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

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.