Raman Spectroscopic Investigation of Maghemite Ferrofluids Modified by the Adsorption of Zinc Tetrasulfonated Phthalocyanine

Abstract

Maghemite (γ-Fe2O3) ferrofluids modified by the adsorption of tetrasulfonated zinc phthalocyanine (ZnPcTs) have been characterized by resonance Raman, Fourier transform infrared (FTIR), and fluorescence spectroscopies. While no signal from the adsorbate was observed from the FTIR measurements, the Raman results have shown good signal-to-noise ratios for the adsorbate at surface coverages down to 0.2. A fluorescence signal from the ZnPcTs in solution was observed only for ZnPcTs concentrations corresponding to a monolayer coverage or for higher ZnPcTs concentrations. Langmuir adsorption plots using Raman and fluorescence data have shown that the ZnPcTs adsorbs on the maghemite surface with an adsorption equilibrium constant of ca. 102, suggesting that electrostatic forces govern the adsorption process. It has been observed that for coverages above a monolayer the colloidal solution becomes unstable and aggregation occurs. Experiments performed with a modified Gouy method have shown that the ZnPcTs-modified maghemite ferrofluids have magnetic properties comparable to those of the maghemite ferrofluid and are stable for at least 24 h after sample preparation.