Synthesis\nand Characterization of Zinc Phthalocyanine-Cellulose\nNanocrystal (CNC) Conjugates: Toward Highly Functional CNCs

We\nreport highly fluorescent cellulose nanocrystals (CNCs) formed\nby conjugating a carboxylated zinc phthalocyanine (ZnPc) to two different\ntypes of CNCs. The conjugated nanocrystals (henceforth called ZnPc@CNCs)\nwere bright green in color and exhibited absorption and emission maxima\nat ∼690 and ∼715 nm, respectively. The esterification\nprotocol employed to covalently bind carboxylated ZnPc to surface\nhydroxyl group rich CNCs was expected to result in a monolayer of\nZnPc on the surface of the CNCs. However, dynamic light scattering\n(DLS) studies indicated a large increase in the hydrodynamic radius\nof CNCs following conjugation to ZnPc, which suggests the binding\nof multiple ZnPc molecular layers on the CNC surface. This binding\ncould be through co-facial π-stacking of ZnPc, where ZnPc metallophthalocyanine\nrings are horizontal to the CNC surface. The other possible binding\nmode would give rise to conjugated systems where ZnPc metallophthalocyanine\nrings are oriented vertically on the CNC surface. Density functional\ntheory based calculations showed stable geometry following the conjugation\nprotocol that involved covalently attached ester bond formation. The\nconjugates demonstrated superior performance for potential sensing\napplications through higher photoluminescence quenching capabilities\ncompared to pristine ZnPc.