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.

Carbazole-thiophene based fluorescent probe for selective detection of Cu2+ and its live cell imaging

Highly sensitive and specific imaging of copper ion (Cu²⁺) in living cells is essential for better understanding the physiological and metabolic processes. We develop a novel fluorescent probe based on carbazole-thiophene for specific Cu²⁺ detection in living cells. Job's plot and density functional theory (DFT) confirmed a stoichiometric ratio of 2:1 between the probe molecules and Cu²⁺. This probe exhibits strong fluorescence in aqueous media, while its fluorescence intensity significantly decreased in the presence of Cu²⁺. An in vitro assay shows that the fluorescent probe has rapid response within 5 s and high sensitivity for the detection of Cu²⁺ in the range from 1 to 10 μM with a detection limit of 0.29 μM. Live cell studies reveal that the fluorescent probe has good cell-membrane permeability and can successfully visualize the fluctuation of the intracellular Cu²⁺ concentration. In addition, the fluorescent probe has low cytotoxicity, which may provide a new tool for monitoring other analytes in living cells.

Coumarin based hydrazone as an ICT-based fluorescence chemosensor for the detection of Cu2+ ions and the application in HeLa cells

We have constructed a new coumarin based fluorescence probe BENZPYR with ICT character through condensation of N, N-diethylamino-3-acetyl coumarin with 2-hydrazinobenzothiazole. The absorbance and fluorescence spectral characteristics of BENZPYR revealed that the chemosensor can specifically detect for Cu²⁺ ions over other different metal ions and the lowest limit of detection was found in nano molar range. The turn off sensor of BENZPYR is related to chelation enhanced quenching (CHEQ) and intramolecular charge transfer (ICT) processes were serve as excellent fluorescent detection of Cu²⁺ ions in DMF medium. Fluorescence microscopy experiments revealed that probe BENZPYR may have application as a fluorophore to detect the Cu²⁺ in living cells. The simulated DFT analysis of electronic and structural properties and also UV-vis absorption spectra are in well accordance with the experimental UV-vis absorption spectra.

Preparation of Macrometallocycle and Selective Sensor for Copper Ion

Two bis-imidazolium salts 1,8-bis[2'-(N-R-imidazoliumyl)acetylamino]naphthalene chloride (L1H4·Cl2: R = Et; L2H4·Cl2: R = nBu), as well as their four NHC metal complexes [L1H2Ag]Cl (1), [L1Ni] (2), [L2Ni] (3) and [L1H2Hg(HgCl4)] (4) have been synthesized. In each of the cationic moieties of complexes 1 or 4, there is a groove-like 14-membered macrometallocycle, and each macrometallocycle is consisted of one biscarbene ligand L1H2 and one metal ion (silver(I) ion for 1 and mercury(II) ion for 4). Three 6-membered cycles are contained in each molecule of complexes 2 or 3. Additionally, the selective recognition of macrometallocycle 1 for Cu2+ was studied with the methods of fluorescence and ultraviolet spectroscopy, 1H NMR titrations, MS and IR spectra. The experimental results display macrometallocycle 1 can discriminate Cu2+ from other cations effectively.

A Simplified and Turn-on Fluorescence Chemosensor Based on Coumarin Derivative for Detection of Aluminium(III) Ion in Pure Aqueous Solution

A novel chemosensor L based on coumarin Schiff-base was synthesized and investigated. Sensor L showed remarkable selectivity for Al(3+) in Tris-HCl aqueous buffer solution (pH 7.2), and the selectivity was not affected by the presence of a large excess of other competitive ions. The sensor responded rapidly to Al(3+) in aqueous solutions with a 2:1 stoichiometry. Meanwhile, it indicated significant improvement of quantum efficiency and ideal fluorescent lifetime.

Facile rhodamine-based colorimetric sensors for sequential detections of Cu(ii) ions and pyrophosphate (P2O74−) anions

Two rhodamine hydrazine derivatives Rh1 and Rh2 with catechol and ether functionalities have been synthesized and utilized towards sequential colorimetric detections of Cu(ii) and pyrophosphate (PPi) ions in a semi-aqueous medium.

A novel 8-hydroxyquinoline-pyrazole based highly sensitive and selective Al(iii) sensor in a purely aqueous medium with intracellular application: experimental and computational studies

A new 8-hydroxyquinoline-pyrazole based probe senses Al³⁺ and hence Al³⁺–8Q-NH-Pyz complex mediated F⁻ in a purely aqueous medium.

Novel pyrazoline-based fluorescent probe for detecting thiols and its application in cells

A new compound, N-(4-(1,5-diphenyl-4,5-dihydro-1H-pyrazol-3-yl)phenyl)-acrylamide (probe L), was designed and synthesized as a highly sensitive and selective fluorescent probe for recognizing and detecting thiol from other amino acids. On being mixed with thiol in buffered DMSO:HEPES=1:1 solution at pH 7.4, the probe exhibited the blue emission at 474 nm. This probe is very sensitive and displayed a linear fluorescence off-on response to thiol. The fluorescence emission of the probe is pH independent in the physiological pH range. Living cell imaging of HeLa cells confirmed its cell permeability and its ability to selectively detect thiol in cells. The structure of the probe was characterized by IR, NMR and HRMS spectroscopy analysis.

Optical sensor: a promising strategy for environmental and biomedical monitoring of ionic species

In this review, we cover the recent developments in fluorogenic and chromogenic sensors for Cu²⁺, Fe²⁺/Fe³⁺, Zn²⁺and Hg²⁺.

A new probe based on rhodamine B and benzothiazole hydrazine for sensing hypochlorite in living cells and real water samples

We have developed a novel fluorescent probe (RBT) based on rhodamine B and benzothiazole hydrazine units for the detection of hypochlorite in living cells and real water samples with excellent selectivity and sensitivity.

Aminoquinoline based highly sensitive fluorescent sensor for lead(II) and aluminum(III) and its application in live cell imaging

We have synthesized a new probe 5-((anthracen-9-ylmethylene) amino)quinolin-10-ol (ANQ) based on anthracene platform. The probe was tested for its sensing behavior toward heavy metal ions Hg(2+), Pb(2+), light metal Al(3+) ion, alkali, alkaline earth, and transition metal ions by UV-visible and fluorescent techniques in ACN/H2O mixture buffered with HEPES (pH 7.4). It shows high selectivity toward sensing Pb(2+)/Al(3+) metal ions. Importantly, 10-fold and 5- fold fluorescence enhancement at 429 nm was observed for probe upon complexation with Pb(2+) and Al(3+) ions, respectively. This fluorescence enhancement is attributable to the prevention of photoinduced electron transfer. The photonic studies indicate that the probe can be adopted as a sensitive fluorescent chemosensor for Pb(2+) and Al(3+) ions.

Vitamin B6 cofactor based fluorescent probe for sensing an anion (F−) and cation (Co2+) independently in a pure aqueous medium

A new highly selective and sensitive bifunctional fluorescent probe for Co²⁺ and F⁻ ions has been derived from a vitamin B₆ cofactor and the response mechanism has been analyzed using DFT calculations.

A sensitive dual colorimetric and fluorescence system for assaying the activity of alkaline phosphatase that relies on pyrophosphate inhibition of the peroxidase activity of copper ions

A novel and highly sensitive optical assay for the accurate determination of alkaline phosphatase activity is developed by utilizing the peroxidase activity of Cu²⁺ ions.

Triazole based ratiometric fluorescent probe for Zn2+ and its application in bioimaging

An efficient fluorescent chemosensor 4-((2-hydroxynaphthalen-1-yl)methyleneamino)-3-phenyl-1H-1,2,4-triazole-5(4H)-thione, based on triazole has been designed by condensing 2-hydroxy-1-napthaldehyde with amine, appended to 1,2,4-triazole unit. The probe displays excellent selectivity and sensitivity in both absorbance and fluorescence detection of Zn2+ over other essential metal ions. The nature of fluorescence behavior of receptor upon addition of Zn2+ has been obtained from Density Functional Theory calculations. Imaging experiment indicates that probe works effectively for intracellular Zn2+ imaging with good cell permeability and biocompatibility.