The modification of Laponite® with silver or copper and investigation of their antibacterial activity

Abstract

Silver and copper modified Laponite® RD were synthesised either by ion exchange or isomorphous substitution into Laponite® RD sheet. Six different concentrations of silver-exchanged Laponite® RD (AGLAP1-6) or copper exchanged Laponite® RD (CULAP1-6) were produced in this work via ion exchange and three different concentrations of silver incorporated Laponite® RD (AL1-3) and a concentration of copper incorporated Laponite® RD (CL1) were synthesized via isomorphous substitution. The silver or copper modified Laponite® RD were characterised with X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), energy dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), magic angle spinning nuclear magnetic resonance (MAS NMR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The XRD showed changes in the phase purity of pure and modified Laponite® RD. The amount of silver or copper exchanged or incorporated into Laponite® RD interlayer or sheet were determined by EDX. The TEM image analysis indicated that the Ag+ and Cu2+ ions were in the nanometer range with a modal size of 13±1 and 15±1nm for silver and copper-exchanged Laponite® RD, 10 and 18nm for silver and copper incorporated Laponite® RD respectively. The antibacterial activity was investigated by exposing Escherichia coli K12W-T, Staphylococcus aureus NCIM B6571 and Pseudomonas aeruginosa NCIMB8295 in tryptone soya broth to the different silver-Laponite® RD and copper-Laponite® RD nanocomposites for a period of 72 hours. All concentrations of silver-Laponite® RD and copper-Laponite® RD reduced the growth of all the bacterial spcies. There was no growth of P. aeruginosa after 6 hours, E. coli after 8 hours and S. aureus after 24 hours with silver-exchanged Laponite® RD containing 2.35 wt. % Ag (AGLAP6); P. aeruginosa after 8 hours, E. coli iv after 24 hours and S. aureus after 24 hours with silver-exchnaged Laponite® RD containing 1.76 wt. % Ag (AGLAP4). Similarly total inhibition was obtained in the medium supplemented with silver incorporated Laponite® RD containing 1.10 wt. % Ag (AL3) for P. aeruginosa at 6 hours, both E. coli and S. aureus at 8 hours; with silver incorporated Laponite® RD 0.58 wt. % Ag (AL2) for P. aeruginosa at 8 hours, both E. coli and S. aureus at 24 hours. To access the persistency and slow release of the antibacterial agents the release profile was compared with AgNO3 and CuSO4. While all the silver-Laponite® RD and copper-Laponite® RD showed slow and consistent release over the whole investigation period, AgNO3 and CuSO4 were abrupt, signifying that Laponite® RD is an efficient slow release material. In addition, silver ions eluted more quickly from the silver incorporated Laponite® RD than the silver-exchanged Laponite® RD. Unmodified Laponite® RD showed no antibacterial property to any of the bacteria tested, but was responsible for the consistent and controlled slow release of the metal nanoparticles into the tryptone soya broth, a phenomenon potentially utilizable in bandages where release for long duration is crucial.