Chemical warfare agents put at stake human life and global safety. These compounds are extremely toxic, and their efficient detection is crucial.
By combining experiments and theory, we realized a new sensor, based on manganese oxide and gold nanoparticles, which has ultra-low detection limits and impressive selectivity towards an important simulant of the vesicant nitrogen mustard gas.
The manganese oxide nanomaterials were synthesized by chemical vapor-deposition (CVD), starting from a Mn(II) molecular complex. This compound can be easily vaporized, and gives manganese-oxide materials of high purity. Then, the manganese oxide surface was partially covered by gold nanoparticles.
Scheme of the synthesis of the Au-decorated manganese oxide nanomaterials
The material was then tested in the detection of a nitrogen mustard gas simulant (named dipropylene glycol monomethyl ether, DPGME). The results were exciting: the sensor showed high efficiency and selectivity, with a detection limit of 0.6 ppb.
All this work was done by our awesome experimental colleagues. However, I want to show you that also theoretical modeling (#compchem) has done its part here, by answering the question: how does this system work?
At molecular level, the sensing action depends on the contact of the analyte molecules with the active part of the sensor – the gold/manganese oxide layer. By using a density functional approach, we have seen that the molecule strongly binds to both gold and metal oxide, as shown in the picture below.
The mustard gas simulant DPGME (cyan,white,and red spheres) binds to both the Mn3O4, surface (ball-and-sticks) and gold (orange spheres). The top part of the figure shows the response of the chemoresistive sensor to the adsorbed DPGME molecules.
To check if this “molecular recognition” ability of the sensor was specific only to the target molecule, we modeled also the contact of ethanol with the sensor.
We found that while the mustard gas simulant is in intimate contact with both gold and Mn3O4, ethanol interacts only with the oxide surface, but not with gold. This can explain the higher response and selectivity of DPGME with respect to ethanol.
In short, the new materials have a low fabrication cost and remarkable sensing capabilities. The reason of their impressive performances in the detection of the mustard gas simulant is that the target molecule is anchored to both manganese oxide and gold.
I was really proud to present this work at the #RSCPoster2020 event – and I thank the organizers for this fantastic (and fun) opportunity.
By the way, if you still don’t know what a #RSCPoster conference is, follow #RSCPoster on Twitter, have a look to the many many great posters presented this year, and consider taking part to the 2021 edition!
Here’s our little contribution to this wonderful global event
Our poster presented at the 2020 #RSCPosterTwitter Conference
If you’d like more information this work – including the technical details of both modeling and experiments – these can be found in our published paper (see here for a free green open access version).
Overwhelmed with the increasing flow of new scientific discoveries and related literature? You’re not alone. We live in the information overload era: too much to read, too little time, and life is short. Probably we’d need more readable, shorter papers too. Why writing a long one? Perhaps, it might connect disciplines which speak different languages but have much in common. Like material science and mineral science.
Let’s start from the first one.
You can make materials for solar cells, optical devices or medical sensors by trapping molecules or nanoparticles inside a “host”. Once there, molecules are no longer free to move, like in a gas or a liquid. This process, called “confinement”, brings to life new properties, which were not present in the individual molecules and are very useful in technology. Energy transfer or information storage, for instance, are made possible by the organization of the confined molecules.
The regular cavities of zeolites do a great job in organizing guest molecules
Tiny smart objects such as molecular machines, motors and diodes, make good use of self-organization processes, which create order from apparent disorder by exploiting interactions between molecules. This task gets easier when molecules are confined in regular pores. Think of a buzzing swarm of bees, first frantically hovering in the air, and then accommodated in a honeycomb.
Similar to honeycombs, regular patterns of pores like those in zeolites can orderly accomodate small molecules or clusters. But if you want to entrap, say, enzymes, peptides, or large nanoparticles, you must use materials with larger pores. Some porous silicas have large honeycomb channels, while the cavities of metal organic frameworks display an amazing variety of size and shape. With those nice architectures awaiting to be filled, ordering molecules might appear like an easy task.
As you imagine, things are more complex. Perfect order cannot be achieved. All cavities would need to be uniformly occupied by the guests. This is going to be very unlikely, because molecules move a lot even when they’re confined… like bees in a hive.
Molecules in nanocavities are sort of like bees in a honeycomb: they form an organized colony (Artwork: Andrea Stangoni)
About bees, I had direct experience… as a child, I used to observe my dad opening up his hives to inspect them. This gave me the chance to “study” the behaviour of these awesome creatures inside their honeycomb.
Bees do not occupy all hexagonal holes in the frame, and move continuously around, without any apparent pattern. Hence they’re not perfectly ordered. In spite of this, the colony is amazingly organized, and performs an impressive number of complex tasks…. not just honey production!
Similarly, guest molecules confined in porous cages are not rigorously ordered. Yet they are organized, and the resulting host-guest materials can perform useful functions, which were absent in the free molecules. They can, for example, absorb and transfer photons like the antenna systems of plants and bacteria.
Now, the question is: can we improve the organization of the molecules and the performances of the materials? Well, first we should know how the molecules occupy the cavities, their orientation, spacing and so on. Are the guests aligned? Are they attached to the pore walls? What happens if water enters the pores? To find those answers, you should use several different techniques: each experiment will give you some pieces to compose the puzzle. And yes, computational chemistry helps a lot to figure our what happens inside the pores. Yet this remains a very difficult problem.
This is where mineral science might help.
Regular patterns of cages are very common in the mineral world. Not long ago, for example, geologists found in Antartica a mineral with the same structure of zeolite Z-SM5, a well-known and widely used artificial industrial catalyst. That was indeed a big surprise! Natural zeolites are indeed amazing: their pores contain impressively stable structures formed by small molecules and cations. Just look at this water wire:
Water wire found in the channels of a natural zeolite
Contrary to what you’d expect, this chain is incredibly resistant to heat and pressure. First found in a rare mineral, it was named “one-dimensional ice”. But actually, our water wire “melts” at about 340 C inside the mineral framework! This is a great example of organized structure made by Nature. You can find many others: the most famous ones are perhaps gas hydrates. Several silica minerals have hydrate structures, which are also very common in man-made porous materials. Indeed, we should pay more attention to the close links between natural and artificial host-guest materials.
Natural porous minerals, the intriguing organization of their guests, and their response to mechanical stress can be an awesome source of inspiration in the quest of more robust and efficient materials. High pressure experiments with zeolites (and also some MOF’s) have already brought us new organized materials, along with many curious facts. But there’s so much yet to be discovered.
Perhaps, the problem with us (me included) and with our scientific era is that we don’t take enough time to relate with other disciplines. I’ve been so lucky to work with many awesome colleagues from the mineral, chemical and material science communities over the years, and it’s thanks to them that I wrote this review. One thing I learnt is that we should always try building bridges and strenghtening links between different fields because there’s nothing to lose, all to gain from a deeper exchange of ideas.
TheTwitter Poster Conference is an annual event organized by the Royal Society of Chemistry, which consists of sharing chemical research using tweets. You may take part either by tweeting an image of your poster or by commenting on other poster (doing both is better, in my view). As in a traditional conference, you see great research, are asked interesting questions, meet old friends, and come in contact with new colleagues or potential collaborators. Only, at the twitter conference this happens 24 hours non-stop on global scale; so, it’s a good idea to get there prepared!
I enjoyed so much my 2017 participation that I couldn’t miss the 2018 edition. Of course, it was awesome, and I am grateful to the organizers, the sessions’ chairs, and all participants, particularly those with whom I interacted. Indeed, I have learnt new stuff, seen exciting science, been inspired, without moving from my office and paying any conference fees. Really cannot ask for more. Many thanks to all of you!
Tweeting posters is not trivial. For optimal readability, you should keep into account, for example, that mobile phones have small screens, and that Twitter images are resized and cropped down – so, it would be better to prepare the poster in landscape format. These and other useful tips can be found in this excellent post. I came across it when the event was over, but it would surely be useful in the future.
Below you can find my poster, illustrating the fruitful collaboration between calculations and diffraction experiments at high-pressure conditions.
Besides water and ethanol in ferrierite (discussed in this post), the poster shows our new work on a host-guest compound of zeolite L and fluorenone dye under high pressure. These host-guest materials have excellent optical properties, useful for many applications, from solar cells to sensing in medical technology. Knowing their structure and working principles could help improve their performances – that’s why we try so hard to understand dye-zeolite composites at molecular level.
Basically fluorenone inside the channels of zeolite L forms a molecular ladder, which is very stable at room conditions because the carbonyl groups of the dye interact very strongly with the potassium cations of the zeolite.
Is this peculiar structure also stable under GPa pressures?
According to experiments and simulations, the answer is apparently yes! Our composite maintains its structure, and the interactions between the dye and the zeolite cations become stronger. The exceptional resilience of this material to compression highlights its outstanding mechanical properties. These are important to extend the application of dye-zeolite composites beyond room-pressure conditions.
Our work on ethanol and water in ferrierite, published here and blogged in my previous post, has been recently covered by MRS Bulletin in an excellent news article – “High pressure and small spaces create order from disorder” by science writer Tim Palucka. Some time ago, I had a very pleasant communication with Tim about the main ideas and results of the paper. That interview also helped me a lot to understand how science communication is done professionally. The piece by Tim really does a great job in explaining the scientific background, the main findings and the perspectives of our research – and, of course, all of us are so happy about it!
MRS Bulletin contains other interesting news articles, which are very useful to get a first impression about what’s going on in the many diverse areas of materials science – we’re very proud to be featured there! Big thanks, therefore, to MRS Bulletin and Dr. Palucka for the awesome coverage, and to Prof. Gion Calzaferri for commenting on our work as an external expert. A pdf version of the news article is freely available at MRS Bulletin (Volume 42, Issue 3, pp. 176-177, DOI: https://doi.org/10.1557/mrs.2017.38 ), while the illustration showing the arrangement of water and ethanol in the zeolite is just here below:
To understand how a motor works you have to know how it is put together and how it can be disassembled. This is true at the molecular level as well. Amazingly complex molecular motors and machines are fabricated everyday, but how do they break down into their constituent pieces? Attracted by this question, we modeled two such species – called ‘rotaxanes’ – and made them break apart.
Typically, rotaxanes are made by two molecules: a ring-shaped one, the “wheel”, and an approximately linear one, the “axle”. What is great about them, is that you can modify the interactions between the ring and the axle by using an external signal. Which means that you can control the movements of these components through light, for example.
A ring (R)
An axle (EE)
Another axle (ZZ)
Using the three components shown above – one ring and two axles – you can build two different rotaxanes: R-EE, and R-ZZ. Both disassemble – or ‘dethread’ – when the axle exits from the ring. This process occurs differently for the two species: While the EE-one (video 1) dethreads in one single step:
the dethreading of the ZZ-axle (video 2) passes through an intermediate and a transition state:
Analyzing the two simulations, we identified the interactions between the molecular components which control the process at the molecular level. For example, we have seen that the elliptic shape of the ring opening is very important, as it can recognize the two different configurations of the terminal groups of the axles. This insight might help in the construction of light-powered molecular devices, of which these rotaxanes are important building blocks.
The results of this work are discussed in our paper (1), which is part of the ChemPhysChem special issue on molecular machines. You may get a nice picture of the field by taking a look at this issue. Its contributions well illustrate the concept that molecular machines -especially natural ones – are incredibly complex and fascinating systems, and we are just beginning to understand their inner workings.
Molecular Dreams 