Yaman Boluk

Nanocomposites of nanocrystalline cellulose for enzyme immobilization

We describe the synthesis, characterization and use of a composite material made of a renewable source and metallic nanoparticles for biosensing applications. Nanocrystalline cellulose (NCC) is a product isolated from natural cellulose fibers, which is of approximately 100 nm long and 10 nm wide in size. We augmented the surface area and tailored the chemical affinity of NCC by optimally dressing it with gold nanoparticles (AuNPs). The deposition of AuNPs on NCC was controlled by using cationic polyethylenimine (PEI) at different pHs. AuNPs were thiol-functionalized using different linkers prior to enzyme immobilization. The enzyme (glucose oxidase or GOx) was conjugated on the composite by carbodiimide coupling, and subsequent activation of linker-carboxylic acid group. Our results showed that GOx was attached to the surface of the NCC nanocomposite. Moreover, the amount of GOx loaded onto the support depended on the length of the thiol-linker used. The lower value (20.3 mg/mg of support) was obtained with the longer thiol-linker (11 carbon chain) compared to 25.2 mg/mg of support for the smaller thiol-linker (3 carbon chain).

Structure of poly(N-isopropylacrylamide) brushes and steric stability of their grafted cellulose nanocrystal dispersions

Thermo-responsive poly(N-isopropylacrylamide) (poly(NIPAAm)) brushes were grafted from the surface of cellulose nanocrystals (CNC) via living radical polymerization (LRP) using different initiator and monomer concentrations. The dry film thickness of the poly(NIPAAm) layer around CNC was calculated based on Scanning Electron Microscopy (SEM) and dynamic light scattering (DLS) measurements. The wet film thicknesses of grafted poly(NIPAAm) brushes in water were calculated to be 15 and 9nm for NIPAAm-CNC-1 and NIPAAm-CNC-2, respectively. Grafted chain densities and wet film thicknesses at below and above the critical temperature (T=34°C) of polyNIPAAm were calculated by applying mean-field analytical theory. The non-ionic poly(NIPAAm) brushes screened the surface charges of CNC particles, leading to a significant decrease in the absolute zeta potential values for the poly(NIPAAm) grafted CNCs compared to the unmodified and initiator modified CNC samples. Nevertheless, the colloidal stability of poly(NIPAAm) grafted CNC particles were still maintained by steric stabilization below the critical temperature On the other side, hydrophobic attractions among poly(NIPAAm) grafted CNC rods above 34°C lead to coagulation and phase separation. While both poly(NIPAAm) grafted CNC samples showed thermo-responsive behavior, the reversibility of this temperature triggered property was dependent on grafting density.

Cationic poly(2-aminoethylmethacrylate) and poly(N-(2-aminoethylmethacrylamide) modified cellulose nanocrystals: synthesis, characterization, and cytotoxicity.

Cellulose nanocrystals (CNCs) continue to gain increasing attention in the materials community as sustainable nanoparticles with unique chemical and mechanical properties. Their nanoscale dimensions, biocompatibility, biodegradability, large surface area, and low toxicity make them promising materials for biomedical applications. Here, we disclose a facile synthesis of poly(2-aminoethylmethacrylate) (poly(AEM)) and poly(N-(2-aminoethylmethacrylamide) (poly(AEMA)) CNC brushes via the surface-initiated single-electron-transfer living radical polymerization technique. The resulting modified CNCs were characterized for their chemical and morphological features using a combination of analytical, spectroscopic, and microscopic techniques. Zeta potential measurements indicated a positive surface charge, and further proof of the cationic nature was confirmed by gold deposition as evidenced by electron microscopy. The cytotoxicity of these cationic modified CNCs was evaluated utilizing a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay in two different cell lines, J774A1 (mouse monocyte cells) and MCF-7 (human breast adenocarcinoma cells). The results indicated that none of the cationic modified CNCs decreased cell viability at low concentrations, which could be suitable for biomedical applications.

Unique viscoelastic behaviors of colloidal nanocrystalline cellulose aqueous suspensions

Unique rheological and phase behaviors of rod-like nanocrystalline cellulose (NCC) suspensions in aqueous media are revealed in the present article. Specifically, the NCC aqueous suspension remained isotropic in a wide NCC concentration range in which the suspension underwent transition from dilute solution to gel, and the relative viscosity of the NCC suspension could be well fitted by the Sato-Teramoto theory in the full concentration range tested. Correspondingly, both zero-shear viscosity and complex viscosity increased monotonically with NCC concentration, and no maximum value was observed along the curves of zero-shear viscosity or complex viscosity versus NCC concentration, indicating a deviation from the lyotropic system. However, a shear-induced birefringence phenomenon was observed, indicating the NCC suspension formed a temporary ordered structure in the external force field but was unable to form an anisotropic (liquid crystalline) phase. The Cox-Merz rule was not applicable for the NCC suspension as a result of oriented domains, i.e., rod-like NCC particles. Moreover, time-concentration superposition was successfully applied to both the storage and loss modulus, attributed to the isotropic feature of the NCC suspension in the tested concentration range. The reason why this NCC suspension remained isotropic could be because of the strong electrostatic repulsions between NCC particles and the weak tendency or driving force of anisotropy formation as a result of the small aspect ratio of NCC particles, Na+ counterions and large amounts of negative charges along the NCC particles. The results suggested that not all the rod-like particles were able to form an anisotropic phase in aqueous suspension, but dominated by various factors.

Changes in wettability of heat-treated wood due to artificial weathering

Effect of artificial weathering on the wettability of three heat-treated North American wood species (jack pine, aspen, and birch) is studied from the point of view of the structural and chemical changes taking place on the wood surface. Weathering increases wettability of all three heat-treated woods by water. Changes in wettability during artificial weathering differ according to heat treatment procedure and wood species and are likely due to combination of structural and chemical changes of the surfaces. Scanning electron microscopic analysis indicates that cracks form due to degradation taking place during weathering. As a result, water has easier entry into the cell wall, which consequently increases wettability. IR spectra suggest that the OH/CH2 ratio for heat-treated specimens is inversely proportional to the contact angle regardless of the type of wood species. The presence of cellulose-rich layer on wood surface and increasing amount of amorphous cellulose transformed from crystallized cellulose due to weathering result in increase in hydroxyl; consequently, it increases heat-treated wood wettability.

Effect of surface preparation on the wettability of heat-treated jack pine wood surface by different liquids

The objectives of this study are to quantitatively evaluate, using a wetting model, the wettability of three probe liquids with different properties on heat-treated jack pine surfaces prepared by three different types of machining (sanding, planing and sawing) and to compare with those of untreated wood surfaces. The results indicate that the heat-treated wood is wetted less than the untreated wood due to degradation of wood components (hemicelluloses, lignin and cellulose) during heat treatment and it absorbs less liquid. The heat-treated wood becomes most hydrophobic when wood surfaces are sanded by 180-grit paper compared to those prepared by other machining process. Heat-treated wood surfaces are strongly acidic similar to those of untreated wood. Consequently, the basic probe liquid, formamide, shows the highest spreading and penetration rate (K-value) on wood surfaces.ZusammenfassungZiel dieser Studie war es, die Benetzbarkeit mit drei Versuchsflüssigkeiten mit unterschiedlichen Eigenschaften von thermisch behandeltem Jack Pine Holz, dessen Oberflächen unterschiedlich bearbeitet worden waren (schleifen, hobeln, sägen) quantitativ anhand eines Modells zu bestimmen und mit unbehandelten Holzoberflächen zu vergleichen. Die Ergebnisse zeigen, dass thermisch behandeltes Holz aufgrund des Abbaus von Holzbestandteilen (Hemicellulose, Lignin und Cellulose) bei der thermischen Behandlung weniger stark benetzt wurde als unbehandeltes Holz und dass es weniger Flüssigkeit aufnimmt. Thermisch behandeltes Holz, dessen Oberfläche mit Schleifpapier der Körnung 180 bearbeitet wurde, ist im Vergleich zu anders bearbeitetem Holz am hydrophobsten. Thermisch behandelte Holzoberflächen sind stark acidisch, ähnlich wie unbehandeltes Holz. Folglich weist die basische Testflüssigkeit Formamid die höchste Ausbreitungs- und Eindringrate (K-Wert) auf Holzoberflächen auf.

Adhesive surface interactions of cellulose nanocrystals from different sources

Adhesion plays an important role in the final properties of nanocomposites. This study explored the surface interaction of cellulose nanocrystals (CNCs) and the effect of CNC sources on adhesion between individual CNCs and the Si tip of an AFM cantilever using a force mapping technique called FMap. The adhesion between CNCs and a Si tip from five different sources has been studied: cotton, Whatman filter paper, hemp, softwood chemical kraft pulp, and softwood-dissolving pulp (alistaple). Mica was used as the background substrate to act as an internal standard. This study’s findings suggest that adhesion is not the same for all CNCs. Transmission electron microscopy and atomic force microscopy were used to determine the size and shape of each CNC. The experimental quantitative data showed that adhesion between CNCs and the Si tip has a close correlation with the diameter of the CNCs. X-ray photoelectron spectroscopy confirmed the presence of sulfate groups on the surface of the CNCs and a correlation between adhesion and surface chemistry of the CNCs was observed.

Investigation of the scaling law on gelation of oppositely charged nanocrystalline cellulose and polyelectrolyte.

The sol-gel transition in the mixture system of oppositely charged polyelectrolyte (quaternized hydroxyethylcellulose ethoxylate, QHEC) and nanocrystalline cellulose (NCC) induced by electrostatic adsorption interaction was investigated by rheological means. Winter and Chambon theory was validated to be applicable for the critical gel point determination, and critical gel point have been successfully determined. With QHEC concentration increasing, more NCC were needed to form a critical gel, and smaller loss tangent and relaxation exponent (n) values at the gel point were observed, indicating the elastic nature of mixture was enhanced with QHEC increase. Gel strength behaved as a function of both QHEC and NCC concentrations, suggesting the gel network at the critical point was composed of entanglements and association of QHEC macromolecular chains, as well as the electrostatic adsorption interaction between QHEC chains and NCC rods. The calculated number of NCC rods per junction decreased from 0.30 to 0.01 when the QHEC concentration increased from 1.0wt% to 3.0wt%, indicating the electrostatic adsorption interaction between the NCC rods and QHEC chains was less significant to gel formation at higher QHEC concentrations. Therefore, the exponents of scaling law η0∝ϵ(-γ) and Ge∝ϵ (z) for the QHEC/NCC mixtures revealed that the scaling law n=z/(z+γ) between n, γ, and z was only feasible at highest QHEC concentration, since the intermolecular interaction (electrostatic adsorption interaction in this article) was so weak that can be neglected and the critical gel network was dominated by QHEC chain entanglements and association.

Interactions between cellulose nanocrystals and anionic and neutral polymers in aqueous solutions

Physical structures of aqueous cellulose nanocrystal (CNC) suspensions in anionic polyelectrolyte carboxymethyl cellulose (CMC) and non-ionic poly(ethylene oxide) (PEO) were investigated by studying their cross polarized, polarized optical microscope (POM) images and dynamic light scattering, zeta potential, 1H spin–lattice relaxation nuclear magnetic resonance (NMR) data. The presence of anionic CMC and nonionic PEO in CNC suspensions led to two different kind of interactions. Semi-dilute CNC suspensions showed first gel-like behavior then phase separation by adding only semi-dilute un-entangled CMC polymer solutions, whereas the addition of PEO didn’t cause any significant change. POM images showed the phase transitions of CNC suspensions in the presence of CMC solutions from the isotropic state to nematic and chiral nematic phases. Dynamic light scattering, zeta potential and 1H spin–lattice relaxation NMR data presented further arguments to explain polymer-CNC interactions in CMC and PEO solutions. 1H NMR solvent relaxation technique determined the adsorption and depletion interactions between polymers and CNC. The minima in spin–spin specific relaxation rate constant showed the depletion of CNC nanoparticles in CMC. It is believed that the depletion flocculation was the case for the effects of CMC polymer chains in CNC suspensions. PEO was adsorbed on CNC surfaces and caused only weak depletion interactions due to the presence of soft particles.

Electrolyte effect on gelation behavior of oppositely charged nanocrystalline cellulose and polyelectrolyte

The electrolyte (NaCl) influences on the sol-gel transition of the complex solution composed of oppositely charged nanocrystalline cellulose (NCC) and polyelectrolyte (quaternized hydroxyethylcellulose ethoxylate, QHEC) were investigated by the rheological means in the present paper. Winter and Chambon theory was applicable to describe the sol-gel transition, and the critical gel points have been successfully determined. When increasing the NaCl concentration, more NCC were needed to form a critical gel due to the screening of the electrostatic interaction, and the larger loss tangent and relaxation exponent (n) values at the gel point demonstrated a less elastic nature of the complex solution with more NaCl. The results indicated the gel network was composed of entanglements and association of QHEC (as polymer network), as well as the electrostatic adsorption interaction between QHEC chains and NCC rods (as cross-linking). With the addition of NaCl, the screening effect led to the enhancement of the entanglements and weakening of the electrostatic adsorption, however, the gel strength decreased with increasing the NaCl amount, suggesting the electrostatic adsorption interaction played a more dominant role than the entanglements when the gel was formed. Moreover, the exponents of the scaling law η0∝ɛ(-γ) and Ge∝ɛ(z) of the QHEC/NCC/NaCl solution revealed that the scaling law n=z/(z+γ) between n, γ, and z was only feasible at the highest NaCl concentration, as a result of that the intermolecular electrostatic interaction was completely screened, indicating the scaling law was only feasible when intermolecular interaction was small enough to be neglected.