Sometimes scientists need to control the process of mixing liquids in such small containers that it will not be possible to lower even the thinnest needle or even hair there. Meanwhile, it is very important to control the diffusion rate of molecules in so-called microreactors in order to create new effective drugs, conduct some biological experiments, and even quickly diagnose diseases. Scientists from ITMO University and their colleagues from the Czech Academy of Sciences proposed to solve the problem by using light energy.
Today biologists, chemists, pharmacists are increasingly using microreactors, also called laboratories on a chip. The tiny containers, dotted with grooves inside, range in size from a few cubic millimeters to a few cubic centimeters - no larger than a matchbox. Nevertheless, these small devices make it possible to perform rapid blood tests, mix microscopic doses of substances to obtain highly effective drugs, and conduct experiments on cells.
However, when working with microreactors, there is one difficulty: scientists have practically no influence on the speed of mixing or, in scientific terms, the diffusion of liquids and reagents that enter such a laboratory on a chip. Scientists from ITMO University, together with colleagues from the Czech Republic, have proposed a methodology that can solve this problem. They decided to use the so-called light pressure to mix liquids.
Back in the late 19th century, British scientist James Maxwell put forward the idea that light can exert pressure on physical objects. Soon the Russian scientist Pyotr Lebedev showed this in practice. However, the force of this pressure is very small, and in those days it was not used. Now, a whole branch of physics is engaged in this area - optomechanics (for the development of which in 2018 the Nobel Prize was received by Professor Arthur Ashkin). With the help of light, they capture living cells, move the smallest particles of matter and, as it turned out, the same forces can be used to stir liquids. The work of scientists is published in the journal Advanced Science.
Based on the latest advances in optomechanics, scientists from St. Petersburg have developed a nanoantenna, which is a tiny cube of silicon about two hundred nanometers in size. This device, invisible to the eye, is able to control the light wave that hits it. "Our nanoantenna converts circularly polarized light into an optical vortex," explains Alexander Shalin, professor at ITMO's Novy Phystech University, "the light energy swirls around it."
In addition to nanoantennas, scientists proposed to launch a certain amount of gold nanoparticles into the liquid. The particles captured by the optical vortex begin to spin around the silicon cube, thus acting as the very "spoon" for mixing the reagents. Moreover, the size of this system is so small that it can increase diffusion by a factor of 100 at one end of the microreactor, practically without affecting what happens at the other.
“Gold is a chemically inert material that does not react well,” says co-author Adria Canos Valero, “and it is also non-toxic. In addition, we needed to ensure that only spin forces and radiation pressure act on the nanoparticles, but not the attraction to the nanoantenna, otherwise the particles would simply stick to it. This effect is observed for gold particles of a certain size if an ordinary green laser is shining on the system. We have considered other metals, but, for example, for silver, these effects are observed only in the ultraviolet spectrum, which is less convenient."
Material provided by ITMO University Press Service
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Vasily Makarov