Study on antibacterial alginate-stabilized copper nanoparticles by FT-IR and 2D-IR correlation spectroscopy

The objective of this study was to clarify the intermolecular interaction between antibacterial copper nanoparticles (Cu NPs) and sodium alginate (NaAlg) by Fourier transform infrared spectroscopy (FT-IR) and to process the spectra applying two-dimensional infrared (2D-IR) correlation analysis. To our knowledge, the addition of NaAlg as a stabilizer of copper nanoparticles has not been previously reported. It is expected that the obtained results will provide valuable additional information on: (1) the influence of reducing agent ratio on the formation of copper nanoparticles in order to design functional nanomaterials with increased antibacterial activity, and (2) structural changes related to the incorporation of Cu NPs into the polymer matrix.

Cu NPs were prepared by microwave heating using ascorbic acid as reducing agent and NaAlg as stabilizing agent. The characterization of synthesized Cu NPs by ultraviolet visible spectroscopy, transmission electron microscopy (TEM), electron diffraction analysis, X-ray diffraction (XRD), and semiquantitative analysis of the weight percentage composition indicated that the average particle sizes of Cu NPs are about 3-10 nm, they are spherical in shape, and consist of zerovalent Cu and Cu₂O. Also, crystallite size and relative particle size of stabilized Cu NPs were calculated by XRD using Scherrer's formula and FT from the X-ray diffraction data. Thermogravimetric analysis, differential thermal analysis, differential scanning calorimetry (DSC), FT-IR, second-derivative spectra, and 2D-IR correlation analysis were applied to studying the stabilization mechanism of Cu NPs by NaAlg molecules. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of stabilized Cu NPs against five bacterial strains (Staphylococccus aureus ATCC 6538P, Escherichia coli ATCC 25922 and O157: H7, and Salmonella enterica serovar Typhimurium ATCC 13311 and 14028) were evaluated with macrodilution, agar dilution plate count, and well-diffusion methods.

On the basis of the semiquantitative analysis, there was a direct correlation between the reducing agent ratio and the percentage of zerovalent Cu. This was confirmed with the statistical analysis of population of Cu NPs from TEM micrographs. At lower reducing agent ratios, two phases coexist (Cu₂O and zerovalent Cu) due to incomplete reduction of copper ions by the reducing agent; however, at higher reducing agent ratios, the Cu NPs consist mainly of zerovalent Cu. Crystallite size and relative particle size of stabilized Cu NPs showed considerable differences in results and tendencies in respect to TEM analysis. However, the relative particle size values obtained from FT of XRD data agreed well with the histograms from the TEM observations. From FT results, the relative particle size and reducing agent ratio of stabilized Cu NPs showed an inverse correlation. The incomplete reduction of copper ions at lower reducing agent ratios was also confirmed by DSC studies. FT-IR and 2D-IR correlation spectra analysis suggested the first event involved in the stabilization of Cu NPs is their electrostatic interaction with -C=O of carboxylate groups of NaAlg, followed by the interaction with the available O-C-O⁻, and finally with the -OH groups. Bacterial susceptibility to stabilized nanoparticles was found to vary depending on the bacterial strains. The lowest MIC and MBC of stabilized Cu NPs ranged between 2 mg/L and 8 mg/L for all studied strains. Disk-diffusion studies with both E. coli strains revealed greater effectiveness of the stabilized Cu NPs compared to the positive controls (cloxacillin, amoxicillin, and nitrofurantoin). S. aureus showed the highest sensitivity to stabilized Cu NPs compared to the other studied strains.

Cu NPs were successfully synthesized via chemical reduction assisted with microwave heating. Average particle size, polydispersity, and phase composition of Cu NPs depended mainly on the reducing agent ratio. Likewise, thermal stability and antibacterial activity of stabilized Cu NPs were affected by their phase composition. Because of the carboxylate groups in polymer chains, the structural changes of stabilized Cu NPs are different from those of NaAlg. NaAlg acted as a size controller and stabilizing agent of Cu NPs, due to their ability to bind strongly to the metal surface. Our study on the stabilizing agent-dependent structural changes of stabilized NPs is helpful for wide application of NaAlg as an important biopolymer.