Fluorescent supramolecular self-assembly gels and their application as sensors: A review

Chromophores that exhibit aggregation-induced emission (i.e., aggregation-induced emission luminogens [AIEgens]) emit intense fluorescence in their aggregated states, but show negligible emission as discrete molecular species in solution due to the changes in restriction and freedom of intramolecular motions. As solvent-swollen quasi-solids with both a compact phase and a free space, gels enable manipulation of intramolecular motions. Thus, AIE-active gels have attracted significant interest owing to their various distinctive properties and promising application potential. Herein, a comprehensive overview of AIE-active gels is provided. The fabrication strategies employed are detailed, and the applications of AIEgens are summarized. In addition, the gel functions arising from the AIE moieties are revealed, along with their structure-property relationships. Furthermore, the applications of AIE-active gels in diverse areas are illustrated. Finally, ongoing challenges and potential means to address them are discussed, along with future perspectives on AIE-active gels, with the overall aim of inspiring research on novel materials and ideas.

Aggregation-Induced Emission-Active Gels: Fabrications, Functions, and Applications

We demonstrate a versatile and easily fabricated paper-based CO2 sensor. The sensor consists of a specially designed fluorescent color-shift chromophore infused into standard filter paper. The emission color of the resulting fluorescent paper changes upon exposure to CO2 due to the formation of carbonic acid, which underlies the sensing mechanism. By using a ratiometric method, the undesirable effects of photobleaching can be eliminated, leading to a stable and repeatable sensor performance. These multiuse sensors have a response time on the order of 1 min and feature low detection limits for a paper-based CO2 gas sensor, suggesting possible low-cost applications in smart buildings or other facilities in which CO2 levels are required to be continuously monitored.

An Ultrasensitive Fluorescent Paper-Based CO2 Sensor.

Biomolecule-based hydrogels have potential use in a wide range of applications such as controlled drug release, tissue engineering, and biofabrication. Herein, driven by specific interactions between ds-DNA (double-stranded DNA) and Zn2+ based metal-complexes, we report that the use of DNA as cross-linkers can enhance interactions between self-assembling Zn2+ complexes containing terpyridine and sugar groups in the generation of bioinspired hydrogels from solutions or suspensions. The gelation process is fast and straightforward without tedious steps and happens at room temperature. Such a hydrogelation process of different Zn2+ complexes endows the visualized and selective DNA analogue discrimination. Several experiments suggest that the strong intercalation binding of Zn2+ complexes with ds-DNA results in the unzipping of ds-DNA into ss-DNA (single-stranded DNA), which further behave as linkers to enhance the intermolecular interactions of self-assembling Zn2+ complex molecules via coordination interactions. This work demonstrates an efficient and universal strategy to prepare hydrogels based on biomolecular recognition. Moreover, the DNA responsive behaviors of Zn2+ complexes are further compared with that of solutions and cells.

Instant hydrogel formation of terpyridine-based complexes triggered by DNA via non-covalent interaction.

Self-healing gels triggered by ultrasound with color-tunable emission based on ion recognition.

It is imperative for various applications to develop new sensing systems in the visualization and detection of CO 2 . Here, several aroylhydrazone-derivatized organogelators have been prepared and developed as CO 2 sensors via the anion-activated strategy. These new compounds can gel some of polar and apolar organic solvents to form stable organogels which have been well characterized by field emission scanning electron microscopy (FE-SEM), FT-IR spectroscopy, concentration- and temperature-dependent 1 H NMR spectra, PXRD, and UV–vis spectroscopy. In addition, they can show gelation-induced fluorescence enhancement performances which have been proven to derive from the aggregation-induced emission (AIE) effect. In particular, the fluoride anion-triggered solutions of the organogelators can permit naked-eye sensing of CO 2 in air via the solution-to-gel transition. Besides, the developed gelation systems can also be used to provide quantitative means for CO 2 stream through re-gelation induced fluorescence enhancement. Moreover, the reversibility of these sensing systems for the capture and release of CO 2 enhances their applicable potential.

Dual-channel sensing of CO2: Reversible solution-gel transition and gelation-induced fluorescence enhancement

A novel gelator that contained both Schiff base and L-lysine moieties was synthesized and its gelation behavior was tested. This gelator can form gels in various organic solvents. The resulting gel can be applied as a fascinating platform for visual recognition of enantiomeric 1-(2-hydroxynaphthalen-1-yl)naphthalen-2-ol (BINOL) through selective gel collapse. In addition, the mechanism for the reaction of the gel with chiral BINOL was investigated by scanning electron microscope and 1H nuclear magnetic resonance.

Visual chiral recognition of 1,1′-binaphthol through enantioselective collapse of gel based on an amphiphilic Schiff-base gelator

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Dual-channel sensing of CO2: Reversible solution-gel transition and gelation-induced fluorescence enhancement

Visual chiral recognition of 1,1′-binaphthol through enantioselective collapse of gel based on an amphiphilic Schiff-base gelator