Better Bonds with Water

Chemistry based on molecular interactions in water could yield novel materials that stand up to pressure but are much more adaptive than those created with traditional methods. Dr. Boris Rybtchinski and his team in the Institute’s Organic Chemistry Department (Faculty of Chemistry) recently applied this original approach to produce a unique nanoparticle filter that not only simplifies the size-sorting process but also comes apart for cleaning and is recyclable. Their findings appeared in Nature Nanotechnology.

 

Almost all functional materials produced today are held together by strong, irreversible bonds known as covalent bonds. These bonds are what make such materials as polymers strong, but under normal circumstances they lack the ability to change, making them difficult to recycle or even dispose properly. In contrast, so-called supramolecular systems are held together by noncovalent interactions. Supramolecular systems are easily self-assembled and are adaptive – they can be self-healing, for instance – so they are easy to fabricate and recycle. Until now, however, what these systems gained in flexibility, they lost in strength.

 

Rybtchinski and his team, including Ph.D. students Elisha Krieg and Elijah Shirman, and staff scientists Drs. Haim Weissman and Eyal Shimoni, have been looking at a noncovalent attachment between molecules known as hydrophobic bonding. Hydrophobic molecules are “water-hating”: When placed in water they bond together, something like coalescing oil droplets. An analysis of the chemical forces reveals that hydrophobic bonds could be relatively strong yet adaptive, not to mention environmentally friendly and cost-efficient. But are these bonds sturdy enough for producing useful new materials that can compete with the existing covalent ones?

 

Supramolecular systems are good candidates for such specialized applications as nanoparticle filters. Existing filters – made to retrieve particles just a few billionths of a meter across – are expensive and difficult to use, and they tend to clog and break. A recyclable filter – one whose bonds break and reform – could overcome these problems.

 

The researchers created molecules with a large hydrophobic component and poured a water solution of them onto standard, inexpensive filter material with very large pores. Instead of running through the filter, the molecules bonded into a sponge-like three-dimensional network filled with even, nanometer-sized spaces.

 

The network turned out to be an excellent nanoparticle filter. When the scientists passed a solution containing gold nanoparticles through the nanoscale network, only those smaller than five nanometers (a critical size for many applications) progressed beyond the supramolecular membrane. To retrieve the nanoparticles and reuse the filter, the team simply dissolved the filter’s hydrophobic bonds with common alcohol. Repeating the process over and over, they found that the filter network could be easily dissolved and reconstituted for further rounds of sorting – without any loss of performance or efficiency.

 

 

Next the scientists wanted to see if their network could be even more specific in sorting the particles. This time they created a slightly thicker 3-D structure. After pouring the nanoparticle solution through, they inspected the network under the Institute’s electron microscope. As anticipated, the smaller particles had penetrated farther into the material while the larger ones were caught closer to the surface, and these could easily be separated according to size – enabling very precise sorting.

 

Rybtchinski believes that, with a few adjustments, recyclable filtering networks could eventually present a more efficient, greener alternative to some of the particle-sorting methods used today. The methodology may also be promising for separating such biomolecules as proteins and DNA. “This method could be quite cost-efficient and easy to use. There is practically no waste involved. Best of all, we have demonstrated a completely new application for non-covalent bonds: We’ve shown they can be robust and at the same time easily reversible, enabling novel noncovalent materials that are more versatile and environmentally friendly than their covalent counterparts.”

 

Yeda, the Institute’s technology transfer arm, has filed for a patent for the noncovalent membranes.