Synthetic fluorescent sensors for studying the cell biology of metals

Single-molecule porphyrin-metal ion interaction and sensing application

It remains a significant challenge to study the interactions between metal ions and porphyrin molecules at single ion level. Here, we constructed a nanopore-based sensing for label-free and real-time analysis of the interaction between Cu²⁺ and 5,10,15,20-tetrakis(4-sulfonatophenyl)-porphyrin (TPPS). The results demonstrate that emerging electronic signatures of the Cu²⁺-TPPS complex that is completely different form the original free TPPS were observed in the α-hemolysin (α-HL) nanopore. Based on the distinctive electronic signal patterns between TPPS and Cu²⁺-TPPS complex, the unique nanopore sensor can achieve a highly sensitive detection of Cu²⁺ in aqueous media. The frequency of signature events showed a linear response toward the concentration of Cu²⁺ in the range of 0.03 µM - 1.0 μM, with a detection limit of 16 nM (S/N = 3). The sensing system also exhibited high selectivity against other metal ions, and the feasibility of this approach for practical applications was demonstrated with the determination of Cu²⁺ in running water.

A six-membered-ring incorporated Si-rhodamine for imaging of copper(ii) in lysosomes

The regulation of copper homeostasis in lysosomes of living cells is closely related to various physiological and pathological processes. Thus, it is of urgent need to develop a fluorescent probe for selectively and sensitively monitoring the location and concentration of lysosomal Cu(2+). Herein, a six-membered ring, thiosemicarbazide, was incorporated into a Si-rhodamine (SiR) scaffold for the first time, affording a SiR-based fluorescent probe SiRB-Cu. Through the effective Cu(2+)-triggered ring-opening process, the probe exhibits fast NIR chromogenic and fluorogenic responses to Cu(2+) within 2 min as the result of formation of a highly fluorescent product SiR-NCS. Compared with a five-membered ring, the expanded ring retains great tolerance to H(+), ensuring the superior sensitivity with a detection limit as low as 7.7 nM and 200-fold enhancement of relative fluorescence in the presence of 1.0 equiv. of Cu(2+) in pH = 5.0 solution, the physiological pH of lysosome. Moreover, the thiosemicarbazide moiety acts not only as the chelating and reactive site, but also as an efficient lysosome-targeting group, leading to the proactive accumulation of the probe into lysosomes. Taking advantage of these distinct properties, SiRB-Cu provides a functional probe suitable for imaging exogenous and endogenous lysosomal Cu(2+) with high imaging contrast and fidelity.

An internal charge transfer-DNA platform for fluorescence sensing of divalent metal ions

Replacement of guanine (G) nucleobases within G-quadruplex (GQ) folding oligonucleotides with push-pull fluorescent 8-arylvinyl-dG residues provides diagnostic emission signalling for divalent metal ion binding.

Assessing the intracellular fate of rhodium(ii) complexes

Rhodium(ii)-fluorophore conjugates have strong rhodium-based fluorescence quenching that can be harnessed to report on a conjugate's cellular uptake and the intracellular decomposition rate. Information gleened from this study allowed the design of an improved STAT3 metalloinhibitor.

Antifibrotic effect of xanthohumol in combination with praziquantel is associated with altered redox status and reduced iron accumulation during liver fluke-associated cholangiocarcinogenesis

Cholangiocarcinoma (CCA) caused by infection of the liver fluke Opisthorchis viverrini, (Ov) is the major public health problem in northeast Thailand. Following Ov infection the subsequent molecular changes can be associated by reactive oxygen species (ROS) induced chronic inflammation, advanced periductal fibrosis, and cholangiocarcinogenesis. Notably, resistance to an activation of cell death in prolonged oxidative stress conditions can occur but some damaged/mutated cells could survive and enable clonal expansion. Our study used a natural product, xanthohumol (XN), which is an anti-oxidant and anti-inflammatory compound, to examine whether it could prevent Ov-associated CCA carcinogenesis. We measured the effect of XN with or without praziquantel (PZ), an anti-helminthic treatment, on DNA damage, redox status change including iron accumulation and periductal fibrosis during CCA genesis induced by administration of Ov and N-dinitrosomethylamine (NDMA) in hamsters. Animals were randomly divided into four groups: group I, Ov infection and NDMA administration (ON); group II, Ov infection and NDMA administration and PZ treatment (ONP); the latter 2 groups were similar to group I and II, but group III received additional XN (XON) and group IV received XN plus PZ (XONP). The results showed that high 8-oxodG (a marker of DNA damage) was observed throughout cholangiocarcinogenesis. Moreover, increased expression of CD44v8-10 (a cell surface in regulation of the ROS defense system), whereas decreased expression of phospho-p38^MAPK (a major ROS target), was found during the progression of the bile duct cell transformation. In addition, high accumulation of iron and expression of transferrin receptor-1 (TfR-1) in both malignant bile ducts and inflammatory cells were detected. Furthermore, fibrosis also increased with the highest level being on day 180. On the other hand, the groups of XN with or without PZ supplementations showed an effective reduction in all the markers examined, including fibrosis when compared with the ON group. In particular, the XONP group, in which a significant reduction DNA damage occurred, was also found to have iron accumulation and fibrosis compared to the other groups. Our results show that XN administered in combination with PZ could efficiently prevent CCA development and hence provide potential chemopreventive benefits in Ov-induced cholangiocarcinogenesis.

Cellulose-Based Sensor Containing Phenanthroline for the Highly Selective and Rapid Detection of Fe2+ Ions with Naked Eye and Fluorescent Dual Modes

Iron ions play a vital role in many biological processes, and their concentrations are responsible for human health. Therefore, it is essential to detect the concentration of iron ions by a rapid, accurate, highly selective, and practical method. Herein, we have synthesized a cellulose-based fluorescent sensor (Phen-MDI-CA) for the highly selective and rapid detection of Fe²⁺ ions via chemically bonding 1,10-phenanthroline-5-amine (Phen) onto cellulose acetate (CA) using 4,4'-methylene diphenyl diisocyanate (MDI) as a cross-linker. Benefiting from the anchoring and diluting effect of a cellulose skeleton, the resultant Phen-MDI-CA displays excellent fluorescence properties in both solution and solid state. More interestingly, a cellulose-based polymer chain significantly improves the sensitivity of phenanthroline to Fe²⁺ ions. Upon meeting Fe²⁺ ions, a red, insoluble, and nonfluorescent Fe-(Phen-MDI-CA) complex appears immediately; thus, Phen-MDI-CA can work as a multimode chromogenic sensor for the highly selective, sensitive, and rapid detection of Fe²⁺ ions. In the instrument-free visual mode, the detection limit for Fe²⁺ ions is 50 ppb, and in fluorescence mode, the detection limit is 2.6 ppb. To our knowledge, this is the first time that such a low detection limit for Fe²⁺ ions in aqueous media has been observed by the naked eye. In addition, Phen-MDI-CA has good solubility and processability in common organic solvents, which facilitates its use in different material forms, e.g., printing ink, coating, and film. Therefore, the Fe²⁺-responsive and chromogenic Phen-MDI-CA exhibits a huge potential in the detection and extraction of Fe²⁺ ions.

Activity-based sensing fluorescent probes for iron in biological systems

Iron is an essential nutrient for life, and its capacity to cycle between different oxidation states is required for processes spanning oxygen transport and respiration to nucleotide synthesis and epigenetic regulation. However, this same redox ability also makes iron, if not regulated properly, a potentially dangerous toxin that can trigger oxidative stress and damage. New methods that enable monitoring of iron in living biological systems, particularly in labile Fe²⁺ forms, can help identify its contributions to physiology, aging, and disease. In this review, we summarize recent developments in activity-based sensing (ABS) probes for fluorescence Fe²⁺ detection.

A low affinity nanoparticle based fluorescent ratiometric probe for the determination of Zn(ii) concentrations in living cells

A ratiometric polymeric fluorescent probe for Zn(ii) was developed capable of measuring Zn(ii) concentrations in aqueous solution between 0 and 5 mM and was also capable of discriminating between resting and high Zn(ii) levels in living cells using confocal fluorescence microscopy.

Bipyridine bisphosphonate-based fluorescent optical sensor and optode for selective detection of Zn2+ ions and its applications

In this study, tetrabutyl 2,2′-bisbipyridine-5,5′-diylbis(methylene) diphosphonate (L) as fluorescence optical sensor was developed for selective determination of Zn²⁺.

Mammalian cells: a unique scaffold forin situbiosynthesis of metallic nanomaterials and biomedical applications

Nanoscale materials biosynthesis by using mammalian scaffold is green and highly biocompatible.

Facile construction of a hyperbranched poly(acrylamide) bearing tetraphenylethene units: a novel fluorescence probe with a highly selective and sensitive response to Zn2+

Thermo-responsive hyperbranched copoly(bis(N,N-ethyl acrylamide)/(N,N-methylene bisacrylamide)) (HPEAM-MBA) was synthesized by using reversible addition–fragmentation chain-transfer polymerization (RAFT). Interestingly, the zinc ion (Zn²⁺) was found to have a crucial influence on the lowest critical solution temperature (LCST) of the thermo-responsive polymer. The tetraphenylethylene (TPE) unit was then introduced onto the backbone of the as-prepared thermo-responsive polymer, which endows a Zn²⁺-responsive “turn-off” effect on the fluorescence properties. The TPE-bearing polymer shows a highly specific response over other metal ions and the “turn-off” response can even be tracked as the concentration of Zn²⁺ reduces to 2 × 10⁻⁵ M. The decrement of fluorescence intensity was linearly dependent on the concentration of Zn²⁺ in the range of 4–18 μmol L⁻¹. The flexible, versatile and feasible approach, as well as the excellent detection performance, may generate a new type of Zn²⁺ probe without the tedious synthesis of the moiety bearing Zn²⁺ recognition units.

A novel fluorescent HPEAM-TPEAH, possessing a highly selective and sensitive response to Zn²⁺, was synthesized using RAFT.

Bis-reaction-trigger as a strategy to improve the selectivity of fluorescent probes

Equipping a fluorophore with two ONOO⁻-specific reaction triggers yielded a probe able to report various degrees of nitrosative stress in live cells.

Highly selective and sensitive recognition of Zn(ii) by a novel coumarinyl scaffold following spectrofluorometric technique and its application in living cells

A biocompatible coumarinyl Schiff base scaffold (H2L), a nontoxic probe and a fluorescent sensor to Zn²⁺, with an LOD of 11 nM, was used for living cell imaging.

One-step eco-friendly approach for the fabrication of synergistically engineered fluorescent copper nanoclusters: sensing of Hg2+ ion and cellular uptake and bioimaging properties

Schematic illustration for one-step green synthetic approach for fabrication of synergistically engineered CuNCs.

Design and application of a tripodal on–off type chemosensor for discriminative and selective detection of Fe2+ ions

A simple and cost effective tris 2(amino ethyl) amine based chemosensor is synthesized via a single-step procedure.

Morphological evolution of Co phosphate and its electrochemical and photocatalytic performance

Cobalt phosphates with different compositions and morphologies were prepared via a one-step hydrothermal method.

Cation-responsive turn-on fluorescence and absence of heavy atom effects of pyridyl-substituted triarylmethyl radicals

Pyridyl-substituted triarylmethyl radicals showed cation-responsive turn-on fluorescence and did not suffer from quenching by internal and external heavy atom effects.

Multi-metal-dependent nucleic acid enzymes

Nucleic acid enzymes require metal ions for activity, and many recently discovered enzymes can use multiple metals, either binding to the scissile phosphate or also playing an allosteric role.

Crystal structure of gluconate bound iron(iii) complex: synthesis, characterization and redox properties of the complex in aqueous solution

The synthesis, characterization and redox properties of the first single crystal X-ray characterized, water soluble bis-gluconato-tetra-iron(iii) containing complex has been reported.

Genetically encoded fluorescent sensors for measuring transition and heavy metals in biological systems

Great progress has been made in expanding the repertoire of genetically encoded fluorescent sensors for monitoring intracellular transition metals (TMs). This powerful toolkit permits dynamic and non-invasive detection of TMs with high spatial-temporal resolution, which enables us to better understand the roles of TM homeostasis in both physiological and pathological settings. Here we summarize the recent development of genetically encoded fluorescent sensors for intracellular detection of TMs such as zinc and copper, as well as heavy metals including lead, cadmium, mercury, and arsenic.

Colorimetric and fluorescent probes for real-time naked eye sensing of copper ion in solution and on paper substrate

In this paper, BT ((E)-2-(4-(4-(bis(pyridin-2-ylmethyl)amino)styryl)-3-cyano-5,5-dimethylfuran-2(5H)-ylidene)malononitrile) with strong donor-π-acceptor structure was synthesized, which showed both colorimetric and fluorescent sensing ability toward Cu²⁺ with high selectivity and sensitivity. Job plot and mass spectra measurement revealed a 1 : 1 coordination mode between Cu²⁺ and probe BT in ethanol/HEPES (1 : 4 v/v) buffer (pH 7.2) solution, and the binding constant was calculated to be 3.6 × 10⁴ M^-1. The colour of BT solution (10 µM) immediately turned from purple red to yellow and the red fluorescence was quenched obviously when a certain amount of Cu²⁺ was added, which enabled a dual-channel detection of Cu²⁺. A paper strip pre-stained with BT solution was further fabricated and it also showed excellent sensing ability toward Cu²⁺ with a detection limit as low as 10^-6 M with the naked eye, which represents better portability and operation simplicity that is favourable for on-site analysis of Cu²⁺ in water.

Intramolecular substitution uncages fluorogenic probes for detection of metallo-carbapenemase-expressing bacteria

This work reports a novel caging strategy for designing fluorogenic probes to detect the activity of β-lactamases. The caging strategy uses a thiophenyl linker connected to a fluorophore caged by a good leaving group-dinitrophenyl. The uncaging proceeds in two steps through the sulfa-releasing and subsequent intramolecular substitution. The length of the linker has been examined and optimized to maximize the rate of intramolecular reaction and thus the rate of fluorescence activation. Finally based on this strategy, we prepared a green fluorogenic probe CAT-7 and validated its selectivity for detecting metallo-carbapenemases (VIM-27, IMP-1, NDM-1) in carbapenem-resistant Enterobacteriaceae (CRE) lysates.

AIE Nanoparticles with High Stimulated Emission Depletion Efficiency and Photobleaching Resistance for Long-Term Super-Resolution Bioimaging

Stimulated emission depletion (STED) nanoscopy is a typical super-resolution imaging technique that has become a powerful tool for visualizing intracellular structures on the nanometer scale. Aggregation-induced emission (AIE) luminogens are ideal fluorescent agents for bioimaging. Herein, long-term super-resolution fluorescence imaging of cancer cells, based on STED nanoscopy assisted by AIE nanoparticles (NPs) is realized. 2,3-Bis(4-(phenyl(4-(1,2,2-triphenylvinyl)phenyl)amino)phenyl) fumaronitrile (TTF), a typical AIE luminogen, is doped into colloidal mesoporous silica to form fluorescent NPs. TTF@SiO₂ NPs bear three significant features, which are all essential for STED nanoscopy. First, their STED efficiency can reach more than 60%. Second, they are highly resistant to photobleaching, even under long-term and high-power STED light irradiation. Third, they have a large Stokes' shift of ≈150 nm, which is beneficial for restraining the fluorescence background induced by the STED light irradiation. STED nanoscopy imaging of TTF@SiO₂ -NPs-stained HeLa cells is performed, exhibiting a high lateral spatial resolution of 30 nm. More importantly, long-term (more than half an hour) super-resolution cell imaging is achieved with low fluorescence loss. Considering that AIE luminogens are widely used for organelle targeting, cellular mapping, and tracing, AIE-NPs-based STED nanoscopy holds great potential for many basic biomedical studies that require super-resolution and long-term imaging.

Deep-Reaching Hydrodynamic Flow Confinement: Micrometer-Scale Liquid Localization for Open Substrates With Topographical Variations

We present a method for nonintrusive localization and reagent delivery on immersed biological samples with topographical variation on the order of hundreds of micrometers. Our technique, which we refer to as the deep-reaching hydrodynamic flow confinement (DR-HFC), is simple and passive: it relies on a deep-reaching hydrodynamic confinement delivered through a simple microfluidic probe design to perform localized microscale alterations on substrates as deep as 600 μm. Designed to scan centimeter-scale areas of biological substrates, our method passively prevents sample intrusion by maintaining a large gap between the probe and the substrate. The gap prevents collision of the probe and the substrate and reduces the shear stress experienced by the sample. We present two probe designs: linear and annular DR-HFC. Both designs comprise a reagent-injection aperture and aspiration apertures that serve to confine the reagent. We identify the design parameters affecting reagent localization and depth by DR-HFC and study their individual influence on the operation of DR-HFC numerically. Using DR-HFC, we demonstrate localized binding of antihuman immunoglobulin G (IgG) onto an activated substrate at various depths from 50 to 600 μm. DR-HFC provides a readily implementable approach for noninvasive processing of biological samples applicable to the next generation of diagnostic and bioanalytical devices.

Cesium Lead Halide Perovskite Quantum Dots as a Photoluminescence Probe for Metal Ions

Perovskite structured CsPbX₃ (X = Cl, Br or I) quantum dots (QDs) have attracted great attention in the past few years for appealing application potentials in photovoltaic and optoelectronic devices. In this report, the CsPbX₃ QDs are shown to perform as a new probe for metal ions with high sensitivity, high selectivity and instant response by the quenching or enhancing of the photoluminescence (PL). Through experimental and calculation efforts, the probing mechanisms are investigated. A wide probing window for Cu²⁺ and Yb³⁺ ions ranging from 2 × 10^-9 to 2 × 10^-6 m is exhibited for CsPbBr₃ QDs. In practice, the CsPbBr₃ QDs are successfully applied for fast probing Cu²⁺ ions in edible oils and vehicle lubricating oils with the precision consistent to the values measured by inductively coupled plasma (ICP). Thus, it provides a promising powerful tool in detecting certain metal ions in biological and industrial organic solution systems.

Aggregation-Induced Emission (AIE) Fluorophore Exhibits a Highly Ratiometric Fluorescent Response to Zn2+ in vitro and in Human Liver Cancer Cells

Two novel organic fluorophores, containing bis-naphthylamide and quinoline motifs, have been designed and synthesized. One of the fluorophores contains an isobutylene unit and exhibits a significant aggregation-induced emission (AIE) and a remarkable highly selective ratiometric fluorescence response towards Zn²⁺ in solution as well as in human liver cancer cells. The AIE behavior of this fluorophore was fully verified by fluorescence and UV/Vis spectroscopy, quantum yield calculations, and single-crystal X-ray diffraction, which revealed an intricate crystal packing system. Conversely, a fluorophore that lacks the isobutylene moiety did not exhibit any significant fluorescent properties as a result of its more flexible molecular structure that presumably allows free intramolecular rotational processes to occur.

Fluorescent Protein-Based Turn-On Probe through a General Protection-Deprotection Design Strategy

We demonstrated a general protection-deprotection strategy for the design of fluorescent protein biosensors through the construction of a turn-on Hg²⁺ sensor. A combination of fluorescent protein engineering and unnatural amino acid mutagenesis was used. Unlike previously reported fluorescent protein-based Hg²⁺ sensors that relied on the binding of Hg²⁺ to the sulfhydryl group of cysteine residues, a well-established chemical reaction, oxymercuration, was transformed into biological format and incorporated into our sensor design. This novel Hg²⁺ sensor displayed good sensitivity and selectivity both in vitro and in live bacterial cells. Over 60-fold change in fluorescence signal output was observed in the presence of 10 μM Hg²⁺, while such a change was undetectable when nine other metal ions were tested. This new design strategy could expand the repertoire of fluorescent protein-based biosensors for the detection of small-molecule analytes.

Imaging Endogenous Metal Ions in Living Cells Using a DNAzyme-Catalytic Hairpin Assembly Probe

DNAzymes are a promising platform for metal ion detection, and a few DNAzyme-based sensors have been reported to detect metal ions inside cells. However, these methods required an influx of metal ions to increase their concentrations for detection. To address this major issue, the design of a catalytic hairpin assembly (CHA) reaction to amplify the signal from photocaged Na+ -specific DNAzyme to detect endogenous Na+ inside cells is reported. Upon light activation and in the presence of Na+ , the NaA43 DNAzyme cleaves its substrate strand and releases a product strand, which becomes an initiator that trigger the subsequent CHA amplification reaction. This strategy allows detection of endogenous Na+ inside cells, which has been demonstrated by both fluorescent imaging of individual cells and flow cytometry of the whole cell population. This method can be generally applied to detect other endogenous metal ions and thus contribute to deeper understanding of the role of metal ions in biological systems.

Design and synthesis of the BODIPY-BSA complex for biological applications

A quinoxaline-functionalized styryl-BODIPY derivative (S1) was synthesized by microwave-assisted Knoevenagel condensation. It exhibited fluorescence enhancement upon micro-encapsulation into the hydrophobic cavity of bovine serum albumin (BSA). The S1-BSA complex was characterized systematically using ultraviolet (UV)-visible absorption, fluorescence emission, kinetics, circular dichroism and time-resolved lifetime measurements. The binding nature of BSA towards S1 was strong, and was found to be stable over a period of days. The studies showed that the S1-BSA complex could be used as a new biomaterial for fluorescence-based high-throughput assay for kinase enzymes.

Detection of Zn2+ release in nitric oxide treated cells and proteome: dependence on fluorescent sensor and proteomic sulfhydryl groups

Nitric oxide (NO) is both an important regulatory molecule in biological systems and a toxic xenobiotic. Its oxidation products react with sulfhydryl groups and either nitrosylate or oxidize them. The aerobic reaction of NO supplied by diethylamine NONOate (DEA-NO) with pig kidney LLC-PK₁ cells and Zn-proteins within the isolated proteome was examined with three fluorescent zinc sensors, zinquin (ZQ), TSQ, and FluoZin-3 (FZ-3). Observations of Zn²⁺ labilization from Zn-proteins depended on the specific sensor used. Upon cellular exposure to DEA-NO, ZQ sequestered about 13% of the proteomic Zn²⁺ as Zn(ZQ)₂ and additional Zn²⁺ as proteome·Zn-ZQ ternary complexes. TSQ, a sensor structurally related to ZQ with lower affinity for Zn²⁺, did not form Zn(TSQ)₂. Instead, Zn²⁺ mobilized by DEA-NO was exclusively bound as proteome·Zn-TSQ adducts. Analogous reactions of proteome with ZQ or TSQ in vitro displayed qualitatively similar products. Titration of native proteome with Zn²⁺ in the presence of ZQ resulted in the sole formation of proteome·Zn-ZQ species. This result suggested that sulfhydryl groups are involved in non-specific proteomic binding of mobile Zn²⁺ and that the appearance of Zn(ZQ)₂ after exposure of cells and proteome to DEA-NO resulted from a reduction in proteomic sulfhydryl ligands, favoring the formation of Zn(ZQ)₂ instead of proteome·Zn-ZQ. With the third sensor, FluoZin-3, neither Zn-FZ-3 nor proteome·Zn-FZ-3 was detected during the reaction of proteome with DEA-NO. Instead, it reacted independently with DEA-NO with a modest enhancement of fluorescence.

Near-Infrared Photothermally Activated DNAzyme-Gold Nanoshells for Imaging Metal Ions in Living Cells

DNAzymes have enjoyed success as metal ion sensors outside cells. Their susceptibility to metal-dependent cleavage during delivery into cells has limited their intracellular applications. To overcome this limitation, a near-infrared (NIR) photothermal activation method is presented for controlling DNAzyme activity in living cells. The system consists of a three-stranded DNAzyme precursor (TSDP), the hybridization of which prevents the DNAzyme from being active. After conjugating the TSDP onto gold nanoshells and upon NIR illumination, the increased temperature dehybridizes the TSDP to release the active DNAzyme, which then carries out metal-ion-dependent cleavage, resulting in releasing the cleaved product containing a fluorophore. Using this construct, detecting Zn2+ in living HeLa cells is demonstrated. This method has expanded the DNAzyme versatility for detecting metal ions in biological systems under NIR light that exhibits lower phototoxicity and higher tissue penetration ability.