A year for nanoscience

Biocompatible Copper Oxide Nanoparticle Composites from Cellulose and Chitosan: Facile Synthesis, Unique Structure, and Antimicrobial Activity

Copper in various forms has been known to have bactericidal activity. Challenges to its application include preventing mobilization of the copper, to both extend activity and avoid toxicity, and bioincompatibility of many candidate substrates for copper immobilization. Using a simple ionic liquid, butylmethylimmidazolium chloride as the solvent, we developed a facile and green method to synthesize biocompatible composites containing copper oxide nanoparticles (CuONPs) from cellulose (CEL) and chitosan (CS) or CEL and keratin (KER). Spectroscopy and imaging results indicate that CEL, CS, and KER remained chemically intact and were homogeneously distributed in the composites with CuONPs with size of 22 ± 1 nm. Electron paramagnetic resonance (EPR) suggests that some 25% of the EPR-detectable Cu(II) is present as a monomeric species, chemically anchored to the substrate by two or more nitrogen atoms, and, further, adopts a unique spatially oriented conformation when incorporated into the [CEL + CS] composite but not in the [CEL + KER] composite. The remaining 75% of EPR-detectable Cu(II) exhibited extensive spin-spin interactions, consistent with Cu(II) aggregates and Cu(II) on the surface of CuONPs. At higher levels of added copper (>59 nmol/mg), the additional copper was EPR-silent, suggesting an additional phase in larger CuONPs, in which S > 0 spin states are either thermally inaccessible or very fast-relaxing. These data suggest that Cu(II) initially binds substrate via nitrogen atoms, from which CuONPs develop through aggregation of copper. The composites exhibited excellent antimicrobial activity against a wide range of bacteria and fungi, including methicillin-resistant Staphylococcus aureus; vancomycin-resistant Enterococcus; and highly resistant Escherichia coli, Streptococcus agalactiae, Pseudomonas aeruginosa, Stenotrophomonas maltophilia, and Candida albicans. Expectedly, the antibacterial activity was found to be correlated with the CuONPs content in the composites. More importantly, at CuONP concentration of 35 nmol/mg or lower, bactericidal activity of the composite was complemented by its biocompatibility with human fibroblasts.

One-Pot Synthesis of Biocompatible Silver Nanoparticle Composites from Cellulose and Keratin: Characterization and Antimicrobial Activity

A novel, simple method was developed to synthesize biocompatible composites containing 50% cellulose (CEL) and 50% keratin (KER) and silver in the form of either ionic (Ag⁺) or Ag⁰ nanoparticles (Ag⁺NPs or Ag⁰NPs). In this method, butylmethylimmidazolium chloride ([BMIm⁺Cl^-]), a simple ionic liquid, was used as the sole solvent and silver chloride was added to the [BMIm⁺Cl^-] solution of [CEL+KER] during the dissolution process. The silver in the composites can be maintained as ionic silver (Ag⁺) or completely converted to metallic silver (Ag⁰) by reducing it with NaBH₄. The results of spectroscopy [Fourier transform infrared and X-ray diffraction (XRD)] and imaging [scanning electron microscopy (SEM)] measurements confirm that CEL and KER remain chemically intact and homogeneously distributed in the composites. Powder XRD and SEM results show that the silver in the [CEL+KER+Ag⁺] and [CEL+KER+Ag⁰] composites is homogeneously distributed throughout the composites in either Ag⁺ (in the form of AgClNPs) or Ag⁰NPs form with sizes of 27 ± 2 or 9 ± 1 nm, respectively. Both composites were found to exhibit excellent antibacterial activity against many bacteria including Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, methicillin-resistant S. aureus (MRSA), and vancomycin-resistant Enterococus faecalis (VRE). The antibacterial activity of both composites increases with the Ag⁺ or Ag⁰ content in the composites. More importantly, for the same bacteria and the same silver content, the [CEL+KER+AgClNPs] composite is relatively more toxic than [CEL+KER+Ag⁰NPs] composite. Experimental results confirm that there was hardly any Ag⁰NPs release from the [CEL+KER+Ag⁰NPs] composite, and hence its antimicrobial activity and biocompatibility is due not to any released Ag⁰NPs but rather entirely to the Ag⁰NPs embedded in the composite. Both AgClNPs and Ag⁰NPs were found to be toxic to human fibroblasts at higher concentration (>0.72 mmol), and for the same silver content, the [CEL+KER+AgClNPs] composite is relatively more toxic than the [CEL+KER+Ag⁰NPs] composite. As expected, by lowering the Ag⁰NPs concentration to 0.48 mmol or less, the [CEL+KER+Ag⁰NPs] composite can be made biocompatible while still retaining its antimicrobial activity against bacteria such as E. coli, S. aureus, P. aeruginosa, MRSA, and VRE. These results, together with our previous finding that [CEL+KER] composites can be used for the controlled delivery of drugs such as ciprofloxacin, clearly indicate that the [CEL+KER+Ag⁰NPs] composite possesses all of the required properties for it to be successfully used as a high-performance dressing to treat chronic ulcerous infected wounds.