Osman Bakr's nanoparticles paper published in Nature Communications
All In One: Characterizing Nanoparticle Properties
Nanoparticles, some of the most advanced yet minute pieces of technology produced, hold opportunities in many fields. They already have a variety of applications, from drug delivery and anticancer therapies to solar power and lithium-ion batteries. Some nanoparticles are built to self-assemble into intricate structures, emulating natural systems with the eventual goal of creating devices with new functionality or extremely high performance.
But making such small particles with uniform characteristics is difficult, and most processes generate particles with varied properties, altering a material's expected performance.
To determine – in a single experiment- the composition and variation in samples of these modern molecules, Osman Bakr, Assistant Professor of Materials Science and Engineering at King Abdullah University of Science and Technology, enlisted the aid of an instrument almost a century old coupled with new mathematical modeling. This work was published recently in a Nature Communications paper, "Determination of nanoparticle size distribution together with density or molecular weight by 2D analytical ultracentrifugation."
Such characterization will aid in optimizing nanoparticle production, thus optimizing performance in the particles' critical applications.
Importance of nanoparticle purity
"Nanoparticles are becoming integral parts of drug delivery, biomedicine, solar cells, electronic devices, you name it," Bakr says. "So whenever you want to be able to optimize these devices, you need to optimize the particles you are using."
And to do that, the nanoparticle composition must be determined. "You need to know what you have, otherwise you cannot predict the behavior of your device because you're dealing with nanoparticles of unknown properties," Bakr adds.
Even a fraction of a percent of nanoparticles with characteristics that deviate from the majority can, for example, affect the flow of electrons, the optical behavior, or the magnetic behavior in a device.
Without a method to determine the existence of impurities, researchers cannot characterize those impurities or, more importantly, even recognize that they're there and affecting the performance of devices.
Challenges to nanoparticle characterization
Nanoparticle characterization typically involves several experiments to determine a sample's composition. Electron microscopy is usually used to determine size and shape of some nanoparticles, but only a small fraction of the particles in a batch can be looked at. Impurities may not be detected, and the range of nanoparticle characteristics may not be represented.
"It's very difficult to measure the density of particles that are very small. Even if you do, they have a huge density distribution," Bakr says. "They're not homogenous—they come in different compositions and different densities."
One method to count them all
Now, with the process described in his recent paper, Bakr is able to deduce size, mass, and density distributions of entire nanoparticle samples—all in a single experiment.
Bakr tested his experimental method at KAUST's Analytical Chemistry Core Lab, one of several core facilities at the University that support the work of faculty, researchers, and students in all areas. He used high-purity, well-defined nanoparticle samples provided by colleagues at École Polytechnique Fédérale de Lausanne (EPFL), in Switzerland, and Carnegie Mellon University, in the US. Testing known samples allowed Bakr to verify the validity of his process and the measurements he obtained, so he can be confident that values acquired from unknown samples are accurate.
Using an analytical ultracentrifuge—invented in 1925 by Theodor Svedberg, who was awarded the Nobel Prize in Chemistry in 1926 for his work using the machine—as well as making a mathematical assumption that the nanoparticles are hydrodynamically spherical, Bakr is able to calculate nanoparticle size, mass, and density by observing how the particles sediment during centrifugation.
"Surprisingly, mathematically, assuming a nanoparticle is spherical gives us the right result," Bakr says. "These particles are not even close to being spheres—they're very faceted." One nanoparticle tested has an aspect ratio of 1.3, meaning it's slightly elongated, but its size and density were measured within 4% accuracy.
"Our method is an approximation, but it's a pretty good approximation," Bakr says.
A distribution of composition
Knowing the compositional variation in nanoparticles can help determine separation methods to remove heterogeneities. "One of the major obstacles right now in nanomaterials-based devices is that there's always a variation in nanomaterials," Bakr says. "It's very difficult to control the composition, so when you integrate them into a device, it does not perform as well as you'd like it to."
Bakr's next focus is developing methods based on ultracentrifugation to better separate and purify nanoparticles. This research could lead to better characterization and purification of nanoparticles that would be used in solar devices, increasing the devices' efficiency. Such improvements would reduce the cost of energy generated by the devices, which is important to drive the growing global alternative energy sector.