Super silk and mine-detectors

Modern materials get solve long-standing problems, when they get a helping hand from plants.

Two of the newest materials are carbon nanotubes and graphene, both rather exotic forms of carbon. One of the oldest materials known to Man is silk, a complex material constructed primarily of carbon, hydrogen, oxygen, and nitrogen. What happens when the ancient meets the modern? And what does this item have to do with plants? Let’s deal with the easy stuff first.

Silkmoth cocoon
Silkmoth cocoon. Photo: Gerd A.T. Müller / Wikipedia

Silk is a proteinaceous material produced by the larvae of the silk moth (Bombyx mori). Commonly termed silkworms, the larvae feed on … leaves of the white mulberry tree (Morus alba). Plant connection sorted.

Traditionally, silk is used to make some of the most exotic fabrics and clothing items. Strong though it is, even stronger silk is desirable – which I’m assuming would lead to longer-lasting, tougher-wearing clothing. The carbon nanotube and graphene connection comes from work by Qi Wang et al.* who fed these materials to… silkworms. In contrast to ‘regular’ silk, the ‘carbon-enhanced’ silk produced under this unusual dietary regime was twice as tough and withstood 50% more stress before breaking. Furthermore, carbonising the silk by heating to 1050 °C gives it the ability to conduct electricity – unlike normal silk. This latter property opens up the possibility of producing biodegradable medical implants, and eco-friendly wearable electronics. Slick work that silkworm’s silk work.

Even more explosive work was announced by Min Hao Wong et al. who have integrated single-walled carbon nanotubes (SWCNTs) into spinach (Spinacia oleracea). Although I have to confess that the details seem rather complicated for a humble botanist, what they’ve produced is plants that can ‘serve as self-powered pre-concentrators and autosamplers of analytes in ambient groundwater and as infrared communication platforms that can send information to a smartphone’. In particular, the plants can detect nitroaromatics, chemicals associated with high explosives. Such bioengineered plants can therefore be used to indicate presence of unexploded devices below soil level, and which may not readily be detected by visual inspection of an area.

As potential ‘bomb detectors’ this work certainly caught the eye of the science news sites with headlines such as ‘Carbon nanotubes turn spinach plants into a living bomb detector’. But, if plants can be used in this way, it may be seen as a safer alternative than sending in humans with mine detectors. However, to detect these compounds they first have to be taken up by the plant and transported internally.

So, maybe we all should handle spinach with extra care from now on (as if their oxalic acid content wasn’t already cause for some concern and reflection)!

[Ed. – In news related to the second item above, Long Zhang et al. report the transformation of grasses with bacterial genes that lead to a breakdown of wastes from explosives and munitions. Hexahydro-1,3,5-triniitro-1,3,5-trizaine (RDX) is released into the environment when many explosives are used, and is a concern as a contaminant of ground water. Although plants can take-up RDX from the soil, they don’t break it down. It therefore remains a potential threat to the environment. Transforming perennial switchgrass (Panicum virgatum) and creeping bentgrass (Agrostis stolonifera) with bacterial enzymes gave them the ability to degrade absorbed RDX to less harmful compounds which don’t pose such an environmental threat. Arguably, this is a great step forward in dealing with such dangerous materials, and an intriguing example of phytoremediation.]

* Based appropriately enough at Tsinghua University in Beijing, China, given that China is regarded as the ancestral home of silk.

References

Wang, Q., Wang, C., Zhang, M., Jian, M., & Zhang, Y. (2016). Feeding Single-Walled Carbon Nanotubes or Graphene to Silkworms for Reinforced Silk Fibers. Nano Letters, 16(10), 6695–6700. https://doi.org/10.1021/acs.nanolett.6b03597

Wong, M. H., Giraldo, J. P., Kwak, S.-Y., Koman, V. B., Sinclair, R., Lew, T. T. S., … Strano, M. S. (2016). Nitroaromatic detection and infrared communication from wild-type plants using plant nanobionics. Nature Materials, 16(2), 264–272. https://doi.org/10.1038/nmat4771

Ju, K.-S., & Parales, R. E. (2010). Nitroaromatic Compounds, from Synthesis to Biodegradation. Microbiology and Molecular Biology Reviews, 74(2), 250–272. https://doi.org/10.1128/MMBR.00006-10

Zhang, L., Routsong, R., Nguyen, Q., Rylott, E. L., Bruce, N. C., & Strand, S. E. (2016). Expression in grasses of multiple transgenes for degradation of munitions compounds on live-fire training ranges. Plant Biotechnology Journal. https://doi.org/10.1111/pbi.12661