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Magnetic vs. non-magnetic colloids – A comparative adsorption study to quantify the effect of dye-induced aggregation on the binding affinity of an organic dye

Williams, Tyler A., Lee, Jenny, Diemler, Cory A., Subir, Mahamud
Journal of colloid and interface science 2016 v.481 pp. 20-27
adsorption, aqueous solutions, binding capacity, colloids, light microscopy, magnetism, malachite green, moieties, nanoparticles, sorption isotherms, ultraviolet-visible spectroscopy
Due to attractive magnetic forces, magnetic particles (MPs) can exhibit colloidal instability upon molecular adsorption. Thus, by comparing the dye adsorption isotherms of MPs and non-magnetic particles of the same size, shape and functional group it should be possible to characterize the influence of magnetic attraction on MP aggregation.For a range of particle densities, a comparative adsorption study of malachite green (MG⁺) onto magnetic and non-magnetic colloids was carried out using a combination of a separation technique coupled with UV–vis spectroscopy, optical microscopy, and polarization dependent second harmonic generation (SHG) spectroscopy.Significant MP aggregation occurs in aqueous solution due to MG⁺ adsorption. This alters the adsorption isotherm and challenges the determination of the adsorption equilibrium constant, Kads. The dye-induced aggregation is directly related to the MG⁺ concentration, [MG⁺]. A modified Langmuir equation, which incorporates loss of surface sites due to this aggregation, accurately describes the resulting adsorption isotherms. The Kads of 1.1 (±0.3)×10⁷ and a loss of maximum MP surface capacity of 2.8 (±0.7)×10³M⁻¹ per [MG⁺] has been obtained. Additionally, SHG has been established as an effective tool to detect aggregation in nanoparticles.