Antimicrobial metal–organic frameworks incorporated into electrospun fibers
Introduction
Metal–organic frameworks (MOF) are a remarkable class of materials in which organic bridging ligands are connected by metal ions to form one-, two-, and three dimensional coordination networks [1]. The key advantages of MOF compared to inorganic microporous structures such as zeolites, is their highly tunable composition, which can be achieved by using different metals or changing the organic linker. The initial interest on MOF came from their very high surface areas and hence an extremely high capacity to capture gases in energy-related technologies [2], [3]. Many other potential applications have been proposed for MOF in fields like heterogeneous catalysis, gas purification and sensing [4], [5]. The focus on gas-related processes has been recently complemented with novel biological applications based on the capacity of coordination polymers for the controlled release of bioactive molecules, either physisorbed within the pore structure or behaving themselves as linkers in pro-drug form [6]. New developments are increasingly focused on the development of materials of complex chemical functionalization in order to impart tailored chemical and physical properties [7].
The release of metal ions contained in the structure of MOF makes them attractive antimicrobial materials for applications in which a tunable antibiotic is required. Silver ion releasing compounds are a well known family of antimicrobials and several silver containing MOF have been reported up to date to this end. Berchel et al. reported a silver-based MOF with a 3-phosphonobenzoate ligand that acts as a source of silver ions and showed antibacterial activity against Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa [8]. Liu et al. reported another silver-based metal–organoboron framework with antibacterial activity against Gram-negative and Gram-positive human pathogens [9]. Hindi et al. [10] and Slenters et al. [11] prepared other silver-based coordination compounds with proved or suggested biocidal capacity. Silver, however, is an expensive metal and the indiscriminate use of it in many consumer goods is suspected to promote not only a general toxic risk but also bacterial resistance [12]. Silver was also incorporated in different types of nanofibers in order to incorporate antimicrobial effect. Sheikh et al. prepared silver doped polyurethane nanofibers via electrospinning in which silver nanoparticles were obtained from silver nitrate by in situ reduction with N,N-dimethylformamide [13]. Shi et al. prepared silver nanoparticle-filled nylon 6 nanofibers by electrospinning, the electrospinning solvent behaving as reducing agent for in situ conversion of AgNO3 into silver nanoparticles. [14]. Several groups reported the use of electrospinning to prepare hybrid nanomaterials for antimicrobial applications by loading chitosan-based fibers with silver nanoparticles [15], [16].
In a recent paper Aguado et al. reported high antibacterial activity of a cobalt imidazolate MOF [17]. The results showed that the controlled release of cobalt gave rise to a long-lasting antibacterial activity with the advantage with respect to silver that it is a relatively inexpensive element, still active for bacteria but less toxic than silver [18]. Zhuang et al. also proposed a cobalt MOF with tetrakis[(3,5-dicarboxyphenyl)-oxamethyl] methane acid as ligand, which demonstrated activity for the inactivation of E. coli [19]. Concerning other metals, Sancet et al. prepared surface-anchored copper MOF with dual functionality, intended to combine sensing and controlled release of an antimicrobial metal ion [20]. The material was tested using the marine bacteria Cobetia marina, and displayed good surface response to adhering microorganisms. It is interesting to point out that in most cases, the linker used in the preparation of MOF reservoirs was not commercially available, the synthesis of it requiring several reaction-separation steps. In this work, we tested a cobalt-based MOF prepared using a simple, relatively cheap and commercially available ligand [21].
Electrospinning is a simple method for generating nanofibers from a wide variety of materials, including many dissolved or melted polymers, using a high-voltage power supply. Electrospun fibers have been proven useful in a number of fields such as water filtration, the design of sensors, the manufacturing of special clothing and many biomedical applications such as wound dressings or scaffolds for tissue engineering [21], [22]. Electrospun fibers have attracted considerable attention due to their remarkable properties, which include small diameter and relatively high surface-to-volume ratio, even though the preparation of porous polymer nanofibers with high surface areas is still a challenge [23]. The incorporation of particles into electrospun polymer nanofibers has also been explored by researchers working in drug delivery applications and in water treatment technologies [24], [25].
The purpose of this study was to prepare and test a biocidal composite material consisting of a cobalt-based MOF embedded in an electrospun polymeric matrix based on polylactic acid (PLA). PLA is derived from renewable resources and displays higher natural hydrophilicity than conventional thermoplastic polymers as a result of better access of water to the polar oxygen linkages in the backbone. This fact has been shown to improve water fluxes and reduce the biofouling tendency of membranes made of PLA [26]. PLA also displays a good spinnability. The composite Co–MOF–PLA material is intended for use in antimicrobial applications such as the preparation of antibacterial tissues or the production of membranes for water treatment.
Section snippets
Materials
Transparent PLA (trade name: ‘PLA Polymer 2002D’) was purchased in the form of pellets from NatureWorks LLC, UK with a melt index of 5–7 g/10 min (at 210 °C/2.16 kg), a molecular weight of ∼121,400 g/mol, a melting temperature of 160 °C and a d-content of 4% (96% l-lactide content). Polyvinylpyrrolidone (PVP), molecular weight 360,000 from Sigma–Aldrich was used as dispersant. Dichloromethane (DCM, 99.5%), used as solvent for PLA, and acetone (99.8%) were reagent grade, obtained from Sigma–Aldrich
Characterization of mats
Cobalt metal–organic Co-SIM-1 was characterized by XRD in order to assess their crystallinity. Fig. S2 (Supplementary information, SI) shows a typical diffractogram, which is good agreement with the corresponding pattern. Fig. S3 (SI) is an image of Co-SIM-1 particles showing that most of them have a primary particle size below 1 μm.
Electrospun mats were prepared with three different Co-SIM-1 loadings: 2, 4.5 and 6 wt.%. (A picture of a typical mat is shown in Fig. S1.) The MOF content of
Conclusion
A cobalt-based metal–organic framework, Co-SIM-1, was successfully included in a PLA electrospun mat with MOF loadings in the 2–6 wt.% range. The suspension was stabilized during all the electrospinning injection time using a solution of PVP 2.5 wt.%. PLA fibers displayed diameters in the 1–2 μm range in which Co-SIM-1 particles consisted of aggregates of several microns formed by aggregation of some tens of primary particles. All Co-SIM-1 aggregates were completely covered by PLA in composite
Acknowledgments
This work has been financed by the Dirección General de Universidades e Investigación de la Comunidad de Madrid, Research Network 0505/AMB-0395 and the Spanish Ministry of Science (CTM2013-45775). One of the authors, J.Q., would like to thank the University of Alcalá for the award of a pre-doctoral grant.
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