Kraft lignin/silica–AgNPs as a functional material with antibacterial activity
Graphical abstract
Introduction
The term nanosilver refers to pure silver in the form of atomic clusters less than 100 nm in diameter [1], [2], [3]. Silver with this morphology is commonly applied as a heterogeneous catalyst of oxidation [3], [4], [5]. Silver nanoparticles also have much stronger antibacterial properties than bulk silver [1], [2], [6] and are currently considered the most promising nanomaterial suitable for medical applications [7]. Various medical devices, such as wound dressings and surgical instruments, are commercially available with nanosilver coatings [8], [9]. Silver nanoparticles (AgNPs) can also be used as an additive in highly advanced materials, for example as a filler in blood-contacting nanocomposite biomaterial (for cardiovascular implants) [10], or as an antimicrobial agent in polymers with self-sterilizing surfaces [11].
The toxicity of silver nanoparticles is mainly induced by biologically active silver ions, which are released in the presence of water due to surface oxidation of silver atoms [12], [13]. The concentration of Ag+ ions in a water–nanosilver colloid depends on the surface area of the average silver particle, and therefore smaller silver nanoparticles are far more toxic than larger ones [3]. It is commonly known that Ag+ ions interact with bacterial cells, which results in the suppression of DNA replication or other life-sustaining processes and can also cause disintegration of the peptidoglycan cellular wall, leading to cell lysis for both Gram-positive and Gram-negative species [14], [15]. The same ions released from AgNPs also have a harmful influence on eukaryotic species, especially organisms living in aquatic environments, where the presence of small amounts of nanosilver can cause significant damage to the entire biocenosis [16]. Therefore, further research is needed to determine the maximum quantity of silver nanoparticles which can be introduced into the environment without inflicting damage on non-target organisms [16], [17].
In order to reduce environmental pollution and obtain functional materials with antibacterial properties, silver nanoparticles can be successfully immobilized on various supports. Utilizing the excellent adsorption capability of silica, it is possible to obtain silica–nanosilver systems as a result of uncomplicated operations [18], [19]. The system is highly stable due to the strong chemical bonding between the silica support and silver atoms, which has been demonstrated by the XPS technique [18]. Nanosilver immobilized on a silica support can also be applied as a catalyst of CO oxidation to CO2 [19], or as a functional filler for antimicrobial and self-cleaning polymers that provide a high level of surface disinfection [10], [18]. It is also possible to obtain nanosilver–polymer blend without any filler, however it may result in not necessarily needed yellow tint of product [20] and spontaneous desorption of AgNPs from the surface, if nanosilver is not bonded chemically. Textile fibers functionalized with nanosilver or nanogold will also be dyed in process of immobilization [21]. The great merit of the AgNP immobilization is that it presumably nullifies its negative influence on eukaryotic cells; however, only few AgNP-support systems has been analyzed. The investigation of nanosilver-functionalized polymethylmethacrylate bone cement has demonstrated suitable cytocompatibility with primary human mesenchymal stem cells and osteoblasts [22]. Foregoing properties were confirmed in further research. However, cytocompatibility of immobilized AgNPs worsened significantly after the addition of gentamicin to the system [23].
Nanosilver, alongside nanogold, can also be immobilized on unbleached paper fibers and mechanical pulp. It has been shown that lignin is the main adsorbent of nanoparticles, hence they cannot be adsorbed on bleached fibers. Specific parts of the lignin molecule, especially the phenolic ring and possibly also the methoxy groups, cause the reduction of Ag+ ions to metallic silver, thus silver nanoparticles are formed on the lignin molecules. The material obtained in the process has been shown to have very good antibacterial properties even when only a very small quantity of silver nanoparticles is added [24].
Silica and silica/lignin hybrids are functional materials used as potential adsorbents. It has been demonstrated that a suitably functionalized silica surface can adsorb dyes [25], [26] and harmful organic compounds [27], including ions of environmentally harmful metals [28], as well as other compounds such as hydrogen peroxide [29], enzymes [30] and natural macromolecules [31]. Silica/lignin systems can also act as adsorbents, as has been shown, for example, in the case of adsorption of cadmium(II) and nickel(II) ions [32].
Considering that both silica and lignin can be used as adsorbents, an attempt was made to combine them into a hybrid material, which was then used for the first time as a support for silver nanoparticles. Chemical nature of hybrid-AgNP bond prevents desorption of nanosilver and its uncontrollable emission. Addition of lignin improves adsorptive properties of the material and also significantly increases cost-efficiency of its production in comparison to antimicrobial materials based on pure silica. Utilization of surface-modified polymer filler instead of modifying monomers requires also far less quantity of AgNPs to prepare material with similar antimicrobial activity, as nanosilver trapped inside the polymer matrix does not have any physical contact with microbes. The properties of the resulting materials were thoroughly analyzed, and the results, as well as a brief description of potential applications, as a functional products with antibacterial activity, are reported here.
Section snippets
Preparation of silica/lignin hybrid materials
Prior to the preparation of the hybrid materials, the commercial silica Syloid 244 (W.R. Grace and Company, USA) was subjected to surface modification in order to enhance its affinity to lignin. The BET surface area of the tested unmodified silica equals 262 m2/g. A detailed description of the modification of silica with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (Sigma–Aldrich, Germany) is given in [33], [34].
The process of silica/lignin hybrid synthesis began with the activation of 3.0 g of
Dispersive-morphological properties
Two silica/lignin hybrid systems were obtained, with different ratios of biopolymer to inorganic material. A TEM photograph of the product obtained using 5 parts by weight of lignin per 100 parts silica is shown in Fig. 1a. The image indicates the presence of single primary particles of silica, exhibiting a tendency towards aggregation and agglomeration (the particle size distribution of the silica recorded by the Zetasizer Nano ZS instrument was 39–68 nm and 1720–2300 nm), which were grafted
Conclusions
In this study, silica/lignin hybrid materials were first obtained – the silica surface, suitably modified using N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and kraft lignin, activated with a solution of strong oxidizing agent, were bound together in a chemically permanent fashion. Next, silver nanoparticles were grafted onto the surface of the silica/lignin hybrids. The manner in which they were bound to the hybrid produced the desired effectiveness, as was confirmed initially by TEM
Acknowledgment
The study was financed within the National Science Centre Poland funds according to decision no. DEC-2013/09/B/ST8/00159.
Authors are thankful to Adam Piasecki, Ph.D. (Institute of Materials Science and Engineering, Faculty of Mechanical Engineering and Management, Poznan University of Technology) for technical assistance in the EDS measurements of this work.
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