Real-time monitoring of azide-alkyne cycloaddition using scanning tunneling microscope fixed junction

Scanning tunneling microscopy-based single-molecule junction techniques, such as break junction (STM-BJ) and fixed junction (STM-FJ), have been widely used to study molecular electronic properties and transient events. However, their application to monitoring chemical reactions remains challenging, because each molecular species involved needs to be chemically modified in order to form stable single-molecule junctions with electrodes. Here, using our previously developed macrocycle-assisted, anchor-free STM-FJ platform, we demonstrate the real-time monitoring of a cucurbit[6]uril (CB6)-catalyzed azide-alkyne cycloaddition reaction in aqueous solution. By modifying one gold electrode with CB6, anchor-free reactants and products can be captured within the macrocyclic cavity and incorporated into stable single-molecule junctions, thereby converting molecular events into electrical readouts with sub-millisecond temporal resolution. Current-time traces reveal a clear evolution of conductance states corresponding to different molecular species. A short-lived high-conductance state was observed at the early stage of the reaction and is attributed to a ternary precursor complex in which both reactants are simultaneously encapsulated. This state gradually disappeared as the conductance signature of the CB6-triazole product becomes dominant, indicating the completion of the reaction. These results show the macrocycle-modified STM-FJ junction is a powerful platform for monitoring chemical reactions and biological systems in aqueous environments.