An in Situ Study of Metal Complexation by an Immobilized Synthetic Biopolymer Using Tapping Mode Liquid Cell Atomic Force Microscopy

Near-field scanning optical microscopy and tapping mode, liquid cell atomic force microscopy were used to study the conformational changes in simple short-chain silica-immobilized biopolymer, poly(L-cysteine) (PLCys), as the polymer was exposed to reducing, metal-rich, and acidic environments, respectively, to simulate on-line metal preconcentration. In a reducing environment (0.01 M dithiothreitol in pH 7.0 ammonium acetate buffer), the PLCys features resembled islands on the surface of the glass, 36 ± 7 nm in height and 251 ± 60 nm in diameter. Upon exposure to metal (Cd2+ buffered at pH 7.0), the PLCys islands broke up into smaller metal binding clusters whose features were lower in height, 22 ± 5 nm, and diameter, 213 ± 53 nm. Exposure to 0.01 M HCl used for metal stripping resulted in protonation of the polymer chains and further reduction in the polymer height to 12 ± 5 nm. These changes in molecular structure have given new insight into the mechanisms involved to achieve strong binding as well as rapid, quantitative release of bound metals to flexible shortchain synthetic biopolymers.