Amperometric Detection of dsDNA Using an Acridine-Orange-Modified Glucose Oxidase.

In the present "genomic era" and in the developing world of DNA chips, DNA detection based on intercalation of specific molecules is of particular interest because the detection process is largely independent of the sequence of the target DNA. In this work, an acridine-orange-based intercalation compound, which was tethered to deglycosylated glucose oxidase was synthesized ad hoc and investigated for its ability to interact with dsDNA. Amperometric detection of DNA hybridization was achieved by signal amplification based on the catalytic oxidation of glucose by DNA-bound glucose oxidase. A clear distinction between dsDNA and ssDNA was achieved by careful design of a DNA-modified electrode surface and prevention of nonspecific adsorption of the acridine-orange-modified enzyme by implementing a potential-assisted immobilization method.

Amperometric Detection of dsDNA using an Acridine-Orange-Modified Glucose Oxidase.

Invited for this month's cover is the group of Prof. Dr. Wolfgang Schuhmann, Dr. Daliborka Jambrec and Dr. Adrian Ruff at Ruhr-Universität in Bochum, Germany. The cover picture shows a novel procedure for the preferential post-hybridization labeling of double-stranded DNA based on the intercalating compound acridine orange, which was covalently bound to glucose oxidase. Labeling with a highly active biocatalyst allows for a simple and sequence-independent amplification of the signal proportional to the amount of hybridized DNA that may be coupled with other amplification strategies. Read the full text of the article at 10.1002/cplu.201700279.

Design of an Os Complex-Modified Hydrogel with Optimized Redox Potential for Biosensors and Biofuel Cells.

Multistep synthesis and electrochemical characterization of an Os complex-modified redox hydrogel exhibiting a redox potential ≈+30 mV (vs. Ag/AgCl 3 M KCl) is demonstrated. The careful selection of bipyridine-based ligands bearing N,N-dimethylamino moieties and an amino-linker for the covalent attachment to the polymer backbone ensures the formation of a stable redox polymer with an envisaged redox potential close to 0 V. Most importantly, the formation of an octahedral N6-coordination sphere around the Os central atoms provides improved stability concomitantly with the low formal potential, a low reorganization energy during the Os(3+/2+) redox conversion and a negligible impact on oxygen reduction. By wiring a variety of enzymes such as pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase, flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase and the FAD-dependent dehydrogenase domain of cellobiose dehydrogenase, low-potential glucose biosensors could be obtained with negligible co-oxidation of common interfering compounds such as uric acid or ascorbic acid. In combination with a bilirubin oxidase-based biocathode, enzymatic biofuel cells with open-circuit voltages of up to 0.54 V were obtained.

Wiring of the aldehyde oxidoreductase PaoABC to electrode surfaces via entrapment in low potential phenothiazine-modified redox polymers.

Phenothiazine-modified redox hydrogels were synthesized and used for the wiring of the aldehyde oxidoreductase PaoABC to electrode surfaces. The effects of the pH value and electrode surface modification on the biocatalytic activity of the layers were studied in the presence of vanillin as the substrate. The enzyme electrodes were successfully employed as bioanodes in vanillin/O2 biofuel cells in combination with a high potential bilirubin oxidase biocathode. Open circuit voltages of around 700 mV could be obtained in a two compartment biofuel cell setup. Moreover, the use of a rather hydrophobic polymer with a high degree of crosslinking sites ensures the formation of stable polymer/enzyme films which were successfully used as bioanode in membrane-less biofuel cells.

Thermoresponsive amperometric glucose biosensor.

The authors report on the fabrication of a thermoresponsive biosensor for the amperometric detection of glucose. Screen printed electrodes with heatable gold working electrodes were modified by a thermoresponsive statistical copolymer [polymer I: poly(ω-ethoxytriethylenglycol methacrylate-co-3-(N,N-dimethyl-N-2-methacryloyloxyethyl ammonio) propanesulfonate-co-ω-butoxydiethylenglycol methacrylate-co-2-(4-benzoyl-phenoxy)ethyl methacrylate)] with a lower critical solution temperature of around 28 °C in aqueous solution via electrochemically induced codeposition with a pH-responsive redox-polymer [polymer II: poly(glycidyl methacrylate-co-allyl methacrylate-co-poly(ethylene glycol)methacrylate-co-butyl acrylate-co-2-(dimethylamino)ethyl methacrylate)-[Os(bpy)2(4-(((2-(2-(2-aminoethoxy)ethoxy)ethyl)amino)methyl)-N,N-dimethylpicolinamide)](2+)] and pyrroloquinoline quinone-soluble glucose dehydrogenase acting as biological recognition element. Polymer II bears covalently bound Os-complexes that act as redox mediators for shuttling electrons between the enzyme and the electrode surface. Polymer I acts as a temperature triggered immobilization matrix. Probing the catalytic current as a function of the working electrode temperature shows that the activity of the biosensor is dramatically reduced above the phase transition temperature of polymer I. Thus, the local modulation of the temperature at the interphase between the electrode and the bioactive layer allows switching the biosensor from an on- to an off-state without heating of the surrounding analyte solution.

Coupling of an enzymatic biofuel cell to an electrochemical cell for self-powered glucose sensing with optical readout.

A miniaturized biofuel cell (BFC) is powering an electrolyser invoking a glucose concentration dependent formation of a dye which can be determined spectrophotometrically. This strategy enables instrument free analyte detection using the analyte-dependent BFC current for triggering an optical read-out system. A screen-printed electrode (SPE) was used for the immobilization of the enzymes glucose dehydrogenase (GDH) and bilirubin oxidase (BOD) for the biocatalytic oxidation of glucose and reduction of molecular oxygen, respectively. The miniaturized BFC was switched-on using small sample volumes (ca. 60 µL) leading to an open-circuit voltage of 567 mV and a maximal power density of (6.8±0.6) µW cm(-2). The BFC power was proportional to the glucose concentration in a range from 0.1 to 1.0 mM (R(2)=0.991). In order to verify the potential instrument-free analyte detection the BFC was directly connected to an electrochemical cell comprised of an optically-transparent SPE modified with methylene green (MG). The reduction of the electrochromic reporter compound invoked by the voltage and current flow applied by the BFC let to MG discoloration, thus allowing the detection of glucose.

Poly(benzoxazine)s Modified with Osmium Complexes as a Class of Redox Polymers for Wiring of Enzymes to Electrode Surfaces.

Benzoxazine-based redox polymers bearing Os complexes are synthesized and used as an immobilization matrix for glucose oxidase (GOx) as a model system for a reagentless biosensor. The polymers are formed by electrochemically induced anodic polymerization of the corresponding benzoxazine monomers modified with Os complexes. The precursors are synthesized in a Mannich-type reaction between bisphenol A, formaldehyde, and the corresponding Os complexes or ligands, which contain free amino groups. The Os complexes are redox active within the polymer films, and thus, can be used as redox relays for the electron transfer between the electrode surface and the prosthetic group within the enzyme. Entrapment of GOx within the poly(benzoxazine) film is achieved successfully, as shown by the biocatalytic activity of the poly(benzoxazine)/GOx films upon the addition of glucose.

Self-powered wireless carbohydrate/oxygen sensitive biodevice based on radio signal transmission.

Here for the first time, we detail self-contained (wireless and self-powered) biodevices with wireless signal transmission. Specifically, we demonstrate the operation of self-sustained carbohydrate and oxygen sensitive biodevices, consisting of a wireless electronic unit, radio transmitter and separate sensing bioelectrodes, supplied with electrical energy from a combined multi-enzyme fuel cell generating sufficient current at required voltage to power the electronics. A carbohydrate/oxygen enzymatic fuel cell was assembled by comparing the performance of a range of different bioelectrodes followed by selection of the most suitable, stable combination. Carbohydrates (viz. lactose for the demonstration) and oxygen were also chosen as bioanalytes, being important biomarkers, to demonstrate the operation of the self-contained biosensing device, employing enzyme-modified bioelectrodes to enable the actual sensing. A wireless electronic unit, consisting of a micropotentiostat, an energy harvesting module (voltage amplifier together with a capacitor), and a radio microchip, were designed to enable the biofuel cell to be used as a power supply for managing the sensing devices and for wireless data transmission. The electronic system used required current and voltages greater than 44 µA and 0.57 V, respectively to operate; which the biofuel cell was capable of providing, when placed in a carbohydrate and oxygen containing buffer. In addition, a USB based receiver and computer software were employed for proof-of concept tests of the developed biodevices. Operation of bench-top prototypes was demonstrated in buffers containing different concentrations of the analytes, showcasing that the variation in response of both carbohydrate and oxygen biosensors could be monitored wirelessly in real-time as analyte concentrations in buffers were changed, using only an enzymatic fuel cell as a power supply.

A redox hydrogel protects hydrogenase from high-potential deactivation and oxygen damage.

Hydrogenases are nature's efficient catalysts for both the generation of energy via oxidation of molecular hydrogen and the production of hydrogen via the reduction of protons. However, their O2 sensitivity and deactivation at high potential limit their applications in practical devices, such as fuel cells. Here, we show that the integration of an O2-sensitive hydrogenase into a specifically designed viologen-based redox polymer protects the enzyme from O2 damage and high-potential deactivation. Electron transfer between the polymer-bound viologen moieties controls the potential applied to the active site of the hydrogenase and thus insulates the enzyme from excessive oxidative stress. Under catalytic turnover, electrons provided from the hydrogen oxidation reaction induce viologen-catalysed O2 reduction at the polymer surface, thus providing self-activated protection from O2. The advantages of this tandem protection are demonstrated using a single-compartment biofuel cell based on an O2-sensitive hydrogenase and H2/O2 mixed feed under anode-limiting conditions.

Engineered electron-transfer chain in photosystem 1 based photocathodes outperforms electron-transfer rates in natural photosynthesis.

Photosystem 1 (PS1) triggers the most energetic light-induced charge-separation step in nature and the in vivo electron-transfer rates approach 50 e(-) s(-1) PS1(-1). Photoelectrochemical devices based on this building block have to date underperformed with respect to their semiconductor counterparts or to natural photosynthesis in terms of electron-transfer rates. We present a rational design of a redox hydrogel film to contact PS1 to an electrode for photocurrent generation. We exploit the pH-dependent properties of a poly(vinyl)imidazole Os(bispyridine)2Cl polymer to tune the redox hydrogel film for maximum electron-transfer rates under optimal conditions for PS1 activity. The PS1-containing redox hydrogel film displays electron-transfer rates of up to 335±14 e(-) s(-1) PS1(-1), which considerably exceeds the rates observed in natural photosynthesis or in other semiartificial systems. Under O2 supersaturation, photocurrents of 322±19 µA cm(-2) were achieved. The photocurrents are only limited by mass transport of the terminal electron acceptor (O2). This implies that even higher electron-transfer rates may be achieved with PS1-based systems in general.

Redox hydrogels with adjusted redox potential for improved efficiency in Z-scheme inspired biophotovoltaic cells.

The improvement of Z-scheme inspired biophotovoltaics is achieved by fine tuning the properties of redox hydrogels applied as immobilization and electron conducting matrices for the photosystem-protein complexes. The formal potentials of the redox hydrogels are adjusted to the respective redox sites in the photosystems for optimized electron transfer without substantial voltage loss. The anode is based on photosystem 2 (PS2) integrated in a phenothiazine modified redox hydrogel with a formal potential of -1 mV vs. SHE, which is 59 mV more positive than the QB acceptor site in PS2. The cathode is based on photosystem 1 (PS1) contacted via an Os-complex based redox hydrogel with a formal potential of 395 mV vs. SHE, i.e. 28 mV more negative than the primary P700 electron acceptor of PS1. The potential difference between the two redox hydrogels is 396 mV. An open circuit voltage (VOC) of 372.5 ± 2.1 mV could be achieved for the biophotovoltaic cell. The maximum power output is 1.91 ± 0.56 µW cm(-2) and the conversion efficiency (η) is 4.5 × 10(-5), representing a 125-fold improvement in comparison to the previously proposed device exploiting the photosynthetic Z-scheme for electrical energy production.

Coupling osmium complexes to epoxy-functionalised polymers to provide mediated enzyme electrodes for glucose oxidation.

Newly synthesised osmium complex-modified redox polymers were tested for potential application as mediators in glucose oxidising enzyme electrodes for application to biosensors or biofuel cells. Coupling of osmium complexes containing amine functional groups to epoxy-functionalised polymers of variable composition provides a range of redox polymers with variation possible in redox potential and physicochemical properties. Properties of the redox polymers as mediators for glucose oxidation were investigated by co-immobilisation onto graphite with glucose oxidase or FAD-dependent glucose dehydrogenase using a range of crosslinkers and in the presence and absence of multiwalled carbon nanotubes. Electrodes prepared by immobilising [P20-Os(2,2'-bipyridine)2(4-aminomethylpyridine)Cl].PF6, carbon nanotubes and glucose oxidase exhibit glucose oxidation current densities as high as 560µAcm(-2) for PBS containing 100mM glucose at 0.45V vs. Ag/AgCl. Films prepared by crosslinking [P20-Os(4,4'-dimethoxy-2,2'-bipyridine)2(4-aminomethylpyridine)Cl].PF6, an FAD-dependent glucose dehydrogenase, and carbon nanotubes achieve current densities of 215µAcm(-2) in 5mM glucose at 0.2V vs. Ag/AgCl, showing some promise for application to glucose oxidising biosensors or biofuel cells.

Direct electron transfer of Trametes hirsuta laccase adsorbed at unmodified nanoporous gold electrodes.

The enzyme Trametes hirsuta laccase undergoes direct electron transfer at unmodified nanoporous gold electrodes, displaying a current density of 28µA/cm(2). The response indicates that ThLc was immobilised at the surface of the nanopores in a manner which promoted direct electron transfer, in contrast to the absence of a response at unmodified polycrystalline gold electrodes. The bioelectrocatalytic activity of ThLc modified nanoporous gold electrodes was strongly dependent on the presence of halide ions. Fluoride completely inhibited the enzymatic response, whereas in the presence of 150mM Cl(-), the current was reduced to 50% of the response in the absence of Cl(-). The current increased by 40% when the temperature was increased from 20°C to 37°C. The response is limited by enzymatic and/or enzyme electrode kinetics and is 30% of that observed for ThLc co-immobilised with an osmium redox polymer.

A new synthesis route for Os-complex modified redox polymers for potential biofuel cell applications.

A new synthesis route for Os-complex modified redox polymers was developed. Instead of ligand exchange reactions for coordinative binding of suitable precursor Os-complexes at the polymer, Os-complexes already exhibiting the final ligand shell containing a suitable functional group were bound to the polymer via an epoxide opening reaction. By separation of the polymer synthesis from the ligand exchange reaction at the Os-complex, the modification of the same polymer backbone with different Os-complexes or the binding of the same Os-complex to a number of different polymer backbones becomes feasible. In addition, the Os-complex can be purified and characterized prior to its binding to the polymer. In order to further understand and optimize suitable enzyme/redox polymer systems concerning their potential application in biosensors or biofuel cells, a series of redox polymers was synthesized and used as immobilization matrix for Trametes hirsuta laccase. The properties of the obtained biofuel cell cathodes were compared with similar biocatalytic interfaces derived from redox polymers obtained via ligand exchange reaction of the parent Os-complex with a ligand integrated into the polymer backbone during the polymer synthesis.