Enrichment and Distribution of Pb²⁺ Ions in Zwitterionic Poly(cysteine methacrylate) Brushes at the Solid-Liquid Interface

We prepared cysteine-based (PCysMA) polyzwitterionic brushes, and surface zeta potential investigations on pH-responsiveness of these PCysMA brushes confirm their zwitterionic character at intermediate pH range, while at pH values either below pH 3.50 or above pH 8.59, they exhibit polyelectrolyte character. Under acid (pH < 3.50) or base (pH > 8.59) conditions, they possess either cationic or anionic character, respectively. In the zwitterionic region, these PCysMA brushes show positive surface zeta potential in the presence of Pb(CH₃COO)2 solutions of various concentrations. The results are in line with microscopic investigations using anomalous X-ray reflectivity (AXRR) carried out along the absorption edge of Pb²⁺ ions. By varying the photon energies around the absorption L₃ edge of lead (13037 eV), the Pb²⁺ concentration normal to the silicon substrates, as a function of depth inside PCysMA brushes, could be revealed at the nanoscale. Both zeta potential and AXRR measurements confirm the enrichment of Pb²⁺ ions inside PCysMA brushes, indicating the potential of PCysMA to be used as a water purification material.