Fluorescence studies of polymer adsorption. 3. Adsorption of pyrene-end-labeled poly(ethylene glycol) on colloidal polystyrene particles

Abstract

The adsorption of pyrene-end-labeled poly(ethy1ene glycol) (Py-PEG-Py) onto aqueous colloidal polystyrene (PS) particles is investigated by photostationary fluorescence, fluorescence decay, and photostationary fluorescence depolarization experiments. All of the spectroscopic observations on this system suggest the association of hydrophobic pyrene moieties with hydrophobic particles: (i) the complete disappearance of excimers accompanied by a slight wavelength shift to lower energy in the monomer emission spectrum, (ii) a significant red-shift of about 3.5 nm in the monomer excitation spectrum, (iii) an increase in the average lifetime of the fluorescence decay, and (iv) increased averaged anisotropy measured in fluorescence depolarization experiments of chromophores 1,6-diphenyl-l,3,5-hexatrien(eD PH) and pyrene upon addition of PS particles. This association with the surface contrasts with previous observations of Py-PEG-Py adsorbing on hydrophilic silica particles (Char, K.; Gast, A. P.; Frank, C. W. Langmuir 1988, 4, 989). Excitation spectra of pyrene groups at various particle concentrations are much more sensitive to their state of binding than emission spectra. Qualitative displacement experiments and studies of the effect of molecular weight of Py-PEG-Py both suggest that the binding of hydrophobic pyrene to the PS surface is stronger than that of PEG segments to the surface.