Nitrite enhanced detection from saliva by simple geometrical modifications of paper-based micromixers

Dysregulation of nitric oxide (NO) and it's two relatively stable metabolites, nitrite, and nitrate, in SARS-CoV-2, are reported in infected populations, especially for nitrates levels > 68.4 mu mol/L. In this paper, we measure the abnormal presence of nitrite in the saliva by developing a cheap mu PAD for colorimetric detection through the modified Griess reaction. This includes a diazotization reaction between nitrite and Griess reagent, including Sulfanilamide and N-Naphthyl-ethylenediamine in an acidic medium, causing a pink Azo compound. The modifications are suggested by a numerical method model that couples the mass flux with the porosity medium equations (convection, diffusion and, dispersion) that improves the mixing process. The mixing index was quantified from the concentration deviation method via simulation of a homogeneous two-phase flow in a porous environment. Five mu PAD designs were fabricated to verify the simulation results of mixing enhancement on the Griess reactants in saliva samples. The investigated geometries include straight, helical, zig-zag, square wave, and inclined jagged shapes fabricated by direct laser writing, suitable for low cost, mass fabrication. Inclined jagged micromixer exhibited the best performance with up to 40% improvement compared with the simple straight geometry. Deliberate geometrical modifications, exemplified here in a jagged micromixer on paper, cut the limit of detection (LOD) by at least half without impacting the linear detection range.

Automated summary

This study developed a cheap mu PAD for colorimetric detection of nitrite in saliva to detect SARS-CoV-2 infection. By using the modified Griess reaction and numerical simulations, the researchers found that inclined jagged micro-mixers showed the best performance in enhancing the mixing process, leading to significant improvement in detection sensitivity.