A Novel Enzymatic Glucose Biosensor and Nonenzymatic Hydrogen Peroxide Sensor Based on (3‐Aminopropyl) Triethoxysilane Functionalized Reduced Graphene Oxide

In the present study, a novel enzymatic glucose biosensor using glucose oxidase (GOx) immobilized into (3‐aminopropyl) triethoxysilane (APTES) functionalized reduced graphene oxide (rGO‐APTES) and hydrogen peroxide sensor based on rGO‐APTES modified glassy carbon (GC) electrode were fabricated. Nafion (Nf) was used as a protective membrane. For the characterization of the composites, Fourier transform infrared spectroscopy (FTIR), X‐ray powder diffractometer (XRD), and transmission electron microscopy (TEM) were used. The electrochemical properties of the modified electrodes were investigated using electrochemical impedance spectroscopy, cyclic voltammetry, and amperometry. The resulting Nf/rGO‐APTES/GOx/GC and Nf/rGO‐APTES/GC composites showed good electrocatalytical activity toward glucose and H₂O₂, respectively. The Nf/rGO‐APTES/GC electrode exhibited a linear range of H₂O₂ concentration from 0.05 to 15.25 mM with a detection limit (LOD) of 0.017 mM and sensitivity of 124.87 μA mM⁻¹ cm⁻². The Nf/rGO‐APTES/GOx/GC electrode showed a linear range of glucose from 0.02 to 4.340 mM with a LOD of 9 μM and sensitivity of 75.26 μA mM⁻¹ cm⁻². Also, the sensor and biosensor had notable selectivity, repeatability, reproducibility, and storage stability.     

An Amperometric Glucose Biosensor Based on Poly (Pyrrole‐2‐Carboxylic Acid)/Glucose Oxidase Biocomposite

A newly developed amperometric glucose biosensor based on graphite rod (GR) working electrode modified with biocomposite consisting of poly (pyrrole‐2‐carboxylic acid) (PCPy) particles and enzyme glucose oxidase (GOx) was investigated. The PCPy particles were synthesized by chemical oxidative polymerization technique using H₂O₂ as initiator of polymerization reaction and modified covalently with the GOx (PCPy‐GOx) after activation of carboxyl groups located on the particles surface with a mixture of N‐(3‐dimethylaminopropyl)‐N′‐ethylcarbodiimide hydrochloride (EDC) and N‐hydroxysuccinimide (NHS). Then the PCPy‐GOx biocomposite was dispersed in a buffer solution containing a certain amount of bovine serum albumin (BSA). The resulting biocomposite suspension was adsorbed the on GR electrode surface with subsequent solvent airing and chemical cross‐linking of the proteins with glutaraldehyde vapour (GR/PCPy‐GOx). It was determined that the current response of the GR/PCPy‐GOx electrodes to glucose measured at +300 mV vs Cl reference electrode was influenced by the duration of the PCPy particles synthesis, pH of the GOx solution used for the PCPy particles modification and the amount of immobilized PCPy‐GOx biocomposite. An optimal pH of buffer solution for operation of the biosensor was found to be 8.0. Detection limit was determined as 0.039 mmol L⁻¹ according signal to noise ratio (S/N: 3). The proposed glucose biosensor was tested in human serum samples.     

Direct Electrochemistry and Electrocatalysis Behaviors of Glucose Oxidase Based on Hyaluronic Acid-Carbon Nanotubes-Ionic Liquid Composite Film

Multi‐walled carbon nanotubes (MWNTs) were dispersed in the ionic liquid [BMIM][BF₄] to form a uniform black suspension. Based on it, a novel glucose oxidase (GOx)‐hyaluronic (HA)‐[BMIM][BF₄]‐MWNTs/GCE modified electrode was fabricated. UV‐vis spectroscopy confirmed that GOx immobilized in the composite film retained its native structure. The experimental results of EIS indicated MWNTs, [BMIM][BF₄] and HA were successfully immobilized on the surface of GCE and [BMIM][BF₄]‐MWNTs could obviously improve the diffusion of ferricyanide toward the electrode surface. The experimental results of CV showed that a pair of well‐defined and quasi‐reversible peaks of GOx at the modified electrode was exhibited, and the redox reaction of GOx at the modified electrode was surface‐confined and quasi‐reversible electrochemical process. The average surface coverage of GOx and the apparent Michaelis‐Menten constant were 8.5×10⁻⁹ mol/cm² and 9.8 mmol/L, respectively. The cathodic peak current of GOx and the glucose concentration showed linear relationship in the range from 0.1 to 2.0 mmol/L with a detection limit of 0.03 mmol/L (S/N=3). As a result, the method presented here could be easily extended to immobilize and obtain the direct electrochemistry of other redox enzymes or proteins.     

Preparation of Close‐Packed Silver Nanoparticles on Graphene to Improve the Enzyme Immobilization and Electron Transfer at Electrode in Glucose/O2 Biofuel Cell

In this study, chemical reduced graphene‐silver nanoparticles hybrid (AgNPs @CR‐GO ) with close‐packed AgNPs structure was used as a conductive matrix to adsorb enzyme and facilitate the electron transfer between immobilized enzyme and electrode. A facile route to prepare AgNPs @CR‐GO was designed involving in β ‐cyclodextrin (β ‐CD ) as reducing and stabilizing agent. The morphologies of AgNPs were regulated and controlled by various experimental factors. To fabricate the bioelectrode, AgNPs @CR‐GO was modified on glassy carbon electrode followed by immobilization of glucose oxidase (GOx ) or laccase. It was demonstrated by electrochemical testing that the electrode with close‐packed AgNPs provided high GOx loading (Γ =4.80 × 10⁻¹⁰ mol•cm⁻²) and fast electron transfer rate (k _s=5.76 s⁻¹). By employing GOx based‐electrode as anode and laccase based‐electrode as cathode, the assembled enzymatic biofuel cell exhibited a maximum power density of 77.437 μW •cm⁻² and an open‐circuit voltage of 0.705 V.     

An Electrochemical ATP Biosensor with Enzymes Entrapped within a PEDOT Film

This paper reports on the electrochemical behavior of an ATP biosensor that utilizes glucose oxidase (GOx) and hexokinase (Hex) immobilized within the electroactive polymer, polyethylenedioxythiophene (PEDOT). This biosensor design detects ATP indirectly at 0.85 V vs. Ag/AgCl based on the oxidation current for enzymatically generated H₂O₂, and at −0.20 V; a potential at which improved analyte selectivity is achieved. The detection figures of merit at both detection potentials are a response time of 15±1 s, an experimental detection limit of 10.0±0.2 μmol L⁻¹ (S/N=3), and a sensitivity in the range of 100–500 mA M⁻¹cm⁻².     

A MnOOH‐Polyaniline Nanocomposite Modified Gold Electrode for Electrochemical Sensing of Nitrite

A novel non‐enzymatic nitrite sensor was fabricated by immobilizing MnOOH‐PANI nanocomposites on a gold electrode (Au electrode). The morphology and composition of the nanocomposites were investigated by transmission electron microscopy (TEM ) and Fourier transform infrared spectrum (FTIR ). The electrochemical results showed that the sensor possessed excellent electrocatalytic ability for NO₂⁻ oxidation. The sensor displayed a linear range from 3.0 μmol•L⁻¹ to 76.0 mmol•L⁻¹ with a detection limit of 0.9 μmol•L⁻¹ (S/N = 3), a sensitivity of 132.2 μA •L•mol⁻¹•cm⁻² and a response time of 3 s. Furthermore, the sensor showed good reproducibility and long‐term stability. It is expected that the MnOOH‐PANI nanocomposites could be applied for more active sensors and used in practice for nitrite sensing.     

Glucose Microbiosensor Based on MnO2 and Glucose Oxidase Modified Carbon Fiber Microelectrode

A glucose microbiosensor has been developed using electrochemical codeposition of glucose oxidase (GOx) along with MnO₂ as mediator, onto a single carbon fiber microelectrode. A two‐step deposition of only MnO₂ initially and then of MnO₂ in the presence of GOx has been introduced to ensure appropriate activity of the mediator. Several parameters such as deposition potential and time, concentration levels etc. have been characterized and optimized. A thin Nafion film was applied as an immobilization/encapsulation and interference‐free protective layer. The proposed microbiosensor was employed as an amperometric glucose detector at pH 7.5 at an operating potential of +0.58 V (vs. Ag/AgCl). The microbiosensor is characterized by a well‐reproducible amperometric response, linear signal‐to‐glucose concentration range from 1.5 mmol L^?1 to 15 mmol L^?1, and a limit of detection (S/N=3) of 0.8 mmol L^?1. The microbiosensor exhibits good stability over more than ten hours of continuous measurement.     

Photodynamic Therapy of Oligoethylene Glycol‐Dendronized Reduction‐Sensitive Porphyrins

Graphene oxide (GO ) and its functionalized derivatives have attracted increasing attention in medical treatment. Herein, a reduction sensitive PEI‐GO ‐SS ‐TPP was synthesized for photodynamic therapy. More than 80% porphyrin release was observed in the presence of 10 mmol•L⁻¹ DTT in one day. The confocal laser scanning microscopy confirmed that the cell uptake efficiency of PEI‐GO‐SS‐TPP was remarkably enhanced as compared to free porphyrin which was significantly dependent on incubation time. For photodynamic therapy, GSH‐OEt could effectively increase the photodynamic therapy efficiency of PEI‐GO ‐SS ‐TPP . Compared with free porphyrin, the toxicity from PEI‐GO ‐SS ‐TPP is much higher with a low IC₅₀ (2.1 µg/mL ) value. All results indicate that the PEI‐GO ‐SS ‐TPP PSs are promising for photodynamic therapy.     

Biosensor Based on Self‐Assembling Glucose Oxidase and Dendrimer‐Encapsulated Pt Nanoparticles on Carbon Nanotubes for Glucose Detection

A novel amperometric glucose biosensor based on layer‐by‐layer (LbL) electrostatic adsorption of glucose oxidase (GOx) and dendrimer‐encapsulated Pt nanoparticles (Pt‐DENs) on multiwalled carbon nanotubes (CNTs) was described. Anionic GOx was immobilized on the negatively charged CNTs surface by alternatively assembling a cationic Pt‐DENs layer and an anionic GOx layer. Transmission electron microscopy images and ζ‐potentials proved the formation of layer‐by‐layer nanostructures on carboxyl‐functionalized CNTs. LbL technique provided a favorable microenvironment to keep the bioactivity of GOx and prevent enzyme molecule leakage. The excellent electrocatalytic activity of CNTs and Pt‐DENs toward H₂O₂ and special three‐dimensional structure of the enzyme electrode resulted in good characteristics such as a low detection limit of 2.5 μM, a wide linear range of 5 μM–0.65 mM, a short response time (within 5 s), and high sensitivity (30.64 μA mM^?1 cm^?2) and stability (80% remains after 30 days).     

Nonenzymatic H2O2 Electrochemical Sensor Based on SnO2‐NPs Coated Polyethylenimine Functionalized Graphene

This paper demonstrated using polyethylenimine (PEI)‐functionalized graphene (Gr) incorporating tin oxide (SnO₂) hybrid nanocomposite as a platform for nonenzymatic H₂O₂ electrochemical sensor. The results of UV‐vis spectroscopy and X‐ray diffraction (XRD) confirmed the simultaneous formation of tin oxide (SnO₂) nanocomposite and reduction of graphene oxide (GO). Transmission electron microscopy (TEM) images showed a uniform distribution of nanometer‐sized tin oxide nanoparticles on the grapheme sheets, which could be achieved using stannous chloride (SnCl₂) complex instead of tin oxide as precursor. The electrochemical measurements, including cyclic voltammetry (CV) and amperometric performance (I‐t), showed that the PEI‐functionalized Gr supported SnO₂ (SnO₂‐PEI‐Gr) exhibited an excellent electrocatalytic activity toward the H₂O₂. The corresponding calibration curve of the current response showed a linear detection range of 9×10⁻⁶∼1.64×10⁻³ mol L⁻¹, while the limit of detection was estimated to be 1×10⁻⁶ mol L⁻¹. Electrochemical studies indicated that SnO₂ and functionalized Gr worked synergistically for the detection of H₂O₂.     

Amperometric Glucose Biosensor Based on Electrochemically Deposited Gold Nanoparticles Covered by Polypyrrole

Graphite rod (GR) modified with electrochemicaly deposited gold nanoparticles (AuNPs) and adsorbed glucose oxidase (GOx) was used in amperometric glucose biosensor design. Enzymatic formation of polypyrrole (Ppy) on the surface of GOx/AuNPs/GR electrode was applied in order to improve analytical characteristics and stability of developed biosensor. The linear glucose detection range for Ppy/GOx/AuNPs/GR electrode was dependent on the duration of Ppy‐layer formation and the linear interval was extended up to 19.9 mmol L⁻¹ after 21 h lasting synthesis of Ppy. The sensitivity of the developed biosensor was determined as 21.7 μA mM⁻¹ cm⁻², the limit of detection – 0.20 mmol L⁻¹. Ppy/GOx/AuNPs/GR electrodes demonstrated advanced good stability (the t _1/2 was 9.8 days), quick detection of glucose (within 5 s) in the wide linear interval. Additionally, formed Ppy layer decreased the influence of electroactive species on the analytical signal. Developed biosensor is suitable for the determination of glucose in human serum samples.     

A Glucose Biosensor Based on Immobilization of Glucose Oxidase into 3D Macroporous TiO2

3D macroporous TiO₂ inverse opals have been derived from a sol‐gel procedure using polystyrene colloidal crystals as templates. EDS and SEM showed a face‐centered cubic (FCC) structure TiO₂ inverse opal was obtained. Glucose oxidase (GOx) was successfully immobilized on the surface of indium‐tin oxide (ITO) electrode modified by TiO₂ inverse opal (TiO_2(IO)). Electrochemical properties of GOx/TiO_2(IO)/ITO electrode were characterized by using the three electrodes system. The result of cyclic voltammetry showed that a couple of stable and well‐defined redox peaks for the direct electron transfer of GOx in absence of glucose, and the redox peak height enhanced in presence of 0.1 μM glucose. Compare with the ordinary structured GOx/TiO₂/ITO electrode, inverse opal structured GOx/TiO_2(IO)/ITO electrode has a better respond to the glucose concentration change. Under optimized experimental conditions of solution pH 6.8 and detection potential at 0.30 V versus saturated calomel electrode (SCE), amperometric measurements were performed. The sensitivity and the detection limit of glucose detection was 151 μA cm^?2 mM^?1 and 0.02 μM at a signal‐to‐noise ratio of 3, respectively. The good response was due to the good biocompatibility of TiO₂ and the large effective surface of the three‐dimensionally ordered macroporous structure.     

Glucose biosensor based on graphite electrodes modified with glucose oxidase and colloidal gold nanoparticles

The electrochemistry of glucose oxidase (GOx) immobilized on a graphite rod electrode modified by gold nanoparticles (Au-NPs) was studied. Two types of amperometric glucose sensors based on GOx immobilized and Au-NPs modified working electrode (Au-NPs/GOx/graphite and GOx/Au-NPs/graphite) were designed and tested in the presence and the absence of N-methylphenazonium methyl sulphate in different buffers. Results were compared to those obtained with similar electrodes not containing Au-NPs (GOx/graphite). This study shows that the application of Au-NPs increases the rate of mediated electron transfer. Major analytical characteristics of the amperometric biosensor based on GOx and 13 nm diameter Au-NPs were determined. The analytical signal was linearly related to glucose concentration in the range from 0.1 to 10 mmol L^?1. The detection limit for glucose was found within 0.1 mmol L^?1 and 0.08 mmol L^?1 and the relative standard deviation in the range of 0.1–100 mol L^?1 was 0.04–0.39%. The τ_1/2 of V _max characterizes the storage stability of sensors: this parameter for the developed GOx/graphite electrode was 49.3 days and for GOx/Au-NPs/graphite electrode was 19.5 days. The sensor might be suitable for determination of glucose in beverages and/or in food.     

Lab‐on‐a‐Chip Biosensor for Glucose Based on a Packed Immobilized Enzyme Reactor

In this work, the development of a packed immobilized enzyme reactor (IMER) and its integration to a capillary electrophoresis microchip is described. The present microchip design differs from others, in the fact that the same design could be used with or without the particles and, just by changing the material used to pack the IMER, different analytes can be detected. The applied procedure involves the separation of the target analyte by capillary electrophoresis (CE), which is then coupled to a post‐column IMER that produces H₂O₂. The H₂O₂ produced is finally detected downstream at the surface of a working electrode. Glucose was detected above 100 μM by packing particles modified with glucose oxidase at the end of the separation channel. The analytical performance of the microchip‐CE has been demonstrated by performing the separation and detection of glucose and noradrenaline. Additions of fructose showed no effect on either the peak position or the peak magnitude of glucose. The microchip‐CE‐IMER was also used to quantify glucose in carbonated beverages with good agreement with other reports.     

Speciation analysis of vanadium in natural water samples by electrothermal vaporization inductively coupled plasma optical emission spectrometry after separation/preconcentration with thenoyltrifluoroacetone immobilized on microcrystalline naphthalene

A sensitive and simple method for low temperature electrothermal vaporization inductively coupled plasma optical emission spectrometry (ETV-ICP-OES) determination of V(IV) and V(V) after separation/preconcentration by a micro-column packed with immobilized thenoyltrifluoroacetone (TTA) on microcrystalline naphthalene has been developed. Thenoyltrifluoroacetone was used as both a chelating agent for micro-column separation/preconcentration and a chemical modifier for ETV-ICP-OES determination of vanadium. Both vanadium species could be trapped by micro-column at pH 4.0, and the vanadate (VO₂⁺) ion could be collected selectively at pH 2.4. Solid material loaded with analyte in the micro-column was dissolved with 100 μL of acetone containing 2.0 mmol L⁻¹ TTA and the vanadium was determined subsequently by ETV-ICP-OES. The concentration of vanadyl (VO²⁺) ion was calculated by subtracting the vanadate concentration from the total concentration of vanadium. Under the optimized experimental conditions, the detection limit (3σ) for the preconcentration of 5 mL of aqueous solution is 0.068 μg L⁻¹ for both species and the relative standard deviations were 4.3% for vanadium(V) and 4.8% for vanadium(IV) (c=10 μg L⁻¹, n=7), respectively. The method was applied successfully to the determination of vanadium(IV) and vanadium(V) in natural water samples.     

Preconcentration and Successful Selective Detection of Traces of Diclofenac in Water using a Nanostructured Modified Carbon Paste Electrode

A highly sensitive, simple and low cost sensor for the quantification of the diclofenac has been constructed. This sensor consists of a carbon paste nano-structured by Multi-Walled Carbon Nanotubes (G-MWCNT)-CPE. Scanning electron microscopy (SEM) and voltammetry technique were used to characterize the electrode material and to determine the analytical performances of the sensor in comparison with those obtained at a G-CPE. The electrochemical oxidation of diclofenac on both G-CPE and (G-MWCNT)-CPE electrodes is mainly controlled by adsorption, presenting a maximum peak current intensity in H₂SO₄ 0.5 mol L⁻¹. The carbon nanotubes, as well as they provide higher conductivity of the paste, act as spacers between the flake graphite particles and avoid their stacking in order to make the surface of graphite particles more accessible to DCF adsorption. The voltammetric measurements of diclofenac on (G-MWCNT)-CPE provide a large quantification range from 0.02 to 1 μmol L⁻¹, a detection limit of 0.004 μmol L⁻¹ and quantification limit of 0.014 μmol L⁻¹ under the optimized operating conditions (H₂SO₄, 0.25 M+KCl 0.25 M, scan rate of 30 mV s⁻¹, preconcentration time 18 min. and MWNTC% (30 %)). The (G-MWCNT)-CPE sensor was successfully applied to natural water samples, just acidified with sulfuric acid (pH<1). These samples were doped with diclofenac in sub-micromolar range and the developed method was validated with excellent recoveries (within a maximum of 3 % difference from 100 %) for all samples indicating no interference effects of the water matrix.     

Simultaneous Determination of Hydroquinone and Catechol by Differential Alternative Pulses Voltammetry

The second order voltammetric technique of high resolution, Differential Alternative Pulses Voltammetry (DAPV), was applied for the simultaneous determination of hydroquinone (HQ) and catechol (CC) on bare spectroscopic graphite electrode. Well resolved anodic and cathodic peaks situated on both sides of the zero line were obtained, while the differential pulse voltammograms were overlapped. The linear concentration range for HQ and CC quantification by DAPV was extended up to 20 μmol L⁻¹ for both the isomers. The sensitivity of the determination was found to be 6.00 μA L μmol⁻¹ and 3.61 μA L μmol⁻¹, while the limit of detection reached was 0.2 μmol L⁻¹ and 0.5 μmol L⁻¹ for HQ and CC, respectively. No interference was observed from the commonly coexisting organic species such as resorcinol, phenol and p‐benzoquinone. The great resolution power of DAPV permitted obtaining excellent results without any electrode modification and any mathematical data processing.     

Tellurium Film Electrodes Deposited on Carbon and Mesoporous Carbon Screen‐printed Substrates for Anodic Stripping Voltammetric Determination of Copper

The performance of screen‐printed electrodes modified in situ with tellurium film for the anodic stripping voltammetric (ASV) determination of Cu(II) is reported. It was found that two types of screen‐printed substrates, namely carbon and mesoporous carbon, were optimal for this application. The selected in situ tellurium film modified electrodes were applied for the square wave ASV determination of copper at μg L⁻¹ concentration levels. Well‐defined and reproducible Cu oxidation stripping peaks were produced at a potential more negative than the anodic dissolution of tellurium. The highest sensitivity of Cu determination was achieved in 0.05 M HCl containing 50 μg L⁻¹ Te(IV) after 300 s of accumulation at −0.5 V. Using the optimized procedure, a linear range from 2 to 35 μg L⁻¹ of Cu(II) was obtained with a detection limit of 0.5 μg L⁻¹ Cu(II) (S/N=3) for 300 s of deposition time. Both sensors, carbon TeF‐SPE and mesoporous carbon TeF‐SPE, were successfully applied for the quantification of Cu in a certified reference surface water sample.     

Visible Light Photoelectrochemical Sensor for Acetaminophen Determination using a Glassy Carbon Electrode Modified with BiVO4 Nanoparticles

A new and simple photoelectrochemical (PEC) sensor using a glassy carbon electrode (GCE) modified with bismuth vanadate (BiVO₄) nanoparticles and dihexadecyl phosphate (DHP) film was useful for acetaminophen (AC) determination. In 0.2 mol L⁻¹ phosphate buffer (pH=9), the GCE without modification exhibited the smaller photocurrent (0.86 μA) when compared with GCE modified with 1.0 mg mL⁻¹ or 2.0 mg mL⁻¹ BiVO₄ nanoparticles suspension (5.9 and 34 μA, respectively). Based on the photocurrent signal generated through the interaction between GCE, BiVO₄ and the energy of visible light a chronoamperometric method for AC determination was developed. The AC linear range concentration from 0.099 to 0.99 μmol L⁻¹ and limits of detection and quantification of 0.027 and 0.091 μmol L⁻¹, respectively, was obtained. The proposed method was applied to the AC determination in commercial drugs and tap water with satisfactory accuracy and precision. Moreover, the PEC construction was easy and had a short response time, which might confer higher sample throughput for the method.     

Fabrication of β‐Cyclodextrin/Glycine/Carbon Nanotubes Electrochemical Neurotransmitters Sensor – Application in Ultra‐sensitive Determination of DOPAC in Human Serum

In this work, an electrochemical sensor was constructed by applying two successive thin layers of glycine‐carbon nanotubes mixture and β‐cyclodextrin (CNTs‐Gly)/CD over glassy carbon electrode surface for some neurotransmitters determination. A host‐guest interaction between CD and neurotransmitters molecules is expected and resulted in enhanced sensitivity, selectivity and stability of sensor response. Other components of the sensor are crucial for the unique electrochemical response. Carbon nanotubes allowed large surface area for glycine distribution that provided hydrogen bonding to CD moieties and contributed to facilitated charge transfer. It was possible to determine 3,4‐dihydroxy phenyl acetic acid (DOPAC) in the linear range of 0.1 μmol L⁻¹ to 80 μmol L⁻¹ with detection limit of 9.40 nmol L⁻¹, quantification limit of 31.5 nmol L⁻¹ and sensitivity of 4.16 μA/μmol L⁻¹. The proposed sensor was applied in synthetic cerebrospinal fluids samples using random standard addition method. Also, the proposed sensor was used to determine DOPAC in presence of common interferences and acceptable recovery results were achieved for its analysis in real blood serum. Figures of merit for (CNTs‐Gly)/CD composite in terms of precision, robustness, repeatability and reproducibility were reported.