New evidence for Cu-decorated binary-oxides mediating bacterial inactivation/mineralization in aerobic media
Graphical abstract
The photo-generated ROS and Cu/Cu-ions damage the E. coli cell envelope and subsequently enter the cytoplasm leading to cell killing.
Introduction
Resistance of bacteria to antibiotics is a serious health problem leading specially during the last decade to an increase in the hospital-acquired infections (HAI). Therefore, the development of innovative antibacterial surfaces presenting long-term effective operational lifetimes, biocompatibility and mechanical resistance is a timely research subject. Antibacterial Cu-colloids have been reported during the last 3 decades showing their antibacterial activity [1]. Bacteria, yeasts, and viruses have been abated on metallic copper surfaces, and the term “contact killing” has been coined for this process. The use of copper has shown a potential antibacterial response in health care settings. Contact killing was observed to take place at a rate of at least 7–8 logs per hour, and no live microorganisms were recovered from copper surfaces after prolonged incubation. Copper has recently been registered at the U.S. Environmental Protection Agency as the first solid antimicrobial material. Cu-cell have been shown to damage the cell envelope through the release of Cu-ions. [2]. Recent work has shed light on mechanistic aspects of contact killing. These findings will be reviewed here and juxtaposed with the toxicity mechanisms of ionic copper. The merit of copper as a hygienic material in hospitals has been extensively reported to preclude/decrease viral, nosocomial infections caused by antibiotic resistant bacteria by textile fabrics impregnated by colloidal Cu by Borkow and Gabbay [3]. More recently Cu-ions have been reported to be biocidal binding to specific sites in the DNA-phosphate destroying the DNA double helix or damaging the bacterial cell wall. The cell wall envelope proteins and lipids were damaged by contact with Cu-surfaces, but the contact killing was not necessarily related to lethal damages to the DNA. When the bacterial cell wall is damaged, the Cu-ions enter the cytoplasm causing metabolical disruption [4].
Sol-gel commercial preparations using Ag have been widely reported during the last few years. These sol-gel suspensions were subsequently annealed on heat resistant substrates [6]. But the thicknesses of these Ag-films and in some cases of Cu-films were not reproducible, they were not mechanically stable, they exhibited low adhesion and could be wiped off by a cloth or by hand contact [6]. The colloid deposition on substrates require temperatures of few hundred degrees for an adequate adherence to the substrate and this will not work on low thermal resistant substrates like PES. This moved us to work on the preparation of sputtered antibacterial films to overcome the shortcomings of colloidal loaded films and this is one of the main points addressed in this study.
This study reports new findings by TiO2-ZrO2 and TiO2-ZrO2-Cu films on PES following our first study on these materials [7]. These materials were reported recently as catalysts and for their use in optical/electronic devices. The films were reported by sol-gel and later calcined on substrates resisting higher temperatures [8], [9]. By inductively coupled plasma mass-spectrometry (ICP-MS) we will show that the TiO2-ZrO2 and Cu decorated TiO2-ZrO2 release Ti, Zr and Cu at ppb levels far below the amounts permitted by sanitary regulations and therefore they can be used safely as antibacterial agents in a mammalian cell context [10], [11]. The low level found for Ti and Cu also suggests that the Escherichia coli disinfection occurs through an oligodynamic effect [12], [13]. The present study addresses new features for films activated by low intensity solar light leading to bacterial inactivation compared to other antibacterial films [14], [15]. The bacterial inactivation/mineralization in aerobic media is reported and the identification of the OH-radical was determined quantitatively. A bacterial inactivation mechanism is suggested based on the ATR-FTIR spectroscopic and other experimental data found during this study.
Section snippets
Preparation of binary oxides films by sputtering, quantitative analysis of the film content by X-ray fluorescence (XRF) and CO2 measurements
Thin Ti and Zr films were sputtered on PES by direct current magnetron sputtering (DCMS) at 200 mA and 300 V in a reactive oxygen atmosphere using a mixed target of Ti and Zr (50%–50%) obtained from K. Lesker, Hastings, UK. The substrate-to-target distance was 10 cm and targets were 2 inches in diameter. Cu sputtering on the TiO2-ZrO2 layers was applied for times up to 10 s. The PES used was Dacron, type 54 spun, plain weave ISO 105-F04 (EMPA) used for color fastness determinations. The nominal
DRS of TiO2-ZrO2 and scavenging of the radicals generated by TiO2-ZrO2 and TiO2-ZrO2-Cu under light irradiation
Fig. 1 shows the diffuse reflectance spectra of TiO2, ZrO2 and TiO2-ZrO2 composites sputtered films for 8 min on PES. It is readily seen that the TiO2-ZrO2 spectrum in Fig. 1 presents a different spectrum compared to the TiO2 and ZrO2. This effect is due to networking in the binary oxide TiO2-ZrO2 but a definitive structure for this composite has not been reported [7], [8], [9]. Rtimi et al., have reported recently lower bacterial inactivation kinetics by TiO2 and ZrO2 each by itself when
Conclusions
This study presents new evidence related to the bacterial inactivation/mineralization on TiO2-ZrO2 and TiO2-ZrO2-Cu films. E. coli inactivation on TiO2-ZrO2 and TiO2-ZrO2-Cu released ppb quantities of Ti, Zr and Cu. These quantities were below the toxicity limit set by sanitary regulations and suggest disinfection proceeding through an oligodynamic effect. The role of the decorated/intra-gap Cu on the mechanism of bacterial inactivation is a controversial matter and more work is necessary to
Acknowledgments
We thank the EPFL and Swiss National Science Foundation (SNF) Project (200021-143283/1) for financial support. We also thank the COST Action MP 1106 for discussions during the course of this study.
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