Silver Nanoparticle Impact on Bacterial Growth: Effect of pH, Concentration, and Organic Matter

Abstract

Silver nanoparticles (Ag NPs) are widely used as antibacterial agents. This antibacterial property carries with it a potential environmental risk once these NPs are discharged into the environment. This study investigated the impact on Pseudomonas fluorescens over a 24 h exposure of well characterized Ag NPs at pH values of 6-9, in the presence and absence of Suwannee River humic acids (SRHA). Ag NPs were characterized by size, aggregation, morphology, dissolution, and surface properties under all conditions. Solubility was low (less than 2%) for all Ag NP concentrations (2-2000 ppb) and under all conditions was less than 40 ppb (0.38 microM). SRHA caused a partial disaggregation of Ag NP aggregates by nanoscale film formation, with individual NPs stabilized by charge and entropically driven steric effects. Dissolved Ag reduced bacterial growth entirely at 2000 ppb (19 microM) under all conditions and adversely affected growth at 200 ppb (1.9 microM) under some conditions, indicating some toxicity. The Ag NPs showed similar toxicity at 2000 ppb (19 microM) in the absence of SRHA and at pH 9 only i.e. SRHA mitigated bactericidal action. Solubility and interactions with SRHA indicate that there was a specific nanoparticle effect which could not be explained by the effect of dissolved Ag.

Metal‐based nanoparticles in soil: Fate, behavior, and effects on soil invertebrates

“…Jiang et al [36] found that the hydrodynamic diameter of TiO 2 NP aggregates varied with the solution pH, being greatest at the isoelectric point (pH 6.0). Fabrega et al [46] reported that Ag NPs stabilized by citrate coating showed no variation in aggregate size over the pH range 6 to 9, consistent with the findings of El Badawy et al [47] for similarly stabilized Ag NPs. Here, the invariant aggregate size may be explained by the relatively small variation in zeta potential of the particles in this pH range [47].…”

“…Here, the invariant aggregate size may be explained by the relatively small variation in zeta potential of the particles in this pH range [47]. Small aggregates of Ag NPs adsorbed humic acid, and this adsorption resulted in disaggregation (or disagglomeration) of the NPs [46].…”

Analysis of engineered nanomaterials in complex matrices (environment and biota): General considerations and conceptual case studies

“…Electron microscopy is presently the predominant technique to investigate the presence, aggregation state, location, and composition of ENPs in ecotoxicological tests and on/in organisms [68][69][70] (see case study II and also Dudkiewicz et al [71], which gives a comprehensive overview of EM techniques for ENP analysis in food matrices). Both the high-resolution TEM and the field emission gun-equipped SEM or scanning transmission electron microscopy systems provide the necessary spatial resolution to visualize particles down to a few nanometers in diameter.…”

Ecotoxicity test methods for engineered nanomaterials: Practical experiences and recommendations from the bench

“…For example, dissolved organic matter can be critical in controlling metal bioavailability to aquatic organisms [27]. Conversely, humic and fulvic acids are also known to reduce the toxicity of ENMs [28,29] and slow the dissolution of metal-based ENMs [30]. Moreover, natural dispersants are usually not well characterized, and the properties of natural organic matter often used in ENM experiments, such as Suwannee River humic acid, may not be representative of the wide range of organic matter in different environments.…”

“…For example, for a 14-or 21-d plant growth test, very unstable NPs may completely dissolve during the test. This may be the case for AgNPs in liquid media [29], but it may not be a realistic representation of the fate of AgNPs in soil. For example, Shoults-Wilson et al [82] demonstrated little or no oxidative dissolution of AgNPs during a 56-d earthworm reproduction assay.…”