Application of the response surface methodology for the evaluation of Staphylococcus aureus inhibition with Ag/TiO2 nanoparticles

Abstract

The present work shows the implementation of the response surface methodology, fed by an experimental central composite design (CCD) to find the conditions that allow maximizing the inhibition of the microorganism Staphylococcus aureus with nanoparticles of TiO2 silanized with 3-aminopropyltriethoxysilane (APTES) and doped with Ag. In addition, poly(lactic) acid composites were prepared with these Ag/TiO2 nanoparticles with the aim to confer their antimicrobial effect. The independent variables considered were pH, AgNO3/TiO2 ratio (% w/w), and TiO2 nanoparticles concentration (g/250 mL), and as the variable of response, the length of the diameter of the halo or zone of inhibition presented by the microorganism (mm). Statistical analysis found that maximization of S. aureus inhibition occurs at intermediate levels with a value of 10 for pH and 5 g of TiO2 solids, while for the concentration of AgNO3 high levels are required, greater than 10% w/w. Likewise, the statistical significance was determined using the Student's t test and the p value; it was found that the significant effect corresponds to the concentration of AgNO3, so a second experimental CCD design equiradial with two factors was considered, estimating AgNO3 concentration and TiO2 amount, the pH at constant 10 value. The second experimental design indicated that maximization in S. aureus inhibition occurs at an AgNO3 concentration between 20 and 25% w/w with high amounts of TiO2 solids (7–8 g), with a resulting zone of inhibition between 26 and 28 mm. The quadratic model obtained, which represents the relationship between the lengths of the zone of inhibition with the variables considered, shows an adjustment of experimental data with a coefficient of determination (R2) of 0.82.