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.

In this paper, the contact configuration of single molecular junction is controlled through side group, which is explored by electrochemical jump-to-contact STM break junction. The conductance values of 2-methoxy-1,3-benzenedicarboxylic acid (2-M-1,3-BDC) is around 10 –3.65 G 0 , which is different from that of 5-methoxy-1,3-benzenedicarboxylic acid (5-M-1,3-BDC) with 10 –3.20 G 0 . Interestingly, the conductance value of 2-M-1,3-BDC is the same as that of 1,3-benzenedicarboxaldehyde (1,3-BDCA), while single molecular junctions of 5-M-1,3-BDC and 1,3-benzenedicarboxylic acid (1,3-BDC) give out similar conductance value. Since 1,3-BDCA binds to the Cu electrode through one oxygen atom, the dominated contact configuration for 1,3-BDC is through two oxygen atoms. The different conductance values between 2-M-1,3-BDC and 5-M-1,3-BDC can be attributed to the different contact configurations caused by the position of the side group. The current work provides a feasible way to control the contact configuration between the anchoring group and the electrode, which may be useful in designing future molecular electronics.