An MIT research team is in the process of creating fluorescent polymer gels capable of changing color when they are heated, exposed to acid, shaken, or disrupted in some form. With this kind of response, these innovative materials hold promise as effective sensors for identifying modifications in structures, the environment or in fluids.
Fluorescent gels could be used as sensors for detecting structural failure in energy-related equipment. These gels were synthesized by mixing a polymer (polyethylene glycol) and a ligand (terpyridine) with two lanthanides — europium and terbium — in varying ratios. The red sample at the far left contains only europium, while the blue-green sample at the far right contains only terbium. A white-light-emitting gel forms when the mixing ratio of terbium to europium is 96 to 4. (Photo credit: Pangkuan Chen)
The researchers mix a commonly used polymer with a chemical that can join the two together and a metal that fluoresces. The binder and the metal immediately self-assemble when mixed into a solvent, and grab the polymer molecules and draw them together to develop into a gel.
The researchers experimented with a variety of metals so as to be able to manipulate the color of light it emits and physical properties of the gel. The gels produced a color-coded response to a number of mild external stimuli during a sequence of tests, and later returned to their pre-stressed color and state.
Natural organisms exhibit certain extraordinary behaviors. For example, the mussel creates well-built fibers that allow it to attach firmly to rocks, boats, and other underwater surfaces. However, those fibers are unstable. When pulled, the stiff fibers become stretchy, and let go. The fibers then return back to their initial stiff state, “self-repairing” any injury that may have occurred. In comparison, manmade materials are usually not highly dynamic, and when they crack, the damage is permanent.
Niels Holten-Andersen, assistant professor in the Department of Materials Science and Engineering (MSE) and the Doherty Professor in Ocean Utilization, has been interested for a long time in mussel fibers and the component that is crucial to their achievement — the metal coordination complex.
This structure comprises a single metal ion, which is a charged particle, with many chemically bound arms, or “ligands,” radiating outward. The ligands comprised organic which are carbon-containing, molecules and can attach to other molecules, thus allowing the complex to act as a crosslink that binds materials together.
Given that capacity, the metal coordination complex has a pivotal role to play in several biological systems such as the human body, where it catalyzes enzyme-controlled reactions controlled by enzymes and binds oxygen to blood’s hemoglobin factor.
Holten-Andersen says that the way nature assembles materials has always captivated him for instance, the creative assembling of sugars, proteins, and fatty acids to form intricate dynamic structures.
We can’t copy nature’s materials. For example, it’s difficult to synthesize proteins in the lab. But we can see how nature builds its materials and why they work the way they do. We can then try to mimic the way nature has done it but using simple, inexpensive building blocks that we know how to make.
To Holten-Andersen, polymer building blocks looked like a fine bet.
“We know how to make simple, cheap, green polymers in large quantities,” he says. About four years ago, he planned to develop polymer gels bound together by metal coordination complexes constructed on transition metals — a group of elements he had often seen in biological scenarios.
To begin with the results showed potential. The metals, polymer molecules, and ligands instantaneously self-assembled into gels, and the emitted colors and mechanical properties of the gels relied upon the transition metal used.
Feeling positive after these results, Holten-Andersen made up his mind to try another group of metals - the lanthanides. Similar to the transition metals, the lanthanides — commonly called as the rare-earth elements — offer a number of appealing and complicated behaviors.
But they have one extra intriguing feature – ability to fluoresce. When lanthanide is exposed to UV light, and it becomes excited and releases light at a characteristic wavelength.
By using the lanthanides, we could still control the properties of our gels, but now we’d have light emission that would reflect any changes in those properties. With those two features intimately coupled, any time the physical properties were disturbed — say, by a change in the temperature of the nearby air or the pH of the surrounding water — the color emitted would change.
This sort of polymer gel could report on its own state and act as an exceptional sensor. For instance, it could be applied as a coating that monitors the structural reliability of cables, pipes, and other underwater structures vital to offshore gas and oil and wind farm companies.
Testing in liquids
Before beginning to test polymers, Holten-Andersen was keen on verifying — as others had revealed — that blending ligands and lanthanide ions in a solvent would generate light-emitting fluids.
Accordingly, the team, - using the Laboratory for Bio-Inspired Interfaces, and consisting of Pangkuan Chen, postdoc in MSE; Qiaochu Li and Scott Grindy, both MSE graduate students; and rising senior Rebecca Gallivan of MSE and rising junior Caroline Liu of mechanical engineering - mixed terpyridine, a commercially available ligand material, with certain lanthanides in a solvent.
The mixtures generated liquids that fluoresce under UV light in the typical colors of the three lanthanides - red for europium, blue for lanthanum, and green for terbium. Those outcomes validate that the complexes formed as anticipated.
However, a mixture releasing pure white light would form a much better sensor. It is easier to view white light turn faintly green than it is to view green light turn faintly less green. The researchers discovered much to their surprise that producing a white-light-emitting fluid was easy. Since white light is essentially a blend of several colors, they simply needed to combine together their red, blue, and green light-emitting fluids. By putting together equal portions of the three colored fluids, a glowing white liquid was produced.
Next, the team exposed their white-light-emitting fluid to a sequence of external stimuli to observe if they would obtain a color-coded response. The result was positive.
For instance, the emitted light gradually changed color when they mildly heated the fluid from room temperature to 55°C. When they allowed it to cool down, the white light returned. The metal ions and ligand split apart when they were heated and then rejoined when they were allowed to cool. The fluid also established as sensitive to a number of changes in pH.
“So we found that this simple blue-red-green approach to making a white-light-emitting system indeed leads to materials that respond to a variety of stimuli, and with that response comes a change in color,” says Holten-Andersen.
Therefore, the fluids may act as excellent sensors for identifying chemical differences within a liquid or for monitoring velocity gradients in fluid flow experiments — differences in flows that now has to be established indirectly by simulation.
Adding the polymer
In the subsequent series of tests, the team tried adding their lanthanide ions and terpyridine ligands into a commonly used polymer known as polyethylene glycol, or PEG. The polymer molecules coupled with ligands, at the start of the experiments, were free-floating in a solvent.
We then mixed in one of our lanthanide metals, and after some gentle shaking, the mixture changed from a fluid to a fluorescent gel.
The ligands and metal ions had self-assembled, joining the polymers together. Once more, they discovered that gels based on a variety of lanthanides emitted varied colors, and integrating them in a range of ratios generated shifts in color.
A series of gels were created from terbium and europium. The sample that was all europium was red; the one that was all terbium, was blue-green; and those in between were created with a range of ratios of the two. Bright white luminescence emerges in the second sample, when the blending ratio of terbium to europium is 96 to 4. The samples illustrate the ease of designing “metallogels” with a wide range of colors.
Similar to the fluids, the gels were established as sensitive detectors of changes in pH and temperature. But possibly the most spectacular response resulted when the gels were sonicated, that is, disturbed by exposure to high-frequency sound waves. The changes can be seen in the white-light-emitting gel when soaked in an ultrasonic bath. The gel is moderately broken down into a fluid in the sample taken after 5 minutes.
The gel that stays retains its white luminescence, while the fluid emits blue light — which is due to the now-split ligands. After another 11 minutes of disturbance, the sample conversion of the gel to fluid is total, and the blue light of the ligands shines forth.
And once more, when some time is given, the white-light gel reassembles.
When we let the blue fluid rest overnight, the polymers found each other again, and it turned back into a gel and made white light. That was very exciting for us because it really shows in principle that as a proof-of-concept, our approach works under these conditions. We can make a material that emits white light, reports its own failure, and then recovers. So it’s a self-reporting material that’s also self-healing.
Currently, Holten-Andersen and his team are analyzing the application of their materials as coatings that can sense failure in structures as well as alterations in temperature and pH— a capability that will be extremely helpful in several environmental and energy systems. The present research is focused on coatings for underwater cables used to supply electric power from ocean wind turbines to the shore.
The team is also planning further primary studies. Area of interest is in producing materials with the capacity to change in response to a variety of external stimuli and then autonomously repair by returning to their former state. If such self-healing materials were available, it would minimize the need to fabricate their replacements in the future.
Holten-Andersen states that knowing the method to assemble self-healing materials requires knowing the way those materials fail and repair in real-world scenarios, which is challenging to study.
He anticipates their novel materials might help. The chemical bonds in the metal-coordinate crosslinks possess an extraordinary ability to split and then re-form — and to broadcast that activity with alterations in light emission. Based on those light alterations and high-resolution imagery, the researchers were guided to get unique insights into where, when, and how the material splits and rejoins.
Holten-Andersen highlights that the researchers still have plenty to discover from nature.
We’re just scratching the surface in understanding nature, given the technology we now have to look at it.
For example, he feels that man is far from discovering all the metal coordination complexes that are used by nature.
They could happen in other natural materials with extraordinary properties such as spider silk, which is elastic, tough, flexible, and an extremely strong material.
“It’s hard to see these metals,” he says. “They appear in tiny concentrations and a single molecule at a time. But I think metal coordination complexes are much more prevalent in nature’s materials than we are currently aware of.” And coordination complexes are only one of the several tricks that nature has created over time to help organisms cope with tough environmental conditions.
The MIT Energy Initiative (MITEI) Seed Fund Program and MIT Sea Grant via the Doherty Professorship in Ocean Utilization sponsored this research. Student researcher Caroline Liu received support from the Energy Undergraduate Research Opportunities Program through MITEI with funding from Lockheed Martin, a Sustaining Member of MITEI.