Engineered Thermochromic Inks from Reversible to Irreversible Color Transitions for Dynamic Anticounterfeiting and Logic-Enabled Sensing.

The development of thermochromic inks with engineered reversible or irreversible color transitions by a single ink formulation poses significant challenges. Herein, we address this challenge by engineering dual-mode thermochromic inks through a microencapsulated ternary system comprising 6'-(diethylamino)-1',3'-dimethylfluoran (DDF), bisphenol AF (BPAF), and 2-(4-benzoxyphenyl)ethyl decanoate (DPE). The reversible ink achieves a pronounced color difference (Δ* = 17.96-25.64) through controlled lactone ring opening/closing mechanisms. Microencapsulation with urea-formaldehyde shells enhances thermal stability and enables solid-solid phase transitions, ensuring durability for practical applications. Furthermore, by mixing reversible thermochromic inks with various pigments, an irreversible variant is developed, which permanently changes color at low temperatures without recovering when heated. Coated on flexible substrates, the inks demonstrate programmable logic operations (NOT, PASS1) and vivid two-color transitions (e.g., blue-to-brown, green-to-red), highlighting their potential in tamper-proof anticounterfeiting, real-time frozen food monitoring, smart packaging, and information encryption. This work establishes a universal method for designing multifunctional thermochromic materials with ultra-low-temperature responsiveness, bridging the gap between advanced security technologies and intelligent sensing systems.