Synthesis and characterization of mesoporous CaCO3@PSS microspheres as a depot system for sustained Methylene Blue delivering

Abstract

Polystyrene sulfonate (PSS)-doped calcium carbonate microparticles (CaCO3@PSS) were synthesized and used for the encapsulation of Methylene Blue (MB). SEM and TEM images suggest the formation of spherical particles with about 2 μm diameters and raspberry-like structure. Brunauer–Emmett–Teller method was used to analyze the specific surface area and the pore size distribution of the CaCO3@PSS microparticles. The sorption of MB from aqueous solution onto CaCO3@PSS microspheres was studied and equilibrium isotherm determined. A physisorption process was indicated by the values of sorption energy (BD) 2.39 × 10−7 mol2/J2, mean free energy (ED) 1.5 kJ/mol and heat of sorption (B) 28.8 J/mol estimated from the Dubinin-Radushkevich and Temkin isotherms. The Langmuir model precisely describes the isotherm and the maximum adsorption capacity was 149.3 mg/g indicating homogeneous nature of adsorption sites and monolayer coverage of MB on the CaCO3@PSS microparticle inner and outer surfaces. The pseudo-second-order model can better describe the adsorption kinetics with a maximum adsorption attained within 5 h. The intra-particle diffusion process was identified as the main mechanism controlling the rate of the dye sorption. The MB release from CaCO3@PSS microspheres was found to be pH-responsive. The presence of negatively charged PSS− groups in pores of CaCO3@PSS microparticles prevents the initial burst release of MB, decreasing the release rate of MB. The experimental results indicate high potential of CaCO3@PSS microspheres as carriers for encapsulation and controlled release of cationic dye molecules, such as photosensitizer MB, used in photodynamic therapy.