Argon Enclosed Droplet Based 3D Microfluidic Device Online Coupled with Time-Resolved ICPMS for Determination of Cadmium and Zinc in Single Cells Exposed to Cadmium Ion

Time-resolved (TRA)-ICPMS has become a booming subfield of single-cell analysis tools in recent years, while generation of single cells remains the major challenge. Microfluidic devices reveal their great capability and potential in encapsulation of single cells into water droplets. However, current strategies to pinch off droplets require a specific oil phase, which is not compatible to conventional ICPMS and makes the signal of cells in the water phase susceptible. Herein, we built a 3D water-in-gas microfluidic device (3D W/G MFD) with commercially available components, producing single cell droplet enclosed by argon gas. By simply tuning the flow rate of gas and water, the droplets were generated to encapsulate single cells, which significantly reduced the probability of the single signal coming from multiple cells by 1 or 2 orders of magnitude compared to direct injection. The developed oil-free 3D W/G MFD was more friendly to online coupling with TRA-ICPMS than water-in-oil devices. The effect of Cd2+ on HepG2 cells was studied by single cell detecting total Zn with 3D W/G MFD-TRA-ICPMS, and the variation of labile Zn was explored by flow cytometry with an N-(6-methoxy-8-quinolyl)-p-toluenesulfonamide probe. To the best of our knowledge, this work pioneered the exploration of variation in cellular metal content and speciation at the single-cell level, compensating for the deficiency of speciation analysis based on TRA-ICPMS and providing new insights into exploring the complexity of biology.