Complete suppression of the fluorophore fluorescence by combined effect of multiple fluorescence quenching groups: A fluorescent sensor for Cu with zero background signals

The reaction-based fluorescent sensors have attracted increasing attention in the past decades. However, the application of these sensors for accurate sensing was significantly retarded by the background fluorescence from the sensors themselves. In this work, we demonstrated a novel strategy that the background fluorescence of the sensor could be completely eliminated by the combined effect of multiple fluorescence quenching groups. Based on this new strategy, as proof-of-principle study, a fluorescent sensor (CuFS) for Cu²⁺ was judiciously developed. In CuFS, three types of fluorescence quenching groups were directly tethered to a commonly used coumarin fluorophore. The fluorescence of coumarin fluorophore in CuFS was completely suppressed by the combined effect of these fluorescence quenching groups. Upon treatment with 22 μM Cu²⁺, sensor CuFS achieved a dramatic fluorescence enhancement (fluorescence intensity enhanced up to 811-fold) centered at 469 nm. The detection limits was determined to be 12.3 nM. The fluorescence intensity enhancement also showed a good linearity with the Cu²⁺ concentration in the range of 12.3 nM to 2 μM. By fabricating test strips, sensor CuFS can be utilized as a simple tool to detect Cu²⁺ in water samples. Furthermore, the fluorescent sensor was successfully applied in detecting different concentration of Cu²⁺ in living cells.     

Ratiometric Fluorescent Paper-Based Sensor Based on CdTe Quantum Dots and Graphite Carbon Nitride Hybrid for Visual and Rapid Determination of Cu2+ in Drinks

A simple and effective ratiometric fluorescence sensor of CdTe QDs/GCNNs for on-site and rapid analysis of Cu²⁺ has been established by mixing physically CdTe QDs and graphite carbon nitride (GCNNs). Two emissions peaks of CdTe QDs at 572 nm and GCNNs at 436 nm are both excitated at 340 nm. Under a UV lamp, fluorescent of traffic yellow CdTe QDs is linearly quenched by Cu²⁺ (as the detection signal), while blue GCNNs remains unchanged (as the reference), resulting in a distinguishable color change gradually from pink yellow to blue. The limit of detection (LOD) of this new sensor for Cu²⁺ is as low as 0.47 ng mL⁻¹ with 1.4 % RSD. The established method has been successfully applied to detection of Cu²⁺ in various drinks with satisfactory results. Moreover, a paper-based sensor, which has been prepared by soaking cellulose acetate membrane in CdTe QDs/GCNNs sensor solution, has a wide semiquantitative detection range for Cu²⁺ (0.01 ~ 5.0 μg mL⁻¹). It has realized successfully on-site and rapid determination of Cu²⁺ in red wine without any pretreatment procedure and is of great promotion and application value in determination of Cu²⁺ in liquid samples.     

Silica Meets Tannic Acid: Designing Green Nanoplatforms for Environment Preservation

Hybrid tannic acid-silica-based porous nanoparticles, TA-SiO₂ NPs, have been synthesized under mild conditions in the presence of green and renewable tannic acid biopolymer, a glycoside polymer of gallic acid present in a large part of plants. Tannic acid (TA) was exploited as both a structuring directing agent and green chelating site for heavy metal ions recovery from aqueous solutions. Particles morphologies and porosity were easily tuned by varying the TA initial amount. The sample produced with the largest TA amount showed a specific surface area an order of magnitude larger than silica nanoparticles. The adsorption performance was investigated by using TA-SiO₂ NPs as adsorbents for copper (II) ions from an aqueous solution. The effects of the initial Cu²⁺ ions concentration and the pH values on the adsorption capability were also investigated. The resulting TA-SiO₂ NPs exhibited a different adsorption behaviour towards Cu²⁺, which was demonstrated through different tests. The largest adsorption (i.e., ~50 wt% of the initial Cu²⁺ amount) was obtained with the more porous nanoplatforms bearing a higher final TA content. The TA-nanoplatforms, stable in pH value around neutral conditions, can be easily produced and their use would well comply with a green strategy to reduce wastewater pollution.     

Selective sensing of Cu2+ ion by naphthalene based Schiff base

Research is going on to synthesize materials for acting as sensors of metal ions and are also published in different journals but the cause for sensing has not been clearly explained. In this paper, the reason for selective sensing of Cu²⁺ ions has been explained. A novel Schiff base fluorescent probe 3-[(2-hydroxy-naphthalen-1-ylmethylene)-hydrazono]-butan-2-one oxime (NPTH) was designed and synthesized to use as Cu²⁺ sensor. From our recent experiments, it has been found that only Cu²⁺ among different metal ions has been sensed by the synthesized ligand. The reason for selection and sensing of Cu²⁺ by the ligand was established by different spectroscopic techniques. The detection limit of NPTH was calculated as low as 4.11 ​× ​10^-4M for Cu²⁺. Therefore, these results indicate that sensor NPTH has great prospective to detect Cu²⁺ ion in environmental analysis systems. Density functional theory (DFT) calculations have been done to ascertain the ground state geometry of NPTH.     

A rigid conjugated pyridinylthiazole derivative and its nanoparticles for divalent copper fluorescent sensing in aqueous media

A rigid conjugated pyridinylthiazole derivative (1) and two bithiazole derivatives with similar structures (2, 3) were synthesized and characterized. Their optical properties were investigated through spectral analysis. By applying the three compounds to Cu²⁺ ions detection, it was shown that compound 1 could be employed as a selective and sensitive Cu²⁺ ions fluorescent chemosensor. For aqueous assay, the nanoparticles of compound 1 were prepared in aqueous media. Compared to the monomer, 1 nanoparticles were more fluorescence sensitive to Cu²⁺ ions. Its binding mode with Cu²⁺ ions was correlated well with Langmuir equation. Compound 1 nanoparticles exhibit a dynamic working range for Cu²⁺ ions from 0.02 to 0.50 μM with a detection limit of 10 nM. The proposed chemosensor has been used for the direct measurement of Cu²⁺ content in drinking water samples with satisfying results.     

Highly chemoselective colorimetric/fluorometric dual-channel sensor with fast response and good reversibility for the selective and sensitive detection of Cu2+

A novel colorimetric and fluorometric dual-channel sensor DA with a favorable optical property and high specificity via a facile synthesis for Cu²⁺ was developed. DA showed a remarkably rapid response and high selectivity for Cu²⁺ over other metal ions with low detection limit of 15.1 nM. The sensing mechanism of DA for Cu²⁺ was based on the chelation-enhanced fluorescence quenching (CHEQ) mechanism, and further confirmed by optical measurements, FTIR, HRMS and DFT calculations. Importantly, DA for sensing Cu²⁺ possessed excellent sensing performances including colorimetric and fluorometric dual-mode detection, fast response, good reversibility, wide pH response range and strong anti-interference ability. Moreover, the DA could be not only applied to quantitatively detect Cu²⁺ in environmental water, food and drink samples, but also show highly colorimetric detection of Cu²⁺ on test strips and silica, indicating its possibility to be utilized as a convenient and low-cost sensor for environment and food monitoring.     

A new coumarin-based fluorescent probe for selective recognition of Cu2+ and S2− in aqueous solution and living cells

A new coumarin-based fluorescent probe CMOH was easily synthesized for detection of Cu²⁺ and S²⁻ in aqueous media and living cells. CMOH displayed high sensitivity (detection limit = 3.2 nM) and selectivity to Cu²⁺ with a non-fluorescence complex CMOH-Cu²⁺ formation via a 1:1 binding mode. According to displacement approach, the fluorescence of CMOH-Cu²⁺ was recovered in the presence of S²⁻ and acted as a sensitive sensor with a low detection limit of 11.4 nM. This ‘‘on-off-on’’ process can be accomplished within 1 min and repeated at least 5 times. What's more, CMOH exhibited good permeability, low cytotoxicity and can be used as a suitable tool to detect changes of Cu²⁺ and S²⁻ in biosystem.     

An ultrasensitive fluorescence sensor for determination of trace levels of copper in blood samples

A novel SBA-15-based fluorescent sensor, SBA-PI: mesoporous SBA-15 structure modified with iminostilbene groups, was designed, synthesized, and characterized by Fourier transform-infrared spectroscopy (FT-IR), ultraviolet–visible spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TGA), low-angle X-ray diffraction techniques (low-angle XRD), and N₂ adsorption–desorption techniques. The SBA-PI as a sensor with a selective behavior for detection of Cu²⁺ comprises iminostilbene carbonyl as the fluorophore group. The SBA-PI sensor displays an excellent fluorescence response in aqueous solutions and the fluorescence intensity quenches remarkably upon addition of Cu²⁺. Other common interfering ions even at high concentration ratio showed either no or very small changes in the fluorescence intensity of SBA-PI in the absence of Cu²⁺. A limit of detection of 8.7 × 10⁻⁹ M for Cu²⁺ indicated that this fluorescence sensor has a high sensitivity and selectivity toward the target copper (II) ion. The fabricated Cu²⁺ sensor was successfully applied for the determination of the Cu²⁺ in human blood samples without any significant interference. With the selective analysis of Cu²⁺ ions down to 0.9 nM in blood, the sensor is a promising and a novel detection candidate for Cu²⁺ and can be applied in the clinical laboratory. A reversibility and accuracy in the fluorescence behavior of the sensor was found in the presence of I^¯ that was described as a masking agent for Cu²⁺.

Graphical abstract

     

Amino-functionalized mesoporous silica modified glassy carbon electrode for ultra-trace copper(II) determination

This paper described a facile and direct electrochemical method for the determination of ultra-trace Cu²⁺ by employing amino-functionalized mesoporous silica (NH₂-MCM-41) as enhanced sensing platform. NH₂-MCM-41 was prepared by using a post-grafting process and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and fourier transform infrared (FTIR) spectroscopy. NH₂-MCM-41 modified glassy carbon (GC) electrode showed higher sensitivity for anodic stripping voltammetric (ASV) detection of Cu²⁺ than that of MCM-41 modified one. The high sensitivity was attributed to synergistic effect between MCM-41 and amino-group, in which the high surface area and special mesoporous morphology of MCM-41 can cause strong physical absorption, and amino-groups are able to chelate copper ions. Some important parameters influencing the sensor response were optimized. Under optimum experimental conditions the sensor linearly responded to Cu²⁺ concentration in the range from 5 to 1000 ng L⁻¹ with a detection limit of 0.9 ng L⁻¹ (S/N = 3). Moreover, the sensor possessed good stability and electrode renewability. In the end, the proposed sensor was applied for determining Cu²⁺ in real samples and the accuracy of the results were comparable to those obtained by inductively coupled plasma optical emission spectrometry (ICP-OES) method.     

Rapid detection of trace Cu2+ using an l‐cysteine based interdigitated electrode sensor integrated with AC electrokinetic enrichment

Copper is an indispensable trace element for human health. Too much or too little intake of copper ion (Cu²⁺) can lead to its own adverse health conditions. Therefore, detection of Cu²⁺ is always of vital importance. In this work, a simple sensor was developed for rapid detection of trace Cu²⁺ in water, in which L‐cysteine (Cys) as a molecular probe was self‐assembled on a gold interdigital electrode to form a monolayer for specific capture of Cu²⁺. The interfacial capacitance of interdigital electrode was detected to indicate the target adsorption level under an AC signal working as the excitation to induce directed movement and enrichment of Cu²⁺ to the electrode surface. This sensor reached a limit of detection of 4.14 fM and a satisfactory selectivity against eight other ions (Zn²⁺, Hg²⁺, Pb²⁺, Cd²⁺, Mg²⁺, Fe²⁺, As³⁺, and As⁵⁺). Testing of spiked tap water was also performed, demonstrating the sensor's usability. This sensor as well as the detection method shows a great application potential in fields such as environmental monitoring and medical diagnosis.     

L-cysteine-regulated in situ formation of Prussian blue/Turnbull’s blue nanoparticles as the colorimetric probe for the detection of copper ion

A fast, simple, sensitive, and selective colorimetric method for the detection of Cu²⁺ was developed using Prussian blue/Turnbull’s blue nanoparticles (PBNPs/TBNPs) as the probe. The colorimetric sensor is based on the following principle. Cu²⁺ can induce the aggregation of L-cysteine (L-cys) modified-PBNPs/TBNPs (L-cys-PBNPs/TBNPs), resulting in an obvious red shift of its maximum absorption peak. Thus, the concentration of Cu²⁺ can be determined based on the peak shift in the UV–Vis spectra. The optimal pH, concentration of L-cys, reaction temperature between L-cys-PBNPs/TBNPs and Cu²⁺, the formation time of L-cys-PBNPs/TBNPs, and the reaction time between L-cys-PBNPs/TBNPs and Cu²⁺ of the method were determined to be pH 4.5, 2.0 mM, 20 °C, 5.0 min, and 2.0 min, respectively. A good linearity for the colorimetric determination of Cu²⁺ at the range of 0.25–2.5 μM (R² = 0.986) was obtained, with a limit of detection (LOD) of 0.12 μM. Moreover, the negligible response of other metal ions demonstrates good selectivity and specificity of the sensor. In addition, the method was employed in the detection of Cu²⁺ in lake water samples, and the spiked recoveries are in the range of 96.7–106.6% with a relative standard deviation less than 7.4%. Therefore, the colorimetric method is applicable for Cu²⁺ detection in real water samples of high sensitivity and selectivity.     

Sequential detection of Cu2+ and cysteine using an imidazole‐based chemosensor in aqueous solution

A highly substituted imidazole‐based colorimetric and fluorogenic chemosensor, 2‐methoxy‐4‐(4,5‐diphenyl‐1H‐imidazol‐2‐yl)phenol (L), for the detection of Cu²⁺ ion and subsequent colorimetric detection of an amino acid, cysteine, was investigated. L exhibited a distinct color change from colorless to red in the presence of Cu²⁺ in an aqueous medium. The L‐Cu²⁺ complex can also be used to detect cysteine by the naked eye over a series of amino acids. The receptor L behaves as a highly selective colorimetric and fluorescent sensor for Cu²⁺ ions at concentrations as low as 4.33 and 2.25 μM, respectively. These values are much less compared to the WHO recommended limit of 30 μM for Cu²⁺ in drinking water. From Job's plot and the ESI‐MS spectrum, a 1:1 stoichiometric complex between L and Cu²⁺ ions can readily be reckoned. This binding was also substantiated by the EPR spectrum and magnetic susceptibility measurements. Additionally, the binding of L with Cu²⁺ ions was also manifested in the detection of B16F10 cells. This was substantiated through fluorescence microscopy. The spectrum of the L‐Cu²⁺ entity was also attempted to reproduce theoretically. The probable structure of this was also propounded through Density Functional Theory.     

Synthesis of a tetrapyrazinoporphyrazine-based fluorescent sensor for detection of Cu<Superscript>2+</Superscript> ion

In this study, a novel tetrapyrazinoporphyrazine-based fluorescent sensor for Cu²⁺ ion was synthesized and fully characterized by ¹H NMR, MALDI-TOF MS, and elemental analysis. A tetrapyrazinoporphyrazine substituent was used as the reporting group and a 2,2′-dipyridylamine moiety was employed as the recognition group. The effects of various metal ions on the absorption and emission spectra of the designed molecule were investigated, demonstrating that this compound shows selectivity and sensitivity toward the Cu²⁺ ion. Upon addition of Cu²⁺ ion, a fluorescent intensity of the tetrapyrazinoporphyrazine-based sensor decreased gradually at 655 nm. To confirm a selective binding ability of 2,2′-dipyridylamine moiety toward Cu²⁺ ion, a comparative study was performed using tetrapyrazinoporphyrazine derivative with bromine instead of 2,2′-dipyridylamine moiety. Furthermore, association constant and detection limit value of the synthesized compound toward Cu²⁺ ion were derived from a repetitive titration experiment. Also, a reversibility of the prepared sensor was confirmed through additional test using compound and ethylenediaminetetraacetic acid.     

Synthesis and characterization of the water-soluble silica-coated ZnS:Mn nanoparticles as fluorescent sensor for Cu ions

Silica-coated ZnS:Mn nanoparticles were synthesized by coating hydrophobic ZnS:Mn nanoparticles with silica shell through microemulsion. The core–shell structural nanoparticles were confirmed by X-ray diffraction (XRD) patterns, high-resolution transmission electron microscope (HRTEM) images and energy dispersive spectroscopy (EDS) measurements. Results show that each core–shell nanoparticle contains single ZnS:Mn nanoparticle within monodisperse silica nanospheres (40 nm). Photoluminescence (PL) spectroscopy and UV–vis spectrum were used to investigate the optical properties of the nanoparticles. Compared to uncoated ZnS:Mn nanoparticles, the silica-coated ZnS:Mn nanoparticles have the improved PL intensity as well as good photostability. The obtained silica-coated ZnS:Mn nanoparticles are water-soluble and have fluorescence sensitivity to Cu²⁺ ions. Quenching of fluorescence intensity of the silica-coated nanoparticles allows the detection of Cu²⁺ concentrations as low as 7.3 × 10⁻⁹ mol L⁻¹, thus affording a very sensitive detection system for this chemical species. The possible quenching mechanism is discussed.     

A multifunctional upconversion nanoparticles probe for Cu2+ sensing and pattern recognition of biothiols

Lanthanide-doped upconversion nanoparticles (Ln-UCNPs) are a new type of nanomaterials with excellent fluorescence properties, which are well applied in fluorescent biosensing. Herein we developed a multifunctional probe based on the surface engineering of core-shell structure UCNPs with polyacrylic acid (PAA). The developed PAA/UCNPs probe could be highly selective to detect and respond to Cu²⁺ at different pH. Cu²⁺ could easily combine with the carboxylate anion of PAA to quench the fluorescence of UCNPs. Therefore, we creatively proposed a fluorescent array sensor (PAA/UCNPs-Cu²⁺), in which the same material acted as the sensing element by coupled with pH regulation for pattern recognition of 5 thiols. It could also easily identify the chiral enantiomer of cystine (l-Cys-and d-Cys), and distinguish their mixed samples with different concentrations, and more importantly, it could be combined with urine samples to detect actual level of homocysteine (Hcys) to provide a new solution for judging whether the human body suffers from homocystinuria.     

Voltammetric Simultaneous Determination of Cu2+, Cd2+ and Pb2+ in Full Aqueous Medium Using Organic Nanoparticles of Disulfide Based Receptor

Disulfide based receptor was prepared using single step condensation reaction and suspended into organic nanoparticles to extend its practical application in aqueous samples. The prepared nanoparticles were used for the simultaneous recognition of three different metallic species (Cu²⁺, Cd²⁺, and Pb²⁺) in aqueous media through voltammetric studies. These metals can be determined simultaneously and without interferences from any of the other potential interferent metal ions, as different signals are displayed in cyclic as well as differential pulse voltammograms, with a detection limit of 193.0 nM for Cu²⁺, 52.0 nM for Cd²⁺ and 32.0 nM for Pb²⁺. The study was extended to real sample analysis by preparing the artificial mixtures of said metal ions.     

A triphenylamine based multi-analyte chemosensor for Hg and Cu ions in MeCN/H2O

A novel triphenylamine based oxidative chemosensor TOC was synthesized. The chromogenic and fluorogenic behaviors of TOC towards Hg²⁺ and Cu²⁺ ions in a binary mixture of MeCN/H₂O (9/1) were dramatically different. TOC displays colorimetric ‘naked eye’ recognition of Hg²⁺ and fluorogenic ‘turn on’ response towards Cu²⁺ via a unique cyclization reaction using two different detection modes. Moreover, TOCAZOL obtained from the oxidative cyclization reaction of TOC with Cu(ClO₄)₂ can be used as a selective fluorescent sensor toward Hg²⁺ ion.     

A fast,highly selective and sensitive dansyl-based fluorescent sensor for copper (II) ions and its imaging application in living cells

Since the copper ions (Cu²⁺) play a fatal role in many foundational physiological processes, it is important to develop a simple, highly sensitive and selective sensor for Cu²⁺ detection in living systems. Herein, an intramolecular charge transfer (ICT) and dansyl-based fluorescent chemosensor 1 was designed, synthesized and characterized for the sensitive and selective quantification of Cu²⁺. It exhibited remarkable fluorescence quenching upon addition of Cu²⁺ over other selected metal ions, attributed to the complex formation between 1 and Cu²⁺ with the association constant 6.7 × 10⁵ M^?1. The sensor 1 showed a fast and linear response towards Cu²⁺ in the concentration range from 0 to 12.5 × 10^?6 mol L^?1 with the detection limit of 2.5 × 10^?7 mol L^?1. This detection could be carried out in a wide pH range of 5.0–14. Furthermore, sensor 1 can be used for detecting Cu²⁺ in living cells.     

Action of a Fluorescent Chemosensor for Cu2+ Encapsulated in Silica Xerogel

The fluorescent chemosensor of the type Ant-NH-O-O-NH-Ant for Cu²⁺ ions has been designed by means of a supramolecular approach, as follows: two anthracene (Ant) fragments as fluorophore subunits have been linked by a noncyclic NH-O-O-NH quadridentate ligand as a receptor. The interaction of Cu²⁺ - receptor is signalled through the enhancement of the anthracene fluorescence when the receptor, i.e., the dioxodiamine chain subunit of the sensor is able to stop a photoinduced electron-transfer mechanism. The experiments with the chemosensor encapsulated in silica xerogel by the sol-gel processing are described.     

A coumarin-based fluorescent probe for selective detection of Cu2+ in water

Fluorescent Red GK, a commercially available coumarin-based dye, was developed as a “turn-off” fluorescent probe for detection of Cu²⁺ in aqueous solution. It exhibited high selectivity and sensitivity at room temperature. Upon addition of Cu²⁺, the strong fluorescence of Fluorescent Red GK was severely quenched and its color changed from orange to colorless under illumination with a UV lamp; the color of the solution also changed from pink to colorless. So, it can be used as a specific colorimetric and fluorescent probe for Cu²⁺ with a detection limit as low as 0.0634?μM.