We next performed the assay of cocaine dissolved in aqueous
solution with this paper device. Into the PEI-UCNPs and NH2-
ACA-1 immobilized test zones, fixed amounts of AuNPs-ACA-2
and cocaine solutions with varying concentrations were added
and kept for 40 min. The gradual decrease of luminescence
intensity along with the increase of cocaine concentration can
be either distinguished by naked eyes or collected by a
smartphone camera. Using the extracted RGB intensities, we
calculated the luminescence quenching efficiency of this system
((F0 − F)/F0, in which F and F0 are the green-channel
intensities of the paper device in the presence and absence of
cocaine, respectively). We found that it was dependent on the
concentration of cocaine over a wide dynamic range (Figure
2a), and a linear dependence was obtained with a quantification