In vivo ratiometric Zn2+ imaging in zebrafish larvae using a new visible light excitable fluorescent sensor

Luminescent Metal Complexes as Emerging Tools for Lipid Imaging

Fluorescence microscopy is a key tool in the biological sciences, which finds use as a routine laboratory technique (e.g., epifluorescence microscope) or more advanced confocal, two-photon, and super-resolution applications. Through continued developments in microscopy, and other analytical methods, the importance of lipids as constituents of subcellular organelles, signalling or regulating molecules continues to emerge. The increasing recognition of the importance of lipids to fundamental cell biology (in health and disease) has prompted the development of protocols and techniques to image the distribution of lipids in cells and tissues. A diverse suite of spectroscopic and microscopy tools are continuously being developed and explored to add to the "toolbox" to study lipid biology. A relatively recent breakthrough in this field has been the development and subsequent application of metal-based luminescent complexes for imaging lipids in biological systems. These metal-based compounds appear to offer advantages with respect to their tunability of the photophysical properties, in addition to capabilities centred around selectively targeting specific lipid structures or classes of lipids. The presence of the metal centre also opens the path to alternative imaging modalities that might not be applicable to traditional organic fluorophores. This review examines the current progress and developments in metal-based luminescent complexes to study lipids, in addition to exploring potential new avenues and challenges for the field to take.

Complexes of Zn(II) with a mixed tryptanthrin derivative and curcumin chelating ligands as new promising anticancer agents

In this study, two novel curcumin (H-Cur)-tryptanthrin metal compounds-[Zn(TA)Cl₂], i.e., Zn(TA), and [Zn(TA)(Cur)]Cl, i.e., Zn(TAC)-were synthesized and investigated using 5-(bis-pyridin-2-ylmethyl-amino)-pentanoic acid (6,12-dioxo-6,12-dihydro-indolo[2,1-b]quinazolin-8-yl)-amide (TA) and H-Cur as the targeting and high-activity anticancer chemotherapeutic moieties, respectively. They were then compared with the di-(2-picolyl)amine (PA) Zn(II) complex [Zn(PA)Cl₂], i.e., Zn(PA). When compared with Zn(PA) and cisplatin, the IC₅₀ values of Zn(TA) and Zn(TAC) indicated that the compounds had high cytotoxicity against A549/DDP cancer cells, implying that the H-Cur-tryptanthrin Zn(II) compounds have the potential for use as anticancer drugs. We propose the use of synthesized theragnostic H-Cur-tryptanthrin Zn(II) complexes with nuclear-targeting and DNA-damaging capabilities as a simple therapeutic strategy against tumors. The Zn(TA) and Zn(TAC) complexes could be traced via red fluorescence and were found to accumulate in the cell nuclei and induce DNA damage, cell cycle arrest, mitochondrial dysfunction, and cell apoptosis both in vitro and in vivo. In addition, Zn(TAC) exhibited a higher antiproliferative effect on A549/DDP than Zn(TA) and Zn(PA), which was undoubtedly associated with the key roles of the novel tryptanthrin derivative TA and H-Cur in the Zn(TAC) complex.

Tracking Labile Copper Fluctuation In Vivo/Ex Vivo: Design and Application of a Ratiometric Near-Infrared Fluorophore Derived from 4-Aminostyrene-Conjugated Boron Dipyrromethene

Specimen differences, tissue-dependent background fluorescence and scattering, and deviated specimen position and sensor concentration make optical imaging for labile copper fluctuation in animals questionable, and a signal comparison between specimens is infeasible. We proposed ratiometric optical imaging as an alternative to overcome these disadvantages, and a near-infrared (NIR) ratiometric sensor, BDPS1, was devised therefore by conjugating boron dipyrromethene (BODIPY) with 4-aminostyrene and modifying the 4-amino group as a Cu⁺ chelator. BDPS1 possessed an excitation ratiometric copper-sensing ability to show the ratio of NIR emission (710 nm) upon excitation at 600 nm to that at 660 nm, F_ex600/F_ex660, increasing from 2.8 to 10.7. This sensor displayed still the opposite copper response of its internal charge transfer (ICT; 670 nm) and local (581 nm) emission bands. Ratiometric imaging with this sensor disclosed a higher labile copper region near the nucleus apparatus, and HEK-293T cells were more sensitive to copper incubation than MCF-7 cells. Dual excitation ratiometric imaging with this sensor realized tracking of labile copper fluctuation in mice, and the whole-body imaging found that tail intravenous injection of CUTX-101, a therapeutical agent for Menkes disease, led to a distinct labile copper increase in the upper belly. The ex vivo imaging of the resected viscera of mice revealed that CUTX-101 injection enhanced the labile copper level in the liver, intestine, lung, and gall bladder in sequence, yet the kidney, heart, and spleen showed almost no response. This study indicated that modifying BODIPY as an extended ICT fluorophore, with its electron-donating group being derived as a metal chelator, is an effective design rationale of NIR ratiometric sensors for copper tracking in vivo/ex vivo.

Recent Endeavors on Molecular Imaging for Mapping Metals in Biology

Abstract

Transition metals such as zinc, copper and iron play vital roles in maintaining physiological functions and homeostasis of living systems. Molecular imaging, including two-photon imaging (TPI), bioluminescence imaging (BLI) and photoacoustic imaging (PAI), could act as non-invasive toolkits for capturing dynamic events in living cells, tissues and whole animals. Herein, we review the recent progress in the development of molecular probes for essential transition metals and their biological applications. We emphasize the contributions of metallostasis to health and disease, and discuss the future research directions about how to harness the great potential of metal sensors.

Graphic Abstract

A FRET-based fluorescent Zn2+ sensor: 3D ratiometric imaging, flow cytometric tracking and cisplatin-induced Zn2+ fluctuation monitoring

Monitoring labile Zn²⁺ homeostasis is of great importance for the study of physiological functions of Zn²⁺ in biological systems. Here we report a novel ratiometric fluorescent Zn²⁺ sensor, CPBT, which was constructed based on chelation-induced alteration of FRET efficiency. CPBT was readily cell membrane permeable and showed a slight preferential localization in the endoplasmic reticulum. With this sensor, 3D ratiometric Zn²⁺ imaging was first realized in the head of zebra fish larvae via Z-stack mode. CPBT could track labile Zn²⁺ in a large number of cells through ratiometric flow cytometric assay. More interestingly, both ratiometric fluorescence imaging and flow cytometric assay demonstrated that the labile Zn²⁺ level in MCF-7 cells (cisplatin-sensitive) decreased while that in SKOV3 cells (cisplatin-insensitive) increased after cisplatin treatment, indicating that Zn²⁺ may play an important role in cisplatin induced signaling pathways in these cancer cells.

A Zn²⁺ sensor exhibiting 3D ratiometric imaging and flow cytometric ability was constructed based on the FRET mechanism, and cisplatin-induced endogenous labile Zn²⁺ fluctuations were monitored in real time.

Evaluating the merit of a diethylamino coumarin-derived thiosemicarbazone as an intramolecular charge transfer probe: efficient Zn(ii) mediated emission swing from green to yellow

We report the synthesis and photophysical properties of a coumarin based probe (1E)-1-(1-(7-(diethylamino)-2-oxo-2H-chromen-3-yl) ethylidene) thiosemicarbazide (DIDOT). DIDOT shows a polarity dependent change in the emission maxima in the solution phase. This is explained by the increased dipole moment in the excited state by an intramolecular charge transfer (ICT) process. DIDOT can successfully detect Zn(ii) in aqueous methanol by a shift in the charge transfer emission maxima from approximately 506 to approximately 535 nm. This shift led to a change in the color of the emission from green to yellow under UV-light. The mechanism of Zn(ii) detection has been delineated using electrospray ionization-mass spectrometry (ESI-MS), Fourier-transform infrared spectroscopy (FTIR) and fluorescence time resolved studies coupled with theoretical calculations. The increment in the charge transfer in the Zn(ii) complex of DIDOT over the bare receptor as a consequence of conformational locking was determined to be the underlying cause of the cation detection phenomenon. The limit of detection and binding constant values of DIDOT towards Zn(ii) were approximately 3 × 10^-8 M and 2.35 × 10⁵ M^-1 respectively. Finally, the practical utility of DIDOT has been demonstrated by successful detection and quantification of Zn(ii) in spiked water samples.

A dual functional turn-on non-toxic chemosensor for highly selective and sensitive visual detection of Mg2+ and Zn2+: the solvent-controlled recognition effect and bio-imaging application

A new dual functional turn-on chemosensor, 2,6-diformyl-4-methylphenol-di(isoquinolinyl-1-hydrazone) (HL), has been developed, which could highly selectively discriminate Mg²⁺ and Zn²⁺ in different solvent systems. The chemosensor HL exhibits rapid visual turn-on fluorescence enhancing recognition toward Mg²⁺/Zn²⁺, which is not interfered by other cations, especially for respective congeners Ca²⁺/Cd²⁺. The remarkable fluorescence enhancement (71-fold or 11-fold) was observed after adding Mg²⁺ in acetonitrile or Zn²⁺ in DMF-H₂O solvent systems. Additionally such a solvent medium-controlled platform could achieve the quantitative determination of Mg²⁺ and Zn²⁺ quantitation with low detection limits of 2.97 × 10^-8 M and 3.07 × 10^-7 M, respectively. Furthermore, the turn-on fluorescence sensing mechanism is also investigated by ¹H NMR, FT-IR and ESI-MS spectroscopy. Density functional theory (DFT) calculations derive optimized geometries of HL and its complexes. Notably, non-toxic HL also can be successfully applied as a visual probe for the practical determination of Mg²⁺/Zn²⁺ in MCF-7 cells, Zebrafish larvae, syrup and water samples, which might provide extensive application in biology and medicine fields.

Novel fluorescent probes for relay detection copper/citrate ion and application in cell imaging

Two novel fluorescent probes, 2‑(2'‑hydroxyphenyl)‑4‑(2'‑hydroxymethyl‑8‑quinolinamino)methyloxazole (L1), and 2‑(2'‑hydroxyphenyl)‑4‑(2'‑methyl‑8‑quinolinamino)methyloxazole (L2), exhibited colorimetric and "turn off" fluorometric response to Cu²⁺ ion in DMSO/H₂O solution (v/v = 1/1, 0.01 M, Tris-HCl buffer, pH 7.20) and the corresponding detection limit were found to be 2.14 × 10^-8 and 2.70 × 10^-8 M, which were much lower than drinking water permission concentrations by the United States Environmental Protection Agency (U.S. EPA) and World Health Organization (WHO). The L1-Cu²⁺ and L2-Cu²⁺ complexes ensemble detected citrate anions (CA) sequentially through fluorescence recovery response due to the extrusion of Cu²⁺ ion from the complexes. The binding processes were investigated by UV-vis, fluorescence, IR and DFT calculation. Furthermore, the vivo sensitivity experiments of Cu²⁺ ion and CA was demonstrated through fluorescence imaging in living cells.

A unique benzimidazole-naphthalene hybrid molecule for independent detection of Zn2+ and N3- ions: Experimental and theoretical investigations

Single crystal X-ray structurally characterized benzimidazole-naphthalene hybrid (NABI) functions as a unique dual analyte sensor that can detect Zn²⁺ cation and N₃^- anion independently. The NABI forms chelate with Zn²⁺ to inhibit internal charge transfer (ICT) and CHN isomerisation resulting chelation enhanced fluorescence (CHEF). On the other hand, the sensing of N₃^- is based on formation of supramolecular H-bonded rigid assembly. The association constant of NABI for Zn²⁺ and N₃^- ions are 19 × 10⁴ M^-1 and 11 × 10² M^-1, respectively. Corresponding limit of detections (LOD) are 6.85 × 10^-8 and 1.82 × 10^-7 M, respectively. NABI efficiently detects intracellular Zn²⁺ and N₃^- ions with no cytotoxicity on J774A.1cells under fluorescence microscope. DFT studies unlock underlying spectroscopic properties of free NABI and Zn²⁺/N₃^- bound forms.

A fluorogenic probe based on chelation–hydrolysis-enhancement mechanism for visualizing Zn2+ in Parkinson's disease models

Developing efficient methods for real-time detection of Zn²⁺ level in biological systems is highly relevant to improve our understanding of the role of Zn²⁺ in the progression of Parkinson's disease (PD).

A simple quinolone Schiff-base containing CHEF based fluorescence 'turn-on' chemosensor for distinguishing Zn2+ and Hg2+ with high sensitivity, selectivity and reversibility

A new simple 'dual' chemosensor MQA ((E)-2-methoxy-N-((quinolin-2-yl)methylene)aniline) for distinguishing Zn²⁺ and Hg²⁺ has been designed, synthesized and characterized. The sensor showed excellent selectivity and sensitivity with a fluorescence enhancement to Zn²⁺/Hg²⁺ over other commonly coexisting cations (such as Na⁺, Mg²⁺, Al³⁺, K⁺, Mn²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Ga³⁺, Cd²⁺, In³⁺ and Pb²⁺) in DMSO-H₂O solution (1/99 v/v), which was reversible with the addition of ethylenediaminetetraacetic acid (EDTA). The detection limit for Zn²⁺/Hg²⁺ by MQA both reached the 10^-8 M level. The 1 : 1 ligand-to-metal coordination patterns of the MQA-Zn2+ and MQA-Hg2+ were calculated through a Job's plot and ESI-MS spectra, and were further confirmed by X-ray crystal structures of complexes MQA-Zn2+ and MQA-Hg2+. This chemosensor can recognize similar metal ions by coherently utilizing intramolecular charge transfer (ICT) and different electronic affinities of various metal ions. DFT calculations have revealed that the energy gap between the HOMO and LUMO of MQA has decreased upon coordination with Zn(ii)/Hg(ii).

A biomimetic fluorescent chemosensor for highly sensitive zinc(ii) detection and its application for cell imaging

To fabricate a novel biomimetic fluorescent chemosensor, PSaAEMA-co-PMPC was synthesized via atom transfer radical polymerization, and this copolymer could be used for the detection of zinc(ii) and cell imaging. A series tests with various metal ions verified the specific fluorescence response behavior. This novel biomimetic fluorescent chemosensor exhibits excellent selectivity for Zn²⁺ ions over a wide range of tested metal ions in an aqueous solution. Moreover, cytotoxicity and bio-imaging tests were conducted to study the potential bio-application of the chemosensor. Owing to the biomimetic portion (phosphorylcholine), this copolymer possesses outstanding biocompatibility and could clearly image cells. The results indicated that PSaAEMA-co-PMPC has great potential for application in zinc(ii) detection and cell imaging.

To fabricate a novel biomimetic fluorescent chemosensor, PSaAEMA-co-PMPC was synthesized via atom transfer radical polymerization, and this copolymer could be used for the detection of zinc(ii) and cell imaging.

Studies on a multifunctional chromo-fluorogenic sensor for dual channel recognition of Zn2+ and CN− ions in aqueous media: mimicking multiple molecular logic gates and memory devices

A new effectual naphthalene-based azomethine receptor has been systematically designed and synthesized as a selective colorimetric and fluorescent chemosensor for dual channel detection of cations and anions in aqueous environments.

A zinc fluorescent sensor used to detect mercury (II) and hydrosulfide

A zinc sensor based on quinoline and morpholine has been synthesized. The sensor selectively fluoresces in the presence of Zn²⁺, while not for other metal ions. Absorbance changes in the 350nm region are observed when Zn²⁺ binds, which binds in a 1:1 ratio. The sensor fluoresces due to Zn²⁺ above pH values of 6.0 and in the biological important region. The Zn²⁺-sensor complex has the unique ability to detect both Hg²⁺ and HS^-. The fluorescence of the Zn²⁺-sensor complex is quenched when it is exposed to aqueous solutions of Hg²⁺ with sub-micromolar detection levels for Hg²⁺. The fluorescence of the Zn²⁺-sensor complex is also quenched by aqueous solutions of hydrosulfide. The sensor was used to detect Zn²⁺ and Hg²⁺ in living cells.

A highly selective ratiometric fluorescent probe for the cascade detection of Zn2+ and H2PO4− and its application in living cell imaging

A simple ratiometric sensor (L1) for the cascade detection of Zn²⁺ and H₂PO₄⁻ with high selectivity was reported based on the intermolecular charge transfer (ICT) mechanism.

Near-Infrared Fluorescent Probes with Large Stokes Shifts for Sensing Zn(II) Ions in Living Cells

We report two new near-infrared fluorescent probes based on Rhodol counterpart fluorophore platforms functionalized with dipicolylamine Zn(II)-binding groups. The combinations of the pendant amines and fluorophores provide the probes with an effective three-nitrogen-atom and one-oxygen-atom binding motif. The fluorescent probes with large Stokes shifts offer sensitive and selective florescent responses to Zn(II) ions over other metal ions, allowing a reversible monitoring of Zn(II) concentration changes in living cells, and detecting intracellular Zn(II) ions released from intracellular metalloproteins.

A Colorimetric and Fluorescent Chemosensor for the Selective Detection of Cu2+ and Zn2+ Ions

A new bi-functional chemosensor 1 based on 3,5-dichlorosalicylaldehyde and 2-(methylthio)aniline has been synthesized. It can detect Cu²⁺ with a color change from pale yellow to dark yellow in aqueous solution. The selective mechanism of 1 for Cu²⁺ was proposed to be the enhancement of the intramolecular charge transfer (ICT) band, which was explained by theoretical calculations. The sensor 1 could be used to detect and quantify Cu²⁺ in water samples. In addition, the sensor 1 displayed "turn-on" fluorescence response only to Zn²⁺, based on an effect of chelation-enhanced fluorescence (CHEF). Therefore, 1 can serve as a 'single sensor for two different targets' with dual modes.

ESIPT blocked CHEF based differential dual sensor for Zn2+ and Al3+in a pseudo-aqueous medium with intracellular bio-imaging applications and computational studies

A novel H3SAL-NH probe exhibits ESIPT from the phenolic OH group to the azomethine N atom in the excited state. This is blocked in the presence of Zn²⁺ and Al³⁺ giving turn on fluorescence which is useful for imaging in live HepG2 cells.

A highly selective fluorescent chemosensor based on a quinoline derivative for zinc ions in pure water

A water-soluble fluorescent sensor with a low detection limit could be used to detect and quantify Zn²⁺in water samples.

A new ICT and CHEF based visible light excitable fluorescent probe easily detects in vivo Zn2+

A new chelator and ICT donor based visible light excitable Zn²⁺ sensor was designed and developed by integrating quinoline and 2-hydroxy-3-(hydroxymethyl)-5-methylbenzaldehyde.

Photoluminescence imaging of Zn2+in living systems

Advances in PL imaging techniques, such as confocal microscopy, two photon microscopy, lifetime and optical imaging techniques, have made remarkable contributions in Zn²⁺tracking.

A novel chromo- and fluorogenic dual sensor for Mg2+and Zn2+with cell imaging possibilities and DFT studies

A diformyl-p-cresol-8-aminoquinoline based probe exhibits dual colorimetric and fluorogenic properties on selective binding towards Mg²⁺and Zn²⁺. This probe could be made selective towards Mg²⁺over Zn²⁺in the presence of TPEN.