The photo-/thermo-chromism of spiropyran in alkanes as a temperature abuse indicator in the cold chain of vaccines

The optimal storage temperature for vaccines ranges from 2 °C to 8 °C. Electronic thermometers can provide real-time and remote temperature monitoring but are not applicable for individual packages. However, it is known that few alkanes have melting points in this range. The photo-induced conversion of colorless spiropyran to blue merocyanine can be thermally reversed, resulting in decoloration. By dispersing spiropyran in a mixture of n-tetradecane and n-pentadecane with the melting point of 8 °C, an accurate and low-cost thermochromism indicator to record temperature abuse has been designed. The results showed that the thin label based on this system has taken a permanent decoloration within 10 min of the temperature exceeding 8 °C.