Uncovering the Formation of Color Gradients for Glucose Colorimetric Assays on Microfluidic Paper-Based Analytical Devices by Mass Spectrometry Imaging.

This study describes the use of mass spectrometry imaging with matrix-assisted laser desorption/ionization (MALDI) and desorption electrospray ionization (DESI) to understand the color gradient generation commonly seen in microfluidic paper-based analytical devices (μPADs). The formation of color gradients significantly impacts assay sensitivity and reproducibility with μPADs but the mechanism for formation is poorly understood. The glucose enzymatic assay using potassium iodide (KI) as a chromogenic agent was selected to investigate the color gradient generated across a detection spot. Colorimetric measurements revealed that the relative standard deviation for the recorded pixel intensities ranged between 34 and 40%, compromising the analytical reliability. While a variety of hypotheses have been generated to explain this phenomenon, few studies have attempted to elucidate the mechanisms associated with its formation. Mass spectrometry imaging using MALDI and DESI was applied to understand the nonuniform color distribution on the detection zone. MALDI experiments were first explored to monitor the spatial distribution of the glucose oxidase and horseradish peroxidase mixture, before and after lateral flow assay with and without KI. MALDI(+)-TOF data revealed uniform enzyme distribution on the detection spots. On the other hand, after the complete assay DESI(-) measurements revealed a heterogeneous shape indicating the presence of iodide and triiodide ions at the zone edge. The reaction product (I) is transported by lateral flow toward the zone edge, generating the color gradient. Mass spectrometry imaging has been used for the first time to prove that color gradient forms as result of the mobility small molecules and not the enzyme distribution on μPAD surface.