Estimating pH at the Air/Water Interface with a Confocal Fluorescence Microscope

Abstract

One way to determine the pH at the air/water interface with a confocal fluorescence microscope has been proposed. The relation between the pH at the air/water interface and that in a bulk solution has been formulated in connection with the adsorption equilibrium and the dissociation equilibrium of the dye adsorbed. Rhodamine B (RhB) is used as a surfaceactive fluorescent pH probe. The corrected fluorescence spectrum of RhB molecules at the air/water interface with the surface density of 1.0 nmol m -2 level shows pH-dependent shifts representing an acid-base equilibrium. Two ways to determine the unknown acid-base equilibrium constant of RhB molecules at the air/water interface have been discussed. With surface-tension measurements, the adsorption properties, maximum surface density, and adsorption equilibrium constants were estimated for both cationic and zwitterionic forms of RhB molecules at the air/water interface.

The chemical reactions in electrosprays of water do not always correspond to those at the pristine air–water interface

“…≈ 0.5 nm , separates the gas-phase (vapor) from the condensed phase (water), two drastically different regions in terms of hydrationreactions spontaneous in one phase are forbidden in the other 19 . In fact, the chemical activities of species at the air-water interface can depart significantly from those in the bulk, as has been demonstrated by surface-specific techniques, including vibrational second harmonic generation and sum frequency generation 18,20,21 , and polarization-modulated infrared absorption reflection spectroscopy 8,11 , and indirect approaches, including NMR 22 and confocal fluorescence microscopy 23 . Even though vibrational spectroscopy-based techniques report directly on thermodynamic properties of the air-water interface, they suffer from interpretational ambiguities and limitations due to low signal-to-noise ratios [24][25][26][27][28][29][30] .…”

Licofelone-DPPC Interactions: Putting Membrane Lipids on the Radar of Drug Development

“…The packing density of the DPPC monolayer may also decrease due to the excess of negative charges introduced in the air-water interface due to the licofelone insertion into the DPPC monolayer. Indeed, the bulk pK a of licofelone is 4.8 (according to the MarvinSketch calculator from ChemAxon) and its surface pK a is not expected to increase more than 1 pK a unit according to literature [35,36,37]. Thus, licofelone has acidic properties (Figure 1), being mainly deprotonated at pH 7.4.…”

A library of Rhodamine6G-based pH-sensitive fluorescent probes with versatile in vivo and in vitro applications

“…While most previous efforts have focused on rhodamine B, we chose R6G as the base compound, due to its higher fluorescence quantum yield and straightforward nucleophilic substitution at the aromatic ester. Unlike modification of rhodamine B, which has been reported to require expensive catalysts, multi-step syntheses, and laborious purification strategies, [55][56][57][58][59][60] the pH-sensitive compounds shown in Scheme 2 were synthesized at low cost using methods easily scalable for industrial production, and adaptable to versatile biologic applications.…”