Fluorescent Chemosensors Based on Spiroring-Opening of Xanthenes and Related Derivatives

Bioorthogonally Assisted Phototherapy: Recent Advances and Prospects

Photoresponsive materials offer excellent spatiotemporal control over biological processes and the emerging phototherapeutic methods are expected to have significant effects on targeted cancer therapies. Recent examples show that combination of photoactivatable approaches with bioorthogonal chemistry enhances the precision of targeted phototherapies and profound implications are foreseen particularly in the treatment of disperse/diffuse tumors. The extra level of on-target selectivity and improved spatial/temporal control considerably intensified related bioorthogonally assisted phototherapy research. The anticipated growth of further developments in the field justifies the timeliness of a brief summary of the state of the art.

A naphthalimide functionalized fluoran with AIE effect for ratiometric sensing Hg2+ and cell imaging application

The pollution caused by mercury ions (Hg²⁺) poses a potential threat to public health. Therefore, monitoring Hg²⁺ concentration in the environment is necessary and significant. In this work, a naphthalimide functionalized fluoran dye NAF has been prepared, which shows a new red-shift in emission at 550 nm with the maximum intensity in a mixture of water-CH₃CN (v/v = 7/3) due to aggregating induced emission (AIE) effect. Meanwhile, NAF can be employed as a Hg²⁺ ions sensor, which displays a selective and sensitive response to Hg²⁺ ions by the reduced fluorescence of naphthalimide fluorophore and increased fluorescence of fluoran group, respectively, showing ratiometric fluorescence signal changes with more than 65-fold emission intensity ratio increase and naked eyes visible color change. In addition, the response time is fast (within 1 min) and the sensing can be conducted in a wide pH range (4.0-9.0). Moreover, the detection limit has been evaluated to be 5.5 nM. The sensing mechanism may be attributed to the formation of a π-extended conjugated system due to the Hg²⁺ ions-induced conversion of spironolactone to the ring-opened form, partially accompanied by the fluorescence resonance energy transfer (FRET) process. Significantly, NAF exhibits suitable cytotoxicity to living HeLa cells, which allows it to be utilized for ratiometric imaging of Hg²⁺ ions assisted by confocal fluorescence imaging.

Highly selective chemosensor for the sensitive detection of Hg2+ in aqueous media and its cell imaging application

The deleterious toxicity of Hg²⁺ on ecological and biological system makes it crucial for the precise monitoring of Hg²⁺. Herein, we prepared a novel "turn-on" chemosensor N'-(4-(methylthio)butan-2-ylidene) rhodamine B hydrazide (denoted as MTRH) by a simple two-step reaction. MTRH exhibited an ultra-low detection limit (LOD) in fluorescence measurement of Hg²⁺ in pure aqueous media, which was estimated to be 1.3 × 10^-9 mol·L^-1. Moreover, the proposed chemosensor holds the ability of visualizing Hg²⁺ by the distinct color change of the solution. The corresponding recognition mechanism was investigated by Job's plots, mass spectrometry and DFT calculation analysis. Importantly, the characteristics such as high sensitivity, low cytotoxicity and good biocompatibility of MTRH exhibited in the application of detecting Hg²⁺ in real water sample and bioimaging of intracellular Hg²⁺ prove that MTRH is a promising tool to evaluate the levels of Hg²⁺ in complex biological systems.

Near-infrared fluorophore IR-61 delays postovulatory aging of mouse oocytes through suppressing oxidative stress mediated by mitochondrial protection

Postovulatory aging can trigger deterioration of oocyte quality and subsequent embryonic development, and thus reduce the success rates of assisted reproductive technology (ART). The molecular mechanisms underlying postovulatory aging, and preventative strategies, remain to be explored. The near-infrared fluorophore IR-61, a novel heptamethine cyanine dye, has the potential for mitochondrial targeting and cell protection. In this study, we found that IR-61 accumulated in oocyte mitochondria and reduced the postovulatory aging-induced decline in mitochondrial function, including mitochondrial distribution, membrane potential, mtDNA number, ATP levels, and mitochondrial ultrastructure. In addition, IR-61 rescued postovulatory aging-caused oocyte fragmentation, defects in spindle structure, and embryonic developmental potential. RNA sequencing analysis indicated that the postovulatory aging-induced oxidative stress pathway might be inhibited by IR-61. We then confirmed that IR-61 decreased the levels of reactive oxygen species and MitoSOX, and increased GSH content in aged oocytes. Collectively, the results indicate that IR-61 may prevent postovulatory aging by rescuing oocyte quality, promoting successful rate in ART procedure.

Rapid detection of carbamate nerve agent analogues using dually functionalized gold nanoclusters

Carbamate nerve agents (CMNAs) are a type of lethal cholinesterase inhibitor with one or more quaternary amine centres and aromatic rings. CMNAs have been recently added to the Annex on Chemicals of the Chemical Weapons Convention (CWC) and Schedules of Controlled Chemicals of China. In this study, a rapid, sensitive and selective method was developed for the fluorescence detection of ambenonium chloride (AC) through host-guest and electrostatic dual interactions between AC and cyclodextrin/11-mercaptoundecanoic acid (CD/MUA) dually functionalized gold nanoclusters (AuNCs). Through this method, AC was detected with a limit of detection of 10.0 ng/mL. Method evaluation showed high selectivity towards AC over other related compounds. The practical applicability was verified, as satisfactory recoveries were obtained for AC spiked in river water and urine, as well as Proficiency Test samples from Organisation for the Prohibition of Chemical Weapons (OPCW). In addition, a fluorescence sensing array comprising four AuNCs was designed to distinguish six carbamates and structurally similar compounds. This method provides a potential approach for the rapid, sensitive and selective recognition and detection of CMNAs.

THQ-Xanthene: An Emerging Strategy to Create Next-Generation NIR-I/II Fluorophores

Near-infrared fluorescence imaging is vital for exploring the biological world. The short emissions (<650 nm) and small Stokes shifts (<30 nm) of current xanthene dyes obstruct their biological applications since a long time. Recently, a potent and universal THQ structural modification technique that shifts emission to the NIR-I/II range and enables a substantial Stokes shift (>100 nm) for THQ-modified xanthene dyes is established. Thus, a timely discussion of THQ-xanthene and its applications is extensive. Hence, the advent, working principles, development trajectory, and biological applications of THQ-xanthene dyes, especially in the fields of fluorescence probe-based sensing and imaging, cancer theranostics, and super-resolution imaging, are introduced. It is envisioned that the THQ modification tactic is a simple yet exceptional approach to upgrade the performance of conventional xanthene dyes. THQ-xanthene will advance the strides of xanthene-based potentials in early fluorescent diagnosis of diseases, cancer theranostics, and imaging-guided surgery.

Domino protocol for the synthesis of diversely functionalized derivatives of a novel fused pentacyclic antioxidant/anticancer fluorescent scaffold: Pyrazolo[5'',1'':2',3']pyrimido[4',5':5,6][1,4]thiazino[2,3-b]quinoxaline

Rising to the challenge of formidable multi-step reaction needed for the synthesis of polycyclic compounds, an efficient one-pot two-step procedure for the synthesis of densely functionalized novel pyrazolo[5″,1'':2',3']pyrimido[4',5':5,6] [1,4]thiazino[2,3-b]quinoxalines from synthetically accessible starting materials 6-bromo-7-chloro-3-cyano-2-(ethylthio)-5-methylpyrazolo[1,5-a]pyrimidine, 3-aminoquinoxaline-2-thiol and some readily accessible alkyl halides was established. The domino reaction pathway involves cyclocondensation/N-alkylation sequence in K₂CO₃/N,N-dimethyl formamide under heating condition. DPPH free radical scavenging activity of all synthesized pyrazolo[5″,1'':2',3']pyrimido[4',5':5,6][1,4]thiazino[2,3-b]quinoxalines was evaluated to determine their antioxidant potentials. IC₅₀ values were recorded in the range of 29-71 μM. N-benzyl substituted derivative represented the most effective antioxidant activity as well as antiproliferative activity against MCF-7 cells. Moreover, fluorescence in solution for these compounds exhibited strong red emission in the visible region (λ_flu. = 536-558 nm) with good to excellent quantum yields (61-95%). Due to their interesting fluorescence properties, these novel pentacyclic fluorophores can be used as fluorescent markers and probes for studies in biochemistry and pharmacology.

Recent progress in the development of fluorescent probes for imaging pathological oxidative stress

Oxidative stress is closely related to the physiopathology of numerous diseases. Reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species (RSS) are direct participants and important biomarkers of oxidative stress. A comprehensive understanding of their changes can help us evaluate disease pathogenesis and progression and facilitate early diagnosis and drug development. In recent years, fluorescent probes have been developed for real-time monitoring of ROS, RNS and RSS levels in vitro and in vivo. In this review, conventional design strategies of fluorescent probes for ROS, RNS, and RSS detection are discussed from three aspects: fluorophores, linkers, and recognition groups. We introduce representative fluorescent probes for ROS, RNS, and RSS detection in cells, physiological/pathological processes (e.g., Inflammation, Drug Induced Organ Injury and Ischemia/Reperfusion Injury etc.), and specific diseases (e.g., neurodegenerative diseases, epilepsy, depression, diabetes and cancer, etc.). We then highlight the achievements, current challenges, and prospects for fluorescent probes in the pathophysiology of oxidative stress-related diseases.

Colorimetric and near-infrared spectrophotometric monitoring of bisulfite using glyoxal modified chromenylium-cyanine chemosensor: Smartphone and paper strip applications for on-site food and beverages control

Detection of bisulfite (HSO₃^-) in food and beverages has vital importance because the excessive amount leads to ill effects on the human body. Colorimetric and fluorometric chromenylium-cyanine-based chemosensor CyR was synthesized and applied for high selective and sensitive analysis of HSO₃^- in red wine, rose wine and, granulated sugar with high recovery ranges and very fast response time without any interference from other competitive species. The limits of detection (LOD) for the UV-Vis and fluorescence titrations were found as 11.5 μM and 3.77 μM, respectively. The on-site and very rapid methods based on paper strips and smartphone which depend on the color changes from yellow to green have been successfully developed to analyze HSO₃^- concentration (10^-5-10^-1 M for paper strip and 163-1205 μM for smartphone). CyR and the bisulfite-adduct formed in the nucleophilic addition reaction with HSO₃^- were verified by FT-IR, ¹H NMR, and MALDI-TOF results as well as Single-Crystal X-Ray Crystallography for CyR.

Influence of ortho group in rhodamine B hydrazide based Schiff base for selective recognition of Cu2+ and Fe3+ ions: A mechanistic approach by DFT and colorimetric studies

Herein we have implemented a computational approach in designing sensor molecules for the selective recognition of Cu²⁺ and Fe³⁺ ions. Seven rhodamine B hydrazide-based Schiff base derivatives were designed and analysed their chemosensing properties against Cu²⁺ and Fe³⁺ ions in ethanol solution theoretically. The theoretical calculations revealed that the selective recognition of Cu²⁺ and Fe³⁺ ions takes place via spirolactam ring-opening and there is a pivotal role of ortho substituents and N-heteroatoms. The two best chemosensors were synthesised and used for the detection of Cu²⁺ and Fe³⁺ ions by colorimetric methods.

Breaking the Photostability and pH Limitation of Halo-Fluoresceins through Chitosan Conjugation

Halo-fluoresceins are widely used in cell and tissue staining, intracellular sensing, and photodynamic therapy, but their notorious photo-instability and pH dependence restrict their applications, especially in long-term visible light exposure and acidic environments. To overcome these limitations, here a strategy is proposed of conjugating chitosan with the carboxyl group of halo-fluorescein (CS-halofluorescein). The cross-linked polymer chains and the hydrogen-bonding networks of chitosan help shielding out ¹ O₂ from direct attacking the encapsulated halo-fluoresceins, leading to a two orders of magnitude lower photobleaching rate. Meanwhile, the condensation of primary amines of chitosan with the carboxyl group on halo-fluorescein blocks the pH-dependent intramolecular spirocyclization, leading to pH-inert fluorescein derivatives. The greatly improved photostability and pH inertness of CS-halofluoresceins can be harvested for aerobic photoredox synthesis and photodynamic bacteria inactivation in extremely acidic media. Moreover, food additive nature of chitosan and erythrosine (TIF) and excellent film-forming property of chitosan allow coating-based light-assisted preservation of perishable fruits, leading to appreciably extended shelf life of fruits (e.g., perishable strawberry, rt: > 3 days; 4 °C: > 5 days).

Xanthene-based functional dyes: towards new molecules operating in the near-infrared region

Xanthene-based functional dyes have diverse applications in life science and materials science. A current challenge is to develop new dyes with suitable physicochemical properties, including near-infrared (NIR) operation, for advanced biological applications such as medical diagnostics and molecular imaging. In this review, we first present an overview of xanthene-based functional dyes and then focus on synthetic strategies for modulating the absorption and fluorescence of dyes that operate in the NIR wavelength region with bright emission and good photostability. We also introduce our work on aminobenzopyranoxanthenes (ABPXs) and bridged tetra-aryl-p-quinodimethanes (BTAQs) as new xanthene-based far-red (FR)/NIR absorbing/emitting molecules whose absorption/fluorescence wavelengths change in response to external stimuli.

Asymmetric Synthesis of Triphenylmethanes via Organocatalytic Regio- and Enantioselective Friedel-Crafts Alkylation of Aniline Derivatives

Herein, we described a highly regio- and enantioselective Friedel-Crafts alkylation of aniline derivatives with in situ generated ortho-quinone methides enabled by chiral phosphoric acid, furnishing a wide range of enantioenriched triarylmethanes bearing three similar benzene rings in high yields (up to 98%) with excellent stereoselectivities (up to 98% ee). Furthermore, the large-scale reactions and diversified transformations of product demonstrate the practicality of the protocol. Density functional theory calculations elucidate the origin of the enantioselectivity.

Differential response for multiple ions: a smart probe to construct optically tunable molecular logic systems

A rhodamine-based optical probe has been designed through a one-pot synthetic protocol involving phenanthroline as a binding motif. The compound showed a bright pink coloration specifically upon the addition of Cu²⁺ and Hg²⁺ ions. However, the appearance of bright red fluorescence was observed only in the presence of Hg²⁺. Considering both, we can detect and discriminate these two ions even at ppb level concentration. Furthermore, these in situ generated metal complexes were utilized for the selective recognition of CN^- and I^- ions. Pre-coated TLC plates were developed for rapid on-site detection of these toxic ions even in remote places. Finally, on a single molecular probe based on differential opto-chemical interactions with different ions (Cu²⁺, Hg²⁺, CN^- and I^-), we were able to design numerous trivial (OR, NOR) and non-trivial (INHIBIT, IMPLICATION, COMPLEMENT, TRANSFER, NOT-TRANSFER) logic gates. Most fascinatingly, we can switch the logic response from one type to another by simply tuning only the optical output channel.

A general highly efficient synthesis of biocompatible rhodamine dyes and probes for live-cell multicolor nanoscopy

The development of live-cell fluorescence nanoscopy is powered by the availability of suitable fluorescent probes. Rhodamines are among the best fluorophores for labeling intracellular structures. Isomeric tuning is a powerful method for optimizing the biocompatibility of rhodamine-containing probes without affecting their spectral properties. An efficient synthesis pathway for 4-carboxyrhodamines is still lacking. We present a facile protecting-group-free 4-carboxyrhodamines' synthesis based on the nucleophilic addition of lithium dicarboxybenzenide to the corresponding xanthone. This approach drastically reduces the number of synthesis steps, expands the achievable structural diversity, increases overall yields and permits gram-scale synthesis of the dyes. We synthesize a wide range of symmetrical and unsymmetrical 4-carboxyrhodamines covering the whole visible spectrum and target them to multiple structures in living cells - microtubules, DNA, actin, mitochondria, lysosomes, Halo-tagged and SNAP-tagged proteins. The enhanced permeability fluorescent probes operate at submicromolar concentrations, allowing high-contrast STED and confocal microscopy of living cells and tissues.

Monitoring Hg2+ and MeHg+ poisoning in living body with an activatable near-infrared II fluorescence probe

Noninvasively imaging mercury poisoning in living organisms is critical to understanding its toxicity and treatments. Especially, simultaneous fluorescence imaging of Hg²⁺ and MeHg⁺in vivo is helpful to disclose the mysteries of mercury poisoning. The key limitation for mercury imaging in vivo is the low imaging signal-to-background ratio (SBR) and limited imaging depth, which may result in unreliable detection results. Here, we designed and prepared a near-infrared II (NIR II) emissive probe, NIR-Rh-MS, leveraging the "spirolactam ring-open" tactic of xanthene dyes for in situ visualization of mercury toxicity in mice. The probe produces a marked fluorescence signal at 1015 nm and displays good linear responses to Hg²⁺ and MeHg⁺ with excellent sensitivity, respectively. The penetration experiments elucidate that the activated NIR-II fluorescence signal of the probe penetrates to a depth of up to 7 mm in simulated tissues. Impressively, the probe can monitor the toxicity of Hg²⁺ in mouse livers and the accumulation of MeHg⁺ in mouse brains via intravital NIR-II imaging for the first time. Thus, we believe that detecting Hg²⁺ and MeHg⁺ in different organs with a single NIR-II fluorescence probe in mice would assuredly advance the toxicologic study of mercury poisoning in vivo.

A Selective and ''Off-On'' Fluorescent Chemosensor Based on Fluorescein for Al3+: Synthesis, Characterization, Spectroscopy Analyses, and DFT Calculation

An efficient fluorescent cation chemosensor based on fluorescein L4 was well prepared and identified with spectroscopy analyses. UV-vis and fluorescence measurements examined the analyte complexation of the L4 with various cations, demonstrating a clear tendency to Al³⁺ ion. In the Job plot study, a stoichiometry ratio of a complex between L4 and Al³⁺ ion was determined to be 1: 2 (L4: Al³⁺). A stoichiometry ratio of complex between L4 and Al³⁺ ion was determined to be 1: 2 (L4: Al³⁺) using the Job plot. The association constant (K_a) of the L4-Al³⁺ complex was found 2.8 × 10⁷ M^-2. The obtained limit of detection (LOD) value (1.37 × 10^-6 M for Al³⁺) exhibited the considerable sensitivity of the chemosensor L4 to Al³⁺ ion. DFT/TD-DFT calculations have also been employed to support the binding mode and photophysical properties of the complexation of chemosensor L4 to Al³⁺ ion and also to investigate the enhancement of L4 fluorescence by Al³⁺ ion.

Aza-phenol Based Macrocyclic Probes Design for "CHEF-on" Multi Analytes Sensor: Crystal Structure Elucidation and Application in Biological Cell Imaging

Metal bound macrocyclic compounds found in biological systems inspired us to design and synthesize two Robson-type macrocyclic Schiff-base chemosensors, H ₂ L1 (H ₂ L1=1,11-dimethyl-6,16-dithia-3,9,13,19-tetraaza-1,11(1,3)-dibenzenacycloicosaphane-2,9,12,19-tetraene-1,11-diol) and H ₂ L2 (H ₂ L2=1,11-dimethyl-6,16-dioxa-3,9,13,19-tetraaza-1,11(1,3)-dibenzenacycloicosaphane-2,9,12,19-tetraene-1,11-diol). Both the chemosensors have been characterized with different spectroscopic techniques. They act as multianalyte sensor and exhibit "turn-on" fluorescence toward different metal ions in 1X PBS (Phosphate Buffered Saline) solution. In presence of Zn²⁺, Al³⁺, Cr³⁺ and Fe³⁺ ions, H ₂ L1 exhibits ∼6-fold enhancement of emission intensity, while H ₂ L2 shows ∼6-fold enhancement of emission intensity in the presence of Zn²⁺, Al³⁺ and Cr³⁺ ions. The interaction between the different metal ion and chemosensor have been examined by absorption, emission, and ¹H NMR spectroscopy as well as by ESI-MS⁺ analysis. We have successfully isolated and solved the crystal structure of the complex [Zn(H ₂ L1)(NO₃)]NO₃ (1) by X-ray crystallography. The crystal structure of 1 shows 1:1 metal:ligand stoichiometry and helps to understand the observed PET-Off-CHEF-On sensing mechanism. LOD values of H ₂ L1 and H ₂ L2 toward metal ions are found to be ∼10^-8 and ∼10^-7 M, respectively. Large Stokes shifts of the probes against analytes (∼100 nm) make them a suitable candidate for biological cell imaging studies. Robson type phenol based macrocyclic fluorescence sensors are very scarce in the literature. Therefore, the tuning of structural parameters as the number and nature of donor atoms, their relative locations and presence of rigid aromatic groups can lead to the design of new chemosensors, which can accommodate different charged/neutral guest(s) inside its cavity. The study of the spectroscopic properties of this type of macrocyclic ligands and their complexes might open a new avenue of chemosensors.

Redox-Reversible Near-Infrared Fluorescent Probe for Imaging of Acute Kidney Oxidative Injury and Remedy

Drug-induced acute kidney injury (DIAKI) is associated with high morbidity and mortality. It remains a diagnostic and therapeutic dilemma due to failure of providing unambiguous real-time feedback on nephrotoxicity, which is regarded as a serious problem in clinics. Herein, we report a reversible fluorescence probe, NRN, to monitor the ONOO^-/GSH in an acute kidney injury model. The NRN near-infrared fluorescent probe features a big Stokes shift (83 nm), which was oxidized by ONOO^- and reduced by succussive glutathione (GSH) with excellent selectivity and good sensitivity (detection limit: 418 nM and 0.28 mM, respectively). Taking the reversibility of NRN toward ONOO^- and GSH, real-time evaluations in vivo with cisplatin (CP) alone and CP combined with acetaminophen-stimulated acute kidney injury and the following remedy process with l-carnitine were realized for the first time. The experiments revealed that acute kidney injury caused by combined drugs might be more serious and irreversible under certain conditions. Therefore, NRN could act as a potential tool for understanding oxidative stress-related DIAKI disease processes.

A Novel D-π-A Type Fluorescent Probe for Cu2+ Based on Styryl-Pyridinium Salts Conjugating Di-(2-picolyl)amine (DPA) Units

A novel D-π-A type fluorescent probe L(NO₃) for Cu (II) sensing was designed and fully characterized. The probe consists of a styryl-pyridine cation fluorescent group and a di-(2-picolyl)amine (DPA) receptor unit, which are linked by a phenyl group to form an electron donor-π-acceptor (D-π-A) conjugate system, especially the introduction of a nitrate counter anion for significantly enhanced water solubility of the probe. Fluorescence titration studies of the probe L(NO₃) showed a higher selectivity for Cu²⁺ than other metal ions, and the emission spectrum was strongly quenched upon binding. The competitive binding assay and the low detection limit (0.932 µM) showed that the probe L(NO₃) had strong anti-interference ability and excellent Cu²⁺ detection performance. The binding ratio of probe L(NO₃) and Cu²⁺ was determined from Job's plot to be 1:1, which is consistent with the results obtained from X-ray crystal structures. Meanwhile, the probe showed instantaneous chemical reversibility when titrated with EDTA solution, indicating potential recycling properties of the probe. In addition, the design of inexpensive fluorescent test strips can perform the on-site and real-time detection Cu²⁺ with a color recognition application.

Light Harvesting Nanoprobe for Trace Detection of Hg2+ in Water

The continuously increasing flow of toxic heavy metals to the environment due to intensive industrial activity and tightening requirements with regard to the content of metal ions in drinking and discharged waters urges the development of affordable and sensitive devices to the field control of pollutants. Here, we report a new thiated Rhodamine-lactam probe for Hg2+ detection and demonstrate how its sensitivity can be increased via the incorporation of the probe molecules into the optically transparent siloxane-acrylate coatings on polymethyl methacrylate and, alternatively, into the water-dispersible light-harvesting FRET nanoparticles (NPs), in which dye cations are separated by fluorinated tetraphenylborate anions. We have shown that the optimization of the FRET NPs composition had allowed it to reach the antenna effect of ~300 and fabricate "off/on" sensor for Hg2+ ion determination in aqueous solutions with the detection limit of ~100 pM, which is far below the maximum permissible concentration (MPC) of mercury in drinking water recommended by the World Health Organization. Although this work is more proof-of-concept than a ready-to-use analytical procedure, the suggested approaches to fabrication of the FRET NPs based on the popular rhodamine-lactam platform can be used as a background for the development of low-cost portable sensing devices for the extra-laboratory determination of hazardous metal ions.

Optical properties, bioimaging and theoretical calculation of a Zn(II) complex based on triphenylamine derivative

A luminescent material with various optical properties based on a triphenylamine Zn(II) complex is described. The ultraviolet-visible absorption, one-photon excited fluorescence (OPEF) and two-photon excited fluorescence (TPEF) of the complex indicate that the material has good OPEF and TPEF properties. And the results of one- and two-photon HepG2 cells imaging experiments show the potential of the complex in fluorescence microscopy bioimaging. The experimental Stokes shift and the FWHM (full-width at half-maximum) in different solvents were correlated with the rMPI polarity of the solvent, and the perfect Boltzmann curves were obtained, where the Boltzmann correlation between Stokes shift and solvent polarity is reported for the second time. But the Boltzmann correlation between FWHM and solvent polarity is reported for the first time. In addition, the computational results indicate that, the covalent bond within the salt ZnBr₂ is strengthened by the coordination, and the newly formed coordination bond Zn-N is stronger than the original covalent bond Zn-Br.

CDs-Peroxyfluor Conjugation for Ratiometric Fluorescence Detection of Glucose and Shortening Its Detection Time from Reaction Dynamic Perspective

A ratiometric fluorescence probe based on the conjugation of peroxyfluor-NHS (PF) and carbon dots (CDs) was designed for selective and rapid detection of glucose. When glucose was catalytically oxidized by glucose oxidase (GOx), the product H₂O₂ would react with colorless and non-fluorescent peroxyfluor moiety to give the colored and fluorescent fluorescein moiety which would absorb the energy of CDs emission at 450 nm due to the Förster Resonance Energy Transfer (FRET) and generate a new emission peak at 517 nm. The reaction between PF and H₂O₂ was slow with a rate constant of about 2.7 × 10^-4 s^-1 under pseudo-first-order conditions (1 uM PF, 1 mM H₂O₂), which was unconducive to rapid detection. Given this, a short time detection method was proposed by studying the kinetics of the reaction between PF and H₂O₂. In this method, the detection time was fixed at three minutes. The linear detection of glucose could be well realized even if the reaction was partially done. As glucose concentration increased from 0.05 mM to 5 mM, the fluorescence intensity ratio (I₅₁₇/I₄₅₀) after 3 minutes' reaction of CDs-PF and glucose oxidation products changed linearly from 0.269 to 1.127 with the limit of detection (LOD) of 17.19 μM. In addition, the applicability of the probe in blood glucose detection was verified.

Phosphorescent Carbon Dots as Long-Lived Donors To Develop an Energy Transfer-Based Sensing Platform

Employing long-lived luminescent materials to design a chemical sensing platform can eliminate real-time excitation and background fluorescence. However, the realization of long-lived emissions in aqueous media was limited to transition-metal complexes, doped quantum dots, organic crystals, and inorganic persistent phosphors, which suffer from the drawbacks of large size, expensive elements, and poor dispersibility. In this work, phosphorescent carbon dots (CDs) were covalently immobilized in a silica matrix (CDs@SiO₂) to achieve afterglow emission in an aqueous dispersion. CDs@SiO₂ with long lifetime (∼1.6 s) was utilized as an energy donor to fabricate nonradiative energy transfer systems with various organic dyes through the surface micelle self-assembly method. Benefiting from the high energy transfer efficiency between CDs@SiO₂ and organic dyes, multicolor afterglow emissions were successfully obtained in aqueous media. As a proof of concept, a ratiometric phosphorescent probe using CDs@SiO₂ as a donor and Hg²⁺-responsive rhodamine derivative as an acceptor was designed. Hg²⁺ triggered the energy transfer process between the donor-acceptor pair, leading to the sensitive detection of Hg²⁺ ions. The work presented here provides opportunities to develop chemical sensors with low background interferences and easily recognizable signals.

A Recent Update on Rhodamine Dye Based Sensor Molecules: A Review

Herein we have discussed such important modified rhodamine compounds which have been used as chemosensors for the last 7-8 years. This review covered some chemosensors for the detection of metal ions like Al(III), Cu(II), Hg(II), Co(II), Fe(III), Au(III), Cr(III), and some anion like CN-. The selectivity, sensitivity, photophysical properties (i.e., UV-Vis spectral studies, fluorescence studies giving special emphasis to absorption wavelength in UV-Vis spectra and excitation and emission wavelength in fluorescence spectra), binding affinity, the limit of detection, and the application of those chemosensors are described clearly. Here we have also discussed some functionalized rhodamine-based chemosensors that emit in the near-infrared region (NIR) and can target lysosomes and detect lysosomal pH. Their versatile applicability in the medicinal ground is also delineated. We have focused on the photophysical properties of spirolactam rhodamine photoswitches and applications in single-molecule localization microscopy and volumetric 3D light photoactivable dye displays. The real-time detection of radical intermediates has also been exemplified.

Brønsted acid catalyzed mechanochemical domino multicomponent reactions by employing liquid assisted grindstone chemistry

Here, we have demonstrated a metal-free energy-efficient mechanochemical approach for expedient access to a diverse set of 2-amino-3-cyano-aryl/heteroaryl-4H-chromenes, tetrahydrospiro[chromene-3,4'-indoline], 2,2'-aryl/heteroarylmethylene-bis(3-hydroxy-5,5-dimethylcyclohex-2-enone) as well as tetrahydro-1H-xanthen-1-one by employing the reactivity of 5,5-dimethylcyclohexane-1,3-dione/cyclohexane-1,3-dione with TsOH⋅H₂O as Brønsted acid catalyst under water-assisted grinding conditions at ambient temperature. The ability to accomplish multiple C-C, C=C, C-O, and C-N bonds from readily available starting materials via a domino multicomponent strategy in the absence of metal-catalyst as well as volatile organic solvents with an immediate reduction in the cost of the transformation without necessitates complex operational procedures, features the significant highlights of this approach. The excellent yield of the products, broad functional group tolerances, easy set-up, column-free, scalable synthesis with ultralow catalyst loading, short reaction time, waste-free, ligand-free, and toxic-free, are other notable advantages of this approach. The greenness and sustainability of the protocol were also established by demonstrating several green metrics parameters.

Schiff Bases: A Versatile Fluorescence Probe in Sensing Cations

Metal cations such as Zn²⁺, Al³⁺, Hg²⁺, Cd²⁺, Sn²⁺, Fe²⁺, Fe³⁺ and Cu²⁺ play important roles in biology, medicine, and the environment. However, when these are not maintained in proper concentration, they can be lethal to life. Therefore, selective sensing of metal cations is of great importance in understanding various metabolic processes, disease diagnosis, checking the purity of environmental samples, and detecting toxic analytes. Schiff base probes have been largely used in designing fluorescent sensors for sensing metal ions because of their easy processing, availability, fast response time, and low detection limit. Herein, an in-depth report on metal ions recognition by some Schiff base fluorescent sensors, their sensing mechanism, their practical applicability in cell imaging, building logic gates, and analysis of real-life samples has been presented. The metal ions having biological, industrial, and environmental significance are targeted. The compiled information is expected to prove beneficial in designing and synthesis of the related Schiff base fluorescent sensors.

A near-infrared GPX4 fluorescent probe for non-small cell lung cancer imaging

Glutathione peroxidase 4 (GPX4) is overexpressed in non-small cell lung cancer (H1299) cells. In this work, a near-infrared fluorescent probe ENBO-ML210 was developed. In vitro and in vivo imaging results showed that ENBO-ML210 could target and visualize GPX4 in H1299 cells, exhibiting potential for the diagnosis of non-small lung cancer.

Photochemical Mechanisms of Fluorophores Employed in Single-Molecule Localization Microscopy

Decoding cellular processes requires visualization of the spatial distribution and dynamic interactions of biomolecules. It is therefore not surprising that innovations in imaging technologies have facilitated advances in biomedical research. The advent of super-resolution imaging technologies has empowered biomedical researchers with the ability to answer long-standing questions about cellular processes at an entirely new level. Fluorescent probes greatly enhance the specificity and resolution of super-resolution imaging experiments. Here, we introduce key super-resolution imaging technologies, with a brief discussion on single-molecule localization microscopy (SMLM). We evaluate the chemistry and photochemical mechanisms of fluorescent probes employed in SMLM. This Review provides guidance on the identification and adoption of fluorescent probes in single molecule localization microscopy to inspire the design of next-generation fluorescent probes amenable to single-molecule imaging.

A rhodamine based chemodosimeter for the detection of Group 13 metal ions

A new rhodamine derivative, HL-CIN, derived from a reaction between N-(rhodamine-6G)lactam-ethylenediamine (L1) and trans-cinnamaldehyde, is reported here for the colorimetric and fluorogenic sensing of Group 13 trivalent cations, namely Al³⁺, Ga³⁺, In³⁺ and Tl³⁺. The absorption intensity of the probe increases significantly at 530 nm whereas the fluorescence intensity enhances massively at 558 nm upon interaction with these metal ions. Other relevant metal ions could not impart any noticeable color change or fluorescence enhancement. The quantum yield or fluorescence life time of HL-CIN increases considerably in the presence of these Group 13 metal ions. Different spectral studies such as ESI-mass, FT-IR, ¹H and ¹³C NMR spectra, establish that HL-CIN undergoes hydrolysis in the presence of the trivalent cations and a rhodamine species in its ring opened form (i.e. N-(2-aminoethyl)-2-((6Z)-3-(ethylamino)-6-(ethylimino)-2,7-dimethyl-6H-xanthen-9-yl)benzamide, (L2)) along with cinnamaldehyde are produced. The rhodamine species in its ring opened form (L2) is responsible for the color change and strong increment in the absorbance and fluorescence of HL-CIN with Group 13 cations. Interaction between L1 and these metal ions could not produce the same outcome. It has been used in test paper strips and to detect these cations in real samples.

A pyridyl functionalized rhodamine chemodosimeter for selective fluorescent detection of mercury ions and cell imaging

Rhodamines gain sustained attention owing to their great potential for probe design applications. Herein, the facile preparation of a new pyridyl functionalized rhodamine dye PR is reported, which has stable fluorescence signal in water with maximum emission peak at 594 nm and Stokes shift of 81 nm. Based on dye PR, a new fluorescent probe PRHg has been developed by modifying the spirolactone of PR with hydrazine hydrate so as to produce spirolactam recognizing group for sensing of Hg²⁺. PRHg exhibits high selectivity and sensitivity towards Hg²⁺ in water/ethanol (v/v = 4/1, pH = 7.0) by a specific Hg²⁺-binding promoted spirolactam ring opening and hydrolyzing process. And, the detection limit for Hg²⁺ is evaluated to be 8.5 nM. Besides, the probe can respond to Hg²⁺ within 40 min and over a wide pH range from 4.0 to 10.0. Moreover, PRHg (40 µM) performs low cytotoxicity to HeLa cells (over 91.0 % cell survival rate), which allows the probe to be employed for tracing intracellular Hg²⁺ by fluorescence imaging.

Development of a NIR iridium(III) complex for self-calibrated and luminogenic detection of boron trifluoride

Boron trifluoride (BF3) is a potential environmental pollutant, and excess exposure to it may cause human diseases. However, the sensitive, rapid and accurate detection of BF3 for on-site purposes is still a challenge. In this work, we developed the first NIR iridium(III)-based probe with dual emission and a Stokes shift of 370 nm for self-calibrated and luminogenic detection of BF3. This probe exhibited a strong luminescence enhancement at around 650 nm to BF3 (0-100 μM) with almost no change in luminescence at 475 nm, displaying a 220-fold I650 nm/I475 nm enhancement at 100 μM of BF3 with a detection limit of 0.35 μM. Moreover, the probe showed a fast response time of less than 5 s to BF3 along with an obvious color change under UV irradiation for visual detection. Importantly, the desirable photophysical properties of the iridium(III)-based probe can be harnessed for time-resolved detection of BF3 in the presence of the fluorescence background. The applicability of the probe was further verified in an organic solvent waste-spiked system and on a glass pane. This work will provide a solid basis for the development of sensitive and on-site BF3 sensing toolkits for environmental monitoring.

Dual-Mode Optical Sensor Array for Detecting and Identifying Perillaldehyde in Solution Phase and Plant Leaf with Smartphone

Promising techniques for detecting and quantifying active components in the plants and foods have received global concern in smart agriculture. Dual-mode optical assays are becoming more attractive and popular thanks to robust and reliable analysis parameters. We herein unveil a novel turn-on and dual-mode sensor array comprising three kinds of reactive indicators including ring-closed rhodamine-hydrazine, squaraine-hydrazine, and 2,4-dinitrophenylhydrazine for evaluating perillaldehyde. Significant colorimetric and fluorescent changes were triggered through reacting primary amine/hydrazine with the active aldehyde group in perillaldehyde, thus turning on the chromogenic responses of all the indicators. Optimal colorimetric sensing showed good responses to perillaldehyde ranged up to 100 mM in ethanol. Dramatic fluorescence enhancement was also exhibited, illustrating good selectivity as well as high sensitivity (detection limit ∼20.0 μM). Inspired by rapid chemical reactions and distinct optical changes, distinct sensor array strips loaded with the optimal solid-state reactive indicators were developed for evaluating the perillaldehyde content in the perilla frutescence leaves. Smartphone-enabled readout system and digital data processing were further performed for chemometric analysis. A good correlation was obtained and the semiquantitative evaluation of the perillaldehyde content could be achieved within 15 min, possessing the significant features of naked-eye recognition, easy operation, and disposability. To the best of our knowledge, present work demonstrated the use of chromogenic sensing strips to evaluate the active perillaldehyde content in solution and vapor phases for the first time. Taken together, these characteristics also indicate that the present turn-on sensor array has great potential applications in the precise detection and evaluation of perillaldehyde in the forthcoming smart agriculture.

A quinoline-rhodamine hybrid probe for ratiometricly sensing of Hg2+ in water and cell imaging application

To develop efficient tools for monitoring toxicant Hg²⁺ in aqueous solution attracts great attention because the abnormal distribution of Hg²⁺ in environment poses great threat to human health. We here report the preparation of a novel quinoline-rhodamine hybrid fluorescent probe P7RHg for ratiometricly sensing of Hg²⁺ in water, with a spirolactam-thiosemicarbazide reaction group. Upon treatment by Hg²⁺, the ratio of fluorescence intensity (F₆₀₀/F₄₆₀) exhibits nearly 90-fold enhancement, presenting two well-resolved emission peaks (140 nm). Meanwhile, the specific Hg²⁺-induced desulfurization provides probe P7RHg an excellent selectivity to Hg²⁺, with a detection limit of 8.6 nM. Moreover, the low cytotoxicity allows P7RHg to be employed for tracing Hg²⁺ in living cells by confocal fluorescence imaging.

AIE+ESIPT Active Hydroxybenzothiazole for Intracellular Detection of Cu2+: Anticancer and Anticounterfeiting Applications

Here, in the present work, a new hydroxybenzothiazole derivative (HBT 2) with AIE+ESIPT features was synthesized by Suzuki-Miyora coupling of HBT 1 with 4-formylphenylboronic acid. The AIE and ESIPT features were confirmed by optical, microscopic (AFM) and dynamic light scattering (DLS) techniques. The yellow fluorescent aggregates of HBT 2 can specifically detect Cu²⁺/Cu⁺ ions with limits of detection as low as 250 nM and 69 nM. The Job's plot revealed the formation of a 1:1 complex. The Cu²⁺ complexation was further confirmed by optical, NMR, AFM and DLS techniques. HBT 2 was also used for the detection of Cu²⁺ ions in real water samples collected from different regions of Punjab. HBT 2 was successfully used for the bio-imaging of Cu²⁺ ions in live A549 and its anticancer activity was checked on different cancer cell lines, such as MG63, and HeLa, and normal cell lines such as L929. We successfully utilized HBT 2 to develop security labels for anticounterfeiting applications.