A Schiff-base receptor based on phenolphthalein derivate appended 2-furoic hydrazide: Highly sensitive fluorogenic “turn on” chemosensor for Al3+

Fluorescence chemosensing and bioimaging of metal ions using schiff base probes working through photo-induced electron transfer (PET)

Though metal ions like copper, iron, zinc, etc. are essential, but their dyshomeostasis is associated with several disorders. Therefore, fast, sensitive, and cost-effective monitoring of these cations will have a significant impact. Many recently reported small organic molecules were able to detect a specific metal ion because of certain variations in the electron/charge transfer processes occurring in those molecules after binding with metal ions. In this context, Schiff base molecules were widely used as fluorescence turn-on/turn-off probes for the detection of metal ions like Al3+, Cu2+, Zn2+, Fe3+, Ag+, heavy metal ions, etc. In this article, we have reviewed the recent developments in fluorimetric chemosensing of metal ions by Schiff bases based on the photo-induced electron transfer (PET) process. A variety of examples have been discussed in which PET was used as a cation recognition mechanism. Particular focus is placed on the molecular probes used for sensing, including their design, selectivity, sensitivity, and in some cases their potential bioimaging applications.

A cyanobiphenyl-based fluorescent ''lighting-up'' sensor for highly selective and sensitive recognition of Al3+: Theoretical, practical and bioimaging studies

The recognition of toxic Al³⁺ in foods and biosystems has of great interest to researchers. Herein, a novel cyanobiphenyl-based chemosensor CATH (E)-N'-((4'-cyano-4-hydroxy-[1,1'-biphenyl]-3-yl)methylene)thiophene-2-carbohydrazide was fabricated and shown to recognize Al³⁺ in HEPES buffer:EtOH (90:10, v:v, pH = 7.4) by ''lighting-up'' fluorescence sensing. The CATH evidenced high sensitivity (LOD = 13.1 nM) and excellent selectivity to Al³⁺ over competing cations. The Job's plot, TOF-MS and theoretical computation studies were performed to probe the binding mechanism of Al³⁺ to CATH. Additionally; CATH was successfully utilized to practical applications and employed to recover of Al³⁺ from different food samples. More importantly, it was employed to intracellular Al³⁺ detection in living cells including THLE2 and HepG2.

A new naphthalimide-picolinohydrazide derived fluorescent "turn-on" probe for hypersensitive detection of Al3+ ions and applications of real water analysis and bio-imaging

The development of high-selective chemosensors for trace Al³⁺ detection in the ecosystem is crucially importance due to its detrimental effects. In this work, a simple Schiff-base fluorescent probe NPP derived from naphthalimide and picolinohydrazide was rationally designed and prepared for efficient detection of Al³⁺. NPP exhibited prominent sensing behaviors toward Al³⁺ with low detection limit (LOD) (39 nM), rapid response time (1 min), strong binding affinity (4.02 × 10⁴), good anti-interference characteristics and visual detection. Binding ratio of NPP-Al³⁺ complex was determined to be 1:1 by Job's plot analysis. In addition, the chelation mechanism of NPP with Al³⁺ ions was proposed and substantiated by the density functional theory (DFT) and time-dependent density functional theory (TD-DFT), IR spectrum and ¹H NMR titration experiments. Furthermore, this "signal-on" probe NPP was efficiently utilized as a promising indicator for Al³⁺ detection in environmental and biological samples.

A reversible and efficient probe for dual mode recognition of Al3+ and Cu2+ with logic gate behaviour: Crystal structure, theoretical and in-vivo bio-imaging investigations

This work presents the synthesis, characterization, crystal structure and spectroscopic investigations of isophthalohydrazide based probe. Among various tested metal ions, the probe selectively detects Al³⁺ and Cu²⁺ in aqueous ethanol via fluorometric and colorimetric methods, respectively. It displays a fluorescence "turn-on" response with Al³⁺ and visual colour change from colourless to yellow with Cu²⁺. Sensing mechanism is explored with UV-Vis, fluorescence spectroscopy and ¹H NMR titration, and confirmed with computational results. Suppression of CN isomerization and photo-induced electron transfer (PET) along with chelation enhanced fluorescence emission (CHEF) result in "turn-on" fluorescence with Al³⁺ while ligand to metal charge transfer (LMCT) accounts for visual colour change with Cu²⁺. Job's plot and HRMS confirm 1:2 (L:M) stoichiometry. The probe also exhibits efficient reversibility and reproducibility with EDTA which are successfully mimicked with combinatorial logic gate and truth table. Additionally, solid state applications and bio-imaging investigation on gut tissue of Drosophila 3rd instar larvae are performed.

Naphthalimide appended isoquinoline fluorescent probe for specific detection of Al3+ ions and its application in living cell imaging

Herein, a novel Schiff base fluorescent probe NIQ based on naphthalimide and iso-quinoline units has been readily prepared for the selective detection of Al³⁺ ions. The obviously visible color changes and prominent fluorescence enhancement were observed upon the addition of Al³⁺ to NIQ, which could be attributed to the complexation of NIQ with Al³⁺ and thus leading to the inhibition of photo-induced electron transfer (PET) and the chelation-enhanced fluorescence (CHEF) progress. The limit of detection (LOD) was 52 nM that was far below the standard recommended by the WHO. Binding ratio (1:1) of NIQ with Al³⁺ ions was supported by Job's plot. The binding constant of NIQ for Al³⁺ were calculated to be 3.27 × 10⁵ M^-1 on the basis of benesi-Hildebrand plot. The plausible binding mechanism for NIQ towards Al³⁺ ions was evidenced by the density functional theory (DFT) and ¹H NMR titration experiment. Furthermore, this "turn-on" probe NIQ has been successfully applied as a biomarker for imaging the Al³⁺ ions in living cells.

A novel reversible fluorescent probe based on naphthalimide for sequential detection of aluminum (Al3+) and fluoride (F-) ions and its applications

A new Schiff base fluorescent probe NBP derived from the one-step condensation strategy of 2-butyl-6-hydroxy-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinoline-5-carbaldehyde and N-(2-(hydrazinecarbonyl)phenyl)benzamide was synthesized and characterized. NBP exhibited high selectivity toward Al³⁺ along with naked-eye color changes and prominent fluorescence enhancement. The limit of detection (LOD) of NBP toward Al³⁺ was detected to be 80 nM. The binding ratio of NBP with Al³⁺ ions was obtained as 1 : 2 on the basis of Job's plot with the association constant K_a value of 4.22 × 10¹⁰ M^-1/2. The plausible complexation mechanism of NBP toward Al³⁺ ions was validated by the density functional theory (DFT) and IR spectrum. In addition, in situ formed "NBP + Al³⁺" could be utilized as the second sensor for selective recognition of F^-via fluorescence quenching with a low detection limit (44 nM). Furthermore, the cell imaging experiments of probe NBP in HeLa cells have successfully demonstrated that NBP could serve as an indicator for monitoring Al³⁺ ions in living cells. On top of that, NBP could be used to prepare simple test paper strips for quickly and qualitatively detecting a trace amount of Al³⁺ ions in a visible manner.

Solvation effect on the ESIPT mechanism of nitrile-substituted ortho-hydroxy-2-phenyl-oxazolines

In this study, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were used to unveil the solvation effects on the excited-state intramolecular proton transfer (ESIPT) of nitrile-substituted ortho-hydroxy-2-phenyl-oxazoline (NOHPO) molecules in three different solvents. According to the functional analysis of the reduced density gradient, hydrogen bond in low-polar solvents is stronger compared to that in high-polar solvents, indicating that proton transfer (PT) can be influenced by the polarity of the solvent. Moreover, the geometric parameters and infrared vibration spectrum of NOHPO in different types of solvents in the S0 and S1 state were compared, which confirmed the above results. By analyzing electronic spectra and frontier molecular orbitals, it was found that the spectral properties were affected by different polar solvents. Molecular electrostatic potential surface calculations proved that PT took place between the H2 atom and N3 atom, and the natural population analysis and Hirshfeld charge reveal the charge distribution after photoexcitation. To investigate the ESIPT progress intensively, the potential energy curves of NOHPO in three types of solvents were established. The findings revealed that NOHPO could transform from enol to keto form in the S1 state spontaneously, and ESIPT progress was promoted with the decrease in polarity.

Phenolphthalein Conjugated Schiff Base as a Dual Emissive Fluorogenic Probe for the Recognition Aluminum (III) and Zinc (II) Ions

In this study, a new phenolphthalein derivative, FFIZNA, has been planned and successfully prepared in an uncomplicated way. The probe FFIZNA could selectively monitor Al³⁺ and Zn²⁺ among other relevant cations with diverse colors through a turn-on emission response in EtOH:HEPES (9/1;v/v) media owing to the chelation enhanced fluorescence (CHEF), prevention of ESIPT, -C=N- isomerization and PET of the probe FFIZNA. The interactions of Al³⁺ and Zn²⁺ with the probe FFIZNA were confirmed by emission spectroscopy, Job's plot and ¹H-NMR titration substantiated 1:2 reaction stoichiometry between FFIZNA and Al³⁺ and Zn²⁺. The time-response study displayed that the emission of FFIZNA with Al³⁺ and Zn²⁺, rapidly boosted and reached the stable value in less than 3.0 and 4.0 min, respectively. Therefore, the FFIZNA has successfully been utilized to the dual recognition of Al³⁺ and Zn²⁺ in solutions. Phenolphthalein conjugated schiff base as a dual emissive fluorogenic probe for the detection aluminum (III) and zinc (II) ions.

A new 7-diethylamino-4-hydroxycoumarin based reversible colorimetric/fluorometric probe for sequential detection of Al3+/PPi and its potential use in biodetection and bioimaging applications

A new 7-diethylamino-4-hydroxycoumarin appended acylhydrazone probe was prepared and utilized for the sequential detection of Al³⁺/PPi in a reversible off–on–off emissive manner. The various practical applications of the probe were established.

1,8-Naphthalimide appended propiolate-based fluorescent sensor for selective detection of cysteine over glutathione and homocysteine in living cells

A novel 1,8-naphthalimide-based fluorogenic chemoprobe NASP was designed and developed as a sensor platform for the selective detection of biologically important cysteine over glutathione and homocysteine in living-cells.