Design of L-Cysteine and Acrylic Acid Imprinted Polypyrrole Sensors for Picomolar Detection of Lead Ions in Simple and Real Media

104 MHz surface acoustic wave sensors, functionalized with two polypyrrole (PPy) imprinted polymers, were designed for lead ion detection in liquid media. The main difference between the two ionic imprinted polymers (IIP) is related to the nature of the chelating agent: L-Cysteine (L-Cys) for the former and acrylic acid (AA) for the latter. Performances of the IIPs based sensors were compared in terms of sensitivity, limits of detection (LOD), dissociation constants and selectivity. The gravimetric results indicated that both designed sensors can detect picomolar concentrations of lead ions. These values are, in fact, significantly inferior to the World Health Organization (WHO) maximum accepted limit in drinking water, and among the lowest LODs ever reported in the literature. The relevant dissociation constants, calculated from the most appropriate fits of the calibration curves, were estimated at: Kd $_{\textsf {1}} = (\textsf {1.1} \pm \textsf {0.4})\times \textsf {10}^{-\textsf {10}}$ M and Kd $_{\textsf {2}}= (\textsf {1.1} \pm \textsf {0.8})\times \textsf {10}^{-\textsf {5}}$ M for L-Cys-IIP/Pb ²⁺ and Kd $_{\textsf {1}}= (\textsf {1.1} \pm \textsf {0.3})\times \textsf {10}^{-\textsf {13}}$ M and Kd $_{\textsf {2}}= (\textsf {5.9} \pm \textsf {1.8}) \times \textsf {10}^{-\textsf {10}}$ M for AA-PPy-IIP/ Pb ²⁺ , thus confirming the strong binding between the designed IIPs and lead ions. Selectivity tests were investigated with mercury, cadmium, copper, zinc, and nickel divalent ions, which largely coexist with lead ions in soils, rivers, and wastewaters.