pH敏感高分子是智能高分子的重要分支, 基于其体积、质量或者弹性等参数随pH值变化的性质, 可以应用在生物、化学和微/纳机械等多个领域. 现有研究多集中在应用领域, 缺乏从更深层次的机理上的研究, 因此亟需在分子水平上对pH敏感高分子的环境敏感机理进行研究. 本工作基于控制变量实验法, 利用基于原子力显微镜的单分子力谱技术(SMFS)对pH敏感型高分子聚丙烯酸(PAA)在pH变化前后的单分子力学弹性进行对比研究. 结果表明, 随着pH值增加, PAA单链构象经历了从塌缩到完全伸展的改变, 并获得了不同构象之间的能量差, 由此提出了一种全新的分子马达(开关)的设计概念. 本工作的研究结果有望为设计多重响应新型聚合物和智能传感器提供理论基础和数据支持.

As an important part of smart materials, the volume, mass, or elasticity of pH-sensitive polymers can shift with pH values. Based on the feature, the pH-sensitive polymers can be used in many fields such as biology, chemistry and micro/ nano electromechanical system. Previous researches are based on the development and utilization of the known properties of functional smart materials, the mechanism of pH-sensitivity at the single-molecule level is still unclear. Based on the single molecule force spectroscopy, the single-chain mechanics of a typical pH-sensitive polymer, polyacrylic acid (PAA), in the buffer solution with different pH values have been studied. In order to reduce the influence of other factors on the experimental results, the buffer solution used in the experiment is disodium hydrogen phosphate citric acid with adjusted salt ion concentration of 0.5 mol/L, and the pH value is the only variable in the experiment (from 2 to 8). PAA is dissolved in deionized water (DI water) to a concentration of 5 mg/L, then used for the polymer physisorption on a silanized glass substrate, which has undergone piranha solution cleaning and then immersed in the (3-aminopropyl)- triethoxysilane (APTES) solution (2 mmol/L, CH₂Cl₂ solution) for 20 mins. After that, the sample is rinsed with abundant DI water to remove the loosely adsorbed polymer and dried by air flow. The single molecule force spectroscopy experiments were carried on a commercial atomic force microscope (Asylum Research, MFP-3D). The experimental results show that as the pH value increases, the single PAA chain undergoes from collapsed conformation to fully extended conformation, and the energy required for the transformation between different conformations during the stretching process is calculated. In addition, according to the chain shrinkage rate under different loads, the shrinkage work of single PAA chain under the change of pH is calculated. Based on the characteristic of the conformation changes of single PAA chain with pH value, a new molecular motor (switch) design concept was proposed. It can be expected that the result of this work can provide theoretical basis and data support for the design of multi response polymer and novel smart sensors.