Development of All-Solid-State Potentiometric Sensors for Monitoring Carbendazim Residues in Oranges: A Degradation Kinetics Investigation

Monitoring fungicide residues in orange fruits is vital, as fungicides for orange cultivation are increasingly used to prevent yield loss. At the same time, increasing restrictions are added by regulatory organizations. For facile on-site monitoring of the fungicide carbendazim (MBC), five ion-selective potentiometric sensors are proposed and compared. The first two sensors were prepared with a precipitation-based technique using molybdate (sensor 1) and tetraphenylborate (TPB) (sensor 2), respectively. Furthermore, two ionophore-based sensors were prepared using β-cyclodextrin as ionophore together with TPB (sensor 3) and tetrakis(4-chlorophenyl)borate (TpClPB) (sensor 4) as ion-exchanger. Further incorporation of multi-walled carbon nanotubes (MWCNTs) between the graphite rod and the sensing membrane of sensor 4 (sensor 5) further improved the stability and significantly lowered the limit of detection (LOD). Their performance was evaluated according to IUPAC recommendations, revealing linear response in the concentration range 1 × 10−4–1 × 10−2 M, 1 × 10−5–1 × 10−2 M, 1 × 10−5–1 × 10−3 M, 1 × 10−6–1 × 10−3 M, and 1 × 10−7–1 × 10−3 M with a Nernstian slope of 54.56, 55.48, 56.00, 56.85, and 57.34 mV/decade, respectively. The LOD values for the five sensors were found to be 7.92 × 10−5, 9.98 × 10−6, 9.72 × 10−6, 9.61 × 10−7, and 9.57 × 10−8 M, respectively. The developed potentiometric sensors were successfully applied to determine the residue and degradation rate of MBC in orange samples. After the researched fungicide was applied to the orange trees, the preharvest interval (PHI) could be calculated based on the MBC degradation kinetics determined in the tested orange samples.