Selecting nanopropellers from randomness (w/video)
(Nanowerk Spotlight) Steerable nanodevices are envisioned for a multitude of applications. For example, magnetic nanodevices can be controlled via external magnetic fields (see for instance: "Artificial nano swimmers"). So far, scientist mainly have used costly synthetic routes to design and synthesize such devices.
Now, though, a team of scientists from the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany, have shown that a very simple route based on solution chemistry can also lead to such steerable machines.
"Our approach was based on the fact that colleagues always design propellers that are all the same and based on a biomimetic approach, namely based on that fact that bacteria propel a flagellum (or several flagella) that are helical when used," Faivre explains to Nanowerk. "We decided to test if random aggregates would do the job, and yes, very surprisingly they do. And since our synthesis approach is so simple, it is very inexpensive."
He points out that the propellers all have different properties since they are all different in term of morphology. "But we also show that with a simple theoretical model, we can sort the propellers based on given parameters we set, e.g. based on their size," he says.
Differently sized nanopropellers: A=0.88µm, B=1.22µm and C=1.15µm (Reprinted with permission from American Chemical Society)
He also notes that the propellers are much smaller than what other groups have reports so far. The smallest propellers the team observed were around 300 nm in size. For comparison, the smallest propellers previously reported measured about 1.5 µm along their longest dimension.
The synthesis method used by the Max Planck team is based on hydrothermal carbonization (HTC), which has been used previously to coat iron oxide nanostructures. Here, iron oxide nanoparticles are suspended in glucose solution and heated to 180°C for 24 hours. The HTC reaction leads to an efficient carbon coating of the iron oxide, effectively fixing the iron oxide nanoparticles in aggregates of varied shapes. A weak homogeneous rotating magnetic field is then used to select nanopropellers from the reaction product by letting them propel to the top of a glass vial.
"The direction of motion of the nanopropellers can be controlled, since they always move parallel to the vector of rotation of the magnetic field," says Faivre. "The speed of the nanopropeller can also be controlled, by varying the frequency of the actuating field."
With specific experimental conditions the researchers can predominantly select structures that are smaller than 1 µm in all dimensions. The nanopropellers are actuated, selected, and imaged in a custom-built open-frame microscope. Further characterization is performed by electron microscopy.
This video shows a nanopropeller actuated by a magnetic field of 1 mT, rotating at 100 Hz. These conditions are below the critical 19 frequency for this nanopropeller. This nanostructure is slightly larger than the diffraction limit of the used microscope and a slightly elongated shape is apparent. The propulsion speed was measured as 6.1 µm per second. A lasercutter marking (vertical stripe) can be seen in the background.
Many potential applications, like for instance drug delivery, rely on functionalizing the propellers' surface. "The carboxylic groups present on HTC carbon allow such surface modification," says Faivre. "We demonstrate this ability by fluorescently labeling our nanostructures. Our functionalization technique is versatile and can enable a wide range of applications such as controlled assembly in solution or triggered release."
The next idea that the team has is to look at the distribution in speed and to understand what are the parameters that make a given propeller faster than its cohorts.
"The problem is that the smaller the propellers gets, the harder it is to visualize them," Faivre points out. "So on the one hand, miniaturization is needed, but on the other hand, we will not be able to characterize the devices if they are too small. Thus, we first need to develop new analytical tools e.g. electron microscopy in liquid for future studies."