Selective determination of L-dopa in the presence of ascorbic acid and uric acid using a 3D graphene foam

Three-dimensional interconnected network graphene foam (GF) was synthesized by chemical vapor deposition. The GF was transferred onto indium tin oxide glass, acting as an electrode for the selective determination of L-dopa in the presence of ascorbic and uric acid. Using differential pulse voltammetry (DPV) method, the oxidation peak current is well linear with L-dopa concentration in the range of 0.05–1 μM with a sensitivity of 2.64 μA μM^?1 and in the range of 1–40 μM with a sensitivity of 1.82 μA μM^?1. The detection limit of this electrode for L-dopa is about 20 nM. The proposed electrode can also effectively avoid the interference of ascorbic acid and uric acid, making the proposed sensor suitable for the accurate determination of L-dopa in human urine fluids. This electrode will have a wide range of potential application prospect in electrochemical detection.     

Green biosynthesis of silver nanoparticles and nanomolar detection of p-nitrophenol

Green biosynthesis of nanoparticles and their applications in sensor field is of great interest to the researchers. We report herein a simple green approach for the synthesis of silver nanoparticles (Ag-NPs) using Acacia nilotica Willd twig bark and its application for the detection of 4-nitro phenol (4-NP). The synthesized Ag-NPs were characterized by Transmission electron microscopy, X-ray diffraction and elemental analysis. The size of synthesized Ag-NPs was in the range of 10–50 nm. The Ag-NPs modified electrode shows a high sensitivity and selectivity towards the sensing of 4-NP. The fabricated modified electrode shows a low detection limit of 15 nM on the wider linear response range from 100 nM to 350 μM with the sensitivity of 2.58?±?0.05 μAμM^?1 cm^?2. In addition, the fabricated sensor shows good repeatability and reproducibility.

Electrochemical behavior of methyl parathion and its sensitive determination at a glassy carbon electrode modified with ordered mesoporous carbon

Ordered mesoporous carbon (OMC) was synthesized and used to modify the surface of a glassy carbon (GC) electrode. Due to the unique properties of OMC, a decrease in the overvoltage of the reduction potential of methyl parathion (MP) (to ca. 219 mV) and a 76-fold increase in the peak current are observed (compared with a bare GC electrode). The absorption capacity of the surface of the electrode for MP was determined by chronocoulometry. The results show that the Г value of the modified electrode (2.34?×?10^–9 mol cm^–2) is 9.5 times as large as that of the GC electrode (2.47?×?10^–10 mol cm^–2). The new electrode exhibits synergistic electrocatalytic and accumulative effects on MP. MP can be determined by linear sweep voltammetry (LSV) which displays a linear relationship between peak current and MP concentration in the range from 0.09 to 61 μM, with a detection limit as low as 7.6 nM (at an S/N of 3) and after an accumulation at 0 V for 5 min. The electrode was successfully applied to the determination of MP in spiked lake water samples.

Construction of a biointerface for glucose oxidase through diazonium chemistry and electrostatic self-assembly technique

In this study, a new procedure for the fabrication of biosensors was developed. The method is based on the covalent attachment of nitrophenyl groups to the electrode surface via diazonium salt reaction followed by their conversion to amine moieties through electrochemical reduction and electrostatic layer-by-layer (LbL) assembly technique. In this procedure, highly stable iron oxide (Fe₃O₄) nanoparticles (IONPs), chitosan (CHIt), GOx, and Nile blue (NB) were assembled on the surface of aminophenyl modified glassy carbon electrode (AP/GCE) by LbL assembly technique. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize the interfaces. The surface coverage of the active GOx and Michaelis–Menten constant (K _M) of the immobilized GOx were Γ?=?3.38?×?10^?11 mol cm^?2 and 2.54 mM, respectively. The developed biosensor displayed a well-defined amperometric response for glucose determination with high sensitivity (8.07 μA mM^?1) and low limit of detection (LOD) of 19.0 μM. The proposed approach allows simple biointerface regeneration by increasing pH which causes disruption of the ionic interactions and release of the electrostatic attached layers. The biosensor can then be reconstructed again using fresh enzyme. Simple preparation, good chemical and mechanical stabilities, and easy surface renewal are remarkable advantages of the proposed biosensor fabrication procedure.     

A highly sensitive method for determination of paracetamol by adsorptive stripping voltammetry using a carbon paste electrode modified with nanogold and glutamic acid

A reliable and simple sensor was fabricated by modifying a carbon paste electrode with nanosized gold particles and poly (glutamic acid) for determination of paracetamol (PAR). The modified electrode exhibited an effective catalytic response to the oxidation and reduction of PAR with good reproducibility and stability. The determination was carried out by differential pulse adsorptive stripping voltammetry after a 30 s accumulation time with an open circuit potential and under stirring. The calibration curve is linear in the range from 0.05 to 70 μM of PAR (with a correlation coefficient of 0.9990), and the sensitivity is 1.51 μA·μM^-1. The modified electrode was used to detect PAR in commercial tablets.     

Facile Fabrication of Highly‐Dispersed Nickel Nanoparticles with Largely Enhanced Electrocatalytic Activity

Supported nickel nanoparticles with high dispersion have been prepared by partial reduction of NiAl‐layered double hydroxide (NiAl‐LDH) precursors, which exhibit significant electrocatalytic behavior towards glucose. XRD and XPS results confirm that the nickel nanoparticles are successfully synthesized. TEM images reveal that the nickel nanoparticles are highly dispersed in the NiAl‐LDH matrix with a size of 6±0.3 nm. The resulting nanocomposite modified electrode displays significant electrocatalytic performance to glucose with a broad linear response range (8.0×10^?5–2.0×10^?3 M), low detection limit (3.6 µM), high sensitivity (339.2 µA/mM), selectivity and excellent reproducibility as well as repeatability.     

Graphene ceramic composite as a new kind of surface-renewable electrode: application to the electroanalysis of ascorbic acid

This study introduces a new surface-renewable electrode based on a sol–gel derived graphene ceramic composite. The electrode was prepared by dispersing graphene nanosheets into a solution of the sol–gel precursors containing methyl triethoxysilane in methanol and hydrochloric acid. During hydrolysis of methyl triethoxysilane, the graphene nanosheets are trapped in the gel. After moulding and drying the composite, it can be used as a surface-renewable electrode to which we refer as a graphene ceramic composite electrode (GCCE). Cyclic voltammograms of the hexacyanoferrate(II/III) model redox system at the GCCE were compared to those obtained with a conventional carbon ceramic electrode and showed a highly improved electron transfer rate at the GCCE. The electrocatalytic oxidation of ascorbic acid as a model analyte was then studied at working potential of 50 mV and over the 3–84 μM concentration range. It revealed a sensitivity of 6.06 μA μM^?1 cm^?2 and a detection limit of 0.82 μM. The GCCE was successfully applied to the determination of ascorbic acid in orange juice and urine samples. Advantages such as good mechanical and chemical stability, ease of fabrication, and reproducible preparation make the GCCE a potentially useful and widely applicable renewable electrode for use in routine analysis. Fig. 1

(Left) FESEM image and photograph of the graphene ceramic composite electrode (GCCE); (right) the cyclic voltammogram of the renewable GCCE in 5 mM K₃[Fe(CN)₆] solution containing 0.1 M KNO₃ at scan rate of 100 mV s^?1      

Voltammetric determination of catechol and hydroquinone using nitrogen-doped multiwalled carbon nanotubes modified with nickel nanoparticles

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.

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Graphical abstract Voltammetric determination of catechol and hydroquinone using nitrogen-doped multiwalled carbon nanotubes modified with nickel nanoparticles.

     

Electrocatalytic study of an electrode modified with Reactive Blue 4 dye covalently immobilized on amine-functionalized silica

In the present work, we investigated the immobilization and electrochemical behavior of Reactive Blue 4 dye on 3-aminopropyl-functionalized silica. The electrochemical behavior of the modified electrode and the electro-oxidation of dipyrone were studied by cyclic voltammetry. The modified electrode showed a well-defined redox coupling with a formal potential of 0.45 V (vs. saturated calomel reference electrode) assigned to anthraquinone/anthrahydroquinone redox process (pH?=?2). The modified electrode also demonstrated electrocatalytic activity and an increased peak current towards the oxidation of dipyrone at a reduced overall potential. The electrocatalytic process was found to be highly dependent on the pH of the supporting electrolyte. The voltammetric responses for dipyrone were linear in the concentration range of 49.9 to 440 μmol L^?1 at a pH of 2.0 with a detection limit and sensitivity of 22.0 μmol L^?1 and 0.0278 μA mmol L^?1, respectively.     

Electrochemical reduction and quantification of menadione in sodium dodecyl sulfate micellar media

Menadione cyclic voltammograms show a pair of redox steps on glassy carbon electrode in 0.1 M H₃PO₄ with potential separation of 343 mV. Cationic, nonionic, and anionic surfactants micellar media significantly decrease the menadione peak potential separation. Statistically significant increase of menadione reduction current (3- and 4.4-fold) has been observed in Triton X 100 and sodium dodecyl sulfate (SDS) micellar media, respectively. Electrochemical reduction of menadione in 9 mM SDS micellar media is reversible diffusion-controlled one-electron process corresponding to formation of relatively stable semiquinone anion radical. The linear dynamic ranges of menadione determination are 7–560 and 600–2,550 μM with the limits of detection and quantification of 1.66 and 5.53 μM, respectively. The current concentration sensitivity is (8.6?±?0.2)?×?10³ μA μM^?1. The voltammetric method for the determination of menadione in pharmaceutical “Aekol” based on preliminary extraction with ethanol has been developed.     

A highly sensitive nonenzymatic glucose sensor based on nickel oxide–carbon nanotube hybrid nanobelts

In this study, we demonstrated a highly sensitive electrochemical sensor for the determination of glucose in alkaline aqueous solution by using nickel oxide single-walled carbon nanotube hybrid nanobelts (NiO–SWCNTs) modified glassy carbon electrode (GCE). The hybrid nanobelts were prepared by the deposition of SWCNTs onto the Ni(SO₄)_0.3(OH)_1.4 nanobelt surface, followed by heat treatment at different temperatures ranging from 400 °C to 600 °C. The NiO–SWCNTs hybrid nanobelts modified electrode prepared at 500 °C displays enhanced electrocatalytic activity towards glucose oxidation, revealing a synergistic effect between the NiO and the deposited SWCNTs. The as-fabricated nonenzymatic glucose sensor exhibits excellent glucose sensitivity (2,980 μA cm^?2 mM^?1), lower detection limit (0.056 μM, signal/noise [S/N] ratio?=?3), and wider linear range (0.5–1,300 μM). Moreover, the sensor has been successfully used for the assay of glucose in serum samples with good recovery, ranging from 96.4 % to 102.4 %.     

Pt-polyaniline nanocomposite on boron-doped diamond electrode for amperometic biosensor with low detection limit

Boron-doped diamond electrodes covered with a nanostructured Pt nanoparticle-polyaniline composite have been fabricated and employed as sensitive amperometric sensors with low detection limit. A highly conductive boron-doped diamond thin film (BDD) was prepared by chemical vapor deposition, and its morphology was characterized by scanning electron microscopy and transmission electron microscopy. The nanostructured composite layer was grown on the BDD electrode by electrochemical deposition of polyaniline and Pt nanoparticles. Glucose oxidase (GOx) was then adsorptively immobilized on the modified BDD electrode. The biosensor displays a large surface area, high catalytic activity of the Pt nanoparticles, efficient electron mediation through the conducting polymer, and low background current of the electrode. The biosensor exhibits an excellent response to glucose, with a broad linear range from 5.9 μM to 0.51 mM, a sensitivity of 5.5 μA·mM^?1, a correlation coefficient (R) of 0.9947, and a detection limit of 0.10 μM. The apparent Michaelis-Menten constant (K _M ^app ) and the maximum current density of the electrode are 4.1 mM and 0.021 mA, respectively. This suggests that the immobilized GOx possesses a higher affinity for glucose at the lower K _M ^app , and that the enzymatic reaction rate constitutes the rate-limiting step of the response.     

A glassy carbon electrode modified with bismuth nanotubes in a silsesquioxane framework for sensing of trace lead and cadmium by stripping voltammetry

Single-walled bismuth nanotubes (sw-BiNTs) were self-assembled with octa(3-aminopropyl) silsesquioxane as a framework and to govern morphology. Deposited on a glassy carbon electrode (GCE), the sw-BiNTs were used for the simultaneous analysis of Pb(II) and Cd(II) by square wave stripping voltammetry. The sw-BiNTs were prepared by (a) coordination interaction between the amino groups of the silsesquioxane and the Bi(III) ions, and by (b) reduction with sodium borohydride. Transmission electron microscopy images revealed single-walled tubular structures with diameters of ~4–6 nm, and with lengths of several hundreds nanometers. GCEs modified with such sw-BiNTs perform much better than bare GCEs in stripping analysis of Pb(II) and Cd(II). The effects of adsorption quantity of sw-BiNTs, solution pH, pulse amplitude, and pulse width were optimized. The modified electrode was then used for the analysis of Pb(II) and Cd(II) in a linear response range from 0.4 to 6 μM with a sensitivity of 4.692 μA μM^?1 and 3.835 μA μM^?1, and detection limits of 1 nM and 5 nM, respectively. The method was successfully applied to the analysis of Pb(II) and Cd(II) in toy leachates, and the results were in good agreement with those obtained with atomic absorption spectrometry. Sensitivity and detection limits were compared with other voltammetric methods, and the sw-BiNTs are deemed to be an attractive alternative for practical applications. Other features of the electrode include low costs, a well reproducible nanostructure, and ease of scale-up of the fabrication process.

Simultaneous sensing of L-tyrosine and epinephrine using a glassy carbon electrode modified with nafion and CeO2 nanoparticles

An electrochemical sensor was developed and tested for detection of L-tyrosine in the presence of epinephrine by surface modification of a glassy carbon electrode (GCE) with Nafion and cerium dioxide nanoparticles. Fabrication parameters of a surfactant-assisted precipitation method were optimized to produce 2–3 nm CeO₂ nanoparticles with very high surface-to-volume ratio. The resulting nanocrystals were characterized structurally and morphologically by X-ray diffractometery (XRD), scanning and high resolution transmission electron microscopy (SEM and HR-TEM). The nanopowder is sonochemically dispersed in a Nafion solution which is then used to modify the surface of a GCE electrode. The electrochemical activity of L-tyrosine and epinephrine was investigated using both a Nafion-CeO₂ coated and a bare GCE. The modified electrode exhibits a significant electrochemical oxidation effect of L-tyrosine in a 0.2 M Britton-Robinson (B-R) buffer solution of pH 2. The electro-oxidation peak current increases linearly with the L-tyrosine concentration in the molar concentration range of 2 to 160 μM. By employing differential pulse voltammetry (DPV) for simultaneous measurements, we detected two reproducible peaks for L-tyrosine and epinephrine in the same solution with a peak separation of about 443 mV. The detection limit of the sensor (signal to noise ratio of 3) for L-tyrosine is ~90 nM and the sensitivity is 0.20 μA μM^?1, while for epinephrine these values are ~60 nM and 0.19 μA μM^?1. The sensor exhibited excellent selectivity, sensitivity, reproducibility and stability as well as a very good recovery time in real human blood serum samples.

Simultaneous electrochemical determination of L-tyrosine and epinephrine in blood plasma with Nafion-CeO₂/GCE modified electrode showing a 443 mV peak-to-peak potential difference between species oxidation peak currents.     

Simultaneous Determination of Dopamine and Uric Acid by Electrocatalytic Oxidation on a Carbon Paste Electrode Using Pyrogallol Red as a Mediator

A sensitive and selective electrochemical method for the simultaneous determination of dopamine (DA) and uric acid (UA) was developed using a pyrogallol red modified carbon paste electrode. Under the optimized conditions, the peak current was linearly dependent on 1.0–700.0 μmol L^?1 DA and 50.0–1000.0 μmol L^?1 UA. The detection limits for DA and UA were 0.78 μmol L^?1 and 35 μmol L^?1, respectively. Finally, this method was also examined for the determination of DA and uric acid in real samples such as drugs and urine.     

Voltammetric Determination of Carcinogenic Derivatives of Pyrene Using a Boron-Doped Diamond Film Electrode

Based on the study of voltammetric behavior of carcinogenic 1-nitropyrene (1-NP), 1-aminopyrene (1-AP), and 1-hydroxypyrene (1-HP), optimum conditions have been found for the determination of these analytes by differential pulse voltammetry (DPV) at a boron-doped diamond film electrode. The optimum medium was methanol-Britton–Robinson buffer (BR buffer) pH 3.0 (70:30) for 1-NP and 1-AP, and methanol-BR buffer pH 5.0 (70:30) for 1-HP. Concentration dependences of the DPV response were measured in the range 1 · 10^?6–1 · 10^?4 mol dm^?3 (R = ?0.9998) with the limit of detection (LOD) 3 · 10^?7 mol dm^?3 for 1-NP, 1 · 10^?7–1 · 10^?5 mol dm^?3 (R = 0.9971) with LOD 6 · 10^?8 mol dm^?3 for 1-AP, and 1 · 10^?7–1 · 10^?5 mol dm^?3 (R = 0.9934) with LOD 1 · 10^?7 mol dm^?3 for 1-HP. Simultaneous determination of 1-NP and 1-AP in a mixture was tested in the methanol-BR buffer pH 3.0 (70:30) medium as well. The content of 1-AP in the concentration range from 1 · 10^?6 to 1 · 10^?4 mol dm^?3 had no effect on the sensitivity of the determination of 1-NP, and vice versa. Due to the close peak potentials of 1-AP and 1-HP, the direct determination of their mixture using voltammetric methods is impossible.     

Thiol Modified Chitosan Self-Assembled Monolayer Platform for Nucleic Acid Biosensor

A self-assembled monolayer (SAM) of thiol modified chitosan (SH-CHIT), with thioglycolic acid (TGA) as a modifier to bestow thiol groups, has been prepared onto gold (Au)-coated glass plates for fabrication of the nucleic acid biosensor. The chemical modification of CHIT via TGA has been evidenced by Fourier transform infrared spectroscopy (FT-IR) studies, and the biocompatibility studies reveal that CHIT retains its biocompatible nature after chemical modification. The electrochemical studies conducted onto SH-CHIT/Au electrode reveal that thiol modification in CHIT amino end enhances the electrochemical behavior indicating that it may be attributed to delocalization of electrons in CHIT skeleton that participates in the resonance process. The carboxyl group modified end of DNA probe has been immobilized onto SH-CHIT/Au electrode using N-ethyl-N′-(3-dimethylaminopropyl)carbodimide (EDC) and N-hydroxysuccinimide (NHS) chemistry for detection of complementary, one-base mismatch and non-complementary sequence using electrochemical and optical studies for Mycobacterium tuberculosis detection. It has been found that DNA-SH-CHIT/Au bioelectrode can specifically detect 0.01 μM of target DNA concentration with sensitivity of 1.69?×?10^?6 A μM^?1.     

Determination of Methimazole on a Multiwall Carbon Nanotube Titanium Dioxide Nanoparticle Paste Electrode

A multiwall carbon nanotube titanium dioxide nanoparticle modified carbon paste electrode was used for direct determination of methimazole. The nanoparticles in the carbon paste increased the surface area of the electrode and improved the sensitivity by enhancing the peak current. The electrochemical behavior of methimazole on the modified electrode was investigated. Experimental variables such as pH and electrode composition were optimized. At the optimum pH 7.0 (universal buffer), the modified electrode had a linear dynamic range of 0.5 to 100.0 μmol L^{? 1} (0.1–11.4 ppm) with a limit of detection of 0.17 μmol L^{? 1} . The application of the electrode for the determination of methimazole in pharmaceuticals and blood serum was investigated.     

Very sensitive differential pulse adsorptive stripping voltammetric determination of 4-nitrophenol at poly (diphenylamine)/multi-walled carbon nanotube-β-cyclodextrin-modified glassy carbon electrode

In this work, a glassy carbon electrode (GCE) modified with poly (diphenylamine)/multi-walled carbon nanotubes-β-cyclodextrin (PDPA/MWCNT-β-CD) film was constructed and used for the determination of 4-nitrophenol (4-NP). Diphenylamine was successfully electropolymerised onto MWCNT-β-CD-modified GCE by cyclic voltammetry in monomer solution and 5 mol L^?1 H₂SO₄. The surface morphology of PDPA/MWCNT-β-CD film was characterised using scanning electron microscopy and electrochemical impedance spectroscopy. After adsorption of 4-NP on PDPA/MWCNT-β-CD at 0.2 V for 150 s, it showed a well-defined reduction peak in phosphate buffer solution at pH = 7. The PDPA/MWCNT-β-CD film enhanced the reduction peak current due to the complex formation between β-CD and 4-NP, presence of conductive polymer film as electron transfer mediator and also ability of MWCNTs for strong adsorptive and catalytic effect. Peak current increased linearly with 4-NP concentration in the range of 0.1 to 13.9 µg L^?1. The detection limit was obtained as 0.02 µg L^?1, which is better than other reported detection limits for the determination of 4-NP. The results showed that modified electrode has good sensitivity and selectivity. This sensor was used for the determination of 4-NP in water samples.     

Synthesis and characterization of activated carbon–ethylenediamine–cobalt(II) tetracarboxyphthalocyanine conjugate for catalytic oxidation of ascorbic acid

We report on the synthesis and characterization of activated carbon–ethylenediamine–cobalt(II) tetracarboxyphthalocyanine conjugate (AC–CONHCH₂CH₂NH₂–CoPc) and its electrocatalytic behavior for oxidation of ascorbic acid. Ultraviolet–visible (UV–Vis), Fourier-transform infrared (FTIR), and electrochemical impedance spectroscopies, and cyclic and square-wave voltammetry were used to characterize the electrode modifiers and modified glassy carbon electrode. The limit of detection was found to be 0.26 µm using 3δ notation. The linear dynamic range was from 1.5 × 10^?4 to 1 × 10^?2 M with electrode sensitivity of 0.01 A mol^?1 L cm^?2. A Tafel slope of 200.8 mV decade^?1 was found. The concentration of ascorbic acid in the tablet was 0.034 M. Oxalic acid showed no interference in ascorbic acid determination.