Molecular level inelastic electron tunneling spectroscopy of protoporphyrin using scanning tunneling microscope

Protoporphyrin IX (PPIX) is a crucial biomarker for tumor recurrence and to detect glioma at an early stage.Conventional optical detection methods for PPIX lack molecular-level sensitivity due to interference from its fluorescence. Inelastic electron tunneling spectroscopy (IETS) can be used as a probing technique to develop a hyper-sensitive biosensor for PPIX. We employ a scanning tunneling microscope (STM) to investigate the vibrational levels of single molecules at room temperature. The IETS-STM measurements of PPIX on highly oriented pyrolytic graphite (HOPG) showed the self-assembly of the molecules with a molecular array spacing of 1.71 ± 0.02 nm, and the spectra exhibited distinguishable peaks even at elevated temperatures. Analysis of peak broadening around 400 cm^-1 conducted at temperatures ranging from 77 K to 300 K showed deviation from the expected theoretical limit of thermal broadening. The impact of temperature and voltage on tunneling electrons were also analyzed to gain insight into the drift of the electrons through PPIX molecules.