Plasmonic droplet screen for single-cell secretion analysis.

Single-cell secretion analysis technologies are needed to elucidate the heterogeneity of cellular functionalities. Although ligand binding assays in microwells provide a promising approach for measuring single-cell secretions, their throughput is limited. Recently, droplet assays have been developed for high-throughput single-cell screening. However, because washing steps are difficult to perform with droplets, there are still challenges in measuring secretions using droplet assays. In this study, a plasmonic droplet screen approach is developed for one-step washing-free multiplex detection of single-cell secretions. Individual cells are encapsulated with antibody-conjugated gold nanorods (AuNRs) in droplets to evaluate their secretion levels. The shift in the plasmon resonance peak reflects the amount of secreted protein without needing additional indicator and washing steps. The plasmonic signals from a continuous flow of single-cell droplets are collected by dark-field spectroscopy (∼100-150 cells min). This platform is tested by screening interleukin-8 (IL-8) and vascular endothelial growth factor (VEGF) secreted from suspended leukemia cells and adherent breast cancer cells. Overall, this novel strategy shows the potential and flexibility of high-efficiency multiplex single-cell secretion analysis.