A new phosphorescent material remembers mechanical stress and resets with ultrasound, enabling reusable optical sensors to track hidden structural damage in aircraft components.
(Nanowerk Spotlight) When structural engineers examine aircraft components for signs of fatigue or hidden damage, they typically rely on instruments like ultrasonic scanners, X-ray systems, or visual inspection. These methods work well for detecting existing cracks, but they share a fundamental limitation: they cannot reveal the history of mechanical stresses a component has endured.
A wing spar that has weathered thousands of flight cycles looks identical to one fresh from the factory unless damage has already manifested. What if materials could remember their mechanical past and display it visibly, like a bruise revealing where the body took a hit?
Materials scientists have pursued this goal with mechano-responsive luminescent materials, substances that convert mechanical force into optical signals. When stressed, these materials emit or change light in detectable ways.
Traditional phosphors based on compounds like zinc sulfide doped with manganese produce light under stress, but their signals vanish almost instantly once the force is removed. They function as real-time indicators but cannot store information about past events.
Other approaches using organic molecules embedded in polymers have achieved longer-lasting signals, but with a different problem: the changes are permanent. Once written, the luminescent information cannot be erased or reset, limiting reusability. Many systems also suffer from irreversible degradation under repeated loading, photobleaching from extended light exposure, or thermal breakdown in harsh environments.
A research team based in China has now developed a material that overcomes these obstacles. Published in the journal Advanced Science (“Reversible Stress‐memory Phosphorescent Carbon Nanodots via Supramolecular Confinement Engineering for Aerospace Monitoring”), the open-access study describes carbon nanodots embedded within a cyclodextrin crystalline framework. These composite materials exhibit room-temperature phosphorescence, meaning they glow after the excitation light switches off, with properties that respond to mechanical stress in a reversible and memorable fashion.
The system hinges on hydrogen bonding between the carbon nanodots and the cyclodextrin molecules that surround them. Cyclodextrins are ring-shaped sugar molecules with hollow, hydrophobic cavities that can trap small guest molecules. Pharmaceutical companies have used them for decades to improve drug solubility and stability.
Phenomenon and mechanism of the cyclodextrin-trapped carbon nanodots. a,b) Schematic illustration of the mechano-responsive phosphorescence mechanism (a) and corresponding energy level transition diagram (b). (Image: Reproduced from DOI:10.1002/advs.202521219, CC BY)
When carbon nanodots lodge within this crystalline matrix, the surrounding hydrogen-bond network creates a rigid microenvironment. This rigidity suppresses the molecular vibrations and rotations that would otherwise dissipate the energy of excited electrons as heat rather than light. The result is phosphorescence lasting hundreds of milliseconds, visible for several seconds after ultraviolet excitation stops.
Mechanical stress disrupts this arrangement. When force acts on the material, the hydrogen bonds between cyclodextrin molecules and the carbon nanodots distort or break. The local environment becomes less rigid, allowing the nanodots to vibrate and rotate more freely. These enhanced molecular motions open non-radiative decay pathways, meaning the excited electrons lose their energy as heat instead of emitting photons. Phosphorescence intensity drops proportionally with applied stress.
The researchers built a custom compression device to quantify this relationship. At zero mechanical stress, the phosphorescence lifetime measured approximately 0.36 s. At 4 MPa of applied pressure, this dropped to 96 ms. Phosphorescence intensity followed a similar decline. Crucially, removing the stress did not restore the original emission. Even after 30 minutes at ambient conditions, the quenched state persisted. The material had memorized its mechanical history.
This memory effect arises because the stressed state represents a metastable configuration. The thermal energy available at room temperature cannot push the hydrogen-bond network back over the energy barrier to its original arrangement. The system remains trapped in its new local energy minimum, retaining the structural imprint of past forces.
Recovery requires external energy input. The team demonstrated that ultrasonic treatment could reset the material. Sound waves propagating through the sample generate localized mechanical vibrations and stress waves that provide enough energy to overcome the barrier and restore the original hydrogen-bonded framework.
After 40 minutes of ultrasonic treatment, phosphorescence intensity and lifetime recovered substantially. Transmission electron microscopy confirmed that the crystalline structure reverted from a disrupted, amorphous state to its original ordered form.
The intrinsic photophysical properties of the carbon nanodots remained unchanged throughout these cycles. Fluorescence spectra, which reflect the electronic structure of the nanodots themselves, showed no shift before or after stress. Quantum yield measurements fluctuated only within a narrow range regardless of pressure. The modulation arises entirely from changes in the surrounding matrix, not from damage to the luminescent particles.
The researchers subjected samples to five complete cycles of mechanical stressing followed by ultrasonic recovery. Phosphorescence intensity remained consistent throughout, demonstrating the robustness and repeatability of the system.
To test practical applicability, the team incorporated the cyclodextrin-trapped carbon nanodots into a polyvinyl alcohol film. This flexible luminescent film retained the stress-responsive phosphorescence behavior and survived 500 bending cycles without significant loss of emission intensity. Under varying stress levels from 0 to 10 MPa, the film displayed progressive phosphorescence quenching that correlated directly with applied force.
The researchers envision structural health monitoring for aerospace components as the primary application. They propose embedding such films on aircraft wing surfaces, where they would function as distributed optical sensors. During flight, any sudden stress event such as a micro-impact would induce localized phosphorescence quenching. Imaging systems could capture these intensity variations, translating them into digital stress maps.
Regions showing pronounced quenching would flag accumulated fatigue or potential failure points. Because the phosphorescence persists after excitation stops and the memory effect preserves stress history, monitoring could occur without continuous illumination or real-time data streaming.
The work represents a departure from conventional approaches. Most mechano-responsive luminescent materials rely on covalent bonds that break irreversibly under stress or produce transient signals that cannot be recorded. By building on non-covalent hydrogen-bond interactions, the cyclodextrin-carbon nanodot system achieves both memory and reversibility without permanent chemical damage.
This research establishes a framework for designing smart materials capable of non-destructive structural monitoring in demanding environments. The combination of low toxicity inherent to carbon nanodots, tunable emission properties, and reversible mechano-responsive behavior positions these materials for potential integration into composite structures where tracking mechanical history could enhance both safety assessment and maintenance efficiency.
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