1,3-Alternate Calix[4]arene as a Homobinuclear Ditopic Fluorescent Chemosensor for Ag+ Ions

Selective Chromogenic Chemosensors for Arsenite Anion: A Facile Approach to Analyzing Arsenite in Honey, Milk, and Water Samples

In this study, two chemosensors, N5R1 and N5R2, based on 5-(4-nitrophenyl)-2-furaldehyde, with varying electron-withdrawing groups, were synthesized and effectively employed for the colorimetric selective detection of arsenite anions in a DMSO/H2O solvent mixture (8 : 2, v/v). Chemosensors N5R1 and N5R2 exhibited a distinct color change upon binding with arsenite, accompanied by a spectral shift toward the near-infrared region (Δλmax exceeding 200 nm). These chemosensors established stability between a pH range 6-12. Among them, N5R2 displayed the lowest detection limit of 17.63 ppb with a high binding constant of 2.6163×105 M-1 for arsenite. The binding mechanism involved initial hydrogen bonding between the NH binding site and the arsenite anion, followed by deprotonation and an intramolecular charge transfer (ICT) mechanism. The mechanism was confirmed through UV and 1H NMR titrations, cyclic voltammetric studies, and theoretical calculations. The interactions between the sensor and arsenite anions were further analyzed using global reactivity parameters (GRPs). Practical applications were demonstrated through the utilization of test strips and molecular logic gates. Real water samples, honey, and milk samples were successfully analyzed by both chemosensors for the sensing of arsenite.

Highly selective Cu2+ detection with a naphthalimide-functionalised pillar[5]arene fluorescent chemosensor

Ligand 1, a rim-differentiated pillar[5]arene macrocycle modified with five naphthalimide groups through click chemistry, serves as an effective ratiometric fluorescent chemosensor for Cu2+. In contrast to the monomeric naphthalimide control compound 2, which shows only monomer emission, ligand 1 demonstrates dual emission characteristics encompassing both the monomer and excimer of the naphthalimide moieties. The binding properties of ligand 1 toward 15 different metal ions were systematically investigated in CH2Cl2/CH3CN (v/v, 1 : 1) by UV-vis and fluorescence spectroscopy. Remarkably, ligand 1 exhibits exceptional selectivity for Cu2+ ions. Upon complexation with Cu2+, the excimer emission of ligand 1 diminishes, concomitant with an enhancement of its monomer emission. The binding ratio for 1·Cu2+ was determined to be 1 : 1, with an association constant of (3.39 ± 0.40) × 105 M-1 calculated using a nonlinear least-squares curve-fitting method. Furthermore, the limit of detection (LOD) was found to be 185 ± 7 nM. Our results from 1H NMR titration, high-resolution mass spectrometry analysis and density functional theory calculations of 1·Cu2+ suggest synergistic coordination between Cu2+ and the triazole groups on ligand 1.

Synthesis of Upper-Rim Sulfanylpropyl- and p-Methoxyphenylazo-Substituted Calix[4]arenes as Chromogenic Sensors for Hg2+ and Ag+ Ions

A series of calix[4]arenes with upper-rim sulfanylpropyl and p-methoxyphenylazo groups (compounds 8-10) were synthesized and found to be effective chromogenic sensors for selectively detecting Hg2+, Hg+, and Ag+ ions among 18 screened metal perchlorates. In comparison to previously reported diallyl- and dithioacetoxypropyl-substituted calix[4]arenes (5, 6, 14, 15, and 16) and the newly synthesized compound 7, the distal (5,17)-disulfanylpropyl-substituted di-p-methoxyphenylazocalix[4]arene 9 demonstrated superior performance with a limit of detection of 0.028 μM for Hg2+ ions in a chloroform/methanol (v/v = 399/1) cosolvent. Job's plot revealed 1:1 binding stoichiometry for all these upper-rim sulfanylpropyl- and p-methoxyphenylazo-substituted calix[4]arenes 8-10 with Hg2+ ions, and Benesi-Hildebrand plots from ultraviolet/visible (UV-vis) titration spectra were used for the determination of their association constants. Our findings indicated that the distal orientation of two p-methoxyphenylazo and two sulfanylpropyl groups in calix[4]arenes 8-10 is more favorable for binding Hg2+ ions than the proximal (5,11-) orientation; moreover, the adjacent sulfanylpropyl groups exhibited superior coordination as ligands compared to the allyl and thioacetoxypropyl groups. Notably, compounds 8-10 displayed a comparable trend in their association with Ag+ ions, albeit with 1 order of magnitude lower binding constants and a distinct binding mode compared to Hg2+ ions. UV-vis spectroscopy, Job's plots, high-resolution mass spectrometry, and 1H nuclear magnetic resonance titration studies are presented and discussed.

Fluorescent cyclophanes and their applications

With the serendipitous discovery of crown ethers by Pedersen more than half a century ago and the subsequent introduction of host-guest chemistry and supramolecular chemistry by Cram and Lehn, respectively, followed by the design and synthesis of wholly synthetic cyclophanes-in particular, fluorescent cyclophanes, having rich structural characteristics and functions-have been the focus of considerable research activity during the past few decades. Cyclophanes with remarkable emissive properties have been investigated continuously over the years and employed in numerous applications across the field of science and technology. In this Review, we feature the recent developments in the chemistry of fluorescent cyclophanes, along with their design and synthesis. Their host-guest chemistry and applications related to their structure and properties are highlighted.

A highly selective chromogenic and fluorogenic chemodosimeter for dual detection of Cu2+ based on a redox-active calix[4]arene with isoxazolylchloroanthracene

Calix[4]arene 1 with 25,27-diisoxazolylchloroanthryl groups is a chromogenic and fluorogenic sensor for Cu2+ with a LOD of 1.67 μM by fluorescence. Calix[4]diquinone was obtained in high yield through the redox reaction of ligand 1 with Cu(ClO4)2.

Ratiometric fluorescent detection of silver nanoparticles in aqueous samples using peptide-based fluorogenic probes with aggregation-induced emission characteristics

The quantification of silver nanoparticles and Ag⁺ contamination in the aquatic ecosystem has attracted considerable interest. Benzoimidazolyl-cyanovinylene (1) was synthesized as an aggregation-induced emission fluorophore, and a fluorescent peptidyl probe (2 and 3) bearing this fluorophore was developed. The fluorescent peptidyl probes coordinated with Ag⁺ selectively among various metal ions, leading to a ratiometric response to Ag⁺ in pure aqueous solutions. Furthermore, an "in situ" protocol was developed to quantify AgNPs using 2 with H₂O₂ as an oxidizing reagent. The fluorescent detection method for Ag⁺ and AgNPs showed promising detection properties such as high selectivity, high sensitivity, fast response, visible light excitation, well-operations in pure aqueous solution, and large fluorescent signal change. The detection limits of Ag⁺ (0.64 ppb) and AgNPs (1.1 ppb) were significantly low. According to the binding mode study, Ag⁺ induced the formation of a 2:1 complex between 2 and Ag⁺ and the chirality of the peptide part of the probe was not critical for this process. The formation of aggregates of the probe triggered by Ag⁺ from AgNPs induced a significant change in fluorescence. Furthermore, the amounts of spiked AgNPs in groundwater and tap water were quantified using the fluorescent detection method with 2.

1,3-Alternate Calix[4]arene Functionalized With Pyrazole and Triazole Ligands as a Highly Selective Fluorescent Sensor for Hg2+ and Ag+ Ions

We report here the synthesis of a 1,3-alternate calix[4]arene 8, with bis-pyrazolylmethylpyrenes on the one end and bis-triazolylmethylphenyls on the other end, as a homoditropic fluorescent sensor for both Hg²⁺ and Ag⁺ ions. Calix[4]arene 3, with lower-rim bis-pyrazolylmethylpyrenes in cone conformation, was also synthesized as a control compound. UV-Vis and fluorescence spectra were used for metal ions screening, and we found that both ligands 8 and 3 showed strong excimer emission of pyrenes when they are as a free ligand in CHCl₃/MeOH (v/v, 3:1) solution; however, they both showed a high selectivity toward Hg²⁺ and Ag⁺ ions with strong fluorescence quenching and yet with different binding ratios. The fluorescence of ligand 8 was strongly quenched by Hg²⁺ but was only partially quenched by Ag⁺ ions; however, the fluorescence of ligand 3 was strongly quenched by Hg²⁺, Ag⁺, and Cu²⁺ ions. Job plot experiments showed that ligand 8 formed a 1:2 complex with both Hg²⁺ and Ag⁺ ions; ligand 3 formed a 1:1 complex with Hg²⁺, but it formed a 2:3 complex with Ag⁺. The binding constant of ligand 3 with Hg²⁺ and Ag⁺ ions was determined by the Benesi-Hildebrand plot of UV-vis titration experiments to be 2.99 × 10³ and 3.83 × 10³ M⁻¹, respectively, while the association constant of ligand 8 with Hg²⁺ and Ag⁺ was determined by Hill plot to be 1.46 × 10¹² and 9.24 × 10¹¹ M⁻², respectively. Ligand 8 forms a strong complex with either two Hg²⁺ or two Ag⁺ ions using both the upper and lower rims of the 1,3-alternate calix[4]arene as the binding pockets; hence, it represents one of the highly selective fluorescent sensors for the homoditropic sensing of Hg²⁺ and Ag⁺ ions.

Conformational Regulation of Multivalent Terpyridine Ligands for Self-Assembly of Heteroleptic Metallo-Supramolecules

A two-ligand system composed of the predesigned multivalent and complementary terpyridine-based ligands was exploited to construct heteroleptic metallo-supramolecules and to investigate the self-assembly mechanism. Molecular stellation of the trimeric hexagon [Cd₆L²₃] gave rise to the exclusive self-assembly of the star hexagon [Cd₁₈L¹₆L³₃] through complementary ligand pairing between the ditopic and octatopic tectons. To understand how the intermolecular heteroleptic complexation influenced the self-assembly pathway, the star hexagon was truncated into two triangular fragments: [Cd₁₂L¹₃L⁴₃] and [Cd₁₂L¹₃L⁵₃]. In the self-assembly of [Cd₁₂L¹₃L⁴₃], the conformational movements of hexatopic ligand L⁴ could be regulated by L¹ to promote the subsequent coordination event, which was the key step to the successful multicomponent self-assembly. In contrast, the formation of [Cd₁₂L¹₃L⁵₃] was hampered by the geometrically mismatched intermediates.

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.

Synthesis of the porphyrin-calix[4]arene conjugates via Pd-catalyzed amination and their evaluation as fluorescent chemosensors

The synthesis of the porphyrin-calix[4]arene conjugates was carried out using the Pd(0)-catalyzed amination of Zn(II) meso-(3-bromophenyl)porphyrinate with bis(3-aminopropoxy)substituted calix[4]arenes (in cone and 1,3-alternate conformations). One of the conjugates was demetalated to give free porphyrin base derivative. The investigation of the fluorescence of the conjugates was studied in the presence of 18 metal perchlorates. The zinc porphyrinate derivatives were found to quench fluorescence in the presence of Cu(II), Al(III) and Cr(III) cations as well as on protonation. Metal-free conjugate was shown to act as a molecular probe for Zn(II), Cu(II) and Cd(II) cations due to strong and different changes of the emission caused by these metals.

Anthryl-1,2,4-oxadiazole-substituted calix[4]arenes as highly selective fluorescent chemodosimeters for Fe(3+)

Fluorescent chemosensors 1 and 2, with 1,2,4-oxadiazoles as the binding ligands and anthracene as the fluorophore, were synthesized through sequential 1,3-dipolar cycloaddition reactions of 25,27-dioxyacetonitrilecalix[4]arenes 8 and 11. The fluorescence of 1 was severely quenched by both Fe(3+) and Cu(2+) , whereas that of 2 was selectively quenched only by Fe(3+) . Control compound 4 was also selectively quenched by Fe(3+) , which implied the importance of anthryl-1,2,4-oxadiazole core; furthermore, it was shown to give various oxidation products such as oxanthrone 13, anthraquinone 14, and imidazolyl oxanthrone 15. In addition to product separation and identification, the fluorescent quenching mechanism of these 9-anthryl-1,2,4-oxadiazolyl derivatives by Fe(3+) is also discussed. Furthermore, it should be noted that the oxadiazole-substituted anthracene 4 and calix[4]arene 2 are Fe(3+) -selective fluorescent chemodosimeters without the interference by Cu(2+) .

The leading role of cation–π interactions in polymer chemistry: the control of the helical sense in solution

Cation–π interactions determine the helical sense adopted by a polyphenylacetylene bearing (R)-α-methoxy-α-phenylacetamide as a pendant group (poly-1).

NHC macrometallocycles of mercury(ii) and silver(i): synthesis, structural studies and recognition of Hg(ii) complex 4 for silver ion

Seven NHC macrometallocycles (Hg(ii) and Ag(i)) were prepared, and the recognition of Ag⁺ using (NHC)₂–Hg–I₂ (4) as a host was studied.

A Highly Selective and Sensitive Turn-On Fluorescent Chemosensor Based on Rhodamine 6G for Iron(III)

Recently, more and more rhodamine derivatives have been used as fluorophores to construct sensors due to their excellent spectroscopic properties. A rhodamine-based fluorescent and colorimetric Fe(3+) chemosensor 3',6'-bis(ethylamino)-2-acetoxyl-2',7'-dimethyl-spiro[1H-isoindole-1,9'-[9H]xanthen]-3(2H)-one (RAE) was designed and synthesized. Upon the addition of Fe(3+), the dramatic enhancement of both fluorescence and absorbance intensity, as well as the color change of the solution, could be observed. The detection limit of RAE for Fe(3+) was around 7.98 ppb. Common coexistent metal ions showed little or no interference in the detection of Fe(3+). Moreover, the addition of CN(-) could quench the fluorescence of the acetonitrile solution of RAE and Fe(3+), indicating the regeneration of the chemosensor RAE. The robust nature of the sensor was shown by the detection of Fe(3+) even after repeated rounds of quenching. As iron is a ubiquitous metal in cells and plays vital roles in many biological processes, this chemosensor could be developed to have applications in biological studies.

A novel biosensor based on single-layer MoS2 nanosheets for detection of Ag(+)

In this work, we use for the first time single layer MoS2 as the fluorescence quencher to design a detection method for Ag(+) with excellent robustness, selectivity and sensitivity. To maintain the ultrathin MoS2, bulk MoS2 materials have been exfoliated by intercalation with lithium followed by reaction with water. As-prepared two-dimensional MoS2 not only has good water-solubility but also obtains high fluorescence quenching efficiency within 5 min. Importantly, the detection limit of this assay for Ag(+) (1 nM) was lower than the maximum limitation guided by the United States Environmental Protection Agency (EPA) and the World Health Organization (WHO). Further, this new Ag(+) probe was demonstrated in monitoring Ag(+) in lake water samples with satisfactory results.

Calixarene-based chemosensors by means of click chemistry

Click chemistry, a new strategy for organic chemistry, has been widely used in the chemical modification of calixarenes because of its reliability, specificity, biocompatibility, and efficiency. Click-derived triazoles also play a critical role in sensing ions and molecules. This in-depth review provides an overview of calixarene-based chemosensors that incorporate click-derived triazoles, and their three characteristics (chromogenic, fluorescence, and wettability) are reviewed.

Pyrene pyridine-conjugate as Ag selective fluorescent chemosensor

A new pyrene pyridine conjugate (PPC) has been developed as a selective fluorescent sensor for Ag⁺ion.

Functionalized gold nanoparticles as nanosensor for sensitive and selective detection of silver ions and silver nanoparticles by surface-enhanced Raman scattering

We have developed a surface-enhanced Raman scattering (SERS) nanosensor firstly for Ag ions and Ag nanoparticles detection based on 2-mercaptoisonicotinic acid (2MNA)-functionalized Au nanoparticles. Ag(+) can coordinate with 2MNA resutling in a variation of its SERS spectrum, which is used as a criterion to determine Ag(+) in a solution. This sensor exhibits a detection limit no more than 25 nM and has a high selectivity against other metal ions. More importantly, it can be directly applied in real sample detection.

A label-free oligonucleotide based thioflavin-T fluorescent switch for Ag+ detection with low background emission

A novel fluorescent Ag(+) sensor was developed based on the label-free silver (I) specific oligonucleotide (SSO) and Thioflavine T (ThT) monomer-excimer switch. C-rich SSO which contain C-C mismatched base pairs can selectively bind to Ag(+) ions and the formed duplexes which constructed by C-Ag(+)-C structure are thermally stabilized without largely altering the double helical structure. ThT give very weak fluorescent in bulk solution and/or in the presence of SSO. However ThT shows high fluorescence in the presence of SSO and Ag(+) at the same time mainly because ThT excimer, which has the high quantum yield, formed and stabilized in the minor or major groove. Based on the discovery, we developed the novel Ag(+) sensor. Under the optimum condition, the selectivity of this system for Ag(+) over other metal ions in aqueous solution is remarkably high, and Ag(+) can be quantified over the dynamic range of 30-450 nM, with a limit of detection of ~16 nM and a linear correlation coefficient of 0.995.