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

Abstract

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 μmol/L. In this paper, we measure the abnormal presence of nitrite in the saliva by developing a cheap μPAD 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 μPAD 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.

Application of Paper-Based Microfluidic Analytical Devices (µPAD) in Forensic and Clinical Toxicology: A Review

“…In fact, in several cases of fatal or nonfatal intoxication, nitrite has been used, either accidentally or deliberately, in suicidal or homicidal contexts [133]. Most of the rapid methods for the direct detection of nitrite using paper-based devices have so far been performed using the Griess colorimetric reaction, or a modified version of the reaction [36,[134][135][136][137][138] in which citric acid was replaced by phosphoric acid or hydrochloric acid [139][140][141][142]. Because of the importance of a high sensitivity, the approach proposed by Zhang et al looks particularly interesting because the authors, integrating an electrokinetic stacking mechanism, increased the analytical sensitivity of the device up to 160-fold, which was able to detect nitrite down to 0.9 µmol/L [134].…”