A lysosome targetable fluorescent probe for palladium species detection base on an ESIPT phthalimide derivative

K, Koner AL. Review on Lysosomal Metal Ion Detection Using Fluorescent Probes

Metal ions are indispensable and play an important role in living systems. Metal ions coordinated to metalloenzymes pocket activate the bound substrate and labile metal ions maintaining the ionic balance. The amount of metal ions present in various subcellular compartments of the cells is highly regulated for maintaining cellular homeostasis. An imbalance in the metal ion concentration is related to several diseases and results in serious pathological conditions. Mostly the internalized metal ions are processed in the lysosomal compartment of the cell. A delicate regulation of metal ions in the lysosomal compartment can modulate the lysosomal pH and inhibit hydrolytic enzymes, which ultimately causes lysosomal storage disorders. In the past decade, the understanding and regulation of lysosomal metal ions based on fluorometric methods have gained significant attention. In this review, we have comprehensively summarized the development of various fluorescent reporters over the past five years for a selective and sensitive estimation of lysosomal metal ion concentration. We believe this consolidated and timely review will help researchers working in the areas associated with lysosomal metal ions.

Effects of substitution and conjugation on photophysical properties of ESIPT-based fluorophores with the core of 4-aminophthalimide

4-Aminophthalimide is a highly fluorescent signaling unit with excellent photophysical properties and wide application foregrounds. Based on this, a range of theoretical investigations are conducted on the fluorescent probe (E)-5-((2-hydroxybenzylidene) amino) isoindoline-1, 3-dione (HID) with the core of 4-aminophthalimide using density functional theory (DFT) and time-containing density functional theory (TD-DFT) methods in this paper. The optimized configurations, vertical excitation and emission energies, electronic characteristics and excited-state intramolecular proton transfer (ESIPT) behaviors of the probe HID are discussed in detail. Furthermore, to enhance the luminescent properties of HID, five novel compounds have been designed based on the structure of HID by introducing amino, methoxy and naphthalene groups (-NH2, -OMe and C10H8). Our work thoroughly explores how the property and position of substituents and conjugation affect photophysical characteristics and ESIPT processes. We find that the ESIPT dynamics can be modulated by the substitution and conjugation effects. Specifically, the introduction of amino and methoxy groups at the ortho-position and the introduction of the naphthalene group promote the ESIPT behavior of HID1, whereas the introduction of amino and methoxy groups at the meta-position exhibits the contrary impact. Therefore, we boldly infer that the introduction of electron-donating groups in the ortho-position and the introduction of the conjugated group make the ESIPT process more effortless to occur, whereas the introduction of substituents with opposing natures in the meta-position makes the ESIPT process more difficult to occur. In addition, the ionization potentials (IP), electron affinities (EA) and reorganization energies (λh and λe) of molecules are calculated to assess their potential as luminescent materials. Our work not only reveals the luminescence and ESIPT mechanism of the probe HID1, but also proposes to modulate the ESIPT process through the substitution and conjugation effects. In particular, the designed molecules have better photoelectric properties as a result of their red-shifted absorption and fluorescence spectra, smaller energy gaps, larger transferred charges and greater charge transferred distances, which offers some valuable ideas for the experimental development of more efficient organic luminescent materials with ESIPT properties.

A Reaction-Based ESIPT Fluorescent Probe for the Detection of Hg2+ with Large Stokes Shift

A novel reaction-based fluorescent probe 1 for Hg2+ was designed and synthesized. 1 was almost nonfluoresent due to inhibition of the ESIPT process between hydroxy group and imid carbonyl oxygen by diphenylphosphinothioate group. After reacting with Hg2+, the fluorescence intensity of 1 exhibited significant enhancement owing to recovery of the ESIPT process via Hg2+-promoted desulfurization-hydrolysis of the diphenylphosphinothioate moiety and cleavage of the P-O bond. 1 not only showed rapid response, high sensitivity, excellent selectivity for Hg2+ over other metal ions, but also could detect Hg2+ with large Stokes shift (165 nm), which was attributed to the ESIPT process. Moreover, the reaction mechanism was fully validated by absorption spectra, fluorescence spectra, fluorescence color as well as ESI-MS analysis. 1 is the reaction-based ESIPT fluorescent probe for the detection of Hg2+ with large Stokes shift, rapid response, high sensitivity and selectivity.

A mitochondrion-targeted fluorescent probe based on ESIPT phthalimide for the detection of Hg2+ with large Stokes shift

A novel mitochondrion-targeted Hg2+-specific fluorescent probe 1 based on ESIPT phthalimide was designed and synthesized for the first time. Owing to the blockage of the ESIPT process between the hydroxy group and the carbonyl oxygen of the imide by the diphenylphosphinothioate group, 1 was almost nonfluorescent. After reacting with Hg2+, 1 exhibited a dramatic fluorescence enhancement due to the recovery of the ESIPT process through Hg2+-induced desulfurization-hydrolysis of the diphenylphosphinothioate moiety and the cleavage of the P-O bond. 1 showed a large Stokes shift, rapid response and high sensitivity and selectivity for Hg2+ over other metal ions. Moreover, 1 was successfully employed to image Hg2+ in the mitochondria of living cells.

Fluorescent Organic Small Molecule Probes for Bioimaging and Detection Applications

The activity levels of key substances (metal ions, reactive oxygen species, reactive nitrogen, biological small molecules, etc.) in organisms are closely related to intracellular redox reactions, disease occurrence and treatment, as well as drug absorption and distribution. Fluorescence imaging technology provides a visual tool for medicine, showing great potential in the fields of molecular biology, cellular immunology and oncology. In recent years, organic fluorescent probes have attracted much attention in the bioanalytical field. Among various organic fluorescent probes, fluorescent organic small molecule probes (FOSMPs) have become a research hotspot due to their excellent physicochemical properties, such as good photostability, high spatial and temporal resolution, as well as excellent biocompatibility. FOSMPs have proved to be suitable for in vivo bioimaging and detection. On the basis of the introduction of several primary fluorescence mechanisms, the latest progress of FOSMPs in the applications of bioimaging and detection is comprehensively reviewed. Following this, the preparation and application of fluorescent organic nanoparticles (FONPs) that are designed with FOSMPs as fluorophores are overviewed. Additionally, the prospects of FOSMPs in bioimaging and detection are discussed.

A highly sensitive sensor for colorimetric detection of palladium(II) in lysosomes and its applications

Considering the scarcity of palladium ion probes with subcellular organelle targeting, especially probes with near-infrared (NIR) emission wavelength fluorophores, our group has been working to overcome this problem and looking forward to providing potential practical tools for exploring the toxicity of palladium ions at the subcellular level. In this paper, a novel colorimetric and NIR fluorescent probe, BHCy-Pd, for the specific detection of palladium ions (Pd²⁺) in lysosomes via an internal charge-transfer (ICT) mechanism was designed and synthesized. As expected, BHCy-Pd exhibited a rapid, selective, and sensitive response for palladium with an ultralow limit of detection at 5.9 nM, accompanied by a distinct color change from purple to blue. Furthermore, BHCy-Pd can be made into a simple test strip for rapid and easy detection of Pd²⁺ in practical applications. Importantly, BHCy-Pd is capable of specific distribution in lysosomes, and thus can detect Pd²⁺ in real-time, thereby providing a potential tool for studying the cytotoxicity of Pd²⁺ ions at the subcellular level.

A highly selective turn-on fluorescence probe with large Stokes shift for detection of palladium and its applications in environment water and living cells

Nowadays, palladium has been widely used in many fields, which facilitates all aspects of our life. However, it may cause water and soil pollution and bring irreversible damage to the environment and organisms. Developing a fluorescence probe for rapid, highly sensitive and selective detection of palladium is still a poser. In this work, we designed and synthesized a novel fluorescence probe (RHS) for specific detection of palladium. Based on Pd⁰-mediated Tsuji-Trost reaction, the fluorescence probe was constructed by a rhodol derivative as thefluorophore and an allyl carbonate moiety as the specific palladium reactive site. The probe displayed excellent properties for detecting palladium, such as high selectivity and sensitivity, rapid response (20 min) and large Stokes shift (155 nm). The detection limit was determined to be as low as 0.140 μM with a linear range from 20 to 80 μM. After addition of palladium in RHS solution, the color of the solution turned from yellow to blue, indicating palladium could be monitored by the naked eyes. Moreover, probe RHS was successfully applied to palladium detection in environmental water samples. Importantly, with low cytotoxicity and good biocompatibility, the probe could monitor palladium in living cells.

A phenanthrene[9,10-d]imidazole-phenol-based fluorescent probe combining ESIPT and AIE for the “turn-on” detection of Cu2+ with green-emission and improved Stokes’ shift, and its application

The “turn-on” probe PIA(OH)-Py responds to Cu2+ in living cells and can determine the concentration of Cu2+ in blood samples.

Theoretical investigation on the ESIPT mechanism and fluorescent sensing mechanism of 2-(2'-hydroxyphenyl) thiazole-4-carboxaldeyde in methanol

2-(2'-Hydroxyphenyl) thiazole-4-carboxaldeyde (aldehyde 1) and hemiacetal 2 were selected to study the mechanism of excited-state intramolecular proton transfer and the detecting of Al³⁺ ion in methanol by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Our theoretical results are in good agreement with the experimental values. The intramolecular H-bond is enhanced in the first excited-state based on the analyses of structural parameters, frontier molecular orbitals and electronic spectra. The stronger intramolecular H-bond is more favorable for ESIPT process. In order to further demonstrate the proton transfer process, we constructed the potential energy curves of probe 1 and 2 in both ground- and excited-states, and concluded that proton transfer processes in probe 1 and 2 are apt to happen in the S₁ state. In addition, the Mayer bond order, energy gap and absorption and fluorescence spectra were applied to interpret the process of detection of Al³⁺ ion.

A near-infrared rhodamine-based lysosomal pH probe and its application in lysosomal pH rise during heat shock

Heat shock is a potentially fatal condition characterized by high body temperature (>40 °C), which may lead to physical discomfort and dysfunctions of organ systems. Acidic pH environment in lysosomes can activate enzymes, thus facilitating the degradation of proteins in cellular metabolism. Owing to the lack of a practical research tool, it remains difficult to exploit relationship between heat shock and lysosome. Herein, a NIR lysosomal pH chemosensor (NRLH) was developed. One typical lysosome-locating group, morpholine, was incorporated into NRLH. The fluorescence intensity showed pH-dependent characteristics and responded sensitively to pH fluctuations in the pH range of 3.0-5.5. NRLH with a pKa of 4.24 displayed rapid response and high selectivity for H⁺ among common species. We also demonstrated NRLH was capable of targeting lysosomes. Importantly, NRLH was applied in cellular imaging and the data revealed that lysosomal pH increased but never decreased during the heat shock. Therefore, NRLH may act as an effective molecular tool for exploring the mechanisms of heat-related pathology in bio-systems.