TEMPO-mediated Oxidation Research Articles

Relationship between Length and Degree of Polymerization of TEMPO-Oxidized Cellulose Nanofibrils

The influence of 2,2,6,6-tetrametylpiperidine-1-oxyl (TEMPO)-mediated oxidation of wood cellulose and the mechanical disintegration of oxidized cellulose in water on degree of polymerization determined by viscosity measurement (DP(v)) and the apparent length of the TEMPO-oxidized cellulose nanofibrils (TOCNs) was investigated. DP(v) values decreased from 1270 to 500-600 with increasing addition of NaClO in the TEMPO-mediated oxidation stage. The DP(v) values were further decreased by mechanical fibrillation in water. There is a linear relationship between the average fibril length and DP(v); the lengths of TOCNs can be approximated from DP(v) using 0.5 M copper ethylenediamine as a solvent of both the cellulose and oxidized celluloses in TOCNs. Based on the cellulose fibril models and TEMPO oxidation mechanism, the depolymerization behavior of TOCNs is tentatively explained in terms of distribution of disordered regions in wood cellulose fibrils and formation of C6-aldehydes in cellulose fibrils during TEMPO-mediated oxidation.

Structure and properties of tempo-oxidized cotton fibers

In this paper, the influence of the catalytic oxidation using water soluble and stable nitroxyl radical 2,2?,6,6?-tetramethylpiperidine-1-oxyl (TEMPO) on structure and properties of cotton fibers was studied. In particular, the selective TEMPO-mediated oxidation has become very interesting way for introduction of functional groups into cellulose fibers with the aim to obtain oxycellulose fibers with specific properties. Unmodified and modified fibers were characterized in terms of weight loss values, introduced functional groups and crystallinity index. Also, oxidized fibers were characterized in terms of the sorption, morphological, and physico-mechanical properties. The TEMPO-oxidized cotton fibers show a minimum increase of fineness (from 1.32 to 1.28 dtex) and increase of crystallinity index (up to 91.9%), while the tensile strength of fibers decreases (up to 10.82 cN/tex). By the TEMPO-mediated oxidation of cotton fibers significant amount of carboxyl groups (up to 0.795 mmol/g cell) can be introduced into cellulose fibers. Introduced hydrophilic carboxyl groups increases the sorption properties of oxidized fibers, that can be used directly or for further chemical modification.

Effect of nanocellulose isolation techniques on the formation of reinforced poly(vinyl alcohol) nanocomposite films

Three techniques including acid hydrolysis (AH), 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation (TMO) and ultrasonication (US) were introduced to isolate nanocellulose from microcrystalline cellulose, in order to reinforce poly(vinyl alcohol) (PVA) films. Important differences were noticed in fiber quality of nanocellulose and film properties of PVA nanocomposite films. The TMO treatment was more efficient in nanocellulose isolation with higher aspect ratio, surface charge (–47 mV) and yields (37%). While AH treatment resulted in higher crystallinity index (88.1%) and better size dispersion. The fracture surface, thermal behavior and mechanical properties of the PVA nanocomposite films were investigated by means of scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and tensile testing. The results showed that both the TMO-derived and AH-derived nanocellulose could be dispersed homogeneously in the PVA matrices. AH/PVA films had higher elongation at break (51.59% at 6 wt% nanocellulose loading) as compared with TMO/PVA, while TMO/PVA films shown superior tensile modulus and strength with increments of 21.5% and 10.2% at 6wt% nanocellulose loading. The thermal behavior of the PVA nanocomposite films was higher improved with TMO-derived nanofibrils addition.

Antimicrobial TEMPO-oxidized hemp fibers with incorporated silver particles

In this paper, the preparation of the antimicrobial silver loaded hemp fibers were carried out by selective TEMPO-mediated oxidation, i.e. oxidation with sodium hypochlorite and catalytic amount of sodium bromide and 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO), followed by silver sorption from aqueous silver nitrate solution. The most suitable experimental conditions for the selective TEMPO-mediated oxidation were determined by changing oxidation conditions: concentration of sodium hypochlorite and duration of sorption. The obtained results showed that the maximum sorption capacity (0.703 mmol of silver per gram of fibers) of modified hemp fibers was obtained for the sample modified with 9.67 mmol NaClO per gram of fibers, during 4 hours. SEM microphotographs of the modified hemp fibers with incorporated silver showed uniformly distributed silver particles on the surface of fibers, with isometric shapes and sizes from 10 to 100 nm, despite the fact that silver was sorbed from ionic solution. The antibacterial activity of the TEMPO-oxidized hemp fibers with silver particles was confirmed in vitro against two strains: Staphylococcus aureus (ATCC 25923) and Escherichia coli (ATCC 25922), and the antifungal activity against fungi Candida albicans (ATCC 24433). The best antimicrobial activity of silver-loaded TEMPO-oxidized hemp fibers was showed against strain Staphylococcus aureus.

TEMPO-mediated oxidation of MCC at high temperature: Synthesis and characterization of high-hydrophilic polymer

Microcrystalline cellulose was reacted with catalytic amounts of 2, 2, 6, 6-tetramethyl-1-piperidine oxoammonium salt (TEMPO), sodium hypochlorite and sodium bromide in Na2CO3/NaHCO3 buffer solution at different temperatures (30 °C, 40 °C, 50 °C). The oxidation procedures included first oxidation and second oxidation. The yield of cellouronic acid produced in the second oxidation was higher than the yield of cellouronic acid produced in the first oxidation at the same oxidation temperature. Moreover, an interesting “high-hydrophilic” phenomenon appeared at higher temperature during microcrystalline cellulose second oxidation (30 °C, 40 °C, 50 °C). Properties associated with the chemical characteristics are discussed by XRD, FTIR, 13C-NMR and Laser Particle Analyzer in view of its interesting high-hydrophilic effects.

Production of cellulose nanocrystals using hydrobromic acid and click reactions on their surface

Cellulose nanocrystals (CNCs) were prepared by acidic hydrolysis of cotton fibers (Whatman #1 filter paper). In our efforts to select conditions in which the hydrolysis media does not install labile protons on the cellulose crystals, a mineral acid other than sulfuric acid (H2SO4) was used. Furthermore, in our attempts to increase the yields of nanocrystals ultrasonic energy was applied during the hydrolysis reaction. The primary objective was to develop hydrolysis reaction conditions for the optimum and reproducible CNC production. As such, the use of hydrobromic acid (HBr) with the application of sonication as a function of concentration (1.5–4.0 M), temperature (80–100 °C), and time (1–4 h) was examined. Applying sonic energy during the reaction was found to have significant positive effects as far as reproducible high yields are concerned. Overall, the combination of 2.5 M HBr, 100 °C, and 3 h associated with the sonication during the reaction generated the highest nanocrystal yields. In addition to the optimization study three types of surface modifications including TEMPO-mediated oxidation, alkynation, and azidation were used to prepare surface-activated, reactive CNCs. Subsequently, click chemistry was employed for bringing together the modified nanocrystalline materials in a unique regularly packed arrangement demonstrating a degree of molecular control for creating these structures at the nano level.

Regulation of Postprandial Blood Metabolic Variables by TEMPO-Oxidized Cellulose Nanofibers

Wood cellulose was converted to individual nanofibers of approximately 4 nm width and 380-570 nm average length by TEMPO-mediated oxidation. The TEMPO-oxidized cellulose nanofibers (TOCNs) were orally administered with glucose and glyceryl trioleate to mice and postprandial responses of blood glucose, insulin, glucose-dependent insulinotropic polypeptide (GIP), and triglycerides were studied. Both blood insulin and GIP concentrations were decreased by TOCN with a carboxyl content and aspect ratio of 1.2 mmol g(-1) and 120, respectively, in dose-dependent manners (0-0.3 mg g(-1) body weight). Of the TOCNs examined, that with a carboxyl content and aspect ratio of 1.2 mmol g(-1) and 120, respectively, was the most effective in reducing postprandial blood glucose, plasma insulin, GIP, and triglyceride concentrations. Thus, TOCNs were found to exhibit characteristic biological activities when administered to mice and may have potential applications in biomedical fields for human health.

Convergent synthesis of the tetrasaccharide repeating unit of the O-antigen of Shigella boydii type 9

A convenient synthesis of the tetrasaccharide repeating unit of the O-antigen of Shigella boydii type 9 has been achieved in excellent yield using a [2 + 2] block glycosylation strategy. TEMPO-mediated selective oxidation of the primary alcohol of the tetrasaccharide derivative 8 to the carboxylic group followed by deprotection of the functional groups furnished target tetrasaccharide 1 as its 4-methoxyphenyl glycoside in high yield.

Heterometallic Copper(II)–Potassium 3D Coordination Polymers Driven by Multifunctionalized Azo Derivatives of β-Diketones

New 3D Cu(II)-K coordination polymers (CPs), [CuK(μ7-L1)(H2O)]n·nH2O (1), and [CuK(μ5-L2)(H2O)]n·2nH2O (2), were easily generated by self-assembly from copper(II) nitrate, potassium hydroxide, and azo derivatives of β-diketones, namely 3-(2-hydroxy-3-sulfo-5-chlorophenylhydrazo)pentane-2,4-dione (H3L1) or 3-(2-hydroxy-3-sulfo-5-nitrophenylhydrazo)pentane-2,4-dione (H3L2). The single-crystal structures of 1 and 2 disclose similar 3D metal–organic networks, driven by the 5-connected potassium, 6-connected L1, or 5-connected L2 nodes. The topological analysis of 1 reveals a rare binodal 5,6-connected underlying net with an unprecedented topology defined by the point (Schläfli) symbol of (3·45·53·6)(32·46·53·64), while 2 features a uninodal 5-connected net with the bnn (hexagonal boron nitride) topology and the point symbol of (46·64). Both networks possess voids filled by crystallization water molecules, the reversible escape and binding of which was studied by thermal analyses. Upon a dehydration–adsorption cycle, 1 undergoes structural changes, although without loss of crystallinity, while 2 basically preserves its initial structure, as confirmed by X-ray powder diffraction analyses. Besides, 1 and 2 act as catalyst precursors for the aerobic TEMPO-mediated selective oxidation of benzyl alcohol to benzaldehyde in aqueous medium.

Photoresponsive Cellulose Nanocrystals

In this communication a method for the creation of fluorescent cellulose nanoparticles using click chemistry and subsequent photodimerization of the installed side-chains is demonstrated. In the first step, the primary hydroxyl groups on the surface of the CNCs were converted to carboxylic acids by using TEMPO-mediated hypohalite oxidation. The alkyne groups, essential for the click reaction, were introduced into the surface of TEMPO-oxidized CNCs via carbodiimide-mediated formation of an amide linkage between monomers carrying an amine functionality and carboxylic acid groups on the surface of the TEMPO-oxidized CNCs. Finally, the reaction of surface-modified TEMPO-oxidized cellulose nanocrystals and azido-bearing coumarin and anthracene monomers were carried out by means of a click chemistry, i.e., Copper(I)-catalyzed Azide-Alkyne Cycloaddition (CuAAC) to produce highly photo-responsive and fluorescent cellulose nanoparticles. Most significantly, the installed coumarin and/or anthracene side-chains were shown to undergo UV-induced [2+2] and [4+4] cycloaddition reactions, bringing and locking the cellulose nanocrystals together. This effort paves the way towards creating, cellulosic photo responsive nano-arrays with the potential of photo reversibility since these reactions are known to be reversible at varying wavelengths.

Quantitative Electron Microscopy of Cellulose Nanofibril Structures from Eucalyptus and Pinus radiata Kraft Pulp Fibers

This work comprises the structural characterization of Eucalyptus and Pinus radiata pulp fibers and their corresponding fibrillated materials, based on quantitative electron microscopy techniques. Compared to hardwood fibers, the softwood fibers have a relatively open structure of the fiber wall outer layers. The fibrillation of the fibers was performed mechanically and chemi-mechanically. In the chemi-mechanical process, the pulp fibers were subjected to a TEMPO-mediated oxidation to facilitate the homogenization. Films were made of the fibrillated materials to evaluate some structural properties. The thicknesses and roughnesses of the films were evaluated with standardized methods and with scanning electron microscopy (SEM), in backscattered electron imaging mode. Field-emission SEM (FE-SEM) and transmission electron microscopy (TEM) were performed to quantify the nanofibril morphology. In this study, we give additional and significant evidences about the suitability of electron microscopy techniques for quantification of nanofibril structures. In addition, we conclude that standard methods are not suitable for estimating the thickness of films having relatively rough surfaces. The results revealed significant differences with respect to the morphology of the fibrillated material. The differences are due to the starting raw material and to the procedure applied for the fibrillation.

An expeditious synthesis of quercetin 3- O-β- d-glucuronide from rutin

We report the synthesis of the major human metabolite of quercetin, quercetin 3- O-β- d-glucuronide, from rutin (quercetin-3-rutinoside), which is commercially available at low cost. This straightforward synthesis is based on the key intermediate 3′,4′,5,7-tetra- O-benzyl-quercetin which is obtained in only two steps by the total benzylation of rutin followed by acid hydrolysis of the rutinoside residue. Glycosylation of the free 3 hydroxyl group by 1-bromo-3,4,6-tetra- O-acetyl-α- d-glucopyranoside yields the protected glucoside. TEMPO-mediated oxidation of primary alcohol on the deprotected glucoside gives access to the benzylated glucuronide. Removal of the benzyl groups which protect the quercetin hydroxyl groups by H 2 (10% Pd/C) yields quercetin 3- O-β- d-glucuronide.

TEMPO-mediated oxidation of lignocellulosic fibers from date palm leaves

The TEMPO-mediated oxidation of date palm lignocellulosic fibers (DPLF) from Phoenix dactylifera – i.e. cellulosic fibers still containing their initial lignin and hemicellulose components – was performed under conditions leading to a high degree of oxidation. The resulting carboxylated fibers were shown to still contain high residual contents of lignin (12%, w/w) and hemicelluloses (34%, w/w). The ultrastructure of the oxidized DPLF was revealed by scanning electron microscopy, energy dispersive X-ray and X-ray photoelectron spectroscopy. These techniques not only confirmed the surface carboxylation of the DPLF, but also gave some insight on the oxidation heterogeneity when going from the surface to the core of the fibers. Finally, in relation with the chemical data and the morphology of the substrate, it has been possible to propose a heterogeneous oxidation kinetic model, which was applied with success not only to DPLF but also to the cellulose whiskers extracted from the rachis of the palm from P. dactylifera.

TEMPO-Mediated Oxidation of Cellulose and Preparation of Cellulose Nanofibrils

TEMPO-Mediated Oxidation of Cellulose and Preparation of Cellulose Nanofibrils

Effects of TEMPO oxidation system on kinetic constants of cotton fibers

The kinetics of the TEMPO-mediated oxidation of cotton fibers were studied. It was revealed that the oxidation reaction of the cotton fibers by TEMPO/NaBr/NaClO system can be approximately described as two pseudo-first-order reaction kinetics that are based on the cellulose microstructure, namely the kinetic processes of the primary wall and the secondary wall. In the concentration range used in this study, the rate constant k was directly proportional to the concentration of TEMPO. As to NaBr, the rate constant was proportional to the concentration in a relatively lower range, while it tended to level off at higher concentration, but the oxidation reaction rate increased with concentration when the concentration was above 1.0 mmol/g. The pH value had a great impact on the oxidation rate; the optimum pH was controlled from 10 to 11. The effect of temperature on the rate constant could be well described by the Arrhenius equation, and the apparent activation energy measured was about 56.66kJ/mol. The X-ray diffraction pattern, which indicates the crystallinity of cotton fibers, was nearly constant during the oxidation.

Regiospecific synthesis of quercetin O-β- d-glucosylated and O-β- d-glucuronidated isomers

Quercetin, the polyphenolic compound, which has the highest daily intake, is well known for its protective effects against aging diseases and has received a lot of attention for this reason. Both quercetin 3- O-β- d-glucuronide and quercetin 3′- O-β- d-glucuronide are human metabolites, which, together with their regioisomers, are required for biological as well as physical chemistry studies. We present here a novel synthetic route based on the sequential and selective protections of the hydroxyl functions of quercetin allowing selective glycosylation, followed by TEMPO-mediated oxidation to the glucuronide. This methodology enabled us to synthesize the five O-β- d-glucosides and four O-β- d-glucuronides of quercetin, including the major human metabolite, quercetin 3- O-β- d-glucuronide.

Structure and Mechanical Properties of Wet-Spun Fibers Made from Natural Cellulose Nanofibers

Cellulose nanofibers were prepared by TEMPO-mediated oxidation of wood pulp and tunicate cellulose. The cellulose nanofiber suspension in water was spun into an acetone coagulation bath. The spinning rate was varied from 0.1 to 100 m/min to align the nanofibers to the spun fibers. The fibers spun from the wood nanofibers had a hollow structure at spinning rates of >10 m/min, whereas the fibers spun from tunicate nanofibers were porous. Wide-angle X-ray diffraction analysis revealed that the wood and tunicate nanofibers were aligned to the fiber direction of the spun fibers at higher spinning rates. The wood spun fibers at 100 m/min had a Young's modulus of 23.6 GPa, tensile strength of 321 MPa, and elongation at break of 2.2%. The Young's modulus of the wood spun fibers increased with an increase in the spinning rate because of the nanofiber orientation effect.

Evaluation of antioxidant capacity of ulvan-like polymer obtained by regioselective oxidation of gellan exopolysaccharide

Gellan is an exopolysaccharide produced in high yield by the non-pathogenic bacterium Sphingomonas elodea ATCC 31461. In this study, four carboxylated derivatives of gellan (OG-1, OG-2, OG-3 and OG-4) with different uronics acid content were prepared using the TEMPO (nitroxyl radical 2,2,6,6-tetramethylpiperidine-1-oxyl radical) mediated oxidation and their antioxidant activities were investigated including scavenging activity of hydroxyl, superoxide anion and 1,1-diphenyl-2-picryl-hydrazyl radicals. The results of chemical analysis and 13C NMR spectrums indicated that the modification was successful. In addition, certain derivative exhibited stronger antioxidant activity compared to that of native ones. The high uronic acids derivative (OG-4) showed the most excellent antioxidant activity in three assays. These results suggested the potential of TEMPO-mediated oxidation in developing water-soluble antioxidative polysaccharides from gellan still taking advantage of its low-cost production. The original carbohydrate structure of OG-4 derivative might find use as surrogates of ulvan, a cell-wall polysaccharides extracted from green seaweeds Ulva sp.

Viscoelastic Evaluation of Average Length of Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation

Dynamic viscoelasticity measurements were performed for aqueous dispersions of cellulose nanofibers prepared by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation and subsequent mechanical disintegration in water. The frequency dependence of the storage and loss moduli of 0.02% (w/v) dispersions of TEMPO-oxidized cellulose nanofibers in water showed terminal relaxation behavior at relatively lower angular frequencies. This strongly suggests that each cellulose nanofiber in the dispersion behaves as a semiflexible rod-like macromolecular chain or colloidal particle. Furthermore, a clear boundary was observed between the terminal relaxation and rubbery plateau regions. The longest viscoelastic relaxation time, τ, was estimated from the angular frequency, corresponding to the boundary point, and the average length of the cellulose nanofibers, L, was estimated using the equation τ = πη(s)L(3)/[18k(B)T ln(L/d)]. The equation gave a value of L = 2.2 μm, which was in good agreement with TEM observations.

TEMPO-oxidized cellulose nanofibers

Native wood celluloses can be converted to individual nanofibers 3-4 nm wide that are at least several microns in length, i.e. with aspect ratios>100, by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation and successive mild disintegration in water. Preparation methods and fundamental characteristics of TEMPO-oxidized cellulose nanofibers (TOCN) are reviewed in this paper. Significant amounts of C6 carboxylate groups are selectively formed on each cellulose microfibril surface by TEMPO-mediated oxidation without any changes to the original crystallinity (∼74%) or crystal width of wood celluloses. Electrostatic repulsion and/or osmotic effects working between anionically-charged cellulose microfibrils, the ζ-potentials of which are approximately -75 mV in water, cause the formation of completely individualized TOCN dispersed in water by gentle mechanical disintegration treatment of TEMPO-oxidized wood cellulose fibers. Self-standing TOCN films are transparent and flexible, with high tensile strengths of 200-300 MPa and elastic moduli of 6-7 GPa. Moreover, TOCN-coated poly(lactic acid) films have extremely low oxygen permeability. The new cellulose-based nanofibers formed by size reduction process of native cellulose fibers by TEMPO-mediated oxidation have potential application as environmentally friendly and new bio-based nanomaterials in high-tech fields.