image: Multiscale tracking strategy integrating molecular, microscopic, and macroscopic approaches, comparing conventional (left) and AIE-enabled (right) monitoring systems.
Credit: ©Science China Press
Complex multiscale dynamics are inherent to materials, where the interconnection across molecular, microscopic, and macroscopic levels is key to understanding their fundamental mechanisms. A prime example is the film formation of polymer emulsions—essential materials in coatings and adhesives—which involves multi-stage dynamics like water evaporation and particle fusion. However, real-time and precise tracking of these processes has remained challenging. Conventional methods often rely on multiple technologies, leading to difficulties in data integration and limited spatiotemporal resolution.
"To address these challenges, our study introduces an innovative monitoring platform leveraging aggregation-induced emission (AIE) technology," explains Dr. Jin Wang, first author of the study published in National Science Review and a researcher at The Hong Kong University of Science and Technology (HKUST). "This platform seamlessly integrates molecular sensing, microscopic imaging, and macroscopic detection into a unified system." AIE fluorogens possess a unique property: their fluorescence emission intensifies upon restriction of molecular motion, enabling high-contrast, in situ, and real-time observation of dynamic changes across multiple scales during film formation. This groundbreaking work establishes a new paradigm for multiscale research in complex systems, with far-reaching implications spanning materials science and life sciences.
At the molecular scale, this study leverages the unique motion-restricted emission mechanism of the AIE probe TPE-4S-Na to achieve real-time tracking of polymer emulsion film formation. As the emulsion transitions from wet to dry, the probe's freedom of movement becomes restricted by polymer chains, causing its photoluminescence quantum yield to surge from 0.5% to 20.2%. Real-time fluorescence monitoring successfully resolved the process into three distinct stages: initial water evaporation (0-11 min), rapid solidification (11-27 min), and final stabilization (27-41 min). This molecular-level monitoring provides unprecedented insight into local drying states and the evolution of wet-dry interfaces.
At the microscopic scale, super-resolution fluorescence microscopy unveiled detailed film formation dynamics. At 25°C, distinct particle fusion was observed, with the AIE probe revealing capillary force-driven deformation through characteristic fluorescence patterns. In contrast, at 5°C, particles remained separate without fusion. Notably, a "coffee-ring effect" was visualized through alternating bright/dark fluorescent rings in multilayer films, where fluorescence intensity closely matched thickness variations measured by AFM (227 nm difference at edges). This real-time microscopic monitoring provides direct evidence for understanding defect formation, offering critical guidance for coating performance optimization.
The strategy was further extended to macroscopic-scale monitoring by correlating fluorescence intensity with grayscale values, enabling the development of a low-cost monitoring platform using ordinary cameras. This system provides a clear, real-time visualization of the drying front as it progresses from the edges to the center. Grayscale analysis effectively identifies three distinct zones—wet, particle-packing, and dry—while also delineating three kinetic stages consistent with molecular-scale observations. This affordable approach eliminates the need for expensive equipment or specialized sample preparation, presenting a practical solution for quality control in industrial coating processes.
For practical applications in coatings, the strategy was successfully demonstrated on industrial-scale, eco-friendly waterborne wood coatings. The technique clearly visualized the three-zone drying process—wet, particle-packing, and fully dried—as it progressed vertically on a 30 × 20 cm wooden panel. The drying kinetics revealed a slow initial phase (before 27 min), a rapid-drying stage (27–85 min), and completion at 93 min. Furthermore, the method proved adaptable to various polyurethane and polyacrylate emulsions, showcasing its robust potential for real-time quality control in diverse industrial coating applications.
“This study pioneers a unified monitoring platform based on AIE technology, seamlessly bridging molecular, microscopic, and macroscopic scales,” emphasizes Prof. Ben Zhong Tang, co-corresponding author of the paper from The Chinese University of Hong Kong, Shenzhen (CUHK-Shenzhen), and HKUST. By harnessing the motion-restricted emission of water soluble AIEgens, the researchers integrated molecular sensing, particle dynamics tracking, and industrial-scale coating inspection into a cohesive framework. Successfully validated in practical applications such as waterborne wood coatings, this strategy not only provides fresh insights into film-forming mechanisms but also offers a versatile and accessible solution for real-time quality control. The established paradigm holds significant potential for analyzing complex dynamic processes across diverse fields, from materials science to biomedical research.