Fig. 4. Photos of the hydrogels immersed in (a) methylene blue (1 × 10⁻² M, 1 × 10⁻³ M and 1 × 10⁻⁴ M) (b) eosin y solution (1 × 10⁻² M, 1 × 10⁻³ M and 5 × 10⁻⁵ M) (c) TPPS₄ (1 × 10⁻³ M, 1 × 10⁻⁴ M and 5 × 10⁻⁶ M) and (d) TMPyP (1 × 10⁻³ M, 1 × 10⁻⁴ M and 1 × 10⁻⁵ M) for 24, 48 and 120 h. Highest concentration is always on the top, lowest on the bottom.