RGO/Ni₂O₃ Heterojunction-Based Reusable, Flexible Device for Cr(VI) Ion Detection in Water

Rapid industrialization has compelled the use of water quality monitors for maintaining drinking water standards. The present article demonstrates RGO/Ni ₂ O ₃ hierarchical nanostructures as a sensing layer for flexible sensors to detect Cr(VI) ions in water. The device showed a maximum response of 6.61 times for 50 ppm of Cr(VI) ions (response time ~3.24 s with a calculated limit of detection (LOD) of ~0.494 ppb). The sensors were highly repeatable (four identical devices were tested) and reusable. The sensor was observed to degrade under repeated compression due to possible rearrangement of grain boundaries. An anomaly in the sensor response characteristics with increasing Cr(VI) ion concentration was observed and explained mathematically on the basis of kinetics studies based on batch adsorption experiments. The studies revealed that the adsorption was governed by only surface defects at lower concentrations (< 1 ppm), while at higher concentrations (>1 ppm) a synergistic activity of both the surface heterogeneity and the effect of adsorbed ions was observed providing a nonlinear response characteristics. To explain the effect of compression on the sensing response, a “stress-assisted sensing model” was proposed which explained the unexpected behavior of the device. The device demonstrated high specificity toward Cr(VI) ions over other heavy metal cations commonly found in drinking water. Thus, the RGO/Ni ₂ O ₃ nanostructured sensing layer demonstrated promising sensing performance as flexible micro sensors for futuristic electronics.