Abstract
Chemical force microcopy, a variation of atomic force microscopy, opened the door to visualize chemical nano-properties of various materials in their natural state. The key function of this method is given by translating adhesion forces between a functionalized tip and the sample to chemical surface behavior. In force titration, these adhesion forces are studied in different pH buffers, which allows estimating the pKa value of the analyzed surface. Herein, we report the use of this method to study natural and chemically treated wood surfaces, which are of interest in sustainable material design. First, we show varying adhesion phenomena of OH- and COOH-functionalized tips on native spruce wood cells. Then, we demonstrate how peak force tapping with chemically functionalized tips can be used to estimate the pKa value of gold substrates (pKa ≈ 5.2) and different wood cell wall layers with high spatial resolution. Additionally, the swelling behavior of wood samples is analyzed in varying pH buffers. With the applied method, chemical surface properties of complex natural substrates can be analyzed.Graphical abstract
Highlights
Products based on cellulose are of interest in sustainable material design, due to its excellent mechanical properties and high availability (Gibson 2012)
Adhesion forces were acquired with OH-tips and COOH-tips on the secondary wood cell wall as indicated in Fig. 1d by 64 simple force-distance measurements in an area of 1 9 1 lm2
Different adhesion properties were observed depending on the tip functionality and the selected surrounding, i.e. ambient air, water or phosphate buffers ranging from pH 2–12
Summary
Products based on cellulose are of interest in sustainable material design, due to its excellent mechanical properties and high availability (Gibson 2012). Contact angle measurements are applied to calculate the surface free energy and for the determination of its acid and basic components (Gindl and Tschegg 2002; Gindl et al 2004) In another method, the pKa value of modified wood is identified by recording the pH of a solvent, in which wood flakes were stored (Balaba and Subramanian 1984). Potentiometric titration or conductometric titration studies the surface charges of wood pulp (Bhardwaj et al 2004; Lloyd and Horne 1993; Katz and Beatson 1984) The challenge in these characterizations arises from the complexity of wood: the composition varies and the components are located heterogeneously in the wood scaffold (Rowell et al 2012). The previously mentioned characterization methods are not sensitive to these local variances, which, are required for the optimal design of modern cellulose-based materials
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