Efficient On-Off Ratiometric Fluorescence Probe for Cyanide Ion Based on Perturbation of the Interaction between Gold Nanoclusters and a Copper(II)-Phthalocyanine Complex

A ratiometric fluorescence and colorimetry dual-signal sensing strategy based on o-phenylenediamine and AuNCs for determination of Cu2+ and glyphosate

A ratiometric fluorescence sensing strategy has been developed for the determination of Cu2+ and glyphosate with high sensitivity and specificity based on OPD (o-phenylenediamine) and glutathione-stabilized gold nanoclusters (GSH-AuNCs). Water-soluble 1.75-nm size GSH-AuNCs with strong red fluorescence and maximum emission wavelength at 682 nm were synthesized using GSH as the template. OPD was oxidized by Cu2+, which produced the bright yellow fluorescence oxidation product 2,3-diaminophenazine (DAP) with a maximum fluorescence emission peak at 570 nm. When glyphosate existed in the system, the chelation between glyphosate and Cu2+ hindered the formation of DAP and reduced the fluorescence intensity of the system at the wavelength of 570 nm. Meanwhile, the fluorescence intensity at the wavelength of 682 nm remained basically stable. It exhibited a good linear relationship towards Cu2+ and glyphosate in water in the range 1.0-10 µM and 0.050-3.0 µg/mL with a detection limit of 0.547 µM and 0.0028 µg/mL, respectively. The method was also used for the semi-quantitative determination of Cu2+ and glyphosate in water by fluorescence color changes visually detected by the naked eyes in the range 1.0-10 µM and 0.30-3.0 µg/mL, respectively. The sensing strategy showed higher sensitivity, more obvious color changes, and better disturbance performance, satisfying with the detection demands of Cu2+ and glyphosate in environmental water samples. The study provides a reliable detection strategy in the environment safety fields.

Small Molecule Optical Probes for Detection of H2S in Water Samples: A Review

Hydrogen sulfide (H2S) is closely linked to not only environmental hazards, but also it affects human health due to its toxic nature and the exposure risks associated with several occupational settings. Therefore, detection of this pollutant in water sources has garnered immense importance in the analytical research arena. Several research groups have devoted great efforts to explore the selective as well as sensitive methods to detect H2S concentrations in water. Recent studies describe different strategies for sensing this ubiquitous gas in real-life water samples. Though many of the designed and developed H2S detection approaches based on the use of organic small molecules facilitate qualitative/quantitative detection of the toxic contaminant in water, optical detection has been acknowledged as one of the best, attributed to the simple, highly sensitive, selective, and good repeatability features of the technique. Therefore, this review is an attempt to offer a general perspective of easy-to-use and fast response optical detection techniques for H2S, fluorimetry and colorimetry, over a wide variety of other instrumental platforms. The review affords a concise summary of the various design strategies adopted by various researchers in constructing small organic molecules as H2S sensors and offers insight into their mechanistic pathways. Moreover, it collates the salient aspects of optical detection techniques and highlights the future scope for prospective exploration in this field based on the limitations of the existing H2S probes.

Efficient cyanide sensing using plasmonic Ag/Fe3O4 nanoparticles

In the line of our previous studies, we have reported a developed sensitive and selective probe for cyanide detection based on Ag/Fe3O4 nanoparticles (NPs) with an extremely low limit of detection at the level of ng per milliliter. Herein, we report the improvement of the easy-to-make magnetic silver nanoparticle-based sensor system for cyanide determination in an extended calibration range with higher selectivity and precision. As far as our knowledge is concerned, the detectable linear range from 1.0 nM to 160 μM (0.026 ng mL-1 to 4.16 μg mL-1) of the improved simple highly precise technique represents the widest assay that has been reported so far. The method is based on strong enhancement of scattered light of the plasmonic nanoparticles and simultaneously cyanide fluorescence quenching. Although the fluorescence of cyanide is highly selective and precise, its intensity is poor. On the other hand, the strongly enhanced Rayleigh signal has a low repeatability. We proposed a method to remove the interference and obtained an effective factor that is directly proportional to cyanide concentration utilizing both above signals simultaneously. In this work, Ag/Fe3O4 NPs have been synthesized easily using a green preparation method and the NPs were consequently characterized using powder XRD, UV-Vis absorption spectroscopy, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). A combination of absorption, Rayleigh and fluorescence characteristics were used for detection of cyanide in real samples and an overview of recently reported sensors for cyanide was also provided.

Glutamic acid-capped iron oxide quantum dots as fluorescent nanoprobe for tetracycline in urine

The fabrication of iron oxide quantum dots (IO-QDs) modified with glutamic acid (Glu) under controllable conditions is reported. The IO-QDs have been characterized by transmission electron microscopy, spectrofluorometry, powder X-ray diffraction, vibrating sample magnetometry, UV-Vis spectroscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy. The IO-QDs exhibited good stability towards irradiation, temperature elevations, and ionic strength, and the quantum yield (QY) of IO-QDs was calculated to be 11.91 ± 0.09%. The IO-QDs were furtherly measured at an excitation wavelength of 330 nm with emission maxima at 402 nm, which were employed to detect tetracycline (TCy) antibiotics, including tetracycline (TCy), chlortetracycline (CTCy), demeclocycline (DmCy), and oxytetracycline (OTCy) in biological samples. The results indicated that TCy, CTCy, DmCy, and OTCy in urine samples show a dynamic working range between 0.01 and 80.0 μM; 0.01 and 1.0 μM; 0.01 and 10 μM; and 0.04 and 1.0 μM, respectively, with detection limits of 7.69 nM, 120.23 nM, 18.20 nM, and 67.74 nM, respectively. The detection was not interfered with by the auto-fluorescence from the matrices. In addition, the obtained recovery in real urine samples suggested that the developed method could be used in practical applications. Therefore, the current study has prospect to develop an easy, fast, eco-friendly, and efficient new sensing method for detecting tetracycline antibiotics in biological samples.

Synthesis of intrinsic dual-emission type N,S-doped carbon dots for ratiometric fluorescence detection of Cr (VI) and application in cellular imaging

In this paper, intrinsic dual-emission fluorescent carbon dots (CDs) doped with N and S atoms have been firstly fabricated. The characterization results show that CDs are successfully synthesized with two separate fluorescence emissions at 468 nm and 628 nm, respectively. The strong and selective interaction of Cr (VI) ions with CDs lead to obvious fluorescence decrease of CDs at 468 nm, which is caused by a mixed quenching mechanism. At the same time, the fluorescence at 628 nm increase. Interestingly, the CDs solution show obvious color change under the daylight and UV light, so visualization detection of Cr (VI) can be realized in water samples. Based on the data of the emission intensity ratios of F₄₆₈/F₆₂₈, Cr (VI) can be detected from 3.8 to 38.9 μM combined with the linear correlation coefficient of 0.998, and the lowest detection concentration is 47.2 nM. The platform is satisfactorily applied to the detection of Cr (VI) ions in water samples. In addition, the CDs could be applied as fluorescent probes for cell imaging with dual fluorescent emission.

Recent advances in the visual detection of ions and molecules based on gold and silver nanoclusters

Gold and silver nanoclusters (Au/AgNCs) exhibit excellent application potential in optical biosensors because of their low toxicity, excellent biocompatibility, and unique optical properties. Au/AgNCs-based visual analysis methods have emerged as powerful tools for detecting various targets with convenient readout. In this review, the applications of Au/AgNCs in the visual detection and bioimaging of metal ions, inorganic anions, small molecules, and biomacromolecules in various devices are summarized. Furthermore, this review also discusses the future perspectives of the field.

An Overview on Coinage Metal Nanocluster-Based Luminescent Biosensors via Etching Chemistry

The findings from the synthetic mechanism of metal nanoclusters yield the etching chemistry based on coinage metal nanoclusters. The utilization of such chemistry as a tool that can alter the optical properties of metal nanoclusters has inspired the development of a series of emerging luminescent biosensors. Compared with other sensors, the luminescent biosensors have the advantages of being more sensitive, saving time and saving cost. We reviewed topics on the luminescent sensors based on the etching of emissive coinage metal nanoclusters. The molecules possessing varied etching ability towards metal nanoclusters were categorized with discussions of corresponding etching mechanisms. The understanding of etching mechanisms favored the discussions of how to use etching methods to detecting biochemical molecules. The emerging luminescent biosensors via etching chemistry also provided challenges and new opportunities for analytical chemistry and sensors.

Synthesis of gold nanoclusters-loaded lysozyme nanoparticles for ratiometric fluorescent detection of cyanide in tap water, cyanogenic glycoside-containing plants, and soils

The modification of protein-stabilized gold nanoclusters with fluorophores has been intensively applied for the ratiometric detection of biomolecules, metal ions, and anions. This study developed a straightforward strategy to prepare lysozyme nanoparticle-encapsulated gold nanoclusters (LysNP-AuNCs) as a dual-emission probe for the ratiometric sensing of cyanide through fluorescence resonance energy transfer (FRET) without the conjugation of additional fluorophores. The reduction of gold ion precursors with lysozyme generated lysozyme-stabilized AuNCs under an alkaline pH, which were demonstrated to self-assemble into nanoaggregates during the formation of AuNCs. The aggregated lysozyme molecules on the AuNCs were treated with glutaraldehyde, triggering the conversion of the aggregated lysozymes into blue-emitting lysozyme nanoparticles. As a result, the AuNCs were well distributed inside a single lysozyme nanoparticle, as demonstrated by transmission electron microscopy. The presence of cyanide triggered the etching of the AuNCs in the LysNP-AuNCs, leading to the suppression of FRET from lysozyme nanoparticle to AuNCs. The LysNP-AuNC probe was implemented for FRET detection of cyanide with a linear range of 3-100 μM. Additionally, the selectivity of the LysNP-AuNC probe for cyanide toward other anions was remarkably high. The practicality of the proposed probe was evaluated by quantifying cyanide in tap water and soils and monitoring the liberation of hydrogen cyanide from cyanogenic glycoside-containing foods.

Discovery of a highly selective and ultra-sensitive colorimetric fluorescent probe for malononitrile and its applications in living cells and zebrafish

A selective and ultra-sensitive colorimetric fluorescent probe was discovered to detect malononitrile in living cells and zebrafish.

A visualized ratiometric fluorescence sensing system for copper ions based on gold nanoclusters/perovskite quantum dot@SiO2 nanocomposites

Excessive copper ions (Cu²⁺) cause serious environmental pollution and even endanger the health of organisms. Fluorescence chemosensing materials are widely used in the detection of metal ions due to their simple operation and high sensitivity. In this study, SiO₂-encapsulated single perovskite quantum dot (PQD@SiO₂) core-shell nanostructures which show strong, stable, and green fluorescence are synthesized and composited with gold nanoclusters (AuNCs) which show Cu²⁺-sensitive and red light-emitting fluorescence to obtain a visualized ratiometric fluorescence sensor (AuNCs/PQD@SiO₂) for the detection of Cu²⁺. In the visualized detection of Cu²⁺, the green fluorescence emitted from the ion-insensitive PQD@SiO₂ component is used as a reference signal and the red fluorescence emitted by ion-sensitive AuNC component is adopted as a sensing signal. In the presence of Cu²⁺, the red fluorescence is quenched whereas the green fluorescence remains stable, which results in a visualized fluorescence color change from orange-red to yellow and finally to green with increasing Cu²⁺ concentration. The significant change in the fluorescence color of AuNCs/PQD@SiO₂ in response to Cu²⁺ enables a rapid, sensitive, and visualized detection of Cu²⁺. Further accurate and sensitive ratiometric fluorescence analysis of Cu²⁺ can be accomplished by measuring the ratio of fluorescence intensities at 643 and 520 nm (I₆₄₃/I₅₂₀) at a certain Cu²⁺ level. The developed AuNCs/PQD@SiO₂-based sensor has been validated by its satisfactory application in the detection of Cu²⁺ in human serum and environmental water samples.

Intrinsic dual-emissive carbon dots for efficient ratiometric detection of Cu2+ and aspartic acid

Herein, novel intrinsic dual-emitting carbon dots (CDs) are prepared through a one-step hydrothermal treatment of glucose and 3-nitroaniline in sulfuric acid solution and utilized for ratiometric determination of Cu²⁺ and aspartic acid (Asp). The CDs exhibited an interesting pH-switchable emission behavior displaying an intrinsic dual-emitting peak with emission maxima at 400 and 610 nm at pH 4.0-5.0. The presence of Cu²⁺ intensively quenched the first emission peak at 400 nm, but it had a negligible effect on the second emission peak. The ratiometric signal displayed a high selectively for Cu²⁺ over other metal ions and provided a linear response over the concentration range of 0.01-1.00 μM with a detection limit of 7.0 nM. Moreover, at pH 4.0, Asp was able to restore the quenched fluorescence of the CDs-Cu²⁺ system with a much more successful performance than other amino acids. This on-off-on fluorescence behavior provided a selective ratiometric fluorescence method for the determination of Asp in the concentration range of 0.2-15 μM. The acceptable detection results for Cu²⁺ in a river water sample (compared to Inductively Coupled Plasma (ICP) method) and for Asp in human serum samples confirmed the potential application of this ratiometric nanoprobe for sensing in real samples.

Ratio fluorescence detection of tetracycline by a Eu3+/NH2-MIL-53(Al) composite

Tetracycline detection has been a great concern because of its overuse and difficulty in degrading. Here, a detection method with ratio fluorescence was developed by synthesizing Eu³⁺ doped nanocomposites with NH₂-MIL-53(Al) nanosheets. After adding tetracycline, the fluorescence intensity at 616 nm characteristic emission peak of Eu³⁺ was sensitized by the antenna effect generated from coordinating Eu³⁺ with tetracycline, but the fluorescence of NH₂-MIL-53(Al) at 433 nm was quenched by the fluorescence resonance energy transfer between the Eu³⁺-tetracycline composition and NH₂-MIL-53(Al). Therefore, the efficient detection of tetracycline was achieved based on this change of ratio fluorescence signal. The experimental results show that Eu³⁺/NH₂-MIL-53(Al) has excellent selectivity, a wider linear range and a lower detection limit for detecting tetracycline. This method can afford favorable ideas for developing advanced chemical and biological sensors.

Tetracycline detection has been a great concern because of its overuse and difficulty in degrading.

Visual and quantitative detection of CN- ion in aqueous media by an HBT-Br and thiazolium conjugated fluorometric and colorimetric probe: Real samples and useful applications

Cyanide released from mostly industrial production is a highly toxic chemical. Its heavy industrial use and transportation increase the danger of human exposure. Since it can often lead to rapid death, selective, sensitive and on-site and rapid monitoring techniques for the detection of cyanide are essential. Therefore, we report an efficient cyanide probe based on thiazolium conjugated HBT-Br derivative. It has striking color change toward cyanide blue to yellow under daylight or colorless to cyan under UV-light. Interaction modes of the probe are based on ESIPT and ICT processes. It is selectively able to react free aqueous cyanide with detection limit of 1.79 μM, lower than physiologically lethal blood levels, >20 μM. Smartphone assisted and test kit applications are able to detect cyanide, qualitatively. Moreover, real samples such as tap or lake water and apricot seeds including cyanide are successfully determined through the present probe. Therefore, the designed probe displays excellent practical potential toward cyanide.

Ratiometric and sensitive cyanide sensing using dual-emissive gold nanoclusters

The detection of cyanide anion (CN^-), a highly toxic pollutant, has attracted growing attention in the past years. In this work, a nanosensor composed of hyperbranched polyethyleneimine (hPEI)-assisted dual-emissive gold nanoclusters (DE-Au NCs) is proposed for ratiometric detection of CN^- based on surface valence state-driving etch. The ratiometric color change of fluorescence is based on a fact that the red-emissive Au NCs with a high content of surface Au(I) can be easily etched by CN^-, while the blue-emissive Au NCs with nearly neutral character can resist CN^-. Because of the specific gold-CN^- chemistry and electrostatic attraction between the positively charged hPEI protecting layer and the negatively charged CN^-, the DE-Au NC-based nanosensor provides high selectivity toward CN^- over other anions with a limit of detection of 10 nM. Practical application of the proposed DE-Au NC nanosensor is verified by satisfying recoveries of CN^- determination in river water and urine samples. Graphical abstract.

Enzyme-Like Properties of Gold Clusters for Biomedical Application

In recent years, the rapid development of nanoscience and technology has provided a new opportunity for the development and preparation of new inorganic enzymes. Nanozyme is a new generation of artificial mimetic enzyme, which like natural enzymes, can efficiently catalyze the substrate of enzyme under mild conditions, exhibiting catalytic efficiency, and enzymatic reaction kinetics similar to natural enzymes. However, nanozymes exist better stability than native enzymes, it can still maintain 85 % catalytic activity in strong acid and alkali (pH 2~10) or large temperature range (4~90°C). This provides conditions for designing complex catalytic systems. In this review, we discussed the enzymatic attributes and biomedical applications of gold nanoclusters, including peroxidase-like, catalase-like, detection of heavy metal ions, and therapy of brain and cancer etc. This review can help us understand the current research status nanozymes.

Pillar[5]arene-based supramolecular AIE hydrogel with white light emission for ultrasensitive detection and effective separation of multianalytes

A novel pillar[5]arene-based supramolecular AIE hydrogel (PDG) with white light emission was constructed. The PDG could be used for ultrasensitive detection and effective separation of multianalytes, and as fluorescent display materials.

Dual-emission ratiometric nanoprobe for visual detection of Cu(II) and intracellular fluorescence imaging

Copper is an essential mineral nutrient for the human body. However, excessive levels of copper accumulated in the body can cause some diseases. Therefore, it is great significant to establish a sensitive bioprobe to recognize copper ions (Cu²⁺) in vivo. In our work, nitrogen-doped carbon dots (N-CDs) and gold nanoclusters (Au NCs) are selected as luminescent nanomaterials and the Au NCs/N- CDs nanohybrids is successfully synthesized by coupling method. The Au NCs/N-CDs exhibited characteristic dual-emission peaks at 450 and 620 nm when excited by a single-wavelength of 380 nm. When different amounts of Cu²⁺ are introduced, the fluorescence intensity of the Au NCs is gradually weakened and fluorescence intensity of the N-CDs is almost unchanged, which can facilitate the visual detection of Cu²⁺. The Au NCs/N-CDs nanohybrid possesses good selectivity to Cu²⁺ with a limit of detection (LOD) is 3.5 μM and linear detection range of 10-150 μM. Visualization detection of Cu²⁺ is implemented by using nanoprobe in water samples. Furthermore, the ratiometric nanoprobe is utilized to the toxicity test of liver cancer cells, indicating excellent biocompatibility and low toxicity. This nanoprobe has been used to the intracellular fluorescence imaging. Moreover, this method is expected to be used to monitor the changes of Cu²⁺ concentration in hepatocytes.

Ratiometric detection of tetracycline based on gold nanocluster enhanced Eu3+ fluorescence

Tetracycline (TC), a widely-used antibiotic to treat bacterial infections, combines with Eu³⁺ to form a stable EuTC complex that has a low fluorescence (FL) yield because of Eu³⁺ coordination with water molecules. We report a simple, label-free ratiometric fluorescent platform for sensitively and selectively sensing of TC, using L-histidine caped gold nanoclusters (His-AuNCs) as a FL indicator and an enhancer of Eu³⁺ FL. The His-AuNCs were prepared via chemical reduction of Au(III) by L-histidine, which was a reducer and a protecting agent. The His-AuNCs exhibited good photostability, outstanding stability toward high ionic strengths, storage stability, and favorable optical properties. In the absence of TC, the AuNCs-Eu³⁺ system displays strong FL emission at 475 nm (F₄₇₅) from the His-AuNCs and weak FL at 620 nm from Eu³⁺ (F₆₂₀) with excitation at 375 nm. TC quenches the His-AuNC FL and greatly enhances the Eu³⁺ emission. This is attributed to an enhancement of the EuTC complex fluorescence by the His-AuNCs. Thus, a ratiometric F₆₂₀/F₄₇₅ FL signal can be used for TC detection by simply mixing AuNCs and Eu³⁺. Under the optimized conditions, the linear range from 10 nM to 60 μM and a detection limit of 4 nM (S/N = 3) for TC were obtained. The application of the assay platform for the detection of TC in environmental and biological samples was demonstrated. The sensing platform has advantages of easy preparation, rapid response, high sensitivity, and good selectivity.

A nanoneedle-based reactional wettability variation sensor array for on-site detection of metal ions with a smartphone

An enhancement of the reactional wettability variation (RWV) sensing strategy is achieved based on the wettability switch of a nanoneedle surface. The sensor unit is formed by coating hydrophobic azoimidazole compounds, as the responder compounds onto the originally hydrophilic surface of cobalt hydroxide nanoneedles. The complexation reaction between metal ions and azoimidazole ligands etches the hydrophobic coating and switches the surface wettability, making the surface hydrophilic again. This switch is revealed by a decrease in the static contact angle (CA) and an increase in the sliding angle of the surface. The reactivity is tuned by the derivatization and conformational manipulation of the azoimidazole compounds. A sensor array composed of six as-tuned sensor units is constructed to distinguish among the species and concentrations of Fe³⁺, Ni²⁺ and La³⁺ at a low limit of 10^-6 M using the chemometric method of principal component analysis (PCA). In addition, a new on-site detection strategy is developed based on PCA of the sliding angle, which can be measured conveniently and swiftly with a smartphone app and a commercially available setup. The application of the general RWV strategy is envisioned to open new possibilities for on-site detection.

Ratiometric fluorescent nanoprobes for visual detection: Design principles and recent advances - A review

Signal generation techniques for visual detection of analytes have received a great deal of attention in various sensing fields. These approaches are considered to be advantageous when instrumentation cannot be employed, such as for on-site assays, point-of-care tests, and he althcare diagnostics in resource-constrained areas. Amongst various visual detection approaches explored for non-invasive quantitative measurements, ratiometric fluorescence sensing has received particular attention as a potential method to overcome the limitations of intensity-based probes. This technique relies on changes in the intensity of two or more emission bands (induced by an analyte), resulting in an effective internal referencing which improves the sensitivity of the detection. The self-calibration, together with the unique optophysical properties of nanoparticles (NPs) have made the ratiometric fluorescent nanoprobes more sensitive and reliable, which in turn, can result in more precise visual detection of the analytes. Over the past few years, a vast number of ratiometric sensing probes using nanostructured fluorophores have been designed and reported for a wide variety of sensing, imaging, and biomedical applications. In this work, a review on the NP-based ratiometric fluorescent sensors has been presented to meticulously elucidate their development, advances and challenges. With a special emphasis on visual detection, the most important steps in the design of fluorescent ratiometric nanoprobes have been given and based on different classes of analytes, recent applications of fluorescent ratiometric nanoprobes have been summarized. The challenges for the future use of the technique investigated in this review have been also discussed.

A novel long-alkyl-chained acylhydrazone-based supramolecular polymer gel for the ultrasensitive detection and separation of multianalytes

By rationally introducing multi-self-assembly driving forces and coordination binding sites into the same molecule, a designed functional gelator, G, was synthesized. Next, a novel supramolecular polymer material, OGV (1% DMSO), was constructed and used for the ultrasensitive detection and separation of multianalytes in gel states. Interestingly, OGV showed a fluorescent ultrasensitive response for the Hg2+ and Fe3+ ions in water. Moreover, by introducing these metal ions into the OGV, stable metal ion-coordinated supramolecular metallogels (HgG and FeG) were formed, which could sense CN- and H2PO4- in water with high selectivity and sensitivity.

Determination of Hg2+ and Cu2+ ions by dual-emissive Ag/Au nanocluster/carbon dots nanohybrids: Switching the selectivity by pH adjustment

An innovative dual-emissive ratiometric nanohybrid probe comprised of red-emitting a (Ag/Au)@insulin nanoclusters (NCs) and blue-emitting carbon dots (CDs) was designed for sensitive and selective ratiometric determination of Hg²⁺ and Cu²⁺ ions.The fluorescence intensity of CDs (λ_ex = 340 nm; λ_em = 420 nm) was unaffected in the presence of the metal ions tested, whereas the red emitting NCs (λ_ex = 340 nm; λ_em = 640 nm) was strongly quenched by both Cu²⁺ and Hg²⁺ ions. Interestingly, the selectivity of the probe toward these two ions was simply switched by controlling the pH of probe solution without using any chelating agent. The probe selectively responded to Hg²⁺ ions at acidic condition (pH = 4.0), Cu²⁺ ions at basic condition (pH = 10.0), and Hg²⁺-Cu²⁺ mixtures at pHs within this range. The respective detection limitsfor determination of Cu²⁺ and Hg²⁺ ions at their specific pH conditions were estimated as 5 nM and 7 nM, over linear ranges of 20-600 nM and 20-2000 nM, respectively. The fabricated ratiometric probe also showed distinguished fluorescence color changes to visual detection of these ions. Finally, the probe was successfully applied to determination of Hg²⁺ and Cu²⁺ ions in tap and mineral water samples.

Bimetallic AuCu nanoclusters-based florescent chemosensor for sensitive detection of Fe3+ in environmental and biological systems

Assays of ferric ion (Fe³⁺) with high sensitivity and selectivity have been required to evaluate its amount in environmental and biological systems. Herein, a novel fluorometric penicillamine-capped bimetallic gold-copper nanoclusters (PA-AuCu bi-MNCs) sensor was constructed for facile, environmentally friendly and quantitative detection of Fe³⁺ through inner filter effect (IFE) mechanism. One-step green synthetic approach was applied for the synthesis of AuCu bi-MNCs by using d-penicillamine (D-PA) as template and stabilizer. In the presence of Fe³⁺, the emission of the PA-AuCu bi-MNCs was hindered that caused selective quenching of the fluorescence intensity. The response to Fe³⁺ allows for two linear dynamic ranges of 5.0 × 10^-7 M-7.0 × 10^-6 M and 7.0 × 10^-6 M-1.0 × 10^-4 M with a detection limit of 0.1 μM, which is approximately 53 times lower than the maximum level (5.37 μM) of Fe³⁺ in drinking water that had been reported by the World Health Organization. The independency of the system from most of the interferences is the important feature of this work. Beside the appropriate selectivity of the proposed method, it shows a considerable operation in various environmental samples including rain water, three types of river water and also in human blood serum as a biological matrix.

Dye-sensitized upconversion nanocomposites for ratiometric semi-quantitative detection of hypochlorite in vivo

Ratiometric fluorescent sensors, which can provide a built-in correction for environmental effects, have attracted significant attention for analytical sensing and optical imaging with the potential to provide a precise and quantitative analysis. Herein, we report a strategy based on dye-sensitized upconversion for the design of dual-excitation upconverion ratiometric probes possessing same emission peaks under a large separation in the excitation spectra (980 nm and 808 nm). Specifically, effective enhancement of upconversion luminescence could be attributed to Cy787 dyes present on the surface of nanoparticles, and it subsequently decreased upon the addition of ClO- under an 808 nm irradiation, whereas the signal under 980 nm excitation remained essentially constant, thus allowing for quantitative ratiometric monitoring of ClO-. The rationally designed dye-sensitized upconverion nanosystem exhibits excellent sensitivity for ClO- with a quantification limit of 3.6 nM in aqueous solutions. We have also demonstrated that the designed nanoprobe is a promising material for semi-quantitative detection of ClO- in an arthritis mouse model.

The presence of a single-nucleotide mismatch in linker increases the fluorescence of guanine-enhanced DNA-templated Ag nanoclusters and their application for highly sensitive detection of cyanide

Fluorescence of DNA-templated silver nanoclusters can be enhanced by more than 100-fold by placing the nanoclusters in proximity to guanine-rich DNA sequences after hybridization. We found that the fluorescence of the guanine-enhanced silver nanoclusters is not increased with the guanine-rich DNA sequence closer to the silver nanoclusters. By studying the different numbers of mismatches in the linker sequences, we found that the presence of a single-nucleotide mismatch in the linker increases fluorescence more than the complementary nucleotide. Further study indicated the mismatch position of the linker sequence also affects the fluorescence of the hybridized DNA-Ag NCs. The evidence reported here indicated that the mismatch of the linker sequence affects the fluorescence enhancement of guanine-enhanced silver nanoclusters. We also found that DNA-Ag NCs is an excellent fluorescence sensor for cyanide, as cyanide effectively quenches the fluorescence of NCs at a very low concentration with high selectivity. Cyanide in the range from 0.10 μM to 0.35 μM could be linearly detected, with a detection limit of 25.6 nM.

Blue emitting copper nanoclusters as colorimetric and fluorescent probe for the selective detection of bilirubin

Hurdles to develop point of care diagnostic methods restrict the translation of progress in the health care sector from bench side to bedside. In this article a simple, cost effective fluorescent as well as colorimetric nanosensor was developed for the early and easy detection of hyperbilirubinemia. A stable, water soluble bovine serum albumin stabilised copper nanocluster (BSA CuNC) was used as the fluorescent probe which exhibited strong blue emission (404nm) upon 330nm excitation. The fluorescence of the BSA CuNC can be effectively quenched by the addition of bilirubin by the formation of copper-bilirubin complex. Meanwhile the copper-bilirubin complex resulted in an observable colour change from pale violet to green facilitating colorimetric detection. The prepared sensor displayed good selectivity and sensitivity over other co-existing molecules, and can be used for quantifying bilirubin with a detection limit down to 257fM. Additionally, the as-prepared probe was coated on a paper strip to develop a portable paper strip sensor of bilirubin. Moreover, the method was successfully applied in real sample analysis and obtained promising result.

Gold nanocluster-based ratiometric fluorescent probes for hydrogen peroxide and enzymatic sensing of uric acid

A method is described for ratiometric fluorometric assays of H₂O₂ by using two probes that have distinct response profiles. Under the catalytic action of ferrous ion, the 615 nm emission of protein-stabilized gold nanoclusters (under 365 nm photoexcitation) is quenched by H₂O₂, while an increased signal is generated with a peak at 450 nm by oxidizing coumarin with the H₂O₂/Fe(II) system to form a blue emitting fluorophore. These decrease/increase responses give a ratiometric signal. The ratio of the fluorescences at the two peaks are linearly related to the concentration of H₂O₂ in the range from 0.05 to 10 μM, with a 7.7 nM limit of detection. The detection scheme was further coupled to the urate oxidase catalyzed oxidation of uric acid which proceeds under the formation of H₂O₂. This method provides an simple and effective means for the construction of ratiometric fluorometric (enzymatic) assays that involve the detection of H₂O₂. Graphical abstract Under catalysis by ferrous ion, hydrogen peroxide quenches the luminescence of gold nanoclusters (AuNCs) and oxidizes coumarin into a fluorescent derivative, which rendered fluorescence ON and OFF at two distinct wavelengths for ratiometric measurements.

A novel "turn-on" fluorescent probe based on triphenylimidazole-hemicyanine dyad for colorimetric detection of CN- in 100% aqueous solution

A new colorimetric fluorescent probe (MCy) for CN^- is designed and synthesized based on triphenylimidazole-hemicyanine dyad. The probe shows high selectivity towards CN^- in 100% aqueous solution even in the presence of other competitive anions such as F^-, Cl^-, AcO^-, NO₂^-, H₂PO₄^-, I^-, SCN^-, SO₄^2-, NO₃^-, HCO₃^-, Br^- and S^2-. Upon nucleophilic addition of CN^- to the indolium group, the probe displays large blue-shift in UV-vis spectrum and remarkable "turn-on" fluorescence owing to the disruption of intramolecular charge transfer. Meanwhile, the probe also undergoes a naked-eye discernible color change from orange to light pink in the sunlight as well as an obvious fluorescence color change from non-emissive to bright blue under a UV lamp. The detection limit of CN^- in aqueous solution can be as low as 20.6nM, which is much lower than the permissible level of CN^- in drinking water according to the WHO. In addition, the probe has been successfully used to determine CN^- in real water samples as well as to fabricate test paper kit for CN^- detection.

A Mitochondria-Specific Fluorescent Probe for Visualizing Endogenous Hydrogen Cyanide Fluctuations in Neurons

An ability to visualize HCN in mitochondria in real time may permit additional insights into the critical toxicological and physiological roles this classic toxin plays in living organisms. Herein, we report a mitochondria-specific coumarin pyrrolidinium-derived fluorescence probe (MRP1) that permits the real-time ratiometric imaging of HCN in living cells. The response is specific, sensitive (detection limit is ca. 65.6 nM), rapid (within 1 s), and reversible. Probe MRP1 contains a benzyl chloride subunit designed to enhance retention within the mitochondria under conditions where the mitochondria membrane potential is eliminated. It has proved effective in visualizing different concentrations of exogenous HCN in the mitochondria of HepG2 cells, as well as the imaging of endogenous HCN in the mitochondria of PC12 cells and within neurons. Fluctuations in HCN levels arising from the intracellular generation of HCN could be readily detected.

A cyanide-triggered hydrogen-bond-breaking deprotonation mechanism: fluorescent detection of cyanide using a thioacetohydrazone-functionalized bispillar[5]arene

Bispillar[5]arene-based fluorescent sensor was used for fluorescent detection of cyanide anions through deprotonation accompanied by intermolecular hydrogen bond breakage.

Simple construction of ratiometric fluorescent probe for the detection of dopamine and tyrosinase by the naked eye

A simple and effective method for constructing a ratiometric fluorescent probe for the detection of dopamine and tyrosinase was developed.

Cobalt metal-mixed organic complex-based hybrid micromaterials: ratiometric detection of cyanide

A micrometer-sized hybrid crystalline material was facilely fabricated from the synergistic coordination chemistry of two types of 2-hydroxyazobenzene building blocks and cobalt ions. This hybrid crystalline material functioned as both receptor and signal reporter in cyanide (CN⁻) sensing with ratiometric absorption responses at 456 nm and 537 nm. The mechanism of CN⁻ sensing involves partial ligand dissociation accompanied by the formation of a new organic metal–CN⁻ adduct. We believe that this protocol would be valuable in achieving the expected selectivities and sensitivities for a wide variety of analytes in many chemical and biological systems in the future.

A cobalt metal-mixed organic complex-based hybrid micromaterial has been presented to detect CN⁻ with ratiometric absorption responses (A_{456 nm}/A_{537 nm}).

pH-Regulated Synthesis of Trypsin-Templated Copper Nanoclusters with Blue and Yellow Fluorescent Emission

In this article, a simple protocol to prepare water-soluble fluorescent copper nanoclusters (CuNCs) using trypsin as a stabilizer and hydrazine hydrate as a reducing agent was reported. It was found that the pH of the reaction solution was critical in determining the fluorescence of CuNCs. CuNCs with blue and yellow fluorescent emission were obtained under basic and acidic conditions, respectively. Although the detailed formation mechanisms of these CuNCs required further analysis, the synthetic route was promising for preparing different fluorescent metal NCs for applications. With good water solubility and excellent photostability, the yellow-emitting CuNCs could serve as a fluorescence probe for detection of Hg²⁺ based on the aggregation-induced quenching mechanism. The fluorescence quenching efficiency had fantastic linearity to Hg²⁺ concentrations in the range of 0.1-100 μM, with a limit of detection of 30 nM. Additionally, the yellow-emitting CuNCs exhibited negligible cytotoxicity and were successfully applied to bioimaging of HeLa cells.

Synthesis of Dicyanovinyl-Substituted 1-(2-Pyridyl)pyrazoles: Design of a Fluorescent Chemosensor for Selective Recognition of Cyanide

A fluorescence "turn-off" probe has been designed and successfully applied to detect cyanide (CN^-) based on a Michael-type nucleophilic addition reaction and intramolecular charge transfer (ICT) mechanism. For this research, a family of 3-aryl-4-(2,2-dicyanovinyl)-1-(2-pyridinyl)pyrazoles as donor-π-acceptor (D-π-A) systems have been synthesized in 58-66% overall yield, by a three-step synthesis sequence starting from p-substituted acetophenones. The substituted p-methoxyphenyl showed good fluorescence emission and large Stokes shifts in different solvents due to its greater ICT. Likewise, this probe evidenced high selectivity and sensitivity and fast recognition for CN^- with a detection limit of 6.8 μM. HRMS analysis, ¹H NMR titration experiments, and TD-DFT calculations were performed to confirm the mechanism of detection and fluorescence properties of the chemodosimeter of CN^-. Additionally, fluorescent test paper was conveniently used to detect cyanide in aqueous solution.