Spectroscopic Probing of Mixed-Mode Adsorption of Ru(bpy)₃²⁺ to Silica

Abstract

Evanescent wave excitation of fluorescence was used to study the adsorption of Ru(bpy)(3)(2+) from aqueous solution to three types of surfaces: bare silica, a dimethylethylsiloxane (C(2)) monolayer on silica, and a dimethyloctadecylsiloxane (C(18)) monolayer on silica. The solution pH was varied to investigate the nonpolar and electrostatic contributions to the free energy of adsorption for each surface. The pH dependence of the adsorption showed that the pK(a) is the same for each of the three surfaces, consistent with earlier conclusions that the acidity of derivatized silica surfaces is due to areas of exposed silica. The free energies of adsorption for the bare silica surface, -26.2(±0.2) kJ/mol at pH 8, was attributed to electrostatic interactions alone. The free energy of adsorption for the C(2) and C(18) surfaces, -28.5(±0.1) and -31.5(±0.1) kJ/mol, respectively, were found to have both electrostatic and nonpolar contributions, with the latter being larger by 50% for the C(2) surface and 100% for the C(18) surface. Using Gouy-Chapman theory, the surface charge densities on each of the three surfaces, calculated from the electrostatic interaction energy of Ru(bpy)(3)(2+), were found to be within the range of literature values: 8.8(±0.1) × 10(-)(7) mol/m(2) for bare silica and 1.7(±0.1) × 10(-)(7) mol/m(2) for both the C(18) and C(2) surfaces. The results demonstrate that a cationic dye can be used to probe the silanol activity of chemically modified silica surfaces. The results support the picture that these chemically modified silica surfaces are acidic due to molecular-scale areas of contact between the bare silica substrate and the aqueous phase.

Deposition and meniscus alignment of DNA–CNT on a substrate

“…The surface was treated with octyl-dimethyl-chlorosilane following a previously described procedure [23]. The silicon wafers (3 in diameter, 500 um thick, 1-20 ohm cm resistivity, N-type Phos., 1-0-0 from Silicon Quest International, Santa Clara, CA) were immersed in 70% H 2 SO 4 /30% H 2 O 2 (here and below %vol-ume) solution for 30 min.…”

“…doi:10.1016/j.jcis.2008. 10.073 functionalized, is negatively charged in aqueous solution [23,24]. Negatively charged DNA-CNT are normally repelled electrostatically and deposit only slightly.…”

The effect of conditioning of fused‐silica capillaries on their electrophoretic performance

“…The peak asymmetry of the probe, measured at 10% of peak height [27], was used to assess its ability to interact electrostatically with the fused-silica surface [19,20]. Unless stated otherwise, a 50 mM Ru(bpy) 3 Cl 2 solution in BGE was used.…”

“…The peaks of the zones tend to migrate more rapidly as concentration increases, resulting in unsymmetrical bands with extended tails. This behavior, described earlier by Wirth and coworkers [19,20] for certain LC bonded silicas with high concentrations of acidic silanols, is characteristic of a nonlinear isotherm distribution [28]. To diminish this effect, we used short injections of the probe solution (1 s) at fairly low concentrations (typically, 50 mM).…”

“…Thus, in addition to EOM measurements we used tris(2,2 0 -bipyridine) ruthenium(II) chloride as a probe to sense its interactions with the conditioned silica surface at varying pH. The Ru(bpy) 3 21 cation, a rather unexplored probe in CE work, was selected because its double charge provides strong electrostatic attraction towards the negatively charged silica surface, while lacking significant acidbase activity [19,20]. 0 -bis(3-propanesulfonic acid) (PIPPS) were purchased from GFS Chemicals (Columbus, OH).…”