Depositing gold nanoparticles in titanium dioxide nanotube arrays may be just what is needed to boost bacteria-killing properties in some implants, according to research out of the Shanghai Institute of Ceramics at the Chinese Academy of Sciences (SICCAS).

The Chinese researchers explained that although titanium dioxide is able to kill bacteria itself, its antimicrobial properties are dependent on a light source. When the metal is exposed to light, it becomes energetically excited by absorbing photons. This generates electron-hole pairs, turning titania into a potent electron acceptor that can destabilize a bacterium's cellular membrane processes by usurping their electron transport chain's terminal acceptor. The membrane is gradually destabilized by this thievery, causing the cell to leak out until it dies.

In the absence of light, the bacteria-killing efficacy of titanium dioxide is limited. Gold nanoparticles, though, can continue to act as antibacterial terminal electron acceptors in darkness due to a phenomenon called localized surface plasmon resonance. Surface plasmons are collective oscillations of electrons that occur at the interface between conductors and dielectrics - such as between gold and titanium dioxide. The localized electron oscillations at the nanoscale cause the gold nanoparticles to become excited and pass electrons to the titanium dioxide surface, thus allowing the particles to become electron acceptors.

The scientists have described the results of tests with a new antibacterial material they developed based on gold nanoparticles in "Plasmonic Gold Nanoparticles Modified Titania Nanotubes for Antibacterial Application" (Li, et al.) in the journal Applied Physics Letters.

"Implant-associated infections have become a stubborn issue that often causes surgery failure," said Xuanyong Liu, primary investigator of a team of researchers at the Shanghai Institute of Ceramics at the Chinese Academy of Sciences (SICCAS). Designing implants that can kill bacteria while supporting bone growth, Liu said, is an efficient way to enhance in vivo osteointegration.

Liu and his team electrochemically anodized titanium to form titanium dioxide nanotube arrays, and then further deposited the arrays with gold nanoparticles in a process called magnetron sputtering. The researchers then allowed Staphylococcus aureus and Escherichia coli to grow separately on the arrays .They report that both organisms were highly unsuccessful, exhibiting profuse membrane damage and cell leakage.

"The findings may open up new insights for the better designing of noble metal nanoparticles-based antibacterial applications," Liu said.

Further research for Liu and his colleagues includes expanding the scope of experimental bacteria used and evaluating the arrays' in vivo efficacy in bone growth and integration.

Stephen Levy is a contributor to Qmed and MPMN.

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