RESUMO
This communication is focused on the synthesis, characterization and experimental proof of the mechanism of antimicrobial activity of powders from the molybdenum-tungsten-oxygen (Mo-W-O) system. Materials with a discrete ratio of Mo/W ranging from 100% MoO3 to 100% WO3 with a stepwise increase of 5-10 wt % W were synthesized by the spray drying method following calcination. Spherical hollow particles with a broad size distribution were formed and the composition influenced the crystalline phases in such a way that either pure and/or mixed oxides (Mo0.6W0.4O3) were obtained. A good correlation between composition variation and phases present on the antimicrobial activity is obtained and provides a detailed screening of the activity efficiency versus compositional transition. Antimicrobial tests were performed against a model Gram-negative bacterium (Escherichia coli). Furthermore, the mechanism of antimicrobial activity is proven by correlating the medium acidification via pH measurements to the bacteria lifespan at low pH values. The mechanism is additionally supported by the bacterial growth when a buffered nutrient medium was used, together with the evidence that the powder particles have no disruptive effect on the cell wall. Consequently, an extended mechanism is proposed for the mixed oxide, relating both the structure and solubility results. Solubility measurements displayed a steep decrease in metal ions concentration with the addition of W. A narrow compositional range was identified (80 to 60 wt % Mo) where the antimicrobial activity was present, which is concurrent with a very strong decrease in solubility. Materials within this range show adequate features for being implemented into hybrid systems consisting of inorganic materials-polymers/varnishes that can be used for touch surfaces in healthcare settings.
RESUMO
Developing novel compounds with antimicrobial properties can be an effective approach to decreasing the number of healthcare-associated infections, particularly in the context of medical devices and touch surfaces. A variety of molybdate powders (Ag2MoO4, CaMoO4, CuMoO4 and Cu3Mo2O9) were synthesized and characterized, and Escherichia coli was used as a model gram-negative bacterium to demonstrate their antimicrobial properties. Optical density measurements, bacterial colony growth, and stained gel images for protein expression clearly showed that silver- and copper molybdates inhibit bacterial growth, whereas CaMoO4 exhibited no bactericidal effect. All tests were performed in both daylight and darkness to assess the possible contribution of a photocatalytic effect on the activity observed. The main mechanism responsible for the antibacterial effect observed for Ag2MoO4 is related to Ag+ release in combination with medium acidification, whereas for compounds containing copper, leaching of Cu2+ ions is proposed. All these effects are known to cause damage at the cellular level. A photocatalytic contribution to the antibacterial activity was not clearly observable. Based on the pH and solubility measurements performed for powders in contact with various media (ultrapure water and bacterial growth medium), silver molybdate (Ag2MoO4) was identified as the best antibacterial candidate. This compound has great potential for further use in hybrid powder-polymer/varnish systems for touch surfaces in healthcare settings.
