Phenoxazine‐based Near‐infrared Fluorescent Probes for the Specific Detection of Copper (II) Ions in Living Cells

A near-infrared fluorescent probe with thiadiazole unit as key skeleton for ICT and ESIPT mechanism and effective detection of Cu2

Fluorescent probe L-I was synthesized to demonstrate that 1,3,4-thiadiazole is an attractive moiety and could be utilized as positive hydrogen bond acceptor for excited state intramolecular proton transfer (ESIPT) processes, guider of electrons movement for intramolecular charge transfer (ICT) process and identify group for mental ions. Furthermore, dicyanoisophorone framework was employed to improve the fluorescence characteristics and near-infrared (NIR) fluorescent emission at 695 nm accompanied by a Stoke's shift as large as 260 nm was obtained. L-I could selectively detect Cu2+ over other analytes taking advantages of high sensitivity, fast response within 30 s and low detection limit (0.026 μM). More important, L-I exhibited good performance for detection of Cu2+ in actual water samples, food products, traditional Chinese medicine and for cell imaging which demonstrates practical significance in the fields of environmental monitor, food safety and biotechnology.

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.

Versatile Fluorescence Lifetime-Based Copper Probe to Quantify Mitochondrial Membrane Potential and Reveal Its Interaction with Protein Aggregation

Mitochondria play a crucial role in maintaining cellular homeostasis, and the depolarization of mitochondrial membrane potential (MMP) is an important signal of apoptosis. Additionally, protein misfolding and aggregation are closely related to diseases including neurodegenerative diseases, diabetes, and cancers. However, the interaction between MMP changes and disease-related protein aggregation was rarely studied. Herein, we report a novel "turn-on" fluorescent probe MitoRhB that specifically targets to mitochondria for Cu2+ detection in situ. The fluorescence lifetime (τ) of MitoRhB exhibits a positive correlation with MMP changes, allowing us to quantitatively determine the relative MMP during SOD1 (A4 V) protein aggregation. Finally, we found that (1) the increasing concentrations of copper will accelerate the depolarization of mitochondria and reduce MMP; (2) the depolarization of mitochondria can intensify the degree of protein aggregation, suggesting a new routine of copper-induced cell death mediated through abnormal MMP depolarization and protein aggregation.

Rational synthesis of 1,3,4-thiadiazole based ESIPT-fluorescent probe for detection of Cu2+ and H2S in herbs, wine and fruits

Here, 1,3,4-thiadiazole unit was employed as novel excited state intramolecular proton transfer (ESIPT) structure to prepare favorable fluorescent probe. High selectivity and rapid response to Cu2+ was obtained and the settling reaction was also used to recover ESIPT characteristics of probe to achieve sequential detection of H2S. Remarkable color change of solution from colorless to bright yellow and fluorescence emission from green to dark realized the visual detection of Cu2+ by naked eyes and transition of probe into portable fluorescent test strips. As expected, L-E could be utilized to quantitatively sense Cu2+ and H2S in different actual water and food samples including herbs, wine and fruits. The limits of detection for Cu2+ and H2S were as low as 34.5 nM and 38.6 nM. Also, probe L-E achieved real-time, portable, on-site quantitative detection of Cu2+ via a colorimeter and a smartphone platform with limit of detection to 90.3 nM.

Acylhydrazone Functionalized Triphenylamine-Based Fluorescent Probe for Cu2+: Tunable Structures of Conjugated Bridge and Its Practical Application

Novel fluorescent probes were constructed for the convenient and rapid analysis of Cu2+ ions, taking advantages of the the triphenylamine backbone as chromophore and acylhydrazone group as the Cu2+ recognition site. Especially, probe T2 could act as a dual-channel probe towards Cu2+ through both fluorescent and colorimetric method. Through the fluorescent method, the detection limit of probe T2 was calculated to be as low as 90 nmol/L and there was a good linear relationship between the intensity change and the concentration of Cu2+ ions. By virtue of the two-phase liquid-liquid extraction method, probe T2 could be successfully applied in practical extraction and separation of Cu2+. Furthermore, by applying a "turn-off-turn-on" circle, compound T2 could act as a sensitive probe towards S2- anions through the indirect approach and the detection limit of complex T2-Cu2+ for S2- anion was found to be 110 nmol/L.

Current and Future of "Turn-On" Based Small-Molecule Copper Probes for Cuproptosis

Increasing evidence shows that abnormal copper (Cu) metabolism is highly related to many diseases, such as Alzheimer's disease, Wilson's disease, hematological malignancies and Menkes disease. Very recently, cuproptosis, a Cu-dependent, programmed cell death was firstly described by Tsvetkov et al. in 2022. Their findings may provide a new perspective for the treatment of related diseases. However, the concrete mechanisms of these diseases, especially cuproptosis, remain completely unclear, the reason of which may be a lack of reliable tools to conduct highly selective, sensitive and high-resolution imaging of Cu in complex life systems. So far, numerous small-molecular fluorescent probes have been designed and utilized to explore the Cu signal pathway. Among them, fluorescence turn-on probes greatly enhance the resolution and accuracy of imaging and may be a promising tool for research of investigation into cuproptosis. This review summarizes the probes developed in the past decade which have the potential to study cuproptosis, focusing on the design strategies, luminescence mechanism and biological-imaging applications. Besides, we put forward some ideas concerning the design of next-generation probes for cuproptosis, aiming to tackle the main problems in this new field. Furthermore, the prospect of cuproptosis in the treatment of corresponding diseases is also highlighted.

Recent progress of TP/NIR fluorescent probes for metal ions

Metal ions are of significance in various pathological and physiological processes. As such, it is crucial to monitor their levels in organisms. Two-photon (TP) and near-infrared (NIR) fluorescence imaging has been utilized to monitor metal ions because of minimal background interference, deeper tissue depth penetration, lower tissue self-absorption, and reduced photodamage. In this review, we briefly summarize recent progress from 2020 to 2022 of TP/NIR organic fluorescent probes and inorganic sensors in the detection of metal ions. Additionally, we present an outlook for the development of TP/NIR probes for bio-imaging, diagnosis of diseases, imaging-guided therapy, and activatable phototherapy.

A Novel D-π-A Type Fluorescent Probe for Cu2+ Based on Styryl-Pyridinium Salts Conjugating Di-(2-picolyl)amine (DPA) Units

A novel D-π-A type fluorescent probe L(NO₃) for Cu (II) sensing was designed and fully characterized. The probe consists of a styryl-pyridine cation fluorescent group and a di-(2-picolyl)amine (DPA) receptor unit, which are linked by a phenyl group to form an electron donor-π-acceptor (D-π-A) conjugate system, especially the introduction of a nitrate counter anion for significantly enhanced water solubility of the probe. Fluorescence titration studies of the probe L(NO₃) showed a higher selectivity for Cu²⁺ than other metal ions, and the emission spectrum was strongly quenched upon binding. The competitive binding assay and the low detection limit (0.932 µM) showed that the probe L(NO₃) had strong anti-interference ability and excellent Cu²⁺ detection performance. The binding ratio of probe L(NO₃) and Cu²⁺ was determined from Job's plot to be 1:1, which is consistent with the results obtained from X-ray crystal structures. Meanwhile, the probe showed instantaneous chemical reversibility when titrated with EDTA solution, indicating potential recycling properties of the probe. In addition, the design of inexpensive fluorescent test strips can perform the on-site and real-time detection Cu²⁺ with a color recognition application.

A New Cellulose-Based Fluorescent Probe for Specific and Sensitive Detection of Cu2+ and Its Applications in the Analysis of Environmental Water

In this work, a novel fluorescent probe CMC−GE−AQ with an effective sensitive detection ability for Cu²⁺ was synthesized and constructed by using carboxymethyl cellulose (CMC) as the skeleton and 8-aminoquinoline (AQ) as the fluorophore. This probe exhibited a highly specific “turn-off” fluorescence response to Cu²⁺, and the fluorescence color changed from bright orange to colorless after adding Cu²⁺. The probe could selectively detect Cu²⁺ in a complex environment and its detection limit (LOD), the binding constant (K_a) and the numbers of binding sites (n) were calculated to be 6.4 × 10⁻⁸ mol L⁻¹, 1.7 × 10⁶ mol⁻¹ L and 1.2, respectively. The sensing detection mechanism was confirmed by X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. In addition, the probe CMC−GE−AQ was successfully applied to detect Cu²⁺ in real water samples, and CMC−GE−AQ-based fluorescent microspheres can serve as a convenient tool for the detection of Cu²⁺.

The Schiff Base Probe With J-aggregation Induced Emission for Selective Detection of Cu2

Here, three Schiff bases 3a-c, differing by the substitutions (-H, -Cl, and -N(CH₃)₂) on the phenyl ring, have been designed and synthesized via the reaction of ortho-aminophenol with benzaldehyde, 2,4-dichlorobenzaldehyde and para-dimethylamine benzaldehyde in 1:1 molar ratio with favourable yields of 89-92%, respectively. Their structural characterizations were studied by FT-IR, NMR, MALDI-MS and elemental analysis. The fluorescence behaviours of compounds 3a and 3b exhibited a severe aggregation caused quenching (ACQ) effect in EtOH/water system. On the contrary, compound 3c had an obvious J-aggregation induced emission (AIE) feature in EtOH/water mixture (v/v = 1:1), and exhibited excellent sensitivity and anti-interference towards Cu²⁺ with the limit of detection (LOD) of 1.35 × 10^-8 M. Job's plot analysis and MS spectroscopic study revealed the 2:1 complexation of probe 3c and Cu²⁺. In addition, probe 3c was successfully applied to the determination of Cu²⁺ in real aqueous samples.

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.

A novel 1,8-naphthalimide-based Cu2+ ion fluorescent probe and its bioimaging application

A new 1,8-naphthalimide-based Schiff base compound, named as (Z)-2-butyl-6-(((2-hydroxyphenyl)imino)methyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (BHBD), has been simply synthesized with high yields. BHBD can be employed as a "turn-on" fluorescent probe for Cu²⁺ ion with high sensitivity, high selectivity and relatively low detection limit (0.48 × 10^-6 M). The fluorescence emission of BHBD is very weak in H₂O/THF (v/v: 7/3) mixture, which is significantly enhanced after addition of Cu²⁺ ion. The proposed mechanism is verified by ¹H NMR, Job's plot and TOF-MS experiments. Anti-interference experiment, cytotoxicity assay and pH influence results indicated that BHBD meets the requirements of bioimaging. Therefore, BHBD has been successfully applied in detecting Cu²⁺ ion in HeLa cells.

A Self-Assembled Nanoindicator from Alizarin Red S-Borono-Peptide for Potential Imaging of Cellular Copper(II) Ions

Recently, smart nanomaterials from peptide self-assembly have received extensive attention in the field of biological and medical applications. Through rationally designing the molecular structure, we constructed a borono-peptide that self-assembled into well-defined nanofibers. Relying on the specific recognition between the vicinal diol compound and boronic acid, a novel alizarin red S (ARS)-borono-peptide (BP) spherical nanoindicator was fabricated, accompanying with the emission of strong fluorescent signal. The fluorescent nanoindicator displayed an intense response to copper(II) ions and underwent the fluorescent "turn-off" due to the strong binding-induced displacement. Originating from the high selectivity toward copper(II) ions, good biocompatibility and cancer cell targeting, the nanoindicator offered the opportunity to image copper(II) ions in cancer cells via fluorescent change.