Antimicrobial propensity of ultrananocrystalline diamond films with embedded silver nanodroplets

Abstract

Ultrananocrystalline diamond (UNCD) layers exhibit excellent mechanical properties and combine chemical inertness with good biological compatibility. Therefore, UNCD is considered a promising material for coating of implants. In this work we present the preparation of thin UNCD films with embedded silver nanodroplets that provide antimicrobial property, addressing another important topic concerning implant surgery, namely the risk of a life threatening bacterial infection. UNCD layers were prepared by microwave plasma-assisted chemical vapor deposition on a silicon substrate. Afterwards, a thin film of silver was deposited on top and treated by rapid thermal annealing (RTA) leading to dewetting and formation of silver nanodroplets on the surface. A second UNCD deposition with a short duration between 5 and 30 min was applied for capping the silver nanodroplets with a thin layer. The sample surfaces were characterized after each step by atomic force microscopy and scanning electron microscopy. The composition of the final samples, including the depth of the incorporated Ag nanodroplets, was analyzed by Auger electron spectroscopy. The impact of the silver layer thickness and the RTA temperature on the nanodroplet morphology was investigated. It was found that after 10 min of capping deposition the silver particles were completely covered with UNCD. In order to study the release of silver ions, the UNCD/Ag/UNCD samples were submerged in deionized water for 7 days at 37 °C, followed by detection of the silver concentration in the aqueous samples by inductively coupled plasma mass spectrometry. The determined concentration was strongly dependent on the thickness of the capping UNCD layer, exhibiting the highest silver content for the sample with the thinnest capping layer. Thus, the UNCD layer thickness can be utilized to control the amount of Ag ions released into the surrounding environment. The antibacterial properties were investigated with bacterial assays of the Gram-negative Escherichia coli and Gram-positive Bacillus subtilis bacteria that were exposed to the samples. All silver containing samples showed significant antimicrobial propensity, whereas the different capping thicknesses affected the time-course dependent antibacterial efficiency. Prime noveltyThin UNCD films were provided with supreme antibacterial propensity by embedding of silver nanodroplets. The thickness of the UNCD capping layer was varied and its influence on the release of silver in water was investigated. The UNCD/Ag/UNCD samples showed significant and controlled by the capping thicknesses time-dependent antibacterial efficiency against Gram-negative E. coli and Gram-positive B. subtilis bacteria.