Fast Growth of AgMOF—Micro and Nano Crystals as Antibacterial Coating for Ti-Based Implants

Bacterial infections are quite common in post-operative recovery for prosthetic bone replacements due to the bacteria capability of adapting to commonly used drugs. This leads to rejection of the prostheses and a subsequent return to the operating room for the replacement of the prosthesis. The study of new antibacterial materials and of their anchoring on implants surfaces has, therefore, a key role in scientific research. Various inorganic and organic materials have been used for this purpose, and, among them, MOF (Metal Organic Framework) possess great potentiality. They are composed by organic ligands and metallic subunits, bond together to obtain a framework. They present a series of active sites, micro and nano porous structures and highly modulable surface areas. Silver-based inorganic materials gained interests in biomedical applications thanks to Ag⁺ ions antibacterial properties.<br/>We synthesized a silver-based MOF using a new synthetic method through which less toxic solvents, lower temperatures, and much shorter times are involved compared to commonly used approaches. We chose 1,4-benzene dicarboxylic acid as ligand because of its low toxicity and the weak bond with Ag⁺ that allow slow release of Ag⁺ ions in solution. The developed approach makes it possible to control the growth of specific crystalline phases in solution and on the implant surface as well.<br/>We developed a fast low temperature synthetic method using dimethyl sulfoxide as solvent. Starting from AgNO₃, and using ammonia to stabilize Ag⁺ ions, we obtained a mixed crystalline phase in 3h and the pure product in 5h at 70°C. To reduce synthesis times, we used two modulators (KCl and benzoic acid). In these conditions, the pure product was obtained in 3h with both modulators at 70°C. The silver ions release has been studied in water solution in neutral conditions and at different pH levels to verify a possible increased release during infections, in which pH values drop. Antibacterial properties were then evaluated to obtain the Minimum Inhibiting Concentration (MIC) and the Minimum Bactericidal Concentration(MBC) . For gram-negative MIC and MBC were observed at 7.8 μg/ml and for gram-positive MIC and MBC were observed at 31.3 μg/ml and 62.5 μg/ml respectively.<br/>We also grow this material on a classic titanium scaffold for bone transplantation. To achieve surface growth, we treated titanium scaffolds and functionalized its surface with organic ligands like p-phosphono benzoic acid. By immersing the scaffold in an Ag⁺ ion solution it was possible to obtain preferential crystallization sites for the MOF. Using similar synthesis methods, we obtained a continuous coverage of the Ti substrate.