to a relatively corrosive environment for copper, depending on the concentration of Cl− [19]. Meanwhile, it easily promotes pitting corrosion and further deterioration on the surface [20]. This could, on the contrary, contributes to the copper antibacterial efficiency. Because whenever the corrosion is enhanced, a faster dissolution process of the antibacterial substance (copper ions) is anticipated. For example, a latest study that evaluated antibacterial property of copper-silver surface has discovered the constructive effect of chloride addition in dissolving antibacterial copper ions [21]. Other experiments also suggest that Cu-Fenton chemistry, as one of the antibacterial mechanisms, could be accelerated by Cl− [22], resulting in a better biofilm removal. On the whole, a different antibacterial efficiency is therefore highly expected in Cl−-containing environment.
Other than affecting the corrosion behaviour in media, it is noteworthy that NaCl deposits could induce atmospheric corrosion on dry surfaces. At high humidity levels, for instance, deliquescence of NaCl could happen, initiating micro-droplets formation on metallic surfaces and introducing further corrosion [23]. In lower humidity circumstances (below than 79% RH, deliquescence point of NaCl), where NaCl does not dissolve though, adsorption of water on the surface could form several layers that act as bulk water [24]. Diffusion of Cl− could thus occur in this quasi-aqueous condition resulting in corrosion attack on copper [25]. Either way, tracking this atmospheric corrosion effect after antibacterial efficiency test may have significant implication for long-term product design [26].
Lately, we commenced focusing on buffers by comparing PBS and Na-4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (Na-HEPES) in terms of their impacts on evaluating the antibacterial performance of metallic copper and Cu2O surface [27]. Corrosion behaviours introduced by PBS caught our attention, and therefore further experiments were designed to reveal the roles of bacteria [28] and to discuss the corrosion attacks and oxide growth [29]. Since PBS contains NaCl, this study narrows down to find out if NaCl plays an important factor as expected, in respect to corrosion. The primary objective is to explore how 0.9% saline affects the antibacterial efficiency test on metallic copper surface. Afterwards, the corrosion behaviours were compared among saline with different Cl− or Escherichia coli (E. coli) concentrations. Finally, the following atmospheric aging effects resulting from the residual NaCl crystals on copper surface were also characterised.
Section snippets
Materials
Copper (99.99%, K09, Wieland) coupons were ground with a silicon carbide sandpaper (stepped down to grit number P600), then cleaned with ethanol in an ultrasonic bath, finally dried by air.
Solutions
Saline with various final concentrations (0.45%, 0.9%, and 1.8%) was prepared with NaCl (VWR, Germany) and pure water for analysis (Merck, Germany). PBS was prepared with NaH2PO4. 1H2O (Merck, Germany, final concentration 0.01 M), NaCl (VWR, Germany, final concentration 0.14 M) and pure water for analysis,
Corrosion of copper surface in 0.9% NaCl and pure water
Before analysis of the actual antibacterial efficiency test, this study first focuses on the corrosive effects introduced by 0.9% NaCl (this concentration is hereafter simply referred to as “saline”) without bacteria. For instance, Fig. 1 (a-c) shows the morphological change and formation of corrosion product on the copper surface after 3 h exposure to this environment. Uniformly distributed sub-micron corrosion products are evident in SE image. Meanwhile, at some locations, much lower
Conclusion
NaCl as a common component in many popular buffers, was examined in this work in view of its roles in copper corrosion. Some concluding remarks are summarised as below:
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Ground copper coupon undergoes severe localised corrosion attacks in the environment of 0.9% saline, regardless of whether E. coli is added or not.
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A layer of Cu2O grows on copper surface when being exposed to pure 0.9% saline. In the case of its E. coli suspension, formation of Cu2O has been inhibited at least in 3 h.
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A faster
Data availability
The data that support the findings of this study are available from the corresponding author on request.
CRediT authorship contribution statement
Jiaqi Luo: Conceptualization, Validation, Investigation, Writing - original draft, Visualization, Project administration. Christina Hein: Validation, Investigation, Resources. Jean-François Pierson: Writing - review & editing, Supervision, Funding acquisition. Frank Mücklich: Resources, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was supported by Erasmus Mundus Joint European Doctoral Programme in Advanced Materials Science and Engineering (DocMASE, 512225-1-2010-1-DE-EMJD, European Commission) and the PhD-Track-Programme (PhD02-14, Franco-German University). The ICP-MS experiments were supported by Dr. Ralf Kautenburger from the chair of Inorganic Solid State Chemistry. J. L. particularly thank Prof. Tomáš Prošek from University of Chemistry and Technology, Prague, for his inspiring perspective during
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