Anti-adhesion and antibacterial activity of silver nanoparticles supported on graphene oxide sheets
Graphical abstract
Introduction
Microorganisms are widespread in nature, and many can adhere to surfaces, forming so-called biofilms [1]. These microorganisms are found in many different environments (ocean, soil, skin, and air), and their adherence to surfaces constitutes a survival strategy [2]. Bacterial communities can anchor on surfaces and produce biofilms, which are commonly surrounded by an extracellular polymeric matrix of exopolysaccharide (EPS) [3]. The dynamic of biofilm formation occurs in different steps; the primary phase corresponds to the adhesion of the microorganisms to the surface, which can in turn act as a support for microbial growth, a source of nutrients or a supplier of the optimal oxygen required by aerobic microorganisms [2]. The presence of biofilms can be harmful to several industrial processes because they cause mechanical blockage in fluid systems, disturbances of heat transfer processes and corrosion in metallic surfaces [4].
In particular, the presence of biofilms in the food industry can lead to severe hygienic problems, economic losses due to food spoilage and even serious diseases when pathogens are involved [5]. In a hospital environment, biofilms may contaminate medical implant devices [6], thus increasing the risk of the transmission of infectious diseases and offering higher microbial resistance to antibiotics [2], [6], as the cells living in the form of biofilms are usually more resistant to antimicrobial compounds than planktonic cells [1].
Pseudomonas aeruginosa is a classic example of a microorganism with the ability to grow and form biofilms on non-sterile solid surfaces [3]. Pseudomonas sp. represent a large and versatile group of bacteria that have adapted to live in soil, seawater and fresh water [7], and due to their excellent adaptability, they are found in food processing environments, including drains, floors, meat surfaces, and dairy products [5]. In hospitals, the presence of P. aeruginosa has been associated with the occurrence of lung infections and cystic fibrosis [6]. The formation of biofilms by P. aeruginosa is related to the presence of both flagellar and pili-mediated motilities [8], the production of large amounts of EPS [4] and the quorum sensing system that controls the cell-cell signaling processes [2].
The removal and eradication of biofilms is generally achieved by mechanical force, acid- or alkaline-based detergents or chemical disinfectants. However, the efficiency of these chemical products is strongly affected by factors such as pH, temperature, solubility, concentration and exposure time [5]. In this context, there is a strong desire to develop new strategies to control biofilm formation on food and medical products. According to Romero and Kolter [2], even after many efforts dedicated to develop new anti-adhesion agents, initiatives to improve existing products and standard protocols are encouraged.
Nanotechnology has emerged as an alternative tool for developing products with new properties to meet the increasing demand of the industrial sectors for advanced functional materials. In particular, nanostructured materials such as carbon nanotubes and graphene have been developed as model systems in nanoscience and nanotechnology. Graphene-based materials are a special class, and they have attracted a large amount of attention because of their large superficial area and exceptional electronic, mechanical and thermal properties [9]. Graphene is a two-dimensional material composed of a hexagonal sp2-hybridized carbon network [10], [11], while graphene oxide (GO) is a chemically modified graphene featuring hydroxyl, carboxyl and epoxy functional groups [12]. This functionalized form of graphene is able to produce stable dispersions in water that consist primarily of single-layer GO sheets [13]. This material has been used as a promising building block for preparing new composites [13]. On the other hand, it is well-known that silver nanoparticles possess antimicrobial activity and have been used as biocide agents in health, food and textile applications [14]. Accordingly, silver nanoparticles assembled on graphene oxide sheets (GO–Ag) have been exploited as novel antibacterial systems [15], [16]. Liu et al. [15] and Das et al. [16] demonstrated antibacterial activity for GO–Ag nanocomposites against Gram-negative Escherichia coli. However, the potential for these nanocomposites to prevent biofilm formation has not been explored. Moreover, the antibacterial activity of GO itself is controversial and demands further investigation. For instance, Liu et al. [17] compared the antibacterial activity of four types of graphene-based materials toward E. coli. GO was reported to exhibit the highest antibacterial activity against E. coli when compared to graphite, graphene and reduced graphene oxide. In contrast, Ruiz et al. [18] conducted a study to characterize the antimicrobial activity of GO, and the results showed that the bacteria grew faster when GO was added to the culture medium. Furthermore, the presence of graphene oxide stimulated the bacterial growth by acting as a surface for their attachment and proliferation. Here, we propose the synthesis and chemical characterization of GO decorated with silver nanoparticles. We also highlight the role of oxidation debris fragments in this process. The GO–Ag nanocomposite showed promising antibacterial activity against P. aeruginosa, and this result raises the possibility of applying GO–Ag nanomaterials as anti-adhesion agents, an application that has not been considered until now.
Section snippets
Graphene oxide
The sample of graphene oxide was purchased from Cheap Tubes Company (Brattleboro, USA). The graphene oxide was produced by the modified Hummers method [19], which consists of a graphite oxidation process using concentrated sulfuric acid (H2SO4) and potassium permanganate (KMnO4). The singled-layered sheets were between 0.7 and 1.2 nm thick, with a size distribution of 300–800 nm.
Preparation and characterization of the GO–Ag nanocomposite
To prepare a colloidal suspension, 12.5 mg of GO was dispersed in 20 mL of deionized water followed by sonication in an
Characterization of the GO and GO–Ag nanocomposite
The nanocomposite formed by graphene oxide and silver nanoparticles was prepared using sodium citrate as a stabilizing agent. The resulting dispersion containing the GO–Ag nanocomposite showed a black-green color. Fig. 1 shows the UV–vis spectrum of the GO and GO–Ag nanocomposite. The UV–vis spectrum of the GO exhibits two characteristic bands at 230 nm, which correspond to the electronic π–π* transitions of CC aromatic bonds, and a shoulder at 305 nm, assigned to the n–π* transitions of CO bonds
Conclusions
We have demonstrated the preparation of a GO–Ag nanocomposite using AgNO3 as a silver precursor and sodium citrate as a stabilizing/reducing agent. The silver nanoparticles supported on GO sheets showed a spherical-like morphology and an average size of 7.5 nm, whereas silver nanoparticles synthesized in the absence of GO were variable in shape, with an average size of 60 nm. We showed that the nucleation sites required to grow silver nanoparticles were attributed to debris oxidation fragments
Acknowledgments
A.F.F, D.S.T.M, A.C.M.M and O.L.A acknowledge the São Paulo State Research Foundation (FAPESP), the National Council for Technological and Scientific Development (CNPq) and the National Institute of Science, Technology and Innovation in Complex Functional Materials (INOMAT/INCT) for their financial support. A.G.S.F acknowledges the financial support from INCT Nanobiosimes (MCT-CNPq) and the Capes-Nanobiotec network. S.M.M and A.B acknowledge the CNPq for their financial support.
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