Multilayer Density Analysis of Cellulose Thin Films

Abstract

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.

Self‐Assembly of Soft Cellulose Nanospheres into Colloidal Gel Layers with Enhanced Protein Adsorption Capability for Next‐Generation Immunoassays

“…All SPR measurements were performed at 20 °C under the constant flow of 100 µL min −1 . The thickness of the adsorbed layer was determined with Equation () [ 56 ] where d is the thickness, l d is the characteristic evanescent electromagnetic field decay length (estimated as 0.37 of the light wavelength 670 nm), Δ angle is the change in the SPR angle, m is the sensitivity factor for the sensor (109.94° RIU −1 , obtained after calibration), n 0 is the refractive index of the bulk solution (1.334 RIU), and n a is the refractive index of the adsorbed substance (for NPcat 1.410 [ 57 ] ).…”

“…The mass of the adsorbed layer per unit area was determined with Equation () [ 58 ] where Δ m is the adsorbed mass, d is the thickness, and ρ is the packing density of the NPcat (estimated to be 1.39 g cm −3[ 57 ] ).…”

“…where d is the thickness, l d is the characteristic evanescent electromagnetic field decay length (estimated as 0.37 of the light wavelength 670 nm), Δ angle is the change in the SPR angle, m is the sensitivity factor for the sensor (109.94° RIU −1 , obtained after calibration), n 0 is the refractive index of the bulk solution (1.334 RIU), and n a is the refractive index of the adsorbed substance (for NPcat 1.410 [57] ). The mass of the adsorbed layer per unit area was determined with Equation (3) [58]…”

“…where Δm is the adsorbed mass, d is the thickness, and ρ is the packing density of the NPcat (estimated to be 1.39 g cm −3 [57] ).…”

Current Opportunities and Challenges in Biopolymer Thin Film Analysis—Determination of Film Thickness

“…In principle also the density can be determined at the interfaces. Sampl et al (2019) showed that the outermost layer of cellulose thin films (3 nm) has a different refractive index and density than the bulk film. Combining SPR with QCM-D, swelling studies can be performed that allow the monitoring of water uptake inside different biopolymer thin films (Kontturi et al, 2013).…”