Enzyme-modulated cleavage of dsDNA for supramolecular design of biosensors

Supramolecular docking and immobilization of biotinylated dsDNA onto a self-assembled monolayer of avidin have been measured using impedance spectroscopy and quartz crystal microbalance technique. The formation of the serial assembly was first achieved by linearizing circular plasmid dsDNA using BamH I endonuclease enzyme. This was followed by a bisulfite-catalyzed transamination reaction in order to biotinylate the dsDNA. The reaction is single-strand specific, and it specifically targets unpaired cytosine bases generated during the enzyme cleavage. The biotinylated dsDNA was then used as a ligand at a gold electrode containing avidin. The process was monitored by ac impedance spectroscopy that was used to probe the changes in interfacial electron-transfer resistance upon binding and a microgravimetric quartz crystal microbalance that reflected in situ mass changes on the dsDNA-functionalized substrates. Our results demonstrated that this approach could be employed for the determination of small-molecular-weight organics such as cisplatin, daunomycin, bisphenol A, chlorinated phenols, and ethidium bromide. A detection limit in the magnitude of ca. 10 nM was achieved. This immobilization technique provides a generic approach for dsDNA-based sensor development and for the monitoring of DNA-analyte interactions.

A sensitive fluorescence monitor for the detection of activated Ras: total chemical synthesis of site-specifically labeled Ras binding domain of c-Raf1 immobilized on a surface

BACKGROUND

The Ras.GDP-Ras.GTP cycle plays a central role in eukaryotic signaling cascades. Mutations in Ras which stabilize activated Ras.GTP lead to a continuous stimulation of downstream effectors and ultimately to cell proliferation. Ras mutants which increase the steady-state concentration of Ras.GTP are involved in about 30% of all human cancers. It is therefore of great interest to develop a biosensor which is sensitive to Ras.GTP but not to Ras.GDP.

RESULTS

The Ras binding domain (RBD) of c-Raf1 was synthesized from two unprotected peptide segments by native chemical ligation. Two fluorescent amino acids with structures based on the nitrobenz-2-oxa-1,3-diazole and coumaryl chromophores were incorporated at a site which is close to the RBD/Ras.GTP binding surface. Additionally, a C-terminal tag consisting of His6 was introduced. The Kd values for binding of the site-specifically modified proteins to Ras.GTP are comparable to that of wild-type RBD. Immobilization of C-terminal His6 tag-modified fluorescent RBD onto Ni-NTA-coated surfaces allowed the detection of Ras.GTP in the 100 nM range. Likewise, Ras.GTP/Q61L (an oncogenic mutant of Ras with very low intrinsic GTP hydrolysis activity) can also be detected in this assay system. Ras.GDP does not bind to the immobilized RBD, thus allowing discrimination between inactive and activated Ras.

CONCLUSIONS

The site-specific incorporation of a fluorescent group at a strategic position in a Ras effector protein allows the detection of activated Ras with high sensitivity. This example illustrates the fact that the chemical synthesis of proteins or protein domains makes it possible to incorporate any kind of natural or unnatural amino acid at the position of choice, thereby enabling the facile preparation of specific biosensors, enhanced detection systems for drug screening, or the synthesis of activated proteins, e.g. phosphorylated proteins involved in signaling pathways, as defined molecular species.

Benchmark data from the literature for evaluation of new glucose sensing technologies

New glucose sensors based on various technologies are being developed to provide information for improved therapy in diabetes. There is a need to establish rational performance standards for these sensors. Frequently sampled, direct blood glucose recordings representative of blood glucose excursions in diabetes are the "gold standard." An extensive literature search revealed a limited number of diabetic and nondiabetic blood glucose recordings suitable for this purpose. Certain blood glucose recordings reflect the diversity of glycemic dynamics and provide sufficient challenge for evaluation of sensor systems. These recordings were converted into an accessible electronic format. An example is given of the use of these benchmark data to estimate aliasing error, or the error due to insufficient sampling frequency, based on a hypothetical sensor system having some properties of conventional "fingerstick" systems. Discrete sampling systems accumulate substantial aliasing error as the sampling period increases.

Electrochemical biosensors: recommended definitions and classification

Two Divisions of the International Union of Pure and Applied Chemistry (IUPAC), namely Physical Chemistry (Commission 1.7 on Biophysical Chemistry formerly Steering Committee on Biophysical Chemistry) and Analytical Chemistry (Commission V.5 on Electroanalytical Chemistry) have prepared recommendations on the definition, classification and nomenclature related to electrochemical biosensors: these recommendations could, in the future, be extended to other types of biosensors. An electrochemical biosensor is a self-contained integrated device, which is capable of providing specific quantitative or semi-quantitative analytical information using a biological recognition element (biochemical receptor) which is retained in direct spatial contact with an electrochemical transduction element. Because of their ability to be repeatedly calibrated, we recommend that a biosensor should be clearly distinguished from a bioanalytical system, which requires additional processing steps, such as reagent addition. A device that is both disposable after one measurement, i.e. single use, and unable to monitor the analyte concentration continuously or after rapid and reproducible regeneration, should be designated a single use biosensor. Biosensors may be classified according to the biological specificity-conferring mechanism or, alternatively, to the mode of physico-chemical signal transduction. The biological recognition element may be based on a chemical reaction catalysed by, or on an equilibrium reaction with macromolecules that have been isolated, engineered or present in their original biological environment. In the latter cases. equilibrium is generally reached and there is no further, if any, net consumption of analyte(s) by the immobilized biocomplexing agent incorporated into the sensor. Biosensors may be further classified according to the analytes or reactions that they monitor: direct monitoring of analyte concentration or of reactions producing or consuming such analytes; alternatively, an indirect monitoring of inhibitor or activator of the biological recognition element (biochemical receptor) may be achieved. A rapid proliferation of biosensors and their diversity has led to a lack of rigour in defining their performance criteria. Although each biosensor can only truly be evaluated for a particular application, it is still useful to examine how standard protocols for performance criteria may be defined in accordance with standard IUPAC protocols or definitions. These criteria are recommended for authors. referees and educators and include calibration characteristics (sensitivity, operational and linear concentration range, detection and quantitative determination limits), selectivity, steady-state and transient response times, sample throughput, reproducibility, stability and lifetime.

Amperometric sensor for L-ascorbic acid determination based on MnO2 bulk modified screen printed electrode

A simple biosensor constructed by bulk-modification of carbon ink with manganese dioxide as a mediator was investigated for its ability to serve as amperometric detector for L-ascorbic acid in hydrodynamic mode. The sensor could be operated at pH 5.0 (0.05 M phosphate buffer) and exhibited excellent reproducibility and stability. Optimization of measurement parameters such as applied working potential and pH value were studied in detail. The screen printed electrode exhibited a linear amperometric increase with the concentration of L-ascorbic acid from 50 mg L(-1) to 250 mg L(-1) and gave a (LOD = 3sigma) detection limit of 0.2 mg L(-1) (1.172 micromol L(-1)). The manganese dioxide modified screen printed electrode shows long term stability.

Continuous monitoring of arterial blood gases and pH during intraoperative rapid blood administration using a Paratrend sensor

BACKGROUND AND OBJECTIVES

The aim of this study was to determine the effects of rapid transfusion of packed red cells on the arterial blood gases and acid-base status of the recipient.

MATERIALS AND METHODS

We studied 16 patients (mean age 66.3+/-9.9 years) who received rapid transfusion of 632.8+/-287.2 g of packed red cells in CPDA-1, stored before use for a period of 15.2+/-4.4 days. During transfusion, monitoring of pH, PCO2 and PO2 was continuous using an intra-arterial multiparameter sensor (Paratrend 7, Biomedical Sensors, UK).

RESULTS

The rate of the transfusion was 73.1+/-9.6 g/min and the duration of observation was 35.8+/-12.8 min. Arterial pH decreased from 7.446+/-0.023 to 7.385+/-0.034 (p<0.001) and PCO2 increased from 32.31+/-1.35 to 36.41+/-1.86 mmHg (p<0.001). Delta pH and delta PCO2 showed significant correlation to the weight and the age of the transfused blood (p<0.001 for both dependent variables). The rate of pH change was positively but insignificantly correlated to the rate of the transfusion. Base excess was significantly decreased and end-tidal CO2 (PetCO2) was increased from 25.8+/-2.0 to 28.1+/-2.3 mmHg (p<0.05), significantly correlating to the amount and age of the administered component (p<0.05). PetCO2 was not elevated when PCO2 changes were minimal. Alterations in PO2 were not specific and our clinical impression was that they were related to unmeasured parameters.

CONCLUSION

Our findings suggest that the fall in pH and the elevation in PCO2 which occur during rapid transfusion of packed red cells may go undetected or be misinterpreted if the acid-base status of the recipient is not monitored continuously. These alterations are mainly of metabolic character and depend on the amount and age of the transfused component. Our data suggest that arterial sampling is essential during massive transfusions.

A three-cascaded-enzymes biosensor to determine lactose concentration in raw milk

The increasing demand for on-line measurement of milk composition directs science and industry to search for practical solutions, and biosensors may be a possibility. The specific objective of this work was to develop an electrochemical biosensor to determine lactose concentration in fresh raw milk. The sensor is based on serial reactions of three enzymes--beta-galactosidase, glucose oxidase, and horseradish peroxidase--immobilized on a glassy carbon electrode. The sequential enzymatic reactions increase the selectivity and sensitivity of the sensor. The sensor requires dilution of the raw milk and the addition of 5-aminosalicylic acid. Lactose concentrations in raw milk measured by the sensor were in good agreement with those measured by a reference laboratory using infrared technology. The results were obtained in milk samples that varied in fat and protein composition. From the results, we conclude that an electrochemical biosensor for determination of lactose concentration in fresh raw milk can be developed, and that the biosensor presented in this study maintained the qualities required for further development into an online sensor in the milking parlor.

Whole-blood glucose and lactate. Trilayer biosensors, drug interference, metabolism, and practice guidelines

OBJECTIVE

To assess the effects of 30 of the most commonly used critical care drugs on measurements obtained with trilayer electrochemical biosensors on a reference analyzer (ABL625-GL), to determine metabolic changes in glucose and lactate in vitro, and to formulate guidelines for whole-blood analysis of these 2 analytes.

DESIGN

Serial measurements were taken of changes in glucose and lactate levels caused by metabolism in whole blood in vitro over time. A parallel control study of drug interference with measurements of glucose and lactate in whole blood and of dose-response relationships in whole-blood samples and in plasma samples also was conducted.

RESULTS

At room temperature, whole-blood metabolism decreased glucose levels -2.3% at 15 minutes, -4.6% at 30 minutes, and -6.4% at 45 minutes. Metabolism increased lactate levels 11.4% at 15 minutes, 20.6% at 30 minutes, and 26.7% at 45 minutes in vitro. Paired differences between drug-spiked and control samples were calculated to determine interference (corrected for metabolism). The threshold for determination of interference was +/-2 SD from within-day precision, equal to +/-0.18 and +/-0.10 mmol/L for glucose and lactate, respectively. Only mannitol (C(6)H(14)O(6)) interfered with glucose and lactate measurements. At a concentration of 24 mg/mL, mannitol decreased whole-blood glucose levels by an average of 0.711 mmol/L (12.8 mg/dL) and whole-blood lactate levels by 0.16 mmol/L (1.4 mg/dL). Mannitol interference with measurements may have resulted from suppression of hydrogen peroxide formation in the enzymatic reactions in the biosensors, repartitioning of water between erythrocytes and plasma, or from other mechanisms.

CONCLUSIONS

Most critical care drugs had no significant effects on the trilayer electrochemical biosensors. Whole-blood analysis should be performed within 15 minutes for lactate and within 30 minutes for glucose because of metabolism in vitro. Mannitol effects on glucose measurements may be clinically significant in mannitol-induced acute renal failure and therefore should be considered for appropriate diagnosis and treatment of critically ill patients.

Hydrophilic sensor membrane based on cation-selective protic chromoionophore

The first potassium optode based on a protic chromoionophore immobilized in a hydrogel matrix is presented. The highly selective protic chromoionophore consists of a cryptohemispherand moiety and a trinitroanilino chromophore part. The acidifying power of potassium ions over sodium ions is 0.6 pH units. This correlates with the findings in solution. In contrast to several crown and aza-crown based chromophores the highly pre-organized moiety allows ion detection even in aqueous environment. The detection limit for potassium ions at pH 7.7 is 5 microM.

[Quantitative analysis of calibration dependence of biosensors]

Three-parameter Hill's equation, which is used in enzyme kinetics, was shown to applicable to calibration curves of both potentiometric (glucose, pesticides, urea, etc.) and amperometric (surfactants, biphenyl, etc.) biosensors. Possible causes of errors of analyte concentration measurements are discussed.

Preliminary study of real-time fiber optic based protein C biosensor

Deficiency of protein C (PC), one of the human body's key anticoagulants, can lead to massive thrombotic complications. There is a diagnostic need to perform real-time assays, in order to quickly identify and treat this disease. An immuno-optical biosensor for the diagnosing of PC deficiencies and monitoring of PC concentrations is being developed for this purpose. Monoclonal antibody against PC (anti-PC) is immobilized on the surface of a tapered quartz fiber that is enclosed in a glass tube (capacity approximately 200 microL). Following sample injection, PC within a sample binds to the anti-PC in a highly specific reaction. The system is then probed with a fluorophore-tagged secondary antibody against PC. Excitation light is applied through the fiber, and the fluorescence intensity is correlated with the PC concentration in the sample. This study presents (1) a feasibility, direct binding assay, (2) a comparison of methods to immobilize anti-PC upon the fiber (direct immobilization vs an avidin-biotin bridge), and (3) effectiveness of an elution step to regenerate the fiber. PC-deficient patients typically have a concentration range less than 2.5 microg/mL. It was found that the sensor could detect PC levels down to 0.1 microg/mL in pure buffer with minimal optimization. Avidin-biotin immobilization of the primary antibody produced enhanced signals, up to 470% of the original intensities. Preliminary fiber regeneration tests achieved nearly a 50% increase in fiber lifetime with the use of a CaCl(2) elution step. Ultimately, further development may lead to automation and the use of the system as a multi-blood factor analyzer.

Improving biosensor analysis

The quality of optical biosensor data must be improved in order to characterize the mechanism and rate constants associated with molecular interactions. Many of the artifacts associated with binding data can be minimized or eliminated by designing the experiment properly, collecting data under optimum conditions and processing the data with reference surfaces. It is possible to globally fit high-quality biosensor data with simple bimolecular reaction models, which validates the technology as a biophysical tool for interaction analysis.

Mediated, amperometric biosensor for glucose-6-phosphate monitoring based on entrapped glucose-6-phosphate dehydrogenase, Mg2+ ions, tetracyanoquinodimethane, and nicotinamide adenine dinucleotide phosphate in carbon paste

In this study, an amperometric carbon paste biosensor is developed for glucose-6-phosphate (G6P) monitoring which is based on entrapped Mg2+ ions, G6P dehydrogenase, NADP+ polyethylenimine (PEI) and the electroactive mediator, tetracyanoquinodimethane (TCNQ). The calibration line had a slope of 1.55 x 10(-5) A. M-1 with a correlation coefficient of 0.9965. The limit of detection (defined as three times the standard deviation of the response of the electrode to blank phosphate buffer injections (noise)) of the G6P biosensor was 5.0 x 10(-5) M. The application of this biosensor for monitoring G6P in human blood using the standard addition method is also demonstrated. A two-parameter empirical equation which adequately describes the deactivation of the biosensor steady-state response with time is also proposed.

Effects of high oxygen concentrations on microbial biosensor signals. Hyperoxygenation by means of perfluorodecalin

Amperometric biosensors register oxygen depletion in response to analyte catabolism, and thus are limited by the availability of dissolved oxygen. Microbial sensors containing immobilized cells of Gluconobacter oxydans were hyperoxygenated to 400% of control levels and the effects on sensor responses to glucose were determined. Oxygenated perfluorodecalin (a completely fluorinated organic substance) was as effective in hyperoxygenation as direct sparging with O2, increasing sensor base medium oxygen concentrations from 9.3 to 37 mg/l. Hyperoxygenation enhanced maximal biosensor response amplitudes, particularly at high cell loading densities. Maximal response rates were also improved, although less dramatically. Results suggest that hyperoxygenation may be a new general approach for modulating biosensor responses.

Calibration of glucose biosensors by using pre-steady state kinetic data

A new method for biosensor calibration and data processing, allowing the prediction of steady state parameters from the analysis of transient response curves (Rinken et al., 1996. Analytical Letters 29, 859), has been evaluated in the case of an oxygen sensor based two-substrate enzyme electrode for glucose determination. The electrochemical glucose biosensor was prepared by covering the surface of oxygen sensor with glucose oxidase (EC 1.1.3.4) immobilized in nylon mesh. This decreased the oxygen flow to the sensor in the presence of glucose and resulted in time-dependent decrease of the biosensor signal. Except the lag period of the response in the beginning of the assay, the oxygen consumption by the immobilized enzyme was described by an exponential function: [formula: see text] The parameter C, which corresponded to the steady-state output of the biosensor, was found to be the most suitable for glucose determination. The non-linear fitting for data of over 1000 independent experiments to the equation above always revealed correlation coefficients greater than 0.97. The calculation of the steady state parameter from the transient phase data makes the analysis fast and precise, especially for sensors with thick membranes, being convenient to use in the case of enzyme electrodes. The theoretical essence of the parameter C also gives valuable information for the optimal design of biosensors.

Evaluation of a lactate sensor for rapid repeated measurements of blood lactate concentration

In critically ill patients, serial measurements of blood lactate may indicate adequacy of therapy and predict development of multi-organ failure. We studied the accuracy, precision, and repeatability of the newly developed 800 Series Lactate Sensor (Ciba Corning Diagnostic Corp., Medfield, U.S.A.). Lactate levels determined with the sensor were compared with the standard laboratory method (Abbott TDX) in 75 paired arterial blood samples from 20 patients. Agreement between methods was determined and the mean coefficient of variation calculated for repeated measurements. The bias of the sensor was -0.38 mmol/l (CI -0.23 to -0.53), and the precision +/- 0.67. The coefficient of variation for repeated measurements was 1.95% with the sensor, and 11.5% with the TDX (P = 0.007). The new sensor offers a more reproducible, rapid method of measuring lactate, vital for serial measurements. The relatively wide limits of agreement between the methods reflect the greater variability of the TDX assay.

[Real time observation of binding of measles virus to Vero cells and neutralization of measles virus by human immunoglobulin using optical biosensor--a new real time diagnosis system for viral infections]

An optical biosensor to monitor molecular intractions was developed. This system made possible to observe reactions with a few minutes. We tried to apply this system for rapid diagnosis of viral infections. Measles virus propagated in our laboratory and crude rabbit anti-measles antiserum were used. A commercially available human immunoglobulin was used as a representative of patient sera. A crude rabbit anti-measles antiserum was fixed on the reaction surface of aminosilane coated cuvette, and specific binding of measles virus to this antibody was monitored. Specific purified IgG antibody was known to give as high sensitivity as the conventional culture method. But the sensitivity by crude antibody was found to be 1/10 in the sensitivity. This might be caused by the masked effects on specific antibody to the contaminated high amounts of non-specific proteins. None purified polyclonal antisera for most of the viruses are commercially available, and 10-20 times highly concentrated antibody solutions could be used for this titrations. Estimated anti-measles antibody titer of the commercial available human immunoglobulin by biosensor system was found to be the same as that by the conventional culture method. It was suggested that 1 X 10(3) virus particles/ml in the test solutions could be detectable and titerable by using commercially available specific anti-viral antibody in real time, and the viral infections could be diagnosed within an hour.

Assessment of the lactate biosensor methodology

The rapid determination of lactate level is useful for clinical emergencies, as in the case of shock conditions or during surgical operations, as well as in numerous cases of respiratory failure, in cardiac or paediatric pathology and during exercise tests. Moreover, it is of prognostic significance in critically ill patients. Photometric methods are slow and, even when performed in good conditions, will give results only 30 min after blood collection, during which time the clinical condition of the patient may change. In this study, we have assessed the lactate biosensor, a method that yields lactate measurements in less than 1 min with only 100 microL of biological fluid. In order to test the validity of this method, we performed comparisons between the Sigma classical enzymatic reference method and two commercially available biosensors: the Ciba-Corning biosensor 865 and the Yellow Springs lactate biosensor. Lactate measurements were performed in heparinized arterial blood samples without antiglycolitic agent (n=71). In order to cover a wide range of lactate levels, samples came from patients admitted to the intensive care unit for severe conditions and patients addressed for bicycle exercise testing. Each whole blood sample was processed in duplicate by both biosensors. For plasma measurement, subsamples of whole blood were centrifuged and the resulting plasma were processed by the biosensors and the Sigma method. Two parameters that can potentially influence lactate measurement were also investigated: haematocrit and total protein levels. The data showed that measurements performed on plasma are satisfactory for both biosensors. For whole blood, the Ciba-Corning device gives accurate results but the Yellow Springs apparatus seriously underestimates lactate levels. This underestimation is strongly influenced by the haematocrit level, so that a correction factor can be calculated (based on the haemoglobin level), which allows accurate "corrected" results to be obtained for whole blood with the Yellow Springs analyser.

Fast temporal response fiber-optic chemical sensors based on the photodeposition of micrometer-scale polymer arrays

Fiber-optic chemical sensor microarrays for the detection of pH and O2 have been developed with subsecond response times. Sensor microarrays are fabricated by the covalent immobilization (pH sensor arrays) or the physical entrapment (O2 sensor arrays) of fluorescent indicators in photodeposited polymer matrices on optical imaging fibers. Polymer microarrays are comprised of thousands of individual elements photodeposited as hemispheres such that each element of the sensor array is coupled directly to a discrete optical element of the imaging fiber and is not in contact with other neighboring elements. Because of the hemispherical shape and the individuality of the array elements, diffusion of analyte to the sensor elements is dominated by radial diffusion, resulting in a rapid response time. pH-sensitive arrays based on fluorescein respond to a 1.5-unit pH change within 300 ms, while the O2-sensitive arrays respond to O2 changes within 200 ms (90% of steady state response).

Kinetic analysis of biosensor data: elementary tests for self-consistency

The validity of the most common kinetic interpretation of biosensor data can be quickly assessed with the aid of two simple tests for self-consistency, requiring only back-of-the-envelope calculations. A search of the recent literature reveals that many published results fail these tests qualitatively.

Assay development for a portable fiberoptic biosensor

The fiberoptic biosensor with tapered optical probes has been developed to perform rapid and sensitive fluoroimmunoassays. A number of assays for biologic analytes were developed using a laboratory breadboard device that employed a large, 514 nm argon ion laser. These assays, with limits of detection of 5-50 ng/ml for protein antigens, showed promise for clinical use because of their demonstrated lack of matrix effects from plasma, seru, or blood. However, such a large device was impractical for on-site diagnostics, so a new, portable, multichannel biosensor was developed. To test this new biosensor, which uses 635 nm laser diodes, the assays were converted to use the cyanine dye, Cy5. The detection antibodies were labeled with Cy5 and assays performed to detect the F1 antigen of Yersinia pestis and the protective antigen of Bacillus anthracis. The limit of detection was found to improve by a factor of 10 for each assay. The portable biosensor was then evaluated in a blind test containing F1 antigen spiked into 30 of 173 serum samples. One hundred percent detection was achieved for samples with 100 ng/ml or more F1 antigen, with a specificity of 88%.

Biosensor markets: opportunities and obstacles

Currently, glucose sensing for the control of diabetes is the only high-volume market for biosensors. Yet, a variety of lower-volume niches are available for exploitation in medical diagnostics and the pharmaceutical and other industries. There are problems waiting for biosensor solutions, but the balance between market opportunities and technical and/or financial obstacles will control the future expansion of biosensor technology.

Modifications to a carbon paste glucose-sensing enzyme electrode and a reduction in the electrochemical interference from L-ascorbate

A glucose-sensing enzyme electrode was prepared by incorporating polyethylene glycol-modified glucose oxidase, horseradish peroxidase and 1,1'-dimethylferrocene into a carbon paste. The modification of glucose oxidase with polyethylene glycol was effective for increasing the enzyme activity in the carbon paste owing to the enhanced affinity of the polyethylene glycolmodified enzyme for the hydrophobic carbon paste matrix. In contrast, however, the enzyme activity of the polymer-modified peroxidase was lower than the unmodified peroxidase in the carbon paste matrix because of a severe loss of the enzyme activity during the modification with polyethylene glycol. Hence, the enzyme pair of polyethylene glycol-modified glucose oxidase and unmodified peroxidase was used for preparing the enzyme electrode. The reductive current response of the electrode to glucose was recorded at -0.2 V vs. Ag/AgCl. After the addition of glucose (100 microM), the current increased immediately and reached a plateau (delta = -0.12 microA) within 30 s. The current response was linear up to a glucose concentration of 500 microM and the detection limit was 20 microM (S/N = 5). Interference from ascorbate was very small: the current response to 1 mM glucose (-1.1 microA) was slightly reduced to -0.9 microA when 1 mM ascorbate was added to the glucose-containing solution. In biological and food samples, the concentration of ascorbate is generally quite low compared with the glucose concentration. The interference from ascorbate could actually be ignored for the purpose of determining glucose in soft drinks.