Portable device for protease detection. (a) Structural illustration of the flow-cell device on a face covering for colorimetric sensing. The vents allow the transmitted respiratory droplets to be concentrated on the absorbent pad. The reagent yielding color change is packed in the reservoir. More details about the flow strip can be found in Table S1. (b) A white-light image of the assembled sensing trip, equipped with a BSPP-AuNP dispersion in the blister pack, ZY7 peptide on the test lane (left, dash box), and YF15 peptide on the positive-control lane (right, dash box). Scale bar of 1 cm is shown. (c) The sensing strip’s LoD for Mpro is estimated to be 30–40 nM. Monodispersed BSPP-AuNPs appear as pink-red, while the clustered gold aggregates by peptide fragments appear as violet-blue. (d) Strip testing of Mpro marker on COVID-negative participants (NP#, n=10). All test lanes show pink-red in the absence of SARS-CoV-2 protease. SEM images of non-aggregating AuNPs on the red lane (e) and clustered AuNPs (f) on the purple lane. (g) Sensor testing on aqueous EBC matrices collected from COVID-negative subjects (n=10). No absorbance ratio change/false positive was noticed.