Real-Time 3D Analysis of Ionic Colloidal Crystallization

Background and Motivation

In the study of molecular crystals, the structure is typically identified through scattering techniques, as we cannot directly observe the internal structure. Due to their larger volume, micrometer-sized colloidal particles allow us to observe their crystallization process in real-time through optical microscopy. However, this process is still limited by the lack of an “X-ray view” in practice. To address this issue, researchers have developed a system of refractive index-matched fluorescently labeled colloidal particles, demonstrating the stable formation of ionic crystals in aqueous solutions and proving that their structure can be controlled by size ratio and salt concentration.

Research Source

This research was conducted by Shihao Zang, Adam W. Hauser, Sanjib Paul, Glen M. Hocky, and Stefano Sacanna from the Department of Chemistry at New York University (NYU) and was published in the journal Nature Materials in 2024. The DOI of the paper is: https://doi.org/10.1038/s41563-024-01917-w.

Research Process

1. Synthesis and Labeling of Colloidal Particles

The research team first designed and developed a method for synthesizing positively and negatively charged colloidal particles using surfactant-free emulsion polymerization, ensuring monodispersity and stability by using different initiators. Positively charged particles were synthesized using the initiator 2,2′-azobis(2-methylpropionamidine) dihydrochloride (AIBA) and a quaternary ammonium salt-containing comonomer for charge stabilization. Negatively charged particles were synthesized using potassium persulfate (KPS) as the initiator. These particles were fluorescently labeled, allowing for clear identification in confocal laser scanning microscopy (CLSM).

2. Assembly and Imaging of Ionic Colloidal Crystals

The research employed the PACs method (Polymer-Attenuated Coulombic Self-Assembly) to assemble multi-component colloidal crystals in water and imaged them using CLSM. By adjusting the size ratio and salt concentration, crystals with different structures, such as CsCl and Cu3Au structures, could be formed. Notably, the use of a near-refractive index-matched colloidal particle system (e.g., PFpMA) in a 40% dimethyl sulfoxide (DMSO) solution rendered the particle system nearly transparent in CLSM, eliminating laser scattering issues.

3. 3D Reconstruction and Crystal Structure Identification

Using the Z-axis scanning data obtained from CLSM, the research team performed 3D reconstruction and summarized particle coordinates using the Trackpy software package. Finally, the simulated X-ray diffraction patterns were compared with experimental and simulated results to confirm the crystal structure.

Main Research Results

1. 3D Particle Localization and Crystal Structure Identification: By utilizing CLSM and the Trackpy software package, the research achieved precise localization of colloidal particle positions and reconstructed the 3D internal structure. The specific crystal structure types were confirmed by generating simulated X-ray diffraction patterns.

2. Defect Dynamics Analysis: The study enabled the observation of dynamic changes in defects within the crystals, such as vacancies and anti-site defects. Through 3D dynamic tracking, the behavior of these defects remaining stationary or moving during the crystal melting process was revealed.

3. Analysis of Crystal Twinning Structures: Using CLSM data and 3D reconstruction, the research team could identify twin boundaries within the crystals and understand, through simulations, how these twinning structures influence the macroscopic morphology of the crystals.

Research Conclusions and Value

This research demonstrated the real-time 3D analysis of ionic colloidal crystals using refractive index-matched fluorescently labeled colloidal particles and confocal laser scanning microscopy (CLSM). The study unveiled the complex internal structures and dynamic defects within colloidal crystals, providing new insights into understanding defects in real crystals, the crystal melting process, and the twinning mechanism of crystal structures.

The application of this technique is not limited to the existing colloidal model systems but can be extended to the study of broader ionic solids and complex structures, offering a powerful tool for the field of materials science, particularly in the exploration of new functional materials. Furthermore, by developing more precise low-refractive index core-shell colloidal particles, future work aims to further improve particle tracking accuracy, facilitating in-depth research on the thermodynamic and kinetic control parameters of the crystallization process.

Research Highlights

1. Novel Refractive Index-Matched Colloidal Particle System: Synthesized new near-refractive index-matched positively and negatively charged colloidal particles, solving the issue of laser scattering in traditional optical microscopy.

2. Real-Time Dynamic 3D Imaging: Achieved the first real-time dynamic 3D imaging of the internal structure of ionic colloidal crystals, revealing the formation and motion mechanisms of defects within the crystals.

3. Precise Identification of Various Crystal Structures: By simulating X-ray diffraction patterns, the study not only identified CsCl and Cu3Au-type crystal structures experimentally but also recognized previously unreported crystal structures.

4. Advancement in Practical Applications: The research methodology has widespread potential applications in the field of materials science, particularly in designing new optoelectronic materials and understanding crystal growth mechanisms.

This research not only fills the gap in the study of 3D internal structure and dynamics of colloidal crystals but also provides a new tool and methodology, driving further advancements in colloidal science and materials research.