Fluolab【Angew.Chem.】北京理工大学贺志远、蔡政旭、陈笑非||探测“隐形冰”的黑科技：SBR提升152倍，近红外余辉让覆冰无所遁形
通讯作者： Xiaofei Chen, Zhengxu Cai, Zhiyuan He
文章概要
引言
积冰现象严重威胁着航空飞行、电力输送及风力发电等基础设施的安全，特别是那些透明且与基底颜色高度一致的“隐形冰”，极难通过肉眼或传统的反射光学手段察觉。尽管现有的电学和热学传感器能提供初步预警，却无法直观展示冰层的空间分布和动态演变。传统的荧光成像虽具潜力，但在复杂的积冰环境下常面临严重的背景噪声、环境光干扰以及聚集诱导淬灭问题。为了攻克这一难题，研究团队提出了一种离子特异性诱导的近红外磷光（FIP）成像策略。通过利用新型有机磷光探针与冰-水界面的协同作用，实现了在极端环境下对隐形冰的高对比度、深层渗透及定量可视化监测。
Molecular structures of PP4P-X and counterion-dependent photophysical properties. (a) Chemical structure of the cationic PP4P framework and the exchangeable counterions in PP4P-X (X = F−, Br−, I−, NO3−, and SCN−). Photographs show the solid PP4P-X powders under ambient light, all exhibiting an intrinsic orange color. (b–f) Normalized UV–vis absorption spectra in ethanol (1 × 10−5 M), steady-state photoluminescence (PL) spectra at 300 K and delayed emission spectra at 77 K (delay time: 50 µs, = 460 nm) for PP4P-F (b), PP4P-Br (c), PP4P-I (d), PP4P-NO3 (e), and PP4P-SCN (f) in solution and in the solid state.
主要实验及结论
研究人员设计并合成了一系列基于芳基取代吡咯并吡咯（PP4P-X） 结构的离子型探针，涵盖了氟、溴、碘等多种反离子。实验发现，虽然这些探针在冷冻状态下都会表现出荧光增强，但近红外磷光（λ = 750 nm）却表现出极强的离子特异性。其中，PP4P-F（氟离子）探针在冷冻诱导下的磷光增强最为显著，实现了从无发射到强磷光信号的跨越。通过原位低温共聚焦显微镜和分子动力学模拟发现，氟离子由于其极强的亲水性和与冰晶格的互不相容性，在冰层生长过程中会被排斥并高度富集于冰-水界面。这种界面锚定效应促使探针分子紧密聚集，通过协同的离子-π和π-π相互作用抑制了分子的非辐射衰减，从而高效触发并稳定了三线态激子的磷光发射。
Time-dependent photoluminescence evolution and macroscopic imaging of PP4P-F during a freezing-thawing cycle. (a) Steady-state PL spectra of an aqueous PP4P-F solution (1 mM) at selected time points during isothermal freezing at 243 K. (b) Corresponding delayed-emission spectra (delay time: 50 µs), showing the emergence and growth of NIR phosphorescence (∼750 nm). (c, d) Time-dependent intensity traces of fluorescence (c) and phosphorescence (d) over the full freezing-thawing cycle. Fluorescence increases and reaches a plateau as freezing proceeds, while phosphorescence appears, strengthens, and stabilizes during ice formation and vanishes upon thawing at 300 K. (e) IVIS Spectrum macroscopic images of fluorescent (Fluo.) and phosphorescent (Phos.) signals under 460 nm excitation. The emission bands propagate across the sample following the movement of the freezing front, yielding strong contrast after complete freezing and disappearing after thawing. Scale bar, 2 cm; signal units: p·s−1·cm−2·sr−1; color map, relative radiant efficiency.
Ion specificity of freezing-induced luminescence and in situ imaging at the ice-water interface. (a) Steady-state PL (top) and delayed-emission spectra (bottom; delay time, 50 µs) of PP4P-X (X = F−, Br−, I−, NO3−, and SCN−) aqueous solutions (1 mM) after freezing at 243 K, showing universal fluorescence enhancement but prominent NIR phosphorescence only for PP4P-F and PP4P-Br. (b) / intensity ratios for fluorescence (blue) and delayed emission (red). Fluorescence enhancement is highest for PP4P-SCN and lowest for PP4P-F, whereas phosphorescence enhancement is most pronounced for PP4P-F, followed by PP4P-Br. (c, d) In situ low-temperature CLSM images of the advancing ice-water interface: bright-field (top), fluorescence channel (490–560 nm, middle), and phosphorescence channel (650–720 nm, bottom). (c) PP4P-F exhibits strong interfacial enrichment and aggregation at the freezing front, accompanied by intense fluorescence and gradually rising phosphorescence. (d) PP4P-I shows partial interfacial accumulation and fluorescence enhancement but only weak phosphorescence. Scale bar, 50 µm.
在应用测试中，PP4P-F展现了卓越的抗干扰能力。在聚氨酯海绵、生物组织以及冻土等强散射不透明介质中，该探针通过时间门控技术有效消除了背景荧光干扰，其信噪比（SBR）在冷冻状态下提升了惊人的152倍，能够清晰勾勒出数厘米深度的冰层轮廓。更具实战价值的是，在航空风洞实验中，研究人员将该探针涂覆在飞机机翼模型表面，FIP成像系统精准捕捉到了冰层的萌生位置、向下游扩散的路径以及冰层的增厚过程。实验数据表明，磷光强度与激光测量的冰层厚度（0.5–10 mm量级）呈现出高度的一一对应关系，证明了该方法在复杂动态环境下进行定量监测的可靠性。
Counterion-dependent modulation of ice crystal growth kinetics and interfacial structure. (a) Morphology of single ice crystals formed in pure water and in 1 mM PP4P-X (X = F−, Br−, I−, NO3−, and SCN−) aqueous solutions under mild supercooling ( = 0.1°C). Pure water and most PP4P-X solutions yield disk-like crystals, whereas PP4P-F and PP4P-Br produce well-defined hexagonal ice. (b) Average growth rates of single ice crystals in the different PP4P-X solutions, illustrating the influence of counterions on ice growth kinetics. (c, d) Snapshots from molecular dynamics simulations of ice growth in NaF (c) and NaI (d) solutions at 245 K. Na+, F−, and I− are shown as blue, pink, and purple spheres, respectively; probe molecules are shown in yellow. (e) Time evolution of the number of water molecules in the ice phase during simulations in NaF and NaI solutions, revealing slower ice growth in the presence of F−. (f) Increase in potential energy (ΔEX) of the ice crystal upon embedding F− or I−, showing a larger energetic penalty for F− incorporation and thus a stronger tendency for F− to be expelled to the ice-water interface.
NIR phosphorescence imaging and penetration capability in complex media. (a) Phosphorescence imaging of polyurethane (PU) sponges at room temperature (RT) and in the frozen state (Ice). Left, PP4P-F-free control; right, samples loaded with PP4P-F. The thickness of the overlaid PU layer increases from 0 to 10 mm. Excitation, 460 nm, 30 s; acquisition, IVIS Spectrum bioluminescence mode; signal unit: p·s−1·cm−2·sr−1. (b) Phosphorescence signal-to-background ratio (SBRfrozen/SBRinitial) of sponges with different ice fractions before and after freezing, showing strong amplification of PP4P-F phosphorescence with increasing ice content. (c, d) Multimodal comparison of hydrogels, foods, biological tissues, and frozen soils before and after freezing (RT/Ice). (c) Photographs under natural light showing negligible visual changes upon freezing. (d) Delayed NIR phosphorescence images displaying an off-on transition from non-emissive to strongly emissive, enabling high-contrast ice detection and localization in opaque or structurally complex samples. (e) Comparison of phosphorescence and fluorescence SBRfrozen/SBRinitial across different sample types before and after freezing, demonstrating the universal freeze-activated emission response of PP4P-F and the superior contrast of time-gated phosphorescence. (f) Time-resolved phosphorescence and fluorescence SBRfrozen/SBRinitial intensity profile during tissue cryopreservation and thawing, where signal evolution reflects the degree of internal ice formation and subsequent melting, enabling dynamic monitoring and visualization of deep-seated ice phases.
Wind-tunnel demonstration of aircraft icing imaging and thickness measurement using the FIP strategy. (a) Schematic of the wind-tunnel icing setup showing the PP4P-F-coated model wing, time-gated NIR phosphorescence imaging system and laser scanner. (b) Natural-light photographs of the model aircraft at 0, 10, and 20 min of icing show only subtle visual changes despite progressive ice accumulation. (c) Corresponding NIR phosphorescence images, with color indicating phosphorescence intensity, reveal the onset, downstream propagation and thickening of the ice layer along the wing. (d) Ice-thickness maps obtained by laser scanning for the same regions, with color representing ice-layer thickness, closely match the spatial distribution of phosphorescence intensity. (e) Reconstructed two-dimensional ice-thickness distribution across the wing surface based on laser-scanning data. (f) Ice-thickness profiles extracted along directions normal to the leading edge at 0, 10, and 20 min show position-dependent variations and progressive growth of the ice layer.
总结及展望
本研究成功建立了一种基于“离子调控-冰界面协同”的有机磷光材料设计策略，解决了传统光学手段难以探测隐形冰的痛点。这种近红外磷光成像技术不仅具有非接触、原位和空间分辨率高的优势，还能提供定量化的厚度信息。该技术在航空防冰诊断、能源设施安全监测以及生物样本冷冻保存评估等领域具有广阔的应用前景。未来，通过进一步优化激发装置和探针的封装技术，该系统有望在更大规模的工业场景中部署，为防冰减灾提供一套精准高效的视觉导航工具。