Excited state intermolecular proton transfer and caging of salicylidine-3,4,7-methyl amine in cyclodextrins

An Inorganic Fluorescent Chemosensor: Rational Design and Selective Mg2+ Detection

A Zn²⁺ based complex, 3, displays greatly increased fluorescence emission in the presence of Mg²⁺. Fluorescent and computational studies suggest that 3 selectively interacts with Mg²⁺ due to optimal cavity size formation between two uncoordinated pyrazole side arms. This work thus represents a new approach to the development of fluorescent chemosensors.

An Indole-Based Fluorescent Chemosensor for Detecting Zn2+ in Aqueous Media and Zebrafish

An indole-based fluorescent chemosensor IH-Sal was synthesized to detect Zn²⁺. IH-Sal displayed a marked fluorescence increment with Zn²⁺. The detection limit (0.41 μM) of IH-Sal for Zn²⁺ was greatly below that suggested by the World Health Organization. IH-Sal can quantify Zn²⁺ in real water samples. More significantly, IH-Sal could determine and depict the presence of Zn²⁺ in zebrafish. The detecting mechanism of IH-Sal toward Zn²⁺ was illustrated by fluorescence and UV–visible spectroscopy, DFT calculations, ¹H NMR titration and ESI mass.

A naphthyl thiourea-based effective chemosensor for fluorescence detection of Ag+ and Zn2

A naphthyl thiourea-based effective chemosensor HNC, (E)-2-(2-hydroxy-3-methoxybenzylidene)-N-(naphthalen-1-yl)hydrazine-1-carbothioamide, was synthesized. HNC showed quick responses toward Ag⁺ and Zn²⁺ through marked fluorescence turn-on in different solvent conditions, respectively. Binding proportions of HNC to Ag⁺ and Zn²⁺ were found to be 2:1 and 1:1, respectively. Detection limits of HNC for Ag⁺ and Zn²⁺ were calculated as 3.82 and 0.21 μM. Binding processes of HNC for Ag⁺ and Zn²⁺ were represented using Job's plot, DFT, ¹ H NMR titration, and ESI-MS.

A Benzothiazole-Based Fluorescence Turn-on Sensor for Copper(II)

A new benzothiazole-based chemosensor BTN (1-((Z)-(((E)-3-methylbenzo[d]thiazol-2(3H)-ylidene)hydrazono)methyl)naphthalen-2-ol) was synthesized for the detection of Cu²⁺. BTN could detect Cu²⁺ with "off-on" fluorescent response from colorless to yellow irrespective of presence of other cations. Limit of detection for Cu²⁺ was determined to be 3.3 μM. Binding ratio of BTN and Cu²⁺ turned out to be a 1:1 with the analysis of Job plot and ESI-MS. Sensing feature of Cu²⁺ by BTN was explained with theoretical calculations, which might be owing to internal charge transfer and chelation-enhanced fluorescence processes.

Adduct under Field-A Qualitative Approach to Account for Solvent Effect on Hydrogen Bonding

The location of a mobile proton in acid-base complexes in aprotic solvents can be predicted using a simplified Adduct under Field (AuF) approach, where solute–solvent effects on the geometry of hydrogen bond are simulated using a fictitious external electric field. The parameters of the field have been estimated using experimental data on acid-base complexes in CDF₃/CDClF₂. With some limitations, they can be applied to the chemically similar CHCl₃ and CH₂Cl₂. The obtained data indicate that the solute–solvent effects are critically important regardless of the type of complexes. The temperature dependences of the strength and fluctuation rate of the field explain the behavior of experimentally measured parameters.

A hydrazono-quinoline-based chemosensor sensing In3+ and Zn2+via fluorescence turn-on and ClO−via color change in aqueous solution

A multi-target sensor, based on hydrazono-quinoline, was developed for fluorescence turn-on detection of In³⁺ and Zn²⁺ and colorimetric detection of ClO⁻.

Thermodynamic Complexing of Monocyclopentadienylferrum (II) Intercalates with Double-Walled Carbon Nanotubes

By employing the methods of molecular mechanics, semi-empirical quantum-chemical РМ3 and Monte-Carlo, the positioning of monocyclopentadienylferrum (II) molecules in double-walled (5,5)@(10,10) carbon nanotubes (CNT) depending on their concentration and temperature has been studied. The molecules have been found out to form stable bonds with CNT walls, with a tendency between intercalate stability and the CNT structure. The temperature growth (over ~500 K) causes gradual bond ruining followed by extrusion of interwall intercalate. Further temperature increase up to 600-700 K is characterised with intercalate external surface desorption, stabilising the whole system and keeping the interwall intercalate only. The CNT's UV-spectrum (5,5)@(10,10) depending on the intercalate concentration and association constant of the "double-walled CNT-intercalate" system have been calculated. A combination of unique optical, electrical and magnetic behaviour of cyclopentadienyl complexes with their ability to form high-stable intercalate with CNT opens a prospect of their applying in nanotechnology.

Hairpin oligonucleotides anchored terbium ion: a fluorescent probe to specifically detect lead(II) at sub-nM levels

A terbium based fluorescent probe was synthesized by coordinating terbium ions with a designed oligonucleotides (5'-ATATGGGGGATAT-3', termed GH5). GH5 improved the fluorescence of terbium ions by four orders of magnitude. The fluorescence enhancement of terbium ions by different oligonucleotides sequences indicated that the polyguanine loop of the hairpin GH5 is key to enhance terbium ion emission. The quantum yield of Tb-GH5 probe was 10.5% and the probe was photo-stable. The result of conductivity titration indicated that the stoichiometry of the probe is 3.5 Tb: 1 GH5, which is confirmed by fluorescence titration. This probe had high sensitivity and specificity for the detection of lead ions. The fluorescence intensity of this probe was linear with respect to lead concentration over a range 0.3-2.1 nM (R(2) = 0.99). The limit of detection for lead ions was 0.1 nM at a signal-to-noise ratio of 3.

Quinoline driven fluorescence turn on 1,3-bis-calix[4]arene conjugate-based receptor to discriminate Fe3+ from Fe2+

The synthesis and characterization of a triazole linked quinoline appended calix[4]arene conjugate, L, and its fluorescence turn on receptor property for Fe(3+) have been demonstrated. The selective and sensitive discrimination of Fe(3+) has been shown using fluorescence and absorption titration experiments. The Fe(3+) binding to L has been further shown by ITC and ESI MS. The mode of binding of Fe(3+) by calix[4]arene conjugate has been shown by absorption, (1)H NMR and visual color change and the species were modeled based on DFT computations. The {L + Fe(3+)} has been shown to label cells with fluorescence imaging. Moreover the utility of this conjugate has been demonstrated by the combination logic gate system.

Phenylene-ethynylene trication as an efficient fluorescent signal transducer in an aptasensor for potassium ion

A tricationic phenylene-ethynylene (N(3+)) fluorophore is investigated as a fluorescent transducer in homogeneous aptasensing system for potassium ion (K(+)) assay in aqueous media. The enhancement of the fluorescent signal of N(3+) by three K(+) aptamers consisting of 12, 15, and 21 nucleotides are observed and used for the determination of N(3+)-aptamer binding affinities. The binding affinities increase with the length of the aptameric oligonucleotides and are proven to be important to the sensitivity and selectivity of the aptasensors. The enhanced fluorescent signal of each N(3+)-aptamer solution is selectively quenched by K(+) due to the ability of K(+) in stabilizing the G-quadruplex structure of the aptamer. Among three aptamers, the 15-base aptamer provides optimal sensitivity and selectivity over other ions such as Li(+), Na(+), NH(4)(+), Mg(2+), Ca(2+) and Sr(2+). The sensing system shows the detection limit of 1 μM of K(+) in clean buffered solution and 30 μM of K(+) in the solution containing 4800-fold excess of Na(+), with wide linear dynamic ranges of micro- to millimolar concentration. This label-free fluorescence aptasensor is conveniently and effectively applicable for analysis of K(+) in urine samples.

Theoretical and experimental study of the inclusion complexes of the 3-carboxy-5,6-benzocoumarinic acid with cyclodextrins

The 3-carboxy-5,6-benzocoumarinic acid (BzCum) presents important emission properties that could make it suitable to be used as a fluorescence marker for proteins. Since the guest-cyclodextrin interaction can be used as a model for the ligand-protein interaction, the host-guest complexes of BzCum with alpha-, beta-, gamma-, 2-hydroxypropyl-beta-cyclodextrin (2-HP-beta-CD) and 2-hydroxypropyl-gamma-cyclodextrin (2-HP-gamma-CD) were investigated by means of steady-state fluorescence measurements. As the guest may exist either in anionic or in neutral form, the experiments were performed at two different pH values, 9.48 and 2.11. The association constants and the stoichiometry of the complexes were estimated by non-linear regression analysis. A theoretical study of the complexes was made using molecular mechanics (MM) calculation methods. The electrostatic and van der Waals contributions to the energy were estimated and a discussion was made on this basis.

Inclusion complex of 2-naphthylamine-6-sulfonate with beta-cyclodextrin: intramolecular charge transfer versus hydrogen bonding effects

The photophysical characteristics of the ground and excited states of 2-naphthylamine-6-sulfonate (2-NA-6-S) were investigated in different solvents and in beta-cyclodextrin (beta-CD). The spectral shifts are well correlated with Kamlet-Taft relationship. Multiple linear regression analysis indicated that both non-specific dipolar interaction and specific hydrogen bonding interactions play competitive roles in determining the position of the absorption maximum, while the dipolar interaction is the dominating parameter in determining the emission maximum. For the Stokes shift, both the nonspecific interaction and the hydrogen donation property of the solvent are participating equally. The molecular encapsulation of 2-NA-6-S by beta-CD in aqueous solution has been studied by different spectroscopic techniques. Fluorescence measurements show that the dielectric constant of beta-CD experienced by the included 2-NA-6-S is intermediate between water and methanol. The changes observed in the absorption and fluorescence spectra of 2-NA-6-S upon inclusion in beta-CD allowed the association constant to be calculated and found to be 465+/-100 and 495+/-100 M-1, respectively. The changes observed for the chemical shifts of 2-NA-6-S and beta-CD 1H NMR spectra and the corresponding 1H NMR spectra of their mixture confirmed the formation of the inclusion complex and showed that 2-NA-6-S is encapsulated in beta-CD cavity in a tilted equatorial approach.

Spectroscopic studies on the inclusion complex of 2-naphthol-6-sulfonate with beta-cyclodextrin

The photophysical properties of 2-naphthol-6-sulfonate (2-NOH-6-S) in various solvents and in aqueous beta-cyclodextrin solution have been investigated. The fluorescence quantum yields in non-aqueous solvents are approximately 0.20+/-0.02, while in water the yield is higher. The fluorescence quantum yield in water was found to depend on the pH value of the medium and increases as the pH increase up to a pH value of 4.0 where it comes to be constant. Absorption and fluorescence measurements show 1:1 inclusion of 2-NOH-6-S in the beta-cyclodextrin cavity. The association constant of 2-NOH-6-S-beta-cyclodextrin complex based on fluorescence measurements was calculated using Benesi-Hildbrand relationship and found to be 330+/-30M(-1). (1)H NMR studies are used to confirm the inclusion and to provide information on the geometry of 2-NOH-6-S inside the cavity of beta-cyclodextrin.

Proton-Transfer Reaction of 4-Methyl 2,6-Diformyl Phenol in Cyclodextrin Nanocage

We report here on the steady-state and time-resolved fluorescence studies on proton-transfer (PT) reaction of 4-methyl 2,6-diformyl phenol (MFOH) in confined nanocavities in three solvents, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), and water. Though DMSO and DMF individually interact with MFOH in a similar fashion, their modes of interaction get significantly modified in the presence of cyclodextrin (CD) nanocages. In DMSO, in the ground state, the solvated molecular anion of MFOH forms 1:1 inclusion complex with beta- or gamma-CD and attains greater stability compared to the normal form. In DMF, the solvated molecular anion gets converted to the H-bonded complex within the CD cavity resulting in a 50-nm blue shift in the absorption spectra. In the excited state, the anionic species gets more stabilized in DMSO while in DMF it is significantly destabilized in the presence of CDs. However, in case of water, MFOH gets trapped inside the water cages so that the CDs fail to complex with it effectively. There are also no changes in the excited-state lifetimes in water in the presence of CDs, but in case of DMSO and DMF, because of restricted rotation of the formyl group within the CD cavity, the contribution of the shorter lifetime components reduce significantly increasing the larger components. Some theoretical calculations at the AM1 level of approximation have also been carried out to demonstrate how the dipolar nature of the solvent influences excited-state PT in confined media.