Composite materials based on Zinc sulfide and Zinc oxide: structural and biocidal properties

Abstract

Introduction. The widespread use of drugs with antimicrobial action has led to the formation of microorganism resistance against wide range of antibiotics. One of the approaches to dissolving this problem is the substances modification by inorganic bioactive ions in oder to initiate a controlled reaction in the bone tissues and provision of antimicrobial activity. It is known that ZnO-based materials have a pronounced biocompatibility, they are characterized by high limit strength, absolute mechanical hardness, as well as the ability to withstand the harsh operating conditions.  The aim of this work is the study of structural and biocidal properties of composite material based on zinc oxide and zinc sulfide (ZnS-ZnO) and its complex with an organic substance - sodium alginate (ZnS-ZnO-Alg) for use in biomedical purpose. Materials and methods. For the synthesis of ZnS-ZnO composite 50 ml 0.2M solution zinc nitrate was added to the 50 ml 0.2M thiourea CS (NH ₂) ₂ solution and stirred in a shaker for 60 minutes. The formation of the compound took place when added to a mixture of 25 mas.% solution of ammonia with the subsequent heating at 80 ^oC for 30 minutes. Synthesis of the metalorganic complex of ZnS-ZnO-Alg was performed by above mentioned procedure, but to the thiourea solution was previously added 1 ml of 3 mas.% solution of sodium alginate under ultrasonic mixing.. For the next research composites were dried or lyophilized. Study of antibacterial activity of the ZnS-ZnO and ZnS-ZnO-Alg particles was carried out with the use of nutrient mediums: Muller Hinton, meat-pepton nutrient (MPN). As the reference cultures were used E. coli ATCC 25922, S. aureus ATCC 25923, S. aureus ATSS 29213, S. aureus ATSS-6538, C albicans ATCC 885-653. Determination of the minimum bactericidal concentration (MBC) was carried out by a modified serial diluted method in liquid nutrient broth followed plating on solid Muller Hinton nutrient medium. In addition, the study of the sensitivity of the above listed microorganisms to the experimental samples was carried out by the method of diffusion in Agar in the modification of the wells. The crystallinity and structure of precipitates were examined using an X-ray diffractometer DRON 3. The elemental composition of synthesized samples was studied by an X ray fluorescence (XRF) analysis using ElvaX Light SDD spectrometer. Results and discussion. X-ray structural analysis indicate that in the composite material, synthesized both in the presence of sodium alginate (ZnS-ZnO-Alg) and without sodium alginate adding (ZnS-ZnO) exist two phases: ZnS and ZnO. Based on RFA calculations show that ZnS-ZnO sample contains up to 50 wt. % zinc oxide. Zinc oxide content in the ZnS-ZnO-Alg is about 25 wt.%. The MBC was determined by above described method placing the liquid from each tube with a sample on a Mueller-Hinton solid culture medium. MBC of ZnS- ZnO-Alg samples against all studied microorganism strains was 1,25 mg/ml. MBC of ZnS- ZnO samples ranged from 5 mg/ml for C. albicans to 12.5 mg/ml for E. coli. It is possible that due to small sample solubility in experimental conditions and small ion diffusion of the active substance there was no full contact with the whole bacterial cell volume. As ZnS-ZnO-Alg samples differe from ZnS- ZnO samples by smaller crystallite size and greater solubility, they exhibit a marked antimicrobial effect. At the same time in direct contact of the entire surface of the sample with bacterial cells under condition of the modifying method of diffusion into agar, both types of samples showed high antimicrobial activity. Obtained data can be explained by two major mechanisms of the antimicrobial action ZnO and ZnS: a) toxic effects of zinc ions in the cell membrane of bacteria; b) toxicity ROS (reactive oxygen spices), formed with the participation of ZnO and ZnS, on components of the bacterial cell. Antibacterial activity is the result of the formation such ROS, as hydrogen peroxide (H₂O₂), peroxide anion (O^2-), hydroxyl radicals (OH^-). These particles damage cellular components such as DNA, lipids and proteins. Conclusion. The composites ZnS- ZnO and ZnS-ZnO-Alg have been obtained by the applied method of synthesis. X-ray structural analysis of samples proved the presence of ZnO and ZnS phase with defined structure: ZnS has a cubic crystal structure type sphalerite (JCPDS 5-566) with average crystallite size of 23 nm and ZnO - hexagonal structure (JCPDS 80-75 ) with an average size of about 35 nm. The introduction of sodium alginate to the reaction mixture during synthesis reduces the size of ZnS and ZnO crystallites to 10 nm and 12 nm, respectively. In the ZnS-ZnO-Alg samples, synthesized in presence of sodium alginate, the ZnS phase content increased for 25wt.% compared with the ZnO phase, which was confirmed by XRF. Microbiological studies have shown the presence of antimicrobial activity of samples against Gram-positive S. aureus, Gram-negative E. coli and fungi C. albicans. The estimated values for the integral antimicrobial activity, calculated by the vector theory, are for ZnS-ZnO and ZnS-ZnO-Alg 1,57 and 1,9 respectively. It means that both types of samples have average antimicrobial activity.