Imaging the Dynamics of Metal Ion-Coupled Electron Transfer on Material Surfaces with Redox/Photo Active Chelators

Metal ions on surfaces of various materials as bulk matrices, doped structural units, or functionalized active sites play critical roles in the establishment of physical and chemical properties. Characterization of surface-bound metal ions and metal ion-coupled electron transfer are urgently needed for the determination of material structures as well as for understanding the relationship to macroscopic properties and technological applications. We present here a mass spectrometric (MS) technique that allows the monitoring of metal ion-coupled electron transfer along with spatial distributions, identities, quantities, valences, redox activities, and associated anions. It is based on the coordination of metal ions with chelators that are redox/photo active. Upon the irradiation of a focused laser beam, metal ions on material surfaces that are covered with chelators are evaporated, ionized, and detected with MS. This technique clearly reveals ligand-metal/metal-ligand and ligand-bridged electron transfers through MS or tandem MS/MS experiments. MS images of metal ions on material surfaces with the spatial resolution down to the sub-micrometer level have been obtained. It has been applied to the monitoring of hot electron transfer, leftover positive metal ions in localized surface plasmon resonance, and photocatalytic activities of crystalline facets of TiO₂.