Synthesis and Characterization of Zinc Phthalocyanine-Cellulose Nanocrystal (CNC) Conjugates: Toward Highly Functional CNCs.

We report highly fluorescent cellulose nanocrystals (CNCs) formed by conjugating a carboxylated zinc phthalocyanine (ZnPc) to two different types of CNCs. The conjugated nanocrystals (henceforth called ZnPc@CNCs) were bright green in color and exhibited absorption and emission maxima at ∼690 and ∼715 nm, respectively. The esterification protocol employed to covalently bind carboxylated ZnPc to surface hydroxyl group rich CNCs was expected to result in a monolayer of ZnPc on the surface of the CNCs. However, dynamic light scattering (DLS) studies indicated a large increase in the hydrodynamic radius of CNCs following conjugation to ZnPc, which suggests the binding of multiple ZnPc molecular layers on the CNC surface. This binding could be through co-facial π-stacking of ZnPc, where ZnPc metallophthalocyanine rings are horizontal to the CNC surface. The other possible binding mode would give rise to conjugated systems where ZnPc metallophthalocyanine rings are oriented vertically on the CNC surface. Density functional theory based calculations showed stable geometry following the conjugation protocol that involved covalently attached ester bond formation. The conjugates demonstrated superior performance for potential sensing applications through higher photoluminescence quenching capabilities compared to pristine ZnPc.