“Rocking” Brownian Motor Pushes Nanoparticles Around

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IBM’s nanomotor can sort and separate nanoparticles, could be lab-on-a-chip game-changer


Image: IBM

The constant movement of atoms and molecules as they bounce off one another in a liquid or a gas is known as Brownian motion and it seems to have held back molecular nanotechnology (MNT) in which molecular-scale machines work in unison to create macroscale objects.

Now researchers at IBM Zurich have developed a new technology that uses Brownian motion to create a motor for sorting, separating and moving nanoparticles without the need for a flowing fluid. The IBM scientists described their research in the journal
Science
, and they believe that it could eventually lead to lab-on-a-chip applications for environmental sciences or biochemistry.

While Brownian motors have been developed in the past, they have mainly focused on what is known as Brownian “flashing” ratchets. These so-called flashing motors get their name because they “flash” the energy landscape (or the potential). In other words, they switch it off for a certain amount of time and then switch it on again for another duration.

If the potential is off, the particles diffuse in all directions. When switched on, the asymmetry of the potential leads to an asymmetric collection of particles onto the potential minimum. If, for instance, more particles are collected from the left side than from the right, this leads to an effective motion of particles to the right. 

What the IBM scientists have done is make what’s termed a “rocking” Brownian motor. It gets this name because the force on the particles via an electric field can be interpreted as a “rocking” or tilting of the energy landscape. Most importantly, with this kind of Brownian motor the energy landscape is fixed.

Most previous implementations of rocking Browning motors used dielectric forces between pre-patterned electrodes on a sample surface to create the switchable asymmetric potential. Unfortunately for this design, these forces are too weak if particles become smaller than one micron. This limited their use to plastic particles larger than one micron.

“Our motor is the first rocking Brownian motor that works for nanoparticles,” said Christian Schwemmer, a post-doc physicist at IBM Research-Zurich and co-author of the paper. “Our implementation works with the electrostatics of charged surfaces, and operates down to particle sizes of 5 nanometers at least. Therefore, particles like DNA, proteins and other biologically relevant entities become accessible.”

The key to device is its 3D patterned surface, according to Armin Knoll a nanoscale systems scientist at IBM Research Zurich and co-author of the research.

Knoll describes the pattern as a 3D saw-tooth pattern with the teeth in the vertical direction. The researchers employed scanning probe lithography to produce the saw-tooth pattern down 1 nm in depth precision.

After patterning is completed, a droplet of a water-based nanoparticle suspension is deposited on the pattern and afterwards covered by a glass coverslip. The distance of the glass to the surface of the patterned film is adjusted to be ~150nm.

Knoll explained that in water all surfaces are negatively charged (glass, pattern, particles) and the particles are repelled from the surfaces. “It costs them more energy to squeeze in between the two surfaces where the gap is small (at the top ridge of the teeth),” he added.

This is the surface pattern’s energy landscape, according to Knoll, and because of it the nanoparticles follow the 3D pattern. While this energy landscape has the shape of a saw tooth (a ratchet), Knoll cautions that the particles would not start to move in one direction, even if there is Brownian motion. To get this movement an oscillating electric field has to be applied.

“The field induces a flow of the water in the nanochannel that oscillates with the field,” said Knoll. “By drag forces the water pushes the particles back and forth in a rocking motion. The saw-tooth energy landscape rectifies this motion because it is much harder for the nanoparticles to move over the steep slope than over the shallow slope of the saw-tooth energy landscape.”

In this design, the particles start to flow along the shallow slope direction. In other words, the oscillating electric field moves the particles back and forth and the saw-tooth landscape hinders the particles from moving backwards.

While the device the IBM researchers have come up with looks to have lab-on-a-chip applications, the technology has two features that differentiate it from other lab-on-chip devices.

“The first feature is that our motor allows for a directed particle transport without net fluid flow,” said Schwemmer. “The second one is that it reaches unprecedented resolution in particle separation.”

In the model the researchers developed to predict the capability of the device, the researchers found that it was capable of separating two nanoparticle populations with merely 1 nm difference in size. So, in practice, 40 nm and 41 nm particles could be sorted into different directions. Because of this the researchers believe it can be used to detect ultra small quantities, such as nanoscale pollutants in drinking water.

Knoll believes the ability of the motors to size-selectively transport nanoparticles without a net fluid flow will make the devices ideal for particle sorting and separation. “The fact that our motor does not require a fluid flow is a huge advantage because flow-based devices need to be operated at high pressures,” added Knoll.

As a separation device, it has an extremely small footprint and requires only a few volts of electric potential, which makes it ideal for mobile or handheld lab-on-chip devices, according to Knoll. In contrast, electrophoresis—which is commonly used to separate proteins or nucleic acids—requires well above 100 Volts.

To become a viable commercial device, the device will have to be integrated into a microfluidic chip that will enable control of the input and output ports. It will also have to be bigger, in the centimeter range in order to reach interesting throughputs for applications. Finally, for real word applications, surfaces will need to be covered to guard against unwanted deposition of biomaterials.



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