Fluorogenic and chromogenic chemosensors and reagents for anions

ESIPT fluorescence turn-on sensors for detection of short chain inorganic polyphosphate in water

We introduce two water-soluble excited state intramolecular proton transfer (ESIPT) based fluorescent turn-on probes responding to inorganic polyphosphates. These ESIPT probes enable specific detection of short-chain inorganic polyphosphates over a range of different condensed phosphates. The probes are weakly emissive in their off-state due to the blocking of ESIPT by Cu2+ coordination. Removal of the copper ion through decomplexation by the analyte accesses the on-state. The probes detect polyphosphates over other biologically occurring phosphates, pyrophosphate, and nucleotides such as ATP, ADP, GTP. An optimal fluorescence response is observed with the short-chain polyphosphate polyP8. Furthermore, the probe shows selectivity towards linear polyphosphates over cyclic metaphosphates. The rapid 'turn-off-turn-on' fluorescence responses upon consecutive addition of Cu2+ and polyP8 are reversible, further highlighting sensor performance in an aqueous environment. One of the sensors is then used to monitor polyP digestion by an exopolyphosphatase (PPX).

A Potent Bis-Heteroleptic Ruthenium(II) Complex-Based Chalcogen Bonding Receptor for Selective Sensing of Phosphates

The incorporation of a selenoimidazolium-based chalcogen bond (ChB) donor into a bis-heteroleptic Ru(II) complex (Ru-Se) has been designed for the first time to explore its anion-sensing properties and understand its selectivity to specific classes of anions. Photophysical studies demonstrate the receptor's selectivity toward phosphates, while 1H NMR displays its ability to recognize both I- and H2PO4- among the different halides and oxoanions through ChB interaction in CH3CN and dimethyl sulfoxide-d6 solvents, respectively. Additionally, microscopic studies such as DLS and TEM reveal that the selective turn-on sensing of H2PO4- and HP2O73- compared to I- is driven by supramolecular aggregation behavior. Hence, the successful fabrication of a selenium ChB-based Ru(II) complex makes it a promising candidate for anion monitoring in supramolecular chemistry.

Submicromolar fluorescence 'turn-on' detection of fluoride anions using meso-(tetra-aryl) calix[4]pyrrole

Meso-(tetra-aryl) picket calix[4]pyrrole 1 featuring p-fluorophenyl groups at all four meso-positions in a cis-configuration has been synthesized and characterized unambiguously using single-crystal X-ray diffraction analysis. This pre-organized system possesses a deep binding pocket created by the four aryl groups so that anions can be accommodated through anion-π interactions and four-point N-H⋯anion hydrogen bonds. Single-crystal X-ray diffraction analysis of the CsF and TEAF (TEA = tetraethylammonium) complexes of receptor 1 unequivocally confirms the formation of 1 : 1 complexes, revealing the binding modes in the solid state. The anion binding studies conducted in acetonitrile confirmed that the receptor forms stable 1/1 complexes with fluoride anions and coumarin anion 2- in solution. The host-coumarin ensemble 3 was then applied for the detection of fluoride anions through a fluorescent dye displacement assay (FDDA). The highly fluorescent coumarin anion becomes completely non-fluorescent upon complexation with receptor 1, and its fluorescence is recovered in a concentration-dependent manner upon the addition of fluoride anions. This system proves to be an excellent 'turn-on' sensor for detecting fluoride anions with high selectivity and sensitivity, with a detection limit for fluoride anions estimated at 177 nM.

Selective binding and fluorescence sensing of Zn(II)/Cd(II) using macrocyclic tetra-amines with different fluorophores: insights into the design of selective chemosensors for transition metals

Selective binding and optical sensing of Zn(II) and Cd(II) by L1, HL2, L3, H2L4 and H2L5 receptors were analysed in aqueous solutions by coupling potentiometric, UV-vis absorption and fluorescence emission measurements, with the aim to determine the effect of complex stability on selective signalling of metals with similar electronic configurations. All receptors share the same cyclic tetra-amine binding unit attached to a single quinoline (Q) or 8-hydroxyquinoline (8-OHQ) unit (L1 and HL2, respectively), two Q or 8-OHQ moieties (L3 and H2L4, respectively), and, finally, two Q and two acetate groups (H2L5). The crystal structures of the Cd(II) and Zn(II) complexes show that L3 and H2L4 feature a cavity in which the larger Cd(II) complex is better fitted than the Zn(II) complex, leading to the formation of more stable Cd(II) complexes. In turn, Zn(II) forms more stable complexes with L1 and HL2, owing to its high tendency to give 5-coordinated complexes. Considering optical selectivity, Zn(II) gives the most emissive complex with L3, while the corresponding Cd(II) complex is basically quenched. The gathered structure of the Zn(II) complex, in which the two Q units are associated with one another-a structural motif not observed in the [CdL3]2+ complex-leads to poor solvation of the Q units, favouring complex emission. Among 8-OHQ-containing receptors, the most emissive complex is formed by Cd(II) with HL2, containing a single 8-OHQ moiety. H2L4 forms non-emissive complexes: the presence of two coordinating 8-OHQ moieties weakens metal interactions with the tetra-amine unit, favouring PET to the excited fluorophore that quench the emission.

A New Bis-Urea Based Cage Receptor for Anions: Synthesis, Solid State Structures and Binding Studies

The synthesis and characterization of a novel bis-urea-based cage receptor for anions (3S,15S)-3,15,20,25-tetramethyl-1,4,6,12,14,17,20,25-octaazatricyclo[15.5.5.17.11]octacosa-7(28),10-diene-2,5,13,16-tetraone (L) is reported. L is a macro-bicyclic ligand built on the 1,7-dimethyl-1,4,7,10-tetraazacyclododecane scaffold to obtain a cage topology in which two ureido moieties have been inserted as binding sites for anions. L can interact with anion guests (G) via H-bonding; in particular, it binds both spherical (Cl-) and V-shaped anions (AcO-) as well as more complex carboxylate anions, such as the norfloxacin (Nor-). NMR experiments highlight that the interaction between L and G mainly occurs at the ureido moieties. L forms L-G adducts of 1 : 1 ([LG]-) and 1 : 2 ([LG2]2-) stoichiometry with Cl- and AcO-. Otherwise, in the case of Nor- only the formation of the [LG]- complex is observed. L shows higher formation constants values for [LAcO]- (2.9) and [LNor]- (3.6) than [LCl]- suggesting a stronger interaction with the carboxylate anions. In the solid state, three crystal structures of the HL⋅G species were obtained (G=Cl-, AcO-, ClO4 -) highlighting the H-bonding interaction between the chloride, acetate or perchlorate anions and the -NH functions of the ureido fragment. The comparison between the two parent open chain receptors (Lb-c) and L has been reported and discussed.

Ion Sensing via Modulation of Charge Transfer in Donor-pi-Acceptor Molecules: Structure, Mechanism & Photophysical Aspects

This review article highlights the importance of novel charge transfer (CT) sensing approach for the detection of ions which are crucial from environmental and biological point of view. The importance, principles of charge transfer, ion sensing, its different types, and its basic process will all be covered here. The strategy has been reported with enormous sensitivity and fast signaling response owing to the fact that strong electronic connection communication exists between donor (D) and acceptor (A) part. Important discoveries made since 2010 will be examined. Herein, we will showcase the binding constants, conditions employed for sensing, and limit of detection of crucial ions via CT based sensors that researchers have bough forth for real-time applications. Additionally, the focus will be on the mechanistic aspects and signaling response as a result of the interaction between ion and sensor molecule.

Smartphone-based detection and discrimination of amine vapors by a single dye-adsorbed material

Smartphone-assisted analysis has become widely utilized for detecting various species in recent years. In such studies, multiple dyes should be employed to ensure selectivity and analyte discrimination. In our research, we have demonstrated the capability of a specially synthesized dye to selectively detect and discriminate liquid amine vapors. The developed material employs meso-toluene-α,β,α',β'-tetrabromoBODIPY immobilized on a thin-layer chromatography plate, exhibiting structure-specific color changes in response to amine vapors. The hue values of these colors, observed under both ambient and UV light, enable discrimination even among closely related amine structures. A mobile application has also been developed for the rapid interpretation of test results.

An Investigation into Anion Sensing of the Molecular Aggregate of 4-(Pyrrol-1-yl)pyridine and Its Derivatives

Recently, 4-(pyrrol-1-yl)pyridine has been found to act as a supramolecular chemodosimeter, sensing nitrite ions in an aqueous solution with naked eye detection and a low limit of detection of 0.330 ppm. This work explores the anion-sensing properties of related derivatives, 4-(2,5-dimethyl-pyrrol-1-yl)pyridine and 4-(2,4-dimethyl-pyrrol-1-yl)pyridine, and provides a comparison with the parent compound. These molecules are determined to be effective sensors for nitrite ions with limits of detection of 1.06 ppm and 1.05 ppm, respectively. The high sensitivity and selectivity to nitrite remain even in the presence of competing anions such as SO32-, NO32-, PO43-, SO42-, Cl-, F-, I-, Br-, AcO-, and CN-. Analogous to the 4-(pyrrol-1-yl)pyridine system, the sensing mechanism appears to be the result of changes in the supramolecular aggregate system upon the interaction of an anion; this is further explored through dynamic light scattering, the Tyndall effect, and NMR spectroscopy. The two methylated derivative systems reported herein, 4-(2,5-dimethyl-pyrrol-1-yl)pyridine and 4-(2,4-dimethyl-pyrrol-1-yl)pyridine, are shown to affect the size of the supramolecular system and provide further insight into the unique mechanism of action.

Electrofluorochromism based on the valence change of europium complexes in electrochemical devices with Prussian blue as the counter electrode

The electrofluorochromism of Eu complexes based on the valence change between Eu3+ and Eu2+ is demonstrated in a two-electrode electrochemical device consisting of Prussian blue (PB) as the counter electrode. This study aims to improve the electrofluorochromic (EFC) performance of luminescence switching between Eu3+ and Eu2+ by enhancing the electrochemical reactivity of the EFC device. By introducing a PB film as a counter electrode in a two-electrode device, the redox reaction of Eu3+/2+ is promoted because of charge compensation by the counter PB film. The increase in the reaction charge enables faster changes in the photoluminescence from Eu3+ to Eu2+ and an increase in the blue luminescence intensity from the Eu2+ state. This approach achieves a lowered driving voltage, accelerates the electrochemical redox reaction of the Eu complex, and enhances the reversibility of the valence change of the Eu ion.

Syntheses of Variants of π(σ) Aromatic Modified N-Methyl N-Confused Porphyrinoids with Adaptive Properties

Retrosynthetically designed and syntheses of three unprecedented core modified N-confused porphyrinoids exhibiting vis-NIR absorption with tunable aromaticity is reported. Controlled modification of types of oxidants (chloranil vs. DDQ) has led to the isolation of 18π-aromatic porphyrinoid 7 (upon chloranil oxidation) while DDQ oxidation has led to 18 e σ-aromatic porphyrinoids 8 and 9. All the three SN3 hybrid N-confused porphyrinoids 7-9 have been thoroughly characterized via solution state spectroscopic measurements and in-depth DFT studies for concluding π-aromaticity of 7 and σ-aromaticity of 8 and 9. While 7 could recognize the fluoride anion with high selectivity via deprotonation rather than an anion recognition based mechanism, acetate anion binding studies clearly revealed NH⋅⋅⋅AcO- H-bonding interaction in the host-guest complex [7-AcO-] supporting an anion recognition based mechanism. Porphyrinoids 8 and 9 remain unsusceptible to anion recognition. The conformational preorganization and anion induced deprotonation leading to conformational reorganization have been extensively studied by solution state spectroscopic techniques and in depth DFT level theoretical calculations.

Conjugated hypercrosslinked polymers for in situ imprinting, selective sorption, and fluorescent turn-on sensing of oxalic acid

Hypercrosslinked polymer (HCP) is a subclass of porous organic polymer possessing abundant microporosity, tailor-made functionality, and excellent stability. It features low-cost and easily direct knitting synthesis, facilitating the construction of π-conjugated frameworks with fluorescent properties by properly selecting building blocks (BBs) and linkers. Simultaneous imprinting of target molecules into the conjugated HCPs will create selective sorbents and sensors. We prepared several BBs to be polymerized with a terephthaloyl chloride (TCL) linker through Friedel-Crafts acylation in the presence of some imprinting molecules to clarify the best collocation for the advancement of imprinted polymer. With the highest increase in fluorescence intensity (F), the conjugated HCP comprised of dibenzofuran (DBF) and TCL was selected as contact with oxalic acid (OA). The OA-imprinted DBF-TCL (MICHP) was characterized by FTIR-approved structures, amorphous SEM images, TGA degradation at 390 °C, blue-shift emission, prolonged lifetime, and aggregation-caused quenching. The increase in F was proportional to OA concentration (0.17-20.0 μM, RSD = 1.6%, LOD = 0.03 μM) in THF/H2O (pH 7.0) containing MICHP (0.2 mg/3 mL) and 6.0 min equilibrium. The F increase arose from inhibiting the quenching of photo-induced electron transfer because of protonation and association of OA with imprinted cavities. Langmuir-Freundlich isotherm precisely modeled the imprinted cavity affinity for OA with binding sites of 114.5 μmol/g and heterogeneity of 0.939. The cavities distinctly recognized OA and malonic acid interferant, presenting imprinting factor (4.76 vs. 1.35), specific sorption ratio (79.0% vs. 25.7%), and relative selectivity coefficient (3.935 vs. 0.779), which sustained the precise measurements of OA in tomato, taro, and urine. This study approved a cheap and easy strategy to implant fluorescent and imprinting functions in HCPs using as sorbent and sensor through Friedel-Crafts acylation of electrophilic crosslinker and nucleophilic BB, especially those with heterocyclics.

Novel Discrete and Imprinted Fluoride-Selective Sensors: Bridging the Gap from DMSO to Aqueous Samples

Fluoride in drinking water has beneficial or harmful health effects depending on its concentration. This highlights the need for new low-cost and portable sensors capable of in situ monitoring of F- ions. Unfortunately, achieving high levels of water compatibility and fluoride specificity remains a challenge. Here, four new urea-based discrete sensors are prepared and characterized. The sensors containing anthracenyl- (5) and 9H-fluorenyl- (7) signaling units exhibit intense luminescent emissions in dimethyl sulfoxide, the former being particularly sensitive and selective to fluoride. In water, 5 displays a superior sensitivity (871 M-1) and a detection limit (8 µm) below international guidelines, albeit with cross-sensitivity to H2PO4‾. To enhance the performance, 5 and 7 are embedded into a fluoride-imprinted polymeric matrix to give solid-state sensors (5P and 7P, respectively). 5P shows good sensitivity (360 M-1) and specificity in water. Besides, it has a low detection limit (35 µm) and a response linear range (118-6300 µm) encompassing the limit established by the Environmental Protection Agency (211 µm). Furthermore, 5P also displays good reusability and adequate recovery values in real-sample testing (102 ± 2%), constituting the first example of a low-cost anion-imprinted polymeric probe tailored for the selective sensing of fluoride in aqueous samples.

A Bis(Acridino)-Crown Ether for Recognizing Oligoamines in Spermine Biosynthesis

Oligoamines in cellular metabolism carry extremely diverse biological functions (i.e., regulating Ca2+-influx, neuronal nitric oxide synthase, membrane potential, Na+, K+-ATPase activity in synaptosomes, etc.). Furthermore, they also act as longevity agents and have a determinative role in autophagy, cell growth, proliferation, and death, while oligoamines dysregulation is a key in a variety of cancers. However, many of their mechanisms of actions have just begun to be understood. In addition to the numerous biosensing methods, only a very few simple small molecule-based tests are available for their selective but reversible tracking or fluorescent labeling. Motivated by this, we present herein a new fluorescent bis(acridino)-crown ether as a sensor molecule for biogenic oligoamines. The sensor molecule can selectively distinguish oligoamines from aliphatic mono- and diamino-analogues, while showing a reversible 1:2 (host:guest) complexation with a stepwise binding process accompanied by a turn-on fluorescence response. Both computational simulations on molecular docking and regression methods on titration experiments were carried out to reveal the oligoamine-recognition properties of the sensor molecule. The new fluorescent chemosensor molecule has a high potential for molecular-level functional studies on the oligoamine systems in cell processes (cellular uptake, transport, progression in cancers, etc.).

Designing Stimuli-Responsive Supramolecular Gels by Tuning the Non-Covalent Interactions of the Functional Groups

The physical characteristics of a supramolecular gel are greatly influenced by the nature and arrangement of functional groups in the gelator. This work focuses on the impact of the functional groups, specifically the hydroxyl group, on the stimuli-responsive properties of a gel. We used a C3-symmetric benzene-1,3,5-tricarboxamide (BTA) platform, which was attached to the methyl ester of phenylalanine (MPBTA) and tyrosine (MTBTA). The gelation studies revealed that MPBTA gelled in alcohols, non-polar aromatic solvents, and aqueous mixtures (1:1, v/v) of high-polar solvents, whereas MTBTA gelled only in an aqueous mixture of DMF (1:1, v/v). The mechanical and thermal strength of the gels were evaluated by rheological and Tgel studies, and the results indicated that MPBTA gels were stronger than MTBTA gels. The gels were characterized by powder X-ray diffraction and scanning electron microscopy (SEM). The analysis of stimuli-responsive properties revealed that MPBTA gels were intact in the presence of sodium/potassium salts, but the MTBTA gel network was disrupted. These results indicate that the elegant choice of functional groups could be used to tune the constructive or destructive stimuli-responsive behavior of gels. This study highlights the significant role of functional groups in modulating the stimuli-responsive properties of supramolecular gels.

Rational design of peptide-based fluorescent probe for sequential recognitions of Cu(II) ions and glyphosate: Smartphone, test strip, real sample and living cells applications

A new peptide-based fluorescent probe named DMDH with easy-to-synthesize, excellent stability, good water solubility and large Stokes shift (225 nm) was synthesized for highly selective sequential detections of copper ions (Cu2+) and glyphosate (Glyp). DMDH demonstrated great detection performance towards Cu2+via strong fluorescence quenching, and forming non-fluorescence DMDH-Cu2+ ensemble. As a new promising cascade probe, the fluorescence of DMDH-Cu2+ ensemble was significantly recovered based on displacement approach after glyphosate was added. Interestingly, the limit of detections (LODs) for Cu2+ and glyphosate were 40.6 nM and 10.6 nM, respectively, which were far lower than those recommended by the WHO guidelines for drinking water. More importantly, DMDH was utilized to evaluate Cu2+ and glyphosate content in three real water samples, demonstrating that its effectiveness in water quality monitoring. Additionally, it is worth noting that DMDH was also applied to analyze Cu2+ and glyphosate in living cells in view of significant cells permeability and low cytotoxicity. Moreover, DMDH soaked in filter paper was used to create qualitative test strips and visually identify Cu2+ and glyphosate through significant color changes. Furthermore, smartphone RGB color recognition provided a new method for semi-quantitative testing of Cu2+ and glyphosate in the absence of expensive instruments.

A New, Rapid, Colorimetric Chemodosimeter, 4-(Pyrrol-1-yl)pyridine, for Nitrite Detection in Aqueous Solution

With increasing concerns over environmental impact and overall health of both the environment and its people, a need to quantify contaminants is of the utmost importance. Chemosensors with low detection limits and a relative ease of application can address this challenge. Nitrite ions are known to be detrimental to both the environment and human health. A new colorimetric chemodosimeter has been prepared from the homolytic photochemical cleavage of a reaction between pyrrole and pyridine. The product, 4-(pyrrol-1-yl)pyridine, yields a limit of detection of 0.330 (±0.09) ppm for the detection of nitrite in aqueous solution, employing a colorimetric change from yellow to pink. It is also highly selective for nitrite when various competitive anions such as SO3 2-, NO3 -, PO4 3-, SO4 -2, Cl-, F-, I-, Br-, AcO-, and CN- are present in great excess. The molecule's especially high sensitivity to nitrite is apparently the result of a complex supramolecular mechanism, characterized by both dynamic light scattering of the aggregate and the Tyndall effect. Consequently, this new sensor provides a simple, low-cost way to rapidly detect nitrite anions in aqueous solution.

Fluorometric and colorimetric sensor for selective detection of cyanide anion by dibenzosuberenone-based dihydropyridazine in aqueous solution

A new chemosensory based on deprotonation and intramolecular charge transfer (ICT) was developed to detect cyanide in food samples. Deprotonation was facilitated by increasing the acidity of the NH proton in the dibenzosuberenone-based dihydropyridazine chemosensor Pz3 with -CN substituents. Addition of cyanide to acetonitrile and aqueous acetonitrile solution (1/9) of Pz3 resulted in their significant color change from colorless to purple in visible light, accompanied by a strong red shift in the absorption spectrum. Meanwhile, the near-infrared (NIR) emission (ex. 525 nm, em. 670 nm) of Pz3- resulting from deprotonation showed fluorescence switching behavior to detect the cyanide anion. While the acidic NH protons interact with basic anions as F-, CN-, OAc- and H2PO4- in organic solution (MeCN), just CN ions interact with in aqueous organic solutions (H2O-MeCN 1/9 HEPES pH 7.4). The limit of detection of cyanide from the fluorescence spectrum is 80 nM, which is well below the value determined for drinking water by World Health Organization (WHO). The interference effect of cations and anions showed that Pz3 could play an important role in the determination of waste NaCN. In addition, Pz3 successfully carried out the selective detection of cyanide in food samples such as bitter almonds and sprouting potatoes.

Interplay between anion-receptor and anion-solvent interactions in halide receptor complexes characterized with ultrafast infrared spectroscopies

The competition between host-guest binding and solvent interactions is a crucial factor in determining the binding affinities and selectivity of molecular receptor species. The interplay between these competing interactions, however, have been difficult to disentangle. In particular, the development of molecular-level descriptions of solute-solvent interactions remains a grand experimental challenge. Herein, we investigate the prototypical halide receptor meso-octamethylcalix[4]pyrrole (OMCP) complexed with either chloride or bromide anions in both dichloromethane (DCM) and chloroform (trichloromethane, TCM) solvent using ultrafast infrared transient absorption and 2D IR spectroscopies. OMCP·Br- complexes in both solvents display slower vibrational relaxation dynamics of the OMCP pyrrole NH stretches, consistent with weaker H-bonding interactions with OMCP compared to chloride and less efficient intermolecular relaxation to the solvent. Further, OMCP·Br- complexes show nearly static spectral diffusion dynamics compared to OMCP·Cl-, indicating larger structural fluctuations occur within chloride complexes. Importantly, distinct differences in the vibrational spectra and dynamics are observed between DCM and TCM solutions. The data are consistent with stronger and more perturbative solvent effects in TCM compared to DCM, despite DCM's larger dielectric constant and smaller reported OMCP·X- binding affinities. These differences are attributed to the presence of weak H-bond interactions between halides and TCM, in addition to competing interactions from the bulky tetrabutylammonium countercation. The data provide important experimental benchmarks for quantifying the role of solvent and countercation interactions in anion host-guest complexes.

Boron-Salphen Conjugate based Molecular Probe Exhibiting Fluorescence On-Off-On Response in Detection of Cu2+ and ATP through Displacement Approach

Synthesis and photophysical properties of a fluorescent probe HBD is described. Probe upon interaction with metal ions, anions and nucleoside pyrophosphates (NPPs) showed fluorescence quenching with Cu2+ due to chelation enhanced quenching effect (CHEQ). Moreover, interaction of ensemble HBD.Cu2+ with anions and NPPs showed fluorescence "turn-On" response with ATP selectively. "On-Off-On" responses observed with Cu2+ and ATP is attributed to an interplay between ESIPT and TICT processes. Cyclic voltammogram of probe exhibited quasi-reversible redox behaviour with three oxidation and two reduction potentials and the change in band gaps of probe suggested the interaction with Cu2+ and ATP. The 2 : 1 and 1 : 1 binding stoichiometry for an interaction between probe and Cu2+ (LOD, 62 nM) and ensemble, HBD.Cu2+ with ATP (LOD, 0.4 μM) respectively are realised by Job's plot and HRMS data. Cell imaging studies carried out to detect Cu2+ and ATP in HeLa cells. Also, the output emission observed with Cu2+ and ATP is utilized to construct an implication (IMP) logic gate. Test paper strips showed naked-eye visible color responses to detect Cu2+ and ATP. In real water samples probe successfully detected copper (0.03 μM) between 5-6.5 ppb level (ICP-MS method).

Recent advances in AIEgen-based chemosensors for small molecule detection, with a focus on ion sensing

Since the aggregation-based emission (AIE) phenomenon emerged in 2001, numerous chemical designs have been built around the AIE concept, displaying its utility for diverse applications, including optics, electronics, energy, and biosciences. The present review critically evaluates the broad applicability of AIEgen-based chemical models towards sensing small analytes and the structural design strategies adjusting the mode of action reported since the last decade. Various AIEgen models have been discussed, providing qualitative and quantitative estimation of cationic metal ions and anionic species, as well as biomolecular, cellular, and organelle-specific probes. A systematic overview of the reported structural design and the underlying working mode will pave the way for designing and developing the next generation of AIEgens for specific applications.

Influence of halogen-halogen interactions in the self-assembly of pillar[5]arene-based supramolecular polymers

Halogen-halogen interactions play a pivotal role in the formation and stability of supramolecular assemblies. Herein, we investigate the assembly dynamics and dissociation pathways of linear supramolecular polymers based on pillar[5]arene-mediated by guest halogen-halogen interactions (C-X × X-C) in both the solution and solid states. The structure of the solid-state supramolecular assembly was determined by single-crystal X-ray diffraction analysis. The binding affinities of four different 1,4-dihalobutane guests with pillar[5]arene were investigated by 1H NMR spectroscopic titration and isothermal titration calorimetry (ITC). The formation of the halogen-bonded linear supramolecular polymer in solution was demonstrated using diffusion-ordered spectroscopy (DOSY) and ITC. Our findings highlight the dependence of the dissociation process on halogen nature within the encapsulated guest, revealing that the process is entropically driven (TΔS = 27.12 kJ mol-1) and enthalpically disfavored (ΔH = 9.99 kJ mol-1). Moreover, the disassembly of supramolecular polymers promoted by N-containing compounds was investigated using 1H NMR spectroscopy and ITC, revealing that the process is driven both enthalpically (ΔH = -2.64 kJ mol-1) and entropically (TΔS = 15.70 kJ mol-1). Notably, the data suggest the formation of N⋯I bonding interactions at both ends of the inclusion guest, elucidating the intricate interplay of halogen interactions and host-guest chemistry in supramolecular polymer systems.

Colorimetric and Fluorometric N-Acylhydrazone-based Chemosensors for Detection of Single to Multiple Metal Ions: Design Strategies and Analytical Applications

The development of optical sensors for metal ions has gained significant attention due to their broad applications in biology, the environment, and medicine. Colorimetric and fluorometric detection methods are particularly valued for their simplicity, cost-effectiveness, high detection limits, and analytical power. Among various chemical probes, the hydrazone functional group stands out for its extensive study and utility, owing to its ease of synthesis and adaptability. This review provides a comprehensive overview of N-acylhydrazone-based probes, serving as highly effective colorimetric and fluorometric chemosensors for a diverse range of metal ions. Probes are categorized into single-ion, dual-ion, and multi-ion chemosensors, each further classified based on the detected metal(s). Additionally, the review discusses detection modes, detection limits, association constants, and spectroscopic measurements.

A near-infrared (NIR) organic probe for rapid naked-eye detection of fluoride ions in aqueous medium

A dihydroxy-substituted tricyanopyrroline-based near-infrared (NIR) colorimetric probe (TCP-34Hy) is developed for the effective detection of fluoride ions in aqueous medium in the presence of Ca2+ ions. The naked-eye detection (drastic color change) selectively makes the NIR colorimetric sensor more efficient. The probe can detect fluoride ions from inorganic sources (NaF/KF) as well as organic sources (TBAF) in water. The water samples from different sources, such as tap, pond, river, and sewage, have been tested successfully for fluoride ion detection using the probe (TCP-34Hy). Investigations of different kinds of soil samples (clay soil, red soil, and river sand) after pre-treatment with NaF are also productive for detection of fluoride ions in water. The probe can also detect fluoride ions in toothpaste in aqueous medium. The role of Ca2+ is discussed and explained on the basis of experimental results. Additionally, the probe (TCP-34Hy) shows high selectivity towards fluoride ion detection with a LOD of 2.3 μM (0.052 ppm) in aqueous medium.

Anion Binding to Ammonium and Guanidinium Hosts: Implications for the Reverse Hofmeister Effects Induced by Lysine and Arginine Residues

Anions have a profound effect on the properties of soluble proteins. Such Hofmeister effects have implications in biologics stability, protein aggregation, amyloidogenesis, and crystallization. However, the interplay between the important noncovalent interactions (NCIs) responsible for Hofmeister effects is poorly understood. To contribute to improving this state of affairs, we report on the NCIs between anions and ammonium and guanidinium hosts 1 and 2, and the consequences of these. Specifically, we investigate the properties of cavitands designed to mimic two prime residues for anion-protein NCIs─lysines and arginines─and the solubility consequences of complex formation. Thus, we report NMR and ITC affinity studies, X-ray analysis, MD simulations, and anion-induced critical precipitation concentrations. Our findings emphasize the multitude of NCIs that guanidiniums can form and how this repertoire qualitatively surpasses that of ammoniums. Additionally, our studies demonstrate the ease by which anions can dispense with a fraction of their hydration-shell waters, rearrange those that remain, and form direct NCIs with the hosts. This raises many questions concerning how solvent shell plasticity varies as a function of anion, how the energetics of this impact the different NCIs between anions and ammoniums/guanidiniums, and how this affects the aggregation of solutes at high anion concentrations.

An Iridium Complex as Bidentate Halogen Bond-Based Anion Receptor Featuring an IncreasedOptical Response

We present a luminescent Ir(III) complex featuring a bidentate halogen bond donor site capable of strong anion binding. The tailor-made Ir(III)(L)2 moiety offers a significantly higher emission quantum yield (8.4 %) compared to previous Ir(III)-based chemo-sensors (2.5 %). The successful binding of chloride, bromide and acetate is demonstrated using emission titrations. These experiments reveal association constants of up to 1.6×105 M-1. Furthermore, a new approach to evaluate the association constant by utilizing the shift of the emission was used for the first time. The experimentally observed characteristics are supported by quantum chemical simulations.

Rational and controllable syntheses of variants of modified N-confused N-fused porphodimethenes and a porphotrimethene with adaptive properties

Retrosynthetic design and synthesis with structural isolation of two unprecedented core modified N-confused N-fused porphodimethene-like porphyrinoids possessing a [5.5.5.5] tetracyclic ring with tunable photophysical properties is reported. The solid-state X-ray crystal structure reveals the expected cis geometry for the meso-sp3 carbons. Controlled chemical oxidation of the porphodimethene analogue 11 bearing the N-confused pyrrole moiety to the corresponding porphotrimethene 12 has been achieved in quantitative yield, while 10 bearing the N-methyl N-confused pyrrole moiety remained unsusceptible to chemical oxidation. All three S2N3 hybrid N-confused N-fused porphodi(tri)methene-like porphyrinoids 10-12 could recognize the fluoride anion with high selectivity; however, porphodimethene 10 and porphotrimethene 12 do so via deprotonation rather than an anion recognition based mechanism as has been anticipated in the case of porphodimethene 11.

New type of tin(IV) complex based turn-on fluorescent chemosensor for fluoride ion recognition: elucidating the effect of molecular structure on sensing property

A novel type of chemosensor based on tin(IV) complexes incorporating hydroxyquinoline derivatives has been designed and investigated for selectively detecting fluoride ions. Sn(meq)2Cl2 (meq = 2-methyl-8-quinolinol) (complex 1) exhibits a significant enhancement in luminescence upon the introduction of fluoride ions. This enhancement greatly surpasses that observed with Snq2Cl2 and Sn(dmqo)2Cl2 (q = 8-hydroxyquinnoline; dmqo = 5,7-dimethyl-8-quinolinol). Furthermore, complex 1 displays excellent sensitivity and selectivity for fluoride detection in comparison to halides and other anions. As a result, complex 1 serves as an outstanding turn-on fluorescent chemosensor, effectively sensing fluoride ions. The Benesi-Hilderbrand method and Job's plot confirmed that complex 1 associates with F- in a 1 : 2 binding stoichiometry. Also, complex 1 exhibited a large binding constant (pKb = 10.4 M-2) and a low detection limit (100 nM). To gain a deeper insight into the photophysical properties and the underlying mechanism governing the formation of the tin(IV) fluoride complex via halide exchange, we successfully synthesized partially fluorinated Sn(meq)2F0.67Cl1.33 (2) and fully fluorinated Sn(meq)2F2 (3), all of which were characterized through computational studies, thereby elucidating their photophysical properties. DFT studies reveal that converting Sn(meq)2Cl2 to Sn(meq)2F2, an endergonic process, leads to greater stability due to reducing steric hindrance about the metal center. Furthermore, the fluorinated complex significantly increases dipole moment, resulting in high affinity toward the F- ion.

An Indole-based Chromofluorogenic Probe for Detection of Trivalent Al3+, Ga3+, In3+ and Fe3+ Ions

An easily synthesizable indole-derived chromofluorogenic probe InNS has been demonstrated for recognition of trivalent metal ions (i. e., Al3+, Ga3+, In3+ and Fe3+). Both UV-Vis and emission spectral studies have been employed to assess the cation sensing ability of InNS in semi-aqueous medium. This probe exhibited a chromogenic response for these metal ions, and the related change was accompanied with the appearance of a new absorption near 376 nm. An obvious color change from pale yellow to dark yellow could also be noticed upon addition of the aforementioned metal ions to the probe's solution. Distinctively from the UV-Vis analysis, the fluorescence behavior of InNS was completely different; it displayed a 'turn-on' fluorescence response for only Al3+ among all the studied cations. The detection limit and the association constant (Ka) for Al3+ were determined to be 12.5 nM and 6.85×106 M-1, respectively. A potential 1 : 1 binding mode of Al3+-InNS has been established based on Job's plot, 1H NMR and DFT analyses. The reversibility experiment was conducted using strongly chelating EDTA ion, and a corresponding logic gate has been devised. In terms of practical applications, the InNS has been utilized to detect Al3+ in human breast carcinoma (MCF-7) cell lines displaying promising 'turn-on' bioimaging experiments.

TDDFT Study on the ESIPT Properties of 2-(2'-Hydroxyphenyl)-Benzothiazole and Sensing Mechanism of a Derived Fluorescent Probe for Fluoride Ion

The level of fluoride ions (F-) in the human body is closely related to various pathological and physiological states, and the rapid detection of F- is important for studying physiological processes and the early diagnosis of diseases. In this study, the detailed sensing mechanism of a novel high-efficiency probe (PBT) based on 2-(2'-hydroxyphenyl)-benzothiazole derivatives towards F- has been fully investigated based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. F- attacks the O-P bond of PBT to cleavage the dimethylphosphinothionyl group, and the potential products were evaluated by Gibbs free energy and spectroscopic analyses, which ultimately identified the product as HBT-Enol1 with an intramolecular hydrogen bond. Bond parameters, infrared vibrational spectroscopy and charge analysis indicate that the hydrogen bond is enhanced at the excited state (S1), favoring excited state intramolecular proton transfer (ESIPT). The mild energy barrier further evidences the occurrence of ESIPT. Combined with frontier molecular orbital (FMO) analysis, the fluorescence quenching of PBT was attributed to the photoinduced electron transfer (PET) mechanism and the fluorescence turn-on mechanism of the product was attributed to the ESIPT process of HBT-Enol1.

Synthesis, characterization, optical properties, biological activity and theoretical studies of a 4 nitrobenzylidene) amino) phenyl)imino)methyl)naphthalen-2-ol -based fluorescent Schiff base

A new Schiff base, 1-(E)-(4-((E) 4nitrobenzylidene) amino) phenyl)imino) methyl)naphthalen-2-ol (4NMN), was prepared from the reaction of p-phenylenediamine with 2-hydroxy-1-naphthaldehyde and 4-nitrobenzaldehyde and characterized with spectroscopic analysis. UV-VIS and NMR. Frontier molecular orbitals, molecular electrostatic potential, and chemical reactivity descriptors of the synthesized compound were studied using molecular modeling methods. The antibacterial and antifungal activities of the Schiff base were studied for its minimum inhibitory concentration. The compound showed a higher effect on yeast than against bacteria. Density functional theory (DFT) calculations were performed to study the mechanism of reaction for the synthesis of 4NMN, and the results were consistent with the experimental findings. 4NMN exhibited moderate antibacterial and antifungal activities and demonstrated higher inhibition potential against different resistant strains compared to the reference drug gentamycin. The absorption and fluorescence spectra of 4NMN were measured in different solvents, and the effect of relative polarity and acidity on the medium was observed. An inner filter effect was observed at high concentrations, and the compound showed considerable fluorescence enhancement with increasing medium viscosity and fluorescence quenching by the addition of traces of Cr1+ and Cu2+. Additionally, molecular docking studies were conducted to investigate the efficiency of antibacterial and antifungal targets.

Advances in ionic liquids as fluorescent sensors

Ionic liquids (ILs) are a class of liquid salts with characteristics such as a low melting point, an ionic nature, non-volatility, and tunable properties. Because of their adaptability, they have a significant influence in the field of fluorescence. This paper reviews the primary literature on the use of ILs in fluorescence sensing technologies. The kind of target material is utilized to classify the fluorescence sensors made with the use of ILs. They include using ILs as probes for metals, nitro explosives, small organic compounds, anions, and gases. The efficacy of an IL-based fluorescence sensor depends on the precise design to guarantee specificity, sensitivity, and a consistent reaction to the desired analyte. The precise method can differ depending on the chemical properties of the IL, the choice of fluorophore, and the interactions with the analyte. Overall, the viability of the aforementioned materials for chemical analysis is evaluated, and prospective possibilities for further development are identified.

A reversible fluorescent sensor for continuous detection of fluoride ion and trace water in chemical reagents

Continuously monitorable fluorescence sensors can provide fast, immediate, in-field detection of analytes without tedious process. A simple fluorescent sensor (BN) constructed from naphthol Schiff base was developed for reversible monitoring of F- and trace water. Sensor BN showed specific selectivity toward F- over other anions giving rise to a fluorescence "turn-on" response. After added F-, the BN solution caused a dramatically observable color change from non-fluorescence to blue-green, and the limit of detection reached 78.5 nM. The Job's and 1H NMR analysis confirmed that the recognition mechanism could be concluded to F- caused deprotonation of sensor BN by hydrogen bonding interaction. Moreover, the deprotonated form BN∙F obtained by using F- was acted as excellent sensitivity sensor for trace water detection with instant response through reprotonation. After addition of trace water, the emission color and spectral signal of BN∙F reverted to the original BN sate with the limit of detection of 0.0011 %. The reversible detection characteristic was conducive to the development of an inkless writing and encryption device. And importantly, BN∙F was utilized as a promising fluorescent sensor in the quantitative determination of water content in routinely chemical reagents.

Fluorescent Probes Based on Charge and Proton Transfer for Probing Biomolecular Environment

Fluorescent probes for sensing fundamental properties of biomolecular environment, such as polarity and hydration, help to study assembly of lipids into biomembranes, sensing interactions of biomolecules and imaging physiological state of the cells. Here, we summarize major efforts in the development of probes based on two photophysical mechanisms: (i) an excited-state intramolecular charge transfer (ICT), which is represented by fluorescent solvatochromic dyes that shift their emission band maximum as a function of environment polarity and hydration; (ii) excited-state intramolecular proton transfer (ESIPT), with particular focus on 5-membered cyclic systems, represented by 3-hydroxyflavones, because they exhibit dual emission sensitive to the environment. For both ICT and ESIPT dyes, the design of the probes and their biological applications are summarized. Thus, dyes bearing amphiphilic anchors target lipid membranes and report their lipid organization, while targeting ligands direct them to specific organelles for sensing their local environment. The labels, amino acid and nucleic acid analogues inserted into biomolecules enable monitoring their interactions with membranes, proteins and nucleic acids. While ICT probes are relatively simple and robust environment-sensitive probes, ESIPT probes feature high information content due their dual emission. They constitute a powerful toolbox for addressing multitude of biological questions.

Design of molecular sensors and switches based on luminescent ruthenium-terpyridine complexes bearing active methylene and triphenylphosphonium motifs as anion recognition sites: experimental and DFT/TD-DFT investigation

Synthesis, characterization and thorough investigation of the photophysical and electrochemical properties of a new category of emissive homo- and heteroleptic Ru(II)-complexes derived from the [4'-(p-triphenylphosphonium methyl phenyl)-2,2':6',2''-terpyridine]bromide (tpy-PhCH2PPh3Br) ligand have been executed in this work. Incorporation of the PhCH2PPh3+Br- group at the terpyridine motif appropriately adjusts the triplet metal-to-ligand charge transfer (3MLCT) and metal-centered (3MC) excited states so that the complexes luminesce at room temperature (RT) having lifetimes within the range of 6.82-9.63 ns. The RT emission characteristics of the complexes get further enhanced via aggregation phenomena through the use of different solvent/non-solvent mixtures (DMSO/H2O and DMSO/PhCH3 mixtures). Temperature dependent emission spectral measurements indicate that the emission intensity, quantum yield and lifetime increase upon dropping down the temperature, thereby designated as the on-state, while the increase of temperature causes a reduction of the said properties, indicating the off-state and the process is fully reversible. Taking advantage of the active methylene group coupled with a phosphonium motif, anion sensing characteristics of the complexes are investigated systematically in DMSO through the use of various optical channels and spectroscopic tools. The complexes are very much sensitive to fluoride and to a lesser extent acetate and dihydrogen phosphate among the studied anions. In essence, the complexes function as sensors for temperature and fluoride ion. Computational investigations were also executed via density functional theory (DFT) and time-dependent (TD)-DFT to obtain a clear understanding of the electronic structures of the metalloreceptors, appropriate assignment of the spectral bands and their mode of interaction with selected anions.

Advances in three dimensional metal enhanced fluorescence based biosensors using metal nanomaterial and nano-patterned surfaces

Metal enhanced fluorescence (MEF) is a phenomenon that increases fluorescence signal through placement of metal near a fluorophore. For biosensing applications, MEF-based biosensors are becoming increasingly popular as it enables highly sensitive detection of molecules, important for early diagnosis. The structure and size of the metal influence the optical properties through enhancing the fluorophore photostability and light absorption and emission. In recent years, many metal nanostructures have been fabricated and examined for their effectiveness in developing MEF-based biosensors. This review focuses on the latest applications of three-dimensional nanostructures and nano-patterned surfaces used to develop and improve fluorescence sensing via MEF. Current reviews mostly discussed the applications of two dimensional MEF and metal-nanoparticles-based MEF with a focus on fabrication of nanoparticles and metal substrates. In this article, we focused more on the effect of the metal nanostructure and size on MEF and then provided an in-depth summary of the performance of the state-of-the-art three dimensional MEF-based biosensors. While more work is needed to demonstrate applicability for complex samples, it is evident that with the use of metal nanoparticles and three dimensional nano-patterns, the assay sensitivity of fluorescence-based detection can be greatly improved, making it suitable for use in early disease diagnostics.

Rational development of a peptide-based probe for fluorescence and colorimetric dual-mode detection of Cu2+ and S2- ions: Real application in cell imaging and test strips

A new dual-mode probe FAM-SSH with fluorescence and colorimetric properties was developed by solid-phase peptide synthesis, comprising 5-carboxy fluorescein (5-FAM) as a fluorophore, and tripeptide (Ser-Ser-His) as a recognition group. FAM-SSH not only displayed highly selective detection of Cu2+ based on fluorescence quenching mode, but also achieved colorimetric recognition of Cu2+ in solution, wherein a color change was observable to the naked eye. Additionally, the FAM-SSH-Cu2+ ensemble was highly selective for S2- over a wide pH range (7.0-12.0), characterized by a fluorescence enhanced response and colorimetric recognition, which was caused by the release of FAM-SSH and the precipitation of CuS. Moreover, the limit of detection (LOD) values for Cu2+ and S2- were 55.5 nM and 31.1 nM, respectively. Results of sample analyses and cell imaging experiments indicated that FAM-SSH has exciting field practicability and good cell permeability, and would be further useful for detection and imaging in environmental systems and living cells. Finally, test strips were produced by immersing them in FAM-SSH solution, thereby creating a method for portable visual detection. More importantly, a smartphone-assisted visual sensing platform was also developed for semi-quantitative Cu2+ and S2- detection with LOD values of 0.48 μM and 1.22 μM, respectively.

Diphenylpyrrole-Strapped Calix[4]pyrrole Extractant for the Fluoride and Chloride Anions

The anion binding features of diphenylpyrrole-strapped calix[4]pyrrole 1 have been investigated by means of 1 H NMR spectroscopy and ITC (isothermal titration calorimetry), as well as single crystal X-ray diffraction analyses. Receptor 1 bearing an auxiliary pyrrolic NH donor and solubilizing phenyl groups on the strap was found to bind F- , Cl- , and Br- as their tetrabutylammonium salts with high affinity in DMSO-d6 . In addition, receptor 1 was found to extract the fluoride anion (as both its tetraethylammonium (TEA+ ) and tetrabutylammonium (TBA+ ) salts), as well as the chloride anion into chloroform-d from an aqueous source phase. Cation metathesis using TBAI or the use of a dual host approach involving crown ethers enabled receptor 1 to extract simple alkali metal fluoride or chloride salts from water. Quantitative binding of NaF by receptor 1 was observed in 20 % D2 O-DMSO-d6 allowing for the direct determination of the NaF concentration in an unknown sample.

Enantioselective and Chemoselective Optical Detection of Chiral Organic Compounds without Resorting to Chromatography

Enantiorecognition and resolution are of essential importance in many diverse areas of science. Whenever there arises a need to analyze/investigate enantiomers in different situations chromatography stands up in our minds immediately. Nevertheless, chemoselective and enantioselective recognition/discrimination (without going for separation) constitutes a different perception and requirement. The techniques using chiroptical sensing cause detection based on molecular interactions induced in different manners. Enantioselective sensing of monosaccharides in γ-cyclodextrin assembly and by diboronic acid based fluorescent sensors, application of bi-naphthol and H8 BINOL based sensors and dendrimers, metal-to-ligand charge transfer transitions in CD, exciton-coupled circular dichroism, surface enhanced Raman spectroscopy, and enantioselective indicator displacement sensor arrays for enantioselective recognition/detection of chiral organic compounds, such as amines, amino acids/alcohols, and hydroxycarboxylic acids have been discussed in progressive manner with mechanistic explanations, wherever available. Besides, the chiroptical vs LC approach has been discussed. The present paper is focused on certain different non-chromatographic optical techniques and aims to extend an understanding and a view to consider such techniques which have been successful in selective detection, and determination of absolute configuration and enantiomeric excess, (without resorting to separation vis-à-vis LC) and that have potential use in high-throughput chiral assay and combinatorial search for asymmetric catalysts and reagents.

2,4,5,7-Tetranitrofluorenone Oximate for the Naked-Eye Detection of H-Bond Donors and the Chiroptical Sensing of Enantiopure Reagents

Hydrogen bonding greatly influences rates and equilibrium positions of chemical reactions, conformations, and sometimes even stereochemistry. This study reports on tetranitrofluorenone oximate, a novel dye capable of naked-eye detection of hydrogen-bond donating species (HBDs) and of rapid determination of H-bond donation strength by hypsochromic shift monitoring. In addition, the molecule possesses atropisomeric conformations, of M and P configuration, as evidenced in solid and solution state studies by X-ray diffraction and electronic circular dichroism (ECD), respectively. In the latter case, enantiopure bis-thioureas were the most effective HBDs to promote a chiral induction (diastereoselective recognition, Pfeiffer effect); the ECD results being rationalized by time-dependent density functional theory (TDDFT) calculations. Based on these experiments, bis-thioureas were used as chiral reagents in asymmetric 1,3-dipolar cycloadditions of structurally-related nitrones; the ECD sensing of the stereoinduction between bis-thioureas and the oximate serving as an indirect method of selection of the most effective HBD for asymmetric synthesis.

A single probe for solvent dependent optical recognition of iron(II/III) and arsenite: discrimination between iron redox states with single crystal X-ray structure evidence

The detection and discrimination of Fe2+ and Fe3+ ions have been investigated using a simple probe (L), produced by the condensation of ethylenediamine and 3-ethoxysalicyaldehyde. Single crystal X-ray structures demonstrate that L interacts with Fe2+ and Fe3+. In aqueous-DMSO media, the L recognises AsO2- by fluorescence and colorimetry techniques. The AsO2- aided PET inhibition and H-bond assisted chelation enhanced fluorescence (CHEF) boost fluorescence by 91-fold. The L can detect 0.354 ppb Fe2+, 0.22 ppb Fe3+ and 0.235 ppt AsO2-.

Electrochemically regulated luminescence of europium complexes with β-diketone in polyether matrices

This study investigates the electrochemical modulation of luminescence color, i.e., electrofluorochromism, of an Eu complex in a polyether solvent. The electrofluorochromic (EFC) reaction of the Eu complex occurred via a reversible redox reaction between Eu3+ and Eu2+. Initially, the intrinsically stable Eu3+ complex showed intense red photoluminescence (PL) induced by f-f transitions. After the electrochemical reduction of Eu3+ to Eu2+, broad blue PL was observed attributed to the d-f transitions in the Eu2+ complex. This distinct blue luminescence from the Eu2+ complex was attributed to the effective stabilization of the Eu2+ state by the polyether solvent. The dynamic EFC reaction that changes the valence state of the Eu ion can be potentially applied to novel chemical sensors, security devices, and display devices.

Biological and Sensing Applications of a Few 1,3,4-Oxadiazole Based Donor-Acceptor Systems

1,3,4-Oxadiazole pharmacophore is still considered a viable biologically active scaffold for the synthesis of more effectual and broad-spectrum antimicrobial agents. Therefore, the present study is based on five 1,3,4-oxadiazole target structures, viz., CAROT, CAROP, CARON (D-A-D-A systems) and NOPON and BOPOB (D-A-D-A-D systems) bearing various bioactive heterocyclic moieties relevant to potential biological activities. Three of the compounds, CARON, NOPON and BOPOB were assessed in-vitro for their efficacy as antimicrobial agents against gram positive (Staphylococcus aureus and Bacillus cereus) and gram negative (Escherichia coli and Klebsiella pneumonia) bacteria; and two fungi, Aspergillus niger and Candida albicans; also, as an anti-tuberculosis agent against Mycobacterium tuberculosis. Most of the tested compounds displayed promising antimicrobial activity, especially CARON which was then analyzed for the minimum inhibitory concentration (MIC) studies. Similarly, NOPON portrayed the highest anti-TB activity among the studied compounds. Consequently, to justify the detected anti-TB activity of these compounds and to recognize the binding mode and important interactions between the compounds and the ligand binding site of the potential target, these compounds were docked into the active binding site of cytochrome P450 CYP121 enzyme of Mycobacterium tuberculosis, 3G5H. The docking results were in good agreement with the result of in-vitro studies. In addition, all the five compounds were tested for their cell viability and have been investigated for cell labeling applications. To conclude, one of the target compounds, CAROT was used for the selective recognition of cyanide ion by 'turn-off' fluorescent sensing technique. The entire sensing activity was examined by spectrofluorometric method and MALDI spectral studies. The limit of detection obtained was 0.14 µM.

19 F-Labeled Probes for Recognition-Enabled Chromatographic 19 F NMR

The NMR technique is among the most powerful analytical methods for molecular structural elucidation, process monitoring, and mechanistic investigations; however, the direct analysis of complex real-world samples is often hampered by crowded NMR spectra that are difficult to interpret. The combination of fluorine chemistry and supramolecular interactions leads to a unique detection method named recognition-enabled chromatographic (REC) 19 F NMR, where interactions between analytes and 19 F-labeled probes are transduced into chromatogram-like 19 F NMR signals of discrete chemical shifts. In this account, we summarize our endeavor to develop novel 19 F-labeled probes tailored for separation-free multicomponent analysis. The strategies to achieve chiral discrimination, sensitivity enhancement, and automated analyte identification will be covered. The account will also provide a detailed discussion of the underlying principles for the design of molecular probes for REC 19 F NMR where appropriate.

Recent developments in the creation of a single molecular sensing tool for ternary iron (III), chromium (III), aluminium (III) ionic species: A review

Rational design of a molecular sensing tool is an important topic in molecular recognition, signalling, and optoelectronics that has piqued the interest of chemists, biologists, and environmental scientists. Approximately 150 years have passed since the beginning of the fluorescent chemosensor sector. Due to the paramagnetic properties of Cr3+ and Al3+ , it is tough to prepare a photoluminescence plug-in detector. Most dye-based Al3+ sensors must be utilized in organic or mixed solvents for robust hydration of Al3+ in water. The sophisticated molecular design of sensors, conversely, allows for the detection of these metal ions in aqueous medium. The design of chemosensors using various fluorophores and their mechanisms of action have been thoroughly discussed. A literature survey covering the design of chemosensors and their mechanisms of action have been thoroughly discussed covering the period 2010-2022 and that was carried out including innovative and exemplary activities from numerous groups throughout the world that have significantly contributed to this sector. The most important advantages of these probes are their aqueous solubility and quick response with outstanding selectivity and sensitivity for temporal distribution with high fidelity of metals in living cells.

Aza-phenol Based Macrocyclic Probes Design for "CHEF-on" Multi Analytes Sensor: Crystal Structure Elucidation and Application in Biological Cell Imaging

Metal bound macrocyclic compounds found in biological systems inspired us to design and synthesize two Robson-type macrocyclic Schiff-base chemosensors, H ₂ L1 (H ₂ L1=1,11-dimethyl-6,16-dithia-3,9,13,19-tetraaza-1,11(1,3)-dibenzenacycloicosaphane-2,9,12,19-tetraene-1,11-diol) and H ₂ L2 (H ₂ L2=1,11-dimethyl-6,16-dioxa-3,9,13,19-tetraaza-1,11(1,3)-dibenzenacycloicosaphane-2,9,12,19-tetraene-1,11-diol). Both the chemosensors have been characterized with different spectroscopic techniques. They act as multianalyte sensor and exhibit "turn-on" fluorescence toward different metal ions in 1X PBS (Phosphate Buffered Saline) solution. In presence of Zn²⁺, Al³⁺, Cr³⁺ and Fe³⁺ ions, H ₂ L1 exhibits ∼6-fold enhancement of emission intensity, while H ₂ L2 shows ∼6-fold enhancement of emission intensity in the presence of Zn²⁺, Al³⁺ and Cr³⁺ ions. The interaction between the different metal ion and chemosensor have been examined by absorption, emission, and ¹H NMR spectroscopy as well as by ESI-MS⁺ analysis. We have successfully isolated and solved the crystal structure of the complex [Zn(H ₂ L1)(NO₃)]NO₃ (1) by X-ray crystallography. The crystal structure of 1 shows 1:1 metal:ligand stoichiometry and helps to understand the observed PET-Off-CHEF-On sensing mechanism. LOD values of H ₂ L1 and H ₂ L2 toward metal ions are found to be ∼10^-8 and ∼10^-7 M, respectively. Large Stokes shifts of the probes against analytes (∼100 nm) make them a suitable candidate for biological cell imaging studies. Robson type phenol based macrocyclic fluorescence sensors are very scarce in the literature. Therefore, the tuning of structural parameters as the number and nature of donor atoms, their relative locations and presence of rigid aromatic groups can lead to the design of new chemosensors, which can accommodate different charged/neutral guest(s) inside its cavity. The study of the spectroscopic properties of this type of macrocyclic ligands and their complexes might open a new avenue of chemosensors.

Converting pH probes into "turn-on" fluorescent receptors for anions

Recognition of anions by synthetic receptors is an integral part of supramolecular chemistry continuing to expand and find new application areas in our daily life. Many applications require visualization of anion recognition events, and the generated analytical signal is used to quantify anions in solution. Transferring a binding event to a measured signal is a challenging task. The design of a synthetic receptor must involve not only the perfectly positioned binding sites with complementary noncovalent interactions for a guest but should also realize the sensing mechanism that generates a strong analytical response upon guest binding. This feature article outlines the design concept for the construction of "turn-on" fluorescent receptors for anions involving fluorescent pH probes. Applications of this concept for the construction of synthetic fluorescent receptors for inorganic anions and nucleotides are described. Features of the obtained receptors and possible competing binding and sensing processes in solution are analyzed to understand the scope and limitations of the approach.

Cyclo[2]carbazole[2]pyrrole: a preorganized calix[4]pyrrole analogue

A cyclo[2]carbazole[2]pyrrole (2) consisting of two carbazoles and two pyrroles has been synthesized by directly linking the carbazole 1- and 8-carbon atoms to the pyrrole α-carbon atoms. Macrocycle 2 is an extensively conjugated 16-membered macrocyclic ring that is fixed in a pseudo-1,3-alternate conformation. This provides a preorganized anion binding site consisting of two pyrrole subunits. ¹H NMR spectroscopic analysis revealed that only the two diagonally opposed pyrrole NH protons, as opposed to the carbazole protons, take part in anion binding. Nevertheless, cyclo[2]carbazole[2]pyrrole 2 binds representative anions with higher affinity in CD₂Cl₂ than calix[4]pyrrole (1), a well-studied non-conjugated tetrapyrrole macrocycle that binds anions via four pyrrolic NH hydrogen bond interactions. On the basis of computational studies, the higher chloride anion affinity of receptor 2 relative to 1 is rationalized in terms of a larger binding energy and a lower host strain energy associated with anion complexation. In the presence of excess fluoride or bicarbonate anions, compound 2 loses two pyrrolic NH protons to produce a stable dianionic macrocycle [2-2H]^2- displaying a quenched fluorescence.