Lecture Series SCIENCE WITH IMPACT: Drop by drop: Multiplexed and multimodal analysis of cells and chemical reactions

Droplet microfluidics is a particularly powerful method for screening applications e.g. for protein engineering and single-cell studies. Monodisperse aqueous droplets of pico- or nanoliter volume immersed in a hydrophobic fluid are formed in a microfluidic device at kHz frequencies. In recent years, droplet microfluidics has been employed for analysis of bioanalytical assays and chemical synthesis, such as kinetic studies of enzymes, cell-free protein synthesis, protein crystallization, nanoparticle formation and many more. In the field of single-cell analysis including single-cell sequencing, droplet microfluidics is nowadays a well-approved method and alternative to cytometry. In the first part of the presentation, I will show applications of droplet microfluidics, where the compartmentalization is of high importance, e.g., when secreted compounds of cells are analyzed.

Most assays in nL droplets, however, are based on fluorescence spectroscopy, which limits the choice of assays and multiplexing capability. Mass spectrometry, on the other hand, allows for label-free detection and identification of multiple components. Recently, we have interfaced droplet microfluidics with matrix-assisted laser desorption/ionization (MALDI)-MS. Instead of the standard MALDI targets where the sample is pipetted in 384 wells, we have fabricated custom-made, transparent, indium-tin oxide coated targets, on which thousands of aqueous nL-droplets reside on the surface, covered by fluorinated oil. Analysis of this droplet array is performed by optical and fluorescence microscopy as well as by means of a MALDI-MS imaging system (Bruker rapifleX). We employ the method for single-cell studies at high throughput, e.g. analysis of biosynthesized enzymes, which will be discussed in the second part of the presentation. First steps towards extremely high throughput measurements will be introduced with our recently fabricated plates hosting up to 300 000 droplets, which poses additional challenges for automated data acquisition and analysis.