Near-Field-Induced Ionization on Photo-Excited Gold Nanoparticles

Gold nanoparticles (AuNPs) have been widely utilized as an effective substrate for triggering molecular ionization upon photoexcitation. However, its underlying mechanism(s) remains unclear. Here, by blocking heat transfer and electron transfer on photo-excited AuNPs with a thin layer of silica shell (i.e., Au@SiO2 NPs), the critical role of the enhanced E-field in ionizing nearby molecules was evidenced. The enhanced E-field of the Au core could enhance the protonation of amino acids (via short-range interaction) by polarizing the analyte molecules. In contrast, the polarizability of the Au core showed a more pronounced effect to enhance the metal cationization process (Na+ and K+ adduction via long-range interaction) by polarizing both the analyte molecules and metal ions and was successfully applied to the sensitive analysis of amino acids in human serum.