Fig. 4

The use of PMA–LA invisible ink to fabricate paper-based smiley sensors. a Photographs of three paper-based UV sensors with increasing exposure time and the corresponding cumulative effective doses of UVA, B, and C. Only selected time points of photographs are shown for clarity. The sensor response at the UVB MED doses of skin types I–VI is highlighted. The 0 s time point represents invisible smileys drawn on filter papers using PMA–LA ink, followed by drying. b–d Reflectance spectra of smileys shown in a on exposure to UVA, B, and C, respectively, with increasing UVR doses as indicated by arrows. In comparison to 16 dose points in a, reflectance spectra in b–d correspond to all the 35 tested doses. e UVR dose-dependent colorimetric response of PMA–LA smiley sensors demonstrating the sensor’s ability to differentiate UVA, B, and C even at extremely low dosages, as reflected from the logarithmic X-axis. Each data point represents an average of the colorimetric response obtained from four independent sensors and associated standard deviation. f–h The precision of PMA–LA smiley sensors at each of the tested UV doses, as calculated from the data presented in e