Review on Recent Advances in Metal Ions Sensing Using Different Fluorescent Probes

Polymerization-induced emission and selective detection to Fe3+/ Fe2+ of triazine-containing polyureas

In this study, four polyureas with triazine moiety (PUAs) were successfully synthesized through the polymerization of triazine-containing diamine and diisocyanate. The intramolecular aggregation of triazine rings and urea groups along the macromolecular backbone gives PUAs a significant polymerization-induced emission (PIE). Among the four PUAs, PUA-LP shows a significant fluorescent emission at 450 nm, compared to non/weak fluorescent 2,4-diamino-6-phenyl-1,3,5-triazine and L-Lysine diisocyanate ethyl ester monomers. Additionally, the external factors such as solution concentration, excitation wavelength, and precipitants also play a crucial role in the fluorescence of PUAs. As expected, PUA-LP can selectively recognize and detect Fe3+/Fe2+ ions even in the presence of 12 other metal ions and 10 anions. The limit of detection of PUA-LP to Fe3+/Fe2+ is as low as 1.02 μM (0.06 mg/L) and 0.86 μM (0.05 mg/L), respectively, and far below 0.3 mg/L of the allowable national standard for drinking water by WHO. Furthermore, the quenching mechanism of Fe3+/Fe2+ to PUA-LP is attributed to static quenching caused by the coordination of Fe3+/Fe2+ ions with a coordination ratio of 2:1. Based on PIE, the fluorescent PUA-LP was made into an observable and portable testing paper for detecting Fe3+/Fe2+ ions. Finally, we measured the recovery rate of the actual tap water samples and compared the performance of PIE-active PUA-LP with the other reported fluorescent probes to Fe3+/Fe2+ ions.

Selective fluorescence sensors for Cu2+ and Hg2+ ions using acridinium-polymer complexes

The photophysical properties of two acridinium derivatives, 9-phenyl-10-methylacridinium (Ph-Arc+) and 9-mesityl-10-methyl acridinium (Mes-Acr+) were investigated in aqueous solutions of poly(methacrylic acids) (PMAA) at various pH values with various polymer to dye (R/D) ratios. In acidic conditions, PMAA effectively turned on the emission of both acridinium derivatives. Micromolar concentrations of copper (II) ions can selectively turn off the emission of dye/PMAA mixtures with a linear response range from 0.50 to 10.0 µM and a detection limit as low as 0.38 µM. In near neutral conditions, Mes-Acr+ in PMAA solutions displayed extremely weak emission. However, trace amounts of Hg2+ ions can instantly turn on the emission with a low detection limit of 43.6 nM and a linear range between 1.0 to 10.0 µM. This analytical method is fast, cost-effective, and environmentally friendly, as it is conducted in 100 % aqueous solution with commercial acridinium derivatives and a biocompatible polymer.

A 'click' based fluorescent probe mimicking the IMPLICATION logic gate for Cu(II) and Pb(II) sensing: DFT and molecular docking studies

'Click' derived 1,2,3-triazole appended scaffolds are intriguing candidates for selective metal ion recognition because of their stereospecificity and efficiency. The presented report uses the 'click' approach to introduce a glyoxal bis-(2-hydroxyanil)-based chemosensor probe (GT) via the CuAAC pathway, which can selectively detect Cu(II) and Pb(II) ions, both of which are among the most hazardous and perturbing environmental pollutants. NMR spectroscopy, IR spectroscopy, and mass spectrometry (LCMS) were used to successfully characterize the synthesized probe. The discerning recognition behaviour of the probe for Cu(II) and Pb(II) ions was established through a chemosensing investigation using fluorescence and UV-vis spectroscopy, wherein the fluorescence spectral analysis demonstrated the probe to mimic the IMPLICATION logic gate. Furthermore, the metal-ligand interaction was also validated by 1H NMR and IR spectroscopy of the synthesized GT-metal complex, and UV-vis spectroscopy was also employed to analyze the effect of time and temperature on the capacity of the probe to bind with Cu(II) and Pb(II) ions. Furthermore, the sensor's atherosclerosis-inhibition potential was investigated in silico utilizing docking analysis with tribbles-1 protein, and a density functional theory (DFT) study enhanced the understanding of its structure using the B3LYP functional and the 6311G++(d,p) basis set.

Development of a Pyrone-Fused Tricyclic Scaffold-based Ratiometric Fluorescent Probe for Al3+ Detection

Aluminum (Al3+) is environmentally abundant and can harm living organisms in various ways, such as by inhibiting root growth, damaging faunal nervous systems, and promoting tumor cell proliferation. However, the dynamics of Al3+ in living organisms are largely unknown; thus, detecting Al3+ in the environment and organisms is crucial. Fluorescent probes are useful tools for the selective detection of metal ions. In particular, ratiometric fluorescent probes exhibit a detection response at two different maximum fluorescence emission wavelengths; which is advantageous for avoiding the influence of background fluorescence. A novel pyrone-fused tricyclic scaffold-based ratiometric fluorescent probe for detecting Al3+, ethyl 11-imino-1-oxo-3-phenyl-1H,11H-pyrano[4,3-b] quinolizine-5-carboxylate (PQ), was developed in this study. The PQ fluorescence blue shifted from 505 to 457 nm upon the addition of Al3+. The blue shift was accompanied by a change in the fluorescence color of the PQ solution from green to blue. Fluorescence titration experiments demonstrated that the fluorescence intensity ratio at the two peaks of interest (457/505 nm) increased in a concentration-dependent manner upon the addition of Al3+. Moreover, this study demonstrated that a PQ-soaked paper displays a visible color change under ultraviolet light upon exposure to Al3+. The above results suggest that PQ is an effective ratiometric probe for the detection of Al3+ in the environment. Future studies will be conducted to introduce various substituents and develop fluorescent probes by leveraging the fluorescence property of a pyrone-fused tricyclic scaffolds.

A Dicyanoisoflurone-based Near-infrared Fluorescence Probe for Highly Sensitive Detection of Hg2

Due to its high toxicity, long durability, easy absorption by aquatic organisms, and significant bioaccumulation, Hg2+ has caused substantial environmental damage and posed serious threats to human health. Therefore, effective detection of Hg2+ is of utmost importance. In this study, a turn-on fluorescent probe based on dicyanoisoflurone was developed for the detection of Hg2+. The probe exhibited near-infrared fluorescence signal at 660 nm upon excitation by 440 nm UV light in a mixture of CH3CN and HEPES buffer (4:1, v/v, 10 mM, pH = 7.5), with selective binding to Hg2+ in a molar ratio of 1:1. This binding event was accompanied by a visible color change from light yellow to orange. By utilizing the enhanced fluorescence signal change, this probe enables highly sensitive analysis and detection of Hg2+ with excellent selectivity (association constant = 1.63 × 104 M- 1), large Stokes shift (220 nm), high sensitivity (detection limit as low as 5.6 nM), short reaction time (30 s), and a physiological pH range of 7.5-9.5. The probe was successfully employed for detecting of Hg2+ in real water and living cells.

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.

Ultrasensitive detection of Ag+and Ce3+ions using highly fluorescent carboxyl-functionalized carbon nitride nanoparticles

The fluorescence quenching of carboxyl-rich g-C3N4nanoparticles was found to be selective to Ag+and Ce3+with a limit of detection as low as 30 pM for Ag+ions. A solid-state thermal polycondensation reaction was used to produce g-C3N4nanoparticles with distinct green fluorescence and high water solubility. Dynamic light scattering indicated an average nanoparticle size of 95 nm. The photoluminescence absorption and emission maxima were centered at 405 nm and 540 nm respectively which resulted in a large Stokes shift. Among different metal ion species, the carboxyl-rich g-C3N4nanoparticles were selective to Ag+and Ce3+ions, as indicated by strong fluorescence quenching and a change in the fluorescence lifetime. The PL sensing of heavy metal ions followed modified Stern-Volmer kinetics, and CNNPs in the presence of Ag+/Ce3+resulted in a higher value ofKapp(8.9 × 104M-1) indicating a more efficient quenching process and stronger interaction between CNNP and mixed ions. Sensing was also demonstrated using commercial filter paper functionalized with g-C3N4nanoparticles, enabling practical on-site applications.

Fast, Selective and Sensitive Fluorescence Detection of Levofloxacin, Fe3+ and Cu2+ Ions in 100% Aqueous Solution Via Their Reciprocal Recognition

The fluorescence detection of ions and pharmaceutical effluents by using organic chemosensors is a valuable surrogate to the currently existing expensive analytical methods. In this regard, the design of multi-functional chemosensors to recognize desirable guests is of utmost importance. In this study, we first show that levofloxacin (LVO) is able to use as a fluorescent chemosensor for the detection of biologically important Cu2+ (turn-off) and Fe3+ (turn-on) ions via independent signal outputs in 100% aqueous buffer solutions. Next, using the reciprocal recognition of LVO and Fe3+ provides a unique emission pattern for the detection of LVO. This approach exhibited a high specificity to LVO among various pharmaceutical samples, namely acetaminophen (AC), azithromycin (AZ), gemifloxacin (GEM) and ciprofloxacin (CIP) and also showed great anti-interference property in urine. The attractive features of this sensing system are availability, easy-to-use, high sensitivity (limit of detection = 18 nM for Cu2+, 22 nM for Fe3+ and 0.12 nM for LVO), rapid response (5 s) with an excellent selectivity.

Advancements in the development of fluorescent chemosensors based on CN bond isomerization/modulation mechanistic approaches

The CN bond isomerization/modulation as a fluorescence signalling mechanism was explored by studying the photophysical properties of conformationally restricted molecules. From the beginning, the CN bond isomerization method has attracted the attention of researchers owing to its simplicity, high selectivity, and sensitivity in fluorescence evaluation. Continuous developments in the field of sensing using CN bond-containing compounds have been achieved via the customization of the isomerization process around the CN bond in numerous ways, and the results were obtained in the form of specific discrete photophysical changes. CN isomerization causes significant fluorescence enhancement in response to detected metal cations and other reactive species (Cys, Hys, ClO-, etc.) straightforwardly and effectively. This review sheds light on the process of CN bond isomerization/modulation as a signalling mechanism depending on fluorescence changes via conformational restriction. In addition, CN bond isomerization-based fluorescent sensors have yet to be well reviewed, although several fluorescent sensors based on this signalling mechanism have been reported. Therefore, CN-based fluorescent sensors are summarized in this review.

A novel isophorone-based fluorescent probe for recognizing Al3+ and its bioimaging in plants

Aluminium ions (Al3+) are widely present in industries and daily life and are closely related to human health and environmental protection. Therefore, it is crucial to detect their concentration. In this paper, a convenient and reliable small molecule fluorescent probe based on a dicyanoisophorone Schiff base and 2-pyridinecarbohydrazide has been developed. The probe is capable of selectively detecting Al3+ with the advantages of near-infrared emission (maximum emission wavelength of 625 nm), good selectivity, high sensitivity (detection limit of 2.18 × 10-7 M) and fast response time (15 s). It has good potential for rapid detection and visual tracking of Al3+ in aqueous solutions and plant bodies.

Portable sensors equipped with smartphones for organophosphorus pesticides detection

Organophosphorus pesticides (OPs) play an important role in agricultural production and the accurate detection of OP residues is essential to ensure food safety. Portable sensors are expected to be a potential device due to their high detection efficiency, easy-to-use processes and low cost. Due to the widespread popularity and powerful capabilities of smartphones, smartphone-based sensing systems have rapidly developed into ideal tools for portable detection, however, a systematic review on the detection of OPs is still lacking. Therefore, a comprehensive overview of sensors equipped with smartphones for OP detection in recent year is provided; this overview includes their sensing signals (colorimetric, fluorescent, chemiluminescent and electrochemical signals), detection mechanism, analysis applications, advantages/disadvantages and perspectives. Moreover, the progress of sensors equipped with smartphones for the detection of OPs in food is thoroughly summarized. This review contributes to food safety and the development of efficient and reliable methods for smartphone-based OPs detection.

Rapid, Sensitive, and Selective "ON-OFF" Detection of Fe3+ Ions Using Novel Acetalophanes and Their Applications in Real Samples

Three novel acetalophanes 1a-c have been designed, synthesized and characterized. The receptors 1b-c, featuring bulky anthracene groups, displayed significant selectivity for Fe3+ ions, resulting in a turn-off fluorescence mode in a DMF-buffer solution. Conversely, the non-steric probe 1a could serve as a versatile sensor for the simultaneous detection of Fe3+ and Cu2+ ions in MeOH-buffer solution. The sensing mechanism for the capability of 1a was demonstrated to be different, as evidenced by the addition of cyanide ions. The probes with Fe3+ exhibited a sensing mechanism that resulted in the deprotection of acetals to the corresponding starting materials, as confirmed by 1H NMR, IR spectra and TLC analysis. The attractive features of these practical and efficient sensors are selectivity, sensitivity (limit of detection = 0.15 µM by 1a, 0.16 µM by 1b and 0.14 µM by 1c), rapid response (less than 5 s). The on-site monitoring of various real samples, including well water, apricot, and green tea, proved to be successful for the quantitative and cost-effective detection of Fe3+. The method demonstrated good precision, even in the presence of other interfering materials.

Extraction-less green spectrofluorimetric method for determination of mercury using an Astra Phloxine fluorophore: Comprehensive experimental and theoretical studies

A new sensitive and selective extraction-less method for the spectrofluorimetric determination of Hg2+ has been developed using the polymethine dye Astra Phloxine (AP) with requirements for green analytical chemistry. The principle of this method for analytical use is the formation of an ion associate (IA) in the presence of mercury ions and AP, which causes a change in the electronic structure of the dye cation and its subsequent fluorescence quenching. The IA created during the process was sufficiently stable in aqueous solutions and did not require the use of surfactants or organic solvents, which are typically used for similar analytical systems. The system indicated high selectivity towards Hg2+ in the presence of at least 100-fold higher concentrations of Co2+, Fe2+, Mg2+, Ni2+, Zn2+, Ca2+, Ba2+, K+, Na+. The calibration curve was linear in the range from 0.003 to 0.1 mg L-1. Precision and accuracy were expressed as intra-day and inter-day analysis with RSD values of 2.9 - 4.8 % and recovery of 97.1 - 110.0 %. The proposed method was used for the determination of Hg2+ on the model and real water samples. Theoretical calculations were used to predict the IA structure and to explain the experimentally observed fluorescence spectra. A few theoretical methods were tested for predicting IA emission energies, and the CIS(D) method was found to be the most accurate.

Molecular recognition of carbonate ion using a novel turn-on fluorescent probe

A novel turn-on fluorescent probe 3 was synthesized by condensing salicylaldehyde and nicotinic hydrazide for the selective detection of CO32- in aqueous medium. Probe 3 exhibited a turn-on fluorescence response toward CO32- with excellent selectivity, sensitivity (DL = 2.76 μM), and good reversibility. The binding constant (K) of probe 3 with CO32- was calculated to be 5 × 103 M-1 (log K 3.69). The 1:1 stoichiometry of the complex between probe 3 and CO32- ions was confirmed by Job's plot and ESI-MS spectra. Deprotonation and hydrogen-bonding interactions are involved in the recognition of CO32- ion, which was also suggested by 1H NMR, ESI-MS spectra, and Density Functional Theory (DFT) calculations. Moreover, an INHIBIT type molecular logic gate was constructed by using 3:CO32- and CH3COOH as inputs and current signal as output. Owing to the practical applications, probe 3 demonstrated its efficiency in quantifying CO32- ion in real water samples through standard addition method, thus showcasing its potential in real environment. Further, the MTT assay indicated very low cytotoxicity (IC50 = 1 mM) of probe 3 and also the cell imaging experiments demonstrated the effective sensing of CO32- ions with probe 3 in the biological systems.

Acylhydrazone Functionalized Triphenylamine-Based Fluorescent Probe for Cu2+: Tunable Structures of Conjugated Bridge and Its Practical Application

Novel fluorescent probes were constructed for the convenient and rapid analysis of Cu2+ ions, taking advantages of the the triphenylamine backbone as chromophore and acylhydrazone group as the Cu2+ recognition site. Especially, probe T2 could act as a dual-channel probe towards Cu2+ through both fluorescent and colorimetric method. Through the fluorescent method, the detection limit of probe T2 was calculated to be as low as 90 nmol/L and there was a good linear relationship between the intensity change and the concentration of Cu2+ ions. By virtue of the two-phase liquid-liquid extraction method, probe T2 could be successfully applied in practical extraction and separation of Cu2+. Furthermore, by applying a "turn-off-turn-on" circle, compound T2 could act as a sensitive probe towards S2- anions through the indirect approach and the detection limit of complex T2-Cu2+ for S2- anion was found to be 110 nmol/L.

An Eco-Friendly, Interference, and Solvent Free Surfactant-Assisted Dual-Wavelength β-CorrectionSpectrometric Method for Total Determination and Speciation of Cu2+ Ions in Water

Spectral interference through the presence of uninformative variables, excess reagents, and complications in the refinement of the analyte signal is common in the quest to identify complex species in real samples. Therefore, an economical green, facile, and sensitive strategy has been developed for Cu2+ detection using the anionic surfactant sodium dodecylsulphate- (SDS-) assisted dual-wavelength β-correction spectrophotometric strategy combined with the chromogenic reagent zincon (ZI). The low limits of detection (LOD) and quantification (LOQ) of Cu2+ using ordinary (single wavelength) spectrophotometry were 0.19 (3.02) and 0.63 (10.0) μgmL-1, and these values were improved to 0.08 (1.27) and 0.26 μgmL-1 (4.12 μM)) using β-correction (dual wavelength) spectrophotometry, respectively. The LOD and LOQ were improved from 0.08 (1.27) and 0.26 (4.12) μgmL-1 to 0.02 (0.32) and 0.08 μgmL-1 (1.27 μM) using SDS-assisted dual-β-correction spectrometry, respectively. Ringbom, s, and the corrected absorbance (Ac) versus Cu2+ concentration plots were linear over the concentration range 1.10-2.4 (17.4-38.1) and 0.50-2.40 μgmL-1 (7.94-38.1 μM), respectively. Sandell's sensitivity index of 3.0 × 10-3 μg/cm2 was achieved. The selectivity was further confirmed via monitoring the impact of common diverse ions and surfactants on the corrected absorbance. Total determination and Cu2+ speciation in water were favorably implemented and validated by ICP-OES at 95% (P=0.05). Satisfactory Cu2+ recoveries in tap (92.2-98.0%) and mineral (105-111.0%) water samples were achieved. The sensing system is simple, reliable, sensitive, and selective for Cu2+ detection.

Pyrene-acylhydrazone-based Turn-on Fluorescent Probe for Highly Sensitive Detection Cu2+ and Application in Bioimaging

The development of highly selective and sensitive, low detection limits, and biocompatible turn-on copper ion fluorescent probes is of great significance for the environment and life sciences. In this study, a novel turn-on fluorescent probe T based on pyrene-acylhydrazone was synthesized via an efficient one-step condensation reaction and characterized by 1H NMR, 13C NMR and HRMS. The probe T exhibited high selectivity with a low detection limit of 0.304 nM towards Cu2+ in DMSO/H2O (v/v = 1 : 1) medium by a PET-TICT dual interplaying sensing mechanisms. Job's plot analysis and HRMS data confirmed the 1 : 1 binding stoichiometry between T and Cu2+ with an association constant of 5.7×103 M-1. Additionally, the binding model was investigated by 1H NMR titration and FT-IR spectra. Furthermore, probe T exhibits low cellular toxicity and excellent membrane permeability, and has been successfully applied for fluorescent imaging of copper ions in live HT-22 cells.

Pyrene-Based Cationic Fluorophores with High Affinity for BF4-, PF6-, and ClO4- Anions: Detection and Removal

Anions play an indispensable role in the balance and regulation of the ecological environment and human health; however, excess anions can cause serious ecological and environment problems. Therefore, the detection and removal of excess anions in aqueous solution is not only a technological problem but also crucial for environmental protection. Herein, a set of water-soluble pyrene-based cationic fluorophores were synthesized, which exhibit high sensitivity for the detection of the anions BF4-, PF6-, and ClO4- via electrostatic interactions. Such fluorescent probes exhibit "turn-on" emission characteristics even at low concentrations of anions due to anion-π+ interactions. Moreover, these fluorescence probes act as efficient precipitating agents for the removal of the BF4-, PF6-, and ClO4- anions from an aqueous environment. This work opens up new avenues for future research on pyrene-based fluorophores as turn-on fluorescence probes for anion detection and as excellent precipitating agents in environmental settings.

Benzil Bis-Hydrazone Based Fluorescence 'Turn-on' Sensor for Highly Sensitive and Selective Detection of Zn(II) Ions

In this study, a novel Benzil Bis-Hydrazone (BBH) sensor with two C = N-N = C moieties was designed and synthesized based on the condensation reaction between benzil-dihydrazone (b) and cinnamaldehyde. The BBH probe in dimethylsulfoxide showed extremely weak fluorescence. However, the same solution exhibited an intensive fluorescence enhancement (152-fold) with the introduction of Zn(II) ions. In contrast, no or negligible fluorescence changes were observed when other ions were added. The fluorogenic behavior of BBH towards the examined cations indicated an excellent selectivity of the BBH sensor for Zn(II) cations without any interference from other cations like Fe(II), Mg(II), Cu(II), Co(II), Mn(II), Cr(III), Hg(II), Sn(II), Al(I), La(III), Ca(II), Ba(II), Na(I), K(I), and especially Cd(II). Moreover, the UV-vis spectrophotometric titrations revealed that a 1:1 stoichiometric complex BBH-Zn(II) was formed during the Zn(II) sensing and the binding constant value for this complex was calculated to be equal to 106.8. Further, in order to show the affinity of the BBH sensor for Zn(II) cations, it was deemed necessary to determine the limit of detection (LOD) which was found to equal to 2.5 10-4 M.

Colorimetric Chemosensor for Cu2+ and Fe3+ Based on a meso-Triphenylamine-BODIPY Derivative

Optical chemosensors are a practical tool for the detection and quantification of important analytes in biological and environmental fields, such as Cu2+ and Fe3+. To the best of our knowledge, a BODIPY derivative capable of detecting Cu2+ and Fe3+ simultaneously through a colorimetric response has not yet been described in the literature. In this work, a meso-triphenylamine-BODIPY derivative is reported for the highly selective detection of Cu2+ and Fe3+. In the preliminary chemosensing study, this compound showed a significant color change from yellow to blue-green in the presence of Cu2+ and Fe3+. With only one equivalent of cation, a change in the absorption band of the compound and the appearance of a new band around 700 nm were observed. Furthermore, only 10 equivalents of Cu2+/Fe3+ were needed to reach the absorption plateau in the UV-visible titrations. Compound 1 showed excellent sensitivity toward Cu2+ and Fe3+ detection, with LODs of 0.63 µM and 1.06 µM, respectively. The binding constant calculation indicated a strong complexation between compound 1 and Cu2+/Fe3+ ions. The 1H and 19F NMR titrations showed that an increasing concentration of cations induced a broadening and shifting of the aromatic region peaks, as well as the disappearance of the original fluorine peaks of the BODIPY core, which suggests that the ligand-metal (1:2) interaction may occur through the triphenylamino group and the BODIPY core.

Investigation on non-radioactive behavior of an acylhydrazone-based fluorescent probe: Coexistence of PET and TICT mechanisms

A fluorescent probe (E)-((2,4-dihydroxybenzyl)diazenyl)(pyridin-2-yl)methanone (HL) to effectively and selectively detect Al³⁺ was designed and synthesized in the experiment. Herein, we explained the excited state dynamics mechanism of HL by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The potential energy surfaces (PESs) proved that the excited-state intramolecular proton transfer (ESIPT) process hardly occurs due to the high reaction barriers, so the fluorescence quenching behavior of HL was not based on ESIPT. The frontier molecular orbitals (FMOs) and spectral properties were analyzed to better understand the origination of fluorescence quenching. It was found that an electron on C = N in HL could be transferred to the fluorophore during excitation in the absence of Al³⁺, accompanied by the PET process. The excited state could undergo a twisted intramolecular charge transfer (TICT) process, releasing non-radiative decay. After binding to Al³⁺, the photo-induced electron transfer (PET) process has no longer occurred, and the TICT process is eliminated, resulting in a significant fluorescence enhancement. Therefore, the calculation results well explain the quenching and enhancement behaviors of fluorescence before and after the reaction with Al³⁺.

A Highly Selective and Sensitive Sequential Recognition Probe Zn2+ and H2PO4- Based on Chiral Thiourea Schiff Base

A series of novel chiral thiourea fluorescent probes HL₁-HL₆ were designed and synthesized from (1R,2R)-1,2-diphenylethylenediamine, phenyl isothiocyanate, and different substituted salicylic aldehydes. All of the compounds were confirmed by ¹H NMR, ¹³C NMR, and HRMS. They exhibit high selectivity and sensitivity to Zn²⁺ in the presence of nitrate ions with the detection limit of 2.3 × 10^-8 M (HL₅). Meanwhile, their zinc (II) complexes (L-ZnNO₃) showed continuous response to H₂PO₄^- in acetonitrile solution. The identification processes could further be verified by supramolecular chemistry data analysis, X-ray single-crystal diffraction analysis, and theoretical study. The research provides reliable evidence for an explanation of the mechanism of action of thiourea involved in coordination, which is important for the application of thiourea fluorescent probes. In short, the sensors HL₁-HL₆ based on chiral thiourea Schiff base will be promising detection devices for Zn²⁺ and H₂PO₄^-.

Exploration of spectroscopic, computational, fluorescence turn-off mechanism, molecular docking and in silico studies of pyridine derivative

The present work reports pyridine-based chalones using spectroscopic techniques to use pyridine derivative analysis. The solvatochromic behavior of 3DPP in non-polar, polar protic and aprotic solvents has been investigated experimentally. The photophysical property of the compound in diverse solvents is attributed to the intra-molecular charge transfer interactions. The dipole moment of 3DPP is estimated theoretically and experimentally using various solvatochromic methods. It is observed that there is a bathochromic shift in the emission spectra of 3DPP, which confirms the π → π* transition. Fluorescence quenching of 3DPP is studied. The type of fluorescence quenching mechanism is found to be collisional quenching. A study of FRET theory on 3DPP was carried out with metal ions. There is a considerable energy transfer between 3DPP and metal ions. NLO behaviors of the compound have been revealed with the help of Kurtz-Perry powder technique. Additionally, the title molecule is docked, carried ADMET studies and drug-like activity using in silico tools. It is probed for antifungal activity through bioinformatics kit which showed potential information.

Molecular probes for fluorescent sensing of metal ions in non-mammalian organisms

While metal ions play an important role in the proper functioning of all life, many questions remain unanswered about exactly how different metals contribute to health and disease. The development of fluorescent probes, which respond to metals, has allowed greater understanding of the cellular location, concentration and speciation of metals in living systems, giving a new appreciation of their function. While the focus of studies using these fluorescent tools has largely been on mammalian organisms, there has been relatively little application of these powerful tools to other organisms. In this review, we highlight recent examples of molecular fluorophores, which have been applied to sensing metals in non-mammalian organisms.

Controlling the Redox Catalytic Activity of a Cyclic Selenide Fused to 18-Crown-6 by the Conformational Transition Induced by Coordination to an Alkali Metal Ion

trans-3,4-Dihydroxyselenolane (DHS), a water-soluble cyclic selenide, exhibits selenoenzyme-like unique redox activities through reversible oxidation to the corresponding selenoxide. Previously, we demonstrated that DHS can be applied as an antioxidant against lipid peroxidation and a radioprotector by means of adequate modifications of the two hydroxy (OH) groups. Herein, we synthesized new DHS derivatives with a crown-ether ring fused to the OH groups (DHS-crown-n (n = 4 to 7), 1-4) and investigated their behaviors of complex formation with various alkali metal salts. According to the X-ray structure analysis, it was found that the two oxygen atoms of DHS change the directions from diaxial to diequatorial by complexation. The similar conformational transition was also observed in solution NMR experiments. The ¹H NMR titration in CD₃OD further confirmed that DHS-crown-6 (3) forms stable 1:1 complexes with KI, RbCl and CsCl, while it forms a 2:1 complex with KBPh₄. The results suggested that the 1:1 complex (3·MX) exchanges the metal ion with metal-free 3 through the formation of the 2:1 complex. The redox catalytic activity of 3 was evaluated using a selenoenzyme model reaction between H₂O₂ and dithiothreitol. The activity was significantly reduced in the presence of KCl due to the complex formation. Thus, the redox catalytic activity of DHS could be controlled by the conformational transition induced by coordination to an alkali metal ion.

A Selective and ''Off-On'' Fluorescent Chemosensor Based on Fluorescein for Al3+: Synthesis, Characterization, Spectroscopy Analyses, and DFT Calculation

An efficient fluorescent cation chemosensor based on fluorescein L4 was well prepared and identified with spectroscopy analyses. UV-vis and fluorescence measurements examined the analyte complexation of the L4 with various cations, demonstrating a clear tendency to Al³⁺ ion. In the Job plot study, a stoichiometry ratio of a complex between L4 and Al³⁺ ion was determined to be 1: 2 (L4: Al³⁺). A stoichiometry ratio of complex between L4 and Al³⁺ ion was determined to be 1: 2 (L4: Al³⁺) using the Job plot. The association constant (K_a) of the L4-Al³⁺ complex was found 2.8 × 10⁷ M^-2. The obtained limit of detection (LOD) value (1.37 × 10^-6 M for Al³⁺) exhibited the considerable sensitivity of the chemosensor L4 to Al³⁺ ion. DFT/TD-DFT calculations have also been employed to support the binding mode and photophysical properties of the complexation of chemosensor L4 to Al³⁺ ion and also to investigate the enhancement of L4 fluorescence by Al³⁺ ion.

Spatiotemporal Imaging of Zinc Ions in Zebrafish Live Brain Tissue Enabled by Fluorescent Bionanoprobes

The zebrafish is a powerful model organism to study the mechanisms governing transition metal ions within whole brain tissue. Zinc is one of the most abundant metal ions in the brain, playing a critical pathophysiological role in neurodegenerative diseases. The homeostasis of free, ionic zinc (Zn²⁺) is a key intersection point in many of these diseases, including Alzheimer's disease and Parkinson's disease. A Zn²⁺ imbalance can eventuate several disturbances that may lead to the development of neurodegenerative changes. Therefore, compact, reliable approaches that allow the optical detection of Zn²⁺ across the whole brain would contribute to our current understanding of the mechanisms that underlie neurological disease pathology. We developed an engineered fluorescence protein-based nanoprobe that can spatially and temporally resolve Zn²⁺ in living zebrafish brain tissue. The self-assembled engineered fluorescence protein on gold nanoparticles was shown to be confined to defined locations within the brain tissue, enabling site specific studies, compared to fluorescent protein-based molecular tools, which diffuse throughout the brain tissue. Two-photon excitation microscopy confirmed the physical and photometrical stability of these nanoprobes in living zebrafish (Danio rerio) brain tissue, while the addition of Zn²⁺ quenched the nanoprobe fluorescence. Combining orthogonal sensing methods with our engineered nanoprobes will enable the study of imbalances in homeostatic Zn²⁺ regulation. The proposed bionanoprobe system offers a versatile platform to couple metal ion specific linkers and contribute to the understanding of neurological diseases.

A novel water-soluble flavonol-based fluorescent probe for highly specific and sensitive detection of Al3+ and its application in onion and zebrafish

A novel and simple turn-on fluorescence probe (HD) for Al³⁺ detection was successfully developed based on flavonol derivatives. This probe exhibited a significantly enhanced fluorescence response toward Al³⁺ in aqueous solution which could be observed by naked-eye from poor fluorescence to strong light green emission. The probe HD displays highly specific detection for Al³⁺ over other competitive metal ions, and the detection limit of probe HD for Al³⁺ was determined to be 2.57 × 10^-8 M, which are much lower than the World Health Organization (WHO) guideline value for drinking food/water. The binding stoichiometry of probe HD with Al³⁺ was determined to be 1:1 according to Job's plot and ESI-HRMS analysis, and the binding constant was calculated to be 2.01 × 10⁴ M^-1. The probe HD exhibited high selectivity, high sensitivity, good anti-interface ability, and wide pH application range as well as the quantitative determination in the detection of Al³⁺. The coordination mechanism of probe HD with Al³⁺ was supported by density functional theory (DFT) calculations and HRMS analysis. In addition, the probe HD was found to have good cell permeability and could be applied for live-cell imaging to detect Al³⁺ in onions and zebrafish.

Real scenario of metal ion sensor: is conjugated polymer helpful to detect hazardous metal ion

Metal ions from natural and anthropogenic sources cause pollution to society and the environment is major concern in the present scenario. The deposition and contamination of metal ions in soil and water affect the biogeochemical cycles. Thus, it threatens the everyday life of living and non-living organisms. Reviews on the detection of metal ions through several techniques (Analytical methods, electrochemical techniques, and sensors) and materials (Nanoparticles, carbon dots (quantum dots), polymers, chiral molecules, metal-organic framework, carbon nanotubes, etc.) are addressed separately in the present literature. This review reveals the advantages and disadvantages of the techniques and materials for metal ion sensing with crucial factors. Furthermore, it focus on the capability of conjugated polymers (CPs) as metal ion sensors able to detect/sense hazardous metal ions from environmental samples. Six different routes can synthesize this type of CPs to get specific properties and better metal ion detecting capability in vast research areas. The metal ion detection by CP is time-independent, simple, and low cost compared to other materials/techniques. This review outlines recent literature on the conjugated polymer for cation, anion, and dual ion sensors. Over the last half decades published articles on the conjugated polymer are discussed and compared.

Simple Fluorescence Sensing Approach for Selective Detection of Fe3+ Ions: Live-Cell Imaging and Logic Gate Functioning

A pyrene-based fluorescent chemosensor APSB [N-(pyrene-1-ylmethylene) anthracen-2-amine] was designed and developed by a simple condensation reaction between pyrene carboxaldehyde and 2-aminoanthracene. The APSB fluorescent sensor selectively binds Fe³⁺ in the presence of other metal ions. Apart from this, APSB shows high selectivity and sensitivity toward Fe³⁺ ion detection. The detection limit for APSB was 1.95 nM, and the binding constant (K _b) was obtained as 8.20 × 10⁵ M^-1 in DMSO/water (95/5, v/v) medium. The fluorescence quantum yields for APSB and APSB-Fe³⁺ were calculated as 0.035 and 0.573, respectively. The function of this fluorescent sensor APSB can be explained through the photo-induced electron transfer mechanism which was further proved by density functional theory studies. Finally, a live-cell image study of APSB in HeLa cells was also carried out to investigate the cell permeability of APSB and its efficiency for selective detection of Fe³⁺ in living cells.

Turn-On Fluorescence Chemical Sensing through Transformation of Self-Trapped Exciton States at Room Temperature

Most of the current fluorescence sensing materials belong to the turn-off type, which make it hard to detect toxic substances such as benzene, toluene, and xylene (BTX) due to the lack of active chemical sites, thereby limiting their development and practical use. Herein, we show a guest-host mechanism stemming from the confined emitter's self-trapped exciton (STE) states or electron-phonon coupling to achieve turn-on fluorescence. We designed a luminescent guest@metal-organic framework (LG@MOF) composite material, termed perylene@MIL-68(In), and established its E-type excimeric emission properties in the solid state. Upon exposure to BTX, especially xylene, we show that the E-excimer readily converts into the Y-excimer due to nanoconfinement of the MOF structure. Such a transformation elevates the fluorescence intensity, thus realizing a turn-on type fluorescent sensor for detecting BTX solvents. Our results further demonstrate that controlling the STE states of perylene at room temperature (vs the previous report of <50 K) is possible via nanoscale confinement, paving the way to enabling turn-on type luminescent sensors for engineering practical applications.

Phosphorescent Ir(III) Complexes for Biolabeling and Biosensing

Cyclometalated Ir(III) complexes exhibit strong phosphorescence emission with lifetime of submicroseconds to several microseconds at room temperature. Their synthetic versatility enables broad control of physical properties, such as charge and lipophilicity, as well as emission colors. These favorable properties have motivated the use of Ir(III) complexes in luminescent bioimaging applications. This review examines the recent progress in the development of phosphorescent biolabels and sensors based on Ir(III) complexes. It begins with a brief introduction about the basic principles of the syntheses and photophysical processes of cyclometalated Ir(III) complexes. Focus is placed on illustrating the broad imaging utility of Ir(III) complexes. Phosphorescent labels illuminating intracellular organelles, including mitochondria, lysosomes, and cell membranes, are summarized. Ir(III) complexes capable of visualization of tumor spheroids and parasites are also introduced. Facile chemical modification of the cyclometalating ligands endows the Ir(III) complexes with strong sensing ability. Sensors of temperature, pH, CO₂, metal ions, anions, biosulfur species, reactive oxygen species, peptides, and viscosity have recently been added to the molecular imaging tools. This diverse utility demonstrates the potential of phosphorescent Ir(III) complexes toward bioimaging applications.

Highly stable and recoverable humidity sensor using fluorescent quantum dot film

Fluorescent sensors are resistant to electromagnetic interference and are electrically insulated, allowing for highly accurate measurements. Quantum dots (QDs) serve as outstanding sensing materials owing to the unique optical properties such as tunable photoluminescence (PL), excellent visible light activity, and high chemical and physical stability. In this paper, we develop an optical humidity sensor based on a QDs nanocomposite film. The film is made of polyvinyl alcohol (PVA), SiO₂ microsphere (SM), and QDs through the layer-by-layer self-assembly method. The mechanism of humidity detection is moisture-induced quenching of the QDs fluorescence intensity. The results reveal that our sensor shows a good linear response to relative humidity in the range of 5% to 97%, a fast response-recovery time of 25 s and 20 s, and good repeatability for more than 50 cycles as well as high stability for over 180 days. Possessing the remarkable property, optical humidity sensors are envisaged for great potential applications in environmental monitoring.

Syntheses of novel pyridine-based low-molecular-weight luminogens possessing aggregation-induced emission enhancement (AIEE) properties

Novel pyridine-based fluorescing compounds, viz. pyrido[1,2-a]pyrrolo[3,4-d]pyrimidines 3a,b and N-methyl-4-((pyridin-2-yl)amino)maleimides 4a–e, were selectively prepared by a one-pot reaction between a functionalized maleimide and 2-aminopyridines with electron-donating or electron-withdrawing groups at position 5 and were investigated photophysically and computationally. The photophysical studies revealed that all the synthesized compounds exhibited fluorescence in organic solvents, while N-methyl-4-((pyridin-2-yl)amino)-substituted maleimide derivatives 4a–e, which are based on an acceptor–donor–acceptor (A–D–A) system, exhibited aggregation-induced emission enhancement (AIEE) properties in aqueous media. Compounds 4a and 4e, bearing electron-withdrawing groups (Br and CF₃, respectively) showed 7.0 and 15 times fluorescence enhancement. Time-dependent density functional theory (TD-DFT) calculations were performed to gain better insight into the electronic nature of the compounds with and without AIEE properties.

Optical Chemical Sensing of Iodide Ions: A Comprehensive Review for the Synthetic Strategies of Iodide Sensing Probes, Challenges, and Future Aspects

Among several anions, iodide (I^- ) ions play a crucial role in human biological activities. In it's molecular form (I₂ ), iodine is utilized for several industrial applications such as syntheses of medicines, fabric dyes, food additives, solar cell electrolytes, catalysts, and agrochemicals. The excess or deficiency of I^- ions in the human body and environmental samples have certain consequences. Therefore, the selective and sensitive detection of I^- ions in the human body and environment is vital for monitoring their overall profile. Amongst various analytical techniques for the estimation of I^- ions, optical-chemical sensing possesses the merits of high sensitivity, selectivity, and utilizing the least amount of sensing materials. The distinctive aims of this manuscript are (i) To comprehensively review the development of optical chemical sensors (fluorescent & colorimetric) reported between 2001-2021 using organic fluorescent molecules, supramolecular materials, conjugated polymers, and metal-organic frameworks (MOFs). (ii) To illustrate the design and synthetic strategies to create specific binding and high affinity of I^- ions which could help minimize negative consequences associated with its large size and high polarizability. (iii) The challenges associated with sensitivity and selectivity of I^- ions in aqueous and real samples. The probable future aspects concerning the optical chemical detection of I^- ions have also been discussed in detail.

A New Cationic Fluorescent Probe for HSO3- Based on Bisulfite Induced Aggregation Self-Assembly

In comparison with the numerous studies that have centered on developing molecular frameworks for the functionalization of fluorescent materials, less research has addressed the influence of the side chains, despite such appendages contributing significantly to the properties and applications of fluorescent materials. In this work, a new series of cationic fluorescent probes with AIE characteristics have been developed, which exhibit unique sensitivity for charge-diffusion anions, namely HSO₃⁻, via the interactions of ions and the cooperation of the controllable hydrophobicity. The impact of the alkyl chain length attached at the cationic probes suggested that the fluorescent intensity and sensitivity of the probes could be partially enhanced by adjusting their aggregation tendency through the action of the hydrophobic effect under aqueous conditions. DLS and SEM images indicated that different particle sizes and new morphologies of the probes were formed in the anion-recognition-triggered self-assembly process, which could be attributed to the composite effect of electrostatic actions, Van der Waals forces and π-π stacking.

Green-synthesized, pH-stable and biocompatible carbon nanosensor for Fe3+: An experimental and computational study

Brightly fluorescent Carbon Dots (CDs) were synthesized by green hydrothermal method using commonly available biomass (Aloe vera) as carbon precursor. Their physiochemical and optical characterization was done by standard microscopic and spectroscopic techniques. Photophysical features of their aqueous dispersion were investigated in detail. The influence of wide pH range (2–12), high ionic load (2M) and temperature on their photoluminescence behavior was investigated. Their in-vitro cytotoxicity examination was conducted on Human Cervical Cancer Cells (HeLa) using MTT assay. Testing of their ion-recognition property for common metal ions was done in aqueous medium. These CDs exhibited preferential interaction with Fe³⁺ over other tested metal ions, without any functionalization. Interaction between CDs and Fe³⁺ was analyzed in the light of Density Functional Theory (DFT). The work demonstrates that these CDs are acting as nanoprobe for Fe³⁺ and sensing it at ultra-trace level (5 nM).

Recent development in fluorescent probes for copper ion detection

Abstract:

Copper is the third most common heavy metal and an indispensable component of life. Variations of body copper levels, both structural and cellular, are related to a number of disorders; consequently, pathophysiological importance of copper ions demands the development of sensitivity and selective for detecting these organisms in biological systems. In recent years, the area of fluorescent sensors for detecting copper metal ions has seen revolutionary advances. Consequently, closely related fields have raised awareness of several diseases linked to copper fluctuations. Further developments in this field of analysis could pave the way for new and innovative treatments to combat these diseases. This review reports on recent progress in the advancement of three fields of fluorescent probes; chemodosimeters, near IR fluorescent probes, and ratiometric fluorescent probes. Methods used to develop these fluorescent probes and the mechanisms that govern their reaction to specific analytes and their applications in studying biological systems, are also given.

Carbon dots with polarity-tunable characteristics for the selective detection of sodium copper chlorophyllin and copper ions

Compared to water-soluble carbon dots (CDs) the properties and applications of hydrophobic CDs are rarely addressed. In this study, a one-pot, simple chemical oxidation approach has been applied to synthesize hydrophobic carbon dots (TO-CDs) at room temperature from triolein (TO) in concentrated sulfuric acid solution. Sodium copper chlorophyllin (SCC) quenches the fluorescence of TO-CDs by a photoinduced electron transfer process. Upon excitation at 400 nm, the fluorescence intensity of TO-CDs probe at 500 nm shows a linear response against the SCC concentration ranging from 1.0 to 10 μM, with a limit of detection (LOD) of 0.61 μM. Quantitation of SCC in flavored drinks shows percentage recovery (%R) vaues of 98-103% and relative standard deviation (RSD) values less than 6.5%. The hydrophobic TO-CDs can be converted into hydrophilic TO-CDs through hydrolysis in NaOH solution. The presence of sulfonyl groups on the hydrophilic TO-CDs enhances the coordination ability of the CDs toward Cu²⁺ ions, leading to fluorescence quenching which allows for the detection of Cu²⁺ ions with LOD of 0.21 μM and a linear range of 0.5-10 μM. The hydrophilic TO-CD probe possesses high selectivity toward Cu²⁺ ions (tolerance at least ten-fold comparative to other metal ions). The assay has been validated with the analysis of spiked soil samples, with %R values of Cu concentration of 97.8-99.0% and RSDs below 2.0%. The surface tunable CD probes demonstrate their potential for the rapid screening of Cu²⁺ ions in environmental samples and SCC in foods.

A Simple Colorimetric Assay for Sensitive Cu2+ Detection Based on the Glutathione-Mediated Etching of MnO2 Nanosheets

In this paper, we developed a quick, economical and sensitive colorimetric strategy for copper ions (Cu²⁺) quantification via the redox response of MnO₂ nanosheets with glutathione (GSH). This reaction consumed MnO₂ nanosheets, which acted as a catalyst for the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to a blue product (oxTMB). In the presence of Cu²⁺, the GSH was catalyzed to GSSG (oxidized glutathione), and the solution changed from colorless to deep blue. Under the optimum conditions, the absorption signal of the oxidized product (oxTMB) became proportional to Cu²⁺ concentration in the range from 10 to 300 nM with a detection limit of 6.9 nM. This detection system showed high specificity for Cu²⁺. Moreover, the system has been efficaciously implemented for Cu²⁺ detection in actual tap water samples. The layered-nanostructures of MnO₂ nanosheets make it possess high chemical and thermal stability. TMB can be quickly oxidized within 10 min by the catalyzing of MnO₂ nanosheets with high oxidase-like activity. There is no need of expensive reagents, additional H₂O₂ and complicated modification processes during the colorimetric assay. Therefore, the strategy primarily based on MnO₂ nanosheets is promising for real-time, rapid and highly sensitive detection of Cu²⁺ under practical conditions.