pH-Controlled Gating in Polymer Brushes

A crucial function in the survival of biological cells is the ability to regulate the boundary between the local environment and the interior of the cell. A mechanism that has evolved for this purpose is the inclusion of channels in the cell membrane, which permit the selective transport of molecules in and out of the system. The channels are lined with chains that are highly sensitive to variations in the surrounding environment, such as changes in pH or salt concentration. In response to such fluctuations, the chains can change their conformations and thereby cause the pores to open or close. An intriguing challenge lies in designing synthetic polymer systems that can mimic this gating behavior. Of particular interest is fabricating smart polymer channels that sense and respond to changes in the environment. Applications for such systems would be manifold, including controlled release, selective filtration, electronic devices, or sensors. In this paper, the authors use a two-dimensional self-consistent mean-field (SCF) theory to design a polymer channel that can be made to open and close by varying the pH of the solvent. These studies provide a theoretical framework for understanding the mechanism by which polymers can act as pH-dependent valves. The distinct advantage of performing the calculations in two dimensions is that the authors can determine how both the vertical and lateral properties of the grafted layer are affected by surrounding solvent conditions. By systematically varying the relevant parameters, they determine the optimal design criteria for synthesizing smart polymer pores.