Multifunctional Core–Shell Nanodiamond Stationary Phase for Open-Tubular Capillary Electrochromatography: Synergistic Interactions and Exceptional Joule-Heat Dissipation

Open-tubular capillary electrochromatography (OT-CEC) holds significant promise for microscale separations but is frequently constrained by trade-offs among phase stability, sample loadability, and the detrimental effects of Joule heating. Herein, we introduce carboxylated nanodiamond (C-ND) as a multifunctional stationary phase to address these challenges. A robust C-ND coating was covalently immobilized onto the capillary inner wall, exploiting the intrinsic core-shell architecture of nanodiamond, which consists of a sp³ diamond core and a sp² graphitic shell, further functionalized with a carboxyl-rich surface. The column exhibited exceptional separation performance for a diverse analyte, ranging from neutral polycyclic aromatic hydrocarbons to charged pharmaceuticals, achieving a maximum efficiency of 249,158 plates/m. Systematic mechanistic investigations revealed that this superior performance stems from a synergistic interplay of hydrophobic, π-π, and electrostatic interactions, dynamically modulated by adjusting pH and ionic strength. Significantly, the column demonstrated an outstanding capability to mitigate Joule heating, maintaining stable operation with minimal current fluctuation (<9%) up to 30 kV without external thermostatting. This thermal resilience, attributed to the phonon-conductive diamond core and the three-dimensional coating architecture, preserved high efficiency and peak integrity even under elevated voltages and high ionic strengths. Furthermore, the column exhibited high loadability, excellent repeatability (RSD < 3.6%), and reliable accuracy in real-sample analysis. This work establishes C-ND as a benchmark stationary phase for OT-CEC and provides a strategic blueprint for designing advanced separation materials where structural durability, multimodal selectivity, and thermal management are engineered as an integral whole.