Photoactivatable Sensors for Detecting Mobile Zinc

Bright photoactivatable probes based on triphenylethylene for Cu2+ detection in tap water and tea samples

Photoactivatable probes can switch fluorescence on from a weak or nonemission state to improve the sensitivity of the sensing system. In this work, we successfully constructed three highly emissive photoactivatable probes, 2-DP, 1-2-DP and 2-2-DP, for Cu2+ detection. Under UV irradiation, the photoluminescence quantum yields of 2-DP, 1-2-DP and 2-2-DP display approximately 52.4-, 11.5- and 49.2-fold enhancement, respectively. Cu2+ selectively quenches the bright photoactivated fluorescence, resulting in an approximately 38-fold fluorescence reduction. The highly selective fluorescence response to Cu2+ yields an excellent low detection limit of 5.8 nM. Moreover, the photoactivatable probes were successfully applied for Cu2+ determination in tap water and tea samples with recovery ranges of 95%-105% and 97%-106%, respectively. This work provides a more sensitive and efficient methodology for Cu2+ detection in heavy metal pollution and food safety.

A near-infrared light-activated nanoprobe for simultaneous detection of hydrogen polysulfide and sulfur dioxide in myocardial ischemia-reperfusion injury

Ischemia-reperfusion-induced cardiomyocyte mortality constitutes a prominent contributor to global morbidity and mortality. However, early diagnosis and preventive treatment of cardiac I/R injury remains a challenge. Given the close relationship between ferroptosis and I/R injury, monitoring their pathological processes holds promise for advancing early diagnosis and treatment of the disease. Herein, we report a near-infrared (NIR) light-activated dual-responsive nanoprobe (UCNP@mSiO2@SP-NP-NAP) for controllable detection of hydrogen polysulfide (H2Sn) and sulfur dioxide (SO2) during ferroptosis-related myocardial I/R injury. The nanoprobe's responsive sites could be activated by NIR and Vis light modulation, reversibly alternating for at least 5 cycles. We employed the nanoprobe to monitor the fluctuation levels of H2Sn and SO2 in H9C2 cardiomyocytes and mice, revealing that H2Sn and SO2 levels were up-regulated during I/R. The NIR light-activated dual-responsive nanoprobe could be a powerful tool for myocardial I/R injury diagnosis. Moreover, we also found that inhibiting the initiation of the ferroptosis process contributed to attenuating cardiac I/R injury, which indicated great potential for treating I/R injury.

A Semisynthetic Bioluminescence Sensor for Ratiometric Imaging of Metal Ions In Vivo Using DNAzymes Conjugated to An Engineered Nano-Luciferase

DNA-based probes have gained significant attention as versatile tools for biochemical analysis, benefiting from their programmability and biocompatibility. However, most existing DNA-based probes rely on fluorescence as the signal output, which can be problematic due to issues like autofluorescence and scattering when applied in complex biological materials such as living cells or tissues. Herein, we report the development of bioluminescent nucleic acid (bioLUNA) sensors that offer laser excitation-independent and ratiometric imaging of the target in vivo. The system is based on computational modelling and mutagenesis investigations of a genetic fusion between circular permutated Nano-luciferase (NLuc) and HaloTag, enabling the conjugation of the protein with a DNAzyme. In the presence of Zn2+ , the DNAzyme sensor releases the fluorophore-labelled strand, leading to a reduction in bioluminescent resonance energy transfer (BRET) between the luciferase and fluorophore. Consequently, this process induces ratiometric changes in the bioluminescent signal. We demonstrated that this bioLUNA sensor enabled imaging of both exogenous Zn2+ in vivo and endogenous Zn2+ efflux in normal epithelial prostate and prostate tumors. This work expands the DNAzyme sensors to using bioluminescence and thus has enriched the toolbox of nucleic acid sensors for a broad range of biomedical applications.

Dual-key-and-lock AIE probe for thiosulfate and Ag+ detection in mitochondria

Ag⁺ ion detection has attracted much attention due to its important role in chemical and biological processes, as well as its potential threat to the environment and human health. Herein, we firstly constructed a dual-key-and-lock sensing strategy for Ag⁺ detection based on three-component co-assembly. An aggregation-induced emission luminogen (AIEgen), namely triphenylamine-thiophene-pyridinium (abbreviated to TPA-T-Py), showed unique co-assembly capability with Ag⁺ and S₂O₃^2- in PBS buffer (pH 7.4, 0.01 M). Cell imaging further proved that mitochondria can be lit up by TPA-T-Py under the dual key stimulation, which was successfully used for Ag⁺ and S₂O₃^2- detection in vitro. In brief, we provide a promising strategy for the construction of dual-lock imaging agents with organelle-targeting ability.

A Timm-Nissl multiplane microscopic atlas of rat brain zincergic terminal fields and metal-containing glia

The ability of Timm's sulphide silver method to stain zincergic terminal fields has made it a useful neuromorphological marker. Beyond its roles in zinc-signalling and neuromodulation, zinc is involved in the pathophysiology of ischemic stroke, epilepsy, degenerative diseases and neuropsychiatric conditions. In addition to visualising zincergic terminal fields, the method also labels transition metals in neuronal perikarya and glial cells. To provide a benchmark reference for planning and interpretation of experimental investigations of zinc-related phenomena in rat brains, we have established a comprehensive repository of serial microscopic images from a historical collection of coronally, horizontally and sagittally oriented rat brain sections stained with Timm's method. Adjacent Nissl-stained sections showing cytoarchitecture, and customised atlas overlays from a three-dimensional rat brain reference atlas registered to each section image are included for spatial reference and guiding identification of anatomical boundaries. The Timm-Nissl atlas, available from EBRAINS, enables experimental researchers to navigate normal rat brain material in three planes and investigate the spatial distribution and density of zincergic terminal fields across the entire brain.

Mobile zinc as a modulator of sensory perception

Mobile zinc is an abundant transition metal ion in the central nervous system, with pools of divalent zinc accumulating in regions of the brain engaged in sensory perception and memory formation. Here, we present essential tools that we developed to interrogate the role(s) of mobile zinc in these processes. Most important are (a) fluorescent sensors that report the presence of mobile zinc and (b) fast, Zn-selective chelating agents for measuring zinc flux in animal tissue and live animals. The results of our studies, conducted in collaboration with neuroscientist experts, are presented for sensory organs involved in hearing, smell, vision, and learning and memory. A general principle emerging from these studies is that the function of mobile zinc in all cases appears to be downregulation of the amplitude of the response following overstimulation of the respective sensory organs. Possible consequences affecting human behavior are presented for future investigations in collaboration with interested behavioral scientists.

Fluorescent Quinoline-Based Supramolecular Gel for Selective and Ratiometric Sensing Zinc Ion with Multi-Modes

A gelator 1 containing functional quinoline and Schiff base groups that could form organogels in DMF, DMSO, acetone, ethanol and 1,4-dioxane was designed and synthesized. The self-assembly process of geator 1 was characterized by field emission scanning electron microscopy (FESEM), UV-vis absorption spectroscopy, fluorescence emission spectroscopy, Fourier transform infrared spectroscopy(FTIR), X-ray powder diffraction (XRD) and water contact angle. Under non-covalent interactions, gelator 1 self-assembled into microbelts and nanofiber structures with different surface wettability. Weak fluorescence was emitted from the solution and gel state of 1. Interestingly, gelator 1 exhibited good selectivity and sensitivity towards Zn2+ in solution and gel states along with its emission enhancement and change. The emission intensity at 423 nm of solution 1 in 1,4-dioxane was slightly enhanced, and a new emission peak appeared at 545 nm along with its intensity sequentially strengthened in the titration process. The obvious ratiometric detection process was presented with a limit of detection (LOD) of 5.51 μM. The detection mechanism was revealed by a theoretical calculation and NMR titration experiment, which was that Zn2+ induced the transition from trans- to cis- of molecule 1 and further coordinated with 1. This study will introduce a new method for the construction of functional self-assembly gel sensors for the detection of Zn2+.

Dual-Ligand Near-Infrared Luminescent Lanthanide-Based Metal-Organic Framework Coupled with In Vivo Microdialysis for Highly Sensitive Ratiometric Detection of Zn2+ in a Mouse Model of Alzheimer's Disease

Zinc, which is the second most abundant trace element in the human central nervous system, is closely associated with Alzheimer's disease (AD). However, attempts to develop highly sensitive and selective sensing systems for Zn²⁺ in the brain have not been successful. Here, we used a one-step solvothermal method to design and prepare a metal-organic framework (MOF) containing the dual ligands, terephthalic acid (H₂BDC) and 2,2':6',2″-terpyridine (TPY), with Eu³⁺ as a metal node. This MOF is denoted as Eu-MOF/BDC-TPY. Adjustment of the size and morphology of Eu-MOF/BDC-TPY allowed the dual ligands to produce multiple luminescence peaks, which could be interpreted via ratiometric fluorescence to detect Zn²⁺ using the ratio of Eu³⁺-based emission, as the internal reference, and ligand-based emission, as the indicator. Thus, Eu-MOF/BDC-TPY not only displayed higher selectivity than other metal cations but also offered a highly accurate, sensitive, wide linear, color change-based technique for detecting Zn²⁺ at concentrations ranging from 1 nM to 2 μM, with a low limit of detection (0.08 nM). Moreover, Eu-MOF/BDC-TPY maintained structural stability and displayed a fluorescence intensity of at least 95.4% following storage in water for 6 months. More importantly, Eu-MOF/BDC-TPY sensed the presence of Zn²⁺ markedly rapidly (within 5 s), which was very useful in practical application. Furthermore, the results of our ratiometric luminescent method-based analysis of Zn²⁺ in AD mouse brains were consistent with those obtained using inductively coupled plasma mass spectrometry.

Photocontrollable Fluorescence Imaging of Mitochondrial Peroxynitrite during Ferroptosis with High Fidelity

Ferroptosis, a new regulatory cell death modality, underlies the pathogenesis of a broad range of disorders. Although much efforts have been made to uncover the molecular mechanisms, some mechanistic details of ferroptosis still remain poorly understood. Particularly, the functional relevance of mitochondrial reactive oxygen species (ROS) in ferroptosis is still highly controversial, which is partially due to the fact that it still remains puzzled how the mitochondrial ROS level varies during ferroptosis. The conventional mitochondria-targeted probes may react with cytosolic ROS and show fluorescence variation before entering mitochondria, thus probably giving a false result on the mitochondrial ROS level and leading to the misjudgment on its biofunction. To circumvent this issue, we rationally designed a photocontrollable and mitochondria-targeted fluorescent probe to in situ visualize the mitochondrial peroxynitrite (ONOO^-), which is the ROS member and mediator of ferroptosis. The photoactivated probe was endowed with a highly specific and sensitive fluorescence response to ONOO^-. Notably, the response activity could be artificially regulated with light irradiation, which ensured that all the probe molecules passed through the cytosol in the locked status and were then photoactivated after reaching mitochondria. This photocontrolled fluorescence imaging strategy eliminated the interference of ONOO^- outside the mitochondria, thus potentially afforded improved fidelity for mitochondrial ONOO^- bioimaging in live cells and animal models. With this probe, for the first time, we revealed the mitochondrial ONOO^- flux and its probable biological source during erastin-induced ferroptosis. These results suggest a tight correlation between mitochondrial ONOO^-/ROS and ferroptotic progression, which will further facilitate the comprehensive exploration and manipulation of ferroptosis.

A Molecular Logic Gate for Developing "AND" Logic Probes and the Application in Hepatopathy Differentiation

Accurate diagnosis and therapy are challenging because most diseases lack a single biomarker that distinguishes them from other disorders. A solution would enhance targeting accuracy by using AND-gated combinations of two disease-associated stimuli. Here, we report a novel "AND" molecular logic gate, enabling a double-controlled release of intact functional molecules. Benefiting from a significant difference in intramolecular cyclization rate, cargo release occurs notably faster with the presence of both stimuli. According to this finding, several AND logic probes have been developed that respond to a broad scope of stimuli and show remarkably improved signal-to-background contrast compared to those of monoresponsive probes. In addition, an AND logic probe that is responsive to monoamine oxidase (MAO) and leucine aminopeptidase (LAP) has been constructed for hepatopathy diagnosis. It works efficiently in living cells and mouse models. Of note, this probe can successfully differentiate cirrhotic from hepatitis B by testing the blood samples from patients.

Recent advances in dual-target-activated fluorescent probes for biosensing and bioimaging

Fluorescent probes have been powerful tools for visualizing and quantifying multiple dynamic processes in living cells. However, the currently developed probes are often constructed by conjugation a fluorophore with a recognition moiety and given signal-output after triggering with one singly target interest. Compared with the single-target-activated fluorescent probes mentioned above, the dual-target-activated ones, triggering with one target under stimulus (such as photoirradiation, microenvironment) or another targets, have the advantages of advoiding nonspecific activation and "false positive" results in complicated environments. In recent years, many dual-target-activated fluorescent probes have been developed to detect various biologically relevant species. In view of the importance of a comprehensive understanding of dual-target- activated fluorescent probes, a thorough summary of this topic is urgently needed. However, no comprehensive and critical review on dual target activated fluorescent probes has been published recently. In this review, we focus on the dual-target-activated fluorescent probes and briefly outline their types and current state of development. In each type, the chemical structure, proposed responsive mechanism and application of probes are highlighted. At last, the challenges and prospective opportunities of every type were proposed.

Aza-Crown-Based Macrocyclic Probe Design for "PET-off" Multi-Cu2+ Responsive and "CHEF-on" Multi-Zn2+ Sensor: Application in Biological Cell Imaging and Theoretical Studies

The work represents a rare example of an aza-crown-based macrocyclic chemosensor, H₂DTC (H₂DTC = 1,16-dihydroxy-tetraaza-30-crown-8) for the selective detection of both Zn²⁺ and Cu²⁺ in HEPES buffer medium (pH 7.4). H₂DTC exhibits a fluorescence response for both Zn²⁺ and Cu²⁺ ions. The reversibility of the chemosensor in its binding with Zn²⁺ and Cu²⁺ ions is also examined using a Na₂EDTA solution. H₂DTC exhibits a chelation-enhanced fluorescence (CHEF) effect in the presence of Zn²⁺ ions and a quenching effect (CHEQ) in the presence of paramagnetic Cu²⁺ ions. Furthermore, the geometry and spectral properties of H₂DTC and the chemosensor bound to Zn²⁺ have been studied by DFT and TDDFT calculations. The limit of detection (LOD) values are 0.11 × 10^-9 and 0.27 × 10^-9 M for Cu²⁺ and Zn²⁺, respectively. The formation constants for the Zn²⁺ and Cu²⁺ complexes have been measured by pH-potentiometry in 0.15 M NaCl in 70:30 (v:v) water:ethanol at 298.1 K. UV-vis absorption and fluorometric spectral data and pH-potentiometric titrations indicate 1:1 and 2:1 metal:chemosensor species. In the solid state H₂DTC is able to accommodate up to four metal ions, as proved by the crystal structures of the complexes [Zn₄(DTC)(OH)₂(NO₃)₄] (1) and {[Cu₄(DTC)(OCH₃)₂(NO₃)₄]·H₂O}_n (2). H₂DTC can be used as a potential chemosensor for monitoring Zn²⁺ and Cu²⁺ ions in biological and environmental media with outstanding accuracy and precision. The propensity of H₂DTC to detect intracellular Cu²⁺ and Zn²⁺ ions in the triple negative human breast cancer cell line MDA-MB-468 and in HeLa cells has been determined by fluorescence cell imaging.

Intelligent demethylase-driven DNAzyme sensor for highly reliable metal-ion imaging in living cells

The accurate intracellular imaging of metal ions requires an exquisite site-specific activation of metal-ion sensors, for which the pervasive epigenetic regulation strategy can serve as an ideal alternative thanks to its orthogonal control feature and endogenous cell/tissue-specific expression pattern. Herein, a simple yet versatile demethylation strategy was proposed for on-site repairing-to-activating the metal-ion-targeting DNAzyme and for achieving the accurate site-specific imaging of metal ions in live cells. This endogenous epigenetic demethylation-regulating DNAzyme system was prepared by modifying the DNAzyme with an m⁶A methylation group that incapacitates the DNAzyme probe, thus eliminating possible off-site signal leakage, while the cell-specific demethylase-mediated removal of methylation modification could efficiently restore the initial catalytic DNAzyme for sensing metal ions, thus allowing a high-contrast bioimaging in live cells. This epigenetic repair-to-activate DNAzyme strategy may facilitate the robust visualization of disease-specific biomarkers for in-depth exploration of their biological functions.

Novel Prostate Cancer Biomarkers: Aetiology, Clinical Performance and Sensing Applications

The review initially provides a short introduction to prostate cancer (PCa) incidence, mortality, and diagnostics. Next, the need for novel biomarkers for PCa diagnostics is briefly discussed. The core of the review provides details about PCa aetiology, alternative biomarkers available for PCa diagnostics besides prostate specific antigen and their biosensing. In particular, low molecular mass biomolecules (ions and metabolites) and high molecular mass biomolecules (proteins, RNA, DNA, glycoproteins, enzymes) are discussed, along with clinical performance parameters.

Extended sawhorse waveform for stable zinc detection with fast-scan cyclic voltammetry

Zinc (Zn(II)) is a divalent cation involved in regulating intracellular signal transduction and gene expression through transcription factor activity, and can act as a metal neurotransmitter by modulating synaptic activity and neuronal plasticity. Previous research has demonstrated spatial heterogeneity of Zn(II) in the brain, has estimated extracellular concentrations of Zn(II) across various brain regions, and has measured rapid intracellular changes in Zn(II) concentration during glutamate flux. Despite this work, quantification of rapid extracellular Zn(II) release from neurons, on a millisecond time scale, in real time has remained difficult with existing technologies. Here, we have developed an electrochemical waveform, called the "extended sawhorse waveform (ESW)," for fast-scan cyclic voltammetry detection at carbon-fiber microelectrodes which enabled rapid and stable Zn(II) monitoring over time. This waveform was developed to overcome existing challenges in monitoring metallotransmitters stably over time electrochemically by introducing a brief cleaning step to facilitate rapid cleaning of the electrode surface in between scans. The ESW scans from 0.5 V down to -1.0 V, up to 1.45 V for 3 ms (cleaning step), and back to 0.5 V at a scan rate of 400 V/s. Repeated introductions of Zn(II) at the electrode using a traditional waveform cause plating which ultimately deteriorates the sensitivity over time; however, using the ESW, significant improvements in stability were observed. Overall, we provide a unique approach to monitor and quantitate rapid Zn(II) signaling in the brain at carbon electrodes which will impact our ability to advance fundamental knowledge of Zn(II) involvement in extracellular signaling pathways in the brain.

Dual-locked spectroscopic probes for sensing and therapy

Optical imaging probes allow us to detect and uncover the physiological and pathological functions of an analyte of interest at the molecular level in a non-invasive, longitudinal manner. By virtue of simplicity, low cost, high sensitivity, adaptation to automated analysis, capacity for spatially resolved imaging and diverse signal output modes, optical imaging probes have been widely applied in biology, physiology, pharmacology and medicine. To build a reliable and practically/clinically relevant probe, the design process often encompasses multidisciplinary themes, including chemistry, biology and medicine. Within the repertoire of probes, dual-locked systems are particularly interesting as a result of their ability to offer enhanced specificity and multiplex detection. In addition, chemiluminescence is a low-background, excitation-free optical modality and, thus, can be integrated into dual-locked systems, permitting crosstalk-free fluorescent and chemiluminescent detection of two distinct biomarkers. For many researchers, these dual-locked systems remain a 'black box'. Therefore, this Review aims to offer a 'beginner's guide' to such dual-locked systems, providing simple explanations on how they work, what they can do and where they have been applied, in order to help readers develop a deeper understanding of this rich area of research.

BODIPY-attached zinc(II) complexes of curcumin drug for visible light assisted photo-sensitization, cellular imaging and targeted PDT

Boron-dipyrromethene (BODIPY) based photosensitizers as porphyrinoids and curcumin as natural product possess exciting photophysical features suitable for theranostic applications, namely, imaging and photodynamic therapy (PDT). Limited aqueous solubility and insufficient physiological stability, however, reduce their efficacy significantly. We have designed a novel strategy to deliver these two unusable cytotoxins simultaneously in cancer cells and herein, report the synthesis, characterization and imaging-assisted photocytotoxicity of three zinc(II) complexes containing N³-donor dipicolylamine (dpa) ligands (L^1-3) and O,O-donor curcumin (Hcur) viz. [Zn(L¹)(cur)]Cl (1), [Zn(L²)(cur)]Cl (2) and [Zn(L³)(cur)]Cl (3), where L² and L³ have pendant fluorescent BODIPY and non-emissive di-iodo-BODIPY moieties. Metal chelation imparted remarkable biological stability (pH ∼7.4) to the respective ligands and induces significant aqueous solubility. These ternary complexes could act as replacements of the existing metalloporphyrin-based PDT photosensitizers as their visible-light photosensitizing ability is reinforced by the dual presence of blue light absorbing curcumin and green light harvesting BODIPY units. Complex 2 having emissive BODIPY unit L² and curcumin, showed mitochondria selective localization in HeLa, MCF-7 cancer cells and complex 3, the di-iodinated analogue of complex 2, exhibited type-I/II PDT activity via inducing apoptosis through mitochondrial membrane disruption in cancer cells while being significantly nontoxic in dark and to the healthy cells.

Colorimetric/fluorescent/Raman trimodal sensing of zinc ions with complexation-mediated Au nanorod

Accurate and selective in-field detection of metal ions in complex media has gained wide interests due to the complexed matrices and weak affinity towards sensing surface. Herein, we develop a first trimodal method for sensing of Zn²⁺ in complex matrices by stimuli-responsive N-[6-piperazinyl-2-pyridinyl]-N-(2-pyridinylmethyl)-2-Pyridinemethanamine dithiocarbamates (DPY) modified gold nanorods (GNRs-DPY). The presence of Zn²⁺ triggers the aggregation of GNRs-DPY, leading to increment of color and fluorescence intensity of the sensing system, which could be visually discerned with bare eye. Moreover, the intensive electromagnetic enhancement among "hot spots" of GNRs, generated during self-aggregation of the GNRs-DPY caused by Zn²⁺, lowers the detection limit of SERS assay to 6 × 10^-3 pM. It is noteworthy that GNRs-DPY based sensing platform not only enables distinguishing Zn²⁺ from Cd²⁺, with simplicity and rapidity, but also demonstrates as trimodal nanoprobe for sensitive and selective quantitative determination of Zn²⁺ in different matrices. Therefore, the GNRs-DPY provides a new strategy for accurate and credible on-spot determination of Zn²⁺ in complicated specimens, as well as offering multiple applications in point-of-care monitoring.

Fluorescence activation, patterning and enhancement with photogenerated radicals, a prefluorescent probe and silver nanostructures

We designed a switchable fluorophore activated by UVA light and a radical initiator, for optical lithography with concomitant metal-enhanced fluorescence by silver nanoparticles.

A highly selective pyridoxal-based chemosensor for the detection of Zn(ii) and application in live-cell imaging; X-ray crystallography of pyridoxal-TRIS Schiff-base Zn(ii) and Cu(ii) complexes

In a simple, one-step reaction, we have synthesized a pyridoxal-based chemosensor by reacting tris(hydroxymethyl)aminomethane (TRIS) together with pyridoxal hydrochloride to yield a Schiff-base ligand that is highly selective for the detection of Zn(ii) ion. Both the ligand and the Zn(ii) complex have been characterized by ¹H & ¹³C NMR, ESI-MS, CHN analyses, and X-ray crystallography. The optical properties of the synthesized ligand were investigated in an aqueous buffer solution and found to be highly selective and sensitive toward Zn(ii) ion through a fluorescence turn-on response. The competition studies reveal the response for zinc ion is unaffected by all alkali and alkaline earth metals; and suppressed by Cu(ii) ion. The ligand itself shows a weak fluorescence intensity (quantum yield, Φ = 0.04), and the addition of zinc ion enhanced the fluorescence intensity 12-fold (quantum yield, Φ = 0.48). The detection limit for zinc ion was 2.77 × 10⁻⁸ M, which is significantly lower than the WHO's guideline (76.5 μM). Addition of EDTA to a solution containing the ligand–Zn(ii) complex quenched the fluorescence, indicating the reversibility of Zn(ii) binding. Stoichiometric studies indicated the formation of a 2 : 1 L₂Zn complex with a binding constant of 1.2 × 10⁹ M⁻² (±25%). The crystal structure of the zinc complex shows the same hydrated L₂Zn complex, with Zn(ii) ion binding with an octahedral coordination geometry. We also synthesized the copper(ii) complex of the ligand, and the crystal structure showed the formation of a 1 : 1 adduct, revealing 1-dimensional polymeric networks with octahedral coordinated Cu(ii). The ligand was employed as a sensor to detect zinc ion in HEK293 cell lines derived from human embryonic kidney cells grown in tissue culture which showed strong luminescence in the presence of Zn(ii). We believe that the outstanding turn-on response, sensitivity, selectivity, lower detection limit, and reversibility toward zinc ion will find further application in chemical and biological science.

The synthesis, characterization, X-ray crystallography, and live-cell imaging of pyridoxal-TRIS Schiff-base ligand which is selective as a luminescence sensor to detect Zn(ii) ion, and the corresponding Zn(ii) and Cu(ii) complexes are described.

A FRET-based fluorescent Zn2+ sensor: 3D ratiometric imaging, flow cytometric tracking and cisplatin-induced Zn2+ fluctuation monitoring

Monitoring labile Zn²⁺ homeostasis is of great importance for the study of physiological functions of Zn²⁺ in biological systems. Here we report a novel ratiometric fluorescent Zn²⁺ sensor, CPBT, which was constructed based on chelation-induced alteration of FRET efficiency. CPBT was readily cell membrane permeable and showed a slight preferential localization in the endoplasmic reticulum. With this sensor, 3D ratiometric Zn²⁺ imaging was first realized in the head of zebra fish larvae via Z-stack mode. CPBT could track labile Zn²⁺ in a large number of cells through ratiometric flow cytometric assay. More interestingly, both ratiometric fluorescence imaging and flow cytometric assay demonstrated that the labile Zn²⁺ level in MCF-7 cells (cisplatin-sensitive) decreased while that in SKOV3 cells (cisplatin-insensitive) increased after cisplatin treatment, indicating that Zn²⁺ may play an important role in cisplatin induced signaling pathways in these cancer cells.

A Zn²⁺ sensor exhibiting 3D ratiometric imaging and flow cytometric ability was constructed based on the FRET mechanism, and cisplatin-induced endogenous labile Zn²⁺ fluctuations were monitored in real time.

Zinc modulates synaptic transmission by differentially regulating synaptic glutamate homeostasis in hippocampus

A subset of presynaptic glutamatergic vesicles in the brain co-releases zinc (Zn²⁺ ) with glutamate into the synapse. However, the role of synaptically released Zn²⁺ is still under investigation. Here, we studied the effect of Zn²⁺ on glutamate homeostasis by measuring the evoked extracellular glutamate level (EGL) and the probability of evoked action potential (P_EAP ) at the Zn²⁺ -containing or zincergic mossy fiber-CA3 synapses of the rat hippocampus. We found that the application of Zn²⁺ (ZnCl₂ ) exerted bidirectional effects on both EGL and P_EAP : facilitatory at low concentration (~1 µM) while repressive at high concentration (~50 µM). To determine the action of endogenous Zn²⁺ , we also used extracellular Zn²⁺ chelator to remove the synaptically released Zn²⁺ . Zn²⁺ chelation reduced both EGL and P_EAP , suggesting that endogenous Zn²⁺ has mainly a facilitative role in glutamate secretion on physiological condition. We revealed that calcium/calmodulin-dependent protein kinase II was integral to the mechanism by which Zn²⁺ facilitated the release of glutamate. Moreover, a glutamate transporter was the molecular entity for the action of Zn²⁺ on glutamate uptake by which Zn²⁺ decreases glutamate availability. Taken together, we show a novel action of Zn²⁺ , which is to biphasically regulate glutamate homeostasis via Zn²⁺ concentration-dependent synaptic facilitation and depression. Thus, co-released Zn²⁺ is physiologically important for enhancing weak stimulation, but potentially mitigates excessive stimulation to keep synaptic transmission within optimal physiological range.

Optical writing and single molecule reading of photoactivatable and silver nanoparticle-enhanced fluorescence

We designed a hybrid nanoparticle-molecular system composed of silver nanostructures (AgNP) and a fluorogenic boron dipyrromethene (BODIPY) that can be selectively activated by UVA or UVC light in the presence of an appropriate photoacid generator (PAG). Light irradiation of the PAG encourages the release of p-toluenesulfonic, triflic or hydrobromic acid, any of which facilitate optical 'writing' by promoting the formation of a fluorescent species. Metal-enhanced fluorescence (MEF) by AgNP was achieved through rational design of the nano-molecular system in accordance with the principles of radiative decay engineering. In addition to increasing signal to noise, AgNP permitted shorter reaction times and low irradiance - all of which have important implications for applications of fluorescence activation in portable fluorescence patterning, bioimaging and super-resolution microscopy. Single molecule fluorescence microscopy provided unique insights into the MEF mechanism which were hidden by ensemble-averaged measurements, demonstrating that single molecule 'reading' is a valuable tool for characterizing particle-molecule interactions such as those responsible for the relative contributions of increased excitation and plasmophoric emission toward overall MEF. This work represents a step forward in the contemporary design of synergistic nano-molecular systems, and showcases the advantage of fusion between classic spectroscopic techniques and single molecule methods in terms of improved quantitative understanding of fluorophore-nanoparticle interactions, and how these interactions can be exploited to the fullest extent possible.

Substrate-Photocaged Enzymatic Fluorogenic Probe Enabling Sequential Activation for Light-Controllable Monitoring of Intracellular Tyrosinase Activity

Tyrosinase (TYR) is a crucial enzyme involved in melanogenesis, and its overexpression is closely associated with melanoma. To precisely monitor intracellular TYR activity, remote control of a molecule imaging tool is highly meaningful but remains to be explored. In this work, we present the first photocaged tyrosinase fluorogenic probe by caging the substrate of the enzymatic probe with a photolabile group. Because of the sequential light and enzyme-activation feature, this probe exhibits photocontrollable "turn on" response toward TYR with good selectivity and high sensitivity (detection limit: 0.08 U/mL). Fluorescence imaging results validate that the caged probe possesses the capability of visualizing intracellular endogenous tyrosinase activity in a photocontrol fashion, thus offering a promising molecule imaging tool for investigating TYR-related physiological function and pathological role. Moreover, our sequential activation strategy has great potential for developing more photocontrollable enzymatic fluorogenic probes with spatiotemporal resolution.

Sanger's Reagent Sensitized Photocleavage of Amide Bond for Constructing Photocages and Regulation of Biological Functions

Photolabile groups offer promising tools to study biological processes with high spatial and temporal control. In the investigation, we designed and prepared several new glycine amide derivatives of Sanger's reagent and demonstrated that they serve as a new class of photocages for Zn²⁺ and an acetylcholinesterase (AChE) inhibitor. We showed that the mechanism for photocleavage of these substances involves initial light-driven cyclization between the 2,4-dinitrophenyl and glycine methylene groups to form acyl benzimidazole N-oxides, which undergo secondary photoinduced decarboxylation in association with rupture of an amide bond. The cleavage reactions proceed with modest to high quantum yields. We demonstrated that these derivatives can be used in targeted intracellular delivery of Zn²⁺, fluorescent imaging by light-triggered Zn²⁺ release, and regulation of biological processes including the enzymatic activity of carbonic anhydrase (CA), negative regulation of N-methyl-d-aspartate receptors (NMDARs), and pulse rate of cardiomyocytes. The successful proof-of-concept examples described above open a new avenue for using Sanger's reagent-based glycine amides as photocages for the exploration of complex cellular functions and signaling pathways.

Rhodol-based fluorescent probes for the detection of fluoride ion and its application in water, tea and live animal imaging

Herein, we presented two novel turn-on colorimetric and fluorescent probes based on a F^- triggered SiO bond cleavage reaction, which displayed several desired properties for the quantitative detection for F^-, such as high specificity, rapid response time (within 3 min) and naked-eye visualization. The fluorescence intensity at 574 nm (absorbance at 544 nm) of the solution was found to increase linearly with the concentration of F^- (0.00-30.0 μM) with the detection limit was estimated to be 0.47 μM/0.48 μM. Based on these excellent optical properties, the probes were employed to monitor F^- in real water samples and tea samples with satisfactory. Furthermore, it was successfully applied for fluorescent imaging of F^- in living nude mice, suggesting that it could be used as a powerful tool to predict and explore the biological functions of F^- in physiological and pathological processes.

A selective purine-based fluorescent chemosensor for the “naked-eye” detection of zinc ions (Zn2+): applications in live cell imaging and test strips

In this work, a novel purine based probe PTAHN was successfully designed and synthesized. PTAHN displayed high selectivity towards Zn²⁺via turn-on fluorescence. What's more, PTAHN can be proficiently employed for imaging Zn²⁺ in living cells.

Unravelling electron transfer in peptide-cation complexes: a model for mimicking redox centres in proteins

We provide evidence that bound zinc promotes electron transfer in a peptide by changing the electronic properties of the peptide.

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.

DNAzyme-Mediated Genetically Encoded Sensors for Ratiometric Imaging of Metal Ions in Living Cells

Genetically encoded fluorescent proteins (FPs) have been used for metal ion detection. However, their applications are restricted to a limited number of metal ions owing to the lack of available metal-binding proteins or peptides that can be fused to FPs and the difficulty in transforming the binding of metal ions into a change of fluorescent signal. We report herein the use of Mg²⁺ -specific 10-23 or Zn²⁺ -specific 8-17 RNA-cleaving DNAzymes to regulate the expression of FPs as a new class of ratiometric fluorescent sensors for metal ions. Specifically, we demonstrate the use of DNAzymes to suppress the expression of Clover2, a variant of the green FP (GFP), by cleaving the mRNA of Clover2, while the expression of Ruby2, a mutant of the red FP (RFP), is not affected. The Mg²⁺ or Zn²⁺ in HeLa cells can be detected using both confocal imaging and flow cytometry. Since a wide variety of metal-specific DNAzymes can be obtained, this method can likely be applied to imaging many other metal ions, expanding the range of the current genetically encoded fluorescent protein-based sensors.

Targeted photoswitchable imaging of intracellular glutathione by a photochromic glycosheet sensor

The development of photochromic fluorescence sensors with dynamic and multiple-signaling is beneficial to the improvement of biosensing/imaging precision. However, elaborate designs with complicated molecular structures are always required to integrate these functions into one molecule. By taking advantages of both redox-active/high loading features of two-dimensional (2D) manganese dioxide (MnO₂) and dynamic fluorescence photoswitching of photochromic sensors, we here design a hybrid photochromic MnO₂ glycosheet (Glyco-DTE@MnO₂) to achieve the photoswitchable imaging of intracellular glutathione (GSH). The photochromic glycosheet manifests significantly turn-on fluorescence and dynamic ON/OFF fluorescence signals in response to GSH, which makes it favorable for intracellular GSH double-check in targeted human hepatoma cell line (HepG2) through the recognition between β-D-galactoside and asialoglycoprotein receptor (ASGPr) on cell membranes. The dynamic fluorescence signals and excellent selectivity for detection and imaging of GSH ensure the precise determination of cell states, promoting its potential applications in future disease diagnosis and therapy.

Light-Activated Nanoprobes for Biosensing and Imaging

Fluorescent nanoprobes are indispensable tools to monitor and analyze biological species and dynamic biochemical processes in cells and living bodies. Conventional nanoprobes have limitations in obtaining imaging signals with high precision and resolution because of the interference with biological autofluorescence, off-target effects, and lack of spatiotemporal control. As a newly developed paradigm, light-activated nanoprobes, whose imaging and sensing activity can be remotely regulated with light irradiation, show good potential to overcome these limitations. Herein, recent research progress on the design and construction of light-activated nanoprobes to improve bioimaging and sensing performance in complex biological systems is introduced. First, recent innovative strategies and their underlying mechanisms for light-controlled imaging are reviewed, including photoswitchable nanoprobes and phototargeted nanosystems. Subsequently, a short highlight is provided on the development of light-activatable nanoprobes for biosensing, which offer possibilities for the remote control of biorecognition and sensing activity in a precise manner both temporally and spatially. Finally, perspectives and challenges in light-activated nanoprobes are commented.

Diketopyrrolopyrrole-based fluorescence probes for the imaging of lysosomal Zn2+ and identification of prostate cancer in human tissue

A series of diketopyrrolopyrrole-based fluorescent probes (DPP-C2, LysoDPP-C2, LysoDPP-C3, and LysoDPP-C4) have been developed for the detection of low pH and Zn2+ in an AND logic fashion. The chelation of Zn2+ or the protonation of a morpholine moiety within these probes results in a partial increase in the fluorescence intensity, an effect ascribed to suppression of one possible photo-induced electron transfer (PET) pathway. In contrast, a large increase in the observed fluorescence intensity is observed at low pH and in the presence of Zn2+; this is rationalized in terms of both possible PET pathways within the probes being blocked. Job plots, fluorescence titration curves, and isothermal titration calorimetry proved consistent with a 1 : 1 Zn2+ complexation stoichiometry. Each probe demonstrated an excellent selectivity towards Zn2+ and the resulting Zn2+ complexes demonstrated pH sensitivity over the 3.5-9 pH range. Fluorescence imaging experiments confirmed that LysoDPP-C4 was capable of imaging lysosomal Zn2+ in live cells. Little evidence of cytotoxicity was seen. LysoDPP-C4 was successfully applied to the bioimaging of nude mice, wherein it was shown capable of imaging the prostate. Histological studies using a human sample revealed that LysoDPP-C4 can discriminate cancerous prostate tissue from healthy prostate tissue.

Fluorescein derived Schiff base as fluorimetric zinc (II) sensor via 'turn on' response and its application in live cell imaging

A novel Schiff base L composed of fluorescein hydrazine and a phenol functionalized moiety has been designed and prepared via cost-effective condensation reaction. The L is utilized for selective sensing of Zn²⁺ over other environmental and biological relevant metal ions in aqueous alcoholic solution under physiological pH range. The binding of Zn²⁺ to the receptor L is found to causes ~23 fold fluorescence enhancement of L. The 1:1 binding mode of the metal complex is established by combined UV-Vis, fluorescence, and HRMS (high-resolution mass spectroscopy) spectroscopic methods. The binding constant (K_a) for complexation and the limit of detection (LOD) of Zn²⁺ is calculated to be 2.86 × 10⁴ M^-1 and 1.59 μM, respectively. Further photophysical investigations including steady-state, time-resolved fluorescence analysis and spectral investigations including NMR (nuclear magnetic resonance), IR (infrared spectroscopy) suggest introduction of CHEF (chelation enhance fluorescence) with the suppression of CN isomerization and PET (photo-induced electron transfer) mechanism for the strong fluorescent response towards Zn²⁺. Finally, the sensor L is successfully employed to monitor a real-time detection of Zn²⁺ by means of TLC (thin layer chromatography) based paper strip. The L is used in the cell imaging study using African green monkey kidney cells (Vero cells) for the determination of exogenous Zn²⁺ by Immunofluorescence Assay (IFA) process.