Regular ArticleHigh-density polystyrene-grafted silver nanoparticles and their use in the preparation of nanocomposites with antibacterial properties
Graphical abstract
Introduction
Polymer nanocomposites (PNCs) play an essential role in the design of novel functional materials since they offer possibility of combining the unique properties of nanoparticles with useful characteristics of polymers and yielding potential performance well beyond that of each individual components. Generally speaking, they contain filler particles, with dimensions in the nanometer range, dispersed in polymer matrix [1]. There are three main categories of PNCs differing in the manner used to combine the nanoparticles and polymers: (1) non-polymer coated nanoparticles distributed in polymer matrix, (2) nanoparticles (NPs) attached to a polymeric micro/nanostructures, (3) polymer-grafted NPs, which form a core@shell structure, dispersed inside a polymer matrix [2], [3], [4]. It is very important to emphasize that only in the last case, the hybrid materials with well-controlled distribution of NPs in polymer matrix may be attained. It has been shown in numerous papers, that the ability to precisely control the spatial distribution of nanostructures inside polymer matrix is crucial to obtain stable hybrid materials with desired properties [5].
Silver polymer nanocomposites (AgPNCs) have attracted especially great deal of attention in recent years because of extraordinary optical, electronic, catalytic and mainly antibacterial properties of silver nanoparticles (AgNPs) and consequently a plethora of their applications in various fields of science, technology, pharmacy and medicine [6], [7], [8], [9], [10]. Firstly, the intensive localized surface plasmon resonance (LSPR) observed for AgNPs can be tuned to any wavelength in the visible spectrum [11], [12], [13], [14]. Because of the LSPR frequency depends strongly on the size and shape of nanoparticles as well as properties of the surrounding environment, AgNPs can be successfully used for the fabrication of chemical and biochemical sensors [15], bioimaging devices [16] and plasmonic circuitry elements [17]. Furthermore, AgNPs have found an important applications in SERS detection [18] and catalysis [6]. Nevertheless, the use of AgNPs as biocidal agents is the most intensively studied and simultaneously the most commercialized. It is commonly believed that unlike conventional antibiotics, AgNPs do not provoke bacterial resistance and at the same time do not cause significant tissue toxicity [7], [19], [20]. In addition, the broad-spectrum of biocidal activity of AgNPs encourage their use in many various biomedical applications [8], [19].
Rahaman and co-workers have recently shown that the AgNPs can be also successfully employed in surface modification of membranes for water purification technologies. The developed modified membranes decorated with AgNPs exhibit both antimicrobial and antifouling properties [21], [22], [23].
Nowadays, AgNPs are used as additives in many consumer products e.g. textiles, packaging, household equipment, cosmetics, coatings for electronic devices, providing them antimicrobial properties [7], [24], [25]. However, for the most practical applications, AgNPs should be entrapped in the suitable matrix capable of maintaining their properties and simultaneously ensuring easy processability as well as good environmental stability [2].
Polystyrene (PS) belongs to the group of thermoplastics and it has found an extremely wide range of applications. The wide applicability of the PS results from the fact that it possesses many useful physical properties, including visible-range transparency, good environmental stability, high mechanical strength and, very importantly, it can be easy processed using typical polymer processing techniques such as injection molding, extrusion or thermoforming. Furthermore, PS can be several times mechanically recycled into new products without any damage to its physical properties [26]. PS is commonly used for the production of packages, including the licensed for contact with food and pharmaceuticals, numerous consumer goods, housing electronic equipment, inner lining of freezers and refrigerators, foam insulation boards [27]. It has also found many medical applications including fabrication of surgical instruments, dental tools, dressing materials, prosthesis, etc. [28]. Needless to say, that for all these applications, but especially for biomedical usage, the implementation of easy-processable nanocomposites exhibiting antibacterial activity is highly desirable.
Despite the wide potential applicability of PNCs containing AgNPs entrapped in the polystyrene matrix (Ag/PS) there are only a few reports on fabrication and antibacterial activity of such kind of materials to date. The main limitation for their preparation is immiscibility of hydrophilic, electrostatic stabilized AgNPs in hydrophobic polymer which can lead to uncontrolled spatial distribution of nanostructures in polymer matrix, their agglomeration and even aggregation [5], [29], [30].
Lin et al. [31] used the method commonly used for the preparation of PNCs, based on the mixing polymer solution with nanoparticles dispersion (in tetrahydrofuran) and subsequently rapid precipitation of solids from the obtained mixture, for the fabrication of Ag/PS. However, transmission electron microscopy (TEM) revealed that the nanoparticles were partially aggregated in the polymer matrix. Meanwhile, it is well-known that the nanoparticles aggregation leads to a substantial deterioration and even losing their properties. Furthermore, the silver content, in prepared through this method composites, was arbitrarily limited by possible degree of silver nanoparticles dispersion in selected solvent and was in the range 1–10% [31].
Vodnik et al. used oleylamine modified AgNPs for preparation of Ag/PS via direct mixing of their chloroform solutions followed by solvent evaporation. Owing to the hydrophobic character of oleylamine a better dispersion of AgNPs in the polystyrene matrix could be achieved [32].
Ag/PS nanocomposites have also been fabricated by polymerization of styrene carried out in the presence of AgNPs, with surfactants under influence of ultrasound radiation [33], [34], [35]. Antibacterial activity of these materials has not been studied.
Awad et al. [36] reported antibacterial activity of Ag/PS nanocomposites against Gram-positive and Gram-negative bacteria which have been prepared by mixing of electrostatic coated silver nanoparticles (average size 98 nm in diameter) with polystyrene solution in toluene, at elevated temperature. Thermal stability of the prepared nanocomposites and their possible processability have not been tested.
An and co-workers [37] reported synthesis and antibacterial activity of silver/polystyrene nanohybrids with core@shell structure. The AgNPs stabilized by ionic liquid were injected into the mixture of styrene, ethanol and polyvinylpyrrolidone (PVP) and subsequently the polymerization of styrene was initiated. According to the author’s opinion, during the polymerization, the polystyrene chains were gradually deposited on a silver core and core@shell nanostructures have been realized in such a way. Nevertheless, the nanoparticles with rather weakly defined and loosely adsorbed polystyrene shell can be prepared by using the proposed method.
Grafting of metal nanoparticles with polymers, also called polymer brushes formation, enables the synthesis of nanohybrids with core@shell architecture and covalently attached polymer layer. There are two possible routes towards this kind of nanostructures: “grafting to” approach based on the reaction between the end-functionalized polymers and nanoparticles prepared in advance and “grafting from” approach consisting of the surface initiated polymerization wherein the polymerization initiator is immobilized on the surface of the substrate and the polymer chains grow in situ from the surface of nanoparticle [3], [30], [38], [39].
So far, polystyrene grafted silver nanoparticles (Ag@PS) have been fabricated by using “grafting to” approach with thiolate-terminated PS prepared by living anionic polymerization (LAP) [40], [41]. Besides the undoubted and evident advantages of anionic polymerization, such as ability to form polymers with precisely designed molecular weight (MW), narrow molecular weight distribution (MWD), nano-structured morphologies and chain-end functionality, there is one major and serious disadvantage – high sensitivity to moisture and impurities. Consequently, LAP necessitates the use of specialised glassware, rigorous purification and drying of all reagents [42].
Nitroxide-mediated radical polymerization (NMRP), one of the living/controlled radical polymerization (CRP) [43], [44], [45] techniques, is a powerful alternative to LAP. This is because the NMRP enables the design of polymers with narrow MWD and complex architecture, at the same time exhibits the significantly lower sensitivity to moisture and impurities than LAP. Besides, compatibility with both aqueous and organic media and tolerance towards a wide range of functional groups are important benefits of this type of CRP. In addition, NMRP, based on reversible termination occurring between a growing propagating polymer chain and nitroxide radicals, is purely thermal process thus a catalyst is not required making the purification procedure of final product simplified which is crucial for biomedical applications of the synthesized polymers [46], [47], [48], [49].
To the best of our knowledge, grafting of polystyrene chains onto silver nanoparticles surface via NMRP and antibacterial activity of the nanocomposites fabricated in this manner have not been reported hitherto.
Recently, our group proposed a novel route for the synthesis of nanostructures with gold core and well-defined polymer shell using NMRP. The developed procedure consisted in a late injection of nitroxide-coated gold nanoparticles (prepared by using disulphide bisnitroxide as the capping agent [50], [51], [52]) into a polymerization system mediated by nitroxide radicals [53]. More recently, we have also shown that the nitroxide-coated silver nanoparticles (N-AgNPs) exhibit very high antibacterial activity against both Gram-positive and Gram-negative bacteria [54].
In the work described herein we propose N-AgNPs as precursors for the fabrication of new type of polymer brushes, covalently attached to nanostructure surface. The brushes prepared, in such a manner, are simultaneously high dense and flexible (in outer sphere) so as to enable easy penetration of free polymer chains thus providing good miscibility of inorganic filler in polymer matrix. Due to the favorable interactions between prepared polystyrene brushes attached to silver surface via nitroxide linker and polystyrene molecules in applied polymer matrix entirely homogeneous, thermally stable nanocomposites exhibiting effective antibacterial activity might be prepared by simply mixing of components in the appropriate ratios. We present the extensive physicochemical characteristic of the obtained materials and their antibacterial activity against two pathogenic bacteria: Pseudomonas aeruginosa (Gram-negative representative) and Staphylococcus aureus (Gram-positive representative).
Section snippets
Materials
NaBH4, AgNO3, 4-hydroxy-TEMPO (TEMPOL), and all solvents were purchased from Sigma-Aldrich (puriss ≥97%), and used as received, styrene (St, Sigma-Aldrich ≥99%) was dried over MgSO4 and passed through a basic aluminium oxide column before use (Merck, 0.06 mm). Benzoyl peroxide (BPO) was recrystallized twice from the chloroform-methanol mixture and dried in a desiccator; bis(N-oxy-2,2,6,6-tetramethylpiperidyl)-4,5-dithiooctanoate (DiSS) was synthesized according to the procedure described in the
Results and discussion
Polystyrene grafted silver nanoparticles Ag@PS have been synthesized using nitroxide-coated silver nanoparticles (N-AgNPs) as precursors according to Scheme 1. We adapted the protocol developed by our group for the preparation of core@shell gold@polystyrene nanostructures consisting in the late injection of nanoparticles covered by radicals into NMRP system. As has been shown in our earlier work this protocol allows to obtain a tailor-made nanohybrids via precisely designed macroradicals
Conclusions
In summary, we propose a novel approach to prepare perfectly homogeneous polystyrene/silver nanocomposites exhibiting antibacterial activity. The developed procedure consists of two main steps: (1) synthesis of polystyrene grafted silver nanoparticles (Ag@PS) by using nitroxide-coated silver nanoparticles (N-AgNPs) as precursors, (2) preparation of nanocomposites by mixing of Ag@PS with narrow-dispersity polystyrenes and next thermoforming at 140 °C. The nanosilver content in the resulting
Acknowledgement
This work has been supported by Project DEC-2011/01/B/ST5/03941 from National Science Center. TEM images have been obtained using the equipment purchased within CePT Project No. POIG.02.02.00-14-024/08-00.
The authors thank Dr. Katarzyna Zawada (Medical University of Warsaw, Faculty of Pharmacy with the Laboratory Medicine Division) for performing ESR measurements and helpful discussions and Katarzyna Markowska, MSc, (University of Warsaw, Faculty of Biology, Institute of Microbiology,
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