Recent Advancements and Future Prospects in NBD-Based Fluorescent Chemosensors: Design Strategy, Sensing Mechanism, and Biological Applications

In recent years, the field of Supramolecular Chemistry has witnessed tremendous progress owing to the development of versatile optical sensors for the detection of harmful biological analytes. Nitrobenzoxadiazole (NBD) is one such scaffold that has been exploited as fluorescent probes for selective recognition of harmful analytes and their optical imaging in various cell lines including HeLa, PC3, A549, SMMC-7721, MDA-MB-231, HepG2, MFC-7, etc. The NBD-derived molecular probes are majorly synthesized from the chloro derivative of NBD via nucleophilic aromatic substitution. This general NBD moiety ligation method to nucleophiles has been leveraged to develop various derivatives for sensing analytes. NBD-derived probes are extensively used as optical sensors because of remarkable properties like excellent stability, large Stoke's shift, high efficiency and stability, visible excitation, easy use, low cost, and high quantum yield. This article reviewed NBD-based probes for the years 2017-2023 according to the sensing of analyte(s), including cations, anions, thiols, and small molecules like hydrogen sulfide. The sensing mechanism, designing of the probe, plausible binding mechanism, and biological application of chemosensors are summarized. The real-time application of optical sensors has been discussed by various methods, such as paper strips, molecular logic gates, smartphone detection, development of test kits, etc. This article will update the researchers with the in vivo and in vitro biological applicability of NBD-based molecular probes and challenges the research fraternity to design, propose, and develop better chemosensors in the future possessing commercial utility.

A Colorimetric Distinct Color Change Cu(II) 4-{[1-(2,5-dihydroxyphenyl)ethylidene]amino}-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one Chemosensor and its Application as a Paper Test Kit

In the current research work "4-{[1-(2,5-dihydroxyphenyl)ethylidene]amino}-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one" chemosensor (C1) synthesized by condensation reaction using "4-amino-1,2-dihydro-1,5-dimethyl-2-phenylpyrazol-3-one" and "2,5-dihydroxy actophenone" was used as the effective sensor of metal ion. The C1 shows absorption peak at 326 nm due to the C = C bond (π-π* transition), while the absorption peak at 364 nm is caused by the C = O bond (n-π* transition). In the presence of copper, C1 only demonstrated a redshift in absorption peak from 364 to 425 nm. Even in the presence of other competing metal ions, the hypsochromic shift of the absorption band and the quenching of the fluorescence emission intensity were different for detecting Cu²⁺, in CH₃OH-H₂O (v/v = 6:4). The capacity of the C1 to bind with Cu²⁺ was further proved using DFT simulations. The complex C1 + Cu²⁺ has a HOMO-LUMO energy gap of 2.8002 eV, which is lesser than C1 (2.9991 eV) showing improvement in the stability of the C1 + Cu²⁺ complex. Using the Benesi-Hildebrand and Scatchard plots, calculated Kb values were to be 47,340 and 48369 M^-1 respectively, showing the creation of stable complexation between Cu²⁺ and C1 with 1:1 stoichiometry. The limit of detection (LOD) for Cu²⁺ ion was 649 nM. Strip sheets were also built and tested to detect varying amounts of Cu²⁺ in aqueous solution, and their color change suggested that they might be used for on-site Cu²⁺ detection in polluted water.

Magnesium: Biochemistry, Nutrition, Detection, and Social Impact of Diseases Linked to Its Deficiency

Magnesium plays an important role in many physiological functions. Habitually low intakes of magnesium and in general the deficiency of this micronutrient induce changes in biochemical pathways that can increase the risk of illness and, in particular, chronic degenerative diseases. The assessment of magnesium status is consequently of great importance, however, its evaluation is difficult. The measurement of serum magnesium concentration is the most commonly used and readily available method for assessing magnesium status, even if serum levels have no reliable correlation with total body magnesium levels or concentrations in specific tissues. Therefore, this review offers an overview of recent insights into magnesium from multiple perspectives. Starting from a biochemical point of view, it aims at highlighting the risk due to insufficient uptake (frequently due to the low content of magnesium in the modern western diet), at suggesting strategies to reach the recommended dietary reference values, and at focusing on the importance of detecting physiological or pathological levels of magnesium in various body districts, in order to counteract the social impact of diseases linked to magnesium deficiency.

8-Hydroxyquinoline Fluorophore for Sensing of Metal Ions and Anions

Among various known hydroxyquinolines, 8-hydroxyquinoline (8-HQ) is the most prevalent moiety due to excellent property for the formation of the complex with different metal ions and anions, and utilized in a wide variety of applications in pharmacological and medicinal fields. 8-Hydroxyquinoline moiety and its analogues acts as fluorophoric ligands on complex formation with alkali and alkaline as well as transition metal ions and anions, thus, considered as an ideal building block in metallo-supramolecular chemistry for recognition, separation, and quantitative investigation of cations. 8-Hydroxyquinoline moiety is also used in various applications for the advancement of novel fluorescent chemosensors in a wide variety of areas viz., material chemistry, bioorganic chemistry, molecular imaging, analytical chemistry, molecular recognition, medical and biological science communities. The present review emphasises on the progress of sensing properties of 8-HQ centred small-molecule fluorescent chemosensors towards several metal ions viz., Fe³⁺ , Al³⁺ , Ag⁺ , Hg²⁺ , Cu²⁺ , Pd²⁺ , Zn²⁺ , Cr³⁺ , Cd²⁺ , Mn²⁺ , Ca²⁺ , and K⁺ and anions such as F^- , CN^- and PPi, from 2008 to 2020, because of their sensitivity and selectivity in terms of diverse colour changes for different species. This critical and comprehensive review might facilitate the improvement of more prevailing chemosensors for future exciting and broad applications.

Systematic approach of chromone skeleton for detecting Mg2+, ion: Applications for sustainable cytotoxicity and cell imaging possibilities

The systematic studies of chromone appended novel chemosensors, favored to Mg²⁺ ion detection, these were analyzed and characterized by different spectroscopic techniques such as NMR, mass spectroscopy, FTIR and optical techniques. The binding demeanor of the ligands was executed with the library of metal ions and shown the good coordination with Mg²⁺ ion to the ligand's cavity. Both ligands demonstrated good binding behavior with Mg²⁺ ion. The ligands represented 1: 1 stoichiometry with Mg²⁺ ions through Job's plot. The low limit of detection of Mg²⁺ ion was determined as 2.56 × 10^-6 and 1.28 × 10^-6 for L_a and L_b respectively. No interference was occurred in Inference study by foreign metal ions that supported the specific detection of Mg²⁺ ion among the other metal ions. Further, the cytotoxicity assay test of these chromone appended ligands revealed that both ligands and their respective compound with Mg²⁺ ion shown negligible toxicity with HeLa cancer cell line. Further, due to the fluorescence properties of the ligands, with or without Mg²⁺ ion was successfully tested in bioimaging experiment of HeLa cancer cell lines and found that ligands with Mg²⁺ ions represented good imaging with HeLa cancer cell.

Near-Infrared Fluorescent Probes for Imaging of Intracellular Mg2+ and Application to Multi-Color Imaging of Mg2+, ATP, and Mitochondrial Membrane Potential

The magnesium ion (Mg²⁺) is an essential cation to maintain proper cellular activities. To visualize the dynamics and functions of Mg²⁺, there is a great need for the development of Mg²⁺-selective fluorescent probes. However, conventional Mg²⁺ fluorescent probes are falling behind in low selectivity and poor fluorescence color variation. In this report, to make available a distinct color window for multi-color imaging, we designed and synthesized highly Mg²⁺-selective and near-infrared (NIR) fluorescent probes, the KMG-500 series consisting of a charged β-diketone as a selective binding site for Mg²⁺ and a Si-rhodamine residue as the NIR fluorophore, which showed photoinduced electron transfer (PeT)-type OFF-ON response to the concentration of Mg²⁺. Two types of KMG-500 series probes, tetramethyl substituted Si-rhodamine KMG-501 and tetraethyl substituted Si-rhodamine KMG-502, were synthesized for the evaluation of cell permeability. For intracellular application, the membrane-permeable acetoxymethyl derivative KMG-501 (KMG-501AM) was synthesized and allowed to stably stain cultured rat hippocampal neurons during imaging of intracellular Mg²⁺. On the other hand, KMG-502 was cell membrane permeable without AM modification, preventing the probe from staying inside cells during imaging. KMG-501 distributed mainly in the cytoplasm and partially localized in lysosomes and mitochondria in cultured rat hippocampal neurons. Mg²⁺ increase in response to the FCCP uncoupler inducing depolarization of the mitochondrial inner membrane potential was detected in the KMG-501 stained neurons. For the first time, KMG-501 succeeded in imaging intracellular Mg²⁺ dynamics with NIR fluorescence. Moreover, it allows one to simultaneously visualize changes in Mg²⁺ and ATP concentration and also mitochondrial inner membrane potential and their interactions. This probe is expected to be a strong tool for multi-color imaging of intracellular Mg²⁺.

Overexpression of the mitochondrial Mg channel MRS2 increases total cellular Mg concentration and influences sensitivity to apoptosis

The mechanism of action of the mitochondrial Mg channel MRS2 and its involvement in cell viability remain unclear. Deletion of MRS2 has been reported to abolish Mg influx into mitochondria, to induce functional defects in mitochondrial organelles, and to result in cell death. We evaluated whether MRS2 expression had an impact on total Mg cellular content by inducing the overexpression of MRS2 in HEK-293 cells. We observed a remarkable increase of total intracellular Mg concentration in cells overexpressing MRS2 compared with control cells. In order to investigate whether and in what manner the detected Mg increment was involved in the MRS2 influence on cell viability, we treated MRS2-overexpressing cells with two known apoptotic inducers. We found that cells overexpressing the MRS2 channel became less responsive to these pharmacological insults. Our experimental evidence indicates that the MRS2 channel controls overall intracellular Mg levels, the alteration of which might have a role in the molecular signaling leading to apoptotic cell death.

Cytotoxic Effects of Artemisia annua L. and Pure Artemisinin on the D-17 Canine Osteosarcoma Cell Line

Artemisia annua has been used for centuries in Traditional Chinese Medicine. Although used as an antimalarial drug, its active compound artemisinin and the semisynthetic derivatives have also been investigated for their anticancer properties, with interesting and promising results. The aims of this research were to evaluate (i) the cytotoxicity and the antiproliferative effect of pure artemisinin and a hydroalcoholic extract obtained from A. annua on the D-17 canine osteosarcoma cell line and (ii) the intracellular iron concentration and its correlation with the cytotoxic effects. Both artemisinin and hydroalcoholic extract induced a cytotoxic effect in a dose-dependent manner. Pure artemisinin caused an increase of cells in the S phase, whereas the hydroalcoholic extract induced an evident increase in the G₂/M phase. A significant decrease of iron concentration was measured in D-17 cells treated with pure artemisinin and hydroalcoholic extract compared to untreated cells. In conclusion, although preliminary, the data obtained in this study are indicative of a more potent cytotoxic activity of the hydroalcoholic extract than pure artemisinin, indicating a possible synergistic effect of the phytocomplex and a mechanism of action involving iron and possibly ferroptosis. Considering the similarities between human and canine osteosarcomas, progress in deepening knowledge and improving therapeutic protocols will probably be relevant for both species, in a model of reciprocal translational medicine.

Fluorescence lifetime imaging of intracellular magnesium content in live cells

The first detailed analysis of FLIM applications for Mg cell imaging is presented. We employed the Mg-sensitive fluorescent dye named DCHQ5, a derivative of diaza-18-crown-6 ethers appended with two 8-hydroxyquinoline groups, to perform fluorescence lifetime imaging in control and Mg deprived SaOS-2 live cells, which contain different concentrations of magnesium. We found that the lifetime maps are almost uniform all over the cells and, most relevantly, we showed that the ratio of the amplitude terms is related to the magnesium intracellular concentration.

Magnesium Is a Key Regulator of the Balance between Osteoclast and Osteoblast Differentiation in the Presence of Vitamin D₃

Magnesium (Mg) is crucial for bone health. Low concentrations of Mg inhibit the activity of osteoblasts while promoting that of osteoclasts, with the final result of inducing osteopenia. Conversely, little is known about the effects of high concentrations of extracellular Mg on osteoclasts and osteoblasts. Since the differentiation and activation of these cells is coordinated by vitamin D₃ (VD3), we investigated the effects of high extracellular Mg, as well as its impact on VD3 activity, in these cells. U937 cells were induced to osteoclastic differentiation by VD3 in the presence of supra-physiological concentrations (>1 mM) of extracellular Mg. The effect of high Mg concentrations was also studied in human bone-marrow-derived mesenchymal stem cells (bMSCs) induced to differentiate into osteoblasts by VD3. We demonstrate that high extra-cellular Mg levels potentiate VD3-induced osteoclastic differentiation, while decreasing osteoblastogenesis. We hypothesize that Mg might reprogram VD3 activity on bone remodeling, causing an unbalanced activation of osteoclasts and osteoblasts.

Magnesium Deprivation Potentiates Human Mesenchymal Stem Cell Transcriptional Remodeling

Magnesium plays a pivotal role in energy metabolism and in the control of cell growth. While magnesium deprivation clearly shapes the behavior of normal and neoplastic cells, little is known on the role of this element in cell differentiation. Here we show that magnesium deficiency increases the transcription of multipotency markers and tissue-specific transcription factors in human adipose-derived mesenchymal stem cells exposed to a mixture of natural molecules, i.e., hyaluronic, butyric and retinoid acids, which tunes differentiation. We also demonstrate that magnesium deficiency accelerates the osteogenic differentiation of human bone marrow-derived mesenchymal stem cells. We argue that magnesium deprivation generates a stressful condition that modulates stem cell plasticity and differentiation potential. These studies indicate that it is possible to remodel transcription in mesenchymal stem cells by lowering extracellular magnesium without the need for genetic manipulation, thus offering new hints for regenerative medicine applications.

Single cell versus large population analysis: cell variability in elemental intracellular concentration and distribution

The quantification of elemental concentration in cells is usually performed by analytical assays on large populations missing peculiar but important rare cells. The present article aims at comparing the elemental quantification in single cells and cell population in three different cell types using a new approach for single cells elemental analysis performed at sub-micrometer scale combining X-ray fluorescence microscopy and atomic force microscopy. The attention is focused on the light element Mg, exploiting the opportunity to compare the single cell quantification to the cell population analysis carried out by a highly Mg-selective fluorescent chemosensor. The results show that the single cell analysis reveals the same Mg differences found in large population of the different cell strains studied. However, in one of the cell strains, single cell analysis reveals two cells with an exceptionally high intracellular Mg content compared with the other cells of the same strain. The single cell analysis allows mapping Mg and other light elements in whole cells at sub-micrometer scale. A detailed intensity correlation analysis on the two cells with the highest Mg content reveals that Mg subcellular localization correlates with oxygen in a different fashion with respect the other sister cells of the same strain. Graphical abstract Single cells or large population analysis this is the question!

Systematic approach in Mg2+ ions analysis with a combination of tailored fluorophore design

A systematic study of a series of diaza-18-crown-6 8-hydroxyquinoline (DCHQ) chemosensors, devoted to Mg(II) ion detection, was performed. Functionalization of DCHQ by peripheral substituents allowed the development of novel all-solid-state optodes via inclusion inside PVC-based polymeric films. The influence on the DCHQ-based optode response of the lipophilic sites functionalization and of the nature of the plasticizer, was investigated. Fluorimetric studies on optodes sensitivity towards a number of different metal cations (Ca²⁺, Na⁺, K⁺, Li⁺, Co²⁺, Cd²⁺, Pb²⁺, Cu²⁺, Hg²⁺, Zn²⁺) and NH₄⁺ were carried out. The results demonstrated the suitability of the DCHQ optodes to perform fast monitoring (<10s) of magnesium (II) ions. Emission light signal was sufficiently brilliant to be captured by a low-cost computer webcam. The phenyl-substituted DCHQ-Ph derivative showed the best performance with a wide range for Mg(II) ion determination between 2.7 × 10^-7 and 2.2 × 10^-2 mol/L. It was possible, therefore, to determine the concentrations of Mg(II) in commercial fertilizer samples by DCHQ-Ph-based optodes with acceptable results: recoveries of 96.2-104.9% and relative standard deviation (RSD, n = 6) less than 5%. Moreover, in comparison to single sensors, the use of an array composed of five optodes (the ones showing the best performances in the preliminary tests) has allowed to reduce the RSD of magnesium determination in real samples (down to 3.7% with respect to 5.5% for single optodes) and to achieve a detection limit (estimated by s/n = 3 method) as low as 4.6 × 10^-7 mol/L.

Where is it and how much? Mapping and quantifying elements in single cells

The biological function of a chemical element in cells not only requires the determination of its intracellular quantity, but also the spatial distribution of its concentration. Different strategies can be employed to quantify and map the intracellular concentration of elements in single cells. The assessment of the intracellular elemental concentration, which is the relevant information, requires the measurement of cell volume. This challenging and demanding task requires combining different techniques allowing gathering of both morphological and compositional information on the same cell. Moreover, the need to analyse samples more similar to their natural state requires complex hardware equipment, and supplementary efforts in preparation protocols. Nevertheless, the response to the question: "where is it and how much?" is worth all these efforts. This review aims at providing an insight into the recent and most advanced techniques and strategies for quantifying and mapping chemical elements in single cells. We describe and discuss indirect detection techniques (label based) which make use of fluorescent dyes, and direct ones (label free), such as particle induced X-ray emission, proton backscattering spectrometry, scanning transmission ion spectrometry, nano-secondary ion mass spectrometry, X-ray fluorescence microscopy, complemented by X-ray imaging.

Human CNNM2 is not a Mg(2+) transporter per se

CNNM2 is associated with the regulation of serum Mg concentration, and when mutated, with severe familial hypomagnesemia. The function and cellular localization of CNNM2 and its isomorphs (Iso) remain controversial. The objective of this work was to examine the following: (1) the transcription-responsiveness of CNNM2 to Mg starvation, (2) the cellular localization of Iso1 and Iso2, (3) the ability of Iso1 and Iso2 to transport Mg(2+), and (4) the complex-forming ability and spectra of potential interactors of Iso1 and Iso2. The five main findings are as follows. (1) Mg-starvation induces CNNM2 overexpression that is markedly higher in JVM-13 cells (lymphoblasts) compared with Jurkat cells (T-lymphocytes). (2) Iso1 and Iso2 localize throughout various subcellular compartments in transgenic HEK293 cells overexpressing Iso1 or Iso2. (3) Iso1 and Iso2 do not transport Mg(2+) in an electrogenic or electroneutral mode in transgenic HEK293 cells overexpressing Iso1 or Iso2. (4) Both Iso1 and Iso2 form complexes of a higher molecular order. (5) The spectrum of potential interactors of Iso1 is ten times smaller than that of Iso2. We conclude that sensitivity of CNNM2 expression to extracellular Mg(2+) depletion depends on cell type. Iso1 and Iso2 exhibit a dispersed pattern of cellular distribution; thus, they are not exclusively integral to the cytoplasmic membrane. Iso1 and Iso2 are not Mg(2+) transporters per se. Both isomorphs form protein complexes, and divergent spectra of potential interactors of Iso1 and Iso2 indicate that each isomorph has a distinctive function. CNNM2 is therefore the first ever identified Mg(2+) homeostatic factor without being a Mg(2+) transporter per se.

A highly specific ‘turn-on’ fluorescent detection of Mg2+ through a xanthene based fluorescent molecular probe

Two acid hydrazones incorporating xanthene as a metal chelating centre (B-XAN and N-XAN) have been synthesized, characterized and evaluated for the optical sensing of Mg²⁺ and Al³⁺.

Magnesium homeostasis in colon carcinoma LoVo cells sensitive or resistant to doxorubicin

Neoplastic cells accumulate magnesium, an event which provides selective advantages and is frequently associated with TRPM7 overexpression. Little is known about magnesium homeostasis in drug-resistant cancer cells. Therefore, we used the colon cancer LoVo cell model and compared doxorubicin-resistant to sensitive cells. In resistant cells the concentration of total magnesium is higher while its influx capacity is lower than in sensitive cells. Accordingly, resistant cells express lower amounts of the TRPM6 and 7, both involved in magnesium transport. While decreased TRPM6 levels are due to transcriptional regulation, post-transcriptional events are involved in reducing the amounts of TRPM7. Indeed, the calpain inhibitor calpeptin markedly increases the levels of TRPM7 in resistant cells. In doxorubicin-sensitive cells, silencing TRPM7 shifts the phenotype to one more similar to resistant cells, since in these cells silencing TRPM7 significantly decreases the influx of magnesium, increases its intracellular concentration and increases resistance to doxorubicin. On the other hand, calpain inhibition upregulates TRPM7, decreases intracellular magnesium and enhances the sensitivity to doxorubicin of resistant LoVo cells. We conclude that in LoVo cells drug resistance is associated with alteration of magnesium homeostasis through modulation of TRPM7. Our data suggest that TRPM7 expression may be an additional undisclosed player in chemoresistance.

Solvent-dependent "turn-on" fluorescence chemosensor for Mg(2+) based on combination of C=N isomerization and inhibition of ESIPT mechanisms

A fluorescent chemosensor (L) for Mg(2+) has been synthesized and characterized, which exhibits turn-on fluorescence response for Mg(2+) only in alcohol solvent (methanol or ethanol) with high sensitivity and selectivity. But in both nonpolar and polar solvents (cyclohexane, DCM, DMSO or MeCN), L showed negligible fluorescent response for Mg(2+). In order to discover the unique phenomenon, optical measurements, liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) and a high performance liquid chromatography with a fluorescence detector (HPLC-FLD) of L and L with Mg(2+) ions in solvents were studied. In alcohol solvent, [L+alcohol molecule] was formed and the mechanism aspect of L concerning the remarkable fluorescence response for Mg(2+) has been discussed.

Synthesis and characterization of a Mg2+-selective fluorescent probe

A new Mg²⁺-selective fluorescent probe P was synthesized and characterized. With optimal conditions, the proposed probe P showed good selectivity to Mg²⁺ compared to other common metal ions, and worked in a wide linear range of 5.0 × 10⁻⁷–6.0 × 10⁻⁶ M with a detection limit of 1.7 × 10⁻⁷ M Mg²⁺ in ethanol-water solution (9:1, v/v, 20 mM HEPES, pH = 10.0).