The authors described gold nanoclusters (AuNCs) for use on an “on ? off ? on” NIR fluorescent probe for the determination of citrate and Cu(II) ion. The AuNCs were prepared by a microwave-assisted method using BSA as both the stabilizing and reducing agent. The resulting BSA-capped AuNCs display NIR fluorescence peaking at 680 nm under 500 nm excitation, a quantum yield of ~6.0%, an average size of 2.8 ± 0.5 nm, water-dispersibility, stability and biocompatibility. The on?off probe for Cu(II) is based on the interaction between Cu(II) and BSA which causes the fluorescence of the BSA?AuNCs to be quenched. The quenched fluorescence is recovered on addition of vitamin C (VC), obviously due to complexation of Cu(II) by citrate. The probe was employed to image Cu(II) and citrate in HeLa cells and in aqueous solutions. The method works in the 20 nM to 0.1 mM concentration range for Cu(II), and in the 8 nM to 120 μM concentration range for VC.
Graphical abstract Schematic presentation of the gold nanocluster based probe whose fluorescence is quenched by Cu(II) ions and then restored by addition of vitamin C. This is demonstrated for both aqueous solutions and living cells.
Nanocomposites consisting of gold nanoclusters and graphene oxide (AuNC/GO) were prepared and investigated with respect to the design of new sensors for hydrogen peroxide (H2O2). The AuNC/GO hybrid nanomaterials were deposited on a gold electrode by the layer-by-layer assembly method, where they showed enhanced photoelectrical and sensing properties. The presence of graphene oxide improves the photoinduced electron separation efficiency of the AuNCs, as well as the catalytic effect of AuNCs on the electroreduction of H2O2. Compared to an electrode modified with AuNCs only, the new electrodes display a more than ten-fold enhanced photocurrent at a working voltage of -500 mV (vs. Ag/AgCl), higher sensitivity for H2O2 (25.76 nA?mM?1), lower LOD (2 μM) and extended linear range (from 30 μM to 5 mM). The sensors were applied to the determination of H2O2 extracted from living human umbilical vein endothelial cells stimulated by angiotensin II.
Graphical abstract Graphene oxide (GO) not only improves the photoinduced charge separation efficiency of fluorescent gold nanoclusters (AuNCs) based photoelectrochemical sensors, but also enhances the catalytic property of AuNCs on the detection of hydrogen peroxide (H2O2).
The authors describe a fluorescence immunoassay for galectin-4, a candidate biomarker for various cancers. Glucose oxidase was encapsulated into a zeolitic imidazolate framework to give a composite (GOx/ZIF-8 composite) that acts as a signal-transduction tag via a biomimetic mineralization process. After modification of the composite with streptavidin, it binds biotinylated antibody against galectin-4. In the immunoassay, the response to galectin-4 results from the enzymatic oxidation of glucose. This reaction produces hydrogen peroxide (H2O2) that reacts with iron(II) ions to generate hydroxy radical (?OH), which leads to the quenching of the fluorescence of gold nanoclusters (AuNCs). Accordingly, the fluorescence quenching of AuNCs depends on the concentration of target galectin-4. The GOx/ZIF-8 composite has a high loading capacity for GOx at uncompromised enzymatic activity. The fluorescence of AuNCs is sensitively quenched by ?OH radicals. Galectin-4 can be detected by this method in concentrations as low as 10 pg·mL?1. It is expected that this kind of enzyme/MOF composite-based immunoassay has a wide scope in that it may be adapted to other low-abundance proteins and biomarkers.
Graphical abstract Schematic of a fluorescence immunoassay for galectin-4, a candidate biomarker for various cancers. It is based on a composite consisting of glucose oxidase and a metal-organic framework (GOx/ZIF-8 composite) as well as gold nanoclusters (AuNC)-iron(II) system.
The authors describe a rapid and sensitive method for the determination of the activity of scavenging hydrogen peroxide in which glucose oxidase–stabilized gold nanoclusters (AuNCs) were employed as a fluorescent nanoprobe. The AuNCs are synthesized by a biomineralization process and display an intense blue fluorescence peaking at 450 nm and a quantum yield of 1.1% under 360–nm excitation. The Fenton reaction induces quenching of fluorescence, and this effect can be used to determine H2O2 in the 0.5 to 10 μmol?L?1 concentration range. The substances displaying H2O2 scavenging activity prevent quenching and thus restore fluorescence. The intensity of restored fluorescence is directly related to the H2O2 scavenging activity of the antioxidant. The method was applied to the determination of the H2O2 scavenging activity of the model antioxidants ascorbic acid and tartaric acid which gave IC50 values of 7.4 and 19.1 μmol?L?1, respectively.
Graphical abstract Blue-emitting gold nanoclusters (AuNCs) were prepared by using GOx as both the reducing and stabilizing agents. The Fenton reaction induces quenching of fluorescence of the AuNCs, and is employed for fluorometric measurement of the H2O2 scavenging activity of the model antioxidants ascorbic acid and tartaric acid.
The authors report on a one-pot approach for synthesizing highly fluorescent protamine-stabilized gold nanoclusters. These are shown to be a viable nanoprobe for selective and sensitive fluorometric determination of lead(II) via quenching of fluorescence via Pb(II)-Au(I) interaction. Under optimized conditions, fluorescence measured at excitation/emission peaks of 300/599 nm drops in the 80 nM–15 μM lead(II) concentration range. The detection limit is 24 nM, and relative standard deviations (for n?=?11) at concentrations of 0.10, 4.0 and 15 μM are 1.6, 2.5 and 1.9%, respectively. The relative recoveries of added lead(II) in the water samples ranged from 97.9?±?2.29% to 101.2?±?1.83%.
Graphical abstract Lead(II) ions are found to be able to selectively and sensitively quench the fluorescence of the protamine-gold nanoclusters (PRT-AuNCs). Thereby, an inexpensive, selective and sensitive lead(II) assay was established.
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.
Carbon nanodots modified with triethylenetetramine (referred to as TCDs) are shown to be viable fluorescent probes for relay recognition of Cu(II) ion and glutathione (GSH). The assay is based on an “on-off-on” mechanism where the “on-off” effect, i.e. quenching by Cu(II) by up to 67%, is exploited to quantify it in concentrations as low as 3.4 nM. The unique quenching of fluorescence (measured at excitation/emission wavelengths of 380/470 nm; quantum yield 16%) is attributed to the fairly selective capture of Cu(II) by the amino and amide groups on the surface of the TCDs. On addition of GSH to the quenched TCD/Cu(II) complex, fluorescence is restored. This effect enables GSH to be quantified in the 0.2 to 175 μM concentration range, with a 0.11 μM detection limit. The turn-on response to GSH is highly selective over other natural amino acids and common anions. Furthermore, the TCDs were successfully applied to image Cu(II) and GSH in living yeast cells.
Graphical Abstract Carbon nanodots modified with triethylenetetramine show strong blue fluorescence which is quenched by Cu(II) but restored on addition of glutathione. Both Cu(II) (down to 3.4 nM) and glutathione (down to 110 nM) can be detected via these effects.
A selective phosphorescent on-off-on probe with long decay lifetime has been designed for the detection of pyrophosphate ions (PPi). The detection scheme is based on the use of europium(III)-modulated Mn(II)-doped ZnS quantum dots capped with N-acetyl-L-cysteine. Both the aggregation of quantum dots and electron transfer induced by Eu(III) ions cause phosphorescence to be quenched (“off” state). Phosphorescence is, however, restored on addition of PPi to the system (“on” state). The effect is attributed to the removal of Eu(III) from the carboxy groups on the surface of the quantum dots owing to the stronger interaction between PPi and Eu(III). A linear relationship exists between phosphorescence intensity (best measured at excitation/emission wavelengths of 316/594 nm) and PPi concentration in the 400 nM to 6000 nM with a detection limit of 145 nM. An additional attractive feature is provided by the long-lived phosphorescence (1920 μs) of the quantum dots. It can be used to eliminate interference by short-lived fluorescence in biological samples by performing time resolved measurements. The probe was applied to the determination of PPi in spiked in urine samples and gave recoveries in the range from 98 to 105% with RSDs of <2.0%.
Graphical abstract Schematic of a long-lived phosphorescent on-off-on probe for the sensitive and selective detection of pyrophosphate ions (PPi). It is based on the use of Eu(III)-modulated Mn(II)-doped ZnS quantum dots (QDs). Phosphorescence is quenched of QDs after the addition of Eu3+but restored after the addition of PPi.
A simple method is described for the determination of copper(II) ions based on the cathodic electrochemiluminescence (ECL) of lucigenin which is quenched by Cu(II). The blue ECL is best induced at ?0.45 V (vs. Ag/AgCl) at a scan rate of 50 mV·s?1. Under optimum conditions, the calibration plot is linear in the 3.0 to 1000 nM Cu(II) concentration range. The limit of detection is 2.1 nM at a signal-to-noise ratio of 3. Compared to other analytical methods, the one presented here is simple, fast, selective and cost-effective. It has been successfully applied in the analysis of copper ions in spiked tap water samples with recoveries ranging from 93.0% (at 50 nM concentration) to 105.7% (at 150 nM).
Graphical abstract The inhibitory effect of Cu(II) on the cathodic electrochemiluminescence of lucigenin enables determination of Cu(II) with a 2.1 nM detection limit.
The authors describe a fluorescent probe for sulfide that is based on carboxy-functionalized semiconducting polymer dots (P-dots). The dots were prepared from carboxy-functionalized poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,1′-3-thiadiazole)] (referred to as COOH-PFBT) via co-precipitation. The P-dots aggregate on addition of Cu(II) ions and their green fluorescence (with excitation/emission peaks at 455/540 nm) is then quenched. Fluorescence is restored on addition of sulfide to the aggregates due to the formation of CuS. This quenching-recovery (“off-on”) mechanism forms the basis for a new sulfide detection scheme. Fluorescence increases linearly in the 1.25 to 75.0 μM sulfide concentration range, with a 0.45 μM detection limit. Good selectivity over other anions is demonstrated. The method shows recoveries ranging between 98.6% and 105.7% when applied to the determination of sulfide in spiked real water samples.
Graphical abstract Schematic of a fluorescent off-on sensor for (hydrogen) sulfide based on the use of semiconducting polymer dots (PFBT-COOH) whose fluorescene is quenched by Cu(II) ion but restored on addition of (hydrogen) sulfide.
The authors describe a fluorometric glucose assay that is based on the use of MnO2 nanosheets and copper nanoclusters (CuNCs) acting as nanoprobes. The CuNCs were synthesized by using bovine serum albumin as a template by chemical reduction of copper(II) sulfate. On addition of MnO2 nanosheets to a colloidal solution of CuNCs, the fluorescence of CuNCs (measured at excitation/emission wavelengths of 335/410 nm) is quenched. However, in the presence of enzymatically generated H2O2, the MnO2 nanosheets are reduced to form Mn(II) ions. As a result, fluorescence intensity recovers. The glucose assay is based on the enzymatic conversion of glucose by glucose oxidase to generate H2O2 and glucuronic acid. The calibration plot is linear in the 1 μM to 200 μM glucose concentration range, and the detection limit is 100 nM. The method was successfully applied to the determination of glucose in spiked human serum samples.
Graphical abstract A sensitive fluorescent bioassay is reported for the detection of glucose based on the hydrogen peroxide-induced decomposition of a quencher system composed of MnO2 nanosheets and copper nanoclusters (CuNCs).
The paper describes a voltammetric method for the quantitation of the activity of telomerase extracted from cancer cells. A thiolated single-stranded telomerase substrate primer was firstly immobilized on a gold electrode. In the presence of a mixture of telomerase and deoxynucleotide triphosphates, the primer becomes elongated and contains repetitive nucleotide sequences (TTAGGG)n. After hybridization with blocker DNA, gold nanoparticles are added and captured by the elongated single-stranded DNA. This reduces the charge transfer resistance of the gold electrode. The telomerase activity is then quantified via differential pulse voltammetry, typically at 0.12 V (vs. SCE). The method is PCR-free, rapid, and convenient. It was applied to the detection of HeLa cells via the telomerase activity of lysed cells. The detection range was from 500 to 50,000 cells/mL and the detection limit was as low as 500 cells/mL.
Graphical abstract A telomerase substrate (TS) primer is immobilized on a gold electrode as the sensing interface to detect the activity of telomerase extracted from cancer cells. Unmodified gold nanoparticles (AuNPs) are utilized which change the electrochemical responses.
Glutathione coated gold and silver nanoclusters (GSH-Au/AgNCs) were synthesized by one-pot reduction methods and are found to be viable fluorescent nanoprobes for cysteine (Cys) and arginine (Arg), with good selectivity over other amino acids. The GSH-Au/AgNCs have two emissions at 616 nm and 412 nm when excited at 360 nm. With the increased concentration of Cys, the ratio of the emission intensities (I616/I412) linearly decreases with Cys in concentration ranging from 0.05 to 10 μM and from 10 to 50 μM, respectively. With increased concentrations of Arg, the ratio of I616/I412 linearly decreases with Arg concentration ranging from 0 to 50 μM and from 50 to 100 μM, respectively. The probe was applied to the determination of Cys and Arg in spiked samples of serum and urine where it gave good recoveries.
Graphical abstract Glutathione-coated gold and silver nanoclusters (GSH-Au/AgNCs) were synthesized by one-pot reduction and are found to be viable fluorescent nanoprobes for cysteine (Cys) and arginine (Arg).
Gold nanoclusters (AuNCs) stabilized with bovine serum albumin were utilized as a fluorescent probe for ferrous ion. The detection scheme is based on the quenching of the fluorescence of the modified AuNCs by hydroxyl radical (•OH) that is generated in the Fenton reaction between Fe(II) and H2O2. Fe(II) can be quantified in the 0.08 to 100 μM concentration range, and the limit of detection is as low as 24 nM. The method also displays good accuracy and high sensitivity when employed to the determination of Fe(II) in rat cerebrospinal fluids (CSFs). When applied to CSFs of a rat model of Alzheimer’s disease, it revealed enhanced levels of Fe(II) compared to a control, thereby showing the important physiological role of iron(II) in this disease.
BSA-stabilized gold nanoclusters (BSA-AuNCs) were utilized for the determination of ferrous ion in rat cerebrospinal fluids. The method is based on the quenching of the fluorescence by hydroxyl radical (•OH) which is generated in the Fenton reaction between Fe(II) and H2O2.
The authors describe a highly chemiluminescent metal-organic framework (MOF) that was obtained by loading the pores of MIL-101(Cr) with luminol. Immobilization is based on Lewis acid-base interactions between the coordinatively unsaturated metal sites of MIL-101(Cr) and the amino groups of luminol. The luminol-loaded MOF displays strong chemiluminescence (CL) in the presence of hydrogen peroxide (H2O2) in alkaline solution. Pyrophosphate (PPi), in turn, acts as a quencher of this CL. These findings have been exploited in the design of a CL based method for the determination of either H2O2 or PPi. The assays for H2O2 works in the 3 to 100 μM concentration range and has a detection limit of 0.5 μM. The quenchometric assay for PPi works in the 5 to 70 μM concentration range and has a 1.2 μM detection limit.
Graphical abstract Luminol-embedded metal-organic frameworks (MOFs) with high chemiluminescent activity were prepared and used for the sensitive determination of either H2O2 or pyrophosphate ions.
This study describes an amperometric sensor for hydrogen peroxide (H2O2) that uses an ITO glass electrode which was modified with a nanocomposite consisting of electrochemically reduced graphene oxide and gold nanoclusters (AuNCs). The sensor was used to quantify extracellular H2O2 released from human neuroblastoma cells of type SH-SY5Y. The calibration plot, established best at a working voltage of ?0.4 V (vs. Ag/AgCl) is linear in the 40 nmol?L?1 to 2 μmol?L?1 concentration range, and the detection limit is 20 nmol?L?1 (at a signal-to-noise ratio of 3). The method was further applied to study bupivacaine-induced cell damage and the protective effects of α-lipoic acid. The study indicated that pretreatment of the cells with lipoic acid retards cell damage induced by bupivacaine. The sensor can be easily fabricated, is disposable and highly sensitive. The sensor is perceived to represent an alternative for studying the interactions of drugs with cells, and as an effective tool to quantify cell-secreted H2O2.
Graphical abstract One-step electrochemical synthesis of graphene oxide and gold nanoclusters on an ITO electrode for studying the release of H2O2 from SH-SY5Y cells and for evaluation of drug-induced cell damage
This paper describes a CdTe quantum dot-based fluorescence resonance energy transfer (FRET) based assay for the detection of the breast cancer biomarker microRNA. The method relies on energy transfer between DNA-templated silver nanoclusters (AgNCs) and CdTe QDs. Interaction between double strand oligonucleotide and QDs can be detected qualitatively through gel analysis and quantitatively by the signal amplification from AgNCs to QDs via FRET, best measured at an excitation wavelength of 350 nm and at emission wavelengths of 550 and 590 nm. Three microRNAs (microRNA-21, microRNA-155 and Let-7a) were quantified to verify the feasibility of the method, and a high sensitivity for microRNAs was achieved. Fluorescence intensity increases linearly with the log of the concentration of microRNA 155 in the 5.0 pM to 50 nM range, with a 1.2 pM detection limit.
Graphical abstract Schematic presentation of a quantum dot-based (QD-based) fluorescence resonance energy transfer technique for the detection of microRNA (miRNA). The method relies on energy transfer between DNA-templated silver nanoclusters (AgNCs) and QDs.
Gold-silver nanoclusters (Au-AgNCs) were synthesized by simultaneous chemical reduction of Au(III) and Ag(I) ions in one pot, using bovine serum albumin as both a template and a reductant. The Au-AgNCs have an average size of 2.4 nm and display strong red fluorescence (with an emission peak at 610 nm on excitation at 360 nm). The fluorescence quantum yield can reach 18.6%. Fluorescence is strongly quenched by hypochlorite, while other common anions have minor (or no) effects on fluorescence. Based on these findings, a fluorometric method was developed for the determination of hypochlorite. The method has a linear response in the 0.7 to 15 μM concentration range, with a limit of detection as low as 80 nM. It was successfully applied to the determination of hypochlorite in (spiked) tap water.
Graphical abstract Gold-silver nanoclusters with strong red fluorescence were synthesized by simultaneous chemical reduction of Au(III) and Ag(I) ions in one pot, and a sensitive and selective method for the detection of hypochlorite was developed based on the quenching of the fluorescence of the nanoclusters.
It is known that the binding of certain proteins to small molecules in ssDNA/small-molecule chimeras protects the conjugated ssDNA from degradation by exonuclease I (Exo I). This has resulted in numerous methods to specifically detect the interaction between small molecules and proteins. We are presenting here an approach that utilizes the terminal protection strategy in combination with the formation of ssDNA-templated silver nanoclusters (AgNCs), thereby providing a fluorometric tool for the detection of such interactions. A C-rich ssDNA (type 5′-CCCCACCCCT-3′) was labelled with biotin at the 3′ end. In the absence of streptavidin (SA), the biotinylated ssDNA is hydrolyzed in the 3′ to 5′ direction by Exo I to form mononucleotides. The formation of the AgNCs is prevented due to the lack of the DNA scaffold, and this results in weak fluorescence. Conversely, in the presence of SA, the specific binding of SA to the biotinylated ssDNA protects the ssDNA from digestion. As a result, fluorescent AgNCs are being formed. Fluorescence is measured at excitation/emission wavelengths of 625/705 nm. The calibration plot for SA is linear in the 6 to 600 nM concentration range, with a 2.6 nM detection limit. The assay is simple, sensitive and affordable. Conceivably, the method may also be used to detect the binding of other small molecules to proteins.
Graphical abstract A fluorescent sensing platform for small molecule-protein interaction assay has been developed based on terminal protection strategy and ssDNA-templated silver nanoclusters (AgNCs).
The authors report on a disposable sensor for the differential pulse anodic stripping voltammetric (DPASV) determination of the ions Zn(II), Pb(II) and Cu(II). Simultaneous detection is accomplished by using a screen-printed carbon electrode (SPCE) co-modified with an in-situ plated bismuth (Bi)) film and gold nanoparticles (AuNPs). The synergistic effect of the Bi film, and the large surface and good electrical conductivity of the AuNPs strongly assist in the co-deposition of the three ions. Four well-defined and fully separated anodic stripping peaks, at 540 mV for Zn(II), 50 mV for Pb(II), 140 mV for Bi(III) and 295 mV for Cu(II), all vs. Ag/AgCl, can be seen. The modified SPCE was characterized by scanning electron microscopy, X-ray diffraction, cyclic voltammetry and electrochemical impedance spectroscopy. Under the optimized conditions, the sensor has a good response to these ions. The detection limits (at an S/N ratio of 3) are 50 ng·L?1 for Zn(II), 20 ng·L?1 for Pb(II), and 30 ng·L?1 for Cu(II). The method was applied to the determination of the 3 ions in spiked lake water samples.
Graphical abstract Schematic of screen-printed carbon electrode (SPCE) co-modified with a bismuth film and gold nanoparticles for electrochemical simultaneous determination of Zn(II), Pb(II) and Cu(II) by differential pulse anodic stripping voltammetric (DPASV).
