We describe a single-step solvothermal method for the preparation of nanocomposites consisting of graphene oxide and Fe3O4 nanoparticles (GO/Fe3O4). This material is shown to be useful as a magnetic sorbent for the extraction of flavonoids from green tea, red wine, and urine samples. The nanocomposite is taking advantage of the high surface area of GO and the magnetic phase separation feature of the magnetic sorbent. The nanocomposite is recyclable and was applied to the extraction of flavonoids prior to their determination by HPLC. The effects of amount of surfactant, pH value of the sample solution, extraction time, and desorption condition on the extraction efficiency, and the regeneration conditions were optimized. The limits of detection for luteolin, quercetin and kaempferol range from 0.2 to 0.5 ng∙ mL−1 in urine, from 3.0 to 6.0 ng∙mL−1 in green tea, and from 1.0 to 2.5 ng∙mL−1 in red wine. The recoveries are between 82.0 and 101.4 %, with relative standard deviations of <9.3 %.
The article describes a method for magnetic solid-phase extraction (MSPE) of trace amounts of natural substances in complex samples by using graphene oxide (GO)-Fe3O4nanoparticles as the sorbent.
High molecular-weight silk peptide (SP) was used to functionalize the surface of nanosheets of reduced graphene oxide (rGO). The SP-rGO nanocomposite was then mixed with mouse anti-human prostate specific antigen monoclonal antibody (anti-PSA) and coated onto a glassy carbon electrode to fabricate an immunosensor. By using the hexacyanoferrate redox system as electroactive probe, the immunosensor was characterized by voltammetry and electrochemical impedance spectroscopy. The peak current, measured at the potential of 0.24 V (vs. SCE), is distinctly reduced after binding prostate specific antigen (PSA). Response (measured by differential pulse voltammetry) is linearly related to PSA concentration in the range from 0.1 to 5.0 ng · mL−1 and from 5.0 to 80.0 ng∙mL−1, and the detection limit is 53 pg∙mL−1 (at an SNR of 3). The immunosensor was successfully applied to the determination of PSA in clinical serum samples, and the results were found to agree well with those obtained with an enzyme-linked immunosorbent assay.
Nanosheets of reduced graphene oxide were functionalized with silk peptide and used to immobilize anti-PSA to fabricate an immunosensor for PSA.
We describe a nanostructured immunosensor for the cardiovascular biomarker netrin 1. A glassy carbon electrode was consecutively modified with multi-walled carbon nanotubes (MWCNTs), nafion (to retain the MWCNTs), thionine-coated gold nanoparticles (Thi@AuNPs), and monoclonal antibodies against netrin 1. The modified electrode was characterized by transmission electron microscopy, cyclic voltammetry, differential pulse voltammetry, UV-visible spectrophotometry and X-ray diffraction. The presence of Thi@AuNPs warrants direct and convenient immobilization of the antibody. This immunoelectrode enables netrin 1 to be determined, best at a voltage of −300 mV (vs. SCE), with a limit of detection of 30 fg mL−1 (at an S/N ratio of 3) after a 50 min incubation time. The detection range extends from 0.09 to 1800 pg∙mL−1. The method is simple, sensitive, specific and reproducible. We presume this stable and reproducible biosensor to be useful for the early detection of cardiovascular diseases.
A high sensitivity immunoassay was developed for the detection of netrin 1 based on multi-walled carbon nanotubes, thionine and gold nanoparticles. Its excellent performance is ascribed to the good conductivity of MWCNTs and the combination of materials.
A nanoporous carbon derived from an aluminum-based metal-organic framework was deposited on stainless steel wires in a sol–gel matrix. The resulting fibers were applied to the solid-phase microextraction of the polycyclic aromatic hydrocarbons (PAHs) naphthalene, acenaphthene, fluorene, phenanthrene and anthracene from water and soil samples. The fiber was then directly inserted into the GC injector and the PAHs were quantified by GC-MS. The effects of salt addition, extraction temperature, extraction time, sample volume and desorption conditions on the extraction efficiency were optimized. A linear response to the analytes was observed in the 0.1 to 12 μg∙L−1 range for water samples, and in the 0.6 to 30 μg∙kg−1 for soil samples, with the correlation coefficients ranging from 0.9934 to 0.9985. The limits of detection ranged from 5.0 to 20 ng∙L−1 for water samples, and from 30 to 90 ng∙kg−1 for soil samples. The recoveries of spiked samples were between 72.4 and 108.0 %, and the precision, expressed as the relative standard deviations, is <12.8 %.
A MOF derived nanoporous carbon coated fiber for use in solid-phase microextraction was prepared via sol–gel technology. The coated fiber has a porous, rough and wrinkled structure, and shows a high thermal stability, good extraction repeatability and long lifetime. The established HS-SPME-GC-MS method is suitable for the determination of the PAHs from water and soil samples.
A sensitive, specific and rapid colorimetric aptasensor for the determination of the plasticizer bisphenol A (BPA) was developed. It is based on the use of gold nanoparticles (AuNPs) that are positively charged due to the modification with cysteamine which is cationic at near-neutral pH values. If aptamers are added to such AuNPs, aggregation occurs due to electrostatic interactions between the negatively-charged aptamers and the positively-charged AuNPs. This results in a color change of the AuNPs from red to blue. If a sample containing BPA is added to the anti-BPA aptamers, the anti-BPA aptamers undergo folding via an induced-fit binding mechanism. This is accompanied by a conformational change, which prevents the aptamer-induced aggregation and color change of AuNPs. The effect was exploited to design a colorimetric assay for BPA. Under optimum conditions, the absorbance ratio of A 527/A 680 is linearly proportional to the BPA concentration in the range from 35 to 140 ng∙mL−1, with a detection limit of 0.11 ng∙mL−1. The method has been successfully applied to the determination of BPA in spiked tap water and gave recoveries between 91 and 106 %. Data were in full accordance with results obtained from HPLC. This assay is selective, easily performed, and in our perception represents a promising alternative to existing methods for rapid quantification of BPA.
The negatively-charged anti-BPA aptamers can absorb onto the positively-charged cysteamine-capped AuNPs (cysteamine-AuNPs) via electrostatic interactions, which can cause the aggregation of AuNPs accompanied by a red-to-blue color change. In the presence of BPA, the specific binding of BPA to the aptamers induces the conformation changes of anti-BPA aptamers, which can release the aptamers from cysteamine-AuNPs and thus prevent the aggregation and color change of cysteamine-AuNPs.
A label-free and single-step method is reported for rapid and highly sensitive detection of bisphenol A (BPA) in aqueous samples. It utilizes an aptamer acting as a probe molecule immobilized on a commercially available array of interdigitated aluminum microelectrodes. BPA was quantified by measuring the interfacial capacitance change rate caused by the specific binding between bisphenol A and the immobilized aptamer. The AC signal also induces an AC electrokinetic effect to generate microfluidic motion for enhanced binding. The capacitive aptasensor achieves a limit of detection as low as 10 fM(2.8 fg ⋅ mL − 1) with a 20 s response time. The method is inexpensive, highly sensitive, rapid and therefore provides a promising technology for on-site detection of BPA in food and water samples.
A. AC electrokinetics effect plays a vital role in BPA detection by introducing microfluidic movement to accelerate the molecular transport to the electrode surface.
B. The ACEK capacitive aptasensor has a limit of detection as low as 10 fM (2.8 fg ⋅ mL − 1) with a 20-s response time.
We describe an aptamer-based colorimetric assay for chloramphenicol (CAP) based on the ability of anti-single-stranded DNA antibody (anti-ssDNA Ab) to recognize ssDNA, and the catalytic ability of PowerVision (PV), which is a polymeric conjugate of horseradish peroxidase and antibody with a high enzyme-to-antibody ratio. The complementary DNA of the aptamer (cDNA) was immobilized on magnetic gold nanoparticles (Fe3O4@Au) and used as a capture probe (AuMNPs-cDNA). The ssDNA Ab and PV were conjugated to AuNPs to form signal tags that recognize ssDNA with anti-ssDNA Ab to form beads containing the amplified probe (AuMNPs-cDNA@anti-ssDNA Ab/PV-AuNPs). The PV on their surface catalyzes the oxidation of the substrate 3,3’,5,5’-tetramethylbenzidine to produce a color change which is quantified by absorptiometry at 652 nm. The assay has a linear calibration plot for CAP in the 0.01 to 100 ng mL−1 range, with a detection limit as low as 3 pg mL−1. The method was successfully employed to detect CAP in real samples. Results were consistent with data obtained using a conventional enzyme-linked immunosorbent assay.
PowerVision- labeled gold nanoparticles acting as signal tag catalyze the H2O2-mediated oxidation of TMB for color development, which can be observed by bare eyes and quantified by ultraviolet-visible spectroscopy.
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
We have studied the direct electrochemistry of glucose oxidase (GOx) immobilized on electrochemically fabricated graphite nanosheets (GNs) and zinc oxide nanoparticles (ZnO) that were deposited on a screen printed carbon electrode (SPCE). The GNs/ZnO composite was characterized by using scanning electron microscopy and elemental analysis. The GOx immobilized on the modified electrode shows a well-defined redox couple at a formal potential of −0.4 V. The enhanced direct electrochemistry of GOx (compared to electrodes without ZnO or without GNs) indicates a fast electron transfer at this kind of electrode, with a heterogeneous electron transfer rate constant (Ks) of 3.75 s−1. The fast electron transfer is attributed to the high conductivity and large edge plane defects of GNs and good conductivity of ZnO-NPs. The modified electrode displays a linear response to glucose in concentrations from 0.3 to 4.5 mM, and the sensitivity is 30.07 μA mM−1 cm−2. The sensor exhibits a high selectivity, good repeatability and reproducibility, and long term stability.
Graphical representation for the fabrication of GNs/ZnO composite modified SPCE and the immobilization of GOx
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.
A nanocomposite consisting of reduced graphene oxide decorated with palladium-copper oxide nanoparticles (Pd-CuO/rGO) was synthesized by single-step chemical reduction. The morphology and crystal structure of the nanocomposite were characterized by field-emission scanning electron microscopy, high resolution transmission electron microscopy and X-ray diffraction analysis. A 3-electrode system was fabricated by screen printing technology and the Pd-CuO/rGO nanocomposite was dropcast on the carbon working electrode. The catalytic activity towards glucose in 0.2 M NaOH solutions was analyzed by linear sweep voltammetry and amperometry. The steady state current obtained at a constant potential of +0.6 V (vs. Ag/AgCl) showed the modified electrode to possess a wide analytical range (6 μM to 22 mM), a rather low limit of detection (30 nM), excellent sensitivity (3355 μA∙mM−1∙cm−2) and good selectivity over commonly interfering species and other sugars including fructose, sucrose and lactose. The sensor was successfully employed to the determination of glucose in blood serum.
A highly sensitive nonenzymatic electrochemical sensor was fabricated using a Pd-CuO composite with reduced graphene oxide. The sensor has a wide detection range and was used to sense glucose in blood serum
We describe a novel magnetic nanosorbent that consists of nanowires consisting of a core of metallic iron and an iron (III) oxide shell. These nanowires were then deposited on graphene oxide to form a composite of the type Fe@Fe2O3/GO. Specifically, the magnetic composite is formed via electrostatic interaction between negatively charged GO nanosheets and positively charged Fe@Fe2O3 nanowires in aqueous solution. The material was successfully applied to the extraction of the endocrine-disrupting phenols bisphenol A, triclosan and 2,4-dichlorophenol from water samples. Compared to neat graphene oxide, the composite material exhibits improved properties in terms of microextraction where both the hydrophilic graphene oxide and the Fe@Fe2O3 nanowires participate in the adsorption of the hydrophilic analytes. The amount of adsorbent, pH of water sample, extraction time and desorption time, type and volume of desorption solution were optimized. Following extraction for the absorbent, the phenols were quantified by HPLC. The three phenols can be determined in 0.5 to 100 ng∙mL−1 concentration range, with limits of detection (at an S/N ratio of 3) ranging from 0.08 to 0.10 ng∙mL−1. The repeatability was investigated by evaluating the intra- and inter-day precisions with relative standard deviations of lower than 7.5 % (n = 5). The recoveries from spiked real water samples were in the range from 84.8 to 92.0 %. The results indicate that the novel material can be successfully applied to the extraction and analysis of phenols from water samples.
Scheme 1 procedure for the synthesis of Fe@Fe203/G0
We report on a novel graphene-based nanoarchitecture modified with plasma-polymerized propargylamine (G-PpPG) and its application in electrochemical sensors for DNA. Films of G-PpPG were characterized by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The presence of graphene enhances the electrochemical activity of the films, and the high density of amino groups (deposited at a low plasma input power) on their surface assists in the immobilization of probe DNA on the water-swollen polymeric network. By contrast, the degree of hybridization of the total complementary target DNA to the probe DNA remains unchanged when G-PpPG nanofilms prepared at higher input power. No substantial non-specific adsorption of totally mismatched target DNA on the polymer films is observed because of the complete coverage of the probe DNA. The detection limit for total complementary target DNA is approximately 1.84 nmol · L−1. The dynamic range extends from 0.1 to 1,000 nmol · L−1. The new nanocomposite may also be used to immobilize other probe DNA sequences, and this makes the approach potentially applicable to the detection of other oligomers.
Preparing the DNA sensor made from the graphene-based nanoarchitecture modified by using PpPG (G-PpPG) includes the following processes: (a) Modifying the Au electrode with the graphene nanosheet, (b) depositing the PpPG film onto the Au electrode coated with graphene, (c) immobilizing the probe DNA onto the G-PpPG film, and (d) hybridizing the MM0 target with the G-PpPG film immobilized with P1
We describe an electrochemical immunoassay for the Cry1Ab toxin that is produced by Bacillus thuringiensis. It is making use of a nanobody (a heavy-chain only antibody) that was selected from an immune phage displayed library. A biotinylated primary nanobody and a HRP-conjugated secondary nanobody were applied in a sandwich immunoassay where horseradish peroxidase (HRP) is used to produce polyaniline (PANI) from aniline. PANI can be easily detected by differential pulse voltammetry at a working voltage as low as 40 mV (vs. Ag/AgCl) which makes the assay fairly selective. This immunoassay for Cry1Ab has an analytical range from 0.1 to 1000 ng∙mL-1 and a 0.07 ng∙mL-1 lower limit of detection. The average recoveries of the toxin from spiked samples are in the range from 102 to 114 %, with a relative standard deviation of <7.5 %. The results demonstrated that the assay represented an attractive alternative to existing immunoassays in enabling affordable, sensitive, robust and specific determination of this toxin.
Nanobodies specific to Cry1Ab toxin were isolated from an immunized camel. A biotinylated primary nanobody and a HRP-conjugated secondary nanobody were applied in a sandwich immunoassay with horseradish peroxidase being used to produce polyaniline, which can be easily detected by differential pulse voltammetry.
We report on the time-resolved detection of the three fluoroquinolone (FQs) antibiotics ciprofloxacin (CIP), enrofloxacin (ENR) and flumequine (FLU). On addition of terbium(III) ions, the terbium(III)-FQs chelates are formed in-situ in an on-capillary derivatization reaction of a microfluidic system. The laser-induced terbium(III)-sensitized luminescence of the chelates is measured at excitation/emission wavelengths of 337/545 nm. The analytes can be separated and quantified within less than 4 min. A solid phase extraction step for analyte preconcentration can be included prior to chelation and microchip capillary electrophoresis. The analytical ranges of the calibration graphs for CIP, ENR and FLU are from 10.6 to 60.0, 10.3 to 51.0, and 11.5 to 58.8 ng mL−1, respectively, and the detection limits are 3.2, 3.1 and 3.6 ng mL−1, respectively. The precision was established at two concentration levels of each analyte and revealed relative standard deviations in the range from 3.0 to 10.2 %. The method was applied to the analysis of FQ-spiked water samples.
We report on the time-resolved detection of the three fluoroquinolone antibiotics. The analytes can be separated and quantified within less than 4 min. A solid phase extraction step for analyte preconcentration can be included prior to chelation and microchip capillary electrophoresis.
We describe a turn-on electrochemical biosensor for the detection of methyltransferases (MTases) causing DNA adenine methylation. This biosensor is based on insertion, methylation-resistant cleavage, signal enrichment caused by gold nanoparticles (AuNPs), and a signal probe-dragging strategy. A double-stranded DNA (dsDNA) containing identical MTase and methylation-resistant endonuclease (Mbo I) sites was immobilized on the surface of a gold electrode via Au-S covalent binding. The surface was subsequently treated with MTase and Mbo I and then washed. Results revealed that the surface of the electrode contains methylated dsDNA and 12-base nucleotides residual. Depending on biotin-streptavidin interactions that enabled signal probes and nucleotide residue hybridization and AuNP enrichment, a large number of signal probes labeled with ferrocene (Fc) are captured by the electrode. Under optimal conditions, the differential pulse voltammetry signals of Fc tags (at a working voltage of 0.24 V vs. Ag/AgCl) are linearly related to the log of the MTase activity in the 0.1 to 40 U·mL−1 range. The dynamic range extends from 0.05 to 50 U·mL−1, and the limit of detection is 0.024 U·mL−1 (at an S/N ratio of 3). The assay is well reproducible and highly selective. In our perception, this strategy provides a promising approach for simple, sensitive and selective detection of Dam MTase and may be extended to the determination of other MTase by exchanging the corresponding DNA.
Proximity-based electrochemical biosensor for highly sensitive detection of DNA adenine methylation methyltransferase (Dam MTase) activity using methylation-resistant cleavage coupled with gold nanoparticle based cooperative signal amplification.
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 describe the electrochemical preparation of bismuth nanoribbons (Bi-NRs) with an average length of 100 ± 50 nm and a width of 10 ± 5 μm by a potentiostatic method. The process occurs on the surface of a glassy carbon electrode (GCE) in the presence of disodium ethylene diamine tetraacetate that acts as a scaffold for the growth of the Bi-NRs and also renders them more stable. The method was applied to the preparation of Bi-NRs incorporated into reduced graphene oxide. This nanocomposite was loaded with the enzyme glucose oxidase onto a glassy carbon electrode. The resulting biosensor displays an enhanced redox peak for the enzyme with a peak-to-peak separation of about 28 mV, revealing a fast electron transfer at the modified electrode. The loading of the GCE with electroactive GOx was calculated to be 8.54 × 10−10 mol∙cm−2, and the electron transfer rate constant is 4.40 s−1. Glucose can be determined (in the presence of oxygen) at a relatively working potential of −0.46 V (vs. Ag|AgCl) in the 0.5 to 6 mM concentration range, with a 104 μM lower detection limit. The sensor also displays appreciable repeatability, reproducibility and remarkable stability. It was successfully applied to the determination of glucose in human serum samples.
A potentiostatic method was used to prepare reduced graphene oxide and bismuth nanoribbons nanocomposite on a glassy carbon electrode. This nanocomposite was loaded with enzyme glucose oxidase to fabricate a glucose biosensor.
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 describe the preparation of nanoporous carbon using a metal-organic framework (MOF) as a template and furfuryl alcohol as the source for carbon. The MOF consists of a zeolitic framework (ZIF-8) that was obtained from 2-methylimidazole and Zn(II) ions. ZIF-8 was soaked with furfuryl alcohol which then was carbonized at 900 °C. The resulting nanoporous carbon (MOF-C) exhibits a high specific surface area and a large pore volume. It was used as a dispersive solid-phase adsorbent for the preconcentration of the benzoylurea insecticides diflubenzuron, triflumuron, hexaflumuron and teflubenzuron from water and tangerine samples. Under optimized conditions, the methods exhibits excellent extraction performance. The insecticides can be quantified via HPLC with UV detection in the 0.5 to 100 ng mL−1 concentration range in case of spiked tap water, and in the 2.0 to 200 ng g−1 concentration range in case of tangerines. The limits of detection range from 0.10 to 0.23 ng mL−1 in case of water samples, and from 0.34 to 0.71 ng g−1 for tangerine sample (at an S/N ratio of 3). Mean recoveries range from 91.7 to 107.9 %, with relative standard deviations of <7.1 %. The results indicate that the method was efficient for the preconcentration of trace levels of benzoylurea insecticides from water and tangerine samples. Conceivably, this new adsorbent has a large potential with respect to the enrichment of other organic pollutants from various kinds of samples.
Nanoporous carbon was prepared using metal-organic framework (zeolitic imidazolate framework, ZIF-8) as template with furfuryl alcohol as carbon precursor, and was used as dispersive solid phase extraction adsorbent for the enrichment of some benzoylurea insecticides from water and tangerine samples.