Silver-functionalized mesoporous silica nanoparticle coatings: Optimal thermal stability and ionic activity for antimicrobial applications

Silver-loaded mesoporous silica nanoparticles were synthesized through a multiple-step process and utilized as a constitutive element to develop a compact, uniform and stable nanocomposite coating. Firstly, MCM-41 nanoparticles were produced through a typical sol-gel process, consisting of the hydrolytic condensation of tetraethoxysilane in the presence of hexadecyl cetyltrimethylammonium bromide. Subsequently, the surface of mesoporous silica nanoparticles was modified through a silanization process using aminopropyltriethoxysilane. The resulting amino-functionalized mesoporous nanoparticles were then immersed in an anhydrous silver nitrate solution to induce the adsorption of silver ions. The Ag+ ions were adsorbed by the mesoporous nanoparticles following the Langmuir model, resulting in a highly stable nanocomposite, with an Ag/SiO2 ratio of 86.4 +/- 2.8 mg g-1. The thermal stability of the constitutive silver-loaded nanoparticles and the early thermal development of silver nanoparticles, confined within the mesoporous structure, were analyzed through Fourier transform infrared spectroscopy, UV-visible spectroscopy, X-ray diffraction and transmission electron microscopy, determining that the thermal degradation occurs above 200 degrees C and silver ions undergo a progressive transformation into metallic silver nanoparticles. The obtained silver-loaded silica nanoparticles were incorporated into an epoxy-functionalized sol-gel precursor, forming a compact nanocomposite coating with unaffected adhesion and structural consolidation. The internal structure of the compact nanocomposite coating was analyzed through scanning electron microscopy revealing a satisfactory dispersion within the embedding material. The release of Ag+ ions across the coating structure was verified through electrochemical impedance spectroscopy determining its ionic conductivity, which diminished by around sixty percent, from 232 +/- 42-88 +/- 46 nS cm-1, after 30 minutes of immersion in deionized water at 37 degrees C. These results allow us to foresee the potential application as a progressive-release biocide material for intensive applications in critical areas, such as hospitals or medical devices, where it is crucial to maintain a sterile environment.