We have studied the direct electrochemistry of glucose oxidase (GOx) immobilized on electrochemically fabricated graphite nanosheets (GNs) and zinc oxide nanoparticles (ZnO) that were deposited on a screen printed carbon electrode (SPCE). The GNs/ZnO composite was characterized by using scanning electron microscopy and elemental analysis. The GOx immobilized on the modified electrode shows a well-defined redox couple at a formal potential of −0.4 V. The enhanced direct electrochemistry of GOx (compared to electrodes without ZnO or without GNs) indicates a fast electron transfer at this kind of electrode, with a heterogeneous electron transfer rate constant (Ks) of 3.75 s−1. The fast electron transfer is attributed to the high conductivity and large edge plane defects of GNs and good conductivity of ZnO-NPs. The modified electrode displays a linear response to glucose in concentrations from 0.3 to 4.5 mM, and the sensitivity is 30.07 μA mM−1 cm−2. The sensor exhibits a high selectivity, good repeatability and reproducibility, and long term stability.
Graphical representation for the fabrication of GNs/ZnO composite modified SPCE and the immobilization of GOx
We describe the electrochemical preparation of bismuth nanoribbons (Bi-NRs) with an average length of 100 ± 50 nm and a width of 10 ± 5 μm by a potentiostatic method. The process occurs on the surface of a glassy carbon electrode (GCE) in the presence of disodium ethylene diamine tetraacetate that acts as a scaffold for the growth of the Bi-NRs and also renders them more stable. The method was applied to the preparation of Bi-NRs incorporated into reduced graphene oxide. This nanocomposite was loaded with the enzyme glucose oxidase onto a glassy carbon electrode. The resulting biosensor displays an enhanced redox peak for the enzyme with a peak-to-peak separation of about 28 mV, revealing a fast electron transfer at the modified electrode. The loading of the GCE with electroactive GOx was calculated to be 8.54 × 10−10 mol∙cm−2, and the electron transfer rate constant is 4.40 s−1. Glucose can be determined (in the presence of oxygen) at a relatively working potential of −0.46 V (vs. Ag|AgCl) in the 0.5 to 6 mM concentration range, with a 104 μM lower detection limit. The sensor also displays appreciable repeatability, reproducibility and remarkable stability. It was successfully applied to the determination of glucose in human serum samples.
A potentiostatic method was used to prepare reduced graphene oxide and bismuth nanoribbons nanocomposite on a glassy carbon electrode. This nanocomposite was loaded with enzyme glucose oxidase to fabricate a glucose biosensor.
Titanium dioxide nanorods (TNR) were grown on a titanium electrode by a hydrothermal route and further employed as a supporting matrix for the immobilization of nafion-coated horseradish peroxidase (HRP). The strong electrostatic interaction between HRP and TNR favors the adsorption of HRP and facilitates direct electron transfer on the electrode. The electrocatalytic activity towards hydrogen peroxide (H2O2) was investigated via cyclic voltammetry and amperometry. The biosensor exhibits fast response, a high sensitivity (416.9 μA·mM−1), a wide linear response range (2.5 nM to 0.46 mM), a detection limit as low as 12 nM, and a small apparent Michaelis-Menten constant (33.6 μM). The results indicate that this method is a promising technique for enzyme immobilization and for the fabrication of electrochemical biosensors.
A TiO2 nanorod film was directly grown on Ti substrate by a hydrothermal route, and was further employed for a supporting matrix to immobilize horseradish peroxidase as a biosensor electrode. The as-prepared hydrogen peroxide biosensor based on Nafion/HRP/TNR/Ti electrode exhibited fast response and excellent electrocatalytic activity toward H2O2, i.e., a high sensitivity (416.9 μA mM−1), a wide linear range (2.5 × 10−8 to 4.6 × 10−4 M) with a low detection limit (0.012 μM) and a small apparent Michaelis-Menten constant (33.6 μM).
An ultrasensitive electrochemical glucose biosensor has been developed by depositing C60-fullerene functionalized with tetraoctylammonium bromide (C60-TOAB+) on the surface of a glassy carbon electrode (GCE). The glucose-binding protein concanavalin A (Con A) was then linked to the surface. Binding of glucose by Con A affects the electroactivity of the reversible redox couple C60/C60 −, and this finding forms the basis for a quantitative glucose assay over the 10 to 10 mM concentration range and with a lower detection limit of 3.3 nM (at an S/N ratio of 3). The sensitivity of this sensor allowed glucose to be determined in saliva. This biosensor possesses excellent selectivity, outstanding reproducibility and good long-term stability.
An ultrasensitive electrochemical glucose biosensor has been developed by depositing C60-fullerene functionalized with tetraoctylammonium bromide (C60-TOAB+) on the surface of a glassy carbon electrode (GCE). The glucose-binding protein concanavalin A (Con A) was then linked to the surface. Binding of glucose by Con A affects the electroactivity of the reversible redox couple C60/C60 −, and this finding forms the basis for a quantitative glucose assay over the 10 to 10 mM concentration range and with a lower detection limit of 3.3 nM (at an S/N ratio of 3). The sensitivity of this sensor allowed glucose to be determined in saliva.
Alloy nanoparticles of the type PtxFe (where x is 1, 2 or 3) were synthesized by coreduction with sodium borohydride in the presence of carbon acting as a chemical support. The resulting nanocomposites were characterized by scanning electron microscopy and X-ray diffraction. The nanocomposite was placed on a glassy carbon electrode, and electrochemical measurements indicated an excellent catalytic activity for the oxidation of glucose even a near-neutral pH values and at a working voltage as low as 50 mV (vs. SCE). Under optimized conditions, the sensor responds to glucose in the 10.0 μM to 18.9 mM concentration range and with a 3.0 μM detection limit (at an S/N ratio of 3). Interferences by ascorbic acid, uric acid, fructose, acetamidophenol and chloride ions are negligible.
Nonenzymatic sensing of glucose is demonstrated at neutral pH values and low working potential using a glassy carbon electrode modified with platinum-iron alloy nanoparticles on a carbon support.
A nanocomposite consisting of coral-like gold nanostructures on reduced graphene oxide (RGO) was synthesized with the assistance of dimethylbiguanide (DMBG). It was then fabricated on a glassy carbon electrode, coating with cysteamine in order to enable the immobilization of acetylcholinesterase (AChE) as a model enzyme whose activity of hydrolyzing the substrate of acetylthiocholine is inhibited by the pesticide triazophos. The biosensor has response to acetylthiocholine in the 0.3 ~ 300 μM concentration range at 0.65 V (vs. SCE). The inhibition of the enzyme by triazophos can be determined in concentrations of up to 210 ppb, with a detection limit of 0.35 ppb of triazophos (S/N = 3). The biosensor is highly reproducible and acceptably stable.
Coral-like gold nanostructures supported on reduced graphene oxide were synthesized with the assistance of dimethylbiguanide to fabricate an acetylcholinesterase (AChE) biosensor, which exhibited high reproducibility and good stability, providing a good platform for the detection of organophosphorus pesticides.
We are presenting a sensor for hydrogen peroxide (H2O2) that is based on the use of a heterostructure composed of Pt nanoparticles (NPs) and carbon nanofibers (CNFs). High-density Pt NPs were homogeneously loaded onto a three-dimensional nanostructured CNF matrix and then deposited in a glassy carbon electrode (GCE). The resulting sensor synergizes the advantages of the conducting CNFs and the nanoparticle catalyst. The porous structure of the CNFs also favor the high-density immobilization of the NPs and the diffusion of water-soluble molecules, and thus assists the rapid catalytic oxidation of H2O2. If operated at a working voltage of −0.2 V (vs. Ag/AgCl), the modified GCE exhibits a linear response to H2O2 in the 5 μM to 15 mM concentration range (total analytical range: 5 μM to 100 mM), with a detection limit of 1.7 μM (at a signal-to-noise ratio of 3). The modified GCE is not interfered by species such as uric acid and glucose. Its good stability, high selectivity and good reproducibility make this electrode a valuable tool for inexpensive amperometric sensing of H2O2.
The Pt NPs/CNF heterostructure-based H2O2 sensor synergizes the advantages of both the conducting carbon nanofibers and the nanoparticle catalyst. The 3D structure of the nanofibers favor high density immobilization of the nanoparticles and penetration by water-soluble molecules, which assists the catalyic oxidation of H2O2. The sensor shows outstanding performance in terms of detection range, detection limit, response time, stability and selectivity.
We have synthesized nitrogen-doped graphene nanoribbons (N-GrNRs) by unzipping multi-walled carbon nanotubes (CNTs) under strongly oxidizing conditions and subsequent doping with nitrogen by a low-temperature hydrothermal method. The N-GNRs were characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy, and assembled on a disposable screen-printed carbon electrode to give a sensor for H2O2 that was characterized by cyclic voltammetry, electrochemical impedance spectroscopy, chronocoulometry and chronoamperometry. The nano-modified electrode displays enhanced electron transfer ability, and has a large active surface and a large number of catalytically active sites that originate from the presence of nitrogen atoms. This results in a catalytic activity towards H2O2 reduction at near-neutral pH values that is distinctly improved compared to electrodes modified with CNTs or unzipped (non-doped) CNTs only. At a working potential of −0.4 V (vs. Ag/AgCl), the amperometric responses to H2O2 cover the 5 to 2785 μM concentration range, with a limit of detection as low as 1.72 μM. This enzyme-free electrochemical sensor exhibits outstanding selectivity and long-term stability for H2O2 detection.
Nitrogen-doped graphene nanoribbons (N-GrNRs) were expediently synthesized for highly sensitive and selective detection of H2O2.
Platinum nanoparticles (Pt-NPs) with sizes in the range from 10 to 30 nm were synthesized using protein-directed one-pot reduction. The model globular protein bovine serum albumin (BSA) was exploited as the template, and the resulting BSA/Pt-NPs were studied by transmission electron microscopy, energy dispersive X-ray spectroscopy, and resonance Rayleigh scattering spectroscopy. The modified nanoparticles display a peroxidase-like activity that was exploited in a rapid method for the colorimetric determination of hydrogen peroxide which can be detected in the 50 μM to 3 mM concentration range. The limit of detection is 7.9 μM, and the lowest concentration that can be visually detected is 200 μM.
Pt-NPs were synthesized using BSA-directed one-pot reduction and BSA/Pt-NPs composite can effectively catalyze the oxidation of TMB producing blue solution in the presence of H2O2.
We describe an anodic stripping voltammetric (ASV) method for glucose sensing that widely expands the typical amperometric i-t response of glucose sensors. The electrode is based on a working electrode consisting of a glassy carbon electrode modified with Pt-Pd nanoparticles (NPs; in an atomic ratio of 3:1) on a reduced graphene oxide (rGO) support. The material was prepared via the spontaneous redox reaction between rGO, PdCl4 2− and PtCl4 2− without any additional reductant or surfactant. Unlike known Pt-based sensors, the use of Pt3Pd NPs results in an ultrasensitive ASV approach for sensing glucose even at near-neutral pH values. If operated at a working voltage as low as 0.06 V (vs. SCE), the modified electrode can detect glucose in the 2 nM to 300 μM concentration range. The lowest detectable concentration is 2 nM which is much lower than the LODs obtained with other amperometric i-t type sensing approaches, most of which have LODs at a μM level. The sensor is not interfered by the presence of 0.1 M of NaCl.
We describe an anodic stripping voltammetric method for glucose sensing that widely expands the typical amperometric i-t response of glucose sensors (2 nM to 300 μM). The electrode is based on a glassy carbon electrode modified with Pt-Pd nanoparticles on a reduced graphene oxide (rGO) support.
We describe a sensitive and selective biosensor for the environmental metabolite 2-hydroxyfluorene (2-HOFlu). It is based on electrochemical impedance spectroscopy and was obtained by assembling a thiolated single-stranded DNA on a gold electrode via S-Au covalent bonding. It is then transformed to a K+-stabilized G-quadruplex-hemin complex which exhibits peroxidase-like activity to catalyze the oxidation of 2-HOFlu by H2O2. This results in the formation of insoluble products that are precipitated on the gold electrode. As a result, the charge transfer resistance (R CT) between the solution and the electrode surface is strongly increased within 10 min as demonstrated by using the ferro/ferricyanide system as a redox probe. The difference in the charge transfer resistances (ΔR CT) before and after incubation of the DNA film with 2-HOFlu and H2O2 serves as the signal for the quantitation of 2-HOFlu with a 1.2. nM detection limit in water of pH 7.4. The assay is highly selective over other selected fluorene derivatives. It was exploited to determine 2-HOFlu in spiked lake water samples where it displayed a detection limit of 3.6 nM. Conceivably, this method has a wide scope in that it may be applied to other analytes for which respective G-quadruplexes are available.
A G-quadruplex DNAzyme based impedimetric biosensor for sensitive detection of 2-hydroxyfluorene using hemin as a peroxidase enzyme mimic was constructed with a detection limit of 1.2 nM in water and 3.6 nM in spiked lake water samples.
Magnetic Fe3O4@SiO2 core shell nanoparticles containing diphenylcarbazide in the shell were utilized for solid phase extraction of Hg(II) from aqueous solutions. The Hg(II) loaded nanoparticles were then separated by applying an external magnetic field. Adsorbed Hg(II) was desorbed and its concentration determined with a rhodamine-based fluorescent probe. The calibration graph for Hg(II) is linear in the 60 nM to 7.0 μM concentration range, and the detection limit is at 23 nM. The method was applied, with satisfying results, to the determination of Hg(II) in industrial waste water.
Gold nanoclusters (AuNCs) stabilized with bovine serum albumin were utilized as a fluorescent probe for ferrous ion. The detection scheme is based on the quenching of the fluorescence of the modified AuNCs by hydroxyl radical (•OH) that is generated in the Fenton reaction between Fe(II) and H2O2. Fe(II) can be quantified in the 0.08 to 100 μM concentration range, and the limit of detection is as low as 24 nM. The method also displays good accuracy and high sensitivity when employed to the determination of Fe(II) in rat cerebrospinal fluids (CSFs). When applied to CSFs of a rat model of Alzheimer’s disease, it revealed enhanced levels of Fe(II) compared to a control, thereby showing the important physiological role of iron(II) in this disease.
BSA-stabilized gold nanoclusters (BSA-AuNCs) were utilized for the determination of ferrous ion in rat cerebrospinal fluids. The method is based on the quenching of the fluorescence by hydroxyl radical (•OH) which is generated in the Fenton reaction between Fe(II) and H2O2.
A nanocomposite consisting of reduced graphene oxide decorated with palladium-copper oxide nanoparticles (Pd-CuO/rGO) was synthesized by single-step chemical reduction. The morphology and crystal structure of the nanocomposite were characterized by field-emission scanning electron microscopy, high resolution transmission electron microscopy and X-ray diffraction analysis. A 3-electrode system was fabricated by screen printing technology and the Pd-CuO/rGO nanocomposite was dropcast on the carbon working electrode. The catalytic activity towards glucose in 0.2 M NaOH solutions was analyzed by linear sweep voltammetry and amperometry. The steady state current obtained at a constant potential of +0.6 V (vs. Ag/AgCl) showed the modified electrode to possess a wide analytical range (6 μM to 22 mM), a rather low limit of detection (30 nM), excellent sensitivity (3355 μA∙mM−1∙cm−2) and good selectivity over commonly interfering species and other sugars including fructose, sucrose and lactose. The sensor was successfully employed to the determination of glucose in blood serum.
A highly sensitive nonenzymatic electrochemical sensor was fabricated using a Pd-CuO composite with reduced graphene oxide. The sensor has a wide detection range and was used to sense glucose in blood serum
This study describes the facile synthesis of platinum nanoparticle-containing porous carbons (Pt/C) by carbonization of freeze-dried agarose gels containing potassium tetrachloroplatinate under a nitrogen atmosphere at 800 °C. By adjusting the ratio between agarose and platinate in the freeze-dried gels, the Pt content in the final Pt/C products could be systematically varied from 0–10 wt.%. Transmission electron microscopy, inductively coupled plasma atomic emission spectrometry, X-ray photoelectron spectroscopy, Raman spectroscopy, and nitrogen physisorption measurements revealed that the Pt/C materials obtained by this method possess high surface areas (350–500 m2 g−1), narrow Pt nanoparticle size distributions (6 ± 3 nm) and nanocrystalline graphite –like carbon character. By immobilization of glucose oxidase on the surface of a 4 wt.% Pt/C electrocatalyst prepared by this route, a very sensitive amperometric glucose biosensor was obtained (response time <2 min, sensitivity 1.9 mA M−1; and a linear response with glucose concentration up to 10 mM). The simplicity and versatility of the described synthetic method suggests its application to the preparation of carbon supported noble metal catalysts including palladium/C and gold/C.
This study describes the facile synthesis of platinum nanoparticle-containing porous carbons (Pt/C) by carbonization of freeze-dried agarose gels containing potassium tetrachloroplatinate. The Pt/C materials exhibited excellent electrocatalytic activities, as demonstrated by their successful integration into amperometric glucose biosensor