【JACS】芘官能化共价有机框架中客体诱导的振动锁定与荧光开关效应及其对全氟辛酸的高灵敏识别（检测限 2.7 μg/L）
【JACS】芘官能化共价有机框架中客体诱导的振动锁定与荧光开关效应及其对全氟辛酸的高灵敏识别（检测限 2.7 μg/L）
文章标题：Guest-Induced Vibration Locking and Fluorescence Switching in a Pyrene-Functionalized Covalent Organic Framework (COF) for Perfluorooctanoic Acid (PFOA) Recognition
文章作者：Gobinda Das, Thirumurugan Prakasam, Nour Alkhatib, Sudhir Kumar Sharma, Hoda Al-assaad, Farah Benyettou, Rasha G. AbdulHalim, Hamzeh Sabouni, Kareem Mazen, Ramesh Jagannathan, Charles J.-A. Jackson, Felipe Gándara, Andreas Mavrandonakis, Mark A. Olson, Samer Aouad, Mohamad El-Roz, Serdal Kirmizialtin, Sabu Varghese, Ali Trabolsi
一、 研究动因：室温振动锁定机制在多孔框架环境污染物检测中的突破
点亮型发光共价有机框架传感器在快速、高选择性检测分子污染物方面极具前景，利用客体限制主体分子振动以抑制非辐射跃迁、增强辐射跃迁已成为前沿信号放大策略。然而，以往的客体诱导运动锁定主要局限于金属有机框架、氢键有机框架等高柔性体系，在共价有机框架中的研究极少；且先前报道的振动锁定机制多依赖于极低温度环境，在室温下的实际应用受到严重限制。此外，全氟和多氟烷基物质（尤其是全氟辛酸）具有极强的环境持久性和生物累积性，如何在室温水相中将其高效识别并转化为直观的光学响应是一项紧迫课题。为此，本研究旨在构筑一种具有悬垂发色团的柔性共价有机框架，通过精准的分子锁钥匹配在室温下实现全氟辛酸的高选择性荧光开启检测。
Fig.1 (a) Synthesis and chemical structure of PY-TTA COF prepared via imine condensation of 4,4′,4′′-(1,3,5-triazine-2,4,6-triyl)trianiline (TTA, orange) and 4-(pyren-2-yl)pyridine-2,6-dicarbaldehyde (PY, blue) in 1,4-dioxane under microwave (MW) irradiation at 110 °C for 2 h. (b) Schematic illustration of the PY-TTA COF pore architecture, highlighting the pendent pyrene chromophores projecting into the pore interior and the corresponding fluorescence response of the COF suspension toward PFOA and representative control analytes, including octanoic acid (OA), perfluorohexanoic acid (PFHA), and perfluorobutanoic acid (PFBA).
二、 研究实施方案与关键实验结果
本研究采用微波辅助席夫碱缩合反应作为合成路线，将4,4’,4”-(1,3,5-三嗪-2,4,6-三基)三苯胺与4-(芘-2-基)吡啶-2,6-二甲醛在1,4-二氧六环中于110℃反应2小时，成功制备了具有悬垂侧基的柔性共价有机框架（产率54%）。与传统将芘单元刚性嵌入骨架的结构不同，本文首次在共价有机框架孔道内部引入伸向孔腔的悬垂芘发色团；在未结合客体时，侧基芘具有较大的旋转振动自由度导致荧光微弱，而当与尺寸形状互补的全氟辛酸结合时，孔道作为“锁”、客体作为“钥匙”，通过氢键作用、亚胺/吡啶位点的质子转移以及碳-氟与π体系的相互作用协同锁定分子运动，显著提升辐射跃迁效率。
实验通过固态核磁共振（包含碳-13、氟-19、氮-15同核/异核相关谱及二维谱学）、粉末X射线衍射、高分辨透射电镜、比表面及孔径分析与半经验量子化学计算相结合的方式进行系统表征。研究对比了全氟癸酸、全氟己酸、全氟丁酸、全氟辛烷磺酸、辛酸、氟化四丁基铵以及氯化钠等无机盐与天然有机物模拟物。结果表明，该材料在水中对全氟辛酸表现出极高的选择性荧光开启响应，检出限低至 2.7 μg/L，荧光平均寿命从 1.81 ns 延长至 2.96 ns，且在 3分钟内 即可达到稳定检测平台。固态下吸附全氟辛酸后发射峰由 552 nm 红移至 600 nm（显现绿转红的光学突变）。对照实验证明，无芘对照骨架在吸附全氟辛酸后几乎无荧光响应，证实悬垂芘的振动限制是信号放大的核心来源。
在执行流程与稳定性验证方面，样品经历水悬浮分散、污染物加入荧光测试、饱和氯化钠/水/甲醇溶液解吸再生的闭环流程。全氟辛酸结合使材料表面接触角由 113.0° 提高至 139.2°，孔径由 1.8 nm 缩减至 1.3 nm，水蒸气吸附迟滞环消失；循环测试表明，该材料在水相中连续经历 6次吸附-解吸循环 后仍保持优异的荧光开启响应与骨架晶态结构。此外，在宽酸碱度范围（pH 1至11）测试中，单纯的酸碱环境并未引起类似幅度的荧光突变，进一步排除了单纯宏观酸效应干扰，证实了分子识别特异性。
Fig.2 (a) Scanning electron microscopy (SEM) and (b) transmission electron microscopy (TEM) images showing the spiked spherical morphology of the PY-TTA COF. (c) Particle size distribution histogram indicating an average particle size of 240 ± 1 nm. (d, e) High-resolution TEM (HRTEM) images showing the ordered crystalline domains and layer-stacked framework. (f) Lattice fringe analysis confirming an interlayer spacing of 0.34 nm, consistent with π-π stacking between adjacent layers. (g, h) Tapping-mode atomic force microscopy (AFM) images showing well-defined spherical particles bearing nanoscale protrusions, consistent with the spiked morphology observed by HRTEM.
Fig.3 (a) Room-temperature dynamic vapor adsorption–desorption isotherms of PY-TTA COF and PY-TTA COF@PFOA showing characteristic stepped uptake and hysteresis consistent with guest-responsive structural flexibility. (b) MD simulation snapshots of the stacking structure of PY-TTA COF in the absence (top) and the presence (bottom) of water molecules.
Fig.4 Guest-triggered structural and optical transformations of PY-TTA COF upon PFOA adsorption. (a) Fluorescence ″turn-on″ response of PY-TTA COF to increasing PFOA concentrations. Inset: photographs of aqueous PY-TTA COF dispersions under 365 nm UV light before and after PFOA addition. (b) Solid-state luminescence spectra of PY-TTA COF and PY-TTA COF@PFOA, showing a distinct redshift in emission upon PFOA exposure. Insets: corresponding solid-state emission color changes. (c) N2 adsorption–desorption isotherms (77 K) of PY-TTA COF (green) and PY-TTA COF@PFOA (red), highlighting changes in pore accessibility and BET surface area following PFOA adsorption. (d) Water contact angle measurements of PY-TTA COF and PY-TTA COF@PFOA demonstrating increased hydrophobicity following PFOA adsorption and recovery of the original surface wettability after NaCl treatment
Fig.5 Solid-state NMR characterization of PFOA binding to PY-TTA COF. (a) Simplified schematic illustration showing the interaction of PFOA molecules with the PY-TTA COF framework. (b) Stacked one-dimensional fast-MAS 13C CP/MAS solid-state NMR spectra of PY-TTA COF (black, bottom) and PY-TTA COF@PFOA (red, top). (c) Stacked one-dimensional 19F solid-state NMR spectra of pure PFOA (bottom) and PY-TTA COF@PFOA (top). (d) Overlay of the two-dimensional fast MAS 13C-1H HETCOR solid-state NMR spectra of PY-TTA COF (black contours) and PY-TTA COF@PFOA (red contours). Asterisks (*) mark spinning sidebands.
三、 创新与提升：微环境协同锁钥设计引领响应型发光材料研发
本工作核心创新在于打破了传统共价有机框架发色团刚性嵌入骨架的固有模式，首创性地利用孔内悬垂芘基团的构象柔性与特异性协同作用，在室温水相中构建了基于“氢键-质子转移-氟-π堆积”的客体诱导分子振动锁定传感机制。这一策略成功攻克了以往发光共价有机框架缺乏室温构象锁定调控机制以及对环境微污染物识别灵敏度不高的难题，实现了 2.7 μg/L 的低检测限、3分钟内的超快响应 以及 6次循环的优异可逆性。论文同时指出，当前的检测限虽然满足初步快速筛查需求，但尚未达到纳克级超痕量合规监测标准，未来可通过孔道功能基团的进一步协同修饰以拓展其在超微量环境监测与智能分离领域的深度应用。
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