Developing Red-Emissive Ruthenium(II) Complex-Based Luminescent Probes for Cellular Imaging

A Review on Recent Development of Phenothiazine-Based Chromogenic and Fluorogenic Sensors for the Detection of Cations, Anions, and Neutral Analytes

This review provides an in-depth examination of recent progress in the development of chemosensors, with a particular emphasis on colorimetric and fluorescent probes. It systematically explores various sensing mechanisms, including metal-to-ligand charge transfer (MLCT), ligand-to-metal charge transfer (LMCT), photoinduced electron transfer (PET), intramolecular charge transfer (ICT), and fluorescence resonance energy transfer (FRET), and elucidates the mechanism of action for cation and anion chemosensors. Special attention is given to phenothiazine-based fluorescence probes, highlighting their exceptional sensitivity and rapid detection abilities for a broad spectrum of analytes, including cations, anions, and small molecules. Phenothiazine chemosensors have emerged as versatile tools widely employed in a multitude of applications, spanning environmental and biomedical fields. Furthermore, it addresses existing challenges and offers insights into future research directions, aiming to facilitate the continued advancement of phenothiazine-based fluorescent probes.

An aggregation induced emission chalcone fluorescent probe with large Stokes shift for biothiols detection

The homeostasis of biothiols is closely related to the health of organisms. In view of the important role of biothiols, a fluorescent probe (7HIN-D) for the detection of intracellular biothiols was developed based on a simple chalcone fluorophore 7HIN with "ESIPT + AIE" characteristics. The probe 7HIN-D was obtained by introducing a biothiols specific DNBS (2,4-dinitrobenzenesulfonyl) unit as a fluorescence quencher to the fluorophore 7HIN. The nucleophilic substitution reaction between biothiols and probe 7HIN-D will release the DNBS unit and the fluorophore 7HIN, which exhibits a "turn on" AIE fluorescence with a large Stokes shift of 113 nm. The probe 7HIN-D displays high sensitivity and good selectivity to biothiols, and the detection limits value of probe 7HIN-D for GSH, Cys and Hcy were 0.384 μmol/L, 0.471 μmol/L and 0.638 μmol/L, respectively. In addition, the probe has been successfully used for fluorescence detection of endogenous biothiols in living cells due to its excellent performance, good biocompatibility and low cytotoxicity.

Recent development of small-molecule fluorescent probes based on phenothiazine and its derivates

Fluorescence probes, as analytical tools with the ability to perform rapid and sensitive detection of target analytes, have made outstanding contributions to environmental analysis and bioassays. Considering the expanding developments in these areas, fluorophores play a key role in the de-sign of fluorescence probes. Compared to classical fluorophores, phenothiazines with elec-tron-rich characteristics have been widely applied to construct electron donor-acceptor dyes, which exhibit outstanding performance in both fluorimetric and colorimetric analysis. In addition, these probes also exhibit the pronounced ability in both solution and solid-state, achieving portable detection for environmental analysis. In this review, we summarize recent advances in the performance of phenothiazine-based fluorescent probes for detecting various analytes, especially in cations, anions, ROS/RSS, enzyme and other small molecules. The general design rules, response mechanisms and practical applications of the probes are analyzed, followed by a discussion of exiting challenges and future research perspectives. It is hoped that this review will provide a few strategies for the development of phenothiazine-based fluorescent probes.

Determination and Imaging of Small Biomolecules and Ions Using Ruthenium(II) Complex-Based Chemosensors

Luminescence chemosensors are one of the most useful tools for the determination and imaging of small biomolecules and ions in situ in real time. Based on the unique photo-physical/-chemical properties of ruthenium(II) (Ru(II)) complexes, the development of Ru(II) complex-based chemosensors has attracted increasing attention in recent years, and thus many Ru(II) complexes have been designed and synthesized for the detection of ions and small biomolecules in biological and environmental samples. In this work, we summarize the research advances in the development of Ru(II) complex-based chemosensors for the determination of ions and small biomolecules, including anions, metal ions, reactive biomolecules and amino acids, with a particular focus on binding/reaction-based chemosensors for the investigation of intracellular analytes' evolution through luminescence analysis and imaging. The advances, challenges and future research directions in the development of Ru(II) complex-based chemosensors are also discussed.

Luminescent monomeric and dimeric Ru(ii) acyclic carbene complexes as selective sensors for NH3/amine vapor and humidity

A new class of luminescent bis(bipyridyl) Ru(ii) pyridyl acyclic carbene complexes with environmentally-sensitive dimerization equilibrium have been developed. Owing to the involvement of the orbitals of the diaminocarbene ligand in the emissive excited state, the phosphorescence properties of these complexes are strongly affected by H-bonding interactions with various H-bonding donor/acceptor molecules. With the remarkable differences in the emission properties of the monomer, dimer, and H-bonded amine adducts together with the change of the dimerization equilibrium, these complexes can be used as luminescent gas sensors for humidity, ammonia, and amine vapors. With the responses to amines and humidity and the corresponding change in the luminescence properties, a proof-of-principle for binary optical data storage with a reversible concealment process has been described.

Resorufin-based responsive probes for fluorescence and colorimetric analysis

The fluorescence imaging technique has attracted increasing attention in the detection of various biological molecules in situ and in real-time owing to its inherent advantages including high selectivity and sensitivity, outstanding spatiotemporal resolution and fast feedback. In the past few decades, a number of fluorescent probes have been developed for bioassays and imaging by exploiting different fluorophores. Among various fluorophores, resorufin exhibits a high fluorescence quantum yield, long excitation/emission wavelength and pronounced ability in both fluorescence and colorimetric analysis. This fluorophore has been widely utilized in the design of responsive probes specific for various bioactive species. In this review, we summarize the advances in the development of resorufin-based fluorescent probes for detecting various analytes, such as cations, anions, reactive (redox-active) sulfur species, small molecules and biological macromolecules. The chemical structures of probes, response mechanisms, detection limits and practical applications are investigated, which is followed by the discussion of recent challenges and future research perspectives. This review article is expected to promote the further development of resorufin-based responsive fluorescent probes and their biological applications.

A dicyanoisophorone-based highly sensitive and selective near-infrared fluorescent probe for sensing thiophenol in water samples and living cells

Thiophenol (PhSH) is an important sulfhydryl compound in organic synthesis, but it is also a volatile environmental pollutant with high toxicity to organisms. Herein, we reported a novel near-infrared (NIR) probe (1) for turn-on fluorescence detection of PhSH. The probe was prepared by coupling 2,4-dinitrophenyl (DNP) to a dicyanoisophorone-based fluorophore (2). PhSH can specifically perform a nucleophilic aromatic substitution on probe 1 and result in the release of fluorophore 2, thus achieving a turn-on fluorescence response (λ_em = 693 nm). A dramatic color change from red (λ_abs = 525 nm) to blue (λ_abs = 668 nm) was also observed. This fluorescent assay displayed a large Stokes shift (∼133 nm) and a high sensitivity for PhSH, as well as a low detection limit (34 nM). Moreover, probe 1 was successfully applied to monitor PhSH in real water samples and image PhSH in living cells.

Responsive Metal Complex Probes for Time-Gated Luminescence Biosensing and Imaging

The development of reliable bioanalytical probes for selective and sensitive detection of particular analytes in biological systems is essential for better understanding the roles of the analytes in their native contexts. In the last two decades, luminescent metal complexes have greatly contributed to the development of such probes for biosensing and imaging due to their unique spectral and temporal properties, controllable cell membrane permeability, and cytotoxicity. Conjugating an analyte-activatable moiety to the metal complex luminophores allows the production of responsive metal complex probes for this analyte detection. Owing to their long-lifetime emissions, the responsive metal complex probes are accessible to the technique of time-gated luminescence (TGL) detection and imaging. With a delay time after pulsed excitation, the TGL technique allows for collection of only long-lived luminescence from responsive metal complex probes, while filtering out short-lived background autofluorescence, providing a background-free approach for the detection and imaging of the analyte at subcellular and/or molecular levels. Responsive metal complex probes, therefore, have emerged as complementary sensing and imaging tools of organic dye-based fluorescent probes for the in situ detection of analytes in complicated biological environments.In this Account, we describe the advances in the development of metal complex probes and their applications for TGL bioassays with particular focus on our efforts made in this field. We first introduce the photophysical/-chemical properties of luminescent metal complexes, including lanthanide (europium and terbium) and transition metal (ruthenium and iridium) complexes. The luminescence lifetimes (τ) of lanthanide and transition metal complexes are at micro/millisecond (μs/ms) and hundreds/thousands nanosecond (ns) levels, respectively. The emission lifetimes are significantly longer than the autofluorescence lifetime (τ < 10 ns) of biological samples. Such long-lived luminescence of these metal complexes enables our research on demonstrating responsive probes for background-free TGL detection of some reactive biomolecules, such as reactive oxygen/nitrogen species (ROS/RNS) and biothiols.We conclude this Account by outlining the future directions to further develop new generation responsive TGL probes for promoting their practical applications. The responsive TGL probes are expected to be translated for biomedical and/or (pre)clinical investigations of biomolecules in situ. Reversibility, lower toxicity, ability of excitation at longer wavelength, and potential to be translated are key criteria for the development of next-generation probes. We also anticipate that further development of responsive TGL probes will contribute to the bioassay in more challenging biological systems, such as plants that have significant higher background autofluorescence than animals.

Tuning the Luminescent Properties of Ruthenium(II) Amino-1,10-Phenanthroline Complexes by Varying the Position of the Amino Group on the Heterocycle

Eight 1,10-phenanthrolines bearing one or two 2-(1-adamantyloxy)ethylamino substituents attached to different positions of the heterocyclic core were prepared according to S_N Ar or palladium-catalyzed amination reactions. Their reaction with cis-Ru(bpy)₂ Cl₂ (bpy=2,2'-bipyridine) was investigated and Ru(bpy)₂ (L)(PF₆ )₂ (phen=1,10-phenanthroline) (L=amino-substituted 1,10-phenanthroline) complexes were obtained in good yields. The electronic structure and emissive properties of these complexes are strongly dependent on the position of the amino substituent in the heterocycle. Emission bands of the complexes bearing 2- and 4-substituted 1,10-phenanthroline ligands are red-shifted (up to 56 nm) and less intense compared to that of the parent [Ru(phen)(bpy)₂ ](PF₆ )₂ . In contrast, the introduction of the substituent in 3- or 5-position of 1,10-phenanthroline ring induces only small decrease of luminescence and the brightness of the complex with the 3-substituted ligand is comparable to that of the parent complex.

A highly sensitive Ru(ii) complex-based phosphorescent probe for thiophenol detection with aggregation-induced emission characteristics

A novel AIEE-active Ru(ii) based phosphorescent probe was designed for the detection of thiophenol in aqueous solution and on test paper.

Recent Progress in the Development of Fluorescent Probes for Thiophenol

Thiophenol (PhSH) belongs to a class of highly reactive and toxic aromatic thiols with widespread applications in the chemical industry for preparing pesticides, polymers, and pharmaceuticals. In this review, we comprehensively summarize recent progress in the development of fluorescent probes for detecting and imaging PhSH. These probes are classified according to recognition moieties and are detailed on the basis of their structures and sensing performances. In addition, prospects for future research are also discussed.

Enantioselective and Differential Fluorescence Lifetime Imaging of Nucleus and Nucleolus by the Two Enantiomers of Chiral Os(II) Polypyridyl Complex

The nucleolus is an important subnuclear structure, but very few dyes are available for nucleolar imaging. Here we show that the Λ-enantiomer of [Os(phen)₂(dppz)]Cl₂ can differentially distinguish the nucleolus from nucleus in living cells with tetrachlorophenolate as counteranion, while the Δ-enantiomer can do so in fixed cells by FLIM imaging. Further studies with three specific metabolic inhibitors for nucleolar protein synthesis found that the lifetime changes of the two enantiomers in the nucleolus can reflect the alteration of the cellular microenvironment, which is related to the general pathological status of the nucleolus. We then observed dynamical architecture changes of the nucleolus, chromosome and spindle apparatus during cell differentiation by these two enantiomers. The chiral Os(II) complex shows many advantages as compared to the commercially available nucleolus dye Syto 9: it displays a much larger Stokes shift value with a near-red emission and a longer lifetime, it can image spindle apparatus during mitosis, and more importantly, it is enantioselective.

A Novel Ruthenium-based Molecular Sensor to Detect Endothelial Nitric Oxide

Nitric oxide (NO) is a key regulator of endothelial cell and vascular function. The direct measurement of NO is challenging due to its short half-life, and as such surrogate measurements are typically used to approximate its relative concentrations. Here we demonstrate that ruthenium-based [Ru(bpy)₂(dabpy)]²⁺ is a potent sensor for NO in its irreversible, NO-bound active form, [Ru(bpy)₂(T-bpy)]²⁺. Using spectrophotometry we established the sensor’s ability to detect and measure soluble NO in a concentration-dependent manner in cell-free media. Endothelial cells cultured with acetylcholine or hydrogen peroxide to induce endogenous NO production showed modest increases of 7.3 ± 7.1% and 36.3 ± 25.0% respectively in fluorescence signal from baseline state, while addition of exogenous NO increased their fluorescence by 5.2-fold. The changes in fluorescence signal were proportionate and comparable against conventional NO assays. Rabbit blood samples immediately exposed to [Ru(bpy)₂(dabpy)]²⁺ displayed 8-fold higher mean fluorescence, relative to blood without sensor. Approximately 14% of the observed signal was NO/NO adduct-specific. Optimal readings were obtained when sensor was added to freshly collected blood, remaining stable during subsequent freeze-thaw cycles. Clinical studies are now required to test the utility of [Ru(bpy)₂(dabpy)]²⁺ as a sensor to detect changes in NO from human blood samples in cardiovascular health and disease.

"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.

Reaction-based fluorescent probe for the selective and sensitive detection of thiophenols with a large Stokes shift and its application in water samples

Although widely used in organic synthesis, pharmaceuticals and agrochemicals, thiophenol has brought about a series of ecological problems due to its high toxicity.

A novel fluorescence turn-on probe for the selective detection of thiophenols by caged benzooxazolidinoindocyanine

Based on a strategy involving 2,4-dinitrophenyl ether and functionalized caged benzooxazolidinoindocyanine, a fluorescence turn-on probe for the selective detection of thiophenols was developed.

Highly sensitive iridium(iii) complex-based phosphorescent probe for thiophenol detection

An iridium(iii) complex could serve as a highly sensitive, long-lifetime phosphorescence thiophenol probe, obtained through rational design.

Installing an additional emission quenching pathway in the design of iridium(III)-based phosphorogenic biomaterials for bioorthogonal labelling and imaging

We report the synthesis, characterization, photophysical and electrochemical behaviour and biological labelling applications of new phosphorogenic bioorthogonal probes derived from iridium(III) polypyridine complexes containing a 1,2,4,5-tetrazine moiety. In contrast to common luminescent cyclometallated iridium(III) polypyridine complexes, these tetrazine complexes are almost non-emissive due to effective Förster resonance energy transfer (FRET) and/or photoinduced electron transfer (PET) from the excited iridium(III) polypyridine unit to the appended tetrazine moiety. However, they exhibited significant emission enhancement upon reacting with (1R,8S,9s)-bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN-OH) (ca. 19.5-121.9 fold) and BCN-modified bovine serum albumin (BCN-BSA) (ca. 140.8-1133.7 fold) as a result of the conversion of the tetrazine unit to a non-quenching pyridazine derivative. The complexes were applied to image azide-modified glycans in live cells using a homobifunctional crosslinker, 1,13-bis((1R,8S,9s)-bicyclo[6.1.0]non-4-yn-9-ylmethyloxycarbonylamino)-4,7,10-trioxatridecane (bis-BCN).

Steering graphene quantum dots in living cells: lighting up the nucleolus

A novel graphene quantum dot capable of lighting up the nucleoli of living cells has been developed.

Functional Materials Based on Metal-Containing Polymers

Since the dawn of human civilization, there has been a demand for materials that include ceramics, metals, and polymers. Increasing demand as well as the need for enhanced performance has driven material scientists to research metal-containing polymers as complements of these materials. Consequently, metal-containing polymers that integrate the excellent thermal, electronic, optical, and magnetic properties of metals with the lightweight, low cost, and in some cases, the chemical stability of organic-based polymers have been designed, and used as catalysts, sensors, ceramic precursors, magnetic materials, and electrical conductors. This chapter provides an overview of some of these functional metal-containing polymers.

Exploitation and application of a highly sensitive Ru(II) complex-based phosphorescent chemodosimeter for Hg2+ in aqueous solutions and living cells

A novel Ru(II) complex-based phosphorescent probe Rubpy-1 was designed and synthesized conveniently by incorporating of chemodosimeter into the luminophor, which exhibits good water solubility, longer excitation wavelength, and rapid turn-on phosphorescent response only toward Hg(2+) in aqueous system under physiological pH. The spectral response mechanism and Hg(2+)-promoted structure change of the chemodosimeter were analyzed in detail by theoretical calculations and electrospray ionization mass spectrometry. When time-resolved photoluminescence techniques were used, the Rubpy-1 could eliminate effectively the signal interference from the short-lived background fluorescence in complicated media, accompanied by the significant improvement of the signal-to-noise ratio and the accuracy of the detection. Furthermore, Rubpy-1 showed low cytotoxicity and excellent membrane permeability toward living cells, which was successfully applied to monitor intracellular Hg(2+) effectively by confocal luminescence imaging.

Two dinuclear Ru(II) polypyridyl complexes with different photophysical and cation recognition properties

Two dinuclear Ru(II) polypyridyl complexes functionalized with vacant coordination sites have been designed and synthesized. Their photophysical properties and interactions with various metal ions have been investigated at room temperature. The two complexes exhibit different UV/Vis absorption and emission intensities. When titrated with various metal ions, complex [{Ru(bpy)2}2(μ2-L(1))](4+) exhibits a notable fluorescence quenching in the presence of Cu(2+) in H2O-CH3CN media (1:1, v/v); its analogous complex [{Ru(bpy)2}2(μ2-L(2))](4+) exhibits no cation selectivity, the fluorescence intensity of complex [{Ru(bpy)2}2(μ2-L(2))](4+) has been enhanced by several transition metal ions due to prevention of the photo-induced electron transfer process. The fluorescence titration spectra and Benesi-Hildebrand expression reveal the formation of a 1:1 bonding mode between [{Ru(bpy)2}2(μ2-L(1))](4+) and Cu(2+) ion with the association constant of 5.50×10(4) M(-1).

Oximated ruthenium tris-bipyridyl complex: synthesis and luminescent response specifically for ClO− in water containing multiple ions

A novel Ru(bpy)₃²⁺ complex capable of a selective response to ClO⁻ in pH 4–11 water in the presence of other anions and cations is reported.

A functional ruthenium(ii) complex for imaging biothiols in living bodies

A functional ruthenium(ii) complex that can act as a luminescent probe for imaging biothiols in living bodies has been successfully developed.

A novel heterobimetallic Ru(ii)–Gd(iii) complex-based magnetoluminescent agent for MR and luminescence imaging

A novel heterobimetallic ruthenium(II)–gadolinium(III) complex, Ru–Gd, has been developed for luminescence and an vivo T₁-weighted MR imaging agent.

A selective, long-lived deep-red emissive ruthenium(II) polypyridine complexes for the detection of BSA

A selective, label free luminescence sensor for bovine serum albumin (BSA) is investigated using ruthenium(II) complexes over the other proteins. Interaction between BSA and ruthenium(II) complexes has been studied using absorption, emission, excited state lifetime and circular dichroism (CD) spectral techniques. The luminescence intensity of ruthenium(II) complexes (I and II), has enhanced at 602 and 613 nm with a large hypsochromic shift of 18 and 5 nm respectively upon addition of BSA. The mode of binding of ruthenium(II) complexes with BSA has analyzed using computational docking studies.

A heterodinuclear RuIr metal complex for direct imaging of rRNA in living cells

A novel dual luminescence heterodinuclear RuIr complex for RNA detection was developed, which was successfully used to image rRNA in living cells.

Development of a ruthenium(II) complex-based luminescent probe for hypochlorous acid in living cells

A novel Ru(II) complex, [Ru(bpy)2(DNPS-bpy)](PF6)2 (bpy: 2,2'-bipyridine, DNPS-bpy: 4-(2,4-dinitrophenylthio)methylene-4'-methyl-2,2'-bipyridine), has been designed and synthesized as a highly sensitive and selective luminescence probe for the recognition and detection of hypochlorous acid (HOCl) in living cells by exploiting a "signaling moiety-recognition linker-quencher" sandwich approach. The complex possesses large stokes shift (170 nm), long emission wavelength (626 nm), and low cytotoxicity. Owing to the effective photoinduced electron transfer (PET) from Ru(II) center to the electron acceptor, 2,4-dinitrophenyl (DNP), the red-emission of bipyridine-Ru(II) complex was completely withheld. In aqueous media, HOCl can trigger an oxidation reaction to cleave the DNP moiety from the Ru(II) complex, which results in the formation of a highly luminescent bipyridine-Ru(II) complex derivative, [Ru(bpy)2(COOH-bpy)](PF6)2 (COOH-bpy: 4'-methyl-2,2'-bipyridyl-4-carboxylic acid), accompanied by a 190-fold luminescence enhancement. Cell imaging experimental results demonstrated that [Ru(bpy)2(DNPS-bpy)](PF6)2 is membrane permeable, and can be applied for capturing and visualizing the exogenous/endogenous HOCl molecules in living cell samples. The development of this Ru(II) complex probe not only provides a useful tool for monitoring HOCl in living systems, but also strengthens the application of transition metal complex-based luminescent probes for bioimaging.