【Angew.Chem.】2.12纳米碳点揭秘：聚集诱导发光材料实现智能水凝胶色彩跨越与高安全性信息加密
文章标题：Uncovering Aggregation‐Induced Emission in Carbon Dots for Color‐Changing Hydrogels and Information Encryption
通讯作者：Patrick Théato
文章概要
引言
自碳点问世以来，利用柠檬酸与尿素这一经典前驱体组合探索其形成机制与发光行为已成为主流。然而，科学界此前鲜有报道该体系下具备聚集诱导发光（AIE） 特性的碳点。本研究成功打破了这一局限，通过调控反应环境，制备出一种对溶剂极性具有动态响应的新型亲水性红光碳点。这一发现不仅深入阐明了碳点由于分子间相互作用导致的光学变化机理，也为开发下一代智能光功能材料、防伪印刷以及多维度信息加密系统提供了崭新且高效的平台。
(A) The preparation of R-CDs; (B) The preparation of PVA-based fluorescent hydrogels.
主要实验及结论
研究人员在二甲基甲酰胺（DMF）溶剂环境中，通过溶热法将柠檬酸与尿素进行反应，成功制备出平均粒径仅为2.12纳米且分布均匀的红光碳点（R-CDs）。深入的结构表征表明，DMF在高温高压下分解生成的中间体积极参与了反应，为碳点引入了丰富的吡啶氮、吡咯氮和石墨氮结构，促进了超大共轭π体系的构建。这种独特的共轭网络使碳点在水分散状态下呈现微弱的蓝光，而一旦加入DMF，则会因强烈的分子间相互作用迅速聚集并激发极强的红光发光中心。通过超快飞秒瞬态吸收光谱分析，研究团队首次从飞秒超快动力学角度证实，这种J-型聚集状态能有效抑制非辐射衰减，使激子寿命大幅延长，从而在宏观上表现出极其鲜艳的红光发射。
(A) TEM image of R-CDs (water as the dispersant). The image in the upper right corner is an HRTEM (High Resolution Transmission Electron Microscopy) picture, and the image in the lower right corner shows the size of R-CDs; (B) and (C) TEM imagine of R-CDs (ethanol as the dispersant); (D) FTIR spectra of multicolor CDs; (E) 1H NMR spectra of multicolor CDs (solvent: D2O, 500 MHz, 25°C); (F) 13C NMR spectra of multicolor CDs (solvent: D2O, 500 MHz, 25°C); (G), (H) and (I) are DEPT-135 NMR spectra of multicolor CDs; (J), (K) and (L) are DOSY NMR spectra of multicolor CDs (solvent: D2O, 500 MHz, 25°C).
High-resolution XPS survey spectra of (A) B-CDs, (B) G-CDs and (C) R-CDs; high-resolution XPS C 1s spectra of (D) B-CDs, (E) G-CDs and (F) R-CDs; high-resolution XPS N 1s spectra of (G) B-CDs, (H) G-CDs and (I) R-CDs; high-resolution XPS O 1s spectra of (J) B-CDs, (K) G-CDs and (L) R-CDs.
Characterizations of the optical properties of R-CDs: (A) UV–vis spectra of B-CDs, G-CDs, and R-CDs; (B) FL spectrum of R-CDs; (C) chromaticity diagram of R-CDs; (D) FL spectra of R-CDs (excitation wavelength: 260–440 nm); (E) FL spectra of R-CDs (excitation wavelength: 450–520 nm); (F) FL spectra of R-CDs (excitation wavelength: 530–630 nm).
Transient absorption spectra for R‑CDs. (A) DMF as solvent, excitation at 420 nm (OD = 0.81; Epulse = 0.30 µJ). (B) DMF as solvent, excitation at 600 nm (OD = 1.42; Epulse = 0.30 µJ). (C) Water as solvent, excitation at 360 nm (OD = 1.10; Epulse = 0.30 µJ). (D) Normalized single‑transient kinetics for the DMF samples (420 nm and 600 nm excitation) together with the global‑analysis fit (colored lines). (E) Normalized single‑transient kinetics for the water sample (360 nm excitation) together with the global‑analysis fit (colored lines).
基于这一独特的溶剂响应色变特性，团队深入拓展了其在多级信息安全加密和智能水凝胶中的应用。将该碳点配制成打印墨水或书写荧光液，可在滤纸等基底上实现信息的可逆擦除与显影。在喷涂DMF后，隐藏的二维码或文字会因碳点聚集发光而清晰显现，待溶剂挥发或遇水后信息再度隐藏，且该材料展现出长达30天以上的优异环境稳定性与自主瞬态自擦除特性。此外，将该红光碳点与聚乙烯醇（PVA）高分子网络融合，成功构筑出具有快速响应能力的荧光色变智能水凝胶。利用碳点在水凝胶基质中随乙酸等溶剂刺激发生的快速荧光转变，成功实现了多维数字密码的动态隐藏与精确读取，为防伪技术提供了更高级别的安全保障。
(A) Image of R-CDs in different DMF concentrations (under daylight, UV 254 nm and UV 365 nm); (B) the aggregation state of R-CDs in different volume ratios (water/DMF: 100/0, 80/20, 60/40, 40/60, 20/80 and 0/100); (C) chromaticity diagram of R-CDs in different DMF concentration; (D) (E) (F) FL spectra of R-CDs in different DMF concentration under different excitation wavelength (365, 450 and 600 nm); (G) intensity change of the emission wavelengths at 450 and 640 nm for R-CDs in different DMF concentrations at an excitation wavelength of 365 nm; (H) intensity change of emission wavelength at 500 and 640 nm for R-CDs in different DMF concentrations at an excitation wavelength of 450 nm; (I) intensity change of the emission wavelength at 640 nm for R-CDs in different DMF concentrations at an excitation wavelength of 600 nm.
(A) Image of R-CDs in different solvents (under daylight, UV 254 nm and UV 365 nm); (B) chromaticity diagram of R-CDs in different solvents; (C) FL spectrum of R-CDs in different solvents (under excitation wavelength of 365 nm); (D) UV–vis spectrum of R-CDs in different solvents and (E) optimal absorption wavelength and absorbance; (F) particle size of R-CDs in different solvents (measurement by DLS); (G) schematic diagram of the aggregation-induced emission of R-CDs.
Application of R-CDs: (A) Color change of writing on filter paper; (B) writing-based information encoding on filter paper; (C) printing-based information encoding on different paper substrates (the left is normal A4 paper, the middle is notebook paper, the right is filter paper); (D) process of appearance and disappearance of printed information encoding; (E) photographs of solvent-responsive fluorescence color change of PVA-based fluorescent hydrogel under 365 nm excitation; (F) number information coding of PVA-based fluorescent hydrogels.
总结及展望
本研究成功开辟了经典柠檬酸-尿素体系在聚集诱导发光领域的新应用。通过微观分子层面的π–π堆积调控与宏观溶剂效应的协同作用，实现了碳点在J-型与H-型聚集状态间的动态转换，赋予了材料出色的光学可调性与环境敏感度。这种兼具高生物相容性、优异稳定性和灵敏溶剂响应的双重发光碳点材料，未来在智能传感、高级防伪标签以及动态信息加密存储等尖端技术领域蕴藏着巨大的产业化应用潜力。