John D. Fortner

Facile Aerosol Synthesis and Characterization of Ternary Crumpled Graphene–TiO2–Magnetite Nanocomposites for Advanced Water Treatment

In this work, the synthesis and characterization of multifunctional crumpled graphene-based ternary nanocomposite photocatalysts for advanced water treatment applications is described. Currently, a major hurdle for the scale-up and optimization of aqueous, graphene-based photocatalysts is restacking of graphene nanosheets due to strong π-π interactions. To overcome this hurdle, a fast and facile aerosol technique to synthesize monomeric, aggregation-resistant, crumpled graphene-based photocatalysts was developed. The aerosol route utilizes water evaporation-induced confinement forces to effectively crumple graphene oxide and subsequently encapsulate commercially available TiO2 and magnetite nanoparticles. The as-synthesized crumpled graphene-TiO2-magnetite (GOTIM) ternary core-shell nanostructures are shown to possess superior aqueous-based photocatalytic properties (over a 20-fold enhancement in some cases) compared to TiO2 alone. Total GOTIM photocatalytic reactivity is confirmed to also include efficient photoreduction reaction pathways, in addition to expected oxidation routes typical of TiO2-based photocatalysts, significantly expanding photocatalytic application potential compared to TiO2 alone. Reaction kinetics and proposed mechanisms (both oxidative and reductive) are described for a model organic compound, here as methyl orange. Further, with the addition of hole scavengers such as EDTA, and/or lowering the O2 concentration, we demonstrate enhancement of photocatalyzed reduction reactions, suggesting potential for directed, controlled reduction applications. In addition to robust aqueous stability, low-field magnetic susceptibility is demonstrated, allowing for low-energy, in situ material separations, which are critical for material recycling and reuse.

Engineered crumpled graphene oxide nanocomposite membrane assemblies for advanced water treatment processes.

In this work, we describe multifunctional, crumpled graphene oxide (CGO) porous nanocomposites that are assembled as advanced, reactive water treatment membranes. Crumpled 3D graphene oxide based materials fundamentally differ from 2D flat graphene oxide analogues in that they are highly aggregation and compression-resistant (i.e., π-π stacking resistant) and allow for the incorporation (wrapping) of other, multifunctional particles inside the 3D, composite structure. Here, assemblies of nanoscale, monomeric CGO with encapsulated (as a quasi core-shell structure) TiO2 (GOTI) and Ag (GOAg) nanoparticles, not only allow high water flux via vertically tortuous nanochannels (achieving water flux of 246 ± 11 L/(m(2)·h·bar) with 5.4 μm thick assembly, 7.4 g/m(2)), outperforming comparable commercial ultrafiltration membranes, but also demonstrate excellent separation efficiencies for model organic and biological foulants. Further, multifunctionality is demonstrated through the in situ photocatalytic degradation of methyl orange (MO), as a model organic, under fast flow conditions (tres < 0.1 s); while superior antimicrobial properties, evaluated with GOAg, are observed for both biofilm (contact) and suspended growth scenarios (>3 log effective removal, Escherichia coli). This is the first demonstration of 3D, crumpled graphene oxide based nanocomposite structures applied specifically as (re)active membrane assemblies and highlights the materials platform potential for a truly tailored approach for next generation water treatment and separation technologies.

Bioaccumulation of 14C60 by the earthworm Eisenia fetida.

Carbon fullerenes, including buckminsterfullerene (C(60)), are increasingly available for numerous applications, thus increasing the likelihood of environmental release. This calls for information about their bioavailability and bioaccumulation potential. In this study, (14)C-labeled C(60) and (14)C-phenanthrene (positive control) were added separately to soils of varying composition and organic carbon content (OC), and their bioaccumulation in the earthworm Eisenia fetida was compared. Biota-sediment accumulation factors (BSAF) were measured after 24 h depuration in soils with high C(60) dosages (60, 100, and 300 mg-C(60) kg(-1) dry soil), which exceed the soil sorption capacity, as well as in soils with a low C(60) dose (0.25 mg kg(-1)) conducive to a high fraction of sorbed molecular C(60). The BSAF value for the low-dose soil (0.427) was 1 order of magnitude lower than for less hydrophobic phenanthrene (7.93), inconsistent with the equilibrium partition theory that suggests that BSAF should be constant and independent of the K(OW) value of the chemical. Apparently, the large molecular size of C(60) hinders uptake and bioaccumulation. Lower BSAF values (0.065-0.13) were measured for high-dose soils, indicating that C(60) bioaccumulates more readily when a higher fraction of molecular C(60) (rather than larger precipitates) is available. For the high-dose tests (heterogeneous C(60) system), soil OC content did not significantly affect the extent of C(60) bioaccumulation after 28 d of incubation, although higher OC content resulted in faster initial bioaccumulation. For low-dose soils, C(60) BSAF decreased with increasing soil OC, as commonly reported for hydrophobic chemicals due to partitioning into soil OC. There was no detectable transformation of (14)C(60) in either soil or worm tissue. Overall, the relatively low extent but rapid bioaccumulation of C(60) in E. fetida suggests the need for further studies on the potential for trophic transfer and biomagnification.

Aqueous Aggregation and Surface Deposition Processes of Engineered Superparamagnetic Iron Oxide Nanoparticles for Environmental Applications

Engineered, superparamagnetic, iron oxide nanoparticles (IONPs) have significant potential as platform materials for environmental sensing, imaging and remediation due to their unique size, physicochemical and magnetic properties. To this end, controlling the size and surface chemistry of the materials is crucial for such applications in the aqueous phase, and in particular, for porous matrixes with particle-surface interaction considerations. In this study, superparamagnetic, highly monodispersed 8 nm IONPs were synthesized and transferred into water as stable suspensions (remaining monodispersed) by way of an interfacial oleic acid bilayer surface. Once stabilized and characterized, particle-particle and model surface interactions (deposition and release) were quantitatively investigated and described systematically as a function of ionic strength (IS) and type with time-resolved dynamic light scattering (DLS), zeta potential, and real-time quartz crystal microbalance with dissipation monitoring (QCM-D) measurements. The critical coagulation concentration (CCC) for oleic acid bilayer coated iron oxide nanoparticles (OA-IONPs) were determined to be 710 mM for NaCl (matching DLVO predictions) and 10.6 mM for CaCl2, respectively. For all conditions tested, surface deposition kinetics showed stronger, more favorable interactions between OA-IONPs and polystyrene surfaces compared to silica, which is hypothesized to be due to increased particle-surface hydrophobic interactions (when compared to silica surfaces).

Tetracycline resistance gene maintenance under varying bacterial growth rate, substrate and oxygen availability, and tetracycline concentration.

Neither amplification nor attenuation of antibiotic resistance genes (ARG) in the environment are well understood processes. Here, we report on continuous culture and batch experiments to determine how tetracycline (TC), aerobic vs anaerobic conditions, bacterial growth rate, and medium richness affect the maintenance of plasmid-borne TC resistance (Tet(R)) genes. The response of E. coli (a model resistant strain excreted by farm animals) versus Pseudomonas aeruginosa (a model bacterium that could serve as a reservoir for ARGs in the environment) were compared to gain insight into response variability. Complete loss of the Tet(R) RP1 plasmid (56 kb) occurred for P. aeruginosa in the absence of TC, and faster loss was observed in continuous culture at higher growth rates. In contrast, E. coli retained its smaller pSC101 plasmid (9.3 kb) after 500 generations without TC (albeit at lower levels, with ratios of resistance to 16S rDNA genes decreasing by about 2-fold). A higher rate of ARG loss was observed in P. aeruginosa when grown in minimal growth medium (M9) than in richer Luria broth. Faster ARG loss occurred in E. coli under anaerobic (fermentative) conditions than under aerobic conditions. Thus, in these two model strains it was observed that conditions that ease the metabolic burden of plasmid reproduction (e.g., higher substrate and O2 availability) enhanced resistance plasmid maintenance; such conditions (in the presence of residual antibiotics) may be conducive to the establishment and preservation of ARG reservoirs in the environment. These results underscore the need to consider antibiotic concentrations, redox conditions, and substrate availability in efforts to evaluate ARG propagation and natural attenuation.

Measuring the grafting density of nanoparticles in solution by analytical ultracentrifugation and total organic carbon analysis.

Many of the solution phase properties of nanoparticles, such as their colloidal stability and hydrodynamic diameter, are governed by the number of stabilizing groups bound to the particle surface (i.e., grafting density). Here, we show how two techniques, analytical ultracentrifugation (AUC) and total organic carbon analysis (TOC), can be applied separately to the measurement of this parameter. AUC directly measures the density of nanoparticle-polymer conjugates while TOC provides the total carbon content of its aqueous dispersions. When these techniques are applied to model gold nanoparticles capped with thiolated poly(ethylene glycol), the measured grafting densities across a range of polymer chain lengths, polymer concentrations, and nanoparticle diameters agree to within 20%. Moreover, the measured grafting densities correlate well with the polymer content determined by thermogravimetric analysis of solid conjugate samples. Using these tools, we examine the particle core diameter, polymer chain length, and polymer solution concentration dependence of nanoparticle grafting densities in a gold nanoparticle-poly(ethylene glycol) conjugate system.

Transport of Sulfide-Reduced Graphene Oxide in Saturated Quartz Sand: Cation-Dependent Retention Mechanisms.

We describe how the reduction of graphene oxide (GO) via environmentally relevant pathways affects its transport behavior in porous media. A pair of sulfide-reduced GOs (RGOs), prepared by reducing 10 mg/L GO with 0.1 mM Na2S for 3 and 5 days, respectively, exhibited lower mobility than did parent GO in saturated quartz sand. Interestingly, decreased mobility cannot simply be attributed to the increased hydrophobicity and aggregation upon GO reduction because the retention mechanisms of RGOs were highly cation-dependent. In the presence of Na(+) (a representative monovalent cation), the main retention mechanism was deposition in the secondary energy minimum. However, in the presence of Ca(2+) (a model divalent cation), cation bridging between RGO and sand grains became the most predominant retention mechanism; this was because sulfide reduction markedly increased the amount of hydroxyl groups (a strong metal-complexing moiety) on GO. When Na(+) was the background cation, increasing pH (which increased the accumulation of large hydrated Na(+) ions on grain surface) and the presence of Suwannee River humic acid (SRHA) significantly enhanced the transport of RGO, mainly due to steric hindrance. However, pH and SRHA had little effect when Ca(2+) was the background cation because neither affected the extent of cation bridging that controlled particle retention. These findings highlight the significance of abiotic transformations on the fate and transport of GO in aqueous systems.

Graphene Oxides in Water: Correlating Morphology and Surface Chemistry with Aggregation Behavior

Aqueous aggregation processes can significantly impact function, effective toxicity, environmental transport, and ultimate fate of advanced nanoscale materials, including graphene and graphene oxide (GO). In this work, we have synthesized flat graphene oxide (GO) and five physically crumpled GOs (CGO, with different degrees of thermal reduction, and thus oxygen functionality) using an aerosol method, and characterized the evolution of surface chemistry and morphology using a suite of spectroscopic (UV-vis, FTIR, XPS) and microscopic (AFM, SEM, and TEM) techniques. For each of these materials, critical coagulation concentrations (CCC) were determined for NaCl, CaCl2, and MgCl2 electrolytes. The CCCs were correlated with material ζ-potentials (R(2) = 0.94-0.99), which were observed to be mathematically consistent with classic DLVO theory. We further correlated CCC values with CGO chemical properties including C/O ratios, carboxyl group concentrations, and C-C fractions. For all cases, edge-based carboxyl functional groups are highly correlated to observed CCC values (R(2) = 0.89-0.95). Observations support the deprotonation of carboxyl groups with low acid dissociation constants (pKa) as the main contributors to ζ-potentials and thus material aqueous stability. We also observe CCC values to significantly increase (by 18-80%) when GO is physically crumpled as CGO. Taken together, the findings from both physical and chemical analyses clearly indicate that both GO shape and surface functionality are critical to consider with regard to understanding fundamental material behavior in water.

In Situ Photocatalytic Synthesis of Ag Nanoparticles (nAg) by Crumpled Graphene Oxide Composite Membranes for Filtration and Disinfection Applications

Graphene oxide (GO) materials have demonstrated considerable potential in next-generation water treatment membrane-based technologies, which include antimicrobial applications. GO antimicrobial properties can be further enhanced by preloading or chemically generating surface-associated nanoscale silver particles (nAg). However, for these systems, enhanced antimicrobial functionality decreases over time as a function of Ag mass loss via dissolution (as Ag(+)). In this work, we demonstrate facile photocatalytic in situ synthesis of nAg particles by crumpled GO-TiO2 (GOTI) nanocomposites as an approach to (re)generate, and thus maintain, enhanced antimicrobial activity over extended operation times. The described photocatalytic formation process is highly efficient and relatively fast, producing nAg particles over a size range of 40 to 120 nm and with active (111) planes. Additionally, we show in situ surface-based photocatalyzed synthesis of nAg particles at the surface of GOTI nanocomposite membrane assemblies, allowing for simultaneous filtration and disinfection. With ca. 3 log inactivation for both Escherichia coli and Bacillus subtilis, the described membrane assemblies with in situ formed nAg demonstrate enhanced antimicrobial activity compared to the GOTI membrane surface or the support membrane alone. Under typical conditions, the working and operational time (Ag dissolution time) is calculated to be over 2 orders of magnitude higher than the loading (synthesis) time (e.g., 123 h versus 0.5 h, respectively). Taken together, results highlight the described material-based process as a potentially novel antifouling membrane technology.

Ultralow protein adsorbing coatings from clickable PEG nanogel solutions: Benefits of attachment under salt-induced phase separation conditions and comparison with PEG/albumin nanogel coatings

Clickable nanogel solutions were synthesized by using the copper catalyzed azide/alkyne cycloaddition (CuAAC) to partially polymerize solutions of azide and alkyne functionalized poly(ethylene glycol) (PEG) monomers. Coatings were fabricated using a second click reaction: a UV thiol-yne attachment of the nanogel solutions to mercaptosilanated glass. Because the CuAAC reaction was effectively halted by the addition of a copper-chelator, we were able to prevent bulk gelation and limit the coating thickness to a single monolayer of nanogels in the absence of the solution reaction. This enabled the inclusion of kosmotropic salts, which caused the PEG to phase-separate and nearly double the nanogel packing density, as confirmed by quartz crystal microbalance with dissipation (QCM-D). Protein adsorption was analyzed by single molecule counting with total internal reflection fluorescence (TIRF) microscopy and cell adhesion assays. Coatings formed from the phase-separated clickable nanogel solutions attached with salt adsorbed significantly less fibrinogen than other 100% PEG coatings tested, as well as poly(L-lysine)-g-PEG (PLL-g-PEG) coatings. However, PEG/albumin nanogel coatings still outperformed the best 100% PEG clickable nanogel coatings. Additional surface cross-linking of the clickable nanogel coating in the presence of copper further reduced levels of fibrinogen adsorption closer to those of PEG/albumin nanogel coatings. However, this step negatively impacted long-term resistance to cell adhesion and dramatically altered the morphology of the coating by atomic force microscopy (AFM). The main benefit of the click strategy is that the partially polymerized solutions are stable almost indefinitely, allowing attachment in the phase-separated state without danger of bulk gelation, and thus producing the best performing 100% PEG coating that we have studied to date.