Metal Ion and Guest-Mediated Spontaneous Resolution and Solvent-Induced Chiral Symmetry Breaking in Guanine-Based Metallosupramolecular Networks.

Two-dimensional (2D) chirality has been actively studied in view of numerous applications of chiral surfaces such as in chiral resolutions and enantioselective catalysis. Here, we report on the expression and amplification of chirality in hybrid 2D metallosupramolecular networks formed by a nucleobase derivative. Self-assembly of a guanine derivative appended with a pyridyl node was studied at the solution-graphite interface in the presence and absence of coordinating metal ions. In the absence of coordinating metal ions, a monolayer that is representative of a racemic compound was obtained. This system underwent spontaneous resolution upon addition of a coordinating ion and led to the formation of a racemic conglomerate. The spontaneous resolution could also be achieved upon addition of a suitable guest molecule. The mirror symmetry observed in the formation of the metallosupramolecular networks could be broken via the use of an enantiopure solvent, which led to the formation of a globally homochiral surface.

Colloidal Stability and Aggregation Mechanism in Aqueous Suspensions of TiO2 Nanoparticles Prepared by Sol-Gel Synthesis.

Understanding the colloidal stability and aggregation behavior of TiO2 nanoparticles in aqueous suspension is a prerequisite to tune supracolloidal structure formation. While the aggregation mechanism for dried TiO2 nanopowders is well documented, there is still work to be done to understand TiO2 nanoparticle aggregation in suspension. Therefore, this work focuses on the colloidal stability and aggregation mechanism of TiO2 nanoparticle aqueous suspensions prepared using a straightforward one-step sol-gel-based approach over a concentration range of 0.5-5 wt %. Fully crystalline nanoparticles consisting primarily of anatase were obtained. After assessing the colloidal stability of the as-prepared suspensions, small-angle X-ray scattering coupled with fractal analysis was carried out. This analysis showed, for the first time, how the TiO2 nanoparticle aggregation mechanism─predicted by the diffusion limited cluster-cluster aggregation (DLCA) and diffusion limited particle-cluster aggregation (DLA) theories─depends directly on the starting concentration in the aqueous suspensions. We found that concentrated suspensions favored DLA, while dilute suspensions tend to follow the DLCA mechanism. The effect of the aggregation mechanism on the aggregate shape is also discussed.

Stimuli-Induced Nonequilibrium Phase Transitions in Polyelectrolyte-Surfactant Complex Coacervates.

Polyelectrolyte-surfactant complexes (PESCs) are important soft colloids with applications in the fields of personal care, cosmetics, pharmaceutics, and much more. If their phase diagrams have long been studied under pseudoequilibrium conditions, and often inside the micellar or vesicular regions, understanding the effect of nonequilibrium conditions, applied at phase boundaries, on the structure of PESCs generates an increasing interest. In this work we cross the micelle-vesicle and micelle-fiber phase boundaries in an isocompositional surfactant-polyelectrolyte aqueous system through a continuous and rapid variation of pH. We employ two microbial glycolipid biosurfactants in the presence of polyamines, both systems being characterized by their responsiveness to pH. We show that complex coacervates (Co) are always formed in the micellar region of both glycolipids' phase diagram and that their phase behavior drives the PESC stability and structure. However, for glycolipid forming single-wall vesicles, we observe an isostructural and isodimensional transition between complex coacervates and a multilamellar walls vesicle (MLWV) phase. For the fiber-forming glycolipid, on the contrary, the complex coacervate disassembles into free polyelectrolyte coexisting with the equilibrium fiber phase. Last but not least, this work also demonstrates the use of microbial glycolipid biosurfactants in the development of sustainable PESCs.

Effect of Sugars on the Real-Time Adsorption of Expansin on Cellulose.

Biological pretreatment is a safe and environmentally friendly method for disrupting recalcitrant lignocellulose structures. Expansin and expansin-like proteins are used to open up the cellulose structure and display significant synergism when mixed with cellulases that catalyze the breakdown of (hemi)cellulose into sugars. However, the adsorption behavior of expansin in the presence of sugar products is yet unknown. In this work, we monitored the effects of various sugars on the real-time adsorption of Bacillus subtilis expansin (BsEXLX1) onto cellulose films using a quartz crystal microbalance with dissipation. Cellobiose and xylose at low concentrations enhanced BsEXLX1 adsorption, whereas they disrupted adsorption at higher concentrations. Arabinose and mannose continuously inhibited expansin adsorption with increasing concentration. No obvious influence of glucose and galactose on BsEXLX1 adsorption was found. Contact angle measurements and atomic force microscopy of cellulose upon BsEXLX1 adsorption in the presence of sugars showed that both hydrophilicity and roughness increased with BsEXLX1 treatment. These results give us the ability to modulate and control expansin adsorption and provide insights into effective expansin use during enzymatic hydrolysis of lignocellulose in biorefineries.

Phase Behavior and Polymorphism of Saturated and Unsaturated Phytosterol Esters.

This study investigated how the physicochemical characteristics of phytosterol esters are influenced by the chain length and degree of unsaturation of the fatty acid ester moiety. Saturated and unsaturated phytosterol esters (PEs) were synthesized by the esterification of different types of fatty acids (stearic, palmitic, lauric, oleic, and linoleic acid) to ß-sitosterol. The non-isothermal crystallization and melting behavior of the pure PEs were analyzed. It was proven by X-ray diffraction that saturated ß-sitosteryl esters and ß-sitosteryl oleate formed a bilayer crystal structure. The lamellar spacings of the bilayer structure decreased with decreasing fatty acid chain length and with an increasing degree in unsaturation. The degree of unsaturation of the fatty acid chain of the ß-sitosteryl esters also influenced the type of subcell packing of the fatty acid moieties in the bilayer structure, whether or not a metastable or stable liquid crystalline phase was formed during cooling. Furthermore, it was found that the melting temperature and enthalpy of the ß-sitosteryl esters increased with an increasing fatty acid chain length while they decreased with an increasing degree of unsaturation. The microscopic analyses demonstrated that ß-sitosteryl oleate formed much smaller spherulites than their saturated ß-sitosteryl analogues.

Electric-Field-Mediated Reversible Transformation between Supramolecular Networks and Covalent Organic Frameworks.

By using an oriented electric field in a scanning tunneling microscope, one can locally control the condensation of boronic acids at the liquid/solid interface. The phase transition between self-assembled molecular networks and covalent organic frameworks is controlled by changing the polarity of the applied bias. The electric-field-induced phase transformation is reversible under ambient conditions.

Anisotropic Diffusion and Phase Behavior of Cellulose Nanocrystal Suspensions.

In this paper, we use dynamic light scattering in polarized and depolarized modes to determine the translational and rotational diffusion coefficients of concentrated rodlike cellulose nanocrystals in aqueous suspension. Within the range of studied concentrations (1-5 wt %), the suspension starts a phase transition from an isotropic to an anisotropic state as shown by polarized light microscopy and viscosity measurements. Small-angle neutron scattering measurements also confirmed the start of cellulose nanocrystal alignment and a decreasing distance between the cellulose nanocrystals with increasing concentration. As expected, rotational and translational diffusion coefficients generally decreased with increasing concentration. However, the translational parallel diffusion coefficient was found to show a local maximum at the onset of the isotropic-to-nematic phase transition. This is attributed to the increased available space for rods to move along their longitudinal axis upon alignment. This increased parallel diffusion coefficient thus confirms the general idea that rodlike particles gain translational entropy upon alignment while paying the price for losing rotational degrees of freedom. Once the concentration increases further, diffusion becomes more hindered even in the aligned regions due to a reduction in the rod separation distance. This leads once again to a decrease in translational diffusion coefficients. Furthermore, the relaxation rate for fast mode translational diffusion (parallel to the long particle axis) exhibited two regimes of relaxation behavior at concentrations where significant alignment of the rods is measured. We attribute this unusual dispersive behavior to two length scales: one linked to the particle length (at large wavevector q) and the other to a twist fluctuation correlation length (at low wavevector q) along the cellulose nanocrystal rods that is of a larger length when compared to the actual length of rods and could be linked to the size of aligned domains.

Engineered Three-Dimensional Microenvironments with Starch Nanocrystals as Cell-Instructive Materials.

Naturally, cells reside in three-dimensional (3D) microenvironments composed of biopolymers that guide cellular behavior via topographical features as well as through mechanical and biochemical cues. However, most studies describing the influence of topography on cells' behavior are performed on rigid and synthetic two-dimensional substrates. To design systems that more closely resemble native microenvironments, herein we develop 3D nanocomposite hydrogels consisting of starch nanocrystals (SNCs) embedded in a gelatin matrix. The incorporation of different concentrations of SNCs (0.05, 0.2, and 0.5 wt %) results in an increase of compressive modulus when compared to hydrogels without SNCs, without affecting the swelling ratio, thus providing a tunable system. Confirming the cytocompatibility of the novel composites, the viability of encapsulated L929 fibroblasts is >90% in all hydrogels. The cellular metabolic activity and DNA content are similar for all formulations and increase over time, indicating that the fibroblasts proliferate within the hydrogels. After 4 d of culture, Live/Dead staining and F-actin/nuclei staining show that the encapsulated fibroblasts develop an elongated morphology in the hydrogels. On the other hand, encapsulated chondrogenic progenitor ATDC5 cells also maintain a viability around 90% but display a round morphology, especially in the hydrogels with SNCs, indicating a potential application of the materials for cartilage tissue engineering. We believe that topographical and mechanical cues within 3D microenvironments can be a powerful tool to instruct cells' behavior and that the developed gelatin/SNC nanocomposite warrants further study.

Thermodynamic Study of Ion-Driven Aggregation of Cellulose Nanocrystals.

The thermodynamics of interactions between cations of the second group of the periodic table and differently negatively charged cellulose nanocrystals was investigated using isothermal titration calorimetry (ITC). The interaction of cations with the negatively charged CNCs was found to be endothermic and driven by an increase in entropy upon adsorption of the ions, due to an increase in degrees of freedom gained by the surface bound water upon ion adsorption. The effect was pH-dependent, showing an increase in enthalpy for cellulose suspensions at near-neutral pH (6.5) when compared to acidic pH (2). Sulfated cellulose nanoparticles were found to readily interact with divalent ions at both pH levels. The adsorption on carboxylate nanocrystals was found to be pH dependent, showing that the carboxylic group needs to be in the deprotonated form to interact with divalent ions. For the combined system (sulfate and carboxylate present at the same time), at neutral pH, the adsorption enthalpy was higher than the value obtained from cellulose nanocrystals containing a single functional group, while the association constant was higher due to an increased favorable entropic contribution. The higher entropic contribution indicates a more restricted surface-bound water layer when multiple functionalities are present. The stoichiometric number n was nearly constant for all systems, showing that the adsorption depends almost completely on the ion valency and on the amount of ionic groups on the CNC surface, independent of the type of functional group on the CNC surface as long as it is deprotonated. In addition, we showed that the reduction in Gibbs free energy drives the ionotropic gelation of nanocellulose suspensions, and we show that ITC is able to detect gel formation at the same time as determining the critical association concentration.

Toward Improved Understanding of the Interactions between Poorly Soluble Drugs and Cellulose Nanofibers.

Cellulose nanofibers (CNFs) have interesting physicochemical and colloidal properties that have been recently exploited in novel drug-delivery systems for tailored release of poorly soluble drugs. The morphology and release kinetics of such drug-delivery systems heavily relied on the drug-CNF interactions; however, in-depth understanding of the interactions was lacking. Herein, the interactions between a poorly soluble model drug molecule, furosemide, and cationic cellulose nanofibers with two different degrees of substitution are studied by sorption experiments, Fourier transform infrared spectroscopy, and molecular dynamics (MD) simulation. Both MD simulations and experimental results confirmed the spontaneous sorption of drug onto CNF. Simulations further showed that adsorption occurred by the flat aryl ring of furosemide. The spontaneous sorption was commensurate with large entropy gains as a result of release of surface-bound water. Association between furosemide molecules furthermore enabled surface precipitation as indicated by both simulations and experiments. Finally, sorption was also found not to be driven by charge neutralization, between positive CNF surface charges and the furosemide negative charge, so that surface area is the single most important parameter determining the amount of sorbed drug. An optimized CNF-furosemide drug-delivery vehicle thus needs to have a maximized specific surface area irrespective of the surface charge with which it is achieved. The findings also provide important insights into the design principles of CNF-based filters suitable for removal of poorly soluble drugs from wastewater.

Real-Time Adsorption of Exo- and Endoglucanases on Cellulose: Effect of pH, Temperature, and Inhibitors.

Effective regulation of cellulase adsorption is key to improving the efficiencies of the two major bottlenecks of lignocellulose hydrolysis and cellulase recovery. In this work, we investigated the effect of inhibitors, pH, and temperature on the adsorption of exo- and endoglucanases (Cel7A and Cel7B, respectively) on cellulose using quartz crystal microgravimetry with dissipation. The addition of glucose and cellobiose can both inhibit the hydrolysis activity of Cel7A, whereas only cellobiose can inhibit that of Cel7B. Notably, the adsorption was favored by acidic conditions (pH ≤ 4.8) and low temperature, whereas alkaline conditions (pH 9 and 10) facilitated enzyme desorption, which is useful to guide the process of cellulase recovery. The adsorption and hydrolysis activity of Cel7A and Cel7B were both higher at 45 °C than at 25 °C. These findings pave the way to effective regulation of cellulase adsorption and thus improve lignocellulose conversion and cellulase recovery.

Effect of Gelation on the Colloidal Deposition of Cellulose Nanocrystal Films.

One of the most important aspects in controlling colloidal deposition is manipulating the homogeneity of the deposit by avoiding the coffee-ring effect caused by capillary flow inside the droplet during drying. After our previous work where we achieved homogeneous deposition of cellulose nanocrystals (CNCs) from a colloidal suspension by reinforcing Marangoni flow over the internal capillary flow (Gençer et al. Langmuir 2017, 33 (1), 228-234), we now set out to reduce the importance of capillary flow inside a drying droplet by inducing gelation. In this paper, we discuss the effect of gelation on the deposition pattern and on the self-assembly of CNCs during droplet drying. CNC films were obtained by drop casting CNC suspensions containing NaCl and CaCl2 salts. A mixed methodology using rheological and depolarized dynamic light scattering was applied to understand the colloidal behavior of the CNCs. In addition, analysis of the mixture's surface tension, viscosity, and yield stress of the suspensions were used to gain deeper insights into the deposition process. Finally, the understanding of the gelation behavior in the drying droplet was used to exert control over the deposit where the coffee-ring deposit can be converted to a dome-shaped deposit.

Thermodynamics of the interactions of positively charged cellulose nanocrystals with molecules bearing different amounts of carboxylate anions.

Negatively charged ions readily interact with the surface of positively charged pyridinium-grafted cellulose nanocrystals. In this work we investigated the thermodynamics of these interactions using isothermal titration calorimetry. We investigated the effect of the ionic charge, using carboxylate salts with different valence (1-4), and compared it with sodium sulfate as another delocalized ionic charge. Experiments performed using cellulose nanocrystals with three different degrees of substitution of pyridinium grafts showed that the number of ions adsorbed onto the surface of cellulose nanocrystals is directly linked to the number of grafted cationic functionalities. The adsorption of anions onto pyridinium-grafted nanocrystals was further found to be endothermic and driven by an increase in entropy upon adsorption of the anions due to the release of surface bound water. The association constant increased with the net charge of the anions from a low value for monovalent sodium acetate. Both entropy and enthalpy increased linearly with the net charge of the anions, demonstrating that a stronger depolarization occurred when anions of higher net charge were adsorbed. The values of the stoichiometric number determined were lower than the values relative to processes involving ions in solution, suggesting a more complex mode of interaction, involving hydrogen-bonding and bridge formation. However, the number of adsorbed ions was directly correlated with the amount of cationic surface grafts. This makes it possible to control the amount of surface interactions directly with the degree of substitution on the nanocrystal surfaces. Comparison with zeta potential measurements showed that zeta-potential measurements can be used as a direct method to determine the stoichiometry of binding between the positive surface grafts and the carboxylate salts.

Influence of the Particle Concentration and Marangoni Flow on the Formation of Cellulose Nanocrystal Films.

Cellulose nanocrystals (CNCs), ribbonlike crystalline nanoparticles, are a biobased material that can be a great alternative to obtaining films with tunable optical properties. Iridescent and light-diffracting films can be readily obtained via the drying of a suspension of these cellulose nanocrystals. The characteristics of the particle deposition process together with the self-assembly in the precluding suspension has a direct effect on the optical properties of the obtained films. Particle deposition onto a substrate is affected by the flow dynamics inside sessile droplets and usually yields a ring-shaped deposition pattern commonly referred to as the coffee-ring effect. We set out to measure and describe the drying kinetics under different conditions. We found that the Marangoni flow inside the droplet was too small to counteract the capillary flow that deposits CNCs at the edges, resulting in the coffee-ring effect, irrespective of the atmospheric humidity. By varying the amount of ethanol in the atmosphere, we were able to find a balance between (1) colloidal stability in the droplet, which is reduced by ethanol diffusion into the droplet, and (2) increasing Marangoni flow relative to capillary flow inside the droplet by changing the droplet surface tension. We could thus make iridescent films with a uniform thickness.

Thermodynamic Study of the Interaction of Bovine Serum Albumin and Amino Acids with Cellulose Nanocrystals.

The interaction of bovine serum albumin (BSA) with sulfated, carboxylated, and pyridinium-grafted cellulose nanocrystals (CNCs) was studied as a function of the degree of substitution by determining the adsorption isotherm and by directly measuring the thermodynamics of interaction. The adsorption of BSA onto positively charged pyridinium-grafted cellulose nanocrystals followed Langmuirian adsorption with the maximum amount of adsorbed protein increasing linearly with increasing degree of substitution. The binding mechanism between the positively charged pyridinum-grafted cellulose nanocrystals and BSA was found to be endothermic and based on charge neutralization. A positive entropy of adsorption associated with an increase of the degree of disorder upon addition of BSA compensated for the unfavorable endothermic enthalpy and enabled formation of pyridinium-g-CNC-BSA complexes. The endothermic enthalpy of adsorption was further found to decrease as a function of increasing degree of substitution. Negatively charged cellulose nanocrystals bearing sulfate and/or carboxylic functionalities were found to not interact significantly with the BSA protein. To investigate in more detail the role of single amino acids in the adsorption of proteins onto cellulose nanocrystals, we also studied the interaction of different types of amino acids with CNCs, i.e., charged (lysine, aspartic acid), aromatic (tryptophan, tyrosine), and polar (serine) amino acids. We found that none of the single amino acids bound with CNCs irrespective of surface charge and that therefore the binding of proteins with CNCs appears to require larger amino acid sequences that induce a greater entropic contribution to stabilize binding. Single amino acids are thus not adsorbed onto cellulose nanocrystals.

Binary Mixed Homopolymer Brushes Tethered to Cellulose Nanocrystals: A Step Towards Compatibilized Polyester Blends.

This article reports on the successful preparation and characterization of cellulose nanocrystals (CNCs) surface-modified with polylactide (PLA) and poly(butylene succinate) (PBS) binary mixed homopolymer brushes. Their synthesis was designed as a three-step procedure combining polyester synthesis and surface-modification of CNCs with simultaneous polyester grafting via a heterogeneous copper(I)-catalyzed azide-alkyne cycloaddition reaction. For comparison, single homopolymer brushes tethered to CNCs (PLLA-g-CNC and PBSBDEMPAM-g-CNC) were obtained applying the same procedure. The hairy nanoparticles were characterized in terms of chemical composition and thermal properties. Spectroscopic analyses suggested "rippled" microphase separation of both immiscible homopolyesters in the mixed brushes, while others showed impeded homopolyester crystallization after surface-grafting. A synergistic relationship between the polyesters and CNCs was also suggested, i.e., the polyester grafting increases the CNC thermal resistance, while CNC presence imparts char formation. The as-obtained binary homopolymer brushes tethered to nanoparticles makes these surface-modified cellulosic nanomaterials attractive as compatibilization/reinforcement agents for PLA/PBS blends.

Effect of cellulose nanowhiskers on surface morphology, mechanical properties, and cell adhesion of melt-drawn polylactic Acid fibers.

Polylactic acid (PLA) fibers were produced with an average diameter of 11.2 (± 0.9) µm via a melt-drawing process. The surface of the PLA fibers was coated with blends of cellulose nanowhiskers (CNWs) (65 to 95 wt %) and polyvinyl acetate (PVAc). The CNWs bound to the smooth PLA fiber surface imparted roughness, with the degree of roughness depending on the coating blend used. The fiber tensile modulus increased 45% to 7 GPa after coating with 75 wt % CNWs compared with the uncoated PLA fibers, and a significant increase in the fiber moisture absorption properties at different humidity levels was also determined. Cytocompatibility studies using NIH-3T3 mouse fibroblast cells cultured onto CNWs-coated PLA surface revealed improved cell adhesion compared with the PLA control, making this CNW surface treatment applicable for biomedical and tissue engineering applications. Initial studies also showed complete cell coverage within 2 days.

Thermodynamics of interactions between cellulose nanocrystals and monovalent counterions.

Alkali and quaternary ammonium cations interact with negatively charged cellulose nanocrystals (CNCs) bearing sulfated or carboxylated functional groups. As these are some of the most commonly occurring cations CNC encounter in applications, the thermodynamic parameters of these CNC-counterion interactions were evaluated with isothermal titration calorimetry (ITC). Whereas the adsorption of monovalent counterions onto CNCs was thermodynamically favourable at all evaluated conditions as indicated by a negative Gibbs free energy, the enthalpic and entropic contributions to the CNC-ion interactions were found to be strongly dependent on the hydration characteristics of the counterion and could be correlated with the potential barrier to water exchange of the respective ions. The adsorption of chaotropic cations onto the surface was exothermic, while the interactions with kosmotropic cations were endothermic and completely entropy-driven. The interactions of CNCs with more bulky quaternary ammonium counterions were more complex, and the mechanism of interaction shifted from electrostatic interactions with surface charged groups of CNCs towards adsorption of alkyl chains onto the CNC hydrophobic planes when the alkyl chain length increased.

Covalent immobilization of N-heterocyclic carbenes on pristine carbon substrates: from nanoscale characterization to bulk catalysis.

To control the synthesis of designer catalysts on graphitic materials up to the nanometer scale, methods should be provided that combine both nanoscale characterization and bulk scale experiments. This work reports the grafting of N-heterocyclic carbene (NHC)-type catalysts on graphite, both at nanometer and bulk scale, as it allows increased insights into the nature of the immobilized catalysts.

Dimensions of Cellulose Nanocrystals from Cotton and Bacterial Cellulose: Comparison of Microscopy and Scattering Techniques.

Different microscopy and scattering methods used in the literature to determine the dimensions of cellulose nanocrystals derived from cotton and bacterial cellulose were compared to investigate potential bias and discrepancies. Atomic force microscopy (AFM), small-angle X-ray scattering (SAXS), depolarized dynamic light scattering (DDLS), and static light scattering (SLS) were compared. The lengths, widths, and heights of the particles and their respective distributions were determined by AFM. In agreement with previous work, the CNCs were found to have a ribbon-like shape, regardless of the source of cellulose or the surface functional groups. Tip broadening and agglomeration of the particles during deposition cause AFM-derived lateral dimensions to be systematically larger those obtained from SAXS measurements. The radius of gyration determined by SLS showed a good correlation with the dimensions obtained by AFM. The hydrodynamic lateral dimensions determined by DDLS were found to have the same magnitude as either the width or height obtained from the other techniques; however, the precision of DDLS was limited due to the mismatch between the cylindrical model and the actual shape of the CNCs, and to constraints in the fitting procedure. Therefore, the combination of AFM and SAXS, or microscopy and small-angle scattering, is recommended for the most accurate determination of CNC dimensions.