A Mitochondrial-Targeted Two-Photon Probe for Zinc Ion

Manipulating Intramolecular Charge Transfer in Terpyridine Derivatives Towards "Turn-On" Fluorescence Chemosensors for Zn2

A series of donor-acceptor (D-A) terpyridine derivatives with various intramolecular charge transfer interactions have been successfully synthesized bearing phenyl, methoxyphenyl, N-butyldiphenylamine (DPA), and triphenylamine (TPA) as electron-donors and terpyridine (TPY), 2,6-di(pyrazin-2-yl)pyridine (PYDPZ), and N,N-dimethylated PYDPZ (PYDPZ-2CH3) as electron acceptors. Upon the introduction of pyrazine rings instead of pyridine ones and further selective N,N'-dimethylation, the intramolecular D-A interactions are significantly enhanced, resulting in the remarkable reduced intramolecular charge transfer (ICT) transitions and quenched PL emissions in CH2Cl2 solution. However, their ICT emissions are clearly recovered upon adding Zn2+. Especially, for double positively charged compound 12, the Zn2+-induced "turn-on" green emission (λmax, em=518 nm, Φ=0.24) with excellent sensitivity (I/I0=77) and selectivity (IZn/ICd=23) is detected. Under the optimized experimental condition (EtOH-H2O (9 : 1, v/v), containing 0.05 M HEPES buffer (pH=7.4)), this probe is used to quantitative determination of Zn2+ in a water sample with an average recovery of 97.5 %. Furthermore, the probe is applied to imaging of Zn2+ ions in HeLa cells and its mechanism is confirmed by the molecular dynamics simulation, in which lower coulomb potential for membrane surface adsorption and energy barrier for membrane translocation can be found.

Targeted imaging of lysosomal zinc ions with a tetrahedral DNA framework fluorescent reporter

Abnormal levels of zinc ions within endo-lysosomes have been implicated in the progression of Alzheimer's disease (AD), yet the detection of low-concentration zinc ions at the organelle level remains challenging. Here we report the design of an endo-lysosome-targeted fluorescent reporter, Znluorly, for imaging endogenous zinc ions. Znluorly is constructed from an amphiphilic DNA framework (DNF) with programmable size and shape, which can encapsulate zinc-responsive fluorophores within its hydrophobic nanocavity. We find that the tetrahedral DNFs of 20 bp in the edge length are effectively located within endo-lysosomes, which can detect zinc ions with a detection limit of ∼31.9 nM (a sensitivity that is ∼2.5 times that of the free fluorophore). Given the organelle-targeting ability and high zinc sensitivity of Znluorly, we employ it to detect endogenous endo-lysosomal zinc ions in neuron cells. We monitor the dynamics of zinc levels in AD model cells and zebrafish, corroborating the positive correlation between zinc levels and AD hallmarks including Aβ aggregates and learning/memory impairments. Our study provides a generalizable strategy for organelle-specific theranostic applications.

Advances in Organic Fluorescent Probes for Intracellular Zn2+ Detection and Bioimaging

Zinc ions (Zn2+) play a key role in maintaining and regulating protein structures and functions. To better understand the intracellular Zn2+ homeostasis and signaling role, various fluorescent sensors have been developed that allow the monitoring of Zn2+ concentrations and bioimaging in live cells in real time. This review highlights the recent development of organic fluorescent probes for the detection and imaging of intracellular Zn2+, including the design and construction of the probes, fluorescent response mechanisms, and their applications to intracellular Zn2+ detection and imaging on-site. Finally, the current challenges and prospects are discussed.

FRET Luminescent Probe for the Ratiometric Imaging of Peroxynitrite in Rat Brain Models of Epilepsy-Based on Organic Dye-Conjugated Iridium(III) Complex

Epilepsy is a chronic neurological disorder characterized by recurrent seizures globally, imposing a substantial burden on patients and their families. The pathological role of peroxynitrite (ONOO-), which can trigger oxidative stress, inflammation, and neuronal hyperexcitability, is critical in epilepsy. However, the development of reliable, in situ, and real-time optical imaging tools to detect ONOO- in the brain encounters some challenges related to the depth of tissue penetration, background interference, optical bleaching, and spectral overlapping. To address these limitations, we present Ir-CBM, a new one-photon and two-photon excitable and long-lived ratiometric luminescent probe designed specifically for precise detection of ONOO- in epilepsy-based on the Förster resonance energy transfer mechanism by combining an iridium(III) complex with an organic fluorophore. Ir-CBM possesses the advantages of rapid response, one-/two-photon excitation, and ratiometric luminescent imaging for monitoring the cellular levels of ONOO- and evaluating the effects of different therapeutic drugs on ONOO- in the brain of an epilepsy model rat. The development and utilization of Ir-CBM offer valuable insights into the design of ratiometric luminescent probes. Furthermore, Ir-CBM serves as a rapid imaging and screening tool for antiepileptic drugs, thereby accelerating the exploration of novel antiepileptic drug screening and improving preventive and therapeutic strategies in epilepsy research.

Probing the dynamic crosstalk of lysosomes and mitochondria with structured illumination microscopy

Structured illumination microscopy (SIM) is a super-resolution technology for imaging living cells and has been used for studying the dynamics of lysosomes and mitochondria. Recently, new probes and analyzing methods have been developed for SIM imaging, enabling the quantitative analysis of these subcellular structures and their interactions. This review provides an overview of the working principle and advances of SIM, as well as the organelle-targeting principles and types of fluorescence probes, including small molecules, metal complexes, nanoparticles, and fluorescent proteins. Additionally, quantitative methods based on organelle morphology and distribution are outlined. Finally, the review provides an outlook on the current challenges and future directions for improving the combination of SIM imaging and image analysis to further advance the study of organelles. We hope that this review will be useful for researchers working in the field of organelle research and help to facilitate the development of SIM imaging and analysis techniques.

A novel diarylethene-based fluorescence sensor for Zn2+ detection and its application

A series of fluorometric sensors of Zn2+ have been synthesized due to the significant function of Zn2+ in the human body and environment. However, most of probes reported for detecting Zn2+ have high detection limit or low sensitivity. In this paper, an original Zn2+ sensor, namely 1o, was synthesized by diarylethene and 2-aminobenzamide. When Zn2+ was added, the fluorescence intensity of 1o increased by 11 times within 10 s, along with a fluorescence color change from dark to bright blue, and the detection limit (LOD) was calculated to be 0.329 μM. According to Job's plot curves, the binding mode of 1o and Zn2+ was measured as 1:1, which was further proved by 1H NMR spectra, HRMS and FT-IR spectra. The logic circuit was designed to take advantage of the fact that the fluorescence intensity of 1o can be controlled by Zn2+, EDTA, UV and Vis. In addition, Zn2+ in actual water samples were tested, in which the recovery rate of Zn2+ was between 96.5 % and 109 %. Furthermore, 1o was successfully made into a fluorescent test strip, which could be used to detect Zn2+ in the environment economically and conveniently.

Precise Spatiotemporal Identification of Mitochondrial H2S Fluctuations through Exploiting an On-Demand Photoactivated Probe

Various signal molecules participate in complex biological processes in mitochondria. However, most currently available probes have problems in elucidating the functions of these active species in mitochondria due to the inability to light up these probes exclusively at the desired mitochondrial location, thereby compromising the specificity and accuracy. In this study, we present an on-demand photoactivation approach to the molecular design of optimized probes for precise spatiotemporal identification of mitochondrial H2S fluctuations. The designed probe with native yellow fluorescence can monitor the process into mitochondria but maintains nonfluorescent response to H2S during cellular delivery, providing the accurate timing of accumulation in mitochondria. On-demand photoactivation exclusively at the desired mitochondrial location affords a significant aggregation-enhanced and emissive response to H2S with lighting up red fluorescence at 690 nm, which is the only way to get such an emissive phenomenon and greatly improves the specificity and accuracy of targeting mitochondrial H2S. By using this photocontrolled fluorescence responsiveness to H2S, precise spatiotemporal identification of mitochondrial H2S fluctuations is successfully performed. Our work could facilitate advances toward interrogating the physiological and pathological consequences of mitochondrial H2S in various biological events.

Novel fluorescent probe for sequential recognition of Zn2+ and pyrophosphate in aqueous based on aggregation-induced emission

A new fluorescent probe (E)-4-(4-([2,2':6',2''-terpyridin]-4'-yl)styryl)-1-dodecylpyridin-1-ium (TPy-SD), with the aggregation-induced emission (AIE) property in aqueous solution, has been synthesized and characterized. The new probe, TPy-SD exhibited excellent selectivity and sensitivity towards Zn²⁺ with a relatively low detection limit (1.76 × 10^-7 M). The addition of Zn²⁺ is thought to disrupt the AIE property of TPy-SD, thereby leading to a fluorescence blue shift. Interestingly, the complex of probe TPy-SD with Zn²⁺ (Zn (II) TPy-SD), with molar ratio of 1:1, can be used as a simple, sensitive, and rapid means for the detection of pyrophosphates (PPi) in solution (water/DMSO = 99:1). As evidenced by transmission electron microscopy (TEM), dynamic light scattering (DLS) and fluorescence emission spectroscopy, this detection is thought to be due to the strong affinity between PPi and Zn²⁺, which brings out Zn²⁺ from the coordination cavity of chemical sensor TPy-SD, thus realizing the detection and recognition of PPi. Therefore, the new AIE fluorescent probe can be used as a dual probe for the detection of cations and anions.

Fluorescent Probes for Biological Species and Microenvironments: from Rational Design to Bioimaging Applications

Some important biological species and microenvironments maintain a complex and delicate dynamic balance in life systems, participating in the regulation of various physiological processes and playing indispensable roles in maintaining the healthy development of living bodies. Disruption of their homeostasis in living organisms can cause various diseases and even death. Therefore, real time monitoring of these biological species and microenvironments during different physiological and pathological processes is of great significance. Fluorescent-probe-based techniques have been recognized as one of the most powerful tools for real time imaging in biological samples. In this Account, we introduce the representative works from our group in the field of fluorescent probes for biological imaging capable of detecting metal ions, small bioactive molecules, and the microenvironment. The design strategies of small molecule fluorescent probes and their applications in biological imaging will be discussed. By regulating the design strategy and mechanism (e.g., ICT, PeT, and FRET) of the electronic and spectral characteristics of the fluorescent platforms, these chemical probes show high selectivity and diverse functions, which can be used for imaging of various physiological and pathological processes. Through the exploration of the rational response mechanism and design strategy, combined with a variety of imaging techniques, such as super-resolution imaging, photoacoustic (PA) imaging, etc., we have realized multimode imaging of the important biological analytes from the subcellular level to the in vivo level, which provides powerful means to study the physiological and pathological functions of these species and microenvironments. This Account aims to offer insights and inspiration for the development of novel fluorescent probes for biological imaging, which could provide powerful tools for the study of chemical biology. Overall, we represent a series of turn-on/turn-off/ratiometric fluorescent/PA probes to visually and dynamically trace biological species and microenvironments in cells and even in vivo that seek higher resolution and depth molecular imaging to improve diagnostic methods and clarify new discoveries related to chemical biology. Our future efforts will be devoted to developing multiorganelle targeted fluorescent probes to study the mechanism of subcellular organelle interaction and employing various dual-mode probes of NIR II and PA imaging to investigate the development of related diseases and treat the related diseases at subcellular and in vivo levels.

A sensitive zinc probe operating via enhancement of excited-state intramolecular charge transfer

Novel highly sensitive fluorescent probes for zinc cations based on the diketopyrrolopyrrole scaffold were designed and synthesized. Large bathochromic shifts (≈80 nm) of fluorescence are observed when the Zn²⁺-recognition unit (di-(2-picolyl)amine) is bridged with the fluorophore possessing an additional pyridine unit able to participate in the coordination process. This effect originates from the dipolar architecture and the increasing electron-withdrawing properties of the diketopyrrolopyrrole core upon addition of the cation. The new, greenish-yellow emitting probes, which operate via modulation of intramolecular charge transfer, are very sensitive to the presence of Zn²⁺. Introduction of a morpholine unit in the diketopyrrolopyrrole structure induces a selective six-fold increase of the emission intensity upon zinc coordination. Importantly, the presence of other divalent biologically relevant metal cations has negligible effects and typically even at a 100-fold higher concentration of Mg²⁺/Zn²⁺, the effect is comparable. Computational studies rationalize the strong bathochromic shift upon Zn²⁺-complexation. Decorating the probes with the triphenylphosphonium cation and morpholine unit enables selective localization in the mitochondria and the lysosome of cardiac H9C2 cells, respectively.

An efficient turn-on fluorescence chemosensor system for Zn(II) ions detection and imaging in mitochondria

Mitochondria-targetable fluorescent chemosensors, Rhodamine-B and rhodamine 6G bearing syringaldehyde based receptors were designed and synthesized for efficient chemosensing of Zinc(II) ions. The probes showed the very selective naked eye color change to pink from colorless upon addition of Zinc(II) ions, further these probes showing turn-on fluorescence enhancement with Zn(II) ions by opening of rhodamine spirolactam. The probes are very sensitive towards Zn(II) ions among other ions. These probes RBS and R6S will be applicable to detect zinc ions upto the low level concentration 0.18 and 0.19 nano molar respectively. The affinity of these sensors RBS and R6S for Zinc (II) ions was found to be in the range of 1.12 × 10⁴ M^-1 and 7.28 × 10⁴ M^-1 respectively. ¹H-nmr titrations of the probes with Zn(II) ions clearly indicating the spiroring opening of the spirolactam. DFT calculations supporting that the perceived photophysical changes of the probes on appendage of the zinc ions. Probes RBS and R6S are useable for selective staining mitochondria. Both of the probes are applicable to reveal labile Zn(II) in live Hela and MCF-7 cells via fluorescence imaging. RBS and R6S are also finding application on quantification of Zinc(II) ions inside mitochondria via fluorescence imaging.

Bioinspired large Stokes shift small molecular dyes for biomedical fluorescence imaging

Long Stokes shift dyes that minimize cross-talk between the excitation source and fluorescent emission to improve the signal-to-background ratio are highly desired for fluorescence imaging. However, simple small molecular dyes with large Stokes shift (more than 120 nanometers) and near-infrared (NIR) emissions have been rarely reported so far. Here, inspired by the chromophore chemical structure of fluorescent proteins, we designed and synthesized a series of styrene oxazolone dyes (SODs) with simple synthetic methods, which show NIR emissions (>650 nanometers) with long Stokes shift (ranged from 136 to 198 nanometers) and small molecular weight (<450 daltons). The most promising SOD9 shows rapid renal excretion and blood-brain barrier passing properties. After functioning with the mitochondrial-targeted triphenylphosphonium (TPP) group, the resulting SOD9-TPP can be engineered for head-neck tumor imaging, fluorescence image-guided surgery, brain neuroimaging, and on-site pathologic analysis. In summary, our findings add an essential small molecular dye category to the classical dyes.

Dual-mode chemosensor for the fluorescence detection of zinc and hypochlorite on a fluorescein backbone and its cell-imaging applications

Fluorescein coupled with 3-(aminomethyl)-4,6-dimethylpyridin-2(1H)-one (FAD) was synthesized for the selective recognition of Zn²⁺ over other interfering metal ions in acetonitrile/aqueous buffer (1 : 1). Interestingly, there was a significant fluorescence enhancement of FAD in association with Zn²⁺ at 426 nm by strong chelation-induced fluorescence enhancement (CHEF) without interrupting the cyclic spirolactam ring. A binding stoichiometric ratio of 1 : 2 for the ligand FAD with metal Zn²⁺ was proven by a Jobs plot. However, the cyclic spirolactam ring was opened by hypochlorite (OCl^-) as well as oxidative cleavage of the imine bond, which resulted in the emission enhancement of the wavelength at 520 nm. The binding constant and detection limit of FAD towards Zn²⁺ were determined to be 1 × 10⁴ M^-1 and 1.79 μM, respectively, and the detection limit for OCl^- was determined as 2.24 μM. We introduced here a dual-mode chemosensor FAD having both the reactive functionalities for the simultaneous detection of Zn²⁺ and OCl^- by employing a metal coordination (Zn²⁺) and analytes (OCl^-) induced chemodosimetric approach, respectively. Furthermore, for the practical application, we studied the fluorescence imaging inside HeLa cells by using FAD, which demonstrated it can be very useful as a selective and sensitive fluorescent probe for zinc.

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.

Organelle-Level Labile Zn2+ Mapping Based on Targetable Fluorescent Sensors

Although many Zn²⁺ fluorescent probes have been developed, there remains a lack of consensus on the labile Zn²⁺ concentrations ([Zn²⁺]) in several cellular compartments, as the fluorescence properties and zinc affinity of the fluorescent probes are greatly affected by the pH and redox environments specific to organelles. In this study, we developed two turn-on-type Zn²⁺ fluorescent probes, namely, ZnDA-2H and ZnDA-3H, with low pH sensitivity and suitable affinity (K_d = 5.0 and 0.16 nM) for detecting physiological labile Zn²⁺ in various cellular compartments, such as the cytosol, nucleus, ER, and mitochondria. Due to their sufficient membrane permeability, both probes were precisely localized to the target organelles in HeLa cells using HaloTag labeling technology. Using an in situ standard quantification method, we identified the [Zn²⁺] in the tested organelles, resulting in the subcellular [Zn²⁺] distribution as [Zn²⁺]_ER < [Zn²⁺]_mito < [Zn²⁺]_cyto ∼ [Zn²⁺]_nuc.

A NIR Emitting Cyanine with Large Stokes' Shift for Mitochondria and Identification of their Membrane Potential Disruption

An NIR emitting (λ_em ≈730 nm) cyanine probe ExCy was synthesized in good yields by extending the π-conjugation length (i. e., with furan moiety) to the donor-accepter system. ExCy exhibited a large Stokes' shift (Δλ≈100 nm) due to strong intramolecular charge transfer (ICT), and high fluorescence quantum yield (Φ_fl ≈0.47 in DCM). Due to its low fluorescence in an aqueous environment (Φ_fl ≈0.007 in H₂ O), the probe exhibited the potential of achieving a large fluorescence turn-on upon entering a hydrophobic cellular environment. Fluorescence confocal microscopy studies revealed that ExCy was readily excitable with a far-red laser line (i. e., 640 nm) while the corresponding emission was collected in the NIR region. ExCy exhibited excellent selectivity towards live cell mitochondria according to the co-localization studies. The probe also exhibited high photostability, long-term imaging ability and wash-free staining ability, when being applied to live cells. Our studies indicated that the mitochondrial localization of ExCy was dependent on the membrane potential of the mitochondria. ExCy was successfully utilized as a mitochondrial membrane potential dysfunction indicator to visually identify cells with mitochondrial dysfunction via fluorescence confocal microscopy. ExCy was further examined for potential in vivo imaging of zebrafish.

A red-emissive two-photon fluorescent probe for mitochondrial sodium ions in live tissue

A cyclocyanine (CC)-based organic small molecule two-photon (TP) fluorescent probe (CCNa1) was developed for mitochondrial sodium ion sensing. CCNa1 exhibits a low solvatochromic shift and strong TP fluorescence enhancement at 575 nm upon binding to Na⁺ and is insensitive to other metal ions and to pH. CCNa1 demonstrated fast cell loading ability, biocompatibility, and sensitive response to mitochondrial Na⁺ influx in live cells and mouse brain tissue.

Real-time detection and imaging of exogenous and endogenous Zn2+ in the PC12 cell model of depression with a NIR fluorescent probe

Depression is closely related to overactivation of N-methyl-d-aspartic acid (NMDA) receptors, and Zn2+ is a vital NMDA receptor modulator involved in the pathophysiological and physiological processes of depression. Therefore, quantitative and real-time detection of Zn2+ is very important for understanding the pathogenesis of depression. In this work, a near-infrared (NIR) fluorescent probe ISO-DPA was designed and synthesized for Zn2+ detection with a large Stokes shift (185 nm), high quantum yield (up to 44%), high sensitivity (LOD = 0.106 μM) and good pH stability. The probe showed rapid response within 10 s, accompanied by a distinct fluorescence change from faint to bright pink with the fluorescence intensity increasing 4.5-fold. Moreover, the sensing mechanism of ISO-DPA towards Zn2+ was supported by MALDI-TOF-MS and Job's plot. The probe ISO-DPA could detect instantaneous variation of exogenous and endogenous Zn2+ in PC12 cells. The bioimaging results reveal the increase of the endogenous Zn2+ concentration in PC12 cells under the oxidative stress induced by glutamate and confirm that overactivation of NMDA receptors results in an increase of the Zn2+ level. All the results proved that ISO-DPA is an excellent probe for detecting Zn2+ in solution and living cells and could help us better understand Zn2+ associated pathogenesis of depression.

Optically superior fluorescent probes for selective imaging of cells, tumors, and reactive chemical species

Fluorescent chemical probes have become powerful tools to study biological events in living cells. They provide a great opportunity to quantitatively and qualitatively analyze the physiological and biochemical properties of living cells in real time. The ability of researchers to manipulate these probes for a desired specific purpose has turned many heads in the scientific community. Despite a slow start, fluorescent probe research has seen exponential growth over the last decade in the world. This change required some adventurous and creative scientists from different fields-like biology, medicine, and chemistry-to come together to facilitate the constant expansion of this field. This review article introduces some fundamental concepts related to fluorescent probe designing and development. It also summarizes various fluorescent probes with superior optical properties used in fields like cell biology, cellular imaging, medical research, and cancer diagnosis. It is hoped that this article will encourage more young and creative scientists to contribute their talents to this field.

Red/NIR neutral BODIPY-based fluorescent probes for lighting up mitochondria

Two mitochondrial-targeted fluorescent probes, 2 and 3, based on a BODIPY scaffold and one or two piperidinyl groups were easily synthesized by Knoevenagel condensation. The biological imaging applications of the two probes 2 and 3 in live HepG2 cells reveal that these two probes display excellent mitochondrial imaging ability. Thus, these probes appear as promising tools for visualization of mitochondrial within live cells.

Precipitated Fluorophore-Based Probe for Accurate Detection of Mitochondrial Analytes

Mitochondria-targeted fluorescent probes are highly important to obtain mitochondrial function information. However, the accuracy of the current mitochondria-targeted fluorescent probes is unsatisfactory owing to the following two reasons. In the first case, some probes that always have a mitochondria-targeting group, thus, would react with the analytes outside of mitochondria and enter mitochondria with the generated fluorophore signal, which leads to a false-positive result. In the other case, after response to the analytes in mitochondria, some probes could diffuse from mitochondria to other organelles, thus triggering a false-negative result. To avoid the two problems, herein, we develop a precipitated fluorophore-based probe, which precipitates in situ after reacting with analytes, for the accurate detection of mitochondrial analytes. The probe was modified with HQPQ, a novel solid-state fluorophore that is insoluble in water. As a proof of concept, we designed and synthesized a probe (HQPQ-B) for H₂O₂ detection. Based on the different mitochondria-targeting capacities of quinoline salts and quinolone, HQPQ loses the mitochondria-targeting ability after reacting with analytes outside of mitochondria, thus avoiding a false-positive result. On the contrary, when the probe first localized in mitochondria and then reacted with analytes, HQPQ would precipitate and remain in mitochondria without diffusing to other sites, thus avoiding a false-negative result. Therefore, HQPQ enables the accurate detection of mitochondrial analytes. We believe that the novel strategy based on HQPQ will be a general strategy for accurate detection of mitochondrial analytes without interference from other sites, which enables an accurate study on mitochondrial function.

Carbon Nanohorns/Pt Nanoparticles/DNA Nanoplatform for Intracellular Zn2+ Imaging and Enhanced Cooperative Phototherapy of Cancer Cells

Superfluous zinc ion (Zn²⁺) in living cells has been identified as a potential tumor biomarker for early cancer diagnosis and cancer progression monitoring. In this paper, we developed a novel carbon nanohorns/Pt nanoparticles/DNA (CNHs/Pt NPs/DNA) nanoplatform based on the clamped hybridization chain reaction (c-HCR) process for intracellular Zn²⁺ imaging and enhanced cooperative phototherapy of cancer cells. Cross-shaped DNAzyme (c-DNAzyme), hairpin DNA1, hairpin DNA2, and aptamer DNA were adsorbed onto the surfaces of CNHs/Pt NPs, and the fluorescence of carboxytetramethyl-rhodamine was also quenched. After entering the living cells, the c-DNAzyme was cleaved to output trigger DNA in the existence of intracellular Zn²⁺ and initiate the c-HCR process for fluorescence amplification. Compared with the single HCR process triggered by a single DNAzyme, the c-HCR process could further improve the amplification efficiency and sensitivity. In addition, such a nanoprobe possesses a catalysis-enhanced photodynamic effect by Pt NP generation of oxygen in a tumor microenvironment and increases the photothermal effect by loading of Pt NPs on CNHs, indicating that this is a promising biological method for cancer diagnosis and cancer cell therapy.

A DCM-based NIR sensor for selective and sensitive detection of Zn2+ in living cells

Zinc ion is the 2nd abundant transition metal element in human's body. It is responsible for many physiological and biological functioning in the body, such as growth of people, immunity, endocrine, etc. The deficiency of zinc could result in an increasing risk for growth retardation, neurological disorder and infectious disease. Thus, developing a nondestructive method for detecting Zn²⁺ in living systems is important. Here we reported a 2-(2-methyl-4H-ylidene)- malononitrile (DCM)-based NIR probe DF-Zn for selective and sensitive detection of Zn²⁺. The probe DF-Zn is cell-permeable and stable at broad pH range. DF-Zn showed a fast response to Zn²⁺, big stock's shift, and "nude-eye" recognition for Zn²⁺. Moreover, the selective binding of probe DF-Zn to Zn²⁺ was reversible. With the addition of EDTA in buffer solution, reversible response of probe to Zn²⁺ could be observed in MCF-7 cells imaging.

Subcellular localised small molecule fluorescent probes to image mobile Zn2

Zn2+, as the second most abundant d-block metal in the human body, plays an important role in a wide range of biological processes, and the dysfunction of its homeostasis is related to many diseases, including Type 2 diabetes, Alzheimer's disease and prostate and breast cancers. Small molecule fluorescent probes, as effective tools for real-time imaging, have been widely used to study Zn2+ related processes. However, the failure to control their localisation in cells has limited their utility somewhat, as they are generally incapable of studying individual processes in a specific cellular location. This perspective presents an overview of the recent developments in specific organelle localised small molecule fluorescent Zn2+ probes and their application in biological milieu, which could help to extend our understanding of the mechanisms that cells use to respond to dysfunction of zinc homeostasis and its roles in disease initiation and development.

Dual-Functionalisation of Fluorophores for the Preparation of Targeted and Selective Probes

A key current challenge in biological research is the elucidation of the that roles chemicals and chemical reactions play in cellular function and dysfunction. Of the available cellular imaging techniques, fluorescence imaging offers a balance between sensitivity and resolution, enabling the cost-effective and rapid visualisation of model biological systems. Importantly, the use of responsive fluorescent probes in conjunction with ever-advancing microscopy and flow cytometry techniques enables the visualisation, with high spatiotemporal resolution, of both specific chemical species and chemical reactions in living cells. Ideal responsive fluorescent probes are those that contain a fluorophore tethered to both a sensing unit, to ensure selectivity of response, and a targeting group, to control the sub-cellular localisation of the probe. To date, probes that are both targeted and selective are relatively rare and most localised probes are discovered serendipitously rather than by design. A challenge in this field is therefore the identification of suitable fluorophore scaffolds that can be readily attached to both sensing and targeting groups. Here we review current strategies for dual-functionalisation of fluorophores, highlighting key examples of targeted, responsive probes.

Designing Stimuli-Responsive Upconversion Nanoparticles that Exploit the Tumor Microenvironment

Tailoring personalized cancer nanomedicines demands detailed understanding of the tumor microenvironment. In recent years, smart upconversion nanoparticles with the ability to exploit the unique characteristics of the tumor microenvironment for precise targeting have been designed. To activate upconversion nanoparticles, various bio-physicochemical characteristics of the tumor microenvironment, namely, acidic pH, redox reactants, and hypoxia, are exploited. Stimuli-responsive upconversion nanoparticles also utilize the excessive presence of adenosine triphosphate (ATP), riboflavin, and Zn²⁺ in tumors. An overview of the design of stimulus-responsive upconversion nanoparticles that precisely target and respond to tumors via targeting the tumor microenvironment and intracellular signals is provided. Detailed understanding of the tumor microenvironment and the personalized design of upconversion nanoparticles will result in more effective clinical translation.

Recent advances in two-photon absorbing probes based on a functionalized dipolar naphthalene platform

Two-photon microscopy (TPM) techniques have been highlighted over the past two decades throughout various fields, including physics, chemistry, biology, and medicine. In particular, the two-photon near-infrared excitation of fluorophores or molecular probes emitting fluorescence have ushered in a new biomedical era, specifically in the deep-tissue imaging of biologically relevant species. Non-linear two-photon optics enables the development of 3D fluorescence images via focal point excitation of biological samples with low photo-damage and photo-bleaching. Many studies have disclosed the relationship between the chemical structure of fluorophores and their two-photon absorbing properties. In this review, we have summarized the recent advances in two-photon absorbing probes based on a functionalized electron donor (D)-acceptor (A) type dipolar naphthalene platform (FDNP) that was previously reported between 2015 and 2019. Our systematic outline of the synthesis, photophysical properties, and examples of two-photon imaging applications will provide useful context for the future development of new naphthalene backbone-based two-photon probes.

Stepwise dual targeting and dual responsive polymer micelles for mitochondrion therapy

Methods to selectively destroy mitochondria of tumor cells and induce cell apoptosis with nanomedicine constitute challenges in cancer therapy. In the present study, we develop cell membrane/mitochondria dual targeting and pH/redox dual responsive nanoparticles for mitochondrion therapy. The nanoparticles are fabricated by the self-assembly of triphenylphosphonium (TPP) grafted poly(ethylene glycol)(PEG)-poly(d,l-lactide)(PLA) copolymers (TPP-PEG-ss-PLA) using disulfide bonds as the intermediate linkers. To shield the surface positive charge of the nanoparticles from TPP composition, chondroitin sulfate (CS) is employed to coat the nanoparticles, and this prolongs blood circulation while endowing an active targeting ability to the cell membrane. In acidic lyso-somes/endosomes, the negatively charged CS layer falls away to expose the TPP component. Subsequently, in the cyto-plasm, the nanoparticles can anchor to the mitochondrial outer membrane by TPP-mediated targeting, thereby inducing a decrease in the membrane potential and opening of the permeability transition pore. Thus, the overproduction of ROS in the mitochondria promotes cell apoptosis. The released DOX directly diffuse into the mitochondria, thereby resulting in mito-chondrial DNA damage. Therefore, the nanoparticles exhibit significant potential in terms of a new avenue for mitochondrion therapy in cancer treatment.

The cross-talk modulation of excited state electron transfer to reduce the false negative background for high fidelity imaging in vivo

In practice, high fidelity fluorescence imaging in vivo faces many issues, for example: (1) the fluorescence background of the probe is bleached by the wide intensity scale of fluorescence microscopy, displaying an inherent false negative background (FNB); and (2) the dosage of the probe has to be increased to achieve sufficient intensity for in vivo imaging, causing a vicious cycle that exacerbates the FNB. Herein, we constructed a fluorophore (F)-electron donor (D)-electron regulator (R) system, and thereby developed a dual modulation strategy for the de novo design of high fidelity probes. Using cross-talk modulation, the probe allows: (1) enhanced ESET (excited state electron transfer) from F to D, which minimizes the inherent FNB based on synergistic PET (photo induced electron transfer); and (2) the inhibition of PET and weakening of ESET from F to D to maximize the reporting intensity to further reduce the FNB, which is additionally enhanced by an overdose of the probe. To test the implementation, we constructed a 7-hydroxy-2-oxo-2H-chromene-3-carbaldehyde (HPC) series of probes, with HPC (F) as the fluorophore, 2-hydrazinylpyridine, which was screened as an electronically adjustable donor (D), and electronic regulators (R). In particular, HPC-7 and HPC-8 provided cell/zebrafish imaging with negligible background even using the rather low fluorescence scale of microscopy (a region for revealing hidden background). Interestingly, with the specificity of HPC for reporting zinc, we achieved probe HPC-5, which possesses both an ultralow inherent FNB and optimal reporting intensity for tissue and in vivo imaging, enabling the in vivo imaging of zinc in mice for the first time. Under this high-fidelity mode, the fluorescence monitoring of zinc ions during the development of liver cancer in mice was successfully performed. We envision that the dual modulation strategy with the F-D-R system could provide a useful concept for the de novo design of practical probes.

A new strategy to improve the water solubility of an organic fluorescent probe using silicon nanodots and fabricate two-photon SiND-ANPA-N3 for visualizing hydrogen sulfide in living cells and onion tissues

A water-soluble fluorescent probe based on SiNDs for H₂S detection can be used in both fully aqueous media and living cells.

Endoplasmic reticulum targeting fluorescent probes to image mobile Zn2

Zn2+ plays an important role in the normal function of the endoplasmic reticulum (ER) and its deficiency can cause ER stress, which is related to a wide range of diseases. In order to provide tools to better understand the role of mobile Zn2+ in ER processes, the first custom designed ER-localised fluorescent Zn2+ probes have been developed through the introduction of a cyclohexyl sulfonylurea as an ER-targeting unit with different Zn2+ receptors. Experiments in vitro and in cellulo show that both probes have a good fluorescence switch on response to Zn2+, high selectivity over other cations, low toxicity, ER-specific targeting ability and are efficacious imaging agents for mobile Zn2+ in four different cell lines. Probe 9 has been used to detect mobile Zn2+ changes under ER stress induced by both tunicamycin or thapsigargin, which indicates that the new probes should allow a better understanding of the mechanisms cells use to respond to dysfunction of zinc homeostasis in the ER and its role in the initiation and progression of diseases to be developed.

An alternative modular 'click-SNAr-click' approach to develop subcellular localised fluorescent probes to image mobile Zn2+

Zn2+ is involved in a number of biological processes and its wide-ranging roles at the subcellular level, especially in specific organelles, have not yet been fully established due to a lack of tools to image it effectively. We report a new and efficient modular double 'click' approach towards a range of sub-cellular localised probes for mobile zinc. Through this methodology, endoplasmic reticulum, mitochondria and lysosome localised probes were successfully prepared which show good fluorescence responses to mobile Zn2+in vitro and in cellulo whilst a non-targeting probe was synthesized as a control. The methodology appears to have wide-utility for the generation of sub-cellular localised probes by incorporating specific organelle targeting vectors for mobile Zn2+ imaging.

Multifunctional Fe3O4@Polydopamine@DNA-Fueled Molecular Machine for Magnetically Targeted Intracellular Zn2+ Imaging and Fluorescence/MRI Guided Photodynamic-Photothermal Therapy

For the precise treatment of tumors, it is necessary to develop a theranostic nanoplatform that has both diagnostic and therapeutic functions. In this article, we designed a new theranostic probe for fluorescence imaging of Zn²⁺ and fluorescence/MRI guided magnetically targeted photodynamic-photothermal therapy. The fluorescence imaging of Zn²⁺ was based on an endogenous ATP-driven DNA nanomachine that could perform repetitive stand displacement reaction. It modifies all units on a single PDA/Fe₃O₄ nanoparticle containing a hairpin-locked initiated strand activated by a target molecule in cells, a two-stranded fuel DNA triggered by ATP, and a two-stranded DNA track responding to an initiated strand and fuel DNA. After entering the cell, the intracellular target Zn²⁺ initiates the nanomachine via an autocatalytic cleavage reaction, and the machine programmatically and gradually runs on the assembled DNA track via fuel DNA driving and the intramolecular toehold-mediated stand displacement reaction. The Fe₃O₄ core first exhibits magnetic targeting, increasing the ability of nanoparticles to enter tumor cells at the tumor site. The Fe₃O₄ could also be employed as a powerful magnetic resonance imaging (MRI) contrast agent and guided therapy. Using 808 nm laser and 635 nm laser irradiation together at the tumor site, the PDA nanoshell produced an excellent photothermal effect and the TMPyP4 molecules entering the cell generated reactive oxygen species, followed by cell damage. A series of reliable experiments suggested that the Fe₃O₄@PDA@DNA nanoprobe showed superior fluorescence specificity, MRI, a remarkable photothermal/photodynamic therapy effect, and favorable biocompatibility. This theranostic nanoplatform offered a split-new insight into tumor fluorescence and MRI diagnosis as well as effective tumor therapy.

Mitochondria-Targeted New Blue Light-Emitting Fluorescent Molecular Probe

Discovery of a nontoxic fluorescent molecular probe to "light up" specific cellular organelles is extremely essential to understand dynamics of intracellular components. Here, we report a new nontoxic mitochondria-targeted linear bithiazole compound, containing trifluoroacetyl terminal groups, which emits intense blue fluorescence and stained mitochondria of various cells. Interestingly, the power of fluorescence is completely off when the bithiazole unit is stapled by a carbonyl bridge.

DNA-Templated Silver Nanocluster/Porphyrin/MnO2 Platform for Label-Free Intracellular Zn2+ Imaging and Fluorescence-/Magnetic Resonance Imaging-Guided Photodynamic Therapy

Developing a theranostic platform that integrates diagnosis and treatment in one single nanostructure is necessary for efficient tumor treatment. Here, we presented a novel theranostic nanoprobe for nonlabeled fluorescence imaging of Zn²⁺ and 635 nm red light-triggered photodynamic therapy (PDT) by a multifunctional DNA-templated silver nanocluster/porphyrin/MnO₂ nanoplatform. MnO₂ nanosheets adsorbed hairpin DNA-silver nanoclusters (AgNCs) and porphyrin (P) by facile physisorption, which accelerate the transfection of nanoprobes and P into tumor cells. After entering the cells, the biodegradation of MnO₂ nanosheets by glutathione and acidic hydrogen peroxide released AgNCs for label-free Zn²⁺ fluorescence imaging by the hairpin DNA-fueled dynamic self-assembly of three-way DNA junction architectures, and the released Mn²⁺ could act as an effective magnetic resonance imaging (MRI) contrast agent. In addition, MnO₂ was decomposed in the acidic H₂O₂-ample environment and produced O₂ to overbear hypoxia-related PDT resistance, highly efficient PDT was obtained by excess singlet oxygen (¹O₂) release of P-AgNCs-MnO₂ nanoprobes under light irradiation compared with free P. In vitro and in vivo studies confirmed that P-AgNCs-MnO₂ exhibited high fluorescence specificity, excellent PDT effect, and good biocompatibility and could be used as a contrast agent for MRI. This theranostic platform provided a new avenue for the fluorescence and MRI diagnosis of tumors and efficient tumor treatment.

NIR-emitting benzothiazolium cyanines with an enhanced stokes shift for mitochondria imaging in live cells

A series of benzothiazolium-based hemicyanines (3a-3f) have been synthesized. Evaluation of their photophysical properties shows that they exhibit improved photophysical characteristics. In comparison with the available commercial MitoTrackers, the new probes revealed an enhanced Stokes shift (Δλ ∼ 80 nm) and minimized aggregation for increased sensitivity. The synthesized probes are found to exhibit excellent selectivity for mitochondrial staining in an oligodendrocyte cell line. Probes show almost no fluorescence in aqueous environments, while the fluorescence is increased by ∼10-fold in organic solvents, making it possible for mitochondrial imaging without the need for post-staining washing. Since the absorption peaks of probes are close to the laser wavelengths of 561 and 640 nm on a commercial confocal microscope, e.g.3a exhibits λabs ∼ 620 nm and λem ∼ 702 nm, they could be useful probes for mitochondrial tracking in live cells.

Facile synthesis of homochiral compounds integrating circularly polarized luminescence and two-photon excited fluorescence

A pair of novel chiral one-dimensional compounds with the general formula [Cd(NO3)2L2]·THF (where L is R- or S-2,2'-bis(methoxymethoxy)-6,6'-bis(4-pyridyl)-1,1'-binaphthyl) for the first time show circularly polarized luminescence, two-photon excited fluorescence, and second harmonic generation activity.

Aromatic secondary amine-functionalized fluorescent NO probes: improved detection sensitivity for NO and potential applications in cancer immunotherapy studies

Tumor-associated macrophages (TAMs), constituting up to 50% of the solid tumor mass and commonly having a pro-tumoral M2 phenotype, are closely associated with decreased survival in patients. Based on the highly dynamic properties of macrophages, in recent years the repolarization of TAMs from pro-tumoral M2 phenotype to anti-tumoral M1 phenotype by various strategies has emerged as a promising cancer immunotherapy approach for improving cancer therapy. Herein, we present an aromatic secondary amine-functionalized Bodipy dye 1 and its mitochondria-targetable derivative Mito1 as fluorescent NO probes for discriminating M1 macrophages from M2 macrophages in terms of their difference in inducible NO synthase (iNOS) levels. The two probes possess the unique ability to simultaneously respond to two secondary oxides of NO, i.e., N2O3 and ONOO-, thus being more sensitive and reliable for reflecting intracellular NO than most of the existing fluorescent NO probes that usually respond to N2O3 only. With 1 as a representative, the discrimination between M1 and M2 macrophages, evaluation of the repolarization of TAMs from pro-tumoral M2 phenotype to anti-tumoral M1 phenotype, and visualization of NO communication during the immune-mediated phagocytosis of cancer cells by M1 macrophages have been realized. These results indicate that our probes should hold great potential for imaging applications in cancer immunotherapy studies and relevant anti-cancer drug screening.

Ultra-fast zinc ion detection in living cells and zebrafish by a light-up fluorescent probe

As the second most abundant transition metal after iron in biological systems, Zn²⁺ takes part in various fundamental life processes such as cellular metabolism and apoptosis, neurotransmission. Thus, the development of analytical methods for fast detection of Zn²⁺ in biology and medicine has been attracting much attention but still remains a huge challenge. In this report, we develop a novel Zn²⁺-specific light-up fluorescent probe based on intramolecular charge transfer combined with chelation enhanced fluorescence induced by structural transformation. Addition of Zn²⁺ in vitro can induce a remarkable color change from colorless to green and a strong fluorescence enhancement with a red shift of 43 nm. Moreover, the probe shows an extremely low detection limit of 13 nM and ultra-fast response time of less than 1 s. The Zn²⁺ sensing mechanism was fully supported by TDDFT calculations as well as HRMS and ¹H NMR titrations. The recognition of Zn²⁺ in living Hela cells as well as the MTT assay demonstrate that the probe can rapidly light-up detect Zn²⁺ in vivo with low cytotoxicity and good cell-permeability. Furthermore, the probe can also be successfully applied to bioimaging Zn²⁺ in living zebrafish.