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1.
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).
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2.
The authors report that sulfide ions are capable of inhibiting the peroxidase-like activity of copper nanoclusters (CuNCs). The catalytic activity of CuNCs toward the oxidation of the chromogenic substrate 3,3′,5,5′-tetramethylbenzidine by H2O2 is remarkably decreased in the presence of sulfide. Based on this finding, a colorimetric assay was developed for the rapid determination of sulfide. Best operated at a wavelength of 652 nm, it has a 0.5 μM detection limit. The method is highly selective and has been successfully applied to the quantification of sulfide in environmental water samples.
Graphical abstract The catalytic activity of CuNCs toward the oxidation of 3,3′,5,5′-tetramethylbenzidine by H2O2 is remarkably decreased in the presence of sulfide ions. This finding has been applied to design a method for colorimetric quantification of sulfide ions in environmental samples.
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3.
Stable copper nanoclusters (CuNCs) were prepared by utilizing D-penicillamine as both the stabilizer and reductant. The emission of the CuNCs (with excitation/emission peaks at 390/645 nm) is largely stabilized by coating with poly(sodium-p-styrenesulfonate) (PSS). Cytochrome c (Cyt c) quenches the fluorescence of the PSS-coated CuNCs, and this effect was exploited to design a quenchometric fluorometric assay for Cyt c. If trypsin is added to the loaded CuNCs, it will hydrolyze Cyt c to form peptide fragments, and fluorescence is gradually restored. A highly sensitive and fluorometric turn-off-on assay was constructed for sequential detection of Cyt c and trypsin. The linear ranges for Cyt c and trypsin are from 8.0 nM to 680 nM, and from 0.1 to 6.0 μg mL?1, and the lower detection limits are 0.83 nM and 20 ng mL?1 for Cyt c and trypsin, respectively.
Graphical abstract Schematic illustration of the fluorometric assay for trypsin based on the electron transfer between poly(p-styrenesulfonate)-protected copper nanoclusters (PSS-CuNCs) and cytochrome c (Cyt c).
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4.
Patients with prostate cancer and systemic lupus erythematosus exhibit reduced DNase I activity, and patients with myocardial infarction exhibit increased DNase I activity. So the assay of DNase I is of high importance. A colorimetric assay is described here for the determination of the activity of DNase. It is based on strand scission of dsDNA as catalyzed by DNase I. The products of digestion (nucleoside monophosphates) can better stabilize citrate capped AuNPs than dsDNA. In the absence of DNase I, the AuNPs aggregate in presence of NaCl and then display a blue color. In the presence of the analyte (DNase I), AuNPs do not aggregate but rather remain dispersed and display a red color. These findings were exploited to design a DNase I activity assay that is based on the measurement of ratio of absorbances at 520 nm (red) to 650 nm (blue). The detection limit for DNase I activity is found to be 7.1 U?L?1. In our perception, this assay has a large potential with respect to diagnoses of DNase I activity-related diseases and in drug screening.
Graphical Abstract A method is described for the determination of the activity of DNase I. It is based on the capability of nucleoside monophosphates (dNMPs; formed by DNase-catalyzed scission of dsDNA) to stabilize red gold NPs against NaCl-induced aggregation. AuNPs stabilized with dsDNA, in contrast, readily aggregate in presence of NaCl to form blue clusters.
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5.
The authors report that the peroxidase-like activity of Au@Pt core-shell nanohybrids (Au@PtNHs) is selectively inhibited by cysteine. This finding has led to  a highly sensitive colorimetric assay for cysteine that is based on the nanohybrid-catalyzed oxidation of TMB by H2O2 to form a blue product. The method has a detection limit of 5.0 nM and a linear range from 10 nM to 20 μM. The assay is highly selective over other amino acids. It was successfully applied to the determination of cysteine in an injection containing a mixture of amino acids.
Graphical abstract The peroxidase-like activity of Au@Pt core-shell nanohybrids (Au@PtNHs) is selectively inhibited by cysteine, enabling the determination of cysteine.
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6.
A composite consisting of chitosan containing azidomethylferrocene covalently immobilized on sheets of reduced graphene oxide was drop-casted on a polyester support to form a screen-printed working electrode that is shown to enable the determination of nitrite by cyclic voltammetry and chronoamperometry. Both reduction and oxidation of nitrite can be accomplished due to the high electron-transfer rate of this electrode. Under optimal experimental conditions (i.e. an applied potential of 0.7 V vs. Ag/AgCl in pH 7.0 solution), the calibration plot is linear in the 2.5 to 1450 μM concentration range, with an ~0.35 μM limit of detection (at a signal-to-noise ratio of 3). The sensor was successfully applied to the determination of nitrite in spiked mineral water samples, with recoveries ranging between 95 and 101 %.
Graphical abstract We describe the design of ferrocene-functionalized reduced graphene oxide electrode and its electrocatalytic properties towards the determination of nitrite. Compared to a reduced graphene oxide electrode, the sensor exhibits enhanced electrocatalytic activity towards both oxidation and reduction of nitrite.
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7.
The authors describe a method for the fabrication of a nanohybrid composed of carbon dots (C-dots) and gold nanoparticles (AuNPs) by in-situ reduction of C-dots and hydroauric acid under alkaline conditions. The process does not require the presence of surfactant, stabilizing agent, or reducing agent. The hybrid material was deposited in a glassy carbon electrode (GCE), and the modified GCE exhibited good electrocatalytic activity toward the oxidation of nitrite due to the synergistic effects between carbon dots and AuNPs. The findings were used to develop an amperometric sensor for nitrite. The sensor shows a linear response in the concentration range from 0.1 μmol?L-1 to 2 mmol?L-1 and a low detection limit of 0.06 μmol?L-1 at the signal-to-noise ratio of 3.
Graphical abstract Fabrication, characterization and electrochemical behavior of a glassy carbon electrode modifid with carbon dots and gold nanoparticles for sensing nitrite in lake water.
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8.
A toehold-aided DNA recycling amplification technology was developed based on the combination of toehold-aided DNA recycling and the hemin/G-quadruplex label. The dsDNA formed between aptamer and DNA1 was first immobilized on magnetic beads. On addition of target analyte (exemplified here for riboflavin), the aptamer-riboflavin complex is formed and DNA1 is released by the beads. After magnetic separation, the supernatant containing the released DNA1 is added to a solution containing the hairpin capture DNA on magnetic beads. DNA1 will hybridize with the hairpin capture DNA via toehold binding and branch migration. This process will open the hairpin structure, and an external toehold is formed in the newly formed dsDNA. On addition of reporter DNA containing the G-quadruplex, it will interact with the formed dsDNA via toehold binding and branch migration, resulting in the releasing of DNA1 and capturing of reporter DNA on the magnetic beads. The released DNA1 will bind to another hairpin capture DNA which can start another round of DNA1 recycling. Chemiluminescence (CL) is generated by the G-quadruplex-hemin-luminol CL reaction system. Under optimal conditions, the calibration plot is linear in the 0.1 to 700 nM riboflavin concentration range, with a 30 pM detection limit (at a signal-to-noise ratio of 3). The method was successfully applied to the quantitation of riboflavin in spiked urine samples.
Graphical abstract Toehold-aided DNA recycling coupled with hemin G-quadruplex for target detection.
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9.
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.
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10.
We describe a colorimetric assay for the determination of the activity of cellulase and xylanase. Following enzymatic hydrolysis, reductive saccharides are produced which are capable of directly reducing auric acid to form gold nanoparticles (AuNPs). The AuNPs are of fuchsia color and possess a strong plasmonic absorption band at 550 nm. Reaction conditions such as temperature, reaction time, and pH of the solution were optimized. A linear relationship between the concentrations of saccharide and the plasmon absorption of gold nanoparticles at 550 nm allowed for quantitative detection of the saccharides formed in solution, from which the hydrolase activity can be calculated. The detection limits for cellulase and xylanase are 0.14 and 0.080 IU mL?1. The results were compared with those of the 3,5-dinitrosalicylic acid method and showed the established method to be reliable and accurate.
Graphical abstract Following enzymatic hydrolysis, reductive saccharides are produced which are capable of directly reducing auric acid to form colloidal gold. The plasmon absorption of the colloidal gold is directly proportional to the amount of reductive saccharides, which can be used to calculate the activity of hydrolase indirectly.
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11.
The article reports an aptamer based assay for cocaine by employing graphene oxide and exonuclease III-assisted signal amplification. It is based on the following scheme and experimental steps: (1) Exo III can digest dsDNA with blunt or recessed 3-terminus, but it has limited activity to ssDNA or dsDNA with protruding 3-terminus; (2) GO can absorb the FAM-labeled ssDNA probe and quench the fluorescence of probe, while the affinity between FAM-labeled mononucleotide and GO is negligible; (3) Cocaine aptamer can be split into two flexible ssDNA pieces (Probe 1 and Probe 2) without significant perturbation of cocaine-binding abilities; (4) The triple complex consisting of Probe 1, Probe 2 and cocaine can be digested by Exo III with the similar efficiency as normal dsDNA. Cocaine aptamer is split into two flexible ssDNA pieces (Probe 2 and 3′-FAM-labeled Probe 1). Cocaine can mediate the cocaine aptamer fragments forming a triplex. The triple complex has unique characteristic with 3′-FAM-labeled blunt end at the Probe 1 and 3′-overhang end at Probe 2. If exonuclease III is added, it will catalyze the stepwise removal of fluorescein (FAM) labeled mononucleotides from the 3-hydroxy termini of the special triplex complex, resulting in liberation of cocaine. The cocaine released in this step can produce a new cleavage cycle, thereby leading to target recycling. Through such a cyclic bound-hydrolysis process, small amounts of cocaine can induce the cleavage of a large number of FAM-labeled probe 1. The cleaved FAM-labeled mononucleotides are not adsorbed on the surface of graphene oxide (GO), so a strong fluorescence signal enhancement is observed as the cocaine triggers enzymatic digestion. Under optimized conditions, the assay allows cocaine to be detected in the 1 to 500 nM concentration range with a detection limit of 0.1 nM. The method was applied to the determination of cocaine in spiked human plasma, with recoveries ranging from 92.0 to 111.8 % and RSD of <12.8 %.
Graphical abstract Aptamer based fluorescent cocaine assay based on graphene oxide and exonuclease III-assisted signal amplification
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12.
A rapid and sensitive aptamer-based assay is described for kanamycin, a veterinary antibiotic with neurotoxic side effects. It is based on a novel FRET pair consisting of fluorescent carbon dots and layered MoS2. This donor-acceptor pair (operated at excitation/emission wavelengths of 380/440 nm) shows fluorescence recovery efficiencies reaching 93 %. By taking advantages of aptamer-induced fluorescence quenching and recovery, kanamycin can be quantified in the of 4–25 μM concentration range, with a detection limit of 1.1 μM. The method displays good specificity and was applied to the determination of kanamycin in spiked milk where it gave recoveries ranging from 85 % to 102 %, demonstrating that the method serves as a promising tool for the rapid detection of kanamycin in milk and other animal-derived foodstuff.
Graphical Abstract A fluorometric aptasensor was developed for the determination of kanamycin. It is based on a novel FRET pair of carbon dots and layered MoS2. The fluorescence recovery efficiency reached 93 % with a good sensitivity, specificity and recoveries in spiked milk.
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13.
A composite consisting of carbon nanotubes (CNT) and copper nanoparticles (CuNPs) was prepared by a chemical reduction method, and its structure characterized by scanning electron microscopy, transmission electron microscopy energy dispersive spectroscopy and FT-IR spectrometry. The hybrid composite was deposited on the surface of a disposable gold electrode that was manufactured from a commercial digital versatile gold disc by a drop casting method. The electrochemical properties of the modified electrode were investigated by cyclic voltammetry and differential pulse voltammetry. The sensor showed an excellent electrocatalytic activity towards oxidation of paracetamol (PA). The calibration plot (with current typically measured at 0.41 V vs. Ag/AgCl) is linear in the 0.5 to 80 μM concentration range, and the detection limit is as low as 10 nM. The sensor was successfully applied to the determination of PA in spiked water and tablet samples where it gave recoveries ranging between 95.25 and 100.5 %.
Graphical abstract Carbon nanotubes (CNT) -copper nanoparticles (CuNPs) hybrid composite was synthesized by a facile method then the nanohybrid was used as a modifier for the DVD gold electrode for improving its performance toward paracetamol electrooxidation. Cyclic voltammetry and differential pulse voltammetry were used for characterization and determination of paracetamol, respectively.
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14.
The article describes the preparation of chitosan-coated hemoglobin (Hb-CS) microcapsules by (a) preparing a CaCO3 precipitate containing Hb, (b) crosslinking Hb with glutaraldehye, (c) coating the particles with chitosan, and (d) preparing Hb-CS microcapsules by removing the CaCO3 template with a solution of disodium EDTA. The morphology and electrochemical properties of the Hb-CS microcapsules were investigated by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. An oxygen sensor was obtained by immobilizing the Hb-CS microcapsules on the surface of a glassy carbon electrode (GCE) first modified with gold nanoparticles. The application of Hb-CS microcapsules facilitates electron transfer on the surface of GCE and warrants the integrity and biological activity of Hb. The oxygen sensor, operated best at a working voltage of ?0.335  V (vs. SCE), displays a low limit of detection (30 nM). The Hb-CS microcapsules also are shown to release loaded oxygen to an anaerobic aqueous environment within 300 min.
Graphical abstract The hemoglobin-chitosan microcapsule shows an excellent ability of electrocatalysis and carrying of oxygen.
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15.
We describe a label-free electrochemical immunosensor for the carcinoembryonic antigen (CEA). It is based on a nanocomposite consisting of electrochemically reduced graphene oxide, gold nanoparticles (AuNPs), and poly(indole-6-carboxylic acid). Coupled to nanoparticle-amplification techniques and modified with ionic liquid (IL), this immunoassay shows high sensitivity and good selectivity for CEA. At the best working voltage of 0.95 V (vs. Ag/AgCl), the lower detection limit is 0.02 ng·mL?1, and the response to CEA is linear in the range from 0.02 to 90 ng·mL?1. The method was applied to the determination of CEA in spiked serum samples and gave recoveries in the range from 98.5 % to 102 %.
Graphical abstract A label-free electrochemical immunosensor was fabricated for the carcinoembryonic antigen (CEA) with a detection limit of 0.02 ng·mL?1. It is based on a nanocomposite consisting of electrochemically reduced graphene oxide (erGO), gold nanoparticles (Au NP), and poly(indole-6-carboxylic acid) (PICA).
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16.
A strategy was developed for the voltammetric determination of the antibiotic drug levofloxacin (LV) based on a glassy carbon electrode modified with a composite consisting of poly(o-aminophenol) and graphene quantum dots (PoAP/GQD) that was fabricated by electropolymerization. The PoAP/GQD composite provides a large surface area and sensing interface and strongly promotes the oxidation current of LV. Under optimal conditions, the modified GCE displays an oxidation peak current (best measured at a working voltage of 1.05 V vs. SCE) that is linearly related to the levofloxacin concentration in the range from 0.05 to 100 μM, and the detection limit is 10 nM (at an S/N of 3). The method was applied to the determination of levofloxacin in spiked milk samples where is gave recoveries between 96.0 and 101.0 %.
Graphical Abstract We describe a one-step electrochemical polymerization method to synthesize a layer of conductive film of poly(o-aminophenol) and graphene quantum dots (PoAP/GQD) onto a glassy carbon electrode (GCE) surface. The composite film exhibited high electro catalytic activity for the quantitative determination of levofloxacin by stripping voltammetry.
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17.
The 16 kDa heat shock protein (16 kDa HSP) against Mycobacterium tuberculosis (MT), expressed during the growth phase of MT, is a potential target in diagnostic tests for tuberculosis (TB). We describe here a method for impedimetric determination of the antigen by using a nanogapped dielectric surface consisting of a silver support coated with a thin finger-shaped coating made from zinc oxide and gold and patterned through a lift-off process. The electrode was characterized by scanning electron microscopy, field emission scanning electron microscopy, atomic force microscopy, and energy-dispersive X-ray spectroscopy. Surface chemical functionalization and immobilization of antibody against the 16 kDa HSP was evidenced by FTIR. In order to improve the detection limit, the antigen was conjugated to 10 nm gold nanoparticles. The resulting biosensor is capable of detecting the 16 kDa HSP in concentrations as low as 100 fM. The method covers a wide analytical range that extends from 100 fM to 1 nM.
Graphical abstract Schematic presentation of the nanogapped impedimetric immunosensor for the diagnosis of tuberculosis
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18.
The authors describe a method for amperometric determination of thiodiglycol (TDG), the main hydrolysis product of sulfur mustard. The electrode consists of a mixture of graphene nanosheets, silver nanoparticles and the ionic liquid octylpyridinium hexafluorophosphate. Electrochemical oxidation of TDG was performed by cyclic voltammetry at pH 4 and revealed a pair of well-defined redox peaks at potentials of 0.43 and 0.19 V (vs. Ag/AgCl). Amperometric detection was accomplished over a dynamic range that is linear in the 10–3700 μM concentration range. The detection limit (at an S/N of 3) is 6 μM. The electrode was applied to the determination of TDG in (spiked) waste water and gave recoveries that ranged from 98.2 to 103.3 %.
Graphical abstract The article describes an amperometric sensor for the determination of thiodiglycol, the main hydrolysis product of sulfur mustard. The electrode was constructed by using graphene nanosheets, silver nanoparticles and an ionic liquid electrode, and it was successfully applied to the determination of thiodiglycol in (spiked) waste water samples.
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19.
The authors describe an aptamer based assay for determination of ractopamine (RAC) by using PicoGreen (PG) as a fluorescent probe specific for dsDNA. In the absence of RAC, the aptamer forms a duplex structure with a complementary sequence that results in enhanced PG fluorescence. Upon binding to RAC, the aptamer undergoes a structural switch. This reduces the number of DNA duplexes formed and causes a reduction of fluorescence intensity of PG as measured at excitation/emission wavelengths of 480/520 nm. Under optimized conditions, the dynamic calibration plot covers the 50 pM to 50 μM concentration range, with a 50 pM detection limit. This meets the safety supervision regulations of the European Commission in terms of residue limits of RAC in food. The method displays high selectivity over other β-adrenergic agonists including clenbuterol, dopamine and salbutamol. The assay was successfully applied to samples of swine urine at spiking levels of 7.4 nM, 22.2 nM and 37 nM. Average recoveries ranged from 95 to 110%, with an RSD of <1.5%. The method is expected to represent a promising tool for simple, rapid and sensitive on-site detection of RAC in animal products.
Graphical abstract An aptamer based fluorescent assay for determination of ractopamine was developed with a dynamic range of 50 pM to 50 μM. The average recovery from spiked urine samples ranged from 95 to 110%, with an RSD of <1.5%.
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20.
A carbon ceramic electrode (CCE) was fabricated from a composite consisting of sol-gel, ceramic graphite, multi-walled carbon nanotubes and the natural carotenoid crocin. The resulting sensor is shown to allow for the determination of NADH at a rather low working potential of 0.22 V (vs. Ag/AgCl). The heterogeneous electron transfer rate constant (ks) and the surface coverage of the modified electrode are 16.8 s?1 and 22 pmol·cm?2, respectively. The sensor shows excellent and linear response in solutions of pH 7.0 over the 0.5 to 100 μM NADH concentration range, a 0.1 μM detection limit, and a sensitivity of 251.3 nA·μM?1·cm?2.
Graphical abstract Schematic of the preparation of a carbon ceramic electrode modified with electropolymerized crocin on multi-walled carbon nanotubes. This sensor has a strongly decreased oxidation overpotential for NADH.
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