Physicists in China have developed a technique for simultaneously measuring the mass and temperature of a single nanoparticle. The technique, which involves levitating the nanoparticle in an optical trap, applying a sinusoidal electrostatic force to it and analysing its subsequent trajectory, will help scientists determine how the properties of nanoparticles change in response to changes in temperature.

Nanoparticles are found in a wide range of products, including cosmetics, paints, food products and pharmaceuticals. To optimize their performance in these diverse applications, it is essential to characterize and control their properties, but current methods of doing this have significant limitations.

The mass of a nanoparticle, for example, is usually estimated based on density data and particle size analyses. The values obtained using this approach are not very accurate, however, and the method does not provide information on the properties of individual nanoparticles or the differences between them.

In recent years, researchers have developed several techniques that aim improve on these estimations. Of these techniques, schemes that rely on optical levitation are among the most promising. In a typical levitation set-up, a calibrated optical field is used as a reference to infer the mass of a particle down to the femtogram (10-18 kg) range. Even this improved technique, however, does not provide any information about how a nanoparticle’s mass varies with temperature – an important parameter since the mass of most materials changes as their temperature increases.

A reference scale

Physicists at the University of Science and Technology of China have now shown that they can track variations in mass, centre-of-mass temperature and other properties of a 165-nm diameter silica particle by using a known AC driving force as a reference scale. Their technique relies on the fact that the particle’s charge and the electric field are calibrated at the position at which the particle is levitated in an optical potential trap. This approach allows the precise magnitude of the electric force acting on the particle to be determined.

“The mass of the particle is then obtained by analysing the trajectory of the particle when subjected to the known electric field force,” explains team member Yu Zheng.  “The temperature of the particle is determined using the thus-calculated mass and a thermal motion scale. This scale is governed by the equipartition theorem, which in classical statistical mechanics relates the temperature of a system to its overall energy.”

Using this technique, the researchers were able to observe a sudden loss of the nanoparticle’s mass when the air pressure falls below a certain point. This phenomenon cannot be explained by the simple effect of water molecules desorbing from the nanoparticles’ surfaces and thus cannot be observed by conventional desorption analysis tools, such as thermal desorption spectrometry.

The researchers now plan to add a heating laser to their set up so they can control the heating of the levitated nanoparticles more precisely. “This will enable us to thermogravimetrically analyse individual particles,” Zheng tells Physics World. “Indeed, preliminary findings from our study have already shown that the variations in mass of an individual nanoparticle with temperature reveal nuanced information that conventional thermogravimetric analyses fail to capture.”

The present study is detailed in Chinese Physics B.

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