Mercaptoethylpyrazine promoted electrochemistry of redox protein and amperometric biosensing of uric acid

Electrochemistry of microperoxidase-11 (MPx-11) anchored on the mixed self-assembled monolayer (SAM) of 2-(2-mercaptoethylpyrazine) (PET) and 4,4'-dithiodibutyric acid (DTB) on gold (Au) electrode and the biosensing of uric acid (UA) is described. MPx-11 has been covalently anchored on the mixed SAM of PET and DTB on Au electrode. MPx-11 on the mixed self-assembly exhibits reversible redox response characteristic of a surface confined species. The heterocyclic ring of PET promotes the electron transfer between the electrode and the redox protein. The apparent standard rate constant kapps obtained for the redox reaction of MPx-11 on the mixed monolayer is approximately 2.15 times higher than that on the single monolayer of DTB modified electrode. MPx-11 efficiently mediates the electrocatalytic reduction of H2O2. MPx-11 electrode is highly sensitive to H2O2 and it shows linear response for a wide concentration range. The electrocatalytic activity of the MPx-11 electrode is combined with the enzymatic activity of uricase (UOx) to fabricate uric acid biosensor. The bienzyme assembly is highly sensitive towards UA and it could detect UA as low as 2 microM at the potential of -0.1 V. The biosensor shows linear response with a sensitivity of 3.4+/-0.08 nA cm(-2) microM(-1). Ascorbate (AA) and paracetamol (PA) do not significantly interfere in the amperometric sensing of UA.     

Simultaneous voltammetric detection of dopamine and uric acid at their physiological level in the presence of ascorbic acid using poly(acrylic acid)-multiwalled carbon-nanotube composite-covered glassy-carbon electrode

The use of poly(acrylic acid) (PAA)-multiwalled carbon-nanotubes (MWNTs) composite-coated glassy-carbon disk electrode (GCE) (PAA-MWNTs/GCE) for the simultaneous determination of physiological level dopamine (DA) and uric acid (UA) in the presence of an excess of ascorbic acid (AA) in a pH 7.4 phosphate-buffered solution was proposed. PAA-MWNTs composite was prepared by mixing of MWNTs powder into 1 mg/ml PAA aqueous solution under sonication. GCE surface was modified with PAA-MWNTs film by casting. AA demonstrates no voltammetric peak at PAA-MWNTs/GCE. The PAA-MWNTs composite is of a high surface area and of affinity for DA and UA adsorption. DA exhibits greatly improved electron-transfer rate and is electro-catalyzed at PAA-MWNTs/GCE. Moreover, the electro-catalytic oxidation of UA at PAA-MWNTs/GCE is observed, which makes it possible to detect lower level UA. Therefore, the enhanced electrocatalytic currents for DA and UA were observed. The anodic peak currents at approximately 0.18 V and 0.35 V increase with the increasing concentrations of DA and UA, respectively, which correspond to the voltammetric peaks of DA and UA, respectively. The linear ranges are 40 nM to 3 microM DA and 0.3 microM to 10 microM UA in the presence of 0.3 mM AA. The lowest detection limits (S/N=3) were 20 nM DA and 110 nM UA.     

Zinc oxide/redox mediator composite films-based sensor for electrochemical detection of important biomolecules

Electrochemical oxidation of serotonin (SN) onto zinc oxide (ZnO)-coated glassy carbon electrode (GCE) results in the generation of redox mediators (RMs) that are strongly adsorbed on electrode surface. The electrochemical properties of zinc oxide-electrogenerated redox mediator (ZnO/RM) (inorganic/organic) hybrid film-coated electrode has been studied using cyclic voltammetry (CV). The scanning electron microscope (SEM), atomic force microscope (AFM), and electrochemical techniques proved the immobilization of ZnO/RM core/shell microparticles on the electrode surface. The GCE modified with ZnO/RM hybrid film showed two reversible redox peaks in acidic solution, and the redox peaks were found to be pH dependent with slopes of −62 and −60 mV/pH, which are very close to the Nernst behavior. The GCE/ZnO/RM-modified electrode exhibited excellent electrocatalytic activity toward the oxidations of ascorbic acid (AA), dopamine (DA), and uric acid (UA) in 0.1 M phosphate buffer solution (PBS, pH 7.0). Indeed, ZnO/RM-coated GCE separated the anodic oxidation waves of DA, AA, and UA with well-defined peak separations in their mixture solution. Consequently, the GCE/ZnO/RMs were used for simultaneous detection of DA, AA, and UA in their mixture solution. Using CV, calibration curves for DA, AA, and UA were obtained over the range of 6.0 × 10⁻⁶ to 9.6 × 10⁻⁴ M, 1.5 × 10⁻⁵ to 2.4 × 10⁻⁴ M, and 5.0 × 10⁻⁵ to 8 × 10⁻⁴ M with correlation coefficients of 0.992, 0.991, and 0.989, respectively. Moreover, ZnO/RM-modified GCE had good stability and antifouling properties.     

Overoxidized polypyrrole film directed single-walled carbon nanotubes immobilization on glassy carbon electrode and its sensing applications

In this paper, the films of overoxidized polypyrrole (PPyox) directed single-walled carbon nanotubes (SWNTs) have been electrochemically coated onto glassy carbon electrode (GCE). Electroactive monomer pyrrole was added into the solution containing sodium dodecyl sulfate (SDS) and SWNTs. Then, electropolymerization was proceeded at the surface of GCE, and a novel kind of conducting polymer/carbon nanotubes (CNTs) composite film with the orientation of CNTs were obtained correspondingly. Finally, this obtained polypyrrole (PPy)/SWNTs film modified GCE was oxidized at a potential of +1.8 V. It can be found that this proposed PPyox/SWNTs composite film modified GCE exhibited excellent electrocatalytic properties for some species such as nitrite, ascorbic acid (AA), dopamine (DA) and uric acid (UA), and could be used as a new sensor for practical applications. Compared with previous CNTs modified electrodes, SWNTs were oriented towards the outside of modified layer by PPyox and SDS, which made the film easily conductive. Moreover, this proposed film modified electrode was more stable, selective and applicable.     

Nonenzymatic electrochemical detection of glucose using well-distributed nickel nanoparticles on straight multi-walled carbon nanotubes

A nonenzymatic electrochemical sensor device was fabricated for glucose detection based on nickel nanoparticles (NiNPs)/straight multi-walled carbon nanotubes (SMWNTs) nanohybrids, which were synthesized through in situ precipitation procedure. SMWNTs can be easily dispersed in solution after mild sonication pretreatment, which facilitates the precursor of NiNPs binding to their surface and results in the homogeneous distribution of NiNPs on the surface of SMWNTs. The morphology and component of the nanohybrids were characterized by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD), respectively. Cyclic voltammetry (CV) and amperometry were used to evaluate the catalytic activity of the NiNPs/SMWNTs nanohybrids modified electrode towards glucose. It was found that the nanohybrids modified electrode showed remarkably enhanced electrocatalytic activity towards the oxidation of glucose in alkaline solution compared to that of the bare glass carbon electrode (GCE), the NiNPs and the SMWNTs modified electrode, attributing to the synergistic effect of SMWNTs and Ni²⁺/Ni³⁺ redox couple. Under the optimal detection conditions, the as-prepared sensors exhibited linear behavior in the concentration range from 1 μM to 1 mM for the quantification of glucose with a limit of detection of 500 nM (3σ). Moreover, the NiNPs/SMWNTs modified electrode was also relatively insensitive to commonly interfering species such as ascorbic acid (AA), uric acid (UA), dopamine (DA), galactose (GA), and xylose (XY). The robust selectivities, sensitivities, and stabilities determined experimentally indicated the great potential of NiNPs/SMWNTs nanohybrids for construction of a variety of electrochemical sensors.     

An amperometric uric acid biosensor based on modified Ir-C electrode

The level of uric acid (UA) has a high relationship with gout, hyperuricemia and Lesch-Nyan syndrome. The determination of UA is an important indicator for clinics and diagnoses of kidney failure. An amperometric UA biosensor based on an Ir-modified carbon (Ir-C) working electrode with immobilizing uricase (EC 1.7.3.3) was developed by thick film screen printing technique. This is the first time to report the utilization of an uricase/Ir-C electrode for the determination of UA by using chronoamperometric (CA) method. The high selectivity of UA biosensor was achieved due to the reduction of H(2)O(2) oxidation potential based on Ir-C electrode. Using uricase/Ir-C as the sensing electrode, the interference from the electroactive biological species, such as ascorbic acid (AA) and UA (might be directly oxidized on the sensing electrode) was slight at the sensing potential of 0.25 V (versus Ag/AgCl). UA was detected amperometrically based on uricase/Ir-C electrode with a sensitivity of 16.60 microAmM(-1) over the concentration range of 0.1-0.8 mMUA, which was within the normal range in blood. The detection limit of UA biosensor was 0.01 mM (S/N=6.18) in pH 7 phosphate buffer solution (PBS) at 37 degrees C. The effects of pH, temperature, and enzymatic loading on the sensing characteristics of the UA biosensor were also investigated in this study.     

Electrochemical synthesis of polyaniline nano-networks on p-aminobenzene sulfonic acid functionalized glassy carbon electrode Its use for the simultaneous determination of ascorbic acid and uric acid

A composite film of polyaniline (PAN) nano-networks/p-aminobenzene sulfonic acid (ABSA) modified glassy carbon electrode (GCE) has been fabricated via an electrochemical oxidation procedure and applied to the electro-catalytic oxidation of uric acid (UA) and ascorbic acid (AA). The ABSA monolayer at GCE surface has been characterized by X-ray photo-electron spectroscopy (XPS) and electrochemical techniques. Atomic force microscopy (AFM), field emission scanning electron microscope (SEM), electrochemical impedance spectroscopy (EIS), UV-visible absorption spectra (UV-vis) and cyclic voltammetry (CV) have been used to investigate the PAN-ABSA composite film, which demonstrates the formation of the composite film and the maintenance of the electroactivity of PAN in neutral and even in alkaline media. Due to its different catalytic effects towards the electro-oxidation of UA and AA, the modified GCE can resolve the overlapped voltammetric response of UA and AA into two well-defined voltammetric peaks with both CV and differential pulse voltammetry (DPV), which can be used for the selective and simultaneous determination of these species in a mixture. The catalytic peak currents are linearly dependent on the concentrations of UA and AA in the range of 50-250 and 35-175mumoll(-1) with correlation coefficients of 0.997 and 0.998, respectively. The detection limits for UA and AA are 12 and 7.5mumoll(-1), respectively. Besides the good stability and reproducibility of PAN-ABSA/GCE due to the covalent attachment of ABSA at GCE surface, the modified electrode also exhibits good sensitivity and selectivity.     

Simultaneous voltammetric determination of norepinephrine, ascorbic acid and uric acid on polycalconcarboxylic acid modified glassy carbon electrode

A novel polycalconcarboxylic acid (CCA) modified glassy carbon electrode (GCE) was fabricated by electropolymerization and then successfully used to simultaneously determine ascorbic acid (AA), norepinephrine (NE) and uric acid (UA). The characterization of electrochemically synthesized Poly-CCA film was investigated by atomic force microscopy (AFM), electrochemical impedance spectroscopy (EIS) and voltammetric methods. It was found that the electrochemical behavior of the polymer-modified electrode depended on film thickness, i.e., the electropylmyerization time. Based on the electrochemical data, the charge transfer coefficient (alpha) and the surface coverage (Gamma) were calculated. This poly-CCA modified GCE could reduce the overpotential of ascorbic acid (AA), norepinephrine (NE) and uric acid (UA) oxidation in phosphate buffer solution (pH 6.0), while it increases the peak current significantly. The current peak separations of AA/NE, NE/UA and AA/UA on this modified electrode are 91mV, 256mV and 390mV in CV at 100mVs(-1), respectively. Therefore, the voltammetric responses of these three compounds can be well resolved on the polymer-modified electrode, and simultaneously determination of these three compounds can be achieved. In addition, this modified electrode can be successfully applied to determine AA and NE in injection and UA in urine samples without interferences.     

Formation of electrochemically reduced graphene oxide on melamine electrografted layers and its application toward the determination of methylxanthines

The current study describes the electrografting of 2,4-diamino-1,3,5-triazine (AT) groups at the surfaces of glassy carbon electrode (GCE) and indium tin oxide (ITO) through in situ diazotization of melamine. The presence of AT groups at the surface of the electrode was confirmed by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). Furthermore, graphene oxide (GO) was self-assembled on AT grafted GCE. The oxygen functional groups present on the surface of GO were electrochemically reduced to form an electrochemically reduced graphene oxide (ERGO) on AT grafted electrode surface. Raman spectra show the characteristic D and G bands at 1340 and 1605 cm⁻¹, respectively, which confirms the successful attachment of GO on AT grafted surface, and the ratio of D and G bands was increased after the electrochemical reduction of GO. EIS shows that the electron transfer reaction of [Fe(CN)₆]^3−/4− was higher at the ERGO modified electrode than at bare, AT grafted, and GO modified GCEs. The electrocatalytic activity of ERGO was investigated toward the oxidation of methylxanthines. It shows excellent electrocatalytic activity toward these methylxanthines by not only shifting their oxidation potentials toward less positive potentials but also enhancing their oxidation currents.     

Simultaneous determination of catecholamines, uric acid and ascorbic acid at physiological levels using poly(N-methylpyrrole)/Pd-nanoclusters sensor

An interesting electrochemical sensor has been constructed by the electrodeposition of palladium nanoclusters (Pd_nano) on poly(N-methylpyrrole) (PMPy) film-coated platinum (Pt) electrode. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy were used to characterize the properties of the modified electrode. It was demonstrated that the electroactivity of the modified electrode depends strongly on the electrosynthesis conditions of the PMPy film and Pd_nano. Moreover, the modified electrode exhibits strong electrocatalytic activity toward the oxidation of a mixture of dopamine (DA), ascorbic acid (AA), and uric acid (UA) with obvious reduction of overpotentials. The simultaneous analysis of this mixture at conventional (Pt, gold [Au], and glassy carbon) electrodes usually struggles. However, three well-resolved oxidation peaks for AA, DA, and UA with large peak separations allow this modified electrode to individually or simultaneously analyze AA, DA, and UA by using differential pulse voltammetry (DPV) with good stability, sensitivity, and selectivity. This sensor is also ideal for the simultaneous analysis of AA, UA and either of epinephrine (E), norepinephrine (NE) or l-DOPA. Additionally, the sensor shows strong electrocatalytic activity towards acetaminophen (ACOP) and other organic compounds. The calibration curves for AA, DA, and UA were obtained in the ranges of 0.05 to 1 mM, 0.1 to 10 μM, and 0.5 to 20 μM, respectively. The detection limits (signal/noise [S/N] = 3) were 7 μM, 12 nM, and 27 nM for AA, DA, and UA, respectively. The practical application of the modified electrode was demonstrated by measuring the concentrations of AA, DA, and UA in injection sample, human serum, and human urine samples, respectively, with satisfactory results. The reliability and stability of the modified electrode gave a good possibility for applying the technique to routine analysis of AA, DA, and UA in clinical tests.     

Selective detection of uric acid in the presence of ascorbic acid at physiological pH by using a beta-cyclodextrin modified copolymer of sulfanilic acid and N-acetylaniline

A beta-cyclodextrin (CD) modified copolymer membrane of sulfanilic acid (p-ASA) and N-acetylaniline (SPNAANI) on glassy carbon electrode (GCE) was prepared and used to determine uric acid (UA) in the presence of a large excess of ascorbic acid (AA) by differential pulse voltammetry (DPV). The properties of the copolymer were characterized by X-ray photoelectron spectra (XPS) and Raman spectroscopy. The oxidation peaks of AA and UA were well separated at the composite membrane modified electrode in phosphate buffer solution (PBS, pH 7.4). A linear relationship between the peak current and the concentration of UA was obtained in the range from 1.0 x 10(-5) to 3.5 x 10(-4)mol L(-1), and the detection limit was 2.7 x 10(-6)mol L(-1) at a signal-to-noise ratio of 3. Two hundred and fifty-fold excess of AA did not interfere with the determination of UA. The application of the prepared electrode was demonstrated by measuring UA in human serum samples without any pretreatment, and the results were comparatively in agreement with the spectrometric clinical assay method.     

Polyethylene terephthalate membrane as a support for covalent immobilization of uricase and its application in serum urate determination

A method is described for covalent immobilization of uricase onto polyethylene terephthalate (PET) membrane with a conjugation yield of 4.44 μg/cm² and 66.6% retention of initial activity of free enzyme. The enzyme exhibited an increase in optimum pH from pH 7.0 to 8.5 and K_m for uric acid from 0.075 mM to 0.13 mM but slight decrease in temp. for maximum activity from 37 °C to 35 °C after immobilization. A colorimetric method for determination of serum uric acid was developed using immobilized uricase, which is based on measurement of H₂O₂ by a color reaction consisting of 3,5-dichlorobenzene sulphonic acid (DHBS), 4-aminoantipyrine and peroxidase as chromogenic system. Minimum detection limit of the method was 0.05 mM. Analytical recovery of added uric acid (5 mg/dl and 10 mg/dl) was 94.3% and 89.8%, respectively. Within and between batch coefficient of variation (CV) were <3.2% and <4.3%, respectively. A good correlation (r = 0.98) was found between uric acid values by standard enzymic colorimetric method and the present method. The immobilized uricase was reused 100 times during the span of 60 days without any considerable loss of activity, when stored in reaction buffer at 4 °C. The support chosen for the present study was biocompatible, antimicrobial, inert, impact resistant, light weight and had good shelf life.     

Construction of amperometric uric acid biosensor based on uricase immobilized on PBNPs/cMWCNT/PANI/Au composite

A chitosan-glutaraldehyde crosslinked uricase was immobilized onto Prussian blue nanoparticles (PBNPs) absorbed onto carboxylated multiwalled carbon nanotube (c-MWCNT) and polyaniline (PANI) layer, electrochemically deposited on the surface of Au electrode. The nanohybrid-uricase electrode was characterized by scanning electron microscopic (SEM), Fourier transform infrared spectroscopy (FTIR) and cyclic voltammetry. An amperometric uric acid biosensor was fabricated using uricase/c-MWCNT/PBNPs/Au electrode as working electrode, Ag/AgCl as standard and Pt wire as auxiliary electrode connected through a potentiostat. The biosensor showed optimum response within 4 s at pH 7.5 and 40 °C, when operated at 0.4 V vs. Ag/AgCl. The linear working range for uric acid was 0.005-0.8 mM, with a detection limit of 5 μM. The sensor was evaluated with 96% recovery of added uric acid in sera and 4.6 and 5.4% within and between batch of coefficient of variation respectively and a good correlation (r = 0.99) with standard enzymic colorimetric method. This sensor measured uric acid in real serum samples. The sensor lost only 37% of its initial activity after its 400 uses over a period of 7 months, when stored at 4 °C.     

ZnO–CuxO/polypyrrole nanocomposite modified electrode for simultaneous determination of ascorbic acid,dopamine, and uric acid

Novel zinc oxide (ZnO) nanosheets and copper oxide (Cu_xO, CuO, and Cu₂O) decorated polypyrrole (PPy) nanofibers (ZnO–Cu_xO–PPy) have been successfully fabricated for the simultaneous determination of ascorbic acid (AA), dopamine (DA), and uric acid (UA). The morphology and structure of ZnO–Cu_xO–PPy nanocomposites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. Compared with the bare glassy carbon electrode (GCE), PPy/GCE, Cu_xO–PPy/GCE, and ZnO–PPy/GCE, ZnO–Cu_xO–PPy/GCE exhibits much higher electrocatalytic activities toward the oxidation of AA, DA, and UA with increasing peak currents and decreasing oxidation overpotentials. Cyclic voltammetry (CV) results show that AA, DA, and UA could be detected selectively and sensitively at ZnO–Cu_xO–PPy/GCE with peak-to-peak separation of 150 and 154 mV for AA–DA and DA–UA, respectively. The calibration curves for AA, DA, and UA were obtained in the ranges of 0.2 to 1.0 mM, 0.1 to 130.0 μM, and 0.5 to 70.0 μM, respectively. The lowest detection limits (signal/noise = 3) were 25.0, 0.04, and 0.2 μM for AA, DA, and UA, respectively. With good selectivity and sensitivity, the current method was applied to the determination of DA in injectable medicine and UA in urine samples.     

Fabrication of a glucose sensor based on a novel nanocomposite electrode

The direct electrocatalytic oxidation of glucose in alkaline medium at nanoscale nickel hydroxide modified carbon ionic liquid electrode (CILE) has been investigated. Enzyme free electro-oxidation of glucose have greatly been enhanced at nanoscale Ni(OH)(2) as a result of electrocatalytic effect of Ni(+2)/Ni(+3) redox couple. The sensitivity to glucose was evaluated as 202 microA mM(-1)cm(-2). From 50 microM to 23 mM of glucose can be selectively measured using platelet-like Ni(OH)(2) nanoscale modified CILE with a detection limit of 6 microM (S/N=3). The nanoscale nickel hydroxide modified electrode is relatively insensitive to electroactive interfering species such as ascorbic acid (AA), and uric acid (UA) which are commonly found in blood samples. Long-term stability, high sensitivity and selectivity as well as good reproducibility and high resistivity towards electrode fouling resulted in an ideal inexpensive amperometric glucose biosensor applicable for complex matrices.     

Nonenzymatic glucose sensor based on ultrasonic-electrodeposition of bimetallic PtM (M = Ru, Pd and Au) nanoparticles on carbon nanotubes–ionic liquid composite film

We report here for the first time on the fabrication of highly dispersed PtM (M = Ru, Pd and Au) nanoparticles on composite film of multi-walled carbon nanotubes (MWNTs)–ionic liquid (IL, i.e., trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide) by using ultrasonic-electrodeposition method. The PtM nanoparticles are characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction, and we find that they are well-dispersed and exhibit alloy properties. Electrochemical experiments show that the PtRu(1:1, i.e., ratio of c(H₂PtCl₆)/c(RuCl₃))–MWNT–IL nanocomposite modified glassy carbon electrode (PtRu(1:1)–MWNT–IL/GCE) has smaller electron transfer resistance and larger active surface area than PtRu(1:1)/GCE, PtRu(1:1)–MWNT/GCE, PtPd(1:1)–MWNT–IL/GCE and PtAu(1:1)–MWNT–IL/GCE. The PtRu(1:1)–MWNT–IL/GCE also presents stronger electrocatalytic activity toward the glucose oxidation than other electrodes. At −0.1 V, the electrode responds linearly to glucose up to 15 mM in neutral media, with a detection limit of 0.05 mM (S/N = 3) and detection sensitivity of 10.7 μA cm⁻² mM⁻¹. Meanwhile, the interference of ascorbic acid, uric acid, acetamidophenol and fructose is effectively avoided. The as-made sensor was applied to the determination of glucose in serum and urine samples. The results agreed closely with the results obtained by a hospital. This novel nonenzyme sensor thus has potential application in glucose detection.     

Selective electrochemical sensor for folic acid at physiological pH using ultrathin electropolymerized film of functionalized thiadiazole modified glassy carbon electrode

This paper demonstrated the selective determination of folic acid (FA) in the presence of important physiological interferents, ascorbic acid (AA) and uric acid (UA) at physiological pH using electropolymerized film of 5-amino-2-mercapto-1,3,4-thiadiazole (p-AMT) modified glassy carbon (GC) electrode. Bare GC electrode fails to determine the concentration of FA in the presence of AA and UA due to the surface fouling caused by the oxidized products of AA and FA. However, the p-AMT film modified electrode not only separates the voltammetric signals of AA, UA and FA with potential differences of 170 and 410 mV between AA–UA and UA–FA, respectively but also shows higher oxidation current for these analytes. The p-AMT film modified electrode displays an excellent selectivity towards the determination of FA even in the presence of 200-fold AA and 100-fold UA. Using amperometric method, we achieved the lowest detection of 75 nM UA and 100 nM each AA and FA. The amperometric current response was increased linearly with increasing FA concentration in the range of 1.0 × 10⁻⁷–8.0 × 10⁻⁴ M and the detection limit was found to be 2.3 × 10⁻¹⁰ M (S/N = 3). The practical application of the present modified electrode was successfully demonstrated by determining the concentration of FA in human blood serum samples.     

Voltammetric biosensors for the determination of formate and glucose-6-phosphate based on the measurement of dehydrogenase-generated NADH and NADPH

This paper describes the development of a modified electrode for the electrocatalytic oxidation of beta-nicotinamide adenine dinucleotide (beta-NADH) and beta-nicotinamide adenine dinucleotide phosphate (beta-NADPH) using electropolymerised 3,4-dihydroxybenzaldehyde (3,4-DHB). Two voltammetric biosensors using enzyme-immobilised membranes were constructed for the determination of formic acid and glucose-6-phosphate (G6P), respectively. The formic acid biosensor based on the combination of formate dehydrogenase (FDH)-modified membrane with 3,4-DHB-coated glassy carbon electrode is one to two orders more sensitive (LOD, 5.0x10(-5) M) than previously reported electrochemical biosensors. Similarly, lower detection limit (4.0x10(-5) M) for the measurement of G6P was achieved using glucose-6-phosphate dehydrogenase (G6PDH) in the presence of beta-NADP(+). The interference of uric acid and ascorbate was minimised by incorporating an additional membrane modified with uricase and ascorbate oxidase, respectively. The biosensing scheme developed in this study can be adopted universally with a number of dehydrogenases for the detection of different substrates.     

Aptamer-graphene oxide for highly sensitive dual electrochemical detection of Plasmodium lactate dehydrogenase

A 90 mer ssDNA aptamer (P38) enriched against Plasmodium falciparum lactate dehydrogenase (PfLDH) through SELEX process was immobilized over glassy carbon electrode (GCE) using graphene oxide (GO) as an immobilization matrix, and the modified electrode was investigated for detection of PfLDH. The GO was synthesized from powdered pencil graphite and characterized by XRD based on the increased interlayer distance between graphitic layers from 0.345 nm for graphite to 0.829 nm for GO. The immobilization of P38 on GO was confirmed by I_D/I_G intensity ratio in Raman spectra where, the ratio were 0.67, 0.915, and 1.35 for graphite, GO and P38-GO, respectively. The formation of the P38 layer over GO-GCE was evident from an increase in the surface height in AFM analysis of the electrode from ∼3.5 nm for GO-GCE to ∼27 nm for P38-GO-GCE. The developed aptasensor when challenged with the target, a detection of as low as 0.5 fM of PfLDH was demonstrated. The specificity of the aptasensor was confirmed through a voltametric measurement at 0.65 V of the reduced co-factor generated from the PfLDH catalysis. Studies on interference from some common proteins, storage stability, repeatability and analysis of real samples demonstrated the practical application potential of the aptasensor.     

Nanocrystalline diamond modified gold electrode for glucose biosensing

Boron-doped diamond has drawn much attention in electrochemical sensors. However there are few reports on non-doped diamond because of its weak conductivity. Here, we reported a glucose biosensor based on electrochemical pretreatment of non-doped nanocrystalline diamond (N-NCD) modified gold electrode for the selective detection of glucose. N-NCD was coated on gold electrode and glucose oxidase (GOx) was immobilized onto the surfaces of N-NCD by forming amide linkages between enzyme amine residues and carboxylic acid groups on N-NCD. The anodic pretreatment of N-NCD modified electrode not only promoted the electron transfer rate in the N-NCD thin film, but also resulted in a dramatic improvement in the reduction of the dissolved oxygen. This performance could be used to detect glucose at negative potential through monitoring the current change of oxygen reduction. The biosensor effectively performs a selective electrochemical analysis of glucose in the presence of common interferents, such as ascorbic acid (AA), acetaminophen (AP) and uric acid (UA). A wide linear calibration range from 10 microM to 15 mM and a low detection limit of 5 microM were achieved for the detection of glucose.