2.2. Preparation of Bluish Materials and Ion Complexations

First, 1.0 g and 0.5 g Alg were sufficiently dissolved in DW to adjust the total weight to 10 g over 24 h. Next, 0.11 g IdC was dissolved in 10 g DW to saturate the solution. Then, total weight of Alg–IdC mixture was fixed to 5 g by adding 0.12 g (a drop) of saturated IdC (aq.) to 4.88 g viscous 5 wt% Alg aqueous solution to achieve a IdC concentration of 2.4 wt% in the Alg–IdC mixtures. The following various concentrations (Figure 1A) were adjusted via changing the ratio of IdC and Alg solution usage. The pre-mixtures were then vortexed and stabilized for more than 12 h at ambient temperature. Next, 0.01 N AgNO₃ (aq.) solution and saturated IdC (aq.) solution were mixed for 2 h. The IdC (aq.) solution mixed with 0.01 N silver ions (aq.) or 0.01 N calcium ion (aq.) was slowly added to the 5 wt% Alg aqueous solution, followed by stirring the solution for 2 h at ambient temperature to attain ion complexation.

(A) Coloration of 5 wt% Alg aqueous solution with saturated IdC (aq.) based on corresponding ratio under the room light. (B) Iridescence revealed by concentration gradient Alg–IdC under sunlight. (C) Structural coloration of calcium alginate salt containing 2.4 wt% saturated IdC.