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Graphene quantum dots (GQDs) functionalized with chitosan (GQDs-CS) were used for the first time as a suitable nanostructured sensing film for efficient immobilization of methylene blue (MB) through amino-hydroxyl reaction to prepare a novel non-enzymatic hydrogen peroxide sensor using a glassy carbon electrode (GCE). The synthesized hybrid nanostructures were characterized by X-ray diffraction, field emission scanning electron microscopy, cyclic voltammetry, FT-IR, UV–vis, photoluminescence, and energy dispersive X-ray spectroscopy techniques. Cyclic voltammograms showed that the GQDs-CS/MB/GCE exhibited a significant electrocatalytic activity for the reduction of H2O2. The calculated kcat is 4.45 × 104 cm3mol−1s−1. The calibration graph for H2O2 constructed by amperometry (−0.6 V vs. SCE) at the modified electrode showed two different linear ranges (1.0 × 10−6–2.9 × 10−3 M and 2.9–11.78 mM) with a sensitivity of 10.115 μA/mM for the lower linear range and a calculated detection limit of 0.7 μM (S/N = 3). The response time of the sensor for H2O2 detection was 3s. The electrochemical response of GQDs-CS/MB/GCE is not influenced by potential interferents (ascorbic and uric acids, dopamine, caffeine, glucose, and various inorganic salts). This modified electrode exhibited suitability for the non-enzymatic H2O2 sensing in food and water samples.

MicroRNAs (miRNAs) are considered reliable molecular biomarkers being definitively linked to the development of relevant diseases including cardiovascular, diabetes, tissue injury and cancer. Therefore, accurate and sensitive quantification of miRNAs may result in correct disease diagnosis establishing these small noncoding RNA transcripts as valuable biomarkers. Electrochemical biosensing approaches appeared in the last 5 years for miRNAs determination based on the use of bioreceptors with affinity for RNA duplexes (viral proteins and antibodies) are ideal choices to overcome some limitations of conventional quantification strategies. In this review some of the main aspects of the interest in miRNA determination, disadvantages of conventional methodologies and unique features offered by the use of this type of versatile bioreceptors coupled to electrochemical transduction are discussed. The relevant characteristics of the reported approaches are critically commented and the research opportunities and future development trends outlined.

Alzheimer's disease (AD) is a common neurodegenerative disorder associated with massive degradation of neuronal cell structure due to aging. In recent years, neurofilament light chain (NfL) arises as one of the most promising blood biomarkers for AD. Bioassays available for NfL determination in blood (ELISA kits or Simoa assays) require multiple steps, specialized personnel and expensive instrumentation of application limited to centralized and high-resource environments. With the aim of overcoming these drawbacks, this work reports the first magnetic microbeads (MBs)-based electrochemical bioplatform developed so far for the determination of NfL. The developed platform relies in a sandwich type immunoassay involving an HRP-secondary antibody to enzymatically label the detector antibody on the surface of commercial MBs functionalized with carboxylic acids. Amperometric transduction is carried out at screen-printed carbon electrodes (SPCEs) upon deposition of the magnetic bioconjugates on the working electrode surface using the H2O2/hydroquinone (HQ) system. This MBs-based immunoplatform provides a LOD value of 3.0 pg mL−1. The analytical performance is appropriate for the determination of NfL in plasma and brain tissues from AD patients using simple and short protocols (60 min) and requiring low amount of samples (5 µL raw plasma and 0.1 µg tissue extracts).

An electrochemical immunosensor was prepared for the simultaneous determination of two hormones, ghrelin (GHRL) and peptide YY (PYY), which play important roles in the regulation of hunger and saciety. Dual screen-printed carbon electrodes modified with reduced graphene oxide (rGO) were used as scaffolds for the immobilization of the corresponding capture antibodies. Grafting of the diazonium salt of 4-aminobenzoic acid (4-ABA) on the modified electrode surfaces allowed covalent immobilization of antibodies. Competitive immunoassays were employed and the affinity reactions were monitored by differential pulse voltammetry upon addition of 1-naphthyl phosphate. Under the optimized working conditions, linear current vs. log [hormone] plots extending between 10−3 and 100 ng/mL GHRL (r2 = 0.990), and 10−4 and 10 ng/mL PYY (r2 = 0.992) were obtained. These ranges are adequate for the determination of both hormones at clinical levels in serum and saliva. An excellent analytical performance in terms of detection limit, reproducibility of the measurements, storage stability and selectivity against other proteins was achieved. The usefulness of the approach was demonstrated by its application to spiked human serum and saliva.

Made available in DSpace on 2018-11-26T15:27:46Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-08-20 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Spanish Ministry of Science and Innovation Community of Madrid, Programme NANOAVANSENS A novel reduced graphene oxide/Sb2O5 hybrid nanomaterial was prepared by a one-pot reaction process. The nanomaterial was characterized by different spectroscopic, microscopic and electrochemical techniques, demonstrating that the graphene sheets were doped with a Sb2O5 thin film. A glassy carbon electrode coated with the hybrid material was further employed as support for the covalent immobilization of laccase to develop an electrochemical biosensor for estriol. The enzyme biosensor showed high sensitivity (275 mA/M) and fast analytical response (4 s) for the hormone, with a limit of detection of 11 nM in the range of 25 nM to 1.03 p,M. The biosensor showed high selectivity for the analytical detection of estriol. (C) 2015 Elsevier Ltd. All rights reserved. Univ Complutense Madrid, Fac Chem, Dept Analyt Chem, E-28040 Madrid, Spain State Univ Sao Paulo, Inst Chem, BR-13560970 Sao Carlos, SP, Brazil Mackenzie Presbiterian Univ, Sch Engn, BR-01302907 Sao Paulo, SP, Brazil State Univ Sao Paulo, Inst Chem, BR-13560970 Sao Carlos, SP, Brazil FAPESP: 2011/23047-7 FAPESP: 2014/02457-0 Spanish Ministry of Science and Innovation: CFQ2011-24355 Spanish Ministry of Science and Innovation: CTQ2012-34238 Spanish Ministry of Science and Innovation: CTQ2014-58989 Community of Madrid, Programme NANOAVANSENS: S2013/MIT-3029

A composite electrode consisted of gold nanoparticles (AuNPs), multi-walled carbon nanotubes (MWCNTs) and Teflon, to which peroxidase (HRP) and ferrocene (Fc) were incorporated as auxiliary enzyme and redox mediator, respectively, was constructed. The enzymes creatininase, creatinase and sarcosine oxidase were then co-immobilized onto the surface of the resulting HRP/Fc/AuNPs/MWCNTs/Teflon electrode for the preparation of a creatinine biosensor. Amperometry in stirred solutions using a detection potential of 0.0 V vs Ag/AgCl allowed a linear calibration plot to be obtained in the 0.003–1.0 mM creatinine concentration range with a limit of detection of 0.1 μM (S/N = 3). The apparent Michaelis-Menten constant for creatininase was KMap = 4.1 ± 0.4 mM. The developed biosensor was validated with good results by determining creatinine in human serum and correlating with the spectrophotometric Jaffe's method.

A new layer-by-layer supramolecular approach for the construction of self-assembled nanoarchitectures of polyamidoamine (PAMAM) dendrimers and peroxidase on gold surface is reported. The methodology is based on the supramolecular self-assembly of alternated layers of adamantane-modified horseradish peroxidase and β-cyclodextrin-branched PAMAM G-5 dendrimers on a gold electrode, previously coated with β-cyclodextrin-modified cysteamine core PAMAM G-4 dendron. The formation of layer-by-layer assemblies (up to three dendrimer/peroxidase bilayers) was studied by SPR, quartz crystal microbalance, AFM and cyclic voltammetry. The analytical applicability of these architectures was evaluated by constructing a H 2 O 2 biosensor. The electroanalytical response of the biosensor towards H 2 O 2 increased with the number of enzyme layers. The bioelectrode constructed with three enzyme layers showed a low detection limit of 160 nM, a sensitivity of 602 μA/M cm 2 and retained 63% of its initial activity after 30 days of storage in wet conditions.

Gold electrodes were functionalized with an electropolymerized matrix of Au nanoparticles modified with 2-mercaptoethanesulfonic acid, 3-mercaptophenyl boronic acid and p -aminothiophenol. The resulting nanostructured electroconductive matrix was used as support for the oriented immobilization of horseradish peroxidase to construct a reagentless amperometric biosensor for H 2 O 2 . The electrode, poised at 0.0 mV, exhibited a rapid response within 8 s and a linear calibration range from 5 μM to 1.1 mM H 2 O 2 . The sensitivity of the biosensor was determined as 498 μA/M cm 2 , and its detection limit was 1.5 μM H 2 O 2 at a signal-to-noise ratio of 3. The electrode retained 95% and 72% of its initial activity after 21 and 40 days of storage at 4 °C.

A novel ethanol biosensor based on the bulk incorporation of alcohol dehydrogenase (ADH) into a colloidal gold (Au coll )–multiwalled carbon nanotubes (MWCNTs) composite electrode using Teflon as binding material is reported. The composite Au coll –MWCNTs–Teflon electrode exhibited significantly improved electrooxidation of NADH when compared with other carbon composite electrodes, including those based on carbon nanotubes. Amperometric measurements for NADH at +0.3 V showed significant differences in sensitivity between Au coll –MWCNTs–Teflon and MWCNTs–Teflon composite electrodes. Incorporation of ADH into the bulk electrode material allowed the construction of a mediatorless ethanol biosensor. Both the enzyme loading and the NAD + concentration in solution were optimized. The ADH–Au coll –MWCNTs–Teflon biosensor allowed a limit of detection for ethanol of 4.7 μmol l −1 , which is remarkably better than those reported for other CNTs-based ADH biosensors. The apparent Michaelis–Menten constant was 4.95 mmol l −1 , which is much lower than that reported by immobilization of ADH onto a gold electrode. Both repeatability of the ethanol amperometric measurements, reproducibility with different biosensors, lifetime and storage ability can be, in general, advantageously compared with other ADH–CNTs biosensors. The biosensor was applied for the rapid determination of ethanol in commercial and certified beer samples.

The preparation of poly-(3-methylthiophene)—multi-walled carbon nanotubes hybrid composite electrodes is reported. The hybrid electrode shows a synergic effect of the electrocatalytic properties, and high active surface area of both the conducting polymer and carbon nanotubes, which gives rise to a remarkable improvement of oxidation of NADH with respect to polymer-modified electrodes, and CNTs-modified electrodes. SEM showed that carbon nanotubes served as nanosized backbone for P3MT electropolymerization. The amperometric NADH detection at +300 mV provided fast responses, a range of linearity between 5.0 × 10−7 and 2.0 × 10−5 mol l−1, and a detection limit of 1.7 × 10−7 mol l−1, which compares advantageously with those reported for other NADH CNT-based amperometric sensors. Furthermore, the direct electrochemistry of cytochrome c and FAD at the hybrid electrode is also checked.

The construction and characterization of a new carbon-felt electrode design of small dimensions is reported. The electrode was checked by testing the electrochemical behaviour of the Fe(CN) 6 3−/4− model system, as well as of several phenolic compounds with xenoestrogenic properties. The use of a carbon-felt cylinder electrode whose heigh was insulated with a poly(ethylene) cover and with two exposed bases of 2.0 mm diameter, resulted in an enhancement of the Fe(CN) 6 3−/4− voltammetric peak current with respect to a conventional glassy carbon electrode with a similar outer surface, which suggests a high contribution of the redox probe solution trapped in the three-dimensional structure of the electrode. The contribution of the electrolysis of the redox probe trapped in the electrode matrix to the voltammetric response, as well as that of the mass transfer from bulk solution, were investigated. The voltammetric behaviour of phenolic compounds with xenoestrogenic properties showed adsorption onto the carbon-felt electrode. Penetration of these compounds into the electrode at open circuit was demonstrated. Two possible applications of the new electrode design are outlined: flow analysis with electrochemical detection of phenolic endocrine disruptors and the possibility of using it for removal of these compounds.

The electrochemical behavior of the N , N -dialkyldithiocarbamate pesticide thiram at a Au microdisk electrode (10 μm φ) and at cylindrical carbon fiber microelectrodes (CFMEs) (8 μm φ, 8 mm length) in low-permitivity organic solvents such as toluene and ethyl acetate is reported. The obtained cyclic voltammograms were very different in shape depending on the microelectrode and the solvent used, as well as on thiram concentration. Analytically useless responses were obtained with the gold microelectrode. However, a good fitting of the voltammetric signals at cylindrical CFMEs with the theoretical model proposed by Aoki was found. The influence of variables such as the concentration of thiram and of the background electrolyte, the length of the CFMEs and the potential scan rate was tested by CV. The oxidation responses obtained by square-wave voltammetry at carbon fiber microelectrodes in ethyl acetate were found to be analytically suitable to develop a method for the determination of thiram at low concentration levels. A linear calibration graph in the 1.0×10 −6 –6.0×10 −4 mol l −1 concentration range was obtained, with a slope of (1.2±0.1)×10 4 μA l mol −1 . The limit of detection was 4.3×10 −7 mol l −1 thiram, and the relative standard deviation at a 5.0×10 −5 mol l −1 concentration level ( n =10) was of 1.6%. A good reproducibility of the electroanalytical measurements was found with no need of cleaning or pretreatment procedures of the CFMEs surface. The SWV method was applied with good results to the determination of thiram in spiked grapes, by measuring directly the net current in the analyte extract in ethyl acetate. Commercial fungicides and pharmaceutical preparations containing thiram were also analysed.

A NADH amperometric microsensor based on cylindrical carbon fiber microelectrodes (CFMEs) coated with poly(3-methylthiophene) (P3MT) is reported. Cyclic voltammograms for NADH showed an oxidation response at the P3MT-coated CFME 320 mV lower than that obtained at a conventional glassy carbon electrode. A higher resistance to the electrode fouling was observed when P3MT-coated CFMEs were compared with P3MT-coated GC electrodes of conventional size. A potential of +0.45 V was selected for the detection of NADH by amperometry in stirred solutions. The steady-state current was reached very rapidly (5–7 s), and the same modified micro-electrode could be used during a whole working day without any cleaning treatment. A linear calibration graph for NADH was obtained in the range 1.0×10 −6 –10×10 −4 mol l −1 with a detection limit of 7.6×10 −7 mol l −1 . The influence of the presence of high concentrations of NAD + or ascorbic acid on the NADH amperometric response was also tested. An amperometric flow-cell adapted for the use of microelectrodes was designed. Improved signal-to-background ratios were obtained in this case, which allowed NADH amperometric measurements to be carried out at potentials lower than +0.45 V. Taking into account the influence of an excess of NAD + on the NADH response, a potential value of +0.30 V was selected. The stability of the polymer coating on the CFME was shown to be good in the carrier stream, and the analytical characteristics of the calibration graph were slightly better than those obtained by amperometry under batch conditions. A flow-injection method for the amperometric-enzymatic determination of L-lactate has been developed by using a reticulated vitreous carbon-based enzyme (lactate dehydrogenase) reactor inserted into the flow cell. Practically constant lactate responses were obtained over the first 8 d. A good selectivity for lactate was achieved against citric, malic, tartaric, and acetic acids, and ethanol.