Microscopic structure of the polymer-induced liquid precursor for calcium carbonate

Yifei Xu,Nico A J M Sommerdijk,Koen C H Tijssen,Arno P M Kentgens,Anat Akiva,Heiner Friedrich,Paul H H Bomans

doi:10.1038/s41467-018-05006-w

Yifei Xu, Nico A J M Sommerdijk + Show 5 more

Open Access

https://doi.org/10.1038/s41467-018-05006-w

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Abstract

Many biomineral crystals form complex non-equilibrium shapes, often via transient amorphous precursors. Also in vitro crystals can be grown with non-equilibrium morphologies, such as thin films or nanorods. In many cases this involves charged polymeric additives that form a polymer-induced liquid precursor (PILP). Here, we investigate the CaCO3 based PILP process with a variety of techniques including cryoTEM and NMR. The initial products are 30–50 nm amorphous calcium carbonate (ACC) nanoparticles with ~2 nm nanoparticulate texture. We show the polymers strongly interact with ACC in the early stages, and become excluded during crystallization, with no liquid–liquid phase separation detected during the process. Our results suggest that “PILP” is actually a polymer-driven assembly of ACC clusters, and that its liquid-like behavior at the macroscopic level is due to the small size and surface properties of the assemblies. We propose that a similar biopolymer-stabilized nanogranular phase may be active in biomineralization.

Highlights

Many biomineral crystals form complex non-equilibrium shapes, often via transient amorphous precursors
The presence of charged polymers such as poly(aspartic acid)[18], poly(acrylic acid)[17,24], poly(allylamine hydrochloride)[19], and double-stranded DNA23 can stabilize amorphous calcium carbonate (ACC)[25], and under specific conditions lead to the formation of a liquid-like precursor, which has been explored in the formation of crystals with non-equilibrium morphologies[18,19,20,21,22]
polymer-induced liquid precursor (PILP) consists of ACC NPs with a ~2 nm nanoparticulate texture, and no other product was detected before the formation of this phase in our experiments (Supplementary Figs. 8, 9)

Summary

Introduction

Many biomineral crystals form complex non-equilibrium shapes, often via transient amorphous precursors. It was reported that droplets (Ø 100 nm–5 μm) of this so-called polymer-induced liquid precursor (PILP)[26] can coalesce and form thin films on solid substrates[18,19,20], or infiltrate into nanopores[20,21], where they transform into solid ACC and subsequently crystallize to calcite or vaterite[18], with their morphologies preserved The formation of these non-equilibrium morphologies have been attributed to the liquid-like nature of PILP, being able to wet the solid substrates, or to be capillarilly absorbed into the nanopores[18,21]. Beyond the importance for achieving control over the morphology of crystalline materials, understanding the PILP process may lead to mechanistic insights into the formation processes of biominerals

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