A bioelectrochemical polypyrrole-containing Fe(CN)6(3-) interface for the design of a NAD-dependent reagentless biosensor

Detection of lactate in human sweat via surface-modified, screen-printed carbon electrodes

Real-time and continuous monitoring of lactate levels in sweat has been used as an indicator of physiological information to evaluate exercise outcomes and sports performance. We developed an optimal enzyme-based biosensor to detect the concentrations of lactate in different fluids (i.e., a buffer solution and human sweat). The surface of the screen-printed carbon electrode (SPCE) was first treated with oxygen plasma and then surface-modified by lactate dehydrogenase (LDH). The optimal sensing surface of the LDH-modified SPCE was identified by Fourier transform infrared spectroscopy and electron spectroscopy for chemical analysis. After connecting the LDH-modified SPCE to a benchtop E4980A precision LCR meter, our results showed that the measured response was dependent on the lactate concentration. The recorded data exhibited a broad dynamic range of 0.1-100 mM (R² = 0.95) and a limit of detection of 0.1 mM, which was unachievable without the incorporation of redox species. A state-of-the-art electrochemical impedance spectroscopy (EIS) chip was developed to integrate the LDH-modified SPCE for a portable bioelectronic platform in the detection of lactate in human sweat. We believe the optimal sensing surface can improve the sensitivity of lactate sensing in a portable bioelectronic EIS platform for early diagnosis or real-time monitoring during different physical activities.

Versatile enzymatic assays by switching on the fluorescence of gold nanoclusters

Herein we present a general and turn-on strategy for enzymatic bioassays on the basis of redox state dependent emission of gold nanoclusters (AuNCs). The photoluminescence of AuNCs was quenched obviously by the oxidative ferricyanide while unaffected by its corresponding reduced state, i.e., ferrocyanide. The distinctive quenching abilities for AuNCs by the redox couple (ferricyanide/ferrocyanide) enabled their utility as new fluorescent sensing platforms to detect redox-related phenomena. The proposed protocols were conducted by using the model oxidoreductases of glucose oxidase (GOx) and the enzyme cascade of lactate dehydrogenase (LDH)/diaphorase to catalytically convert ferricyanide to ferrocyanide, which switched on fluorescence of the detection systems. The detection limit for glucose and lactate was found to be as low as 0.12 and 0.09 μM, respectively. This work features the first use of the redox couple of ferricyanide/ferrocyanide in fluorescent bioanalysis, which enables versatile, signal on and highly sensitive/selective detections as compared to the state of the art fluorescently enzymatic sensing platforms. Importantly, considering the significance of ferricyanide/ferrocyanide involves in numerous other oxidoreductases mediated biocatalysis, this protocol has wide versatility that enables combination with oxidoreductases related reactions for biosensing.

Screen-Printed Carbon Electrodes Modified with Graphene Oxide for the Design of a Reagent-Free NAD+-Dependent Biosensor Array

A facile approach for the construction of reagent-free electrochemical dehydrogenase-based biosensors is presented. Enzymes and cofactors (NAD⁺ and Fe(CN)₆^3-) were immobilized by modification of screen-printed carbon electrodes with graphene oxide (GO) and an additional layer of cellulose acetate. The sensor system was exemplarily optimized for an l-lactate electrode in terms of GO concentration, working potential, and pH value. The biosensor exhibited best characteristics at pH 7.5 in 100 mM potassium phosphate buffer at an applied potential of +0.250 V versus an internal pseudo Ag reference electrode. Thereby, sensor performance was characterized by a linear working range from 0.25 to 4 mM and a sensitivity of 0.14 μA mM^-1. The detection principle was additionally evaluated with three other dehydrogenases (d-lactate dehydrogenase, alcohol dehydrogenase, and formate dehydrogenase, respectively). The developed reagentless biosensor array enabled simultaneous and cross-talk free determination of l-lactate, d-lactate, ethanol, and formate.

d-Lactate electrochemical biosensor prepared by immobilization of thermostable dye-linked d-lactate dehydrogenase from Candidatus Caldiarchaeum subterraneum

A stable d-lactate electrochemical sensing system was developed using a dye-linked d-lactate dehydrogenase (Dye-DLDH) from an uncultivated thermophilic archaeon, Candidatus Caldiarchaeum subterraneum. To develop the system, the putative gene encoding the Dye-DLDH from Ca. Caldiarchaeum subterraneum was overexpressed in Escherichia coli, and the expressed product was purified. The recombinant enzyme was a highly thermostable Dye-DLDH that retained full activity after incubation for 10 min at 70°C. The electrode for detection of d-lactate was prepared by immobilizing the thermostable Dye-DLDH and multi-walled carbon nanotube (MWCNT) within Nafion membrane. The electrocatalytic response of the electrode was clearly observed upon exposure to d-lactate. The electrode response to d-lactate was linear within the concentration range of 0.03-2.5 mM, and it showed little reduction in responsiveness after 50 days. This is the first report describing a d-lactate sensing system using a thermostable Dye-DLDH.

Spectrally matched upconverting luminescent nanoparticles for monitoring enzymatic reactions

We report on upconverting luminescent nanoparticles (UCLNPs) that are spectrally tuned such that their emission matches the absorption bands of the two most important species associated with enzymatic redox reactions. The core-shell UCLNPs consist of a β-NaYF4 core doped with Yb(3+)/Tm(3+) ions and a shell of pure β-NaYF4. Upon 980 nm excitation, they display emission bands peaking at 360 and 475 nm, which is a perfect match to the absorption bands of the enzyme cosubstrate NADH and the coenzyme FAD, respectively. By exploiting these spectral overlaps, we have designed fluorescent detection schemes for NADH and FAD that are based on the modulation of the emission intensities of UCLNPs by FAD and NADH via an inner filter effect.

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.

Tryptophan repressor-binding proteins from Escherichia coli and Archaeoglobus fulgidus as new catalysts for 1,4-dihydronicotinamide adenine dinucleotide-dependent amperometric biosensors and biofuel cells

The tryptophan (W) repressor-binding proteins (WrbA) from Escherichia coli (EcWrbA) and Archaeoglobus fulgidus (AfWrbA) were investigated for possible use in 1,4-dihydronicotinamide adenine dinucleotide (NADH) dependent amperometric biosensors and biofuel cells. EcWrbA and AfWrbA are oligomeric flavoproteins binding one flavin mononucleotide (FMN) per monomer and belonging to a new family of NAD(P)H:quinone oxidoreductases (NQOs). The enzymes were covalently linked to a low potential Os redox polymer onto graphite in the presence of single-walled carbon nanotube (SWCNT) preparations of varying average lengths. The performance of the enzyme modified electrodes for NADH oxidation was strongly depending on the average length of the applied SWCNTs. By blending the Os redox polymer with SWCNTs, the electrocatalytic current could be increased up to a factor of 5. Results obtained for AfWrbA modified electrodes were better than those for EcWrbA. For NADH detection, a linear range between 5 microM and 1 mM, a lower limit of detection of 3 microM, and a sensitivity of 56.5 nA microM(-1) cm(-2) could be reached. Additionally spectroelectrochemical measurements were carried out in order to determine the midpoint potentials of the enzymes (-115 mV vs NHE for EcWrbA and -100 mV vs NHE for AfWrbA pH 7.0). Furthermore, an AfWrbA modified electrode was used as an anode in combination with a Pt black cathode as a biofuel cell prototype.

Highly sensitive sensor for detection of NADH based on catalytic growth of Au nanoparticles on glassy carbon electrode

In this work, an electrochemical dihydronicotinamide adenine dinucleotide (NADH) sensor based on the catalytic growth of Au nanoparticles (Au NPs) on glassy carbon electrode was developed. Catalyzed by Au NPs immobilized on pretreated glassy carbon electrode, the reduction of AuCl(4)(-) in the presence of hydroquinone and cetyltrimethyl ammonium chloride led to the formation of enlarged Au NPs on the electrode surface. Spectrophotometry and high-resolution scanning electronic microscope (SEM) analysis of the sensor morphologies before and after biocatalytic reaction revealed a diameter growth of the nanoparticles. The catalytic growth of Au NPs on electrode surface remarkably facilitated the electron transfer and improved the performance of the sensor. Under optimal conditions, NADH could be detected in the range from 1.25 x 10(-6) to 3.08 x 10(-4) M, and the detection limit was 2.5 x 10(-7) M. The advantages of the proposed sensor, such as high precision and sensitivity, fast response, low cost, and good storage stability, made it suitable for on-line detection of NADH in complex biological systems and contaminant degradation processes.

Electrochemical determination of levetiracetam by screen-printed based biosensors

This work shows an easy and fast electrochemical method for Levetiracetam (LEV) determination, which is a novel antiepileptic. Most of the methods used up to now for its determination required a pre-treatment of the sample. It is shown here that the developed Peroxidase based biosensors avoid this kind of drawbacks. Screen-printed carbon electrodes have been used as transducers for the Peroxidase immobilization by pyrrole electropolymerization. Experimental variables that can affect LEV chronoamperometric response, such as hydrogen peroxide concentration, pH and applied potential, have been optimized in order to perform a selective LEV determination. Under these conditions, the performance of the biosensors has been tested. The residual standard deviation (RSD) of the slopes of different calibration curves was 9.77% (n=4 and alpha=0.05) for the reproducibility and 7.73% (n=4 and alpha=0.05) in the case of the repeatability. An average limit of detection of 9.81x10(-6) M (alpha=beta=0.05) was obtained. The biosensors have been finally applied to the determination of LEV in complex matrices, such as pharmaceutical drugs and spiked human plasma samples, yielding successful results.

Microplate-compatible biamperometry array for parallel 48-channel amperometric or coulometric measurements

We report a new reusable electrochemical array for parallel biamperometric measurements that has been designed for use with standard microplates. The 48-channel array uses half of the available 96 wells and has 48 pairs of Pt wire electrodes. Applications to the quantitation of a variety of oxidizable species, including acetaminophen, ascorbic acid, hydroquinone, trolox, and uric acid, are demonstrated in assays that use potassium ferricyanide as an oxidant to produce a mixture of ferri- and ferrocyanide. Hydrogen peroxide quantitation is also demonstrated, based on an assay in which ferrocyanide is oxidized, again to produce a mixture of ferri- and ferrocyanide. Detection limits (signal-to-noise ratio (S/N) = 3) in these assays range from 1 (acetaminophen, R2 = 0.994) to 8 microM (ascorbic acid, R2 = 0.967), and linearity was observed to analyte concentrations of at least 100 microM. We also demonstrate the application of the biamperometric array to enzymatic assays, using the glucose oxidase reaction as an example; following a 20 min enzyme reaction time, a detection limit of 0.1 mM glucose was obtained. These results indicate that applications to other oxidase-based assays are feasible in this high-throughput format. The new electrochemical array employs standard, inexpensive microplates, and the biamperometric measurements are simple, precise, and rapid, requiring only 2 min for 48 parallel measurements.

Assembling Amperometric Biosensors for Clinical Diagnostics

Clinical diagnosis and disease prevention routinely require the assessment ofspecies determined by chemical analysis. Biosensor technology offers several benefits overconventional diagnostic analysis. They include simplicity of use, specificity for the targetanalyte, speed to arise to a result, capability for continuous monitoring and multiplexing,together with the potentiality of coupling to low-cost, portable instrumentation. This workfocuses on the basic lines of decisions when designing electron-transfer-based biosensorsfor clinical analysis, with emphasis on the strategies currently used to improve the deviceperformance, the present status of amperometric electrodes for biomedicine, and the trendsand challenges envisaged for the near future.

CYP450 biosensors based on gold chips for antiepileptic drugs determination

Three-electrode configuration chips containing a Pt, Au and a screen-printed Ag/AgCl as counter, working and reference electrode, respectively, have been developed. Selective determination of Phenobarbital (PB) has been carried out by Cytochrome P450 2B4 (CYP450) immobilization into a polypyrrole matrix onto the gold working electrode. Chronoamperometric experiments show a PB diffusion coefficient of 2.42x10(-6)cm(2)s(-1), a reproducibility and repeatability in terms of residual standard deviation (RSD) of 13% and 5.51%, respectively, and a limit of detection (LOD) of 0.289 micromol dm(-3) (alpha=beta=0.05) for the developed CYP450-biosensor chip. Its performance has been showed by the determination of PB in pharmaceutical drugs. HPLC has been used as reference technique.

Fluorescent sensing layer for the determination of L-malic acid in wine

An enzymatic method for determining L-malic acid in wine based on an L-malate sensing layer with nicotinamide adenine dinucleotide (NAD+), L-malate dehydrogenase (L-MDH) and diaphorase (DI), immobilized by sol-gel technology, was constructed and evaluated. The sol-gel glass was prepared with tetramethoxysilane (TMOS), water and HCl. L-MDH catalyzes the reaction between L-malate and NAD+, producing NADH, whose fluorescence (lambdaexc=340 nm, lambdaem=430 nm) could be directly related to the amount of L-malate. NADH is converted to NAD+ by applying hexacyanoferrate(III) as oxidant in the presence of DI. Some parameters affecting sol-gel encapsulation and the pH of the enzymatic reaction were studied. The sensing layer has a dynamic range of 0.1-1.0 g/L of L-malate and a long-term storage stability of 25 days. It exhibits acceptable reproducibility [sr(%) approximately 10] and allows six regenerations. The content of L-malic acid was determined for different types of wine, and polyvinylpolypyrrolidone (PVPP) was used as a bleaching agent with red wine. The results obtained for the wine samples using the sensing layer are comparable to those obtained from a reference method based on UV-vis molecular absorption spectrometry, if the matrix effect is corrected for.

A noninterference polypyrrole glucose biosensor

In order to eliminate the interference of impurities, such as ascorbic acid, a noninterference polypyrrole glucose biosensor was constructed with a four-electrode cell consisting of a polypyrrole film electrode, a polypyrrole-glucose oxidase electrode, a counter electrode and a reference electrode. The pure catalytic current of glucose oxidase (GOD) can be obtained from the difference between response currents of two working electrodes with and without GOD. The effects of potential, pH and temperature on analytical performance of the glucose biosensor were discussed. The optimum pH and apparent activation energy of enzyme-catalyzed reaction are 5.5 and 25 kJ mol(-1), respectively. The response current of the biosensor increases linearly with the increasing glucose concentration from 0.005 to 20.0 mmol dm(-3). The results show the glucose biosensor with under 2% of relative deviation has good ability of anti-interference. The glucose biosensor was also characterized with FT-IR and UV-vis spectra.

Sensitive Amperometric Immunosensing Using Polypyrrolepropylic Acid Films for Biomolecule Immobilization

An electrochemical immunosensor was constructed using an electropolymerized pyrrolepropylic acid (PPA) film with high porosity and hydrophilicity. A high density of carboxyl groups of PPA was used to covalently attach protein probes, leading to significantly improved detection sensitivity compared with conventional entrapment methods. As a model, anti-mouse IgG was covalently immobilized or entrapped in the PPA film and used in a sandwich-type alkaline phosphatase-catalyzing amperometric immunoassay with p-aminophenyl phosphate as the substrate. With covalent binding, the detection limit for IgG in PBS buffer, pH 7.4, was 100 pg/mL with a dynamic range of 5 orders of magnitude. The covalent bonding mode in the carbonate-bicarbonate buffer, pH 9.6, further brought down the detection limit to 20 pg/mL with remarkable selectivity.

Electrochemistry of immobilized redox enzymes: kinetic characteristics of NADH oxidation catalysis at diaphorase monolayers affinity immobilized on electrodes

In the class of NADH:acceptor oxidoreductases, the diaphorase from Bacillus stearothermophilusis a particularly promising enzyme for sensing NADH, and indirectly a great number of analytes, when coupled with a NAD-dependent dehydrogenase as well as for the design of mono- and multienzyme affinity sensors. The design and rational optimization of such systems require devising immobilization procedures that prevent dramatic losses of the enzymatic activity and a full kinetic characterization of the immobilized enzyme system. Two immobilization procedures are described, which involve recognition of the biotinylated diaphorase by a monolayer of neutravidin adsorbed on the electrode surface either directly or through the intermediacy of a monolayer of biotinylated rabbit immunoglobulin. Thorough kinetic characterization of the two systems is derived from cyclic voltammetric responses. A precise estimate of the enzyme coverages is obtained after comparing the enzyme kinetics of the immobilized and the homogeneous system.