Improved antimicrobial activity of silica–Cu using a heteropolyacid and different precursors by sol–gel: synthesis and characterization

Abstract

Antimicrobial coatings are used to avoid the proliferation of fungi and bacteria inside urban buildings by incorporating low concentrations of antimicrobial agents (biocides). This study is focused on evaluating the antifungal activity of copper (II) supported on a silica matrix, and as counter-cation of a heteropolyacid, obtained from the sol-gel method using various catalysts, against two fungal isolates. The fungi used in this evaluation were Alternaria alternata and Chaetomium globosum isolated by routine microbiological techniques from biodeteriorated paints films. In all cases, fungi were selected due to their ability to grow on the paint films and their negative impact on human health. First, silicas were prepared using tetraethylorthosilicate and methyl trimethoxysilane as precursors and acetic acid, hydrochloric acid as catalysts of the hydrolysis reaction of the alkoxide. The effect of addition of copper (II) in the form of copper nitrate was studied. With respect to the morphology of the silica, most of them are similar, between pure and modified silica, in all cases. Once the stage of the synthesis and characterization of the prepared materials was performed, six of them were selected to evaluate their antifungal activity by agar plate inhibition test against both fungi of interest (A. alternata and C. globosum), and the percent inhibition was determined in each case. Tested Cu-based solids were shown to have a higher antifungal activity because they completely inhibit the growth of both fungi with lower concentrations relative to its control. This work was performed as a preliminary study, in order to guide the selection of a suitable organic biocide from a list of possible antifungal agents. Graphical Abstract

Assessment of waterborne coatings formulated with sol-gel/Ag related to fungal growth resistance

“…The homogeneous solution obtained was vigorously stirred for 2 h and, kept for 7 days at room temperature, for gelation and drying. In the samples prepared with acetic acid, the molar ratios of the reagents were 1:1.16:5:3.7 for precursor/catalyst/ solvent/water respectively, whereas in the samples prepared with HPA, the addition of the catalyst was done in order to reach a final concentration into the silica of 11 wt.% of HPA [25,26], and the molar ratios of the reagents were 1:0.005:5:3.7. In ST-HAg sample, silver acetate was added in the proper amount to occupy the half of the acid sites of HPA (H 1.5 Ag 1.5 ) and, in ST sample an ethanol solution of HPA-Ag, 11 wt.% of HPA-0.3 wt.% of silver acetate, was added to obtain the impregnated solid (HAg/ST sample), then was stirred for 15 min and dried at room temperature.…”

“…When acetic acid or molybdophosphoric acid were used in the synthesis, a variation of E i was observed. ST and ST-H samples presented E i values of 135 mV and 463 mV respectively, showing an increase of the acidity when HPA is used as catalyst, because of an extremely high initial acidic strength (800 mV) of the commercial heteropolyacid (HPA) [26]. When silver was added into the synthesis (ST-HAg sample) the E i value decreased to 99 mV, showing a substitution of HPA protons by silver ions.…”

Fourier Transform Infrared and Raman Characterization of Silica-Based Materials

“…The surface characteristics are another feature that are able to be tuned by the silica coatings. In this field, it is possible to mention the superhydrophobicity that is widely explored with the possibility of self-cleaning surface creation [22,23] and also the addition of antimicrobial properties is of high importance [24,25].…”

Functionalized zirconia compounds as antifungal additives for hygienic waterborne coatings

“…In the samples synthesized with acetic acid, the molar ratio of the reagents was 1:1.16:5:3.7 for precursor:catalyst:solvent:water respectively, whereas in those prepared with PMA, the addition of the catalyst was done in order to reach a final concentration into the silica of 11 wt. % of PMA [29,30], and the molar ratio of the reagents was 1:0.005:5:3.7. The composition and nomenclature of the synthesized solids are given in Table 1 and, in accordance with the compositions the following meanings were adopted: S = silica, Z = zirconia, SZ = silicazirconia, A = acetic acid, H = heteropolyacid and, Ag = silver.…”

“…The potentiometric titration allows the determination of the acidic properties of a dispersion of solid particles by measuring the electrode potential through the membrane. The initial electrode potential (E i ) indicates the maximum acid strength of the sites, and it may be classified according to the following scale: very strong sites, E i > 100 mV; strong sites, 0 < E i < 100 mV; weak sites, -100 < E i < 0 mV; and very weak sites, E i < -100 mV [30,41]. When PMA were used as catalyst in the synthesis compare with acetic acid, an increase of the acidity was observed, and this difference could be attributed to the initial acidic strength extremely high (800 mV) of PMA [30].…”