It has been reported that although the antimicrobial action of silver nanoparticles may share many of the same mechanisms as ionic silver, silver nanoparticles have a much higher antimicrobial efficacy. If the antimicrobial properties are as a result of Ag + ions or free radicals on the surface of silver nanoparticles, it would suggest that each particle acts as a reservoir of antimicrobial elements that can be released gradually. One report has determined that free radicals are released from the surface of silver nanoparticles and, after comparing the antimicrobial effect of silver nanoparticles with samples also containing an antioxidant to eliminate free radicals, concluded that they may be responsible for the antimicrobial activity ( Kim et al., 2007). It has also been reported that the bactericidal properties of silver nanoparticles are actually attributed to Ag + ions that are adsorbed on to the surface of the particles owing to their high oxidation reactivity ( Morones et al., 2005 Lok et al., 2007). This observation was found to correlate with bacterial cell death, which was suggested to be as a result of nanoparticles interacting with the sulfur-containing proteins within the bacteria, causing membrane permeability and damage to their DNA ( Morones et al., 2005 sondi and Salopek-Sondi, 2004). The mechanism of this action has been investigated using transmission electron microscopy (TEM) based techniques, which found that silver nanoparticles smaller than 10 nm can attach to and penetrate the cell membranes of Escherichia coli, a common gram-negative bacterium ( Morones et al., 2005 sondi and Salopek-Sondi, 2004). In addition to ionic and nanocrystalline silver, metallic silver nanoparticles have also been shown to possess antimicrobial properties. Su, in Electrospinning for Tissue Regeneration, 2011 Silver nanoparticles Amorphous and polycrystalline hydroxyapatite and doped polycrystalline layers were non-toxic, while silver-doped amorphous HA layers were mildly toxic from1.2 at.% Ag ( Jelinek et al., 2013b). Crystallinity was not found to have any effect on antibacterial efficacy. An antibacterial efficacy of 100% was observed for silver-doped HA layers of 8.3 and 13.7 at.% Ag concentration. For Gram negative bacteria the efficacy changed from 3.9% to 100% after silver dopation. The Ag + HA layers, with 4.4%, 8.3% and 13.7% of Ag, exhibited excellent antibacterial ability. In biological tests it was found that for Gram positive bacteria silver dopation raised the antibacterial efficacy from 71% to 99.9%. We observed a distinct difference between the Young’s modulus Y and hardness H of amorphous HA and Ag + HA films on the one hand and the polycrystalline equivalents on the other, as well as a decrease in Y and H with silver dopation. Distribution of Ag (a), Ca (b), P (c) elements in Ag + HA layer (SEM magnification 1500 ×, 10 micrometer scale, 4.4 at.% Ag).
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