Real-time frequency-domain terahertz sensing and imaging of isopropyl alcohol–water mixtures on a microfluidic chip

We have developed a real-time chemical sensing methodology that integrates transmission terahertz frequency-domain spectroscopy (THz-FDS) with a quartz-based microuidic subsystem. The initial examination of isopropyl alcohol–water (IPA/water) mixtures at frequencies from 0.570 to 0.630 THz exhibited well-behaved transmission performance, showing the potential to reveal detailed spectral information that is usually absent in THz time-domain spectroscopy (THz-TDS). Linear least-squares regression plots at three selected THz frequencies, i.e. 0.580, 0.590 and 0.625 THz, suggested excellent linear signal responses over the entire IPA concentration range with detection responsivities greater than 4.6712 μV/% with R2 > 0.953. The system can potentially achieve a high detection sensitivity owning to the low THz detector noise. The real-time monitoring of sequentially injected IPA/water mixtures displayed fast response potentially approaching ~10 ns which is suitable for studying ultrafast in situ dynamic chemical and biological phenomena. The THz mapping capability of two-stream laminar flow was also demonstrated. The THz microfluidic sensing strategy represents a versatile approach to enable broadband, high spectral and spatial resolution, as well as fast responses for a wide range of chemical and biological applications including real-time, label-free monitoring, sensing and imaging.