Fig. 1.

Conversion of an NFC tag into a CARD enables wireless rf detection of chemical analytes with a smartphone. NFC-enabled smartphones communicate with NFC tags by simultaneously energizing the NFC tag with an alternating magnetic field (f = 13.56 MHz) through inductive coupling and transferring data by signal modulation. NFC tags are converted into CARDs by disrupting the LCR circuit (step 1) and recompleting the circuit with a stimuli-responsive variable circuit component by drawing (step 2) with solid sensing materials.