Analysis of nicotinamide adenine dinucleotide (NAD+/NADH)-modified electrodes is important for in vitro monitoring of key biological processes. In this work, time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to analyze NAD+/NADH-modified gold electrodes. Interestingly, no obvious characteristic peaks of nicotinamide fragment could be observed in the mass spectra of NAD+/NADH in their neutral sodium pyrophosphate form. However, after acidification, the characteristic peaks for both NAD+ and NADH were detected. This was due to the suppression effect of inner pyrophosphoric salts in both neutral molecules. Besides, it was proved that the suppression by inner salt was intramolecular. No obvious suppression was found between neighboring molecules. These results demonstrated the suppression effect of inner salts in ToF-SIMS analysis, providing useful evidence for the study of ToF-SIMS ionization mechanism of organic molecule-modified electrodes. Graphical Abstract ᅟ.
Keywords: Inner salt; Ionization mechanism; Modified electrode; Nicotinamide adenine dinucleotide; Secondary ion mass spectrometry; Suppression effect.