Ion-Desorption Efficiency and Internal-Energy Transfer in Surface-Assisted Laser Desorption/Ionization: More Implication(s) for the Thermal-Driven and Phase-Transition-Driven Desorption Process

Fundamental factors governing the ion-desorption efficiency and extent of internal-energy transfer to a chemical thermometer, benzylpyridinium ion ([BP]⁺), generated in the surface-assisted laser desorption/ionization (SALDI) process, were systematically investigated using noble metal nanoparticles (NPs), including AuNPs, AgNPs, PdNPs, and PtNPs, as substrates, with an average particle size of 1.7-3.1 nm in diameter. In the correlation of ion-desorption efficiency and internal-energy transfer with physicochemical properties of the NPs, laser-induced heating of the NPs, which are dependent on their photoabsorption efficiencies, was found to be a key factor in governing the ion-desorption efficiency and the extent of internal-energy transfer. This suggested that the thermal-driven desorption played a significant role in the ion-desorption process. In addition, a stronger binding affinity of [BP]⁺ to the surface of the NPs could hinder its desorption from the NPs, and this could be another factor in determining the ion-desorption efficiency. Moreover, metal NPs with lower melting points could also facilitate the ion-desorption process via the phase-transition process, which could lower the activation barrier (ΔG^#) of the ion-desorption process by increasing the entropic change (ΔS^#). The study reveals that high photoabsorption efficiency, weak binding interaction with analyte molecule, and low melting point could be critical for the design of SALDI substrates with efficient ion desorption.