Contributed by Siddharth V. Patwardhan, Ph.D., Senior Lecturer, Department of Chemical and Biological Engineering; Joseph R. H. Manning, Ph.D. Candidate in Chemical Engineering, University of Sheffield
This blog is based on a recent article and associated cover feature: An eco-friendly, tunable and scalable method for producing porous functional nanomaterials designed using molecular interactions, ChemSusChem, 10(8), 1683-1691, 2017. For more information, visit the Green Nanomaterials Research Group.
Nanosilicas have the potential to solve a number of pressing industrial issues, but are locked away because of wasteful and prohibitively expensive synthesis conditions. By contrast, nature produces far more complex silica under ambient conditions. By combining natural silica with computer simulations, we have discovered a method to produce green nanosilica, unlocking their industrial potential once and for all.
As an industrial material, silica is widely used as an inert filler and texturing agent in everyday products ranging from car tires to toothpaste, drug tablets, and powdered cosmetics and foods. Since the 1990s, scientists have been working on new, more complex nanosilica materials to improve upon these applications and to enable more high-value applications, such as soaking up pollutants from the air and water, capturing carbon from industrial exhausts, catalytically cracking crude oil into petroleum products, and storing medicines for slow release in the body. The key difference between currently-used industrial silicas and the new nanosilicas is a tiny pattern of holes on the material’s surface (Figure 1). These holes are a perfect size for the material to act like a sponge and soak up or release molecules exactly when and where they are needed.
But these advantages come with a cost, specifically making the synthesis much more complex and expensive. To build up this spongy structure, a molecular template called a surfactant is used during synthesis (Figure 2). The surfactant helps to direct the shape of the material around it on the nanoscale as it assembles, but using it both slows down the synthesis and increases the energy required for the material assembly. Furthermore, these surfactants need to be removed before the sponge-like structure can be accessed, adding a new step to the process.
Due to how tightly-bound the template molecules are to the structure, the commonest and most effective way to remove them is to destroy them with heat. This has two big environmental and cost drawbacks: First, this requires heating the material to over 500oC for an extended time, which is very energy-intensive; second, once it has been destroyed, the template chemicals cannot be reused to make more nanosilica, increasing the cost significantly as the surfactants are the most expensive reagent in the process. All of this adds up to a more complex and environmentally damaging two-step synthesis, locking away the nanosilicas from seeing widespread use.
This creates a stark contrast with natural silica materials – there are several microorganisms that create highly detailed and complex silica cell walls around themselves (Figure 3). Additionally, this occurs in the ocean, which has much milder conditions compared to those used in the lab.
The way these microorganisms can manage this amazing feat is through specialised proteins that, in addition to acting like a far more complex template than the surfactants used in artificial nanosilicas, also give the chemical reaction a huge speed boost to boot.
So if nature can do that, why can’t we? The simple answer is that we can. Using template molecules whose structure is inspired by the natural proteins, we can produce silica faster, greener and cheaper than current industry methods (Figure 4) while retaining the quality of nanosilicas.
While using such “bio-inspired” templates is an excellent solution to the drawbacks of nanosilica synthesis, there remains the need to remove the templates from the material. Simply aping the methods of purifying other nanosilicas like heat treatment methods negate much of the benefits of adopting this bio-inspired approach, as well as blocking their use for more specialised applications, such as the storage of delicate biomolecules.
Instead, in our most recent study, we took a step back and studied what makes the bio-inspired template so good at its job in the first place. Using computer simulations of the template and silica, we found that the two species are attracted to each other by their opposite charges, which is the source of both the structure direction and speed boost. What we also found is that this attraction is highly dependent on the solution chemistry – simply washing the materials in acidic environment (contrasting with the neutral or slightly alkaline reaction conditions) acted like a switch to unstick the bio-inspired templates from the nanosilica, leaving behind a pure, ready-to-use material, and similarly, a ready-to-reuse template molecule. This is specific to the type of interaction between the template and silica, meaning that this discovery was only possible because we used the bio-inspired templates rather than the surfactant templates whose interactions are much more difficult to switch off.
The new washing technique is a clear improvement over purification by heat treatment, as washing both eliminates the energy costs and allows for the templates to be used as a catalyst rather than a reagent, both of which are important principles of green chemistry. Environmental improvements notwithstanding, the washing method has some significant technical advantages over the previous methods, too. By fine-tuning the strength of the washing acid, we could choose to only remove a certain amount of our template, leading to new composite materials in a much simpler, less laborious way than before (where the surfactant had to be fully removed prior to a separate reintroduction of active chemicals into the structure) (Figure 5).
Overall, this new technique has unlocked the possibility for nanosilicas to be upscaled to industrial levels. By harnessing the power of computer simulations, and applying green principles to the technique design, this study has cut down energy costs of material purification significantly and avoided damage to the template, allowing for it to be reused. Not only that, but the elimination of harsh conditions during all parts of the process enables new applications of nanosilica in carrying fragile enzymes or other biomolecules.
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