Negatively Charged Polysaccharide Stabilizes Amorphous Strontium Sulfate Precursor Phase

Biomineralization describes how living organisms orchestrate the synthesis of functional inorganic materials for structural protection or mechanical support. Mineralization under biological regulation often involves amorphous mineral phases, such as amorphous calcium carbonate or amorphous calcium phosphate, that lack long-range order and transform into stable crystals over time. Unlike calcium-bearing biominerals, little work has been dedicated toward understanding the biomineralization of earth alkaline sulfates. To our knowledge, this study is the first to unambiguously detect a transient amorphous strontium sulfate (ASS) phase using time-resolved X-ray diffraction and Transmission Electron Microscopy (TEM) under cryogenic conditions. Specifically, the incorporation of the negatively charged polysaccharide heparin was found to mediate bioinspired crystallization pathways based on fundamental inorganic-organic interactions, guiding an amorphous-to-crystalline transformation of the strontium sulfate (SrSO4) mineral. The process yielded unusual ellipsoidal SrSO4 microcrystals that partially underwent particle-particle fusion. Nanofocused synchrotron wide-angle X-ray scattering and high-resolution TEM revealed mesocrystalline order from imperfect attachment of ASS nanoclusters, with crystal lattice planes exhibiting gradual tilts in orientation. The nonlinear optical properties of SrSO4 crystals open intriguing avenues for solid-state optical devices used for IR-to-visible light conversion.