Desorption/ionization on silicon (DIOS) mass spectra of model ionic dyes methylene blue (MB+Cl-) and methyl orange (Na+MO-) were studied using p+ type-derived porous silicon (PS) free layers. As-prepared PS (PS-H), the PS thermally oxidized at 300 degrees C (PS-OX), PS with chemically grafted cation-exchanging alkylsulfonic acid (PS-SO(3)H) and anion-exchanging propyl-octadecyldimethylammonium chloride (PS-ODMA+Cl-) groups was tested as ionization platforms. Two mechanisms of the methylene blue desorption/ionization were found: (1) the formation of [MB + H]+* ion due to the reduction/protonation of MB+, which is predominant for PS-H and PS-OX platforms and (2) direct thermal desorption of the MB+ cation, prevailing for PS-SO3H. The fragmentation of the cation is significantly suppressed in the latter case. The samples of PS-SO3H and PS-ODMA+ Cl- efficiently adsorb the dyes of the opposite charge from their solutions via the ion-exchange. Consequent DIOS MS studies allow to detect only low fragmented ions (MB+ and MO-, respectively), demonstrating the potential of the ion-exchange adsorption combined with DIOS MS for the analysis of ionic organic compounds in solutions.
Redox behaviour of four imidazophenazine dye derivatives under mass spectrometric conditions of matrix-assisted laser desorption/ionization (MALDI), laser desorption/ionization (LDI) from metal and graphite surface, electrospray, low temperature secondary ion mass spectrometry (LT SIMS) and fast atom bombardment (FAB) was studied and distinctions in the reduction-dependent spectral patterns were analyzed from the point of view of different quantities of protons and electrons available for reduction in different techniques. The reduction products [M + 2H](+*), [M + 3H](+) and M(-*), [M + H](-) were observed in the positive and negative ion modes, respectively, which permitted to suggest independent occurrence of reduction and protonation/deprotonation processes. LDI from graphite substrate was the only technique that allowed us to obtain abundant negative ions of all dye derivatives. The yield of field ionization (FI) or field desorption (FD) mechanism to ion formation under LDI from rough graphite surface has been addressed. The sensitivity of reduction of the dyes to variation of reduction-initiating agents confirms high redox activity of the dyes essential for their functioning in natural and artificial systems.
The possibility to control the reduction rate of redox-active dyes incorporated into nanostructures is of interest for nanotechnology and biomedicine. We propose a novel mass spectrometric approach to study the aggregation-dependent modulation of cationic dye methylene blue (MB) reduction in the case of its inclusion into negatively charged nanolayers, which is based on detecting the difference in the redox activity of monomers and dimers of the MB cation (Cat + ). A regular reproducible recording of either intact Cat + in the case of MB present in its monomeric form, or one-and two-electron reduction products [Cat + H] + c and [Cat + 2H] + in the case of MB dimer formation, is observed for three different anionic nanostructures with varied content of MB, tested by three mass spectrometric methods: (1) an anionic surfactant sodium dodecyl sulfate (SDS) monolayer with adsorbed MB cations at the liquid-gas interface, probed by fast atom bombardment; (2) dried shells of soap bubbles blown from an SDS and MB aqueous solution, tested by laser desorption/ionization; (3) a nanotextured surface of porous silicon modified by -SO 3 À groups with adsorbed MB cations, studied by a desorption/ionization on silicon technique. The requirement for MB cations to be in the form of dimers or higher aggregates for reduction to be observed under mass spectrometric conditions is justified for the listed systems, where only another MB cation can serve as a source of the electrons and protons (or hydrogen radical Hc) necessary for reduction reaction. The proposed method can be applied to mass spectrometric imaging of stained biological materials, supplying information not only about the localization, but also the MB aggregation state as well.
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