*3.1. Materials*

All the reactants were commercial products. Iron (III) nitrate nonahydrate (Fe(NO3)3·9H2O, 99%), citric acid (C6H8O7, 99%) and lanthanum (III) nitrate hexahydrate (La(NO3)3·6H2O, 99.999%) were purchased from Sigma-Aldrich. 4-chlorophenol (4-CP, ClC6H4OH, ≥99%) and Rhodamine B (RhB, C28H31ClN2O3) were bought from Merck. All chemicals were analytically pure and were used as received without further purification. Deionized water (>18 MΩ cm) obtained from a Sartorius water purification system was used during the whole synthetic procedure to prepare the required metal ion solutions.
