1,8-Naphthyridine-Derived Ni2+/Cu2+-Selective Fluorescent Chemosensor with Different Charge Transfer Processses

Synthesis and optimization of chitosan-incorporated semisynthetic polymer/α-Fe2O3 nanoparticle hybrid polymer to explore optimal efficacy of fluorescence resonance energy transfer/charge transfer for Co(II) and Ni(II) sensing

Initially, four synthetic fluorescent polymers (SFPs) are synthesized from α-methacrylic acid and methanolacrylamide monomers carrying -C(=O)OH and -C(=O)NH subfluorophores, respectively. Among SFPs, ∼1:1 incorporation of subfluorophores in the optimum SFP3 is explored by spectroscopic analyses. Subsequently, chitosan is incorporated in SFP3 to produce five semi-synthetic fluorescent polymers (SSFPs). The maximum incorporation of chitosan in SSFP4 is supported by different spectroscopies. In SSFP4, strong electrostatic interactions among polar functionalities of chitosan and synthetic polymer favor resonance-associated charge transfer (RCT) from SSFP4-(amide) to SSFP4-(canonical). Finally, three hybrid fluorescent polymers (HFPs) are fabricated encapsulating iron-oxide nanoparticle within SSFP4. The maximum proportion of hematite (α-Fe2O3) phase in HFPs is explored by spectroscopic, magnetometric, microscopic, and light scattering studies. HFP2 shows local/RCT/fluorescence resonance energy transfer (FRET) emission at 393/460/570 nm. In HFP2, FRET, RCT, and ratiometric pH-sensing within 3.0-6.5 phenomena are explored by solvent polarity effects, time-correlated single photon counting, quantum yield measurements, alongside I431/I460 vs pH plots. RCT and FRET emissions of HFP2 are utilized for selective sensing of Co(II)/Ni(II) with limits of detection of 4.990 ppb (460 nm)/4.353 ppb (570 nm) and 45.041 ppb (428 nm)/29.617 ppb (527 nm) in organic and aqueous solutions, respectively.

Aggregation/Crystallization-Induced Emission in Naphthyridine-Based Carbazolyl-Modified Donor-Acceptor Boron Dyes Tunable by Fluorine Atoms

Four donor-acceptor boron difluoride complexes based on the carbazole electron donor and the [1,3,5,2]oxadiazaborinino[3,4-a][1,8]naphthyridine acceptor were designed, synthesized, and systematically spectroscopically investigated in solutions, in dye-doped polymer films, and in the solid states. The dyes exhibit an intense blue to red solid-state emission with photoluminescence quantum yields of up to 59 % in pure dye samples and 86 % in poly(methyl methacrylate) films. All boron complexes show aggregation-induced emission and reversible mechanofluorochromism. The optical properties of these dyes and their solid state luminescence can be tuned by substitution pattern, i. e., the substituents at the naphthyridine unit. Exchange of CH3- for CF3-groups does not only increase the intramolecular charge transfer character, but also provides a crystallization-induced emission enhancement.

ESIPT-based dual-emissive perimidine derivative as a rapid and sensitive sensor for Cu2+ and Al3+: Construction of memory device, 2-to-1 encoder and 1-to-2 decoder

An ESIPT based fluorescent perimidine derivative oPSDAN was developed and characterized by ¹H NMR, ¹³C NMR and mass spectroscopy. The study of the photo-physical properties of the sensor unveiled its selectivity and sensitivity towards Cu²⁺ and Al³⁺ ions. The sensing of ions was accompanied by colorimetric change (for Cu²⁺) as well as emission turn-off response. The binding stoichiometries of sensor oPSDAN with Cu²⁺ ion and Al³⁺ ions were determined to be 2:1 and 1:1, respectively. The binding constants and detection limits for Cu²⁺ and Al³⁺ were calculated from the UV-vis and fluorescence titration profiles as, 7.1 × 10⁴ M^-1, 1.9 × 10⁴ M^-1 and 9.89 nM, 1.5 × 10^-8 M, respectively. The mechanism was established by ¹H NMR as well as mass titrations and was supported by DFT and TD-DFT calculations. The UV-vis and fluorescence spectral results were further utilized for construction of memory device, encoder and decoder. Sensor-oPSDAN was also tested for determining Cu²⁺ ions in drinking water.

Synthesis, Computational and cytotoxicity studies of aryl hydrazones of β-diketones: Selective Ni2+ metal Responsive fluorescent chemosensors

A new fluorescent sensor 2-(2-(3-chloro-4-fluorophenyl)hydrazono)-5,5-dimethyl cyclohexane-1,3-dione (A) and 2-(2-(4-chloro-2-nitrophenyl)hydrazono)-5,5-dimethyl cyclohexane-1,3-dione (B) composed of a β-diketones of aryl hydrazones synthesized by simple and cost-effective method. Various analytical tools analyzed the structural investigations of the synthesized substituted β-diketones of aryl hydrazones like FT-IR, ¹H, ¹³C NMR and UV-Vis techniques, Single-crystal X-ray diffraction studies (SCXRD) (for A), Scanning electron microscopy (SEM), and fluorescence spectroscopy. SEM also investigates surface morphology modifications of aryl hydrazones and Ni²⁺ complex. Furthermore, the metal sensing (Chemo sensing) behavior of newly prepared aryl hydrazones of β-diketones derivatives was further studied by fluorescence spectroscopy. The aryl hydrazones sensor materials show admirable fluorescence selectivity with enrichment to Ni²⁺ over different cations in an aqueous ethanol solution with a recognition extremity of 4 μM-7 μM. A joint experimental and theoretical investigation was led on the chemical structure employing a density functional theory (DFT) (B3LYP), engaging a 6-31G basis set. The DFT technique's enhanced geometrical bond angles and lengths exhibited great covenant with the experimental results. The highest occupied molecular (HOMO) orbital and lowest unoccupied (LUMO) molecular orbital energy has been concluded. The cytotoxicity studies show these compounds impede the growth of KB cells highly and from the studies to evaluate their capability to accurately dock aryl hydrazones to antibodies of cancer protein such as 4LRH, 4L9K, 4 EKD and 4GIW cancer proteins.

New thiophene hydrazide dual-functional chemosensor: Colorimetric sensor for Cu2+ & fluorescent sensor for Al3

A new thiophene hydrazide derivative TSB was synthesized and utilized as naked-eye colorimetric sensor for Cu²⁺ by the color changed from colorless to yellow as well as green fluorescent turn on sensor for Al³⁺ in DMSO/H₂O (1/1, V/V) solution. The dual-functional chemosensor TSB for Cu²⁺/Al³⁺ sensing displayed excellent properties of special selectivity, superior sensitivity, outstanding anti-interference performance, instantaneous response, wide pH working range and good reversibility. The detection limits of TSB for Cu²⁺/Al³⁺ were determined as low as 46.5 nM and 32.7 nM, respectively. The 1:1 binding mode of TSB with Cu²⁺/Al³⁺ was proved by spectrometric titrations, Job's plots, FTIR, ¹H NMR and HRMS analysis. Moreover, chemosensor TSB was successfully utilized for detection of Cu²⁺ and Al³⁺ in real environmental water and food samples with high reliability, demonstrating its practical applicability.

One-step synthesis of mitochondrion-targeted fluorescent carbon dots and fluorescence detection of silver ions

Silver ions (Ag+) are the most representative harmful ions found in polluted water and widely used in many industries; excessive ingestion of Ag+ in the human body may result in interaction with different metabolites in the human body and in aquatic microorganisms, leading to many diseases. Therefore, there is a great desire to develop good fluorescent probes for Ag+. Herein, a kind of mitochondrion-targeted fluorescent carbon dot was developed. These carbon dots exhibit 29.5% fluorescence quantum yield in water, good photostability and thermal stability. The as-fabricated carbon dots can quickly detect Ag+ in 100% water solution with good selectivity and anti-interference ability. Further, the carbon dots have been successfully applied to monitor Ag+ in living cells via the dual-channel method.

A Reversible Colorimetric and Fluorescence "Turn-Off" Chemosensor for Detection of Cu2+ and Its Application in Living Cell Imaging

Dual-function chemosensors that combine the capability of colorimetric and fluorimetric detection of Cu²⁺ are still relatively rare. Herein, we report that a 3-hydroxyflavone derivative (E)-2-(4-(dimethylamino)styryl)-3-hydroxy-4H-chromen-4-one (4), which is a red-emitting fluorophore, could serve as a reversible colorimetric and fluorescence “turn-off” chemosensor for the detection of Cu²⁺. Upon addition of Cu²⁺ to 4 in neutral aqueous solution, a dramatic color change from yellow to purple-red was clearly observed, and its fluorescence was markedly quenched, which was attributed to the complexation between the chemosensor and Cu²⁺. Conditions of the sensing process had been optimized, and the sensing studies were performed in a solution of ethanol/phosphate buffer saline (v/v = 3:7, pH = 7.0). The sensing system exhibited high selectivity towards Cu²⁺. The limit of naked eye detection of Cu²⁺ was determined at 8 × 10⁻⁶ mol/L, whereas the fluorescence titration experiment showed a detection limit at 5.7 × 10⁻⁷ mol/L. The complexation between 4 and Cu²⁺ was reversible, and the binding constant was found to be 3.2 × 10⁴ M⁻¹. Moreover, bioimaging experiments showed that 4 could penetrate the cell membrane and respond to the intracellular changes of Cu²⁺ within living cells, which indicated its potential for biological applications.

A dual-responsive colorimetric probe for the detection of Cu2+ and Ni2+ species in real water samples and human serum

The monitoring of heavy transition metals has increasingly attracted great attention because they pollute the environment and have unique physiological functions. Chemosensors are useful tools for monitoring heavy transition metals due to their simple visualization, excellent sensitivity and high selectivity. Herein, we have developed a novel chemosensor for the detection of water-soluble Cu2+ and Ni2+ species with different mechanisms, and low detection limits of 2.1 nM for Cu2+ and 1.2 nM for Ni2+ were obtained. The colorimetric probe CPH has been applied to qualitative and quantitative detection of Cu2+ and Ni2+ species in real samples.

Copper(II) complex as a turn on fluorescent sensing platform for acetylcholinesterase activity with high sensitivity

Acetylcholinesterase (AChE) is an important enzyme associated with many nervous diseases, demonstrating the great need for smarter sensing platform with improved sensitivity, selectivity and simplified operation. A "turn on" fluorometric assay is described herein for AChE activity detection, according to the specific enzyme catalyzed reaction of acetylcholine (ATCh) by AChE, which generates thiocholine (TCh) as the product. The well-designed fluorescent probe HBTP possesses ESIPT (Excited State Intramolecular Proton Transfer) nature, leading to a larger Stokes shift, which could be quenched upon coordination with Cu²⁺. The fluorescence-silent HBTP-Cu²⁺ complex could be broken by TCh generated from reaction of ATCh with AChE, giving rise to HBTP release which originates from competitive coordination of TCh with Cu²⁺. This complex probe HBTP-Cu²⁺ offers a limit detection as low as 0.02 mU mL^-1, which is lower than most reported literatures. Furthermore, both HBTP-Cu²⁺ and HBTP show little toxicity to live cells and is available in visualizing cellular AChE activity.

Synthesis of Fluorescent 1,7-Dipyridyl-bis-pyrazolo[3,4-b:4',3'-e]pyridines: Design of Reversible Chemosensors for Nanomolar Detection of Cu2

An efficient access toward novel tridentate ligands based on 1,7-dipyridinyl-substituted bis-pyrazolo[3,4-b:4',3'-e]pyridines (BPs) and their usefulness as fluorescent probes for cation detection is reported. The synthesis proceeds by a three-step sequence starting from 2-chloropyridine (1), all reactions were performed using microwave radiation under solvent-free conditions, and an overall yield of up to 63% was obtained. Photophysical properties of three representative 1,7-dipyridinyl-BPs (PBPs, 6a-6c) substituted at position 4 with different donor (D) or acceptor (A) groups were investigated. Compounds exhibited large Stokes shift in different solvents and strong blue light emission in both solution and solid state, and quantum yields were as high as 88% for some of them; thus, a twisted intramolecular charge transfer (TICT) fluorescence mechanism characteristic of the 1,4,7-triaryl-BPs was confirmed. The 4-phenyl-substituted probe (Ph-PBP, 6b) was used successfully in the detection of some metals (Cu2+, Co2+, Ni2+, and Hg2+) by fluorescence quenching phenomena, which could be reversed in the presence of ethylenediamine. This probe showed a greater sensitivity toward Cu2+ in concentrations as low as 26 nM, and in the process of "on-off-on" for this fluorescent molecular switch, only 1 equiv of the analyte was used.

A pyrazolopyrimidine based fluorescent probe for the detection of Cu2+ and Ni2+ and its application in living cells

A fluorescent sensor L based on a pyrazolopyrimidine core simultaneously detects Cu²⁺ and Ni²⁺ ions by photoluminescence quenching, even in the presence of other metal cations. Sensor L possesses high association constants of 5.24 × 10³ M^-1 and 2.85 × 10⁴ M^-1 and low detection limits of 0.043 μM and 0.038 μM for Cu²⁺ and Ni²⁺, respectively. The binding stoichiometry ratios of L to Cu²⁺ or Ni²⁺ is 1:1 as determined by Benesi-Hildebrand and Job's plots, and by crystal structures. DFT calculations on L-Cu²⁺ indicated reduced electron donation from the coordinated pyrazolopyridine to the fused pyrimidine and pendant phenyl group which, together with a smaller HOMO-LUMO orbital gap could favour non-radiative decay and explain the observed fluorescence quenching. Sensor L possessed low cytotoxicity and good imaging characteristics for Cu²⁺ and Ni²⁺ in living cells, suggesting potential applications for detecting Cu²⁺ and Ni²⁺in vivo.

Fluorescent ZnII Chemosensor Mediated by a 1,8-Naphthyridine Derivative and It's Photophysical Properties

One of the greatest challenges in using fluorescent chemosensors for highly selective and sensitive transition-metal ions is finding an efficient and simple method for its synthesis. In this study, a highly efficient fluorescence chemosensor for ZnII was developed from N-Boc-L-proline modified 1,8-naphthyridine. The fluorescence intensity of the chemosensor was increased significantly only in the presence of ZnII ion which provided a perceived color change for rapid visual sensing, while other metal ions showed fluorescence quenching or little changes. It was worth noting that the chemosensor L distinguished ZnII from CdII commonly having similar properties. The solvent effect and possible bonding mode for fluorescence enhancement have been also discussed. Results of this study indicated that the Boc-group in l-proline significantly improved the sensitivity and selectivity for ZnII detection performance, as confirmed by comparison experiments and time dependent-density functional theory (TD-DFT) calculations.

Highly selective colorimetric and reversible fluorometric turn-off sensors based on the pyrimidine derivative: mimicking logic gate operation and potential applications

Two simple and efficient energy transfer based fluorogenic receptors exhibiting high sensitivity for the Ni²⁺ ion and its validation using DFT geometry optimization.

Single Chemosensor for Double Analytes: Spectrophotometric Sensing of Cu(2+) and Fluorogenic Sensing of Al(3+) Under Aqueous Conditions

(E)-N-((8-Hydroxy-1,2,3,5,6,7-hexahydropyrido-[3,2,1-ij]-quinolin-9-yl)methylene)-4-tert-butyl -benzhydrazide has been developed as a single, dual-functional chemosensor. The chemosensor showed a good selectivity and sensitivity toward to Al(3+) and Cu(2+) at a low detection limit, respectively. Theoretical calculations have also been carried out to understand the configuration of the complexes.

Resurfaced fluorescent protein as a sensing platform for label-free detection of copper(II) ion and acetylcholinesterase activity

Protein engineering by resurfacing is an efficient approach to provide new molecular toolkits for biotechnology and bioanalytical chemistry. H39GFP is a new variant of green fluorescent protein (GFP) containing 39 histidine residues in the primary sequence that was developed by protein resurfacing. Herein, taking H39GFP as the signal reporter, a label-free fluorometric sensor for Cu(2+) sensing was developed based on the unique multivalent metal ion-binding property of H39GFP and fluorescence quenching effect of Cu(2+) by electron transfer. The high affinity of H39GFP with Cu(2+) (Kd, 16.2 nM) leads to rapid detection of Cu(2+) in 5 min with a low detection limit (50 nM). Using acetylthiocholine (ATCh) as the substrate, this H39GFP/Cu(2+) complex-based sensor was further applied for the turn-on fluorescence detection of acetylcholinesterase (AChE) activity. The assay was based on the reaction between Cu(2+) and thiocholine, the hydrolysis product of ATCh by AChE. The proposed sensor is highly sensitive (limit of detection (LOD) = 0.015 mU mL(-1)) and is feasible for screening inhibitors of AChE. Furthermore, the practicability of this method was demonstrated by the detection of pesticide residue (carbaryl) in real food samples. Hence, the successful applications of H39GFP in the detection of metal ion and enzyme activity present the prospect of resurfaced proteins as versatile biosensing platforms.

Highly sensitive and selective fluorescence detection of copper (II) ion based on multi-ligand metal chelation

A fluorescent probe was synthesized and demonstrated to be highly selective and sensitive in the reaction with copper (II) ion, generating a large variation of the fluorescence intensity in a dose-response manner. The probe contains a dansyl moiety as fluorophore and a multidentate ligand for copper (II) ion recognition. The reaction of the molecular probe with copper (II) ion proceeds rapidly and irreversibly in a 1 to 1 stoichiometric way, leading to the production of stable copper (II) complex, which subsequently results in the quenching of fluorescence. The detection limit for copper (II) ion was measured to be about 2ppb. It was also shown that the probe has high selectivity for copper (II) ion and good anti-interference ability against other transition metal ions. The herein reported very simple and reliable fluorescence probe could be employed for copper (II) ion detection in many aspects.

Twin applications of highly selective Cu2+ fluorescent chemosensor and cytotoxicity of 2-(2-phenylhydrazono)-1H-indene-1,3(2H)-dione and 2-(2-(4-methoxyphenyl)hydrazono)-1H-indene-1,3(2H)-dione: molecular docking and DFT studies

Turn on chemosensor for paramagnetic Cu²⁺ have been developed based on indene 1,3-dione. Their ability to inhibit KB cell, molecular docking and DFT studies also carried out.

Rhodamine-based 'turn-on' fluorescent probe for Cu(II) and its fluorescence imaging in living cells

A novel rhodamine spirolactam derivative 3',6'-Bis(diethylamino)-2-(2-hydroxyethylamino) spiro[isoindoline-1,9'-xanthen]-3-one (RO1) was synthesized, and characterized by high-resolution mass spectrometry (HRMS), X-ray crystallography, Infrared spectroscopy (IR), and (1)H NMR and (13)C NMR spectroscopy. RO1 exhibited highly sensitive and exclusively selective fluorescence response toward Cu(2+) over other metal ions with a detection limit of 0.56ppb in mixed aqueous solution. The fluorescence was pH-independent in the wide range pH 3.1-11.6. The turn-on fluorescence enhancement of the probe is based on Cu(2+) induced ring-opening mechanism of the rhodamine spirolactam. Moreover, by means of fluorescence microscopy experiments, it was demonstrated that RO1 could monitor trace Cu(2+) changes by live cell imaging.

N-(7-Dibromo-methyl-5-methyl-1,8-naphthyridin-2-yl)acetamide-pyrrolidine-2,5-dione (1/1)

In the title co-crystal, C₁₂H₁₁Br₂N₃O·C₄H₅NO₂, the naphthyridine derivative and the pyrrolidine-2,5-dione mol­ecules have crystallographic mirror-plane symmetry with all non-H atoms, except the Br atom, located on the mirror plane. In the crystal, N—H⋯N, N—H⋯O and C—H⋯O hydrogen bonds link the mol­ecules into heterodimers. These dimers are further linked into a one-dimensional structure along [010] by weak C—Br⋯O inter­actions [Br⋯O = 3.028 (5) Å and C—Br⋯O = 158.52 (4)°].