In situ real-time continuous monitoring of soil carbonate ions using a portable CuO-Cu₂O-based electrochemical sensor

Real-time continuous monitoring of soil salinity and alkalinity is essential for sustainable agriculture, yet the in-situ detection of carbonate ions (CO₃^2-), a key indicator of soil alkalinity, remains a challenge. Herein, we designed a portable electrochemical sensor for the real-time continuous detection of free CO₃^2- in soil. The CuO-Cu₂O nanowires were synthesized by in-situ oxidation and topological transformation. The nanowires exhibited excellent chemical and structural stability and resistance to alkali, which were used as sensing material. The CuO-Cu₂O sensor exhibited excellent sensing performance over a wide temperature range (0-40 °C) and pH (pH value: 7-11), and in complex media including aqueous solutions, slurry and soil, which demonstrated that the CuO-Cu₂O sensor possessed excellent resistance to biological interference, immunity to colloidal particle effects and high ion selectivity, highlighting its great potential for practical applications in complex environments. The sensor demonstrated a low detection limit (5.4 ppb), wide detection range (0.01-1600 ppm), high sensitivity (264.98 μA mM^-1 cm^-2), excellent selectivity, and long-term stability. Density functional theory (DFT) calculations showed that CuO-Cu₂O heterostructure-induced electron redistribution triggered a shift of the d-band center toward the Fermi level, strengthened CO₃^2- adsorption, facilitated charge transfer, and then resulted in superior electrochemical response. Furthermore, it was integrated with ESP32 and Internet of Things (IoT) technology into a portable device for real-time monitoring of free CO₃^2- concentration in soil. This work provides a strategy for soil sensors to support precision agriculture and environmental monitoring.