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

Abstract

Reagent-based colorimetric analyzers often heat the fluid under analysis for improved reaction kinetics, whilst also aiming to minimize energy use per measurement. Here, a novel method of conserving heat energy on such microfluidic systems is presented. Our design reduces heat transfer to the environment by surrounding the heated optical cell on four sides with integral air pockets, thereby realizing an insulated and suspended bridge structure. Our design was simulated in COMSOL Multiphysics and verified in a polymethyl methacrylate (PMMA) device. We evaluate the effectiveness of the insulated design by comparing it to a non-insulated cell. For temperatures up to 55 °C, the average power consumption was reduced by 49.3% in the simulation and 40.2% in the experiment. The designs were then characterized with the vanadium and Griess reagent assay for nitrate at 35 °C. Nitrate concentrations from 0.25 µM to 50 µM were tested and yielded the expected linear relationship with a limit of detection of 20 nM. We show a reduction in energy consumption from 195 J to 119 J per 10 min measurement using only 4 µL of fluid. Efficient heating on-chip will have broad applicability to numerous colorimetric assays.

Two chemistries on a single lab-on-chip: Nitrate and orthophosphate sensing underwater with inlaid microfluidics

“…). Finally, the preparation of the nitrate reagent, and of both phosphate reagents, used for each colourimetric assay followed previous works (Murphy et al, 2021;Morgan et al, 2022). Nitrate Reagent was prepared such that 2.5000 g of vanadium (III) chloride, 15 ml of concentrated hydrochloric acid (HCl), 0.1250 g of NEDD (N-(1-Naphthyl)ethylenediamine dihydrochloride, CAS-No.…”

“…Consequently, development of automated in situ sensors have focused on the most abundant fixed forms of N or P that directly limit primary production: nitrate (NO − 3 ) or dissolved orthophosphate (PO 3− 4 ). The gold-standard approaches toward nitrate and dissolved orthophosphate measurement rely on reagent-based colorimetry and have been shown to achieve nanomolar limits of detection (Murphy et al, 2021;Morgan et al, 2022). These techniques have been applied towards benchtop & portable nutrient detection platforms (McCaul et al, 2021), mixing one or several specifically crafted reagents with sample to generate a colored complex.…”

“…The novel NP Sensor combines the "fluid engine" of our previous design with a custom designed lab-on-chip that was built to perform two chemistries within the same instrument. Both nutrients are measured using two different reagent-based colourimetric assays: a modified Griess assay for nitrate detection via vanadium (III) chloride reduction to nitrite (Nightingale et al, 2018;Murphy et al, 2021), and the phosphomolybdenum blue (PMB) assay for phosphate (Grand et al, 2017;Morgan et al, 2022). A detailed discussion on the specificity of these assays in the context of marine environments can be found in the Supplementary Material.…”

Simultaneous Absorbance and Fluorescence Measurements Using an Inlaid Microfluidic Approach

“…The limit of detection (LOD) can be defined as three times the standard deviation using the triple-sigma method [ 27 , 28 , 43 , 44 ]. The LOD for absorbance was calculated using the blanks preceding the 0.1 µM measurements, which ensured minimum crosstalk for evaluating the LOD.…”

“…In summary, the novel optical cell presented here was thoroughly characterized and could be used in conjunction with numerous assays and a wide range of other lab-on-chip technologies. For example, embedded heaters as described in the work of Murphy et al [ 44 ] could be integrated with this fluorescence chamber to realize an in situ RT-PCR chip. Similarly, the optical cell presented here can be readily integrated with on-chip valving solutions to control reagents and fluid flow for sample preparation, such as in the work of Luy et al [ 27 ] or Ogilvie et al [ 52 ], where reagents are combined with seawater samples for on-chip spectroscopy.…”