The authors describe the synthesis of a multifunctional nanocomposite with an architecture of type Fe3O4@SiO2@graphene quantum dots with an average diameter of about 22 nm. The graphene quantum dots (GQDs) were covalently immobilized on the surface of silica-coated magnetite nanospheres via covalent linkage to surface amino groups. The nanocomposite displays a strong fluorescence (with excitation/emission peaks at 330/420 nm) that is fairly selectively quenched by Hg2+ ions, presumably due to nonradiative electron/hole recombination annihilation. Under the optimized experimental conditions, the linear response to Hg2+ covers the 0.1 to 70 μM concentration range, with a 30 nM lower detection limit. The high specific surface area and abundant binding sites of the GQDs result in a good adsorption capacity for Hg2+ (68 mg?g?1). The material, due to its superparamagnetism, can be separated by using a magnet and also is recyclable with EDTA so that it can be repeatedly used for simultaneous detection and removal of Hg2+ from contaminated water.
Graphical abstract A schematic view of preparation process for the Fe3O4@SiO2@graphene quantum dots nanocomposite (denoted as Fe3O4@SiO2@GQDs). The graphene quantum dots were covalently immobilized on the surface of silica-coated magnetite nanospheres (Fe3O4@SiO2) via covalent linkage to surface amino groups.
This article reports on the synthesis of water dispersible carbon quantum dots (CDs) by a one-step hydrothermal method using polyamidoamine (PAMAM) and (3-aminopropyl)triethoxysilane (APTES) as a platform and passivant. The resulting CDs are highly uniform and finely dispersed. The synergistic effect between PAMAM and APTES on the surface of the CDs results in a fluorescence that is much brighter than that of CDs modified with either APTES or PAMAM only. The fluorescence of the co-modified CDs is quenched by Hg(II) ions at fairly low concentrations. Under the optimum conditions, the intensity of quenched fluorescence drops with Hg(II) concentration in the range from 0.2 nM to 10 μM, and the detection limit is 87 fM. The effect of potentially interfering cations on the fluorescence revealed a high selectivity for Hg2+. The fluorescent probe was applied to the determination of Hg(II) in (spiked) waters and milk and gave recoveries between 95.6 and 107 %, with relative standard deviation between 4.4 and 6.0 %.
Graphical abstract Strongly fluorescent carbon quantum dots (CDs) modified with polyamidoamine (PAMAM) and 3-aminopropyltriethoxysilane (APTES) were synthesized by one-step hydrothermal strategy. The resulting co-modified CD s were used as fluorescent probe for sensitive and selective detection of Hg2+.
The authors describe an array for chemiluminescence (CL) based determination of cardiac troponin T (cTnT), an important cardiovascular disease marker. The tracing tag consists of silver nanoparticles (AgNPs) loaded with guanine-rich DNA sequences and detection antibody in a high numerical ratio. The loaded AgNPs were then reacted with hemin to form a hemin/G-quadruplex DNAzyme. A disposable immunosensor array was fabricated by immobilizing capture antibody on corresponding sensing sites on a glass chip. Once a sandwich immunocomplex is formed on the array, the tracing tag catalyzes the CL reaction of the luminol-p-iodophenol and H2O2 system to produce a CL signal, which is collected by a CCD camera. An intuitive CL image is obtained containing all of the spots on the array. Under optimal conditions, the method shows a wide linear range over 4 orders of magnitude (from 0.003 to 270 ng·L?1), a detection limit down to 84 fg·L?1, and a throughput as high as 44 tests·h?1. The results obtained with serum samples are in acceptable agreement with reference values. The AgNP-based tracing tag as well as the immunoassay method shows a promising potential for point-of-care testing for the early clinical diagnosis of cardiovascular disease.
Graphical abstract Schematic presentation of silver nanoparticles (AgNPs) functionalized with hemin/G-quadruplex DNAzyme for highly sensitive chemiluminescence (CL) immunoassay of cardiac troponin T (cTnT) on a glass chip array.
The authors describe an upconversion nanoparticle-based (UCNP–based) fluorometric method for ultrasensitive and selective detection of Cu2+. The UCNPs show a strong emission band at 550 nm under near-infrared excitation at 980 nm. The principle of the strategy is that gold nanoparticles (AuNP) can quench the fluorescence of UCNP. In contrast, the addition of L-cysteine (Cys) can induce the aggregation of AuNP, resulting in a fluorescence recovery of the UCNPs. On addition of Cu2+, it oxidizes Cys to cystine and is reduced to Cu+. The Cu+ thusformed can be oxidized cyclically to Cu2+ by dissolved O2, which catalyzes and recycles the whole reaction. Thus, the aggregation of AuNP is inhibited and the fluorescence recovered by Cys is quenched. Under the optimal condition, the quenching efficiency shows a good linear response to the concentrations of Cu2+ in the 0.4–40 nM range. The limit of detection is 0.16 nM, which is 5 orders of magnitude lower than the U.S. Environmental Protection Agency limit for Cu2+ in drinking water (20 μM). The method has been further applied to monitor Cu2+ levels in real samples. The results of detection are well consistent with those obtained by atomic absorption spectroscopy.
Graphical abstract Gold nanoparticles (AuNP) as a high efficient fluorescence quenching reagent of upconversion nanoparticles (UCNP) were used in a fluorometric method for detection of Cu2+ based on a cyclic catalytic oxidation amplification strategy.
The article describes a bienzyme visual system for aptamer-based assay of Hg(II) at nanomolar levels. The detection scheme is based on the finding that Hg(II) ions captured by aptamer-functionalized magnetic beads are capable of inhibiting the enzymatic activity of uricase and thus affect the formation of H2O2 and the blue product, i.e., oxidized tetramethylbenzidine. This strategy allows for a visual detection of Hg(II) at nanomolar levels without additional amplification procedure. Measuring the absorbance at 650 nm, the logarithmic calibration plot is linear in the concentration range of 0.5–50 nM and the limit of detection (LOD) is 0.15 nM. This is as low as the LOD obtained by atomic fluorescence spectrometry (AFS). The ions K+, Mg2+, Na+, Ca2+, Cu2+, Zn2+, Fe3+, Al3+, Co2+, AsO2?, Ni2+, Cd2+ and Pb2+ do not have a significant effect on color formation. The method was applied to the analysis of (spiked) river water, lake water, mineral water, tap water and certified reference water samples, and the results agreed well with those obtained by AFS or certified values, with recoveries ranging from 97% to 109%. The relative standard deviation for five parallel detections at a 10 nM Hg(II) level is 5.2%.
Graphical abstract A bienzyme-based visual aptasensor was fabricated for label-free detection of nanomolar Hg2+ in water samples without any amplification or enrichment procedure.
The authors introduce an arc ion plating method for the deposition of chromium oxide (Cr2O3) on a steel wire substrate, and its use as a coating for solid phase microextraction. The coating has a micro- and nano-scaled structure after annealing at 700 °C. It is found that Cr2O3 exhibits a good extraction capability for the aromatic hydrocarbons naphthalene, anthracene, fluorene, fluoranthene, and biphenyl. Following desorption by high temperature at 300 °C, the analytes were quantified by gas chromatography (GC). The limits of detection are in the range between 20 and 200 ng·L?1, and calibration plots are linear within a wide range (0.2 to 400 μg·L?1). The coating has excellent mechanical properties, with a hardness is as high as 31.7 GPa, and the adhesion strength between coating and substrate reaches 20.1 N (corresponding to the critical Hertzian contact stress of 10 GPa). This, along with the chemical and thermal stability of the Cr2O3 coating, endows the wire with a long operational life. It was used for at least 100 times without any obvious decline of extraction capability.
Graphical abstract An arc ion plating method was introduced for the deposition of chromium oxide (Cr2O3) on a steel wire substrate, and its use as a coating for solid phase microextraction with high mechanical strength, stability, and long operational lifetime.
A stir foam composed of graphene oxide, poly(ethylene glycol) and natural latex (GO-PEG-NL) was prepared for use in micro-solid phase extraction sorbent of preservative agents and antioxidants from cosmetic products. The extracted analytes were quantified by GC-MS. Under the optimized conditions, the calibration plots are linear in the concentration ranges between 5.0 μg·L?1 to 1.0 mg·L?1 for benzoic acid, of 10.0 μg·L?1 to 1.0 mg·L?1 for 2-methyl-3-isothiazolinone (MI), and between 1.0 μg·L?1 and 1.0 mg·L?1 for both 3-tert-butyl-4-hydroxyanisole (BHA) and 2,6-di-tert-butyl-p-hydroxytoluene (BHT). The LODs are 1.0 μg·L?1 for benzoic acid, 5.0 μg·L?1 for MI and 0.5 μg·L?1 for both BHA and BHT. The stir-foam can be easily prepared, is inexpensive and well reproducible (RSDs <3%, for n?=?6). It can be re-used for up to 12 times after which extraction efficiency has dropped to 90%. The method was successfully applied to the determination of preservatives and antioxidants in cosmetic samples. Recoveries from spiked samples ranged between 94.5?±?2.1% and 99.8?±?1.8%.
Graphical abstract A stir foam was prepared from graphene oxide, poly(ethylene glycol) and natural latex (GO-PEG-NL) and is shown to be a most viable sorbent for the microextraction of trace amounts of preservative agents and antioxidants from cosmetic products.
The authors describe an amperometric biosensor for the determination As(III) and Cd(II) based on the inhibition of the enzyme acetylcholineesterase (AChE). A platinum electrode was modified with ruthenium(II)-tris(bipyridyl), graphene oxide and AChE and then showed redox peaks at 0.06 and 0.2 V vs Ag/AgCl in the presence of acetylthiocholine chloride (ATChCl). Amperometry unveiled a steady-state turnover rate with the release of thiocholine. In the presence of arsenic(III) and cadmium(II), AChE showed an inhibitive response at 0.214 and 0.233 V vs Ag/AgCl, respectively. The electrode exhibits a detection limit and linear range of 0.03 μM and 0.05–0.8 μM for As(III) and 0.07 μM and 0.02–0.7 μM for Cd(II), respectively. Type of inhibition and inhibition constants induced by As(III) and Cd(II) on the catalytic sites of AChE were determined from Dixon and Lineweaver-Burk plots. The modified electrode was applied to the determination of As3+ and Cd2+ in river, tap and waste water, and the results proved that the method is sensitive and can be an alternative to chromatographic and spectroscopic techniques.
An efficient approach is demonstrated for preparing particles consisting of a silver core and a shell of molecularly imprinted polymer (Ag@MIP). The MIP is prepared by using bisphenol A (BPA) as the template and 4-vinylpyridine as the functional monomer. The Ag@MIP fulfills a dual function in that the silver core acts as a SERS substrate, while the MIP allows for selective recognition of BPA. The Ag@MIP is characterized by scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, thermogravimetric analysis and Raman spectroscopy. The Raman intensity of Ag@MIP is higher than that of bare silver microspheres. The detection limit for BPA is as low as 10?9 mol·L?1.
Graphical abstract Schematic illustration of the preparation of silver microspheres coated with a molecularly imprinted polymer (Ag@MIPs) for detecting bisphenol A (BPA) by surface enhanced Raman scattering (SERS).
Copper nanoclusters (Cu-NCs) were prepared by reducing CuCl2 with ascorbic acid in the presence of the short peptide template Cys-Cys-Cys-Asp-Leu. They were characterized by UV-vis absorption and fluorescence spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The Cu-NCs have a size of ~2 nm, can be well dispersed in water and are photostable. Their fluorescence (peaking at 425 nm under 365-nm excitation) is quenched by Fe(III) ions. Based on this finding, a sensitive and selective fluorescence assay for the detection of Fe(III) was developed. Under optimized conditions and a pH value of 2.0, the assay displays a linear response in the 0.05 to 30 μM Fe(III) concentration range, with a detection limit of 20 nM based on an S/N ratio of 3. The assay was successfully applied to the determination of Fe(III) in spiked human serum where is gave recoveries that ranged from 96.2 % to 98.3 %.
Graphical abstract Copper nanoclusters (Cu-NCs) were prepared by reducing CuCl2 with ascorbic acid with peptide as the template. The fluorescence of Cu-NCs is quenched by Fe(III) ions with a linear response in the 0.05 to 30 μM of Fe(III) concentration range.
The authors describe a dye-sensitized photoelectrochemical immunoassay for the tumor marker carcinoembryonic antigen (CEA). The method employs the rhodamine dye Rh123 with red color and absorption maximum at 500 nm for spectral sensitization, and a 3D nanocomposite prepared from graphene oxide and MoS2 acting as the photoelectric conversion layer. The nanocomposite with flower-like 3D architectures was characterized by transmission electron microscopy, scanning electron microscopy, X-ray powder diffraction, and UV-vis diffuse reflectometry. A photoelectrochemical sandwich immunoassay was developed that is based on the use of the nanocomposite and based on the specific binding of antibody and antigen, and by using a secondary antibody labeled with Rh123 and CdS (Ab2-Rh123@CdS). Under optimal conditions and at a typical working voltage of 0 V (vs. Hg/HgCl2), the photocurrent increases linearly 10 pg mL?1 to 80 ng mL?1 CEA concentration range, with a 3.2 pg mL?1 detection limit.
Graphical abstract Flower-like GO-MoS2 complex with high efficiency of electron transport was synthesized to construct photoelectrochemical platform. The sandwich-type immunoassay was built on this platform based on specific binding of antigen and antibody. Carcinoembryonic antigen in sample was detected sensitively by using sensitization of rhodamine dye Rh123 as signal amplification strategy.
A method is described for the colorimetric determination of mercury(II). In the absence of Hg(II), aminopropyltriethoxysilane (APTES) which is positively charged at pH 7 is electrostatically absorbed on the surface of gold nanoparticles (AuNPs). This neutralizes the negative charges of the AuNPs and leads to NP aggregation and a color change from red to blue-purple. However, in the presence of Hg(II), reduced Hg (formed through the reaction between Hg(II) and citrate on the AuNP surface) will replace the APTES on the AuNPs. Hence, the formation of aggregates is suppressed and the color of the solution does not change. The assay is performed by measuring the ratio of absorbances at 650 and 520 nm and can detect Hg(II) at nanomolar levels with a 10 nM limit of detection. The specific affinity between mercury and gold warrants the excellent selectivity for Hg(II) over other environmentally relevant metal ions.
Graphical Abstract Schematic of the method for determination of Hg2+ based on the gold amalgam-induced deaggregation of gold nanoparticles in the presence of APTES with the LOD of 10.1 nM.
Nitrogen-doped multiwalled carbon nanotubes modified with nickel nanoparticles (Ni/N-MWCNT) were prepared by a thermal reduction process starting from urea and Ni(II) salt in an inert atmosphere. The nanocomposite was deposited on a screen printed electrode and characterized by X-ray diffraction, scanning and transmission electron microscopy, nitrogen adsorption, X-ray photoelectron spectroscopy, and thermogravimetric analyses. The performance of the composite was investigated by cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The numerous active metal sites with fast electron transfer properties result in enhanced electrocatalytic activity towards the individual and simultaneous detection of catechol (CC) and hydroquinone (HQ), best at 0.21 V for CC and 0.11 V for HQ (vs. Ag/AgCl). For both targets the detection limit (S/N of 3) was 9 nM (CC) and 11 nM (HQ), and the Ni/N-MWCNT-electrode showed linear response from 0.1–300 μM CC, and 0.3–300 μM HQ. The electrode is selective over many potentially interfering ions. It was applied to the analysis of spiked water samples and gave satisfactory recoveries. It also is sensitive for CC (5.396 μA·μM?1 cm?2) and HQ (5.1577 μA·μM?1 cm?2), highly active, durable, acceptably repeatable and highly reproducible.
Graphical abstract Voltammetric determination of catechol and hydroquinone using nitrogen-doped multiwalled carbon nanotubes modified with nickel nanoparticles.
Hybrids consisting of silver nanoparticles (in varying fractions) and of TiOx/ZnO were prepared via top-down ion beam sputtering (IBS) deposition on silicon substrates. The deposited nanomaterials were characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. It is shown that such composites represent a viable substrate for use in both surface enhanced Raman spectroscopy (SERS) and surface enhanced infrared absorption spectroscopy (SEIRAS), as exemplarily shown for crystal violet as the model analyte. The C-H bending mode at about 1181 cm?1 and the C-N vibration at 1361 cm?1 observed in the SERS and SEIRAS spectra, respectively, have been used as analytical signal. The substrate consisting of TiOx NPs with 33% fraction of silver provides the strongest enhancement in SERS (up to 10,000-fold), while TiOx/AgNPs with thickness of 2 and 1 nm in ion beam sputtering, respectively, provides the best sensitivity in SEIRAS. The substrates also display photocatalytic activity as shown by the degradation of adsorbed crystal violet under ultraviolet irradiation.
Graphical abstract Schematic of the preparation of hybrid substrates consisting of Ag and TiOx/ZnO nanoparticles via ion beam sputtering deposition. They were applied in both surface enhanced Raman and surface enhanced infrared absorption spectroscopies using crystal violet as model analyte, showing enhancements up to >10,000-fold in Raman.
The authors describe a silicon nanoparticle-based fluorometric method for sensitive and selective detection of Cu2+. It is based on the catalytic action of Cu2+ on the oxidation of cysteine (Cys) by oxygen to form cystine and the by-product H2O2. The generated H2O2 is catalytically decomposed by Cu2+ to generate hydroxyl radicals which oxidize and destroy the surface of SiNPs. As a result, the blue fluorescence of the SiNPs is quenched. The method has excellent selectivity due to the dual catalytic effects of Cu2+, which is much better than most previously reported nanomaterial-based assays for Cu2+. Under the optimal conditions, the method has low detection limit (29 nM) and a linear response in a concentration range from 0.05 μM to 15 μM. The method has been successfully applied to the determination of Cu2+ in spiked real water samples, and the results agreed well with those obtained by the Chinese National Standard method (GB/T 7475-1987; AAS).
Graphical abstract Schematic presentation of a fluorometric method for the determination of Cu2+ based on the dual catalytic effects of Cu2+, and the oxidative effect of hydroxy radicals on the surface of silicon nanoparticles (SiNPs). The method has a 29 nM detection limit and good selectivity.
Melamine functionalized cellulose@carbon nanotubes were prepared by the following method: (a) Cellulose was dissolved in a mixture of urea and NaOH solutions and then mixed with multiwall carbon nanotubes; (b) the MWCT-modified cellulose was precipitated by addition of water; (c) the MWCNTs were activated with thionyl chloride, and finally reacted with melamine. The resulting sorbent was used in preconcentration of Co(II) ions prior to their determination by flame atomic absorption spectroscopy. The sorbent was characterized by FTIR and field emission scanning electron microscopy. The effects of the pH value of the medium, contact time, and elution conditions were optimized. The calibration plot is linear in the 3 to 200 μg·L￣1 Co(II) concentration range, the limit of detection is 0.8 μg·L￣1, and the relative standard deviation (at a concentration of 100 μg·L￣1) is 3.5 %. The maximum sorption capacity is 167 mg·g￣1. The method was applied to the preconcentration of cobalt ions from (spiked) food and water samples and gave recoveries between 96 and 102 %.
The family of zearalenones (ZENs) represents a major group of mycotoxins with estrogenic activity. They are produced by Fusarium fungi and cause adverse effects on human health and animal production. The authors describe here a label-free amperometric immunosensor for the direct determination of ZENs. A glassy carbon electrode (GCE) was first modified with polyethyleneimine-functionalized multi-walled carbon nanotubes. Next, gold and platinum nanoparticles (AuPt-NPs) were electro-deposited. This process strongly increased the surface area for capturing a large amount of antibodies and enhanced the electrochemical performance. In a final step, monoclonal antibody against zearalenone was orientedly immobilized on the electrode, this followed by surface blocking with BSA. The resulting biosensor was applied to the voltammetry determination of ZENs, best at a working voltage of 0.18 V (vs SCE). Under optimized conditions, the method displays a wide linear range that extends from 0.005 to 50 ng mL?1, with a limit of detection of 1.5 pg mL?1 (at an S/N ratio of 3). The assay is highly reproducible and selective, and therefore provides a sensitive and convenient tool for determination of such mycotoxins.
Graphical abstract An amperometric immunosensor for the direct determination of ZENs has been developed by immobilizing anti-ZEN monoclonal antibody on multi-walled carbon nanotubest hat were deposited, along with gold and platinum nanoparticles, on a glassy carbon electrode modified with Staphylococcus protein A.
The authors describe a quartz crystal microbalance based aptasensor for the determination of Pb2+. In order to enhance its response, oligonucleotide-embellished gold nanoparticles (AuNPs) were used to amplify the frequency changes. The method is based on the use of specific aptamers immobilized on the surface of the quartz crystal microbalance (QCM) and the binding of Pb2+, which prevents the self-assembly of the AuNPs on the QCM. Trace concentrations of Pb2+ can be determined by monitoring the change in the response frequency of the QCM. The method has a 4 nmol?L?1 detection limit and works in the 5 to 200 nmol?L?1 Pb2+ concentration range. This aptasensor also shows adequate selectivity for Pb2+ over potentially interfering metal ions.
Graphical abstract Schematic of the assay. The specific aptamer SAP does not readily combine with its partial complementary strand CSAP in the presence of Pb(II) ion. Thus, the mass change on the crystal is inversely proportional to the concentration of Pb(II).
We describe a colorimetric assay for mercury(II) ion. It is based on a hybridization chain reaction (HCR) and the use of Fe3O4@Au nanoparticles (NPs). Aptamers specific for Hg(II) were immobilized on the surface of the Fe3O4@AuNPs. The presence of Hg(II) inhibits the HCR process and this enables less Methylene Blue (MB) to intercalate into the dsDNA structure. After magnetic separation of the DNA-loaded NPs carrying Hg(II), the change in the absorbance of the residual MB solution is measured at 663 nm. The respective calibration plot is linear in the 1 to 300 nM concentration range, with a 0.7 nM detection limit (at a signal-to-noise ratio of 3). The method displays excellent selectivity over other metal ions. It was applied to the analysis of Hg(II) in spiked river water.
Graphical abstract Fe3O4@Au nanoparticles (NPs) were fabricated, then aptamers were modified on the surface of Fe3O4@AuNPs. The absence of Hg2+ leads to the formation of dsDNA polymers via hybridization chain reaction (HCR) process on the surface of Fe3O4@AuNPs, Methylene Blue (MB) intercalates into these DNA polymers, which can be easily separated from MB solution by applying a magnet, thereby inducing a color change of the MB solution.
The kinetics of the oxidation of CoIILn complexes {where L = ethylenediaminetetraacetate (EDTA), diethylenetriaminepentaacetate (DTPA), or N-(2-hydroxyethyl)ethylenediaminetriacetate (HEDTA)} by CrVI were studied under pseudo-first-order conditions with [CoIILn] ? [CrVI]. The kinetics showed first-order dependence on [CrVI]. The rate constant, kobs, decreases with increasing concentration of [CrVI]. At constant [H+], ionic strength, and temperature, the rate law is described by Eq. (i)
Both k2 and k3 showed acid-dependent and acid-independent pathways. The direct conversion CoIILn to CoIIILm is ruled out by spectrophotometric and ESR spectroscopic measurements that showed the formation of initial reaction intermediate(s). The rate law is consistent with one-electron and concurrent two-electron transfers leading to the formation of CrV and CrIV, respectively. An inner-sphere process, at least for the first term, leading to the formation of a relatively stable CrV species is almost certain. The kinetic term showing second-order dependence on [CoIILn], most likely, involves concurrent two-electron transfer leading to the formation of CrIV. The type of rate law and the proposed mechanism, reported here, depart from the well-established rate laws observed and mechanisms proposed for the oxidation of one-electron reductants by CrVI.
