The Effect of Temperature and Shear on the Gelation of Cellulose Nanocrystals in Deep Eutectic Solvents.

With the flourishing development of 3D printing technology, the demand for printing materials has been increasing rapidly in recent years. In particular, physical gels formed by cellulose nanocrystals (CNCs) exhibit suitable shear-thinning behavior, high storage moduli, and high yield stresses for extrusion-based printing. While most studies use water as the dispersing medium to form CNC percolated gels, the dispersing behavior of CNCs in alternative solvents, such as deep eutectic solvents (DESs), has not been fully explored. Especially, DESs have low volatility and good ionic conductivity to form functional ionogels. Precise control of the rheological properties and selection of suitable dispersion processes continue to pose significant challenges. In light of this, we have devised a novel dispersion process employing thermal and shear treatments to facilitate the gelation of CNCs within DESs. A crude dispersion of CNCs in the DES underwent thermal treatment to partially remove the surface sulfate ester on CNCs. As a result, the repulsive force between CNCs decreases. A second shear then significantly increases the strength of CNC/DES gels potentially because of the increased rod-rod contacts. This approach enables the formation of high-strength gels at low concentrations of CNCs. Both thermal treatment and a second shear are crucial to forming strong percolated CNC gels. In short, we showed a simple strategy to facilitate the dispersion and gelation of CNCs for direct ink writing.

Construction of an efficient artificial light-harvesting system based on hyperbranched polyethyleneimine and improvement of photocatalytic performance.

An artificial light-harvesting system (ALHS) was developed in aqueous solution by employing the electrostatic co-assembly of a tetraphenylethylene derivative modified with two sulfonate groups (TPE-BSBO) and hyperbranched polyethyleneimine (PEI) as the energy donors, and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (DBT) as the energy acceptors. The ALHS exhibits not only high efficiency in energy transfer and conversion but also a significant enhancement in the generation of reactive oxygen species (ROS), especially superoxide anion radicals (O2˙-), facilitating its utilization in photocatalytic oxidation reactions.

Bioinspired simultaneous regulation in fluorescence of AIEgen-embedded hydrogels.

The development of stimuli-responsive functional fluorescent hydrogels is of great significance for the realization of artificial intelligence. In the present work, we design and synthesize a stimulus-responsive hydrogel embedded with an aggregation-induced emission (AIE) monomer, in which the fluorescence brightness and intensity can be tuned. The hydrogel embedded with tetraphenylethene-grafted-poly[3-sulfopropyl methacrylate potassium salt] (TPE-PSPMA) as the functional element is prepared by the radical polymerization method. Among them, the TPE core exhibits adaptive fluorescence ability through the AIE effect, while the PSPMA chain provides tunable hydrophilic properties under an external stimulus. The effect of different cationic surfactants with different lengths of hydrophobic tails on the fluorescence properties of TPE-PSPMA in solution is systematically investigated. With cationic surfactants, such as cetyltrimethylammonium bromide (CTAB), the fluorescence intensity is gradually tuned from 1059 to 4623. And the fluorescence intensities increase with the growth of hydrophobic tails of surfactants, which results from hydrophobicity-induced electrostatic interactions among surfactants and polymer chains. Furthermore, an obvious tunable fluorescence feature of hydrogel copolymerized TPE-PSPMA is realized, resulting from the change of brightness and the dynamic increase of fluorescence intensity (from 1031 to 3138) for the hydrogel immersed in CTAB solution with different soaking times. Such a typical fluorescence-regulated behavior can be attributed to the AIE of the TPE-PSPMA chain and the electrostatic interaction between the surfactant and the anionic polymer chain. The designed TPE-PSPMA-based hydrogel is responsive to stimuli, inspiring the development of intelligent systems such as soft robots and smart wearables.

3D Printing of Metal-Organic Framework-Based Ionogels: Wearable Sensors with Colorimetric and Mechanical Responses.

Soft ionotronics are emerging materials as wearable sensors for monitoring physiological signals, sensing environmental hazards, and bridging the human-machine interface. However, the next generation of wearable sensors requires multiple sensing capabilities, mechanical toughness, and 3D printability. In this study, a metal-organic framework (MOF) and three-dimensional (3D) printing were integrated for the synthesis of a tough MOF-based ionogel (MIG) for colorimetric and mechanical sensing. The ink for 3D printing contained deep eutectic solvents (DESs), cellulose nanocrystals (CNCs), MOF crystals, and acrylamide. After printing, further photopolymerization resulted in a second covalently cross-linked poly(acrylamide) network and solidification of MIG. As a porphyrinic Zr-based MOF, MOF-525 served as a functional filler to provide sharp color changes when exposed to acidic compounds. Notably, MOF-525 crystals also provided another design space to tune the printability and mechanical strength of MIG. In addition, the printed MIG exhibited high stability in the air because of the low volatility of DESs. Thereafter, wearable auxetic materials comprising MIG with negative Poisson's ratios were prepared by 3D printing for the detection of mechanical deformation. The resulting auxetic sensor exhibited high sensitivity via the change in resistance upon mechanical deformation and a conformal contact with skins to monitor various human body movements. These results demonstrate a facile strategy for the construction of multifunctional sensors and the shaping of MOF-based composite materials.

The effect of temperature on the kinetics of enhanced amide bond formation from lactic acid and valine driven by deep eutectic solvents.

Deep eutectic solvents have been found to facilitate the copolymerization of hydroxy acids and amino acids through an ester-amide exchange reaction, and to drive the formation of amino acid-enriched oligomers with peptide backbones. The complexity of oligomer distribution is significantly reduced in deep eutectic solvents and amide-linked oligomers can be selectively produced. In the present study, we investigated the kinetics of amide bond formation in deep eutectic solvents to understand how the solvents regulate the pathways of complex copolymerization. A mathematical model successfully simulated the reaction of a lactic acid/valine mixture in deep eutectic solvents at different temperatures and provided insight into the activation energy of each step. Our findings indicated that the esterification and the evaporation of hydroxy acids were greatly suppressed in deep eutectic solvents because of the strong interaction between the quaternary ammonium salts and the hydroxy acids. In contrast, the ester-amide exchange reaction in deep eutectic solvents was significantly enhanced by lowering the activation entropies. The synergic effect of reduced esterification and increased exchange leads to amino acid-enriched oligomers with high yield and high selectivity. Furthermore, the reduced evaporation of hydroxy acids in deep eutectic solvents may preserve limited reactants in the prebiotic scenario. These results reveal deep eutectic solvents as sustainable media for the simple synthesis of amide bonds.

3D Printing of Thermal Insulating Polyimide/Cellulose Nanocrystal Composite Aerogels with Low Dimensional Shrinkage.

Polyimide (PI)-based aerogels have been widely applied to aviation, automobiles, and thermal insulation because of their high porosity, low density, and excellent thermal insulating ability. However, the fabrication of PI aerogels is still restricted to the traditional molding process, and it is often challenging to prepare high-performance PI aerogels with complex 3D structures. Interestingly, renewable nanomaterials such as cellulose nanocrystals (CNCs) may provide a unique approach for 3D printing, mechanical reinforcement, and shape fidelity of the PI aerogels. Herein, we proposed a facile water-based 3D printable ink with sustainable nanofillers, cellulose nanocrystals (CNCs). Polyamic acid was first mixed with triethylamine to form an aqueous solution of polyamic acid ammonium salts (PAAS). CNCs were then dispersed in the aqueous PAAS solution to form a reversible physical network for direct ink writing (DIW). Further freeze-drying and thermal imidization produced porous PI/CNC composite aerogels with increased mechanical strength. The concentration of CNCs needed for DIW was reduced in the presence of PAAS, potentially because of the depletion effect of the polymer solution. Further analysis suggested that the physical network of CNCs lowered the shrinkage of aerogels during preparation and improved the shape-fidelity of the PI/CNC composite aerogels. In addition, the composite aerogels retained low thermal conductivity and may be used as heat management materials. Overall, our approach successfully utilized CNCs as rheological modifiers and reinforcement to 3D print strong PI/CNC composite aerogels for advanced thermal regulation.

Cationic Cellulose Nanocrystals-Based Nanocomposite Hydrogels: Achieving 3D Printable Capacitive Sensors with High Transparency and Mechanical Strength.

Hydrogel ionotronics are intriguing soft materials that have been applied in wearable electronics and artificial muscles. These applications often require the hydrogels to be tough, transparent, and 3D printable. Renewable materials like cellulose nanocrystals (CNCs) with tunable surface chemistry provide a means to prepare tough nanocomposite hydrogels. Here, we designed ink for 3D printable sensors with cationic cellulose nanocrystals (CCNCs) and zwitterionic hydrogels. CCNCs were first dispersed in an aqueous solution of monomers to prepare the ink with a reversible physical network. Subsequent photopolymerization and the introduction of Al3+ ion led to strong hydrogels with multiple physical cross-links. When compared to the hydrogels using conventional CNCs, CCNCs formed a stronger physical network in water that greatly reduced the concentration of nanocrystals needed for reinforcing and 3D printing. In addition, the low concentration of nanofillers enhanced the transparency of the hydrogels for wearable electronics. We then assembled the CCNC-reinforced nanocomposite hydrogels with stretchable dielectrics into capacitive sensors for the monitoring of various human activities. 3D printing further enabled a facile design of tactile sensors with enhanced sensitivity. By harnessing the surface chemistry of the nanocrystals, our nanocomposite hydrogels simultaneously achieved good mechanical strength, high transparency, and 3D printability.

Ester-mediated peptide formation promoted by deep eutectic solvents: a facile pathway to proto-peptides.

The ester-amide exchange reaction enables spontaneous formation of prebiotic proto-peptides under mild conditions. However, this reaction also leads to oligomers with a vast sequence diversity of ester and amide linkages. Here, we demonstrate using deep eutectic solvents as a universal strategy to regulate the reaction pathways and promote the formation of amino acid-enriched oligomers with peptide backbones.

3D Printable Strain Sensors from Deep Eutectic Solvents and Cellulose Nanocrystals.

Stretchable and conductive hydrogels have been intensively studied as wearable electronics to monitor the physiological activities of human bodies. However, it remains a challenge to fabricate robust hydrogels as sensors with complex 3D structures. Here, we designed a 3D printable ink from cellulose nanocrystals (CNCs), deep eutectic solvents (DESs), and ionically cross-linked polyacrylic acid (PAA). DESs composed of choline chloride and ethylene glycol served as a nonvolatile medium with high ionic conductivity. The dispersion of CNCs in a mixture of DESs, acrylic acid, and Al3+ ions formed ionogels with a reversible physical network for 3D printing. After the printing process, the ionogel was solidified by the photopolymerization of acrylic acid in the presence of Al3+ ions to form a second ionically cross-linked network. The first physical network of CNCs provides an energy-dissipating mechanism to make a strong and highly stretchable nanocomposite ionogel. When compared to hydrogels, we found that the DES/CNC nanocomposite ionogel was more stable in the air because of the low volatility of DESs. We further used the DES/CNC ink to 3D print an auxetic sensor with negative Poisson's ratios so that the sensor provided a conformal contact with the skin during large deformation. In addition, the auxetic sensor could continuously monitor and identify different motions of the human body by the change in resistance. These results demonstrate a simple and rapid strategy to fabricate stable and sensitive strain sensors from cheap and renewable feedstock.

Surveying the sequence diversity of model prebiotic peptides by mass spectrometry.

The rise of peptides with secondary structures and functions would have been a key step in the chemical evolution which led to life. As with modern biology, amino acid sequence would have been a primary determinant of peptide structure and activity in an origins-of-life scenario. It is a commonly held hypothesis that unique functional sequences would have emerged from a diverse soup of proto-peptides, yet there is a lack of experimental data in support of this. Whereas the majority of studies in the field focus on peptides containing only one or two types of amino acids, here we used modern mass spectrometry (MS)-based techniques to separate and sequence de novo proto-peptides containing broader combinations of prebiotically plausible monomers. Using a dry-wet environmental cycling protocol, hundreds of proto-peptide sequences were formed over a mere 4 d of reaction. Sequence homology diagrams were constructed to compare experimental and theoretical sequence spaces of tetrameric proto-peptides. MS-based analyses such as this will be increasingly necessary as origins-of-life researchers move toward systems-level investigations of prebiotic chemistry.

Kinetics of prebiotic depsipeptide formation from the ester-amide exchange reaction.

In this work, we introduce a kinetic model to study the effectiveness of ester-mediated amide bond formation under prebiotic conditions. In our previous work, we found that a simple system composed of α-hydroxy acids and α-amino acids is capable of forming peptide bonds via esterification followed by the ester-amide exchange reaction. To further understand the kinetic behavior of this copolymerization, we first tracked the growth of initial species from a valine/lactic acid mixture in a closed system reactor. A mathematical model was developed to simulate the reactions and evaluate the rate constants at different temperatures. We found these reactions can be described by the empirical Arrhenius equation even when reaction occurred in the solid (dry) state. Further calculations for activation parameters showed that the ester-mediated pathway facilitates amide bond formation by lowering activation entropies. These results provide a theoretical framework that illustrates why the ester-mediated pathway for peptide bond formation is efficient and why it would have been more favorable on the early Earth, compared to peptide bond formation without the aid of hydroxy acids.

Ester-Mediated Amide Bond Formation Driven by Wet-Dry Cycles: A Possible Path to Polypeptides on the Prebiotic Earth.

Although it is generally accepted that amino acids were present on the prebiotic Earth, the mechanism by which α-amino acids were condensed into polypeptides before the emergence of enzymes remains unsolved. Here, we demonstrate a prebiotically plausible mechanism for peptide (amide) bond formation that is enabled by α-hydroxy acids, which were likely present along with amino acids on the early Earth. Together, α-hydroxy acids and α-amino acids form depsipeptides-oligomers with a combination of ester and amide linkages-in model prebiotic reactions that are driven by wet-cool/dry-hot cycles. Through a combination of ester-amide bond exchange and ester bond hydrolysis, depsipeptides are enriched with amino acids over time. These results support a long-standing hypothesis that peptides might have arisen from ester-based precursors.