A highly K(+)-selective phenylaza-[18]crown-6-lariat-ether-based fluoroionophore and its application in the sensing of K+ ions with an optical sensor film and in cells

Metal-free introduction of primary sulfonamide into electron-rich aromatics

We report herein a direct and practical synthesis of arylsulfonamides from electron-rich aromatic compounds by using in situ generated N-sulfonylamine as the active electrophile. Substrates include derivatives of aniline, indole, pyrrole, furan, styrene and so on. The reaction proceeds under mild conditions and tolerates many sensitive functional groups such as alkyne, acetate, the trifluoromethoxy group or acetoxymethyl ester. Applications of this method for the construction of metal ion sensors and fluorogenic dye have been demonstrated, thus highlighting the potential of this method for probe development.

Highly Selective and Rapid "Turn-On" Fluorogenic Chemosensor for Detection of Salicylic Acid in Plants and Food Samples

Salicylic acid (SA) is one of the chemical molecules, involved in plant growth and immunity, thereby contributing to the control of pests and pathogens, and even applied in fruit and vegetable preservation. However, only a few tools have ever been designed or executed to understand the physiological processes induced by SA or its function in plant immunity and residue detection in food. Hence, three Rh6G-based fluorogenic chemosensors were synthesized to detect phytohormone SA based on the "OFF-ON" mechanism. The probes showed high selectivity, ultrafast response time (<60 s), and nanomolar detection limit for SA. Moreover, the probe possessed outstanding profiling that can be successfully used for SA imaging of callus and plants. Furthermore, the fluorescence pattern indicated that SA could occur in the distal transport in plants. These remarkable results contribute to improving our understanding of the multiple physiological and pathological processes involved in SA for plant disease diagnosis and for the development of immune activators. In addition, SA detection in some agricultural products used probes to extend the practical application because its use is prohibited in some countries and is harmful to SA-sensitized persons. Interestingly, the as-obtained test paper displayed that SA could be imaged by ultraviolet (UV) and was directly visible to the naked eye. Given the above outcomes, these probes could be used to monitor SA in vitro and in vivo, including, but not limited to, plant biology, food residue detection, and sewage detection.

New nanostructured extracellular potassium ion probe for assay of cellular K+ transport

The concentration of potassium ion is an important indicator for human health, and its abnormality is often accompanied by various diseases. However, most tools currently used to study potassium ion transport are low throughput. Herein, we reported a new K⁺ fluorescent nanoprobe CP1-KS with high selectivity and sensitivity to K⁺ (fluorescence enhanced factor was up to 9.91 at 20 mM K⁺). The polymeric fluorescent probe CP1-KS was composed of the small-molecular K⁺ indicator KS and amphiphilic copolymer CP1. This sensor can be easily and uniformly dispersed in cell culture medium and is suitable for high throughput analysis. To assess the utility of the probe CP1-KS in biological field, this probe was employed as an extracellular fluorescent probe to monitor the efflux of K⁺ from cells (E coli, B. Subtilis 168, Hela and MCF-7 cells) under various stimulation including lysozyme, nigericin, digitonin, and ATP. Results demonstrated that CP1-KS is an effective analysis tool for extracellular K⁺ concentration. We believe that the nanoprobe has great potential in antibacterial drug screening, K⁺ ionophore function, K⁺ channel activity, cell membrane permeability analysis or other K⁺ related field in the future.

Effects of Neutral, Anionic and Cationic Polymer Brushes Grafted from Poly(para-phenylene vinylene) and Poly(para-phenylene ethynylene) on the Polymer’s Photoluminescent Properties

The conformation of a fluorescent polymer, in the solid state or in solution, plays a critical role in the polymer’s fluorescent properties. Thus, grafted side chains on a fluorescent polymer can directly influence its optical properties. In this study, the effect of grafted polymeric side chains on the photoluminescent properties of poly(para-phenylene vinylene) (PPV) and poly(para-phenylene ethynylene) (PPE) were investigated. Low- and high-molecular-weight grafts of neutral poly(n-butyl acrylate), cationic poly(trimethylaminoethyl methacrylate) and anionic poly(sulfopropyl acrylate) were grafted onto PPVs and PPEs, and the effect of the grafting on the graft copolymer’s absorption and emission wavelengths, the fluorescence intensity and the quantum yield were studied. The results indicate that in the case of the ionic grafts, contrary to the expectations, the polymers have a reduced quantum yield. This contrasts with the copolymers with uncharged side chains (PnBA), where a major increase in the quantum yield is seen for the self-quenching conjugated pristine polymers. These results reinforce that the molecular conformation of the polymer in a solid or solution plays a critical role in fluorescent polymers photoluminescent properties.

Probing the flux of mitochondrial potassium using an azacrown-diketopyrrolopyrrole based highly sensitive probe

The diketopyrrolopyrrole bearing an aza-18-crown-6 as a binding unit as well as a PPh₃⁺ group is highly sensitive towards K⁺ and localizes selectively in mitochondria of cardiac H9C2 cells. Fast efflux/influx of mitochondrial K⁺ can be observed upon stimulation with nigericin.

Hydrogel-Based Optical Ion Sensors: Principles and Challenges for Point-of-Care Testing and Environmental Monitoring

Hydrogel is a unique family of biocompatible materials with growing applications in chemical and biological sensors. During the past few decades, various hydrogel-based optical ion sensors have been developed aiming at point-of-care testing and environmental monitoring. In this Perspective, we provide an overview of the research field including topics such as photonic crystals, DNAzyme cross-linked hydrogels, ionophore-based ion sensing hydrogels, and fluoroionophore-based optodes. As the different sensing principles are summarized, each strategy offers its advantages and limitations. In a nutshell, developing optical ion sensing hydrogels is still in the early stage with many opportunities lying ahead, especially with challenges in selectivity, assay time, detection limit, and usability.

Switch-On Diketopyrrolopyrrole-Based Chemosensors for Cations Possessing Lewis Acid Character

For the first time diketopyrrolopyrroles (DPPs) have been synthesized directly from nitriles possessing (aza)crown ethers leading to macrocycle-dye hybrids. Depending on the nature of the linkage between DPP and macrocyclic ring, various coordination effects are found. The strong interaction of the cations possessing Lewis acid character such as Li⁺ , Mg²⁺ and Zn²⁺ with 2-aminopyridin-4-yl-DPPs, leading to a bathochromic shift of both emission and absorption, as well as to strong enhancement of fluorescence was rationalized in terms of strong binding of these cations to the N=C-NR₂ functionality. The same effect has been observed for protonation. Depending on the size and the structure of the macrocyclic ring the complexation of cations by aza-crown ethers plays an important but secondary role. The interaction of Na⁺ and K⁺ with 2-aminopyridin-4-yl-DPPs leads to moderate enhancement of fluorescence due to the aza-crown ethers binding. The very weak fluorescence of DPP bearing 2-dialkylamino-pyridine-4-yl substituents is due to the closely lying T₂ state and the resulting intersystem crossing.

Rational Design of a Polymer-Based Ratiometric K+ Indicator for High-Throughput Monitoring Intracellular K+ Fluctuations

Highly selective fluorescent K⁺ sensors are of great importance for monitoring K⁺ fluctuations in various biological processes. In particular, highly efficient ratiometric K⁺ sensors that can emit in dual wavelengths and facilitate the quantitative determination of K⁺ are highly anticipated. Herein, we present the first polymer-based ratiometric fluorescent K⁺ indicator (PK1) for quantitatively detecting K⁺ in aqueous solutions and high-throughput monitoring K⁺ fluctuations in living cells. PK1 was synthesized by conjugating a small molecular K⁺ probe and a red emission reference dye to a hydrophilic polymer skeleton. The newly synthesized PK1 can form highly stable nanoparticles in aqueous solutions and work in 100% water without the aid of any organic solvents or surfactants. PK1 is sensitive to K⁺ with a fluorescence enhancement of sevenfold after interactions with K⁺ at 1000 mM and inert to other metal ions, physiological pH, or dye concentration vibrations. More importantly, the fluorescence intensity ratio at 572 and 638 nm is linearly correlated with log [K⁺] in the range of 2-500 mM (R² = 0.998), which will facilitate the quantitative detection of K⁺. Practical application of PK1 in detecting different K⁺-rich samples demonstrates its great potential in quantitative detection of K⁺. PK1 can be quickly internalized by live cells and shows no obvious cytotoxicity. We also demonstrate that PK1 could be used for monitoring K⁺ fluctuations under different stimulations by using a confocal microscope and especially a microplate reader, which is high throughput and time saving. The rational design of PK1 will broaden the design concept of ratiometric fluorescent K⁺ sensors and facilitate the quantitative detection of K⁺.

A dual-fluorophore sensor approach for ratiometric fluorescence imaging of potassium in living cells

Potassium is the most abundant intracellular metal in the body, playing vital roles in regulating intracellular fluid volume, nutrient transport, and cell-to-cell communication through nerve and muscle contraction. On the other hand, aberrant alterations in K+ homeostasis contribute to a diverse array of diseases spanning cardiovascular and neurological disorders to diabetes to kidney disease to cancer. There is an unmet need for studies of K+ physiology and pathology owing to the large differences in intracellular versus extracellular K+ concentrations ([K+]intra = 150 mM, [K+]extra = 3-5 mM). With a relative dearth of methods to reliably measure dynamic changes in intracellular K+ in biological specimens that meet the dual challenges of low affinity and high selectivity for K+, particularly over Na+, currently available fluorescent K+ sensors are largely optimized with high-affinity receptors that are more amenable for extracellular K+ detection. We report the design, synthesis, and biological evaluation of Ratiometric Potassium Sensor 1 (RPS-1), a dual-fluorophore sensor that enables ratiometric fluorescence imaging of intracellular potassium in living systems. RPS-1 links a potassium-responsive fluorescent sensor fragment (PS525) with a low-affinity, high-selectivity crown ether receptor for K+ to a potassium-insensitive reference fluorophore (Coumarin 343) as an internal calibration standard through ester bonds. Upon intracellular delivery, esterase-directed cleavage splits these two dyes into separate fragments to enable ratiometric detection of K+. RPS-1 responds to K+ in aqueous buffer with high selectivity over competing metal ions and is sensitive to potassium ions at steady-state intracellular levels and can respond to decreases or increases from that basal set point. Moreover, RPS-1 was applied for comparative screening of K+ pools across a panel of different cancer cell lines, revealing elevations in basal intracellular K+ in metastatic breast cancer cell lines vs. normal breast cells. This work provides a unique chemical tool for the study of intracellular potassium dynamics and a starting point for the design of other ratiometric fluorescent sensors based on two-fluorophore approaches that do not rely on FRET or related energy transfer designs.

Development of a molecular K+ probe for colorimetric/fluorescent/photoacoustic detection of K. Anal Bioanal Chem 2020;412:6947-6957. [PMID: 32712812 DOI: 10.1007/s00216-020-02826-y]

The potassium ion (K⁺) plays significant roles in many biological processes. To date, great efforts have been devoted to the development of K⁺ sensors for colorimetric, fluorescent, and photoacoustic detection of K⁺ separately. However, the development of molecular K⁺ probes for colorimetric detection of urinary K⁺, monitoring K⁺ fluxes in living cells by fluorescence imaging, and photoacoustic imaging of K⁺ dynamics in deep tissues still remains an open challenge. Herein, we report the first molecular K⁺ probe (NK2) for colorimetric, fluorescent, and photoacoustic detection of K⁺. NK2 is composed of 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) as the chromophore and phenylazacrown-6-lariat ether (ACLE) as the K⁺ recognition unit. Predominate features of NK2 include a short synthetic procedure, high K⁺ selectivity, large detection range (5-200 mM), and triple-channel detection manner. NK2 shows good response to K⁺ with obvious color changes, fluorescence enhancements (about threefold), and photoacoustic intensity changes. The existence of other metal ions (including Na⁺, Mg²⁺, Ca²⁺, Fe²⁺) and pH changes (6.5-9.0) have no obvious influence on K⁺ sensing of NK2. Portable test strips stained by NK2 can be used to qualitatively detect urinary K⁺ by color changes for self-diagnosis of diseases induced by high levels of K⁺. NK2 can be utilized to monitor K⁺ fluxes in living cells by fluorescent imaging. We also find its excellent performance in photoacoustic imaging of different K⁺ concentrations in the mouse ear. NK2 is the first molecular K⁺ probe for colorimetric, fluorescent, and photoacoustic detection of K⁺ in urine, in living cells, and in the mouse ear. The development of NK2 will broaden K⁺ probes' design and extend their applications to different fields. Graphical abstract.

Organocatalytic enone-azide [3 + 2]-cycloaddition: synthesis of functionally rich C/N-double vinyl 1,2,3-triazoles

An enolate-mediated organocatalytic [3 + 2]-cycloaddition of enones with less reactive vinyl/alkyl/aryl azides is reported at room temperature for short reaction times. The metal-free amine-mediated catalytic conditions of this [3 + 2]-cycloaddition allowed us to synthesize a collection of C/N-double vinyl-1,2,3-triazoles and C-vinyl-1,2,3-triazoles through functionalized enones as quality azidophiles with various azides. It is an efficient catalytic [3 + 2]-cycloaddition for the synthesis of biologically important fully decorated C/N-double vinyl-1,2,3-triazoles with excellent outcomes with reference to the reaction rate, selectivity, operation simplicity, substrate scope, yields, and synthetic applications as demonstrated in the paper. Herein, we illustrated the importance of enolate reactivity with azides compared to enamines by correlation with previous enamine-mediated click reactions in the reaction mechanism section.

Na+ Selective Fluorescent Tools Based on Fluorescence Intensity Enhancements, Lifetime Changes, and on a Ratiometric Response

Over the years, we developed highly selective fluorescent probes for K+ in water, which show K+ -induced fluorescence intensity enhancements, lifetime changes, or a ratiometric behavior at two emission wavelengths (cf. Scheme 1, K1-K4). In this paper, we introduce selective fluorescent probes for Na+ in water, which also show Na+ induced signal changes, which are analyzed by diverse fluorescence techniques. Initially, we synthesized the fluorescent probes 2, 4, 5, 6 and 10 for a fluorescence analysis by intensity enhancements at one wavelength by varying the Na+ responsive ionophore unit and the fluorophore moiety to adjust different Kd values for an intra- or extracellular Na+ analysis. Thus, we found that 2, 4 and 5 are Na+ selective fluorescent tools, which are able to measure physiologically important Na+ levels at wavelengths higher than 500 nm. Secondly, we developed the fluorescent probes 7 and 8 to analyze precise Na+ levels by fluorescence lifetime changes. Herein, only 8 (Kd =106 mm) is a capable fluorescent tool to measure Na+ levels in blood samples by lifetime changes. Finally, the fluorescent probe 9 was designed to show a Na+ induced ratiometric fluorescence behavior at two emission wavelengths. As desired, 9 (Kd =78 mm) showed a ratiometric fluorescence response towards Na+ ions and is a suitable tool to measure physiologically relevant Na+ levels by the intensity change of two emission wavelengths at 404 nm and 492 nm.

Coumarin-Based Small-Molecule Fluorescent Chemosensors

Coumarins are a very large family of compounds containing the unique 2H-chromen-2-one motif, as it is known according to IUPAC nomenclature. Coumarin derivatives are widely found in nature, especially in plants and are constituents of several essential oils. Up to now, thousands of coumarin derivatives have been isolated from nature or produced by chemists. More recently, the coumarin platform has been widely adopted in the design of small-molecule fluorescent chemosensors because of its excellent biocompatibility, strong and stable fluorescence emission, and good structural flexibility. This scaffold has found wide applications in the development of fluorescent chemosensors in the fields of molecular recognition, molecular imaging, bioorganic chemistry, analytical chemistry, materials chemistry, as well as in the biology and medical science communities. This review focuses on the important progress of coumarin-based small-molecule fluorescent chemosensors during the period of 2012-2018. This comprehensive and critical review may facilitate the development of more powerful fluorescent chemosensors for broad and exciting applications in the future.

Reaction engineering and photophysical studies of fully enriched C-vinyl-1,2,3-triazoles

A library of fluorogenic C-vinyl-1,2,3-triazoles were synthesized in very good yields with excellent selectivity by using an organocatalytic formal [3 + 2]-cycloaddition. One of the coumarin-triazoles 4ba has shown excellent fluorescence properties (λ_em = 533 nm).

Investigation of cation binding and sensing by new crown ether core substituted naphthalene diimide systems

Different modes of cation binding lead to very different optical readouts from two structurally similar sensors.

A Ratiometric Fluorescent Probe for K+ in Water Based on a Phenylaza-18-Crown-6 Lariat Ether

This work presents two molecular fluorescent probes 1 and 2 for the selective determination of physiologically relevant K⁺ levels in water based on a highly K⁺ /Na⁺ selective building block, the o-(2-methoxyethoxy)phenylaza-18-crown-6 lariat ether unit. Fluorescent probe 1 showed a high K⁺ -induced fluorescence enhancement (FE) by a factor of 7.7 of the anthracenic emission and a dissociation constant (K_d ) value of 38 mm in water. Further, for 2+K⁺ , we observed a dual emission behavior at 405 and 505 nm. K⁺ increases the fluorescence intensity of 2 at 405 nm by a factor of approximately 4.6 and K⁺ decreases the fluorescence intensity at 505 nm by a factor of about 4.8. Fluorescent probe 2+K⁺ exhibited a K_d value of approximately 8 mm in Na⁺ -free solutions and in combined K⁺ /Na⁺ solution a similar K_d value of about 9 mm was found, reflecting the high K⁺ /Na⁺ selectivity of 2 in water. Therefore, 2 is a promising fluorescent tool to measure ratiometrically and selectively physiologically relevant K⁺ levels.

Fluorescence chemosensor containing 4-methyl-7-coumarinyloxy, acetylhydrazono and N-phenylaza-15-crown-5 moieties for K+ and Ba2+ ions

A new aza-15-crown-5 derivative 1 bearing coumarin and hydrazone moieties was synthesized and characterized. The fluorescent sensing behavior and selectivity of 1 toward different metal ions in ethanol were investigated. There are 4-fold and 2-fold fluorescence enhancements of 1 at λem=406 nm upon addition of five equivalents of K+ and Ba2+, respectively. The fluorescence titration revealed the stoichiometry of 1:1 for 1-K+1-Ba2+ complexes and the binding constants (lgKs) of 4.11 for 1-K+ and 3.61 for 1-Ba2+. Compound 1 can be used as a fluorescent chemosensor for K+ and Ba2+ through discriminable photoresponse signals. The calculated detection limits for K+ and Ba2+ are 2.28 μm and 5.21 μm, respectively.

A highly selective, colorimetric, and environment-sensitive optical potassium ion sensor

Potassium ions (K⁺) play vital roles in many biological processes and thus highly selective sensors for K⁺ are critical for disease diagnosis and health monitoring. Herein, we report a colorimetric K⁺ sensor (KS7) in which a hemicyanine dye was used as a fluorophore and phenylaza-[18]crown-6 lariat ether (ACLE) was utilized as a K⁺ ligand. The maximum absorption peak of KS7 shifted hypsochromically by 77 nm (from 515 to 438 nm) with an isosbestic point at 452 nm upon the addition of K⁺ to its aqueous solution accompanied by a color change from red to yellow. This sensor exhibited a linear response range to K⁺ from 1 to 200 mM, indicating its wide detection range for cellular, urinary, and environmental potassium ions. Further, this sensor is solvent-sensitive, implying its environmental sensitivity. For the demonstration of its applications, we prepared filter paper-based K⁺ test strips, which were used to detect K⁺ in urine conveniently.

Insights in the determination of saxitoxin with fluorogenic crown ethers in water

Abstract

In this contribution, we present new insights and a critical discussion in the optical detection of saxitoxin using fluorophores with crown ethers. Fluorescence enhancement is caused by the reduction of photoinduced electron transfer upon complexation with the analyte. Our attempts to improve this detection method neither did yield a functioning sensor nor were the attempts to reproduce published data in this area successful. Due to the fact that only low concentrations of saxitoxin are available, multiple surrogates were investigated at high concentrations. However, no turn on response was observed. Moreover, a fluorescent decomposition product of saxitoxin that forms under UV light was discovered which was in our opinion misinterpreted as a sensor response by previous publications.

Graphical abstract

Electronic supplementary material

The online version of this article (10.1007/s00706-017-2074-x) contains supplementary material, which is available to authorized users.

30 s Response Time of K+ Ion-Selective Hydrogels Functionalized with 18-Crown-6 Ether Based on QCM Sensor

Potassium detection is critical in monitoring imbalances in electrolytes and physiological status. The development of rapid and robust potassium sensors is desirable in clinical chemistry and point-of-care applications. In this study, composite supramolecular hydrogels are investigated: polyethylene glycol methacrylate and acrylamide copolymer (P(PEGMA-co-AM)) are functionalized with 18-crown-6 ether by employing surface initiated polymerization. Real-time potassium ion monitoring is realized by combining these compounds with quartz crystal microbalance. The device demonstrates a rapid response time of ≈30 s and a concentration detection range from 0.5 to 7.0 × 10^-3 m. These hydrogels also exhibit high reusability and K⁺ ion selectivity relative to other cations in biofluids such as Na⁺ , NH₄⁺ , Mg²⁺ , and Ca²⁺ . These results provide a new approach for sensing alkali metal ions using P(PEGMA-co-AM) hydrogels.

Highly K+ -Selective Fluorescent Probes for Lifetime Sensing of K+ in Living Cells

The new K⁺ -selective fluorescent probes 1 and 2 were obtained by Cu^I -catalyzed 1,3-dipolar azide alkyne cycloaddition (CuAAC) reactions of an alkyne-substituted [1,3]dioxolo[4,5-f][1,3]benzodioxole (DBD) ester fluorophore with azido-functionalized N-phenylaza-18-crown-6 ether and N-(o-isopropoxy) phenylaza-18-crown-6 ether, respectively. Probes 1 and 2 allow the detection of K⁺ in the presence of Na⁺ in water by fluorescence enhancement (2.2 for 1 at 2000 mm K⁺ and 2.5 for 2 at 160 mm K⁺ ). Fluorescence lifetime measurements in the absence and presence of K⁺ revealed bi-exponential decay kinetics with similar lifetimes, however with different proportions changing the averaged fluorescence decay times (τ_f(av) ). For 1 a decrease of τ_f(av) from 12.4 to 9.3 ns and for 2 an increase from 17.8 to 21.8 ns was observed. Variation of the substituent in ortho position of the aniline unit of the N-phenylaza-18-crown-6 host permits the modulation of the K_d value for a certain K⁺ concentration. For example, substitution of H in 1 by the isopropoxy group (2) decreased the K_d value from >300 mm to 10 mm. 2 was chosen for studying the efflux of K⁺ from human red blood cells (RBC). Upon addition of the Ca²⁺ ionophor ionomycin to a RBC suspension in a buffer containing Ca²⁺ , the fluorescence of 2 slightly rose within 10 min, however, after 120 min a significant increase was observed.

Potassium-Ion-Selective Fluorescent Sensors To Detect Cereulide, the Emetic Toxin of B. cereus, in Food Samples and HeLa Cells

We report the development of new chemical probes for cereulide, a toxic metabolite produced by specific strains of Bacillus cereus, through displacement of potassium cations from a preformed specific complex and a subsequent change in the fluorescence emission. For this purpose, we designed fluorescent probes for potassium cations that were suitable for displacement assays with cereulide from organic extracts. The fluorescence detection of natural cereulide in rice samples was achieved by using synthetic cereulide as a reference and a potassium fluorescent reporter, and this was found to be useful as a portable and fast method for the in situ detection of cereulide in food extracts. To study the fate of cereulide in live cells, we designed a procedure that was suitable for live-cell microscopy imaging of HeLa cells by comparing the cellular location of the potassium fluorogenic probe, which stained intracellular endolysosomes, in the absence and presence of cereulide; we concluded that in the presence of cereulide, the fluorescence of the probe was decreased because of complexation of the potassium ions by cereulide.

Emission enhancement of GFP chromophore in aggregated state via combination of self-restricted effect and supramolecular host–guest complexation

The emission response of GFP chromophore in aggregated state is greatly enhanced more than 100-fold due to the inhibition of conformational motion and the reduction of strong π–π interaction.

Recent progress in the design and applications of fluorescence probes containing crown ethers

Crown ethers, discovered by the winner of the Nobel Prize Charles Pedersen, are cyclic chemical compounds that consist of a ring or multiple rings containing several ether groups that are capable of binding various ions.

A Highly Selective Potassium Sensor for the Detection of Potassium in Living Tissues

The development of highly selective sensors for potassium is of great interest in biology. Two new hydrosoluble potassium sensors (Calix-COU-Alkyne and Calix-COU-Am) based on a calix[4]arene bis(crown-6) and an extended coumarin were synthesized and characterized. The photophysical properties and complexation studies of these compounds have been investigated and show high molar extinction coefficients and high fluorescence quantum yields. Upon complexation with potassium in the millimolar concentration range, an increase of one- and two-photon fluorescence emission is detected. A twofold fluorescence enhancement is observed upon excitation at λ=405 nm. The ligands present excellent selectivity for potassium in the presence of various competitive cations in water and in a physiological medium. The photophysical properties are not affected by the presence of a large amount of competing cations (Na⁺ , Ca²⁺ , Mg²⁺ , etc.). Ex vivo measurements on mouse hippocampal slices show that Calix-COU-Alkyne accumulates extracellularly and does not alter the neuronal activity. Furthermore, the sensor can be utilized to monitor slow extracellular K⁺ increase induced by inhibition of K⁺ entry into the cells.

Fluorescent probes and bioimaging: alkali metals, alkaline earth metals and pH

All living species and life forms have an absolute requirement for bio-functional metals and acid-base equilibrium chemistry owing to the critical roles they play in biological processes. Hence, a great need exists for efficient methods to detect and monitor biometals and acids. In the last few years, great attention has been paid to the development of organic molecule based fluorescent chemosensors. The availability of new synthetic fluorescent probes has made fluorescence microscopy an indispensable tool for tracing biologically important molecules and in the area of clinical diagnostics. This review highlights the recent advances that have been made in the design and bioimaging applications of fluorescent probes for alkali metals and alkaline earth metal cations, including lithium, sodium and potassium, magnesium and calcium, and for pH determination within biological systems.

Protonated Hexaazamacrocycles as Selective K(+) Receptors

Protonated hexaazamacrocycle [M•H](+) is able to detect K(+) ions present at ppb level in methanolic solutions containing 10(-5) M of Na(+) ions. The high sensitivity and selectivity of [M•H](+) for K(+) is ascribed to the favorable energy balance between the K(+) ion desolvation and its coordination to the [M•H](+) macrocycle, which allows the formation of the corresponding adduct before the Coulombic explosion of the ESI-MS nanodroplets.

Optical probes for the detection of protons, and alkali and alkaline earth metal cations

Luminescent sensors and switches continue to play a key role in shaping our understanding of key biochemical processes, assist in the diagnosis of disease and contribute to the design of new drugs and therapies. Similarly, their contribution to the environment cannot be understated as they offer a portable means to undertake field testing for hazardous chemicals and pollutants such as heavy metals. From a physiological perspective, the Group I and II metal ions are among the most important in the periodic table with blood plasma levels of H(+), Na(+) and Ca(2+) being indicators of several possible disease states. In this review, we examine the progress that has been made in the development of luminescent probes for Group I and Group II ions as well as protons. The potential applications of these probes and the mechanism involved in controlling their luminescent response upon analyte binding will also be discussed.

Current developments in fluorescent PET (photoinduced electron transfer) sensors and switches

A fluorophore can be combined with a receptor according to a molecular engineering design in order to yield fluorescent sensing and switching devices.

Novel BODIPY-based fluorescence turn-on sensor for Fe3+ and its bioimaging application in living cells

A novel boron-dipyrromethene (BODIPY) based fluorescence turn-on sensor for detecting Fe(3+) in aqueous media is reported with 23-fold fluorescence enhancement. The sensor is comprised of a combination of BODIPY fluorophore and a new Fe(3+)-recognizing cryptand that exhibits high selectivity, sensitivity, and reversibility toward Fe(3+) detection. Cell imaging studies demonstrate that this sensor is capable of sensing Fe(3+) in living cells.