Since ancient times, opals have been considered precious gemstones due to their iridescent colors. The shimmering effect of these stones is due to their nanostructures.
A research team headed by Prof. Dr. Markus Retsch at the University of Bayreuth has created colloidal crystals imitating such structures, which are ideal for building new types of sensors.
These sensors are designed to visibly and continuously capture the temperature in their environment during a specified period. They are, thus, ideal for long-lasting monitoring of temperature-sensitive procedures. The researchers have showcased their findings in the journal Advanced Materials.
Prospective applications are already looking out for these types of sensors.
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. Short-term temperature spikes can endanger the safety and service life of the batteries.
Marius Schöttle, MSc, Doctoral Researcher and Study Lead Author, University of Bayreuth
“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,” added Schöttle.
We have developed a sensor that is sensitive to time and temperature—without the need for complex electronics or special measuring devices. 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.
Prof. Dr. Markus Retsch, Chair of Physical Chemistry I and Study Coordinator, University of Bayreuth
Gradual Colloidal Crystals Derived from Natural Opals
Opals are made up of spherical particles that form superordinate nanostructures. When these extremely symmetrical structures interact with visible light, they make the surfaces shimmer in highly varied colors, the same as the wings of butterflies or certain beetles.
In the last few years, artificial and natural representatives of this group of materials have been progressively studied. At the University of Bayreuth, the research group guided by Prof. Dr. Markus Retsch has at present examined whether nanostructured materials can be created using this construction principle but with a regulated variation of the mixtures of various particles, which have technologically appealing features.
The aim was to create nanostructured films that slowly change their physical properties along a specific direction. This distinctive gradual behavior could be realized by just changing the properties of a binary particle mixture. To do this, the scientists designed an experimental setup that facilitates the creation of such gradual colloidal crystals that are made up of two types of unique particles.
Two types of particles were created in the laboratory that only varied in a single feature: their resulting nanostructures merge at various temperatures so that the materials’ surfaces permanently lose their shimmering colors.
In reality, this irreversible dry sintering method forms a colorless film layer. The team has developed colloidal crystals from both types of particles and exploited their recently developed gradient fabrication method. The structure of the resulting crystals is the same at all times: inside each crystal, the proportion of particles that lose their structures at higher temperatures and are, therefore, more stable, increases constantly towards one side.
Comparative studies have demonstrated that a greater percentage of more stable particles causes an unrushed structural degradation within the crystal and delays the resulting color loss.
Fine-Tuned Crystals as Optical Sensors
At present, Bayreuth researchers have used this discovery to tweak several colloid crystals. A colloid crystal in which the proportion of stable particles alters slowly now assumes the function of a sensor: when the temperature is higher during a specified period, the color loss increases and tends to spread along the gradient direction.
If the periods during a constant temperature are shorter, this process terminates sooner. Since the color losses are permanent in any case, the sensor records the level of a surrounding temperature as a function of time.
Schöttle, M., et al. (2021) Time–Temperature Integrating Optical Sensors Based on Gradient Colloidal Crystals. Advanced Materials. doi.org/10.1002/adma.202101948.