Nanosecond UV Laser Ablation of Gold Nanoparticles: Enhancement of Ion Desorption by Thermal-Driven Desorption, Vaporization, or Phase Explosion

Abstract

The phase transition of surface-assisted laser desorption/ionization (SALDI) substrates has been identified as a driving process for ion desorption in many previous SALDI fundamental studies. Here, the effects of various phase transition stages, including substrate melting, vaporization, and phase explosion, on SALDI ion desorption efficiency and extent of heat transfer were investigated. We employed molecular dynamics to simulate the phase transition (from melting, vaporization, to phase explosion) of gold nanoparticles (AuNPs, ⌀: 2.5 nm) upon laser-induced heating and experimentally probed the corresponding SALDI ion desorption efficiency and extent of heat transfer to a chemical thermometer, benzylpyridinium (BP) salt (using 355 nm solid-state laser, pulse width: 6 ns, laser fluence range: 21.3 to 125.9 mJ/cm2). The results showed that substrate phase explosion has the most significant effect on enhancing the ion desorption efficiency and lowering the extent of heat transfer, which were reflected by an abrupt increase in both the ion desorption efficiency and the survival yield, when the laser fluence exceeded the AuNPs’ phase explosion threshold temperature (5800 K). Compared with phase explosion, vaporization only exhibited a limited effect on the ion desorption efficiency, while the effect of melting was not noticeable and even overridden by the thermal-driven desorption. The significant effect of phase explosion on enhancing the ion desorption efficiency could be attributed to the weaker binding interaction between the BP ions and the Au atoms which were rapidly ablated during the phase explosion stage, and the cooling effect on the BP ions could be due to the adiabatic expansion of the ablation plume during the phase explosion. The study revealed that the SALDI substrate with a lower phase explosion threshold would have a higher potential in enhancing the analytical performance of SALDI-MS.