E-beam fabrication of antibacterial silver nanoparticles on diameter-controlled TiO₂ nanotubes for bio-implants

To reduce the risk of infection, many researchers have applied antibacterial silver to TiO₂ implant surfaces. In this study, silver nanoparticles were applied to TiO₂ nanotubes with e-beam evaporation. In advance, the diameters of the nanotubes were controlled by varying anodization. The objective of this study was to optimize the antibacterial effects of silver nanoparticles, but also maintain the nanotubular surface of biocompatible TiO₂ nanotubes. The surfaces were characterized with field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), high resolution X-ray diffraction (HR-XRD), X-ray photoelectron spectroscopy (XPS) and scanning probe microscopy (SPM). Contact angle measurements and 3 times simulated body fluid immersion (SBF) tests were performed for indirect verification of osteogenic properties. Results showed that the silver nanoparticles were efficiently fabricated on the nanotubes. We found that the contact angle was influenced more by the degree of anatase phase in the TiO₂ than by surface roughness and apatite forming ability was proportionally increased as hydrophilicity was increased. Silver ion release was measured by the inductively coupled plasma mass spectrometry (ICP-MS). All specimens showed more than 0.1ppb of silver concentration which was known to be the level for the antibacterial property. The antibacterial activity test against Staphylococcus aureus (S. aureus) showed that bacterial colonization was effectively inhibited in all experimental groups. Water soluble tetrazolium (WST) assays showed that silver nanoparticles fabricated on large-diameter nanotubes were not cytotoxic. However, smaller diameter of nanotubes showed mild cytotoxicity, due to the excess of silver nanoparticles aggregated on the top surface. Overall, we concluded that large-diameter TiO₂ nanotubes had favorable osteogenic properties, were non-cytotoxic, maintained a nanoporous surface, and displayed antibacterial properties that were highly beneficial for bio-implants.