Potentiometric Biosensor for Highly Toxic Pb (lead) and Cd (cadmium) Detection in Milk Using Layer-by-Layer Immobilization

Milk contamination with heavy metals such as lead (Pb) and Cadmium (Cd) poses a serious threat to public health, particularly in industrial regions. To address the need for rapid and on-site detection of these contaminants, a potentiometric biosensor was developed for the quantification of Pb²⁺ and Cd²⁺ ions in milk and dairy products. The biosensor functions by measuring the change in electrical potential resulting from the inhibition of catalase, an enzyme that catalyzes the decomposition of hydrogen peroxide, by toxic metal ions. The enzyme was immobilized on a graphite substrate using a five-layer Layer-By-Layer (LbL) coating of chitosan and sodium alginate to improve sensor sensitivity and stability. Milk samples were artificially contaminated with lead at concentrations of 0.001 mg/L, 0.1 mg/L, and 1.0 mg/L, and with cadmium at 0.003 mg/L, 0.03 mg/L, and 0.3 mg/L, representing levels below, at, and above the maximum permissible concentrations (MPCs). Catalase activity decreased by 30 and 54% at MPC levels of Pb (0.1 mg/L) and Cd (0.03 mg/L), respectively, and by 60 and 50% at 10× MPC levels. These findings suggest that the developed biosensor is a reliable, low-cost, and field-deployable tool for real-time monitoring of milk contamination, offering practical benefits over conventional laboratory-based methods.