Monomer/dimer dependent modulation of reduction of the cationic dye methylene blue in negatively charged nanolayers as revealed by mass spectrometry

Abstract

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]⁺˙ 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 –SO₃⁻ 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 H˙) 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.