We describe a chemical exfoliation method for the preparation of MoS2 nanosheets. The nanosheets were incorporated into poly(3,4-ethylenedioxythiophene) (PEDOT) by electrodeposition on a glassy carbon electrode (GCE) to form a nanocomposite. The modified GCE is shown to enable simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). Due to the synergistic effect of MoS2 and PEDOT, this electrode displays better properties in terms of electrocatalytic oxidation of AA, DA and UA than pure PEDOT, which is illustrated by cyclic voltammetry and differential pulse voltammetry (DPV). Under optimum conditions and at pH 7.4, the respective sensitivities and best working potentials are as follows: AA: 1.20 A?mM?1?m?2, 30 mV; DA: 36.40 A?mM?1?m?2, 210 mV; UA: 105.17 A?mM?1?m?2, 350 mV. The calculated detection limits for AA, DA and UA are 5.83 μM, 0.52 μM and 0.95 μM, respectively. The modified electrode was applied to the detection of the three species in human urine samples and gave satisfactory results.
Graphical abstract MoS2 nanosheets were prepared by a facile chemical exfoliation method. MoS2 and poly(3,4-ethylenedioxythiophene) nanocomposite modified glassy carbon electrodes were fabricated, which are shown to enable simultaneous determination of ascorbic acid, dopamine and uric acid with high sensitivity and selectivity.
MoS2 thin films with marigold flower-like nanostructures were grown on conductive fluorine-doped tin oxide (FTO) substrates through a one-step hydrothermal synthesis for their application as counter electrodes (CEs) in dye-sensitized solar cells (DSSCs). Different MoS2 thin film samples (A–D) were grown on FTO slides using different concentrations of precursors (sodium molybdate and thioacetamide), while keeping the Mo/S molar ratio constant (1:4.6), in all samples. The effect of varying precursor concentrations (3.2–12.6 mM on MoS2 basis) on the structure of the nanostructured thin films and their performance as DSSC-CEs was investigated. Scanning electron microscopy revealed a material with an infolded petal-like morphology. With increasing precursor concentration, the petal-like structures tended to form bunched nanostructures (100–300 nm) resembling marigold flowers. X-ray diffraction analysis, X-ray photoelectron, and Raman spectroscopy studies showed that the thin films were composed of hexagonal MoS2 with good crystallinity. Hall effect measurements revealed MoS2 to be a p-type semiconductor with a carrier mobility of 219.80 cm2 V?1 s?1 at room temperature. The electrochemical properties of the thin films were examined using cyclic voltammetry and electrochemical impedance spectroscopy. The marigold flower-like MoS2 thin films showed excellent electrocatalytic activity towards the I¯/I3¯ reaction and low charge transfer resistance (Rct) values of 14.77 Ω cm?1, which was close to that of Pt electrode (12.30 Ω cm?1). The maximum power conversion efficiency obtained with MoS2 CE-based DSSCs was 6.32%, which was comparable to a Pt CE-based DSSC (6.38%) under one sun illumination. Similarly, the maximum incident photon-to-charge carrier efficiency exhibited by MoS2 CE-based DSSCs was 65.84%, which was also comparable to a Pt CE-based DSSC (68.38%). The study demonstrated that the marigold flower-like nanostructured MoS2 films are a promising alternative to the conventional Pt-based CEs in DSSCs.
The paper describes a fluorescent method for determination of Au(III) using molybdenum disulfide quantum dots (MoS2 QDs) that were prepared by a hydrothermal route using glutathione as a reductant. The photoluminescence of MoS2 QDs peaks at 416 nm if excited at 340 nm and is temporally stable even in presence of NaCl or when stored in the refrigerator for one year. Its quantum yield is 12.7 %. The blue-green fluorescence of MoS2 QDs is fairly specifically quenched by Au(III) ions and therefore presents a useful nanoprobe for this ion. Fluorescence intensity drops linearly with the concentration of Au(III) in the range from 0.5 to 1000 μM, and the lower detection limit is 64 nM. The quenching mechanism was investigated and it is concluded that the process is due to the reduction of Au(III) and the deposition of Au(0) on the surface of the MoS2 QDs. The nanoprobe was successfully applied to the determination of Au(III) in (spiked) environmental samples. A test stripe for Au(III) was obtained by soaking a piece of paper with a colloidal solution of the MoS2 QDs, and it was found that this stripe, after drying, can also be used to quantify Au(III) via fluorescence.
Graphical abstract Molybdenum disulfide quantum dots (MoS2 QDs) have a high quantum yield and show good stability. MoS2 QDs are shown to be a sensitive fluorescent probe for the determination of Au3+ ions in solution and with a test stripe via fluorescence quenching.
The authors describe a colorimetric method for the determination of Hg(II) ions by exploiting the peroxidase-lile activity of few-layered MoS2 nanosheets (MoS2-NSs). These were prepared by sonication-induced exfoliation of bulk MoS2 crystals in aqueous surfactant solution. The MoS2-NSs were found to acts as a peroxidase mimic that is capable of oxidizing the substrate 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H2O2) to give a blue product with an absorption maximum at 652 nm. The addition of Hg(II) strongly accelerates the kinetics of this reaction. It is shown that the enzyme mimic possesses a high affinity for TMB and a lower pseudo-Michaelis-Menten constant. The stimulating effect of Hg(II) is seriously influenced by the change of surface charge. The use of nanosheets covered with (negatively charged) polystyrene sulfonate results in a decrease in the formation of blue dye, while those covered with (cationic) poly(diallyldimethyl ammonium) ions cause a small increase. Under optimal conditions, the peroxidase-like activity of MoS2-NSs is affected by Hg(II) in the 2.0 to 200 μM concentration range. The method has a detection limit (LOD) of 0.5 μM which is much below the allowed level in cosmetics (1 ppm; ca. 5 μM). The method display excellent sensitivity, selectivity and stability. It was applied to the determination of total mercury in cosmetic samples, and results compared well with results obtained by ICP-AES.
Graphical abstract A spectrophotometric assay for mercury?-?(II) determination is reported that is based on Hg2+-stimulation effect on the 3,3′,5,5′-tetramethylbenzidine (TMB)-H2O2 reaction system catalyzed by MoS2 nanosheets.
The authors describe an electrochemical sensor for hydrogen peroxide (H2O2). It was constructed by consecutive, selective modification of a glassy carbon electrode (GCE) with Prussian Blue (PB), layered molybdenum disulfide (MoS2), and reduced graphene oxide (rGO). The properties of the modified GCE were characterized via high-resolution transmission electron microscopy, UV-vis spectroscopy and X-ray diffraction. The electrochemical properties of the electrode were studied using cyclic voltammetry and electrochemical impedance spectroscopy. The sensor exhibits excellent electrocatalytic activity for the reduction of hydrogen peroxide in comparison to GCEs modified with MoS2-rGO or PB only. Response is linear in the 0.3 μM to 1.15 mM H2O2 concentration range at a working analytical voltage of 0.1 V, with a 0.14 μM detection limit. The electrochemical sensitivity is 2883.5 μA·μM?1·cm?2, and response is fast (<10 s). The sensor is selective, stable and reproducible. This is attributed to the efficient electron transport properties of the MoS2-rGO composite and the high loading with PB.
Graphic abstract Prussian Blue nanoparticles were deposited on MoS2-rGO modified glassy carbon electrode by electrochemical method. This sensor was used for the detection of H2O2 in tap water and river water.
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.
The authors report on a composite based electrocatalyst for methanol oxidation and H2O2 sensing. The composite consists of Pt nanoparticles (NPs), Pd nanoflakes, and MoS2. It was synthesized by chemical reduction followed by template-free electro-deposition of Pt NPs. FESEM images of the Pd nanoflakes on the MoS2 reveal nanorod-like morphology of the Pd NPs on the MoS2 support, whilst FESEM images of the Pt-Pd/MoS2 composite show Pt NPs in high density and with the average size of ~15 nm, all homogeneously electrodeposited on the Pd-MoS2 composite. A glassy carbon electrode (GCE) was modified with the composite to obtain an electrode for methanol oxidation and H2O2 detection. The modified GCE exhibits excellent durability with good catalytic efficiency (the ratio of forward and backward peak current density, If/Ib, is 3.23) for methanol oxidation in acidic medium. It was also used to sense H2O2 at an applied potential of ?0.35 V vs. Ag|AgCl which can be detected with a 3.4 μM lower limit of detection. The sensitivity is 7.64 μA μM?1 cm?2 and the dynamic range extends from 10 to 80 μM. This enhanced performance can be explained in terms of the presence of higher percentage of metallic 1T phase rather than a semiconducting 2H phase in MoS2. In addition, this is a result of the high surface area of MoS2 with interwoven nanosheets, the uniform distribution of the Pt NPs without any agglomeration on MoS2 support, and the synergistic effect of Pt NPs, Pd nanoflakes and MoS2 nanosheets. In our perception, this binder-free nano-composite has promising applications in next generation energy conversion and in chemical sensing.
Graphical abstract A composite consisting of palladium nanoflakes and molybdenum disulfide was decorated with platinum nanoparticles and then placed on a glassy carbon electrode which is shown to be a viable electrocatalyst for both methanol oxidation and detection of hydrogen peroxide.
The authors describe an enzyme-free amperometric method for the determination of glucose at nanomolar levels at near neutral pH values. A hybrid nanostructure composed of molybdenum disulfide and copper sulfide (MoS2-CuS) was prepared using L-cysteine as both the sulfur donor and the reducing agent. The nanohybrid was then immobilized on a glassy carbon electrode (GCE) by incorporating it into a film of poly(vinyl butyral). Transmission electron microscopy and Raman spectroscopy were utilized to characterize the MoS2-CuS nanohybrids. Three modified GCEs (MoS2/GCE, CuS/GCE and MoS2-CuS/GCE) were investigated with respect to their sensitivity to glucose, and the MoS2-CuS/GCE was found to perform best in displaying a limit of detection as low as 0.3 μM in pH 7.2 buffer at an applied potential of +0.18 V (versus Ag/AgCl). The repeatability and intermediate precision are below 7.0% at 0.05, 0.5 and 1.0 mM concentration levels. The method was applied to the determination of glucose in spiked human serum samples, and recoveries were between 92.3 and 110.7%. This detection scheme is rapid and cost-effective. Natural enzymes and additional electron mediators are not required.
The authors describe a highly efficient photoelectrochemical (PEC) scheme for the determination of hydrogen peroxide (H2O2). BiVO4 microrods were hydrothermally synthesized and deposited on fluorine - doped tin oxide (FTO) glass which acts as the working electrode. Scanning electron microscopy, X-ray powder diffraction and Raman spectroscopy were utilized for the characterization of the microrods. On irradiation with visible light, the holes generated in the microrods are capturing electrons from H2O2 to produce a photocurrent at an operating potential of 0 V vs. Ag/AgCl. Under optimal conditions, the photocurrent increases with the concentration of H2O2 in the range from 50 μmol·L?1 to 1.5 mmol·L?1, and the limit of detection is 8.5 μmol·L?1 (at 3σ). A repeatability and intermediate precision of ≤6.6% was accomplished at H2O2 levels of 0.1, 0.5 and 1.0 mmol·L?1. The method was applied to the determination of H2O2 in spiked sterilized milk samples and gave satisfactory results. As the method works at zero potential, the photocurrent can be measured with simple instrumentation such as digital multimeters, and this will enable expensive electrochemical workstations to be replaced in future.
Graphical abstract An enzyme-free photoelectrochemical sensing strategy is described for sensitive determination of hydrogen peroxide in foodstuff using fluorine-doped tin oxide electrode modified with BiVO4 microrods.
Diphenyl diselenide was immobilized on chitosan loaded with magnetite (Fe3O4) nanoparticles to give an efficient and cost-effective nanosorbent for the preconcentration of Pb(II), Cd(II), Ni(II) and Cu(II) ions by using effervescent salt-assisted dispersive magnetic micro solid-phase extraction (EA-DM-μSPE). The metal ions were desorbed from the sorbent with 3M nitric acid and then quantified via microflame AAS. The main parameters affecting the extraction were optimized using a one-at-a-time method. Under optimum condition, the limits of detection, linear dynamic ranges, and relative standard deviations (for n?=?3) are as following: Pb(II): 2.0 ng·mL?1; 6.3–900 ng·mL?1; 1.5%. Cd(II): 0.15 ng·mL?1; 0.7–85 ng·mL?1, 3.2%; Ni(II): 1.6 ng·mL?1,.6.0–600. ng·mL?1, 4.1%; Cu(II): 1.2 ng·mL?1, 3.0–300 ng·mL?1, 2.2%. The nanosorbent can be reused at least 4 times.
Graphical abstract Fe3O4-chitosan composite was modified with diphenyl diselenide as a sorbent for separation of metal ions by effervescent salt-assisted dispersive magnetic micro solid-phase extraction.
Core-shell Au@Ag nanorods (Ag@GNRs) were synthesized and utilized to construct a voltammetric biosensor for trichloroacetic acid (TCA). The biosensor was prepared by immobilizing hemoglobin (Hb) on a glassy carbon electrode (GCE) that was modified with the Ag@GNRs. Cyclic voltammetry revealed a pair of symmetric redox peaks, indicating that direct electron transfer occurs at the Hb on the Ag@GNR-film. The electron transfer rate constant is as high as 2.32 s?1. The good electrocatalytic capability and large surface area of the Ag@GNR-film is beneficial in terms of electron transfer between Hb and the underlying electrode. The modified GCE, best operated at ?0.4 V (vs. SCE), exhibits electrocatalytic activity toward TCA in the 0.16 μM to 1.7 μM concentration range, with a 0.12 μM detection limit (at an S/N ratio of 3).
Graphical abstract Core-shell Au@Ag nanorods (Ag@GNRs) were synthesized and used to immobilize hemoglobin to construct an effective biosensor for trichloroacetic acid.
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.
The authors describe double-shell magnetic nanoparticles functionalized with 2-mercaptobenzothiazole (MBT) to give nanospheres of the type MBT-Fe3O4@SiO2@C). These are shown to be viable and acid-resistant adsorbents for magnetic separation of the heavy metal ions Ni(II), Cu(II) and Pb(II). MBT act as a binding reagent, and the carbon shell and the silica shell protect the magnetic core. Following 12 min incubation, the loaded nanospheres are magnetically separated, the ions are eluted with 2 M nitric acid and then determined by inductively coupled plasma-mass spectroscopy. The limits of detection of this method are 2, 82 and 103 ng L￣1 for Ni(II), Cu(II), and Pb(II) ions, respectively, and the relative standard deviations (for n = 7) are 6, 7.8, and 7.4 %. The protocol is successfully applied to the quantitation of these ions in tap water and food samples (mint, cabbage, potato, peas). Recoveries from spiked water samples ranged from 97 to 100 %.
Graphical abstract Mercaptobenzothiazole-functionalized magnetic carbon nanospheres of type Fe3O4@SiO2@C were synthesized. Then applied for magnetic solid phase extraction of Ni(II), Cu(II) and Pb(II) from water and food samples with LOD of 0.002, 0.082 and 0.103 μg L?1 respectively.
Tungsten disulfide (WS2) nanosheets were obtained by exfoliating WS2 bulk crystals in N-methylpyrrolidone by ultrasonication. Gold nanoparticles (GNPs) were synthesized by in-situ ultrasonication of sodium citrate and HAuCl4 while fabricating the WS2 nanosheets. In this way, the GNPs were self-assembled on WS2 nanosheets to form a GNPs/WS2 nanocomposite through interaction between sulfur and gold atoms. The photoelectrochemical response of WS2 nanosheets is significantly enhanced after integration of the GNPs. The GNPs/WS2 nanocomposite was coated onto a glassy carbon electrode (GCE) to construct a sensing interface which then was modified with an antibody against the carcinoembryonic antigen (CEA) to obtain a photoelectrochemical immunosensor for CEA. Under optimized conditions, the decline in relative photocurrent is linearly related to the logarithm of the CEA concentration in the range from 0.001 to 40 ng mL?1. The detection limit is 0.5 pg mL?1 (at S/N =?3). The assay is sensitive, selective, stable and reproducible. It was applied to the determination of CEA in clinical serum samples.
Graphical abstract Schematic presentation of the fabrication of Au/WS2 nanocomposites by in-situ ultrasonication and the procedure for the CEA photoelectrochemical immunosensor preparation, and the photocurrent response towards the carcinoembryonic antigen.
CdSe is an important semiconductor for photoelectrochemistry. Here, we propose a two-step method for preparing thin films of aggregated CdSe nanoparticles on Cd electrodes. We first anodized the Cd electrode in an aqueous solution of 0.2 M KNO3 at ?0.9 V (vs. Hg|Hg2SO4(s)|K2SO4 (saturated)) into a porous and layered structure covered with Cd(OH)2 precipitation, and then selenized the Cd(OH)2 deposited on the Cd anode in an aqueous solution of 0.2 M Na2SeSO3. The resulting CdSe nanoparticles self-assembled into strawberry-like nanoaggregates. The anodization time and selenization time were optimized separately. Under our experimental conditions, the optimized anodization time was 80 s, whereas the optimized selenization time ranged from 15 to 60 min, corresponding to the partial or complete conversion of the deposited Cd(OH)2 into smaller and larger strawberry-like CdSe nanoaggregates, respectively. The optimized partially and completely selenized films showed photocurrent responses that were enhanced in different ways but demonstrated comparable performances. They presented an anodic photocurrent density as high as 3.2 mA cm?2 at ?0.3 V with good stability under visible light illumination of 100 mW cm?2 in a solution containing a sacrificial reagent of ascorbic acid.
Graphical Abstract Strawberry-like CdSe nanoaggregates were prepared by selenizing the anodization film of Cd(OH)2 on Cd electrode and they demonstrated enhanced photoelectrochemical performance.
The author describes the preparation of a magnetic metal organic framework of type MOF-199 containing magnetite (Fe3O4) nanoparticles carrying covalently immobilized 4-(thiazolylazo) resorcinol (Fe3O4@TAR). This material is shown to represent a viable sorbent for separation and preconcentration of Cd(II), Pb(II), and Ni(II) ions. Box-Behnken design was applied to optimize the parameters affecting preconcentration. Following elution with 0.6 mol L?1 EDTA, the ions were quantified by FAAS. The capacity of the sorbent ranged between 185 and 210 mg g?1. The limits of detection are 0.15, 0.40, and 0.8 ng mL?1 for Cd(II), Ni(II), and Pb(II) ions, respectively. The relative standard deviations are <8.5 %. The method was successfully applied to the rapid extraction of trace amounts of these ions from sea food and agri food.
Graphical abstract (a) A schematic diagram of Fe3O4 functionalization by TAR (4-(thiazolylazo) resorcinol). (b) The schematic illustration of the magnetic metal organic framework-TAR nanocomposite. H3BTC: benzene-1,3,5-tricarboxylic acid; TEA: triethylamine; 3-CPS: 3-chloropropyl triethoxysilane.
Magnesium(II)-doped nickel ferrite (Mg–NiFe2O4) nanoparticles are introduced as a new adsorbent for magnetic solid phase extraction of lead(II) ions from aqueous solutions. The structure and morphology of the adsorbent was characterized by FTIR, X-ray diffraction and scanning electron microscopy. The effects of pH value, amount of adsorbent, type, concentration and volume of the eluent and adsorption/desorption time on the extraction efficiency were studied. Following elution with hydrochloric acid, Pb(II) ions were quantified by flame atomic absorption spectrometry. Under optimized conditions, the calibration graph is linear in the 0.5–125 ng mL?1 Pb(II) ion concentration range. Other figures of merit include (a) a 0.2 ng mL?1 limit of detection, (b) an enrichment factor of 200, (c) an intra-day relative standard deviation (for n =?6 at 50 ng mL?1) of 1.6%, and (d) an inter-day precision of 3.8%. The method was validated by the analysis of the certified reference material, NIST SRM 1566b. It was successfully applied to the determination of Pb(II) ion in spiked water samples, industrial wastewater and acidic lead battery waters.
Graphical abstract Schematic of the synthesis of Mg(II)-doped NiFeO4 nanoparticles and their application as a magnetic sorbent for solid-phase extraction of a Pb(II) ions prior to determination by flame atomic absorption spectrometry (FAAS).
An amino acid derived ionic liquid, Fe3O4 nanoparticles and graphene oxide (GO) were used to prepare a material for the magnetic solid phase extraction (MSPE) of the ions Al(III), Cr(III), Cu(II) and Pb(II). The material was characterized by Fourier transform infrared spectral (FT-IR), scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), magnetic analysis and isoelectric point (pI) analysis. It is shown to be a viable sorbent for the separation of these metal ions. Single factor experiments were carried out to optimize adsorption including pH values, ionic strength, temperature and solution volume. Following desorption with 0.1 M HCl, the ions were quantified by inductively coupled plasma optical emission spectrometry. Under the optimum conditions, the method provides a linear range from 10 to 170 μg· L?1 for Al(III); from 4.0 to 200 μg· L?1 for Cr(III); from 5.0 to 170 μg· L?1 for Cu(II); and from 5.0 to 200 μg· L?1 for Pb(II). The limits of detection (LOD) are 6.2 ng L?1 for Al(III); 1.6 ng L?1 for Cr(III); 0.52 ng L?1 for Cu(II); and 30 ng L?1 for Pb(II). Method performance was investigated by determination of these ions in (spiked) environmental water and gave recoveries in the range of 89.1%–117.8%.
Graphical abstract The graph shows that Al(III), Cr(III), Cu(II), Pb(II) are not adsorbed quantitatively by Fe3O4-SiO2. On the other hand, Cr(III) and Pb(II) are adsorbed quantitatively by Fe3O4-SiO2-GO while Al(III) and Cu(II) are not quantitatively retained. However, 3D–Fe3O4-SiO2-GO-AAIL adsorb all these 4 metal ions quantitatively.
A SERS-based aptasensor for ochratoxin A (OTA) is described. It is making use of Fe3O4@Au magnetic nanoparticles (MGNPs) and of Au@Ag nanoprobes modified with the Raman reporter 5,5-dithiobis-(2-nitrobenzoic acid; DTNB). Au-DTNB@Ag NPs were modified with the OTA aptamer (aptamer-GSNPs) and used as Raman signal probes. The SERS peak of DTNB at 1331 cm?1 was used for quantitative analysis. MGNPs modified with cDNA (cDNA-MGNPs) were used as capture probes and reinforced substrates. When the Au-DTNB@Ag-Fe3O4@Au complexes are formed through oligonucleotide hybridization, the Raman signal intensity of the Raman probe is significantly enhanced. If the OTA concentration in samples increases, more Raman signal probes (aptamer-GSNPs) will dissociate from the cDNA-MGNPs because more OTA aptamer is bound by OTA. This leads to a lower Raman signal after magnetic separation. Under the optimal conditions, the detection limit for OTA is 0.48 pg·mL?1 based on 3σ criterion. This is attributed to the multiple Raman signal enhancement and the good performance of the OTA aptamer. The good recovery and accuracy of the assay was confirmed by evaluating spiked samples of wine and coffee.
Graphical abstract Schematic of an aptamer based SERS assay for OTA by integrating Fe3O4@AuNPs (MGNPs) with Au-DTNB@Ag NPs with multiple signal enhancement. Aptamer modified Au-DTNB@Ag NPs are used as Raman probes, and MGNPs modified with cDNA are used as capture probes and reinforced substrates.
A method is described for the rapid fluorometric determination of dopamine (DA) by using molybdenum disulfide quantum dots (MoS2 QDs) that were fabricated via an ammonium hydroxide etching method. The probe has a fluorescence (with excitation/emission peaks at 267/380 nm) that is quenched by DA with high selectivity over various interferences. This is attributed to a reaction that occurs between DA and the molybdate ions in pH 9 solutions of MoS2 QDs. The formation of organic molybdate complexes and of dopamine-quinone results in strong quenching of the fluorescence of the QDs which is due to both electron transfer and an inner filter effect. Under the optimum conditions, the assay works in the 0.1–100 μM DA concentration range, with two linear ranges and a 10 nM detection limit. The method was applied to the determination of DA in spiked artificial urine samples, where it gave recoveries ranging from 97.6 to 102.2%, demonstrating that the method a promising tool for rapid and selective detection of DA.
Graphical abstract MoS2 QDs are facilely synthesized via the etching effect of ammonium hydroxide for highly selective fluorometric detection of dopamine.
