Building a cost-effective mechanochemical Raman system: improved spectral and time resolution for in situ reaction and rheology monitoring

Abstract

Raman spectroscopy has become an indispensable tool for in operando monitoring in preparative mechanochemistry due to its ability to provide real-time, non-invasive insight into solid-state reactions. While commercial systems based on fiber Raman probes offer ease of use and plug-and-play deployment, their relatively low spectral resolution and lower intensity make them less suitable for the specific demands of mechanochemical reaction monitoring. To address these challenges and make this valuable methodology widely available, we describe a high sensitivity free-optics Raman system, termed mcRS (mechanochemical Raman System), constructed from the ground up using affordable off-the-shelf optical components. The mcRS includes a free-space Raman probe and a custom-designed dispersive spectrometer utilizing only lenses, paired with a low-cost industrial-grade CMOS camera as the detector. By fine-tuning the optics to minimize photon loss and achieving tighter spot sizes on the detector, the mcRS provides a 30% improvement in spectral resolution compared to previous inhouse fiber based systems, for half of the cost, and offers a five-fold reduction in price compared to commercial systems. This is accompanied by a five-fold improvement in time resolution and a novel feature, the possibility to simultaneously collect Raman data and monitor rheology of the sample, which often plays an important role in mechanochemical reactions. We validated the mcRS performance by monitoring the reaction between ZnO and imidazole under Neat Grinding (NG) and Liquid-Assisted Grinding (LAG) conditions, using ethanol as the liquid additive. The enhanced capabilities of the mcRS offer significant advancements in the in situ studies of mechanochemical processes, allowing for differentiation between two Zeolitic Imidazolate Framework products based on subtle differences in the high-frequency modes (3100-3200 cm^-1).