Electrochemical sensor based on nitrogen doped graphene: Simultaneous determination of ascorbic acid, dopamine and uric acid

Doped graphene: synthesis, properties and bioanalysis

We discuss early advances in the preparation of doped graphene and its unique properties as well as its applications in bioanalysis.

Electrodeposition synthesis of reduced graphene oxide–carbon nanotube hybrids on indium tin oxide electrode for simultaneous electrochemical detection of ascorbic acid, dopamine and uric acid

Reduced graphene oxide–carbon nanotube (rGO–CNT) hybrids have been synthesized by electrodeposition of GO stabilized CNT using indium tin oxide (ITO) as working electrode, followed by electrochemical reduction of GO–CNT into rGO–CNT.

A new electrochemical aptasensor based on electrocatalytic property of graphene toward ascorbic acid oxidation

Based on the superior electrocatalytic property of graphene (GN) toward ascorbic acid (AA) oxidation, a new electrochemical aptasensor has been developed. Here, adenosine triphosphate (ATP) is used as the model to demonstrate the performance of the developed aptasensor. Briefly, GN is attached to the thiolated ATP binding aptamer (ABA) modified gold electrode through π-π stacking interaction, resulting in a significant oxidation signal of AA. In the presence of ATP, the formation of ATP-ABA complex leads to the release of GN from sensing interface, resulting in a sharp decrease of the oxidation peak current of AA and an obviously positive shift of the related peak potential. Taking both the change values of the peak current and peak potential of AA oxidation as the response signals, ATP can be detected sensitively. This is the first time to demonstrate the application of GN as the nanocatalyst in an amplified aptasensor. It can be expected that GN, as nanocatalyst, should become the very promising amplifying-elements in DNA-based electrochemical biosensors.

One-step pyrolytic synthesis of nitrogen and sulfur dual-doped porous carbon with high catalytic activity and good accessibility to small biomolecules

As one of promising catalysts that contain high density of active sites, N doped carbons have been extensively researched, while the reports for N, S dual-doped carbon materials are far less exhaustive. Herein, devoid of activation process and template, N, S dual-doped porous carbon (N-S-PC) was prepared for the first time via one-step pyrolysis of sodium citrate and cysteine. Possessing unique porous structure and large pore volume as well as good accessibility, N-S-PC demonstrates significantly improved electrocatalytic activity toward oxidation of ascorbic acid (AA), dopamine (DA), and uric acid (UA). In the coexisting system, the peak potential separation between AA and DA is up to 251 mV, which is much larger than for most of the other carbons. On the basis of large potential separation and high current response, selective and sensitive simultaneous determination of AA, DA, and UA was successfully accomplished by differential pulse voltammetry, displaying a linear response from 50 to 2000 μM, from 0.1 to 50 μM, and from 0.1 to 50 μM with a detection limit (S/N = 3) of 0.78, 0.02, and 0.06 μM. This work highlights the importance of N, S dual doping and hierarchical porous carbons for efficient catalysis.

Nafion covered core-shell structured Fe3O4@graphene nanospheres modified electrode for highly selective detection of dopamine

Nafion covered core-shell structured Fe3O4@graphene nanospheres (GNs) modified glassy carbon electrode (GCE) was successfully prepared and used for selective detection dopamine. Firstly, the characterizations of hydro-thermal synthesized Fe3O4@GNs were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. Then Fe3O4@GNs/Nafion modified electrode exhibited excellent electrocatalytic activity toward the oxidations of dopamine (DA). The interference test showed that the coexisted ascorbic acid (AA) and uric acid (UA) had no electrochemical interference toward DA. Under the optimum conditions, the broad linear relationship was obtained in the experimental concentration from 0.020 μM to 130.0 μM with the detection limit (S/N=3) of 0.007 μM. Furthermore, the core-shell structured Fe3O4@GNs/Nafion/GCE was applied to the determination of DA in real samples and satisfactory results were got, which could provide a promising platform to develop excellent biosensor for detecting DA.

A new aptamer/graphene interdigitated gold electrode piezoelectric sensor for rapid and specific detection of Staphylococcus aureus

A novel aptamer/graphene interdigitated gold electrode piezoelectric sensor was developed for the rapid and specific detection of Staphylococcus aureus (S. aureus) by employing S. aureus aptamer as a biological recognition element. 4-Mercaptobenzene-diazonium tetrafluoroborate (MBDT) salt was used as a molecular cross-linking agent to chemically bind graphene to interdigital gold electrodes (IDE) that are connected to a series electrode piezoelectric quartz crystal (SPQC). S. aureus aptamers were assembly immobilized onto graphene via the π-π stacking of DNA bases. Due to the specific binding between S. aureus and aptamer, when S. aureus is present, the DNA bases interacted with the aptamer, thereby dropping the aptamer from the surface of the graphene. The electric parameters of the electrode surface was changed and resulted in the change of oscillator frequency of the SPQC. This detection was completed within 60min. The constructed sensor demonstrated a linear relationship between resonance frequency shifts with bacterial concentrations ranging from 4.1×10(1)-4.1×10(5)cfu/mL with a detection limit of 41cfu/mL. The developed strategy can detect S. aureus rapidly and specifically for clinical diagnosis and food testing.

Sensitive electrochemical sensors for simultaneous determination of ascorbic acid, dopamine, and uric acid based on Au@Pd-reduced graphene oxide nanocomposites

Sensitive electrochemical sensors were fabricated with reduced graphene oxide-supported Au@Pd (Au@Pd-RGO) nanocomposites by one-step synthesis for individual and simultaneous determination of ascorbic acid (AA), dopamine (DA), and uric acid (UA) with low detection limits and wide concentration ranges. From the Au@Pd-RGO-modified electrodes, well-separated oxidation peaks and enhanced peak currents of AA, DA, and UA were observed owing to the superior conductivity of RGO and the excellent catalytic activity of Au@Pd nanoparticles. For individual detection, the linear responses of AA, DA, and UA were in the concentration ranges of 0.1-1000, 0.01-100, and 0.02-500 μM with detection limits of 0.02, 0.002, and 0.005 μM (S/N = 3), respectively. For simultaneous detection by synchronous change of the concentrations of AA, DA, and UA, the linear response ranges were 1-800, 0.1-100, and 0.1-350 μM with detection limits of 0.28, 0.024, and 0.02 μM (S/N = 3), respectively. The fabricated sensors were further applied to the detection of AA, DA, and UA in urine samples. The Au@Pd-RGO nanocomposites have promising applications in highly sensitive and selective electrochemical sensing.

Improved heterogeneous electron transfer kinetics of fluorinated graphene derivatives

Though graphitic carbons are commercially available for various electrochemical processes, their performance is limited in terms of various electrochemical activities. Recent experiments on layered carbon materials, such as graphene, demonstrated an augmented performance of these systems in all electrochemical activities due to their unique electronic properties, enhanced surface area, structure and chemical stabilities. Moreover, flexibility in controlling electronic, as well as electrochemical activities by heteroatom doping brings further leverage in their practical use. Here, we study the electron transfer kinetics of fluorinated graphene derivatives, known as fluorinated graphene oxide (FGO) and its reduced form, RFGO. Enhanced electron transfer kinetics (heterogeneous electron transfer (HET)) is observed from these fluorinated systems in comparison to their undoped systems such as graphene oxide (GO) and reduced GO. A detailed study has been conducted using standard redox probes and biomolecules revealing the enhanced electro-catalytic activities of FGO and RFGO, and electron transfer rates are simulated theoretically. This study reveals that fluorine not only induces defects in graphitic lattice leading to an enhanced HET process but also can modify the electronic structure of graphene surface.

Optimization of modified carbon paste electrode with multiwalled carbon nanotube/ionic liquid/cauliflower-like gold nanostructures for simultaneous determination of ascorbic acid, dopamine and uric acid

We describe the modification of a carbon paste electrode (CPE) with multiwalled carbon nanotubes (MWCNTs) and an ionic liquid (IL). Electrochemical studies by using a D-optimal mixture design in Design-Expert software revealed an optimized composition of 60% graphite, 14.2% paraffin, 10.8% MWCNT and 15% IL. The optimal modified CPE shows good electrochemical properties that are well matched with model prediction parameters. In the next step, the optimized CPE was modified with gold nanostructures by applying a double-pulse electrochemical technique. The resulting electrode was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and electrochemical impedance spectroscopy. It gives three sharp and well-separated oxidation peaks for ascorbic acid (AA), dopamine (DA), and uric acid (UA). The sensor enables simultaneous determination of AA, DA and UA with linear responses from 0.3 to 285, 0.08 to 200, and 0.1 to 450 μM, respectively, and with 120, 30 and 30 nM detection limits (at an S/N of 3). The method was successfully applied to the determination of AA, DA, and UA in spiked samples of human serum and urine.

A rapid and sensitive method for hydroxyl radical detection on a microfluidic chip using an N-doped porous carbon nanofiber modified pencil graphite electrode

Hydroxyl radicals (˙OH) play an important role in human diseases. Traditional detection methods are time consuming and require expensive instruments. Here, we present a simple and sensitive method for the detection of hydroxyl radicals on a microfluidic chip using an electrochemical technique. Aniline monomer is electrochemically polymerized on the surface of a pencil graphite electrode and carbonized at 800 °C. The resulting N-doped porous carbon nanofiber-modified pencil graphite electrode is embedded into a microfluidic chip directly as a working electrode. 4-Hydroxybenzoic acid (4-HBA) is selected as the trapping agent owing to its unique 3,4-DHBA product and high trapping efficiency. A low detection limit of 1.0 × 10(-6) M is achieved on the microfluidic chip. As a demonstration, the microfluidic chip is successfully utilized for the detection of ˙OH in cigarette smoke. The strong π-π stacking and hydrophobic interactions between the nitrogen-doped carbon materials and the pencil graphite make the modified electrode well-suited for the microfluidic chip.

Electrochemical performance of electrospun free-standing nitrogen-doped carbon nanofibers and their application for glucose biosensing

In spite of excellent electrochemical properties, nitrogen-doped carbon nanofibers (NCNFs) have rarely been studied in the field of electroanalysis. In this work, we investigated the electrochemical properties and biosensing performance of NCNFs prepared by a newly proposed approach. The as-obtained NCNFs present a unique free-standing structure with high flexibility which could be convenient for electrode modification. Electrochemical measurements of typical redox species including [Ru(NH3)6]3+/2+, [Fe(CN)6]3-/4-, [Fe(H2O)6]3+/2+, and dopamine indicate that the NCNFs have a larger surface area and faster electron transfer rate compared with carbon nanofibers (CNFs). The presence of high content of pyrrolic-N and abundant defective sites in NCNFs leads to an obvious positive shift of peak potential for oxygen reduction at NCNFs relative to that obtained at CNFs. The unique structure and properties greatly enhance the electrochemical performance of NCNFs. The glucose biosensor based on glucose oxidase/NCNFs shows linear ranges of 0.2-1.2 mM at -0.42 V and 0.05-3 mM at 0.40 V both with high stability. These results suggest that the NCNFs could be a convenient and stable platform for electrochemical biosensors.

Preparation of N-doped graphene by reduction of graphene oxide with mixed microbial system and its haemocompatibility

A steady, effective and environment friendly method of introducing nitrogen into graphene is by microbial reduction of graphene oxide with mixed microorganisms from the anode chamber of microbial fuel cells (MFC). Using this method, N-doped graphene is easily obtained under mild conditions and by simple treatment processes, with the N/C ratio reaching 8.14%. Various characterizations demonstrate that the as-prepared N-doped graphene has excellent properties and is comparable with, and in some aspects, even better than, pristine graphene (containing only elemental C) prepared by chemical methods. The N-doped graphene (mainly substitution of C in the plane of the graphene sheet) with uniform distribution of N was haemocompatible, nontoxic, and water-dispersible, all of which are desirable properties for biomaterials and attributable to a synergetic metabolic effect of mixed microorganisms.

Sensing of glycoprotein via a biomimetic sensor based on molecularly imprinted polymers and graphene-Au nanoparticles

A novel strategy was proposed for preparing a highly sensitive glycoprotein sensor based on molecularly imprinted polymers (MIP), which was electropolymerized with o-phenylenediamine and 3-aminophenylboronic acid monohydrate in the presence of template molecules (bovine serum albumin (BSA)). Sensitivity improved dramatically owing to the application of a graphene-Au nanoparticles hybrid as the electrode modifier, and the immobilization of a large amount of 6-ferrocenylhexanethiol, as the electroactive species, onto nanoparticles. The quantification of BSA was realized by detecting the electrochemical oxidation signal of 6-ferrocenylhexanethiol, which was bonded onto the electrode. Under optimized conditions, a good relationship was obtained between the response current and logarithm of BSA concentration in the range of 1.0 × 10(-11) to 1.0 × 10(-5) g mL(-1) with a detection limit of 7.5 × 10(-12) g mL(-1) (S/N = 3). The resulting MIP sensor displayed good selectivity, reproducibility and stability.

A review of organic and inorganic biomaterials for neural interfaces

Recent advances in nanotechnology have generated wide interest in applying nanomaterials for neural prostheses. An ideal neural interface should create seamless integration into the nervous system and performs reliably for long periods of time. As a result, many nanoscale materials not originally developed for neural interfaces become attractive candidates to detect neural signals and stimulate neurons. In this comprehensive review, an overview of state-of-the-art microelectrode technologies provided fi rst, with focus on the material properties of these microdevices. The advancements in electro active nanomaterials are then reviewed, including conducting polymers, carbon nanotubes, graphene, silicon nanowires, and hybrid organic-inorganic nanomaterials, for neural recording, stimulation, and growth. Finally, technical and scientific challenges are discussed regarding biocompatibility, mechanical mismatch, and electrical properties faced by these nanomaterials for the development of long-lasting functional neural interfaces.

ZnO nanowire arrays on 3D hierachical graphene foam: biomarker detection of Parkinson's disease

We report that vertically aligned ZnO nanowire arrays (ZnO NWAs) were fabricated on 3D graphene foam (GF) and used to selectively detect uric acid (UA), dopamine (DA), and ascorbic acid (AA) by a differential pulse voltammetry method. The optimized ZnO NWA/GF electrode provided a high surface area and high selectivity with a detection limit of 1 nM for UA and DA. The high selectivity in the oxidation potential was explained by the gap difference between the lowest unoccupied and highest occupied molecular orbitals of a biomolecule for a set of given electrodes. This method was further used to detect UA levels in the serum of patients with Parkinson's disease (PD). The UA level was 25% lower in PD patients than in healthy individuals. This finding strongly implies that UA can be used as a biomarker for PD.

Doped graphene for metal-free catalysis

Graphene has attracted increasing attention in different scientific fields including catalysis. Via modification with foreign metal-free elements such as nitrogen, its unique electronic and spin structure can be changed and these doped graphene sheets have been successfully employed in some catalytic reactions recently, showing them to be promising catalysts for a wide range of reactions. In this review, we summarize the recent advancements of these new and interesting catalysts, with an emphasis on the universal origin of their catalytic mechanisms. We are full of hope for future developments, such as more precisely controlled doping methods, atom-scale surface characterization technology, generating more active catalysts via doping, and finding wide applications in many different fields.

Heteroatom-doped graphene materials: syntheses, properties and applications

Heteroatom doping endows graphene with new or improved properties and greatly enhances its potential for various applications.

Graphene and its nanocomposite material based electrochemical sensor platform for dopamine

In this review, the recent progress in the electrochemical sensing of dopamine with various graphene and their nanocomposite materials modified electrodes are presented.

New β-cyclodextrin entrapped in polyethyleneimine film-modified electrodes for pharmaceutical compounds determination

The electrochemical behavior of ascorbic acid and uric acid on glassy carbon bare electrodes and ones modified with β-cyclodextrin entrapped in polyethyleneimine film has been investigated using square wave voltammetry. The electrode modification was achieved in order to separate the voltammetric peaks of ascorbic acid and uric acid when present in the same solution. On the modified electrodes the potential of the oxidation peak of the ascorbic acid was shifted to more negative values by over 0.3 V, while in the case of uric acid, the negative potential shift was about 0.15 V compared to the bare glassy carbon electrode. When the two compounds were found together in the solution, on the bare electrode only a single broad signal was observed, while on the modified electrode the peak potentials of these two compounds were separated by 0.4 V. When the uric acid concentration remained constant, the peak intensity of the ascorbic acid is increased linearly with the concentration (r² = 0.996) and when the ascorbic acid concentration remains constant, the peak intensity of the uric acid increased linearly with the concentration (r² = 0.992). FTIR measurements supported the formation of inclusion complexes. In order to characterize the modification of the electrodes microscopic studies were performed. The modified electrodes were successfully employed for the determination of ascorbic acid in pharmaceutical formulations with a detection limit of 0.22 μM.