Colour-changing microcrystals could be a cheap and safe way to monitor medicines

A team of Chinese and US researchers has invented a class of microcrystal that changes colour dramatically as the temperature changes.

It’s a feature that could become very useful in tracking medicines.

It can be frustrating to figure out whether the food inside it is still safe to eat if a power outage stops your fridge from working for a few hours – maybe some food at the back stayed cool enough to avoid spoilage, but maybe it didn’t.

This problem is much, much more concerning when industrial fridges and freezers, carrying commercial amounts of food – or medicines – break down.

It’s possible to track the temperature of packs of vaccines with monitors – but these are either expensive electronics or flawed in design.

Commercially available substances that change colour with the temperature tend to fade, and they’re mostly designed to spot whether something gets above 0°C. This is still too warm for things like the Pfizer and Moderna COVID-19 vaccines, which need to be kept at −70 °C and −20 °C, respectively.

New research published in ACS Nano has suggested an alternative, demonstrating materials with robust colour changes that can be tuned to specific temperatures.

Two pictures of vaccine vial, on with green spot on lid and one with no spot
Appearing green on a vial lid (left), this structural color material becomes colorless (right) when warmed. Credit: Adapted from ACS Nano 2023, DOI:10.1021/acsnano.3c00467

The materials are made from tiny grains of silica, each a few hundred nanometres in size: nanoparticles coated with glycerol.

These nanoparticles exhibit structural colour: waves of light bounce off them in a way that makes them gleam in bright reds and greens, like the wings on beetles.

The researchers combined these microcrystals with mixtures of water and ethylene or polyethylene glycol, which melt at different temperatures depending on the different proportions of each chemical.

When the brightly coloured mixtures became hot enough to melt, the microcrystals broke open and all the structural colour vanished, leaving a dull colourless substance.

This wasn’t reversible: even if the mixture froze again, the colour was gone.

The researchers were able to tune their mixtures to change colour at any point between -70°C and 37°C, and to demonstrate their efficacy used these substances to print both a QR code for monitoring temperature history on a “donated” organ (using a model pig’s heart) and flexible labels that could go on top of vaccine vials.

In both cases, the systems were very sensitive, and signalled clearly when temperatures became too warm.

In their paper, the researchers write that their materials “have established a general design rule for next-generation intelligent indicators, holding promise for reliable cold chains”.

“In addition, we anticipate that structural colour liquids will benefit various areas such as wearable sensors, droplet robots, and photonic display,” they add.

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