Microfluidic Device with Room Temperature Ionic Liquids for Detecting Low Concentrations of CO2

Abstract

In this work we report the use of a gas sensor that can be used in inline configuration for applications like organ-on-a-chip. The key element of the device is a microfluidic channel with a thickness of 140 μm and width of 500 μm. This microfluidic channel is sandwiched between a top and bottom substrate, each substrate containing microelectrodes making this a non-planar interdigitated microelectrode (NP-IμE) configuration. Room temperature ionic liquids (RTILs) are used as electrolyte in the microfluidic channel. Due to the dimensions of the microfluidic channel only low volume of RTILs are required as electrolyte. RTILs can be tuned to selectively capture specific gases. This in addition to other favourable properties such as wide electrochemical window, thermal stability and negligible vapor pressure makes it a good candidate as the selective electrolyte in electrochemical gas sensor. The use of NP-IμE makes the electric field penetrate through the channel. This allows us to register the perturbations in signal caused by absorption of CO2 from the entire cross-section of the channel, thereby increasing the sensitivity of the device (as compared to a planar architecture). 1-ethyl-3-methylimidazolium 2-cyanopyrolide ([EMIM][2-CNpyr]) was used as the RTIL, as it has been shown to be a good sorbent of CO2 with high capacity at low partial pressures due to its chemical affinity. The RTILs exposed to different concentrations of CO2 was injected into the microfluidic channel. Cyclic Voltammetry (CV), Differential pulse voltammetry (DPV) and Electrochemical impedance spectroscopy (EIS) experiments were done using our microfluidic device. During the CV and DPV experiments, RTIL exposed to CO2 shows a reduction peak (of CO2). This along with the changes in EIS spectrum (Nyquist plot) can be used to detect the presence of CO2. We used CO2 as a test gas to analyse the sensitivity of our microfluidic device combined with RTIL. In future we will be using this same setup to sense biomarker gases and other toxic gases by changing the RTIL and tuning its affinity towards the target analytes.