Gabriela Biliuta

Oxidized cellulose—Survey of the most recent achievements

The functionalization and particularly the oxidation of cellulose is an intriguing and challenging topic due to the presence of multiple reactive sites, which can undergo specific reactions. The variety of the oxidizing agents used to improve the selectivity and yields of these transformations is illustrated by the steadily growing of the number of publications and patents reported. This paper is focused on the most selective agents for cellulose oxidations, i.e., sodium periodate and stable or non persistent nitroxyl radicals, emphasizing on the most recent developments reported so far.

Comparison study of TEMPO and phthalimide-N-oxyl (PINO) radicals on oxidation efficiency toward cellulose

Regenerated cellulose fibers, type viscose, have been oxidized with sodium hypochlorite and catalytic amounts of sodium bromide by using two different protocols: first, involving the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and second, employing N-hydroxyphthalimide (NHPI). The reactions were carried out at room temperature and pH=10.5 for 2.5h. Viscose oxidized samples were analyzed and compared in terms of the negative charged groups content, as determined by potentiometric titration and methylene blue adsorption, morphologies and crystallinities changes, as well as changes in the degree of polymerization. The highest content of the carboxylic groups and the best preservation of the morphology and molecular weight of the original material have been found in the case of using NHPI/anthraquinone as oxidation mediators. TEMPO-mediated oxidation leads to the highest depolymerization and cause significant degradation of the cellulosic material.

One-shot carboxylation of microcrystalline cellulose in the presence of nitroxyl radicals and sodium periodate

Correction for ‘One-shot carboxylation of microcrystalline cellulose in the presence of nitroxyl radicals and sodium periodate’ by Sergiu Coseri et al., RSC Adv., 2015, 5, 85889–85897.

Bromide-free oxidizing system for carboxylic moiety formation in cellulose chain.

NHPI (N-hydroxyphthalimide) was used to mediate the oxidation of cellulose fibers in the absence of sodium bromide, as traditionally was used in this kind of transformations, solely using sodium hypochlorite (NaOCl) as the primary oxidant. Avoiding the use of NaBr is highly desired from both environmental and corrosion concerns. The non-persistent PINO (phthalimide-N-oxy) radical, the key species in the oxidation reaction, has been in situ generated from NHPI and copper (II) chloride. The reaction was performed at room temperature at pH=10.5. The carboxylic moiety formation was evidenced by FTIR and X-ray photoelectronic spectroscopy (XPS) and the content of the negatively charged groups determined by potentiometric titration. The changes appeared in crystallinity were evidenced by X-ray diffraction technique.

Preparation and characterization of oxidized starch/poly(N,N-dimethylaminoethyl methacrylate) semi-IPN cryogels and in vitro controlled release evaluation of indomethacin

Fabrication of novel semi-interpenetrating polymer network (semi-IPN) cryogels by cross-linking polymerization of N,N-dimethylaminoethyl methacrylate (DMAEM) in the presence of either oxidized potato starch (OPS) or oxidized wheat starch (OWS) and their characterization are presented in the paper. The influence of the nature of entrapped polymer on the properties of the composite cryogels was evaluated by the swelling kinetics, FT-IR spectroscopy, scanning electron microscopy, and response at external stimuli such as temperature, pH, and ionic strength. Indomethacin (IDM), taken as a model anti-inflammatory drug, was easily loaded into the composite cryogels by the solvent sorption-evaporation strategy. The in vitro release of IDM from the semi-IPN cryogels was low in simulated gastric fluid at pH 1.3, irrespective of the nature of the entrapped oxidized starch, and consistent in simulated intestinal fluid (SIF) at pH 7.4, the influence of the entrapped polysaccharide being evident. The release mechanism of IDM from the composite cryogels was discussed based on two kinetic models, finding that the drug release at 37°C was pseudo-Fickian diffusion, regardless the cryogel composition.

Magnetic cellulosic materials based on TEMPO-oxidized viscose fibers

A simple method for preparation of magnetic cellulose fibers by coating (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)-oxidized viscose with oleic-acid-coated or uncoated, freshly prepared magnetic nanoparticles (MNp) is presented. MNp attachment was facilitated by chemical activation of the cellulose fibers through introduction of negatively charged carboxylic groups using the well-established TEMPO-mediated oxidation protocol. The resulting composite materials preserved the intrinsic properties of the cellulose fibers, but gained notable specific features due to the presence of magnetic nanoparticles. The obtained composite materials were characterized using spectral (Fourier-transform infrared spectroscopy) and microscopic (scanning electron microscopy) methods. Thermogravimetric analyses were carried out to evaluate the thermal stability of the magnetic fibers. The magnetic properties were evaluated using vibrating-sample magnetometry.

Selective oxidation of cellulose, mediated by N-hydroxyphthalimide, under a metal-free environment

Three types of cellulosic samples, including microcrystalline, never-dried sulfite pulp, and Cellets, were selected to test the oxidative capacity of a new oxidation protocol involving the redoubtable N-hydroxyphthalimide (NHPI) catalyst. All reactions were performed under mild conditions, at room temperatures and at atmospheric pressure, in the presence of sodium hypochlorite and sodium bromide. The generation of the reactive radicalic species, i.e. phthalimide N-oxyl (PINO) from its parent hydroxylamine, NHPI, in the presence of sodium hypochlorite, has been firstly proved by using the UV-Vis technique, followed next by the reaction with each cellulose type sample for a designated period of time, varying the amount of the NaClO introduced in the reaction. The main finding of this new proposed protocol resides in the elimination of any metal cocatalyst required for the generation of the non-persistent free radical PINO, as has been previously used. The obtained amounts of the carboxylic groups after oxidation, determined using potentiometric titration, are satisfactory, whereas the degree of polymerization of the resulted products has still superior values, as compared to many other methods used today. All the oxidized samples exhibited an increased crystallinity compared to the unoxidized samples as revealed by X-ray diffraction experiments. This new approach not only avoids the introduction of an additional chemical reagent, but also provides an easy and convenient alternative to those existing today for the preparation of oxidized cellulose products having high amounts of carboxylic groups and remarkably, less depolymerized.