Multilayer Density Analysis of Cellulose Thin Films

An approach for the multilayer density analysis of polysaccharide thin films at the example of cellulose is presented. In detail, a model was developed for the evaluation of the density in different layers across the thickness direction of the film. The cellulose thin film was split into a so called “roughness layer” present at the surface and a “bulk layer” attached to the substrate surface. For this approach, a combination of multi-parameter surface plasmon resonance spectroscopy (SPR) and atomic force microscopy (AFM) was employed to detect changes in the properties, such as cellulose content and density, thickness and refractive index, of the surface near layer and the bulk layer. The surface region of the films featured a much lower density than the bulk. Further, these results correlate to X-ray reflectivity studies, indicating a similar layered structure with reduced density at the surface near regions. The proposed method provides an approach to analyse density variations in thin films which can be used to study material properties and swelling behavior in different layers of the films. Limitations and challenges of the multilayer model evaluation method of cellulose thin films were discussed. This particularly involves the selection of the starting values for iteration of the layer thickness of the top layer, which was overcome by incorporation of AFM data in this study

Dynamic and Static Assembly of Sulfated Cellulose Nanocrystals with Alkali Metal Counter Cations

Sulfate groups on cellulose particles such as cellulose nanocrystals (CNCs) provide colloidal stability credit to electrostatic repulsion between the like-charged particles. The introduction of sodium counter cations on the sulfate groups enables drying of the CNC suspensions without irreversible aggregation. Less is known about the effect of other counter cations than sodium on extending the properties of the CNC particles. Here, we introduce the alkali metal counter cations, Li+, Na+, K+, Rb+, and Cs+, on sulfated CNCs without an ion exchange resin, which, so far, has been a common practice. We demonstrate that the facile ion exchange is an efficient method to exchange to any alkali metal cation of sulfate half esters, with exchange rates between 76 and 89%. The ability to form liquid crystalline order in rest was observed by the presence of birefringence patterns and followed the Hofmeister series prediction of a decreasing ability to form anisotropy with an increasing element number. However, we observed the K-CNC rheology and birefringence as a stand-out case within the series of alkali metal modifications, with dynamic moduli and loss tangent indicating a network disruptive effect compared to the other counter cations, whereas observation of the development of birefringence patterns in flow showed the absence of self- or dynamically-assembled liquid crystalline order

Model - free approach to quadrupole spin relaxation in solid Bi-209-aryl compounds

Nuclear Quadrupole Resonance (NQR) experiments were performed for deuterated and non-deuterated triphenylbismuth (BiPh3) to inquire into Bi-209 relaxation mechanisms. The studies are motivated by the idea of exploiting Quadrupole Relaxation Enhancement (QRE) as a novel contrast mechanism for Magnetic Resonance Imaging. From this perspective relaxation features of nuclei possessing quadrupole moment (quadrupole nuclei) are of primary importance for the contrast effect. Spin-spin relaxation rates associated with the NQR lines were described in terms of the Redfield relaxation theory assuming that the relaxation is caused by fluctuations of the electric field gradient tensor at the position of the quadrupole nucleus that are described by an exponential correlation function. The description referred to as a model-free approach is an analogy of the description used for paramagnetic contrast agents. It was demonstrated that for the deuterated compound this approach captures the essential features of Bi-209 relaxation, but it should not be applied for non-deuterated compounds as dipolar interactions between neighbouring protons and the quadrupole nucleus considerably contribute to the relaxation of the last one. Thus, the relaxation scenario for species containing quadrupole nuclei is fundamentally different than for paramagnetic contrast agents and this fact has to be taken into account when predicting contrast effects based on QRE

Interactions and Dissociation Constants of Galactomannan Rendered Cellulose Films with Concavalin A by SPR Spectroscopy

Interactions of biomolecules at interfaces are important for a variety of physiological processes. Among these, interactions of lectins with monosaccharides have been investigated extensively in the past, while polysaccharide-lectin interactions have scarcely been investigated. Here, we explore the adsorption of galactomannans (GM) extracted from Prosopis affinis on cellulose thin films determined by a combination of multi-parameter surface plasmon resonance spectroscopy (MP-SPR) and atomic force microscopy (AFM). The galactomannan adsorbs spontaneously on the cellulose surfaces forming monolayer type coverage (0.60 ± 0.20 mg·m−2). The interaction of a lectin, Concavalin A (ConA), with these GM rendered cellulose surfaces using MP-SPR has been investigated and the dissociation constant KD (2.1 ± 0.8 × 10−8 M) was determined in a range from 3.4 to 27.3 nM. The experiments revealed that the galactose side chains as well as the mannose reducing end of the GM are weakly interacting with the active sites of the lectins, whereas these interactions are potentially amplified by hydrophobic effects between the non-ionic GM and the lectins, thereby leading to an irreversible adsorption

Aspects of structural order in Bi-209-containing particles for potential MRI contrast agents based on quadrupole enhanced relaxation

Quadrupole relaxation enhancement (QRE) has been suggested as the key mechanism for a novel class of field-selective, potentially responsive magnetic resonance imaging contrast agents. In previous publications, QRE has been confirmed for solid compounds containing Bi-209 as the quadrupolar nucleus (QN). For QRE to be effective in aqueous dispersions, several conditions must be met, i.e. high transition probability of the QN at the H-1 Larmor frequency, water exchange with the bulk and comparatively slow motion of the Bi-carrying particles. In this paper, the potential influence of structural order within the compounds (crystallinity') on QRE was studied by nuclear quadrupole resonance (NQR) spectroscopy in one crystalline and two amorphous preparations of Triphenylbismuth (BiPh3). The amorphous preparations comprised (1) a shock-frozen melt and (2) a granulate of polystyrene which contained homogeneously distributed BiPh3 after common dissolution in THF and subsequent evaporation of the solvent. In contrast to the crystalline powder which exhibits strong, narrow NQR peaks the amorphous preparations did not reveal any NQR signals above the noise floor. From these findings, we conclude that the amorphous state leads to a significant spectral peak broadening and that for efficient QRE in potential contrast agents structures with a high degree of order (near crystalline) are required

209Bi Quadrupole Relaxation Enhancement in Solids as a Step Towards New Contrast Mechanisms in Magnetic Resonance Imaging

<p>Motivated by the possibility of exploiting species containing high spin quantum number<br> nuclei (referred to as quadrupole nuclei) as novel contrast agents for Magnetic Resonance<br> Imaging, based on Quadrupole Relaxation Enhancement (QRE) effects, 1H spin-lattice<br> relaxation has been investigated for tris(2-methoxyphenyl)bismuthane and tris(2,6-<br> dimethoxyphenyl)bismuthane in powder. The relaxation experiment has been performed in<br> the magnetic field range of 0.5 T to 3 T (the upper limit corresponds to the field used in many<br> medical scanners). A very rich QRE pattern (several frequency specific 1H spin-lattice<br> relaxation rate maxima) has been observed for both compounds. Complementary Nuclear<br> Quadrupole Resonance experiments have been performed in order to determine the<br> quadrupole parameters (quadrupole coupling constant and asymmetry parameters) for 209Bi.<br> Knowing the parameters, the QRE pattern has been explained on the basis of a quantummechanical<br> picture of the system including single and double-quantum coherences for the<br> participating nuclei (1H and 209Bi). In this way the quantum-mechanical origin of the spintransitions leading to the QRE effects has been explained.</p

Tuning nuclear quadrupole resonance:a novel approach for the design of frequency-selective MRI contrast agents

Abstract The interaction between water protons and suitable quadrupolar nuclei (QN) can lead to quadrupole relaxation enhancement (QRE) of proton spins, provided the resonance condition between both spin transitions is fulfilled. This effect could be utilized as a frequency selective mechanism in novel, responsive T₁ shortening contrast agents (CAs) for magnetic resonance imaging (MRI). In particular, the proposed contrast mechanism depends on the applied external flux density—a property that can be exploited by special field-cycling MRI scanners. For the design of efficient CA molecules, exhibiting narrow and pronounced peaks in the proton T₁ relaxation dispersion, the nuclear quadrupole resonance (NQR) properties, as well as the spin dynamics of the system QN−¹H, have to be well understood and characterized for the compounds in question. In particular, the energy-level structure of the QN is a central determinant for the static flux densities at which the contrast enhancement appears. The energy levels depend both on the QN and the electronic environment, i.e., the chemical bonding structure in the CA molecule. In this work, the NQR properties of a family of promising organometallic compounds containing ²⁰⁹Bi as QN have been characterized. Important factors like temperature, chemical structure, and chemical environment have been considered by NQR spectroscopy and ab initio quantum chemistry calculations. The investigated Bi-aryl compounds turned out to fulfill several crucial requirements: NQR transition frequency range applicable to clinical 1.5- and 3 T MRI systems, low temperature dependency, low toxicity, and tunability in frequency by chemical modification