Dinuclear tricarbonylrhenium(I) complexes: impact of regioisomerism on the photoluminescence properties

Dinuclear Re(I) complexes have proportionally been much less studied than mononuclear analogues. In particular, very little information is available about their solid-state emission properties. In this work, two structural isomers of dinuclear complexes (Bi-Re-metaPhe and Bi-Re-paraPhe), which differ by the relative position of the coordination spheres on a central phenyl ring, were synthesized and compared with each other and with the parent mononuclear compound (Mono-Re-Phe), from a theoretical and experimental point of view. In solution, the electronic, electrochemical and spectroscopic properties of the dinuclear complexes were almost identical, and rather close to those of the monomer. In the solid state, the photoluminescence (PL) efficiency of dimers was not higher than that of the monomer, but a clear mechanoresponsive luminescence (MRL) effect appeared only for the former ones. The positional isomerism influenced the amplitude of this effect, as well as the aggregation-induced emission (AIE) properties in a water-acetonitrile mixture. This study reveals the importance of positional isomerism to modulate the emission properties in the solid state. It also shows the advantage of dinuclear structures to access new MRL-active materials.

Shining New Light on Biological Systems: Luminescent Transition Metal Complexes for Bioimaging and Biosensing Applications

Luminescence imaging is a powerful and versatile technique for investigating cell physiology and pathology in living systems, making significant contributions to life science research and clinical diagnosis. In recent years, luminescent transition metal complexes have gained significant attention for diagnostic and therapeutic applications due to their unique photophysical and photochemical properties. In this Review, we provide a comprehensive overview of the recent development of luminescent transition metal complexes for bioimaging and biosensing applications, with a focus on transition metal centers with a d6, d8, and d10 electronic configuration. We elucidate the structure-property relationships of luminescent transition metal complexes, exploring how their structural characteristics can be manipulated to control their biological behavior such as cellular uptake, localization, biocompatibility, pharmacokinetics, and biodistribution. Furthermore, we introduce the various design strategies that leverage the interesting photophysical properties of luminescent transition metal complexes for a wide variety of biological applications, including autofluorescence-free imaging, multimodal imaging, organelle imaging, biological sensing, microenvironment monitoring, bioorthogonal labeling, bacterial imaging, and cell viability assessment. Finally, we provide insights into the challenges and perspectives of luminescent transition metal complexes for bioimaging and biosensing applications, as well as their use in disease diagnosis and treatment evaluation.

Construction of a Colorimetric and Near-Infrared Ratiometric Fluorescent Sensor and Portable Sensing System for On-Site Quantitative Measurement of Sulfite in Food

Sulfites play imperative roles in food crops and food products, serving as sulfur nutrients for food crops and as food additives in various foods. It is necessary to develop an effective method for the on-site quantification of sulfites in food samples. Here, 7-(diethylamino) quinoline is used as a fluorescent group and electron donor, alongside the pyridinium salt group as an electron acceptor and the C=C bond as the sulfite-specific recognition group. We present a novel fluorescent sensor based on a mechanism that modulates the efficiency of intramolecular charge transfer (ICT), CY, for on-site quantitative measurement of sulfite in food. The fluorescent sensor itself exhibited fluorescence in the near-infrared light (NIR) region, effectively minimizing the interference of background fluorescence in food samples. Upon exposure to sulfite, the sensor CY displayed a ratiometric fluorescence response (I447/I692) with a high sensitivity (LOD = 0.061 μM), enabling accurate quantitative measurements in complex food environments. Moreover, sensor CY also displayed a colorimetric response to sulfite, making sensor CY measure sulfite in both fluorescence and colorimetric dual-signal modes. Sensor CY has been utilized for quantitatively measuring sulfite in red wine and sugar with recoveries between 99.65% and 101.90%, and the RSD was below 4.0%. The sulfite concentrations in live cells and zebrafish were also monitored via fluorescence imaging. Moreover, the sulfite assimilated by lettuce leaves was monitored, and the results demonstrated that excessive sulfite in leaf tissue could lead to leaf tissue damage. In addition, the sulfate-transformed sulfite in lettuce stem tissue was tracked, providing valuable insights for evaluating sulfur nutrients in food crops. More importantly, to accomplish the on-site quantitative measurement of sulfite in food samples, a portable sensing system was prepared. Sensor CY and the portable sensing system were successfully used for the on-site quantitative measurement of sulfite in food.

A ratiometric fluorescence probe for bisulfite detection in live cells and meat samples

The development of reliable probe technology for the detection of bisulfite (HSO3-) in situ in food and biological samples is contributing significantly to food quality and safety assurance as well as community health. In this work, a responsive probe, EHDI, is developed for ratiometric fluorescence detection of HSO3- in aqueous solution, meat samples, and living cells. The probe is designed based on the HSO3- triggered 1,4-addition of electron deficit C = C bond of EHDI. As a result of this specific 1,4-addition, the π-conjugation system was destructed, resulting in blue shifts of the emission from 687 to 440 nm and absorption from 577 to 355 nm. The probe has good water solubility, high sensitivity and selectivity, allowing it to be used for imaging of HSO3- internalization and production endogenously. The capability of probe EHDI for HSO3- was then validated by traditional HPLC technology, enabling accurately detect HSO3- in beef samples. The successful development of this probe thus offers a new tool for investigating HSO3- in situ in food and biological conditions.

A multi-signal mitochondria-targeted fluorescent probe for simultaneously distinguishing biothiols and realtime visualizing its metabolism in cancer cells and tumor models

Biothiols and its metabolite SO2 derivatives play vital roles in various physiological processes. Although a few probes have been designed for monitoring the metabolism of biothiols, developing multi-signal fluorescent probes with practicability for simultaneously distinguishing biothiols (GSH, Cys and Hcy) and real-time visualizing SO2 derivatives is an enormous challenge. To better visualize biothiols metabolism in vitro and vivo, we developed a novel multi-signal NIR fluorescent probe (probe 2) with mitochondria-targeted for distinguishing biothiols and its metabolism, based on an ICT-PET synergetic mechanism. Probe 2 with dual recognition sites distinguishing detected Cys/Hcy (Red-Green), GSH (Green) and SO32- (Blue) via three channels. First probe 2 distinguished Cys and GSH to estimate main biothiols in living cells through the ratio changes of two well-defined emission bands (Red-Green), and then imaged its metabolite SO2 with ratiometric fluorescence (Red-Blue), eliminating the interference by different biothiols. Notably, probe 2 exhibits satisfactory sensitivity (detection limit: 0.21, 0.13, 0.14 and 3.06 μM for Cys, Hcy, GSH and SO32-, respectively), high selectivity, reliability at physiological pH, and rapid fluorescence response (within 10 min). Given these advantages, probe 2 has been successfully applied to the real-time monitor GSH metabolic process in MCF-7 cells and biothiols metabolism in breast cancer, suggesting biothiols metabolic changes might be a diagnostic indicator during cancer treatment. So probe 2 is a convenient and efficient tool for understanding the physiological functions of biothiols and its metabolism.

"One stone, two birds": a mitochondria-targeted fluorescent probe for the detection of viscosity and HSO3- in living cells

The material transport and physiological events of mitochondria need to be supported by a suitable microenvironment. For example, high viscosity will seriously hinder material exchange, and SO2, as the precursor of HSO3-, is an endogenous signal molecule that plays a key role in information transmission. It is very important to detect viscosity and HSO3- in mitochondria. Here, we developed a dual-responsive fluorescent probe (named Hcy-NT) to image the changes in mitochondrial viscosity and HSO3- in a "killing two birds with one stone" manner. Hcy-NT showed an OFF-ON fluorescence signal for the increase in cell viscosity induced by nystatin, while an ON-OFF fluorescence signal for intracellular and endogenous HSO3-. Its limits of detection for HSO3- were calculated by both absorption and fluorescence methods, which were 1.200 and 1.291 μM, respectively. This work provides a valuable tool for the study of viscosity and HSO3- related physiological processes and the diagnosis of potential diseases.

A dual-channel fluorescent nanoprobe for accurate cancer diagnosis by sequential detection of adenosine triphosphate and sulfur dioxide

Cancer is one of the major diseases that seriously endanger the health of all mankind. Accurate diagnosis of early cancer is the most promising way to reduce cancer harm and improve patient survival. However, many developed fluorescent probes for cancer imaging only have the function of identifying one marker, which cannot meet the needs of accurate diagnosis. Here, a fluorescent nanoprobe (CPH@ZIF-90) utilizing ZIF-90 to encapsulate SO₂-sensitive dye (CPH) is synthesized for the sequential detection of ATP and SO₂. The nanoprobe first interacts with ATP to release CPH, thus increasing the fluorescence at 685 nm and realizing the near-infrared (NIR) fluorescence detection of ATP. Then, SO₂ acts on the released CPH through nucleophilic addition, affecting the π-conjugated structure of CPH and resulting in enhanced fluorescence at 580 nm. CPH@ZIF-90 exhibits satisfactory sensitivity and selectivity for sequential detection of ATP and SO₂. Excitedly, CPH@ZIF-90 can sequentially image the endogenous ATP and SO₂ in cells, showing sensitive fluorescence changes in dual channels (red and green). Due to the NIR emission properties of CPH@ZIF-90 and its ability to enrich in tumor, it is applied to monitor ATP and SO₂ in mice and distinguish normal mice from tumor mice. The ability of CPH@ZIF-90 to sequentially detect two cancer-related biomarkers makes it provide meaningful assistance in accurate early diagnosis of cancer.

A mitochondria-targetable fluorescent probe for sulfur dioxide detection and visualisation in living cells

Sulfur dioxide (SO₂) is a one of reactive sulfur species (RSS) that plays significant roles in many physiological processes. While abnormal levels of SO₂ in mitochondria have been related to various diseases. Hence, developing suitable fluorescent probe for monitoring SO₂ is significant in living organisms. In this research, we designed and synthesized a mitochondrial-target probe Mito-NPH featuring the graft of a strong electron-withdrawing 4-pyridiniumylacrylonitrile unit to an electron-donating naphthalenic unit that intramolecular charge transfer (ICT) process happened. The probe Mito-NPH underwent a nucleophilic addition of HSO₃^-/SO₃^2-to give fluorescent emission signal change from red to blue and exhibited specific response toward HSO₃^-/SO₃^2-over other analytes. Moreover, Mito-NPH showed ultrafast response rate (within 10 s) for HSO₃^-. Importantly, cell imaging results demonstrated that the probe can sense endogenous SO₂ in mitochondria.

Phosphorescent Ir(III) Complexes for Biolabeling and Biosensing

Cyclometalated Ir(III) complexes exhibit strong phosphorescence emission with lifetime of submicroseconds to several microseconds at room temperature. Their synthetic versatility enables broad control of physical properties, such as charge and lipophilicity, as well as emission colors. These favorable properties have motivated the use of Ir(III) complexes in luminescent bioimaging applications. This review examines the recent progress in the development of phosphorescent biolabels and sensors based on Ir(III) complexes. It begins with a brief introduction about the basic principles of the syntheses and photophysical processes of cyclometalated Ir(III) complexes. Focus is placed on illustrating the broad imaging utility of Ir(III) complexes. Phosphorescent labels illuminating intracellular organelles, including mitochondria, lysosomes, and cell membranes, are summarized. Ir(III) complexes capable of visualization of tumor spheroids and parasites are also introduced. Facile chemical modification of the cyclometalating ligands endows the Ir(III) complexes with strong sensing ability. Sensors of temperature, pH, CO₂, metal ions, anions, biosulfur species, reactive oxygen species, peptides, and viscosity have recently been added to the molecular imaging tools. This diverse utility demonstrates the potential of phosphorescent Ir(III) complexes toward bioimaging applications.

Fluorescent probes for the detection of disease-associated biomarkers

Fluorescent probes have emerged as indispensable chemical tools to the field of chemical biology and medicine. The ability to detect intracellular species and monitor physiological processes has not only advanced our knowledge in biology but has provided new approaches towards disease diagnosis. In this review, we detail the design criteria and strategies for some recently reported fluorescent probes that can detect a wide range of biologically important species in cells and in vivo. In doing so, we highlight the importance of each biological species and their role in biological systems and for disease progression. We then discuss the current problems and challenges of existing technologies and provide our perspective on the future directions of the research area. Overall, we hope this review will provide inspiration for researchers and prove as useful guide for the development of the next generation of fluorescent probes.

Diketopyrrolopyrrole-based fluorescent probe for endogenous bisulfite detection and bisulfite triggered phototoxicity specific in liver cancer cells

As the main existing form of SO₂ derivatives, bisulfite showed closely relationship to many diseases. In this work, a new fluorescent probe (SDPP-DM) based on thienyl-substituted diketopyrrolopyrrole (SDPP) was designed and synthesized for the detection of endogenous bisulfite. The probe displayed obvious color changes from green to pink towards bisulfite due to the reduced conjugated length caused by the addition to the α,β-unsaturated double bond of its structure, and the change of the fluorescence intensity of SDPP-DM (I/I₀) was about 16 folds. In addition, SDPP-DM was prepared a test strip for bisulfite identified by naked eye through color and fluorescence changes. Besides, SDPP-DM was successfully applied to imaging and discriminating different endogenous bisulfite levels in normal and cancer cells of liver. More importantly, the ROS generation and cell viability tests showed the phototoxicity of SDPP-DM triggered by bisulfite, indicating the specific phototoxicity of SDPP-DM towards liver cancer cells than normal liver cells.

A mitochondria-targeted fluorescent probe for real-time imaging SO2/H2O2

Studies have shown that changes in the redox state of cells might be closely related to pathological and physiological processes. Sulfur dioxide and hydrogen peroxide, as a significant redox couple in living cells, are endogenously produced by cells. Here, we report a long-wavelength fluorescent probe to reversibly monitor sulfur dioxide and hydrogen peroxide. This probe (NBD) displayed high selectivity and sensitivity, which could be accumulated in mitochondria for real-time imaging of SO₂/H₂O₂. These results indicated that NBD would be an ideal tool for monitoring the redox cycle state in living cells.

Revealing Sulfur Dioxide Regulation to Nucleophagy in Embryo Development by an Adaptive Coloration Probe

Understanding signaling molecules in regulating organelles dynamics and programmed cell death is critical for embryo development but is also challenging because current imaging probes are incapable of simultaneously imaging the signaling molecules and the intracellular organelles they interact with. Here, we report a chemically and environmentally dual-responsive imaging probe that can react with gasotransmitters and label cell nuclei in distinctive fluorescent colors, similar to the adaptive coloration of chameleons. Using this intracellular chameleon-like probe in three-dimensional (3D) super-resolution dynamic imaging of live cells, we discovered SO₂ as a critical upstream signaling molecule that activates nucleophagy in programmed cell death. An elevated level of SO₂ prompts kiss fusion between the lysosomal and nuclear membranes and nucleus shrinkage and rupture. Significantly, we revealed that the gasotransmitter SO₂ is majorly generated in the yolk, induces autophagy there at the initial stage of embryo development, and is highly related to the development of the auditory nervous system.

A novel fluorescent probe based on a triphenylamine derivative for the detection of HSO3- with high sensitivity and selectivity

A novel highly active fluorescence chemical sensor (TBQN) for HSO₃^- was synthesized by the Knoevenagel reaction based on triphenylamine-benzothiazole as a new fluorophore. The probe possessed good selectivity toward HSO₃^- and anti-interference ability with common ions. The fluorescence and UV-vis spectra of the TBQN probe were significantly changed after the addition of HSO₃^-. At the same time, the probe solution released obvious green fluorescence. Moreover, the limit of detection for HSO₃^- was calculated to be 3.19 × 10^-8 M. The TBQN probe displayed a rapid response to HSO₃^- and it took about 3 min to complete the recognition. The detection mechanism is the nucleophilic addition reaction between HSO₃^- and -C[double bond, length as m-dash]C- in the probe molecule. The π-conjugation and ICT (intramolecular charge transfer) transition in the TBQN molecule were destroyed by this addition, which resulted in the change of the fluorescence before and after the addition of HSO₃^-. Then, the mechanism was verified by theoretical calculations, ¹H NMR measurements and mass spectroscopy. In addition, the probe showed low cytotoxicity and could be used for biological imaging in RAW264.7 cells.

Enhanced Sulfite-Selective Sensing and Cell Imaging with Fluorescent Nanoreactors Containing a Ratiometric Lipid Peroxidation Sensor

The detection of SO₂ and its derivatives is indispensable for monitoring atmospheric, water quality, and biological fluctuation of oxidative stress and metabolism of biothiols within native cellular contexts. In this article, the brush copolymer nanoreactors containing amine-terminated PDMS were used to encapsulate the fluorescent indicator C11-BDP, forming sulfite-sensitive nanoreactors (ssNRs). Surprisingly, the ssNRs were found to be highly selective to sulfite over a range of reactive oxygen/nitrogen/sulfur species and anions, which was not observed with freely dissolved indicators. The ssNRs showed a rapid response (t₉₅ = 65 s), an excellent detection limit (0.7 μM), and a very high sensitivity (ca. 1000-fold ratiometric intensity change) to sulfite. For cellular studies, the ssNRs exhibited negligible toxicity and could be endocytosed into endosomes and lysosomes. Finally, the ssNRs allowed us to visualize the different responses of three different types of cells (pre-adipocytes, RAW264.7, and HeLa cells) to external stimuli in the culture media with sulfites and lipopolysaccharides.

3D CoPt nanostructures hybridized with iridium complexes for multimodal imaging and combined photothermal-chemotherapy

Combined photothermal-chemotherapy has shown great potential in improving the efficiency of tumor treatment. In this article, we have designed a new type of nanocomposite Ir-CoPt-PVP composed of cobalt/platinum alloy nanoparticles (CoPt) and iridium(III) complex (Ir) for combined photothermal therapy (PTT) and chemotherapy. The obtained CoPt was synthesized by a simple solvothermal method and modified by polyvinyl pyrrolidone (PVP), which exhibited excellent photothermal efficiency and stability, and can also be a bimodal bioimaging contrast agent in photothermal imaging (PTI) and photoacoustic imaging (PAI). Furthermore, the combination therapy has shown obvious tumor cell-growth inhibition in vitro. Overall, the results revealed the great potential of Ir-CoPt-PVP nanocomposites in improving therapeutic efficiency by photothermal-chemotherapy and photothermal/photoacoustic imaging.

Responsive ruthenium complex probe for phosphorescence and time-gated luminescence detection of bisulfite

Sensitive and selective quantification of specific analytes is of great significance in analytical and environmental sciences, as well as in the food industry. Herein, we report the design, synthesis, characterization, and application of a responsive ruthenium(ii) complex probe, Ru-azo, for phosphorescence and time-gated luminescence (TGL) detection of bisulfite, an important additive in the food industry. Upon a specific nucleophilic addition reaction between bisulfite and the azo group of Ru-azo, a new ruthenium(ii) complex, Ru-SO3, was obtained, which resulted in a remarkable increase in phosphorescence intensity, allowing the bisulfite detection to be achieved. In addition, long-lived emissions of Ru-azo (τ = 258 ns) and Ru-SO3 (τ = 261 ns) also enabled the TGL detection of bisulfite in autofluorescence-rich food samples. Through theoretical computations, the photoinduced electron transfer (PET) process within the ruthenium(ii) complex was validated, which unveiled the rationality of the luminescence "off-on" response of Ru-azo to bisulfite. The probe showed advantages of good water solubility, and high sensitivity, selectivity and accuracy for responding to bisulfite, facilitating its application in phosphorescence and TGL detection of bisulfite in aqueous and food samples.

Employing an ICT-FRET Integration Platform for the Real-Time Tracking of SO2 Metabolism in Cancer Cells and Tumor Models

Glutathione (GSH) mediates a wide variety of biological events and human diseases. Although it has been the subject of intense study in recent years, a further understanding of its molecular mechanisms and metabolism routes in living cells has remained limited due to a lack of appropriate analytical tools. Sulfur dioxide (SO₂), an important metabolite of GSH, is usually associated with the symptoms of neurological disorders, cardiovascular diseases, and lung cancer. Herein, a novel multisignal fluorescent probe was rationally designed and exploited for the simultaneous detection of GSH and its metabolite SO₂ via an ICT-FRET synergetic mechanism. The probe shows completely reversed fluorescence responses toward GSH (enhanced red emission) and SO₂ (annihilated red fluorescence) with high selectivity and sensitivity. In particular, the probe displayed completely different fluorescent signals (blue-shift) with SO₂ in the presence of GSH, thereby allowing the imaging of the metabolism process of GSH to SO₂ in two independent channels without spectral cross interference. Given these advantages, this probe has been successfully applied to the real-time monitoring of the SO₂ metabolic process in living cells and mice models, and it has thus been found that GSH can metabolize SO₂ by enzymatic reaction with TST (thiosulfate sulphurtransferase); additionally, SO₂ was transformed into sulfate under SUOX (sulfite oxidase). We anticipate that this research will provide a convenient and efficient tool for understanding the interrelated physiological functions of GSH and SO₂ in more biosystems.

Dinuclear metal complexes: multifunctional properties and applications

Dinuclear metal complexes have enabled breakthroughs in OLEDs, photocatalytic water splitting and CO₂reduction, DSPEC, chemosensors, biosensors, PDT and smart materials.

High cytotoxic and apoptotic effects of platinum(ii) complexes bearing the 4-acridinol ligand

4-Acridinol platinum(ii) complex PtA induces SK-OV-3/DDP cell apoptosis that is mediated by the mitochondrial dysfunction pathway.

Charge-driven tripod somersault on DNA for ratiometric fluorescence imaging of small molecules in the nucleus

Although fluorescence tracing of small bioactive molecules in living cells has been extensively studied, it is still a challenging task to detect their variations in the nucleus mainly due to the impermeable nuclear membrane and nucleic acid interference. Herein, we take advantage of the nucleic acid enriched environment in the nucleus to establish a strategy, named "charge-driven tripod somersault on DNA", for ratiometric fluorescence imaging of small bioactive molecules in the nucleus. Taking SO2 derivatives as a typical target analyte, a tripodal probe has been constructed by conjugating two DNA binding groups containing a SO2 derivative reaction site. Mechanism studies demonstrate that upon encountering and reacting with SO3 2-/HSO3 -, a charge variation occurs at the responsive arm of the tripodal probe, triggering a tripod somersault on DNA, resulting in the conformational rearrangement of the DNA binding modes with DNA-modulated fluorescence change, which allows the second emission feature to emerge. In this strategy, probe-DNA binding is not influenced by RNA or non-specific protein association, thus making it ideal for tracing nucleus-localized analytes. The application of this strategy has realized both in vitro and in vivo ratiometric fluorescence imaging of the variations of endogenous SO2 derivatives in the nucleus for the first time, with high specificity and selectivity. Also, in theory, this strategy opens up a new avenue for the design of fluorescence probes for the nucleus-localized biological analytes.

Theranostic Lysosomal Targeting Anticancer and Antimetastatic Agents: Half-Sandwich Iridium(III) Rhodamine Complexes

Two rhodamine-modified half-sandwich Ir(III) complexes with the general formula [(Cp^x)Ir(ĈN) Cl] were synthesized and characterized, where Cp^x is 1-biphenyl-2,3,4,5-tetramethylcyclopentadienyl (Cp^xbiph). Both complexes showed potent anticancer activity against A549, HeLa, and HepG2 cancer cells and normal cells, and altered ligands had an effect on proliferation resistance. The complex enters cells through energy dependence, and because of the different ligands, not only could it affect the anticancer ability of the complex but also could affect the degree of complex lysosome targeting, lysosomal damage, and further prove the antiproliferative mechanism of the complex. Excitingly, antimetastatic experiments demonstrated that complex 1 has the ability to block the migration of cancer cells. Furthermore, although the complex did not show a stronger ability to interfere with the coenzyme NAD⁺/NADH pair by transfer hydrogenation, the intracellular reactive oxygen species (ROS) content has shown a marked increase. NF-κB activity is increased by ROS regulation, and the role of ROS-NF-κB signaling pathway further induces apoptosis. Moreover, cell flow experiments also demonstrated that complex 1 blocked the cell cycle in S phase, but the complex did not cause significant changes in the mitochondrial membrane potential.

Potential anticancer agent for selective damage to mitochondria or lysosomes: Naphthalimide-modified fluorescent biomarker half-sandwich iridium (III) and ruthenium (II) complexes

In this work, five naphthalimide-modified half-sandwich iridium and ruthenium complexes ([(η⁵-Cp^x)Ir(NˆN)Cl]PF₆, [(η⁶-p-cym)Ru(NˆN)Cl]PF₆) have been presented. The anticancer activities of the complexes against various cancer cell lines were investigated, among them, complexes 2 and 4 showed better anticancer activity than cisplatin, and their anticancer activity is better than complex 5 without fluorophore. In addition, a series of biological tests of complex 2 were performed using flow cytometry, the results indicated that the complex could induce cell death in a variety of ways. By changing of the ligands, the complexes exhibited different photophysical properties, and the mechanism of action of the complexes entering the cell and inducing apoptosis are different. Moreover, complex 2 successfully targeted mitochondria, while complex 4 targeted lysosomes, causing mitochondrial damage and lysosomal damage to induce apoptosis. Excitingly, complex 2 has good antimetastatic ability to cancer cells. Furthermore, complexes 2 and 4 did not have a significant effect on the NADH binding reaction, but they had a moderate binding ability to BSA.

Coumarin-Based Small-Molecule Fluorescent Chemosensors

Coumarins are a very large family of compounds containing the unique 2H-chromen-2-one motif, as it is known according to IUPAC nomenclature. Coumarin derivatives are widely found in nature, especially in plants and are constituents of several essential oils. Up to now, thousands of coumarin derivatives have been isolated from nature or produced by chemists. More recently, the coumarin platform has been widely adopted in the design of small-molecule fluorescent chemosensors because of its excellent biocompatibility, strong and stable fluorescence emission, and good structural flexibility. This scaffold has found wide applications in the development of fluorescent chemosensors in the fields of molecular recognition, molecular imaging, bioorganic chemistry, analytical chemistry, materials chemistry, as well as in the biology and medical science communities. This review focuses on the important progress of coumarin-based small-molecule fluorescent chemosensors during the period of 2012-2018. This comprehensive and critical review may facilitate the development of more powerful fluorescent chemosensors for broad and exciting applications in the future.

Rapid "turn-on" photoluminescence detection of bisulfite in wines and living cells with a formyl bearing bis-cyclometalated Ir(iii) complex

A new photoluminescence (PL) probe based on a formyl bearing bis-cyclometalated Ir(iii) complex, [Ir(ppy)2phen-CHO]+PF6- (1), is synthesized and applied to the selective detection of a bisulfite anion (HSO3-). Probe 1 is prepared using 2-phenylpyridine (ppy) as the C^N main ligand and 1,10-phenanthroline-5-carboxaldehyde (phen-CHO) as the N^N ancillary ligand. Probe 1 displayed excellent selective PL enhancement in response to HSO3- in acetic acid-sodium acetate buffer solution (pH = 5.0). The increase of PL signal is directly proportional to the concentration of HSO3- in the range of 2 μM to 45 μM with a detection limit of 0.9 μM using 50 μM probe 1 and in the range of 0.5 μM to 6 μM with a detection limit of 0.3 μM using 10 μM probe 1. More importantly, probe 1 can respond to HSO3- rapidly within 40 s. Furthermore, probe 1 was successfully applied to detect HSO3- in real white wines and the bioimaging of HSO3- in living cells. The superior properties of probe 1 make it of great potential use for studying the effects of HSO3- in other biosystems.

Visualizing Endogenous Sulfur Dioxide Derivatives in Febrile-Seizure-Induced Hippocampal Damage by a Two-Photon Energy Transfer Cassette

Febrile seizure (FS), a frequently encountered seizure disorder in pediatric populations, can cause hippocampus damage. It has been elucidated that sulfur dioxide (SO₂) content is overproduced during the development of FS and related brain injury. Thus, monitoring in situ the level of endogenous SO₂ in FS-related models is helpful to estimate the pathogenesis of FS-induced brain injury, but the effect detection method remains to be explored. Herein, we developed a two-photon energy transfer cassette based on an acedan-anthocyanidin scaffold, TP-Ratio-SO₂, allowing us to achieve this purpose. TP-Ratio-SO₂ specifically responds to SO₂ derivatives (HSO₃^-/SO₃^2-) in an ultrafast fashion (less than 3 s), and HSO₃^-/SO₃^2- can be sensitively determined with a detection limit of 26 nM. Moreover, it exhibits significant changes in two well-resolved fluorescence emissions (Δλ = 140 nm) by reacting with HSO₃^-/SO₃^2-, behaving as a ratiometric fluorescent sensor. Importantly, ratiometric imaging of endogenous SO₂ derivatives generation in hyperpyretic U251 cells and in a rat model of FS-treated hippocampus damage was successfully carried out by TP-Ratio-SO₂, demonstrating that it may be a promising tool for studying the role of SO₂ in FS-associated neurological diseases.

Delivery of Phosphorescent Anticancer Iridium(III) Complexes by Polydopamine Nanoparticles for Targeted Combined Photothermal-Chemotherapy and Thermal/Photoacoustic/Lifetime Imaging

Recently, phosphorescent iridium complexes have demonstrated great potential as anticancer and imaging agents. Dopamine is a melanin-like mimic of mussel adhesive protein that can self-polymerize to form polydopamine (PDA) nanoparticles that demonstrate favorable biocompatibility, near-infrared absorption, and photothermal effects. Herein, PDA nanoparticles are functionalized with β-cyclodextrin (CD) substitutions, which are further assembled with adamantane-modified arginine-glycine-aspartic acid (Ad-RGD) tripeptides to target integrin-rich tumor cells. The thus formed PDA-CD-RGD nanoparticles can deliver a phosphorescent iridium(III) complexes LysoIr ([Ir(ppy)2(l)]PF6, ppy = 2-phenylpyridine, L = (1-(2-quinolinyl)-β-carboline) to form a theranostic platform LysoIr@PDA-CD-RGD. It is demonstrated that LysoIr@PDA-CD-RGD can be applied for targeted combined cancer photothermal-chemotherapy and thermal/photoacoustic/two-photon phosphorescence lifetime imaging under both in vitro and in vivo conditions. This work provides a useful strategy to construct multifunctional nanocomposites for the optimization of metal-based anticancer agents for further biomedical applications.

Oncosis-inducing cyclometalated iridium(iii) complexes

Oncosis is a non-apoptotic form of programmed cell death (PCD), which differs from apoptosis in both morphological changes and inner pathways, and might hold the key to defeating a major obstacle in cancer therapy - drug-resistance, which is often a result of the intrinsic apoptosis resistance of tumours. However, despite the fact that the term "oncosis" was coined and used much earlier than apoptosis, little effort has been made to discover new drugs which can initiate this form of cell death, in comparison to drugs inducing apoptosis or any other type of PCD. So herein, we present the synthesis of a series of mitochondria-targeting cyclometalated Ir(iii) complexes, which activated the oncosis-specific protein porimin and calpain in cisplatin-resistant cell line A549R, and determined their cytotoxicity against a wide range of drug-resistant cancer types. To the best of our knowledge, these complexes are the very first metallo-components to induce oncosis in drug-resistant cancer cells.

An AIE + ESIPT ratiometric fluorescent probe for monitoring sulfur dioxide with distinct ratiometric fluorescence signals in mammalian cells, mouse embryonic fibroblast and zebrafish

Sulfur dioxide (SO₂) is associated with serious diseases including lung cancer, cardiovascular diseases, and many neurological disorders. However, discrimination of the physiological and pathological functions of SO₂ in different living systems is restricted by the lack of functional molecular tools. To address this critical challenge, herein, we have developed a novel ratiometric probe, TPE-TE, for monitoring SO₂ with distinct ratiometric fluorescence signals in mammalian cells, mouse embryonic fibroblasts, and zebrafish via a combination of an ESIPT mechanism and the aggregate fluorescence method for the first time. The TPE-TE exhibits well-resolved emission peaks, high sensitivity, excellent selectivity, and low cytotoxicity. Moreover, this probe possesses higher sensitivity in an aqueous solution than the current probes. Taking advantage of these prominent features, we have achieved the detection of endogenous and exogenous SO₂ with distinct ratiometric fluorescence signals in mammalian cells and mouse embryonic fibroblast. For the detection of endogenous SO₂, probe-loaded HeLa cells exhibited stronger ratiometric fluorescence signals than HepG2 cells. For the detection of exogenous SO₂, it was found that macrophage cells exhibited stronger ratiometric fluorescence signals than cancer cells for the first time. Interestingly, mouse embryonic fibroblasts incubated with this probe showed unique ratiometric imaging. Moreover, TPE-TE could be suitable for ratiometric SO₂ imaging in living zebrafish.

Sulfur dioxide prodrugs: triggered release of SO2via a click reaction

Sulfur dioxide (SO₂) is being recognized as a possible endogenous gasotransmitter with importance on par with that of NO, CO, and H₂S. Herein we describe a series of SO₂ prodrugs that are activated for SO₂ release via a bioorthogonal click reaction. The release rate can be tuned by adjusting the substituents on the prodrug.

Real-time detection of oxalyl chloride based on a long-lived iridium(iii) probe

A series of luminescent iridium(iii) complexes were designed and evaluated for their ability to detect oxalyl chloride ((COCl)₂) at ambient temperature. In the presence of (COCl)₂, a double amidation reaction takes place at the diamino functionality of complex 1, leading to the switching-on of a long-lived red luminescence with a 9-fold enhanced emission. Complex 1 exhibited high sensitivity and selectivity, with a detection limit for (COCl)₂ at 32 nM. Additionally, complex 1 can be used to detect (COCl)₂ using a simple smartphone, allowing for the portable and real-time monitoring of (COCl)₂.

Lysosome targeted drugs: rhodamine B modified N^N-chelating ligands for half-sandwich iridium(iii) anticancer complexes

We designed and synthesized four rhodamine-modified half-sandwich iridium complexes ([(η⁵-Cp^x)Ir(N^N)Cl]PF₆).

A luminescent bimetallic iridium(iii) complex for ratiometric tracking intracellular viscosity

The luminescent bimetallic iridium probe C10 could distinguish cancer cells from normal cells and track viscosity changes during cell apoptosis.

A series of water-soluble A-π-A' typological indolium derivatives with two-photon properties for rapidly detecting HSO3 -/SO3 2- in living cells

It is believed that HSO₃ ^- and SO₃ ^2- play important roles in several physiological processes. However, probes with two-photon absorption to detect HSO₃ ^- or SO₃ ^2- in living cells are still limited. Herein, a series of novel indolium derivatives (L1-L4) with an A-π-A' structure was designed and synthesized as ratiometric probes to detect HSO₃ ^-/SO₃ ^2-in vitro. L3 and L4 display a colorimetric and ratiometric fluorescence dual response to HSO₃ ^-/SO₃ ^2- with a very fast (∼15 s) and high specificity, as well as low detection limits (∼22 nM). Furthermore, their detection is also carried out by using a two-photon excited fluorescence method. A nucleophilic addition reaction is proposed for the sensing mechanism, which is supported by MS, ¹H NMR, and density functional theory (DFT) investigations. Importantly, L3 was successfully used for detecting intrinsically generated intracellular HSO₃ ^-/SO₃ ^2- in cancerous cells under one- and two-photon excited fluorescence imaging.

Light-Up Mitophagy in Live Cells with Dual-Functional Theranostic Phosphorescent Iridium(III) Complexes

Phosphorescent Ir(III) complexes are expected to be new multifunctional theranostic platforms that enable the integration of imaging capabilities and anticancer properties. Mitophagy is an important selective autophagic process that degrades dysfunctional mitochondria. Until now, the regulation of mitophagy is still poorly understood. Herein, we present two phosphorescent cyclometalated iridium(III) complexes (Ir1 and Ir2) that can accumulate in mitochondria and induce mitophagy. Because of their intrinsic phosphorescence, they can specially image mitochondria and track mitochondrial morphological alterations. Mechanism studies show that Ir1 and Ir2 induce mitophagy by depolarization of mitochondrial membrane potential, depletion of cellular ATP, perturbation in mitochondrial metabolic status, and induction of oxidative stress. Moreover, no sign of apoptosis is observed in Ir1- and Ir2-treated cells under the same conditions that an obvious mitophagic response is initiated. We demonstrate that Ir1 is a promising theranostic agent that can induce mitophagy and visualize changes in mitochondrial morphology simultaneously.