Micromechanical properties of pH-sensitive smart materials

Stimulus-responsive polymer sensors are important components of micro/nano-mechanical systems (M/NEMS), which are widely used in many frontier fields. As an important part of smart materials, the volume, mass, or elasticity of pH-sensitive polymers can shift with pH values, which can be used in many fields such as biology, chemistry and micro/nano electromechanical system. However, few studies on the micromechanical properties of smart materials have been reported until now. In this paper, a comparative study of the single-molecule mechanical elasticity of pH-sensitive polymeric polyacrylic acid (PAA) at pH change was performed using atomic force microscopy-based single-molecule force spectroscopy (SMFS). The results show that the single-chain conformation of PAA undergoes from collapse to full extension with increasing pH and the energy difference between different conformations is obtained, which leads to a novel design concept of a molecular motor (switch). It is expected that our study can provide a theoretical basis and data support for the design of new polymers and smart sensors with multiple responses.