A Highly Selective Mitochondria-Targeting Fluorescent K+Sensor

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⁺.

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.

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.

Dynameric Frameworks with Aggregation-Induced Emission for Selective Detection of Adenosine Triphosphate

Luminogenic materials with aggregation-induced emission (AIE) have attracted considerable interest for applications. If these systems aggregate, the free rotation of their scaffold is restricted, and as a consequence the photoluminescence increases. Herein, the first experimental observation of a "dynameric effect" on AIE is described. A comparison is made of the AIE of molecular and dynameric sensors that exhibit non-linear turn-on switching of fluorescence upon their interaction with adenosine triphosphate (ATP). Confirmation was obtained from the enhanced ATP detection with multivalent dynameric networks compared with a molecular sensor. The dynamic, reversible behaviour of the imine linkages is critical to produce this enhancement, as a static, imine-reduced, polymeric sensor showed decreased AIE activity. The dynameric frameworks showed selectivity for ATP over adenosine diphosphate, and adenosine monophosphate over guanosine triphosphate or cytidine triphosphate. Together, these results will accelerate the systematic discovery of efficient adaptive biomimetic sensors.

Rosamine with pyronine-pyridinium skeleton: unique mitochondrial targetable structure for fluorescent probes

Rosamine-based probes (1a–b) with pyronine-pyridinium skeleton served as mitochondrial targetable probes for detecting independent species (H₂O₂ and H₂S).

On-Surface Synthesis of Adenine Oligomers via Ullmann Reaction

Despite the fact that DNA bases have been well-studied on surface, the on-surface synthesis of one-dimensional DNA analogs through in situ reactions is still an interesting topic to be investigated. Herein, from the interplay of high-resolution scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations, we have delicately designed a halogenated derivative of adenine as precursor to realize the combination of DNA bases and Ullmann reaction, and then successfully synthesized adenine oligomers on Au(111) via Ullmann coupling. This model system provides a possible bottom-up strategy of fabricating adenine oligomers on surface, which may further give access to man-made DNA strands with multiple bases.

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.

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.