Food Quality Monitoring Using A Low-Profile Multiplexed Harmonic Sensor

In this work, we present a low-profile, passive, multiplexed harmonic food sensor for wirelessly monitoring pH and ammonium (
$\text{NH}_{4}^{+}$
) in food products. The sensor incorporates a wideband slotted antenna that receives frequency-hopped interrogation signals (f), along with a pair of narrowband inverted-F antennas, amalgamated with pH and
$\text{NH}_{4}^{+}$
sensing electrodes, to retransmit the modulated second-harmonic signal (2f) back towards the receiver. This unique frequency orthogonality aids in mitigating possible signal interferences, such as clutters and crosstalks, particularly in environments with extensive signal scattering. Our findings indicate that changes in pH and ammonium levels can be individually discerned from the alterations in two peak frequencies within the frequency-hopping spread spectrum (FHSS), demonstrating a sensitivity of 6.2 MHz per unit change in pH and a sensitivity of 3.8 MHz/(mmol/L) for
$\text{NH}_{4}^{+}$
. Moreover, our experiments demonstrate the ability of the sensor to rapidly and precisely assess milk spoilage under practical conditions. This multiplexed food sensor may provide a trajectory towards enhanced product quality, diminished waste, as well as augmented consumer confidence in the food supply chain.