A Facile Electrochemical Method for Simultaneous and On-Line Measurements of Glucose and Lactate in Brain Microdialysate with Prussian Blue as the Electrocatalyst for Reduction of Hydrogen Peroxide

Hybridization of bioelectrochemically functional infinite coordination polymer nanoparticles with carbon nanotubes for highly sensitive and selective in vivo electrochemical monitoring

This study demonstrates the formation of a three-dimensional conducting framework through hybridization of bioelectrochemically active infinite coordination polymer (ICP) nanoparticles with single-walled carbon nanotubes (SWNTs) for highly sensitive and selective in vivo electrochemical monitoring with combination with in vivo microdialysis. The bioelectrochemically active ICP nanoparticles are synthesized through the self-assembly process of NAD(+) and Tb(3+), in which all biosensing elements including an electrocatalyst (i.e., methylene green, MG), cofactor (i.e., β-nicotinamide adenine dinucleotide, NAD(+)), and enzyme (i.e., glucose dehydrogenase, GDH) are adaptively encapsulated. The ICP/SWNT-based biosensors are simply prepared by drop-coating the as-formed ICP/SWNT nanocomposite onto a glassy carbon substrate. Electrochemical studies demonstrate that the simply prepared ICP/SWNT-based biosensors exhibit excellent biosensing properties with a higher sensitivity and stability than the ICP-based biosensors prepared only with ICP nanoparticles (i.e., without hybridization of SWNTs). By using a GDH-based electrochemical biosensor as an example, we demonstrate a technically simple yet effective online electroanalytical platform for continuously monitoring glucose in the brain of guinea pigs with the ICP/SWNT-based biosensor as an online detector in a continuous-flow system combined with in vivo microdialysis. Under the experimental conditions employed here, the dynamic linear range for glucose with the ICP/SWNT-biosensor is from 50 to 1000 μM. Moreover, in vivo selectivity investigations with the biosensors prepared by the GDH-free ICPs reveal that ICP/SWNT-based biosensors are very selective for the measurement of glucose in the cerebral system. The basal level of glucose in the microdialysates from the striatum of guinea pigs is determined to be 0.31 ± 0.03 mM (n = 3). The study offers a simple route to the preparation of electrochemical biosensors, which is envisaged to be particularly useful for probing the chemical events involved in some physiological and pathological processes.

Sensitive detection of glucose in human serum with oligonucleotide modified gold nanoparticles by using dynamic light scattering technique

Dynamic light scattering based sensor for glucose was developed with oligonucleotide functionalized gold nanoparticles (Oligo-AuNPs). Oligonucleotide 5'-SH-(A)(12)-AGACAAGAGAGG-3' (Oligo 1) modified AuNPs and oligonucleotide 5'-CAACAGAGAACG-(A)(12)-HS-3' (Oligo 2) modified AuNPs could hybridize with oligonulceotide 5'-CGTTCTCTGTTGCCTCTCTTGTCT-3' (Oligo 3), which resulted in the aggregation of Oligo-AuNPs probes, and triggered the increase of their average diameter. However, Oligo 3 could be cleaved into DNA fragments by the mixture of glucose, glucose oxidase (GOD) and Fe(2+), and the DNA fragments could not hybridize with Oligo-AuNPs probes. Under the conditions of 3.7 nM Oligo 1-AuNPs, 3.7 nM Oligo 2-AuNPs, 8.0 μg/mL GOD, 100 nM Oligo 3 and 900 nM Fe(2+), the average diameter of Oligo-AuNPs probes decreased linearly with the increasing concentration of glucose over the range from 50 pmol/L to 5.0 nmol/L, with a detection limit of 38 pmol/L (3σ/slope). Moreover, five sugars had no effect on the average diameter of mixture of Oligo-AuNPs probes, GOD and Fe(2+), which demonstrated the good selectivity of the assay.

Rational design of surface/interface chemistry for quantitative in vivo monitoring of brain chemistry

To understand the molecular basis of brain functions, researchers would like to be able to quantitatively monitor the levels of neurochemicals in the extracellular fluid in vivo. However, the chemical and physiological complexity of the central nervous system (CNS) presents challenges for the development of these analytical methods. This Account describes the rational design and careful construction of electrodes and nanoparticles with specific surface/interface chemistry for quantitative in vivo monitoring of brain chemistry. We used the redox nature of neurochemicals at the electrode/electrolyte interface to establish a basis for monitoring specific neurochemicals. Carbon nanotubes provide an electrode/electrolyte interface for the selective oxidation of ascorbate, and we have developed both in vivo voltammetry and an online electrochemical detecting system for continuously monitoring this molecule in the CNS. Although Ca(2+) and Mg(2+) are involved in a number of neurochemical signaling processes, they are still difficult to detect in the CNS. These divalent cations can enhance electrocatalytic oxidation of NADH at an electrode modified with toluidine blue O. We used this property to develop online electrochemical detection systems for simultaneous measurements of Ca(2+) and Mg(2+) and for continuous selective monitoring of Mg(2+) in the CNS. We have also harnessed biological schemes for neurosensing in the brain to design other monitoring systems. By taking advantage of the distinct reaction properties of dopamine (DA), we have developed a nonoxidative mechanism for DA sensing and a system that can potentially be used for continuously sensing of DA release. Using "artificial peroxidase" (Prussian blue) to replace a natural peroxidase (horseradish peroxidase, HRP), our online system can simultaneously detect basal levels of glucose and lactate. By substituting oxidases with dehydrogenases, we have used enzyme-based biosensing schemes to develop a physiologically relevant system for detecting glucose and lactate in rat brain. Because of their unique optical properties and modifiable surfaces, gold nanoparticles (Au-NPs) have provided a platform of colorimetric assay for in vivo cerebral glucose quantification. We designed and modified the surfaces of Au-NPs and then used a sequence of reactions to produce hydroxyl radicals from glucose.

Carbon-ring microelectrode arrays for electrochemical imaging of single cell exocytosis: fabrication and characterization

Fabrication of carbon microelectrode arrays, with up to 15 electrodes in total tips as small as 10-50 μm, is presented. The support structures of microelectrodes were obtained by pulling multiple quartz capillaries together to form hollow capillary arrays before carbon deposition. Carbon ring microelectrodes were deposited by pyrolysis of acetylene in the lumen of these quartz capillary arrays. Each carbon deposited array tip was filled with epoxy, followed by beveling of the tip of the array to form a deposited carbon-ring microelectrode array (CRMA). Both the number of the microelectrodes in the array and the tip size are independently tunable. These CRMAs have been characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, and electrogenerated chemiluminescence. Additionally, the electrochemical properties were investigated with steady-state voltammetry. In order to demonstrate the utility of these fabricated microelectrodes in neurochemistry, CRMAs containing eight microring electrodes were used for electrochemical monitoring of exocytotic events from single PC12 cells. Subcellular temporal heterogeneities in exocytosis (i.e. cold spots vs hot spots) were successfully detected with the CRMAs.

High-salt-tolerance matrix for facile detection of glucose in rat brain microdialysates by MALDI mass spectrometry

Due to its strong ultraviolet absorption, high salt tolerance, and little interference in the low molecular weight region, N-(1-naphthyl) ethylenediamine dihydrochloride (NEDC) has been applied as a matrix to measure the level of glucose in rat brain microdialysates by matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) in combination with in vivo microdialysis. By monitoring the ion signals of (glucose + Cl)(-) in the mass spectra, we achieved a low detection limit of ~10 μM for glucose in 126 mM NaCl, which is a typical component in artificial cerebrospinal fluid, without prior sample purification. It is concluded that NEDC-assisted laser desorption/ionization (LDI) MS is a fast and general method for sensitive detection of small molecules (such as glucose and amino acids) in high ionic strength solutions.

Rational design and one-step formation of multifunctional gel transducer for simple fabrication of integrated electrochemical biosensors

This study demonstrates a new strategy to simplify the biosensor fabrication and thus minimize the biosensor-to-biosensor deviation through rational design and one-step formation of a multifunctional gel electronic transducer integrating all elements necessitated for efficiently transducing the biorecognition events to signal readout, by using glucose dehydrogenase (GDH) based electrochemical biosensor as an example. To meet the requirements for preparing integrated biosensors and retaining electronic and ionic conductivities for electronically transducing process, ionic liquids (ILs) with enzyme cofactor (i.e., oxidized form of nicotinamide adenine dinucleotide) as the anion were synthesized and used to form a bucky gel with single-walled carbon nanotubes, in which methylene green electrocatalyst was stably encapsulated for the oxidation of nicotinamide adenine dinucleotide. With such kind of rationally designed and one-step-formed multifunctional gel as the electronic transducer, the GDH-based electrochemical biosensors were simply fabricated by polishing the electrodes onto the gel followed by enzyme immobilization. This capability greatly simplifies the biosensor fabrication, prolongs the stability of the biosensors, and, more remarkably, minimizes the biosensor-to-biosensor deviation. The relative standard deviations obtained both with one electrode for the repeated measurements of glucose and with the different electrodes prepared with the same method for the concurrent measurements of glucose with the same concentration were 3.30% (n = 7) and 4.70% (n = 6), respectively. These excellent properties of the multifunctional gel-based biosensors substantially enable them to well-satisfy the pressing need of rapid measurements, for example, environmental monitoring, food analysis, and clinical diagnoses.

Effects of applied potential on the mass of non-conducting poly(ortho-phenylenediamine) electro-deposited on EQCM electrodes: comparison with biosensor selectivity parameters

An electrochemical quartz-crystal microbalance (EQCM) was used to determine the mass of poly-(o-phenylenediamine) (PoPD) layers electro-deposited at different applied potentials in neutral buffered monomer solution, conditions that produce the insulating form of the polymer used as a permselective membrane in biosensor applications. There was a systematic increase in the total, steady state PoPD mass deposited for fixed applied potentials from 0.05 to 0.6 V vs. SCE, followed by a plateau up to 0.8 V. Comparison of PoPD mass and permselectivity parameters indicates that the ability of the passivating form of PoPD to block interference species in biosensor applications is not related in a simple way to the mass of material deposited on the surface. Instead, effects of the applied electropolymerisation potential in driving the electro-oxidation of oPD dimers and oligomers formed during the electro-deposition process are likely to have a more direct impact on the selectivity characteristics of the PoPD layer. The results highlight the usefulness of apparent permeabilities, especially of ascorbic acid, in revealing differences between PoPD layers electro-deposited under different conditions.

Online electrochemical measurements of Ca2+ and Mg2+ in rat brain based on divalent cation enhancement toward electrocatalytic NADH oxidation

This study describes a novel electrochemical approach to effective online monitoring of electroinactive Ca(2+) and Mg(2+) in the rat brain based on the current enhancement of divalent cations toward electrocatalytic oxidation of NADH. Cyclic voltammetry for NADH oxidation at the electrodes modified with the polymerized film of toluidine blue O (TBO) reveals that the current of such an electrocatalytic oxidation process is remarkably enhanced by divalent cations such as Ca(2+) and Mg(2+). The current enhancement is thus used to constitute an electrochemical method for the measurements of Ca(2+) and Mg(2+) in a continuous-flow system with the polyTBO-modified electrode as the detector. Upon being integrated with in vivo microdialysis, the electrochemical method is successfully applied in investigating on cerebral Ca(2+) and Mg(2+) of living animals in two aspects: (1) online simultaneous measurements of the basal levels of Ca(2+) and Mg(2+) in the brain of the freely moving rats by using ethyleneglcol-bis(2-aminoethylether) tetraacetic acid (EGTA) as the selective masking agent for Ca(2+) to differentiate the net current responses selectively for Ca(2+) and Mg(2+); and (2) online continuous monitoring of the cerebral Mg(2+) following the global ischemia by using Ca(2+)-masking agent (i.e., EGTA) to completely eliminate the interference from Ca(2+). Compared with the existing methods for the measurements of cerebral Ca(2+) and Mg(2+), the method demonstrated here is advantageous in terms of its simplicity both in instrumentation and in the experimental procedures and near real-time nature, and is thus highly anticipated to find wide applications in understanding of chemical events involved in some physiological and pathological processes.

Optical detection of single cell lactate release for cancer metabolic analysis

Sensitive detection of extracellular lactate concentrations at a single cell level is of importance for studying the metabolic alterations in tumor progression. A unique nanoscale optical fiber lactate sensor was developed to monitor the extracellular lactate concentrations of cancer cells by immobilizing its nanotip with lactate dehydrogenases, which could catalyze lactate conversion to generate NADH for sensitive fluorescence detection. The results demonstrate that the fabricated nanosensor can successfully detect the extracellular lactate concentrations for single HeLa, MCF-7, and human fetal osteoblast (hFOB) cells, showing that the cancer cells have distinctly higher extracellular lactate concentrations than normal cells as that predicted by Warburg effect. The nanosensor was also employed to investigate the effect of a monocarboxylate transporter inhibitor on the lactate efflux from cancer cells. Different lactate efflux inhibition profiles were obtained for HeLa and MCF-7 cell lines. This work demonstrates that the nanosensor has potential for evaluating the effect of metabolic agents on cancer metabolism and survival.

Physiologically relevant online electrochemical method for continuous and simultaneous monitoring of striatum glucose and lactate following global cerebral ischemia/reperfusion

This study demonstrates a new electroanalytical method with a high physiological relevance for simultaneous online monitoring of glucose and lactate in the striatum of the rat brain following global cerebral ischemia/reperfusion. The online analytical method is based on the efficient integration of in vivo microdialysis sampling with an online selective electrochemical detection with the electrochemical biosensors with dehydrogenases, i.e., glucose and lactate dehydrogenases, as recognition elements. The dehydrogenase-based electrochemical biosensors are developed onto the dual split-disk plastic carbon film (SPCF) electrodes with methylene green (MG) adsorbed onto single-walled carbon nanotubes (SWNTs) as the electrocatalyst for the oxidation of dihydronicotiamide adenine dinucleotide (NADH) at a low potential of 0.0 V (vs Ag/AgCl). Artificial cerebrospinal fluid (aCSF) containing NAD(+) is externally perfused from a second pump and online mixed with the brain microdialysates to minimize the variation of pH that occurred following the cerebral ischemia/reperfusion and to supply NAD(+) cofactor and O(2) for the enzymatic reactions of dehydrogenases and ascorbate oxidase, respectively. As a result, the developed online electroanalytical method exhibits a high selectivity against the electrochemically active species endogenously existing in the cerebral systems and a high tolerance against the variation of pH and O(2) following cerebral ischemia/reperfusion. This property, along with the good linearity and a high stability toward glucose and lactate as well as little cross-talk between two biosensors, substantially makes this method possible for the continuous, simultaneous, and online monitoring of glucose and lactate in the rat brain following global cerebral ischemia/reperfusion. This study establishes a new and effective platform for the investigation of the energy metabolism in physiological and pathological processes.

Enzyme immobilization strategies and electropolymerization conditions to control sensitivity and selectivity parameters of a polymer-enzyme composite glucose biosensor

In an ongoing programme to develop characterization strategies relevant to biosensors for in-vivo monitoring, glucose biosensors were fabricated by immobilizing the enzyme glucose oxidase (GOx) on 125 μm diameter Pt cylinder wire electrodes (Pt(C)), using three different methods: before, after or during the amperometric electrosynthesis of poly(ortho-phenylenediamine), PoPD, which also served as a permselective membrane. These electrodes were calibrated with H(2)O(2) (the biosensor enzyme signal molecule), glucose, and the archetypal interference compound ascorbic acid (AA) to determine the relevant polymer permeabilities and the apparent Michaelis-Menten parameters for glucose. A number of selectivity parameters were used to identify the most successful design in terms of the balance between substrate sensitivity and interference blocking. For biosensors electrosynthesized in neutral buffer under the present conditions, entrapment of the GOx within the PoPD layer produced the design (Pt(C)/PoPD-GOx) with the highest linear sensitivity to glucose (5.0 ± 0.4 μA cm(-2) mM(-1)), good linear range (K(M) = 16 ± 2 mM) and response time (< 2 s), and the greatest AA blocking (99.8% for 1 mM AA). Further optimization showed that fabrication of Pt(C)/PoPD-GOx in the absence of added background electrolyte (i.e., electropolymerization in unbuffered enzyme-monomer solution) enhanced glucose selectivity 3-fold for this one-pot fabrication protocol which provided AA-rejection levels at least equal to recent multi-step polymer bilayer biosensor designs. Interestingly, the presence of enzyme protein in the polymer layer had opposite effects on permselectivity for low and high concentrations of AA, emphasizing the value of studying the concentration dependence of interference effects which is rarely reported in the literature.

Contributions by a novel edge effect to the permselectivity of an electrosynthesized polymer for microbiosensor applications

Pt electrodes of different sizes (2 x 10(-5)-2 x 10(-2) cm(2)) and geometries (disks and cylinders) were coated with the ultrathin non-conducting form of poly(o-phenylenediamine), PPD, using amperometric electrosynthesis. Analysis of the ascorbic acid (AA) and H(2)O(2) apparent permeabilities for these Pt/PPD sensors revealed that the PPD deposited near the electrode insulation (Teflon or glass edge) was not as effective as the bulk surface PPD for blocking AA access to the Pt substrate. This discovery impacts on the design of implantable biosensors where electrodeposited polymers, such as PPD, are commonly used as the permselective barrier to block electroactive interference by reducing agents present in the target medium. The undesirable "edge effect" was particularly marked for small disk electrodes which have a high edge density (ratio of PPD-insulation edge length to electrode area), but was essentially absent for cylinder electrodes with a length of >0.2 mm. Sample biosensors, with a configuration based on these findings (25 microm diameter Pt fiber cylinders) and designed for brain neurotransmitter L-glutamate, behaved well in vitro in terms of Glu sensitivity and AA blocking.