Imaging Photon-Induced Near-Field Distributions of a Plasmonic, Self-Assembled Vesicle by a Laser-Integrated Electron Microscope.

Plasmonic polymeric nanoassemblies offer valuable opportunities in photoconversion applications. Localized surface plasmon mechanisms behind such nanoassemblies govern their functionalities under light illumination. However, an in-depth investigation at the single nanoparticle (NP) level is still challenging, especially when the buried interface is involved, due to the availability of suitable techniques. Here, we synthesized an anisotropic heterodimer composed of a self-assembled polymer vesicle (THPG) capped with a single gold NP, enabling an 8-fold enhancement in hydrogen generation compared to the nonplasmonic THPG vesicle. We explored the anisotropic heterodimer at the single particle level by employing advanced transmission electron microscopes, including one equipped with a femtosecond pulsed laser, which allows us to visualize the polarization- and frequency-dependent distribution of the enhanced electric near fields at the vicinity of Au cap and Au-polymer interface. These elaborated fundamental findings may guide designing new hybrid nanostructures tailored for plasmon-related applications.

Effect of Surface Functionality on the Rheological and Self-Assembly Properties of Chitin and Chitosan Nanocrystals and Use in Biopolymer Films.

Chitin nanocrystals (ChNCs) are unique to all other bio-derived nanomaterials in one aspect: the inherent presence of a nitrogen moiety. By tuning the chemical functionality of this nanomaterial, and thus its charge and hydrogen bonding capacity, one can heavily impact its macroscopic properties such as its rheological and self-assembly characteristics. In this study, two types of ChNCs are made using acid hydrolysis (AH-ChNCs) and oxidative (OX-ChNCs) pathways, unto which deacetylation using a solvent-free procedure is utilized to create chitosan nanocrystals (ChsNCs) of varying degree of deacetylation (DDA). These nanocrystals were then studied for their rheological behavior and liquid crystalline ordering. It was found that with both deacetylation and carboxylation of ChNCs, viscosity continually increased with increasing concentrations from 2 to 8 wt %, contrary to AH-ChNC dispersions in the same range. Interestingly, increasing the amine content of ChNCs was not proportional to the storage modulus, where a peak saturation of amines provided the most stiffness. Conversely, while the introduction of carboxylation increased the elastic modulus of OX-ChNCs by an order of magnitude from that of AH-ChNCs, it was decreased by increasing DDA. Deacetylation and carboxylation both inhibited the formation of a chiral nematic phase. Finally, these series of nanocrystals were incorporated into biodegradable pectin-alginate films as a physical reinforcement, which showed increased tensile strength and Young's modulus values for the films incorporated with ChsNCs. Overall, this study is the first to investigate how surface functionalization of chitin-derived nanocrystals can affect their rheological and liquid crystalline properties and how it augments pectin/alginate films as a physical reinforcement nanofiller.

Direct observation by high resolution transmission electron microscopy of gold(III) particle transformation during aging reduction reaction.

We use a gold nanoparticle synthesis as a model system to study the morphological and compositional changes in gold(III) precursor particles, while reduction is taking place during aging after mechanical activation. Scanning transmission electron microscopy coupled with a high-angle annular dark field detector revealed the nanoscale changes in particle morphology, while electron energy loss spectroscopy mapped the changes in the chemical landscape during the reduction process. Tracking a specific region of interest on the sample grid allowed for comparisons to be made of the same particles across a two day monitoring period. High-angle annular dark field images permitted the visualization of particle size reduction of the gold salt while electron energy loss spectroscopy captured the surprising mobility of the lighter chlorine and sodium ions in a solid matrix during the reduction process. This system offers unique insight into precursor particle reactivity in the solid phase, which is relevant for many mechanochemical and aging-based reactions.

Mechanistic study of the increased phototoxicity of titanium dioxide nanoparticles to Chlorella vulgaris in the presence of NOM eco-corona.

Widespread applications and release of photoactive nanoparticles (NPs) such as titanium dioxide (TiO2) into environmental matrices warrant mechanistic investigations addressing toxicity of NPs under environmentally relevant conditions. Accordingly, we investigated the effects of surface adsorbed natural organic matters (NOMs) such as humic acid, tannic acid and lignin on the band gap energy, abiotic reactive oxygen species (ROS) generation, surface chemistry and phototoxicity of TiO2 NPs. Initially, a liquid assisted grinding method was optimized to produce TiO2 NPs with a NOM layer of defined thickness for further analysis. Generally, adsorption of NOM reduced the band-gap energy of TiO2 NPs from 3.08 eV to 0.56 eV with humic acid, 1.92 eV with tannic acid and 2.48 eV with lignin. Light activated ROS generation by TiO2 NPs such as hydroxyl radicals, however, was reduced by 4, 2, 9 times in those coated with humic acid, tannic acid and lignin, respectively. This reduction in ROS despite decrease in band gap energy corroborated with the decreased surface oxygen vacancy (as revealed by X-ray Photoelectron Spectroscopy (XPS)) and quenching of ROS by surface adsorbed NOM. Despite the reduced ROS generation, the NOM-modified TiO2 NPs exhibited an increased phototoxicity to Chlorella vulgaris in comparison to pristine TiO2 NPs. Further analysis suggested that photoactivation of NOM modified TiO2 NPs releases toxic degradation products. Findings from our studies thus provide mechanistic insight into the ecotoxic potential of NOM-modified TiO2 NPs when exposed to light in the environment.

Hazard Profiling of Commercially Relevant Quantum Dot Components Revealed Synergistic Interactions between Heavy Metals and Polymers.

Commercially used quantum dots (QDs) exemplify complex nanomaterials with multiple components, though little is known about the type of interactions between these components in determining the overall toxicity of this material. We synthesized and characterized a functional QD (CdSe/ZnS_P&E) that was identical in structure and composition to a patented and commercially applied QD and the combinations of its components (CdSe, CdSe/ZnS, ZnS, CdSe_P&E, ZnS_P&E, and P&E). Cells exposed to incremental concentrations of these materials were investigated for cell viability and cellular perturbations, contributing to a final common pathway of cell death using high-content screening assays in model human intestinal epithelial cells (HIEC-6). The concentrations that resulted in a loss of 20% cell viability (EC20 values) for each tested component were used for estimating the combination index (CI) to evaluate synergistic or antagonistic effects between the components. Complete QD (core/shell-polymer) showed the highest toxic potential due to synergistic interactions between core and surface functional groups. The cationic polymer coating enhanced cellular uptake of the QD, ensuing lysosome acidification and release of heavy metal ions to the intracellular milieu, and caused oxidative stress and cytotoxicity. Overall, this study advances our understanding of the collective contribution of individual components of a functional QD toward its toxic potential and emphasizes the need to study multilayered nanomaterials in their entirety for hazard characterization.

Applications of Plasmon-Enhanced Nanocatalysis to Organic Transformations.

Localized surface plasmon resonance (LSPR) is a physical phenomenon exhibited by nanoparticles of metals including coinage metals, alkali metals, aluminum, and some semiconductors which translates into electromagnetic, thermal, and chemical properties. In the past decade, LSPR has been taken advantage of in the context of catalysis. While plasmonic nanoparticles (PNPs) have been successfully applied toward enhancing catalysis of inorganic reactions such as water splitting, they have also demonstrated exciting performance in the catalysis of organic transformations with potential applications in synthesis of molecules from commodity to pharmaceutical compounds. The advantages of this approach include improved selectivity, enhanced reaction rates, and milder reaction conditions. This review provides the basics of LSPR theory, details the mechanisms at play in plasmon-enhanced nanocatalysis, sheds light onto such nanocatalyst design, and finally systematically presents the breadth of organic reactions hence catalyzed.

Mechanochemical methods for the transfer of electrons and exchange of ions: inorganic reactivity from nanoparticles to organometallics.

Mechanochemistry by milling, grinding, extrusion or other types of shear and mechanical agitation has shown novel reactivity for a wide range of reactions, not seen in traditional solution-based environments. While the area has been extensively investigated and reviewed in the context of organic and solid-state supramolecular chemistry, less attention has been given to the recent advances in the context of inorganic transformations. Here we provide a perspective of inorganic mechanochemical reactions, focusing on transformations that are based on transfer of charged species: exchange of ions and electrons (redox reactions). These types of mechanochemical transformations typically lead to the formation of new nanoparticles and organometallic complexes. Herein, we provide an overview of mechanochemical reactivity that complements the recent developments in organic synthesis and catalysis.

High-Humidity Shaker Aging to Access Chitin and Cellulose Nanocrystals.

To unlock nature's potential for functional biomaterials, many efforts have been devoted to isolating the nanocrystalline domains within the supramolecular structure of polysaccharides. Yet, low reactivity and yield in aqueous systems along with excessive solvent usage hinders its development. In this report, the first solvent-free pathway to access carboxylated chitin and cellulose nanocrystals with excellent mass balance is described, relying on a new method coined high-humidity shaker aging (HHSA). The method involves a mild grinding of the polysaccharide with ammonium persulfate followed by an aging phase under high-humidity and on a shaker plate. Insights into the mechanism were uncovered, which highlighted the unique role of high humidity to afford a gradual uptake of water by the material up to deliquescence when the reaction is complete. This process was then validated for direct synthesis of nanocrystals from biomass sources including crab and soft wood pulp.

Mechanochemical bottom-up synthesis of phosphorus-linked, heptazine-based carbon nitrides using sodium phosphide.

Herein, we present the bottom-up, mechanochemical synthesis of phosphorus-bridged heptazine-based carbon nitrides (g-h-PCN). The structure of these materials was determined through a combination of powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), 31P magic angle spinning nuclear magnetic resonance (MAS NMR), density functional theory (DFT) and electron energy loss spectroscopy (EELS). Compared to traditional furnace-based techniques, the presented method utilizes milder conditions, as well as shorter reaction times. Both samples of g-h-PCN directly after milling and aging and after an hour of annealing at 300 °C (g-h-PCN300) show a reduction in photoluminescent recombination, as well as a nearly two-time increase in photocurrent under broad spectrum irradiation, which are appealing properties for photocatalysis.

Density Functional Theory Calculations Decipher Complex Reaction Pathways of 6:2 Fluorotelomer Sulfonate to Perfluoroalkyl Carboxylates Initiated by Hydroxyl Radical.

6:2 Fluorotelomer sulfonate (6:2 FTSA) is a ubiquitous environmental contaminant belonging to the family of per- and polyfluoroalkyl substances. Previous studies showed that hydroxyl radical (•OH) efficiently transforms 6:2 FTSA into perfluoroalkyl carboxylates (PFCAs) of different chain lengths (C2-C7), yet the reaction mechanisms were not elucidated. This study used density functional theory (DFT) calculations to map the entire reaction path of 6:2 FTSA initiated by •OH and experimentally verified the theoretical results. Optimal reaction pathways were obtained by comparing the rate constants calculated from the transition-state theory. We found that 6:2 FTSA was first transformed to C7 PFCA and C6F13•; C6F13• was then further reacted to C2-C6 PFCAs. The parallel addition of •OH and O2 to CnF2n+1• was essential to producing C2-C6 PFCAs. The critical step is the generation of alkoxyl radicals, which withdraw electrons from the adjacent C-C groups to result in chain cleavage. The validity of the calculated optimal reaction pathways was further confirmed by the consistency with our experimental data in the aspects of O2 involvement, identified intermediates, and the final PFCA profile. This study provides valuable insight into the transformation of polyfluoroalkyl substances containing aliphatic carbons in •OH-based oxidation processes.

Carboxylated Chitosan Nanocrystals: A Synthetic Route and Application as Superior Support for Gold-Catalyzed Reactions.

In this study, we demonstrate for the first time the fabrication of carboxylated chitosan nanocrystals (ChsNC) with high degree of deacetylation (DDA) at >80% and narrow size distribution. We also studied its application as a sustainable support material for metal-based catalysts. Carboxylated chitin nanocrystals (ChNCs) were initially prepared through partial cleavage of glycosidic bonds in chitin by ammonium persulfate, with concurrent oxidation of chitin C6 primary alcohols to produce carboxylate groups on the surface of the ChNCs. ChsNCs were subsequently prepared using an alkaline deacetylation procedure in the presence of NaBH4 to preserve the nanorod structure of the biomaterial. The resulting nanocrystals feature both carboxyl and amino functional groups. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) spectroscopy were used to determine the morphology and composition of these carboxylated ChNCs and ChsNCs. Subsequently, we tested the ability of the as-made ChsNCs as a biomass-based catalyst support for Au nanoparticles (NPs) using the 4-nitrophenol reduction and the aldehyde-amine-alkyne (A3) coupling reactions to demonstrate its capabilities in regard to the ones of cellulose nanocrystals (CNCs). In particular, Au NPs over ChsNCs featured the highest turnover frequency (TOF) value for the 4-nitrophenol reduction reported for all Au-based catalysts supported on carbon-based systems. Spectroscopic and imaging techniques confirmed the importance of precisely controlling the redox state of Au as it is being deposited to afford a highly disperse active site on the bionano-support.

Nanocellulose, a Versatile Green Platform: From Biosources to Materials and Their Applications.

With increasing environmental and ecological concerns due to the use of petroleum-based chemicals and products, the synthesis of fine chemicals and functional materials from natural resources is of great public value. Nanocellulose may prove to be one of the most promising green materials of modern times due to its intrinsic properties, renewability, and abundance. In this review, we present nanocellulose-based materials from sourcing, synthesis, and surface modification of nanocellulose, to materials formation and applications. Nanocellulose can be sourced from biomass, plants, or bacteria, relying on fairly simple, scalable, and efficient isolation techniques. Mechanical, chemical, and enzymatic treatments, or a combination of these, can be used to extract nanocellulose from natural sources. The properties of nanocellulose are dependent on the source, the isolation technique, and potential subsequent surface transformations. Nanocellulose surface modification techniques are typically used to introduce either charged or hydrophobic moieties, and include amidation, esterification, etherification, silylation, polymerization, urethanization, sulfonation, and phosphorylation. Nanocellulose has excellent strength, high Young's modulus, biocompatibility, and tunable self-assembly, thixotropic, and photonic properties, which are essential for the applications of this material. Nanocellulose participates in the fabrication of a large range of nanomaterials and nanocomposites, including those based on polymers, metals, metal oxides, and carbon. In particular, nanocellulose complements organic-based materials, where it imparts its mechanical properties to the composite. Nanocellulose is a promising material whenever material strength, flexibility, and/or specific nanostructuration are required. Applications include functional paper, optoelectronics, and antibacterial coatings, packaging, mechanically reinforced polymer composites, tissue scaffolds, drug delivery, biosensors, energy storage, catalysis, environmental remediation, and electrochemically controlled separation. Phosphorylated nanocellulose is a particularly interesting material, spanning a surprising set of applications in various dimensions including bone scaffolds, adsorbents, and flame retardants and as a support for the heterogenization of homogeneous catalysts.

Transformation of 6:2 Fluorotelomer Sulfonate by Cobalt(II)-Activated Peroxymonosulfate.

Peroxymonosulfate (PMS)-based advanced oxidation processes generate highly reactive SO4•- and are promising for water treatment. In this study, we investigated the reaction mechanism of 6:2 fluorotelomer sulfonate (6:2 FTS) with Co2+-activated PMS. 6:2 FTS was simultaneously transformed to perfluoroalkyl carboxylic acids (C2-C7 PFCAs) of different chain lengths, with perfluorohexanoic acid (C6) as the predominant one. The mass balance of the intermediates and products versus the initially added 6:2 FTS was close to 100% over the reaction period. Using chemical scavenging methods, we identified that •OH, instead of SO4•-, was the oxidant initiating the reaction of 6:2 FTS. •OH was mainly produced from SO4•- reacting with H2O. Thus, the reactivity of 6:2 FTS was controlled by the factors affecting the production and scavenging of both SO4•- and •OH. Density functional theory calculations showed that •OH oxidizes 6:2 FTS by H-abstraction from ethyl carbons. This is the first study that demonstrates that •OH in Co2+-activated PMS can play a significant role in contaminant transformations. The results indicate that great caution should be taken when PMS or other agents that generate •OH are used for the treatment of water containing 6:2 FTS or its structural analogs.

Palladium nanoparticles supported on chitin-based nanomaterials as heterogeneous catalysts for the Heck coupling reaction.

In this report, chitin and chitosan nanocrystals were used as biomass-based supports for Pd nanoparticles (NPs) used as a heterogeneous catalyst for the Heck coupling reaction. By using a one-pot fabrication method, a Pd salt precursor was directly reduced and deposited onto these nanocrystal catalysts. Characterization of these nanocomposites showed disperse Pd NPs on the surfaces of the chitinous nanocrystals. Heck coupling model reactions revealed full product yield in relatively benign conditions, outcompeting the use of other catalysts supported on biomass-based nanomaterials, including cellulose nanocrystals. These initial results show the potential for using chitinous nanomaterials as effective catalyst supports in cross-coupling reactions.

New Insights into the Degradation Mechanism of Perfluorooctanoic Acid by Persulfate from Density Functional Theory and Experimental Data.

Thermally activated persulfate is a promising oxidant for in situ remediation of perfluorooctanoic acid (PFOA), yet a comprehensive understanding of the degradation mechanism is still lacking. In this study, we used density functional theory (DFT) calculations and experimental data to map entire reaction pathways for the degradation of PFOA by persulfate, with specific considerations on the influence of pH. The DFT results showed that the rate-limiting step was the first electron abstraction from PFOA, yet the generation of SO4•- from the decomposition of persulfate contributed a large part of the free energy of activation (ΔG‡) for the overall reaction. The subsequent steps did not contribute to the ΔG‡. For the electron abstraction from PFOA, we investigated reactions using protonated and deprotonated species of PFOA and SO4•- and showed that the reaction of anionic PFOA with HSO4• was most favorable with a ΔG‡ of 7.2 kJ/mol. This explains why low pH (<3.5) is a sine qua non condition for the degradation of PFOA by persulfate. The overall ΔG‡ derived theoretically based on the pathway involved HSO4• was consistent with the ΔG‡ determined experimentally. This study provides valuable insight into remediation strategies that include persulfate as an oxidizing agent for perfluoroalkyl carboxylic acids.

Mechanochemical amorphization of chitin: impact of apparatus material on performance and contamination.

Herein, we present a study of the impact of the jar and ball medium on the performance in the mechanochemical amorphization of chitin. We measured the crystallinity index of chitin after milling it in a vibration mill in an apparatus made of copper, aluminum, brass, tungsten carbide, zirconia, stainless steel, polytetrafluoroethylene (PTFE), or poly(methyl methacrylate) (PMMA). These materials offer a range of Vickers hardness values and the impact of these parameters is discussed. The role of the size and mass of the balls is also studied in the case of stainless steel. This study also highlights one of the major challenges during milling, which is contamination of the studied samples.

Solid-state mechanochemical ω-functionalization of poly(ethylene glycol).

Poly(ethylene glycol) (PEG) is a linear polymer with a wide range of applications in chemical manufacturing, drug development and nanotechnology. PEG derivatives are being increasingly used to covalently modify small molecule and peptide drugs, as well as bioactive nanomaterials in order to improve solubility in biological serum, reduce immunogenicity, and enhance pharmacokinetic profiles. Herein we present the development of mechanochemical procedures for PEG functionalization without the need for bulk solvents, offering a cleaner and more sustainable alternative to existing solution-based PEG procedures. The herein presented mechanochemical procedures enable rapid and solvent-free derivatization of PEG with tosyl, bromide, thiol, carboxylic acid or amine functionalities in good to quantitative yields and with no polymer chain oligomerization, proving the versatility of the method.

Cellulose Nanocrystals as Chiral Inducers: Enantioselective Catalysis and Transmission Electron Microscopy 3D Characterization.

Cellulose nanocrystals (CNCs), derived from cellulose, provide us with an opportunity to devise more sustainable solutions to current technological challenges. Enantioselective catalysis, especially heterogeneous, is the preferred method for the synthesis of pure chiral molecules in the fine chemical industries. Cellulose has been long sought as a chiral inducer in enantioselective catalysis. We report herein an unprecedentedly high enantiomeric excess (ee) for Pd patches deposited onto CNCs used as catalysts for the hydrogenation of prochiral ketones in water at room temperature and 4 bar H2. Our system, where CNCs acted as support and sole chiral source, achieved an ee of 65% with 100% conversions. Cryo-electron microscopy, high-resolution transmission electron microscopy, and tomography were used for the first time to study the 3D structure of a metal functionalized CNC hybrid. It established the presence of sub-nanometer-thick Pd patches at the surface of CNCs and provided insight into the chiral induction mechanism.