Monitoring Crystallization Processes in Confined Porous Materials by Dynamic Nuclear Polarization Solid-State Nuclear Magnetic Resonance

Researchers from the SACS and CMO teams showed that solid-state NMR, combined with the sensitivity enhancement of dynamic nuclear polarization (DNP), enables efficient detection of metastable, transient species formed at the early stages of crystallization processes occurring in pure water under nanoconfinement. The approach relies on the use of a new class of polarizing agents composed of mesoporous silica materials with face-to-face TEMPO radicals embedded in the walls of the material, developed at the ICR. The results open new perspectives on the possibility of unveiling the mechanistic aspect of nucleation at an atomic level.

Establishing mechanistic understanding of crystallization processes at the molecular level is challenging, as it requires both the detection of transient solid phases and monitoring the evolution of both liquid and solid phases as a function of time. Here, we demonstrate the application of dynamic nuclear polarization (DNP) enhanced NMR spectroscopy to study crystallization under nanoscopic confinement, revealing a viable approach to interrogate different stages of crystallization processes. We focus on crystallization of glycine within the nanometric pores (7−8 nm) of a tailored mesoporous SBA-15 silica material with wall-embedded TEMPO radicals. The results show that the early stages of crystallization, characterized by the transition from the solution phase to the first crystalline phase, are straightforwardly observed using this experimental strategy. Importantly, the NMR sensitivity enhancement provided by DNP allows the detection of intermediate phases that would not be observable using standard solid-state NMR experiments. Our results also show that the metastable β polymorph of glycine, which has only transient existence under bulk crystallization conditions, remains trapped within the pores of the mesoporous SBA-15 silica material for more than 200 days.

Collaboration : Prof. Kenneth D.M. Harris, University of Cardiff (UK)

Fundings :

– ERC StG « STRUCTURE » GA 758498

– A*MIDEX Programme “Investissements d’Avenir” (Grant ANR-11-IDEX-0001-02)