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

Ionic Transistors

Biological voltage-gated ion channels, which behave as life's transistors, regulate ion transport precisely and selectively through atomic-scale selectivity filters to sustain important life activities. By this inspiration, voltage-adaptable ionic transistors that use ions as signal carriers may provide an alternative information processing unit beyond solid-state electronic devices. This review provides a comprehensive overview of the first generation of biomimetic ionic transistors, including their operating mechanisms, device architecture development, and property characterizations. Despite its infancy, significant progress has been made in the applications of ionic transistors in fields such as DNA detection, drug delivery, and ionic circuits. Challenges and prospects of full exploitation of ionic transistors for a broad spectrum of practical applications are also discussed.

[1,3]-Dioxolo[4,5-f]benzodioxole (DBD) Fluorescent Dyes; Synthesis, Properties, and Applications

In this review we give an overview of the syntheses and photophysical properties of the new class of fluorescent dyes based on a [1,3]-dioxolo[4,5-f]benzodioxole core and their derivatives. Starting from commercially available reactants (e.g., sesamol, 1,2,4,5-tetrachlorobenzene) the core units can be prepared in a simple manner. Then, the benzene core can be derivatized via lithiation and their photophysical properties can be adjusted as desired. The obtained fluorophores have an absorption range of 403–520 nm and an emission range of 495–665 nm. This class of fluorescent dyes is also characterized by a long fluorescence lifetime, a high stability towards photobleaching, large Stokes shifts, and small size. Thus, the DBD dyes are optimally suited for optical sensing.

1 Introduction

2 Synthesis

3 Properties

4 Applications

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.

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.

Synthesis, structure and metal ion coordination of novel benzodiazamacrocyclic ligands bearing pyridyl and picolinate pendant side-arms

Complex formation of benzodiazacrown ethers with heavy and transition metal ions was studied using NMR spectroscopy, potentiometry and X-ray crystallography.

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.