Quantifying Polydopamine Shell Hydration in Core–Shell Colloids by Analytical Ultracentrifugation: Implications for Self-Assembly and Structural Color of Photonic Crystals

Polystyrene-polydopamine (PS@PDA) core-shell particles are widely used as building blocks to form colloidal photonic crystals with tunable light absorption. Increasing the PDA shell thickness typically leads to more saturated colors but also diminishes the structural order of the resulting assemblies. To understand this phenomenon, we correlate particle properties in solution with the structural properties of the formed colloidal crystals. In particular, we focus on correlating the water swellability of the PDA shell with the agglomeration behavior upon drying. To this end, we combine density variation sedimentation velocity (SV) and buoyant density gradient equilibrium (DGE) experiments in analytical ultracentrifugation (AUC) to quantify the hydration of the PDA shell. Density variation SV-AUC in H₂O-D₂O mixtures yields the particle's anhydrous density, while DGE-AUC, using an in situ formed sucrose gradient, reveals the buoyant density at the isopycnic position in the measurement cell. Mass balance analysis then enables quantification of the water mass incorporated within the PDA shell. These combined experiments show that PDA shell hydration generally increases with shell thickness and depends on the reaction parameters chosen for the shell formation process. The structural order and photonic properties of colloidal crystals formed from the different PS@PDA particles decrease with increasing shell thickness, indicating that more swollen shells promote aggregation during self-assembly and thus disturb structural order. Advanced AUC experiments therefore provide access to key physicochemical characteristics of functional colloids and establish valuable process-structure-property relations for the design of colloidal photonic crystals.