Significance It is of fundamental importance to establish the microscopic picture of crystal nucleation for both academic and industrial research. Here, we report an in situ time-resolved Raman spectroscopy to study crystallization from solution, one crystal nucleation at a time. The observed dynamics of α-glycine crystallization from water without any additive supports the nonclassical nucleation pathway, where prenucleation aggregates form and convert to a crystal. By the direct comparison of Raman spectrum of the aggregates between the experiment and simulation, we propose the structure of these aggregates as linear hydrogen-bonded networks. We demonstrate the power of studying one nucleation event at a time, which can accelerate the investigation of crystal nucleation by optical spectroscopy.

<p>Crystallization is an important process in a wide range of disciplines from fundamental science to industrial application. Despite the importance of controlling the crystallization and its morphology (<i>e.g.</i> polymorphism), the lack of microscopic description of crystal nucleation often limits the rational approach to its engineering and control. The biggest challenge to experimentally track the nucleus formation is the stochastic and heterogeneous nature of the nucleation occurring at nanometer scale. To overcome this challenge, we developed a method we call “Single Nucleus Spectroscopy” or SNS and use it to follow the formation of single crystal glycine nucleus by Raman spectroscopy at 46 ms time resolution. The spectral evolution was analyzed by non-supervised spectral decomposition algorithm which unraveled the Raman spectrum of prenucleation aggregates. In order to gain microscopic insights into the structure of these aggregates we have established a direct comparison between the experiments and theoretical works. The outcome of our analysis is a new hypothesis of glycine crystal nucleation mechanism.<br></p>

<p>Crystallization is an important process in a wide range of disciplines from fundamental science to industrial application. Despite the importance of controlling the crystallization and its morphology (<i>e.g.</i> polymorphism), the lack of microscopic description of crystal nucleation often limits the rational approach to its engineering and control. The biggest challenge to experimentally track the nucleus formation is the stochastic and heterogeneous nature of the nucleation occurring at nanometer scale. To overcome this challenge, we developed a method we call “Single Nucleus Spectroscopy” or SNS and use it to follow the formation of single crystal glycine nucleus by Raman spectroscopy at 46 ms time resolution. The spectral evolution was analyzed by non-supervised spectral decomposition algorithm which unraveled the Raman spectrum of prenucleation aggregates. In order to gain microscopic insights into the structure of these aggregates we have established a direct comparison between the experiments and theoretical works. The outcome of our analysis is a new hypothesis of glycine crystal nucleation mechanism.<br></p>

<p>Crystallization is an important process in a wide range of disciplines from fundamental science to industrial application. Despite the importance of controlling the crystallization and its morphology (<i>e.g.</i> polymorphism), the lack of microscopic description of crystal nucleation often limits the rational approach to its engineering and control. The biggest challenge to experimentally track the nucleus formation is the stochastic and heterogeneous nature of the nucleation occurring at nanometer scale. To overcome this challenge, we developed a method we call “Single Nucleus Spectroscopy” or SNS and use it to follow the formation of single crystal glycine nucleus by Raman spectroscopy at 46 ms time resolution. The spectral evolution was analyzed by non-supervised spectral decomposition algorithm which unraveled the Raman spectrum of prenucleation aggregates. In order to gain microscopic insights into the structure of these aggregates we have established a direct comparison between the experiments and theoretical works. The outcome of our analysis is a new hypothesis of glycine crystal nucleation mechanism.<br></p>