Protection of 18th century paper using antimicrobial nano-magnesium oxide

https://doi.org/10.1016/j.ibiod.2018.04.004Get rights and content

Highlights

  • Nano-magnesium oxide particles (MgO NPs) of average size of around 10 nm have been synthesised using a simple one-pot synthesis.

  • The MgO NPs offer potent bactericidal activity but are non-toxic to mammalian cells at the same concentrations.

  • A combination of electron microscopy techniques shows how these nanoparticles produce oxidative stress, followed by cell membrane leakage and death.

  • The MgO NPs prevented bacterial colonisation of 18th century papers from the Archives of the Spanish Royal Botanic Garden.

Abstract

Magnesium oxide nanoparticles (MgO NPs) have attracted considerable interest as antimicrobial agents in a wide variety of applications. We report a simple synthetic route towards MgO NPs (average diameter 10 nm) possessing potent antibacterial activity against both Gram-negative and Gram-positive bacteria. Detailed electron microscopy studies show how these particles induce oxidative stress, cell membrane leakage and cell death in bacteria at low NP concentrations, but remain non-toxic to eukaryotic cells. Applying a homogeneous dispersion of these nanoparticles on 18th century paper proved to be a highly effective means of preventing bacterial colonisation without altering the appearance of the paper samples, thus opening the doors to the use of these colourless, low-cost, and scalable nanoparticles for preventing biodeterioration in a range of paper-based objects and surfaces.

Introduction

The enduring threat of microbial contamination to historical and contemporary objects of art in archives and in museums remains to be one of the principal problems of cultural heritage conservation. Microbes can penetrate deep within the microstructures of materials causing material loss from acid corrosion, enzymatic degradation and mechanical attack, all of which induce esthetical spoiling of paintings, sculptures, textiles, ceramics, metals, books and manuscripts alike (Sterflinger and Piñar, 2013). Regular decontamination of infected artefacts, exhibition areas and storage rooms/depots results in significant expenditures for museums, local authorities and private collectors. Ultimately, material loss brought about by prolonged microbial attack can result in loss of the cultural and historical value of paintings, books and manuscripts – the socioeconomic cost of which is inestimable (Sterflinger and Pinzari, 2012; La Russa et al., 2014). Furthermore, microbial contamination in libraries, museums and their storage rooms/depots can also represent a serious threat to the health and occupational safety of restorers, museum personnel and the general public (Skóra et al., 2015).

Metal oxide nanoparticles for example ZnO, MgO, CuO and CaO are being studied as novel inorganic antimicrobial agents for potential applications in food, the environment and healthcare (Hajipour et al., 2012; Dizaj et al., 2014). Nanostructured inorganic materials possess unique tuneable physicochemical properties and, moreover, the combination of their large surface area and dimensions allows them to interact and internalise within cells, respectively, meaning that they display a broad spectrum of antibacterial activity. Moreover, their modular nature means that a library of relatively low cost materials with different sizes, shapes, surface properties, and chemical compositions can be developed leading to a great potential for developing effective antimicrobial agents with high stability under harsh environmental conditions. Despite the advance of this field of research in recent years, the antibacterial mechanism of action of metal oxide nanoparticles is in most cases not entirely understood. Magnesium oxide nanoparticles (MgO NPs) have received significant attention as antibacterial agents in recent years due to their high stability and low cost based on their preparation from economical precursors (He et al., 2016; Tang et al., 2012). The mechanism of antibacterial activity of MgO NPs has been attributed to the production of reactive oxygen species (ROS), which induce oxidative stress and lipid peroxidation in bacteria (Tang and Lv, 2014) as well as non-ROS mediated bacterial toxicity mechanisms (Leung et al., 2014). It is important to note that the antibacterial effect is often species and genus dependent and depends upon the size, shape, chemical composition and surface properties (e.g. hydrophobicity) of the nanoparticles (Raghupathi et al., 2011; Hajipour et al., 2012).

Although MgO NPs are generally regarded as safe (Ge et al., 2011), the application of any antibacterial nanomaterial and its nanotoxicological profile is a major concern. Currently, MgO NPs are used as additives in heavy fuel-oil (Park et al., 2006), for the cleaning of fuel-oil pipelines, avoidance of sludge formation in storage tanks and protection of boilers against corrosion. MgO has been used as a mineral supplement source for magnesium, an essential nutrient for the human body (Srinivasan et al., 2017) and they are also used for diverse applications in medicine, e.g. for the relief of cardiovascular disease and stomach problems and as anti-cancer therapy (Krishnamoorthy et al., 2012). As already mentioned, toxic effects are highly dependent on the physicochemical properties of each individual nanoparticle as well as on the types of cells tested (Reddy et al., 2007). It therefore follows that an extensive evaluation of nanoparticles on different biological systems is needed to determine their toxicity. As an illustration, despite the aforementioned examples showing the low cytotoxicity of MgO NPs, they have been shown to display toxicity on early developmental and larval stages of zebrafish (Ghobadian et al., 2015). One of the great challenges of nanotechnology is the corresponding environmental health and safety implications of the widespread use of nanomaterials, since the properties of engineered nanomaterials are potentially highly hazardous to the human population due to their potential for high ecotoxicity. The widespread use of nanoparticles and their inevitable release into the general environment ultimately means that they will find their way into terrestrial, aquatic and atmospheric environments where their toxicity, behaviour and ultimate fate are largely unknown (Bondarenko et al., 2013).

Recently, there has been a drive for greater use of nanomaterials in the conservation of cultural heritage (Baglioni et al., 2015, La Russa et al., 2012, 2016). Recent examples include their use for protecting stone monuments (Sierra-Fernandez et al., 2017a), textiles (Pietrzak et al., 2017), murals (Baglioni et al., 2012), glass (Shirakawa et al., 2016) and paper (Asghar Ariafar et al., 2017). Both silver (Koizhaiganova et al., 2015) and nano-silver (Li et al., 2017) are studied frequently and some studies have even thoroughly evaluated the sensitivity of museum microbes to nanosilver (Gutarowska et al., 2012), but cheaper more readily available metal-oxide particles have been shown to serve as alternative solutions to biodeterioration issues (Ruffolo et al., 2010 and Sierra-Fernandez et al., 2017b).

The aim of the research presented herein was to study the use of nano-magnesium oxide particles to protect a variety of 18th century papers from the Archives of the Real Jardín Botánico in Madrid (Spain) from microbial contamination. These papers have been selected as a representative sample of different paper qualities that were used as herbarium materials at the end of the 18th century to keep plant specimens (herbarium sheets) dried for other purposes. They are part of the old herbarium paper sheets kept in the RJB archive before plants were stored with standardized herbarium paper sheets (c. 1980). They arrived at the RJB between 1785 and 1800 and have been kept here since, in a rather dry environment, but without further specific preserving measures (controlled humidity and temperature). Furthermore, we wished to demonstrate how a combination of antibacterial assays (solution-based quantification of cell viability) and high resolution electron microscopy imaging (qualitative analysis of the microbial cell morphology and internal structure) could be used together to elucidate conceivable mechanisms of action.

Section snippets

Reagents

Milli-Q water has been used throughout. Magnesium methoxide, 7–8% in methanol, was obtained from Alfa-Aesar; absolute ethanol was purchased from Panreac. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was purchased from Invitrogen. Resazurin sodium salt and TBX agar from Sigma-Aldrich.

Eukaryotic cells

Vero cells (monkey kidney epithelial cells) were purchased from the American Type Culture Collection (ATCC, Manassas, VA, USA, number CCL-81). Dulbecco's modified Eagle's medium (DMEM),

Results and discussion

A sol-gel method was chosen for the synthesis of the MgO NPs since it is represents a simple, cheap, reproducible an scalable synthetic process, which also allows for a reasonable control of the size of the nanoparticles and their subsequent properties. The synthetic conditions were optimized in terms of reactant amounts, pH conditions, reaction time and temperature to obtain the desired product, as reported in the Experimental Section. After the condensation step of the sol-gel process and the

Conclusions

The powerful antibacterial activity of nano-magnesium oxide particles was used to protect various 18th century papers from bacterial colonisation. The combination of antibacterial assays and high resolution electron microscopy imaging has shown how the particles cause oxidative stress, cell membrane leakage and cell death and furthermore, this methodology has verified that the paper surfaces were covered by a homogenous nanoparticle coating which does not alter the aesthetics of the paper. Our

Acknowledgements

Financial support by the Fundación General CSIC (S.G.M., Programa ComFuturo), Fondo Social Europeo-Gobierno de Aragón (J.M.F, L.D.M., I.F.C. & S.G.M.) is gratefully acknowledged. Access to and use of the Advanced Microscopy Laboratory (University of Zaragoza) and to the XRD facility of the Servicio General de Apoyo a la Investigación of the Universidad de Zaragoza is gratefully acknowledged. The authors also thank Mario Soriano (Centro de Investigación Principe Felipe, Valencia, Spain), Rodrigo

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