Novel Tactoid Assemblies of Hydrophilic Quantum Dots: Self-Assembly through Chromonic Liquid Crystal Phase Transitions

The self-assembly of nanoparticles using hydrophilic materials offers significant potential for biological applications, including bioimaging, biosensing, and encapsulation/controlled release. In this study, we investigate how CdSe/ZnS quantum dots (QDs), functionalized with carboxylic acid, self-assemble within a hydrophilic Liquid Crystal host. We used disodium cromoglycate (DSCG), a type of lyotropic chromonic liquid crystal, as the host medium for the self-assembly of quantum dots. DSCG, which consists of aromatic plank-like molecules with ionic groups at the periphery, exhibits a columnar structure upon dissolution in water and presents distinct biphasic textures (tactoid morphologies) in the isotropic-to-nematic transition region. A DSCG concentration of 13.8 wt% in water was selected to achieve a phase transition temperature compatible with biological systems.

By rapidly cooling the system from the isotropic phase at 40°C to the nematic phase at 19°C, we successfully induced the formation of QD-rich spindle-shaped tactoids. These tactoids were only formed under quenching conditions for concentration 025, 0.5, 0.75 mg/mL. The tactoids exhibit birefringent textures with no visible defects under polarized optical microscopy (POM) and show uniform QD distribution, confirmed through fluorescence microscopy. The birefringence of the tactoids changes as they rotate from 0° to 180°, providing insight into the internal alignment of both the QDs and liquid crystal molecules. We observed the distribution of QD inside the Tactoid using Transmission Electron Microscopy. We observe that tactoid formation is dependent on the QD concentration: as QD concentration increases, the number of tactoids also rises, but concentrations above 1 mg/mL result in irregular microstructures.