Biodegradation of Functionalized Nanocellulose.

Nanocellulose has attracted widespread interest for applications in materials science and biomedical engineering due to its natural abundance, desirable physicochemical properties, and high intrinsic mineralizability (i.e., complete biodegradability). A common strategy to increase dispersibility in polymer matrices is to modify the hydroxyl groups on nanocellulose through covalent functionalization, but such modification strategies may affect the desirable biodegradation properties exhibited by pristine nanocellulose. In this study, cellulose nanofibrils (CNFs) functionalized with a range of esters, carboxylic acids, or ethers exhibited decreased rates and extents of mineralization by anaerobic and aerobic microbial communities compared to unmodified CNFs, with etherified CNFs exhibiting the highest level of recalcitrance. The decreased biodegradability of functionalized CNFs depended primarily on the degree of substitution at the surface of the material rather than within the bulk. This dependence on surface chemistry was attributed not only to the large surface area-to-volume ratio of nanocellulose but also to the prerequisite surface interaction by microorganisms necessary to achieve biodegradation. Results from this study highlight the need to quantify the type and coverage of surface substituents in order to anticipate their effects on the environmental persistence of functionalized nanocellulose.

Tunable thermo-reversible bicontinuous nanoparticle gel driven by the binary solvent segregation.

Bicontinuous porous structures through colloidal assembly realized by non-equilibrium process is crucial to various applications, including water treatment, catalysis and energy storage. However, as non-equilibrium structures are process-dependent, it is very challenging to simultaneously achieve reversibility, reproducibility, scalability, and tunability over material structures and properties. Here, a novel solvent segregation driven gel (SeedGel) is proposed and demonstrated to arrest bicontinuous structures with excellent thermal structural reversibility and reproducibility, tunable domain size, adjustable gel transition temperature, and amazing optical properties. It is achieved by trapping nanoparticles into one of the solvent domains upon the phase separation of the binary solvent. Due to the universality of the solvent driven particle phase separation, SeedGel is thus potentially a generic method for a wide range of colloidal systems.

Controlling Bicontinuous Structures through a Solvent Segregation-Driven Gel.

The past decade has seen increased research interest in studying bicontinuous structures formed via colloidal self-assembly due to their many useful applications. A new type of colloidal gel, solvent segregation-driven gel (SeedGel), has been recently demonstrated as an effective approach to arrest bicontinuous structures with unique and intriguing properties, such as thermoreversibility, structural reproducibility, and sensitive temperature response. Here, using a model system with silica particles in the 2,6-lutidine/water binary solvent, we investigate the factors controlling the domain size of a SeedGel system by varying the particle concentration, solvent ratio, and quenching protocol. A phase diagram is identified to produce SeedGels for this model system. Our results indicate that by adjusting the sample composition, it is possible to realize bicontinuous domains with well-controlled repeating distances (periodicities). In addition, the effect of quenching rate on the domain size is systematically investigated, showing that it is a very sensitive parameter to control domain sizes. By further heating SeedGel up into the spinodal region, the structure evolution under high temperatures is also investigated and discussed. These results provide important insights into how to control bicontinuous structures in SeedGel systems.

Evaluating performance, degradation, and release behavior of a nanoform pigmented coating after natural and accelerated weathering.

Pigments with nanoscale dimensions are added to exterior coatings to achieve desirable color and gloss properties. The present study compared the performance, degradation, and release behavior of an acrylic coating that was pigmented by a nanoform of Cu-phthalocyanine after both natural (i.e., outdoor) and accelerated weathering. Samples were weathered outdoors in three geographically distinct locations across the United States (Arizona, Colorado, Maryland) continuously for 15 months. Identically prepared samples were also artificially weathered under accelerated conditions (increased ultraviolet (UV) light intensity and elevated temperatures) for three months, in one-month increments. After exposure, both sets of samples were characterized with color, gloss, and infrared spectroscopy measurements, and selectively with surface roughness measurements. Results indicated that UV-driven coating oxidation was the principal degradation pathway for both natural and accelerated weathering samples, with accelerated weathering leading to an increased rate of oxidation without altering the fundamental degradation pathway. The inclusion of the nanoform pigment reduced the rate of coating oxidation, via UV absorption by the pigment, leading to improved coating integrity compared to non-pigmented samples. Release measurements collected during natural weathering studies indicated there was never a period of weathering, in any location, that led to copper material release above background copper measurements. Lab-based release experiments performed on samples weathered naturally and under accelerated conditions found that the release of degraded coating material after each type of exposure was diminished by the inclusion of the nanoform pigment. Release measurements also indicated that the nanoform pigment remained embedded within the coating and did not release after weathering.

Unveiling the Synergistic Role of Oxygen Functional Groups in the Graphene-Mediated Oxidation of Glutathione.

This is the first report of an atomic-scale direct oxidation mechanism of the thiol group in glutathione (GSH) by epoxides on graphene oxide (GO) at room temperature. The proposed reaction mechanism is determined using a coupled experimental and computational approach; active sites for the reaction are determined through examination of GO surface chemistry changes before and after exposure to GSH, and density functional theory (DFT) calculations determine the reaction barriers for the possible GO-GSH reaction schemes. The findings build on the previously established catalytic mechanism of GSH oxidation by graphenic nanocarbon surfaces and importantly identify the direct reaction mechanism which becomes important in low-oxygen environments. Experimental results suggest epoxides as the active sites for the reaction with GSH, which we confirm using DFT calculations of reaction barriers and further identify a synergism between the adjacent epoxide and hydroxyl groups on the GO surface. The direct oxidation mechanism at specific oxygen sites offers insight into controlling GO chemical reactivity through surface chemistry manipulations. This insight is critical for furthering our understanding of GO oxidative stress pathways in cytotoxicity as well as for providing rational material design for GO applications that can leverage this reaction.

UV-Vis quantification of hydroxyl radical concentration and dose using principal component analysis.

Hydroxyl radicals (∙OH) are powerful oxidizing species formed naturally in the environment or artificially produced to destroy contaminants in water treatment facilities. Their short lifetime and high reactivity, however, present a significant challenge to quantifying their concentration in solution. Herein, we developed a novel method to accurately measure the steady-state ∙OH concentration and total ∙OH dose produced during the UV photolysis of hydrogen peroxide (H2O2) by monitoring the loss of salicylic acid (SA). This information can be acquired using only benchtop UV-Vis spectroscopy, thus expanding measurement capabilities of resource-limited laboratories by eliminating the need for sophisticated instrumentation. To improve the precision with which the rate of SA loss was measured compared to previous methods, we applied principal component analysis (PCA) to fit the UV-Vis spectra collected during SA exposure to ∙OH. For our experimental conditions consisting of 12 mL solutions composed of ≤ 100 mM H2O2 and 0.07 mM SA, the steady-state ∙OH concentration throughout the complete photolysis of H2O2 was 1.33 × 10-11 M ± 1.14 × 10-12 M. This represents more than a ten-fold improvement in reducing the uncertainty of the measurement, with respect to narrowing the 95 % confidence interval, compared to a previous method that employed matrix analysis to process the spectra. Furthermore, the variance of the measured ∙OH concentrations was reduced by a factor of 100 compared to previous methods. Using PCA, the limit-of-detection and limit-of-quantitation for ∙OH are 5.33 × 10-13 M and 1.23 × 10-12 M, respectively. By developing quantitative relationships among ∙OH concentration, H2O2 concentration, and UV exposure time, we also show how to calculate the equivalent exposure to ∙OH generated in natural aquatic environments by indirect photolysis. Finally, we use this methodology to demonstrate that the presence of suspended carbonaceous nanoparticles at concentrations as high as 300 ppm does not affect ∙OH concentration.

Photochemical Transformations of Carbon Dots in Aqueous Environments.

The unique physicochemical and luminescent properties of carbon dots (CDs) have motivated research efforts toward their incorporation into commercial products. Increased use of CDs will inevitably lead to their release into the environment where their fate and persistence will be influenced by photochemical transformations, the nature of which is poorly understood. This knowledge gap motivated the present investigation of the effects of direct and indirect photolysis on citric and malic acid-based CDs. Our results indicate that natural sunlight will rapidly and non-destructively photobleach CDs into optically inactive carbon nanoparticles. We demonstrate that after photobleaching, •OH exposure degrades CDs in a two-step process that will span several decades in natural waters. The first step, occurring over several years of •OH exposure, involves depolymerization of the CD structure, characterized by volatilization of over 60% of nascent carbon atoms and the oxidation of nitrogen atoms into nitro groups. This is followed by a slower oxidation of residual carbon atoms first into carboxylic acids and then volatile carbon species, while nitrogen atoms are oxidized into nitrate ions. Considered alongside related CD studies, our findings suggest that the environmental behavior of CDs will be strongly influenced by the molecular precursors used in their synthesis.

Graphene/polymer nanocomposite degradation by ultraviolet light: The effects of graphene nanofillers and their potential for release.

The ultraviolet (UV)-induced degradation of graphene/polymer nanocomposites was investigated in this study. Specifically, the effect of few-layer graphene nanofillers on the degradation of a thermoplastic polyurethane (TPU) and the release potential of graphene from the degraded nanocomposite surfaces were assessed. Graphene/TPU (G/TPU) nanocomposites and neat TPU were UV-exposed under both dry and humid conditions in the NIST SPHERE, a precisely controlled, high intensity UV-weathering device. Neat TPU and G/TPU were characterized over the time course of UV exposure using color measurements and infrared spectroscopy, for appearance and chemical changes, respectively. Changes in thickness and surface morphology were obtained with scanning electron microscopy. A new fluorescence quenching measurement approach was developed to identify graphene sheets at the nanocomposite surface, which was supported by contact angle measurements. The potential for graphene release from the nanocomposite surface was evaluated using a tape-lift method followed by microscopy of any particles present on the tape. The findings suggest that graphene improves the service life of TPU with respect to UV exposure, but that graphene becomes exposed at the nanocomposite surface over time, which may potentially lead to its release when exposed to small mechanical forces or upon contact with other materials.

Copper release and transformation following natural weathering of nano-enabled pressure-treated lumber.

Commercially available lumber, pressure-treated with micronized copper azole (MCA), has largely replaced other inorganic biocides for residential wood treatment in the USA, yet little is known about how different outdoor environmental conditions impact the release of ionic, nano-scale, or larger (micron-scale) copper from this product. Therefore, we weathered pressure treated lumber for 18 months in five different climates across the continental United States. Copper release was quantified every month and local weather conditions were recorded continuously to determine the extent to which local climate regulated the release of copper from this nano-enabled product during its use phase. Two distinct release trends were observed: In cooler, wetter climates release occurred primarily during the first few months of weathering, as the result of copper leaching from surface/near-surface areas. In warmer, drier climates, less copper was initially released due to limited precipitation. However, as the wood dried and cracked, the exposed copper-bearing surface area increased, leading to increased copper release later in the product lifetime. Single-particle-ICP-MS results from laboratory prepared MCA-wood leachate solutions indicated that a) the predominant form of released copper passed through a filter smaller than 0.45 micrometers and b) released particles were largely resistant to dissolution over the course of 6 wks. Toxicity Characteristic Leaching Procedure (TCLP) testing was conducted on nonweathered and weathered MCA-wood samples to simulate landfill conditions during their end-of-life (EoL) phase and revealed that MCA wood released <10% of initially embedded copper. Findings from this study provide data necessary to complete a more comprehensive evaluation of the environmental and human health impacts introduced through release of copper from pressure treated lumber utilizing life cycle assessment (LCA).

Resonantly Enhanced Nonlinear Optical Probes of Oxidized Multiwalled Carbon Nanotubes at Supported Lipid Bilayers.

With production of carbon nanotubes surpassing billions of tons per annum, concern about their potential interactions with biological systems is growing. Herein, we utilize second harmonic generation spectroscopy, sum frequency generation spectroscopy, and quartz crystal microbalance with dissipation monitoring to probe the interactions between oxidized multiwalled carbon nanotubes (O-MWCNTs) and supported lipid bilayers composed of phospholipids with phosphatidylcholine head groups as the dominant component. We quantify O-MWCNT attachment to supported lipid bilayers under biogeochemically relevant conditions and discern that the interactions occur without disrupting the structural integrity of the lipid bilayers for the systems probed. The extent of O-MWCNT sorption was far below a monolayer even at 100 mM NaCl and was independent of the chemical composition of the supported lipid bilayer.

Potential Environmental Impacts and Antimicrobial Efficacy of Silver- and Nanosilver-Containing Textiles.

For textiles containing nanosilver, we assessed benefit (antimicrobial efficacy) in parallel with potential to release nanosilver (impact) during multiple life cycle stages. The silver loading and method of silver attachment to the textile highly influenced the silver release during washing. Multiple sequential simulated household washing experiments for fabric swatches in deionized water with or without detergent showed a range of silver release. The toxicity of washing experiment supernatants to zebrafish (Danio rerio) embryos was negligible, with the exception of the very highest Ag releases (∼1 mg/L Ag). In fact, toxicity tests indicated that residual detergent exhibited greater adverse response than the released silver. Although washing the fabrics did release silver, it did not affect their antimicrobial efficacy, as demonstrated by >99.9% inhibition of E. coli growth on the textiles, even for textiles that retained as little as 2 µg/g Ag after washing. This suggests that very little nanosilver is required to control bacterial growth in textiles. Visible light irradiation of the fabrics reduced the extent of Ag release for textiles during subsequent washings. End-of-life experiments using simulated landfill conditions showed that silver remaining on the textile is likely to continue leaching from textiles after disposal in a landfill.