REPOSITORY > RESULTS

Abstract

A new approach for the synthesis of silver (Ag), platinum (Pt) and gold (Au) nanoparticles and their composites with hydroxyapatite (HAp) is developed via a sonochemical process. The major goal was to design physicochemical properties of material in order to achieve: (i) intensive antibacterial activity and (ii) maximal reduction of toxicity for humans and their environment.
Silver, with natural ability for antibacterial activity, was investigated for study of the mechanism of antibacterial activity and sources of toxicity. Sonochemically formed HAp/Ag composites have three sources of antibacterial activity: Ag-ions, incorporated within apatite structure, smaller Ag nanoparticles, embedded within apatite plate-like particles, and larger Ag nanoparticles, attached onto the surface of the apatite particles. These metal particles have a cubic and hexagonal structure and they are spherical and hexahedral in shape. Beside the presence of different factors able to affect antibacterial activity of HAp/Ag composite, such as silver state, structure, particles shape and size, antibacterial activity of the composite was amplified only by irradiation by light. It confirmed that reactive oxidative stress driven by reactive radicals is the major mechanism of its action. A non-selectivity of this mechanism is a major source of very high toxicity of this metal which is able to induce necrosis of almost 100% of human cells for 1 h. These results confirm an urgent need for finding a more suitable replacement of this metal, widely present in practice. These findings encouraged us to develop more human-friendly materials with enhanced antibacterial activity.
The first strategy used for development of suitable Ag-replacement concerned activation of platinum, as a bioinert and non-toxic metal. Bio-inspired approach to photocatalysis with a selection of biologically acceptable building blocks that can be activated by non-toxic visible light is used for its activation. Combination of HAp with platinum (Pt0 and Ptn+) provided formation of a novel photocatalitically active material. Thus formed hybrid semiconductor/metallic nanocomposites made of apatite plate-like particles with Pt metallic nanoparticles attached on their surface and two different types of surface adsorbed Pt-complexes belong to innovative solutions for design of functional bioactive materials. This material can be activated by exposure to both, ultraviolet (UV) as well as visible (VIS) light. UV activation is allowed by semiconductive HAp phase, while activity induced by VIS is related to complexes adsorbed on HAp surface. Pt metallic nanoparticles have the ability of electron trapping which provides extension of material’s activity during the period when it is kept in the dark. Thus developed material has an intensive ability for degradation of non-degradable dyes which confirmed its ability to form high concentration of reactive radicals. However, in contrast to Ag, Pt nanoparticles have dual nature able to release and to accept electrons. It regulates radical formation, decreases toxicity but in the same time limits intensity of developed antibacterial activity. Developed material succeeds only in overcoming the toxicity issue of silver.
The second strategy used for development of suitable replacement of Ag-based antibacterial materials was activation of gold. Similar to the platinum, gold is also bioinert and non-toxic metal. The strategy used for its activation concerned surface functionalization. Naturally-sourced amino acids are the most effective in this process. Finally-formed materials contain HAp plate-like particles with functionalized Au nanoparticles attached onto their surface. In this case amino acids are involved into attachment of Au nanoparticles onto HAp surface. Thus designed material has an amphiphilic structure with hydrophobic gold and hydrophilic amino acid. In comparison with Ag and Pt, this material is unable for photocatalytic activity and production of reactive radicals which is partially the source of its non-toxicity. The major mechanism of its action against bacteria is charge. Accordingly, it classifies developed HAp/Au/functionalized material into analogous of the group of the special type of antibacterial agents known as antibacterial peptides. It is able for electrostatic interactions with bacterial cells that destroy the structure and change a permeability of their walls. Positively charged groups of amino acids attached onto Au are prone to interact with negatively charged surface of bacteria. This mechanism is selective since human cells have a completely different surface charge than bacteria which decreased affinity of material to interact with them and ensured its non-toxicity for human cells. Developed material succeeds in overcoming the problem of toxicity of silver and in the same time it has a very strong antibacterial activity which even exceeds intensity of silver activity against bacteria.
Newly developed HAp/Pt and HAp/Au composites are good alternatives to composite with silver. HAp/Pt possesses ability for self-cleaning while HAp/Au has intensive antibacterial activity. Capabilities of developed materials together with green method for their production and high decrease of their toxicity for human cells are significant improvements compared to highly toxic Ag. These properties introduce newly developed composites as novel nanomaterials with a potential to improve health care using smart mechanisms able to “recognize” mammalian cells.