Mass spectrometric monitoring of ligand-bridged hot electron transfer and anaerobic oxidization on auto renewable droplet-based plasmonic nanoreactors under visible light illumination.

The light induced hot-electron on plasmonic nanostructures has been recognized as a breakthrough discovery for photovoltaic and photocatalytic applications. With mass spectrometry, we demonstrate the dynamics of hot electron transfers of anaerobic oxidization reactions on Au decorated TiO2 plasmonic nanoparticles, which were coated on the inner surface of a flask. Those nanoparticles were covered by continuously renewed liquid droplets of solvent and reactants that were transported through a Venturi jet mixer with auto-spray. In addition to intensive mass transfer in such droplet-based nanoreactors, as well as strong adsorption of reactants and rapid desorption of products on materials surfaces, the localized surface plasmon resonance (LSPR) excitation upon visible light illumination, by which accumulated energies of plasmons are transferred to electrons in the conduction band of the material, attributes to the efficient photocatalytic transformation. Mass spectrometric detection of intermediate radical anions and negative ions with stable isotope labeling unambiguously identifies that highly energetic hot electrons can escape from the plasmonic nanostructures, be collected by adsorbed molecules, and initiate bond cleavages. It was demonstrated that losses of two H atoms result in the anaerobic oxidization of each benzyl alcohol molecule to a benzyl aldehyde molecule in the absence of molecular oxygen with more than 90 % yields. The well recyclable plasmonic nanoreactors implicate the injection of transferred electrons eventually back to electronically depleted Au+ positive ions. Bridged by adsorbed molecules, electrons were repeatedly circulated back and forth in plasmonic nanoreactors, where the collected light was eventually converted into chemical energy.

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.

Ion fragmentations via photoelectron activated radical relays and competed hole oxidization on semiconductor nanoparticles for mass spectrometry.

Structural identification is challenging in mass spectrometric imaging because of inadequate sample quantities and limited sampling time in each pixel for tandem mass spectrometry (MS/MS) experiments, which are usually used for the generation of fragment ions. We report herein the observation of a cascade of highly specific chemical bond cleavages via a low-energy photoelectron activated radical relays and a competed hole oxidization on surfaces of (Bi2O3)0.07(CoO)0.03(ZnO)0.9 semiconductor nanoparticles irradiated with the 3rd harmonic (355 nm) of the Nd3+: YAG laser. Distinguished from high energy electron impact (EI), this approach generates gaseous radical anions through the exothermic capture of low-energy tunneling electrons that are not able to cause extensive vibrational excitations. It was found not only original radical center but also secondary or even tertiary radical centers cause specific bond cleavages exclusively on α positions. The original radical center directly activates the cleavages of α-positioned chemical bonds that cause the formation of secondary radical centers. Ion fragmentations proceed along the newly formed radical centers that further activate the cleavages of their α-positioned chemical bonds. Using 8 compounds, we have demonstrated various radical reactions involved in desulfonation, cyclization, and ring contraction reactions as well as competed hole oxidization-generated hydroxyl radical substitution reactions. The interpretable fragment ions provide unambiguous experimental evidences for structural elucidation of drug residues and metabolites in mass spectrometric imaging of tissue slices without tandem mass spectrometry (MS/MS).