Eugenii Katz

Nanoparticle Arrays on Surfaces for Electronic, Optical, and Sensor Applications

Particles in the nanometer size range are attracting increasing attention with the growth of interest in nanotechnological disciplines. Nanoparticles display fascinating electronic and optical properties as a consequence of their dimensions and they may be easily synthesized from a wide range of materials. The dimensions of these particles makes them ideal candidates for the nanoengineering of surfaces and the fabrication of functional nanostructures. In the last five years, much effort has been expended on their organization on surfaces for the construction of functional interfaces. In this review, we address the research that has led to numerous sensing, electronic, optoelectronic, and photoelectronic interfaces, and also take time to cover the synthesis and characterization of nanoparticles and nanoparticle arrays.

Electrocatalytic oxidation of reduced nicotinamide coenzymes at gold and platinum electrode surfaces modified with a monolayer of pyrroloquinoline quinone. Effect of Ca2+ cations

Chemisorption of cystamine and cysteamine was used for functionalization of Au and Pt electrodes, respectively with amino groups. The functionalized electrodes were used for covalent immobilization of pyrroloquinoline quinone (PQQ) as a monolayer by carbodiimide coupling of the PQQ carboxylic groups with the surface amino groups. The electrochemical properties of the PQQ-modified electrodes (formal potential E°, peak currents Ip and peak-to-peak separation ΔE) were changed after addition of Ca2+ cations, probably owing to the formation of a complex between PQQ and Ca2+. Electrocatalytic oxidation of NADH and NADPH was shown at the PQQ-modified electrodes. This process was strongly enhanced in the presence of Ca2+ cations. Cyclic voltammetry, steady-state current, rotating disk electrode and flow-injection measurements were applied to study this electrocatalytic process. The kinetic parameters of this electrocatalytic process were evaluated in the presence or absence of Ca2+ cations assuming the formation of an intermediate charge-transfer complex between the immobilized PQQ and NADH. The only influence of Ca2+ cations on the kinetics of the catalytic process is an equilibrium shift towards the formation of the charge-transfer complex [NADH ⋯ Ca2+ ⋯ PQQ]. The decay of this complex leading to the formation of the final products (NAD+ and PQQH2) was found to be independent of the presence of Ca2+ cations. The process studied is considered as a model for the mechanism of catalysis showed by PQQ- and Ca2+-containing dehydrogenases. These PQQ-modified electrodes were stable enough even in a flow-injection system and can be considered very promising for practical applications.

Enzyme monolayer-functionalized field-effect transistors for biosensor applications

Abstract A gate surface of an ion-selective field-effect transistor was modified with a monolayer enzyme array that stimulates biocatalytic reactions that control the gate potential. Stepwise assemblage of the biocatalytic layer included primary silanization of the Al 2 O 3 -gate with 3-aminopropyltriethoxysilane, subsequent activation of the amino groups with glutaric dialdehyde and the covalent attachment of the enzyme to the functionalized gate surface. Urease, glucose oxidase, acetylcholine esterase and α-chymotrypsin were used to organize the biocatalytic matrices onto the chip gate. The resulting enzyme-based field-effect transistors, ENFETs, demonstrated capability to sense urea, glucose, acetylcholine and N -acetyl- l -tyrosine ethyl ester, respectively. The mechanism of the biosensing involves the alteration of the pH in the sensing layer by the biocatalytic reactions and the detection of the pH change by the ENFET. The major advantage of the enzyme-thin-layered FET devices as biosensors is the fast response-time (several tens of seconds) of these bioelectronic devices. This advantage over traditional thick-polymer-based ENFETs results from the low diffusion barrier for the substrate penetration to the biocatalytic active sites and minute isolation of the pH-sensitive gate surface from the bulk solution.

Concatenated logic gates using four coupled biocatalysts operating in series

The assembly of three concatenated enzyme-based logic gates consisting of OR, AND, XOR is described. Four biocatalysts, acetylcholine esterase, choline oxidase, microperoxidase-11, and the NAD+-dependent glucose dehydrogenase, are used to assemble the gates. Four inputs that include acetylcholine, butyrylcholine, O2, and glucose are used to drive the concatenated-gates system. The cofactor NAD+, and its reduced 1,4-dihydro form, NADH, are used as a reporter couple, and these provide an optical output for the gates. The modulus of the absorbance changes of NADH is used as a readout signal.

A photoactivated ‘molecular train’ for optoelectronic applications: light-stimulated translocation of a β-cyclodextrin receptor within a stoppered azobenzene-alkyl chain supramolecular monolayer assembly on a Au-electrode

Abstract A light-driven molecular shuttle is organized on a gold electrode surface. The assembly consists of a ferrocene-functionalized β-cyclodextrin, Fc-β-CD, molecule threaded on a monolayer-immobilized long alkyl component containing a photoisomerizable azobenzene unit, and terminated with a bulky anthracene group. The Fc-β-CD resides preferentially on the trans-azobenzene component, and photoisomerization of the trans-azobenzene to the cis-azobenzene state causes the translocation of the Fc-β-CD to the alkyl-chain component of the assembly. The state of the molecular shuttle is electronically transduced by chronoamperometry. The interfacial electron transfer rate constants for the oxidation of the ferrocene units of Fc-β-CD in the respective positions are kt=65 s−1 and kc=15 s−1. The light-driven translocation of Fc-β-CD is reversible, and proceeds by the cyclic isomerization of the azobenzene component between the trans and cis states.

A biofuel cell based on pyrroloquinoline quinone and microperoxidase-11 monolayer-functionalized electrodes

Abstract A pyrroloquinoline quinone (PQQ) monolayer-functionalized-Au-electrode and a microperoxidase-11 (MP-11)-modified Au-electrode are used as catalytic anode and cathode in a biofuel cell element, respectively. The cathodic oxidizer is H2O2 whereas the anodic fuel-substrate is 1,4-dihydronicotinamide adenine dinucleotide, NADH. The PQQ-monolayer electrode catalyzes the oxidation of NADH in the presence of Ca2+ ions. The MP-11-functionalized electrode catalyzes the reduction of H2O2. The biofuel cell generates an open-circuit voltage, Voc, of ca. 320 mV and a short-circuit current density, Isc, of ca. 30 μA·cm−2. The maximum electrical power, Wmax, extracted from the cell is 8 μW at an external load of 3 kΩ. The fill factor of the biofuel cell, f=Wmax·Isc−1·Voc−1, is ca. 25%.

Electrochemical study of pyrroloquinoline quinone covalently immobilized as a monolayer onto a cystamine-modified gold electrode

Abstract The electrochemistry of pyrroloquinoline quinone (PQQ) was studied in solubilized and immobilized states on a Au electrode modified with a chemisorbed cystamine monolayer. An electrochemically reversible diffusion-controlled reaction ( k ≈ 1.7 × 10 − 3 cm s − at pH 7.0) is observed for solubilized PQQ on the cystamine-modified electrode under acidic and neutral conditions (pH ⩽ 7) when the surface amino groups are positively charged. However, the electrochemical reduction of PQQ is completely irreversible on a non-modified Au electrode as well as on an electrode surface modified with neutral or negatively charged groups. The cystamine monolayer on the Au electrode surface was used as a basis for the covalent immobilization of PQQ via carbodiimide coupling of the PQQ carboxylic groups with the surface amino groups. The electrochemical reaction of the immobilized PQQ was reversible over a wide pH range (pH 2–11). The PQQ modified electrodes exhibited very high stability. A surface concentration of ca. 1 × 10 −10 mol cm −2 , corresponding to a monolayer, and an electron transfer rate constant k s of ca. 3.3 s −1 (pH 7.0) were evaluated for the PQQ-modified electrode. The total amount of immobilized PQQ can be increased dramatically if an Au electrode with a very high surface roughness is used. The redox potential E ° of −0.125 ± 0.003 V/SCE (pH 7.0) was obtained for both solubilized and immobilized PQQ. It is suggested that the PQQ-modified electrodes developed in this work can be used to prepare biosensors based on PQQ enzymes and to facilitate chemical reactions characteristic of PQQ itself directly on the electrode surface.

Application of bifunctional reagents for immobilization of proteins on a carbon electrode surface: Oriented immobilization of photosynthetic reaction centers

Abstract A new kind of bifunctional reagent was used to immobilize covalently monolayers of photosynthetic reaction centers (RCs) on a carbon electrode surface. Condensed aromatic rings were used as an anchor group for chemisorption on the basal-plane surface of a pyrolytic graphite electrode and chemically active functional groups were used to immobilize the RCs covalently via the amino acid residues of the protein. The RCs were randomly immobilized via lysine residuals when the bifunctional reagent activated for the reaction with amino groups was applied. An oriented immobilization of the RCs via the cysteine residual located at their accepting side was achieved when an electrode surface activated for thiol binding was used. A dramatic difference in the photoinduced currents was observed for different orientations of the RCs immobilized on the electrode surface. The small separation between the quinone sites inside the RCs and the electrode surface in the case of oriented RCs provides efficient non-diffusional electron transfer, and application of an additional solubilized electron transfer mediator does not affect the photocurrent. Electrochemical oxidation of the immobilized electron transfer mediator was shown to be the limiting step of photocurrent formation and a quantum efficiency of ca. 60% (for the absorbed light) was calculated for the photocurrent generation. In the case of randomly oriented RCs the photocurrent was much smaller, but it could be increased by application of a diffusionally mobile electron transfer mediator.

Chronopotentiometry and Faradaic impedance spectroscopy as signal transduction methods for the biocatalytic precipitation of an insoluble product on electrode supports : routes for enzyme sensors, immunosensors and DNA sensors

The biocatalyzed precipitation of an insoluble product produced on electrode supports is used as an amplification path for biosensing. Enzyme-based electrodes, immunosensors and DNA sensors are developed using this biocatalytic precipitation route. Faradaic impedance spectroscopy and chronopotentiometry are used as transduction methods to follow the precipitation processes. While Faradaic impedance spectroscopy leads to the characterization of the electron-transfer resistance at the electrode, chronopotentiometry provides the total resistance at the interfaces of the modified electrodes. A horseradish peroxidase, HRP, monolayer-functionalized electrode is used to sense H(2)O(2) by the biocatalyzed oxidation of 4-chloro-1-naphthol (1), to the insoluble product benzo-4-chlorohexadienone (2). An antigen monolayer electrode is used to sense the dinitrophenyl antibody, DNP-Ab, applying an anti-antibody-HRP conjugate as a biocatalyst for the oxidative precipitation of 1 by H(2)O(2) to yield the insoluble product 2. An oligonucleotide (3) functionalized monolayer electrode is used to sense the DNA-analyte (4), that is one of the Tay-Sachs genetic disorder mutants. Association of a biotin-labeled oligonucleotide to the sensing interface, followed by the association of the avidin-HRP conjugate and the biocatalyzed precipitation of 2 leads to the amplified sensing of 4. The amount of the precipitate accumulated on the conductive support is controlled by the concentration of the respective analytes and the time intervals employed for the biocatalytic precipitation of 2. The electron-transfer resistances of the electrodes covered by the insoluble product (2) are derived from Faradaic impedance measurements, whereas the total electrode resistances are extracted from chronopotentiometric experiments. A good correlation between the total electrode resistances and the electron-transfer resistances at the conducting supports are found. Chronopotentiometry is suggested as a rapid transduction means (a few seconds). The precautions needed to apply chronopotentiometry in biosensors are discussed.

Direct electron transfer between the covalently immobilized enzyme microperoxidase MP-11 and a cystamine-modified gold electrode

A fundamental prerequisite for the development of reagentless amperometric biosensors is the control over electron-transfer processes between immobilized biocatalytical recognition systems, e.g. enzymes, and the surface of a suitable electrode. According to Marcus’ theory [l], the electron-transfer rate is governed by the potential difference, the reorganisation energy and, most significantly for the development of enzyme electrodes, the distance between the involved redox centers. Thus, alternative electron-transfer pathways using free-diffusing redox mediators [2,3] redox-relay loaded polymers (Marcusian wires) [4,5], mediator-modified enzymes [6] and modified electrode surfaces [7,8] have been investigated aiming on a shortening of the distance between the redox sites involved in the electrontransfer reaction in question. However, the most attractive way for the development of even reagentless amperometric enzyme electrodes would be the direct communication between enzyme and electrode surface. Consequently, a relative configuration of the redox sites has to be established with the maximum distance determined according to Marcus’ theory. Especially with peroxidases, direct electron transfer has been demonstrated [9], and horseradish peroxidase has been used successfully as a biocatalytical compound for the construction of biosensors [lo]. Peroxidases adsorbed strongly at special treated carbon surfaces [ill show good electrocatalytic properties for the reduction of H,O, and have been used in combination with various oxidases for the construction of biosensors [12]. Unfortunately, the electron-transfer pathway re-