Although they sound like a recent discovery, nanoparticles have been in use for centuries. For instance, the famous Lycurgus cup, crafted by 4th century Roman artisans, features dichroic glass, with silver and gold nanoparticles scattered throughout, giving the cup a red appearance when illuminated from behind and a green appearance when light is shining on it from the front.
Since then, researchers understanding of nanoparticles has progressed immensely. The creation of nanocubes has been of special interest because of their possible applications as gas sensors and biosensors. Nanoparticles can be formed using either chemical or physical techniques, although physical techniques are beneficial due to the absence of organic contaminants typically introduced by chemical techniques.
However, evenly sized nanocubes are hard to create in sufficient quantities using physical techniques. Recently researchers from the Nanoparticles by Design Unit at the Okinawa Institute of Science and Technology (OIST) Graduate University have discovered a new strategy to conquer this problem. Their research was published in Advanced Functional Materials recently.
The cube shape is not the lowest energy structure for iron nanoparticles, thus, we couldn't rely on equilibrium thermodynamics considerations to self-assemble these nanocubes.
Dr. Jerome Vernieres, First Author of the Publication
Instead, the OIST researchers guided by Prof. Mukhles Sowwan, took advantage of the possibilities offered by a method called magnetron-sputtering inert-gas condensation to develop their iron nanocubes. In this technique, argon gas is heated up first and converted into ionized plasma.
Then, a magnet, properly placed behind a target composed of a desired material, in this case, iron, controls the plasma’s shape, and guarantees that argon ions bombard the target; therefore the name "magnetron". Consequently, iron atoms are sputtered away from the target, causing them to collide with each other and with argon atoms, and form nanoparticles.
Uniform nanocubes can be produced by accurately controlling the plasma by manipulating the magnetic field.
Uniformity is key in sensing applications. We needed a way to control the size, shape, and number of the nanocubes during their production.
Dr. Stephan Steinhauer, OIST
To control the shape and size of these cubes, the team came up with a simple but important observation: iron is magnetic in its own right! The researchers found out that they could use of the intrinsic magnetism of the target itself as a novel way to alter the magnetic field of the magnetron.
In this manner, they were able to control the plasma where the particles are grown, and thus to manipulate the nanocube sizes during formation. "This is the first time uniform iron nanocubes have been made using a physical method that can be scaled for mass production" clarifies Vernieres.
To properly comprehend the mechanics of this method, the OIST team partnered with researchers from the University of Helsinki to formulate theoretical calculations. "The work relied heavily on both experimental methods and theoretical calculations. The simulations were important for us to explain the phenomena we were observing", illuminates Dr. Panagiotis Grammatikopoulos.
Once the team invented a technique to create these uniform iron cubes, the subsequent step was to assemble an electronic device that can use these nanocubes for sensing applications.
We noticed that these cubes were extremely sensitive to the levels of gaseous NO2. NO2 sensing is used for a variety of different purposes, from diagnosis of asthma patients to detecting environmental pollution, so we immediately saw an application for our work.
Dr. Stephan Steinhauer, OIST
The researchers from the Nanoparticles by Design Unit, in partnership with researchers from the Université de Toulouse, then constructed a prototype NO2 sensor that measured the change in electrical resistance of the iron nanocubes because of exposure to NO2 gas.
Because exposure to even a very little quantity of NO2 can create a measurable change in electrical resistance that is significantly larger than for other atmospheric pollutants, the iron nanocube-based sensor is both very specific and sensitive. "These nanocubes have many potential uses. The fact that we can produce a relatively large quantity of uniform nanocubes using an increasingly common synthesis method makes this research highly promising for industrial applications," highlighted Vernieres.