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Optical tweezers with microlenses for handling nanoparticles

An optical tweezer system has been invented that improves stable nanoparticle handling and detection up to 90 ° C. The system, based on the incorporation of microlenses, could facilitate biological applications in cellular environments.

The advance is the work of a group of physicists from the Autonomous University of Madrid (UAM) in Spain.

One of the research lines of NanoBig (Nanomaterials for Bioimaging), a group associated with the Department of Materials Physics of the Autonomous University of Madrid, consists of manipulating microparticles and nanoparticles with biological applications using optical tweezers.

For the group, the manipulation of nanoparticles is especially interesting, since they have properties as sensors of the environment where they are (for example, as biological sensors of temperature, pH or pressure).

But the manipulation of nanoparticles is somewhat complicated: the optical forces used are very small, and any disturbance can destabilize or release the trapped object.

For this reason, much of NanoBig’s efforts have focused on studying the optical forces exerted on nanoparticles and trying to improve their stability.

Now, as a result of these investigations, the group has developed a system of optical tweezers that allows stable manipulation of nanoparticles and their detection up to 90 degrees Celsius (ºC).

Microlensing generating a laser focus that allows the manipulation of nanoparticles. (Image: UAM)

Light has the ability to exert mechanical force on objects with which it interacts. The phenomenon was described by Kepler in 1619 to explain why the tails of comets always point in the opposite direction to the sun.

Today, this force is known as radiation pressure. Its theoretical foundation was described by Maxwell’s theory of electromagnetism in the 19th century.

After the invention of the laser, in 1986, this property of light was used and studied by Arthur Ashkin (1922-2020) to trap individual dielectric particles of nanometric and microscopic size in three dimensions; discovery for which he was awarded the Nobel Prize in Physics in 2018.

Since then the technique has been known as ‘optical tweezers’, with widespread applications in biology, physics, chemistry and materials science, since it allows applying well-defined forces, as well as torsions and manipulation of objects at nano and microscale in a non-invasive way. .

The system developed by the UAM group is based on the incorporation of microlenses, small transparent spheres capable of magnifying the effects of conventional optical tweezers.

The system thus manages to obtain much smaller optical trap sizes, produce an increase in optical forces and increase the intensity of the light recorded.

“The experimental results of the work demonstrated a 7-fold increase in the applied optical forces, a factor 2 increase in the recorded light intensity and a stability of the nanoparticles up to 90ºC”, describe the authors.

In sum, the system allows easier manipulation of the nanoparticles, which will allow its extension to more complex media, such as cellular environments.

The work was carried out by the Optical Trapping laboratory, in charge of Dr. Patricia Haro González of the NanoBig group. It was funded by the Community of Madrid through the Multi-year Agreement with the UAM, in its line of action “Stimulus to research by young doctors”, within the framework of the V Regional Plan for Scientific Research and Technological Innovation (V PRICIT).

The research and development team has published the details of the new system in the academic journal Small, under the title “Nanojet Trapping of a Single Sub ‐ 10 nm Upconverting Nanoparticle in the Full Liquid Water Temperature Range”. (Source: UAM)

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