Here, quercetin-mediated silver nanoparticle (AgNP) formation combined with loop-mediated isothermal amplification (LAMP) was introduced to colorimetrically detect two major infectious pathogens, SARS-CoV-2 and Enterococcus faecium, using a foldable PMMA microdevice. The nitrogenous bases of LAMP amplicons can readily form a complex with Ag+ ions, and the catechol moiety in quercetin, which acted as a reducing agent, could be chelated with Ag+ ions, resulting in the easy electron transfer from the oxidant to the reductant and producing brown-colored AgNPs within 5 min. The introduced method exhibited higher sensitivity than agarose gel electrophoresis due to more active redox centers in quercetin. The detection limit was attained at 101 copies μL-1 and 101 CFU mL-1 for SARS-CoV-2 RNA and E. faecium, respectively. A foldable microdevice made of two pieces of PMMA that fully integrates DNA extraction, amplification, and detection processes was fabricated to establish practical applicability. On one PMMA, DNA extraction was performed in a reaction chamber inserted with an FTA card, and then LAMP reagents were added for amplification. Silver nitrate was added to the reaction chamber after LAMP. On the other PMMA, quercetin-soaked paper discs loaded in the detection chamber were folded toward the reaction chamber for colorimetric detection. An intense brown color was produced within 5 min when heated at 65 °C. The introduced colorimetric assay, which is highly favorable for laboratory and on-site applications, could be a valuable alternative to conventional methods for detecting infectious diseases, given its unique principle, simplicity, and naked-eye detection.
Keywords: colorimetric detection; infectious pathogens; loop-mediated isothermal amplification; quercetin; silver nanoparticles.