Metal-ion-mediated synergistic coordination: construction of AIE-metallogel sensor arrays for anions and amino acids

Metallogel-based six membered sensor arrays show applications in multi-analyte detection and fluorescence encryption.

Recent Trends in the Design, Synthesis, Spectroscopic Behavior, and Applications of Benzazole-Based Molecules with Solid-State Luminescence Enhancement Properties

Molecules that exhibit solid-state luminescence enhancement, i.e. the rare property to be more strongly emissive in the solid state than in solution, find an increasing number of applications in the fields of optoelectronic and nanophotonic devices, sensors, security papers, imaging, and theranostics. Benzazole (BZ) heterocycles are of particular value in this context. The simple enlargement of their π-electron system using a -C=C-Ar or -N=C-Ar moiety is enough for intrinsic solid-state luminescence enhancement (SLE) properties to appear. Their association with a variety of polyaromatic motifs leads to SLE-active molecules that frequently display attractive electroluminescent properties and are sensitive to mechanical stimuli. The excited-state intramolecular proton transfer (ESIPT) process that takes place in some hydroxy derivatives reinforces the SLE effect and enables the development of new sensors based on a protection/deprotection strategy. BZ may also be incorporated into frameworks that are prototypical aggregation-induced enhancement (AIE) luminogens, such as the popular tetraphenylethene (TPE), leading to materials with excellent optical and electroluminescent performance. This review encompasses the various ways to use BZ units in SLE systems. It underlines the significant progresses recently made in the understanding of the photophysical mechanisms involved. A brief overview of the synthesis shows that BZ units are robust building blocks, easily incorporated into a variety of structures. Generally speaking, we try to show how these small heterocycles may offer advantages for the design of increasingly efficient luminescent materials.

Stimuli responsive dynamic transformations in supramolecular gels

Supramolecular gels are formed by the self-assembly of small molecules under the influence of various non-covalent interactions. As the interactions are individually weak and reversible, it is possible to perturb the gels easily, which in turn enables fine tuning of their properties. Synthetic supramolecular gels are kinetically trapped and usually do not show time variable changes in material properties after formation. However, such materials potentially become switchable when exposed to external stimuli like temperature, pH, light, enzyme, redox, and chemical analytes resulting in reconfiguration of gel matrix into a different type of network. Such transformations allow gel-to-gel transitions while the changes in the molecular aggregation result in alteration of physical and chemical properties of the gel with time. Here, we discuss various methods that have been used to achieve gel-to-gel transitions by modifying a pre-formed gel material through external perturbation. We also describe methods that allow time-dependent autonomous switching of gels into different networks enabling synthesis of next generation functional materials. Dynamic modification of gels allows construction of an array of supramolecular gels with various properties from a single material which eventually extend the limit of applications of the gels. In some cases, gel-to-gel transitions lead to materials that cannot be accessed directly. Finally, we point out the necessity and possibility of further exploration of the field.

A fluorescent supramolecular gel and its application in the ultrasensitive detection of CN- by anion-π interactions

Supramolecular gels have been widely reported on account of their unique superiority and application prospects. In this work, we constructed a novel supramolecular gel (HD-G) by using hydroxy-naphthaldehyde decorated with naphthalimide in DMSO solution, which exhibited excellent selectivity and ultrasensitive sensing properties toward CN^- (the lowest detection limit is 1.82 × 10^-10 M). The sensing mechanism of this supramolecular gel takes advantage of π-π stacking interactions and anion-π interactions, which is different from the other familiar methods.

Cyanide Sensing in Water Using a Copper Metallogel through "Turn-on" Fluorescence

The development of fluorescent probes for selective detection of cyanide has gained considerable attention over the past two decades due to benefits like high selectivity as well as sensitivity, fast response, visual output, accurate quantification, and a simplified sample preparation procedure. However, the propensity of supramolecular gels toward fluorescence sensing of cyanide in aqueous medium is not well explored until now. Herein, we report the design and synthesis of a novel copper based metallogel capable of sensing cyanide in water by fluorescence "turn on". Toward this, a terpyridine attached poly(aryl ether) dendrone derivative (G1) is synthesized which forms gel and exhibits Aggregation Induced Emission (AIE). The addition and diffusion of copper ions to the gel resulted in the formation of a nonluminescent copper metallogel (CuG). The copper metallogel could selectively sense cyanide in water by a fluorescence "turn-on" signal due to the regeneration of the AIE active gel. The mechanistic pathways of the sensing have been studied, and the detection limit for sensing was found to be as low as 1.09 μM. A thin film of CuG was prepared by casting the gel and used as a test strip for the visual detection of cyanide in water.

Hyperbranched Poly(amido amine) Entrapped Tetraphenylethene as a Fluorescence Probe for Sequential Quadruple-Target Detection and Its Potential as a Chemical Logic Gate

Fluorescence sensors exhibit great potential as molecular logic gates to perform computation on a nanometer scale. For achieving the more complex artificial intelligence activities, developing complex logic gates using multitarget sensing systems with multi-input characteristics is highly desirable. Herein, a water-soluble quadruple-target fluorescence sensor that embeds a small amount (4.1 wt %) of tetraphenylethene (TPE) units into hyperbranched poly(amido amine) (TPE-HPA) has been designed. The nonfluorescent TPE-HPA could experience the fluorescence "off-on-off-on-off" by sequential addition of sodium hexametaphosphate (SHMP), Fe³⁺, ascorbic acid (AA), and H₂O₂. The as-prepared quadruple-target sensor showed good sensitivity and selectivity to SHMP, Fe³⁺, AA, and H₂O₂, and the limit of detection values were 29 nM, 20 nM, 0.66 μM, and 0.78 μM, respectively. On the basis of the multitarget sensing nature of TPE-HPA, chemical or electrochemical-induced logic gates were constructed, including YES, NOT, OR, NOR, NAND, INHIBIT, IMP, and higher logic systems.

Stimuli-Sensitive Aggregation-Induced Emission of Organogelators Containing Mesogenic Au(I) Complexes

As the luminescence from conventional organic luminophores is typically quenched in constrained environments, the aggregation-induced emission (AIE) phenomenon is of interest for the development of materials that exhibit strong luminescence in condensed phases. Herein, new bismesogenic Au complexes were developed as organogelators and their photophysical properties, including their AIE characteristics, were investigated in organogels and crystals. The crystals of the gold complexes exhibited room-temperature phosphorescence with relatively high quantum yields. Moreover, the gold complexes also showed photoluminescence in the organogels and we demonstrated that the reversible switching of the luminescence intensity was induced by the sol-gel phase transition. The intense photoluminescence in the crystal and gel was induced by the restricted internal motion of the luminophore in the molecular aggregates. However, in the sol, the network structure of the organogel was destroyed and the nonradiative deactivation of the excited states was enhanced. As a result, we can conclude that the switching of the luminescence intensity was induced by changes in the aggregated structures of the molecules. The developed Au-complex-based gelators are excellent candidates for the realization of stimuli-responsive soft and smart luminescent materials.

Crystallographic Elucidation of Stimuli-Controlled Molecular Rotation for a Reversible Sol-Gel Transformation

To get an idea about the most probable microporous supramolecular environment in the gel state, gelator molecule 1 has been crystallized from its gelling solvent (dimethylformamide). Crystal structure analysis of 1 shows a strong π···π stacking interaction between the electron-deficient pentafluorophenyl ring and electron-rich naphthyl ring. The gelling solvent situated in the "molecular pocket" stitches the gelators through weak H-bonding interactions to facilitate the formation of an organogel. Scanning electron microscopy analysis exhibits a ribbonlike fibrous morphology that resembles the supramolecular arrangement of 1 in its crystalline state, as evidenced by powder X-ray diffraction. In the presence of external stimuli (tetrabutylammonium fluoride), the organogel of 1 disassembles into sol. This sol-gel transformation phenomenon has been explained on the basis of X-ray single-crystal analysis. Single crystals obtained from the sol state show that naphthylic -OH of 1 gets deprotonated, resulting in C-C bond rotation that plays a major role in the sol-gel transformation. Gelator 1 exhibits weak green fluorescence in the gel state, whereas it shows highly intense yellow fluorescence in the sol state. Furthermore, a reversible sol-gel transformation associated with changes in the spectroscopic properties has been observed in the presence of acids and fluoride ions, respectively.

A highly sensitive “ON–OFF” optical sensor for the selective detection of cyanide ions in 100% aqueous solutions based on hydrogen bonding and water assisted aggregation induced emission

Nanoparticles N,N′-(pyridine-2,6-diyl)bis(2-(2,4-dichlorophenoxy)acetamide) (1) exhibited an “on–off” emission response toward cyanide (CN⁻) ions in 100% aqueous solutions based on AIE features.

Novel Amino-pillar[5]arene as a Fluorescent Probe for Highly Selective Detection of Au3+ Ions

A novel fluorescent probe, amino-pillar[5]arene (APA), was prepared via a green, effective, and convenient synthetic method, which was characterized by nuclear magnetic resonance (NMR), infrared (IR), and high-resolution mass spectrometry. The fluorescence sensing behavior of the APA probe toward 22 metal ions in aqueous solutions were studied by fluorescence spectroscopy. The results showed that APA could be used as a selective fluorescent probe for the specificity detection of Au3+ ions. Moreover, the detection characteristics were investigated by fluorescence spectral titration, pH effect, fluorescence competitive experiments, Job's plot analysis, 1H NMR, and IR. The results indicated that detection of Au3+ ions by the APA probe could be achieved in the range of pH 1-13.5 and that other coexisting metal ions did not cause any marked interference. The titration analysis results indicated that the fluorescence intensity decreased as the concentration of Au3+ ions increased, with an excellent correlation (R 2 = 0.9942). The detection limit was as low as 7.59 × 10-8 mol·L-1, and the binding ratio of the APA probe with Au3+ ions was 2:1. Therefore, the APA probe has potential applications for detecting Au3+ ions in the environment and in living organisms.

Physically Cross-Linked Hydrogel Based on Phenyl-1,3,5-triazine: Soft Scaffold with Aggregation-Induced Emission

A phenyltriazine compound has been used for the first time as a monomer in the construction of a hydrogel. This physically cross-linked soft material showed blue fluorescence when excited under UV-light. Polymer formation and intermolecular H-bonds arising from triazine moieties operate as aggregation-induced emission (AIE) mechanisms. The combination of soft materials and AIE properties expands the applications of these materials. As a proof of concept, two luminescent dyes have been incorporated into the hydrogel to produce a white-light-emitting material.

Recent Advances in Aggregation-Induced Emission Chemosensors for Anion Sensing

The discovery of the aggregation-induced emission (AIE) phenomenon in the early 2000s not only has overcome persistent challenges caused by traditional aggregation-caused quenching (ACQ), but also has brought about new opportunities for the development of useful functional molecules. Through the years, AIE luminogens (AIEgens) have been widely studied for applications in the areas of biomedical and biological sensing, chemosensing, optoelectronics, and stimuli responsive materials. Particularly in the application of chemosensing, a myriad of novel AIE-based sensors has been developed to detect different neutral molecular, cationic and anionic species, with a rapid detection time, high sensitivity and high selectivity by monitoring fluorescence changes. This review thus summarises the recent development of AIE-based chemosensors for the detection of anionic species, including halides and halide-containing anions, cyanides, and sulphur-, phosphorus- and nitrogen- containing anions, as well as a few other anionic species, such as citrate, lactate and anionic surfactants.

Two macrocycle-based sensors for anions sensing

Two macrocyclic bis-benzimidazolium salts 2 and 4 (23-membered for 2 and 25-membered for 4) were prepared, and their structures were confirmed by X-ray crystallography, 1H NMR and 13C NMR spectroscopy. The research of anion recognitions using 2 or 4 as hosts were carried out with the methods of fluorescence and ultraviolet spectroscopy, 1H NMR titrations, MS and IR spectra. The experiment results show that 2 can detect acetate anion and 4 can detect nitrate anion with favorable selectivity and sensitivity.