Autonomous microswimmers use optical signals to measure temperature as they move, enabling precise thermal mapping in fluid environments without external guidance or contact.
(Nanowerk Spotlight) Measuring temperature at the microscopic scale isn’t just about gauging heat. It is a way to observe life, reactivity, and instability in their most dynamic and fragile forms. Inside a living cell, even minor temperature shifts can signal metabolic changes or drug responses. In microreactors and lab-on-a-chip systems, temperature gradients shape reaction pathways. Yet despite advances in microscale engineering, scientists still lack tools that can move through these environments and simultaneously map temperature with high spatial precision.
Conventional temperature probes, including thermocouples and resistive sensors, are not designed for this scale. They require physical contact and lack the resolution to detect variations in the micrometer range. Luminescent thermometers, which use light-emitting materials to measure temperature, offer a non-contact solution with high precision. But they are passive. They cannot move or adapt to changing environments, and they rely on particles accumulating at a specific location to report local conditions. In dynamic systems like living tissues or flowing microfluidic channels, this limits their usefulness.
Magnetic microswimmers, which are guided externally using magnetic fields, have been used to overcome this limitation. However, these systems often require large control setups and depend on sensing methods that do not work well in deeper tissue. Their movement and measurement are difficult to integrate in real-world biological settings.
A different strategy involves upconverting materials that absorb low-energy light and emit higher-energy photons. These are especially attractive for biomedical applications because they can be excited by near-infrared light, which penetrates biological tissues more effectively than visible light. Among them, sodium yttrium fluoride doped with ytterbium and erbium ions stands out. The erbium ions emit light in a way that varies with temperature, while the ytterbium ions absorb energy efficiently and transfer it to the erbium.
Building on this approach, João M. Gonçalves and Katherine Villa at the The Barcelona Institute of Science and Technology have developed a mobile sensing system that addresses both mobility and temperature sensitivity. Their study, published in Advanced Intelligent Systems (“Thermometric Based‐Microswimmers with Chemical and Optical Engines”), presents a microswimmer that integrates dual propulsion with optical thermometry. The device can move autonomously in fluid environments while providing accurate local temperature readings within a physiologically relevant range.
Each microswimmer consists of a layered structure. At the core is a microparticle of sodium yttrium fluoride doped with ytterbium and erbium. A thin layer of silicon dioxide surrounds the core, which stabilizes the emission and prevents quenching effects. This is followed by a zinc oxide coating that enables light-activated motion. Finally, a platinum layer is applied to one hemisphere of the particle, giving it an asymmetric structure known as a Janus configuration. This asymmetry is essential for propulsion.
Schematic representation of a) multistep synthesis procedure for obtaining the NaYF4:Yb3+,Er3+@SiO2@ZnO-Pt core@shell@shell Janus microswimmers, b) chemical propulsion mechanism, c) UV light propulsion, and d) upconversion luminescent thermometry. (Image: Reprinted from DOI:10.1002/aisy.202500525, CC BY) (click on image to enlarge)
The researchers demonstrated two distinct propulsion mechanisms. In the presence of hydrogen peroxide, the platinum layer catalyzes a chemical reaction that generates oxygen and creates a concentration gradient around the particle. This drives motion through a process known as diffusiophoresis. The microswimmers moved reliably under peroxide concentrations ranging from 0.5 to 3 percent, and motion was confirmed using mean square displacement analysis, which distinguishes active movement from random diffusion.
In a second mode, the particles were activated by ultraviolet light. The zinc oxide coating absorbs the light and generates charge carriers, which then interact with water molecules to produce a flow of protons. This creates an electric field around the particle that results in self-electrophoretic propulsion. The researchers confirmed that the particles moved more rapidly under light than in the dark, and trajectories showed the nonlinear patterns characteristic of self-propelled motion.
While propulsion was critical, the main innovation was the integration of optical thermometry within a mobile platform. Under near-infrared excitation at 980 nanometers, the particles emitted visible green light. The intensity of this light depended on the temperature, specifically through a change in the balance between two emission bands originating from thermally coupled energy levels in the erbium ions. As the temperature increased, the ratio between these two bands shifted in a way that followed a Boltzmann distribution, allowing precise temperature measurement.
The team tested this behavior in a controlled range between 297 and 333 kelvin. They achieved a relative sensitivity of 1.2 percent per kelvin and a temperature uncertainty of 0.2 kelvin, both values consistent with the best reported luminescent thermometers. The particles did not require dyes, which often degrade under light, and their emission was stable over time. The use of near-infrared light also minimized interference from the surrounding medium, making the system compatible with biological environments.
One important concern was whether the propulsion itself would interfere with temperature measurement, either through localized heating from chemical reactions or from the excitation light source. The researchers tested for this by measuring temperature in the presence of peroxide and under UV light. In both cases, they found no significant heating effects. This confirmed that the propulsion methods did not distort the thermometric data.
The researchers also demonstrated that the system could operate in a typical microscopy setting. In one experiment, they heated a dispersion of the microswimmers to 70 degrees Celsius, placed it on a microscope slide, and monitored emission intensity as the sample cooled. The observed changes in light matched those recorded in a spectrometer, showing that temperature could be measured optically even without spectral resolution. Although real-time tracking of spectral changes during movement is not yet possible with the current setup, the proof of concept suggests this will be achievable with improved instrumentation.
The potential applications are wide-ranging. In biological systems, microswimmers could enable targeted measurement of temperature in tissues or even within individual cells. Conventional delivery methods for nanoparticles rely on passive accumulation, which is inefficient and poorly controlled. These active particles could navigate to specific regions and report thermal conditions with much higher spatial and temporal resolution. They could also be used in drug delivery systems that respond to temperature changes, providing feedback on local treatment effects.
In technology settings, such as microfluidic reactors or integrated circuits, the ability to map temperature in real time without installing fixed sensors offers a clear advantage. Current methods often require luminescent films or embedded sensors that can alter the thermal properties of the system. By contrast, these microswimmers can move freely, collecting data without interfering with the environment they are monitoring.
This work by Gonçalves and Villa marks a step toward autonomous sensing systems that can operate at the microscale with precision and flexibility. By combining self-propulsion with reliable luminescent thermometry, the microswimmers create new possibilities for dynamic temperature mapping in both biological and synthetic environments. The concept may serve as a foundation for future developments in mobile sensing, where single particles act not just as probes but as autonomous investigators within complex systems.
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Katherine Villa (The Barcelona Institute of Science and Technology)
, 0000-0003-1917-0299 corresponding author
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