In Situ X-ray Absorption Spectroscopy and Droplet-Based Microfluidics: An Analysis of Calcium Carbonate Precipitation

Droplet-based microfluidicsystems are ideally suited for the investigationof nucleation and crystallization processes. To best leverage thefeatures of such platforms (including exquisite time resolution andhigh-throughput operation), sensitive and in situ detection schemesare needed to extract real-time chemical information about all speciesof interest. In this regard, the extension of conventional (UV, visible,and infrared) optical detection schemes to the X-ray region of theelectromagnetic spectrum is of high current interest, as techniquessuch as X-ray absorption spectroscopy (XAS) provide for the element-specificinvestigation of the local chemical environment. Accordingly, herein,we report for the first time the integration of millisecond droplet-basedmicrofluidics with XAS. Such a platform allows for the sensitive acquisitionof X-ray absorption data from picoliter-volume droplets moving athigh linear velocities. Significantly, the high-temporal resolutionof the droplet-based microfluidic platform enables unprecedented accessto the early stages of the reaction. Using such an approach, we demonstratein situ monitoring of calcium carbonate precipitation by extractingXAS spectra at the early time points of the reaction with a dead timeas low as 10 ms. We obtain insights into the kinetics of the formationof amorphous calcium carbonate (ACC) as a first species during thecrystallization process by monitoring the proportion of calcium ionsconverted into ACC. Within the confined and homogeneous environmentof picoliter-volume droplets, the ACC content reaches 60% over thefirst 130 ms. More generally, the presented method offers new opportunitiesfor the real-time monitoring of fast chemical and biological processes.