Colloidal crystals lose color as the temperature rises

Due to their iridescent colors, opals have been considered particularly precious gemstones since antiquity. The way these stones shimmer is caused by their nanostructures.

A research group led by Markus Retsch at the University of Bayreuth in Germany has now produced colloidal crystals that mimic such nanostructures, for use as a new type of sensor that can visibly and continuously document the temperature in the environment over a defined period. Such sensors could be used for monitoring temperature-sensitive processes. The scientists report the novel colloidal crystal sensor in a paper in Advanced Materials.

Attractive applications are already in sight for this new type of sensor. "For the safe operation of modern high-performance batteries, it is important that they are exposed to only moderate temperatures for many hours of operation," says Marius Schöttle, a doctoral researcher at the University of Bayreuth and lead author of the paper.

"Short-term temperature spikes can endanger the safety and service life of the batteries. With the help of the new sensors, compliance with uniform ambient temperatures can be monitored reliably. Moreover, the sensor is already pre-programmed due to its material composition: it works autonomously and cannot be manipulated afterward."

"We have developed a sensor that is sensitive to time and temperature – without the need for complex electronics or special measuring devices," says Markus Retsch, a professor of physical chemistry and coordinator of the new study. "In addition, the artificial crystals we synthesized represent a new class of materials that are very interesting for fundamental research. It is possible that these colloidal gradients will help us to track down previously inaccessible physical phenomena."

Opals consist of spherical particles that form superordinate nanostructures. Interactions between these highly symmetrical nanostructures and visible light make the surfaces shimmer in the most diverse colors. The same is true for the wings of butterflies and some beetles. In recent years, natural and artificial representatives of this class of materials have been increasingly studied.

At the University of Bayreuth, Retsch and his research team investigated whether nanostructured materials can be produced using the same construction principle, but using mixtures of different particles with technologically attractive properties. Their vision was to realize nanostructured films that gradually change their physical properties along a certain direction.

The researchers found they could produce this unique gradual behavior by simply varying the composition of a binary particle mixture. For this purpose, they developed an experimental set-up that allowed the preparation of colloidal crystals made up of two distinct types of latex particle. These particles differed in the temperature at which they transition from a hard, glassy state to a soft, viscous state, known as the glass-transition temperature.

When these latex particles transition to the soft, viscous state they merge together, causing them to irretrievably lose their iridescent colors. Technically speaking, this irreversible dry sintering process creates a colorless film layer.

The researchers created colloidal crystals from both types of particles, utilizing a newly developed gradient fabrication technique to vary the proportion of the two particles steadily along the length of the crystals. The structure of the crystals is always the same: within each crystal, the proportion of particles with a higher glass-transition temperature, which are more stable, increases continuously towards one side. Comparative studies have shown that a larger percentage of more stable particles causes a slower structural degradation within the crystal and retards the resulting color loss.

The Bayreuth team has now used this discovery to fine-tune various colloidal crystals. A colloidal crystal in which the proportion of stable particles changes steadily can function as a temperature sensor: the higher the temperature over a defined period, the further the color loss spreads along the gradient direction. Since the color losses are irreversible, this means the sensor can document the ambient temperature as a function of time.

This story is adapted from material from the University of Bayreuth, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.