Small-Molecule Fluorescent Probes for Binding- and Activity-Based Sensing of Redox-Active Biological Metals

Although transition metals constitute less than 0.1% of the total mass within a human body, they have a substantial impact on fundamental biological processes across all kingdoms of life. Indeed, these nutrients play crucial roles in the physiological functions of enzymes, with the redox properties of many of these metals being essential to their activity. At the same time, imbalances in transition metal pools can be detrimental to health. Modern analytical techniques are helping to illuminate the workings of metal homeostasis at a molecular and atomic level, their spatial localization in real time, and the implications of metal dysregulation in disease pathogenesis. Fluorescence microscopy has proven to be one of the most promising non-invasive methods for studying metal pools in biological samples. The accuracy and sensitivity of bioimaging experiments are predominantly determined by the fluorescent metal-responsive sensor, highlighting the importance of rational probe design for such measurements. This review covers activity- and binding-based fluorescent metal sensors that have been applied to cellular studies. We focus on the essential redox-active metals: iron, copper, manganese, cobalt, chromium, and nickel. We aim to encourage further targeted efforts in developing innovative approaches to understanding the biological chemistry of redox-active metals.

Development in Fluorescent OFF-ON Probes Based on Cu2+ Promoted Hydrolysis Reaction of the Picolinate Moiety

Anions and cations have a key role in our normal life. Cu²⁺ ion is a crucial trace element accountable for the part of several cellular enzymes and proteins, including cytochrome c oxidase, dopamine monooxygenase, Cu/Zn superoxide dismutase, and ceruloplasmin. WHO has found the extreme acceptable level of Cu²⁺ ions in drinking water is up to 2.0 ppm. Excess use of Cu²⁺ ions is associated with various human genetic disorders. Thus, the visualization of Cu²⁺ ions to avoid its toxic effects in chemical and biological systems is significant. In this review we have summarized sensors based on catalytic hydrolysis of picolinate to detect Cu²⁺ ions. The sensors based on hydrolysis of picolinate are very selective as compared to the other sensors for Cu²⁺ ions detection. We have focused on describing the structure, spectral properties, detection limits, and bioimaging model of the sensors.

A highly selective fluorescent probe for the sensing of Cu2+ based on the hydrolysis of a quinoline-2-carboxylate and its application in cell imaging

A highly selective OFF–ON fluorescent probe is developed for the sensing of Cu2+ based on the hydrolysis of a quinoline-2-carboxylate moiety. The probe is weakly fluorescent due to esterification of the phenolic group. Upon treatment with 1 equiv. of Cu2+, the probe exhibits strong fluorescence at 570 nm. The probe also exhibits high selectivity for Cu2+ over other cations with a low detection limit of 0.2 μM, which is sensitive enough to meet the standard of the World Health Organization for Cu2+ in drinking water (30 μM). Moreover, the probe shows a very low cell cytotoxicity, and imaging experiments demonstrate that the probe can be used for the sensing of Cu2+ in living cells.

A new 1,8-naphthalimide-based fluorescent “turn-off” sensor for detecting Cu2+ and sensing mechanisms

In this article, a new “turn-off” fluorescent sensor N- n-butyl-4-{2-[(ethylimino)methyl]phenol}-1,8-naphthalimide (HL) for CuII ions is synthesized, which contains a 1,8-naphthalimide moiety as the fluorophore and a Schiff base as the recognition group. As expected, it exhibits high selectivity and sensitivity for detecting CuII ions over other common metal ions in acetonitrile–2-(4-(2-hydroxyethyl)-1-piperazinyl)-ethanesulfonic acid (HEPES) (1:1 v/v, pH = 7.4) solution. In addition, the fluorescence intensity for HL showed a good linearity with the concentration of CuII ions in the range of 0.5–5.0 μM. The 2:1 binding stoichiometry between HL and CuII ions was established on the basis of combined fluorescence titrations, a Job’s plot, single-crystal X-ray analysis and mass spectrometry. The quenching response of HL toward CuII ions is attributed to the reverse photoinduced electron transfer mechanism. The proposed sensor HL is preliminarily applied to quantify CuII ions in water samples from the Yellow River and tap water.

A simple fluorescent probe for the fast sequential detection of copper and biothiols based on a benzothiazole derivative

A simple benzothiazole fluorescent chemosensor was developed for the fast sequential detection of Cu²⁺ and biothiols through modulating the excited-state intramolecular proton transfer (ESIPT) process. The compound 1 exhibits highly selective and sensitive fluorescence "on-off" recognition to Cu²⁺ with a 1:1 binding stoichiometry by ESIPT hinder. The in situ generated 1-Cu²⁺ complex can serve as an "on-off" fluorescent probe for high selectivity toward biothiols via Cu²⁺ displacement approach, which exerts ESIPT recovery. It is worth pointing out that the 1-Cu²⁺ complex shows faster for cysteins (within 1min) than other biothiols such as homocysteine (25min) and glutathione (25min). Moreover, the compound 1 displays 160nm Stoke-shift for reversibly monitoring Cu²⁺ and biothiols. In addition, the probe is successfully used for fluorescent cellular imaging. This strategy via modulation the ESIPT state has been used for determination of Cu²⁺ and Cys with satisfactory results, which further demonstrates its value of practical applications.

A highly sensitive and fast responsive naphthalimide-based fluorescent probe for Cu2+ and its application

The response of the probe L to Cu²⁺ is reversible and very fast (20 s). L has a low detection limit of 49 nM and was used for imaging of Cu²⁺ in MCF-7 cells with satisfying results. The sensor L can be analyzed with a molecular logic gate.

A turn-on fluorescent chemodosimeter based on detelluration for detecting ferrous iron (Fe2+) in living cells

A turn-on fluorescent probe for the detection of Fe²⁺ is facilely synthesized via a nucleophile substitution reaction. The fluorescent probe, N-butyl-4-phenyltellanyl-1,8-naphthalimide (Naph-Te), shows excellent selectivity to Fe²⁺ in a mixed solution of acetonitrile and phosphate buffer under aerobic conditions. The coexistence of biological abundant metal ions such as Na⁺, K⁺, Ca²⁺ and Mg²⁺ has little effect on the fluorescence signal. This turn-on response is achieved via a redox-involved reaction triggered by Fe²⁺ at neutral pH and room temperature, which removes the heavy-atom effect of the tellurium atom on the naphthalimide fluorophore to afford a fluorescent product (N-butyl-4-hydroxyl-1,8-naphthalimide). The probe has excellent cell membrane permeability and is further applied successfully to monitor supplementary Fe²⁺ in live HL-7702 cells using a laser confocal fluorescence microscope.

Nanomolar detection of Hcy, GSH and Cys in aqueous solution, test paper and living cells

Based on selective cleavage of naphthalimide-based fluorescent probes by biological thiols, a “turn-on” fluorescent probe toward thiols has been developed.

Luminescent probes for the bioimaging of small anionic species in vitro and in vivo

This comprehensive review examines recent developments in the use of fluorescent/luminescent probes for the bioimaging of anionic species. Images in cover art reproduced with permission from ref. 290 and 306.

One single molecule as a multifunctional fluorescent probe for ratiometric sensing of Fe3+, Cr3+ and colorimetric sensing of Cu2+

The reagent Rh-C, incorporating a rhodamine moiety and a coumarin backbone and prepared via click chemistry, was developed as the first single molecule for detecting Cu²⁺, Fe³⁺ and Cr³⁺. Its response to Cu²⁺ in different solutions is visible to the naked eye and it exhibits a ratiometric fluorescence response to Fe³⁺ in methanol and Cr³⁺ in acetonitrile.