GO/Cysteamine-Modified Screen-Printed Carbon Electrode for Square-Wave Anodic Stripping Voltammetric Determination of Cadmium in Fortified Mango and Avocado Digestates

Cadmium contamination in fruits is a relevant food-safety concern because of its toxicity, environmental persistence, and potential transfer from agricultural soils to edible plant tissues. This study developed a screen-printed carbon electrode modified with graphene oxide and cysteamine (SPCE/GO–cysteamine) to determine Cd(II) by square-wave anodic stripping voltammetry (SWASV). Two surface-modification routes, GO-cysteamine and GO-rGO-cysteamine, and GO concentrations of 0.1, 0.2, and 0.5 mg mL−1 were compared by cyclic voltammetry. The direct GO-cysteamine route at 0.2 mg mL−1 produced the highest CV response among the tested conditions and was selected for Cd(II) determination. Raman spectroscopy FTIR spectroscopy and scanning electron microscopy were used to characterize the graphene-based materials employed during surface optimization. Cd(II) concentrations from 1 to 10 µg L−1 were experimentally evaluated; with a calculated LOQ of 0.74 µg L−1, the entire experimentally evaluated range of 1–10 µg L−1, with a sensitivity of 11.30 µA (µg L−1)−1, R2 = 0.9892, a limit of detection of 0.25 µg L−1 and a limit of quantification of 0.74 µg L−1. Fortified Kent mango and Hass avocado digestates also showed concentration-dependent responses, with R2 values of 0.9799 and 0.9788, respectively, and relative standard deviations below 5%. These results demonstrate the feasibility of the SPCE/GO–cysteamine platform for Cd(II) determination in digested fruit matrices and support its further development as a portable screening approach for food-quality control.