Design of Emission Ratiometric Metal-Ion Sensors with Enhanced Two-Photon Cross Section and Brightness

Mitochondria-targeting two-photon fluorescent probe for sequential recognition of Cu2+ and ATP in neurons and zebrafish

Highly active mitochondria play a significant role in neuron function. Cu2+ and ATP levels in mitochondria regulate neuronal mitochondrial activity. However, mitochondrial activity was often evaluated by mitochondrial membrane potential. Less is known about the dynamics of Cu2+ and ATP in mitochondria. Herein, we developed a two-photon fluorescence probe (MP), which provided a determination of mitochondrial ATP and Cu2+. The fluorescence of MP showed remarkable quenching in the presence of Cu2+ and then gradually recovered in the presence of ATP, which can be used for sequential recognition. MP has high sensitivity to Cu2+ and ATP, with limits of detection (LOD) close to 0.31 nM and 13.6 nM, respectively. Using this useful probe, we monitor the fluctuation of concentrations of Cu2+ and ATP by fluorescence imaging at single neuron and zebrafish.

A sensitive zinc probe operating via enhancement of excited-state intramolecular charge transfer

Novel highly sensitive fluorescent probes for zinc cations based on the diketopyrrolopyrrole scaffold were designed and synthesized. Large bathochromic shifts (≈80 nm) of fluorescence are observed when the Zn²⁺-recognition unit (di-(2-picolyl)amine) is bridged with the fluorophore possessing an additional pyridine unit able to participate in the coordination process. This effect originates from the dipolar architecture and the increasing electron-withdrawing properties of the diketopyrrolopyrrole core upon addition of the cation. The new, greenish-yellow emitting probes, which operate via modulation of intramolecular charge transfer, are very sensitive to the presence of Zn²⁺. Introduction of a morpholine unit in the diketopyrrolopyrrole structure induces a selective six-fold increase of the emission intensity upon zinc coordination. Importantly, the presence of other divalent biologically relevant metal cations has negligible effects and typically even at a 100-fold higher concentration of Mg²⁺/Zn²⁺, the effect is comparable. Computational studies rationalize the strong bathochromic shift upon Zn²⁺-complexation. Decorating the probes with the triphenylphosphonium cation and morpholine unit enables selective localization in the mitochondria and the lysosome of cardiac H9C2 cells, respectively.

Chromis-1, a Ratiometric Fluorescent Probe Optimized for Two-Photon Microscopy Reveals Dynamic Changes in Labile Zn(II) in Differentiating Oligodendrocytes

Despite the significant advantages of two-photon excitation microscopy (TPEM) over traditional confocal fluorescence microscopy in live-cell imaging applications, including reduced phototoxicity and photobleaching, increased depth penetration, and minimized autofluorescence, only a few metal ion-selective fluorescent probes have been designed and optimized specifically for this technique. Building upon a donor-acceptor fluorophore architecture, we developed a membrane-permeant, Zn(II)-selective fluorescent probe, chromis-1, that exhibits a balanced two-photon cross section between its free and Zn(II)-bound form and responds with a large spectral shift suitable for emission-ratiometric imaging. With a Kd of 1.5 nM and wide dynamic range, the probe is well suited for visualizing temporal changes in buffered Zn(II) levels in live cells as demonstrated with mouse fibroblast cell cultures. Moreover, given the importance of zinc in the physiology and pathophysiology of the brain, we employed chromis-1 to monitor cytoplasmic concentrations of labile Zn(II) in oligodendrocytes, an important cellular constituent of the brain, at different stages of development in cell culture. These studies revealed a decrease in probe saturation upon differentiation to mature oligodendrocytes, implying significant changes to cellular zinc homeostasis during maturation with an overall reduction in cellular zinc availability. Optimized for TPEM, chromis-1 is especially well-suited for exploring the role of labile zinc pools in live cells under a broad range of physiological and pathological conditions.

Low-Affinity Zinc Sensor Showing Fluorescence Responses with Minimal Artifacts

The study of the zinc biology requires molecular probes with proper zinc affinity. We developed a low-affinity zinc probe (HBO-ACR) based on an azacrown ether (ACR) and an 2-(2-hydroxyphenyl)benzoxazole (HBO) fluorophore. This probe design imposed positive charge in the vicinity of a zinc coordination center, which enabled fluorescence turn-on responses to high levels of zinc without being affected by the pH and the presence of other transition-metal ions. Steady-state and transient photophysical investigations suggested that such a high tolerance benefits from orchestrated actions of proton-induced nonradiative and zinc-induced radiative control. The zinc bioimaging utility of HBO-ACR has been fully demonstrated with the use of human pancreas epidermoid carcinoma, PANC-1 cells, and rodent hippocampal neurons from cultures and acute brain slices. The results obtained through our studies established the validity of incorporating positively charged ionophores for the creation of low-affinity probes for the visualization of biometals.

Open-Chain Crown-Ether-Derived Two-Photon Fluorescence Probe for Real-Time Dynamic Biopsy of Mercury Ions

A novel two-photon fluorescence probe for Hg2+ derived from bis(styryl)terephthalonitrile, as a two-photon fluorophore, and bis[2-(2-hydroxyethyl sulfanyl) ethyl]amino group (ionophore), as a novel Hg2+ ligand, was developed. The probe possesses small molecule size, large two-photon absorption cross-section (1067 GM) in H2O, non-cytotoxic effect, long wavelength emission at 588 nm, large Stokes shift (121 nm), excellent photostability, high water solubility, good cell permeability, and pH insensitivity in the biologically relevant range. The probe can selectively detect Hg2+ ions in live cells and living tissues without interference from other metal ions and the membrane-bound probes, and its quenching constant is 8.73 × 105 M–1.

Target-Activated Modulation of Dual-Color and Two-Photon Fluorescence of Graphene Quantum Dots for in Vivo Imaging of Hydrogen Peroxide

The development of nanoprobes suitable for two-photon microscopy techniques is highly desirable for mapping biological species in living systems. However, at the current stage, the nanoprobes are restricted to single-color fluorescence changes, making it unsuitable for quantitative detection. To circumvent this problem, we report here a rational design of a dual-emission and two-photon (TP) graphene quantum dot (GQD(420)) probe for imaging of hydrogen peroxide (H2O2). For specific recognition of H2O2 and lighting the fluorescence of TPGQD(420), a boronate ester-functionalized merocyanine (BMC) fluorophore was used as both target-activated trigger and the dual-emission fluorescence modulator. Upon two-photon excitation at 740 nm, TPGQD(420)-BMC displays a green-to-blue resolved emission band in response to H2O2 with an emission shift of 110 nm, and the H2O2 can be determined from 0.2 to 40 μM with a detection limit of 0.05 μM. Moreover, the fluorescence response of the TPGQD(420)-BMC toward H2O2 is rapid and extremely specific. The feasibility of the proposed method is demonstrated by two-photon ratiometrically mapping the production of endogenous H2O2 in living cells as well as in deep tissues of murine mode at 0-600 μm. To the best of our knowledge, this is the first paradigm to rationally design a dual-emission and two-photon nanoprobe via fluorescence modulation of GQDs with switchable molecules, which will extend new possibility to design powerful molecular tools for in vivo bioimaging applications.

New zinc(ii) dyes with enhanced two-photon absorption cross sections based on the imidazolyl ligand

The effective TPA cross sections of Dyes 1–3 revealed that the exceeding TPA behavior may attributed to form the new D–π–A type compound that enhanced the intramolecular ICT progress. Dyes 1–3 were successfully applied in bio-imaging.

A ratiometric two-photon fluorescent probe for hydrazine and its applications

A ratiometric two-photon fluorescent probe TNQ was developed based on quinoline platform to detect hydrazine (N2H4) with high selectivity. TNQ exhibited large two-photon absorption cross sections at 710 nm (250 GM) and excellent ratiometric two-photon fluorescent detection signal for hydrazine. TNQ was also successfully applied to selectively detect hydrazine vapor even at a concentration down to 0.05%. Cell cytotoxicity and bio-imaging studies revealed that probe TNQ was cell-permeable and could be used to detect hydrazine in living cells with low cytotoxicity under two-photon excitation.

Engineering a FRET strategy to achieve a ratiometric two-photon fluorescence response with a large emission shift and its application to fluorescence imaging

Two-photon excitation (TPE) probe-based fluorescence imaging has become one of the most attractive diagnostic techniques to investigate biomolecules and biological events in live cells and tissues. At the current stage most of the TPE-based sensing is reflected by fluorescence intensity changes. Nevertheless the mere altering of intensity could be facilely affected by ambient conditions. On the other hand, TPE probes based on an intramolecular charge transfer (ICT) strategy could solve this problem to some extent with a morphology change-induced emission shift. However their applications are yet constrained due to the inherent limitation of ICT, e.g. the high degree of overlap of two emissions bands and shifts of the TPE maxima. To achieve the desired TPE-based sensing and to circumvent the problems stated above, we adapted a Förster resonance energy transfer (FRET) strategy to develop small molecule ratiometric TPE fluorescent probes. Our FRET-based ratiometric TPE fluorescent probe displays a remarkable emission shift (up to 125 nm) with two well-resolved emission bands. Hence the ratio of these two emission bands could enable the measurement of fluorescence changes more accurately, thus further improving imaging in live cells and deep tissues. To the best of our knowledge, the current reported probe has the largest emission shift among all the small molecule ratiometric TPE fluorescent probes while the maximum TPE wavelength remains unchanged. This work has provided a FRET approach to fabricate novel small molecule ratiometric TPE fluorescent probes that improve imaging in deep tissues.

Design of ratiometric emission probe with visible light excitation for determination of Ca2+ in living cells

An organic salt as a fluorescent probe based on intramolecular charge transfer for Ca(2+) determination is developed. Ca(2+) can be detected by ratiometric emission at 490 and 594 nm with an excitation wavelength of 405 nm. This probe is highly selective for Ca(2+) over other divalent metal cations and displays a large Stokes shift of 189 nm that can avoid interference of the excitation light beam and autofluorescence of biological samples. The dissociation constant for Ca(2+) is 2.25 ± 0.47 μM and pertinent to Ca(2+) detection in cellular resting and dynamic states. The probe demonstrates its application in monitoring Ca(2+) in living cells under confocal microscopic imaging.

Ultra-sensitive and selective Hg2+ chemosensors derived from substituted 8-hydroxyquinoline analogues

A mercury concentration of 15 ppt can be selectively detected, in aqueous medium, thanks to a thiophosphinatylated 8-hydroxyquinoline sensor.

A ratiometric fluorescent molecular probe with enhanced two-photon response upon Zn2+ binding for in vitro and in vivo bioimaging

The ratiometric two-photon (2P) probe GBC shows enhanced 2P activity upon zinc ion binding and has been used for zinc ion imaging in vitro and in vivo.

Zn(II)-coordination modulated ligand photophysical processes - the development of fluorescent indicators for imaging biological Zn(II) ions

Molecular photophysics and metal coordination chemistry are the two fundamental pillars that support the development of fluorescent cation indicators. In this article, we describe how Zn(II)-coordination alters various ligand-centered photophysical processes that are pertinent to developing Zn(II) indicators. The main aim is to show how small organic Zn(II) indicators work under the constraints of specific requirements, including Zn(II) detection range, photophysical requirements such as excitation energy and emission color, temporal and spatial resolutions in a heterogeneous intracellular environment, and fluorescence response selectivity between similar cations such as Zn(II) and Cd(II). In the last section, the biological questions that fluorescent Zn(II) indicators help to answer are described, which have been motivating and challenging this field of research.

Luminescent Fluorene-Based Bis-Pyrazolyl Aniline Ligand for Aluminum Detection

The design, synthesis, and photophysical properties of a new fluorene-based fluorescent chemosensor, 4-((E)-2-(2-(benzo[d]thiazol-2-yl)-9,9-diethyl-9H-fluoren-7-yl)vinyl)-N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)benzenamine (AXF-Al), is described for the detection of Al3+. AXF-Al exhibited absorption at 382 nm and strong fluorescence emission at 542 nm (fluorescence quantum yield, Φ F, of 0.80). The capture of Al3+ by the pyrazolyl aniline receptor resulted in nominal change in the linear absorption (372 nm) but a large hypsochromic shift of 161 nm in the fluorescence spectrum (542 to 433 nm, Φ F = 0.88), from which Al3+ was detected both ratiometrically and colorimetrically. The addition of other metal ions, namely Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg 2+ and Pb2+, produced only minimal changes in the optical properties of this probe. The emission band of this probe was also accessed by two-photon excitation in the near-IR, as two-photon absorption (2PA) is important for potential applications in two-photon fluorescence microscopy (2PFM) imaging. The 2PA cross section of the free fluorenyl ligand AXF-Al was 220 GM at 810 nm and 235 GM at 810 nm for the Al-ligand complex, practically useful properties for 2PFM.

Metal coordination in photoluminescent sensing

Coordination chemistry plays an essential role in the design of photoluminescent probes for metal ions. Metal coordination to organic dyes induces distinct optical responses which signal the presence of metal species of interest. Luminescent lanthanide (Ln(3+)) and transition metal complexes of d(6), d(8) and d(10) configurations often exhibit unique luminescence properties different from organic dyes, such as high quantum yield, large Stokes shift, long emission wavelength and emission lifetimes, low sensitivity to microenvironments, and can be explored as lumophores to construct probes for metal ions, anions and neutral species. In this review, the design principles and coordination chemistry of metal probes based on mechanisms of PeT, PCT, ESIPT, FRET, and excimer formation will be discussed in detail. Particular attention will be given to rationales for the design of turn-on and ratiometric probes. Moreover, phosphorescent probe design based on Ln(3+) and d(6), d(8) and d(10)-metal complexes are also presented via discussing certain factors affecting the phosphorescence of these metal complexes. A survey of the latest progress in photoluminescent probes for identification of essential metal cations in the human body or toxic metal cations in the environment will be presented focusing on their design rationales and sensing behaviors. Metal complex-based photoluminescent probes for biorelated anions such as PPi, and neutral biomolecules ATP, NO, and H(2)S will be discussed also in the context of their metal coordination-related sensing behaviors and design approaches.

11B NMR sensing of d-block metal ions in vitro and in cells based on the carbon-boron bond cleavage of phenylboronic acid-pendant cyclen (cyclen = 1,4,7,10-tetraazacyclododecane)

Noninvasive magnetic resonance imaging (MRI) including the "chemical shift imaging (CSI)" technique based on (1)H NMR signals is a powerful method for the in vivo imaging of intracellular molecules and for monitoring various biological events. However, it has the drawback of low resolution because of background signals from intrinsic water protons. On the other hand, it is assumed that the (11)B NMR signals which can be applied to a CSI technique have certain advantages, since boron is an ultratrace element in animal cells and tissues. In this manuscript, we report on the sensing of biologically indispensable d-block metal cations such as zinc, copper, iron, cobalt, manganese, and nickel based on (11)B NMR signals of simple phenylboronic acid-pendant cyclen (cyclen = 1,4,7,10-tetraazacyclododecane), L(6) and L(7), in aqueous solution at physiological pH. The results indicate that the carbon-boron bond of L(6) is cleaved upon the addition of Zn(2+) and the broad (11)B NMR signal of L(6) at 31 ppm is shifted upfield to 19 ppm, which corresponds to the signal of B(OH)(3). (1)H NMR, X-ray single crystal structure analysis, and UV absorption spectra also provide support for the carbon-boron bond cleavage of ZnL(6). Because the cellular uptake of L(6) was very small, a more cell-membrane permeable ligand containing the boronic acid ester L(7) was synthesized and investigated for the sensing of d-block metal ions using (11)B NMR. Data on (11)B NMR sensing of Zn(2+) in Jurkat T cells using L(7) is also presented.

Balance between fluorescence enhancement and association affinity in fluorescent heteroditopic indicators for imaging zinc ion in living cells

A fluorescent heteroditopic indicator for the zinc(II) ion possesses two different zinc(II) binding sites. The sequential coordination of zinc(II) at the two sites can be transmitted into distinct fluorescence changes. In the heteroditopic ligand system that our group developed, the formations of mono- and dizinc(II) complexes along an increasing gradient of zinc(II) concentration lead to fluorescence enhancement and an emission bathochromic shift, respectively. The extents of these two changes determine the sensitivity and, ultimately, the effectiveness of the heteroditopic indicator in quantifying zinc(II) ion over a large concentration range. In this work, a strategy to increase the degree of fluorescence enhancement upon the formation of the monozinc(II) complex of a heteroditopic ligand under simulated physiological conditions is demonstrated. Fluorination of the pyridyl groups in the pentadentate N,N,N'-tris(pyridylmethyl)ethyleneamino group reduces the apparent pK(a) value of the high-affinity site, which increases the degree of fluorescence enhancement as the monozinc(II) complex is forming. However, fluorination impairs the coordination strength of the high-affinity zinc(II) binding site, which in the triply fluorinated ligand reduces the binding strength to the level of the low-affinity 2,2'-bipyridyl. The potential of the reported ligands in imaging zinc(II) ion in living cells was evaluated. The subcellular localization properties of two ligands in five organelles were characterized. Both benefits and deficiencies of these ligands were revealed, which provides directions for the near future in this line of research.

Zinc homeostasis and signaling in health and diseases: Zinc signaling

The essential trace element zinc (Zn) is widely required in cellular functions, and abnormal Zn homeostasis causes a variety of health problems that include growth retardation, immunodeficiency, hypogonadism, and neuronal and sensory dysfunctions. Zn homeostasis is regulated through Zn transporters, permeable channels, and metallothioneins. Recent studies highlight Zn's dynamic activity and its role as a signaling mediator. Zn acts as an intracellular signaling molecule, capable of communicating between cells, converting extracellular stimuli to intracellular signals, and controlling intracellular events. We have proposed that intracellular Zn signaling falls into two classes, early and late Zn signaling. This review addresses recent findings regarding Zn signaling and its role in physiological processes and pathogenesis.

Molecular and genetic features of zinc transporters in physiology and pathogenesis

Zinc (Zn) is a vital element. It plays indispensable roles in multifarious cellular processes, affecting the expression and activity of a variety of molecules, including transcription factors, enzymes, adapters, channels, growth factors, and their receptors. A disturbance in Zn homeostasis due to Zn deficiency or an excess of Zn absorption can therefore impair the cellular machinery and exert various influences on physiological programs, such as systemic growth, morphogenetic processes, and immune responses, as well as neuro-sensory and endocrine functions. Thus, Zn imbalance becomes pathogenic in humans. Zn homeostasis is controlled by the coordinated actions of Zn transporters, which are responsible for Zn influx and efflux, and intricately regulate the intracellular and extracellular Zn concentration and distribution. In this review, we describe crucial roles of Zn transporters in biological phenomena, focusing in particular on how Zn transporters contribute to cellular events at the molecular, biochemical, and genetic level, with recent progress uncovering the roles of Zn transporters in physiology and pathogenesis.