Selective Encapsulation of Single Cells and Subcellular Organelles into Picoliter- and Femtoliter-Volume Droplets

TLDR

The paper presents a method that combines optical trapping with microfluidic droplet generation to selectively encapsulate a single cell or subcellular organelle in picoliter- or femtoliter-volume aqueous droplets. The technique positions the target by optical trapping, then generates a droplet that stably confines it; rapid laser photolysis freezes the cell’s state and confines the lysate, enabling downstream assays such as β‑galactosidase activity measurement. Encapsulation results in stable confinement, and laser‑induced lysis within the droplet permits rapid enzyme activity readouts, illustrating the platform’s promise for single‑cell and single‑organelle investigations.

Abstract

This paper describes a method, which combines optical trapping and microfluidic-based droplet generation, for selectively and controllably encapsulating a single target cell or subcellular structure, such as a mitochondrion, into a picoliter- or femtoliter-volume aqueous droplet that is surrounded by an immiscible phase. Once the selected cell or organelle is encased within the droplet, it is stably confined in the droplet and cannot be removed. We demonstrate in droplet the rapid laser photolysis of the single cell, which essentially "freezes" the state that the cell was in at the moment of photolysis and confines the lysate within the small volume of the droplet. Using fluorescein di-β-d-galactopyranoside, which is a fluorogenic substrate for the intracellular enzyme β-galactosidase, we also assayed the activity of this enzyme from a single cell following the laser-induced lysis of the cell. This ability to entrap individual selected cells or subcellular organelles should open new possibilities for carrying out single-cell studies and single-organelle measurements.