We describe a near-infrared (NIR) fluorescent thrombin assay using a thrombin-binding aptamer (TBA) and Zn(II)-activated CuInS2 quantum dots (Q-dots). The fluorescence of Zn(II)-activated Q-dots is quenched by the TBA via photoinduced electron transfer, but if thrombin is added, it will bind to TBA to form G-quadruplexes and the Q-dots are released. As a result, the fluorescence intensity of the system is restored. This effect was exploited to design an assay for thrombin whose calibration plot, under optimum conditions, is linear in the 0.034 to 102 nmol L−1 concentration range, with a 12 pmol L−1 detection limit. The method is fairly simple, fast, and due to its picomolar detection limits holds great potential in the diagnosis of diseases associated with coagulation abnormalities and certain kinds of cancer.
We developed a simple near-infrared fluorescence assay using thrombin binding aptamer (TBA) and Zn(II)-activated CuInS2 quantum dots for the highly selective and sensitive detection of thrombin.
We describe the preparation of carbon dots (CDs) from glucose that possess high stability, a quantum yield of 0.32, and low toxicity (according to an MTT assay). They were used, in combination with the fluorogenic zinc(II) probe quercetin to establish a fluorescence resonance energy transfer (FRET) system for the determination of Zn(II). The CDs are acting as the donor, and the quercetin-Zn(II) complex as the acceptor. This is possible because of the strong overlap between the fluorescence spectrum of CDs and the absorption spectrum of the complex. The method enables Zn(II) to be determined in the 2 to 100 μM concentration range, with a 2 μM detection limit. The method was applied to image the distribution of Zn(II) ions in HeLa cells.
Based on the fluorescence resonance energy transfer (FRET) between carbon dots and quercetin (QCT)-Zn2+, the fluorescence indicator was established, which displays high sensitivity and selectivity in the detection of Zn2+. The method was also applied to image the distribution of Zn(II) ions in HeLa cells.
We describe a reversible fluorescent DNA–based INHIBIT logic gate for the determination of silver(I) and iodide ions using graphene oxide (GO) as a signal transducer and Ag(I) and iodide as mechanical activators. The basic performance, optimized conditions, sensitivity and selectivity of the logic gate were investigated and revealed that the method is highly sensitive and selective over potentially interfering ions. The limits of detection for Ag(I) and iodide are 10 nM and 50 nM, respectively. This logic gate was successfully applied to the determination of Ag(I) and iodide in (spiked) tap water and river water. It was also used for the determination of iodide in human urine samples with satisfactory results. Compared to other methods, this INHIBIT logic gate is simple in design and has small background interference.
A simple and reversible fluorescent DNA-based INHIBIT logic gate is designed by using graphene oxide as a signal transducer and silver ions and iodide as mechanical activators.
We describe a method for the preparation of water-soluble gold nanoclusters (Au-NCs) from chloroauric acid using denatured-casein as both a reducing and stabilizing agent. The resulting Au-NCs were characterized by photoluminescence, UV–vis absorption, and X-ray photoelectron spectroscopies, and by transmission electron microscopy. The Au-NCs have an average diameter of 1.7 ± 0.2 nm and exhibit orange-red fluorescence emission peaking at 600 nm (with a Stokes’ shift as large as 237 nm), a quantum yield of 4.3 %, and good stability over the physiologically relevant range of pH values and ionic strength. Cytotoxicity studies showed the Au-NCs to display negligible effects in terms of altering cell proliferation or triggering apoptosis. Fluorescence imaging of HeLa cancer cells was accomplished by loading such cells with the Au-NCs. The fluorescence of the Au-NCs is found to be strongly quenched by Hg(II) ions, and thus the Au-NCs can be used for detecting and, possibly, imaging of Hg(II). An assay was worked out for the determination of Hg(II), and its limit of detection is 1.83 nM, which is 5.5 times lower than the maximum allowed concentration of Hg(II) in drinking water as defined by the US EPA.
Denatured-casein was firstly used as a reductant and stabilizing agent in facile preparation of orange-red fluorescent AuNCs for imaging of HeLa cells and for the quantitation of mercury(II)
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.
We describe a sensitive method for the immunochromatographic determination of aflatoxin B1. It is based on the following steps: 1) Competitive interaction between non-labeled specific primary antibodies and target antigens in a sample and in the test zone of a membrane; 2) detection of the immune complexes on the membrane by using a secondary antibodies labeled with gold nanoparticles. The method enables precise adjustment of the required quantities of specific antibodies and the colloidal (gold) marker. It was applied in a lateral flow format to the detection of aflatoxin B1 and exhibits a limit of detection (LOD) of 160 pg · mL−1 if detected visually, and of 30 pg · mL−1 via instrumental detection. This is significantly lower than the LOD of 2 ng · mL−1 achieved by conventional lateral flow analysis using the same reagents.
Immunochromatography with secondary labeled antibodies caused 10-fold decrease of detection limit
This article describes a fluorescent molecularly imprinted polymer (MIP) capable of selective fluorescent turn-on recognition of the tumor biomarker α-fetoprotein. The technique is making use of amino-modified Mn-doped ZnS quantum dots (QDs) as solid supports, 4-vinylphenylboronic acid and methyl methacrylate as the functional monomers, γ-methacryloxypropyl trimethoxysilane as the grafting agent, and α-fetoprotein as a template. A graft imprint is created on the surface of the QDs. The functional monomers are shown to play an important role in the formation of the binding sites and in preventing nonspecific protein binding. The resulting MIP-QDs display a good linear response to α-fetoprotein in the 50 ng · L−1 to 10 μg · L−1 concentration range, and the limit of detection is 48 ng · L−1. In our perception, the method has a wide scope in that it may be adapted to various other glycoproteins.
Schematic illustration of the synthesis of the MIP-QDs composites
A fluorescent probe for the sensitive and selective determination of sulfide ions is presented. It is based on the use of graphene quantum dots (GQDs) which emit strong and stable blue fluorescence even at high ionic strength. Copper(II) ions cause aggregation of the GQDs and thereby quench fluorescence. The GQDs-Cu(II) aggregates can be dissociated by adding sulfide ions, and this results in fluorescence turn on. The change of fluorescence intensity is proportional to the concentration of sulfide ions. Under optimal conditions, the increase in fluorescence intensity on addition of sulfide ions is linearly related (r 2 = 0.9943) to the concentration of sulfide ions in the range from 0.20 to 20 μM, and the limit of detection is 0.10 μM (at 3 σ/s). The fluorescent probe is highly selective for sulfide ions over some potentially interfering ions. The method was successfully applied to the determination of sulfide ions in real water samples and gave recoveries between 103.0 and 113.0 %.
Graphene quantum dots (GQDs) emit strong blue fluorescence which, however, is quenched by copper(II) ions due to the formation of GQDs-Cu(II) aggregates. Fluorescence is recovered by sulfide ions due to the dissociation of GQDs-Cu(II) aggregates.
A molecularly imprinted polymer (MIP) was prepared by self-polymerization of dopamine in the presence of bovine hemoglobin (BHb) and then deposited on the surface of an electrode modified with gold nanoparticles (AuNPs). Scanning electron microscopy, cyclic voltammetry, and differential pulse voltammetry were employed to characterize the modified electrode using the hexacyanoferrate redox system as an electroactive probe. The effects of BHb concentration, dopamine concentration, and polymerization time were optimized. Under optimized conditions, the modified electrode selectively recognizes BHb even in the presence of other proteins. The peak current for hexacyanoferrate, typically measured at + 0.17 V (vs. SCE), depends on the concentration of BHb in the 1.0 × 10−11 to 1.0 × 10−2 mg mL−1 range. Due to the ease of preparation and tight adherence of polydopamine to various support materials, the present strategy conceivably also provides a platform for the recognition and detection of other proteins.
Gold nanoparticles and molecularly imprinted self-polymerization dopamine were modified on gold electrode surface to recognize and determine bovine hemoglobin. Under the optimized conditions, the modified electrode showed specific adsorption, selective recognition, and sensitive detection of bovine hemoglobin.
We report on the capillary electrophoretic behavior of citrate-capped gold and silver nanoparticles in aqueous medium when applying a ligand-exchange surface reaction with thiols. Gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) of similar size (39 ± 6 and 41 ± 7 nm, respectively) and shape were synthesized, covered with a citrate shell, and characterized by microscopic and spectroscopic techniques. The analysis of these NPs by CE was accomplished by using a buffer solution (pH 9.7; 40 mM SDS, 10 mM CAPS; 0.1 % methanol) containing the anions of thioctic acid or thiomalic acid. These are capable of differently interacting with the surface of the AuNPs and AgNPs and thus introducing additional negative charges. This results in different migration times due to the formation of differently charged nanoparticles.
Capillary electrophoretic behavior of citrate-capped gold and silver nanoparticles (NPs) in aqueous medium when applying a ligand-exchange surface reaction with thiols (thioctic and thiomalic acids), which introduces additional negative charges, has been studied
We report on an electrochemical method for the determination of the activity of trypsin. A multi-functional substrate peptide (HHHAKSSATGGC-HS) is designed and immobilized on a gold electrode. The three His residues in the N-terminal are able to recruit thionine-loaded graphene oxide (GO/thionine), a nanocover adopted for signal amplification. Once the peptide is cleaved under enzymatic catalysis by trypsin (cleavage site: Lys residue), the His residues leave the electrode, and the GO/thionine cannot cover the peptide-modified electrode anymore. Thus, the changes of the electrochemical signal of thionine, typically acquired at a voltage of -0.35 V, can be used to determine the activity of trypsin. A detection range of 1 × 10−4 to 1 U, with a detection limit of 3.3 × 10−5 U, can be achieved, which is better than some currently available methods. In addition, the method is highly specific, facile, and has the potential for the detection of trypsin-like proteases.
Graphene oxide was adopted as a nanocover for the development of a sensitive electrochemical method to detect the activity of trypsin.
Copper nanoclusters (Cu-NCs) were fabricated by chemical reduction of Cu(II) ions using formaldehyde as the reductant and poly(vinyl pyrrolidone) as the protecting agent. The resulting Cu-NCs were characterized by TEM, FT-IR, UV–vis and XPS and fluorescence spectroscopy. The Cu-NCs display a luminescence quantum yield of about 13 %, and the emission peaks shift from 398 to 457 nm on increasing the excitation wavelength from 310 to 390 nm. The Cu-NCs possess a storage stability of at least 2 months and are stable in the presence of high concentrations of salt. Their fluorescence is strongly quenched by hypochlorite, while other common cations, anions and hydrogen peroxide have minor (or no) effects on fluorescence. On this basis, a fluorometric hypochlorite assay was developed that has a 0.1 μM detection limit and a linear range that extends from 1 to 30 μM. The method was successfully used to the determination of hypochlorite in local tap water samples, and the results agreed well with those obtained by a colorimetric method.
Hypochlorite ion is found to quench the fluorescence copper nanoclusters synthesized by reduction of Cu(II) ion by formaldehyde in the presence of poly(vinyl pyrrolidone) serving as protecting agent. This finding resulted in a new fluorescence assay for hypochlorite.
We report on an ultrasensitive fluorescence immunoassay for human chorionic gonadotrophin antigen (hCG). It is based on the use of silica nanoparticles coated with a copolymer (prepared from a fluorene, a phenylenediamine, and divinylbenzene; PF@SiO2) that acts as a fluorescent label for the secondary monoclonal antibody to β-hCG antigen. In parallel, Fe3O4 nanoparticles were coated with polyaniline, and these magnetic particles (Fe3O4@PANI) served as a solid support for the primary monoclonal antibody to β-hCG antigen. The PF@SiO2 exhibited strong fluorescence and good dispersibility in water. A fluorescence sandwich immunoassay was developed that enables hCG concentrations to be determined in the 0.01–100 ng·mL−1 concentration range, with a detection limit of 3 pg·mL−1.
Fluorescence detection of prepared immune reagent nano-composites using the fluorescence cell
We report on a facile immunoassay for porcine circovirus type 2 (PCV2) based on surface enhanced Raman scattering (SERS) using multi-branched gold nanoparticles (mb-AuNPs) as substrates. The mb-AuNPs in the immunosensor act as Raman reporters and were prepared via Tris base-induced reduction and subsequent reaction with p-mercaptobenzoic acid (pMBA). They possess good stability and high SERS activity. Subsequently, the modified mb-AuNPs were covalently conjugated to the monoclonal antibody (McAb) against the PCV2 cap protein to form SERS immuno nanoprobes. These were captured in a microtiterplate via a immunoreaction in the presence of target antigens. The effects of antibody concentration, reaction time and temperature on the sensitivity of the immunoassay were investigated. Under optimized assay conditions, the Raman signal intensity at 1,076 cm−1 increases logarithmically with the concentrations of PCV2 in the concentration ranging from 8 × 102 to 8 × 106 copies per mL. The limit of detection is 8 × 102 copies per mL. Compared to conventional detecting methods such as those based on PCR, the method presented here is rapid, facile and very sensitive.
Acetylcholinesterase (AChE) from Electrophorus electricus was immobilized on the surface of amino-modified magnetic beads (AChE-MB), and its activity evaluated by the quantification of acetylcholine hydrolysis. A reference mixture composed of AChE binders (galanthamine and a probe coumarin, K i = 0.031 ± 0.010 μM) and non-binders (ketamine and propranolol) was used to probe the fishing assay. The performance of the bioconjugation assay was demonstrated with a library of 12 reference coumarins from which two ligands were directly identified by LC-MS/MS in a single assay, demonstrating the usefulness of this approach.
A bioconjugate-screening assay with AChE-modified magnetic beads was developed to direct identification of AChE binders, in mixtures, by LC-MS/MS. A reference mixture of twelve coumarins was used and, the two ligands were identified.
Magnetic Fe3O4@SiO2 core shell nanoparticles containing diphenylcarbazide in the shell were utilized for solid phase extraction of Hg(II) from aqueous solutions. The Hg(II) loaded nanoparticles were then separated by applying an external magnetic field. Adsorbed Hg(II) was desorbed and its concentration determined with a rhodamine-based fluorescent probe. The calibration graph for Hg(II) is linear in the 60 nM to 7.0 μM concentration range, and the detection limit is at 23 nM. The method was applied, with satisfying results, to the determination of Hg(II) in industrial waste water.
We report on the preparation of fluorescent silica nanoparticles (SiNPs) modified with chitosan and lucigenin by using a reverse microemulsion method. The introduction of chitosan to the lucigenin doped SiNPs is shown to improve the fluorescence quantum yield. The modified SiNPs were used as fluorescent markers in an aptamer-based method for selective determination of thrombin. In this protocol, thrombin was sandwiched between streptavidin-coated magnetic beads and the fluorescent SiNPs modified with a thrombin-binding aptamer. The method was successfully applied to the determination of thrombin in human serum and showed a detection limit as low as 0.02 nM. In our perception, the protocol presented here is promising in that such SiNPs may be applied to the sensitive fluorescent detection of other analytes by changing the corresponding aptamer.
The introduction of chitosan to the lucigenin doped SiNPs is shown to improve the fluorescence quantum yield. The modified SiNPs were used as fluorescent markers in an aptamer-based method for selective determination of thrombin. The effect of chitosan concentration on fluorescence intensity of lucigenin/SiO2 nanoparticles (the volume of chitosan solution is 100 μL)
Microparticles were synthesized by suspension copolymerization of the synthetic ionic liquid (IL) 1-allyl-3-methyl-imidazolium bromide with ethylene glycol dimethacrylate. The particles have a regular spherical shape and an average diameter of 65 ± 24 μm. Their affinity for the fluoroquinolone antibiotics ofloxacin (OFL), lomefloxacin (LOM) and ciprofloxacin (CIP) is much higher than that of the blank polymer (not containing an IL), of polymers using methacrylic acid as functional monomer, of hydrophilic-lipophilic balanced sorbents, and of C18 sorbents. The microparticles were applied to the solid-phase extraction and rapid preconcentration of the fluoroquinolones from urine which then were quantified by HPLC. The calibration plot covers the 0.05 to 20 μg mL−1 concentration range, and the average recoveries at three spiking levels range from 93.6 to 103.7 %, with RSD of ≤5.7 %. The method was successfully applied to the determination of fluoroquinolones in spiked urine.
Microparticles covalently functionalized with an ionic liquid ([Amim][Br]) were synthesized by suspension copolymerization and show higher affinity for fluoroquinolones than other sorbents. The microparticles were used as a sorbent for solid-phase extraction and preconcentration of three fluoroquinolones from urine.
This article describes an electrochemical immunosensor for rapid determination of Salmonella pullorum and Salmonella gallinarum. The first step in the preparation of the immunosensor involves the electrodeposition of gold nanoparticles used for capturing antibody and enhancing signals. In order to generate a benign microenvironment for the antibody, the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate was used to modify the surface of a screen-printed carbon electrode (SPCE). The single steps of modification were monitored via cyclic voltammetry and electrochemical impedance spectroscopy. Based on these findings, a sandwich immunoassay was worked out for the two Salmonella species by immobilizing the respective unlabeled antibodies on the SPCE. Following exposure to the analytes, secondary antibody (labeled with HRP) is added to form the sandwich. After adding hydrogen peroxide and thionine, the latter is oxidized and its signal measured via CV. A linear response to the Salmonella species is obtained in the 104 to 109 cfu · mL−1 concentration range, and the detection limits are 3.0 × 103 cfu · mL−1 for both species (at an SNR of 3). This assay is sensitive, highly specific, acceptably accurate and reproducible. Given its low detection limit, it represents a promising tool for the detection of S. pullorum, S. gallinarum, and - conceivably - of other food-borne pathogens by exchanging the antibody.
We describe an electrochemical sandwich assay based on a screen-printed carbon electrode, gold nanoparticles and ILs and capable of detecting Salmonella pullorum and Salmonella gallinarum. The preparation is outlined in the Schematic.
