A microfluidic chip capable of switching W/O droplets to vertical laminar flow for electrochemical detection of droplet contents

• PDMS-glass hybrid channel was wettability-patterned via local silane coating.
• Droplet flow could be switched to vertical laminar flow in the channel.
• On-chip amperometric detection was performed after the flow-profile switching.
• Analytical performance for detection of droplet contents significantly improved.
• The developed chip has been applied to kinetics study of enzyme reaction.

Analysis of droplet contents is a key function involved in droplet-based microfluidic systems. Direct electrochemical detection of droplet contents suffers problems such as relatively poor repeatability, interference of capacitive current and relatively poor detectability. This paper presents a novel hybrid polydimethylsiloxane-glass chip for highly sensitive and reproducible amperometric detection of droplet contents. By wettability-patterning of the channel surface of the hybrid chip, water in oil droplets generated in the upstream part of the central channel can be switched to a two-phase vertical laminar flow (i.e., a continuous oil stream flowing atop a continuous aqueous stream) in the downstream part of the channel. The vertical laminar flow keeps the analyte in the underneath-flowing aqueous stream in direct contact with the sensing electrodes located on the bottom surface of the channel. Therefore, steady-state current signals with high sensitivity (1.2 A M−1 cm−2 for H2O2), low limit of detection (0.12 μM, S/N = 2), and good reproducibility (RSD 1.1% at 0.3 mM H2O2) were obtained. The methods for patterning of the inner channel surface are presented, and the behaviors of the microchip in flow profile switching and amperometric detection are discussed. The application of the developed microchip to enzyme kinetics study is also demonstrated.

The upstream part of the hybrid polydimethylsiloxane-glass channel was hydrophobilized with octadecyltrichlorosilane while the downstream part remained hydrophobic for upper polydimethylsiloxane (PDMS) and hydrophilic for glass bottom, as the insert shows. Once the water in oil (W/O) droplets were transported to the downstream channel, they were switched to an oil-atop-water vertical laminar flow, ensuring direct contact between the analyte in aqueous phase and the electrodes on the glass bottom. The figure shows i–t signals obtained with a K3Fe(CN)6 sample solution that was operated in the flow profiles of two-phase W/O droplet flow (a), two-phase vertical laminar flow converted from W/O droplets (b), and single-phase aqueous flow (c).Figure optionsDownload as PowerPoint slide