1,3-Di-naphthalimide Conjugate of Calix[4]arene as a Sensitive and Selective Sensor for Trinitrophenol and This Turns Reversible when Hybridized with Carrageenan as Beads

A fluorescent naphthalimide conjugate of calix[4]arene (L₁ ) has been synthesized and characterized. The selective and efficient detection of trinitrophenol (TNP) by L₁ among nine other different nitroaromatic compounds was demonstrated using absorption and fluorescence spectroscopy. The minimum detection limit is 29 nM, which is the lowest reported so far by any conjugate of calixarene toward TNP. The fluorescence quenching is associated with a high Stern-Volmer constant of 3.3 ± 0.4 × 10⁵ M^-1. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) data revealed a network structure with pores having a weighted average size of 0.66 ± 0.08 μm for L₁ . When incubated with TNP, the pores were filled with fibril structures, as supported by both SEM and TEM data. In order to demonstrate the real time applications, the L₁ has been coated onto a Whatman filter paper and the imprint of TNP contaminated thumb has been detected upon physical contact. The ¹HNMR titration and the studies carried out using the control molecule support the necessity of both the naphthalimide moiety and the calixarene platform for sensing. In order to mend L₁ as a reversible sensor for TNP, the same is incorporated into carrageenan beads (L₁ @C_b ) and the reversible sensing has been shown for three cycles by reusing the same material upon recovery followed by washing it. The solid-state detection of TNP has also been demonstrated using the lyophilized L₁ @C_b bead powder. The fluorescence intensity of L₁ was quenched upon addition of solid TNP to the lyophilized bead powder of L₁ @C_b as studied by fluorescence microscopy. The computational studies show that one of the arms of the calixarene takes a bent conformation, and the 1:1 TNP complex of L₁ is stabilized by exhibiting differential extents of hydrogen bonding interactions with the two arms owing to their conformational difference. The result of such complexation was already felt through the shifts observed in the experimentally measured ¹HNMR spectra.

Elucidating sensing mechanisms of a pyrene excimer-based calix[4]arene for ratiometric detection of Hg(ii) and Ag(i) and chemosensor behaviour as INHIBITION or IMPLICATION logic gates

This article reports the synthesis and characterisation of two lower rim calix[4]arene derivatives with thiourea as spacer and pyrene or methylene-pyrene as fluorophore. Both derivatives exhibit a fluorimetric response towards Hg²⁺, Ag⁺ and Cu²⁺. Only methylene-pyrenyl derivative 2 allows for selective detection of Hg²⁺ and Ag⁺ by enhancement or decrease of excimer emission, respectively. The limits of detection of 2 are 8.11 nM (Hg²⁺) and 2.09 nM (Ag⁺). DFT and TD-DFT computational studies were carried out and used to identify possible binding modes that explain the observed response during fluorescence titrations. Calculations revealed the presence of different binding sites depending on the conformation of 2, which suggest a reasonable explanation for non-linear changes in fluorescence depending on the physical nature of the interaction between metal centre and conformer. INHIBITION and IMPLICATION logic gates have also been generated monitoring signal outputs at pyrene monomer (395 nm) and excimer (472 nm) emission, respectively. Thus 2 is a potential primary sensor towards Ag⁺ and Hg²⁺ able to configure two different logic gate operations.

Ratiometric Cu2+ Binding, Cell Imaging, Mitochondrial Targeting, and Anticancer Activity with Nanomolar IC50 by Spiro-Indoline-Conjugated Calix[4]arene

A triazole-derivatized, spiro-indoline-linked, 1,3-di-derivative of calix[4]arene (L) has been synthesized to take advantage of its ion-binding capability in the ring-open form. Indeed, the spiro-indoline moiety is well known for its photochromic, acidochromic, and metallochromic properties. Therefore, the L has been explored for Cu2+ binding, cell imaging, and anticancer activity of the corresponding complex since Cu2+ complexes are known for such activity. The conversion from the closed to open form of L is expedited by light or proton, while the metal ion can open as well as stabilize it. The open form of L showed binding of Cu2+ ratiometrically as demonstrated by absorption and fluorescence spectroscopy. This leads to the formation of 1:1 complex with a binding constant of (6.9 ± 2.3) × 105 M-1, with the lowest detection limit being 1.9 nM. In the complex, the Cu2+ is bound by two triazole-N and two phenolic-O groups resulting in a distorted tetrahedral coordination core of CuN2O2 as demonstrated based on density functional theory studies. To form such coordination core, the arms underwent considerable changes in some of the dihedral angles. The binding of Cu2+ to L induces self-assembly of L by varying from simple particles to rodlike structures when bound to Cu2+. The on-off fluorescence intensity of L and its Cu2+-bound species are responsible for imaging cancer cells. The L shows red fluorescence in MDA-MB-231 cancer cells by targeting mitochondria as proved based on the colocalization study carried out using MitoTracker Green. While the L alone is nontoxic to cancer cells, the presence of Cu2+ brings cell death to an extent of 90% with an IC50 value of 165 nM by bringing a substantial quench in the fluorescence of L. A shift of population from G0/G1 and G2M phases to the Sub-G1 phase was observed as the concentration of the complex was increased, indicating cell death as studied by fluorescence-activated cell sorting. Thus, the present work clearly proved that a calix[4]arene functionalized at the lower rim with spiro-indoline moieities when complexed with Cu2+ acts as an efficient anticancer agent and is capable of imaging cancer cells.

Constructing bridged multifunctional calixarenes by intramolecular indole coupling

2,2′-Bisindole bridges can be easily created at (thia)calixarene cores providing the molecules with multiple functionalities for application in supramolecular chemistry.

Physicochemical and Ion-Sensing Properties of Benzofurazan-Appended Calix[4]arene in Solution and on Gold Nanoparticles: Spectroscopy, Microscopy, and DFT Computations in Support of the Species of Recognition

A calix[4]arene conjugate (L) functionalized at the lower rim with a benzofurazan fluorophore (NBD) and at the upper rim with a thioether moiety has been synthesized and characterized by ¹H NMR, ¹³C NMR, and mass spectrometry techniques. Both the absorption and emission spectral data for L in different solvents exhibited progressive changes with an increase in polarity. Ion recognition studies were performed by absorption and fluorescence spectroscopy using 10 different metal ions. Among these, Hg²⁺ exhibited greater changes in these spectra, whereas Cu²⁺ showed only significant changes and all other ions showed no change in the spectral features. Although the Hg²⁺ has dominant influence on the spectral features and provides a detection limit of 56.0 ± 0.6 ppb, the selectivity was hampered because of the presence of the derivatizations present on both the rims of L for ion interaction in solution. Therefore, L was immobilized onto gold nanoparticles (AuNP_L's) so that the upper rim derivatizations anchor onto the gold surface through Au-S interactions, and this leaves out only the lower rim NBD derivatization for interaction with ions selectively. The AuNP_L's were characterized by transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS) analyses. The surface characteristics were analyzed by contact angle measurements. The AuNP_L's exhibit greater selectivity and enhanced sensitivity for Hg²⁺ ions with a lowest detection limit of 48.0 ± 0.8 ppb. The immobilization of L onto AuNPs was reflected in the corresponding fluorescence lifetime values, and the addition of Hg²⁺ to either L or AuNP_L showed fluorescence quenching. The reversible recognition of Hg²⁺ by L was demonstrated by titrating L or AuNP_L with Hg²⁺ followed by tetra-butyl ammonium iodide for several cycles. The structural features of Hg²⁺-bound species were demonstrated by density functional theory computations and were supported by the XPS data. The Hg²⁺ induces aggregated fibrillar morphology into supramolecular L, as demonstrated by microscopy when Hg²⁺ was added either to L or to AuNP_L, supporting aggregation-caused quenching.

Nanoarchitectonics of Small Molecule and DNA for Ultrasensitive Detection of Mercury

Reliable and ultrasensitive detection of mercury ions is of paramount importance for toxicology assessment, environmental protection, and human health. Herein, we present a novel optoelectronic approach based on nanoarchitectonics of small-molecule templated DNA system that consists of an adenine (A)-conjugated small organic semiconductor (BNA) and deoxyribo-oligothymidine (dT_n). This mutually templated dynamic chiral coassembly system (BNAn-dT_n) with tunable chiroptical, morphological, and electrical properties is tapped in to enable ultrasensitive and selective detection of inorganic and organometallic mercury in water. We observe a rapid transformation of the BNA_n-dT_n coassembly into a metallo-DNA duplex [dT-Hg-dT]_n in the presence of mercury, which is utilized for a chiro-optical and conductivity-based rapid and subnanomolar sensitivity (≥0.1 nM, 0.02 ppb) to mercury ions in water (∼100 times lower than United States Environmental Protection Agency tolerance limit). This ultrasensitive detection of inorganic and organometallic mercury is driven by a novel chemical design principle that allows strong mercury thymine interaction. This study is anticipated to inspire the development of future templated DNA nanotechnology-based optoelectronic devices for the rapid and ultrasensitive detection of numerous other toxic analytes.

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⁺.

Thymine Functionalized Graphene Oxide for Fluorescence "Turn-off-on" Sensing of Hg2+ and I- in Aqueous Medium

Selective detection of either mercury (Hg2+) or iodide (I-) ion using fluorescence turn-on or turn-off processes is an important area of research. In spite of intensive research, simultaneous detection of both mercury and iodide using fluorescence turn-off-on processes, high sensitivity and theoretical support concerning the mechanisms are still lacking. In the present work, graphene oxide is functionalized by thymine to realize simultaneous detection of both Hg2+ and I- selectively using fluorescence turn-off-on mechanism. Ultra high sensitivity to the extent of ppb level exploiting large surface area of graphene is achieved. DFT calculations also assist to realize the detailed mechanisms involving this PL quenching and also its regain during sensing of these ions in aqueous solution.