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2. Experimental section
2.1. Materials
The starting materials were ethanol (CSC Jaeklechemie), tetraethoxysilane Si(OC2H5)4 (TEOS, 98%, Sigma–Aldrich), ammonium hydroxide solution in water (25%, Fluka), citric ly900009 monohydrate (CA·H2O 98%, Aldrich), polyethylene glycol (PEG, mw = 10,000, Fluka), nitric acid (10 mol/L, Merck), zinc acetate dihydrate Zn(OOCCH3)2·2H2O (ZnAc2·2H2O, 98%, Sigma–Aldrich), and manganese acetate Mn(OOCCH3)2 (98%, Sigma–Aldrich). All chemicals were used as received without any further purification.
2.2. Synthesis
Silica cores were prepared by a modified Stöber process [29]. The particles were collected by centrifugation washed with water and dispersed in water for further use. The shell material was applied by a polymer-assisted sol–gel method. In a typical reaction Zn(OOCCH3)2·2H2O (731 mg, 3.33 mmol) and Mn(OOCCH3)2 (5.82 mg, 0.03 mmol) were dissolved in 23 ml of a water/ethanol mixture (4/1 in volume) in the presence of varying amounts of nitric acid to adjust the pH value. After the addition of 1.36 g (0.05 g/ml) PEG and/or CA monohydrate (1.42 g, molar ratio of metal ions to CA was 2:1), the solution was stirred for 1 h. A dispersion of SiO2 in water was added and the reaction mixture was stirred for 24 h at room temperature. Solvent was removed by freeze-drying and the resulting precipitate was annealed at different temperatures from 600 °C to 1100 °C.