Nickel as a modifier of calcium oxalate: an in situ liquid cell TEM investigation of nucleation and growth

Despite extensive research on the nucleation and growth of calcium oxalate (CaOx) crystals, there are still several challenges and unknowns that remain. In particular, the role of trace metal elements in the promotion or inhibition of CaOx crystals is not well understood. In the present study, in situ graphene liquid cell transmission electron microscopy (in situ GLC TEM) was used to observe real-time, nanoscale transformations of CaOx crystals in the presence of nickel ions (Ni²⁺). The results showed that Ni²⁺ form Ni-water complexes, acting as a shape-directing species, generating a unique morphology and altering growth kinetics. Transient adsorption of Ni-water complexes resulted in a metastable phase formation of calcium oxalate trihydrate. Atomistic molecular dynamics simulations confirmed that Ni²⁺ acts as a weak inhibitor which slows down the CaOx crystallization, elucidating that Ni²⁺ impacts small-sized CaOx clusters by bringing more water into the clusters. This work highlighted the intricacies behind the effect of Ni²⁺ on CaOx biomineralization that were made possible to discern using in situ GLC TEM.