Short-lived calcium carbonate precursors observed in situ via Bullet-dynamic nuclear polarization

The discovery of (meta)stable pre-nucleation species (PNS) challenges the established nucleation-
and-growth paradigm. While stable PNS with long lifetimes are readily accessible experimentally,
identifying and characterizing early-stage intermediates with short lifetimes remains challenging. We
demonstrate that species with lifetimes ≪ 5 s can be characterized by nuclear magnetic resonance
spectroscopy when boosted by ‘Bullet’ dynamic nuclear polarization (Bullet-DNP). We investigate the
previously elusive early-stage prenucleation of calcium carbonates in the highly supersaturated
concentration regime, characterizing species that form within milliseconds after the encounter of
calcium and carbonate ions and show that ionic pre-nucleation species not only govern the
solidification of calcium carbonates at weak oversaturation but also initiate rapid precipitation events
at high concentrations. Such, we report a transient co-existence of two PNS with distinct molecular
sizes and different compositions. This methodological advance may open new possibilities for
studying and exploiting carbonate-based material formation in unexplored parts of the phase space.