Temperature‐Controlled Conversion of Boc‐Protected Methylene Blue: Advancing Solid‐State Time‐Temperature Indicators

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Abstract
Cold‐chain management is of high importance in preserving perishable products and in retaining quality. A visible marker on packages indicating complete maintenance of the cold chain assures safe consumption of products by end‐users and assists in reducing waste. Time‐temperature indicators (TTIs) are integrated markers that provide information about exposure of packages to adverse temperature and have been gaining increased attention by consumers. Here we present a methylene‐blue‐based derivative, N,N,N′,N′‐tetramethyl‐N10‐Boc‐phenothiazine‐3,7‐diamine (BocPTDA), that can be used as a solid‐state organic TTI dye, exhibiting an irreversible change from colorless to blue green upon heating. The conversion properties, studied using a silicagel‐coated plate, confirmed that BocPTDA undergoes a color change above 20 °C. At temperatures of 4 °C and below, no visible changes are exhibited, making BocPTDA a well‐suited marker for monitoring abrupt temperature deviations indicating improper cold‐chain management. Thus, application of BocPTDA‐based TTI systems on packages could inform consumers about the cold‐chain maintenance, assuring quality and safe consumption.

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