Comparative analysis of in-situ and ex-situ synthesis of coconut husk cellulose nanofiber/copper-based metal-organic frameworks for curcumin uptake and release

As a prominent global producer of coconut (Cocos nucifera L.), Indonesia yields an annual output above 6 million tons of coconut husk waste (CHW). This research uses CHW to fabricate the cellulose nanofibers (CNF) / copper1,3,5-benzene tricarboxylate (Cu-BTC) composite to address the generated environmental issue using in-situ and ex-situ synthesis. The impact of different synthesis methods on composite properties and curcumin uptake/ release performance is examined. The isolated CNF exhibits a fibrous morphology with a ca. 10 - 100 nm diameter, while the Cu-BTC particles are cubical with a side length of 4.1 mu m. All CNF/Cu-BTC composites, resulting from the in-situ synthesis (C1) and ex-situ synthesis (with the crosslinkers of (1) triethylamine (C2); (2) citric acid (C3); (3) glutaraldehyde (C4)), display a flower branch-like topography, with Cu-BTC surrounding the CNF matrix. The C1 shows better dispersity of Cu-BTC on the CNF surface, while the two seem to aggregate in C2, C3 and C4. The curcumin uptakes of CNF, Cu-BTC, C1, C2, C3, and C4 are observed at 40.77 mg/g, 150.67 mg/g, 386.03 mg/g, 688.71 mg/g, 347.35 mg/g, and 294.84 mg/g, respectively. Among the composites, C2 exhibits higher uptake capabilities. This is due to the presence of triethylamine as an organic linker in C2, which provides more binding sites. Compared with the single component (CNF and Cu-BTC), the enhanced curcumin uptake of composites is credited to their three-dimensional interior structure, which increases the number of binding sites. The uptake performance of all composites follows a modified pseudo-first-order law and multilayer mechanism. The curcumin and composite interactions are driven by Van Der Waals interaction, hydrogen bonding for the first layer, and ion exchange for the upper layers. The release study of curcumin from composites shows slow release, and C2 has the highest release, with 36.7 % at pH 7.4 and 39.1 % at pH 5.5.