Nanocluster-Induced Liquid-like Precursor Formation and Crystallization: In Situ Visualization and 3D Reconstruction

Revealing the crystallization mechanism of inorganic materials modulated by organic molecules has broad implications in biomineralization, crystallography, and materials science. However, directly visualizing the participation of organic molecules in the inorganic materials' crystallization process remains a significant challenge. Here, we introduce carboxyl-functionalized gold nanoclusters (Au NCs) as an alternative to polymers for investigating CaCO₃ crystallization via the transient "Au NCs-induced liquid precursor" phase, similar to "polymer-induced liquid precursor". Exploiting the ultrasmall size, high density, and stable spontaneous fluorescence properties of Au NCs, this approach enables direct in situ observation of liquid precursor formation and dynamic association/dissociation using light microscopy. Results show that Au NCs are incorporated into the liquid precursor and the quantity of liquid precursor exhibits a nearly linear increase over time until the depletion of free Ca²⁺ ions in solution due to crystallization of calcite. Subsequently, the dissolution of the liquid precursor provides ions for further crystal growth. The occlusion and 3D spatial distribution of Au NCs within CaCO₃ throughout the crystallization process can also be visualized using confocal fluorescence microscopy, demonstrating preferential adsorption on specific lattice planes of CaCO₃. This study substantially enhances our comprehension of the differential growth rates of various crystallographic faces and the spatial distribution of Au NCs within the crystals.