Direct electrocatalytical H2O2 reduction with hemin covalently immobilized at a monolayer-modified gold electrode

Electron Transfer in Binary Hemin-Modified Alkanethiol Self-Assembled Monolayers on Gold: Hemin's Lateral and Interfacial Interactions

Orientated coupling of redox enzymes to electrodes by their reconstitution onto redox cofactors, such as hemin conjugated to self-assembled monolayers (SAMs) formed on the electrodes, poses the requirements for a SAM design enabling reconstitution. We show that the kinetics of electron transfer (ET) in binary SAMs of alkanethiols on gold composed of in situ hemin-conjugated 11-amino-1-undecanethiol (AUT) and diluting OH-terminated alkanethiols with 11, 6, and 2 methylene groups (MC₁₁OH, MC₆OH, and MC₂OH) depends on both the SAM composition and surface density of hemin, Γ_heme. In AUT/MC₁₁OH SAMs composed of equal linker/diluent lengths, the heterogeneous ET rate constant k_s decreased with the Γ_heme and varied between 70 and 500 s^-1. For shorter diluents, the k_s of 245-330 s^-1 (C₆) and 300-340 s^-1 (C₂) showed a little (if any) Γ_heme dependence. In AUT/MC₁₁OH SAMs, the increasing Γ_heme resulted in the steric crowding of hemin species and their neighboring lateral interactions in the plane of hemin localization, affecting the potential distribution at the SAM/electrode interface and inducing local electrostatic effects interfering with hemin oxidation. In AUT/MC₆OH and AUT/MC₂OH SAMs, hemin discharged at the plane of the closest approach to the gold surface, equal to the diluent length and permeable to electrolyte ions, which lessened those effects. All studied binary SAMs provided steric hindrance for protein reconstitution on the hemin cofactor conjugated to the extended AUT linker. Further use of SAM-modified electrodes with the covalently attached hemin as interfaces for heme proteins' reconstitution should consider SAMs with loosely dispersed redox centers terminating more rigid molecular wires. Such wires place hemin at fixed distances from the electrode surface and thus ensure the interfacial properties required for the effective on-surface reconstitution of proteins and enzymes.

Cu-MOF/hemin: a bionic enzyme with excellent dispersity for the determination of hydrogen peroxide released from living cells

The stability, repeatability and sensitivity of an electrochemical biosensor material are closely connected with the dispersibility of metal organic frameworks (MOFs) in aqueous media. Herein, a nanocomposite based on Cu-MOF/hemin, which is not only highly water-soluble but also simple and efficient in synthesis, was used for the construction of a non-enzymatic sensor to detect hydrogen peroxide (H₂O₂). The Cu-MOF/hemin was characterized via scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS)-mapping, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis (TGA), which indicate that hemin and the Cu-MOF were successfully combined. As a H₂O₂ electrochemical biomimetic enzyme, the Cu-MOF/hemin exhibited excellent electrocatalytic performance, which was confirmed by the electrochemical experiments and chromogenic reactions, and the possible mechanism of the reactions has been deduced. The electrochemical sensor based on the biomimetic enzyme exhibited an extended linear detection range from 0.01-5.0 mM (R = 0.998), low detection limit of 4.14 μM, and high selectivity and stability under the optimized conditions. More importantly, the practical application ability of the sensor was verified by the test of H₂O₂ in human serum samples and it could be used for the real-time detection of H₂O₂ released from living cells with satisfactory results. Therefore, this novel nanocomposite has certain potential in preparing electrochemical sensing platforms for nonenzymatic biosensing and provides a new method for clinical diagnosis and real-time monitoring.

Biomimetic versus enzymatic high-potential electrocatalytic reduction of hydrogen peroxide on a functionalized carbon nanotube electrode

We report the non-covalent functionalization of a multi-walled carbon nanotube (MWCNT) electrode with a biomimetic model of the horseradish peroxidase (HRP) active site.

We report the non-covalent functionalization of a multi-walled carbon nanotube (MWCNT) electrode with a biomimetic model of the horseradish peroxidase (HRP) active site. By modifying the MWCNT electrode surface with imidazole-modified polypyrrole, a new biomimetic complex of HRP was synthesized on the MWCNT sidewalls via the coordination of imidazole (Im) to the metal centre of iron protoporphyrin IX, affording (Im)(PP)Fe^III. Compared to the pi-stacking of non-coordinated (PP)Fe^III on a MWCNT electrode, the (Im)(PP)Fe^III-modified MWCNT electrode exhibits higher electrocatalytic activity with an I_max = 0.52 mA cm^–2 for the reduction of H₂O₂, accompanied by a high onset potential of 0.43 V vs. Ag/AgCl. The performances of these novel surface-confined HRP mimics were compared to those of a MWCNT electrode modified by HRP. Although the enzyme electrode displays a higher electrocatalytic activity towards H₂O₂ reduction, the (Im)(PP)Fe^III-modified MWCNT electrode exhibits a markedly higher operational stability, retaining 63% of its initial activity after one month.

Enhanced electroactivity and substrate affinity of microperoxidase-11 attached to pyrene-linkers π–π stacked on carbon nanostructure electrodes

An exceptionally large electroactively connected microperoxidase-11 (MP-11) with strong affinity for organic peroxide and offering a high electrocatalytic reduction current density of 7.5 mA cm⁻² is achieved for the first time.

Structure orientation of hemin self-assembly layer determining the direct electron transfer reaction

A strategy was proposed to control the hemin orientation via experimental models, which shows heme plane orientation dependent direct electron transfer and electrocatalysis.

Hydrogen peroxide biosensor based on microperoxidase-11 immobilized in a silica cavity array electrode

Hydrogen peroxide biosensor based on the silica cavity array modified indium-doped tin oxide (ITO) electrode was constructed. An array of silica microcavities was fabricated by electrodeposition using the assembled polystyrene particles as template. Due to the resistance gradient of the silica cavity structure, the silica cavity exhibits a confinement effect on the electrochemical reactions, making the electrode function as an array of "soft" microelectrodes. The covalently immobilized microperoxidase-11(MP-11) inside these SiO2 cavities can keep its physiological activities, the electron transfer between the MP-11 and electrode was investigated through electrochemical method. The cyclic voltammetric curve shows a quasi-reversible electrochemical redox behavior with a pair of well-defined redox peaks, the cathodic and anodic peaks are located at -0.26 and -0.15V. Furthermore, the modified electrode exhibits high electrocatalytic activity toward the reduction of hydrogen peroxide and also shows good analytical performance for the amperometric detection of H2O2 with a linear range from 2×10(-6) to 6×10(-4)M. The good reproducibility and long-term stability of this novel electrode not only offer an opportunity for the detection of H2O2 in low concentration, but also provide a platform to construct various biosensors based on many other enzymes.

Peroxidase activity of de novo heme proteins immobilized on electrodes

De novo proteins from designed combinatorial libraries were bound to heme terminated gold electrodes. The novel heme proteins were shown to possess peroxidase activity, and this activity was compared to that of horseradish peroxidase and bovine serum albumin when immobilized in a similar fashion. The various designed proteins from the libraries displayed distinctly different levels of peroxidase activity, thereby demonstrating that the sequence and structure of a designed protein can exert a substantial effect on the peroxidase activity of immobilized heme.

Amperometric enzyme biosensors based on optimised electron-transfer pathways and non-manual immobilisation procedures

Development of reagentless biosensors implies the tight and functional immobilisation of biological recognition elements on transducer surfaces. Specifically, in the case of amperometric enzyme electrodes, electron-transfer pathways between the immobilised redox protein and the electrode surface have to be established allowing a fast electron transfer concomitantly avoiding free-diffusing redox species. Based on the specific nature of different redox proteins and non-manual immobilisation procedures possible biosensor designs are discussed, namely biosensors based on (i) direct electron transfer between redox proteins and electrodes modified with self-assembled monolayers; (ii) anisotropic orientation of redox proteins at monolayer-modified electrodes; (iii) electron-transfer cascades via redox hydrogels; and (iv) electron-transfer via conducting polymers.

Picodroplet-deposition of enzymes on functionalized self-assembled monolayers as a basis for miniaturized multi-sensor structures

We are reporting on a novel approach for structured immobilisation of enzymes on gold surfaces modified with monolayers of functionalised alkylthiols. The formation of enzyme spots is achieved by shooting very small volumes of an appropriate enzyme solution (down to 100 pl) onto a thiol-monolayer modified gold surface using a micro-dispenser. Formation of enzyme patterns is obtained by moving the micro-dispenser relative to the modified gold surface using a micro-positioning device. Enzyme spots with typical lateral dimensions of 100 microm are obtained, but also, more complex structures, e.g. lines or meander structures, can be achieved by multiple droplets dispensed during the concomitant movement of the micro-dispenser. The first enzyme layer on top of the functionalised thiol-monolayer is subsequently covalently immobilised using either carbodiimide activation of carboxilic headgroups at the enzyme or via already introduced activated ester functions at the monolayer. Immobilised enzyme activities of glucose oxidase and lactate oxidase patterns have been characterised by means of scanning electrochemical microscopy. The product of the enzyme-catalysed reaction, H(2)O(2), is detected with an micro-electrode in the presence of either or both substrates, glucose and lactate, leading to a visualisation of the corresponding enzyme pattern and the lateral enzymatic activity.

Oriented immobilization of antibodies onto the gold surfaces via their native thiol groups

The general approach for site-oriented immobilization of antibodies onto gold supports is reported. The immobilization is carried out using the native sulfide groups of immunoglobulin (IgG). To liberate the thiol groups, the intact IgG was split into two half-IgG fragments without destruction of the binding site of the antibody. The immobilization of half-IgG fragments on the gold surface was carried out by simple adsorption. The antigen binding capacity of the half-IgG modified gold supports is similar to that of the gold surfaces with the traditionally linked antibodies and is much higher than for nonspecifically adsorbed intact IgGs. The immobilized antibodies, according to the proposed approach, maintain high antigen binding constants. The immobilization procedure provides orientation of IgG fragments in terms of the similar distance between the binding site of the antibody and the surface of the gold support, which does not cause the distribution of the apparent affinity constants. The high operational stability of half-IgG modified gold electrodes makes them applicable for analytical applications.

Towards amperometric immunosensor devices

In contrast to optical immunosensors, the electrochemical detection of an immunanalytical reaction does require a labeling, but allows an easier discrimination of specific and non-specific binding. We present a concept and first results for a multivalent amperometric immunosensor system which is based on silicon technology. The capture molecule streptavidin, covalently immobilized on silica, allows the immobilization of biotinylated antigens at a defined density. A nanostructured gold electrode serving as a stable network of nanowires is expected to be beneficial for the electrochemical detection of bound ferrocene-labeled antibody molecules. The results presented focus on site-specific immobilization of streptavidin on silica and reduction of non-specific binding of proteins.