Carbon Nanotube-gold Nanoparticle Research Articles

Determination of promethazine in forensic samples using multi-walled carbon nanotube-gold nanoparticle electrochemical sensor.

An electrochemical sensor was developed based on a glassy carbon electrode (GCE) modified with multi-walled carbon nanotubes (MWCNTs) and gold nanoparticles (AuNPs) for the determination of promethazine (PMZ) in 'purple drank', pharmaceutical formulations, and synthetic saliva. The oxidation of PMZ at the modified electrode occurred at a higher cathodic potential and produced a higher sensitivity compared to the unmodified GCE. The morphology of the modified electrode was characterized using field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). The presence of MWCNTs and AuNPs was confirmed. The optimized parameters included the concentration and pH of the supporting electrolyte, amount of modifiers used to fabricate the electrode, deposition potential, and time. Using these optimized conditions, the method has a linear range from 0.5 to 100 μmol L-1, with a R2 value of 0.9991. The limit of detection (3SDblank/slope) was 0.13 μmol L-1. The proposed electrochemical sensor was successfully applied for the determination of PMZ in 'purple drank', pharmaceutical formulations, and spiked synthetic saliva samples. The results obtained from this sensor were in statistical agreement with the values obtained using the reference gas chromatography-flame ionization method.

Disposable Stochastic Platform for the Simultaneous Determination of Calcipotriol and Betamethasone in Pharmaceutical and Surface Water Samples

A disposable stochastic platform based on calix [6]arene modified multi-walled carbon nanotubes-gold nanoparticles screen-printed electrode has been developed for the simultaneous determination of calcipotriol and betamethasone. For both analytes, very wide linear concentration ranges and extremely low limits of quantification (LOQ) were obtained: from 1.0 × 10−15 to 1.0 × 10−3 mol L−1 and with a 1.0 × 10−15 mol L−1 LOQ for calcipotriol, and from 1.0 × 10−16 to 1.0 × 10−2 mol L−1 with a 1.0 × 10−16 mol L−1 LOQ for betamethasone. The applicability of the sensing platform was successfully tested in commercially available topical pharmaceutical gel and surface water samples, obtaining recovery values ranging from 99.10 to 99.99% and relative standard deviation values under 0.05%. The obtained results render the proposed platform a viable, robust, selective, and sensitive tool that can be employed for the determination of the analytes in on-site routine quality control of pharmaceuticals and water quality monitoring.

A cellular nitric oxide sensor based on porous hollow fiber with flow-through configuration

Nitric oxide plays important transmission and regulation roles in the human body, but its in-vitro concentration is extremely low with a short half-life. In this work, we developed a three-dimensional ‘flow-through’ configuration based on polysulfone hollow fiber (PHF) for efficient detection of cell released NO. The PHF served as the substrate for cell culture as well as the base layer of the working electrode. The carbon nanotubes-gold nanoparticles (CNT-AuNPs) composites uniformly wrapped around the PHF as the sensing layer. The CNT provided a large specific surface area, which allowed uniform distribution and high loading of AuNPs, thus enhancing the electrocatalytic activity synergistically. Compared with the conventional flow-by configuration, such configuration resulted in a higher surface area per unit volume and enhanced NO molecule capture efficiency. The CNT-AuNPs PHF sensor showed a low detection limit (91 nM), high stability, selectivity, and biocompatibility. We utilized it for real-time in-situ detection of NO released by human lung cancer cell H1299 under drug stimulation. Furthermore, owing to the unique PHF structure, we performed long-term monitoring of NO release under the treatment of Lipopolysaccharide, Nitroglycerin and Aminoguanidine, which helps to understand the kinetic process of cellular drug response.

Label-Free and Ultrasensitive Electrochemical DNA Biosensor Based on Urchinlike Carbon Nanotube-Gold Nanoparticle Nanoclusters

Nanomaterials have been extensively utilized in biosensing systems for highly sensitive and selective detection of a variety of biotargets. In this work, a facile, label-free, and ultrasensitive electrochemical DNA biosensor has been developed, based on "urchinlike" carbon nanotube-gold nanoparticle (CNT-AuNP) nanoclusters, for signal amplification. Specifically, electrochemical polymerization of dopamine (DA) was employed to modify a gold electrode for immobilization of DNA probes through the Schiff base reaction. Upon sensing the target nucleic acid, the dual-DNA (reporter and linker) functionalized AuNPs were introduced into the sensing system via DNA hybridization. Afterward, the end-modified single-wall carbon nanotubes with DNA (SWCNT-DNA) were attached to the surface of the AuNPs through linker-DNA hybridization that formed 3D radial nanoclusters, which generated a remarkable electrochemical response. Because of the larger contact surface area and super electronic conductivity of CNT-AuNP clusters, this novel designed 3D radial nanostructure exhibits an ultrasensitive detection of DNA, with a detection limit of 5.2 fM (a linear range of from 0.1 pM to 10 nM), as well as a high selectivity that discriminates single-mismatched DNA from fully matched target DNA under optimal conditions. This biosensor, which combines the synergistic properties of both CNTs and AuNPs, represents a promising signal amplification strategy for achieving a sensitive biosensor for DNA detection and diagnostic applications.

Preparation of a Magnetic Multiwalled Carbon Nanotube-Gold Nanoparticle Hybrid Material for the Efficient Extraction of Triazine Herbicides from Rice

A facile and efficient analytical method for determination of four triazine herbicides (atrazine, ametryn, terbutryn, and terbuthylazine) from rice was developed by employing a magnetic multiwalled carbon nanotube-gold nanoparticle material (mMWCNT@Au) as the solid-phase extraction adsorbent, followed by high performance liquid chromatography (HPLC) detection. The mMWCNT@Au was fabricated based on a simple solvothermal reduction method. The prepared material was characterized by transmission electron microscopy, scanning electron microscopy, and energy dispersive spectroscopy. Parameters influencing the extraction efficiency (the type and volume of extraction solvent, mass of mMWCNT@Au, extraction time, volume of desorption solvent and desorption time) were systematically investigated and optimized. Under the optimal extraction and determination conditions, the calibration curves were linear over the concentration range of 2.5 to 1000 μg kg−1 with correlation coefficients in the range of 0.9980 to 0.9998. The limits of detection for atrazine, ametryn, terbutryn, and terbuthylazine were 1.05, 0.97, 1.19, and 1.22 μg kg−1, respectively. The intra-day and inter-day recoveries were in the ranges from 88.3 to 105.4% and 88.2 to 101.8%, respectively. The relative standard deviations ranged from 4.3 to 9.0% and 4.5 to 9.1%, respectively. The proposed method has been shown to be a reliable alternative approach for the determination of triazine herbicides in rice samples.

An electrochemical biosensor for the detection of epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is critically involved in a variety of biological processes. Electrochemical sensing offers potential to develop more effective technology for EMT detection. In this study, by using the unique performance of quantum dot (QD)-nanocomposite materials, we establish an electrochemical biosensor that can specifically detect the change of E-cadherin and analyze different stages of EMT. The signal for EMT is largely magnified due to the transmission of molecular information to the electronic device. In addition, differential pulse voltammetry reveals that the response of the electrochemical signals is rapid and sensitive, due to the synergistic effect of QDs and carbon nanotube-gold nanoparticles. Our study thus suggests that electrochemical sensing is an effective technology for detecting EMT and may have broad applications in analyzing various cell type transitions.

PC software-based portable cyclic voltammetry system with PB-MCNT-GNPs-modified electrodes for E. coli detection

PC software-based portable cyclic voltammetry (PCV) systems have the advantages of portability, high performance, and real-time detection. In this paper, the PCV system used cyclic voltammetry (CV) as the main detection and analysis method and contained the following components: a three-electrode unit, a portable potentiostat, and PC software. The PC software was used as the system control and display, and a dynamic peak position adjustment (DPPA) algorithm for E. coli measurements based on thick biofilm modification on electrodes was designed especially for this system to realize the real-time correspondence between the measured results and the modified electrodes. The performance test results obtained by setting different detection parameters in the PCV system were compared with those of commercial electrochemical workstations. The difference was less than 4.99%, with a relative standard deviation less than 0.20%. An electrochemical biosensor based on a Prussian blue-multiwalled carbon nanotube-gold nanoparticle composite was developed for E. coli detection. After constructing an antibody-BSA-E. coli electrode modification on the sensor, experimental data processed by the DPPA algorithm showed that the logarithm (lg DfE.coli) of the E. coli dilution factor and the peak current response had a linear relationship. The PCV system could quickly and accurately detect E. coli concentrations with dynamic adjustment algorithms for biofilm-modified electrodes. Furthermore, the system could detect the electrochemical activities of various high-sensitivity biomolecules, showing great detection potential for on-site monitoring and meeting the requirements of real-time and portable detection in various food safety fields.

1-Pyrene carboxylic acid functionalized carbon nanotube-gold nanoparticle nanocomposite for electrochemical sensing of dopamine and uric acid.

A highly sensitive, selective and cost effective method is described for sensing dopamine (DA) and uric acid (UA). A glassy carbon electrode (GCE)was modified with a nanocomposite consisting of gold nanoparticle-loaded multi-walled carbon nanotube (CNT) modified with 1-pyrene carboxylic acid (PCA). The stable aqueous dispersion of non-covalently functionalized CNT-PCA is an efficient bioprobe for the ultra sensitive and selective detection of dopamine and uric acid in the presence of the potentially interfering agent ascorbic acid (AA). The presence of PCA on the CNT introduces anionic carboxyl groups which repel ascorbate. The presence of thepyrene group augments high electrocatalytic activity towards oxidation of DA and UA, and the gold nanoparticles contribute to the amplification of the signal. The modified GCE gives an excellent peak current with well distinguishable peaks for AA, DA and UA (near -0.08V, +0.14V, and+0.22V vs Ag/AgCl) in differential pulse voltammetry. Chronoamperometric detection of DA (working potential of 0.16V vs Ag/AgCl) and UA (working potential of 0.3V vs Ag/AgCl) showed linear ranges of 1nM-150μM (LOD 1nM) and 1μM-240μM (LOD 1μM) for DA and UA, respectively. The nanoprobe was validated by monitoring the recovery of spiked DA and UA in human blood serum samples which indicated a recovery within ±2%. Graphical abstract A glassy carbon electrode modified with a gold nanoparticle-loaded multi-walled carbon nanotube (CNT) - 1-pyrene carboxylic acid (PCA) composite was used for the sensitive and selective detection of the dopamine and uric acid.

A novel molecularly imprinted sensing platform based on MWCNTs/AuNPs decorated 3D starfish like hollow nickel skeleton as a highly conductive nanocomposite for selective and ultrasensitive analysis of a novel pan-genotypic inhibitor velpatasvir in body fluids

Herein, a novel ultrasensitive molecularly imprinted sensor (MIS) for selective determination of a new direct acting anti-HCV velpatasvir (VELPR) was developed and fabricated for the first time. The fabricated MIS based on modification of a glassy carbon electrode (GCE) with multi-walled carbon nanotubes-gold nanoparticles (MWCNTs-AuNPs) composite after deposition on electro-synthesized 3D starfish like hollow nickel skeleton (3D SH-Ni S) to increase conductivity and effective surface area of MIS to amplify its signal. After that, the electrode was coated with molecularly imprinted polymer (MIP) by in situ electro-polymerization forming cavity with specific affinity and natural binding sites to VELPR. Differential pulse voltammetry (DPV) was used for selective detection of VELPR in complex matrices whereas, scanning electron microscope (SEM) and cyclic voltammetry (CV) were employed to characterize the fabricated sensor. All experimental factors concerning fabrication and chemical sensing properties were carefully studied and optimized. Under the optimized variables, the fabricated sensor exhibited excellent DPV response to VELPR over the range of 0.649–80.0 ng mL−1 with LOD of 0.21 ng mL−1. In addition to high sensitivity and selectivity, the sensor response to VELPR was highly reproducible and stable. Moreover, the fabricated sensor was successfully applied for the determination of VELPR in complex biological matrices and pharmaceutical tablets.

Facile fabrication of a 3,4,9,10-perylene tetracarboxylic acid functionalized graphene-multiwalled carbon nanotube-gold nanoparticle nanocomposite for highly sensitive and selective electrochemical detection of dopamine.

A novel non-enzymatic electrochemical sensor for highly sensitive and selective detection of dopamine was developed based on a 3,4,9,10-perylene tetracarboxylic acid functionalized graphene-multiwalled carbon nanotube-gold nanoparticle nanocomposite modified glassy carbon electrode (PTCA-RGO-MWCNTs-Au NPs/GCE). The nanocomposite film was prepared by a facile, eco-friendly and controllable route and its morphology was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopic (EDX) analysis, and X-ray diffraction (XRD) spectroscopy, respectively. Cyclic voltammetry and chronoamperometry were used for evaluating the electrochemical behaviors of the prepared sensor. The DA sensor exhibited excellent electrochemical performance toward DA with a sensitivity as high as 0.124 μA mM-1, a wide linear range of 1-100 μM and a low detection limit of 0.07 μM (S/N = 3). Moreover, it showed good selectivity toward DA without any obvious interference by AA and UA. Furthermore, the prepared DA sensor was applied to detect DA in real samples with satisfactory results.

An immunochromatographic assay for carcinoembryonic antigen on cotton thread using a composite of carbon nanotubes and gold nanoparticles as reporters

This paper describes a low-cost, sensitive, visual and rapid immunochromatographic assay method on cotton thread for carcinoembryonic antigen (CEA) detection by using novel carbon nanotube/gold nanoparticles (CNT/GNPs) nanocomposite reporter probe. CEA, a lung cancer protein biomarker, was used as analyte to demonstrate the principle of the immunochromatographic assay on cotton thread biosensor. In the presence of target CEA, the decreasing aggregation amount of CNT/GNPs nanocomposite reporter probes on the test zone induced directly readout by naked eye. Meanwhile, quantitative detection could be performed conveniently with a commercial available scanner. The performance with respect to sensitivity of the method was greatly improved by 2–3 magnitudes comparing with traditional gold nanoparticles (GNPs) or carbon nanotubes (CNTs) as reporter probe. Under optimal conditions, the biosensor was capable of detecting 2.32 ng/mL CEA (S/N ≥ 3) which is sensitive enough for clinical diagnosis. These results indicated the novel CNT/GNPs nanocomposite reporter probe based immunochromatographic assay on cotton thread is particularly suitable for point-of-care (POC) diagnostics in resource-limited regions.

Using carbon nanotubes-gold nanocomposites to quench energy from pinnate titanium dioxide nanorods array for signal-on photoelectrochemical aptasensing

On the basis of the absorption and emission spectra overlap, an enhanced resonance energy transfer caused by excition-plasmon resonance between carbon nanotubes-gold nanoparticles (CNTs-Au) and pinnate titanium dioxide nanorods array (P-TiO2 NA) was obtained. Three-dimensional single crystalline P-TiO2 were prepared successfully on fluorine-doped tin oxide conducting glass (FTO glass), and its optical absorption properties and photoelectrochemical (PEC) properties were investigated. With the synergy of CNTs-Au as energy acceptor, it resulted in the enhancement of energy transfer between excited P-TiO2 NA and CNTs-Au. Upon the novel sandwichlike structure formed via DNA hybridization, the exciton produced in P-TiO2 NA was annihilated and a damped photocurrent was obtained. With the use of carcinoembryonic antigen (CEA) as a model which bonded to its specific aptamer and destroyed the sandwichlike structure, the energy transfer efficiency was lowered, leading to PEC response augment. Thus a signal-on PEC aptasensor was constructed. Under the optimal conditions, the PEC aptasensor for CEA determination exhibited a linear range from 0.001 to 2.5ngmL−1 with a detection limit of 0.39pgmL−1 and was satisfactory for clinical sample detection. Furthermore, the proposed aptasensor shows satisfying performance, such as easy preparation, rapid detection and so on. Moreover, since different aptamer can specifically bind to different target molecules, the designed strategy has an expansive application for the construction of versatile PEC platforms.

An Efficient Approach to Prepare Carbon Nanotube–Gold Nanoparticles Nanocomposites Based on Amphiphilic Copolymer Containing Coumarin

Abstract An efficient approach to prepare carbon nanotube–gold nanoparticle nanocomposites was reported by using amphiphilic copolymer containing coumarin. Amphiphilic copolymer PSVM was synthesized by addition–fragmentation chain transfer (RAFT) polymerization and was employed to functionalize pristine MWNTs (multiwalled nanotubes). Gold nanoparticles (AuNPs) synthesized by an in situ reduction approach were then strongly anchored on the functionalized MWNTs. This technique provided a simple and convenient route to efficiently assemble AuNPs onto the surfaces of MWNTs.

A gold nanoparticle functionalized multiwalled carbon nanotube–poly(o-phenylenediamine) composite film for glucose biosensing applications

A new biosensor based on immobilization of glucose oxidase on a multiwalled carbon nanotube–gold nanoparticle coated glassy carbon electrode (GCE) was fabricated for the electrochemical determination of glucose. First, a carboxylated multiwalled carbon nanotube (cMWCNT) was functionalized with 4-aminothiophenol (ATP) by a coupling agent, N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride, and then gold nanoparticles (AuNPs) were covalently linked to the ATP–cMWCNT via the formation of a covalent Au–S bond. Finally, glucose oxidase was immobilized simultaneously by the electropolymerization of o-phenylenediamine on the AuNP–ATP–cMWCNT coated GCE. The biosensor showed a linear range from 0.05 mM to 8.85 mM with a sensitivity of 27.93 μA mM−1 cm−2 and a detection limit of 0.015 mM for the detection of glucose. The biosensor was successfully applied to estimate the glucose concentration in human blood serum samples. Furthermore, the AuNP–ATP–cMWCNT was investigated for the non-enzymatic detection of hydrogen peroxide by using voltammetric and amperometric techniques, indicating a linear relationship in the range of 5 × 10−3 to 13.10 mM.

A nano self-assembled artificial peroxidase: spectroscopic and electrochemical investigations

A nano-micelle with highly efficient peroxide activity was constructed by self-assembly of sodium dodecyl sulfate micellar, histidine and hematin in 50 mM phosphate buffer at 25 °C. UV–Vis spectrometry methods were utilized for characterization of the nanostructured material or artificial peroxidase (AP). The Michaelis–Menten (K m) and catalytic rate (k cat) constants of the AP were obtained to be 5.5 μM and 0.06 s−1, respectively, in 50 mM phosphate buffer solution at pH 8.0. The catalytic efficiency of AP was evaluated to be 0.011 μM−1 s−1. The AP was also immobilized on a functional multi-wall carbon nanotubes-gold nanoparticles (AuNPs) nano-complex modified glassy carbon electrode (GCE). The transmission electron microscopy method was utilized for the characterization of the nano-materials. The electron-transfer rate constant (k s) and the apparent Michaelis–Menten constant K m app of the AP modified GCE were evaluated to be 1.36 s−1 and 0.19 μM, respectively. For a biosensor without a redox protein, the properties of the AP modified GCE were significant and will further benefit from additional studies and improvement.

Conductivity modulation of carbon nanotubes through hybridization with quantum dots and gold nanoparticles

CdSe/ZnS quantum dots and gold nanoparticles have been grown in-situ onto the surface of carbon nanotubes. The nanohybrids were then, dielectrophoretically aligned between gold microelectrodes and the electrical response was investigated. The conductance of nanotube-quantum dot hybrid was found to be sensitive to incident white light. The electrical resistance of the carbon nanotube-quantum dot nanohybrid decreased from 1.9 × 10 4 Ω (in the dark) to 0.91 × 10 4 Ω after the impact of white light. The electrical resistance of the carbon nanotube-gold nanoparticle nanohybrid was calculated to be 1.86 × 10 3 Ω, which is much smaller than that of carboxylated carbon nanotubes (4.03 × 10 5 Ω). Such a modulation in electrical properties may enhance the use of carbon nanotubes in future nanoscale devices.

Synergic effect of multi-walled carbon nanotubes and gold nanoparticles towards immunosensing of ricin with carbon nanotube–gold nanoparticles–chitosan modified screen printed electrode

An amperometric immunosensor for the specific detection of Ricinus communis is reported. Screen printed electrodes (SPEs) were modified with gold nanoparticles (GNPs) loaded multiwalled carbon nanotubes (MWCNTs)-chitosan (Ch) film. The ratio of MWCNT and GNP was optimised to get best electrochemically active electrode. Sandwich immunoassay format was used for the immunosensing of ricin. The revealing antibodies tagged with the enzyme alkaline phosphatase (ALP) converts the substrate 1-naphthyl phosphate into 1-naphthol that was determined with the amperometric technique. The amperometric current obtained was correlated with the concentration of ricin. The prepared GNP-MWCNT-Ch-SPE showed high stability due to the Ch film, short response time with good reproducibility and increased shelf life of the electrodes immobilised with antibodies. The electrochemical activity of the electrode improved because of optimization of composition of CNTs and gold nanoparticles. Under the optimal conditions, the modified electrode showed a wide linear response to the concentration of ricin in the range of 2.5-25 ng mL(-1) with a limit of detection of 2.1 ng mL(-1) and with a relative standard deviation of 5.1% and storage life of 32 days.

One step electrochemically deposited nanocomposite film of chitosan–carbon nanotubes–gold nanoparticles for carcinoembryonic antigen immunosensor application

A simple and controllable one-step electrodeposition method for the preparation of a chitosan–carbon nanotubes–gold nanoparticles (CS–CNTs–GNPs) nanocomposite film was used to fabricate an immunosensor for detection of carcinoembryonic antigen (CEA). The porous three-dimensional CS–CNTs–GNPs nanocomposite film, which offered a large specific surface area for immobilization of antibodies, exhibited improved conductivity, high stability and good biocompatibility. The morphology of the formed nanocomposite film was investigated by scanning electron microscopy (SEM), and the electrochemical behaviors of the immunosensor were characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Under the optimal conditions, the proposed immunosensor could detect CEA in two linear ranges from 0.1 to 2.0ngmL−1 and from 2.0 to 200.0ngmL−1, with a detection limit of 0.04ngmL−1. The immunosensor based on CS–CNTs–GNPs nanocomposite film as the antibody immobilization matrix could exhibit good sensitivity, stability, and reproducibility for the determination of CEA.

An amperometric uric acid biosensor based on multiwalled carbon nanotube–gold nanoparticle composite

An amperometric uric acid biosensor was fabricated by immobilizing uricase (EC 1.7.3.3) onto gold nanoparticle (AuNP)/multiwalled carbon nanotube (MWCNT) layer deposited on Au electrode via carbodiimide linkage. Determination of uric acid was performed by oxidation of enzymically generated H 2O 2 at 0.4 V. The sensor showed optimal response within 7 s at 40 °C in 50 mM Tris–HCl buffer (pH 7.5). The linear working range of the biosensor was 0.01–0.8 mM. The limit of detection (LOD) was 0.01 mM. The sensor measured uric acid levels in serum of healthy individuals and persons suffering from gout. The analytical recoveries of the added uric acid, 10 and 20 mg L –1, were 98.0% and 96.5%, respectively. Within- and between-batch coefficients of variation were less than 5.6% and less than 4.7%, respectively. A good correlation ( r = 0.998) was obtained between serum uric acid values by the standard enzymic colorimetric method and the current method. A number of serum substances had practically no interference. The sensor was used in more than 200 assays and had a storage life of 120 days at 4 °C.

A novel amperometric biosensor for oxalate determination using multi-walled carbon nanotube-gold nanoparticle composite

An amperometric oxalate biosensor using nanohybrid film of multi-walled carbon nanotubes (MWCNTs) and gold colloidal nanoparticles (GNPs) via carbodiimide chemistry by forming amide linkages between carboxylic acid groups on the CNTs and amine residues of cysteamine self-assembled monolayer (SAM) has been prepared. The c-MWCNTs were immobilized on the gold (Au) electrode and characterized by FTIR. The morphologies of the c-MWCNT/Au and GNPs/MWCNT/Au electrodes were investigated by scanning electron microscopy (SEM) and the electrochemical performance of the Au, c-MWCNT/Au and GNPs/c-MWCNT/Au electrodes were also studied amperometrically. The Cl − and NO 3 − insensitive oxalate oxidase from grain sorghum was finally immobilized on this electrode. The influence of pH, temperature and oxalate concentration on electrode activity was studied. The electrode showed optimum response within 7 s. The electrocatalytic response showed a linear dependence on the oxalic acid concentration ranging from 1 to 800 μM with a detection limit of 1 μM. The K m value for the oxalic acid sensor was 444.44 μM. The enzyme electrode retained 30% of its initial activity after 5 months, when stored at 4 °C. The electrode was employed for measurement of oxalic acid in serum, urine and foodstuffs.