Three-Dimensional Printing of Cytocompatible, Thermally Conductive Hexagonal Boron Nitride Nanocomposites.

Hexagonal boron nitride (hBN) is a thermally conductive yet electrically insulating two-dimensional layered nanomaterial that has attracted significant attention as a dielectric for high-performance electronics in addition to playing a central role in thermal management applications. Here, we report a high-content hBN-polymer nanocomposite ink, which can be 3D printed to form mechanically robust, self-supporting constructs. In particular, hBN is dispersed in poly(lactic- co-glycolic acid) and 3D printed at room temperature through an extrusion process to form complex architectures. These constructs can be 3D printed with a composition of up to 60% vol hBN (solids content) while maintaining high mechanical flexibility and stretchability. The presence of hBN within the matrix results in enhanced thermal conductivity (up to 2.1 W K-1 m-1) directly after 3D printing with minimal postprocessing steps, suggesting utility in thermal management applications. Furthermore, the constructs show high levels of cytocompatibility, making them suitable for use in the field of printed bioelectronics.

Surface Oxidation of Graphene Oxide Determines Membrane Damage, Lipid Peroxidation, and Cytotoxicity in Macrophages in a Pulmonary Toxicity Model.

While two-dimensional graphene oxide (GO) is used increasingly in biomedical applications, there is uncertainty on how specific physicochemical properties relate to biocompatibility in mammalian systems. Although properties such as lateral size and the colloidal properties of the nanosheets are important, the specific material properties that we address here is the oxidation state and reactive surface groups on the planar surface. In this study, we used a GO library, comprising pristine, reduced (rGO), and hydrated GO (hGO), in which quantitative assessment of the hydroxyl, carboxyl, epoxy, and carbon radical contents was used to study the impact on epithelial cells and macrophages, as well as in the murine lung. Strikingly, we observed that hGO, which exhibits the highest carbon radical density, was responsible for the generation of cell death in THP-1 and BEAS-2B cells as a consequence of lipid peroxidation of the surface membrane, membrane lysis, and cell death. In contrast, pristine GO had lesser effects, while rGO showed extensive cellular uptake with minimal effects on viability. In order to see how these in vitro effects relate to adverse outcomes in the lung, mice were exposed to GOs by oropharyngeal aspiration. Animal sacrifice after 40 h demonstrated that hGO was more prone than other materials to generate acute lung inflammation, accompanied by the highest lipid peroxidation in alveolar macrophages, cytokine production (LIX, MCP-1), and LDH release in bronchoalveolar lavage fluid. Pristine GO showed less toxicity, whereas rGO had minimal effects. We demonstrate that the surface oxidation state and carbon radical content play major roles in the induction of toxicity by GO in mammalian cells and the lung.

Optothermally Reversible Carbon Nanotube-DNA Supramolecular Hybrid Hydrogels.

Supramolecular hydrogels (SMHs) are three-dimensional constructs wherein the majority of the volume is occupied by water. Since the bonding forces between the components of SMHs are noncovalent, SMH properties are often tunable, stimuli-responsive, and reversible, which enables applications including triggered drug release, sensing, and tissue engineering. Meanwhile, single-walled carbon nanotubes (SWCNTs) possess superlative electrical and thermal conductivities, high mechanical strength, and strong optical absorption at near-infrared wavelengths that have the potential to add unique functionality to SMHs. However, SWCNT-based SMHs have thus far not realized the potential of the optical properties of SWCNTs to enable reversible response to near-infrared irradiation. Here, we present a novel SMH architecture comprised solely of DNA and SWCNTs, wherein noncovalent interactions provide structural integrity without compromising the intrinsic properties of SWCNTs. The mechanical properties of these SMHs are readily tuned by varying the relative concentrations of DNA and SWCNTs, which varies the cross-linking density as shown by molecular dynamics simulations. Moreover, the SMH gelation transition is fully reversible and can be triggered by a change in temperature or near-infrared irradiation. This work explores a new regime for SMHs with potential utility for a range of applications including sensors, actuators, responsive substrates, and 3D printing.

Identification and Optimization of Carbon Radicals on Hydrated Graphene Oxide for Ubiquitous Antibacterial Coatings.

While the antibacterial properties of graphene oxide (GO) have been demonstrated across a spectrum of bacteria, the critical role of functional groups is unclear. To address this important issue, we utilized reduction and hydration methods to establish a GO library with different oxidation, hydroxyl, and carbon radical (•C) levels that can be used to study the impact on antibacterial activity. Using antibiotic-resistant bacteria as a test platform, we found that the •C density is most proximately associated with bacterial killing. Accordingly, hydrated GO (hGO), with the highest •C density, had the strongest antibacterial effects through membrane binding and induction of lipid peroxidation. To explore its potential applications, we demonstrated that coating of catheter and glass surfaces with hGO is capable of killing drug-resistant bacteria. In summary, •C is the principle surface moiety that can be utilized for clinical applications of GO-based antibacterial coatings.

Toxicological Profiling of Highly Purified Metallic and Semiconducting Single-Walled Carbon Nanotubes in the Rodent Lung and E. coli.

The electronic properties of single-walled carbon nanotubes (SWCNTs) are potentially useful for electronics, optics, and sensing applications. Depending on the chirality and diameter, individual SWCNTs can be classified as semiconducting (S-SWCNT) or metallic (M-SWCNT). From a biological perspective, the hazard profiling of purified metallic versus semiconducting SWCNTs has been pursued only in bacteria, with the conclusion that aggregated M-SWCNTs are more damaging to bacterial membranes than S-SWCNTs. However, no comparative studies have been performed in a mammalian system, where most toxicity studies have been undertaken using relatively crude SWCNTs that include a M:S mix at 1:2 ratio. In order to compare the toxicological impact of SWCNTs sorted to enrich them for each of the chirality on pulmonary cells and the intact lung, we used density gradient ultracentrifugation and extensive rinsing to prepare S- and M-SWCNTs that are >98% purified. In vitro screening showed that both tube variants trigger similar amounts of interleukin 1ß (IL-1ß) and transforming growth factor (TGF-ß1) production in THP-1 and BEAS-2B cells, without cytotoxicity. Oropharyngeal aspiration confirmed that both SWCNT variants induce comparable fibrotic effects in the lung and abundance of IL-1ß and TGF-ß1 release in the bronchoalveolar lavage fluid. There was also no change in the morphology, membrane integrity, and viability of E. coli, in contradistinction to the previously published effects of aggregated tubes on the bacterial membrane. Collectively, these data indicate that the electronic properties and chirality do not independently impact SWCNT toxicological impact in the lung, which is of significance to the safety assessment and incremental use of purified tubes by industry.

High-Concentration Aqueous Dispersions of Nanoscale 2D Materials Using Nonionic, Biocompatible Block Copolymers.

Conditions for the dispersion of molybdenum disulfide (MoS2) in aqueous solution at concentrations up to 0.12 mg mL(-1) using a range of nonionic, biocompatible block copolymers (i.e., Pluronics and Tetronics) are identified. Furthermore, the optimal Pluronic dispersant for MoS2 is found to be effective for a range of other 2D materials such as molybdenum diselenide, tungsten diselenide, tungsten disulfide, tin selenide, and boron nitride.

Aggregation and Stability of Reduced Graphene Oxide: Complex Roles of Divalent Cations, pH, and Natural Organic Matter.

The aggregation and stability of graphene oxide (GO) and three successively reduced GO (rGO) nanomaterials were investigated. Reduced GO species were partially reduced GO (rGO-1h), intermediately reduced GO (rGO-2h), and fully reduced GO (rGO-5h). Specifically, influence of pH, ionic strength, ion valence, and presence of natural organic matter (NOM) were studied. Results show that stability of GO in water decreases with successive reduction of functional groups, with pH having the greatest influence on rGO stability. Stability is also dependent on ion valence and the concentration of surface functional groups. While pH did not noticeably affect stability of GO in the presence of 10 mM NaCl, adding 0.1 mM CaCl2 reduced stability of GO with increased pH. This is due to adsorption of Ca(2+) ions on the surface functional groups of GO which reduces the surface charge of GO. As the concentration of rGO functional groups decreased, so did the influence of Ca(2+) ions on rGO stability. Critical coagulation concentrations (CCC) of GO, rGO-1h, and rGO-2h were determined to be ∼ 200 mM, 35 mM, and 30 mM NaCl, respectively. In the presence of CaCl2, CCC values of GO and rGO are quite similar, however. Long-term studies show that a significant amount of rGO-1h and rGO-2h remain stable in Call's Creek surface water, while effluent wastewater readily destabilizes rGO. In the presence NOM and divalent cations (Ca(2+), Mg(2+)), GO aggregates settle from suspension due to GO functional group bridging with NOM and divalent ions. However, rGO-1h and rGO-2h remain suspended due to their lower functional group concentration and resultant reduced NOM-divalent cation bridging. Overall, pH, divalent cations, and NOM can play complex roles in the fate of rGO and GO.

Differences in the Toxicological Potential of 2D versus Aggregated Molybdenum Disulfide in the Lung.

2D molybdenum disulfide (MoS2 ) has distinct optical and electronic properties compared to aggregated MoS2 , enabling wide use of these materials for electronic and biomedical applications. However, the hazard potential of MoS2 has not been studied extensively. Here, a comprehensive analysis of the pulmonary hazard potential of three aqueous suspended forms of MoS2 -aggregated MoS2 (Agg-MoS2 ), MoS2 exfoliated by lithiation (Lit-MoS2 ), and MoS2 dispersed by Pluronic F87 (PF87-MoS2 )-is presented. No cytotoxicity is detected in THP-1 and BEAS-2B cell lines. However, Agg-MoS2 induces strong proinflammatory and profibrogenic responses in vitro. In contrast, Lit- and PF87-MoS2 have little or no effect. In an acute toxicity study in mice, Agg-MoS2 induces acute lung inflammation, while Lit-MoS2 and PF87-MoS2 have little or no effect. In a subchronic study, there is no evidence of pulmonary fibrosis in response to all forms of MoS2 . These data suggest that exfoliation attenuates the toxicity of Agg-MoS2 , which is an important consideration toward the safety evaluation and use of nanoscale MoS2 materials for industrial and biological applications.

Fate and Transport of Molybdenum Disulfide Nanomaterials in Sand Columns.

Research and development of two-dimensional transition metal dichalcogenides (TMDC) (e.g., molybdenum disulfide [MoS2]) in electronic, optical, and catalytic applications has been growing rapidly. However, there is little known regarding the behavior of these particles once released into aquatic environments. Therefore, an in-depth study regarding the fate and transport of two popular types of MoS2 nanomaterials, lithiated (MoS2-Li) and Pluronic PF-87 dispersed (MoS2-PL), was conducted in saturated porous media (quartz sand) to identify which form would be least mobile in aquatic environments. The electrokinetic properties and hydrodynamic diameters of MoS2 as a function of ionic strength and pH were determined using a zeta potential analyzer and dynamic light scattering techniques. Results suggest that the stability is significantly decreased beginning at 10 and 31.6 mM KCl, for MoS2-PL and MoS2-Li, respectively. Transport study results from breakthrough curves, column dissections, and release experiments suggest that MoS2-PL exhibits a greater affinity to be irreversibly bound to quartz surfaces as compared with the MoS2-Li at a similar ionic strength. Derjaguin-Landau-Verwey-Overbeek theory was used to help explain the unique interactions between the MoS2-PL and MoS2-Li surfaces between particles and with the quartz collectors. Overall, the results suggest that the fate and transport of MoS2 is dependent on the type of MoS2 that enters the environment, where MoS2-PL will be least mobile and more likely be deposited in porous media from pluronic-quartz interactions, whereas MoS2-Li will travel greater distances and have a greater tendency to be remobilized in sand columns.

Graphene oxide enhances cellular delivery of hydrophilic small molecules by co-incubation.

The delivery of bioactive molecules into cells has broad applications in biology and medicine. Polymer-modified graphene oxide (GO) has recently emerged as a de facto noncovalent vehicle for hydrophobic drugs. Here, we investigate a different approach using native GO to deliver hydrophilic molecules by co-incubation in culture. GO adsorption and delivery were systematically studied with a library of 15 molecules synthesized with Gd(III) labels to enable quantitation. Amines were revealed to be a key chemical group for adsorption, while delivery was shown to be quantitatively predictable by molecular adsorption, GO sedimentation, and GO size. GO co-incubation was shown to enhance delivery by up to 13-fold and allowed for a 100-fold increase in molecular incubation concentration compared to the alternative of nanoconjugation. When tested in the application of Gd(III) cellular MRI, these advantages led to a nearly 10-fold improvement in sensitivity over the state-of-the-art. GO co-incubation is an effective method of cellular delivery that is easily adoptable by researchers across all fields.

Interactions of graphene oxide nanomaterials with natural organic matter and metal oxide surfaces.

Interactions of graphene oxide (GO) nanomaterials with natural organic matter (NOM) and metal oxide surfaces were investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). Three different types of NOM were studied: Suwannee River humic and fulvic acids (SRHA and SRFA) and alginate. Aluminum oxide surface was used as a model metal oxide surface. Deposition trends show that GO has the highest attachment on alginate, followed by SRFA, SRHA, and aluminum oxide surfaces, and that GO displayed higher interactions with all investigated surfaces than with silica. Deposition and release behavior of GO on aluminum oxide surface is very similar to positively charged poly-L-lysine-coated surface. Higher interactions of GO with NOM-coated surfaces are attributed to the hydroxyl, epoxy, and carboxyl functional groups of GO; higher deposition on alginate-coated surfaces is attributed to the rougher surface created by the extended conformation of the larger alginate macromolecules. Both ionic strength (IS) and ion valence (Na(+) vs Ca(2+)) had notable impact on interactions of GO with different environmental surfaces. Due to charge screening, increased IS resulted in greater deposition for NOM-coated surfaces. Release behavior of deposited GO varied significantly between different environmental surfaces. All surfaces showed significant release of deposited GO upon introduction of low IS water, indicating that deposition of GO on these surfaces is reversible. Release of GO from NOM-coated surfaces decreased with IS due to charge screening. Release rates of deposited GO from alginate-coated surface were significantly lower than from SRHA and SRFA-coated surfaces due to trapping of GO within the rough surface of the alginate layer.

Deposition and release of graphene oxide nanomaterials using a quartz crystal microbalance.

Interactions of graphene oxide (GO) with silica surfaces were investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). Both GO deposition and release were monitored on silica- and poly-l-lysine (PLL) coated surfaces as a function of GO concentration and in NaCl, CaCl2, and MgCl2 as a function of ionic strength (IS). Under favorable conditions (PLL-coated positive surface), GO deposition rates increased with GO concentration, as expected from colloidal theory. Increased NaCl concentration resulted in a greater deposition attachment efficiency of GO on the silica surface, indicating that deposition of GO follows Derjaguin-Landau-Verwey-Overbeek (DLVO) theory; GO deposition rates decreased at high IS, however, due to large aggregate formation. GO critical deposition concentration (CDC) on the silica surface is determined to be 40 mM NaCl which is higher than the reported CDC values of fullerenes and lower than carbon nanotubes. A similar trend is observed for MgCl2 which has a CDC value of 1.2 mM MgCl2. Only a minimal amount of GO (frequency shift <2 Hz) was deposited on the silica surface in CaCl2 due to the bridging ability of Ca(2+) ions with GO functional groups. Significant GO release from silica surface was observed after adding deionized water, indicating that GO deposition is reversible. The release rates of GO were at least 10-fold higher than the deposition rates under similar conditions indicating potential high release and mobility of GO in the environment. Under favorable conditions, a significant amount of GO was released which indicates potential multilayer GO deposition. However, a negligible amount of deposited GO was released in CaCl2 under favorable conditions due to the binding of GO layers with Ca(2+) ions. Release of GO was significantly dependent on salt type with an overall trend of NaCl > MgCl2 > CaCl2.

Colloidal properties and stability of graphene oxide nanomaterials in the aquatic environment.

While graphene oxide (GO) has been found to be the most toxic graphene-based nanomaterial, its environmental fate is still unexplored. In this study, the aggregation kinetics and stability of GO were investigated using time-resolved dynamic light scattering over a wide range of aquatic chemistries (pH, salt types (NaCl, MgCl2, CaCl2), ionic strength) relevant to natural and engineered systems. Although pH did not have a notable influence on GO stability from pH 4 to 10, salt type and ionic strength had significant effects on GO stability due to electrical double layer compression, similar to other colloidal particles. The critical coagulation concentration (CCC) values of GO were determined to be 44 mM NaCl, 0.9 mM CaCl2, and 1.3 mM MgCl2. Aggregation and stability of GO in the aquatic environment followed colloidal theory (DLVO and Schulze-Hardy rule), even though GO's shape is not spherical. CCC values of GO were lower than reported fullerene CCC values and higher than reported carbon nanotube CCC values. CaCl2 destabilized GO more aggressively than MgCl2 and NaCl due to the binding capacity of Ca(2+) ions with hydroxyl and carbonyl functional groups of GO. Natural organic matter significantly improved the stability of GO in water primarily due to steric repulsion. Long-term stability studies demonstrated that GO was highly stable in both natural and synthetic surface waters, although it settled quickly in synthetic groundwater. While GO remained stable in synthetic influent wastewater, effluent wastewater collected from a treatment plant rapidly destabilized GO, indicating GO will settle out during the wastewater treatment process and likely accumulate in biosolids and sludge. Overall, our findings indicate that GO nanomaterials will be stable in the natural aquatic environment and that significant aqueous transport of GO is possible.