Metachromatic Butterfly Bile Pigments for Multi‐Level Optical Security Films

ABSTRACT Metachromatic optical responses are highly sought after for advanced security technologies, but are rarely achieved using bio‐derived materials. Here, we demonstrate that a naturally occurring metachromatic bile pigment found in butterflies can serve as a metachromatic and fluorescent platform for multi‐level optical security applications. While the photochromatic and fluorescent properties of bile pigments have been reported previously, their metachromatic behavior has not been experimentally confirmed nor exploited in functional materials. In this work, we isolate sarpedobilin, a bile pigment from the cyan wing patches of the butterfly Graphium milon and embed the extract into polymer films. The resulting composites reveal pronounced concentration‐dependent shifts in both transmittance and fluorescence emission. By spatially controlling pigment concentrations, we fabricate structured films that function as optical security labels with three authentication layers: (i) a visible pattern under visible light, (ii) a color‐tunable fluorescence response under UV excitation, and (iii) a spectroscopically verifiable unique signature. This work establishes butterfly‐derived bile pigments as a bio‐based materials platform for multi‐modal optical security technologies.