Time–temperature indicators can be used for food and medical products to visually indicate freshness or to monitor temperature information during transportation and storage.

Younan Xia and co-workers, Washington University, USA, have used the instability of nanocrystals in solution to develop a new class of colormetric indicators. The triangular silver nanoplates used were prone to spontaneous rounding of the corners. The nanoplates displayed localized surface plasmon resonance peaks in the visible region, the positions of which were highly sensitive to the sharpness of the corners.

The time and temperature dependence of the nanoplates’ UV/Vis extinction spectra were monitored by ageing the samples in air at 4 °C, 25 °C, 47 °C and 80 °C. The rate of blue shift had a strong correlation with the temperature, slowing down as the temperature for aging was reduced.

• Nanocrystal-Based Time–Temperature Indicators
  J. Zeng, S. Roberts, Y. Xia,
  Chem. Eur. J. 2010, 16.
  DOI: 10.1002/chem.201002665