Electronic Devices with Protein Layers

TECHNICAL FIELD

The present invention relates generally to electronic devices and methods for producing electronic devices.

In particular, the present invention relates to electronic devices comprising an interface between biological material and conventional materials used in electronics.

BACKGROUND ART

Much effort has been put on design of field effect transistors (FET) suitable for biosensing (BioFET), as discussed in Michael J. Schoning and Arshak Poghossian, Recent advances in biologically sensitive field effect transistors (BioFETs), Analyst, 2002, 127, 1137-1151.

The use of single-walled carbon nanotubes (SWCNTs) in biological FET applications has been demonstrated in Maria Teresa, Martinez; Yu-Chih, Tseng; Nerea Ormategui; Iraida, Loinaz; Ramon, Eritja and Jeffrey Bokor, Label-Free DNA Biosensors Based on Functionalized Carbon Nanotube Field Effect Transistors, Nano Lett., 2009, 9 (2), pp 530-536.

Graphene has become more attractive material in device development because of its superior properties, as described in Priscilla, Kailian, Ang; Wei, Chen; Andrew, Thye; Shen ,Wee and Kian, Ping, Loh, Solution-Gated Epitaxial Graphene as pH Sensor, J. Am. Chem. Soc. 2008, 130, 14392-14393.

Corresponding findings have been presented also in Nihar Mohanty and Vikas Berry, Graphene-Based Single-Bacterium Resolution Biodevice and DNA Transistor: Interfacing Graphene Derivatives with Nanoscale and Microscale Biocomponents Nano Lett., Vol. 8, No. 12, 2008, 4469.

Biosensors have also been disclosed in patent application publications US 2010/0025660 A1 and GB 2 452 857 A.

Most of the materials for biosensing include chemical modification of graphene surface in order to get the desired functionalities on the surface.

Several methods for producing graphene materials have been proposed in the background art. As graphene is naturally present in graphite, several of the proposed methods produce graphene by exfoliating from graphite. Other suggested production methods include deposition of graphene on surfaces.

Liang, X et al. Graphene Transistors Fabricated via Transfer-Printing in Device Active-Areas on Large Wafer, Nano Letters, Vol. 7, No. 12, 3840-3844, 2007, discloses a method that uses pillars on a stamp to cut and exfoliate graphene islands from graphite and then uses transfer printing to place the islands from the stamp into device active-areas on a substrate. The publication also reports transistors fabricated from the printed graphene.

International Patent Application Publication No. WO 2007/097938 A1 discloses graphene layers epitaxially grown on single crystal substrates. A produced device comprises a single crystal region that is substantially lattice-matched to graphene. A graphene layer is deposited on the lattice-matched region by means of molecular beam epitaxy (MBE), for instance.

DISCLOSURE OF INVENTION

In view of the great potential of biological materials as constituents in various electronic devices, there remains need for new production methods with their associated advantages.

Therefore, it is an object of the present invention to provide a new method for producing an electronic device comprising biological and electrical materials and an interface between the biological and electrical materials.

According to an aspect of the invention, the production method comprises utilizing self-assembly of proteins at a desired location when producing the electronic device.

In another embodiment, the proteins form a layer on, and attached to, a surface of hydrophobic material.

In an embodiment, the proteins form a layer on, and attached to, a surface of graphene.

In an embodiment, the proteins form a layer on, and attached to, a surface of silicon.

In an embodiment, the proteins contain hydrophobin proteins that are particularly suitable for forming self-assembled layers on hydrophobic surfaces.

Thus, according to another aspect of the invention, there is also provided an electronic device that includes a protein layer as a structural part thereof and the protein layer contains hydrophobin proteins.

Therefore, the invention provides new methods for producing electronic devices as well as totally new types of electronic devices.

The invention also has several embodiments that may provide certain advantages over the previously known methods and electronic devices, at least in view of some particular applications.

Some embodiments provide a stable protein layer on a surface in the device, such as on a graphene or silicon surface.

In an embodiment, the proteins contain proteins that are particularly suitable for detecting certain molecules.

In embodiments, wherein the surface of the graphene layer is provided with functionalized proteins, the proteins may be used, for example, to deliver the platelets to desired sites or connecting the graphene electrically to an external circuitry. Some of these embodiments also allow the use of biomolecular recognition for placing the nanomaterial at specific and desired positions. Thus, there are also embodiments in which the proteins and graphene in the device are in the form of platelets.

The invention has also several other embodiments providing associated advantages.

BRIEF DESCRIPTION OF DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, the invention is now described with the aid of the examples and with reference to the following drawings, in which:

FIG. 1 presents a schematic cross-section of an electronic device according to an embodiment.

FIG. 2 presents a schematic cross-section of an electronic device according to another embodiment.

FIG. 3 presents a schematic cross-section of an electronic device according to a further embodiment.

FIG. 4 presents a schematic cross-section of an electronic device according to an even further embodiment.

FIG. 5 presents the structure of one protein that can be used according to an embodiment, namely a HFBI protein;

FIG. 6 presents a schematic drawing of proteins adhered on the surface of a graphene layer according to one embodiment;

FIG. 7 is a schematic drawing that depicts a cross-section of a platelet according to one embodiment;

FIG. 8 is a schematic drawing that depicts a cross-section of a platelet according to another embodiment;

FIG. 9 is a schematic drawing that depicts a cross-section of a platelet according to third embodiment;

FIG. 10 is a schematic drawing that depicts a cross-section of a platelet according to fourth embodiment;

FIG. 11 shows a TEM image of a piece of graphene according to an embodiment;

FIG. 12 shows a diffraction pattern measured from the graphene piece of FIG. 11;

FIG. 13 presents intensities of the diffraction peaks measured from the graphene piece of FIG. 11;

FIG. 14 shows protein exfoliated graphene flakes and HOPG pillars in one experiment;

FIG. 15 presents a schematic cross-section of a BioFET according to an embodiment;

FIG. 16 shows electron mobilities in the BioFET of FIG. 15;

FIG. 17 shows images of functionalized graphene sheets according to embodiments;

and

FIG. 18 shows further images of functionalized graphene sheets according to embodiments.

DEFINITIONS

Graphene refers generally to material that consists essentially of a one-atom-thick planar sheet of sp²-bonded carbon atoms. In graphene, the carbon atoms are densely packed in a honeycomb crystal lattice.

Graphene sheet has a similar meaning as graphene but it is used when referring to the sheet-like nature of the graphene material, especially when describing a thickness of an object.

Graphene layer is material that contains graphene and exhibits the functional properties of graphene as relevant for the application in question. Therefore, a graphene layer contains at least one graphene sheet. A graphene layer can contain a plurality of graphene sheets on top of each other, and therefore can have a thickness of several atomic layers.

Layer of graphene is the same as graphene layer.

Platelet is a generally planar film-like object of typically small size and thickness. Size and thickness may be in the order of micrometers or nanometres, for instance.

Protein is a polypeptide molecule comprising a chain of amino acids joined together by peptide bonds.

MODES FOR CARRYING OUT THE INVENTION

The following description presents methods for using hydrophobin proteins as functional layers in graphene-based electrical devices. According to an embodiment, the existing silicon technology can be interfaced with a thin membrane consisting of a self-assembled protein layer. According to an embodiment, such devices can be used as sensors. The route for obtaining such sensing devices would be to functionalize the surface of graphene with recognition elements that contain specificities for analytes that are let in contact with the layer.

According to the embodiments, the manufacture of functionalized parts of electronic devices can be facilitated by using hydrophobins. For example, proteins are first functionalized and then brought into contact with a graphene surface whereby the functionalized proteins can adsorb on the graphene surface by a self-assembly process. In an alternative approach, proteins are functionalised after formation of the self-assembled membrane. During adsorption, hydrophobin does not create chemical bonds to graphene, which can be beneficial for maintaining and controlling the sensitive properties of graphene.

A common approach to the BioFET operation is represented by the so called constant charge (or constant drain current) mode. In the constant charge mode, by setting the drain current at a fixed value using a feedback circuit, the voltage shift that results from the biochemical reaction can be recorded directly. The resulting sensor output signal is then proportional to the voltage shift. The arrangement in the constant charge mode also allows a simultaneous multisensory characterization by using only one common reference electrode. In general the BioFET transducer is very sensitive to any kind of electrical interaction at or nearby the gate insulator/electrolyte interface, thus any biochemical reaction leading to chemical or electrical changes at this interface will result on a signal measured by the biosensor. It is known that among others, the following basic mechanisms of potential generation for BioFETs can be considered: (i) potential changes that are caused by a catalytic reaction product (e.g., between an enzyme and its substrate); (ii) potential changes that are caused by surface polarisation effects or the change of dipole moments (e.g., antigen—antibody affinity reactions or DNA hybridisation, in principle, under certain conditions a FET is able to detect the change of the electric field associated with binding of biomolecules; (iii) potential changes that are coming from living biological systems as a result of more sophisticated biochemical processes (e.g., action potential of nerve cells).

Hydrophobin layer is not only suitable as the interfacial membrane because of its structural and adhesive properties, but also because it can be readily engineered with various functionalities, including recognition elements. The functionalization with recognition elements can be realized by the use of genetic engineering where the desired elements are fused with hydrophobins by combining them into one polypeptide chain. Attachment of the recognition element to a wildtype hydrophobin or an engineered hydrophobin can also be carried out chemically before or after formation of the protein membrane. The resulting fusion proteins are incorporated into the membranes so that the hydrophobin part is interlocked on the interface and the recognition element is free towards the solution. The choice of recognition elements will clearly have a significant role in the functionality of the sensor layer. The recognition unit facing towards solution (or gas phase) can be further functionalized by chemical or physical means if necessary. For instance, nanoparticles, capable of enhancing optical and electrical signals, can be attached to the formed protein layer, which has been described in more detail in Laaksonen P.; Kivioja, J.; Paananen, A.; Kainlauri, M.; Kontturi, K.; Ahopelto, J. and Linder, M.B., Selective nanopatterning using citrate-stabilized au nanoparticles and cystein-modified amphiphilic protein, Langmuir, 2009, 25, 5185.

Protein layers can be prepared at the interface between water and air, water and oil or water and a hydrophobic solid material. Formation of the protein layer/membrane is spontaneous and does not require fixing or other treatments for adhesion. In case of graphene-based sensors, protein layer can be formed straight from solution by immersing the graphene surface in it or by transferring the layer from air/water or oil/water interface. The same procedure can be used also with other hydrophobic surfaces than graphene, such as silicon, graphite or polystyrene, for instance. In order to make mixed layers of proteins with different functionalities, a mixture of hydrophobins is used as the starting solution. Based on the preparation method and properties (sizes) of the proteins, a mixed layer containing predicted amounts of the different molecules can be obtained.

The graphene surface itself can be made using any available method. Furthermore, it is possible to replace the graphene with another suitable material, such as silicon, graphite or polystyrene.

FIG. 1 is a schematic cross-section of a BioFET according to an embodiment. The device of FIG. 1 has been prepared on a substrate 10, which can be a silicon substrate, for instance. In the embodiment of FIG. 1, the substrate 10 is a p-type substrate. The device of FIG. 1 also comprises a dielectric layer 13, which can be SiO₂, for instance. On the surface of the dielectric layer 13, the device comprises a semiconducting layer 12 that forms a channel of the BioFET. The semiconducting layer 12 can be made, for example, of graphene or doped silicon. The channel is connected to a source electrode 14 and a drain electrode 15 that can be made of a suitable metal, for example by evaporation. The device comprises also a layer of functionalized proteins 11 on the surface of the semiconducting layer 12. The functionalized proteins themselves can be any suitable proteins, such as those described in the specification hereafter. Of course, not every protein in the layer of functionalized proteins need be functionalized. Instead, the layer can contain a mixture of functionalized and non-functionalized proteins. Also, it is possible that the proteins per se are not functionalized but the layer of proteins is functionalized by means of constituents brought onto or into the layer of proteins 11. Then, the layer of proteins 11 form a stable substrate for such constituents. Such constituents may include nanoparticles, for instance. The possible nanoparticles include, but are not limited to, metal nanoparticles, semiconductor nanoparticles and chromophores. Thus, also other suitable nanoparticles can be used. Furthermore, any combination of different nanoparticles can be used.

The device of FIG. 1 can be suitably biased by means of a bias voltage connected to the substrate 10. Then, the current in the channel is sensitive to polarization state of the layer of functionalized proteins 11 on the surface of the semiconducting layer 12, or other changes in the charge within the layer of functionalized proteins 11.

In a modification of the device of FIG. 1, the substrate 10 is a silicon layer of a SOI wafer (silicon-on-insulator) whereby it is possible to separate the gate portion of the substrate 10 from other portions of the silicon layer by patterning the silicon layer of the SOI wafer.

FIG. 2 presents a schematic cross-section of a BioFET according to another embodiment. The device of FIG. 2 has been prepared on a non-doped or weakly doped n-type silicon substrate 10. Furthermore, the device of FIG. 2 comprises a p-type well 17 prepared in the substrate 10 at the location of the device. The device of FIG. 2 also comprises a gate electrode 16 connected to the well 17. The gate electrode 16 can be made of a suitable metal, by evaporation in a hole made in the dielectric layer 13, for instance. Other elements of the device of FIG. 2 are as in the above-described device of FIG. 1.

FIG. 3 presents a schematic cross-section of a BioFET according to a further embodiment. The device of FIG. 3 has been prepared on a SOI substrate. The SOI substrate includes an insulator layer 21 and a semiconductor layer 20 on the surface of the insulator layer 21. In the device of FIG. 3, the semiconductor layer 20 is a silicon layer. The channel of the BioFET is formed in the semiconductor layer 20, between a source electrode 14 and a drain electrode 15. The device comprises also a layer of functionalized proteins 11 in direct contact with the semiconductor layer 20 acting as the channel of the BioFET.

FIG. 4 presents a schematic cross-section of a BioFET according to an even further embodiment. The device of FIG. 4 has been prepared on a substrate that includes an insulator layer 21 and a semiconductor layer 20 on the surface of the insulator layer 21. The material of the semiconductor layer 20 can be, for example, silicon, GaAs, InP or any other suitable semiconducting material. The channel of the BioFET is formed in the semiconductor layer 20, between a source electrode 14 and a drain electrode 15. Furthermore, device comprises a hydrophobic layer 24 prepared on the surface of the semiconductor layer 20. The hydrophobic layer 24 covers the channel of the BioFET but holes have been made for the source and drain electrodes 14, 15. Such a hydrophobic layer 24 can be made of polystyrene, for instance. Generally, the material of the hydrophobic layer 24 is hydrophobic insulating material. The hydrophobic layer 24 can be used to form a hydrophobic surface also on non-hydrophobic semiconductors.

Furthermore, the hydrophobic layer 24 can be used to protect the semiconductor layer 20 against oxidation or other factors causing ageing or other harm to the device. Device of FIG. 4 also comprises a layer of functionalized proteins 11 on the surface of the hydrophobic layer 24.

The design features and structures in the devices of FIGS. 1 to 4 can also be combined or changes between the embodiments. For example, it is possible to use the hydrophobic layer 24 disclosed in context of the device of FIG. 4 also in the devices of FIGS. 1 and 2, between the semiconducting layer 12 and the layer of functionalized proteins 11. Then, also non-hydrophobic semiconducting materials can be used in the semiconducting layer 12 of these devices. In a corresponding way, substrates and other features of the above embodiments can be interchanged when making other variants of the BioFETs. Furthermore, it is not essential that the device is a transistor but any other suitable electronic device is possible, the transistor being presented only as an example of possible devices.

Further particulars of the materials and manufacturing methods that can be used in the above embodiments are discussed below.

The embodiments can utilize self-assembly of proteins as a layer on a suitable surfaces, such as hydrophobic surfaces. Such self-assembly process can be made selective by suitable designing the properties of the surfaces. For example, the target area can be made hydrophobic whereas the other exposed surfaces can be made non-hydrophobic or hydrophilic. Such selectivity can be based, for example, on a proper selection of materials. According to an embodiment, hydrophobic materials are materials on which water has contact angle higher than 90 degrees.

The embodiments using graphene as the hydrophobic surface can have the graphene made by any available method, such as one of those described in the above-referred background art. The graphene layers can also be produced by protein-assisted exfoliation out of graphite, which is more specifically described in the specification hereunder. However, the use of the protein-assisted exfoliation method is not a requirement of the present invention but any method of graphene production can be used. The exfoliation method is described in detail as an embodiment because it is also itself new and not publicly known among the practitioners in the present field of technology.

With regard to the other methods of producing graphene layers, we specifically mention that a graphene layer or graphene layers have been manufactured by means a deposition method also on non-carbon surfaces. CVD, ALD, MBE and other such methods as known in the art can be used. For example, the above-referred patent application publication WO 2007/097938 A1 discloses graphene layers epitaxially grown on single crystal substrates.

According to an embodiment relating to exfoliation method, graphene-containing platelets are produced by exfoliating a graphene layer from a surface. The surface can be any surface containing the graphene layer to be exfoliated. The surface may be, for example, a surface of a graphite object or particle. In case the surface forms part of a highly oriented graphite body, the surface can repeatedly produce exfoliated graphene layers. Also the size or area of the produced graphene-containing platelets can be relative large as the domain size is typically high in highly oriented graphite. In case highly oriented pyrolytic graphite (HOPG) of highest quality is used (ZYA grade), domain sizes may be up to 10 μm², thus allowing fabrication of graphene-containing platelets in the corresponding size range. The surface can also be formed by ordinary graphite or graphite powder.

In other words, the surface from which the graphene layer or graphene layers are exfoliated contains at least one one-atom-thick graphene sheet, but it can be formed by an object that contains a plurality of graphene sheets, or even a body of graphite.

In the embodiment, exfoliation is facilitated by treatment with proteins. The treatment can comprise bringing the proteins and the surface into contact by any suitable means. These proteins can be same proteins that are used to form the protein layer of the electronic device. Suitable proteins for both of these purposes are describe in the specification hereafter.

The proteins may be natural proteins from fungi, for instance, or any modified or synthetically produced polypeptide that is functionally equivalent to proteins in achieving the desired effect, i.e. forming a layer on a hydrophobic surface. The proteins may also be fusion proteins. The protein may also be contained in a larger structural unit comprising another part or parts attached to the protein.

It is presently contemplated that the demonstrated function of the proteins is based on adhesive forces created between the proteins and the hydrophobic surface, such as a surface of graphene or silicon. However, the exact mechanisms are not yet known, and the above assumption may prove inaccurate in later studies. The interaction between the proteins and the surface of the graphene layer may be contemplated even without firm binding of the proteins on the surface of the graphene. Proteins are also typically large compared to the thickness of a graphene sheet and can therefore effectively deliver forces to the graphene sheets. In case of exfoliation, such forces influencing the proteins, and thereby also the graphene, may be induced by acoustic or mechanical energy, for instance. Examples of acoustic or mechanical energy include energy present in ultrasonic waves and in such movement as caused by shaking and mixing and in a flow of liquid. Thus, in order to facilitate exfoliation, it is possible to use ultrasonication, which means exposing the surface to ultrasonic waves.

According to an embodiment, the proteins include proteins that contain a part that is more hydrophobic than the rest of the protein's body. In another embodiment, the proteins are proteins that have a hydrophobic part that is capable of adhering to the surface of graphene or other suitable surface. According to a further embodiment, proteins include amphiphilic proteins. Examples of such hydrophobic and amphiphilic proteins include hydrophobins. Also other proteins can exhibit such properties. Such proteins include rodlins, chaplins, repellants, and SapB as for example described in Elliot, M. and Talbot, N. J, Building filaments in the air: aerial morphogenesis in bacteria and fungi, Current opinion in microbiology 2004, 7: 594-601, and Kershaw, M and Talbot, N. J., Hydrophobins and Repellents: Proteins with Fundamental Roles in Fungal Morphogenesis, Fungal Genetics and Biology, 23, 18-33, 1998.

In some particular embodiments, the proteins include hydrophobins. Examples of hydrophobins include HFBI, HFBII, HFBIII, SRHI, SC3, HGFI and other polypeptides that have resemblance in properties or sequence to said polypeptides. Examples of hydrophobins include therefore also other similar polypeptides which have corresponding properties.

One group of hydrophobins are hydrophobins identified from their amino acid sequence by the content and order of Cys residues, which take the form: Y—C(1)-X—C(2)-C(3)-X—C(4)-X—C(5)-XC(6)-C(7)-X—C(8)-Y where X denotes a sequence of more than one amino acid residue, but typically less than one hundred amino acid residues. Y denotes a sequence, which can be variable in length, consisting of any number of amino acid residues or can even be deleted completely. C denotes a Cys residue where C(2) and C(3) typically follow each other in sequence and C(6) and C(7) also typically follow each other directly in sequence.

According to an embodiment, hydrophobins include polypeptides comprising amino acid sequences, which have, for example, at least 40% similarity at the amino acid sequence level to the mentioned hydrophobins HFBI, HFBII, HFBIII, SRHI, SC3 and HGFI. The level of similarity can of course be also higher, such as at least 50%, at least 60%, at least 80%, or at least 90%.

Typical examples of hydrophobins and their structure and properties are described in Linder et al. Hydrophobins: the protein-amphiphiles of filamentous fungi, FEMS Microbiology Reviews, 29, 877-896, 2005.

In nature, hydrophobins have been found as are amphiphilic proteins produced by filamentous fungi. However, recombinant DNA technologies allow the production in a variety of other organisms such as bacteria, archea, yeasts, plant cells, or other higher eucaryotes. Hydrohophobins may also be produced without the use of living cells, either by synthesis or be cell-free production methods. In addition to adhesive property, these hydrophobins have also some further useful properties that can be utilized in some embodiments. For example, hydrophobins of this type are typically able to form protein films, which can be used to support the exfoliated graphene, for instance. The formed protein films may be elastic films in some embodiments. In some embodiments, the protein film may even be formed by an ordered network of proteins, and even by means of self-assembly of the proteins. In some embodiments, such an ordered network of proteins is a monolayer, i.e. contains substantially only one layer of proteins.

Film-forming property of some hydrophobins and their adhesion to surfaces have been demonstrated in publication Szilvay, G. R.; Paananen, A.; Laurikainen, K.; Vuorimaa, E.; Lemmetyinen, H.; Peltonen, J.; Linder, M. B., Self-assembled hydrophobin protein films at the air-water interface: Structural analysis and molecular engineering, Biochemistry, 2007, 46, 2345-2354.

FIG. 5 shows a structure of a HFBI protein that can be used according to an embodiment.

FIG. 6 is a schematic drawing depicting graphene sheets 1 and proteins 2 adhered on the surface of the outermost graphene sheet 1.

The proteins in embodiments can also include fusion proteins that comprise at least two functional parts. One of the functional parts can be selected such that it has ability to adhere to materials such as graphene whereas at least one of the other parts can be selected according to other desired functions. Such other desired functions may relate, for example, to solubility, electrical properties, mechanical properties, chemical properties and/or adhesive properties.

According to an embodiment, at least one of the functional parts in a fusion protein is formed by a hydrophobin or a hydrophobin-like molecule. Examples of such fusion proteins include molecules where some functionality such as solubility, charge, hydrophobicity, chemical reactivity, enzymatic activity, conductivity, or some binding capability has been added to the hydrophobin or hydrophobin like molecule. The addition of this functional group may be performed by chemical coupling, enzymatic modification, post-translational modification, or by using recombinant DNA methods. In a particular embodiment a fusion protein named NCysHFBI can be used. This protein variant contains an added Cys residue, which allows chemical reactions through a sulfhydryl group that it contains.

According to the needs of the application, class I and/or class II hydrophobins can be used. The class I hydrophobins typically form aggregates that are highly insoluble, whereas the aggregates of class II members dissolve more readily. This information can be used when selecting suitable proteins according to the needs of each application.

Examples of class II hydrophobins include HFBI, HFBII, and HFBIII that can be obtained from Trichoderma reesei.

Other sources of hydrophobins than Trichoderma include all filamentous fungi, such as Schizophyllum, Aspergillus, Fusarium, Cladosporium, and Agaricus species. Examples of additional sources are explained in, for example, the above-referred Linder et al. (FEMS Microbiology reviews, 2005).

In some embodiments, the proteins form a layer on the surface of graphene. By this way, it is possible to produce articles that contain graphene and a protein layer. Such articles are called platelets in this document.

The treatment of the surface by proteins may be effected, for example, by preparing a solution containing the proteins and spreading the solution on the surface. The surface can also be immersed in the solution, or otherwise brought into contact with the solution. In one embodiment, the solution is an aqueous solution, and may be purified water or water with added substances to control for example pH or ionic strength. Solvents and non-aqueous components may also be added to the solution.

The surface may be treated by the proteins also without the presence of a solution. For example, it is possible first to prepare a layer of proteins and then bring the surface and the protein layer into contact with each other. Such protein layer can be formed on a surface of a mechanical object, such as a stamp. The protein layer can also be formed at an interface between fluid and fluid, such as at a liquid-liquid, liquid-solid, gas-solid or gas-liquid interface. For example, it is possible first to prepare a solution containing the proteins and then let the proteins assemble at the interface between the solution and the surrounding atmosphere, such as air or selected process gas. Then, the protein layer can be touched with the target surface or with an intermediate object that is used to transfer the protein layer to the target surface. The term “target surface” refers herein to the surface in the device on which the protein layer will be formed or transferred. The term “desired location” refers to the area of a surface on which the self-assembly of the proteins is desired. Therefore, when the protein layer is self-assembling directly on a surface in the device, the “target surface” and “desired location” refer both to the same object. On the other hand, when the ready-made protein layer is transferred onto the “target surface”, the “desired location” refers to another location on which or wherein the proteins formed the self-assembled layer.

Thus, an embodiment of the method comprises forming a layer of proteins and touching the formed layer of proteins with the surface of the graphene in order to adhere the layer of proteins on the surface. After this, the protein layer with the attached graphene can be pressed against a target surface on a substrate in order to stamp the graphene on the substrate, if desired. Therefore, it is also possible to accurately place the formed platelets onto a desired target location and therefore to utilize the platelets in electronic applications, for instance. In another embodiment, the layer of proteins is touched directly with the target surface. In a further embodiment, the target surface forms the interface of which the proteins form the layer.

By means of stamps of other objects it is also possible to provide the platelets with a desired form and thereby to produce desired patterns of graphene. Patterns may be formed also by way of patterning the surface itself from which the graphene is exfoliated.

The above-described methods can be used to manufacture devices or platelets that contain a graphene layer and a protein layer on the surface of the graphene layer. FIGS. 7 to 10 shows schematic drawings depicting cross-sections of such platelets.

Platelet of FIG. 7 consists of a graphene layer 3 and a protein layer 4 on the surface of the graphene layer 3. The graphene layer 3 may comprise a plurality of graphene sheets 1 shown in FIG. 6 or consist of a single graphene sheet 1. The protein layer 4 comprises proteins 2 in at least one layer. Such platelets can be formed, for example, by exfoliation at interfaces as described above.

Platelet of FIG. 8 comprises a graphene layer 3 and protein layers 4 on the both principal surfaces of the graphene layer 3. In case the protein layers 4 are substantially uniform, the graphene is substantially totally protected by the protein layers 4. However, in some embodiments, the protein layers 4 may also contain holes or voids exposing the graphene layer 3. Such platelets can be produced, for example, by exfoliation in solutions wherein also the other principal surface of the graphene layer 3 becomes in contact with the proteins.

Platelet of FIG. 9 comprises a layer structure of a plurality of graphene layers 3 spaced apart and supported by a plurality of protein layers 4. Such platelets can be produced, for example, by using a combination of hydrophobin or hydrophobin-like proteins that have additional functionalities. The additional functionality is chosen so that it forms an interaction with other proteins, for example other hydrophobins or hydrophobin-like proteins that also are bound to graphene sheets. In this way, the proteins associate with each other and form layered structures.

Platelet of FIG. 10 comprises a protein layer 4 and graphene layers 3 on the both principal surfaces of the protein layer 4. Such platelets can be produced, for example, by combining hydrophobins or hydrophobin-like proteins with additional functionalities so that protein associated graphene form multiple layers. In subsequent stages, the exposed protein layers can be removed.

As understood from the above description, the protein layer 4 does not necessarily cover the graphene layer 3 completely even in the platelets. Thus, in some embodiments, as good coverage as possible is advantageous whereas some other embodiments allow considerable irregularities in the protein layer 4. And as also understood from the above description, coverage is not necessarily important at all in some embodiments of the manufacturing method. This is the case when the proteins are used only to facilitate exfoliation but no support function is needed or desired.

Some embodiments allow irregularities also in the graphene layer 3. Thus, also the graphene layer 3 in a platelet may include variations in thickness and any individual graphene sheet 1 may also comprise a plurality of smaller domains of uniform lattice structure.

In the platelets, the thickness of the graphene layer can be, for example, 1 to 10 graphene sheets 1. Typical thicknesses are believed to be within 1 to 5 graphene sheets 1 in most of the electronics applications, but as already stated, the applications and their requirements are various and the properties of the platelets are selected accordingly.

A special case of the platelet is a platelet wherein the graphene layer 3 consists of a single graphene sheet 1. Such platelets may be produces in any configuration presented in FIGS. 7 to 10.

According to an embodiment, the protein layer 4 includes hydrophobins. Again, a special case is a platelet wherein the protein layer 4 is a monolayer of hydrophobins.

The protein layer 4 may also include fusion proteins, or be formed exclusively by fusion proteins. Furthermore, a single protein layer 4 can include different types of the fusion proteins. In platelets of FIGS. 8 and 9, it is also possible that only one of the protein layers 4 contains fusion proteins.

As the proteins in the protein layers 4, any proteins disclosed above in the context of describing embodiments of the manufacturing methods can be used. Also combinations of the disclosed proteins can be used, either within the single the protein layers 4 or in different protein layers 4 of an individual platelet.

The thickness of a platelet can be less than 50 nanometres, for instance. Thicknesses of individual protein layers 4 may range between 1 to 10 nanometres, for instance.

EXAMPLES

Different methods of exfoliating graphene from graphite by adsorption of proteins have been examined, particularly with hydrophobins. In the experiments, it has been found that the methods can provide a new, efficient way for detaching thin flakes of graphene with thickness varying from 1 to less than 10 graphene sheets even in mild conditions, such as temperatures T<100° C. and near neutral pH.

One of the examined methods comprises forming a stable dispersion of graphene by ultrasonication of graphite in the presence of a wild-type protein or a functional fusion protein, and utilizing the product in nanoelectronical components via self-assembly of the biomolecules that can attach to a substrate material.

Also larger areas of graphene material can be formed by binding multiple layers together via functionality or other interaction between the protein coatings.

Small hydrophobic patch embedded in the hydrophilic body of hydrophobins causes them to self-assemble at interfaces between hydrophilic and hydrophobic materials. The structure of the class II hydrophobin, HFBI, used in one of the examples is presented in FIG. 5. A good example of the interfacial self-assembly is the assembly of HFBI at the interface between water and air, where they have shown to form a crystalline lattice. Besides this amphiphilic behaviour, there is also strong tendency for the hydrophobic patch to bind to solid hydrophobic materials. Thus, the layer of hydrophobins used in this example can be transferred and bound on a substrate, if desired. Another property that makes hydrophobin particularly interesting is the strong lateral interaction between the proteins as the interfacial monolayer forms. The strong monolayer membrane of HFBI, which can be only few nanometers in thickness, has been proposed as a potential material in electronics and has shown interesting behaviour when coupled with the conventional materials used in electronics.

The interaction between hydrophobins and hydrophobic surfaces has been studied both on macroscopic, microscopic and even on nanoscopic surfaces. Although the crystallinity of the protein layer has not been verified in every case, the evidence of selective binding to hydrophobic surfaces was clear. In the experiments, the self-assembly of hydrophobins has been expanded as two-dimensional material having surface area at a microscopic level and thickness on nanoscale.

Based on some of the tests, it was considered that the extreme change in wettability of hydrophobic materials, such as graphite, after the treatment with hydrophobins would have been one of the key factors leading to exfoliation of graphene. In earlier attempts to exfoliate graphene from graphite in solutions, the surface energy of graphene-water interface has been lowered by means of solvents or by adding a surfactant to the system as a third phase and thus facilitating the dispersion of graphene sheets to the solution. However, it is believed that the formation of highly ordered protein crystal at the surface of graphene flakes can stable the dispersions further.

Besides stabilization of the exfoliated flakes, the separation of the flakes is considered to play a role in exfoliation of graphene. Yet the exact mechanism is not know, it is probable that the relatively small size of hydrophobin molecules and its high affinity towards graphite surface are also relevant factors. The energetic benefit of having a hydrophilic protein layer on the graphene surface is surprisingly high, because it has been demonstrated to exceed the stacking of graphene sheets in graphite. The exfoliation of graphene was facilitated by ultrasonication, which has been believed to destroy the graphene sheets. However, in our experiments, graphene appears in pieces of several square micrometers, which is sufficiently large for components for electronic devices.

Example of FIGS. 11 to 13

Exfoliation of graphene was carried out by exposing a 0.5-1.0 ml solution containing a small piece (<1 mg) of highly oriented pyrolytic graphite (HOPG) of highest quality (ZYA grade, domain size up to 10 μm²) or Kish graphite, 0.02-0.026 mM protein (HFBI wildtype or a fusion protein NCysHFBI) in 10 mM sodium phosphate buffer at pH 8 or pure deionized water to ultrasonic waves in a tip sonicator. Amplitude of the probe was set to 26 micrometers. Temperature of the solution was controlled by keeping the sample in an ice bath during the sonication. The sample was exposed to sonication for total time of 10 minutes, but small pauses in sonication were kept at approximately one minute intervals to prevent the solution from boiling. Thus, the sonication temperature maintained between 0 and 100° C. After sonication, the sample was centrifuged for 15 minutes at 500 rpm to facilitate sedimentation of the heavier pieces of graphite. The supernatant was used in the further analyses.

The sequences of the HFBI and NCysHFBI used in the examples are as follows:

NCysHFBI

SCPATTTGSSPGPSNGNGNVCPPGLFSNPQCCATQVLGLIGLDCKV

PSQNVYDGTDFRNVCAKTGAQPLCCVAPVAGQALLCQTAVGA

HFBI

SNGNGNVCPPGLFSNPQCCATQVLGLIGLDCKVPSQNVYDGTDFRN

VCAKTGAQPLCCVAPVAGQALLCQTAVGA

Samples of graphene sheets for transmission electron microscopy (TEM) were prepared by pipetting 2×20 microliters of the fresh supernatant on a holey carbon grid. TEM image of a piece of graphene sheet is shown in FIG. 11. The sample was kept on top of a filter paper to allow the absorption of the excess solution to the filter. An example of a graphene layer and diffraction patterns measured from different parts of it is shown in FIG. 12. Electron diffraction patterns measured at different spots of a large folded piece of graphene shows evidence of single layered graphene sheets. FIG. 13 presents the intensities of the diffraction peaks measured at spot i3 that show the peaks that can be labelled with Miller-Bravais indices {1100} and {2110}. The ratio between intensities of the peaks {1100} and {2110} have values larger to 1, which according to calculations, corresponds to the diffraction of electrons from a single sheet of graphene. Pieces on the same size range and similar properties were found all over the samples indicating to an effective method for graphene exfoliation. Also pieces consisting of multilayered graphene and graphite were present in some extent.

By choosing a functional fusion protein comprising a hydrophobin part and another part having a functional group possessing the ability for biomolecular recognition or other binding, it is possible to assemble the graphene sheets on a surface having affinity to the chosen functionality. By this method, it is possible to utilize self-assembly of the chosen function for directing the graphene sheets on a patterned substrate, for instance.

Example of FIG. 14

Exfoliation of graphene from Kish graphite and HOPG pillars was also tested by experiments. Exfoliation of graphene and thin graphite sheets was carried out by exposing pieces of graphite and hydrophobin proteins to ultrasonic waves in an ultrasonic bath (Branson, Bransonic 1510, freq 40 kHz) in aqueous solution for 40 minutes. The proteins used were the wild type HFBI and its fusions (NCysHBFI)₂dimer and HFBI-ZE. Exfoliation was carried out in 0.3-1 ml volumes of protein dissolved in mQ water (Millipore) as 1.0 to 3 mg/ml solutions. Chemically purified Kish graphite was applied as granules that were immersed in protein solution and treated as described above. Lithographically worked wafer made from HOPG was immersed in the solution as a platelet containing micro pillars and the supporting graphite wafer and sonicated in the ultrasonic bath.

After exfoliation, the excess protein was removed from the solution by centrifugation of the exfoliated material. Larger pieces of graphite that were not affected by the treatment were first separated from the dispersed material by a gentle centrifugation with a mini centrifuge (National Labnet Co., Mini centrifuge C-1200). After this, the supernatant was centrifuged at room temperature with 14 000 rpm (Eppendorf, Centrifuge 5417R) for 5 minutes after which the solution was replaced with fresh mQ water. This washing was repeated for three times.

HOPG micropillars were prepared from highly organized pyrolythic graphite (HOPG) (SPI supplier, SPI-1 grade) that was attached after cutting on small pieces to a Si substrate with a silver epoxy. A 120 nm thick layer of PECVD Si₃N₄was deposited at 200° C. Optical lithography with a negative resist (Micro Resist Technology, ma-N 1410) and development (ma-D 533 s) was followed by wet etching of the nitride layer with a BHF solution. The resist mask was removed with acetone and isopropanol. The remained Si₃N₄structures served as a hard mask during the O₂ICP etch patterning of the HOPG. Finally the nitride mask was removed in buffered HF solution.

Then, the samples were prepared for Raman studies. First, pH of the exfoliated graphene solution was adjusted by adding 10 mM Macllvane buffer having pH 3. Pieces of oxidized silicon wafers were immersed in to the solution for 17 h, during which the small flakes of graphene were attached to the silicon oxide due to opposite charges. The surfaces were rinsed with mQ water and dried with nitrogen before Raman measurements.

After these steps, the samples were measured by Raman measurements. FIG. 14 shows comparisons of Raman signals from different graphitic materials. FIG. 14a depicts Raman shift spectra measured from HOPG (solid line), a thin exfoliated HOPG micropillar (dashed line) and exfoliated graphene flake (dotted line). The linear background has been subtracted from all spectra, and the spectra have been normalized according to the intensity of the D′ peak.

FIG. 14
b is an optical microscope image of an exfoliated graphene flake and FIG. 14c is an intensity map of the D′ peak measured from the same graphene flake. FIG. 14d is an optical microscope image of an exfoliated HOPG pillar and FIG. 14e is an intensity map of the D′ peak measured from the same pillar.

In the example of FIG. 14, the protein exfoliated graphene flakes and HOPG pillars were immobilized on a Si wafer with a 90 nm thick SiO₂surface for Raman measurements on a confocal Raman microscope with a 532 nm laser. Area scans around the flakes were performed to analyze the size and composition of the flakes in question. Peak fitting was done to selected spectra and the classification of the graphene flakes was done based on the composition of the graphene D′ peak and the intensity ratios of the graphene D′ peak and G peak in the Raman shift spectrum (for more information, see: Graf D, et at (2007) Spatially resolved Raman spectroscopy of single- and few-layer graphene. Nano Lett 7: 238-242).

Graphene sheets of different thicknesses have special fingerprints in their Raman spectrum that reveal the number of layers in a particular flake (for more information, see: Ferrari A C, et at (2006) Raman spectrum of graphene and graphene layers. Phys Rev Lett 97: 187401/1-187401/4). FIG. 14 shows a comparison of Raman spectra of bulk graphite and graphene samples of different thicknesses. The samples are: highly oriented pyrolytic graphite (HOPG), a protein-exfoliated graphene sheet and a protein-exfoliated HOPG micropillar consisting of several layers of graphene. FIG. 14b shows the optical images and Raman intensity plot of the graphene spectrum D′ peak for the exfoliated graphene flake and the exfoliated thin HOPG micropillar. The HOPG micropillar has characteristics similar to those of HOPG in the Raman spectra. The Raman spectrum of the exfoliated small flake is more similar to the Raman spectrum of graphene than graphite. The number of graphene layers in the thinnest exfoliated graphene sample was determined from the Raman spectra by comparing the relative intensities of the G and D′ peak and the position of the G peak. The results show that the observed graphene flake is either a bilayer or a monolayer of graphene.

Example of FIGS. 15 to 18

The effect of hydrophobin proteins on the electrical properties of a few layer thick graphene was measured on a bottom gated field effect transistor setup. FIG. 15 shows the schematic cross section of the few-layer graphene FET. The substrate was highly doped p-type Silicon wafer with a 300 nm SiO₂layer. The highly p-doped substrate was used as a back gate whereas the 300 nm SiO₂acted as a gate oxide. Graphene was transferred on to the substrate by the scotch tape method and confocal Raman microscopy was used to determine the bilayer nature of the graphene sheet. Au top contacts were evaporated and patterned on top of the graphene. A Pt adhesion layer was used to improve gold adhesion on the surface.

Electrical characterization of the device was performed on the device in the initial state, with the protein and after cleaning the transistor in boiling acetone. A layer of hydrophobin protein, HFBI, was transferred on the graphene based on the hydrophobic-hydrophilic contrast of graphene and SiO₂, in a similar manner as described in D. Graf,', F. Molitor, K. Ensslin, C. Stampfer, A. Jungen, C. Hierold, and, L. Wirtz, Nano Letters 2007 7 (2), 238-24. We measured the gate dependence of the current between the top electrodes in two point mode. The top electrodes had a constant source-drain-voltage of 10 mV and the gate voltage was swept between −100 V and 100 V. The used voltages are high due to the thick gate dielectric. The electrical characteristics were measured before and after the protein monolayer transfer and after removing the protein monolayer with boiling acetone.

FIG. 16 shows the calculated electron mobilities based on the measurement results relating to the transistor of FIG. 15. In the calculation we assume a linear gate voltage dependence of the carrier concentration in graphene. Due to the two point measurement of the current, the calculated values might not be accurate. However, the relative change in mobility between initial clean, protein monolayer covered and after protein removal curves can be observed. It can be seen that the HFBI protein monolayer reduces the mobility of electrons by a measurable amount compared to the initial state.

Two different variants of HFBI were used for to bring different chemical and electrostatic properties to the exfoliated flakes. These were (NCysHFBI)₂dimer and HFBI-ZE. The former has a reactive S—S group linking the two HFBI domains on its surface and the latter has a peptide segment that recognizes and binds a complementary peptide, ZR (for more information, see: Zhang K, Diehl M R & Tirrell D A (2005) Artificial polypeptide scaffold for protein immobilization. J Am Chem Soc 127: 10136-10137). To demonstrate the functionality of the protein layer adsorbed on the graphene surface, flakes exfoliated by (NCysHFBI)₂were labeled with 3 nm gold nanoparticles coated with mercaptosuccinic acid (MSA) (for more information, see: Chen S & Kimura K (1999) Synthesis and characterization of carboxylate-modified gold nanoparticle powders dispersible in water. Langmuir 15: 1075-1082). The flakes exfoliated with HFBI-ZE were labeled with 15 nm gold nanoparticles coated with ZR peptide. MSA nanoparticles bind to NCysHFBI through the added disulphide group and to some extent electrostatic interactions. Binding of peptides ZR and ZE is based on hydrophobic interactions, but it can be prevented by adjusting pH to low values because there are strong electrostatic interactions between the peptides.

Labeling of the graphene sheets was carried out by combining dispersions of exfoliated graphene with the nanoparticles of interest. pH of the nanoparticles solutions was adjusted to the desired value before combining the solutions. Nanoparticles and graphene were let to interact with each other for at least two hours before taking a sample for TEM imaging.

Functionalized protein coated graphene flakes using an engineered protein (NCys-HFBI) and 3 nm gold nanoparticles are shown in FIGS. 17a and 17b. In Figures, it is possible to see details of exfoliated and labeled flakes showing gold nanoparticles bound to their surfaces. Figure c is a control experiment showing a labeled monolayer of NCysHBI on a hydrophobic polymer surface.

FIGS. 17
a and 17b depict details of graphene flakes coated with an (NCysHFBI)₂-nanoparticle layer. The nanoparticles form a monolayer binding very selectively on the graphene flakes and the nanoparticle surface coverage varies from flake to flake. This may imply to an incomplete protein layer on the graphene surface. For comparison, an image of an NCysHFBI monolayer having a complete surface coverage of nanoparticles is presented in FIG. 17c. HFBI-ZE-coated graphene sheets were reacted with ZR-functionalized nanoparticles at pHs 3 and 5 (FIG. 18). The difference in interaction between ZE and ZR at different pHs was clearly demonstrated, since no nanoparticles were bound to the flakes at pH 3, where the peptides repel each other. On the other hand, at pH 5, graphene flakes were visibly labeled with the nanoparticles. This example clearly demonstrates that graphene flakes can be functionalized with hydrophobin fusions and that the protein membrane has maintained its original functionality. FIG. 18 shows graphene/graphite sheets exfoliated from Kish graphite by a mixture of HFBI and HFBI-ZE and labeled with ZR-functionalized Au nanoparticles at pH 3 (electrostatic repulsion) (in FIG. 18a) and at pH 5 (electrostatic attraction) (in FIG. 18b).

A Further Example

We have also prepared redox enzyme/hydrophobin fusion proteins (laccase and glucose oxidase), which can be attached to hydrophobic surfaces and will react selective with specific substrates. Glucose oxidase is particularly interesting for its application for diagnostics and treatment of diabetes. These proteins can also be used in the above embodiments.

Therefore, the above-described embodiments and examples provide considerable benefits. For example, it is possible to construct a quick one-step method for graphene exfoliation. Furthermore, embodiments provide safe methods as no strong chemicals or high temperatures are needed. Embodiments allow safe and easy handling of the material in solution dispersions as no hazardous nanopowders are necessary either. There are also embodiments allowing functionalization of graphene without disturbing the electronic structure and properties. For such embodiments, a wide variety of functionalities are available. Furthermore, there are embodiments allowing combination of biomolecular recognition and silicon technology. It is also possible to create an interface between biological and electrical materials and, by means of some embodiments, also build devices even utilizing by self-assembly of proteins at desired locations.

The above description is only to exemplify the invention and is not intended to limit the scope of protection offered by the claims. The claims are also intended to cover the equivalents thereof and not to be construed literally.

Electronic Devices with Protein Layers

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