Wireless oxygen sensors enabled by near-field communication (NFC) tag. (a) Schematic drawing of a smart packaging that measures oxygen exposure from the inside. A radio frequency reader, in this case, a smartphone, can remotely read the sensor’s state, and access the quality of the package contents without a line of sight. (b) Schematic of poly(4-vinylpyridine)-dispersed SWCNTs and coordinating FeII ions as the sensing element. (c) Sensing responses to various O2 concentrations (diluted in N2). The inset shows the calibration curve of the sensor. Reproduced with permission from Ref. 189. Copyright 2017, American Chemical Society.