Conductance and Configuration of Molecular Gold-Water-Gold Junctions under Electric Fields

Extensive studies have shown that water can mediate electron transfer by forming water bridges. However, the electrical conductance of a single water molecule at room temperature remains unknown. Here, we studied single water molecular junctions via conductance measurements, theoretical analysis, and infrared spectroscopy. We found two states associated with single water molecular conductance, corresponding to two orientations in the molecular junctions. These two states set fundamental limits on water-mediated electron transfer rates. We further switched the orientations via an external electric field and determined the single water dipole moment. Our work provides a molecular-scale foundation to understand water-mediated electron transfer in chemistry and materials science and motivates future investigations of energy transduction at water-electrode interfaces.