Surface Chemistry Sorts Nanoparticles Inside Growing Calcite Crystals

By Dr. Noopur JainReviewed by Susha Cheriyedath, M.Sc.Aug 5 2026

Sulfate-rich spheres and carboxylate-rich vesicles spontaneously occupied different regions of calcite, revealing a route to spatially programmed composite materials and sequential payload release.

Paper: Interfacial chemistry governs nanoparticle self-sorting during biomimetic crystallization. Image credit: AI-generated conceptual image created using ChatGPT/OpenAI    

In a recent 'Article in Press' in the journal Nature Communications, researchers demonstrated that the interfacial chemistry of diblock copolymer nanoparticles governs their spontaneous self-sorting occlusion within biomimetic calcite crystals, enabling programmable nanoscale spatial organization.

Biomimetic Nanoparticle Self-Sorting

Biominerals found in nature, such as bones, teeth, and shells, form hierarchical organic–inorganic composite structures where organic molecules are precisely organized within inorganic mineral matrices at the nanoscale. This intricate spatial arrangement imparts exceptional mechanical properties, including stiffness and toughness.

Achieving similar nanoscale control in synthetic biomimetic materials remains a major challenge primarily due to the complex interplay of organic–inorganic interactions during crystallization. Previous studies predominantly focused on single-component organic additives, but natural biomineralization involves a diverse mixture of proteins, polysaccharides, and other biomolecules that are selectively incorporated into specific domains within the mineral host.

Understanding how nanoscale components with distinct surface chemistries selectively partition, or self-sort, during crystallization could provide key design principles for fabricating advanced organic–inorganic nanocomposites.

This work investigates the role of nanoparticle interfacial chemistry in governing their self-sorting occlusion within calcite crystals, thereby advancing fundamental knowledge of polymer nanoparticles used as controlled models of biological macromolecules during crystallization.

RAFT Synthesis and Characterization

The study synthesized a series of diblock copolymer nanoparticles as analogs of biomacromolecules, differing primarily in surface chemistry, size, and morphology. Using reversible addition–fragmentation chain-transfer (RAFT)-mediated polymerization-induced self-assembly (PISA), spherical sulfate-rich nanoparticles (S56-B500, ~101 nm diameter) and larger carboxylate-rich vesicular nanoparticles (M54-B200, ~306 nm diameter) were prepared, each fluorescently labeled to facilitate imaging.

These nanoparticles were introduced, separately or as binary mixtures, into growing calcite crystals formed via the ammonia diffusion method. Transmission electron microscopy (TEM) was used to characterize nanoparticle morphology and size, while scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), and argon ion beam etching were used to analyze the spatial distribution of occluded nanoparticles within the calcite host.

In situ dynamic light scattering (DLS) monitored changes in nanoparticle size, colloidal stability, and solution concentration during crystallization, allowing the timing of occlusion to be inferred in conjunction with electron microscopy. To probe molecular-level interactions, atomic force microscopy (AFM) force spectroscopy was performed using tips functionalized with polymer stabilizer chains to quantify rupture forces and contour lengths with the growing calcite (10(\bar{1})4) face.

Controlled synthesis of diblock copolymer spheres or vesicles via RAFT-mediated PISA. a, Synthetic protocol for the preparation of rhodamine-functionalized poly(ammonium 2-sulfatoethyl methacrylate)56-block-poly(benzyl methacrylate)500 [S56-B500] spheres; b, Synthetic protocol for the preparation of fluorescein-functionalized poly(methacrylic acid)54-block-poly(benzyl methacrylate)200 [M54-B200] vesicles; c, TEM image of S56-B500 spheres; d, TEM image of M54-B200 vesicles. The dashed circles indicate individual vesicles. Their apparent aggregation is attributed to drying artifacts arising during TEM sample preparation; e, Particle size distributions for S56-B500 spheres and M54-B200 vesicles obtained from dynamic light scattering (DLS). Cartoons in c and d (top right) illustrate the corresponding copolymer morphologies.

Surface Chemistry Drives Occlusion

The incorporation of nanoparticles into calcite crystals exhibited striking, reproducible self-sorting behavior, primarily governed by surface chemistry rather than size or morphology. The smaller sulfate-rich spherical nanoparticles (S56-B500) preferentially incorporated into the internal core of the crystals, forming well-defined domains aligned with calcite facets, whereas the larger carboxylate-rich vesicles (M54-B200) localized predominantly near the crystal surfaces.

When both nanoparticles were present, they spontaneously segregated into distinct crystalline regions, demonstrating spatially selective occlusion resembling biological mineralization. This effect was not due to random partitioning but reflected a combination of calcium-dependent colloidal stability, ionic bridging, and specific nanoparticle–crystal surface interactions.

Dynamic light scattering revealed that M54-B200 vesicles exhibited reversible aggregation behavior in response to changes in calcium ion concentration during crystallization; they aggregated when Ca²⁺ levels were high and redispersed as Ca2+ was depleted, leading to delayed occlusion compared to S56-B500 spheres, which remained colloidally stable throughout.

Atomic force microscopy force spectroscopy provided mechanistic insight, showing that the polymer stabilizer chains on the vesicles (M54) had stronger binding interactions with calcite surfaces than the spheres’ stabilizers (S56), as indicated by longer contour lengths and comparable rupture forces.

Additional experiments varying nanoparticle size and morphology demonstrated that neither property determined occlusion location; rather, surface chemistry remained the primary determinant. For example, sulfate-functionalized vesicles of larger size still occluded preferentially in crystal cores. The conclusion was further supported by the observation that sulfate-like nanoparticle-coated silica microparticles localized in crystal cores, whereas carboxylate-coated metal–organic framework microparticles localized near the crystal surfaces. The corresponding uncoated microparticles were not incorporated into calcite.

The study also demonstrated the proof-of-concept potential of this self-sorting by performing acid-triggered dissolution experiments on the composite crystals. Nanoparticles near the surface were preferentially released first, followed by those embedded deeper, highlighting the potential of such materials for programmed, spatiotemporal delivery of functional nanoscale payloads, although no therapeutic cargo or biological delivery was tested.

These findings illuminate how subtle differences in the surface chemistry of nanoscale components drive their selective partitioning during crystallization. This mechanism may also be relevant to natural biomineralization processes, although substantial further research is required to verify this possibility. The results underscore the value of using well-defined polymer nanoparticle analogs as model systems to unravel complex organic–inorganic interactions.

Programmable Composite Crystals

This research establishes that the interfacial chemistry of nanoscale diblock copolymer nanoparticles is a primary determinant of their self-sorting occlusion within biomimetic calcite crystals. By precisely tailoring nanoparticle surface functionality, the study demonstrates programmable spatial segregation of distinct nanoparticles into preferentially defined crystalline regions.

Calcium-dependent colloidal stability, ionic bridging, and differences in polymer–mineral interactions collectively directed the particles into core and near-surface regions during successive stages of crystal growth.

Beyond providing fundamental insights into organic–inorganic interactions and biomineralization, the findings offer a promising strategy for designing multifunctional composite materials with spatially resolved domains.

Such materials may support the future design of mechanically graded composites and systems for controlled, spatiotemporal release of encapsulated nanoactives, although mechanical enhancement was not evaluated in this study. This control over crystal growth thus represents a useful conceptual advance in biomimetic materials science, with potential applications that require further development and validation.