Dry ice carbonation approach for the synthesis of calcium carbonate-based magnetic composites

Abstract

Multifunctional magnetic composites that integrate biocompatibility, structural tunability and magnetic responsiveness are highly sought after for advanced biomedical applications. Here, we introduce a previously unexplored dry ice-driven carbonation strategy for the synthesis of CaCO₃-coated magnetite (Fe₃O₄@PSS@CaCO₃) microstructures, which can be carried out under both aqueous and completely solvent-free, low-temperature conditions. Unlike conventional CaCO₃ mineralisation approaches that rely on dissolved carbonate salts and elevated temperatures, this method uniquely employs solid CO₂ (dry ice) as a dual-function reagent, serving simultaneously as a controlled carbonate source and an intrinsic cooling medium. This enables a mild and environmentally benign route to complex magnetic CaCO₃ architectures. Strikingly, the choice of reaction medium governs both polymorphism and morphology: aqueous carbonation yields phase-pure rhombohedral calcite microcrystals (≈0.7 µm), whereas the solvent-free dry ice approach produces previously inaccessible acicular microstructures (≈1 µm) comprising a rare coexistence of all three anhydrous CaCO₃ polymorphs (calcite, vaterite, and aragonite) under ambient pressure. Structural, compositional, and morphological features were studied using XRD, SEM, FT-IR, and EDX, while SQUID magnetometry confirmed that all composites retain superparamagnetic behaviour, enabling efficient magnetic manipulation despite CaCO₃ encapsulation. As a proof of concept, the composites were evaluated as magnetically recoverable drug carriers, using methylene blue as a model compound and doxorubicin as a clinically relevant anticancer drug. UV-Vis spectroscopy revealed efficient drug loading and sustained release, governed by the porosity and polymorphic nature of the CaCO₃ shell. By combining the superparamagnetism of Fe₃O₄, the biocompatibility and pH-responsiveness of CaCO₃ and a fundamentally new solid-state carbonation paradigm, this work establishes a versatile and sustainable platform for next-generation magnetic materials with strong potential in targeted drug delivery, bioimaging and other magnetically assisted biomedical applications.