Development and application of a self-referencing glucose microsensor for the measurement of glucose consumption by pancreatic beta-cells

Can Single Cell Respiration be Measured by Scanning Electrochemical Microscopy (SECM)?

Ultramicroelectrode (UME), or, equivalently, microelectrode, probes are increasingly used for single-cell measurements of cellular properties and processes, including physiological activity, such as metabolic fluxes and respiration rates. Major challenges for the sensitivity of such measurements include: (i) the relative magnitude of cellular and UME fluxes (manifested in the current); and (ii) issues around the stability of the UME response over time. To explore the extent to which these factors impact the precision of electrochemical cellular measurements, we undertake a systematic analysis of measurement conditions and experimental parameters for determining single cell respiration rates via the oxygen consumption rate (OCR) in single HeLa cells. Using scanning electrochemical microscopy (SECM), with a platinum UME as the probe, we employ a self-referencing measurement protocol, rarely employed in SECM, whereby the UME is repeatedly approached from bulk solution to a cell, and a short pulse to oxygen reduction reaction (ORR) potential is performed near the cell and in bulk solution. This approach enables the periodic tracking of the bulk UME response to which the near-cell response is repeatedly compared (referenced) and also ensures that the ORR near the cell is performed only briefly, minimizing the effect of the electrochemical process on the cell. SECM experiments are combined with a finite element method (FEM) modeling framework to simulate oxygen diffusion and the UME response. Taking a realistic range of single cell OCR to be 1 × 10-18 to 1 × 10-16 mol s-1, results from the combination of FEM simulations and self-referencing SECM measurements show that these OCR values are at, or below, the present detection sensitivity of the technique. We provide a set of model-based suggestions for improving these measurements in the future but highlight that extraordinary improvements in the stability and precision of SECM measurements will be required if single cell OCR measurements are to be realized.

Enzyme free glucose sensing by amino-functionalized silicon quantum dot

Silicon quantum dots have become one of the most popular nanomaterials in biological applications for their excellent biocompatibility and optical properties. Herein, we synthesized amino-functionalized silicon quantum dots (NH₂@SiQDs) via a simple microemulsion method, in which silicon tetrachloride and allylamine were used as source of silicon and functional group. NH₂@SiQDs exhibits good water-solubility, high fluorescence quantum yield and optical stability. A non-enzymatic biosensor of glucose was developed based on the fluorescence quenching of NH₂@SiQDs in response to glucose. The fluorescence response was linearly proportional to glucose in the concentration range of 1.0 × 10^-6-9.0 × 10^-5 mol/L and the detection limit was determined to be 3.0 × 10^-7 mol/L. The developed glucose sensor was successfully applied in blood glucose analysis of human serum. Satisfactory result that agreed very well with traditional method was obtained.

Micro/nanoelectrochemical probe and chip devices for evaluation of three-dimensional cultured cells

Herein, we present an overview of recent research progress in the development of micro/nanoelectrochemical probe and chip devices for the evaluation of three-dimensional (3D) cultured cells. First, we discuss probe devices: a general outline, evaluation of O₂ consumption, enzyme-modified electrodes, evaluation of endogenous enzyme activity, and the collection of cell components from cell aggregates are discussed. The next section is focused on integrated chip devices: a general outline, electrode array devices, smart electrode array devices, droplet detection of 3D cultured cells, cell manipulation using dielectrophoresis (DEP), and electrodeposited hydrogels used for fabrication of 3D cultured cells on chip devices are discussed. Finally, we provide a summary and discussion of future directions of research in this field.

Real-time characterization of uptake kinetics of glioblastoma vs. astrocytes in 2D cell culture using microelectrode array

Extracellular measurement of uptake/release kinetics and associated concentration dependencies provides mechanistic insight into the underlying biochemical processes. Due to the recognized importance of preserving the natural diffusion processes within the local microenvironment, measurement approaches which provide uptake rate and local surface concentration of adherent cells in static media are needed. This paper reports a microelectrode array device and a methodology to measure uptake kinetics as a function of cell surface concentration in adherent 2D cell cultures in static fluids. The microelectrode array simultaneously measures local concentrations at five positions near the cell surface in order to map the time-dependent concentration profile which in turn enables determination of surface concentrations and uptake rates, via extrapolation to the cell plane. Hydrogen peroxide uptake by human astrocytes (normal) and glioblastoma multiforme (GBM43, cancer) was quantified for initial concentrations of 20 to 500 μM over time intervals of 4000 s. For both cell types, the overall uptake rate versus surface concentration relationships exhibited non-linear kinetics, well-described by a combination of linear and Michaelis-Menten mechanisms and in agreement with the literature. The GBM43 cells showed a higher uptake rate over the full range of concentrations, primarily due to a larger linear component. Diffusion-reaction models using the non-linear parameters and standard first-order relationships are compared. In comparison to results from typical volumetric measurements, the ability to extract both uptake rate and surface concentration in static media provides kinetic parameters that are better suited for developing reaction-diffusion models to adequately describe behavior in more complex culture/tissue geometries. The results also highlight the need for characterization of the uptake rate over a wider range of cell surface concentrations in order to evaluate the potential therapeutic role of hydrogen peroxide in cancerous cells.

Single-stranded DNA functionalized single-walled carbon nanotubes for microbiosensors via layer-by-layer electrostatic self-assembly

In this letter, the facial noncovalent adsorption of single-stranded DNA (ssDNA) provided single-walled carbon nanotubes (SWNTs) with biofunctionality while their superior properties were retained. In this case, we innovatively demonstrated the feasibility of employing the negative surface charge of ssDNA-SWNTs to realize layer-by-layer electrostatic self-assembly. On the basis of such a sandwichlike structure, an applicable glucose microbiosensor with direct electrochemistry and high performance was fabricated. The proposed protocol provided an ideal platform for various sensing applications, and might have profound influence on related nanotechnology.

Quantitative exploration of the contribution of settlement, growth, dispersal and grazing to the accumulation of natural marine biofilms on antifouling and fouling-release coatings

The accumulation of microbial biofilms on ships' hulls negatively affects ship performance and efficiency while also playing a role in the establishment of even more detrimental hard-fouling communities. However, there is little quantitative information on how the accumulation rate of microbial biofilms is impacted by the balance of the rates of cell settlement, in situ production (ie growth), dispersal to surrounding waters and mortality induced by grazers. These rates were quantified on test panels coated with copper-based antifouling (AF) or polymer-based fouling-release (FR) coatings by using phospholipids as molecular proxies for microbial biomass. The results confirmed the accepted modes of efficacy of these two types of coatings. In a more extensive set of experiments with only the FR coatings, it was found that seasonally averaged cellular production rates were 1.5 ± 0.5 times greater than settlement and the dispersal rates were 2.7 ± 0.8 greater than grazing. The results of this study quantitatively describe the dynamic balance of processes leading to the accumulation of microbial biofilm on coatings designed for ships' hulls.

Oscillatory glucose flux in INS 1 pancreatic β cells: a self-referencing microbiosensor study

Signaling and insulin secretion in β cells have been reported to demonstrate oscillatory modes, with abnormal oscillations associated with type 2 diabetes. We investigated cellular glucose influx in β cells with a self-referencing (SR) microbiosensor based on nanomaterials with enhanced performance. Dose-response analyses with glucose and metabolic inhibition studies were used to study oscillatory patterns and transporter kinetics. For the first time, we report a stable and regular oscillatory uptake of glucose (averaged period 2.9±0.6 min), which corresponds well with an oscillator model. This oscillatory behavior is part of the feedback control pathway involving oxygen, cytosolic Ca(2+)/ATP, and insulin secretion (periodicity approximately 3 min). Glucose stimulation experiments show that the net Michaelis-Menten constant (6.1±1.5 mM) is in between GLUT2 and GLUT9. Phloretin inhibition experiments show an EC(50) value of 28±1.6 μM phloretin for class I GLUT proteins and a concentration of 40±0.6 μM phloretin caused maximum inhibition with residual nonoscillating flux, suggesting that the transporters not inhibited by phloretin are likely responsible for the remaining nonoscillatory uptake, and that impaired uptake via GLUT2 may be the cause of the oscillation loss in type 2 diabetes. Transporter studies using the SR microbiosensor will contribute to diabetes research and therapy development by exploring the nature of oscillatory transport mechanisms.

A self referencing platinum nanoparticle decorated enzyme-based microbiosensor for real time measurement of physiological glucose transport

Glucose is the central molecule in many biochemical pathways, and numerous approaches have been developed for fabricating micro biosensors designed to measure glucose concentration in/near cells and/or tissues. An inherent problem for microsensors used in physiological studies is a low signal-to-noise ratio, which is further complicated by concentration drift due to the metabolic activity of cells. A microsensor technique designed to filter extraneous electrical noise and provide direct quantification of active membrane transport is known as self-referencing. Self-referencing involves oscillation of a single microsensor via computer-controlled stepper motors within a stable gradient formed near cells/tissues (i.e., within the concentration boundary layer). The non-invasive technique provides direct measurement of trans-membrane (or trans-tissue) analyte flux. A glucose micro biosensor was fabricated using deposition of nanomaterials (platinum black, multiwalled carbon nanotubes, Nafion) and glucose oxidase on a platinum/iridium microelectrode. The highly sensitive/selective biosensor was used in the self-referencing modality for cell/tissue physiological transport studies. Detailed analysis of signal drift/noise filtering via phase sensitive detection (including a post-measurement analytical technique) are provided. Using this highly sensitive technique, physiological glucose uptake is demonstrated in a wide range of metabolic and pharmacological studies. Use of this technique is demonstrated for cancer cell physiology, bioenergetics, diabetes, and microbial biofilm physiology. This robust and versatile biosensor technique will provide much insight into biological transport in biomedical, environmental, and agricultural research applications.

Electrochemical glucose biosensor of platinum nanospheres connected by carbon nanotubes

BACKGROUND

Glucose biosensors comprised of nanomaterials such as carbon nanotubes (CNTs) and metallic nanoparticles offer enhanced electrochemical performance that produces highly sensitive glucose sensing. This article presents a facile biosensor fabrication and biofunctionalization procedure that utilizes CNTs electrochemically decorated with platinum (Pt) nanospheres to sense glucose amperometrically with high sensitivity.

METHOD

Carbon nanotubes are grown in situ by microwave plasma chemical vapor deposition (MPCVD) and electro-chemically decorated with Pt nanospheres to form a CNT/Pt nanosphere composite biosensor. Carbon nanotube electrodes are immobilized with fluorescently labeled bovine serum albumin (BSA) and analyzed with fluorescence microscopy to demonstrate their biocompatibility. The enzyme glucose oxidase (GO(X)) is immobilized onto the CNT/Pt nanosphere biosensor by a simple drop-coat method for amperometric glucose sensing.

RESULTS

Fluorescence microscopy demonstrates the biofunctionalization capability of the sensor by portraying adsorption of fluorescently labeled BSA unto MPCVD-grown CNT electrodes. The subsequent GO(X)-CNT/Pt nanosphere biosensor demonstrates a high sensitivity toward H(2)O(2) (7.4 microA/mM/cm(2)) and glucose (70 microA/mM/cm(2)), with a glucose detection limit and response time of 380 nM (signal-to-noise ratio = 3) and 8 s (t(90%)), respectively. The apparent Michaelis-Menten constant (0.64 mM) of the biosensor also reflects the improved sensitivity of the immobilized GO(X)/nanomaterial complexes.

CONCLUSIONS

The GO(X)-CNT/Pt nanosphere biosensor outperforms similar CNT, metallic nanoparticle, and more conventional carbon-based biosensors in terms of glucose sensitivity and detection limit. The biosensor fabrication and biofunctionalization scheme can easily be scaled and adapted for microsensors for physiological research applications that require highly sensitive glucose sensing.

Simultaneous single neuron recording of O2 consumption, [Ca2+]i and mitochondrial membrane potential in glutamate toxicity

In order to determine the sequence of cellular processes in glutamate toxicity, we simultaneously recorded O(2) consumption, cytosolic Ca(2+) concentration ([Ca(2+)](i)), and mitochondrial membrane potential (mDeltapsi) in single cortical neurons. Oxygen consumption was measured using an amperometric self-referencing platinum electrode adjacent to neurons in which [Ca(2+)](i) and mDeltapsi were monitored with Fluo-4 and TMRE(+), respectively, using a spinning disk laser confocal microscope. Excitotoxic doses of glutamate caused an elevation of [Ca(2+)](i) followed seconds afterwards by an increase in O(2) consumption which reached a maximum level within 1-5 min. A modest increase in mDeltapsi occurred during this time period, and then, shortly before maximal O(2) consumption was reached, the mDeltapsi, as indicated by TMRE(+) fluorescence, dissipated. Maximal O(2) consumption lasted up to 5 min and then declined together with mDeltapsi and ATP levels, while [Ca(2+)](i) further increased. mDeltapsi and [Ca(2+)](i) returned to baseline levels when neurons were treated with an NMDA receptor antagonist shortly after the [Ca(2+)](i) increased. Our unprecedented spatial and time resolution revealed that this sequence of events is identical in all neurons, albeit with considerable variability in magnitude and kinetics of changes in O(2) consumption, [Ca(2+)](i), and mDeltapsi. The data obtained using this new method are consistent with a model where Ca(2+) influx causes ATP depletion, despite maximal mitochondrial respiration, minutes after glutamate receptor activation.

Dielectrophoretic assembly of insulinoma cells and fluorescent nanosensors into three-dimensional pseudo-islet constructs

Dielectrophoretic forces, generated by radio-frequency voltages applied to micromachined, transparent, indium tin oxide electrodes, have been used to condense suspensions of insulinoma cells (BETA-TC-6 and INS-1) into a 10 x 10 array of three-dimensional cell constructs. Some of these constructs, measuring approximately 150 microm in diameter, 120 microm in height and containing around 1000 cells, were of the same size and cell density as a typical islet of Langerhans. With the dielectrophoretic force maintained, these engineered cell constructs were able to withstand mechanical shock and fluid flow forces. Reproducibility of the process required knowledge of cellular dielectric properties, in terms of membrane capacitance and membrane conductance, which were obtained by electrorotation measurements. The ability to incorporate fluorescent nanosensors, as probes of cellular oxygen and pH levels, into these 'pseudo-islets' was also demonstrated. The footprint of the 10 x 10 array of cell constructs was compatible with that of a 1536 microtitre plate, and thus amenable to optical interrogation using automated plate reading equipment.

Paraquat Increases Cyanide-insensitive Respiration in Murine Lung Epithelial Cells by Activating an NAD(P)H:Paraquat Oxidoreductase

Pulmonary fibrosis is one of the most severe consequences of exposure to paraquat, an herbicide that causes rapid alveolar inflammation and epithelial cell damage. Paraquat is known to induce toxicity in cells by stimulating oxygen utilization via redox cycling and the generation of reactive oxygen intermediates. However, the enzymatic activity mediating this reaction in lung cells is not completely understood. Using self-referencing microsensors, we measured the effects of paraquat on oxygen flux into murine lung epithelial cells. Paraquat (10-100 microm) was found to cause a 2-4-fold increase in cellular oxygen flux. The mitochondrial poisons cyanide, rotenone, and antimycin A prevented mitochondrial- but not paraquat-mediated oxygen flux into cells. In contrast, diphenyleneiodonium (10 microm), an NADPH oxidase inhibitor, blocked the effects of paraquat without altering mitochondrial respiration. NADPH oxidases, enzymes that are highly expressed in lung epithelial cells, utilize molecular oxygen to generate superoxide anion. We discovered that lung epithelial cells possess a distinct cytoplasmic diphenyleneiodonium-sensitive NAD(P)H:paraquat oxidoreductase. This enzyme utilizes oxygen, requires NADH or NADPH, and readily generates the reduced paraquat radical. Purification and sequence analysis identified this enzyme activity as thioredoxin reductase. Purified paraquat reductase from the cells contained thioredoxin reductase activity, and purified rat liver thioredoxin reductase or recombinant enzyme possessed paraquat reductase activity. Reactive oxygen intermediates and subsequent oxidative stress generated from this enzyme are likely to contribute to paraquat-induced lung toxicity.

Measuring metabolism and biophysical flux in the tissue, cellular and sub-cellular domains: Recent developments in self-referencing amperometry for physiological sensing

Ultimately, advances in genomics, proteomics and metabolomics will be realized by combining these approaches with biophysical sensors for understanding the functional and structural (physiological) aspects of sub-cellular systems (cytomics). Therefore, the emergence of the new fields of cytomics and physiomics will require new technologies to probe the functional realm of living cells. While amperometric sensors have been used, their sensitivity and reliability are significantly improved through the development of new strategies and data acquisition systems for the operation of the sensors. This includes the application of the principles of the vibrating or self-referencing microsensor to the operation of amperometric sensors. The development of self-referencing amperometry (SRA) is significant because it effectively converts static concentration sensors into dynamic biophysical sensors that directly monitor physiological flux. SRA has been developed for analytes such as O2, NO, H2O2 and ascorbate. These sensors have been validated against non-biological microscopic flux sources that were theoretically modeled, before being applied to biological research. This new sensor technology has been shown, through research in a wide variety of biological and biomedical research projects, to be an important new tool in the arsenal of the cell biologist. SRA technology has been adapted through SRA-H2O2 and SRA-NADH sensors, for electrochemically coupled enzyme based self-referencing biosensors (SRB) for glucose, glutamate and ethanol. These developments in self-referencing sensor technologies offer great promise in extending electroanalytical chemistry and biosensor technologies from the micro to the nanoscale where researchers can study physiology at the sub-cellular and organellar levels.

Unusually large acrylamide induced effect on the droplet size in AOT/Brij30 water-in-oil microemulsions

Droplet microemulsions are widely used as templates for controlled synthesis of nanometer sized polymer gel beads for use as, e.g., nanobiosensors. Here we examine water-in-oil microemulsions typically used for preparation of sensors. The cores of the microemulsion droplets are constituted by an aqueous component consisting of water, reagent monomer mixture, buffer salts, and the relevant dyes and/or enzymes. The cores are encapsulated by a mixture of the surfactants Brij30 and AOT and the resulting microemulsion droplets are suspended in a continuous hexane phase. The size of the final polymer particles may be of great importance for the applications of the sensors. Our initial working hypothesis was that the size of the droplet cores and therefore the size of the synthesized polymer gel beads could be controlled by the surfactant-to-water ratio of the template microemulsion. In the present work we have tested this hypothesis and investigated how the monomers and the ratio between the two surfactants affect the size of the microemulsion droplets and the microemulsion domain. We find that the monomers in water have a profound effect on the microemulsion domain as well as on the size of the microemulsion droplets. The relation between microemulsion composition and droplet size is in this case more complicated than assumed in standard descriptions of microemulsions [R. Strey, Colloid Polym. Sci. 272 (1994) 1005-1019; I. Danielsson, B. Lindman, Colloids Surf. 3 (1981) 391-392; Y. Chevalier, T. Zemb, Rep. Progr. Phys. 53 (1990) 279-371].

Monitoring in real time with a microelectrode the release of reactive oxygen and nitrogen species by a single macrophage stimulated by its membrane mechanical depolarization

Macrophages are key cells of the immune system. During phagocytosis, the macrophage engulfs a foreign bacterium, virus, or particle into a vacuole, the phagosome, wherein oxidants are produced to neutralize and decompose the threatening element. These oxidants derive from in situ production of superoxide and nitric oxide by specific enzymes. However, the chemical nature and sequence of release of these compounds is far from being completely determined. The aim of the present work was to study the fundamental mechanism of oxidant release by macrophages at the level of a single cell, in real time and quantitatively. The tip of a microelectrode was positioned at a micrometric distance from a macrophage in a culture to measure oxidative-burst release by the cell when it was submitted to physical stimulation. The ensuing release of electroactive reactive oxygen and nitrogen species was detected by amperometry and the exact nature of the compounds was characterized through comparison with in vitro electrochemical oxidation of H2O2, ONOO-, NO*, and NO2(-) solutions. These results enabled the calculation of time variations of emission flux for each species and the reconstruction of the original flux of production of primary species, O2*- and NO*, by the macrophage.

Hypoxic stress in diabetic pregnancy contributes to impaired embryo gene expression and defective development by inducing oxidative stress

We have shown that neural tube defects (NTD) in a mouse model of diabetic embryopathy are associated with deficient expression of Pax3, a gene required for neural tube closure. Hyperglycemia-induced oxidative stress is responsible. Before organogenesis, the avascular embryo is physiologically hypoxic (2-5% O(2)). Here we hypothesized that, because O(2) delivery is limited at this stage of development, excess glucose metabolism could accelerate the rate of O(2) consumption, thereby exacerbating the hypoxic state. Because hypoxia can increase mitochondrial superoxide production, excessive hypoxia may contribute to oxidative stress. To test this, we assayed O(2) flux, an indicator of O(2) availability, in embryos of glucose-injected hyperglycemic or saline-injected mice. O(2) flux was reduced by 30% in embryos of hyperglycemic mice. To test whether hypoxia replicates, and hyperoxia suppresses, the effects of maternal hyperglycemia, pregnant mice were housed in controlled O(2) chambers on embryonic day 7.5. Housing pregnant mice in 12% O(2), or induction of maternal hyperglycemia (>250 mg/dl), decreased Pax3 expression fivefold, and increased NTD eightfold. Conversely, housing pregnant diabetic mice in 30% O(2) significantly suppressed the effect of maternal diabetes to increase NTD. These effects of hypoxia appear to be the result of increased production of mitochondrial superoxide, as indicated by assay of lipid peroxidation, reduced glutathione, and H(2)O(2). Further support of this interpretation was the effect of antioxidants, which blocked the effects of maternal hypoxia, as well as hyperglycemia, on Pax3 expression and NTD. These observations suggest that maternal hyperglycemia depletes O(2) in the embryo and that this contributes to oxidative stress and the adverse effects of maternal hyperglycemia on embryo development.

A Wireless, Remote Query Glucose Biosensor Based on a pH-Sensitive Polymer

This paper describes a wireless, remote query glucose biosensor using a ribbonlike, mass-sensitive magnetoelastic sensor as the transducer. The glucose biosensor is fabricated by first coating the magnetoelastic sensor with a pH-sensitive polymer and upon it a layer of glucose oxidase (GOx). The pH-responsive polymer swells or shrinks, thereby changing mass, respectively, in response to increasing or decreasing pH values. The GOx-catalyzed oxidation of glucose produces gluconic acid, inducing the pH-responsive polymer to shrink, which in turn decreases the polymer mass. In response to a time-varying magnetic field, a magnetoelastic sensor mechanically vibrates at a characteristic resonance frequency, the value of which inversely depends on sensor mass loading. As the magnetoelastic films are magnetostrictive, the vibrations launch magnetic flux that can be remotely detected using a pickup coil. Hence, changes in the resonance frequency of a passive magnetoelastic transducer are detected on a remote query basis, without the use of physical connections to the sensors.The sensitivity of the glucose biosensors decreases with increasing ionic strength; at physiological salt concentrations, 0.6 mmol/L of glucose can be measured. At glucose concentrations of 1-10 mmol/L, the biosensor response is reversible and linear, with the detection limit of 0.6 mmol/L corresponding to an error in resonance frequency determination of 20 Hz. Since no physical connections between the sensor and the monitoring instrument are required, this sensor can potentially be applied to in vivo and in situ measurement of glucose concentrations.