Abstract
We report an experimental approach for high-resolution real-time monitoring of transiently formed species occurring during the onset of precipitation of ionic solids from solution. This is made possible by real-time nuclear magnetic resonance (NMR) monitoring using dissolution dynamic nuclear polarization (D-DNP) to amplify signals of functional intermediates and is supported by turbidimetry, cryogenic electron microscopy, and solid-state NMR measurements. D-DNP can provide drastic signal improvements in NMR signal amplitudes, permitting dramatic reductions in acquisition times and thereby enabling us to probe fast interaction kinetics such as those underlying formation of prenucleation species (PNS) that precede solid–liquid phase separation. This experimental strategy allows for investigation of the formation of calcium phosphate (CaP)-based minerals by 31P NMR—a process of substantial industrial, geological, and biological interest. Thus far, many aspects of the mechanisms of CaP nucleation remain unclear due to the absence of experimental methods capable of accessing such processes on sufficiently short time scales. The approach reported here aims to address this by an improved characterization of the initial steps of CaP precipitation, permitting detection of PNS by NMR and determination of their formation rates, exchange dynamics, and sizes. Using D-DNP monitoring, we find that under our conditions (i) in the first 2 s after preparation of oversaturated calcium phosphate solutions, PNS with a hydrodynamic radius of Rh ≈ 1 nm is formed and (ii) following this rapid initial formation, the entire crystallization processes proceed on considerably longer time scales, requiring >20 s to form the final crystal phase.
Highlights
We report an experimental approach for high-resolution real-time monitoring of transiently formed species occurring during the onset of precipitation of ionic solids from solution
Some authors argue that prenucleation clusters (PNC) do not contradict the classical nucleation and growth theory (CNT), thereby alleviating the need of new theories.[13−16] According to this conception, stable precursors are often described as subcritical nuclei that possess a higher energetic state than the macroscopic crystal (>1 μm) but that remain in thermodynamic equilibrium with the solution.[13−16]
dissolution dynamic nuclear polarization (D-DNP) with real-time nuclear magnetic resonance (NMR) measurements, we developed the capability to directly observe and analyze transient prenucleation species (PNS) that may be present immediately after preparation of oversaturated
Summary
D-DNP with real-time NMR measurements, we developed the capability to directly observe and analyze transient PNS that may be present immediately after preparation of oversaturated. The D-DNP results provide key information about two distinct aspects of PNS formation: (i) the size and motional freedom of the developing solute species (via the 31P line widths) and (ii) the phosphate exchange rate (via the number of 31P signals observed). = 13 mM (Figure 3), indicates at most a 3-fold longer rotational diffusion time, corresponding to a maximum increase by a factor 31/3 (∼1.4) of the hydrodynamic radius Rh according to the Stokes−Einstein equation This enables an estimate of Rh < 0.9 nm for the hydrodynamic radius of the transiently observed Ca2+-binding Pi species that we detect here during the onset of calcium phosphate precipitation. In addition to the return to thermal equilibrium of the hyperpolarized 31P spins, the PPNS signal may decay for t > 2 s if the growth or aggregation of the PNS generates species with molecular weights that are too high to allow detection by solution-state NMR (i.e., postnucleation species).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
