Laser desorption ionization mass spectrometry for biological, forensic, and material analyses

Abstract

Laser desorption ionization mass spectrometry (LDI-MS) is a prevailing technique for qualitative and quantitative chemical analysis, utilizing laser energy to induce the desorption and ionization of solid samples. With the plasmonic nanoparticles (NPs) for efficient laser absorption and energy transfer, the sensitive detection of small molecules can be readily achieved. However, the potential of LDI-MS as an analytical tool is yet to be fully exploited. This study aims at developing new analytical techniques and expanding the application of LDI-MS to different fields, including biological, forensic, and material analyses, to advance LDI-MS as a more comprehensive analytical tool. In this study, plasmonic metal NPs were developed as the LDI-MS reporters by conjugating with antibodies for the detection of trace protein markers in tissue sections and urine by antibody arrays, with the detection limit at low attomoles of proteins. Moreover, the antibody-conjugated AuNP was also developed as a versatile probe for the optical, electron microscopy, plasmon-enhanced fluorescence, and MS imaging of proteins on tissue sections, providing three times enhancement to the fluorescence intensities and achieving duplex fluorescence and MS imaging of massive proteins. Besides tissue imaging, plasmonic NPs were also utilized for the chromogenic and MS visualization of latent fingerprints via the direct coating of Ag-Au alloy NPs. The high absorbance of Ag improved the optical absorption efficiency for the formation of high contrast fingerprint images, while the inertness of Au suppressed NP aggregation to provide colour stability. The alloy NPs with 60%wt of Ag showed optimal physicochemical properties to generate vivid fingerprint images with extended shelf-life. Moreover, the balance of physicochemical properties in NPs was identified to be critical for improving the LDI efficiency, demonstrated by the MS detection of the endogenous compounds in the fingerprint residues. The LDI efficiency of analyte ions from the NP surface was further investigated, and the Coulomb repulsion between the NP and the ions adsorbed was recognized to be one of the dominating factors assisting the desorption at low laser fluence. The creation of a hole-rich AuNP surface by the removal of hot electrons using an electron scavenger, juglone, boosted the desorption efficiency for sixty times. This charge-driven desorption concept was also utilized for the quantification of separated charges in the gold-metal sulfide core-shell nanostructures. A simple-structured cation with high charge density, tetrabutylammonium, was identified to be a highly sensitive probe for the detection of photoexcited holes. Through the direct charge measurement, the charge separation efficiency was recognized to be inversely proportional to the bandgap of metal sulfide shell, suggesting that the bandgap could be one of the most critical factors governing the charge separation process.