US20040245209A1

US20040245209A1 - Method for fabricating a carbon nanotube array and a biochip using the self-assembly of supramolecules and staining of metal compound

Info

Publication number: US20040245209A1
Application number: US10/859,530
Authority: US
Inventors: Hee Jung; Dae Jung; Ki Kwon; Su Lee; Do Choi
Original assignee: Korea Advanced Institute of Science and Technology KAIST
Current assignee: Korea Advanced Institute of Science and Technology KAIST
Priority date: 2003-06-05
Filing date: 2004-06-02
Publication date: 2004-12-09
Also published as: KR100549103B1; CN1601383A; JP3951141B2; JP2005049334A; CN1332270C; KR20040105319A

Abstract

A method for fabricating a carbon nanotube (CNT) nanoarray, which includes the steps of forming a thin film of supramolecules on a substrate on which metal catalyst for CNT synthesis is deposited, inducing the self-assembly of the supramolecules by annealing to form a regular structure, selectively staining the formed regular structure with a metal compound, etching the metal compound-stained thin film to form a nanometer or smaller size pattern, forming a nanopattern of metal catalyst by using the nanopattern of supramolecules stained with the formed metal compounds, and growing carbon nanotubes (CNTs) vertically on the formed metal catalyst nanopattern. A biochip is readily fabricated by binding bioreceptor(s) to CNTs of the CNT nanoarray.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method for fabricating a carbon nanotube (CNT) nanoarray, which comprises the steps of forming a thin film of supramolecules on a substrate, inducing the self-assembly of the supramolecules by annealing to form a regular pattern, selectively staining the formed regular pattern with a metal compound, etching the metal compound-stained thin film to form a nanometer or smaller size metal catalyst pattern, and growing carbon nanotubes (CNTs) vertically on the formed metal catalyst pattern. The invention also relates to a method for fabricating a biochip, including attachment of a bioreceptor to the above fabricated CNT array.

2. Background of the Related Art

Formerly, surface pattern formation has been achieved by photolithography using a polymeric thin film as photoresist, but the realization of a nanometer-sized, highly precise pattern by this method encounters many difficulties, because of limitations in the wavelength of light capable of being used and the necessity for provision of an apparatus and technology suitable for such light wavelength, as well as issues relating to the resolution of the polymer itself.

Since the year 1990, there have been attempts to use new photoresists in photolithography and to increase the resolution of pattern using light of a shorter wavelength. Furthermore, patterning techniques of a completely new concept, such as nanopatterning techniques using soft lithography, started to appear. Such techniques have advantages in that they allow inexpensive patterning and continuous operations. However, their resolution limit is a level of about 100 nm and it is difficult to expect a further increase in resolution leading to an increase in integration density.

Meanwhile, Korean patent No. KR 10/263671B1 discloses a method for forming nanometer-sized fine patterns using supramolecules as a patterning material. In this method, the thickness of fine pattern remaining in a groove is ensured using one additional buffer layer in order to ensure a margin for excessive etching, and a spacer is also formed on the buffer layer in order to reduce the size of the groove. However, the number of process steps is large, and the pattern size is a level of several tens of nanometers.

Korean patent publication No. KR 2002-0089528A discloses a small-sized, self-assembled structure for forming devices that are widely used in the microelectronic industry. The self-assembly method disclosed in this application provides the ability to form arrays in association with a surface, but it is impossible for the self-assembly itself to determine the position of a device-forming material within the boundary along the surface. Thus, in forming a device within the boundary along the surface, an individual positioning technique is necessary, and a suitable positioning technique is used with the self-assembly method, to form a structure capable of functioning as an individual part in integrated electronic circuits. The positioning technique permits one to determine the boundary of a structure by lithography, direct formation methods or other positioning techniques, so that a patterned substrate is formed and a device is assembled on the substrate by self-assembly.

A self-assembled structure can be combined with a structure formed by the conventional chemical or physical deposition technique, and an integrated electronic circuit can comprise integrated optical parts. The self-assembled structure can be formed using nanoparticle dispersion in such a manner that the desired structure is obtained by adjustment according to a material surface state and temperature and concentration conditions. A linker, one end of which is bound to the substrate surface and the other end of which is chemically bound to nanoparticles, is used, and selective binding using the linker can be used to yield a self-assembly process of nanoparticles.

Another selective binding method is to use natural interaction, such as electrostatic and chemical interaction, to perform the self-assembly process of nanoparticles, in which the nanoparticles are deposited in micropores such that they are positioned within the boundary defined by porous regions. The micropores can be found in certain materials, such as inorganic oxides or two-dimensional organic crystals, or suitable micropores can be formed by, for example, ion milling or chemical etching. However, this method is disadvantage in that the process is complex, and a spacing between pattern remains at a level of several tens to hundreds of nanometers.

Furthermore, Korean patent publication No. 2003-0023191A discloses a method for forming a nanometer-sized ultrafine pattern using a self-assembled monomolecular layer. This method comprises the steps of forming a layer of aromatic imine molecules with substituted end groups on a substrate, selectively binding and cutting the substituent groups of the aromatic imine molecule layer, and hydrolyzing the resulting aromatic imine molecule layer, thereby enabling the pattern to be formed in a short time. However, the pattern size according to this method still remains at a level of several tens of nanometers.

Meanwhile, dip-pen nanolithography was reported in which the tip of an atomic force microscope is stained with surfactant molecules having a chemical affinity for a solid substrate, and nanofeatures are formed on the substrate, much like the tip of a pen would write with ink on paper (Piner, R. D. et al., Science, 283:661, 1999). This technique has an advantage in that it is possible to achieve a high-resolution pattern as small as 5 nm in special resolutions using an ultra-sharp tip. However, this technique has a problem in that the pattern must be separately formed in a serial processing manner, so that a long time is required to achieve the desired features, thus making it difficult to directly apply this technique to mass production.

As described above, although various methods, including photolithography and etching methods using ultraviolet light and X-ray, are being introduced, the formation of sub-100 nm patterns has reached intrinsic limitations. In an attempt to resolve this issue, bottom-up methods are being widely studied as a substitute for the existing top-down methods.

The bottom-up methods are based on the formation of microstructures by the self-assembly of molecules, and among such basic technologies, a method of analyzing the microstructure of supramolecules by a scanning electronic microscope has been reported (Hudson, S. D. et al., Science, 278:449, 1997) and it has been reported that the orientation of supramolecules varies depending on the surface property of a substrate (Jung, H. T. et al., Macromolecules, 35:3717, 2002). However, these publications describe only the microstructure analysis of supramolecules and the orientation of supramolecules, respectively.

Studies are being conducted on forming sub-100 nm patterns using block copolymers, e.g., involving the formation of regular patterns using block copolymers and the formation of dot-shaped patterns using metal staining (Park, M. et al., Science, 276:1401, 1997). However, the patterns formed by such methods remain at a level of several tens of nanometers or larger size, since they rely on the molecular chain of the polymers. Also, the use of the block copolymers has problems in that the aspect ratio of the pattern formed is not large, the structure of a thin film is complex, and it is difficult to give an orientation to the structure of the thin film.

Meanwhile, microarray protein chips are of high importance in current researches on diagnostic proteomics. An early array technology (U.S. Pat. No. 5,143,854) that utilized a photolithographic technique for a polypeptide array on the surface of a substrate has recently generated new interest and is the subject of ongoing work. In particular, increasing importance is being attached to development of a microarray-type format in various immunoassays, including antigen-antibody pairs and enzyme-liked immunosorbent assays.

However, it is not easy to make the protein array smaller than the DNA array or to integrate or arrange the protein array into a substantial format having increased sensitivity. The lattice pattern of DNA oligonucleotides can be produced on the surface of a substrate by photolithography, but in the case of a protein consisting of several hundreds of amino acids, more highly integrated lattice patterns with high density (for example, an antibody can comprise about 1,400 amino acids) are required for the exact diagnosis of diseases on the substrate surface. It is not easy to satisfy this requirement.

Another problem with proteins is that they can easily lose their three-dimensional structure during manipulation under denaturing conditions (Bernard, A. et al., Anal. Chem., 73:8, 2001), so that the manipulation of proteins has many limitations.

A solution to such problems requires that the proteins be processed in such manner that they can be arrayed at high resolution without loss of their three-dimensional structure. Towards this objective, various approaches, including inkjet printing, drop-on-demand technology, microcontact printing, and soft lithography have been proposed. However, arrays formed by such methods are characterized by spacing dimensions of several tens of micrometers to several millimeters, and no techniques have been found that produce highly integrated diagnostic protein nanoarrays having a high density character, while maintaining the three-dimensional structure of the protein.

Because of their properties of excellent structural rigidity, chemical stability, ability to act as ideal one-dimensional (ID) “quantum wires” with either semiconducting or metallic behaviors, a large aspect ratio, and empty interior, CNTs exhibit a broad range of potential applications as a basic material of flat panel displays, transistors, energy reservoirs, etc., and as various sensors with nanosize.

The CNT synthesis using known methods of CVD synthesis involves first depositing Fe, Ni, Co or the alloy of these three metals as a metal catalyst on a substrate, etching the deposited substrate with water-diluted HF, mounting the sample on a quartz boat, and then after inserting the quartz boat into the reactor of a CVD device, additionally etching the metal catalyst film using NH ₃gas at 750˜-1050° C. to form fine metal catalyst particles with nanosize. Since the CNT is synthesized on the fine metal catalyst particles, forming the fine metal catalyst particles is an important process in the CVD synthesis method. However, arranging the metal catalyst in a patterned form with regular intervals is impossible in such method. Therefore, it is important to array the metal catalyst for arranging the CNT vertically at regular intervals.

As a solution to such problems, the growth of CNT on a nickel catalyst array fabricated by using e-beam lithography has been reported (Li, J. et al., Nano Letter, 3:597, 2003). However, such approach has many limitations in application to large size substrates and mass-production.

Recently, researches have been conducted to detect both protein-protein and protein-ligand reactions by means of electrochemical changes of CNT after immobilization of a biomaterial (Dai, H. et al., ACC. Chem. Res., 35:1035, 2002; Sotiropoulou, S. et al., Anal. Bioanal. Chem., 375:103, 2003; Erlanger, B. F. et al., Nano Lett., 1:465, 2001; Azamian, B. R. et al., JACS, 124:12664, 2002).

The reasons that CNT attracts public attention as a biochip material and technique include the following: firstly, CNT needs no labeling; secondly, CNT has high sensitivity to electric or electrochemical signal change; and thirdly, CNT is capable of reacting in an aqueous solution without deterioration of a protein because it has chemical functional groups. The application of biological systems to CNT as a well-arranged and new nanomaterial, will create important fusion technologies in various fields, including for example disease diagnosis (hereditary diseases), proteomics and nanobiotechnology.

Many applications of CNT in the bioengineering field have recently appeared. Applications of CNT to biochips, for applications such as glucose biosensors, detection of protein, detection of a specific DNA sequence, and the like, have been proposed (Sotiropoulou, S. et al., Anal. Bioanal. Chem., 375:103, 2003; Chen, R. J. et al., Proc. Natl. Acad. Sci. USA, 100:4984, 2003; Cai, H. et al., Anal. Bioanal. Chem., 375:287, 2003). At the present time, the most universal method for detecting the result of a reaction in a biochip is to use conventional fluorescent materials and isotopes (Toriba, A. et al., Biomed. Chromatogr., 17:126, 2003; Syrzycka, M et al., Anal. Chim. Acta, 484:1, 2003; Rouse, J. H. et al., Nano Lett., 3:59, 2003). However, as novel methods to easily and precisely measure an electrical or electrochemical signal are attempted, there are increased demands for CNT as a new material.

The methods of preparing a high density CNT multiplayer, attaching DNA thereon and detecting complementary DNA, are useful in genotyping, mutation detection, pathogen identification and the like. PNA (peptide nucleic acid: DNA mimic) that is regio-specifically fixed on a single walled CNT and its complementary binding to probe DNA, have been reported (Williams, K. A. et al., Nature, 420:761, 2001). Also, the fixing of an oligonucleotide on a CNT array by a electrochemical method and its use to detect DNA by guanine oxidation has been reported (Li, J. et al., Nano Lett., 3:597, 2003). These methods, however, do not apply CNT to the fabrication and development of biochips.

Recently, a high capacity biomolecule detection sensor using CNT has been disclosed (WO 03/016901 Al). This patent publication describes a multi-channel type biochip produced by arranging a plurality of CNTs on a substrate using a chemical linker and attaching various types of receptors. However, this structure has the substantial disadvantage of relative weakness to environmental changes.

SUMMARY OF THE INVENTION

Accordingly, the present inventors have conducted intensive studies to develop a simpler method for forming a several nanometer-sized ultrahigh density pattern. The inventors have discovered that by forming a metal catalyst pattern of several nanometer or smaller size, utilizing supramolecular self-assembly and selective metal compound staining techniques, and growing CNT vertically on the formed pattern, a CNT array can be produced in a ready and efficient manner.

The invention provides a method for fabricating a CNT array, by steps including arranging CNT on a nanopattern of metal catalyst formed by using supramolecular self-assembly and selective metal compound staining.

The invention additionally provides a method for fabricating a biochip, which includes the attachment of a bioreceptor to a CNT array fabricated in accordance with the invention.

Other aspects, features and embodiments of the invention will be more fully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a self-assembly process of supramolecules. FIG. 1[0031] a shows that disc-shaped dendrimers (1) and fan-shaped supramolecules (2) are self-assembled into cylindrical structures (3) which are then arranged into three-dimensional hexagonal structures (4). FIG. 1b shows that cone-shaped molecules (5) are self-assembled into spherical structures (6) which are arranged into a three-dimensional regular structures (7).
FIG. 2 schematically shows a process for forming a nanopattern and CNT array for the fabrication of a biochip according to one aspect of the present invention. [0032]
FIG. 3 is a schematic diagram showing the process of introducing carboxyl groups and removing the caps on the ends of the grown CNT. [0033]
FIG. 4 is a transmission electron microscope photograph showing that supramolecules are self-assembled into hexagonal pillar-shaped regular structures. [0034]
FIG. 5 is a scanning electron microscope photograph showing a nanopattern formed according to the present invention.[0035]

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

To achieve the above objects, the present invention provides a method for fabricating a carbon nanotube (CNT) nanoarray, which includes the steps of: (a) forming on a substrate a thin film of a metal catalyst selected from the group consisting of Fe, Ni, Co, and alloys of such metals; (b) forming a thin film of supramolecules inducing self-assembly on the thin film of the metal catalyst; (c) self-assembling the supramolecules by annealing to form a regular structure; (d) selectively staining the formed regular structure with a metal compound; (e) removing a portion which is not stained with the metal compound, by etching, wherein the metal compound-stained thin film is used as a mask, thereby forming a nanopattern of supramolecules stained with the metal compound; (f) forming nanopattern of a metal catalyst by ion-milling using the nanopattern of the supramolecule as a mask; and (g) vertically arranging CNTs on the nanopattern of the metal catalyst supramolecules. [0036]
In one embodiment of the present invention, a compound of the following formula ([0037] 1) is used as the supramolecules, but it will be appreciated that any self-assembling supramolecules may also be used, without limitation.
Examples of the self-assembling supramolecules include disc-shaped dendrimers ([0038] 1), fan-shaped supramolecules (2), stick-chain shaped or cone-shaped molecules (5). An example of the fan-shaped supramolecules includes a compound of the following formula (2), an example of the disk-shaped supramolecules includes a compound of the following formula (3), and an example of the cone-shaped supramolecules includes a compound of the following formula (4):
Such supramolecules are formed into a regular structure by physical secondary binding, such as by van der Waals forces, unlike polymers in which monomers are covalently bonded. Such supramolecules are self-assembled by suitable temperature or concentration, external magnetic field or electric field, etc., to form certain fine structures. The supramolecules of formula ([0039] 1) used in the present invention correspond to the fan-shaped dendrimers. As shown in FIG. 1a, such fan-shaped dendrimers are self-assembled into plate-shaped structures (1), which are then assembled into pillar-shaped structures (3), which are formed into a three-dimensional hexagonal structure (4). In addition, as shown in FIG. 1b, the cone-shaped supramolecules (5) are self-assembled into spheres (6), which are then arranged into a three-dimensional regular structure (7).
In the present invention, the thin film in the step (b) is preferably formed by spin-coating, rubbing, or solution spreading, which forms a thin film on the water surface, and the annealing in the above step (c) is preferably performed by heating the supramolecules above their liquid crystal transition temperature and then cooling them slowly. Furthermore, the metal compound staining in the above step (d) is preferably performed by selectively staining the central portion of the thin film with ruthenium tetraoxide(RuO[0040] ₄), and the step (e) is preferably performed by a reactive ion etching method.
Thin films in accordance with the invention include films that have a thickness that does not exceed about 5 mm, which may by way of example include films having a thickness in a range of from about 5 nm to about 5 mm, such as films with a thickness of from about 0.01 μm to about 500 μm. [0041]
In one embodiment of the present invention, the step of forming CNT on the nanopattern of the substrate can be performed by any suitable CNT growth method, e.g., any of CNT growth methods known in the art. In preferred practice, the CNTs are grown vertically on the substrate by plasma chemical vapor deposition, thermal chemical vapor deposition, electrophoresis or mechanical methods (Korean patent publication No. KR 2002-0001260A). [0042]
The present invention in another aspect can additionally include the step of exposing the carboxyl group by plasma treatment of the vertically synthesized CNT array end. With such method, various bioreceptors can be bound chemically after the carboxyl group exposed on the end of CNT array by plasma treatment. [0043]
In yet another aspect, the present invention provides a method for fabricating a biochip, in which a bioreceptor selected from the group consisting of protein, peptide, amino acid, DNA, PNA, enzymatic substrate, ligand, cofactor, carbohydrate, lipid, oligonucleotide, and RNA, is attached to the CNT array fabricated by the above method. [0044]
The step of attaching the bioreceptor to CNTs is performed in any suitable manner, e.g., by either applying a charge of a polarity opposite to the net charge of the bioreceptor to CNTs (KR 2003-0014997A), or by using a binding aid. The binding aid preferably includes a chemical substance having an aldehyde, amine or imine group attached to a carbon group end. [0045]
In one aspect of the present invention, a method is provided for fabricating a biochip which includes binding a bioreceptor having an amine group (NH[0046] ₂) to the CNT array in which the carboxyl group is exposed on an upper end, by forming an amide bond. It is typically preferred to use a coupling agent and a coupling aid for inducing an amide bond in the broad practice of the present invention.
Bioreceptors such as proteins, peptides and amino acids, possess respective intrinsic isoelectric points, and have a net charge with a neutral ion, cation or anion according to the ion intensity or pH of solution. Also, by adjusting the condition of solution to adjust the electrostatic interaction and hydrophobic interaction between such bioreceptors and CNTs with a certain charge, the same or different kinds of bioreceptors can be moved or arranged on the desired positions of a chip. [0047]
In still another aspect, the present invention provides a biochip, in which a bioreceptor selected from the group consisting of proteins, peptides, amino acids, DNAs, PNAs, enzymatic substrates, ligands, cofactors, carbohydrates, lipids, oligonucleotides, and RNAs, is attached to the above CNT array. [0048]
According to the present invention, a protein-specific receptor that binds selectively to a target protein involved in diseases can be selectively attached to the CNT nanoarray on one chip by applying an electric field. Also, a bioreceptor that can interact with various target proteins involved in various diseases can be attached selectively to CNTs by applying electric fields of different polarities from each other to the CNTs. Accordingly, it is possible to diagnose a variety of diseases on one chip in one step at large amounts in a rapid manner. [0049]
As used herein, the term “bio-nanoarray” is defined to include biochips and biosensors, in which a bioreceptor that binds to or reacts with a biomaterial is attached to a nanopattern. [0050]
The present invention is described in greater detail hereinafter. [0051]
In the general practice of the present invention, a thin film is formed on a substrate, of Fe, Ni, Co, or an alloy thereof, as a metal catalyst for growing CNT vertically on a substrate by a suitable technique such as thermal deposition, e-beam deposition, sputter, etc. The formed metal catalyst thin film is formed into a metal catalyst array by use of a nanopattern of supramolecules, as described herein (FIG. 2). [0052]
According to a preferred embodiment of the present invention, supramolecules are first dissolved in a tetrahydrofuran (THF) solvent at a 1-wt % concentration, and the resulting solution is applied on a substrate to form a thin film of supramolecules. In forming the thin film of supramolecules, spin-coating, rubbing, or solution spreading that forms a thin film on the water surface is preferably used. In this embodiment, a silicon wafer having the metal catalyst (Fe, Ni, Co, or an alloy thereof) deposited thereon, is used as the substrate, and the modification of the substrate surface is not carried out (FIG. 2[0053] a).
Thereafter, the supramolecules are heated above their liquid crystal phase transition temperature such that they are self-assembled. Since the supramolecules used in the present invention have a liquid crystal phase transition temperature of about 230° C., they are heated to 240° C. and then cooled slowly. Thus, the supramolecules are self-assembled into pillar-shaped microstructures (FIG. 2[0054] b).
The self-assembly process of supramolecules by annealing according to a preferred embodiment of the present invention will now be described. [0055]
The properties of supramolecules can be modified by annealing, and starting materials suitable for annealing include supramolecules produced by pyrolysis. Also, the supramolecules used as the starting materials can undergo at least one preheating step under different conditions. The additional treatment in annealing the supramolecules formed by laser pyrolysis improves their crystallinity and removes contaminants such as atomic carbon, and possibly can change their stoichiometry by combining additional oxygen, or atoms from gaseous or nongaseous compounds. The supramolecules are preferably heated in an oven so as to provide uniform heating. The treatment conditions are generally mild so that a significant amount of sintered particles are not caused. Thus, the heating temperature is preferably lower than the melting point of both the starting material and the product. If the thermal treatment involves a change in composition, the size and shape of the molecules can be changed even at mild heating temperatures. [0056]
Self-assembled structures are formed on the surface of material/substrate or within the surface. The self-assembled structures are positioned within boundaries in the form of positioned islands, and each of the structures can serve as an element of circuits or devices having a plurality of elements. Particularly, each of the structures may be an element of integrated electronic circuits, and examples of this element include electrical parts, optical devices and photonic crystals. [0057]
In order to form a structure within a predetermined boundary, a process of defining the boundary of the structure and a separate self-assembly process are required for the formation of the self-assembled structure. The process of defining the boundary of the structure utilizes an external force in defining the structure boundary. It is generally impossible to define the structure boundary by the self-assembly process itself. When a composition/material is bound, its self-assembly is based on the natural sensing function of the composition/material, which causes natural ordering in the resulting structure. Generally, although the positioning process can be conducted before or after the self-assembly process, the nature of treatment steps can also indicate certain orders. The net effect results in a self-assembled structure having a region within the boundary, which is covered with nanoparticles, and also a region outside the boundary, which is not covered with the nanoparticles. The process of defining the boundary is linked to the self-assembly process, by either activating the self-assembly process in the boundary or inactivating the region outside the boundary. Generally, to carry out the activating process or the inactivating process, the application of an external force is necessary. [0058]
The fact that supramolecules are self-assembled into a regular structure on a substrate can be confirmed by a transmission electron microscope. A sample was fabricated under the same conditions as described in the present invention, and a photograph of the sample taken by the transmission electron microscope is shown in FIG. 4. The photograph in FIG. 4 suggests that the supramolecules are self-assembled into hexagonal pillar-shaped regular structures. [0059]
The step of staining the regular structures of self-assembled supramolacules with a metal compound according to a preferred embodiment of the present invention will now be described. [0060]
First, RuO[0061] ₄solution and a substrate coated with a thin film of supramolecules are maintained in a glass container with the solution not being in direct contact with the substrate. In this process, while RuO₄vapor in the RuO₄solution is diffused into a gas phase, the supramolecular thin film on the substrate is stained with the Ru metal. The stained RuO₄vapor chemically selectively reacts with certain portions of the thin film.
Although RuO[0062] ₄is used in the present invention, osmium tetraoxide (OsO₄) or other metal compounds capable of selectively staining the structures formed by supramolecules may also be used in the present invention.
According to a preferred embodiment of the present invention, as the substrate coated with the supramolecular thin film is subjected to the metal compound staining process and then an etching process, a portion of the supramolecular thin film on the substrate is removed so that nanopatterned devices are ultimately obtained. In this etching process, any method which is conventionally used in a semiconductor device fabrication process may be used, without limitation (FIG. 2[0063] c). For example, the etching process can be performed by using an etching solution, such as a KCN-KOH mixture solution or an HF aqueous solution, or by reactive ion etching (RIE).
In the embodiment, a nanopattern of metal catalyst is finally formed on a substrate, when ion-milling is performed using the nanopattern stained with the formed metal compound as a mask, to form an array of Fe, Co, Ni or an alloy thereof, for use as a catalyst for CNT synthesis on a substrate. (FIG. 2[0064] d)
CNT can be synthesized by known methods in the art. In one such method, C[0065] ₂H₂, CH₄, C₂H₄, C₂H₆, or CO gas is used as a reacting gas, and plasma chemical vapor deposition, thermal chemical vapor deposition, etc. is used to grow CNT vertically. If CNT is formed by metal catalyst nanopattern, CNT having very small diameter can be formed because the diameter of one pattern is below 10 nm.
The present invention also includes the process of introducing carboxyl groups to the vertically grown CNT ends for binding biomaterials by treating the CNT ends with plasma to open the end caps of the CNT (FIG. 3). [0066]
The CNT nanoarray formed according to an illustrative preferred embodiment of the present invention as described above can be used as important surface substrates in forming a desired array by reacting various bioreceptors with the CNT nanoarray, in order to produce biochips of high integration density and small size. [0067]
Generally, the biochips are fabricated by linking biomolecules directly to a substrate or linking the biomolecules to the substrate by means of linker molecules. For example, when bioreceptors (e.g., DNAs, antibodies or enzymes) must be attached to the CNT in order to produce DNA chips, protein chips or protein sensors, the desired bioarray can be fabricated by reacting carboxyl groups introduced to the CNT ends with amine groups of the above biomaterial and binding those to the CNT ends with by amide bonds. [0068]
A method for fabricating DNA chips as bionanoarray articles according to the present invention, includes the step of attaching a previously prepared probe to the surface of a solid substrate by a spotting method. In this case, an amine group-bound probe is dissolved in 1× to 7×, preferably 2× to 5×, and more preferably 3× SSC buffer solution (0.45 M NaCl, 15 mM C[0069] ₆H₅Na₃O₇, pH 7.0), and then spotted to a CNT end having an exposed carboxyl group by a microarrayer. Then, the probe is bound to the CNT end by the reaction between aldehyde and amine. Here, the concentration of the probe is more than 10 pmol/μl, preferably more than 50 pmol/μl, and more preferably more than 100 pmol/μl. Also, the amine group bound to the probe is reacted with the carboxyl group introduced to the CNT end at a humidity of 70-90%, and preferably 80%, for 4-8 hours, preferably 5-7 hours, and most preferably about 6 hours, so that the probe is bound to the CNT end. An amide coupling agent and EDC/NHS as an aid can be suitably used in the method.

EXAMPLES

The present invention will hereinafter be described in further detail by examples. It will however be obvious to a person skilled in the art that these examples can be modified into various different forms and the present invention is not limited to or by the examples, such examples being presented to further illustrate the broad scope of the present invention. [0070]

Example 1

Synthesis of Supramolecules

The supramolecules of formula (1) used in the present invention were synthesized by a process as shown in reaction scheme (1) below. The result of scanning electron microscopic analysis on such supramolecules confirmed that the supramolecules are regular cylindrical structures of nanometer- or smaller size (FIG. 4). [0071]

Example 2

Modification of Substrate Surface

The above metal catalyst was deposited on a silicon wafer by using thermal deposition, e-beam deposition, sputter, etc. to form a thin film of Fe, Ni, Co, or an alloy thereof, as a metal catalyst for synthesizing CNT (FIG. 2[0072] a).

Example 3

Formation of Thin Film of Supramolecules

The supramolecules synthesized in Example 1 were dissolved in a tetrahydrofuran (THF) solvent. The solution was spin-coated on the silicon wafer of Example 2 to form a thin film (FIG. 2[0073] a). In this Example, the spin-coating was performed at 2,000-3,000 rpm for 10-30 second. During this spin-coating, the thickness of the thin film can be suitably changed.

Example 4

Annealing

The thin film of supramolecules was heated to 240° C. and then cooled slowly, to form regular microstructures (FIG. 3[0074] b). The supramolecules used in the present invention are self-assembled at 240° C., which can vary according to the kinds of supramolecules used. At this temperature, the supramolecules have a sufficient mobility for their self-assembly, and self-assembled into the most stable structures. In the case of the supramolecules used in the present invention, three-dimensional structures in which cylinders are arranged into a hexagonal shape are the most stable structures. FIG. 4 is a transmission electron microscope photograph showing that supramolecules are self-assembled into hexagonal pillar-shaped regular structures.

Example 5

Staining with Ruthenium Tetraoxide (RuO₄)

Since the supramolecules can be stained with RuO[0075] ₄selectively at their central portions, the supramolecules were exposed to RuO₄vapor for several minutes, to stain chemically their central portions with a RuO₄metal compound (FIG. 2c).
When the supramolecules was exposed to RuO[0076] ₄, the RuO₄metal compound was diffused into air so that the supramolecular thin film was stained with the RuO₄metal compound. The RuO₄metal compound chemically reacts with certain reactive groups (e.g., ether bonds, alcohols, benzene rings, and amines), and the supramolecules are stained with the RuO₄metal compound at their portions corresponding to the central portions of the supramolecular cylinders.
The staining metal compound may be replaced with other metal compounds depending on the kinds of supramolecules employed in specific applications. For example, osmium tetroxide (OsO[0077] ₄) chemically reacts with reactive groups such as carbon double bonds, alcohols, ether bonds, amines, etc.

Example 6

Etching

Thereafter, the metal compound-stained thin film of supramolecules was subjected to an etching process, so that dot-shaped structures remained at the central portions of the supramolecular cylinders due to the difference in etch rate between the staining metal compound and the supramolecules (FIG. 2[0078] c). FIG. 5 is a scanning electron microscope photograph showing the configuration of such dot-shaped structures. In this example, the etching was carried out using CF₄gas for about 100 seconds. This etching time cannot be considered to be common in all cases, since it varies depending on the apparatus employed. Thus, a test step for setting etching conditions was required.

Example 7

Method of Forming Nanopattern on Substrate

The metal catalyst thin film was etched using the nanopattern of supramolecules stained with the metal compounds obtained by Examples 1-6, as a mask, thereby forming nanopattern of metal catalyst. In the present invention, the metal catalyst thin film for CNT synthesis was formed between the substrate and the supramolecular thin film and the formed dot-shaped nanopattern serves as a mask in a subsequent etching step (FIG. 2[0079] c).
The lower portions of the film, on which the dot pattern stained with RuO[0080] ₄metal compounds was formed, were not etched, while the intermediate thin film layer whose surface was exposed, was etched, so that the pattern formed by the supramolecules was transferred to the intermediate thin film layer. This varies depending on a material used as the intermediate thin film layer. If the intermediate thin film layer is a metal catalyst thin film layer for CNT synthesis, it can be etched by ion milling, and etching conditions are in accordance with the properties of each thin film layer (FIG. 2d).

Example 8

Fabrication of CNT Array

Reacting gas such as C[0081] ₂H₂, CH₄, C₂H₄, C₂H₆, CO etc. was supplied into a chamber and power having high frequency was applied to both electrodes to cause a glow electric discharge, thereby vertically synthesizing and growing CNT on the metal catalyst nanoarray formed in example 7. The synthesized CNT formed a CNT array on the substrate by the regular arrangement of fixed metal catalyst (FIG. 2e).
Moreover, the vertically grown CNT can be treated with plasma by a method similar to the described in the prior art (Huang, S. et al., [0082] J. Phys. Chem. B, 106:3543, 2002), and carboxyl groups can be introduced to the CNT by removing the caps of the end portions. Then, various bioreceptors can be chemically bound to the CNT.

Example 9

Fabrication of Biochip by Attaching Biomaterial or Bioreceptor to the CNT Array

For attaching bioreceptors to the CNT array fabricated according to example 8, the method of binding by applying a change of polarity opposite to the net charge of the bioreceptor to the CNT (KR 2003-0014997A) or using a binding aid can be used (FIG. 2[0083] f). The preferred binding aid is a chemical substance having an aldehyde, amine or imine group attached to a carbon group end.
Furthermore, a biochip can be fabricated by binding a bioreceptor having an amine group (NH[0084] ₂) by means of an amide bond to the end of the CNT array having an exposed carboxyl group formed in example 8. For this method, it is preferred that EDC(1-ethyl-3-(3-dimethylamini-propyl) carbodiimide hydrochloride) is used as a coupling agent and NHS(N-hydroxysuccinimide), NHSS(N-hydroxysulfosuccinimide) is used as a coupling aid.
As described above, the present invention provides a method for fabricating a CNT array, which involves forming a catalyst pattern for CNT synthesis using a pillar shaped nanopattern which is formed by using the supramolecular self-assembly and the selective metal compound staining as a mask, and arranging CNT on the pattern. In addition, the present invention provides a method for fabricating a biochip by attaching a biomaterial or a biomaterial binding bioreceptor to the CNT nanoarray. [0085]

Claims

What we claim is:

1. A method for fabricating carbon nanotubes (CNT) nanoarray, which comprises the steps of:

(a) forming on a substrate a thin film of a metal catalyst selected from the group consisting of Fe, Ni, Co, and alloys of said metals;

(b) forming a thin film of supramolecules inducing self-assembly on the thin film of the metal catalyst;

(c) self-assembling the supramolecules by annealing to form a regular structure;

(d) selectively staining the formed regular structure with a metal compound;

(e) removing a portion which is not stained with the metal compound by etching wherein the metal compound-stained thin film is used as a mask, thereby forming nanopattern of a supramolecule stained with the metal compound;

(f) forming nanopattern of the metal catalyst by ion-milling using the nanopattern of the supramolecule as a mask; and

(g) arranging CNTs vertically on the nanopattern of the metal catalyst supramolecules.

2. The method of claim 1, wherein the supramolecule is a disc-shaped dendrimer, fan-shaped supramolecules or cone-shaped supramolecules.

3. The method of claim 2, wherein the supramolecule is the compound of the following formula (1):

4. The method of claim 1, wherein step (c) includes heating the supramolecules above their liquid crystal transition temperature and then cooling slowly.

5. The method of claim 1, wherein the metal compound of the step (d) comprises ruthenium tetraoxide (RuO₄).

6. The method of claim 1, which additionally comprises a step of exposing carboxyl functionality by plasma treatment of ends of the vertically arranged CNTs.

7. A method of fabricating a biochip, comprising attaching to a CNT, in a CNT nanoarray fabricated according to the method of claim 1, a bioreceptor selected from the group consisting of proteins, peptides, amino acids, DNA, PNA, enzymatic substrates, ligands, cofactors, carbohydrates, lipids, oligonucleotides, and RNA.

8. The method of claim 7, wherein the bioreceptor is attached to a CNT in said nanoarray by applying an electric field.

9. The method of claim 8, wherein a charge of a polarity opposite to a net charge of the bioreceptor is applied to the CNT.

10. The method of claim 8, wherein the bioreceptor is attached to the CNT using a binding aid.

11. The method of claim 10, wherein the binding aid comprises a chemical substance having an aldehyde, amine or imine group attached to a terminal carbon group.

12. A method for fabricating a biochip, which comprises binding to a terminal carboxyl functionality of a CNT, in a CNT nanoarray fabricated by the method of claim 6, a bioreceptor having an amine group(NH₂).

13. The method of claim 12, which comprises using a coupling agent and a coupling aid for inducing formation of an amide bond in said binding.

14. A biochip fabricated by the method of claim 7, in which a bioreceptor, selected from the group consisting of proteins, peptides, amino acids, DNA, PNA, enzymatic substrates, ligands, cofactors, carbohydrates, lipids, oligonucleotides, and RNA, is attached to a CNT of said CNT nanoarray.

15. A method of detecting reaction between biomaterials and bioreceptors, which comprises using the biochip of claim 14 to effect a bioreceptor/biomaterial reaction.

16. A biochip fabricated by the method of claim 12, in which a bioreceptor, selected from the group consisting of proteins, peptides, amino acids, DNA, PNA, enzymatic substrates, ligands, cofactors, carbohydrates, lipids, oligonucleotides, and RNA, is bound to a CNT in said CNT nanoarray by an amide bond.

17. A method of detecting reaction between biomaterials and bioreceptors, which comprises using the biochip of claim 16 to effect a bioreceptor/biomaterial reaction.