Organosilicon compounds as fluorescent chemosensors for fluoride anion recognition

Fluorinated benzothiadiazole fluorescent probe based on ICT mechanism for highly selectivity and sensitive detection of fluoride ion

Excessive fluoride ion (F-) in the environment can affect health and even endanger life when ingested by the human body. However, most fluoride probes have the disadvantages of low sensitivity and long detection time. Herein, fluorescent probe 3a is successfully synthesized by linking two acetylenyltrimethylsilyl groups at both ends of the fluorinated benzothiadiazole core. After the addition of F- to 3a, the emission at 436 nm is significantly quenched and slightly blue-shifted. It is confirmed by electrospray ionization high-resolution mass spectrometry (ESI-HRMS) and density functional theory calculations (DFT) that these changes are due to the F- triggered Si-C bond cleavage and the subsequent inactivation of intramolecular charge transfer (ICT). The detection limit and response time of probe 3a for F- are 10-8 mol/L and 25 s, respectively. Importantly, fluorescent material 3a can be processed into portable test tools for the visual detection of fluoride ion.

13-8-13-Membered Tricyclic Ladder-Type Siloxanes Hybridized with BINOLs: Synthesis, Characterization, and Fluorescence Sensing of Fluorides

Developing fluorescent chemosensors with sensitivity and high specificity for recognizing fluorides is still challenging. Herein, four innovative compounds based on 13-8-13-membered tricyclic ladder-type siloxanes hybridized with BINOLs (abbreviated as TLS-BINOLs) were prepared through the B(C6F5)3-catalyzed Piers-Rubinsztajn reaction. The well-defined ladder-type structure of the TLS-BINOLs was determined by X-ray crystallographic analysis. Additionally, the fluorescent sensing ability of the TLS-BINOLs toward anions was studied. Our finding revealed that all four ladder-type compounds (TLS-BINOLs) exhibited high specificity in recognizing fluorides through fluoride-triggered structural decomposition. The detection limits for fluorides were determined to be 0.37, 0.35, 0.39, and 0.48 μM for the respective TLS-BINOLs. The nonemissive product induced by the fluorides was also determined using single-crystal X-ray diffraction analysis.

Novel organoboron complexes with robust core: Synthesis, functionalization, and subcellular targeting

The construction of novel organoboron complexes with facile synthesis and unique advantages for biological imaging remains a challenge and thus has garnered considerable attention. Herein, we developed a new molecular platform, boron indolin-3-one-pyrrol (BOIN3OPY) via a two-step sequential reaction. The molecular core is robust enough to allow for post-functionalization to produce versatile dyes. When compared to the standard BODIPY, these dyes feature an N,O-bidentate seven-membered ring center, significantly redshifted absorption, and a larger Stokes shift. This study establishes a new molecular platform that provides more flexibility for the functional regulation of dyes.

Turn-on fluorescent capsule for selective fluoride detection and water purification

It has been a long-standing challenge to develop organic molecular capsules for selective anion binding in water. Here, selective recognition of aqueous fluoride was achieved through triple protonation of a hemicryptophane (L), which is composed of a fluorescent cyclotriveratrylene (CTV) cap and tris(2-aminoethyl)amine (tren) as the anion binding site. Fluoride encapsulation by [3H-L]³⁺ was evidenced by ¹H NMR, ¹⁹F NMR, LC-MS, and X-ray crystallography. In addition, [3H-L]³⁺ exhibited a 'turn-on' fluorescence signal (λ _em = 324 nm) upon fluoride addition. An apparent association constant K _A = (7.5 ± 0.4) × 10⁴ M^-1 and a detection limit of 570 nM fluoride were extracted from the fluorescence titration experiments in citrate buffer at pH 4.1. To the best of our knowledge, [3H-L]³⁺ is the first example of a metal-free molecular capsule that reports on fluoride binding in purely aqueous solutions with a fluorescence response. Finally, the protonated capsule was supported on silica gel, which enabled adsorptive removal of stoichiometric fluoride from water and highlights real-world applications of this organic host-guest chemistry.

Aryl-Phenanthro[9,10-d]imidazole: A Versatile Scaffold for the Design of Optical-Based Sensors

Fluorescent and colorimetric sensors are important tools for investigating the chemical compositions of different matrices, including foods, environmental samples, and water. The high sensitivity, low interference, and low detection limits of these sensors have inspired scientists to investigate this class of sensing molecules for ion and molecule detection. Several examples of fluorescent and colorimetric sensors have been described in the literature; this Review focuses particularly on phenanthro[9,10-d]imidazoles. Different strategies have been developed for obtaining phenanthro[9,10-d]imidazoles, which enable modification of their optical properties upon interaction with specific analytes. These sensing responses usually involve changes in the fluorescence intensity and/or color arising from processes like photoinduced electron transfer, intramolecular charge transfer, intramolecular proton transfer in the excited state, and Förster resonance energy transfer. In this Review, we categorized these sensors into two different groups: those bearing formyl groups and their derivatives and those based on other molecular groups. The different optical responses of phenanthro[9,10-d]imidazole-based sensors upon interaction with specific analytes are discussed.

Perylenemonoimide-Based Colorimetric Probe with High Contrast for Naked-Eye Detection of Fluoride Ions

Excessive fluoride ions (F^-) in drinking water are harmful to the environment and human health. However, most reported probes of F^- can only detect fluorocarbons rather than aqueous F^-. Herein, a colorimetric and fluorescent probe (PMI-OH) based on perylenemonoimide is designed and synthesized for the detection of aqueous F^-, with high sensitivity, good selectivity, and reversibility. The F^- causes deprotonation of PMI-OH, leading to a significant red shift of 222 nm (from 520 to 742 nm) of the absorption band. Upon the addition of fluorocarbons, the fluorescence intensities of PMI-OH show good linearity against the concentrations of F^-, realizing the quantitative detection of fluorocarbons with a limit of detection as low as 0.495 μM. Finally, PMI-OH is applied to detect F^- in drinking water. The color of PMI-OH solution shows remarkable response from pink to green when the concentrations of F^- exceed the upper limit set by the World Health Organization (WHO), realizing rapid and naked-eye detection of aqueous F^-.

A Theoretical Study of the Sensing Mechanism of a Schiff-Based Sensor for Fluoride

In the current work, we studied the sensing process of the sensor (E)-2-((quinolin-8ylimino) methyl) phenol (QP) for fluoride anion (F^-) with a "turn on" fluorescent response by density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. The proton transfer process and the twisted intramolecular charge transfer (TICT) process of QP have been explored by using potential energy curves as functions of the distance of N-H and dihedral angle C-N=C-C both in the ground and the excited states. According to the calculated results, the fluorescence quenching mechanism of QP and the fluorescent response for F^- have been fully explored. These results indicate that the current calculations completely reproduce the experimental results and provide compelling evidence for the sensing mechanism of QP for F^-.

Cyanide Anion Determination Based on Nucleophilic Addition to 6-[(E)-(4-Nitrophenyl)diazenyl]-1′,3,3′,4-tetrahydrospiro[chromene-2,2′-indole] Derivatives

This work provides a novel approach for the instant detection of CN− anions based on chromogenic 6-[(E)-(4-nitrophenyl)diazenyl]-1′,3,3′,4-tetrahydrospiro[chromene-2,2′-indole] derivatives. New colorimetric detectors were synthesized and characterized. These compounds exhibited a substantial color change from orange to magenta and blue when treated with cyanide ions in a CH3CN solution buffered with sodium phosphate and demonstrated high selectivity to CN− anions. Common anions were tested, and they did not interfere with cyanide detection, except for carbonates and hydrosulfites. The simple preparation of a molecular sensor and the easily noticeable color change makes this a practical system for the monitoring of CN− ions. This color change is explained by nucleophilic substitution of the pyrane ring oxygen atom at the indoline C-2 atom by the cyanide anion. This generates the appearance of intensively colored 4-(4-nitrophenylazo)phenolate chromophore and allows for determining very low levels of CN− anion.

The sensing mechanism of a flavone-based ESIPT fluorescent chemodosimeter for selective recognition towards fluoride: a theoretical

The sensing mechanism of 3-hydroxyflavone-based (3-HF) fluorescent chemodosimeter 3-triisopropylsilylflavone (3-TPSF) for detecting fluoride (F⁻) has been theoretically investigated. The calculated Laplacian bond order confirms that the Si–O bond of 3-TPSF is the reaction site of F⁻. The free energy barrier of 18.33 kcal mol⁻¹ indicates that F-triggered desilylation reaction can occur and then form the anionic state of 3-HF (3-HF⁻) with a fluorescence peak at 545 nm. 3-HF⁻ captures H⁺ of the mixed aqueous medium to be transformed into 3-HF with an intramolecular hydrogen bond (O₁–H⋯O₂). The energy barrier of 1.86 kcal mol⁻¹ in the S₁ state obtained from the constructed potential energy curves confirms that the excited state intramolecular proton transfer (ESIPT) in 3-HF occurs to form a tautomer structure, which produces a long-wavelength emission of 549 nm. The fluorescence emitted from both 3-HF⁻ and 3-HF agrees with the experimental value of 530 nm appearing after adding F⁻. Charge transfer analyses indicate that the extent of intramolecular charge transfer in 3-HF⁻ is more intense than that of 3-TPSF, which induces a large Stokes shift of 180 nm. Therefore, the sensing mechanism is attributed to the combination of a large charge transfer feature and ESIPT that are caused by desilylation reaction. The significant fluorescence change makes 3-TPSF a chemodosimeter for detecting F⁻.

The sensing mechanism of 3-hydroxyflavone-based (3-HF) fluorescent chemodosimeter 3-triisopropylsilylflavone (3-TPSF) for detecting fluoride (F⁻) has been theoretically investigated.

Hexaphenylbenzene-based fluorescent probes for the detection of fluoride ions

Novel hexaphenylbenzene derivatives (HPB-1 and HPB-2) were synthesized and their sensing abilities were investigated.

DFT/TD-DFT study to decipher the fluoride induced ring opening process of spiropyran

DFT/TD-DFT methods were used to determine the fluoride anion sensing mechanism of 3',6'-Bis(tert-butyldimethylsilyloxy)spiro[benzo[f]chromene3,9'ffuorene], abbreviated as SP. The description of ring opening in the ground state of SP molecule and its isomerization in open form is presented. It was revealed from the study that in the ground state, SP is the most stable form in contrast with the isomer obtained in the open form. To initiate the ring opening, at first, the fluoride ion attacks as a nucleophile to de-silylate the SP molecule. This attack of fluoride ion may induce Cspiro-O bond cleavage leading to the formation of two anionic species, i.e., MC-D1 and MC-D2 respectively (MC is merocyanin). The mono-de-silylation process was endogenic, which was followed by the ring opening process. Furthermore, the orthogonal geometry of probe SP does not show ICT character, whereas, MC-D1 and MC-D2 displayed ICT character owing to the formation of planar geometry along with an increase in conjugation. The fluorescence property of SP, and most stable isomers of open form (CT, MC-D1, and MC-D2) were predicted theoretically. The calculated emission spectra uncovered that SP may show fluorescence, which could be quenched in presence of fluoride anion.

Nickel(ii)-catalyzed reductive silylation of alkenyl methyl ethers for the synthesis of alkyl silanes

A new one pot protocol has been developed for the reductive silylation of alkenyl methyl ethers using Et₃Si–BPin and HSiEt₃ with nickel(ii) catalyst. Styrene type methyl ethers, multi-substituted vinyl methyl ethers, heterocycles and unconjugated vinyl ethers are all tolerated to form alkyl silanes. Mechanistic study reveals that it is a cascade of a C–O bond silylation and vinyl double bond hydrogenation process. Internal nucleophilic substitution or oxidative addition pathways were both acceptable for C–O bond cleavage. The acquired intermediate alkenyl silanes then proceeded through an unconventional reduction process thus providing alkyl silanes.

A Ni(ii)-catalyzed tandem reaction including vinyl C–O bond silylation and olefin hydrogenation has been developed providing structurally diversified alkyl silanes.

Ratiometric sensing of fluoride ions using Raman spectroscopy

Ratiometric Raman spectroscopy represents a novel sensing approach for the detection of fluoride anions based on alkyne desilylation chemistry. This method enables rapid, anion selective and highly sensitive detection of fluoride in a simple paper-based assay format using a portable Raman spectrometer.

Polymer matrix: A good substrate material for oxygen probes used in pressure sensitive paints

Over the past few years, surface pressure measurement has fundamental importance in many areas, particularly, aerodynamic research. Conventional methods involve pressure taps, but due to the nature of these pressure taps, only pressure information of isolated points on model surface is available, which limit their applications in aerodynamics studies. Recently the newly developed approach, pressure sensitive paint (PSP) has revolutionized such pressure measurements and various PSP materials have been developed for aerodynamics research. Hence, the main focus of this review is to study the interactions of polymers with different oxygen probes and polymeric role as supporting material in the maturation of PSP. In this review, the selected PSP materials are categorically elucidated in terms of their advantages and limitations to give a fair insight about their applicability. Further, we have summarized and articulated such particular optical oxygen sensing materials either that have been used as PSP or have potential to be used as PSP materials.

A Hydrosilylation Approach to Silicon-Bridged Functional Dipyrromethanes: Introducing Silicon to A New Arena

Two silylene-spaced ((E)-vinylsilyl)anthracene-dipyrromethane dyads have been designed and synthesized by RhCl(PPh₃ )₃ -catalyzed hydrosilylation reactions of 5-methyl-5'-(ethynylaryl)dipyrromethanes with (9-Anthryl)-dimethylsilane. The complexation studies of dyads toward different anions have also been performed, which reveal that dyads exhibit a highly selective response towards fluoride anion attributable to both hydrogen-bonding and pentacoordination phenomena. This dual-mode fluoride recognition event is unprecedented and may pave the way for future developments in the areas of porphyrinoids, organosilicon, polymer, and supramolecular chemistry.

Through bond energy transfer (TBET)-operated fluoride ion sensing via spirolactam ring opening of a coumarin–fluorescein bichromophoric dyad

The detection of fluoride ions in a competitive environment often poses several challenges. In this work, we have designed and synthesized a coumarin functionalized fluorescein dyad (R3) which represents an ideal through bond energy transfer (TBET) fluorophore with the coumarin unit as donor and fluorescein unit as acceptor. The bichromophoric dyad demonstrates the detection of fluoride ions in the parts per billion (ppb) concentration level (22.8 ppb) with high selectivity via a TBET emission signal at 548 nm with a diagnostic bright yellow colour fluorescence output. Based on UV-visible, fluorescence, ¹H NMR and DFT studies, it is shown that the fluoride ion induces the opening of the spirolactam ring of the fluorescein moiety and provides a π-conjugation link between the donor and acceptor units enabling a TBET phenomenon with a larger pseudo-Stokes shift of 172 nm. To the best of our knowledge, this is the first report where the fluoride ion is detected via a TBET signal between the coumarin and fluorescein units in a bichromophoric dyad.

A coumarin–fluorescein based bichromophoric dyad detects fluoride ions in parts per billion concentration level via a TBET emission signal at 548 nm with a diagnostic bright yellow fluorescence.

Synthesis and characterization of poly(tetraphenylimidazole)s and their application in the detection of fluoride ions

Here, we describe the synthesis and characterization of a silyl protected tetraphenylimidazole monomer and its homo and co-polymer. The requisite monomer was accessed by Suzuki–Miyaura cross-coupling of 2-(1-(4-bromophenyl)-4,5-diphenyl-1H-imidazol-2-yl)phenol and 4-vinylphenylboronic acid followed by protection of the phenolic group by tert-butyl(chloro)diphenylsilane. The desired polymers were readily synthesized by using free radical polymerization. Both the polymers and monomer were characterized using different analytical techniques including multinuclear NMR, GPC (for polymers), and single crystal X-ray crystallography (for the monomer). By utilizing the greater fluorophilicity of the silyl atom, the polymers were studied as probes for the detection of fluoride ions. The selectivity and sensitivity of the synthesized polymers were investigated in detail.

We describe the synthesis and characterization of a silyl protected tetraphenylimidazole monomer and its polymers. The polymers were studied as probes for the detection of fluoride ions. Both the probes showed high selectivity and sensitivity over other ions tested.

Efficient Optical and UV-Vis Chemosensor Based on Chromo Probes-Polymeric Nanocomposite Hybrid for Selective Recognition of Fluoride Ions

A novel colorimetric sensor based on the TiO₂/poly(acrylamide-co-methylene bis acrylamide-co-2-(3-(4-nitro-phenyl)thioureido)ethyl methacrylate) nanocomposite was synthesized via a surface modification strategy; methacryloxypropyltrimethoxysilane was used to provide reactive vinyl groups on the surface of TiO₂ nanoparticles for the successful surface polymerization of Am (acrylamide), MBA (methylenbisacrylamide), and NPhM (2-(3-(4-nitrophenyl)thioureido)ethyl methacrylate) components. The successful preparation of nanocomposites was confirmed using Fourier transform infrared, ¹H NMR, ¹³C NMR, scanning electron microscopy, transmission electron microscopy, thermogravimetry analysis, and X-ray diffraction methods, and the sensing ability of the probe toward fluoride ions was investigated using naked-eye detection and UV-vis measurement. The interaction of the prepared polymeric nanocomposite with fluoride ions elicited a significant visible change in color from pale yellow to orange and was further affirmed by a clean interconversion of the two absorption bands at 330 and 485 nm. The selective binding ability of the polymeric nanocomposite towards fluoride over other anions, such as I^-, Cl^-, Br^-, AcO^-, H₂PO₄ ^-, and H₂SO₄ ^- was further explored; the prepared chemosensor could detect fluoride ions in acetonitrile with a detection limit of 3 μM.

Crystal structure of 2-(prop-2-yn-1-yloxy)-1-naphthaldehyde, C14H10O2

C14H10O2, triclinic, P1̄ (no. 2), a = 7.498(3) Å, b = 7.973(3) Å, c = 9.660(4) Å, α = 67.211(13)°, β = 84.489(14)°, γ = 72.224(14)°, V = 506.8(4) Å3, Z = 2, Rgt(F) = 0.0394, wR ref(F 2) = 0.1135, T = 100(2) K.

Non-symmetric thieno[3,2-b]thiophene-fused BODIPYs: synthesis, spectroscopic properties and providing a functional strategy for NIR probes

Non-symmetric thieno[3,2-b]thiophene-fused BODIPYs were designed and characterized, and further functionalization through the Knoevenagel condensation reaction to form a NIR sensor was achieved.

A multi-analyte selective dansyl derivative for the fluorescence detection of Cu(ii) and cysteine

A new multi-analyte selective fluorescence chemosensor DA was synthesized by a simple one pot reaction between dansyl chloride and 2-aminobenzohydrazide in the presence of a base.

Förster Resonance Energy-Transfer-Based Ratiometric Fluorescent Indicator for Quantifying Fluoride Ion in Water and Toothpaste

A Förster resonance energy-transfer (FRET)-based ratiometric fluorescent indicator Cou-FITC-Si toward fluoride ion has been designed and synthesized by combining coumarin unit and fluorescein derivative as energy donor and acceptor, respectively. The fluorescein unit is capped with tert-butyldiphenylchlorosilane. The indicator gives out emission responses based on switch-on of the FRET process that triggered by the desilylation mediated by the fluoride ion. The fluorescence emission spectrum of Cou-FITC-Si presents a significant bathochromic shift of 59 nm after the addition of fluoride ion with up to 180-fold increase of the fluorescence intensity ratio. The limit of detection of the Cou-FITC-Si indicator system toward fluoride ion was estimated to be 3.3 ppb. Furthermore, this indicator has been successfully applied for quantifying the fluoride ion of different concentrations from commercially available toothpaste.

Encumbrance in desilylation triggered fluorogenic detection of the fluoride ion - a kinetic approach

Highly selective fluorogenic detection of the fluoride ion becomes viable due to its propensity towards cleaving Si-O and Si-C bonds, the key reactive elements in fluoride selective chemodosimeters. Herein, acridinedione derived, two novel fluorescent probes bearing tertiarybutyldiphenylsilyloxy (TBDPS) and tertiarybutyldimethylsilyloxy (TBDMS) groups were synthesized and their fluoride selective dosimetric action in organic solvents and in mixed aqueous medium was established through steady state and time resolved fluorescence techniques. Unusually, these molecular probes maintain their sensitivity down to 10 ppb in both organic and mixed aqueous medium; hence they can be considered as highly selective and sensitive fluorescent probes for the fluoride anion. By following the kinetics of the desilylation process it is established that the reaction follows second order kinetics with respect to fluoride ion concentration in acetonitrile whereas it becomes first order in mixed aqueous medium owing to its high degree of hydration. Also, the hydrophobic and sterically crowded substitution on the silyl receptor hampers the reaction kinetics only in organic solvents whereas its influence in mixed aqueous medium is relatively very less. However, common inorganic cations (Na⁺) effectively hinder the reaction kinetics through strong ion pair interaction and prolong the response time. Therefore, the indigenous influences of three different factors which encumber the desilylation process were quantitatively enumerated and the prospective application of these fluorescent probes in detecting and validating fluoride ions in various environmental samples is demonstrated.

A fluoride activated methylene blue releasing platform for imaging and antimicrobial photodynamic therapy of human dental plaque

A fluoride activated methylene blue releasing platform was developed for imaging and antimicrobial photodynamic therapy of human dental plaque.

Nanomaterials for the optical detection of fluoride

Overexposure to fluoride ions (F^-) causes serious diseases in human beings. Extensive efforts have been made to develop sensitive and selective approaches for F^- detection and a variety of F^- sensors have been constructed recently. The burgeoning nanotechnology has provided novel materials for F^- analysis due to the extraordinary properties of nanomaterials. In this review, we present the recent advances in different nanomaterials-based approaches for the optical F^- detection via colorimetric, fluorescent and chemiluminescent responses. The materials include gold nanomaterials, CeO₂ nanoparticles, semiconductor quantum dots, carbon quantum dots, metal-organic frameworks, upconversion nanoparticles, micellar nanoparticles, polymer dots, SiO₂ nanoparticles and graphene oxide. The recent trends and challenges in the optical detection of F^- with various nanomaterials are also discussed.

Ligand-Capped CdTe Quantum Dots as a Fluorescent Nanosensor for Detection of Copper Ions in Environmental Water Sample

In this work, as a novel fluorescent nano-sensor, a ligand-capped CdTe QDs (CdTe-L QDs) was designed for the detection and quantification of Cu²⁺ ions in environmental water samples. The synthesized QDs were characterized by transmission electron microscopy (TEM), thermo-gravimetric (TG) analysis, Fourier transform infrared (FTIR), UV-Vis spectrophotometry and fluorescence spectroscopy. Optical properties of the produced nanosensor were monitored by UV-Vis and fluorescence spectrophotometry. It was observed that fluorescence intensity of the produced nano-sensor selectively quenched by adding Cu²⁺ ions in comparison to other metal ions tested. Using CdTe-L QDs, a rapid and facile analytical method was developed to determine Cu²⁺ ions in the concentration range of 5.16 ± 0.07 × 10^- 8 mol L^- 1-1.50 ± 0.03 × 10^- 5 mol L^- 1 with a detection limit of 1.55 ± 0.05 × 10^- 8 mol L^- 1. The nanosensor was successfully applied for the determination of Cu²⁺ ions in various water samples, and the results were compared with the standard method. Graphical Abstract.