Inlaid microfluidic optics: absorbance cells in clear devices applied to nitrite and phosphate detection

A novel type of microfluidic absorbance cell is presented here that inlays black poly(methyl methacrylate) (PMMA) into a clear PMMA substrate to realize an isolated optical channel with microlitre volumes. Optical measurements are frequently performed on microfluidic devices, offering effective, quick, and robust chemical analysis capabilities on small amounts of sample. Many lab-on-chip systems utilize micrometer-sized channels to analyze liquid samples via light-absorbance measurements, but this requires sophisticated coordination of light through a small cross-section, often requiring collimating and beam-steering optics. Here, we detail the fabrication process to realize long path length absorbance cells based on a simple hybrid-material approach. A z-shape microchannel structure crosses a clear-black interface at both ends of the absorbance cell, thereby creating integral optical windows that permit light coupling into a microchannel completely embedded in black PMMA. Furthermore, we have integrated v-groove prisms on either side of the microfluidic channel. The prisms enabled seamless integration with printed circuit boards and permit the optical elements to be located off-chip without use of epoxies or adhesives. Three path lengths, 10.4, 25.4, and 50.4 mm, were created and used to characterize the novel cell design using typical colorimetric measurements for nitrite and phosphate. We compare the attenuation coefficient measured by our optical cells with the literature, showing excellent agreement across nutrient concentrations from 50 nM-50 μM. The measurements were performed with well-known reagent-based methods, namely the Griess assay for nitrite and the molybdovanadophosphoric acid or the 'yellow method' for phosphate. The longest 50.4 mm path length cell had a limit-of-detection of 6 nM for nitrite and 40 nM for phosphate, using less than 12 μl of fluid. The inlaid fabrication method described permits robust and high-performance optical measurements with broad applicability for in situ marine sensors and for numerous lab-on-chip sensors based on colorimetric assays. One such application is shown whereby two inlaid absorbance cells are integrated with four microfluidic check valves to realize a complete lab-on-chip nitrite sensor.