A Study of Adsorption and Molecular Dynamics of Spin-Labeled Molecules on the Surface of Silica Nanoparticles

摘要

Stable nitroxyl radicals of different structures, hydrophobicities, and electric charges were used as spin probes for studying the adsorption and molecular dynamics of the adsorbed molecules on the surface of LEVASIL silica nanoparticles. Neutral hydrophobic probes, namely, spin labeled derivatives of indole, are not adsorbed on the nanoparticles; however, the microviscosity and hydrophobicity of their environment differ from those in aqueous solutions. pH in the LEVASIL suspension is measured using a probe with an amino group. A study of the adsorption of a series of positively charged spin probes with hydrocarbon substituents of different lengths showed that the hydrophobic interactions do not contribute to their binding to the nanoparticle surface. The binding constants, the average number of negatively charged adsorption centers per nanoparticle, and the surface potential were determined from the adsorption isotherms of the radical cations. The rotational mobility parameters of the adsorbed radicals were estimated after analysis of the EPR line shape. A dependence of these parameters on the total spin probe concentration is observed. This is explained by the rapid dynamic equilibrium (chemical exchange) between the adsorbed and unbound spin probes. The chemical exchange rates are estimated. A slow increase in adsorption in the equilibrium nanoparticle suspension (with a characteristic time of～10~4 min) and a change in adsorption isotherms (accompanied by a change in the dynamical parameters of the adsorbed particles) are detected. Slow changes in these parameters are attributed to the existence of sufficiently deep energy traps (pores) where the adsorbed radicals slowly diffuse. The rotational mobility of the radicals in these pores is less than in the surface adsorption centers.