Russian Science Foundation: Between red and bright green
St Petersburg University scientists have proposed new luminescent nanothermometres. They are based on ions of the rare earth elements terbium and europium. Changing the intensity of their glow enables the scientists to determine the temperature from −150°C to +200°C in a non-contact way. This principle is based on the phenomenon of energy transfer between two optically active centres. It improves the thermal sensitivity of the sensor and carry out measurements with an accuracy of tenths of a degree. The research is supported by the grant from the Presidential Programme of the Russian Science Foundation. The research findings can be used for controlling the temperature of devices operating in extreme conditions.
In a wide variety of industries from metallurgy and chemical industry to medicine and molecular biology, temperature is one of the most important parameters that affects the properties of materials and the flow of many processes. Most often, for accurate temperature measurement, contact thermometers are used. They must be brought directly to the analysed surface or put into the necessary medium. For example, devices of this type include mercury thermometers that measure the temperature of the human body.
Contact thermometers are not always applicable. They cannot be used for micro- and nano-objects. They also do not work under high pressure or in aggressive environments. The scientists have developed an alternative to traditional sensors, i.e. non-contact optical thermometers. They consist of materials that glow when they absorb radiation of certain wavelengths. The principle of their operation is that the luminescence spectrum changes even with slight heating and cooling. This ensures that we can measure temperature more accurately.
The scientists from St Petersburg University and Peter the Great St Petersburg Polytechnic University developed fluorescent thermometers based on oxide nanoparticles activated by ions of the rare earth elements terbium and europium. The first of them emits bright green radiation, while the second element emits red radiation. Terbium ions can transfer energy to europium ions, thus increasing the intensity of their luminescence. This enabled the scientists to use this pair for fluorescent thermometry, since the luminescence intensity of terbium (green glow) and europium (red glow) ions changes even with a small change in temperature.
The nanothermometers synthesised by us can be used in a wide temperature range. This significantly expands the scope of their application. Their thermal sensitivity at temperatures above 150°C is higher than the maximum achievable sensitivity of any Boltzmann fluorescent thermometers. This ensures that we can significantly improve the accuracy of temperature measurement. In future, we are planning to increase the sensitivity by increasing the efficiency of energy transfer between active centres.
Ilia Kolesnikov, Candidate of Physics and Mathematics, Principal Investigator of the project, and specialist of the Centre for Optical and Laser Materials Research at St Petersburg University
The synthesis of luminescent particles was carried out using a variation of the sol-gel method developed by the members of the research team of the project. The size of the obtained particles was less than 100 nanometres. This ensured that the scientists could determine the temperature with a submicron spatial resolution. The researchers measured the luminescence spectra of the resulting nanoparticles and then studied how they changed as the temperature rose and fell. It turned out that the luminescence intensity of terbium ions decreased upon heating due to the enhancement of the process of energy transfer to europium ions and thermal quenching. The luminescence intensity of europium ions varied nonmonotonically with increasing temperature. In other words, it increased at temperatures below 370°C and then decreased. This behaviour is associated with the simultaneous influence of two oppositely directed factors, i.e. energy transfer from terbium ions and temperature quenching.
Since each of the active centres had its own dynamics of luminescence change, the scientists mathematically described its dependence on temperature for terbium and europium ions and estimated the temperature without contact in the range from −150°C to +200°C with an accuracy of tenths of a degree.