Influence of the powder dimensions on the antimicrobial properties of modified layered double hydroxide

Nano-hybrids of layered double hydroxides (LDHs), or hydrotalcite-like compounds, containing the antimicrobial anions of o-hydroxybenzoate (o-BzOH), also known as salicylate, were synthesized by a direct coprecipitation method. Elemental analysis indicated the chemical formula [Zn0.65Al0.35(OH)2] (C7H5O3)0.35 ∗ 0.7 H2O with a value of the molar fraction x = MIII/MIII + MII of 0.35. FT-IR spectroscopy and X-ray diffractograms confirmed the presence of o-hydroxybenzoate anions into the clay galleries, and indicated the presence of small size and disordered crystallites. The initial powder dimensions, reported as “surface weighted mean”, D[3,2], were reduced by using high energy ball milling (HEBM) technique for 1 min, 2 min, and 5 min. Also the morphology, investigated by SEM, was influenced by HEBM: big agglomerated particles were found for the pristine samples, while well-defined platelets with uniformand thinner size were present in the milled samples. The release of the o-hydroxybenzoate anion into a saline solution was measured for the initial powder with 25.0 μmofD[3,2] and for the powder with the smallest dimensions (D[3,2] = 8.2 μm). It was observed that the lower the powder dimensions the higher the percentage of active anions released into the saline solution. Also the diffusion parameter was found higher for the lower dimension powders. The antimicrobial activity against Escherichia coli was found much more effective for the sample with the lower powder dimensions, and even a slow but progressive reduction was noted after 24 h. In the presence of particles of 8.2 μm the microbial growth after 96 h of incubation is not evident.