Tip-enhanced Photoinduced Electron Transfer and Ionization on Vertical Silicon Nanowires.

Nanostructured semiconductors are one of the most potent candidates for matrix-free laser desorption/ionization mass spectrometric (LDI-MS) analysis of low molecular weight molecules. Herein, the enhanced photoinduced electron transfer and LDI on the tip of vertical silicon nanowires (SiNWs) array were investigated. Theoretical simulation and LDI detection of indigo and isatin molecules in negative mode revealed that electric field can be enhanced on the tip end of SiNWs, thereby promoting the energy and electron transfer to analytes adsorbed on the tip of SiNWs. Based on this finding, a tip-contact sampling method coupled with LDI-MS detection was established. In this strategy, the tip of SiNWs can be regarded as micro-extraction heads for sampling of molecules when contacting with analytes. Impression of skin, tissue and pericarp on the vertical SiNWs array can effectively transfer endogenous metabolites or exogenous substance onto the tip. Upon laser irradiation, those adsorbed molecules on SiNWs tip can be efficiently ionized and detected in negative ion mode due to the tip-enhanced electron transfer and LDI effect. We believe this work may significantly expand the application of LDI-MS in various fields.