A “Turn-On” fluorescent probe for sensitive and selective detection of fluoride ions based on aggregation-induced emission

A multifunctional near-infrared fluorescent probe based on benzothiazole structure for fluoride-ion detection

Fluoride ions (F-) are one of the essential trace elements for the human body and are widely existed in nature. In this study, we present a novel fluorescent probe (YF-SZ-F) designed and synthesized for the specific detection of F-. The probe exhibits high sensitivity, excellent selectivity, and low cytotoxicity, making it a promising tool for biomedical applications. Imaging experiments conducted on zebrafish and Arabidopsis roots demonstrate the probe's remarkable cellular permeability and biocompatibility, laying a solid foundation for its potential biomedical utility. Furthermore, the probe holds potential for practical applications in environmental monitoring and public health through its capability to detect fluoride ions in water samples and via mobile phone software. This multifaceted functionality underscores the broad applicability and significance of the fluorescent probe, not only in scientific research but also in real-world scenarios, contributing to the development of more convenient and precise methods for fluoride detection.

A V-shaped bis-coumarin based fluorescence probe for F- detection in tea infusions and potable water and bioimaging applications in living systems

Fluorine (F) is a pivotal element in the formation of human dental and skeletal tissues, and the consumption of water and tea constitutes a significant source of fluoride intake. However, prolonged ingestion of water and tea with excessive fluoride content can lead to fluorosis, which poses a serious health hazard. In this manuscript, a novel turn-on fluorescent probe DCF synthesized by bis-coumarin and tert-butyldiphenylsilane (TBDPS) was introduced for detecting F- in potable water and tea infusions. By leveraging the unique chemical affinity between fluoride and silicon, F- triggers the silicon-oxygen bond cleavage in DCF, culminating in a conspicuous emission of yellow fluorescence. Validated through a succession of optical tests, this probe exhibits remarkable advantages in terms of superior selectivity, a low detection limit, a large Stokes shift, and robust interference resistance when detecting inorganic fluoride. Moreover, it can serve as portable test strips for on-site real-time identification and quantitative analysis of F-. Furthermore, the application of DCF for in-situ monitoring and imaging of F- in zebrafish and soybean root tissues proved its significant value for F- detection in both animal and plant systems. This probe potentially functions as an efficient instrument for delving into the toxic mechanisms of fluoride in physiological processes.

Recent advances in AIEgen-based chemosensors for small molecule detection, with a focus on ion sensing

Since the aggregation-based emission (AIE) phenomenon emerged in 2001, numerous chemical designs have been built around the AIE concept, displaying its utility for diverse applications, including optics, electronics, energy, and biosciences. The present review critically evaluates the broad applicability of AIEgen-based chemical models towards sensing small analytes and the structural design strategies adjusting the mode of action reported since the last decade. Various AIEgen models have been discussed, providing qualitative and quantitative estimation of cationic metal ions and anionic species, as well as biomolecular, cellular, and organelle-specific probes. A systematic overview of the reported structural design and the underlying working mode will pave the way for designing and developing the next generation of AIEgens for specific applications.

Bromine atom introduction improves the F- sensing ability of an indolo[3,2-b]carbazole-salicylaldehyde-based fluorescence turn-on sensor

The heavy-atom effect usually quenches fluorescence, but scarcely enhances it. Herein, fluorescence turn-on sensors without or with a bromine atom for F- detection are presented, achieving fast response time within 1 min, and the LODs of 1.9 × 10-7 and 8.5 × 10-8 M, reflecting that halogen atom introduction is beneficial for F- detection ability improvement. The sensing mechanism of -OH unit deprotonation is confirmed based on the results of a 1 : 2 stoichiometric ratio, 1H NMR titration and TD-DFT calculation. The water environment F- detection and spiked recovery experiments demonstrate their potential for real sample detection.

TDDFT Study on the ESIPT Properties of 2-(2'-Hydroxyphenyl)-Benzothiazole and Sensing Mechanism of a Derived Fluorescent Probe for Fluoride Ion

The level of fluoride ions (F-) in the human body is closely related to various pathological and physiological states, and the rapid detection of F- is important for studying physiological processes and the early diagnosis of diseases. In this study, the detailed sensing mechanism of a novel high-efficiency probe (PBT) based on 2-(2'-hydroxyphenyl)-benzothiazole derivatives towards F- has been fully investigated based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. F- attacks the O-P bond of PBT to cleavage the dimethylphosphinothionyl group, and the potential products were evaluated by Gibbs free energy and spectroscopic analyses, which ultimately identified the product as HBT-Enol1 with an intramolecular hydrogen bond. Bond parameters, infrared vibrational spectroscopy and charge analysis indicate that the hydrogen bond is enhanced at the excited state (S1), favoring excited state intramolecular proton transfer (ESIPT). The mild energy barrier further evidences the occurrence of ESIPT. Combined with frontier molecular orbital (FMO) analysis, the fluorescence quenching of PBT was attributed to the photoinduced electron transfer (PET) mechanism and the fluorescence turn-on mechanism of the product was attributed to the ESIPT process of HBT-Enol1.

Reversal of a Fluorescent Fluoride Chemosensor from Turn-Off to Turn-On Based on Aggregation Induced Emission Properties

Here we present a new approach for the development of fluoride chemosensors taking advantage of aggregation induced emission (AIE) properties. Although AIE-based chemosensors have been described, they rely primarily on the analyte causing aggregation and hence fluorescence. We propose a new concept in the use of AIE for the development of fluorescent sensors. Our hypothesis is based on the fact that a turn-off chemosensor in solution can be transformed into turn-on in the solid state if the properties of ACQ and AIE are properly combined between the fluorescent molecules involved. To demonstrate this hypothesis, we have selected a fluorescent chemosensor for the fluoride anion with a conjugated structure of bis(styryl)pyrimidine that, while showing turn-off behavior in solution, becomes turn-on when it is brought to the solid state. We have also combined it with the advantages of a detection system based on the microfluidic paper-based analytical devices (μPAD). The system is fully characterized spectroscopically both in solution and in the solid state, and quantum mechanical calculations were performed to explain how the sensor works. The prepared device presents a high sensitivity, with no interference and with an LoD and LoQ that allow determination of fluoride concentrations in water 2 orders of magnitude below the maximum allowed by WHO.

A new sensor based on thieno[2,3-b]quinoline for the detection of In3+ , Fe3+ and F- by different fluorescence behaviors

Based on thieno[2,3-b]quinoline-2-carbohydrazide and salicylaldehyde, a novel fluorescent probe (L) was designed and synthesized. L could be used as a multifunctional sensor to sequentially detect In³⁺ and Fe³⁺ through fluorescence enhancement and fluorescence quenching in DMF/H₂ O buffer solutions. At the same time, L had good anti-interference ability, which could still detect In³⁺ and Fe³⁺ well in the presence of other metal ions. For F^- , it could be detected by enhancing the fluorescence change caused by the introduction of Al³⁺ . When other anions were present, the detection of F^- would not be interfered. The detection limits of In³⁺ , Fe³⁺ and F^- were 1.16×10^-10 M, 2.03×10^-8 M and 7.98×10^-9 M, respectively. The complexation model and sensing mechanism between L and In³⁺ , Fe³⁺ and F^- were confirmed by calculating structural optimization and energy optimization using Gaussian 09 software.

A novel ICT-based chemosensor for F- and its application in real samples and bioimaging

A novel colorimetric and fluorescent chemosensor MQS-Si with intramolecular charge transfer character has been designed and synthesized. The chemosensor shows exclusively "off-on" fluorescence response toward F^- at 620 nm in HEPES (pH 7.4): DMSO solution (7:3, v/v), which is attributed to the specific cleavage of Si-O bond. The ultrasensitive detection limit for F^- in the fluorescence measurement is down to 30 nM. Application of the chemosensor has been demonstrated by selective detection of F^- in drinking water, urine and serum samples and fluorescence imaging of F^- in living cells and zebrafish, which proves that MQS-Si has a promising application in vitro and in vivo detection of F^- and may be utilized for the diagnosis of fluorosis and esteofluorosis.

Aggregation Induced Emissive Luminogens for Sensing of Toxic Elements

The major findings in the growing field of aggregation induced emissive (AIE) active materials for the detection of environmental toxic pollutants have been summarized and discussed in this Review article. Owing to the underlying photophysical phenomenon, fluorescent AIE active molecules show more impact on sensing applications. The major focus in current research efforts is on the development of AIE active materials such as TPE based organic fluorescent molecules, metal organic framework, and polymers that can be employed for the detection of toxic pollutants such as CN- , NO2- , Hg2+ , Cd2+ , As3+ , As5+ , F- , Pb2+ , Sb3+ ions.

Selective anions mediated fluorescence "turn-on", aggregation induced emission (AIE) and lysozyme targeting properties of pyrene-naphthalene sulphonyl conjugate

We have designed and synthesized a novel pyrene-naphthalene sulphonyl conjugate, 1-((1Z)-(4-((Z)-4-(pyrene-1-yl)methyleneamino)phenylsulfonyl)phenylimino)methyl)naphthalene-2-ol (PSN) through a facile two-step reactions. It was characterized by various spectral techniques. Fluorescence spectral studies showed that compound PSN featured fluorescence enhancement upon increasing the water content in THF. This can be attributed to the phenomena of aggregated induced emission (AIE), which is confirmed by SEM and AFM studies, due to the restriction of CHN isomerization of PSN. The anion sensing of PSN was examined with various anions. Among these anions, H₂PO₄^- and F^- ions were selectively sensing with a low detection limit of 3.52 × 10^-7 M and 7.23 × 10^-7 M, respectively, and an obvious color change from yellow to orange was observed by the naked eye. The mechanism of sensing involved the formation of hydrogen bonding interaction between O-H group of PSN and H₂PO₄^-/ F^- ions. The binding of PSN with LYZ was also examined by docking studies, which shows that H-bonding and hydrophobic interactions play crucial roles for the interaction of LYZ toward PSN.

Excited State Intramolecular Proton Transfer (ESIPT)-Based Sensor for Ion Detection

C-2 and C-5 substituted imidazole skeleton was synthesized through a one-pot two-step strategy. Synthesized molecule emits the light on ESIPT (excited-state intramolecular proton transfer). This molecule was utilized for its proton donor ability, and we have observed that fluoride and cyanide ions can be detected selectively. Different cations and anions were selected to observe the response of the synthesized molecule. However, there were not any fluorometric and colorimetric response except for fluoride and cyanide ions. Detection limits of fluoride and cyanide ions were found to be 9.22 μM and 11.48 μM, respectively. ¹H-NMR spectra for the solution of the sensor and TBAF (tetrabuthylammoniumfluoride) were used for the identification of [L]^-[HF₂]^- species. 3 equiv. TBAF saturated the solution of the sensor in d₆-DMSO, and some of the proton resonances shifted to upfield due to the through-bond effect. The disappearance of NH proton with 0.5 equiv. TBAF or TBACN (tetrabuthylammoniumcyanide) showed that there was a proton abstraction by fluoride and cyanide ions, instead of the hydrogen bond. Solid-state application was utilized, and paper test strips were applied. Emission differences emerged when the sensor loaded strips were reacted with TBAF. Time resolved experiments revealed that solution of the sensor and TBAF in DMSO have multiexponential decay, and one of the lifetime was measured as 13.4 ns.

AIE-based fluorescent sensors for low concentration toxic ion detection in water

Ions, including anions and heavy metals, are extremely toxic and easily accumulate in the human body, threatening the health of humans and even causing human death at low concentrations. It is therefore necessary to detect these toxic ions in low concentrations in water. Fluorescent sensing is a good method for detecting these ions, but some conventional dyes often exhibit an aggregation caused quench (ACQ) effect in their solid state, limiting their large-scale application. Fluorescent probes based on aggregation-induced emission (AIE) properties have received significant attention due to their high fluorescence quantum yields in their nano aggragated states, easy fabrication, use of moderate conditions, and selevtive recognization of organic/inorganic compounds in water with obvious changes in fluorescence. We surmarize the recent advances of AIE-based sensors for low concentration toxic ion detection in water. The detection probes can be divided into three categories: chemical reaction types, chemical interaction types and physical interaction types. Chemical reaction types utilize nucleophilic addition and coordination reaction, while chemical interaction types rely on hydrogen bonding and anion-π interactions. The physical interaction types are composed of electrostatic attractions. We finally comment on the challenges and outlook of AIE-active sensors.

Role of Conical Intersections on the Efficiency of Fluorescent Organic Molecular Crystals

Organic molecular crystals are attractive materials for luminescent applications because of their promised tunability. However, the link between the chemical structure and emissive behavior is poorly understood because of the numerous interconnected factors which are at play in determining radiative and nonradiative behaviors at the solid-state level. In particular, the decay through conical intersection dominates the nonadiabatic regions of the potential energy surface, and thus, their accessibility is a telling indicator of the luminosity of the material. In this study, we investigate the radiative mechanism for five organic molecular crystals which display a solid-state emission, with a focus on the role of conical intersections in their photomechanisms. The objective is to situate the importance of the accessibility of conical intersections with regards to emissive behavior, taking into account other nonradiative decay channels, namely, vibrational decay, and exciton hopping. We begin by giving a brief overview of the structural patterns of the five systems within a larger pool of 13 crystals for a richer comparison. We observe that because of the prevalence of sheet like and herringbone packing in organic molecular crystals, the conformational diversity of crystal dimers is limited. Additionally, similarly spaced dimers have exciton coupling values of a similar order within a 50 meV interval. Next, we focus on three exemplary cases, where we disentangle the role of nonradiative decay mechanisms and show how rotational minimum energy conical intersections in vacuum lead to puckered ones in the crystal, increasing their instability upon crystallization in typical packing motifs. In contrast, molecules with puckered conical intersections in vacuum tend to conserve this trait upon crystallization, and therefore, their quantum yield of fluorescence is determined predominantly by other nonradiative decay mechanisms.

A new ESIPT-based fluorescent probe for the highly sensitive detection of amine vapors

A new ESIPT-based fluorescent probe has been developed as a rapid, highly sensitive, and selective sensor for amine vapors.

A simple colorimetric and ratiometric fluoride ion probe with large color change

Two colorimetric and ratiometric fluoride ion probes SHJ-1 and SHJ-2 based on the acylhydrazone skeleton have been developed. Among the eight anions (F-, Cl-, Br-, I-, ClO4 -, H2PO4 -, HSO4 -, CH3COO-), the present probes showed high selectivity and sensitivity toward fluoride ion detection with obvious color change. Notably, the probe SHJ-1 exhibited a red shift of 145 nm upon fluoride sensing, which is the largest value among fluoride ion probes based on acylhydrazone derivates to date. 1HNMR titration study and theoretical calculations suggested that the strong binding of the probe SHJ-1 to fluoride as well as the further deprotonation may facilitate the intramolecular charge transfer transition. These two probes are 1 : 1 complexed with fluoride ions, and the detection limits were calculated to be 1.24 μM for SHJ-1 and 15.73 μM for SHJ-2.

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.

Synthesis of fluorescent polycarbonates with highly twisted N,N-bis(dialkylamino)anthracene AIE luminogens in the main chain

A synthetic route to embed aggregation-induced-emission-(AIE)-active luminophores in polycarbonates (PCs) in various ratios is reported. The AIE-active monomer is based on the structure of 9,10-bis(piperidyl)anthracene. The obtained PCs display good film-forming properties, similar to those observed in poly(bisphenol A carbonate) (Ba-PC). The fluorescence quantum yield (Φ) of the PC with 5 mol% AIE-active monomer was 0.04 in solution and 0.53 in solid state. Moreover, this PC is also miscible with commercially available Ba-PC at any blending ratio. A combined analysis by scanning electron microscopy and differential scanning calorimetry did not indicate any clear phase separation. These results thus suggest that even engineering plastics like polycarbonates can be functionalized with AIE luminogens without adverse effects on their physical properties.

Highly Sensitive Ratiometric Fluorescent Paper Sensors for the Detection of Fluoride Ions

Two sensitive and ratiometric fluorescent probes (probe I and probe II) were developed for the detection of fluoride ions. Probe I can detect fluoride ions quantitatively within a range of 0-6 μM and a detection limit of 73 nM, while probe II has a range of 0-40 μM and a detection limit of 138 nM. The test strips from probe I are quickly able to recognize F- (5 min) inside of the F- safety level in drinking water (1.0 mg/L, ∼5 μM) under 254 nm ultraviolet light, and the test strips from probe II quickly recognize F- (12 min) in dangerously high F- levels in water (4.0 mg/L, ∼21 μM) under 254 nm ultraviolet light. This combination of fluorescent paper sensors from probe I and probe II can be used as a simple and convenient tool to determine whether water is safe to drink or dangerous.