Creation of new macroscopic shapes from fragments of an initial self-assembled structure. Yellow-arrowed path from 1 to 3: (1) An initial fiber-like structure is created, then broken down with UV light (purple lightning symbol); (2 & 3) because they are tightly packed in the aftermath of the collapsing structure, these fragments co-assemble into other diverse shapes. Gray-arrowed path from 4 to 5: simply mixing the same molecules in a solvent does not create the same variety and size of nanostructures.Researchers at Okinawa Institute of Science and Technology Graduate University (OIST) in Japan have created self-assembling molecules that can be broken down by ultraviolet light to recombine into novel macroscopic shapes.
Traditional chemistry is immensely powerful when it comes to producing very diverse and very complex microscopic chemical molecules. Currently out of reach, however, is the synthesis of large structures up to the macroscopic scale, which would require tremendous amounts of chemicals as well as an elaborate and complicated synthesis technique.
For this purpose, scientists rely instead on ‘self-assembling’ molecules, compounds that can interact with other copies of themselves to spontaneously congregate into spheres, tubes or other desired shapes. Using this approach, researchers at OIST have created new self-assembling molecules that can transform into novel, exotic and previously unobserved shapes when irradiated with UV light, which forces them to rearrange into different ‘metastable’ states. They describe this work in a paper in Chemical Communications.
When designing self-assembly structures, scientists typically aim for the state of lowest energy – or ‘ground state’ – in which the structure would be at its highest stability. The downside to this stability is that breaking down the structure in order to alter its shape becomes very difficult. In this research, OIST scientists inserted a weakness into their ground-state self-assembled structures, resulting in structures that required only a small nudge to collapse. In this case, the nudge comprised using ultraviolet light to snip a specific bond between two atoms within the molecule, splitting the structure into smaller fragments. The fragments are then able to co-assemble into less stable – known as metastable – but novel and exotic shapes.
“This report is about a new concept in material science,” explained Ye Zhang from OIST’s Bioinspired Soft Matter Unit and an author of the study. “We converted a self-assembling phenomenon into co-assembling in a spatially and temporally controllable manner using light. Eventually, we constructed exotic heterogeneous nanostructures inaccessible though conventional synthetic path.”
This new concept led to a fascinating discovery: because the remaining fragments are tightly packed together following their collapse from the initial structure, they can form novel and exotic structures that are not attainable if the same molecules are simply mixed together.
The nanostructures can be imagined as Lego bricks: for example, 2x5 bricks – 2 studs wide and 5 studs long – that self-assemble into a nanofiber. Ultraviolet light will split these 2x5 bricks into two smaller pieces: for example, a 2x3 brick and a 2x2 brick, destroying the entire fiber-like structure. But because these smaller bricks stay close to each other, they can easily recombine into new shapes visible with the naked eye. In contrast, if the 2x3 and 2x2 bricks are simply mixed together in a bucket with varying distances between the bricks, their lack of spatial organization prevents the assembly of such novel nanostructures.
According to Zhang, the ability to create new structures is vital: “In material science, the function is always related to the structure. If you create a different structure, you manipulate the function and even create new applications.” For example, the toxicity of a molecule in a nanofiber shape might be much lower or higher than the same molecule assembled in a spherical shape.
The present research performed at OIST strongly suggests that the initial conditions are the most important critical factor influencing the final shape taken by self-assembling molecules. “If you know how the molecules pack with each other from the parameters of the initial state, then it will give you more clues to aim towards a specific macroscopic shape,” said Zhang.
This shapeshifting ability holds great potential for biological applications, Zhang suggested. “For example you introduce the molecule into a living organism and it adopts a certain structure. Then using light, you break a chemical bond and then the molecule will switch to another structure with the function you want.”
In pharmaceutical design, such a concept would allow a drug to reach its target in a living organism – an organ or a tumor – in an inactive state, thus limiting potential side effects. Once at the target location, however, the drug would reshape itself into a different structure with therapeutic activity.
“For now, using ultraviolet light as we do is not ideal as it is toxic for living cells. The next step for us is to move towards more biocompatible self-assembling structures with better adaptability to living systems,” said Zhang.
This story is adapted from material from OIST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.