Mechanistic study on plasmonic and plexcitonic nanoparticle-assisted laser desorption/ionization mass spectrometry

Abstract

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) is an important analytical technique for biomolecules and small organic molecules. As inorganic substrates act as modulators to absorb and transfer photo energy to analyte molecules, their responses to laser are critical to the analytical performance of SALDI-MS. Plasmonic metal nanoparticle (NP) is an important class of substrate because of their unique characteristics, such as heat confinement and near-field enhancement. There is, however, little understanding of the relationships between these properties and the performance of SALDI-MS. Therefore, this study aimed to investigate SALDI mechanisms of plasmonic metal NPs and self-assembled NP-semiconductor quantum dot (QD) nanostructures systematically.

Laser desorption mechanisms of gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) have been investigated using benzylpyridinium (BP) salts as a chemical thermometer. By combining mass spectrometric measurements with molecular dynamics simulation, effects of laser-induced heating, phase transition, and explosion to the ion desorption were revealed. Phase explosion and Coulomb explosion of the substrate possess the most significant enhancement effect on ion desorption. Due to the adiabatic expansion of ablation plume, they also introduced a cooling effect to the internal energy of desorbed ions. Moreover, both desorption efficiencies and fragmentation degrees of desorbed ions were controlled systematically by adjusting Ag-Au alloy NPs’ metal composition.

Local-electromagnetic-field-enhanced molecular ionization of silica-shelled AuNPs (Au@SiO2 NPs) has also been studied using polycyclic aromatic hydrocarbons (PAHs) as model analytes. The local electromagnetic (EM) field enhancements on the SiO2 shell surfaces were controlled via varying the AuNP core sizes. Insulating SiO2 shells also prevented direct interactions between the AuNPs and the adsorbed PAH molecules. The experimentally measured molecular cation intensities of PAHs were found to increase linearly with the theoretically calculated local E-field enhancement factors. Two possible ionization mechanisms, including plasmon-induced resonance energy transfer (PIRET) and local-electromagnetic-field-enhanced multi-photon excitation, have been proposed to account for this observation. Using gold nanoparticle-silicon dioxide nanoparticle (AuNP-SiO2 NP) self-assembled systems as substrate, the effects of local EM field enhancements on ion desorption and heat transfer were also revealed. As probed by BP ions, a stronger enhanced local EM field between closely packed AuNPs (hot spots) resulted in much more vigorous ion desorption and surface heat transfer. When hot electrons generated on photo-excited AuNPs were captured by electronegative juglone, an obvious lattice cooling has resulted. This indicated that hot electrons played a central role in the surface heating of AuNPs while enhanced local EM field at hot spot could provide extra electrical potential energies to these electrons.

By replacing SiO2 NP with semiconductor QDs, the plasmon-exciton coupling effects in NP-QD plexcitonic nanostructures on heat transfer were also investigated. For the weakly-coupled AuNP-TiO2 QD self-assembled system, the strong enhanced local EM fields at the hot spots could promote heat transfer to adsorbed ions through generating more energetic hot electrons. For the strongly-coupled AuNP-CdS QD self-assembled system, an exceptionally low heat transfer was achieved by trapping of absorbed photo-energy in the form of plexciton. This reduced heat loss effectively and enhanced the ion intensities of important analytes like medicines, human hormones, and proteins by 8 to 255 folds.