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.

A Reaction-based Ratiometric Fluorescent Probe with Large STOKES Shift for Cu2+ in Neat Aqueous Solution

A novel reaction-based ratiometric fluorescent probe 1 for Cu2+ using picolinate as the reaction site and hemicyanine as the fluorophore was developed. 1 displayed maximum absorption peak at 355 nm and fluorescence emission peak at 500 nm, with large Stokes shift of 145 nm. Upon reaction with Cu2+, the maximum absorption and fluorescence emission peaks red-shifted to 390 nm and 570 nm respectively, owing to Cu2+-induced hydrolysis of the picolinate moiety in 1. Meanwhile, the solution of 1 turned from green to orange under a 365 nm UV lamp. 1 not only could detect Cu2+ ratiometrically by the ratios of both absorbance (A390 nm/A355 nm) and fluorescence intensity (F570 nm/F500 nm), but also displayed large Stokes shift, fast response, high sensitivity and excellent selectivity over other metal ions in neat aqueous solution.

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 fluorescence "turn-on" probe for Cu (Ⅱ) based on flavonoid intermediates generated by copper-induced oxidative cyclization and its fluorescence imaging in living cells

A fluorescence "turn-on" probe for Cu (Ⅱ) ions was prepared based on the condensation reaction of coumaraldehyde and 1-hydroxy-2-acetylnaphthalene. A strong fluorescent flavonoid intermediate was formed and verified by the NMR and ESI-MS experiments. The water-soluble and pH dependence experiments were performed to confirm the optimal solvent condition (CH₃CN: HEPES = 1:1, v/v, pH = 7.2-7.4). The dynamic experiments indicated that the formation process of the intermediate catalyzed by Cu(Ⅱ) ions was probably pseudo-first-order reaction process. The probe showed good selectivity toward copper ions and almost no interference except Ag⁺ ions by the selectivity and competitive experiments. The HeLa cells were used in the cell fluorescence imaging tests and it was demonstrated that the probe could be used in the phycological condition and showed weak cytotoxicity by the MTT experiments.

Recent development in fluorescent probes for copper ion detection

Abstract:

Copper is the third most common heavy metal and an indispensable component of life. Variations of body copper levels, both structural and cellular, are related to a number of disorders; consequently, pathophysiological importance of copper ions demands the development of sensitivity and selective for detecting these organisms in biological systems. In recent years, the area of fluorescent sensors for detecting copper metal ions has seen revolutionary advances. Consequently, closely related fields have raised awareness of several diseases linked to copper fluctuations. Further developments in this field of analysis could pave the way for new and innovative treatments to combat these diseases. This review reports on recent progress in the advancement of three fields of fluorescent probes; chemodosimeters, near IR fluorescent probes, and ratiometric fluorescent probes. Methods used to develop these fluorescent probes and the mechanisms that govern their reaction to specific analytes and their applications in studying biological systems, are also given.

Naphthalimide-phenanthroimidazole incorporated new fluorescent sensor for "turn-on" Cu2+ detection in living cancer cells

In recent years, fluorescent sensors have emerged as attractive imaging probes due to their distinct responses toward bio-relevant metal ions. However, the bioimaging application main barrier is the 'turn-off' response toward paramagnetic metal ions such as Cu²⁺ under physiological conditions. Herein, we report a new sensor (2-methyl(4-bromo-N-ethylpiperazinyl-1,8-naphthalimido)-4-(1H-phenanthro[9,10-d]imidazole-2-yl) phenol)NPP with multifunctional (Naphthalimide, Piperazine, Phenanthroimidazole) units for fluorescent and colourimetric detection of Cu²⁺ in an aqueous medium. Both absorption and fluorescence spectral titration strategies were used to monitor the Cu²⁺-sensing property of NPP. The NPP displays a weak emission at ca. 455 nm, which remarkably enhances (⁓3.2-fold) upon selective binding of Cu²⁺ over a range of metal ions, including other paramagnetic metal ions (Mn²⁺, Fe³⁺, Co²⁺). The stoichiometry, binding constant (K_a) and the LOD (limit of detection) of NPP toward Cu²⁺ ions were found to be 1:1, 5.0 (± 0.2) × 10⁴ M^-1 and 6.5 (± 0.4) × 10^-7 M, respectively. We have also used NPP as a fluorescent probe to detect Cu²⁺ in live (human cervical HeLa) cancer cells. The emission intensity of NPP was almost recovered in HeLa cells by incubating 'in situ' the derived Cu²⁺ complex (NPP-Cu²⁺) in the presence of a benchmark chelating agent such as EDTA (ethylenediaminetetraacetate). The fluorescent emission of NPP was reverted significantly in each cycle upon sequencial addition of Cu²⁺ and EDTA to the NPP solution. Overall, NPP is a novel, simple, economic and portable sensor that can detect Cu²⁺ in biological and environmental scenarios.

Colorimetric Chemosensor and Turn on Fluorescence Probe for pH Monitoring Based on Xanthene Dye Derivatives and its Bioimaging of Living Escherichia coli Bacteria

A new turn on fluorescence probe based on 3',6'-dihydroxy-6-methyl-2-((pyridin-2-ylmethylene)amino)-4-(p-tolyl)spiro[benzo[f]isoindole-1,9'-xanthen]-3(2H)-one (BFFPH) derived from benzo[f]fluorescein was prepared. Full characterization of the prepared probe using spectroscopic analysis was described such as IR, NMR and MS spectra. The sensitivity of BFFPH for monitoring of pH change in alkaline medium was studied. BFFPH exhibited a high sensitivity to alkaline pH by two pKa values at 8.82 and 10.66 in UV/vis spectroscopy titration. The pH monitoring was studied in broad range of pH values (2.5-12.2) at two pKa values at 8.72 and 10.73 by recording the effect of pH on the fluorescence intensity of BFFPH. The acid-base reversibility character of the probe was investigated as well as the effect of the pH change on the fluorescence quantum yield. The application of the prepared BFFPH probe for detection of living Escherichia coli (E. coli) bacteria using confocal fluorescence microscope was investigated.

Effect of n-alkyl substitution on Cu(ii)-selective chemosensing of rhodamine B derivatives

Rhodamine B hydrazide-based molecular probes (1-10) were synthesized by derivatization with n-alkyl chains of different lengths at the hydrazide amino end. These probes exhibited selective absorption (A∼557) and fluorescence (I∼580) 'off-on' signal transduction along with a colourless → magenta colour transition in the presence of Cu(ii) ions among all the competitive metal ions investigated. The effective coordination of these probes to Cu(ii) ions under the investigated environment forming [Cu·L]2+ (L = 1-5) and [Cu·L2]2+ (L = 6-10) complexes led to their spiro-ring opening, which in turn was expressed through signatory spectral peaks of ring-opened rhodamine. All these probes exhibited Cu(ii) selectivity in signalling despite structural modifications to the core receptor unit through variation of the nature of the alkyl substituents. However, the sensitivity of the signalling and kinetics of the spiro-ring opening varied and could be correlated with the number of carbon atoms present in the n-alkyl substituents. Structural elucidation with X-ray diffraction and X-ray photoemission spectroscopic analyses provided further insight into the structure-function correlation in their Cu(ii) complexes. These probes with Cu(ii) coordination showed selectivity in signalling, high complexation affinity (log Ka = 4.8-8.8), high sensitivity (LOD = 4.1-80 nM), fast response time (rate = 0.0017-0.0159 s-1) and reversibility with counter anions, which ascertained their potential utility as chemosensors for Cu(ii) ion detection.

Highly selective and sensitive determination of Cu2+ in drink and water samples based on a 1,8-diaminonaphthalene derived fluorescent sensor

A new simple and efficient fluorescent sensor L based on 1,8‑diaminonaphthalene Schiff-base for highly sensitive and selective determination of Cu²⁺ in drink and water has been developed. This Cu²⁺-selective detection over other tested metal ions displayed an obvious color change from blue to colorless easily detected by naked eye. The detection limit is determined to be as low as 13.2 nM and the response time is very fast within 30 s. The 1:1 binding mechanism was well confirmed by fluorescence measurements, IR analysis and DFT calculations. Importantly, this sensor L was employed for quick detection of Cu²⁺ in drink and environmental water samples with satisfactory results, providing a simple, rapid, reliable and feasible Cu²⁺-sensing method.

One-pot synthesis of the stable CdZnTeS quantum dots for the rapid and sensitive detection of copper-activated enzyme

Galactose oxidase is a copper-activated enzyme and have a vital role in metabolism of galactose. Much of the work is focused on determining the amount of galactose in the blood rather than measuring the amount of galactose oxidase to urge the galactosemia patients to restrict milk intake. Here, a simple and effective method was developed for Cu²⁺ and copper-activated enzyme detection based on homogenous alloyed CdZnTeS quantum dots (QDs). Meso- 2,3-dimercaptosuccinic acid (DMSA) was used as the reducing agent for preparing QDs and the highest quantum yield of CdZnTeS QDs was 69.4%. In addition, the as-prepared CdZnTeS QDs show superior fluorescence properties, such as good photo-/chemical stability. The DMSA was the surface ligand of the QDs, containing abundant -SH and -COOH, thus the surface ligands have a high affinity with Cu²⁺. Therefore, this developed probe can be applied for Cu²⁺ and galactose oxidase detection and shows a good sensitivity in the buffer. Then, this probe was successfully used for Cu²⁺ and galactose oxidase detection in real samples with the satisfactory results. The proposed fluorescence quenching strategy gives a new and simple insight for enzyme assay without the enzyme-catalyzed reaction.

A new hydroxynaphthyl benzothiazole derived fluorescent probe for highly selective and sensitive Cu(2+) detection

A new reactive probe, 1-(benzo[d]thiazol-2-yl)naphthalen-2-yl-picolinate (BTNP), was designed and synthesized. BTNP acts as a highly selective probe to Cu(2+) in DMSO/H2O (7/3, v/v, Tris-HCl 10mM, pH=7.4) solution based on Cu(2+) catalyzed hydrolysis of the picolinate ester moiety in BTNP, which leads to the formation of an ESIPT active product with dual wavelength emission enhancement. The probe also possesses the advantages of simple synthesis, rapid response and high sensitivity. The pseudo-first-order reaction rate constant was calculated to be 0.205min(-1). Moreover, application of BTNP to Cu(2+) detection in living cells and real water samples was also explored.

Chloride accelerated Fenton chemistry for the ultrasensitive and selective colorimetric detection of copper

A highly selective, ultrasensitive (visual and instrumental detection limits of 40 nM and 0.1 nM, respectively), environmentally-friendly, simple and rapid colorimetric sensor was developed for the detection of copper(II) in water. This sensor is based on a novel signal-amplification mechanism involving reactive halide species (RHSs) including chlorides or bromides, which accelerate copper Fenton reactions oxidizing the chromogenic substrate to develop colour. The results of this study expand our understanding of copper-based Fenton chemistry.

Highly selective detection of Hg2+ and MeHgI by di-pyridin-2-yl-[4-(2-pyridin-4-yl-vinyl)-phenyl]-amine and its zinc coordination polymer

Di-pyridin-2-yl-[4-(2-pyridin-4-yl-vinyl)-phenyl]-amine and its zinc coordination polymer are used to detect Hg²⁺ and MeHgI with high selectivity.

A reaction-based fluorescent turn-on probe for Cu2+ in complete aqueous solution

FP can detect Cu²⁺ in complete aqueous solution with a rapid response time, high sensitivity, and high selectivity.

A dual-emission fluorescence-enhanced probe for imaging copper(ii) ions in lysosomes

We have developed the first example of a fluorescence-enhanced and lysosome-targeted Cu²⁺ probe (Lys-Cu) with unique dual-channel emissions. The newly synthesized fluorescent probe Lys-Cu, which contains two recognition sites with different sensing mechanisms for Cu²⁺, displays fluorescence-enhanced dual-channel emissions with fluorescence response to Cu²⁺ in the lysosome pH environment. Fluorescence imaging shows that Lys-Cu is membrane-permeable and suitable for visualization of Cu²⁺ in lysosomes of living cells by dual-channel imaging.

A novel biosensor for copper(ii) ions based on turn-on resonance light scattering of ssDNA templated silver nanoclusters

A new ultrasensitive biosensor for copper(ii) ions was first developed based on turn-on resonance light scattering (RLS) of ssDNA templated silver nanoclusters through anti-galvanic reduction (AGR).

Proton donor modulating ESIPT-based fluorescent probes for highly sensitive and selective detection of Cu2+

Two novel ESIPT-based fluorescent probes for Cu²⁺ detection were developed. Altering the linker in probe molecules reversed their sensing behavior. Both probes exhibited high selectivity and sensitivity to Cu²⁺, and can be used for cell imaging.

The role of copper ions in pathophysiology and fluorescent sensors for the detection thereof

Copper ions are crucial to life, and some fundamental roles of copper in pathophysiology have been elucidated using fluorescent sensors.

A novel phosphatidylserine-functionalized AuNP for the visual detection of free copper ions with high sensitivity and specificity

In this paper, we have reported on novel phosphatidylserine-functionalized AuNPs (gold nanoparticles) for the visual detection of Cu²⁺ by employing phosphatidylserine for Cu²⁺ recognition and AuNPs for signal generation. The phosphatidylserine (1,2-dioleoyl-sn-glycero-3-phospho-l-serine, DOPS) was covalently assembled on the AuNP surface to obtain DOPS-functionalized AuNPs. It was demonstrated that DOPS-functionalized AuNPs could specifically bind Cu²⁺ and lead to the aggregation of AuNPs, which gave rise to a colour change from wine-red to blue and a new absorption band around 650 nm. This provides a sensing platform for the simple, rapid and field portable colorimetric detection of Cu²⁺. By using the sensing platform, a selective and sensitive visual biosensor for the detection of Cu²⁺ was developed. The proposed biosensor has outstanding analytical advantages such as good stability, relatively high sensitivity, low cost and short analysis time. It can be used to detect concentrations of Cu²⁺ as low as 30 μM in river water by observation with the naked eye and of 1.55 μM Cu²⁺ in river water by UV-visible spectrophotometry, within 10 min and with a recovery of 98-103% and a relative standard deviation (RSD) < 4% (n = 6). The proposed biosensor is promising for on-site detection of trace Cu²⁺ in clinical diagnosis or environmental monitoring.

A naphthalimide-based bifunctional fluorescent probe for the differential detection of Hg²⁺ and Cu²⁺ in aqueous solution

A novel fluorescent probe NPM based on naphthalimide was designed and synthesized. Interestingly, NPM exhibited highly selective fluorescence turn-on for Hg(2+) and turn-off for Cu(2+) in aqueous solution (10 mM HEPES, pH 7.5). Its fluorescence intensity enhanced in a linear fashion with the concentration of Hg(2+) and decreased in a nearly linear fashion with the concentration of Cu(2+). Thus NPM could be potentially used for the quantification of Hg(2+) and Cu(2+) in aqueous solution. A series of model compounds were rationally designed and synthesized in order to explore the sensing mechanisms and binding modes of NPM with Hg(2+) and Cu(2+).

A ligation DNAzyme-induced magnetic nanoparticles assembly for ultrasensitive detection of copper ions

A novel biosensor for ultrasensitive detection of copper (Cu(2+)) was established based on the assembly of magnetic nanoparticles induced by the Cu(2+)-dependent ligation DNAzyme. With a low limit of detection of 2.8 nM and high specificity, this method has the potential to serve as a general platform for the detection of heavy metal ions.

Rhodamine-based fluorescent off–on sensor for Fe3+ – in aqueous solution and in living cells: 8-aminoquinoline receptor and 2 : 1 binding

A rhodamine-based Fe³⁺ sensor of a rigid 8-aminoquinoline receptor shows a 2 : 1 binding according to 1D and 2D-¹HNMR experiments.

Chemodosimeter approach for nanomolar detection of Cu2+ ions and their bio-imaging in PC3 cell lines

A new rhodamine–azaindole based fluorescence probe for Cu²⁺ has been synthesized which shows fluorescence resonance energy transfer process in acetonitrile. Further, the probe undergoes Cu²⁺ promoted hydrolysis in mixed aqueous media as well as in the intracellular systems.

Sensitive and selective detection of copper ions with highly stable polyethyleneimine-protected silver nanoclusters

Copper is a highly toxic environmental pollutant with bioaccumulative properties. Therefore, sensitive Cu(2+) detection is very important to prevent over-ingestion, and visual detection using unaugmented vision is preferred for practical applications. In this study, hyperbranched polyethyleneimine-protected silver nanoclusters (hPEI-AgNCs) were successfully synthesized using a facile, one-pot reaction under mild conditions. The hPEI-AgNCs were very stable against extreme pH, ionic strength, temperature, and photoillumination and could act as sensitive and selective Cu(2+) sensing nanoprobes in aqueous solutions with a 10 nM limit of detection. In addition, hPEI-AgNCs-doped agarose hydrogels were developed as an instrument-free and regenerable platform for visual Cu(2+) and water quality monitoring.

A two-photon ratiometric fluorescence probe for cupric ions in live cells and tissues

Development of sensitive and selective probes for cupric ions (Cu²⁺) at cell and tissue level is a challenging work for progress in understanding the biological effects of Cu²⁺. Here, we report a ratiometric two-photon probe for Cu²⁺ based on the organic-inorganic hybrids of graphene quantum dots (GQDs) and Nile Blue dye. Meanwhile, Cu-free derivative of copper-zinc superoxide dismutase (SOD) – E₂Zn₂SOD is designed as the unique receptor for Cu²⁺ and conjugated on the surface of GQDs. This probe shows a blue-to-yellow color change in repose to Cu²⁺, good selectivity, low cytotoxicity, long-term photostability, and insensitivity to pH over the biologically relevant pH range. The developed probe allows the direct visualization of Cu²⁺ levels in live cells as well as in deep-tissues at 90–180 μm depth through the use of two-photon microscopy. Furthermore, the effect of ascorbic acid is also evaluated on intracellular Cu²⁺ binding to E₂Zn₂SOD by this probe.