Electrochemical Quantitative Evaluation of the Surface Charge of a Poly(1‐Vinylimidazole) Multilayer Film and Application to Nanopore pH Sensor

Nanopore pH sensing is based on the interaction between the surface charge of the nanopore and ions passing through the nanopore. The nanopore surface charge can be derived from the acid‐base dissociation equilibrium of the modified polyelectrolyte. Various polyelectrolytes have been selected based on the acid dissociation constant of the monomer units, and various techniques have been applied to modify nanopores. However, they have been developed without clear guidelines for characterizing the surface modification status or surface charge. One reason has been the difficulty in accurately estimating the surface charge of nanopores in solution. Thus, in this study, the dissociation constant (pKaapp) of the surface charge of a modified polyelectrolyte nanopore was quantitatively estimated via electrochemical measurements. Previously, the modification status of nanopores has been evaluated using the ion current response. In addition, we monitored in real‐time the polyelectrolyte modification status using a quartz crystal microbalance (QCM). Some polyelectrolytes were difficult to immobilize directly on the nanopore surface, and those polymers could be effectively modified by the layer‐by‐layer (LbL) technique. Therefore, we produced a guideline for the fabrication of a nanopore sensor for pH measurements under physiological conditions by quantitative evaluation of the pKaapp via electrochemical methods, the monitoring of the modification status by QCM, and the development of an effective modification method via the LbL technique.