N-diethylaminosalicylidene based “turn-on” fluorescent Schiff base chemosensor for Al3+ ion: Synthesis, characterisation and DFT/TD-DFT studies

A zwitterionic probe for ratiometric fluorescent detection of aluminium(III) ion in aqueous medium and its application in bioimaging

In the present study, we have synthesized an aminobenzoic acid containing Schiff base (compound 1) and its structure was confirmed through single crystal X-ray study. Importantly, the compound 1 crystallizes in the zwitterionic form, with an anionic carboxylate group (-COO-) and a cationic iminium group (-C = NH+-). The compound 1 is highly soluble in water due to its zwitterionic feature in the solid state. Interestingly, compound 1 acts as a ratiometric fluorescent probe for the selective detection of Al3+ ion in aqueous solution without organic cosolvent. It can also detect Al3+ ion by visual colour change to bluish-green fluorescence under 365 nm UV light. The association constant between compound 1 with Al3+ ion was estimated to be 1.67 × 104 M-1. The lowest detection limit for Al3+ ion was calculated to be 7.05 × 10-8 M in water. Compound 1 in combination with Al3+ ion demonstrated fluorescent imaging potential of the nucleus of in RAW 264.7 murine macrophage cell line. In addition, the sensing model is developed as paper based sensor ''Test Kit' 'for its practical applicability.

Highly selective, sensitive and biocompatible rhodamine-based isomers for Al3+ detection: A comparative study

Selective detection of a metal ion with high selectivity is of great importance to understand its existence and its role in many chemical and biological processes. We report here the synthesis, characterization and Al3+ sensing properties of two rhodamine-based isomers, (E)-2-((2-(allyloxy)benzylidene)amino)ethyl)-3',6'-bis(ethylamine)-2',7'-dimethylspiro[isoindoline-1,9'-xanthen]-3-one (L-2-oxy) and (E)-2-((4-(allyloxy)benzylidene)amino)ethyl)-3',6'-bis(ethylamine)-2',7'-dimethylspiro[isoindoline-1,9'-xanthen]-3-one (L-4-oxy). L-2-oxyand L-4-oxy show pink coloration, significant enhancement in absorbance at 530 nm and fluorescence intensity at 553 nm in the presence of Al3+ among several cations. Quantum yield and lifetime of the probes increase in the presence of Al3+. LOD values have been determined as low as ∼1.0 nM for both the isomers. DFT study suggests that the cation induces opening of spirolactam ring resulting in the changes of the rhodamine dyes. Additional reason could be Chelation Enhanced Fluorescence (CHEF) effect due to the subsequent chelation of the metal ion. Between two isomers, L-2-oxy displays better sensing ability towards Al3+ in terms of fluorescence enhancement, limit of detection, lifetime enhancement. Both the probes have been utilized in cell imaging studies using rat skeletal myoblast cell line (L6 cell line).

Robust Optical Detection of Ga3+ by a Rhodamine- and Coumarin-Based Proficient Probe: Theoretical Investigations and Biological Applications

The present investigation highlights a rhodamine-B- and coumarin-based efficient probe that selectively detects Ga3+ over other metal ions. The active pocket of the ligand for trapping the metal ions and the binding stoichiometry of its Ga3+ complex were discovered by single-crystal X-ray diffraction (SC-XRD) analysis. This binding stoichiometry was further confirmed in the solution state by mass spectrometry and Job's plot. The detection limit was found to be at the nanomolar level. Pyrophosphate being a well-known quencher could easily quench the fluorescence intensity of the RC in the presence of Ga3+ and reversibly recognize Ga3+ in the solution. The spiro ring opening of the ligand after Ga3+ insertion is proposed to be the principal mechanism for the turn-on fluorescence response. This ring opening was confirmed by SC-XRD data and nuclear magnetic resonance (NMR) titration experiments. Both ground- and excited-state calculations of the ligand and complex have been carried out to obtain information about their energy levels and to obtain the theoretical electronic spectra. Furthermore, the live-cell imaging of the probe only and the probe after the addition of Ga3+ have been carried out in HaCaT cells and satisfactory responses were observed. Interestingly, with the help of this probe, Ga3+ can be tracked inside the intracellular organelle such as lysosomes along with other regions of the cell. The article highlights a rhodamine-coumarin-based probe for the detection of Ga3+ over other metal ions with a nanomolar level detection limit. Structural characterization of the ligand and its Ga3+ complex was investigated by SC-XRD. Density functional theory (DFT) and time-dependent DFT (TD-DFT) studies were carried out to explore the excited-state energies and electronic spectra. The application of the probe for the detection of Ga3+ in live cells has been explored, and positive responses were observed.

Spectroscopic and TD-DFT studies on the chromo-fluorogenic detection of cyanide ions in organic and aquo-organic media

A new naked-eye chromogenic and fluorogenic probe KS5 has been developed for the detection of CN- ions in neat DMSO and H2O:DMSO (1:1 v/v) media. The probe KS5 exhibited selectivity towards CN- and F- ions in organic and high selectivity towards CN- ions in aquo-organic media resulting in a colour change from brown to colourless and a turn-on fluorescence response. The probe could able to detect CN- ions via a deprotonation process, which was conceived by consecutive addition of hydroxide and hydrogen ions and confirmed using 1H NMR studies. The limit of detection (LOD) of KS5 towards CN- ions were in the range of 0.07-0.62 µM in both these solvent systems. Suppression of intra-molecular charge transfer (ICT) transition and photoinduced electron transfer (PET) process of KS5 by the added CN- ions are responsible for the chromogenic and fluorogenic changes observed, respectively. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations strongly supported the proposed mechanism along with the optical properties of the probe before and after the addition of CN- ions. To prove the practical applicability, KS5 was successfully utilized to detect CN- ions in cassava powder and bitter almonds as well as to determine CN- ions in various real water samples.

A pyridine modified naphthol hydrazone Schiff base chemosensor for Al3+ via intramolecular charge transfer process

A pyridine modified naphthol hydrazone Schiff base chemosensor, NaPy, was prepared in a two-step process to detect aluminum ion (Al3+) in different samples. The probe shows a turn-off emission response towards Al3+ at a 1:1 binding stoichiometry via intramolecular charge transfer (ICT) mechanism, as validated by density functional theory (DFT) calculations and a series of spectroscopic measurements. The response time is slightly over one minute with a limit of detection (LOD) value of 0.164 µM, demonstrating the great sensitivity of the probe. It is also found that NaPy exhibits high selectivity towards Al3+ and resists interference from seventeen other cations. Application investigations in paper strips, water samples and HeLa cells suggest that NaPy can be used as an efficient probe for sensing Al3+ in real environmental samples and biosystems.

Modulation of aluminum sensing properties of a sulphone group containing chemosensor and its biological applications

A chemosensor with two binding pockets facilitates binding of one metal ion in either of the pockets providing a better chance for the interaction and hence recognition of the cation. We report here a chemosensor, namely 2,2'-(1E)-(5,5'-sulfonylbis(2-hydroxy-5,1-phenylene))bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)dinaphthalen-1-ol (H₄L-naph), for selective sensing of Al³⁺ in DMF- HEPES buffer (1:4, v/v, pH 7.4). It shows almost 100-fold fluorescence enhancement at 532 nm (λ_ex = 482 nm) in the presence of Al³⁺. Its quantum yield and excited state lifetime enhances significantly with the cations. H₄L-naph forms a 1:2 complex with Al³⁺ with an association constant value of 2.18 × 10⁴ M^-2. Fluorescence enhancement may be attributed to CHEFF mechanism and restriction of >CN isomerization. Effect of the presence of naphthyl rings instead phenyl ring of a previously reported probe has resulted shifting of excitation/emission peak towards longer wavelength. The probe has been applied to image Al³⁺ in L6 cells with no significant cytotoxicity.

A naphthol hydrazone Schiff base bearing benzothiadiazole unit for fluorescent detection of Fe3+ in PC3 cells

Naphthol hydrazone derivatives are recognized as efficient chelating agents for both qualitative and quantitative detection of metal ions. Here we design a naphthol hydrazine-based chemosensor with covalently linking a strong electron-withdrawing benzothiadiazole group to modulate the molecular electronic structure, nominated as NtHzBtd. The fluorescent probe performs excellent selectivity and sensitivity towards Fe³⁺ with 1:1 binding stoichiometry, while exhibiting a quick response at 55 s with a relatively low limit of detection of 0.036 µM. A series of spectroscopic measurements in tandem with theoretical calculations suggest that the probe undergoes both intramolecular charge transfer (ICT) and chelation enhanced quenching (CHEQ) processes. Successful color rendering of paper strips and bioimaging in PC3 cells demonstrate the promising applicability of NtHzBtd for portable Fe³⁺ detection in real samples and biosystems.

A New Azo-Schiff Base Dual-mode Chemosensor: Colorimetric Detection of Cobalt Ions and Fluorometric Detection of Aluminum Ions in Aqueous Ethanol Solution

A new Azo-Schiff base ligand (H₂L) was designed and synthesized as a cation chemosensor. The chemosensor H₂L as dual chemosensor showed selective fluorescence recognition of Al³⁺ with a noticeable fluorescence enhancement and colorimetric detection of Co2 + in aqueous ethanol solution. The H₂L exhibits a linear response toward Al³⁺ ions in the concentration range of 1.91 × 10^-8 M to 4.8 × 10^-6 M with a limit of detection of 1.91 × 10^-8 M. The sensing mechanism of sensor H₂L toward Al³⁺ was investigated by 1H NMR and IR spectroscopies. Fluorescence switch based on the control of EDTA and Al³⁺ proved H₂L could act as a reversible chemosensor. The molecular structure of [NiL] complex has been determined by X-ray crystallography.

Theoretical investigation on the fluorescence properties and ESIPT mechanism of the Al3+ ion sensor 1-((2-hydroxynaphthalen-1-yl)methylene)urea(OCN)

A new action mechanism for the fluorescent detection on the Al³⁺ ion of the sensitive 1-((2-hydroxynaphthalen-1-yl)methylene)urea(OCN) is theoretically studied. The extensive theoretical calculations on the OCN and the isomer structure OCN-T are performed. The emission and absorption spectra consistent with the experiment value. The absorption spectra peaks (362 nm and 326 nm) of OCN and OCN-T molecules are attributed to the experimentally observed absorption spectra at 356 nm and 314 nm, respectively. The calculated fluorescence value of the OCN-AL structure is 460 nm, while the OCN-T-AL structure has no fluorescence. These results better explain that OCN and its isomers OCN-T are involved in the absorption, and the detection spectrum signal is emitted from the OCN-AL complex. The OCN and OCN-T molecules are obvious hydrogen bonding systems. The excited state intramolecular proton transfer photochemical behaviors and detecting Al³⁺ ion photophysical changes were explained for the first time at the molecular level. As driving force of excited state intramolecular proton transfer (ESIPT) reaction, the bond parameters and vibrational frequencies of intramolecular hydrogen bond were analyzed by optimizing structures and calculating infrared spectra, analysis of frontier molecular orbitals. To further elucidate the proton transfer reactive paths, the scanned the potential energy curves (PECs) of OCN and OCN-T chemosensor in different electronic states are plotted. This work proposes a reasonable explanation for the detection mechanism of the OCN sensor.

On the synthesis and performance of a simple colorimetric and fluorescent chemosensor for Cu2+ with good reversibility

A colorimetric and fluorescent chemosensor (CS1) for Cu²⁺ based on the mechanism of internal charge transfer (ICT) has been successfully designed and prepared by simple condensation of 4^_(diethylamino)salicylaldehyde and oxalyl dihydrazide. Cu²⁺ in solution (DMSO/H₂O = 7:3, v/v) by 5 mM NaAc-HAc at pH 7.0 was determined through dual channels: (1) "naked-eye" observation, a visually dramatic color change from light green to orange, which can be used for qualitative determination of Cu²⁺; (2) spectrofluorometry, which can quantificationally assay Cu²⁺. It provides a simple-to-use platform for reliable detection of Cu²⁺ at concentrations ranging from 5.0 × 10^-7 to 1.1 × 10^-5 M with detection limit of 1.2 × 10^-7 M, which is nearly 2 × 10² times lower than the maximum allowable level of inorganic Cu²⁺ in drinking water (1.3 ppm, ∼20 µM) permitted by the EPA (Environmental Protection Agency), and the sensing detection of Cu²⁺ ions was reversible.