A Noninvasive Triboelectric Fruit Freshness Sensor with ZnO and PVA Layers

Here, we report the development of a triboelectric fruit freshness sensor (TFFS) as a pathway for a noninvasive hassle-free technique to monitor fruit quality with time. The triboelectric nanogenerator (TENG)-based flexible sensor has been fabricated using a dual-layer combination of a ZnO nanowall film and a PVA layer, which offers a superior output (∼55% higher than bare ZnO) by virtue of triboelectrification at two interfaces. The strategical inclusion of the PVA layer offers a superior humidity sensing ability TENG with a sensitivity of 0.4 V/% RH. Probing the surface of fruits like banana, tomato, etc., through a simple touch, a rising trend of TFFS output is observed that corroborates to the postharvest water loss event. The attainment of the highest output indicates the ripening stage, which is also supported by the visible impression. With preserving days, the declining nature of the output beyond the peak indicates the aging-related surface degradation due to reduction in firmness, moisture settlement, etc. Specifically, the TFFS exhibits a distinct output drop pattern with preserving time for climacteric (∼0.8 V/day) and nonclimacteric fruits (∼0.05 V/day) due to variations in water loss, CO₂ emission during ripening, and senescence stages for the two different classes. Such a noninvasive and real-time sensing capability of the fabricated TFFS makes it a promising technology for monitoring fruit quality throughout the supply chain to reduce the food waste problem.