On Laser Desorption/Ionization Mass Spectrometric Probing of Nanocomposites of MoS₂ With Sulfur-Containing Organic...

A current trend in modern nanotechnology is the combination of inorganic nanomaterials with organic compounds, aiming to obtain nanocomposites whose properties are predetermined by the interactions between their components. In the present investigation, the reflection of noncovalent and covalent interactions in nanocomposites of a promising 2D nanomaterial based on MoS₂ with representatives of biologically significant sulfur-containing organic compounds: amino acid cysteine, tripeptide glutathione, thio-derivative of nitrogen bases thioadenine, and thiol thioglycerol, in the specific features of their laser desorption/ionization (LDI) mass spectra is determined. It turned out that the outcome of the interactions in the composites depended on the type of organic compound. The mutual effect of the nanocomposites' components is revealed in a combined analysis of positive and negative ion LDI mass spectra. MoS₂-catalyzed formation of cysteine, glutathione, and thioadenine oxidation products, being covalent dimers bound by -S-S- disulfide bridge, is documented. Negative ion LDI mass spectra of the nanocomposites contain sets of Mo_xS_yO_z clusters sputtered from the MoS₂ nanosheets, whose patterns differ from those characteristic of pure MoS₂. The general trend is suppression of abundances of clusters with more than one Mo atom, which may be explained by the stabilizing role of organic molecules' adsorption at the surface of MoS₂ flakes. The appearance of naked Mo⁺ ions is observed in the positive ion mode, being most expressed for the composites with thioglycerol and glutathione. The mechanism of Mo⁺ release is proposed: Chemical interactions of thiols with the edges of MoS₂ nanosheets result in the removal of sulfur atoms from the edges, which depletes the molybdenum bonds with the sheet, followed by Mo⁺ splitting off. The observed chemical transformations of sulfur-containing compounds and MoS₂ within the nanocomposites are to be accounted for in the search for applications of their composites.