An Energy Efficient Thermally Regulated Optical Cell for Lab-on-Chip Devices: Applied to Nitrate and Nitrite Detection.

Abstract

Reagent-based colourimetric analyzers that are intended to operate for significant durations in the field typically aim to conserve energy use per measurement. In many cases, the fluid under analysis must be elevated to a temperature above that of the environment to improve reaction kinetics. In these cases, it is important to minimize the amount of energy lost to the environment in the form of heat while thermally controlling the optical cell. Here, a novel method of conserving heat energy on microfluidic systems is presented. The design minimizes heat transfer to the environment by surrounding the heated optical cell on four sides with integral air-pockets, thereby creating an insulated and suspended bridge structure. This unique microfluidic design was simulated in COMSOL and then verified in a polymethyl methacrylate (PMMA) device constructed using rapid prototyping approaches. To evaluate the effectiveness of the insulated bridge approach, the insulated design is compared to a monolithic non-insulated design without air-pockets. One 25 mm absorbance cell was suspended within an insulated bridge, and one 25 mm absorbance cell was created in a slab of monolithic PMMA. To hold the insulated cell at 35 °C, 45 °C, and 55 °C, power required was reduced by 49.3% on average in simulation and 40.2% on average in experiment when compared to the standard cell. Both insulated and non-insulated cell designs were then applied to a commonly used colourimetric assay to measure nitrate. Nitrate was reduced to nitrite using the vanadium (III) chloride method, then converted to a coloured azo dye with the well-established Griess method. The colourimetric reaction kinetics were studied at 22 °C and 41 °C, resulting in 95% colour development at 225 minutes and 20 minutes, respectively. The colourimetric method was applied to insulated and non-insulated designs at 35 °C for nitrate concentrations from 0.25 µM to 50 µM. A reduction in heating energy from 195 J to 119 J was demonstrated while preserving the expected linearity and limits of detection of 20 nM. By suspending the optical cell or inlaid microchannel within a chip, the approach avoids heating the thermal mass of the rest of the device and any interconnects. This design will have broad applicability to numerous chemical protocols that rely on optical absorbance measurements performed in situ for marine environments.