Hydrogels were prepared in a glass vial by adding the required volume of a stock solution of NaDC (200 mM), a pre-solubilised stock solution of OTPh (0.02 mg mL -1 ) in NaDC (100 mM), or a pre-solubilised stock solution of P3HT (0.027 mg mL -1 ) in NaDC (200 mM). Phosphate buffer (0.5 M) was added to each sample and the volume of 5.0 mL was completed with water. When required, pyrene was added to the solutions by injecting the appropriate volume of the pyrene stock solution. All samples were placed for 30 s on a Vortex mixer (Thermolyne Type 37600 Mixer) and were then heated at 60 °C for 15 min. The hot solutions (3.0 mL) were transferred to 10 mm × 10 mm quartz cells and the gels formed as the samples cooled. These samples were kept at rest for 20 h in the dark before any measurements were performed. No precipitation of NaDC and OTPh or P3HT was observed. The final concentrations in the gels were 90 mM for NaDC, 0.006 mg mL -1-for OTPh or 0.005 mg mL -1 for P3HT, 10 µM for pyrene, and 50 mM for the phosphate buffer.