Simultaneous visualization and quantification of copper (II) ions in Alzheimer's disease by a near-infrared fluorescence probe

A novel NIR fluorescent probe for copper(ii) imaging in Parkinson's disease mouse brain

Abnormal copper ion (Cu2+) levels are considered to be one of the pathological factors of Parkinson's disease (PD), but the internal relationship between Cu2+ and PD progression remains elusive. Visualizing Cu2+ in the brain will be pivotal for comprehending the underlying pathophysiological processes of PD. In this work, a near-infrared (NIR) fluorescent probe, DDAO-Cu, capable of detecting Cu2+ with exceptional sensitivity (about 1.8 nM of detection limit) and selectivity, rapid response (<3 min), and deep tissue penetration, was designed for quantification and visualization of the Cu2+ level. It could detect not only Cu2+ in cells but also the changes in the Cu2+ level in the rotenone-induced cell and zebrafish PD models. Moreover, DDAO-Cu can cross the blood-brain barrier to image Cu2+ in the brain of PD model mice. The imaging result showed a significant increase in Cu2+ levels in brain regions of PD model mice, including the cerebral cortex, hippocampus, and striatum. Meanwhile, Cu2+ levels in the substantia nigra region were significantly reduced in PD model mice. It revealed the nuanced relationship of Cu2+ levels in different brain regions in the disease and indicated the pathological complexity of PD. Overall, DDAO-Cu represents a novel and practical tool for investigating Cu2+-related physiological and pathological processes underlying Parkinson's disease.

Luminescence Probes in Bio-Applications: From Principle to Practice

Bioanalysis based on optical imaging has gained significant progress in the last few decades. Luminescence probes are capable of detecting, monitoring, and tracing particular biomolecules in complex biological systems to figure out the roles of these molecules in organisms. Considering the rapid development of luminescence probes for bio-applications and their promising future, we have attempted to explore the working principles and recent advances in bio-applications of luminescence probes, in the hope of helping readers gain a detailed understanding of luminescence probes developed in recent years. In this review, we first focus on the current widely used luminescence probes, including fluorescence probes, bioluminescence probes, chemiluminescence probes, afterglow probes, photoacoustic probes, and Cerenkov luminescence probes. The working principles for each type of luminescence probe are concisely described and the bio-application of the luminescence probes is summarized by category, including metal ions detection, secretion detection, imaging, and therapy.

A ratiometric fluorescent probe revealing the abnormality of acetylated tau by visualizing polarity in Alzheimer's disease

Abnormal neuronal polarity leads to early deficits in Alzheimer's disease (AD) by affecting the function of axons. Precise and rapid evaluation of polarity changes is very important for the early prevention and diagnosis of AD. However, due to the limitations of existing detection methods, the mechanism related to how neuronal polarity changes in AD is unclear. Herein, we reported a ratiometric fluorescent probe characterized by neutral molecule to disclose the polarity changes in nerve cells and the brain of APP/PS1 mice. Cy7-K showed a sensitive and selective ratiometric fluorescence response to polarity. Remarkably, unlike conventional intramolecular charge transfer fluorescent probes, the fluorescence quantum yield of Cy7-K in highly polar solvents is higher than that in low polar solvents due to the transition of neutral quinones to aromatic zwitterions. Using the ratiometric fluorescence imaging, we found that beta-amyloid protein (Aβ) inhibits the expression of histone deacetylase 6, thereby increasing the amount of acetylated Tau protein (AC-Tau) and ultimately enhancing cell polarity. There was a high correlation between polarity and AC-Tau. Furthermore, Cy7-K penetrated the blood-brain barrier to image the polarity of different brain regions and confirmed that APP/PS1 mice had higher polarity than Wild-type mice. The probe Cy7-K will be a promising tool for assessing the progression of AD development by monitoring polarity.

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.

Monoamine oxidase B activatable red fluorescence probe for bioimaging in cells and zebrafish

A real-time and specific for the detection of Monoamine Oxidase B (MAO-B) to investigate the MAO-B-relevant disease development and treatment process is urgently desirable. Here, we utilized MAO-B to catalyze the conversion of propylamino groups to aldehyde groups, which was then quickly followed by a β-elimination process to produce fluorescent probes (FNJP) that may be used to detect MAO-B in vitro and in vivo. The FNJP probe possesses unique properties, including favorable reactivity (Km = 10.8 μM), high cell permeability, and NIR characteristics (λem = 610 nm). Moreover, the FNJP probe showed high selectivity for MAO-B and was able to detect endogenous MAO-B levels from a mixed population of NIH-3 T3 and HepG2 cells. MAO-B expression was found to be increased in cells under lipopolysaccharide-stimulated cellular oxidative stress in neuronal-like SH-SY5Y cells. In addition, the visualization of FNJP for MAO-B activity in zebrafish can be an effective tool for exploring the biofunctions of MAO-B. Considering these excellent properties, the FNJP probe may be a powerful tool for detecting MAO-B levels in living organisms and can be used for accurate clinical diagnoses of related diseases.

Recent Development on Copper-Sensor and its Biological Applications: A Review

Metal ion recognition is one of the most prospective research topics in the field of chemical sensors due to its wide range of clinical, biological and environmental applications. In this context, hydrazones are well known compounds that exhibit metal sensing and several biological properties due to the presence of N=CH- bond. Some of the biological properties includes anti-cancer, anti-tumor, anti-oxidant, anti-microbial activities. Hydrazones are also used as a ligand to detect metal ion as well as to generate metal complexes that exhibit medicinal properties. Thus, in recent years, many attempts were made to develop novel ligands with enhanced metal sensing and medicinal properties. In this review, some of the recent development on the hydrazones and their copper complexes are covered from the last few years from 2015-2023. These includes significance of copper ions, synthesis, biological properties, mechanism and metal sensing properties of some of the copper complexes were discussed.

Are high copper levels related to Alzheimer's and Parkinson's diseases? A systematic review and meta-analysis of articles published between 2011 and 2022

Copper performs an important role in the brain, but in high levels it can be neurotoxic. Further, some authors have described that copper dyshomeostasis could be related with neurodegenerative diseases. Thus, this review was performed to observe whether high copper levels are related to Alzheimer's and Parkinson's diseases (AD and PD), using the literature published recently. Articles that measured copper levels in AD or PD patients was included, as well as they that measured copper levels in models used to mimic these diseases. Also, results about high copper levels effects and its relationship with AD and PD observed in laboratory animals are considered. In summary, 38 and 24 articles with AD and PD patients were included, respectively. Despite of the heterogeneity between the studies in humans, meta-analysis has demonstrated that there is an increase in free and total copper levels in the blood of AD patients compared to controls, and a decrease in copper levels in PD patients. A decrease in the metal content in postmortem brain tissue was observed in AD and PD. In manuscripts using animal models that mimic AD and PD, it was included seven and three articles, respectively. Two of them have reported an increase in copper concentrations in AD model, and one in PD model. Finally, studies with laboratory animals have concluded that high copper levels are related to oxidative stress, neuroinflammation, mitochondrial dysfunction, changes in neurotransmitter levels, cell death, and reduced both cognitive and locomotor activity, which are also described in AD or PD.

A novel sensitive fluorescent probe with double channels for highly effective recognition of biothiols

Biothiols play a crucial role in maintaining redox balance in organisms, and anomalous levels of biothiols in human organs can lead to various sicknesses and biological disorders. This work developed a novel sensitive fluorescent probe TZ-NBD with double channels for highly efficient recognition of biothiols. TZ-NBD adopts 4-Chloro-7-nitrobenzofurazan (NBD-Cl) as the recognition moiety with simultaneous fluorescence output. By incorporating NBD-Cl with the other fluorophore, benzothiazole dihydrocyclopentachromene derivative (TZ-OH), the dual-channel sensitive fluorescence probe TZ-NBD was built. The existence of Cys/ Hcy could significantly trigger both the green and red fluorescent emissions, which were derived from fluorophores amine-substituted NBD and TZ-OH, respectively. While exposing to GSH, only the red-channel fluorescence signal could be detected, indicating the release of TZ-OH. The phenomena was mainly attributed to the fact that sulfur-substituted NBD has nearly no fluorescence, while amine-substituted NBD shows obvious green fluorescence. In our study, TZ-NBD exhibited dual-channel sensitivity, fast response, and excellent selectivity to biothiols in vitro. Moreover, TZ-NBD was favorably utilized for recognition of biothiols in vivo. We believe that the sensitive fluorescence probe with double channels can afford an alternate approach for monitoring biothiols in organisms and would be useful for studying diseases associated with biothiols.

Macrophage membrane-encapsulated nitrogen-doped carbon quantum dot nanosystem for targeted treatment of Alzheimer's disease: Regulating metal ion homeostasis and photothermal removal of β-amyloid

The abnormal aggregation of β-amyloid protein (Aβ) is a major contributor to Alzheimer's disease (AD). Cu2+ homeostasis imbalance can lead to the aggregation of Aβ, resulting in cytotoxic oligomers and fibrous aggregates, causing neuroinflammation and nerve cell damage, ultimately leading to AD. In this study, we synthesized nitrogen-doped carbon quantum dot (CQD), and designed a macrophage membrane (RAW-M) encapsulated CQD nanosystem for the first time. The abundant nitrogen-containing groups on the surface of CQD effectively capture excess Cu2+ and inhibit rapid Aβ aggregation. Additionally, the good photothermal properties of CQD dissolve the formed fiber precipitates under near-infrared light (NIR). In vitro and in vivo studies showed that the nanosystem significantly improved BBB permeability under laser irradiation, enhancing its ability to cross the BBB and overcome traditional anti-AD drug limitations. In vivo investigations conducted on APP/PS1 mice indicate that the nanosystem strongly reduced Aβ deposition, mitigated neuroinflammation, and ameliorates deficits in learning and memory. Overall, our nanocarrier approach adjusts metal ion homeostasis, inhibits abnormal Aβ aggregation, and uses excellent photothermal properties to depolymerize mature Aβ fibrils to protect cells from Aβ neurotoxicity, providing an effective strategy for Aβ-targeted treatment of AD.

Visualization of the elevated levels of hypochlorous acid in Alzheimer's disease with a ruthenium(II) complex-based luminescence probe

Alzheimer's disease (AD) is an age-related neurodegenerative disorder that devastatingly affects people's lives. Accumulating evidence indicates that the pathological progression of AD is inseparably connected with hypochlorous acid (HClO). However, further exploring the biological function remains an open challenging due to a lack of effective tools to image HClO in AD brains. To this end, a ruthenium(II) luminescence probe, Ru-HClO, is developed for quantitative detection and visualization of HClO in nerve cells and AD brains. Ru-HClO shows quenched luminescence due to the PET process (excited electron transfer from Ru(II) center to diaminomaleonitrile) and the CN bond isomerization in the excited state. The HClO-triggered specific cleavage reaction with Ru-HClO cleaves the CN bond to form highly luminescent Ru-COOH. Ru-HClO shows rapid response speed, high sensitivity and selectivity, excellent biocompatibility, which makes the probe to be applied to semi-quantitative analysis of HClO in nerve cells and high-throughput screening of anti-AD drugs in the AD cell model. Moreover, using Ru-HClO as a probe, present work further validated that the elevated levels of HClO secretion were accompanied by the AD progressed. These findings may provide valuable results for figuring out the biological roles that HClO played in AD but also for accelerating anti-AD therapeutic discovery.

A large Stokes shift NIR fluorescent probe for visual monitoring of mitochondrial peroxynitrite during inflammation and ferroptosis and in an Alzheimer's disease model

The excessive formation of peroxynitrite (ONOO-) in mitochondria has been implicated in various pathophysiological processes and diseases. However, owing to short emission wavelengths and small Stokes shifts, previously reported fluorescent probes pose significant challenges for mitochondrial ONOO- imaging in biological systems. In this study, a near-infrared (NIR) fluorescent probe, denoted as DCO-POT, is designed for the visual monitoring of mitochondrial ONOO-, displaying a remarkable Stokes shift of 170 nm. The NIR fluorophore of DCO-CHO is released by DCO-POT upon the addition of ONOO-, resulting in off-on NIR fluorescence at 670 nm. This phenomenon facilitates the high-resolution confocal laser scanning imaging of ONOO- generated in biological systems. The practical applications of DCO-POT as an efficient fluorescence imaging tool are verified in this study. DCO-POT enables the fluorometric visualization of ONOO- in organelles, cells, and organisms. In particular, ONOO- generation is analyzed during cellular and organism-level (zebrafish) inflammation during ferroptosis and in an Alzheimer's disease mouse model. The excellent visual monitoring performance of DCO-POT in vivo makes it a promising tool for exploring the pathophysiological effects of ONOO-.

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.

Construction of a water-soluble fluorescent probe for copper (II) ion detection in live cells and food products

The quality of wine can be affected by excess Cu²⁺ due to the occurrence of oxidation reactions or precipitation. Therefore, it is essential to use simple and effective testing methods to ensure the Cu²⁺ content in wine. In this work, we designed and synthesized a rhodamine polymer fluorescent probe (PEG-R). The water solubility of PEG-R was improved by the introduction of polyethylene glycol, which improved the performance and broadened its application in the food field. The PEG-R was characterized by high sensitivity, selectivity and fast response to Cu²⁺ and was able to complete the response process within 30 s, with approximately 29-fold fluorescence enhancement of the probe after exposure to Cu²⁺, the limit of detection (LOD) was 1.295 × 10^-6 M. The probe can be used for the determination of Cu²⁺ in living cells, zebrafish, white wine and food products, and it was made into practical gels and test strips.

Construction of a novel ESIPT and AIE-based fluorescent sensor for sequentially detecting Cu2+ and H2S in both living cells and zebrafish

The development of effective methods for tracking Cu²⁺ and H₂S in living organisms is urgently required due to their vital function in a variety of pathophysiological processes. In this work, a new fluorescent sensor BDF with excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) features for the successive detection of Cu²⁺ and H₂S was constructed by introducing 3,5-bis(trifluoromethyl)phenylacetonitrile into the benzothiazole skeleton. BDF showed a fast, selective and sensitive fluorescence "turn off" response to Cu²⁺ in physiological media, and the situ-formed complex can serve as a fluorescence "turn on" sensor for highly selective detection of H₂S through the Cu²⁺ displacement approach. In addition, the detection limits of BDF for Cu²⁺ and H₂S were determined to be 0.05 and 1.95 μM, respectively. Encouraged by its favourable features, including strong red fluorescence from the AIE effect, large Stokes shift (285 nm), high anti-interference ability and good function at physiological pH as well as a low toxicity, BDF was successfully applied for the consequent imaging of Cu²⁺ and H₂S in both living cells and zebrafish, making it an ideal candidate for detecting and imaging of Cu²⁺ and H₂S in live systems.

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.

Molecular probes for fluorescent sensing of metal ions in non-mammalian organisms

While metal ions play an important role in the proper functioning of all life, many questions remain unanswered about exactly how different metals contribute to health and disease. The development of fluorescent probes, which respond to metals, has allowed greater understanding of the cellular location, concentration and speciation of metals in living systems, giving a new appreciation of their function. While the focus of studies using these fluorescent tools has largely been on mammalian organisms, there has been relatively little application of these powerful tools to other organisms. In this review, we highlight recent examples of molecular fluorophores, which have been applied to sensing metals in non-mammalian organisms.

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.

Development of a Fluorescein-Based Probe with an "Off-On" Mechanism for Selective Detection of Copper (II) Ions and Its Application in Imaging of Living Cells

Copper is a common metallic element that plays an extremely essential role in the physiological activities of living organisms. The slightest change in copper levels in the human body can trigger various diseases. Therefore, it is important to accurately and efficiently monitor copper ion levels in the human body. Recent studies have shown that fluorescent probes have obvious advantages in bioimaging and Cu²⁺ detection. Therefore, a novel Cu²⁺ probe (N2) was designed and synthesized from fluorescein, hydrazine hydrate and 5-p-nitrophenylfurfural that is sensitive to and can detect Cu²⁺ within 100 s. The response mechanism of the N2 probe to Cu²⁺ was studied by several methods such as Job's plots and MS analysis, which showed that the Cu²⁺ and the N2 probe were coordinated in a complexation ratio of 1:1. In addition, compared with other cations investigated in this study, the N2 probe showed excellent selectivity and sensitivity to Cu²⁺, exhibiting distinct fluorescence absorption at 525 nm. Furthermore, in the equivalent range of 0.1-1.5, there is a good linear relationship between Cu²⁺ concentration and fluorescence intensity, and the detection limit is 0.10 μM. It is worth mentioning that the reversible reaction between the N2 probe and Cu²⁺, as well as the good biocompatibility shown by the probe in bioimaging, make it a promising candidate for Cu²⁺ biosensor applications.

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.

Role of Alginate Composition on Copper Ion Uptake in the Presence of Histidine or Beta-Amyloid

The anomalous interaction between metal ions and the peptide beta-amyloid is one of the hallmarks of Alzheimer's disease. Metal-binding biopolymers, including polysaccharides, can elucidate the fundamental aspects of metal ions' interactions with biological tissue and their interplay in Alzheimer's disease. This work focuses on the role of the alginate composition on Cu(II) adsorption in the presence of histidine or β-amyloid, the peptide associated with the progression of Alzheimer's disease. Alginate samples with different mannuronic/guluronic (M/G) ratios led to similar Cu(II) adsorption capacities, following the Langmuir isotherm and the pseudo-second-order adsorption kinetic models. Although the presence of histidine produced up to a 20% reduction in the copper adsorption capacity in guluronic-rich alginate samples (M/G~0.61), they presented stable bidentate chelation of the metallic ion. Chemical analyses (FTIR and XPS) demonstrated the role of hydroxyl and carboxyl groups in copper ion chelation, whereas both crystallinity and morphology analyses indicated the prevalence of histidine interaction with guluronic-rich alginate. Similar results were observed for Cu(II) adsorption in alginate beads in the presence of beta-amyloid and histidine, suggesting that the alginate/histidine system is a simple yet representative model to probe the application of biopolymers to metal ion uptake in the presence of biological competitors.