Dimitris S. Argyropoulos

Quantitative Phosphorus-31 NMR Analysis of Lignins, a New Tool for the Lignin Chemist

Abstract A novel quantitative method has been developed for the determination of the various types of hydroxyl groups present in lignins. The syringyl, guaiacyl and p-hydroxyphenyl free phenolic groups, as well as the primary, and the secondary hydroxyl groups (belonging to individual erythro and threo forms of the aryl-glycerol β-O-4 ether strctures) can be quantitatively determined from a 31P NMR experiment. This is made possible by phosphitylating lignins with 1,3,2 dioxaphospholanyl chloride, followed by 31P NMR spectroscopy, in the presence of a relaxation reagent (chromium acetylacetonate) and an internal standard. The various aspects leading to the development of this technique are discussed together with relevant statistical information pertaining to the reproducibility and quantitative validity of the method. This simple and novel form of spectroscopy may become a valuable resource to the lignin chemist, because it can supply detailed quantitative information about the structure of a soluble lign...

Regular Linking of Cellulose Nanocrystals via Click Chemistry: Synthesis and Formation of Cellulose Nanoplatelet Gels

Over a number of years work in our laboratory has been developing new chemistry for the use of cellulose nanocrystals (CNCs) as scaffolds for the creation of nanomaterials with novel, stimuli responsive characteristics. Our work takes advantage of the rigid nature of CNCs, the unique nanopattern etched on their surface in the form of regularly spaced primary OH groups, and the fact that these materials have all reducing end groups located on one end. In this communication, a method for the grafting of amine-terminated monomers onto surface-modified CNCs followed by click chemistry is demonstrated. Initially the primary hydroxyl groups on the surface of the CNCs were selectively activated by converting them to carboxylic acids by the use of TEMPO-mediated hypohalite oxidation. Further reactions using the activated TEMPO-oxidized CNCs were carried out via carbodiimide-mediated formation of an amide linkage between precursors carrying an amine functionality and the carboxylic acid groups on the surface of the TEMPO-oxidized CNCs. Subsequently, two sets of CNCs were prepared, containing on their surface an azide derivative and an alkyne derivative, respectively. Finally, the click chemistry reaction, that is, the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition between the azide and the alkyne, surface-activated CNC was employed, bringing together the nanocrystalline materials in a unique regularly packed arrangement demonstrating a degree of molecular control for creating these structures at the nano level.

Photobactericidal Porphyrin-Cellulose Nanocrystals: Synthesis, Characterization, and Antimicrobial Properties

Adherence and survival of pathogenic bacteria on surfaces leading to concomitant transmission to new hosts significantly contributes to the proliferation of pathogens, which in turn considerably increases the threat to human health, particularly by antibiotic-resistant bacteria. Consequently, more research into effective surface disinfection and alternative materials (fabrics, plastics, or coatings) with antimicrobial and other bioactive characteristics is desirable. This report describes the synthesis and characterization of cellulose nanocrystals that were surface-modified with a cationic porphyrin. The porphyrin was appended onto the cellulose surface via the Cu(I)-catalyzed Huisgen-Meldal-Sharpless 1,3-dipolar cycloaddition having occurred between azide groups on the cellulosic surface and porphyrinic alkynes. The resulting, generally insoluble, crystalline material, CNC-Por (5), was characterized by infrared and diffusion (1)H NMR spectroscopies, gel permeation chromatography, and thermogravimetric analysis. Although only suspended, and not dissolved, in an aqueous system, CNC-Por (5) showed excellent efficacy toward the photodynamic inactivation of Mycobacterium smegmatis and Staphylococcus aureus , albeit only slight activity against Escherichia coli . The synthesis, properties, and activity of CNC-Por (5) described herein serve as a benchmark toward our overall objectives of developing novel, potent, bioactive, photobactericidal materials that are effective against a range of bacteria, with potential utilization in the health care and food preparation industries.

Thermal properties of lignin in copolymers, blends, and composites: a review

The need for renewable alternatives to conventional petroleum based polymers has been the motivation for work on biobased composites, blends and materials whose foundations are carbon neutral feedstocks. Lignin, an abundant plant derived feedstock and waste byproduct of the cellulosic ethanol and pulp and paper industry, qualifies as a renewable material. Despite the fact that it is often difficult to blend lignin with other polymers due to its complex structure and reactivity, published research over the past decades, has focused on issues such as lignin miscibility with other polymers, the thermal and mechanical strength behavior of its copolymers and its fractions as well as efforts at tuning its thermal properties via chemical modifications and other means. As such, this study attempts to offer a comprehensive overview that largely discusses the importance of these processes with the aim of finding an effective, cost efficient and environmentally friendly means that may allow the utilization of this important and largely ignored biopolymer.

Spectral Characterization of Eucalyptus Wood

The main difficulties in wood and pulp analyses arise principally from their numerous components with different chemical structures. Therefore, the basic problem in a specific analytical procedure may be the selective separation of the main carbohydrate-derived components from lignin due to their chemical association and structural coexistence. The processing of the wood determines some structural modification in its components depending on the type of wood and the applied procedure. Fourier transform infrared (FT-IR) spectrometry and X-ray diffraction have been applied to analyze Eucalyptus g. wood chips and unbleached and chloritebleached pulp. The differences between samples have been established by examination of the spectra of the fractions obtained by successive extraction (acetone extractives, acetone free extractive samples, hemicelluloses, and lignins) by evaluating the derivative spectra, band deconvolution, etc. The energy and the hydrogen bonding distance have been evaluated. The relationship between spectral characteristics and sample composition has been established, as well as the variation of the degree of crystallinity after pulping and bleaching. The integral absorption and lignin/carbohydrate ratios calculated from FT-IR spectra of the IR bands assigned to different bending or stretching in lignin groups are stronger in the spectrum of eucalyptus chips than those from brown stock (BS) pulp spectra because of the smaller total amount of lignin in the latter. FT-IR spectra clearly show that after chlorite bleaching the structure of the wood components is partially modified or removed. Along with FT-IR data, the X-ray results confirmed the low content of lignin in the pulp samples by increasing the calculated values of the crystalline parameters. It was concluded that FT-IR spectroscopy can be used as a quick method to differentiate Eucalyptus globulus samples.

The effect of isolation method on the chemical structure of residual lignin

Two methods are used for the isolation of residual lignin: acidolytic and enzymatic hydrolysis. Recently a two-step procedure that is a combination of enzymatic and acidic hydrolyses was proposed. In this paper, the structures of residual lignins isolated by these three methods are compared. Enzymatic hydrolysis gave lignin with the highest yield (83%); however, it contained high amounts of carbohydrates and protein. The molar mass of enzymatic lignin was the highest, indicating that no cleavage of lignin occurred. Acidolysis gave a significantly lower lignin yield (40%), but this lignin was practically free from impurities. The β-aryl ether and lignin-carbohydrate linkages cleaved during the isolation, which was manifested in the decreased molar mass of the lignin as well as in increased phenolic hydroxyl group content. The new two-step isolation procedure gave properties between the preparations of enzymatic and acidolytic hydrolyses. The lignin yield was high (78%), but it contained some impurities, although less than the enzymatic lignin. The lignin-carbohydrate linkages cleaved to some extent, but the β-aryl ether linkages remained intact.

31P NMR IN WOOD CHEMISTRY: A REVIEW OF RECENT PROGRESS

Over the past few years substantial efforts in our laboratory have been devoted toward the development of novel NMR techniques for the analysis of soluble and solid lignocellulosic substances. These efforts were undertaken in order to expand the frontiers of application of NMR for the detection of functional groups present in such materials. Our methodology involves the selective phosphorus-tagging of a variety of functional groups present in lignin and carbohydrates, followed by solution and solid state31P NMR spectroscopies. This paper attempts to review the status of this technique by discussing its development for the analysis of soluble and solid lignocellulosic samples.Solution31P NMR can be used to examine soluble lignin and carbohydrate samples after phosphitylation with 1,3,2 dioxaphospholanyl chloride. This is a novel and powerful means to determine the three principal forms of phenolic hydroxyls present in ligninsi.e. p-hydroxyphenyl, guaiacyl, and syringyl structures. In addition, primary hydroxyls, carboxylic acids, and the two diastereomeric forms of arylglycerol-beta-aryl ether units (β-O-4 structures) present in lignins can also be determined from a single31P NMR experiment. When applied to carbohydrates, the technique gave characteristic signals for thealpha andbeta anomers and the epimeric forms of monosaccharides. Completely resolved31P NMR spectra were also obtained when lignin-carbohydrate model compounds were examined.Solid state31P NMR can be used to determine quinone chromophore groups present in solid lignocellulosic samples. The method is based on literature accounts that describe adduct formation between trimethyl phosphite and quinones followed by solid state31P NMR. This reaction when reexamined in our laboratory showed that the presence of carboxylic acids in high yield pulps significantly affected the solid state31P NMR signal intensity. This realization permitted the development of an experimental protocol that allowed solid state31P NMR signals to be received from high yield pulps that contain information only onortho-quinones and coniferaldehyde chromophores. It was thus found that about 0.7ortho-quinone groups are present in every 100 C9 units within the lignin of a black spruce refiner mechanical pulp sample, in agreement with previously applied independent techniques.

Accurate and reproducible determination of lignin molar mass by acetobromination.

The accurate and reproducible determination of lignin molar mass by using size exclusion chromatography (SEC) is challenging. The lignin association effects, known to dominate underivatized lignins, have been thoroughly addressed by reaction with acetyl bromide in an excess of glacial acetic acid. The combination of a concerted acetylation with the introduction of bromine within the lignin alkyl side chains is thought to be responsible for the observed excellent solubilization characteristics acetobromination imparts to a variety of lignin samples. The proposed methodology was compared and contrasted to traditional lignin derivatization methods. In addition, side reactions that could possibly be induced under the acetobromination conditions were explored with native softwood (milled wood lignin, MWL) and technical (kraft) lignin. These efforts lend support toward the use of room temperature acetobromination being a facile, effective, and universal lignin derivatization medium proposed to be employed prior to SEC measurements.

Propensity of Lignin to Associate: Light Scattering Photometry Study with Native Lignins

Many studies of lignins in solution invoke association and aggregation phenomena to explain their solution behavior (e.g., reprecipitation onto pulp fibers, condensation, etc.). Following their colloidal (apparent) molecular weights in solution as a function of time allows us to explore observable dissociation phenomena. These measurements were carried out using multiple angle laser light scattering (MALLS) photometry in the static mode. The challenges and opportunities of measuring the specific refractive index increment (dn/dC) of lignin solutions and determining the kinetics of the dissociation process were thus investigated. Hardwood and softwood representative lignins were isolated, and method for their full dissolution in THF was further developed, which then lead to accurate dn/dC values being obtained as a function of time. When coupled to additional work using light scattering static measurements and Zimm plots for the same solutions, this effort offers insight into the aggregation and ensuing dissociative events that operate within the lignin macromolecules.

Porphyrin‐Cellulose Nanocrystals: A Photobactericidal Material that Exhibits Broad Spectrum Antimicrobial Activity†

Towards our overall objectives of developing potent antimicrobial materials to combat the escalating threat to human health posed by the transmission of surface‐adhering pathogenic bacteria, we have investigated the photobactericidal activity of cellulose nanocrystals that have been modified with a porphyrin‐derived photosensitizer (PS). The ability of these previously synthesized porphyrin‐cellulose‐nanocrystals (CNC‐Por (1)) to mediate bacterial photodynamic inactivation was investigated as a function of bacterial strain, incubation time and illumination time. Despite forming an insoluble suspension, CNC‐Por (1) showed excellent efficacy toward the photodynamic inactivation of Acinetobacter baumannii, multidrug‐resistant Acinetobacter baumannii (MDRAB) and methicillin‐resistant Staphylococcus aureus (MRSA), with the best results achieving 5–6 log units reduction in colony forming units (CFUs) upon illumination with visible light (400–700 nm; 118 J cm−2). CNC‐Por (1) mediated the inactivation of Pseudomonas aeruginosa, although at reduced activity (2–3 log units reduction). Confocal laser scanning microscopy of CNC‐Por (1) after incubation with A. baumannii or S. aureus suggested a lack of internalization of the PS. Research into alternative materials such as CNC‐Por (1) may lead to their application in hospitals and healthcare‐related industries wherein novel materials with the capability of reducing the rates of transmission of a wide range of bacteria, particularly antibiotic resistant strains, are desired.