Christian Doppler Laboratory for Polymer/Metal Interfaces, Max-Planck-Insitut für Eisenforschung GmbH, Max-Planck-Strasse 1, D-40237 Düsseldorf, Germany.
Langmuir. 2010 Jan 19;26(2):815-20. doi: 10.1021/la9022322.
An experimental approach is presented that allows the measurement of interactions of single macromolecules at the electrolyte/single-crystal interfaces under the continuous variation of electrolyte composition. Single polyelectrolyte desorption experiments with poly(acrylic acid) were performed on atomically defined single-crystal ZnO(0001)-Zn surfaces in aqueous electrolytes of varying pH and constant ionic strength. The corresponding characterized single-crystalline surface structures were proven to be stable in the pH range of 4-11, enabling the analysis of desorption forces on both surface terraces and step edges as a function of pH. Thereby, contributions of electrostatic and van der Waals forces as well as contributions of coordinative interfacial bonds could be distinguished. The results showed that carboxylic acid functionalities adsorb weakly to hydroxide-stabilized polar ZnO(0001)-Zn surfaces with forces in the range of 60-80 pN, whereas they strongly bind to the separating step-edges between the polar terraces, most probably via coordinative bonds exhibiting forces of up to 700 pN. Thus, by means of single-molecule desorption spectroscopy individual binding sites could be readily identified by distinct features in the force-distance profiles. Moreover, the measurement of desorption forces on the large atomically flat terraces at varying pH proved that a maximum molecular desorption force occurs at pH 7 as a result of increasing repulsive interactions at pH values above the surface point of zero charge and decreasing electrostatic interactions when shifting the pH in the direction of the pK(A) of the poly(acrylic acid).
提出了一种实验方法，允许在连续变化的电解质组成下测量电解质/单晶界面处单个大分子的相互作用。在原子定义的 ZnO(0001)-Zn 单晶表面上进行了不同 pH 值和恒定离子强度的水溶液电解质中的聚（丙烯酸）单聚电解质解吸实验。证明相应的特征单晶表面结构在 pH 值为 4-11 的范围内是稳定的，从而能够分析 pH 值变化时表面平台和台阶边缘的解吸力。由此，可以区分静电力和范德华力以及配位界面键的贡献。结果表明，羧酸官能团弱吸附在带负电荷的 ZnO(0001)-Zn 表面上，其力在 60-80 pN 范围内，而羧酸官能团强结合在极性平台之间的分离台阶边缘，很可能通过配位键表现出高达 700 pN 的力。因此，通过单分子解吸光谱，可以通过力-距离曲线中的独特特征轻松识别单个结合位点。此外，在不同 pH 值下在大的原子平坦平台上测量解吸力证明，由于在表面等电点以上的 pH 值处排斥相互作用增加，以及在向聚（丙烯酸）的 pK(A)方向移动 pH 值时静电相互作用减小，最大分子解吸力出现在 pH 7 处。
