Ultrathin Cellulose Films Research Articles

Humidity Response of Cellulose Thin Films.

Cellulose–water interactions are crucial to understand biological processes as well as to develop tailor made cellulose-based products. However, the main challenge to study these interactions is the diversity of natural cellulose fibers and alterations in their supramolecular structure. Here, we study the humidity response of different, well-defined, ultrathin cellulose films as a function of industrially relevant treatments using different techniques. As treatments, drying at elevated temperature, swelling, and swelling followed by drying at elevated temperatures were chosen. The cellulose films were prepared by spin coating a soluble cellulose derivative, trimethylsilyl cellulose, onto solid substrates followed by conversion to cellulose by HCl vapor. For the highest investigated humidity levels (97%), the layer thickness increased by ca. 40% corresponding to the incorporation of 3.6 molecules of water per anhydroglucose unit (AGU), independent of the cellulose source used. The aforementioned treatments affected this ratio significantly with drying being the most notable procedure (2.0 and 2.6 molecules per AGU). The alterations were investigated in real time with X-ray reflectivity and quartz crystal microbalance with dissipation, equipped with a humidity module to obtain information about changes in the thickness, roughness, and electron density of the films and qualitatively confirmed using grazing incidence small angle X-ray scattering measurements using synchrotron irradiation.

Ultrathin Films of Cellulose: A Materials Perspective.

A literature review on ultrathin films of cellulose is presented. The review focuses on different deposition methods of the films—all the way from simple monocomponent films to more elaborate multicomponent structures—and the use of the film structures in the vast realm of materials science. The common approach of utilizing cellulose thin films as experimental models is therefore omitted. The reader will find that modern usage of cellulose thin films constitutes an exciting emerging area within materials science and it goes far beyond the traditional usage of the films as model systems.

Trimethylsilylcellulose synthesis revisited

A simplification of the overall method of producing trimethylsilylcellulose solutions in common organic volatile solvents is reported. Microcrystalline cellulose was derivatized aiming its solubilization in tetrahydrofuran and toluene to obtain ultra-thin films of cellulose by spin-coating. The main simplifications are: 1) elimination of a swelling step before dissolution in the binary solvent N,N-dimethylacetamide/LiCl by using microcrystalline cellulose; 2) derivatization of cellulose –OH groups with –Si(CH3)3 groups was successful with no additional catalyst, like trimethylsilyl chloride, the degree of substitution (DS) being 2.2 ± 0.1. The DS at the extreme surface of the films was computed from the X-ray Photoelectron Spectroscopy data: using solely C 1s fitted component areas and, alternatively, from both C 1s and Si 2p areas. The DS estimated with a technique specific of the surface is close to the bulky one obtained from gravimetric measurements reported in the literature for materials synthesized with catalyst. The topographic uniformity is shown by Atomic Force Microscopy.

Erratum to: Ion reduction in metallic nanoparticles nucleation and growth on cellulose films: Does substrate play a role?

The effect of the substrate (GaAs, Si, glass or Au) on the reduction of silver and gold ions in AgNO3 and HAuCl4·3H2O salts aqueous solutions was here studied. The main goal of this study was the understanding of the reduction mechanism of silver and gold ions in interaction with ultrathin cellulose films deposited on different substrates. Surface morphology was observed and measured using atomic force microscopy (AFM) and its chemical composition characterized by X-ray photoelectron spectroscopy (XPS). Results show that, besides the contribution of the cellulose film to the metallic ions reduction and nanoparticle (NP) growth, the substrate also plays an active role. This is clearly evident in the case of silicon, gallium arsenide, and gold substrates, either bare or covered by cellulose films. For bare glass substrates, no NPs were observed contrarily to glass substrates covered by a cellulose film, where NPs appear on the cellulose film. Yet, XPS showed that, in this last case, metallic ion reduction did not occur, at least in the surface region of the film, where oxidized silver was detected, suggesting a weak or absent reduction power of cellulose. In all cases, XPS C 1s spectra of the cellulose did not show any oxidation of the film. Anyway, the images point out the important role of the cellulose covering film on the final distribution of NPs on the surface.

A method for the heterogeneous modification of nanofibrillar cellulose in aqueous media

Cellulosic substrates were modified by using sequential adsorption of functionalized carboxymethyl cellulose (CMC) and “click” chemistry in aqueous media. First, the effect of degree of substitution (DS), and level of functionalization as well as ionic strength of the medium were systematically investigated in situ by using quartz crystal microbalance with dissipation (QCM-D) in terms of the extent of adsorption of propargyl and azido functionalized CMC. It was found that the functionalization of CMC did not prevent its adsorption on cellulose. However, it was only effective in the presence of electrolytes. Moreover, the adsorption was found to be more efficient for the functionalized CMCs with low initial DS. Next, “click” chemistry, copper (I)-catalyzed azide-alkyne cycloaddition reaction (CuAAC), was carried out for covalent attachment of different molecules on the pre-functionalized ultrathin cellulose films. The modified cellulosic surfaces were further characterized using AFM imaging and XPS. Finally, the method was successfully used in modification of nanofibrillar cellulose (NFC) in aqueous media.

Enzymatic Treatment as a Pre‐Step to Remove Cellulose Films from Sensors

AbstractIn this work an enzymatic treatment is proposed as a preparative, cleaning protocol to remove cellulose films from resonators and sensors. Quartz crystal and surface plasmon gold sensors, coated with ultrathin films of cellulose are used in studies of molecular (for example, polymer and surfactant) adsorption. The sensors are usually recycled after removal of the film, with limited success, after one of two treatments, either hot acid or ammoniac solutions. In the proposed, improved protocol a mixture of cellulases from Aspergillus species, are used as a pre‐treatment to facilitate the release of the cellulose film from the surfaces of the sensors. Two concentrations of NaCl solutions were considered in the enzymatic treatment, 1 and 10 mM, at given enzyme solution concentration, temperature and pH. It was found that after 80 min, the water contact angle after treatment with both salt concentration conditions reached a plateau. The average water contact angle after integration of the enzymatic and ammoniac treatments was found to be low enough, between 6.4 and 7.1 deg to allow reuse the sensors. It is concluded that the use of the ammoniac cleaning solution after the enzymatic treatment is a very convenient, safe and less time consuming way to remove the cellulose films from the sensors to be recycled.

Experimental evidence on medium driven cellulose surface adaptation demonstrated using nanofibrillated cellulose

This paper combines theoretical considerations with experimental evidence to explain the behavior of cellulose when exposed to different media. The observations are explained based on the amphiphilic character of the cellulose molecule and fundamental physicochemical phenomena. Nanofibrillated cellulose was chosen to demonstrate the phenomena since due to its high surface area the effects at issue are pronounced. X-Ray photoelectron spectroscopy and contact angle measurements were used to demonstrate the chemical and energetical changes taking place on the cellulose surface, and atomic force microscopy was used to follow nanoscale structural changes. Due to its hydrophilicity cellulose is well dispersed in water. However, when exposed to non-polar media like air or organic solvents cellulose undergoes partly irreversible reorganization like aggregation or surface passivation in order to find the energetically most favorable state. We show that when NFC is dried directly from water it aggregates strongly and accumulates a very high amount of non-cellulosic material on the surface. Very similar effects also occur when using non-polar media like toluene. Hence, both the reactivity and nanoscale structure are lost. In contrast, NFC retains its reactivity and nano-scaled structure in amphiphilic media like dimethyl acetamide as is confirmed with a simple silylation reaction. We conclude that the interfacial phenomenon is general for cellulosic material but has the most practical impact on applications of nanoscaled cellulose or ultrathin cellulose films.

Ultrathin Cellulose Films of Tunable Nanostructured Morphology with a Hydrophobic Component

In recent years, a wide range of different methods to implant nanosized patterns on surfaces have been developed. Spin coating immiscible binary polymer blends is a straightforward method to prepare micro- and nanostructures on thin films. This study utilizes binary blends to effortlessly prepare stable, surface-functionalized cellulose films. Blends of trimethylsilyl cellulose (TMSC) majority phase and polystyrene (PS) minority phase in toluene were spin coated into an ultrathin film, and TMSC was hydrolyzed to cellulose. The films were characterized and quantified using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle measurements, and quartz crystal microbalance (QCM-D). AFM revealed that horizontally phase-separated structures form during spin coating: after the hydrolyzation of TMSC to cellulose, PS protrudes from cellulose as distinct patches. The patches are disk-like structures with a circular radial cross-section and a height of ca. 5-20 nanometers. The smaller the amount of PS in the original spin coating solution, the smaller the PS island dimensions in the films. The results obtained from the XPS measurements support the AFM results. Water contact angle of the PS/cellulose films increases from 61 degrees to 71 degrees when the relative amount of hydrophobic PS is increased from 1:100 to 1:5. Thus by simply varying the ratio of the film components subtly tailored hydrophobic properties can be achieved. The swelling of the films due to exposure to water was studied by QCM-D. The swelling was not affected by the amount of PS in the blend, and at equivalent cellulose content the blends and pure cellulose films exhibited similar swelling characteristics. In addition, the QCM-D evaluation demonstrated that the films are stable over extended periods of time and are suitable for fundamental studies by QCM-D.

Grafting of Porphyrins on Cellulose Nanometric Films

Ultrathin films of cellulose were functionalized with iron protoporphyrin IX (FePP). Spin-coating allows the production of silylated cellulose films in a controlled way. Cellulose regeneration is achieved through the hydrolyzation of the silane groups, exposing the film to acidic vapors. To enhance the reactivity of the cellulose surface to the protoporphyrin, carbonyldiimidazole (CDI) was used as an activator. The effect of different spacers on the porphyrin grafting such as 1,8-diaminooctane and 1,4-phenylenediamine was studied. The highest level of cellulose functionalization with FePP was achieved when both the cellulose film and FePP were activated by CDI and a diaminoalkane was used as a spacer between the surface and the FePP. ATR/MIR (attenuated total reflection in multiple internal reflections) was performed in situ to follow the kinetics of the different chemical reactions with the cellulose surface. ATR/MIR proved again to be a powerful tool for probing the surface reaction. X-ray photoelectron spectroscopy permitted the elemental analysis of the cellulose surface after the chemical modification.

The Physical Action of Cellulases Revealed by a Quartz Crystal Microbalance Study Using Ultrathin Cellulose Films and Pure Cellulases

The effects of fungal cellulases on model cellulose films were studied using a high-resolution quartz crystal microbalance (QCM) sensitive to minute changes of the nanometer thick model cellulose films. It was found that endoglucanases not only produce new end groups but also cause a swelling of the cellulose film. The cellobiohydrolases degraded the films quickly, which was detected as a rapid decrease in the remaining amount of cellulose on the QCM crystal. However, changing viscoelastic properties of the films also indicated a softening of the film during the degradation. A defined mixture of selected cellulases caused a significantly higher rate of degradation than only cellobiohydrolases. Cellulase synergism is discussed with the endoglucanase swelling effects and film softening added.

Cellulose—model films and the fundamental approach

This critical review describes the recent arrival of ultrathin films of cellulose. The methodology of preparation as well as the applications of the films for fundamental research is fully covered. The review places cellulose in a wider scientific context where cellulose research is no longer a field of interest for specialised scientists only. Cellulose and cellulosic materials should interest communities such as biochemists, physical chemists, surface chemists, organic chemists, polymer chemists and also physicists working close the disciplines mentioned. (149 references.).

Interaction forces between cellulose microspheres and ultrathin cellulose films monitored by colloidal probe microscopy—effect of wet strength agents

Colloidal probe microscopy was employed to study forces between cellulose surfaces upon addition of a series of cationic copolymers in aqueous solution, as model compounds for wet strength agents. The content of quaternary ammonium groups and primary amines was systematically varied in the cationic polymers, to distinguish between the importance of electrostatical and H-bonding effects. Cellulose microspheres were glued at the apex of tipless microfabricated cantilevers and used as colloidal probes. Ultra thin cellulose films and cellulose fibres were employed as model surfaces. The cellulose films of a thickness of about 5 nm were spin-coated from cellulose solution onto silicon substrates. The root-mean-square-roughness (RMS) was 0.3-0.8 nm. The cationic model polymers were compared to Servamine, a polymer employed as standard wet strength resin in papermaking industries. Force versus separation measurements showed a detailed picture of adhesion and contact breaking. Relatively strong adhesion of the order of 0.3 mJ/m(2) was observed with Servamine within a range of approximately 10 nm. At larger distances weak bond breaking and elastic chain pulling were identified. When approaching the surface one to two small jump-in's possibly related to strong binding of Servamine and subsequent attraction could be found in the case of Servamine. In contrast, all the model copolymers showed only a weak adhesion of 8-30 micro/m(2), i.e., an order of magnitude less than that of Servamine and subsequent elastic rupture domains. The contour length, persistence length and characteristic rupture distances were calculated by means of applying the WLC model. Measurements against cellulose fibres obtained from the production process proved the relevance of the model systems.

Functional Incorporation of Integrins into Solid Supported Membranes on Ultrathin Films of Cellulose: Impact on Adhesion

Biomimetic models of cell surfaces were designed to study the physical basis of cell adhesion. Vesicles bearing reconstituted blood platelet integrin receptors α IIb β 3 were spread on ultrathin films of cellulose, forming continuous supported membranes. One fraction of the integrin receptors, which were facing their extracellular domain toward the aqueous phase, were mobile, exhibiting a diffusion constant of 0.6 μm 2 s −1. The functionality of receptors on bare glass and on cellulose cushions was compared by measuring adhesion strength to giant vesicles. The vesicles contained lipid-coupled cyclic hexapeptides that are specifically recognized by integrin α IIb β 3. To mimic the steric repulsion forces of the cell glycocalix, lipids with polyethylene glycol headgroups were incorporated into the vesicles. The free adhesion energy per unit area Δ g ad was determined by micro-interferometric analysis of the vesicle’s contour near the membrane surface in terms of the equilibrium of the elastic forces. By accounting for the reduction of the adhesion strength by the repellers and from measuring the density of receptors one could estimate the specific receptor ligand binding energy. We estimate the receptor-ligand binding energy to be 10 k BT under bioanalogue conditions.

Hydration Forces in Ultrathin Films of Cellulose

Ultrathin (d < 20 nm) films of cellulose were fabricated by regeneration of trimethylsilylcellulose films, which were deposited on silicon wafers by the Langmuir−Blodgett (LB) or the spin-coating m...

Orientation selective immobilization of human erythrocyte membranes on ultrathin cellulose films

We report the orientation selective immobilization of human erythrocyte membranes on planar solid supports. The orientation of the immobilized membrane was identified with selective fluorescence labels. When the right-side-out (RSO) ghosts were incubated with planar glass cover slides, no adsorption or rupture of erythrocytes could be observed. To increase the interfacial attraction between cells and the surface, two types of hydrated polymer films were deposited on the glass cover slides; (a) physisorbed films of cationic polylysine, and (b) Langmuir–Blodgett (LB) films of cellulose derivatives. On polylysine films, patches of the ruptured membranes could be observed, but the surface coverage still remained poor. On the other hand, RSO ghosts were likely to coat the surface of cellulose films more continuously. The fluorescence labeling demonstrated that immobilized erythrocyte membranes selectively inverted their native orientation. Tentatively, we interpreted this larger surface coverage on the cellulose film in terms of the “wetting affinity” between the cell surface glycocalix and the polysaccharides. Such ultrathin (thickness 5–10 nm), biological polysaccharide films have a large potential to immobilize native cell membranes without denaturing their structure, membrane orientation, and functions.

Regeneration and Hydroxyl Accessibility of Cellulose in Ultrathin Films

The regeneration of LB films of (trimethylsilyl)cellulose (TMSC) (1) to ultrathin films of cellulose (2) is monitored by X-ray photoelectron spectroscopy (XPS). Subsequent chemical derivatization of these regenerated cellulose films with trifluoroacetic anhydride (TFAA) (3) to give the corresponding cellulose trifluoroacetate (5) was proven and followed by XPS as a function of exposure time in order to study hydroxyl accessibility. The derivatization behavior of regenerated cellulose films is compared with literature data on bulk cellulose materials with varying degrees of crystallinity. In contrast to the latter, the functionalization kinetics of the regenerated cellulose films indicates a low hydroxyl accessibility in conjuction with a high degree of order in these films.

Regeneration, derivatization and utilization of cellulose in ultrathin films

Cellulose films obtained from TISC Langmuir–Blodgett films provide a model system for adsorption studies of dyes and polyelectrolytes. The conversion of multilayers of trimethylsilylcellulose (TMSC), a “hairy‐rod” molecule with short hydrophobic TMS side groups, into ultrathin cellulose films, the derivatization of these films with succinic anhydride, and the use of regenerated and modified cellulose films for adsorption studies are reported with the aim of finding ways of changing the chemical properties of cellulose films.

Silylated Cellulose Materials in Design of Supramolecular Structures of Ultrathin Cellulose Films

Abstract To use the structure-forming potential and the biodegradability of cellulose and nonionic cellulose ethers, we developed synthesis pathways for soluble and regenerable silyl celluloses suitable for the design of advanced materials. A 6-0-silylation of cellulose takes place in a heterogeneous phase reaction in the presence of ammonia-saturated polar aprotic solvents at −15°C with thexyldimethylchlorosilane. After 2,3-di-O-methylation, this type of regioselectively-substituted cellulose derivatives yields sensor matrices for the detection of halohydrocarbons in air. On the other hand, thexyldimethylsilyl celluloses and trimethylsilyl celluloses with degrees of substitution in the 2.6 to 3.0 range form mono-and multilayered supramolecular structures by applying the Langmuir–Blodgett technique and, after desilylation, oriented ultrathin cellulose films.

Ultrathin films of cellulose on silicon wafers

The first preparation of thin films of cellulose itself is reported. Cellulose exhibits excellent wetting behavior, is extremely stable against oxidation and other chemical degradation and in thin‐film form would have a number of applications. The method involves the preparation of LB films of trimethylcellulose (see Figure) and the in situ conversion of these thin films on silicon substrates to cellulose. magnified image