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Most bacteria in natural and engineered environments grow and exist in biofilms. Recent investigations have shown that nanoparticles (NPs) interact with environmental biofilms, but these interactions are still not well characterized. Extracellular polymeric substances (EPS) are polymers secreted by bacteria to establish the functional and structural integrity of biofilms, and EPS porosity is a major contributor to NP access to and diffusion in biofilms. We used a synergistic combination of total internal reflection fluorescence microscopy and image correlation spectroscopy to monitor and map diffusion of fluorescent NPs in alginate yielding a detailed picture of the heterogeneous structure and connectivity of pores within a model EPS polymer. Using different sizes (20, 100, and 200 nm) of carboxylated polystyrene NPs, we examined how NP diffusive behaviors change as a result of calcium-induced cross-linking of the alginate matrix. This study reveals that cross-linking decreases NP diffusion coefficients and pore accessibility in an NP size-dependent manner and that NP movement through alginate matrices is anisotropic and heterogeneous. These results on heterogeneous and size-dependent movement within biofilms have important implications for future studies and simulations of NP-biofilm interactions.

Understanding how surface forces and aqueous ions influence the interactions between nanoparticles (NPs) and supported lipid bilayers (SLBs) is central to all disciplines interested in studying the nano–bio interface. A prevailing understanding is that cationic NPs have higher penetrating capability across cell membranes. In contrast, we report in this study that anionic polystyrene NPs are capable of binding and penetrating SLBs formed by zwitterionic DOPC when charge and hydrophobicity work in concert with specific anions. The preferential deposition of anionic NPs onto the bilayer is rationalized electrostatically by considering the slightly positive charge of DOPC. In addition to charge, NP hydrophobicity played an important role in the subsequent penetration of anionic NPs into SLBs. The extent of NP deposition was modulated by chaotropic anions (NO3−). This study also demonstrates the promise of using a novel combination of surface sensitive techniques, quartz crystal microbalance with dissipation monitoring (QCM-D) and surface-enhanced infrared absorption spectroscopy (SEIRAS), to extend the current understanding of NP–SLB interactions to the molecular scale.

The surfaces of nanoparticles (NPs) are continuously evolving as they are exposed to different environmental conditions. Under the same sulfidation conditions, silver NPs (AgNPs) are generally considered to undergo the same surface modifications (i.e., removal of original ligands and formation of a new silver sulfide (Ag2S) shell). We examined this generalization by studying how different ligands, polyvinylpyrrolidone (PVP) and thiolated polyethylene glycol (PEG), affected 1) the formation of a new Ag2S shell and 2) the subsequent mobility of these transformed AgNPs. The deposition of PEG-AgNPs onto a silica substrate decreased by a factor of 40 after sulfidation, while the deposition of PVP-AgNPs increased by a factor of 15. The decreased deposition of sulfidized PEG-AgNPs was attributed to the removal of PEG and the formation of Ag2S, which created a more negatively charged surface (−15 mV to −30 mV) and consequently greater electrostatic repulsion with the silica substrate. The increased deposition of sulfidized PVP-AgNPs was suggested to be caused by the removal of PVP and the absence of Ag2S, which decreased steric repulsion with the silica substrate. For the first time, this study revealed the unique abilities of two common polymeric coatings of AgNPs to form Ag2S during sulfidation and how Ag2S determined their subsequent mobility. The results of this communication also highlighted the importance of monitoring the continuous evolution of the NP surface for better prediction of NP exposure and toxicity.

Surface properties of engineered silver nanoparticles (AgNPs) are strongly affected by environmental transformation. The fate and transport of these transformed AgNPs is largely unknown and cannot be fully explained by the traditional Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The objective of this study was to investigate the changes in the composition and surface properties of polyvinylpyrrolidone (PVP) capped AgNPs after environmental transformation and their subsequent effects on the flocculation and filtration of these transformed AgNPs during water treatment processes. To study the aggregation and deposition behavior of the transformed particles, PVP-AgNPs exposed to humic acid (HA) and/or sulfidation were characterized, followed by separate flocculation, granular media filtration, and quartz crystal microgravimetry (QCM) experiments. X-ray photoelectron spectroscopy revealed that HA exposure modified the original PVP capping viaadsorption and/or ligand exchange and that sulfidation stripped the PVP from the particle surface as a result of the formation of silver sulfide. Sulfidation thereby reduced the stability of PVP-AgNPs in self-aggregation but enhanced the mobility of AgNPs in granular media filtration and quartz collector deposition. Without unbound macromolecules in the background solution, polymers on the particle surface largely prevent self-aggregation but allow favorable clean bed deposition. This difference between the effects on self-aggregation and granular media filtration is in contrast to traditional DLVO theory. QCM yielded two types of results, the initial rate of deposition and the ultimate deposition, and both gave insights into expected filtration behavior.

Nanotechnology is no longer in its infancy and has made significant advances since the implementation of the National Nanotechnology Initiative (NNI) in 2000. Incorporation of nanotechnology in many fields including information technology, medicine, materials, energy, catalysis and cosmetics has led to an increase in engineered nanomaterial (ENM) production, and consequently, increased nanomaterial use. In comparison, the generation of concrete and consistent evidence related to the environmental health and safety of nanomaterials (NanoEHS) is lacking. The main factors contributing to the slower progress in NanoEHS versusconventional EHS are related to the complexity, property transformations, life cycles and behavior of nanomaterials even in carefully controlled environments. Therefore, new systematic, integrated research approaches in NanoEHS are needed for overcoming this complexity and bridging current knowledge gaps. A workshop on “NanoEHS: Fundamental Science Needs” brought together scientists and engineers to identify current fundamental science challenges and opportunities within NanoEHS. Detailed discussions were conducted on identifying the fundamental properties that are critical in NanoEHS, differentiating between conventional and NanoEHS studies as well as understanding, the effect of dynamic transformations on nanometrology, role of dosimetry and mechanistic data gaps in nanotoxicology. An important realization that even simple nanoscale materials can be complex when considering NanoEHS implications was noted several times during the workshop. Despite this fact, a number of fundamental research areas to further the scientific foundation to address NanoEHS needs are suggested.

As nanoparticles (NPs) enter into biological systems, they are immediately exposed to a variety and concentration of proteins. The physicochemical interactions between proteins and NPs are influenced by the surface properties of the NPs. To identify the effects of NP surface heterogeneity, the interactions between bovine serum albumin (BSA) and gold NPs (AuNPs) with similar chemical composition but different surface structures were investigated. Different interaction modes and BSA conformations were studied by dynamic light scattering, circular dichroism spectroscopy, fluorescence quenching and isothermal titration calorimetry (ITC). Depending on the surface structure of AuNPs, BSA seems to adopt either a “side-on” or an “end-on” conformation on AuNPs. ITC demonstrated that the adsorption of BSA onto AuNPs with randomly distributed polar and nonpolar groups was primarily driven by electrostatic interaction, and all BSA were adsorbed in the same process. The adsorption of BSA onto AuNPs covered with alternating domains of polar and nonpolar groups was a combination of different interactions. Overall, the results of this study point to the potential for utilizing nanoscale manipulation of NP surfaces to control the resulting NP–protein interactions.

While environmental biofilms have recently been implicated as a potential major sink for nanoparticles (NPs), the mechanisms of interactions remain largely unknown. Polysaccharides are a common component of biofilm extracellular polymeric substances (EPS) and an initial point of contact for NPs in early NP–biofilm interactions. In this study, the significance of polysaccharide coatings on the deposition of hematite and silica NPs was examined by quartz crystal microgravimetry (QCM) and in-depth characterization of surface properties. NP deposition was shown to be largely governed by electrostatic forces. However, bulk surface zeta potential values of the tested polysaccharide-coated surfaces were not sufficient in describing the varying extent of NP deposition. Surface charge density and distribution both appeared to contribute to different NP deposition behaviors. These results suggest that nanometer to micrometer spatial characterization of biofilm surface properties, including chemical composition and charge, is necessary to improve our understanding of NP–biofilm interactions.