DE102006062735A1 - electrode device - Google Patents

Abstract

An electrode device (1) comprises an electrode (2) with a plurality of discrete control elements (6) which have at least one electrical property which can be influenced by illumination, an electrically conductive or semiconductive electrode substrate (5) which can interact electrically with the control elements (6), and a lighting means (3) for driving the electrode (3). At least one of the control elements (6) is connected to at least one peptide, protein or protein complex (8) such that charge transfer mediated by the control element (6) can take place between the peptide, protein or protein complex (8) and the electrode (2) ,

Description

background the invention

The The invention relates to an electrode device, in particular for Measuring or influencing electrical or electrochemical parameters. Furthermore it relates to a method for measuring or influencing electrical or electrochemical parameters.

State of technology

A generic electrode device is known from the patent US 4,591,550 (Hafeman et al.). The device comprises a one-piece flat electrode of a semiconductor material immersed in a liquid medium. By means of a likewise immersed in the medium reference electrode and a potentiostat, the electrode is kept at a constant potential. Opposite the electrode is a light emitting diode array that can illuminate different areas of the immersed surface of the semiconductor electrode to produce electron-hole pairs in the respectively illuminated area that are registered, for example, as a photocurrent. The photocurrent depends on the electrochemical state, eg the pH, of the medium in the immediate vicinity of the illuminated area. In this way, the state of the medium is to be determined spatially resolved by sequentially illuminating different surface areas and simultaneously measuring the photocurrent. It is also proposed to bind proteins, in particular enzymes, to the electrode and detect them in a spatially resolved manner.

Similar Devices in which instead of the light-emitting diodes one or more focused laser beams as a light pointer for illuminating the electrode are known among others from "Highly integrated surface potential sensors "(M. George, W.J. Parak, H.E. Gaub, Sensors and Actuators B, 69, 200, Pages 266-275), "Lateral resolution of light-addressable potentiometric sensors: an experimental and theoretical investigation "(W. J. Parak, U.G. Hofmann, H.E. Gaub, J.C. Owicki, Sensors and Actuators A, 63, 1997, pages 47-57), and "Investigation of the spatial resolution of the light-addressable potentiometric sensor "(M. George, W.J. Parak, I. Gerhardt, W. Moritz, F. Kaesen, H. Geiger, I. Eisele, H. E. Gaub, Sensors and Actuators, 86, 2000, pages 187-196).

The patent US 5,751,018 (Alivisatos et al.) Discloses a structure of semiconductor nanoparticles covalently bonded to the surface of a metallic electrode substrate by bridging molecules.

The invention underlying problem

Of the Invention is based on the object improvements in measuring or influencing electrical or electrochemical parameters provide.

Inventive solution

to solution The object of the invention teaches an electrode device with the Features of claim 1, an electrode device having the features of claim 13 and a method having the features of claim 11th

One achievable advantage of the invention is one compared to known Devices improved spatial resolution of the electrode device. This is desirable in many applications. For example, you can at a higher spatial resolution the electrode device more molecules located on the electrode independently be influenced or measured, for example, a protein chip to realize, in which a variety of different to the electrode coupled proteins measured by means of a light pointer and / or to be influenced. In the known electrode device with a one-piece Semiconductor electrode is the spatial resolution among other things by the Diffusion of the electron-hole pairs generated in the electrode by the illumination limited. With the invention, the diffusion of charge carriers can be limited and so the spatial resolution be improved.

One Another achievable advantage of the invention is an enlargement of surface the electrode. This can be particularly beneficial if molecules or molecular complexes to the electrode surface to be connected. For example, if redox proteins on the electrode can be bonded to the electrode device as a sensor to use, can be achieved by the invention, a higher sensitivity become.

It is another achievable with the invention advantage that the biological activity molecules or molecular complexes coupled to the electrode, especially proteins, less or not at all affected.

By changing the electrical property of the electrode, electrical or electrochemical properties of an object adjacent to the electrode can be influenced and / or measured. In biosensing applications, for example, it can be used to detect products of enzymatic turnover reactions or to detect mediators which react with a protein altered by analyte interaction the. It can also be used for direct electrochemical communication of the electrode with a redox-active center of a protein, for example to detect hydrogen peroxide, superoxide radicals, nitric oxide or catecholamines. Another possible area of application is the direct influence of biomolecules, for example as a redox electrode for the electroreduction of cytochrome c or hexacyanoferrate (III).

Construction and training the solution according to the invention

In a preferred embodiment According to the invention, the illumination means may be at least two different ones conditions in which different areas of the electrode are illuminated. Thus, the electrode can by sequentially illuminating the areas spatially resolved be controlled. By change The electrical property in this area can be spatially resolved electrical or electrochemical properties of a to the driven area adjoining object, in particular a molecule or molecular complex as described below, influenced and / or measured. These execution can also be used to on the Elektro de arranged biological Examination objects, e.g. Nerve cells, electrical anywhere to stimulate and to measure their reaction spatially resolved. The device can also be used for electrochemical imaging of surfaces or as spatially resolving optical image sensor can be used.

The preferred by electrical influencing electrical property is the conductivity of the Electrode or the control element.

The preferred extent of the control is in at least one Direction parallel to the electrode surface, preferably in all directions parallel to electrode surface, particularly preferably in all three spatial directions smaller than the diffusion length free charge carrier in the material. Preferably, the extent is less than a tenth, more preferably one hundredth, more preferably one Thousandth, especially preferably one ten-thousandth, especially preferably one hundred thousandths, more preferably one Millionth of the diffusion length. The diffusion length For the purposes of the present invention, the diffusion length is more free charge carrier in a large compared to the diffusion length volume of the material. For example, the diffusion length is for an n-doped silicon material with a specific resistance of 10 to 15 ohms under normal conditions at about 100 to 400 microns. The diffusion length determined by the expert in "Direct measurement of minority carrier diffusion length in planar devices "(A. Boudjani, G. Bassou, et al., Solid State Electronics 38 (2), 1995, pages 471-475) procedure, the entire content of which is hereby incorporated by reference Part of the present disclosure.

Preferred controls are nanoparticles. A nanoparticle in the sense of the present invention has an extent in the nanometer range, ie less than 100 nanometers, preferably less than 10 nanometers, in at least one spatial direction, preferably in two spatial directions, particularly preferably in all three spatial directions. The expansion is preferably between 1 and 6 nanometers, more preferably less than 5 nanometers. The nanoparticles can be globular, but in particular non-globular forms are also suitable, for example rod-shaped or multi-rod tetrapod nanoparticles, which are essentially composed of four rod-shaped nanoparticles converging at their one ends. The nanoparticles are preferably inorganic and colloidal. Nanoparticles which are suitable for the invention and processes for their preparation are described to the skilled person inter alia in "On the development of colloidal nanoparticles toward multifunctional structures and their possible uses for biological applications" (T. Pellegrino, S. Kudera, T. Liedl, A. Munoz Javier, L. Manna, WJ Parak, SMALL 1, 2005, 48-63) and the patent US 5,751,018 (Alivisatos et al.). The entire contents of the above-mentioned documents by reference are part of the present disclosure.

In a preferred embodiment According to the invention, the control elements comprise at least one semiconductor material, preferably a selenium, sulfur or tellurium-based semiconductor material, particularly preferably CdSe, CdS, CdTe, ZnSe ZnS or a combination from them, more preferably CdSe and ZnS, with particular preference a core of CdSe from a shell is surrounded by ZnS (core / shell system). By the excitation with light in the wavelength range The absorption of the control can be in the respectively illuminated area the electrode generates local electron-hole pairs become what can cause a photocurrent. This can for spatially resolved Reading or driving the electrode can be used.

In a preferred embodiment of the invention, the controls are nanoparticles comprising a semiconductor material, for example quantum dots. With quantum dots, it is possible to adjust the position of the absorption edge by the size of the particles. It is an achievable advantage of this embodiment of the invention that the absorption wavelength can be adjusted so that the light that is necessary for the excitation, one with the electrode to as little as possible affects the measuring or influencing object, which, inter alia, can counteract artifacts.

The Controls preferably form a layer, more preferably a monolayer. The layer preferably covers at least one Part of the electrode surface. Preferably, the electrode further comprises an electrically conductive or semi-conductive electrode substrate associated with the controls can interact electrically, thus giving an electrical signal to the controls created and / or can be removed from them. The electrode substrate is preferably a metal or a metal alloy, e.g. Gold or Aluminum. A particularly preferred electrode substrate is a planar gold electrode. The controls are preferably on immobilized to the electrode substrate.

In a preferred embodiment of the invention, the controls are coupled to the electrode substrate such that charge transfer between the control element and the electrode substrate can take place. The controls may be directly or indirectly, covalently or noncovalently bound to the electrode substrate. Preferably, the controls are bound by dithiols to the electrode substrate. Suitable methods and apparatus for coupling the controls to the electrode substrate and control electrode-substrate constructions suitable for carrying out the present invention are also disclosed in the specification US 5,751,018 (Alivisatos et al.), The entire contents of which are incorporated herein by reference.

Control elements according to the invention can also include several materials. In one embodiment of the invention the controls are a metal component. For example, to the Ends of a rod-shaped semiconductor nanoparticle metal beads be arranged. The beads can also at the free ends of a tetrapod-shaped semiconductor nanoparticle be arranged. The semiconductor nanoparticles are preferably CdSe, CdS, CdTe, ZnSe or a combination these materials, the metal beads preferably of gold and are preferably epitaxial to the free ends grown up of the semiconductor nanoparticle. Gold supplier, for example organometallic compounds such as chlorotriphenylphosphine be. Suitable methods and apparatus for growing gold on nanocrystals The skilled person will find, inter alia, in "Selective growth of metal tips semiconductor quantum rods and tetrapods "(T.Mokari et al., Science, 2004, 304, p. Pages 1787-1790), the entire content of which is hereby incorporated by reference Part of the present disclosure. The controls can be on a gold surface be coupled to the elec trene rod substrate by the thiol ends on the gold surface bound dithiols to a gold component of the controls. Tetrapods are preferably with three legs through their metal beads coupled to the electrode substrate, while the fourth leg of the Electrode points away. Since the gold-thiol bond is quite stable, the above execution The invention helps to better bond the controls to the electrode substrate surface. It is an achievable advantage of this embodiment of the invention that higher currents occur between Particles and electrode substrate can flow. Another achievable Advantage is an improved stability of the electrode.

At a preferred control is at least one molecule or molecular complex bound to an electrical or electrochemical property of the molecule or molecular complex to read or influence. The preferred control is a nanoparticle according to the invention, comprising a semiconductor material. Preferred molecules or molecular complexes are peptides, proteins or protein complexes. Preferred to the Control elements of the electrode device according to the invention bound proteins are receptors and enzymes, more preferably Redox proteins such as e.g. Cytochrome c, cytochrome c 'and azurin, but also generally heme proteins and copper proteins. The molecules or molecular complexes can directly or indirectly, covalently or non-covalently bound to the control be.

Preferably, the molecule or molecular complex is coupled to the control such that charge transfer can take place. If the control is initially covered with hydrophobic stabilizer molecules where the molecules or molecular complexes are to be bound, it may be advantageous to make the surface hydrophilic in a first step because many suitable molecules or molecular complexes are hydrophilic. This is preferably done by at least partial ligand exchange, particularly preferably in aqueous solution. Preferred novel ligands are carboxy-terminated thiols and / or hydroxy- and carboxy-terminated mixed layers, respectively. Such ligands have proven to be particularly advantageous in the attachment of cytochrome c. Particularly preferred are mercaptosuccinic acid and mercaptopropionic acid. These ligands preferentially bind with their thiol group to the surface of the control thereby displacing the hydrophobic stabilizer molecules while the carboxyl group of the ligands point into the solution. By means of an EDC activation of the carboxyl group, a protein can be covalently bound via its amino group to the carboxyl group, as described, for example, in "Bioconjugate Techniques" (GT Her Manson, 1996, Academic press, San Diego), the entire contents of which are incorporated herein by reference. Azurine is preferably attached to the controls by aliphatic or aromatic groups, more preferably nanoparticles comprising at least one of CdSe, CdS CdTe, ZnSe and ZnS. It is an achievable advantage of the aforementioned embodiments of the invention that the molecules or molecular complexes can be bound close to the surface of the control.

Thereby, that the control in a preferred embodiment of the invention both with the electrode substrate as well as with the molecule or molecular complex is connected, can be achieved by the one Control mediated charge transfer between molecule and electrode substrate takes place. This charge transfer can be due to the illumination-induced change of a modulated electrical property of the control, preferably be switched on or off. The charge transfer preferably takes place by means of a lighting induced photocurrent in the control instead of. Consequently, a locally switchable Electrode for reading the electrochemical response of a and the Electrode bound molecule or molecular complex will be realized. The illumination intensity is at a preferred Electrode device modulated, e.g. sinusoidal or in pulse form. Especially the electrode is preferably synchronized in the lock-in process read out and / or driven to the signal-to-noise ratio improve.

The Electrode is when used as intended preferably surrounded by a medium, for example an aqueous one Solution, particularly preferably a buffer solution, for example, a phosphate buffer solution. The Reading the molecules is preferably done by applying an electrical potential, e.g. through a potentiostat, between the electrode substrate and the medium and the registration of the current response, preferably by voltammetric or amperemetric analysis. By applying Of the electrical potential, it is also possible oxidation or reduction reactions in the molecule trigger or the molecule to influence in another way.

Leakage through direct contact of the medium with the electrode substrate surface can be counteracted be covered by the electrode substrate surface as close as possible with controls becomes. Also by adding an alkyl thiol, preferably to the free gold surface, but does not bind to a free CdSe surface, leakage currents can be counteracted become. In a preferred embodiment of the invention is the Electrode for suppression of leakage currents covered with a polymer layer. The controls are in this Case preferably Tetrapods, only with their from the electrode protruding leg out of the polymer layer and with the solutions stay in contact. Suitable devices and methods are the Expert, inter alia, known from "Preparation of asymmetric nanostructures through site selective modification of tetrapods "(H. Liu and A. P. Alivisatos, Nano Letters, 2004, 4 (12), pp. 2397-2401), the entire relating thereto Content by reference is part of the present disclosure.

The molecules or molecular complexes Preferably, interact with substances in the solution, especially preferably with electron uptake or release. So are to Example reactive oxygen species such as superoxide suitable analyte molecules for cytochrome c and azurin, hydrogen peroxide comes as an analyte molecule for cytochrome c in question, nitric oxide for Cytochrome c '. It is possible, the electron transfer reactions by means of the electrode device according to the invention demonstrated. In this case, the signal chain starting from the reactive Species over the protein to the electrode using the photosensitive controls be turned on and off locally, thereby reading the protein-analyte interaction can be achieved.

In an execution the invention runs the Signal chain over multiple proteins, protein complexes or peptides, with one at the control bound protein, protein complex or peptide acts as a mediator. This execution The invention can contribute to the fact that an implementation with Proteins possible is whose redox center is embedded deeper in the protein structure is. As a mediator, for example, cytochrome c comes into question, as a second Protein cytochrome c oxidase or the copper protein laccase. In a Aussführung is the second protein, protein complex or peptide in solution, in another is immobilized on the first one. Suitable arrangements and methods, the person skilled in the art knows u.a. from "Inter-protein communication in cyt. c / lactase-modified polyelectrolyte multilayer electrodes "(R.V. Dronov, H. Mohwald, D.G. Kurth, M.K. Beissenhirtz, F. Scheller, Poster session and exhibition, 3rd International Symposium on Sensor Science in Jülich, Germany, from 18 to 21 July 2005, pages 100-101) and "A biofuel cell with electrochemically switchable and tuneable power output "(E. Katz, I. Willner, Journal of the American Chemical Society, 125 (22), 203, pages 6803-6813). The entire content in this regard the above-mentioned documents are incorporated herein by reference Epiphany.

In another embodiment of the invention is a low molecular weight redox mediator on the Control immobilized. This design can contribute to the possibility of coupling to proteins whose redox center is embedded deeper in the protein structure. As redox mediator come, for example, ferrocene or the Chion PQQ in question. The redox mediator is preferably immobilized on the control by bifunctional spacer molecules. Most preferably, it is covalently bonded to a thiol layer of the control.

It is desirable in many applications with a single electrode a variety of different molecules or molecular complexes to read and / or influence, for example, a variety different analyte molecules prove in parallel. In a preferred embodiment of the invention the electrode has at least a first and a second region, wherein in the first region other molecules or molecular complexes are bound to the controls as in the second area. Preferably Each area comprises at least one control element, particularly preferably multiple controls. A preferred electrode forms an array of hundreds, thousands or more discrete areas where each other molecules or molecular complexes are bound. Within each area are preferably one Plurality of identical molecules or molecular complexes bound to the electrode. Basically, the number of areas only by the spatial resolution of Limited electrode device and the size of the molecules. The molecules are preferably applied to the electrode by spotting. An example for Such an array are protein clips that contain proteins, protein complexes or peptides are bound to the electrode, and where the areas preferably punctiform and are arranged in rows and columns on the surface of the electrode. The proteins, protein complexes or peptides are preferably by means of soft lithograph stamped on the electrode. For reading the protein chip is preferably a light spot as a light pointer over the chip surface guided. The preferred point of light has an extension which is at most so big particularly preferably smaller than a range of identical proteins, Protein complexes or peptides. So can different proteins, protein complexes or peptides separately lit and by synchronous application and / or removing signals to and from the electrode and / or read out.

When Every means of lighting comes Facility in question, which selectively areas on the surface of the Electrode can illuminate so that at least one electric Property changes. Particularly preferred light sources include lasers. But also light-emitting diodes, Discharge lamps or conventional incandescent lamps are suitable. A preferred light source is monochromatic, for example a laser or a filter equipped, originally polychromatic light source. In one execution the invention, the wavelength of light is chosen so that it is just sufficient to change the electrical property. In another embodiment of the Invention becomes the wavelength of light or the wavelength range so selected that the absorption by an object adjacent to the electrode, in particular that to be influenced by the electrode and / or is minimized to measured object. These embodiments can contribute to artifacts in the use of the electrode device according to the invention submissions.

The preferred illumination means comprises deflection means for illuminating different areas of the electrode, for example mirrors or prisms. But it is also conceivable to use an array of a plurality of isolated light sources as Beleuchtungsmit tel, eg a light-emitting diode array. A suitable light emitting diode array is in the patent US 4,591,550 (Hafeman et al.), The entire contents of which are incorporated herein by reference. In a preferred embodiment of the invention, at least a portion of the electrode surface is scanned sequentially, eg line by line with a main and sub-scan direction, and synchronously applied and / or removed from the electrode to successively scan the scanned areas of the electrode to control or read.

The above over Controls said applies accordingly also for nanoparticles according to the invention, that do not work as controls.

Summary the drawings

The The invention will be described below with reference to schematic drawings embodiments with more details explained.

It demonstrate:

1 a cross-sectional view of an electrode device according to the invention,

2 a first embodiment of a control according to the invention bound to the electrode substrate and

3 a second embodiment of a control according to the invention bound to the electrode substrate.

description a first embodiment

In the 1 shown spatially resolving electrode device 1 includes an electrode 2 and a lighting means 3 to the electrode surface with a light beam 4 scan. The electrode 2 has a planar gold layer 5 on that with a monolayer of CdSe quantum dots 6 is occupied. The quantum dots 6 are through dithiols 7 to the gold layer 5 bound. For this purpose, the gold surface is functionalized in a first step with dithiol. Then the quantum dots 6 , which were previously covered with a hydrophobic stabilizer molecule, brought into connection with the gold surface. Upon contact with the controls, the thiol at the contact surface displaces the hydrophobic stabilizer molecules in a type of ligand exchange and binds to the control.

The light source of the illuminant 3 is a laser with a wavelength within the absorption range of the quantum dots 6 , The laser beam is focused with a lens system to a point on the electrode surface. By means of a mirror system as deflection means, the point is guided line by line over the electrode surface.

On the of the gold layer 5 opposite side of the quantum dots 6 are cytochrome c proteins 8th by mercaptopropionic acid 9 to the surface of the quantum dots 6 bound. To this end, the surfaces of the quantum dots initially covered with hydrophobic stabilizer molecules become 6 occupied by ligand exchange in aqueous solution with a Mercaptopropionsäure layer. The acid binds with its thiol group to the surface of the quantum dot 6 and with its carboxyl group to the protein 8th ,

The protein-coated electrode is in a 0.1 M phosphate buffer solution 10 , between the gold layer and the buffer solution 10 is by means of a potentiostat 11 and an electrode immersed in the buffer solution 12 an electrical voltage applied. In the circuit formed thereby 13 For example, during line by line scanning of the electrode surface by the light spot, the photocurrent is measured by the light excitation of the quantum dots being illuminated 6 and the charge transfer between proteins and gold layer in the circuit 13 is caused. In a connected computer, the location and the current measured at this location are assigned to each other and linked together.

description two further embodiments

2 shows a second embodiment of an electrode according to the invention, in which instead of the CdSe quantum dots 6 dumbbell Au-CdSe Au nanoparticles to the surface of the gold layer 5 through the mediation of dithiol molecules 7 are bound. A dumbbell-shaped Au-CdSe Au nanoparticle consists of a rod-shaped CdSe nanocrystal 14 with gold spheres epitaxially grown on its ends 15 , The nanoparticle is in all places to which it is not due Dithiolmoleküle 7 bound to the gold surface is with hydrophobic stabilizer molecules 16 covered.

3 shows a third embodiment of the electrode according to the invention, in which instead of the CdSe quantum dots 6 CdSe tetrapods 17 be used. The tetrapods are composed of four rod-shaped CdSe nanocrystals that converge at their one ends. In 3 only three of these rod-shaped nanocrystals are shown. At the four free ends of the tetrapod are epitaxially gold spheres 15 grew up. At three of its ends, two of which are shown, the tetrapod is through the mediation of dithiol molecules 7 to the surface of the gold layer 5 bound. A fourth end points away from the gold layer. The tetrapod is present in all places where it is not due to dithiol molecules 7 bound to the gold surface, with hydrophobic stabilizer molecules 16 covered. To the buffer medium 10 from the gold layer 5 to insulate is the electrode 2 with a polymer layer 18 covered, so that only the point away from the polymer layer leg of the tetrapod in the medium 10 enough.

Claims (11)

Electrode device ( 1 ), comprising: - an electrode ( 2 ) with several discrete controls ( 6 ), which have at least one electrical property that can be influenced by illumination, and an electrically conductive or semiconductive electrode substrate ( 5 ), with the controls ( 6 ) can interact electrically; and - a lighting means ( 3 ) to the electrode ( 2 ), characterized in that at least one of the controls ( 6 ) with at least one peptide, protein or protein complex ( 8th ) that is connected through the control ( 6 ) mediated charge transfer between the peptide, protein or protein complex ( 8th ) and the electrode ( 2 ) can take place. Electrode device ( 1 ) according to claim 1, characterized in that the illumination means ( 3 ), at least two different states can take in which different areas of the electrode ( 2 ) are illuminated to the electrode ( 2 ) locally resolved to control. Electrode device ( 1 ) according to claim 1 or 2, characterized in that the controls ( 6 ) Are nanoparticles. Electrode device ( 1 ) according to one of the preceding claims, characterized in that the controls ( 6 ) have an elongated shape or the shape of a tetrapod. Electrode device ( 1 ) according to one of the preceding claims, characterized in that the controls ( 6 ) comprise a semiconductor material. Electrode device ( 1 ) according to one of the preceding claims, characterized in that the electrode ( 2 ) to an aqueous medium ( 10 ) or from an aqueous medium ( 10 ) is surrounded. Electrode device ( 1 ) according to claim 6, characterized in that an electrical voltage between medium ( 10 ) and electrode ( 2 ) is created. Electrode device ( 1 ) according to one of the preceding claims, characterized in that the electrode ( 2 ) comprises at least a first and a second region, wherein in the first region other peptides, proteins or protein complexes ( 8th ) to the controls ( 6 ) are coupled as in the second area. Electrode device ( 1 ) according to one of the preceding claims, characterized in that at least one peptide, protein or protein complex is coupled to the electrode by means of at least one carboxy-terminated thiol molecule. Electrode device ( 1 ), comprising: - an electrode ( 2 ) with several discrete controls ( 6 ) with at least one electrical property which can be influenced by illumination, and - an illumination means ( 3 ) to the electrode ( 2 ), characterized in that the controls ( 6 ) have an elongated shape or the shape of a tetrapod. Method for measuring or influencing electrical or electrochemical parameters, comprising the steps of: - providing an electrode ( 2 ) with several discrete controls ( 6 ), which have at least one electrical property which can be influenced by illumination, and an electrically conductive or semiconducting electrode substrate ( 5 ), with the controls ( 6 ) can interact electrically; Coupling at least one peptide, protein or protein complex ( 8th ) to at least one control element ( 6 ); - Illumination of the electrode ( 2 ); and - by the control ( 6 ) mediated charge transfer between peptide, protein or protein complex ( 8th ) and the electrode ( 2 ).