Functional Gels from Natural Nanomaterials

Gels are entangled networks where a fluid is physically confined, yielding a material with a unique soft-solid behavior. Evidenced by widespread applications: ranging from simple food additives and drug carriers, to intricate smart materials and soft robotics, these materials have evolved academically into the backbone of many research studies. Gels possess properties that enable their application as molecular receptors, absorbing chemical species from solution or air. To further improve on their applicability, functional units can be installed inside the gel’s structure. During this endeavor, chemically or physically, reversible, responsive, and selective absorption processes can occur. Regardless, supramolecular approaches to embed functional moieties within gels remain rare. To overcome this pitfall, several researchers have proposed the use of cellulose nanocrystals (CNCs), colloidal suspensions modified with inorganic or organic salts, to assemble cellulose-based hydrogels.

Simply put, cellulose nanocrystals spontaneously assemble into gels when mixed with a polyionic organic or inorganic salt. Unfortunately, not much has been reported on this ion-induced gelation strategy to create functional CNC gels with organic host molecules. To bridge this gap, researchers from the University of British Columbia in Canada: Dongjie Zhang (graduate student), Dr. Miguel A. Soto (postdoctoral fellow), Dr. Lev Lewis and led by Professor Mark MacLachlan, in collaboration with Dr. Wadood Hamad at the Bioproducts Innovation Centre of Excellence, FPInnovations developed a novel approach to create CNC-based materials that contain functional ions. Their work is currently published in the research journal, Angewandte Chemie.

The researchers envisioned ionic macrocycles driving gelation of CNCs to generate materials with embedded host molecules. To pursue their concept, the team selected the rigid tetracationic macrocycle, cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺), as the functional ion. Technically, their approach involved mixing of an aqueous suspension of sodium neutralized cellulose nanocrystals with [CBPQT]Cl₄ in solution. The enhancement of the gel stiffness with increasing host concentration was confirmed through rheological studies of the CNC-CBPQT⁴⁺ systems.

The authors reported that addition of [CBPQT]Cl₄ to CNCs caused gelation and embedded an active host inside the material. Moreover, the team established that the fabricated CNC gels could reversibly absorb guest molecules from solution then undergo molecular recognition processes that created colorful host–guest complexes. The researchers even implemented the aforementioned materials in gel chromatography (for guest exchange and separation), and as elements to encode 2- and 3-dimensional patterns, where they anchored well.

In summary, the Canadian scientists employed a supramolecular ion-induced strategy to produce CNC-CBPQT⁴⁺ gels with a cationic host embedded in a cellulose nanocrystal matrix, a supramolecular process that does not require tedious synthesis.  Remarkably, the reported gels could function as receptors to absorb guest species from solution to subsequently undergo reversible assembly of host–guest complexes in their interior. In a statement to Advances in Engineering, Professor Mark MacLachlan, the lead author emphasized that they anticipated their facile, yet robust, gel fabrication strategy will have important applications particularly when creating new functional materials. “These materials could be excellent for developing sensors or sequestering contaminants from the environment,” he added.