Environmental Science: Nano Blog

Jean-Pierre Sauvage, Fraser Stoddart and Ben Feringa have just been awarded the Nobel Prize in Chemistry for the design and synthesis of molecular machines. Ranging from artificial muscles to micromotors, these nanomachines can perform different tasks and present a myriad of applications. In 1983, Sauvage linked two-ring-shaped molecules by a freer mechanical bond. He was then followed by Stoddart who developed, among other things, a molecule-based computer chip, and more recently Ben Feringa who designed a nanocar. These three remarkable scientists were pioneers in the field and many others now benefit from their contributions to science.

Taking advantage of these findings, Uygun and colleagues developed unique micromotors that offer high-speed metal remediation. Microscale machines have been used for accelerated isolation and degradation of toxins and clean-up of oil-contaminated water, among other uses. The continuous autonomous movement of functionalised nanomachines around a contaminated sample leads to enhanced transport of the remediation agent resulting in greatly accelerated decontamination. However, most of these require external fuel, such as hydrogen peroxide, and expensive catalysts, which then prevents their widespread use. Only recently have new fuel-free Mg-based microparticles been developed. They are highly biocompatible as they use water as their sole fuel. Using these new nanomachines, Uygun describes a Mg Janus-micromotor that is functionalised with meso-2,3-dimercaptosuccinic acid (DMSA), which has been recognised as an excellent chelating agent for heavy metals (Figure 1).

Figure 1. Micromotor “on the fly” removing Zn, Cd and Pb.

The micromotors were prepared by half-coating magnesium microparticles with Ti and Au layers and the external gold surface was modified by incubation in DMSA. The removal of its passivation layer will expose the Mg surface allowing an Mg-water redox reaction that generates hydrogen microbubbles leading to an efficient water-propulsion. Moreover, a small number of micromotors can lead to a nearly complete removal of heavy metals within a short period of time and they are not impacted by co-existing metal ions present in complex samples, making these nanomachines an interesting and cost effective option for fast removal of heavy metal pollutants.

By developing these self-propelled water-driven molecular machines, Uygun and colleagues are corroborating the Nobel Prize laureates and spreading the belief of the Swedish Academy of Science: “we are at the dawn of a new industrial revolution of the 21^st century and the future will show how molecular machinery can become an integral part of our lives”.

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About the webwriter

Luiza Cruz is a PhD student in the Barrett Group at Imperial College London. Her work is towards the development of new medicines, using medicinal and natural products chemistry.

Iron oxide nanoparticles are not just a laboratory curiosity but a major presence within the whole of the natural world. Ferrihydrite is the predominant iron oxide nanoparticle found under abiotic conditions owing to its very low surface energy. The essential function of iron storage and transport within living organisms is carried out by the protein ferritin, made possible through the incorporation of iron nanoparticles inside the protein’s internal cavity.

The ability of ferritin to template the formation of iron oxide nanoparticles of defined size has been exploited in the production of systems used in targeted drug delivery, magnetic resonance imaging and nano-electronics. The ability of the iron core in ferritin to adsorb phosphate anions has also been put to use in removing this nutrient down to levels which prevent the development of bacteria on water purification membranes.

Understanding nanoparticles

In spite of being recognised for their essential function and with much work dedicated to the development of exciting applications surrounding them, iron oxide nanoparticles have been so far poorly understood at a fundamental level regarding their structure and reactivity models. Their implicitly small size and low symmetry has made imaging difficult using conventional crystallographic techniques. The properties of nanoparticles are generally dependent on their size and any model attempting to quantify the reactivity displayed by the surface must take this into account. The nature of the surface itself is dependent on the chemical properties of the surrounding environment.

Hiemstra and Zhao have conducted both an experimental and a computational study in order to generate a valid reactivity model for the adsorption of phosphate and arsenate by ferrihydrite and by the ferritin core. Ferrihydrite was modelled and essential properties such as surface area, density of surface reactive groups such as O(H), and surface charge were calculated as a function of the particle size.

The experimental study followed the adsorption of phosphate anions onto freshly prepared ferrihydrite and the effect of phosphate concentration on the formation and properties of iron oxide nanoparticles inside ferritin was analysed. Corroboration of theoretical with experimental data allowed for the development of an anion adsorption model with account for surface reactivity and generated new understanding concerning the formation, growth and aggregation of iron oxide nanoparticles under conditions relevant to environmental applications.

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Reactivity of ferrihydrite and ferritin in relation to surface structure, size, and nanoparticle formation studied for phosphate and arsenate

Tjisse Hiemstra and Wei Zhao

Environ. Sci.: Nano, 2016, Advance Article
DOI: 10.1039/C6EN00061D

The answer is nanomaterials. Particularly Carbon Black (CB) and silica nanomaterials which reinforce rubber increasing its durability, road grip and mileage. Other materials, such as carbon nanotubes (CNTs), offer better performance and ecological benefits via fuel savings. However, these are more expensive therefore their use is still very limited.

Nanomaterial fragments are generally released into the environment, constituting a rather uncontrollable source of emission of nanocomposites. In some countries where release quantification is already required, these emissions have been estimated between 4,000 and 7,000 tons of microplastic fragments. In spite of this immense environmental impact, little is known regarding the release from nanocomposites under mechanical and chemical stresses combined. Challenging the hypothesis of release being induced by a synergy of stresses, Wohlleben and co-workers bring a new sequence to test degradation pathways (Figure 1) as well as a fresh look at appropriate analytical techniques.

Figure 1. Synergetic degradation pathways by combined mechanical and chemical stresses.

In both cases shown in Figure 1, only chemical degradation or mechanical shear does not induce appreciable release of fragments. This happens only when the second stress is introduced, showing that synergetic degradation occurs on the diagonal of the scheme shown above.
In the first case, polyurethane (PU) with different single fillers (blue pathway in Figure 1) was first aged under standard conditions then put under mechanical stress simulating rain conditions (immersion, shaking or sonicating). Ultraviolet spectroscopy (UV-Vis), transmission electron microscopy (TEM) and analytical ultracentrifugation (AUC) or field flow fractionation were used to analyse the results. Images from X-ray photoelectron spectroscopy (XPS) showed that nanofillers remain on the surface after UV and rain weathering, accumulated into dense agglomerates as the polymer matrix was removed by the combined photolysis and hydrolysis.

Moreover, by creating an extended, highly reproducible, very low scatter semi-quantitative method to analyse turbidity of the released fragments, Wohlleben and co-workers were able to affirm that the release was reduced when CNTs were used (Figure 2a). Considering fragments below 150 nm diameter, PU filled with CNTs also showed reduced release (Figure 2b). More importantly, fragments coming from PU with CNTs were mostly organic, showing that the release of nanofiller fragments was suppressed.

Figure 2. a) Turbidity assessment of the released fragments from aged PU surfaces after UV and rain, with increasing mechanical shear: 24 h immersion (light grey), 24 h shaking (dark grey) and 1 h sonicating (black). b) Size-selective analysis (AUC) of fragments in the size range of 5 nm to 150 nm released from aged PU surfaces after UV and rain, with increasing mechanical shear: 24 h immersion (light grey), 24 h shaking (dark grey) and 1 h sonicating (black).

In the second case in Figure 1, natural rubber (NR) nanocomposite were filled with 40% CB and 4% CNT representing an innovative reinforced tire tread and it was compared to NR with 40% CB representing a conventionally reinforced tread and also to neat NB. The test focused on a sequence of mechanical-chemical-mechanical stresses, enabling the simulation of dust aging on dry roads and also the direct run-off into surface waters of the secondary fragments using UV irradiation.

During sanding, there was no noticeable difference between the particle concentration of the three rubber specimen. After aging, the structural differences of the fragments were minimal between wet and dry aging. Moreover, being CB and CNT both relatively more inert to UV degradation, they seem to have accumulated on the surface (less oxidised organic structures were quantified).

Fragments could potentially release smaller fragments and even free nanomaterial. Hence, Wohlleben and colleagues also analysed this scenario and indeed, smaller fragments were formed when a second sanding process was introduced, with no significant differences between NB with both fillers and only CB. However, it clearly showed that dry aging induces stronger secondary fragmentation than submersed aging, these results being in contradiction with the expected combined effect of hydrolysis and photolysis being more aggressive than photolysis only.

In summary, regarding analytical techniques, simple UV-Vis was shown to be the most sensitive technique. Qualitative identification by TEM is essential and analysis of XPS images was also important for a plausibility check.

This study is the first to analyse the combined forces of mechanical fragmentation, environmental aging and again mechanical stresses, showing a stepwise sequence that could continue ad infinitum and be tailored to simulate specific scenarios and provide useful estimates of release rates, enabling more reliable modelling and risk assessments.

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Release from nanomaterials during their use phase: combined mechanical and chemical stresses applied to simple and multi-filler nanocomposites mimicking wear of nano-reinforced tires
Wendel Wohlleben, Jessica Meyer, Julie Muller, Philipp Müller, Klaus Vilsmeier, Burkard Stahlmecke and Thomas A. J. Kuhlbusch

Environ. Sci.: Nano, 2016,3, 1036-1051
DOI: 10.1039/C6EN00094K, Paper