12-07-2022 | By Liam Critchey
While the malleability, strength and functionality of many plastics have helped modern-day society in several ways, their disposal has also been causing a number of issues. We’ve known for a long time that plastics take a long time to break down naturally (which is why there are drives to recycle them), but we’re only just starting to understand the extent to which microplastics and nanoplastics are starting to pollute our environment―especially in marine environments.
While we often see pictures of littered plastics in large landfills, it’s what we can’t see that poses more of a threat. We often hear about the effects of nanoplastics (1-1000 nm) and microplastics (5mm) on fish and marine life, which is now evidenced by these small plastic materials being found internally within different marine species. But it’s not just marine life that is at threat, as land animals are also at risk of ingesting micro and nanoplastics. This was showcased to be a little closer to home in 2022 when it was discovered that nanoplastics have been found in various hospital patients, both in their bloodstream and deep within lung tissue.
Of both types, nanoplastics present a wider threat than microplastics do. While microplastics tend to sediment on the sea floor, nanoplastics are small and light enough to remain suspended in the water, and this means that they get picked up by ocean currents and spread around. Additionally, their high surface-to-volume ratio means that they can absorb a lot of toxic pollutants from the water and act as a medium where pathogens can grow. This makes them potentially more toxic than microplastics, and because they can penetrate tissues much easier as well (due to their smaller size), they pose more of a risk to different living beings.
We don’t know the extent of micro- and nanoplastic pollution worldwide with regards to the number of animals that have ingested them, but it’s thought to be a significant amount. However, given society’s reliance on plastic products in a range of industries―from the simple plastic bag to higher-end shape memory polymers and beyond―it’s unlikely that their use will be stopped, and a lot of damage has already been done from the output over the last few decades anyway.
So, while cutting down on plastic products is certainly one option for preventing nanoplastics from infiltrating the bodies of animals, another approach is to try and remove them before they have a chance to pollute the environment. To do this, scientists need to find a way of simultaneously detecting and eliminating the nanoplastics―preferably with a single system―and a research team has now looked to achieve this using layered MXene-derived materials.
Looking Towards Micro/Nanorobots
There are already several techniques available for characterising nanoplastics―from electron microscopy to nanoparticle tracking analysis (NTA) and mass spectrometry―but they all give different information about the nanoplastics, so analysing nanoplastics in water system ‘on the fly’ has typically been a challenge. Finding a way of providing an on-site screening of nanoplastics in wastewater systems could be a viable strategy for their detection and removal before they enter the main water systems.
However, the small size of nanoplastics renders conventional filters ineffective, but one way to potentially overcome this, as well as the need for complex scientific equipment, could be to use microrobots that can capture these ultra-small plastic particles using electrostatic forces (by using oppositely charged magnetic particles).
Microrobots are already used in different industries in sensing and purification processes. Microrobots can combine the unique properties of nanomaterials with autonomous motion abilities and programmable functionalities to interact with pollutants and purify/remove them. So, in the case of nanoplastics, there is the potential to use microrobots to sense and remove them from the water. Given the need to use them in water systems, it’s thought that light-powered microrobots have the best potential, where the light waves can both induce a motion to the microrobots and instigate the photocatalytic degradation of nanoplastic materials.
MXene Microrobots for Removing Nanoplastics
A team of researchers has now taken to using MXenes as the base material for creating the microrobots. MXenes are a specific class of 2D materials that possess a general chemical formula of Mn+1XnTn―where M is an early transition metal, X is either carbon or nitrogen, and T is a surface terminating functionality group such as an oxygen, fluorine, or a hydroxide group. MXenes have already been used in different water remediation applications, and the wide range of active properties means that they can be altered to provide different properties when needed.
The microrobots built by the researchers offer an ‘on-the-fly’ capture of nanoplastics in water samples. The microrobots were created using photocatalytic and magnetic MXene-derived layers, which were fabricated into a single system to provide multifunctional effects. The layers were created by annealing Ti3C2Tx MXenes (the most common MXene) into photocatalytic titanium dioxide layers (showing the same 2D material layered structure as MXenes), followed by the deposition of platinum and iron oxide nanoparticles to create multifunctional layers.
The microrobots were able to capture nanoplastics in a 3D space, where they were detected through electrochemical means. The microrobots then subsequently moved towards the nanoplastics using a light-powered, negative gravitactic self-propulsion mechanism that has six degrees of freedom. It’s thought that the addition of the platinum layer offered a way to obtain much higher motion speeds than when prototypes were created without it.
Combined with the self-propulsion, the microrobots have a pH-programmable surface charge (which results in a programmable Zeta potential) that has been adjusted to maximise the electrostatic attraction of nanoplastic materials. This means that the microrobots can quickly attract the nanoplastic materials and trap them on both their active surface and in between the layers. Because the microrobots were tailored to attract the opposite charge of the nanoplastics, the microrobots removed 97% of the nanoplastic material from the water in just one minute.
The microrobot devices created are a proof-of-concept study into a potential alternative way of dealing with the issues of nanoplastics in our waterways. The microrobots offer a way of identifying the nanoplastics, moving towards them, and subsequently removing them. Whether microrobots become a viable strategy for tackling the plastic pollution problems remains to be seen, but it’s an interesting approach to a problem that needs solving―and the more potential solutions that come to the fore, the more likely it is that one will be successful and have a global impact.