The dead butterfly Troides Helena was provided by Shanghai Entomological Museum, China. Sodium hydroxide (granular, ≥96%), nitric AM 92016 (liquid, 65–68%), acetylacetone (liquid, 99.0%), isopropyl alcohol (liquid, 99.7%), toluene (liquid, 99.5%), titanium(III) chloride solution (liquid, 15.0%), potassium bromide (granular, 99.0%), silver nitrate (granular, 99.8%), and potassium persulfate (granular, ≥99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd and used without further purification. Ultrapure water with a resistivity of 18.25 MΩ was used throughout the experiments.
2.2. Pretreatment of Troides Helena templates
Firstly, 2.000 g Troides Helena wings, containing inverse V-type scales, were soaked in 3 wt% NaOH solution for 2 h at 25 °C to remove the surface organics and get a hydrophilic surface. Secondly, a large amount of ultrapure water was used to remove most of the NaOH and impurities contained in templates. Thirdly, the wings were further repeatedly soaked in 1.36% HNO3 solution for 30 min and then washed with ultrapure water in turn for three times to neutralize the residual NaOH and remove the excessive HNO3. Finally, templates were exposed in air for 30 min to get rid of excessive water.