AIE-ESIPT based colorimetric and “OFF-ON-OFF” fluorescence Schiff base sensor for visual and fluorescent determination of Cu2+ in an aqueous media

Development of a Coumarin-Based Schiff Base Fluorescent Probe and its Application in Detection of Cu²⁺

A highly practical Schiff base fluorescent probe, (E)-3-amino-N'-((7-(diethylamino)-2-oxo-2 H-chromen-3-yl)methylene)thiophene-2-carbohydrazide (M), with a facile synthetic route has been successfully developed. M has been utilized for the specific detection of Cu2+ in THF/H2O Tris buffer solution (v/v = 9:1, 0.01 M, pH = 7.4) via the fluorescence quenching mechanism. The detection of Cu2+ by M has been largely unaffected by interfering ions and has demonstrated a distinct dual-channel response in both colorimetry and fluorescence. The response time of M towards Cu2+ is remarkably fast, taking only 30 s. Additionally, M exhibits exceptional sensitivity with a limit of detection (LOD) as low as 1.76 × 10- 7 M. The stoichiometric ratio between M and Cu2+ has been determined to be 1:1 through Job's Plot, while the binding constant has been calculated as 1.19 × 104 M- 1 using the Benesi-Hildebrand equation. The structure of M has been elucidated by 1H NMR and ESI-MS analyses, thereby confirming the binding mode between M and Cu2+. Further validation has been achieved through DFT calculations. The test paper based on M has finally been prepared for the rapid and convenient detection of Cu2+. The M has also been utilized for the detection of Cu2+ in real samples, including lake water, onions, and coffee, demonstrating favorable recovery rates. Moreover, successful visual detection has been achieved in food samples such as bean sprouts and rice. The aforementioned examples have collectively illustrated the practical applicability of M in authentic samples.

A substituent-modified new salicylaldehyde-diphenyl-azine based AIEgen: A promising skeleton for copper ion sensor

In this study, we have reported a novel 4-bromo-salicylaldehyde-diphenyl-azine (B-1), a new member of salicylaldehyde-diphenyl-azine (SDPA) family known for its excellent sensing properties. In contrast to the previously reported AIEgens, we found that the bromo-substitution at the 4th position of the salicylaldehyde moiety blue-shifted the emission by 10 and 15 nm as compared to the unsubstituted (Tong et.al 2017) and Bromo at the 5th position (Jain et.al 2023) respectively. Moreover, B-1 crystallizes instantly as the cooling process starts, which was not observed in the previously reported scaffolds. The sensing investigation again demonstrated the precise and ultrasensitive behavior of B-1 for copper ions. B-1 has a very low LOD value i.e. 29.2 x 10-8 M with a high association constant and binds with copper ion in 2:1 mode. This time we also analyzed the practical applicability in the solid phase using cotton swabs and performed the real-time estimation of copper ions in water and biological samples like urine and blood serum. The excellent percentage recovery and the RSD value suggest the precision of the experiments. Further, we also perform the sensing in living cancer HeLa cells. Altogether, we found that the SDPA skeleton is precise and ultrasensitive for copper ions and versatile which can be used variously to detect copper ions in the real world. This research will surely help in developing new specific skeleton-based AIEgens with desirable emission properties and precise applications in the future.

Synthesis of Intrinsically-Fluorescent Aliphatic Tautomeric Polymers for Proton-Conductivity, Dual-State Emission, and Sensing/Oxidation-Reduction of Metal Ions

Herein, fluorescent conducting tautomeric polymers (FCTPs) are developed by polymerizing 2-methylprop-2-enoic acid (MPEA), methyl-2-methylpropenoate (MMP), N-(propan-2-yl)prop-2-enamide (PPE), and in situ-anchored 3-(N-(propan-2-yl)prop-2-enamido)-2-methylpropanoic acid (PPEMPA). Among as-synthesized FCTPs, the most promising characteristics in FCTP3 are confirmed by NMR and Fourier transform infrared (FTIR) spectroscopies, luminescence enhancements, and computational studies. In FCTP3, ─C(═O)NH─, -C(═O)N<, ─C(═O)OH, and ─C(═O)OCH3 subluminophores are identified by theoretical calculations and experimental analyses. These subluminophores facilitate redox characteristics, solid state emissions, aggregation-enhanced emissions (AEEs), excited-state intramolecular proton transfer (ESIPT), and conductivities in FCTP3. The ESIPT-associated dual emission/AEEs of FCTP3 are elucidated by time correlated single photon counting (TCSPC) investigation, solvent polarity effects, concentration-dependent emissions, dynamic light scattering (DLS) measurements, field emission scanning electron microscopy images, and computational calculations. The cyclic voltammetry measurements of FCTP3 indicate cumulative redox efficacy of ─C(═O)OH, ─C(═O)NH─/-C(═O)N<, ─C(─O─)═NH+─/─C(─O─)═N+, and ─C(═N)OH functionalities. In FCTP3, ESIPT-associated dual-emission enable in the selective detection of Cr(III)/Cu(II) at λem1/λem2 with the limit of detection of 0.0343/0.079 ppb. The preferential interaction of Cr(III)/Cu(II) with FCTP3 (amide)/FCTP3 (imidol) and oxidation/reduction of Cr(III)/Cu(II) to Cr(VI)/Cu(I) are further supported by NMR-titration; FTIR and X-ray photoelectron spectroscopy analyses; TCSPC/electrochemical/DLS measurement; alongside theoretical calculations. The proton conductivity of FCTP3 is explored by electrochemical impedance spectroscopy and I-V measurements.

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).

Mechanistic Innovations in Fluorescent Chemosensors for Detecting Toxic Ions: PET, ICT, ESIPT, FRET and AIE Approaches

Fluorescent chemosensors have become vital tools for detecting toxic ions due to their exceptional sensitivity, selectivity, and rapid response times. These sensors function through various mechanisms, each providing unique advantages for specific applications. This review offers a comprehensive overview of the mechanistic innovations in fluorescent chemosensors, emphasizing five key approaches: Photoinduced Electron Transfer (PET), Fluorescence Resonance Energy Transfer (FRET), Intramolecular Charge Transfer (ICT), Aggregation-Induced Emission (AIE), and Excited-State Intramolecular Proton Transfer (ESIPT). We highlight the substantial progress made in developing these chemosensors, discussing their design principles, sensing mechanisms, and practical applications, with a particular focus on their use in detecting toxic ions relevant to environmental and biological contexts.

Synthesis of a super-low detection limit fluorescent probe for Al3+ and its application in fluorescence imaging of zebrafish and cells

A novel fluorescent probe N'-(2-hydroxybenzylidene)-indole-3-formylhydrazine (JHK) was designed and synthesized based on the condensation reaction of indole-3-formylhydrazine and salicylaldehyde. The probe JHK solution could highly selectively recognize Al3+ by the obvious fluorescence enhancement (288-fold) after adding Al3+. And the probe solution with Al3+ had a very high fluorescence quantum yield (89.29 %). The detection limit was calculated to be 1.135 nM, which was significantly lower than many reported detection limits, indicating that the probe JHK had pretty good sensitivity. The ratio of JHK to Al3+ (1:1) and the sensing mechanism were determined by Job's plot, 1H NMR spectra, FTIR spectra, ESI-MS and Gaussian calculation. The probe solution and medium-speed filter paper were successfully used to make test papers for more convenient detection of Al3+. Furthermore, the probe JHK had been successfully applied to the detection of Al3+ in real water, zebrafish and living cells.

A new Schiff base derived from 5-(thiophene-2-yl)oxazole as "off-on-off" fluorescence sensor for monitoring indium and ferric ions sequentially and its application

A new Schiff base sensor (E)-N'-((8-hydroxy-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)methylene)-5-(thiophen-2-yl)oxazole-4-carbohydrazide (TOQ) was synthesized and found to emit yellowish green fluorescence upon introduction of In³⁺. Furthermore, the resulting complex TOQ-In³⁺ was quenched selectively by Fe³⁺. The detection limits of TOQ for In³⁺ and Fe³⁺ were 1.75 × 10^-10 M and 8.45 × 10^-9 M, respectively. The complex stoichiometry of TOQ with target ions was determined to be 1:2 via Job's plot analysis, which further was verified by ESI-MS titration and theoretical calculations. Moreover, TOQ can be used for the determination of target ions in environmental water samples. A portable paper sensor of TOQ was successfully developed for detecting In³⁺ to assess its applicability.

Preparation of temperature-responsive nanomicelles with AIE property as fluorescence probe for detection of Fe3+ and Fe2

Temperature-responsive nanomicelles with aggregation induced emission (AIE) property were prepared by the host-guest complexation of ferrocene functionalized tetraphenyl (TPE-Fc) and β-cyclodextrin-poly (N-isopropylacrylamide) (β-CD-(PNIPAM)₇). The AIE chromophore TPE-Fc bound to the hydrophobic cavity of cyclodextrin serves as the core of micelles, and temperature sensitive PNIPAM serves as the shell to give the micelles good solubility. The size of the nanomicelles is about 100 nm. At the excitation wavelength of 340 nm, the strongest fluorescent emission peak was 421 nm. The introduction of cyclodextrin star polymer increased the fluorescence intensity of nanomicelles, thus improving the recognition of probe to Fe³⁺ and Fe²⁺. The fluorescent probe can quickly detect Fe³⁺ and Fe²⁺ in water within 5 min even in the presence of various interfering ions. The detection limits of Fe³⁺ and Fe²⁺ were 1.04 μM and 0.78 μM, respectively in the range of 10-90 μM. The formation of complex between the probe and Fe³⁺/Fe²⁺ was supported by Job's plot. The probe was successfully applied to the detection of Fe³⁺and Fe²⁺ in actual water sample with a good recovery. In addition, a possible sensing mechanism for the interaction of iron ions with amide bond groups of nanomicelles was proposed.

AIE+ESIPT Active Hydroxybenzothiazole for Intracellular Detection of Cu2+: Anticancer and Anticounterfeiting Applications

Here, in the present work, a new hydroxybenzothiazole derivative (HBT 2) with AIE+ESIPT features was synthesized by Suzuki-Miyora coupling of HBT 1 with 4-formylphenylboronic acid. The AIE and ESIPT features were confirmed by optical, microscopic (AFM) and dynamic light scattering (DLS) techniques. The yellow fluorescent aggregates of HBT 2 can specifically detect Cu²⁺/Cu⁺ ions with limits of detection as low as 250 nM and 69 nM. The Job's plot revealed the formation of a 1:1 complex. The Cu²⁺ complexation was further confirmed by optical, NMR, AFM and DLS techniques. HBT 2 was also used for the detection of Cu²⁺ ions in real water samples collected from different regions of Punjab. HBT 2 was successfully used for the bio-imaging of Cu²⁺ ions in live A549 and its anticancer activity was checked on different cancer cell lines, such as MG63, and HeLa, and normal cell lines such as L929. We successfully utilized HBT 2 to develop security labels for anticounterfeiting applications.

a Versatile Schiff Base Chemosensor for the Determination of Trace Co2+, Ni2+, Cu2+, and Zn2+ in the Water and Its Bioimaging Applications

In this work, a simple and versatile Schiff base chemosensor (L) was developed for the detection of four adjacent row 4 metal ions (Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺) through colorimetric or fluorescent analyses. L could recognize the target ions in solutions containing a wide range of other cations and anions. The recognition mechanisms were verified with a Job's plot, HR-MS assays, and ¹H NMR titration experiments. Then, L was employed to develop colorimetric test strips and TLC plates for Co²⁺. Meanwhile, L was capable of quantitatively measuring the amount of target ions in tap water and river water samples. Notably, L was used for imaging Zn²⁺ in HepG2 cells, zebrafish, and tumor-bearing mice, which demonstrated its potential biological applications. Therefore, L can probably serve as a versatile tool for the detection of the target metal ions in environmental and biological applications.