Novel Hydrogen-Bonding Cross-Linking Aggregation-Induced Emission: Water as a Fluorescent "Ribbon" Detected in a Wide Range

Advances on fluorescence chemosensors for selective detection of water

Water, although an important part of everyday life, is acts as one of the most significant contaminants in various applications such as biomedical monitoring, chemical production, petroleum-based fuel and food processing. In fact, the presence of water in other solvents is a huge concern. For the quantification of trace water content, different methods such as Karl-Fischer, electrochemical, nuclear magnetic resonance, chromatography, and thermogravimetric analysis have been used. Although every technique has its own benefit, each one suffers from several drawbacks that include high detection costs, lengthy procedures and specialized operations. Nowadays, the development of fluorescence-based chemical probes has become an exciting area of research for the quick and accurate estimation of water content in organic solvents. A variety of chemical processes such as hydrolysis reaction, metal ions promoted oxidation reaction, suppression of the -C═N isomerization, protonation and deprotonation reactions, and molecular aggregation have been well researched in the last few years for the fluorescent detection of trace water. These chemical processes eventually lead to different photophysical events such as aggregation-induced emission (AIE), aggregation-induced emission enhancement (AIEE), aggregation-caused quenching (ACQ), fluorescent resonance energy transfer (FRET), charge transfer, photo-induced electron transfer (PET), excited state intramolecular proton transfer (ESIPT) that are responsible for the detection. This review presents a summary of the fluorescence-based chemosensors reported in recent years. The design of water sensors, sensing mechanisms and their potential applications are reviewed and discussed.

Pyrene-based fluorescent chemosensor for rapid detection of water and its applications

This manuscript introduces a pyrene-based Schiff base L by reacting pyrenecarboxaldehyde with 2-aminothiazole in equimolar ratio. The ligand L was characterized by various spectral data and single crystal. The water sensing ability of L was examined in different organic solvents. The weakly emissive L in DMSO showed a fluorescence enhancement upon the addition of water. The water-induced fluorescence enhancement of L was occurred due to the combined effect of aggregation-induced emission (AIE) phenomenon and suppression of photo-induced electron transfer (PET) process. Using L, the water in DMSO can be detected down to 0.50 wt% with a quantification limit of 1.52 wt%. The analytical novelty of the developed sensor L was validated by detecting moisture in a variety of raw food products.

Schiff Bases: A Versatile Fluorescence Probe in Sensing Cations

Metal cations such as Zn²⁺, Al³⁺, Hg²⁺, Cd²⁺, Sn²⁺, Fe²⁺, Fe³⁺ and Cu²⁺ play important roles in biology, medicine, and the environment. However, when these are not maintained in proper concentration, they can be lethal to life. Therefore, selective sensing of metal cations is of great importance in understanding various metabolic processes, disease diagnosis, checking the purity of environmental samples, and detecting toxic analytes. Schiff base probes have been largely used in designing fluorescent sensors for sensing metal ions because of their easy processing, availability, fast response time, and low detection limit. Herein, an in-depth report on metal ions recognition by some Schiff base fluorescent sensors, their sensing mechanism, their practical applicability in cell imaging, building logic gates, and analysis of real-life samples has been presented. The metal ions having biological, industrial, and environmental significance are targeted. The compiled information is expected to prove beneficial in designing and synthesis of the related Schiff base fluorescent sensors.

Pyrene-Based AIE Active Materials for Bioimaging and Theranostics Applications

Aggregation-induced emission (AIE) is a unique research topic and property that can lead to a wide range of applications, including cellular imaging, theranostics, analyte quantitation and the specific detection of biologically important species. Towards the development of the AIE-active materials, many aromatic moieties composed of tetraphenylethylene, anthracene, pyrene, etc., have been developed. Among these aromatic moieties, pyrene is an aromatic hydrocarbon with a polycyclic flat structure containing four fused benzene rings to provide an unusual electron delocalization feature that is important in the AIE property. Numerous pyrene-based AIE-active materials have been reported with the AIE property towards sensing, imaging and theranostics applications. Most importantly, these AIE-active pyrene moieties exist as small molecules, Schiff bases, polymers, supramolecules, metal-organic frameworks, etc. This comprehensive review outlines utilizations of AIE-active pyrene-based materials on the imaging and theranostics studies. Moreover, the design and synthesis of these pyrene-based molecules are delivered with discussions on their future scopes.

Detection of Trace Water Based on Electro-oxidation of Molybdenum Disulfide Nanomaterials to Form Molybdenum Oxysulfide

Molybdenum disulfide nanomaterials nowadays are very popular in electrocatalysis field due to their outstanding catalytic performance toward many electrochemical reactions. However, the electrochemical oxidation reaction of molybdenum disulfide nanomaterials in the range of positive potential has not been studied thoroughly. Herein, we have investigated electro-oxidation of molybdenum disulfide nanomaterials and put forward a new reaction mechanism: molybdenum disulfide nanomaterials are electro-oxidized with water to form molybdenum oxysulfide (MoOS2) and hydrogen ions, leading to the release of hydrogen on the counter electrode. Various characterization methods such as contact angle measurement, scanning electron microscope (SEM), transmission electron microscope (TEM) with energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS), and gas chromatography (GC) were applied to attest the doping of oxygen and the generation of hydrogen. Based on this reaction, we constructed a novel ultrasensitive electrochemical sensor for detecting trace water with the minimum detectable content of 0.0010% (v/v) in various organic solvents and ionic liquids, which is comparable to the Karl Fischer titration, but with much simpler reagent.

Zinc (II) and AIEgens: The "Clip Approach" for a Novel Fluorophore Family. A Review

Aggregation-induced emission (AIE) compounds display a photophysical phenomenon in which the aggregate state exhibits stronger emission than the isolated units. The common term of "AIEgens" was coined to describe compounds undergoing the AIE effect. Due to the recent interest in AIEgens, the search for novel hybrid organic-inorganic compounds with unique luminescence properties in the aggregate phase is a relevant goal. In this perspective, the abundant, inexpensive, and nontoxic d10 zinc cation offers unique opportunities for building AIE active fluorophores, sensing probes, and bioimaging tools. Considering the novelty of the topic, relevant examples collected in the last 5 years (2016-2021) through scientific production can be considered fully representative of the state-of-the-art. Starting from the simple phenomenological approach and considering different typological and chemical units and structures, we focused on zinc-based AIEgens offering synthetic novelty, research completeness, and relevant applications. A special section was devoted to Zn(II)-based AIEgens for living cell imaging as the novel technological frontier in biology and medicine.

Fluorescence sensors for detection of water based on tetraphenylethene–anthracene possessing both solvatofluorochromic properties and aggregation-induced emission (AIE) characteristics

TPE-(An-CHO)4 has been developed as an SFC (solvatofluorochromism)/AIEE (aggregation-induced emission enhancement)-based fluorescence sensor for detection of water over a wide range from low to high water content regions in solvents.

Development of optical sensor for water in acetonitrile based on propeller-structured BODIPY-type pyridine-boron trifluoride complex

A propeller-structured 3,5,8-trithienyl-BODIPY-type pyridine–boron trifluoride complex, ST-3-BF₃, which has three units of 2-(pyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile at the 3-, 5-, and 8-positions on the BODIPY skeleton, was designed and developed as an intramolecular charge transfer (ICT)-type optical sensor for the detection of a trace amount of water in acetonitrile. The characterization of ST-3-BF₃ was successfully determined by FTIR, ¹H and ¹¹B NMR measurements, high-resolution mass spectrometry (HRMS) analysis, thermogravimetry-differential thermal analysis (TG-DTA), photoabsorption and fluorescence spectral measurements, and density functional theory (DFT) calculations. ST-3-BF₃ showed a broad photoabsorption band in the range of 600 to 800 nm, which is assigned to the S₀ → S₁ transition of the BODIPY skeleton with the expanded π-conjugated system over the 2-(pyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile units at the 3-, 5-, and 8-positions onto the BODIPY core. In addition, a photoabsorption band was also observed in the range of 300 to 550 nm, which can be assigned to the ICT band between the 2-(pyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile units at 3-, 5-, and 8-positions and the BODIPY core. ST-3-BF₃ exhibited a characteristic fluorescence band originating from the BODIPY skeleton at around 730 nm. It was found that by addition of a trace amount of water to the acetonitrile solution of ST-3-BF₃, the photoabsorption band at around 415 nm and the fluorescence band at around 730 nm increased linearly as a function of the water content below only 0.2 wt%, which could be ascribed to the change in the ICT characteristics due to the dissociation of ST-3-BF₃ into ST-3 by water molecules. Thus, this work demonstrated that the 3,5,8-trithienyl-BODIPY-type pyridine–boron trifluoride complex can act as a highly-sensitive optical sensor for the detection of a trace amount of water in acetonitrile.

Propeller-structured 3,5,8-trithienyl-BODIPY-type pyridine–boron trifluoride complex, ST-3-BF₃, has been developed as an intramolecular charge transfer (ICT)-type optical sensor for the detection of a trace amount of water in acetonitrile.

Metal-Organic Complexes@Melamine Foam Template Strategy to Prepare Three-Dimensional Porous Carbon with Hollow Spheres Structures for Efficient Organic Vapor and Small Molecule Gas Adsorption

Three-dimensional (3D) porous carbon materials have received substantial attention owing to their unique structural features. However, the synthesis of 3D porous carbon, especially 3D porous carbon with hollow spheres structures at the connection points, still pose challenges. Herein, we first develop a metal-organic complexes@melamine foam (MOC@MF) template strategy, by using hot-pressing and carbonization method to synthesize 3D porous carbon with hollow spheres structures (denoted as NOPCs). The formation mechanism of NOPCs can be attributed to the difference in Laplace pressure and surface energy gradient between the carbonized MOC and carbonized MF. These rare 3D porous carbons exhibit high BET surface area (2453.8 m² g^-1), N contents (10.5%), and O contents (16.3%). Moreover, NOPCs show significant amounts of toluene and methanol at room temperature, reaching as high as 1360 and 1140 mg g^-1. The adsorption amounts of SO₂ and CO₂ for NOPCs are up to 93.1 and 445 mg g^-1. Theoretical calculation indicates surfaces of porous carbon with N and O coexistence could strongly enhance adsorption with high adsorption energy of -65.83 kJ mol g^-1.

Two 8-hydroxyquinoline-based fluorescent chemosensors for ultra-fast and sensitive detection of water content in strong polar organic solvents with large Stokes shifts

It is of great significance to detect the moisture in organic solvents before used in water-sensitive reactions. Herein, two Schiff base quinoline derivatives, 8-hydroxyquinoline-2-carboxaldehyde thiosemicarbazone (HQCT) and 8-hydroxyquinoline-2-carboxaldehyde (pyridine-2-carbonyl)-hydrazine (HQPH), were designed and synthesized by a simple one-step reaction, and used as fluorescent chemosensors for ultra-fast and sensitive detection of water content in strong polar organic solvents. Based on excited-state intramolecular proton transfer (ESIPT) process, HQCT and HQPH exhibited strong fluorescence emissions with large Stokes shifts in dimethyl sulfoxide (DMSO) and N, N-dimethylformamide (DMF) solvents compared to other various organic solvents, and their fluorescence quenching and fluorescent color changes were obviously observed with increasing water content. The experimental results revealed that the hydroxyl groups substituted at the 8-position of HQCT and HQPH played a key role in the fluorescence emission processes. Dynamic light scattering (DLS) and ¹H NMR titration indicated that the sensing mechanism for the detection of water was based on inhibition of the ESIPT by H₂O via forming hydrogen bonds. In the range of 0.0-1.8 wt%, the fluorescence intensity of chemosensors changed as a linear function of water content. The detection limits of water in DMSO by HQCT and HQPH were as low as 0.0220 wt% and 0.0274 wt%, respectively. Moreover, HQCT and HQPH are successfully applied for the detection of moisture content in real commercial organic solvents.

Micromesoporous Nitrogen-Doped Carbon Materials Derived from Direct Carbonization of Metal-Organic Complexes for Efficient CO2 Adsorption and Separation

Metal-organic complexes (MOCs) are considered as excellent precursors to prepare carbon materials, due to the fact that heteroatoms and functional groups can be naturally reserved in the resulting carbon materials through the carbonization. Herein, micromesoporous nitrogen-doped carbons MPNC-1 and MPNC-2 are successfully obtained by direct carbonization (800 °C, KOH activation) of metal-organic complexes DQA-1 and DQA-2. MPNC-1 and MPNC-2 exhibit high BET surface area (2368.9 and 2327.6 m² g^-1), pore volume (1.95 and 1.89 cm³ g^-1), and N contents (17.2% and 12.3%). At 25 °C and 1 bar, MPNC-1 and MPNC-2 show high CO₂ adsorption of 7.53 and 6.58 mmol g^-1, the estimated CO₂/N₂ selectivity are 20.5 and 22.6, indicating excellent promise for practical CO₂ adsorption and separation applications. Theoretical calculation indicates carbon surfaces with pyridinic-N, pyrrolic-N, and graphitic-N coexistence could strongly change the local electronic distribution and electrostatic surface potential, enhancing the CO₂ adsorption with adsorption energy of -58.96 kJ mol g^-1. Theoretical calculation also highlights that CO₂ adsorption mechanism is electrostatic interaction with a large green isosurface between CO₂ molecules and the carbon surface.

Colorimetric and ratiometric fluorescence sensing of water based on 9-methyl pyrido[3,4-b]indole-boron trifluoride complex

In this work, 9-methyl pyrido[3,4-b]indole-boron trifluoride complex, 9-MP-BF3, was designed and developed as a colorimetric and ratiometric fluorescent sensor for the detection of water in the low- and high-water-content regions in solvents. In the low-water-content region, a new photoabsorption band at around 360 nm and a fluorescence band at around 370 nm gradually appeared due to the dissociation of 9-MP-BF3 into 9-methyl pyrido[3,4-b]indole (9-MP) by water molecules with a simultaneous decrease in the photoabsorption band at around 390 nm and the fluorescence band at around 460 nm originating from 9-MP-BF3. In the moderate-water-content region, the photoabsorption band at around 360 nm and the fluorescence band at around 370 nm gradually shifted to a longer wavelength region with an increase in the fluorescence intensity, which could be ascribed to the formation of a hydrogen-bonded complex (9-MP-H2O) with water molecules. Furthermore, in the high-water-content region, two photoabsorption bands at around 305 nm and 390 nm and one fluorescence band at around 460 nm gradually reappeared with simultaneous decrease in the photoabsorption band at around 290 nm and the fluorescence band at around 370 nm, which was attributed to the formation of a hydrogen-bonded proton transfer complex (9-MP-H+) with water molecules. Thus, this work revealed the mechanism of a colorimetric and ratiometric fluorescent sensor based on pyrido[3,4-b]indole-boron trifluoride complex for the detection of water over a wide range from low water content to high water content in solvents.

A Dual Associated-Functional Fluorescent Switch: From Alternate Detection Cycle for Fe(III) and pH to Molecular Logic Operations

With the expansion and deepening of scientific research, dual-functional or multifunctional materials are urgently needed to replace those for single application. Herein, a fluorescence sensing system based on an In(III)-organic complex with in situ Lewis acid sites has been constructed, exhibiting high sensitivity for the detection of Fe(III) ions with a low detection limit of 3.95 μM and a short response time of within 10 s. It is noteworthy that the quenched fluorescence of the Fe(III)-incorporated sample could be reopened linearly with an increase of alkalinity, followed by the reactivation of its functionality to identify Fe(III) ions, forming an alternate detection cycle for Fe(III) and pH with off-on-off fluorescent switch characteristics. Considering its unique molecular recognition capability, an advanced three-input (Fe(III), EDTA, and OH^-) and two-output (B440 and G489) Boolean logic operation comprising BUFF, NOT, OR, and AND logic gates was integrated, possessing potential applications in intelligent multianalyte sensing systems.

From aggregation-induced to solution emission: a new strategy for designing ratiometric fluorescent probes and its application for in vivo HClO detection

The fluorescence of aggregation-induced emission is converted into that from ICT-induced emission in the presence of HClO.

Fluorescent sensor for water based on photo-induced electron transfer and Förster resonance energy transfer: anthracene-(aminomethyl)phenylboronic acid ester-BODIPY structure

An anthracene-(aminomethyl)phenylboronic acid ester-BODIPY (DJ-1) was designed and developed as a fluorescent sensor based on photo-induced electron transfer (PET) and Förster resonance energy transfer (FRET) for the detection of a trace amount of water in solvents, where the anthracene skeleton and BODIPY skeleton are the donor fluorophore and the acceptor fluorophore in the FRET process, respectively. It was found that the addition of water to organic solvents containing DJ-1 causes both the suppression of PET in the anthracene-(aminomethyl)phenylboronic acid ester as the PET-type fluorescent sensor skeleton and the energy transfer from the anthracene skeleton to the BODIPY skeleton through a FRET process, thus resulting in the enhancement of the fluorescence band originating from the BODIPY skeleton. This work demonstrates that the PET/FRET-based fluorescent dye composed of the donor fluorophore possessing PET characteristics and the acceptor fluorophore in the FRET process can act as a fluorescent sensor with a large SS for the detection of a trace amount of water in solvents.

An anthracene-(aminomethyl)phenylboronic acid ester-BODIPY (DJ-1) structure was developed as a fluorescent sensor based on photo-induced electron transfer (PET) and Förster resonance energy transfer (FRET) for detection of water in solvents.

Development of an intramolecular charge transfer-type colorimetric and fluorescence sensor for water by fusion with a juloidine structure and complexation with boron trifluoride

An intramolecular charge transfer-type optical sensor fused with a juloidine structure and complexed with boron trifluoride can detect and determine water over a wide concentration range.

N-Arylated bisferrocene pyrazole for the dual-mode detection of hydrogen peroxide: an AIE-active fluorescent “turn ON/OFF” and electrochemical non-enzymatic sensor

N-Arylated bisferrocene pyrazoles for the dual-mode detection of H₂O₂ by AIE-active fluorescence and non-enzymatic electrochemical methods.

Nitrogen-Doped Microporous Carbons Derived from Pyridine Ligand-Based Metal-Organic Complexes as High-Performance SO2 Adsorption Sorbents

Heteroatom-doped porous carbons are emerging as platforms for gas adsorption. Herein, N-doped microporous carbon (NPC) materials have been synthesized by carbonization of two pyridine ligand-based metal-organic complexes (MOCs) at high temperatures (800, 900, 1000, and 1100 °C). For NPCs (termed NPC-1- T and NPC-2- T, where T represents the carbonization temperature), the micropore is dominant, pyridinic-N and other N atoms of MOC precursors are mostly retained, and the N content reaches as high as 16.61%. They all show high Brunauer-Emmett-Teller surface area and pore volume, in particular, NPC-1-900 exhibits the highest surface areas and pore volumes, up to 1656.2 m² g^-1 and 1.29 cm³ g^-1, respectively, a high content of pyridinic-N (7.3%), and a considerable amount of SO₂ capture (118.1 mg g^-1). Theoretical calculation (int = ultrafine m062x) indicates that pyridinic-N acts as the leading active sites contributing to high SO₂ adsorption and that the higher content of pyridinic-N doping into the graphite carbon layer structure could change the electrostatic surface potential, as well as the local electronic density, which enhanced SO₂ absorption on carbon edge positions. The results show great potential for the preparation of microporous carbon materials from pyridine ligand-based MOCs for effective SO₂ adsorption.

Hot-Pressing Method To Prepare Imidazole-Based Zn(II) Metal-Organic Complexes Coatings for Highly Efficient Air Filtration

Particulate matters (PMs) air pollution has become a serious environmental issue due to its great threat to human health. Herein, metal-organic complexes PBM-Zn1 and PBM-Zn2 coatings (noted as PBM-Zn-Filter) have been produced by the hot-pressing method on various substrates for the first time. Layer-by-layer PBM-Zn-Filters were also obtained through varying hot-pressing cycles. The obtained PBM-Zn-Filters with high robustness show excellent performance in PMs removal. In particular, benefiting from thelarger conjugation system, micropore structure, lower pressure drop, higher electrostatic potential ζ, and electron cloud exposed metal center of PBM-Zn2 (DFT calculations), PBM-Zn2@melamine foam-4 gives the highest removal rates, PM2.5:99.5% ± 1.2% and PM10:99.3% ± 1.1%, and the removal efficiency for capture PM2.5 and PM10 particles in cigarette smoke were both retained at high levels (>95.5%) after 24 h tests. More importantly, a homemade mask is made up by imbedding the PBM-Zn2@melamine foam-4 into a commercial breathing mask, which shows higher removal efficiency, lower pressure drop, smaller thickness, and higher quality factor than two commercial breathing masks, the PMs removal efficiencies for both PM2.5 and PM10 are 99.6% ± 0.5% and 99.4% ± 0.8%, and acceptable air resistance are demonstrated.

Tetraphenylethene– and diphenyldibenzofulvene–anthracene-based fluorescence sensors possessing photo-induced electron transfer and aggregation-induced emission enhancement characteristics for detection of water

RS-1 and RS-2 have been developed as PET/AIEE hybrid fluorescence sensors for detection of water in the low and high water content regions in solvents.

A colorimetric and fluorescent sensor for water in acetonitrile based on intramolecular charge transfer: D–(π–A)2-type pyridine–boron trifluoride complex

A D–(π–A)₂-type pyridine–boron trifluoride complex, YNI-2-BF3, was developed as a colorimetric and fluorescent sensor based on intramolecular charge transfer for the detection of water in solvents.