Single-Molecule Sensing of Interfacial Acid–Base Chemistry

Bronsted acid and base interactions are a cornerstone of chemistry describing a wide range of chemical
phenomena. However, probing such interaction at the solid−
liquid interface to extract the elementary and intrinsic information
at a single-molecule level remains a big challenge. Herein, we
employ an STM break junction (STM-BJ) technique to investigate
the acid−base chemistry of carboxylic acid-based molecules at a Au
(111) model surface and propose a prototype of a single-molecule
pH sensor for the first time. The single-molecule measurements in
different environmental conditions verify that the formation
probability of molecular junctions is determined by the populations of deprotonated -COO− form in a self-assembled monolayer.
Furthermore, the variation of the intensity of the conductance peaks (i.e., junction-forming probability) with the pH of the bulk
solution fits well to the Henderson−Hasselbalch type equation. From the equation, a good linear relation is found between the
degree of dissociation of the immobilized -COOH group and the environmental pH, providing a feasible way to design chemicals
and biosensors and a detector at the single-molecule scale.