A nanocomposite was prepared from graphene-like two-dimensional black phosphorene (BP, an allotrope of phosphorus) and nafion (Nf) treated with isopropanol (IP). A glassy carbon electrode (GCE) modified with this nanocomposite was found to be a viable sensor for voltammetric determination of clenbuterol (CLB). Unlike previously reported pure BP, the BP nanocomposite was stable towards water and oxygen. Its morphology, structure, electrochemically active surface area and electrochemical stability were investigated. The BP-Nf (IP) modified GCE displayed good electrochemical stability and electrocatalytic capacity with a low working potential of 0.94 V (vs. SCE), excellent peak current response for CLB in a linear concentration range of 0.06–24 μM with a detection limit of 3.7 nM (3σ/m) and a sensitivity of 0.14 μA·μM?1·cm?2 under optimal conditions. A sensing mechanism for the electro-oxidation of CLB was suggested and verified by density functional theory calculations under imitation of aqueous solution conditions. The sensor was successfully applied to the determination of CLB in bovine meat and bovine serum samples.
Graphical abstract Highly-stable black phosphorene (BP) nanocomposite based on Nafion (Nf) was used to modify a glassy carbon electrode (GCE). It is shonw to be a viable electrochemical platform for sensitive voltammetric determination of trace clenbuterol (CLB) in bovine beef and bovine serum.
The authors report on a low temperature method for large-scale fabrication of cuprous oxide nanocubes deposited on nitrogen-doped reduced graphene oxide (Cu2O/N-RGO). The material was deposited in a glassy carbon electrode (GCE) where it is found to display enhanced electrocatalytic activity for oxidation of diethylstilbestrol (DES). The morphology and composition of Cu2O/N-RGO were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and energy-dispersive spectroscopy. The results demonstrate that the RGO is doped with 3.5% of nitrogen (atomic ratio), and that nanostructured Cu2O particles with controlled cubical morphology and an average size of about 450 nm have been homogeneously deposited on the surface of N-RGO sheets. The oxidation peak of DES was recorded at 0.315 V (vs. saturated calomel electrode) using differential pulse voltammetry. Under the optimal conditions, the modified GCE displays a linear response in the 0.3 to 150 μM DES concentration range, and the limit of detection is 10 nM. The method was applied to the determination of DES in spiked milk, meat and urine samples and gave excellent selectivity, stability and reproducibility.
Graphic abstract A nanocomposite consisting of Cu2O nanocubes/N-doped reduced graphene oxide (Cu2O/N-RGO) for the electrochemical determination of diethylstilbestrol (DES). The Cu2O/N-RGO modified electrode displays a linear response in the 0.3 to 150 μM DES concentration range. The method was applied to the determination of DES in spiked milk, meat and urine samples
The authors describe an environmentally friendly and fast (~14 min) method for the synthesis of homogeneously distributed fluorescent polydopamine nanodots (PDA-NDs) using KMnO4 as the oxidant. Alkaline phosphatase (ALP) catalyzes the hydrolysis of ascorbic acid 2-phosphate to release free ascorbic acid which undergoes an in-situ redox reaction with KMnO4. Depending on the activity of ALP, more or less KMnO4 is consumed, and this affects the formation of the PDA-NDs. Based on this finding, a sensitive method was worked out to quantify the activity of ALP via real-time formation of fluorescent PDA-NDs. The fluorometric signal (best measured at excitation/emission peaks of 390/500 nm) is linear in the 1 to 50 mU·mL?1 ALP activity range, and the limit of the detection is as low as 0.94 mU·mL?1 (based on 3 σ/m). The method was successfully applied to the determination of ALP activity in spiked human serum and in MCF-7 cell lysates. It was also applied in a method to screen for inhibitors of ALP.
Graphical abstract Schematic of a fluorometric method for the determination of alkaline phosphatase (ALP) activity. The method is based on the in-situ regulation of the formation of fluorescent polydopamine nanodots (PDA-NDs) through the competition between the KMnO4-induced polymerization of dopamine and ALP-directed ascorbic acid 2-phosphate (Asc-2P) hydrolysis. AA: Ascorbic acid.
Magnetic microspheres (Fe3O4) were coated with polydopamine (PDA) and loaded with the metal ions Ti(IV) and Nb(V) to give a material of type Fe3O4@PDA-Ti/Nb. It is shown to be useful for affinity chromatography and for enrichment of phosphopeptides from both standard protein solutions and real samples. For comparison, such microspheres loaded with single metal ions only (Fe3O4@PDA-Ti and Fe3O4@PDA-Nb) and their physical mixtures were also investigated under identical conditions. The binary metal ion-loaded magnetic microspheres display better enrichment efficiency than the single metal ion-loaded microspheres and their physical mixture. Both multiphosphopeptides and monophosphopeptides can be extracted. The Fe3O4@PDA-Ti/Nb microspheres exhibit ultra-high sensitivity (the lowest detection amount being 2 fmol) and selectivity at a low mass ratio such as in case of β-casein/BSA (1:1000).
Graphical abstract Magnetic microspheres (Fe3O4) were coated with polydopamine (PDA) and loaded with the metal ions Ti(IV) and Nb(V) to give a material of type Fe3O4@PDA-Ti/Nb. Results showed its great potential as an affinity probe in phosphoproteome research due to rapid magnetic separation of phosphopeptides, ultrahigh sensitivity and selectivity, and remarkable reusability.
A multifunctional fluorescent probe is synthesized for the determination of adenosine 5′-triphosphate (ATP). The 6-carboxyfluorescein-labeled aptamer (FAM-aptamer) was bound to the surface of magnetite nanoparticles coated with polydopamine (Fe3O4@PDA) by π-π stacking interaction to form the multifunctional probe. The probe has three functions including recognition, magnetic separation, and yielding a fluorescent signal. In the presence of ATP, FAM-aptamer on the surface of the probe binds to ATP and returns to the solution. Thus, the fluorescence of the supernatant is enhanced and can be related to the concentration of ATP. Fluorescence intensities were measured at excitation/emission wavelengths of 494/526 nm. Response is linear in the 0.1–100 μM ATP concentration range, and the detection limit is 89 nM. The probe was applied to the quantitation of ATP in spiked human urine and serum samples, with recoveries ranging between 94.8 and 102%.
Graphical abstract A multifunctional fluorescent probe based on the use of FAM-aptamer and Fe3O4@PDA is described for the determination of ATP in spiked human urine and serum samples. FAM-aptamer: 6-carboxyfluorescein-labeled aptamer; Fe3O4@PDA: magnetite nanoparticles coated with polydopamine. ATP: adenosine 5′-triphosphate.
Carbon quantum dots doped with nitrogen and phosphorus were prepared from adenosine 5′-monophosphate (AMP) in a single simple synthesis step. The nitrogen and phosphorus doped C-dots (N,P-C-dots) were characterized by transmission electron microscopy, Fourier transform infrared spectroscopy, fluorescence spectroscopy, X-ray photoelectron spectroscopy and X-ray powder diffraction. These carbon dots display blue fluorescence, with excitation/emission maxima at 360/430 nm, a quantum yield of 26.5% and an average decay time of 4.3 ns. Fluorescence is strongest at neutral pH values but quenched at very high and very low pH values. It is also quenched by ferric ions which suggests the use of the N,P-C-dots as fluorescent probes for Fe(III). A hemolysis test inferred favorable blood compatibility. The fluorescence of the doped C-dots is excitation wavelength dependent and also is susceptible to 2-photon excitation. The nanoparticles were applied in the fluorescent multicolor bioimaging of A549 (adenocarcinomic alveolar basal epithelial) cells under different excitation wavelengths, typically at 405, 488 and 543 nm. Emission colors ranging from blue to green and red can be adjusted in this way.
Graphical abstract Nitrogen and phosphorus doped carbon dots were synthesized and showed excitation wavelength-dependent behavior. They were applied to multi-color fluorescence imaging of adenocarcinomic alveolar basal epithelial cells.
The authors describe an ultrasonic-assisted headspace method for solid phase micro-extraction (UA-HS-SPME) of 7 polychlorinated biphenyls (PCBs) with codes PCB28, PCB52, PCB101, PCB118, PCB138, PCB153 and PCB180. The coating is based on a poly-dopamine metal-organic framework [PDA-MIL-53(Fe)] on a stainless steel wire. The coating can be prepared and evenly deposited on the stainless fiber by dipping the PDA fiber into a solution of MIL-53(Fe). The assay is also environmentally friendly because water is used as the solvent. The effects of extraction time, addition of salts, pH value and power of ultrasonic power were optimized. The coating is found to possess a high selectivity and adsorption capacity for PCBs compared to commercial SPME fibers such as the divinylbenzene/carboxen/polydimethylsiloxane fibers. Following desorption, the PCBs were quantified by GC-MS. The detection limits are between 50 and 90 pg?g?1 of PCBs in soil. The fibers can be easily prepared, and the batch-to-batch reproducibility (RDS) is <10% (for n = 6). The fibers are inexpensive, re-usable and can be easily manipulated, and particularly well suited for screening polychlorinated biphenyls in soil.
Graphical abstract Schematic of the preparation of an extraction fiber using stainless steel wire as substrate, PDA as adhesive, and MIL-53(Fe) as the adsorbent. It was applied to the extraction of PCBs from soil. The fiber is durable and inexpensive.
A voltammetric sensor is presented for the simultaneous determination of dopamine (DA) and uric acid (UA) in the presence of ascorbic acid (AA). It is based on a gold electrode (GE) modified with carboxyl-functionalized graphene (CFG) and silver nanocube functionalized DA nanospheres (AgNC@PDA-NS). The AgNC@PDA-NS nanocomposite was characterized by scanning electron microscopy and UV-Vis spectroscopy. The electrochemical behavior of the modified electrode was evaluated by electrochemical impedance spectroscopy, cyclic voltammetry and differential pulse voltammetry. The modified electrode displays good electrocatalytic activity towards DA (typically at 0.14 V vs. Ag/AgCl) and UA (typically at 0.29 V vs. Ag/AgCl) even in the presence of ascorbic acid. Response to DA is linear in the concentration range of 2.5 to 130 μM with a detection limit of 0.25 μM. Response to UA is linear in the concentration range of 10 to 130 μM with a detection limit of 1.9 μM. In addition, the sensitivity for DA and UA is 0.538 and 0.156 μA μM?1 cm?2, respectively. The modified electrode also displays good stability, selectivity and reproducibility.
Graphical abstract The gold electrode modified with polydopamine nanospheres functionalized with silver nanocube and carboxylated graphene is used for simultaneous determination of DA and UA in the presence of AA, with wide linear range and low detection limit.
An electrochemical non-enzymatic glucose sensor based on copper nanorods (CuNRs) was developed. The CuNRs were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction spectroscopy, and X-ray photoelectron spectroscopy. The results display a layer of rough cuprous oxide that is formed on the surface of CuNRs. The CuNR- modified glassy carbon electrode exhibits an outstanding capability in terms of nonenzymatic sensing of glucose. The sensor displays high sensitivity (1490 μA?mM?1?cm?2), fast response time (less than 5 s), a low detection limit of 8 nM (S/N = 3), long term stability, and excellent anti-fouling ability. The sensor was applied to the detection of glucose in (spiked) human serum and in black ice tea, with relative standard deviations (for n = 6) of 1.7 % and 1.9 %, respectively.
Graphical abstract The surface of Cu nanorods was covered with cuprous oxide, which increased the surface area of the nanorods and provided more catalytic active sites for the electro-oxidation of glucose. Good linearity and selectivity were obtained in glucose sensing.
The authors describe a sandwich-type electrochemical immunoassay for sensitive determination of the carcinoembryonic antigen (CEA). It is based on the use of iridium nanoparticles (Ir NPs) acting as electrochemical signal amplifier on the surface of a glassy carbon electrode. At first, polydopamine-reduced graphene oxide (PDA-rGO) was employed to immobilize primary antibody (Ab1) against CEA. Secondly, Ir-NPs were used as a support for the immobilization of secondary antibody (Ab2) to afford signal labels. The large surface area of PDA-rGO and the excellent electro-oxidative H2O2-sensing properties of Ir NPs result in a sensitive assay for CEA. Operated best at a working voltage of ?0.6 V (vs. SCE), the assay has a linear range that extends from 0.5 pg?mL?1 to 5 ng·mL?1, and the lower detection limit is 0.23 pg?mL?1. The immunosensor displays satisfactory reproducibility and stability, thus demonstrating a reliable immunoassay strategy for tumor biomarkers. It was applied to the determination of CEA in spiked serum samples.
Graphical abstract Schematic of an amperometric sandwich immunoassay for the carcinoembryonic antigen using a glassy carbon electrode modified with polydopamine, reduced graphene oxide and iridium nanoparticles
A photoelectrochemical wire microelectrode was constructed based on the use of a TiO2 nanotube array with electrochemically deposited CdSe semiconductor. A strongly amplified photocurrent is generated on the sensor surface. The microsensor has a response in the 0.05–20 μM dopamine (DA) concentration range and a 16.7 μM detection limit at a signal-to-noise ratio of 3. Sensitivity, recovery and reproducibility of the sensor were validated by detecting DA in spiked human urine, and satisfactory results were obtained.
Graphical abstract Schematic of a sensitive photoelectrochemical microsensor based on CdSe modified TiO2 nanotube array. The photoelectrochemical microsensor was successfully applied to the determination of dopamine in urine samples.
The authors report on a new approach for the determination of the breast cancer biomarker microRNA-155 (miRNA-155). It is based on the measurement of the fluorescence shift of oligonucleotide-templated copper nanoclusters (DNA-CuNC). A probe DNA was designed that acts as a template for the preparation of CuNC which, under 400 nm excitation, exhibit strong fluorescence enhancement at 490 nm and a 90 nm Stokes shift after binding to target miRNA-155 and formation of a DNA-RNA heteroduplex. Under the optimal conditions, the fluorescence of the DNA-CuNC increases with increasing concentration of miRNA-155 in the range from 50 pM to 10 nM, with a 11 pM detection limit. The assay has excellent selectivity over noncomplementary RNA. The method was applied to the determination of miRNA-155 in the presence of human plasma and saliva.
Graphical abstract Schematic of the detection strategy that relies on the fluorescence shift of DNA-CuNCs resulting from the specific binding of DNA-CuNCs with target miRNA-155. Fluorescence intensities are linearly proportional to the concentrations of target RNA from 50 pM to 10 nM.
The authors report that the peroxidase-like activity of Au@Pt core-shell nanohybrids (Au@PtNHs) is selectively inhibited by cysteine. This finding has led to a highly sensitive colorimetric assay for cysteine that is based on the nanohybrid-catalyzed oxidation of TMB by H2O2 to form a blue product. The method has a detection limit of 5.0 nM and a linear range from 10 nM to 20 μM. The assay is highly selective over other amino acids. It was successfully applied to the determination of cysteine in an injection containing a mixture of amino acids.
Graphical abstract The peroxidase-like activity of Au@Pt core-shell nanohybrids (Au@PtNHs) is selectively inhibited by cysteine, enabling the determination of cysteine.
A composite consisting of chitosan containing azidomethylferrocene covalently immobilized on sheets of reduced graphene oxide was drop-casted on a polyester support to form a screen-printed working electrode that is shown to enable the determination of nitrite by cyclic voltammetry and chronoamperometry. Both reduction and oxidation of nitrite can be accomplished due to the high electron-transfer rate of this electrode. Under optimal experimental conditions (i.e. an applied potential of 0.7 V vs. Ag/AgCl in pH 7.0 solution), the calibration plot is linear in the 2.5 to 1450 μM concentration range, with an ~0.35 μM limit of detection (at a signal-to-noise ratio of 3). The sensor was successfully applied to the determination of nitrite in spiked mineral water samples, with recoveries ranging between 95 and 101 %.
Graphical abstract We describe the design of ferrocene-functionalized reduced graphene oxide electrode and its electrocatalytic properties towards the determination of nitrite. Compared to a reduced graphene oxide electrode, the sensor exhibits enhanced electrocatalytic activity towards both oxidation and reduction of nitrite.
A simple method is described for the determination of copper(II) ions based on the cathodic electrochemiluminescence (ECL) of lucigenin which is quenched by Cu(II). The blue ECL is best induced at ?0.45 V (vs. Ag/AgCl) at a scan rate of 50 mV·s?1. Under optimum conditions, the calibration plot is linear in the 3.0 to 1000 nM Cu(II) concentration range. The limit of detection is 2.1 nM at a signal-to-noise ratio of 3. Compared to other analytical methods, the one presented here is simple, fast, selective and cost-effective. It has been successfully applied in the analysis of copper ions in spiked tap water samples with recoveries ranging from 93.0% (at 50 nM concentration) to 105.7% (at 150 nM).
Graphical abstract The inhibitory effect of Cu(II) on the cathodic electrochemiluminescence of lucigenin enables determination of Cu(II) with a 2.1 nM detection limit.
Carbon polymer dots (CPDs) were prepared by a one-pot aqueous synthetic route from ascorbic acid and diethylenetriamine at room-temperature. The CPDs under 350-nm excitation exhibit blue fluorescence peaking at 430 nm with a quantum yield of 47%. Other features include an average diameter of 5 nm, a fluorescence that is independent of the excitation wavelength, good water dispersibility and photostability, and excellent biocompatibility. The CPDs are shown to be viable fluorescent probes for ferric ion which acts as a strong quencher. The response to Fe(III) is linear in the 0.2 to 10 μM concentration range, and the detection limit is 0.1 μM. The probe was applied to the determination of Fe(III) in environmental waters and to intracellular imaging of ferric ions in HeLa cells.
Graphical abstract Carbon polymer dots (CPDs) are prepared from ascorbic acid and diethylenetriamine (DETA) at room-temperature (RT). The RT-CPDs exhibit excellent optical performance, biocompatibility and selectivity of quenching by ferric ions. This can be applied for determination and intracellular imaging of ferric ion.
The authors describe a strategy for rapid and sensitive determination of phenyl carbamate pesticides in environmental samples. It consists of the following steps: (a) Enrichment and clean-up of the analytes using a C18 microtip based procedure; (b) alkaline hydrolysis of the carbamates (carbofuran, isoprocarb and carbaryl) to form phenol derivatives; and (c) fast separation and amperometric detection in a microfluidic chip (MCs). The microchips were fabricated by using press-transferred carbon black nanoparticles (CB-NPs) as electrochemical sensing nanomaterial. The excellent electrochemical behavior of the CB-NPs coupled to the microchip warrants good separation and allows for the voltammetric determination (best at a working voltage of +0.70 V vs Ag/AgCl) of the carbamates within < 6 min. The authors also describe a rapid procedure for the clean-up and enrichment of the carbamates from real samples by using a C18 microtip. The procedure allowed a 10-fold enrichment of the analytes, and this led to a detection limits in ?the 0.7 to 1.2 μM concentration range. The assay was applied to samples of river, lake and irrigation water that were spiked with carbamates at 50 and 100 μM levels. Recoveries are in the 87 to 108 % range, and RSDs (n = 3) in the 5 to 11 % range. The exploitation of the such nanomaterials coupled to microfluidics and microextraction procedures for real sample analysis in our preception represents a most viable tool for the analysis of complex real samples, for on-site environmental monitoring, and for rapid diagnosis.
This article reviews the progress made in the past 5 years in the field of direct and non-enzymatic electrochemical sensing of glucose. Following a brief discussion of the merits and limitations of enzymatic glucose sensors, we discuss the history of unraveling the mechanism of direct oxidation of glucose and theories of non-enzymatic electrocatalysis. We then review non-enzymatic glucose electrodes based on the use of the metals platinum, gold, nickel, copper, of alloys and bimetals, of carbon materials (including graphene and graphene-based composites), and of metal-metal oxides and layered double hydroxides. This review contains more than 200 refs.
Figure This article reviews the history of unraveling the mechanism of direct electrochemical glucose oxidation and the attempts to successfully develop non-enzymatic electrochemical glucose sensors over the past 5 years.
A dual enhancing strategy has been employed to develop a sandwich type of electrochemical immunoassay for the prostate specific antigen (PSA). The signal is enhanced by using Pt-Cu hierarchical trigonal bipyramid nanoframes (HTBNFs) and a composite consisting of Fe3O4 nanoparticles and reduced graphene oxide in polydopamine that serve to capture the primary antibody (Ab1). This nanocomposite shows better electrical conductivity than Fe3O4 and reduced graphene oxide (RGO), respectively, alone. The Pt-Cu HTBNFs were used to label the secondary antibody (Ab2) and act as tags for signal amplification by virtue of their outstanding electrochemical reduction activity towards H2O2. At a working potential of +0.1 V (vs. SCE), the interference by dissolved oxygen can be avoided. This immunoassay is highly sensitive, with a linear range that extends from 0.1 pg?mL?1 to 5 ng?mL?1 and an ultralow detection limit of 0.03 pg?mL?1.
Graphical abstract Schematic of the dual amplification strategy in the immunosensor for the prostate specific antigen (PSA) that is based on the use of a first antibody (Ab1) conjugated to a Fe3O4-reduced graphene oxide nanocomposite (Fe3O4-RGO), and of Pt-Cu trigonal bipyramid nanoframes as a label for the second antibody (Ab2).
CdSe:Eu nanocrystals were successfully synthesized and characterized by transmission electron microscopy, X-ray powder diffraction, and X-ray photoelectric spectroscopy. The CdSe:Eu nanocrystals showed enhanced green electrochemiluminescence (ECL) intensity when compared to pure CdSe nanocrystals. Further, the nanocrystals were used to design an ECL immunosensor for the detection of carcinoembryonic antigen (CEA) that has a linear response over the 1.0 fg·mL?1 to 100 ng·mL?1 CEA concentration range with a 0.4 fg·mL?1 detection limit. The assay was applied to the determination of CEA in human serum samples.
Graphical abstract Schematic of the assay: GCE-glassy-carbon electrode, Ab- Antibody, BSA- Bovine serum albumin, Ag- Antigen. CdSe:Eu nanocrystals were used to design an ECL immunosensor for the detection of carcinoembryonic antigen.
