Lower Rim 1,3-Diderivative of Calix[4]arene-Appended Salicylidene Imine (H2L): Experimental and Computational Studies of the Selective Recognition of H2L toward Zn2+ and Sensing Phosphate and Amino Acid by [ZnL]

Supramolecular Logic Gates Based on the Conjugates of Calixarenes and Carbohydrates

In the era of application-oriented research, laboratory to real life translation is highly regarded and in great demand. This could mean that molecular science developed for sensing and detecting a variety of chemical species awaits conversion to devices. In that, the molecular logic gates are the most promising ones where the information storage and/or data processing can be easily carried out in terms of molecular inputs and electrical response outputs. This would facilitate the simultaneous execution of a diverse array of molecular sensing functions. The recent progress in molecular logic gates based on supramolecular optical receptors, in particular, fluorescent ones, such as calixarene derivatives and carbohydrate conjugates will have a transformative impact on molecular devices and will encourage this science to yield technology. Therefore, this review provides a critical evaluation of recent publications on molecular logic gates based on the derivatives of calixarenes and glyco-conjugates, including several from our own research group, with the view that the corresponding applications are a beneficiary in laboratory-to-device translation. In addition, this review is also expected to assist young researchers in planning their research focus in the broad area of supramolecular-based logic gates targeting some specific applications.

A highly sensitive fluorescent probe with different responses to Cu2+ and Zn2

We report a highly sensitive fluorescent probe based on p-dimethylaminobenzoyl derivatives (probe L) for the detection of Cu²⁺ and Zn²⁺. In this work, the probe L exhibited a fluorescent turn-on sensing model to Cu²⁺ and Zn²⁺ with a distinct fluorescent color change from colorless to green and yellow respectively. Probe L exhibited high selectivity as a fluorescent Cu²⁺/Zn²⁺ probe with a limit of detection (LOD) of 45 nM/17 nM. The results of ¹H NMR titrations revealed that the response of L to Cu²⁺ and Zn²⁺ was triggered by the interaction of the thiophene unit and the metal ion. Furthermore, the fluorescence titrations and Job's plot curves displayed the binding ratio of 1:2 for Cu²⁺ and 1:1 for Zn²⁺ metal-L complex formation respectively. Density functional theory calculation also demonstrated the possibility of molecular luminescence and the process of metal-L complex formation. Additionally, fluorescent test strips have been prepared for convenient detection of Cu²⁺ and Zn²⁺, which means the convenient and rapid assay in real samples can be achieved.

Calix[4]arene Based Redox Sensitive Molecular Probe for SERS Guided Recognition of Labile Iron Pool in Tumor Cells

Targeting the intracellular "labile" iron pool is turned as a key modulator for cancer progression since the former is responsible for several pathological processes in tumor cells. Herein, we report a nonfluorescent calix[4]arene based triazole appended molecular probe (PTBC) for redox-specific detection of Fe³⁺ under physiological condition by UV-vis, FT-IR, ¹H NMR, HR-MS spectroscopies, ITC, and the binding strategy between Calix[4]arene and Fe³⁺ was modeled by DFT calculations. As a new insight PTBC probe showed significant Raman fingerprint through surface enhanced Raman scattering (SERS) modality revealing the ultrasensitive detection of Fe³⁺ with a LOD of 2 nM. Interestingly, intracellular "iron pool" has been recognized in human lung adenocarcinoma cells (A549) by the PTBC illustrating the distinct Raman mapping. Finally, PTBC imparted cytotoxicity via reactive oxygen species (ROS) generation in cellular milieu signifies its capability as a theranostic molecular probe.

Photoluminescence properties of tetrahedral zinc(ii) complexes supported by calix[4]arene-based salicylaldiminato ligands

The synthesis and photophysical properties of four new hybrid salicylaldiminato-calix[4]arene ligands and their corresponding zinc(ii) complexes are described. The Schiff bases were obtained from condensation reactions between cone-25,27-di(aminoethoxy)-26,28-dihydroxy-calix[4]arene and salicylaldehyde (H2L1) or o-vanillin (H2L2) and 1,3-alt-25,27-di(aminoethoxy)-26,28-di(n-propyloxy)-calix[4]arene and 3,5-di-tert-butyl-salicylaldehyde (H2L3) or o-vanillin (H2L4). Complexation reactions were investigated by ESI-MS, IR, NMR, UV-vis absorption and steady-state and time resolved fluorescence spectroscopy, and X-ray crystallography. All ligands support 1 : 1 complexes (ZnL1-ZnL4), with equilibrium constants derived from absorption spectrophotometry in the range log K11 = 5.5-8.2 (MeCN or MeOH/CH2Cl2, I = 0.01 M). The zinc complexes show blue fluorescence, both in solution as well as in the solid state, with λem, Φf, and τ ranging from 472-504 nm, 0.11-0.60, and 2-9 ns, respectively. The nature of the substituents on the salicylaldiminato fragments was found to be the main parameter that influences the photophysical properties of the zinc complexes. Insights into the electronic nature of the UV-vis transitions were obtained with time dependent density functional theory (TD-DFT) calculations.

A novel, highly sensitive, selective, reversible and turn-on chemi-sensor based on Schiff base for rapid detection of Cu(II)

In this work, a novel optical fluoro-chemisensor was designed and synthesized for copper (II) ions detection. The sensor film is created by embedded N,N-Bis(2-hydroxo-5-bromobenzyl)ethylenediamine in poly vinyl chloride (PVC) film in presence of dioctyl phthalate (DOP) as plasticizer. The receptor Schiff base reveals "off-on" mode with high selectivity, significant sensitivity to Cu(II) ions. The selectivity of optical sensor for Cu(II) ions is the result of chelation enhanced fluorescence (CHEF). The optimal conditions of pH and response time at which higher efficiency of sensor film is performed was found to be 6.8 and 2.48min. The possible interference of other metal ions in solution was examined in presence of different types of metal ions. This film shows high selectivity and ultra-sensitivity with low detection limit LOD (1.1×10^-8M). Thus, these considerable properties make it viable to monitor copper metal ions within very low concentration range (0-15×10^-6M Cu(II)) and highly selective even in the presence of different types of metal ions. The sensor reversibility was achieved by utilizing EDTA solution with concentration of 0.1M solution.

A quinoline-based Cu2+ ion complex fluorescence probe for selective detection of inorganic phosphate anion in aqueous solution and its application to living cells

A quinaldine functionalized probe QP has been designed and synthesized. It exhibited selective turn-off fluorescence response toward Cu²⁺ ion over most of the biologically important ions at physiological pH. The binding ratio of the probe QP and Cu²⁺ ion was determined to be 1:1 through fluorescence titration, Job's plot and ESI-MS. The binding constant (K) of Cu²⁺ to probe QP was found to be 2.12×10⁴M^-1. Further, the Cu²⁺ ensemble of probe QP was found to respond H₂PO₄^- and HPO₄^2- among other important biological anions via fluorescence turn-on response at physiological pH. Fluorescence microscopy imaging using living Hela cells showed that probe QP could be used as an effective fluorescent probe for detecting Cu²⁺ cation and H₂PO₄^- and HPO₄^2- anions in living cells.

Acridine-based complex as amino acid anion fluorescent sensor in aqueous solution

Novel acridine-based fluorescence sensors containing alaninol ligands, L1 and D1, were designed and synthesized. The structure of the compound was characterized by IR, (1)H NMR, (13)C NMR, MS spectra. L1 and D1 possess efficient Cu(2+) cation ON-OFF selective signaling behavior based on ligand-to-metal binding mechanism at physiological pH condition. Additionally, the L1-Cu(II) and D1-Cu(II) complexes could further serve as reversible OFF-ON signaling sensing ensemble to allow ratiometric response to amino acid anion in aqueous solution.

A "turn-on" fluorescent chemosensor for aluminum ion and cell imaging application

A simple and efficient fluorescent chemosensor for Al(3+) is reported in the paper. The chemosensor is obtained by dehydration reaction of 2-hydroxy-1-naphthaldehyde and 2-aminophenol. The chemosensor has high selectivity and sensitivity for Al(3+) and displays fluorescence "off-on" switch signal. The detection limit of the chemosensor for Al(3+) can reach 1.0×10(-7) M in DMSO/H2O (1:9, v/v) solution. The mass spectra and Job's plot analysis confirm the 1:1 stoichiometry between chemosensor and Al(3+). Potential utilization of the probe as an intracellular sensor of Al(3+) in human cancer (HiSa) cells is also examined by confocal fluorescence microscopy.

Sulphur-rich functionalized calix[4]arenes for selective complexation of Hg2+ over Cu2+, Zn2+ and Cd2+

ON–OFF fluorescent chemosensors based on sulphur-rich functionalized calix[4]arenes selectively bind Hg²⁺ over Cu²⁺, Zn²⁺ and Cd²⁺.

Fluorescence switching method for cascade detection of salicylaldehyde and zinc(II) ion using protein protected gold nanoclusters

A new fluorescence switching sensor for cascade detection of salicylaldehyde (SA) and Zinc(II) ion was developed based on bovine serum albumin protected gold nanoclusters (BSA-AuNCs). In the detection, SA interacted with amino groups of BSA-AuNCs, inducing simultaneous formation of fluorescent Schiff base and fluorescence quenching of AuNCs. Zn(II) could further strongly coordinate with the Schiff base ligands, leading to blue-shift and increase of the fluorescence from Schiff base-metal coordination complexes and simultaneous recovery of fluorescence from AuNCs. The new fluorescence switching sensor for Zn(2+) detection has advantages of simplicity, rapidity, naked-eye detection, high sensitivity and selectivity. The linear range of the method for Zn(2+) detection is from 0.1 μM to 100 μM with the limit of detection (LOD) of 29.28 nM. In practical samples, the recoveries of the samples ranged from 99.63% to 100.58%.

A fluorescent chemosensor based on naphthol for detection of Zn(2)

A simple naphthol-based fluorescent receptor 1 was prepared and evaluated for its fluorescence response to heavy metal ions. Receptor 1 exhibits an 'off-on-type' mode with high selectivity in the presence of Zn(2+) ion. The selectivity of 1 for Zn(2+) is the consequence of combined effects of chelation-enhanced fluorescence (CHEF), C = N isomerization and π-π stacking interaction between the two naphthalene rings.

Making pyrophosphate visible: the first precipitable and real-time fluorescent sensor for pyrophosphate in aqueous solution

An in situ generated Zn²⁺ complex of di-2-(picoly) amine BINOL–DPA was presented as a precipitable and real-time fluorescent sensor for PPi with a detection limit of 95 nm, and it could be successfully applied in imaging PPi in living cells.

Synthesis and characterization of a novel, ditopic, reversible and highly selective, “Turn-On” fluorescent chemosensor for Al3+ ion

A ditopic Schiff base fluorescent chemosensor, L (OFF state), selectively senses Al³⁺ by inhibition of ESIPT/GSIPT, CN isomerization and CHEF (ON state).

A cap-type Schiff base acting as a fluorescence sensor for zinc(II) and a colorimetric sensor for iron(II), copper(II), and zinc(II) in aqueous media

A simple and low cost chemosensor is described. This sensor could simultaneously detect three biologically important metal ions through fluorogenic (Zn(2+)) and chromogenic (Fe(2+), Cu(2+), and Zn(2+)) methods in aqueous solution. The sensor could function as a "turn-on" fluorescence receptor only to Zn(2+) ions. In addition, the sensor could be successfully applied to the detection of intracellular Zn(2+). Meanwhile, the sensor displayed an obvious red color upon selective binding with Fe(2+). Therefore, the sensor could serve as a useful tool for the discrimination of Fe(2+) from Fe(3+) in aqueous media. Moreover, the sensor also showed color changes from yellow to colorless upon selective binding with Zn(2+) and Cu(2+), respectively. The detection limit of the sensor for Cu(2+) (1.5 μM) is far below the guidelines of the World Health Organization (30 μM) as the maximum allowable copper concentration in drinking water, and therefore it is capable of being a practical system for the monitoring of Cu(2+) concentrations in aqueous samples. These results provide a new approach for selectively recognizing the most important three trace elements in the human body simultaneously, for Zn(2+) by emission spectra and Fe(2+), Cu(2+), and Zn(2+) by the naked eye.

Recognition of free tryptophan in water by synthetic pseudopeptides: fluorescence and thermodynamic studies

Pseudopeptidic receptors containing an acridine unit have been prepared and their fluorescence response to a series of amino acids was measured in water. Free amino acids, not protected either at the C or the N terminus, were used for this purpose. The prepared receptors display a selective response to tryptophan (Trp) versus the other assayed amino acids under acidic conditions. The macrocyclic nature of the receptor is important as the fluorescence quenching is higher for the macrocyclic compound than for the related open-chain receptor. Notably, under the experimental acidic conditions used, both the receptor and guest are fully protonated and positively charged; thus, the experimental results suggest the formation of supramolecular species that contain two positively charged organic molecular components in proximity stabilized through aromatic-aromatic interactions and a complex set of cation-anion-cation interactions. The selectivity towards Trp seems to be based on the existence of a strong association between the indole ring of the monocharged amino acid and the acridinium fragment of the triprotonated form of the receptor, which is established to be assisted by the interaction of the cationic moieties with hydrogen sulfate anions.

Zinc 2-((2-(benzoimidazol-2-yl)quinolin-8-ylimino)methyl)phenolates: synthesis, characterization and photoluminescence behavior

A series of 2-(2-(1H-benzoimidazol-2-yl)quinolin-8-yliminomethyl)phenol derivatives and their zinc complexes (C1-C5) were synthesized and fully characterized. The molecular structure of the representative complex C2 was determined by single crystal X-ray diffraction, which revealed that the zinc was five-coordinated with the tetra-dentate ligand and a methanol bound to the metal to afford a distorted square-pyramidal geometry. The UV-Vis absorption and fluorescence spectra of the organic compounds and their zinc complexes were measured and investigated in various solvents such as methanol, THF, dichloromethane, and toluene; significant influences by solvents were observed on their luminescent properties; red-shifts for the zinc complexes were clearly observed in comparison to the free organic compounds.

Fluorescent calix[4]arene chemosensor for acidic and basic amino acids in pure aqueous media

A fluorescent calix[4]arene chemosensor 2 displays high sensitivity toward acidic and basic amino acids in aqueous media through multiple H-bond interactions.

A Zn2+ specific triazole based calix[4]arene conjugate (L) as a fluorescence sensor for histidine and cysteine in HEPES buffer milieu

A highly fluorescent Zn(2+) complex of the triazole linked salicyl-imino-thiophenyl conjugate of calix[4]arene, [ZnL] has been demonstrated to be a chemo-sensing ensemble for the recognition of His and Cys among the naturally occurring amino acids in HEPES buffer milieu. The recognition behaviour of the [ZnL] towards these amino acids has been shown on the basis of fluorescence, absorption and visual fluorescent colour changes. The species of recognition were shown by ESI MS titrations, AFM & TEM microscopy and cell studies.

Cd2+ complex of a triazole-based calix[4]arene conjugate as a selective fluorescent chemosensor for Cys

An N,N-Dimethylamine ethylimino-appended triazole-linked calix[4]arene conjugate, L, has been synthesized and characterized, and its Cd(2+) complex has been isolated and characterized. The structure of [CdL] was established by computational calculation using B3LYP/LANL2DZ. Time-dependent density functional theory calculations were performed to demonstrate the electronic properties of [CdL]. This highly fluorescing [CdL] has been used to recognize Cys selectively among the 20 naturally occurring amino acids. [CdL] exhibits a minimum detection limit of 58 ppb for Cys, with reusability and reversibility being imparted to the system during sensing. Thus, the sensing of Cys was well demonstrated using various techniques, viz., fluorescence, absorption, visual color change, electrospray ionization MS, (1)H NMR, and live cell imaging experiments.

Imino-phenolic-pyridyl conjugates of calix[4]arene (L1 and L2) as primary fluorescence switch-on sensors for Zn2+ in solution and in HeLa cells and the recognition of pyrophosphate and ATP by [ZnL2]

Pyridyl-based triazole-linked calix[4]arene conjugates, viz. L(1) and L(2), were synthesized and characterized. These two conjugates were shown to be selective and sensitive for Zn(2+) among the 12 metal ions studied in HEPES buffer medium by fluorescence, absorption, and visual color change with the detection limit of ~31 and ~112 ppb, respectively, by L(1) and L(2). Moreover, the utility of the conjugates L(1) and L(2) in showing the zinc recognition in live cells has also been demonstrated using HeLa cells as monitored by fluorescence imaging. The zinc complexes of L(1) and L(2) were isolated, and the structure of [ZnL(1)] has been established by single-crystal XRD and that of [ZnL(2)] by DFT calculations. TDDFT calculations were performed in order to demonstrate the electronic properties of receptors and their zinc complexes. The isolated zinc complexes, viz. [ZnL(1)] and [ZnL(2)], have been used as molecular tools for the recognition of anions on the basis of their binding affinities toward Zn(2+). [ZnL(2)] was found to be sensitive and selective toward phosphate-bearing ions and molecules and in particular to pyrophosphate (PPi) and ATP among the other 18 anions studied; however, [ZnL(1)] was not sensitive toward any of the anions studied. The selectivity has been shown on the basis of the changes observed in the emission and absorption spectral studies through the removal of Zn(2+) from [ZnL(2)] by PPi. Thus, [ZnL(2)] has been shown to detect PPi up to 278 ± 10 ppb at pH 7.4 in aqueous methanolic (1/2 v/v) HEPES buffer.