Calixarene-mediated host-guest interactions leading to supramolecular assemblies: visualization by microscopy

Host-guest chemistry, particularly of supramolecules, has been an intriguing topic for researchers for a long time due to its multiplicative applications ranging from chemical to biological to materials science. Supramolecules, such as calixarenes, are excellent host molecular systems due to their controllable cavity along with the ease of functionalization both at the lower and upper rims. The host-guest interactions involving calixarenes have been primarily studied using physical methods, such as absorption, fluorescence and nuclear magnetic resonance spectroscopy, isothermal titration calorimetry and mass spectrometry. The corresponding literature as disseminated through review articles triggered broadening of the spectrum of research. Depending upon the nature of the derivatization, the supramolecular conjugates of calixarenes have been shown to form different morphologies of micro and nanometric size as reported in the literature. Pertinent research performed in our own group was based on atomic force microscopy, transmission electron microscopy and scanning electron microscopy studies. The literature reveals that such morphologies are modified in the presence of guest species. Thus, the supramolecular host-guest complexation of calixarenes leading to the formation of various architectures has been studied using both spectroscopy and microscopy techniques to obtain complimentary data. However, there are no review articles that provide discussions on this exciting area of supramolecular science involving microscopy. Therefore, in the present article, for the first time, we have brought together the research reported in the literature during the past decade, including ours, in demonstrating the supramolecular architectures formed from the host-guest interactions extended by the conjugates of calixarenes, and their applications using microscopy. The scope of this article spans across various features of interaction in these systems: (i) in solution, (ii) at the air-water interface and (iii) on solid surfaces. The application domain includes sensing of organic explosives and drugs, exhibiting antibacterial activity, supramolecular self-assembly or co-assembly resulting in gels, micelles and vesicles, and the consequent aggregation-induced emission and a few others.

Unusual absence of FRET in triazole bridged coumarin-hydroxyquinoline, an active sensor for Hg2+ detection

A triazole-bridged coumarin conjugated quinoline sensor has been 'click'-synthesized by Cu(i) catalyzed Huisgen cycloaddition, and it exhibited high selectivity for toxic Hg2+. Surprisingly, no evidence of energy transfer from the quinoline moiety to coumarin has been found, substantiated by time-resolved fluorescence study. The possible binding mode of this sensor to Hg2+ has been established via NMR study, steady-state and time-resolved fluorescence spectroscopy, which is further supported by TDDFT calculations. The sensor has been found to be cell membrane permeable and non-toxic, and hence is suitable for intracellular Hg2+ detection.

Highly selective and sensitive turn-on fluorescent sensor for detection of Al3+ based on quinoline-base Schiff base

A new aluminum ion fluorescent probe (4-(diethylamino)-2-hydroxybenzylidene)isoquinoline-1-carbohydrazide (HL¹) has been conveniently synthesized and characterized. HL¹ exhibited a highly selective and pronounced enhancement for Al³⁺ in the fluorescence emission over other common cations by forming a 2:1 complex, with a recognition mechanism based on excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT). The strong fluorescent emission can be observed even at ppm level concentration of the probe in the presence of Al³⁺ with 41 fold intensity enhancement at 545 nm. HL¹ displays good linear relationship with Al³⁺ in the low concentration and the limit of detection is 8.08 × 10^-8 mol/L. Similar molecules with different substituents on salicylaldehyde phenyl ring were synthesized for studying the structure-activity relationship. Density-functional theory (DFT) calculations are in agreement with the proposed mechanism. It is confirmed that HL¹ could be used to detect Al³⁺ ions in real sample by fluorescence spectrometry and Al³⁺ ions in cells by bioimaging.

A Ratiomeric Fluorescent Sensor for Zn2+ Based on N,N'-Di(quinolin-8-yl)oxalamide

A new ratiometric fluorescent sensor (DQO) based on N,N'-Di(quinolin-8-yl) oxalamide has been designed and synthesized for selective detection of Zn²⁺. The fluorescence ratio (I _{536 nm}/I _{450 nm}) of DQO was enhanced 10-fold when Zn²⁺ was present in a buffer aqueous solution at pH 8.66. The sensor showed linear response toward Zn²⁺ in the concentration range 0-15 μM, and the detection limit was calculated to be 2.4 μM. A Job's plot implied the formation of a DQO/Zn²⁺ complex with 1:1 stoichiometry, and the apparent association constant of DQO/Zn²⁺ complex was computed to be 1.5 × 10⁴ M^-1.

A 1,3-Capped Calix[4] Conjugate Possessing an Amine Moiety as an Anion Receptor: Reversible Anion Sensing Detected by Spectroscopy and Characterization of the Supramolecular Features by Microscopy

A phenylenediamine-capped conjugate of calix[4]arene (Lamino ) was synthesized by reducing its precursor, Limino , with sodium borohydride in methanol. The Lamino sample binds to anions due to the more flexible and bent conformation of the capped aminophenolic binding core, compared to the precursor Limino . The Lamino sample showed selectivity towards H2 PO4 (-) by exhibiting a ratiometric increase in emission by about 11-fold with a detection limit of (1.2±0.2) μm ((116±20) ppb) over 15 anions studied, including other phosphates, such as P2 O7 (4-) , adenosine monophosphate (AMP(2-) ), adenosine diphosphate (ADP(2-) ), and adenosine triphosphate (ATP(2-) ). The Lamino sample shows an increase in the absorbance at λ=315 nm in the presence of H2 PO4 (-) , CO3 (2-) , HCO3 (-) , CH3 CO2 (-) , and F(-) . The (1) H NMR spectroscopic titration of Lamino with H2 PO4 (-) , F(-) , and CH3 CO2 (-) showed major changes in the phenylene-capped and salicyl moieties, and thereby, confirming the aminophenolic region as the binding core. However, the binding strength of these anions followed the trend H2 PO4 (-) >F(-) ≫CH3 CO2 (-) >HSO4 (-) . The heat changes observed by isothermal titration calorimetry support this trend. The Lamino sample showed reversible sensing towards H2 PO4 (-) and F(-) in the presence of Mg(2+) and Ca(2+) , respectively. NOESY studies of Lamino , in comparison with its anionic complexes, revealed that major conformational changes occurred in the capping region to facilitate the binding of anion. ESI-MS and the Job's method revealed 1:1 stoichiometry between Lamino and H2 PO4 (-) or F(-) . In the SEM micrographs of Lamino , the spherical particles are converted into spherical aggregates and further form large agglomerates and even branched sheets in the presence of anions, depending upon their binding strength.

Substituent-dependent fluorescent sensors for zinc ions based on carboxamidoquinoline

A series of carboxamidoquinoline-based fluorescent sensors (the AQZ family) were synthesized and characterized. The AQZ family members were highly soluble in water and showed good selectivity for Zn(2+)via enhanced fluorescence in aqueous buffer solution. Fluorescence signals could be tuned from dual-wavelength ratiometric changes to changes in the intensity of a single wavelength upon binding Zn(2+) through the introduction of different substituents onto the quinoline ring. Concentrations of free Zn(2+) of 10(-5)-10(-6) M could be detected using the sensors. Changes of substituents and their positions on the quinoline ring influenced the sensitivity for Zn(2+), but had little effect on Zn(2+) affinities.