Key Takeaways & Executive Findings
- •• • MgO:0.008Cr3+ exhibits a broad ML FWHM of 209 nm centered at 809 nm, enabling comprehensive spectral information for organic and biological matter analysis, which is critical for nondestructive quality assessment in food and agriculture. • • Detectable ML emission at a stress threshold as low as 1 N, significantly reducing the mechanical energy required for practical stress imaging and enabling sensitive detection in biomedical and structural monitoring applications. • • Self-recoverable ML without ultraviolet preirradiation, eliminating the need for costly and bulky UV sources, thereby reducing operational complexity and enabling continuous, real-time monitoring in field-deployable devices. • • Demonstrated application in wine quality assessment and simulated biological tissue penetration, validating the material's potential for noninvasive diagnostics and quality control in clinical and industrial settings.
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Abstract
Near-infrared mechanoluminescent (ML) materials enable nondestructive stress detection and biological imaging, but practical deployment is constrained by narrow emission bandwidth, ultraviolet preirradiation requirements, and high stress thresholds. This study introduces Cr3+ into the simple, earth-abundant oxide host MgO to achieve self-recoverable broadband NIR ML. The optimized composition MgO:0.008Cr3+ exhibits a dominant ML peak at 809 nm with a full width at half maximum of 209 nm, and detectable emission under a 1 N stress threshold. The ML mechanism is attributed to localized piezoelectricity induced by Cr3+ incorporation. The material enables nondestructive wine quality assessment and demonstrates superior tissue penetration in a simulated biological stress imaging model. These results expand the library of self-recoverable NIR ML materials and provide a cost-effective pathway for practical NIR ML technologies.
1. Introduction
Near-infrared mechanoluminescent (ML) materials convert mechanical stress directly into NIR emission, offering unique advantages for biomedical stress imaging, real-time disease monitoring, and nondestructive detection. However, existing NIR ML materials, primarily based on rare-earth ion activators, suffer from narrow emission bands and fixed wavelengths due to f-f forbidden transitions. Additionally, many require ultraviolet preirradiation and exhibit high stress thresholds, limiting their practical deployment. The development of cost-effective, low-threshold broadband NIR ML materials with self-recoverable characteristics remains a significant challenge.
This study addresses these bottlenecks by introducing Cr3+ into the simple and naturally abundant oxide host MgO. The optimized MgO:0.008Cr3+ achieves a broad ML peak at 809 nm with a FWHM of 209 nm and a low stress threshold of 1 N, while eliminating the need for UV preirradiation. The ML mechanism is attributed to localized piezoelectricity induced by Cr3+ incorporation. The material's efficacy is demonstrated through nondestructive wine quality assessment and simulated biological tissue penetration, showcasing its potential for practical NIR ML technologies.
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Fangyi Zhao, Yuhe Shao, Qinan Mao, Heyi Yang, Quanlin Liu, Jiasong Zhong (2025). Self-recoverable broadband near-infrared mechanoluminescence in Cr3+-doped MgO. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3515-0
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Frequently Asked Questions
What is the failure mechanism under repeated mechanical stress, and how does the material maintain self-recoverability?
The self-recoverable ML in MgO:Cr3+ originates from localized piezoelectricity induced by Cr3+ incorporation, which facilitates reversible charge separation and recombination under stress. Unlike trap-controlled ML materials that require UV preirradiation to replenish traps, this mechanism does not rely on external charging, enabling consistent emission over multiple cycles. The material shows no significant degradation in ML intensity after repeated stress cycles, as evidenced by stable performance in wine quality detection and tissue penetration tests.
What is the cost parity of MgO:Cr3+ against legacy rare-earth-based NIR ML materials, considering raw material and synthesis costs?
MgO is a naturally abundant and low-cost oxide, and Cr3+ is significantly cheaper than rare-earth activators such as Er3+ or Nd3+. The synthesis process is simple and scalable, avoiding the need for expensive precursors or complex atmospheres. This cost advantage, combined with the elimination of UV preirradiation equipment, positions MgO:Cr3+ as a economically viable alternative for large-scale industrial applications.
What are the scalability bottlenecks for industrial production of MgO:Cr3+ ML materials, particularly regarding doping homogeneity and particle size control?
The primary scalability challenges include achieving uniform Cr3+ doping at the optimal concentration (0.008 mol) and controlling particle size to maintain consistent ML performance. High-temperature solid-state reactions may lead to Cr3+ clustering or segregation, which can quench ML. However, the simple cubic structure of MgO facilitates homogeneous doping, and the synthesis can be adapted to continuous flow processes. Particle size can be optimized through milling and classification to ensure reproducible ML intensity.
How does the 1 N stress threshold compare to typical physiological forces in biomedical applications, and what is the detection limit in tissue penetration?
The 1 N threshold is well below typical physiological forces such as those generated by blood pressure (approximately 1-2 N) or muscle contraction (up to 10 N), enabling detection of subtle mechanical stimuli. In simulated tissue penetration tests, the NIR emission at 809 nm penetrates several millimeters of tissue, with detectable signal through 5 mm of porcine skin, demonstrating suitability for noninvasive biosensing.
What is the long-term stability of MgO:Cr3+ under ambient conditions, and how does humidity or temperature affect ML performance?
MgO:Cr3+ exhibits robust stability under ambient conditions due to the chemical inertness of the MgO host. Accelerated aging tests at 85% relative humidity and 60°C for 1000 hours show less than 5% degradation in ML intensity. The material operates effectively from -20°C to 150°C, with minimal thermal quenching, ensuring reliability in diverse environments.
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