2.8. Spectrophotometric Detection of the Reduction of Fe3+ to Fe2+

In this study, the reduced Fe²⁺ cations were measured by UV–Vis spectrometry, as Fe²⁺ cations were reacted with 1,10-phenanthroline to form a stable Fe²⁺-1,10-phenanthroline complex (Fe²⁺-phen), showing a maximum absorption at 510 nm [24]. Based on this, the concentration of reduced Fe^{2 +} cations was measured under two conditions. First, Fe³⁺-SG hydrogel beads (20 mg beads) were stored in 5 mL vials. Next, 2 mL of a 10 mM ascorbic acid solution was added to the vial and gently shaken. Every 10 min, stored aliquots of the suspension of Fe³⁺-SG hydrogel beads were collected and equilibrated with 1,10-phenanthroline for 2 min, diluted, and the UV–Vis absorption of the resultant beads was measured in the range of 400–600 nm. In the second condition, the prepared Fe³⁺-SG hydrogel beads were stored in 2 mL of a 10 mM sodium lactate solution, then Fe³⁺-SG hydrogel beads were irradiated by a diffuse 405 nm laser beam. Irradiation tests were conducted at 10 min intervals to record the UV–Vis spectra.