Kinetically Tunable, Subzero-Active, Visual Time-Temperature Indicators Based on the Permanganate-Oxalate Reaction.

Area of Science:

• Chemical Engineering
• Materials Science
• Biotechnology

Background:

• Biological products and specimens require consistent ultracold storage to maintain integrity.
• Existing time-temperature indicators (TTIs) are insufficient for monitoring ultracold conditions at the individual aliquot level.

Purpose of the Study:

• To develop novel visual time-temperature indicators (TTIs) functional below 0 °C for ultracold storage.
• To adapt the permanganate-oxalate reaction using eutectic perchlorate-based antifreeze solutions for customizable kinetics and robust performance.

Main Methods:

• Adapted the autocatalytic permanganate-oxalate reaction with eutectic compositions of LiClO4, NaClO4, and Mg(ClO4)2 to depress melting points to -18 °C, -37 °C, and -67 °C.
• Characterized eight customized TTIs with run times ranging from five minutes at 25 °C to 7 days at -20 °C.
• Assessed temperature sensitivity, accuracy, reproducibility, and stability under freeze/thaw cycles and extended pre-freezing periods.

Main Results:

• Developed ultracold-active TTIs with customizable kinetics and temperature sensitivity consistent with Arrhenius behavior.
• Achieved high accuracy and reproducibility with within-batch and between-batch run-time precision ≤ 4.8% CV and ≤ 7.5% CV, respectively.
• Demonstrated indicator stability, maintaining color intensity for at least 12 months when stored below melting points, with minimal impact from freeze/thaw cycles.

Conclusions:

• The permanganate-oxalate system in eutectic perchlorate-based antifreeze solutions provides a simple, inexpensive approach for ultracold-active TTIs.
• These TTIs offer customizable kinetics and robust performance for monitoring ultracold conditions.
• The developed TTIs can significantly improve the quality monitoring of biologicals and biospecimens during ultracold storage and handling.