New Highly Selective Colorimetric and Ratiometric Anion Receptor for Detecting Fluoride Ions

π-Extension of Indoles Using Acrolein Linker: Synthesis of Indolo[3,2-a]carbazole-6-carbaldehydes and Racemosin B

A simple method for the synthesis of indolo[3,2-a]carbazole-6-carbaldehydes by the π-extension of indoles with acrolein is reported. The scope of the method is demonstrated with 33 examples. The products exhibit high activities toward advanced synthesis and are proved to be able to produce valuable chemicals, such as natural products, dyes, and organic fluorescent materials. In addition, the alkaloid racemosin B can be prepared by this method in two steps in ∼50% overall yield.

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.

Functionalized Quinoxaline for Chromogenic and Fluorogenic Anion Sensing

This Review article provides a comprehensive analysis of recent examples reported in the field of quinoxaline-based chromogenic and fluorogenic chemosensors for inorganic anions such as fluoride, cyanide, acetate, and phosphate, as well as their utility in biomolecular science. It commences with a discussion of the various structural motifs such as quinoxaline-based oligopyrroles, polymers, sulfonamides, cationic receptors, and miscellaneous receptors bearing mixed recognition sites in the same receptor. Advances are discussed in depth, where the focus of this review is to tackle mainly solution state anion sensing utilizing quinoxaline-based receptors using different spectroscopic techniques with reference to anion selectivity by colorimetric and fluorescence response. The various examples discussed in this Review illustrate how the integration of anion binding elements with the quinoxaline chromophore could result in anion responsive chemosensors. Over the years, it has been observed that structural modification of the quinoxaline moiety with different sets of signaling unit and recognition sites has resulted in a few anion specific chemosensors.

Chemistry and Properties of Indolocarbazoles

The indolocarbazoles are an important class of nitrogen heterocycles which has evolved significantly in recent years, with numerous studies focusing on their diverse biological effects, or targeting new materials with potential applications in organic electronics. This review aims at providing a broad survey of the chemistry and properties of indolocarbazoles from an interdisciplinary point of view, with particular emphasis on practical synthetic aspects, as well as certain topics which have not been previously accounted for in detail, such as the occurrence, formation, biological activities, and metabolism of indolo[3,2- b]carbazoles. The literature of the past decade forms the basis of the text, which is further supplemented with older key references.

Rationally designed upconversion nanoprobe for simultaneous highly sensitive ratiometric detection of fluoride ions and fluorosis theranostics

For many years, fluorosis has been known as a worldwide disease which seriously diminishes quality of life through skeletal embrittlement and hepatic damage. Aiming to develop novel drugs for simultaneous fluorosis diagnosis and therapy, in this work we explore the feasibility of a novel pyrogallic acid-titanium(iv) complex-modified upconversion nanoprobe (UCNP-PA-Ti) for F- capture and real-time quantification. Utilizing the strong interaction between Ti4+ and F-, the modified PA-Ti decomposes in F--containing solution, which not only weakens the FRET but results in upconversion luminescence (UCL) recovery. Both in vitro and in vivo experiments demonstrate a highly sensitive F- UCL response and therapeutic efficiency, which was promising for successful UCL image monitoring and the therapeutic process. Long blood circulation time and low toxicity ensured their safe application for fluorosis theranostics. Our work provides a new possibility for F- concentration detection within fluorosis therapeutic periods and encourages the development of novel drugs for fluorosis theranostics.

A smart ratiometric red fluorescent chemodosimeter for fluoride based on anthraquinone nosylate

A smart ratiometric red fluorescent chemodosimeter (AH) has been explored for specific detection of F⁻ with a lowest detection limit of 3.45 × 10⁻¹⁰ M. The sensing mechanism was worked out as fluoride-triggered deprotonation of imidazole accompanied by deprotection of the nosylate group.

Naked-eye and fluorescence detection of basic pH and F−with a 1,8-naphthalimide-based multifunctional probe

A novel multifunctional colorimetric and fluorescent 1,8-naphthalimide-based probe toward basic pH and F⁻has been developed.

Colorimetric and ratiometric fluorescent chemosensor for fluoride ions based on phenanthroimidazole (PI): spectroscopic, NMR and density functional studies

Phenanthro-imidazole conjugate of biscarbanohyrazone, probe PI was designed, synthesized and characterized for selective detection of fluoride ion.

Bis-ureidoquinoline as a selective fluoride anion sensor through hydrogen-bond interactions

Bis-ureidoquinoline shows a characteristic UV-vis absorbance and turn-on fluorescence changes in the presence of the fluoride anion. Such selective changes probably originate from the hydrogen-bond interactions, as shown by the (1)H NMR titration and DFT calculations. Bis-ureidoquinoline can be used as a fluoride-selective sensor for the detection of fluoride anions under illumination from a laboratory hand-held UV lamp.

Hydrogen bonding interaction between active methylene hydrogen atoms and an anion as a binding motif for anion recognition: experimental studies and theoretical rationalization

Two new reagents, having similar spatial arrangements for hydrogen atoms of the active methylene functionalities, were synthesized and interactions of such reagents with different anionic analytes were studied using electronic spectroscopy as well as by using (1)H and (31)P NMR spectroscopic methods. Experimental studies revealed that these two reagents showed preference for binding to F(-) and OAc(-). Detailed theoretical studies along with the above-mentioned spectroscopic studies were carried out to understand the contribution of the positively charged phosphonium ion, along with methylene functionality, in achieving the observed preference of these two receptors for binding to F(-) and OAc(-). Observed differences in the binding affinities of these two reagents toward fluoride and acetate ions also reflected the role of acidity of such methylene hydrogen atoms in controlling the efficiencies of the hydrogen bonding in anion-Hmethylene interactions. Hydrogen bonding interactions at lower concentrations of these two anionic analytes and deprotonation equilibrium at higher concentration were observed with associated electronic spectral changes as well as visually detectable change in solution color, an observation that is generally common for other strong hydrogen bond donor functionalities like urea and thiourea. DFT calculations performed with the M06/6-31+G**//M05-2X/6-31G* level of theory showed that F(-) binds more strongly than OAc(-) with the reagent molecules. The deprotonation of methylene hydrogen atom of receptors with F(-) ion was observed computationally. The metal complex as reagent showed even stronger binding energies with these analytes, which corroborated the experimental results.

A new ditopic ratiometric receptor for detecting zinc and fluoride ions in living cells

The synthesis, characterization and ion binding properties of a new ditopic ratiometric receptor (1), based on 2-(4,5-dihydro-1H-imidazol-2-yl)phenol and crown ether moieties, have been described. The ditopic ratiometric receptor has been studied in sensing both F(-) and Zn(2+) ions, exhibiting different fluorescent colour changes from cyan green to blue/black observable by the naked eye under UV-light. The addition of Zn(2+) to the solution of 1 induced the formation of a 2 : 2 ligand-metal complex 1-Zn(2+), which displays a remarkable blue shift of the emission maxima of 1 from 455 nm to 400 nm due to the inhibition of excited-state intramolecular proton transfer (ESIPT) mechanism. The sensing processes were monitored by fluorescence/absorption titrations, and further confirmed by Job's plot and (1)H NMR titrations. The crystal structure of 1-Zn(2+) reveals that 1 binds Zn(2+) in four-coordinated modes. Furthermore, 1 is cell permeable and may be applied to detect trace Zn(2+) and F(-) during the development of a living organism.