Properties of APTES-Modified CNC Films.

Sulfated cellulose nanocrystals' (CNCs') facile aqueous dispersibility enables producing films, fibers, and other materials using only water as a solvent but prevents using sulfated CNCs in applications that require water immersion. We report that modifying CNCs with 3-aminopropyl-triethoxysilane (APTES) via a simple, single-pot reaction scheme dramatically improves the hydrolytic stability of CNC films. The effects of APTES modification on CNCs' properties were studied using attenuated total reflectance Fourier transform infrared spectroscopy, atomic force and optical microscopy, thermogravimetric analysis, dynamic light scattering, and ultimate analysis. Substituting a mere 12.6% of the CNCs' available hydroxyl groups with APTES dramatically increased the hydrolytic stability of shear cast films while only having minor impacts on their mechanical properties. In addition, quartz crystal microbalance with dissipation monitoring (QCMD) and multiparametric surface plasmon resonance (MP-SPR) studies showed that the CNC-APTES films also had a greater irreversible binding with carbofuran, a pesticide and emerging contaminant. These results highlight that APTES modification is a promising method for increasing the utility of sulfated CNCs in sensors, adsorbents, and other applications requiring water immersion.

Phase Behavior and Rheological Properties of Size-Fractionated MXene (Ti3C2Tx) Dispersions.

Understanding the dispersion behavior of MXenes is interesting from a fundamental colloid science perspective and critical to enabling the fluid-phase manufacturing of MXene devices with controlled microstructures and properties. However, the polydispersity, irregular shape, and charged surfaces of MXenes result in a complex phase behavior that is difficult to predict through theoretical calculations. As two-dimensional (2D) nanomaterials, MXenes can form lyotropic liquid crystal phases, gels, and aggregates. This work aims to elucidate the effects of MXene (Ti3C2Tx) sheet size on their phase behavior and associated rheological properties. Aqueous dispersions of large sheets with an average lateral dimension of 3.0 µm, small sheets with an average lateral dimension of 0.3 µm, and a bimodal mixture of the two sizes were investigated by using cross-polarized optical microscopy and rheology. At low concentrations, the large MXene dispersions exhibited lyotropic liquid crystal behavior and extended aligned textures, but increasing concentration resulted in the formation of dense flocs. Dispersions of small sheets formed small birefringent domains with increasing concentration but lacked long-range ordering. A bimodal mixture of these sizes enabled the formation of liquid crystalline phases with extended aligned textures with less floc formation. These results provide insights into using polydispersity to tune dispersion microstructure and rheological properties that can be applied to designing dispersions for fluid-phase manufacturing methods, such as direct ink writing.

Anisotropic Nanomaterial Liquid Crystals: From Fiber Spinning to Additive Manufacturing.

There have long been synergistic relationships among the discovery of new anisotropic materials, advancements in liquid crystal science, and the production of manufactured goods with exciting new properties. Ongoing progress in understanding the phase behavior and shear response of lyotropic liquid crystals comprised of one-dimensional and two-dimensional nanomaterials, coupled with advancements in extrusion-based manufacturing methods, promises to enable the scalable production of solid materials with outstanding properties and controlled order across multiple length scales. This Perspective highlights progress in using anisotropic nanomaterial liquid crystals in two extrusion-based manufacturing methods: solution spinning and direct ink writing. It also describes current challenges and opportunities at the interface of nanotechnology, liquid crystalline science, and manufacturing. The intent is to inspire additional transdisciplinary research that will enable nanotechnology to fulfill its potential for producing advanced materials with precisely controlled morphologies and properties.

Antibody Immobilization on Sulfated Cellulose Nanocrystals.

Exploiting cellulose nanocrystals' high aspect ratio and tailorable surface for immunological biosensors has been hindered by the relatively limited research on using commonly available sulfated cellulose nanocrystals (CNCs) for antibody immobilization and by the low hydrolytic stability of dried assemblies produced from sulfated CNCs. Herein, we report a reaction scheme that enables both hydrolytic stability and antibody immobilization through 3-aminopropyl-triethoxysilane and glutaric anhydride chemistry. Immobilization was demonstrated using three model antibodies used in the detection of the cancer biomarkers: alpha-fetoprotein, prostate-specific antigen, and carcinoembryonic antigen. Thermogravimetric analysis coupled with Fourier-transform infrared spectroscopy provided evidence of CNC modification. Quartz crystal microbalance with dissipation monitoring was used to monitor binding during each step of the immobilization scheme as well as binding of the corresponding antigens. The general reaction scheme was tested using both aqueous CNC dispersions and CNC films. Film modification is slightly simpler as it avoids centrifugation and washing steps. However, modifying the dispersed CNCs provides access to their entire surface area and results in a greater capacity for antigen binding.

Additive Manufacturing of Viscoelastic Polyacrylamide Substrates for Mechanosensing Studies.

Polymerized polyacrylamide (PAA) substrates are linearly elastic hydrogels that are widely used in mechanosensing studies due to their biocompatibility, wide range of functionalization capability, and tunable mechanical properties. However, such cellular response on purely elastic substrates, which do not mimic the viscoelastic living tissues, may not be physiologically relevant. Because the cellular response on 2D viscoelastic PAA substrates remains largely unknown, we used stereolithography (SLA)-based additive manufacturing technique to create viscoelastic PAA substrates with tunable mechanical properties that allow us to identify physiologically relevant cellular behaviors. Three PAA substrates of different complex moduli were fabricated by SLA. By embedding fluorescent markers during the additive manufacturing of the substrates, we show a homogeneous and uniform composition throughout, which conventional manufacturing techniques cannot produce. Rheological investigation of the additively manufactured PAA substrates shows a viscoelastic behavior with a 5-10% loss moduli compared to their elastic moduli, mimicking the living tissues. To understand the cell mechanosensing on the dissipative PAA substrates, single live cells were seeded on PAA substrates to establish the basic relationships between cell traction, cytoskeletal prestress, and cell spreading. With the increasing substrate moduli, we observed a concomitant increase in cellular traction and prestress, but not cell spreading, suggesting that cell spreading can be decoupled from traction and intracellular prestress in physiologically relevant environments. Together, additively manufactured PAA substrates fill the void of lacking real tissue like viscoelastic materials that can be used in a variety of mechanosensing studies with superior reproducibility.

Dispersant Effects on Single-Walled Carbon Nanotube Antibacterial Activity.

There is significant interest in understanding whether nanomaterials with outstanding mechanical or electrical properties also possess antibacterial properties. However, assessment of antibacterial activity is a complex problem at the interface of chemistry and microbiology. Results can be affected by many factors including nanomaterial size, surface chemistry, concentration, and the dispersion media. The difficulty of dispersing nanomaterials such as single-walled carbon nanotubes (SWNTs) has resulted in many studies being conducted in the presence of dispersion aides which may themselves contribute to bacterial stress. The recent discovery that a standard microbial growth media, tryptic soy broth (TSB), is an effective SWNT dispersant provides a new opportunity to investigate the potential antibacterial activity of SWNTs using dispersants that range from antibacterial to growth-supporting. The five dispersants chosen for this work were Sodium dodecyl sulfate (SDS), pluronic, lysozyme, DNA, and tryptic soy broth. Staphylococcus aureus and Salmonella enterica were used as the model Gram-positive and Gram-negative bacteria. Activity was measured in terms of colony forming unit (CFU) and optical density measurements. None of the systems exhibited activity against Salmonella. SDS was fatal to Staph. aureus regardless of the presence of SWNTs. The activity of pluronic and lysozyme against Staph. aureus was enhanced by the presence of SWNTs. In contrast, the DNA and TSB dispersions did not have any activity regardless of the presence of SWNTs. These results highlight that the purported antibacterial activity of SWNTs may only be effective against bacteria that are sensitized by the dispersant and suggests the need for additional research on the mechanisms by which SWNT-dispersant interactions can result in antibacterial activity.

Effects of Non-covalent Functionalization and Initial Mixing Methods on SWNT/PP and SWNT/EVOH Composites.

We report that a simple, low-cost type of spray-freeze drying (SFD) significantly improves the dispersion of single-walled carbon nanotubes (SWNTs) in thermoplastic polymers. Conventional SFD requires costly specialized equipment and large amounts of material, both of which are impediments to laboratory research on nanomaterial composites. Our method uses more readily available equipment and can be adapted to use milligrams to grams of material. A household spray bottle containing an aqueous nanomaterial dispersion is used to spray the dispersion into a dish of liquid nitrogen. The resulting material is then lyophilized in a standard laboratory freeze dryer. The usefulness of this simplified method was explored by comparing the properties of polypropylene (PP) composites produced by this method to those produced by a previously reported rotary evaporation method in which the dispersion is vacuum-dried onto the polymer. The role of the initial dispersion state was explored by using pristine SWNTs as well as SWNTs stabilized by two common SWNT stabilizers: polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate. Based on rheological, thermal, and morphological characterization, the porous friable structures produced by SFD resulted in better SWNT dispersion compared to composites produced by a previously reported rotary evaporation method. However, the PP/PVP-SWNT nanocomposites produced by both methods contained large aggregates. To verify that this aggregation behavior was the result of thermodynamic incompatibility between PP and PVP, ethylene vinyl alcohol (EVOH) nanocomposites containing PVP-SWNT were also produced using the SFD method. The results of this research show how a low-cost alternative to SFD along with careful consideration of compatibility is a promising approach to produce nanocomposites.

Getting Everyone to the Fair: Supporting Teachers in Broadening Participation in Science and Engineering Fairs.

Science and Engineering (S&E) fairs are a valuable educational activity and are believed to increase students' engagement and learning in science and engineering. However, due to differences in resources, many schools do not implement fairs to achieve these benefits for their students. This study reports the findings of a program intended to increase the participation of students from low-achieving and under-resourced schools in a regional fair program that feeds into the international fair competition. We found that the number of schools and projects participating in our regional fair increased dramatically since the start of the program. Teachers had mostly positive expectations for the project and expressed buy-in for the effort the project would take. They recruited a diverse pool of students to participate in the school fairs. Quasi-experimental methods allowed us to explore the impact of completing S&E fairs on student gains on science self-efficacy, interest and value perceptions. Controlling for pre-existing differences in these attitudes, we found that students not completing projects showed declines in their science attitudes during the year. Students who completed projects maintained similar attitudes, while those whose projects advanced to the regional fair had substantial gains on all three variables. It is unknown whether this gain can be attributed to the experience of engaging with a quality project, from being the kind of student who completes a quality project, or some other factor. Future research with greater experimental control could address these questions.

Substrate properties as controlling parameters in attached algal cultivation.

There is growing interest in attached algae cultivation systems because they could provide a more cost- and energy-efficient alternative to planktonic (suspended algae) cultivation systems for many applications. However, attached growth systems have been far less studied than planktonic systems and have largely emphasized algae strains of most interest for biofuels. New algal biorefinery pathways have assessed the commercial potentials of algal biomass beyond biofuel production and placed more emphasis on value-added products from that biomass. Therefore, algal strain selection criteria and biomass cultivation methods need to be updated to include additional strains for improved efficiency. One possible way of improving attached cultivation systems is through engineering substrate surface characteristics to boost algal adhesion and enable strain selective algal colonization and growth. This review explores the effect of substrate chemical and topographical characteristics on the cultivation of attached algae. It also highlights the importance of considering algal community structure and attachment mechanisms in investigating attached algae systems using the example of filamentous algae found in algal turf scrubber (ATS™) systems. KEY POINTS : • Attached algal cultivation is a promising alternative to planktonic cultivation. • Performance increase results from tuning surface qualities of attachment substrates. • Attachment adaptation of periphytic algae has innate potential for cultivation.

Microstructure and electrochemical properties of high performance graphene/manganese oxide hybrid electrodes.

Hybrids consisting of 2D ultra-large reduced graphene oxide (RGO) sheets (∼30 µm long) and 1D α-phase manganese oxide (MnO2) nanowires were fabricated through a versatile synthesis technique that results in electrostatic binding of the nanowires and sheets. Two different hybrid (RGO/MnO2) compositions had remarkable features and performance: 3 : 1 MnO2/RGO (75/25 wt%) denoted as 3H and 10 : 1 MnO2/RGO (90/10 wt%) denoted as 10H. Characterization using spectroscopy, microscopy, and thermal analysis provided insights into the microstructure and behavior of the individual components and hybrids. Both hybrids exhibited higher specific capacitance than their individual components. 3H demonstrated excellent overall electrochemical performance with specific capacitance of 225 F g-1, pseudocapacitive and electrochemical double-layer capacitance (EDLC) contributions, charge-transfer resistance <1 Ω, and 97.8% capacitive retention after 1000 cycles. These properties were better than those of 10H; this was attributed 3H's more uniform distribution of nanowires enabling more effective electronic transport. Thermal annealing was used to produce reduced graphene oxide (RGO) that exhibited significant removal of oxygen functionality with a resulting interlayer spacing of 0.391 nm, higher D/G ratio, higher specific capacitance, and electrochemical properties representing more ideal capacitive behavior than GO. Integrating ultra-large RGO with very high surface area and MnO2 nanowires enables chemical interactions that may improve processability into complex architectures and electrochemical performance of electrodes for applications in electronics, sensors, catalysis, and deionization.

Chiral Structure Formation during Casting of Cellulose Nanocrystalline Films.

A Landau-de Gennes formulation coupled with a mass-transfer equation was used to track the evaporation front and the development of chiral microstructures during the casting of sulfuric acid-hydrolyzed cellulose nanocrystal (CNC) films. These simulations are compared to thin-film casting experiments that used analogous processing parameters and environments. The results show that the initial concentration, chiral strength, surface anchoring, speed of drying, and the influence of initial shear alignment all affect the uniformity of the microstructure and the orientation of the chiral director. In this report, we aim to show that under optimal casting conditions, the lateral size of planar microstructural domains that exhibit uniform selective reflection can be achieved on the order of millimeters.

Comparison of Attachment and Antibacterial Activity of Covalent and Noncovalent Lysozyme-Functionalized Single-Walled Carbon Nanotubes.

Carbon nanotube-lysozyme (LSZ) conjugates provide an attractive combination of high strength and antimicrobial activity. However, there has not been a direct comparison of the covalent and noncovalent methods for creating them. In this work, single-walled carbon nanotubes (SWNT) were functionalized with LSZ using both noncovalent adsorption and covalent attachment via N-ethyl-N-(3-dimethylamino-propyl) carbodiimide hydrochloride-N-hydroxysuccinimide (EDC-NHS) chemistry. The amount of attached lysozyme, dispersion stability, and antimicrobial activity was compared. In addition, the mechanical properties of LSZ-SWNT in poly(vinyl alcohol) (PVA) composite films were investigated. Dispersions of covalently bound LSZ-SWNT had better dispersion stability. This was attributed to covalent functionalization enabling sustained SWNT dispersion at a lower LSZ/SWNT ratio. The covalently bound LSZ-SWNT also exhibited a lower initial rate of antibacterial response but were active over a longer time scale. Composite films made from LSZ-SWNT maintained similar activity as the corresponding dispersions. However, the noncovalent LSZ-SWNT films were stronger and more hydrolytically stable than those made from covalent LSZ-SWNT.

3D Printing of Additive-Free 2D Ti3C2Tx (MXene) Ink for Fabrication of Micro-Supercapacitors with Ultra-High Energy Densities.

Recent advances in the development of self-powered devices and miniaturized electronics have increased the demand for on-chip energy storage devices that can deliver high power and energy densities in a limited footprint area. Here, we report the fabrication of all-solid-state micro-supercapacitors (MSCs) through a three-dimensional (3D) printing of additive-free and water-based MXene ink. The fabricated MSCs benefit from the high electrical conductivity and excellent electrochemical properties of two-dimensional (2D) Ti3C2Tx MXene and a 3D interdigital electrode architecture to deliver high areal and volumetric energy densities. We demonstrate that a highly concentrated MXene ink shows desirable viscoelastic properties for extrusion printing at room temperature and therefore can be used for scalable fabrication of MSCs with various architectures and electrode thicknesses on a variety of substrates. The developed printing process can be readily used for the fabrication of flexible MSCs on polymer and paper substrates. The printed solid-state devices show exceptional electrochemical performance with very high areal capacitance of up to ∼1035 mF cm-2. Our study introduces Ti3C2Tx MXene as an excellent choice of electrode material for the fabrication of 3D MSCs and demonstrates 3D printing of MXene inks at room temperature.

Transparent and Homogenous Cellulose Nanocrystal/Lignin UV-Protection Films.

In the context of valorization of lignin produced from the pulp and paper industries, biodegradable UV-protection films were prepared using lignin and cellulose nanocrystals (CNCs). Initially, CNC films were optimized for improving their transparency by studying the effect of various sodium hydroxide (NaOH) concentrations. Maximum (%) transmittance of CNC film was obtained for NaOH addition between 3 and 4 wt %. The optimized CNC suspensions were used for incorporating alkaline lignin (AL) and softwood kraft lignin (SKL) in various concentrations (1-10 wt %). Morphological characterization showed homogeneity of the lignin distribution in CNC/lignin films. Complete UV blocking was achieved at 10 wt % lignin (AL or SKL) in CNC films. Cross-polarized optical microscopy and scanning electron microscopic images of films showed some degrees of global alignment of CNC rods upon addition of NaOH, which remained unaffected by lignin addition. Lignin modification through acetylation reduced the lignin color and improved visible light transmission of films without significantly affecting the UV-absorption properties. Presence of lignin also enhanced the thermal and contact angle stability of the films. This work shows for the first time that CNC aqueous suspensions with and without containing lignin could be tuned through the addition of NaOH to produce transparent and homogenous films, providing a simple and green approach in engineering CNC/lignin UV-protection films.

Orientation Relaxation Dynamics in Cellulose Nanocrystal Dispersions in the Chiral Liquid Crystalline Phase.

A Landau-de Gennes formulation was implemented in dynamic finite element simulations to compare with postshear relaxation experiments that were conducted on cholesteric cellulose nanocrystal (CNC) dispersions. Our study focused on the microstructural reassembly of CNCs in lyotropic dispersions as parameters such as chiral strength and gap confinement were varied. Our simulation results show that homeotropic and/or more complicated three-dimensional helical configurations are possible, depending on the choice of these parameters. We also observed how dynamic banding patterns develop into the hierarchical microstructures that are characterized by an equilibrium pitch length in both the experiments and simulations. This work has immediate relevance for cellulose nanocrystal dispersion processing and provides new insight into fluid phase ordering for tailorable optical properties.

Microelectromechanical Systems from Aligned Cellulose Nanocrystal Films.

Microelectromechanical systems (MEMS) have become a ubiquitous part of a multitude of industries including transportation, communication, medical, and consumer products. The majority of commercial MEMS devices are produced from silicon using energy-intensive and harsh chemical processing. We report that actuatable standard MEMS devices such as cantilever beam arrays, doubly clamped beams, residual strain testers, and mechanical strength testers can be produced via low-temperature fabrication of shear-aligned cellulose nanocrystal (CNC) films. The devices had feature sizes as small as 6 µm and anisotropic mechanical properties. For 4 µm thick doubly clamped beams with the CNC aligned parallel to the devices' long axes, the Young's moduli averaged 51 GPa and the fracture strength averaged 1.1 GPa. These mechanical properties are within one-third of typical values for polysilicon devices. This new paradigm of producing MEMS devices from CNC extracted from waste biomass provides the simplicity and tunability of fluid-phase processing while enabling anisotropic mechanical properties on the order of those obtained in standard silicon MEMS.

New insights into the flow and microstructural relaxation behavior of biphasic cellulose nanocrystal dispersions from RheoSANS.

Cellulose nanocrystals (CNC) have been studied as nanostructured building blocks for functional materials and function as a model nanomaterial mesogen for cholesteric (chiral nematic) liquid crystalline phases. In this study, both rheology and small angle neutron scattering (RheoSANS) were used to measure changes in flow-oriented order parameter and viscosity as a function of shear rate for isotropic, biphasic, liquid crystalline, and gel dispersions of CNC in deuterium oxide (D2O). In contrast to plots of viscosity versus shear rate, the order parameter trends showed three distinct rheological regions over a range of concentrations. This finding is significant because the existence of three rheological regions as a function of shear rate is a long-standing signature of liquid crystalline phases composed of rod-like polymers, but observing this trend has been elusive for high-concentration dispersions of anisotropic nanomaterials. The results of this work are valuable for guiding the development of processing methodologies for producing ordered materials from CNC dispersions and the broader class of chiral nanomaterial mesogens.

Single-Walled Carbon Nanotube Dispersion in Tryptic Soy Broth.

There has been little research on the dispersion of carbon nanotubes in dispersions of standard microbiological media. We report that tryptic soy broth (TSB) containing casein digest disperses single-walled carbon nanotubes (SWNT) at concentrations similar to those achieved in lysozyme (LSZ), one of the best known biomolecular SWNT dispersants. Similar to LSZ, the proposed mechanism for SWNT dispersion in TSB is favorable π-π stacking interactions with l-tryptophan. This is supported by similar SWNT concentrations in both LSZ and TSB supernatants, and the absence of appreciable dispersion in TSB that does not contain a source of l-tryptophan. Since l-tryptophan alone is insufficient to enable dispersion, it was previously hypothesized that LSZ's macromolecular structure created steric hindrance that was critical for SWNT dispersion. These new results show that intermediately sized l-tryptophan containing species can also enable dispersion. In addition, since TSB is a commonly used growth medium for microbiological research, its dispersive ability presents new research avenues for studying the effect of SWNT on prokaryotic cells without the need to oxidize SWNT or add dispersants that may induce microbial stress.

Concentration of lysozyme/single-walled carbon nanotube dispersions.

The dispersion of single-walled carbon nanotubes (SWNT) in aqueous solutions of biological materials enables the production of bulk films and fibers that combine natural biological activity with SWNT's intrinsic mechanical, thermal, and electrical properties. In this work, we report the rheology and phase behavior of concentrated lysozyme (LSZ)/SWNT dispersions. Even at low concentration, the LSZ's globular structure causes a deviation from the rheological behavior expected of rigid rods such as SWNT. With increasing concentration, stabilized SWNT typically form lyotropic liquid crystalline phases. However, in this case, the LSZ results in depletion attraction and the formation of large dense SWNT aggregates surrounded by a LSZ network. At intermediate concentrations, the microstructure and rheological properties are a complex function of the initial dispersion state, the absolute concentrations, and the LSZ to SWNT ratio. The rheological effects of concentrating mixtures comprised of aggregates, a range of bundle sizes, and individual SWNT were compared to the effects of concentrating supernatants comprised solely of individual SWNT and small bundles. In general, lysozyme concentration has the greatest impact on dispersion viscoelasticity. However, the inherent viscosity was a function of SWNT concentration; data from both initial mixtures and supernatants spanning two orders of magnitude in concentration could be collapsed onto a single master curve. This work provides a foundation for exploring the behavior of other globular protein-SWNT dispersions.