Mass spectrometric imaging of organic and metallic metabolites by plasmon-induced interfacial charge-transfer transition (PICTT) on Au sputtered ITO slides.

Protonation/deprotonation is the major ionization mechanism of organic molecules in current electrospray ionization (ESI) and matrix assisted laser desorption ionization mass spectrometric (MALDI MS) imaging. But cellular complexities are far beyond protonated or deprotonated organic molecules. There are tremendous endogenous organic and metallic metabolites regulating oxidation-reduction homeostasis that cannot be protonated or deprotonated. We describe an electron-driven ionization paradigm for mass spectrometric imaging of organic and metallic metabolites based on the charge and energy flow at the plasmonic metal-molecule interface. Enhanced plasmonic electron transfer was observed on Au sputtered ITO glass slides that were made with a physical vapor deposition approach. Plasmon-induced interfacial charge-transfer transition (PICTT) enables the decay of plasmons by direct excitation of electrons from Au atoms to strongly coupled electron receptors in tissue sections that are blotted on Au sputtered ITO slides. The highly reactive plasmonic hot electrons facilitate not only the mass spectrometric imaging of endogenous organic metabolites but also the surface plasmon-driven chemical reactions that can generate coordinative species for the visualization of endogenous metal ions. Beyond the biological application, the PICTT MS technique provides a way to tackle with the nature of electronic excitations at the plasmon-molecule interface that has been challenging because of the lack of a suitable experimental tool to directly monitor the outcomes of the interaction of the electron with an adsorbate.