Visible-Near-Infrared and Fluorescent Copper Sensors Based on Julolidine Conjugates: Selective Detection and Fluorescence Imaging in Living Cells

Colorimetric and Fluorometric N-Acylhydrazone-based Chemosensors for Detection of Single to Multiple Metal Ions: Design Strategies and Analytical Applications

The development of optical sensors for metal ions has gained significant attention due to their broad applications in biology, the environment, and medicine. Colorimetric and fluorometric detection methods are particularly valued for their simplicity, cost-effectiveness, high detection limits, and analytical power. Among various chemical probes, the hydrazone functional group stands out for its extensive study and utility, owing to its ease of synthesis and adaptability. This review provides a comprehensive overview of N-acylhydrazone-based probes, serving as highly effective colorimetric and fluorometric chemosensors for a diverse range of metal ions. Probes are categorized into single-ion, dual-ion, and multi-ion chemosensors, each further classified based on the detected metal(s). Additionally, the review discusses detection modes, detection limits, association constants, and spectroscopic measurements.

A Bifunctional Fluorescence Probe Based on AIE-ICT Strategy for Visual Detection of Cu2+/Co2+ in Complex Matrix

A novel fluorescence chemical sensor-based probe 1-{[(E)-(2-aminophenyl)azanylidene]methyl}naphthalen-2-ol (AMN) was designed and synthesized, which performed a "naked eye" detection ability toward Cu²⁺ and Co²⁺ based on aggregation-induced emission (AIE) fluorescence strategy. It has sensitive detection ability for Cu²⁺ and Co²⁺. In addition, the color changed from yellow-green to orange under the sunlight, realizing the rapid identification of Cu²⁺/Co²⁺, which has the potential of on-site visual detection under the "naked eye". Moreover, different "on" and "off" fluorescence expressions were exhibited under excessive glutathione (GSH) in AMN-Cu²⁺ and AMN-Co²⁺ systems, which could be employed to distinguish Cu²⁺ from Co²⁺. The detection limits for Cu²⁺ and Co²⁺ were measured to be 8.29 × 10^-8 M and 9.13 × 10^-8 M, respectively. The binding mode of AMN was calculated to be 2:1 by Jobs' plot method analysis. Ultimately, the new fluorescence sensor was applied to detect Cu²⁺ and Co²⁺ in real samples (tap water, river water, and yellow croaker), and the results were satisfying. Therefore, this high-efficiency bifunctional chemical sensor platform based on "on-off" fluorescence detection will provide significant guidance for the advance development of single-molecule sensors for multi-ion detection.

Multipronged diagnostic and therapeutic strategies for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and a major contributor to dementia cases worldwide. AD is clinically characterized by learning, memory, and cognitive deficits. The accumulation of extracellular amyloid β (Aβ) plaques and neurofibrillary tangles (NFTs) of tau are the pathological hallmarks of AD and are explored as targets for clinical diagnosis and therapy. AD pathology is poorly understood and there are no fully approved diagnosis and treatments. Notwithstanding the gap, decades of research in understanding disease mechanisms have revealed the multifactorial nature of AD. As a result, multipronged and holistic approaches are pertinent to targeting multiple biomarkers and targets for developing effective diagnosis and therapeutics. In this perspective, recent developments in Aβ and tau targeted diagnostic and therapeutic tools are discussed. Novel indirect, combination, and circulating biomarkers as potential diagnostic targets are highlighted. We underline the importance of multiplexing and multimodal detection of multiple biomarkers to generate biomarker fingerprints as a reliable diagnostic strategy. The classical therapeutics targeting Aβ and tau aggregation pathways are described with bottlenecks in the strategy. Drug discovery efforts targeting multifaceted toxicity involving protein aggregation, metal toxicity, oxidative stress, mitochondrial damage, and neuroinflammation are highlighted. Recent efforts focused on multipronged strategies to rationally design multifunctional modulators targeting multiple pathological factors are presented as future drug development strategies to discover potential therapeutics for AD.

The Povarov Reaction: A Versatile Method to Synthesize Tetrahydroquinolines, Quinolines and Julolidines

The multicomponent Povarov reaction represents a powerful approach for the construction of substances containing N-heterocyclic frameworks. By using the Povarov reaction, in addition to accessing tetrahydroquinolines, quinolines and julolidines in a single step, it is possible to form the following new bonds: two Csp 3–Csp 3 and one Csp 3–Nsp 3, two Csp 2–Csp 2 and one Csp 2–Nsp 2, and four Csp 3–Csp 3 and two Csp 3–Nsp 1, respectively. This short review discusses the main features of the Povarov reaction, including its mechanism, the reaction scope by employing different catalysts and substrates, as well as stereoselective versions.

1 Introduction

2 Mechanism of the Povarov Reaction

3 Tetrahydroquinolines

4 Quinolines

5 Julolidines

6 Concluding Remarks

Enhanced synthesis of oxo-verdazyl radicals bearing sterically-and electronically-diverse C3-substituents

The synthetic viability of the hydrazine- and phosgene-free synthesis of 1,5-dimethyl oxo-verdazyl radicals has been improved via a detailed study investigating the influence of the aryl substituent on tetrazinanone ring formation. Although it is well established that functionalisation at the C3 position of the tetrazinanone ring does not influence the nature of the radical, it is crucial in applications development. The synthetic route involves a 4-step sequence: Schiff base condensation of a carbohydrazide with an arylaldehyde, alkylation, ring closure then subsequent oxidation to the radical. We found that the presence of strong electron-donating substituents and electron rich heterocycles, result in a significant reduction in yield during both the alkylation and ring closure steps. This can, in part, be alleviated by milder alkylation conditions and further substitution of the aryl group. In comparison, more facile formation of the tetrazine ring was observed with examples containing electron-withdrawing groups and with meta- or para-substitution. Density functional theory suggests that the ring closure proceeds via the formation of an ion pair. Electron paramagnetic resonance spectroscopy provides insight into the precise electronic structure of the radical with small variations in hyperfine coupling constants revealing subtle differences.

Fluorescent Biosensors Based on Silicon Nanowires

Nanostructures are arising as novel biosensing platforms promising to surpass current performance in terms of sensitivity, selectivity, and affordability of standard approaches. However, for several nanosensors, the material and synthesis used make the industrial transfer of such technologies complex. Silicon nanowires (NWs) are compatible with Si-based flat architecture fabrication and arise as a hopeful solution to couple their interesting physical properties and surface-to-volume ratio to an easy commercial transfer. Among all the transduction methods, fluorescent probes and sensors emerge as some of the most used approaches thanks to their easy data interpretation, measure affordability, and real-time in situ analysis. In fluorescent sensors, Si NWs are employed as substrate and coupled with several fluorophores, NWs can be used as quenchers in stem-loop configuration, and have recently been used for direct fluorescent sensing. In this review, an overview on fluorescent sensors based on Si NWs is presented, analyzing the literature of the field and highlighting the advantages and drawbacks for each strategy.

A Diverse Micromorphology of Photonic Crystal Chips for Multianalyte Sensing

The diversity by nano/microstructural material or device constructing can provide the exciting opportunity for sensitivity and selectivity to achieve facile and efficient multianalyte recognition for clinical diagnosis, environment monitoring, etc., in complex system analysis. Colloidal poly(styrene-methyl methacrylate-acrylic acid) (poly(St-MMA-AA)) nanoparticle-assembled photonic crystals (PCs) can achieve manipulative 3D structural colors and approach PC sensor chip for high-efficient multianalysis utilizing simple dye. Focusing on the morphology effects of structural color, a PC microchip is designed and constructed with various geometrical micromorphologies. Based on the angle dependence of colloidal-crystal structural color, the stopband distribution is explored on various morphological PC pixels. Selective fluorescent enhancement is realized for stopband-matched PCs, which approach the successful discrimination of metal ions and complex multianalysis of groundwater. Meanwhile, printed droplet-shaping manipulation can achieve a large-scale structural-color sensor array of chips with designable nano/microstructures via colloidal assembly. It will be the critical puzzle piece between macromorphology and microstructure for the structural-color researches.

A novel highly sensitive dual-channel chemical sensor for sequential recognition of Cu2+ and CN− in aqueous media and its bioimaging applications in living cells

A simple and unique dual-channel chemical probe (DH) was designed and synthesized, which not only realized sequential recognition of Cu2+ and CN− by colorimetric and fluorometric methods, but also realized fluorescence detection of CN−.

A new highly selective colorimetric Schiff base chemosensor for determining the copper content in artisanal cachaças

This work demonstrated the feasibility of applying the Schiff base 5-bromo-2-salicyl-beta-alanine as a colorimetric chemosensor for the spectrophotometric quantification of the copper content in artisanal cachaças. For this, the experimental conditions were optimized to obtain an efficient, sensitive, reversible, and highly selective chemosensor to Cu²⁺ ions. The complex stoichiometry was 1:1, with a formation constant of 5.82 × 10² L mol^-1 and molar absorptivity of 5.82 × 10³ mol L^-1 cm^-1. Then, a spectrophotometric analytical method was developed and validated according to the Brazilian legislation. The linearity of the analytical curve was demonstrated by ANOVA, at a confidence level of 95%. The limits of detection and quantification were 0.0659 and 0.200 mg L^-1, respectively. The coefficients of variation for both the intra- and inter-day precisions were lower than 3.83%, and the accuracy presented a mean recovery of 100.55 ± 2.87%. The absence of a matrix effect was confirmed by the standard addition method, and the copper content in three artisanal cachaças from different geographical origins was estimated as lower than 2.93 mg L^-1. This result was in accordance with the Brazilian legislation but reinforces the need to carry out stricter quality control to achieve exportation standards. Therefore, the proposed method can be considered a simple, selective, linear, precise, and accurate tool that involves only a simple complexation reaction through the addition of the chemosensor solution in a buffered medium. As a consequence, the simplicity, practicality, rapidity, and low cost of synthesis of the proposed Schiff base chemosensor are highlighted.

Unambiguous Detection of Elevated Levels of Hypochlorous Acid in Double Transgenic AD Mouse Brain

Alzheimer's disease (AD) is one of the most prevalent forms of dementia. The current diagnosis methods based on the behavior and cognitive decline or imaging of core biomarkers, namely, amyloid-β (Aβ) plaques and neurofibrillary tangles (NFTs), in the brain offer poor to moderate success. Detection and imaging of biomarkers that cause additional traits of pathophysiological aberrations in the brain are invaluable to monitor early disease onset and progression of AD pathology. The pathological hallmark of AD is associated with generation of excessive reactive oxygen species (ROS) in the brain, which aggravate oxidative stress and inflammation. ROS production involves elevated levels of hypochlorous acid (HOCl) and can serve as one of the potential biomarkers for the diagnosis of AD. We report the design, synthesis, and characterization of switchable coumarin-morpholine (CM) conjugates as off-on fluorescence probes for the specific detection of HOCl produced and proximally localized with amyloid plaques. The nonfluorescent thioamide probe CM2 undergoes regioselective transformation to fluorescent amide probe CM1 in the presence of HOCl (∼90-fold fluorescence enhancement and 0.32 quantum yield) with high selectivity and sensitivity (detection limit: 0.17 μM). The excellent cellular uptake and blood-brain barrier (BBB) crossing ability of CM2 allowed unambiguous and differential detection, imaging, and quantification of HOCl in cellular milieu and in the wild type (WT) and AD mouse brains. This study demonstrates the elevated level of HOCl in the AD mouse brain and the potential to expand the repertoire of biomarkers for the diagnosis of AD.

Fluorescent determination of zinc by a quinoline-based chemosensor in aqueous media and zebrafish

A quinoline-based fluorescence sensor QDTD was developed for Zn²⁺. QDTD can detect Zn²⁺ by fluorescence turn-on. Detecting limit (0.27 μM) of QDTD for Zn²⁺ was far below WHO standard (76.0 μM). For the practical application, compound QDTD could be used to determine Zn²⁺ in real samples and applied to the test kit. More importantly, QDTD was expertly applied for Zn²⁺ imaging in HeLa cells and zebrafish with good membrane-permeability. Detection mechanism of Zn²⁺ ion by compound QDTD was suggested through the analytical tools like ¹H NMR titration, ESI-MS, Job plot, fluorescent and UV-vis titration, and theoretical calculations, and through the synthesis and applications of a model compound AAQA.

Two-component ratiometric sensor for Cu2+ detection on paper-based device

The copper(II) ion (Cu²⁺) has played an indispensable role in diverse kinds of functional physiological processes of organisms, which has become of growing interest. Despite the fact that numerous Cu²⁺ test papers using fluorescent probes have been fabricated, sensors featuring the ratiometric property that integrates quenched probes and an inner standard dye are rarely reported. Herein, a two-component ratiometric sensor in a paper-based device is proposed to realize highly selective Cu²⁺ detection. To overcome shortcomings such as low signal-to-noise ratio and incorrect response of the quenching probe, a novel BODIPY-based turn-off probe (P2017) is designed and introduced into the paper-based device with better water solubility and selectivity for Cu²⁺ detection. Furthermore, a reference dye (B001), exhibiting an emission at 690 nm when the excitation wavelength is 480 nm, is also introduced into the paper-based device. These two components can enhance the quality of the signal as P2017 is sensitively quenched by Cu²⁺, while B001 with a photostable property, serving as an internal benchmark, is unable to react with Cu²⁺. The results indicated that the two components provided a new concept for optimizing paper-based device fabrication and developing accurate, simple, and inexpensive Cu²⁺ detection methods, which could be potentially applied to monitor human health and the environment in remote areas. Graphical abstract.

A novel hydrazide-based selective and sensitive optical chemosensor for the detection of Ni2+ ions: applications in live cell imaging, molecular logic gates and smart phone-based analysis

A novel hydrazide-based optical sensor for Ni²⁺ ions was designed, which can be applied for recovery of contaminated water samples, smart phone-based analysis and live cell imaging.

Excited state intramolecular proton transfer in julolidine derivatives: an ab initio study

We have studied, using ab initio tools, a series of recently prepared fluorescent julolidine derivatives, undergoing Excited State Intramolecular Proton Transfer (ESIPT). We show that the computed free energy change in the excited state (ΔGES) can be used to predict the preference for enol, keto, or dual emission. Indeed, two julolidine molecules experimentally show dual emission, consistent with our finding of a small ΔGES. In agreement with experimental outcomes the complexation between the ESIPT centre and BF2 increases the rigidity of the fluorophore and greatly facilitates emission at energies close to the original enol (E*) fluorescence band. The protonation of the imino group also suppresses ESIPT and sole E* emission is obtained. We disclose that chemical substitution can significantly tune the radiationless deactivation of the enol related to the C[double bond, length as m-dash]N bond rotation of the ESIPT centre. While there is a significant barrier for the experimentally studied compounds we have found a strong correlation between the barrier height and the electron donating strength of the phenyl substituent. Strong donors such as amines facilitate the barrierless non-radiative decay from E* back to the ground state, while weak electron donors make the barrier sufficiently high to allow ESIPT. Strong electron accepting groups such as -NO2 further increase this barrier. This work therefore illustrates the fine interplay necessary to design dual emitters.

A highly selective and sensitive fluorescent probe for Cu2+ based on a novel naphthalimide-rhodamine platform and its application in live cell imaging

Copper plays important roles in a variety of fundamental physiological processes. At the cell organelle level, aberrant copper homeostasis in lysosomes can lead to various serious diseases. Herein, a bifluorophore-based, lysosome-targetable Cu²⁺-selective ratiometric fluorescent probe (V) has been synthesized by reasonable design. The probe V shows high selectivity toward Cu²⁺ ions over other cations and exhibits high sensitivity (1.45 nM) for the detection of Cu²⁺ ions. Meanwhile, the probe is cell permeable and suitable for ratiometric visualization of lysosomal Cu²⁺ in the living cell.

A new Schiff-base chemosensor for selective detection of Cu2+ and Co2+ and its copper complex for colorimetric sensing of S2- in aqueous solution

A new Schiff-base colorimetric chemosensor 1 was developed for the detection of Cu²⁺, Co²⁺ and S^2-. Sensor 1 could simply monitor Cu²⁺ and Co²⁺ by a color change from colorless to yellow. The binding modes of 1 to Cu²⁺ and Co²⁺ were determined to be a 2 : 1 complexation stoichiometry through Job's plot and ESI-mass spectrometry analysis. The detection limits (0.02 μM and 0.63 μM) for Cu²⁺ and Co²⁺ were lower than the recommended values (31.5 μM and 1.7 μM) by the World Health Organization (WHO) for Cu²⁺ and the Environmental Protection Agency (EPA) for Co²⁺, respectively. Importantly, 1 could detect and quantify Cu²⁺ in real water samples. In addition, the Cu²⁺-2·1 complex could be used as a highly selective colorimetric sensor for S^2- in the presence of other anions without any interference. Moreover, the sensing mechanisms of Cu²⁺ and Co²⁺ by 1 were explained by theoretical calculations.

Current progress, challenges and future prospects of diagnostic and therapeutic interventions in Alzheimer's disease

The diverse pathological mechanisms and their implications for the development of effective diagnostic and therapeutic interventions in Alzheimer's disease are presented with current progress, challenges and future prospects.

A highly sensitive pyridine-dicarbohydrazide based chemosensor for colorimetric recognition of Cu2+, AMP2−, F− and AcO− ions

A single colorimetric chemosensor for the detection of both Cu²⁺ and AMP²⁻ ions has been developed for the first time.