Tunable Magnetism via Core–Shell to Solid-Solution Transformation in Cu–Ni Nanoparticles for Flow-Based Bioanalysis

Flow-based bioanalysis using magnetic particles has garnered significant attention owing to its potential for the rapid and efficient detection of biomolecules, pathogens, and environmental contaminants. This study investigates Cu–Ni nanoparticles as a promising material for such applications, leveraging their tunable magnetic properties and surface functionalities. The nanoparticles are synthesized using a liquid-phase reduction method, and the deposition processes of Ni and Cu are analyzed through quartz crystal microbalance (QCM)-assisted electrochemical measurements. The results reveal a core-shell structure formation mechanism, where Cu is preferentially deposited first due to its higher electrochemical nobility, followed by Ni. To achieve homogeneous alloy structures, heat treatment is employed to transform the core–shell particles into solid-solution nanoparticles. To mitigate oxidation and aggregation during heat treatment, a silica coating is applied to effectively preserve particle integrity. Structural analyses using X-ray diffraction and transmission electron microscopy confirm the successful transformation of the core–shell structure into a homogeneous solid solution. Magnetic properties are evaluated using a vibrating sample magnetometer, which demonstrates that Cu incorporation systematically reduces the magnetization of Ni. Finally, the bioanalytical feasibility is demonstrated by assessing the ability of the nanoparticles to capture and detect DNA corresponding to the COVID-19 genome as a model target. These findings lay the foundation for designing Cu–Ni nanoparticles with tunable magnetic properties for advanced bioanalytical applications. • In situ QCM monitoring elucidates the formation mechanism of Cu-Ni nanoparticles. • Thermal annealing induces a structural transition from core-shell to solid solution. • Magnetic properties are tunable by controlling the compositions and structures. • Optimized Cu-Ni nanoparticles can be applicable for flow bioanalysis.