Capillary Electrophoresis Separation in the Presence of an Immiscible Boundary for Droplet Analysis

Abstract

This paper demonstrates the ability to use capillary electrophoresis (CE) separation coupled with laser-induced fluorescence for analyzing the contents of single femtoliter-volume aqueous droplets. A single droplet was formed using a T-channel (3 microm wide by 3 microm tall) connected to microinjectors, and then the droplet was fluidically moved to an immiscible boundary that isolates the CE channel (50 microm wide by 50 microm tall) from the droplet generation region. Fusion of the aqueous droplet with the immiscible boundary effectively injects the droplet content into the separation channel. In addition to injecting the contents of droplets, we found aqueous samples can be introduced directly into the separation channel by reversibly penetrating and resealing the immiscible partition. Because droplet generation in channels requires hydrophobic surfaces, we have also investigated the advantages to using all hydrophobic channels versus channel systems with patterned hydrophobic and hydrophilic regions. To fabricate devices with patterned surface chemistry, we have developed a simple strategy based on differential wetting to deposit selectively a hydrophilic polymer (poly(styrenesulfonate)) onto desired regions of the microfluidic chip. Finally, we applied our device to the separation of a simple mixture of fluorescein-labeled amino acids contained within a approximately 10-fL droplet.

Dilution‐Free Analysis from Picoliter Droplets by Nano‐Electrospray Ionization Mass Spectrometry

“…Transferring droplets from oil to aqueous streams while, e.g., keeping the carrier liquids separate has proven difficult. [18] Previous efforts have utilized spatially selective surface coating[18,19] to establish a boundary between the hydrophobic and aqueous phases or synchronized electrical pulses from embedded electrodes to drive the droplets from the oil to an aqueous stream. [20] The latter approach was recently used to analyze droplet contents by electrospray ionization (ESI) MS.[21] The aqueous channel into which the droplets were extracted operated in excess of 4 μL/min and the contents were flowed from the chip through tubing to a conventional, external sheath flow electrospray source.…”

A Microfluidic Chip for ICPMS Sample Introduction

“…The current chip is designed for sample introduction only. However, with slight changes of the design advanced introduction and sample preparation steps cloud be implemented on-chip, e.g., dilution [27][28][29] , ultra fast mixing 30 , chemical reactions 31 , separation [32][33][34] , or cell sorting 35,36 . Under advanced introduction devices we understand for example the introduction of sample and standard droplets sequentially or parallel with a single chip.…”

Electrical power free, low dead volume, pressure-driven pumping for microfluidic applications

“…28 Additionally, droplet generation in microfluidic devices has been used for single cell analysis, 4,29 electrophoretic separation, 30 and DNA analysis. 31 To further demonstrate the capabilities of the pump, we used it to generate water droplets in a continuous phase of silicone oil through hydrodynamic focusing of a stream of water in a cross-shaped microchannel ͓Fig.…”