Trace Analysis and Reaction Monitoring by Nanophotonic Ionization Mass Spectrometry from Elevated Bowtie and Silicon Nanopost Arrays

Silicon nanopost arrays (NAPA) are used in trace analysis by mass spectrometry (MS) because they enable highly efficient ion production from small molecules and thin tissue sections by UV laser desorption ionization (LDI). Such nanophotonic ionization of adsorbates relies on localized interactions between a nanostructured substrate and laser radiation. In LDI from NAPA, only the component of the oscillating electric field vector that is parallel with the posts couples the laser energy into the nanostructure. Enhancements in control over adsorbate ionization and fragmentation are expected if the surface-parallel component can also interact with the nanostructure. Here, an alternative nanophotonic ionization platform is introduced for LDI-MS, the elevated bowtie (EBT) array by adding triangular chromium features on top of silicon post pairs to form bowties. Compared to NAPA, the threshold fluence for ionization from EBT is lower, and at low laser fluences the ionization efficiency is increased by a factor of approximate to 17. The EBT platform with optimized apex angle exhibits a higher survival yield for molecular ions produced from biomolecules and xenobiotics and allows more control over fragmentation by adjusting the fluence. These unique nanophotonic ionization attributes are utilized for trace analysis and reaction monitoring in complex biological samples.