Template driven biopolymer adsorption in surface patterned duplex biocomposites - a dye probe investigation

• Flax fiber-chitosan composites (FFCs) were prepared and characterized • Dye adsorption characterizes chitosan immobilization at solid-liquid interfaces • Chitosan onto flax fiber vs. graphite templates yield variable surface accessibility • Physicochemical properties of biocomposites depend on template & chitosan loading • Structure-function properties of FFCs reveal unique switchable brush-like structure Herein, we re-define the dye adsorption method to probe the microstructure and template driven surface patterning of supramolecular biocomposites that resemble biopolymer brush systems. Flax fiber composites (FFCs) were prepared by surface patterning chitosan onto flax fibers (or graphite) at variable levels. SEM is employed for micron-scale structural characterization, along with Raman/IR spectroscopy. PZC studies indicate that the predominant cationic nature of FFC surfaces occurs at a PZC point below pH 5.3, which supports the use of anionic dyes as selective surface adsorption probes for this investigation. Reactive Black-5 (RB-5) and Methyl Orange (MO) were used as dye probes, which reveal uptake selectivity towards the chitosan fraction of the composites. Incremental chitosan loading of the FFCs yielded a maximum RB-5 dye adsorption capacity (Q e ; 900 mg/g), according to the Sips model with a favorable R 2 value (0.96). Raman and IR spectral analysis confirm that the dye adsorption mechanism hinges upon electrostatic interactions between the positively charged amine groups of chitosan and the negatively charged sulfonate groups of the anionic dyes, as well as hydrogen bonding effects between the hydroxyl groups and amine groups. Hill and Scatchard plots provide support that cooperative dye uptake occurs due to the distinctive biopolymer microstructural effects for the composite (FFC 3.0) at elevated chitosan loading. These concepts are further corroborated through EDX analysis, which provides insight into the preferential deposition of the anionic dyes onto the grooved structural features of FFC 3.0. The role of templates with variable surface chemistry, apolar (graphite) versus hydrophilic (flax fiber), facilitate unique surface-driven assembly of chitosan with unique functional properties that parallel “biopolymer-brush” structures. Switching from a fiber to a graphite substrate yielded greater uptake of RB-5 dye, by ca. 30%. This trend indicates template-driven configurational changes of the surface patterned chitosan. The RB-5 dye uptake results indicate variable surface accessibility of the protonated amine groups of chitosan. We report a first example of dye probes that reveal enhanced chitosan surface properties, attributed to two effects: (i) “biopolymer brush-like” microdomains that emerge with incremental chitosan doping, and (ii) switchable template-assisted chitosan surface patterning onto lignocellulose fiber versus graphite templates.