Patent Application
20100129261
The present invention relates to a spectral sensor of SERS (Surface-Enhanced Raman Scattering) having a well-defined nanostructure and a use thereof for chemical and biological sensing with high reliability, high reproducibility, and ultra high sensitivity.
SERS is a spectroscopic method which utilizes a phenomenon that when molecules are adsorbed on a nanostructure surface of metal such as gold and silver, etc. intensity of Raman scattering is dramatically increased to the level of 106-108 times compared with normal Raman signals. Together with a nanotechnology which is currently being developed fast, SERS sensor can be further developed for high sensitive detection of a single molecule. In addition, it is highly expected that SERS sensor can be used importantly as a medical sensor.
SERS sensor has a great advantage over an electrical nano sensor which gives a sensing signal by the resistance change of a sensor when molecules are adsorbed on the sensor. The reason is that, experimental data measured with a resistance sensor is a scalar value while for SERS sensor a whole spectrum of vector data can be obtained so that the amount of information which can be obtained from a single measurement is much bigger for the latter than the former.
Kneipp and Nie, et. al. have reported for the first time that single molecule SERS detection can be carried out by using aggregated metal nanoparticles. Since then, many studies of SERS enhancement with various nanostructures (nanoparticles, nanoshell, and nanowires) have been reported. In order to utilize SERS as a high sensitive detection method for a biosensor, Mirkin et. al. reported high sensitive DNA analysis by using nanoparticles that are coated with DNAs.
In addition to high sensitive DNA analysis, many studies are actively being carried out to use SERS sensors for early diagnosis of various diseases such as Alzheimer's disease and diabetes, etc.
Thus, it can be said that because SERS provides information of the conformation and vibrational states of the molecules that is obtainable by Raman spectroscopy, SERS is a high selective detection method that give more information on molecules than conventional detection methods such as laser fluorescence analysis, etc. SERS is a powerful analytical method with ultra-high sensitivity for chemical/biological/biochemical sensing.
In spite of such advantages, there are still many problems of SERS to be solved: {circle around (1)} SERS mechanism has not been completely understood, {circle around (2)} synthesis and control of well-defined nanostructures are difficult, and {circle around (3)} reliability and reproducibility of SERS signals depending on the wavelength and the polarization direction of the excitation light need to be improved. Such problems remain as a biggest issue of SERS applications to achieve a development and a commercialization of nano-bio SERS sensors.
In order to solve above problems, studies for optical properties and precise SERS enhancement controls of well defined nanostructures are now more required than ever before.
Moskovits, Halas, and van Duyne et. al. recently showed that SERS enhancement can be controlled and optimized by using a well-defined nanostructures. Moskovits and Yang et. al., respectively reported that SERS enhancement can be controlled by using metal nanowire bundles. In 2006, Moerner et. al. reported a SERS active nanostructure of a nanobowtie fabricated by using electron beam lithography.
Presently, a SERS sensor using nanoparticles is most widely studied. Base structure for SERS which has been suggested by Binger and Bauer, et. al. is an optical structure which is made of metal island film (MIF) on a flat metal surface. MIF consists of metal particles in two-dimensional random array and can be up to several nanometers in length and width, respectively. In this structure, the shape of metal particles can be diverse and the arrangement of metal particles has a random structure that is decided by chance. Thus, it is impossible for MIF to obtain a well-defined structure and reproducibility and reliability cannot be obtained from such SERS sensor. In addition, due to a diverse shape of metal particles, a uniform scattering intensity cannot be obtained.
Problems associated with a SERS sensor are described above in view of MIF structure as an example. However, such problems are general for a SERS sensor which uses metal nanoparticles. Specifically, obtainment of a well-defined structure remains as a difficult subject to achieve because it is impossible to control a shape of metal particles and parameters of metal surface. The size of the metal particles, which is less than 5 nm, remains as an intrinsic limitation.
Instead of metal particles, metal nanowires, especially Ag nanowires have been used in some studies to produce SERS sensor.
Using Langmuir-Blodgett method, Tao et. al. (Nano. Lett. 2003, 3, 1229) produced a monolayer consisting of a great amount of Ag nanowire on Si wafer and carried out a SERS measurement using it (see
Jeong et. al. synthesized a flat array (rafts) of Ag nanowire using a template (J. Phys. Chem. B 2004, 108, 12724). By using Ag nanowire rafts arranged in one direction (see
Aroca et. al. (Anal. Chem. 2005, 77, 378) reported a large-quantity synthesis of Ag nanowires for a SERS substrate at the liquid phase. However, as it is shown in
Schneider et. al. (J. Appl. Phys. 2005, 97, 024308) and Lee et. al. (J. Am. Chem. Soc. 2006, 128, 2200) respectively produced an Ag nanowire array using a template and carried out a SERS measurement while either maintaining the template or removing the template by etching. As a result, it was found that more SERS signals were obtained by removing the template.
Proke et. al. (Appl. Phys. Lett., 2007, 90, 093105) reported SERS enhancement of ZnO and Ga2O3 nanowires coated with Ag, respectively. SERS enhancements of the nanostructures are determined by the shapes of ZnO and Ga2O3 nanowires. Ga2O3 nanowires get entangled but ZnO nanowires do not. Further, it was found that when entangled Ga2O3 nanowires are used, stronger SERS signals can be taken.
The SERS enhancement studies reported by the above-described works by Jeong, Proke, Schneider, and Lee, et. al. and with a dimer of metal particles support the theoretical SERS studies of Brus and Käll on the SERS enhancement where SERS results from the very strong electric field (i.e., hot spot or interstitial field) that is formed between at least two nanoparticles that are in close contact with each other (1-5 nm), instead of between metal particles that are isolated. According to a theoretical calculation based on electromagnetic principle, SERS enhancement of ˜1012 times is expected at the hot spot.
Still, similar to a spectral sensor using metal nanoparticles, a spectral sensor for SERS using nanowires is problematic in terms of controlling shape and quality of the nanowires. In addition, the physical structure of the produced nanowires has not been well-defined and the occurrence of hot spot, which is essential for SERS enhancement, could not be precisely controlled. Thus, reliability and reproducibility are not certain and the SERS signal could not be controllably carried out, making it difficult to develop a sensor using it. Especially for a cluster of nanoparticles, an occurrence, a position and intensity of hot spot may vary depending on the degree of clustering and it is known as a huge problem for maintaining reproducibility and controlling SERS signals.
As explained in the above, leading research groups of van Duyne and Halas, et. al. developed their own nano systems such as nanopattern and nanoshell and improved the reproducibility and the control of SERS enhancement by taking advantage of surface plasmon property of the systems. Currently, they are also trying to develop biosensors using the nanostructures. However, a SERS spectral sensor of nanowires, which is easy to be produced with high quality, high purity, and excellent shape and where individual position and structure of nanowires on a substrate can be controlled and the hot spot can be precisely controlled, has not been developed yet.
Inventors of the present invention recently succeeded in synthesis of the single-crystal Ag nanowire and single-crystal Au nanowire by using a vapor phase method. Single-crystal Ag nanowire has the highest conductivity among metals. Thus, it can be used for developing a nanodevice and an electrical nanosensor using it.
Noble metal nanowires produced without any catalysts by using a vapor phase method have a clean single-crystal surface which can be used for assembled structures of biomolecules on the surface of the nanowire. The nanowires have an excellent shape and they are individually separated to a size that can be precisely controlled even with an optical microscope. Nanowires having such advantages will be very useful for a study to understand a basic mechanism of SERS enhancement such as a change in SERS enhancement due to different wavelengths of light and an interaction between polarization direction and surface plasmon of the nanomaterials.
Inventors of the present invention conducted a research to control SERS enhancements by using nanowires that are produced by a vapor phase method and have a well-defined surface and crystal state, and to enhance SERS signals from chemicals, proteins, and biomolecules such as DNA and to improve reproducibility therefor. As a result, the present invention was completed. If a well-defined and efficient SERS system is manufactured by using the single-crystal nanowires produced by said vapor phase method, a great improvement can be made in development of a biosensor and a sensor for diagnosis of disease.
For solving the above-described problems, the object of the present invention is to provide a SERS spectral sensor which is easily produced, consists of nanowires with high quality, high purity and excellent shape and where the structure and the individual position of the nanowires on a substrate is controlled and the occurrence of hot spot is precisely controlled. Another object of the present invention is to provide a condition for operating a SERS spectral sensor to improve its sensitivity. Still another object of the present invention is to provide a use of the spectral sensor of the present invention for chemical and biological sensing with ultra high sensitivity, high reliability, high reproducibility and high structure specificity.
The spectral sensor for SERS (Surface-Enhanced Raman Scattering) of the present invention is a spectral sensor for determining the presence and the amount of biological or chemical materials in an analyte applied to the sensor, and used in conjunction with laser beam and Raman spectrometer. The spectral sensor of the present invention consists of (i) a substrate, (ii) a noble metal thin film located on top of the said substrate and (iii) single-crystal noble metal nanowires located on top of the said noble metal thin film, wherein a contact point is formed between the said noble metal thin film and the said noble metal nanowires and an enhancement of SERS is achieved by hot spots that are formed on said contact point (hereinafter, it is referred to as ‘spectral sensor Structure A’).
In addition, the spectral sensor of the present invention is a spectral sensor for determining the presence and the amount of biological or chemical materials in an analyte applied to the sensor, and used in conjunction with laser beam and Raman spectrometer. The spectral sensor of the present invention consists of (i) a substrate, and (ii) single-crystal noble metal nanowires located on top of the said substrate, wherein a contact point is formed by a physical contact of the said two noble metal nanowires and an enhancement of SERS is achieved by hot spots that are formed on the said contact point (hereinafter, it is referred to as ‘spectral sensor Structure B’).
In the above-described spectral sensor of the present invention, the structure (or position) of the noble metal nanowires consisting of the SERS sensor is physically adjusted and based on a physical contact (contact point) between two nanowires or a physical contact (contact point) between a single nanowire and the noble metal film that is formed on top of the substrate a controlled hot spot is created.
Substrate which can be used for said spectral sensor Structure A or spectral sensor Structure B can be anyone that is inert to SERS and non-reactive to the noble metals. For spectral sensor Structure A, it is preferably silicon single-crystal substrate, sapphire single-crystal substrate, glass substrate, gypsum substrate or mica substrate, etc. For spectral sensor Structure B, it is preferably silicon single-crystal substrate, sapphire single-crystal substrate, glass substrate, gypsum substrate or mica substrate, etc.
Nobel metal nanowires that are applied to the spectral sensor of the present invention are produced by heat-treating under the stream of inert gas a precursor comprising oxides of noble metal, noble metals or noble metal halides that is placed at front end of a reacting furnace and a semiconducting or nonconducting single-crystal substrate that is placed at rear end of the furnace. As a result, noble metal single-crystal nanowire is formed on the said single-crystal substrate.
The said method for producing noble metal single-crystal nanowire does not use a catalyst, instead it simply uses a precursor including oxides of noble metal, noble metals or noble metal halides to form a noble metal nanowire on the single-crystal substrate. Since noble metal single-crystal nanowires are produced along the drift of the materials in vapor phase without using catalyst, the operation process is simple and reproducible. In addition, it is favorable in that highly pure nanowires having no impurities can be produced.
In addition, according to the said method, temperatures at the front and the rear ends of the furnace are controlled, respectively, and by adjusting the flow rate of the inert carrier gas and a tubular pressure needed during the said heat treatment, driving forces for the metal nucleus formation and its growth, nucleation rate for the nucleus formation and its growth rate on the single-crystal substrate are all controlled. Thus, it is possible to control and to reproduce the size of the noble metal single-crystal nanowire and its density on the substrate, etc. As a result, a high quality noble metal single-crystal nanowire which is free of any defect and has high crystallinity can be obtained.
In this connection, the essential feature of the method of the present invention is the use of a precursor including oxides of noble metal, noble metals or noble metal halides to form a noble metal nanowire using a vapor phase transfer method while no catalyst is used. The most important condition to produce metal nanowires having high purity, high quality and excellent shape is temperatures at the front and the rear ends of the reacting furnace, flow rate of the said inert carrier gas and pressure during the said heat treatment.
The said conditions including heat treatment temperature, flow rate of inert carrier gas and pressure during the heat treatment can be independently varied. However, only when the said three conditions are varied depending on the state of others, noble metal single-crystal nanowires having preferred quality and shape can be obtained.
Preferably, the temperature at the front end of the furnace is maintained to be higher than that at the rear end. Specifically, difference in temperature between the front end and the rear end is within the range of 0 and 700° C. (i.e., the temperature of the front end is about from 0 to 700° C. higher than that of the rear end).
Regarding the flow rate of the inert carrier gas, preferably 100 to 600 sccm gas is introduced from the front end to the rear end. Preferably, the flow rate is between 400 and 600 sccm, and more preferably the flow rate is between 450 and 550 sccm.
The pressure for the said heat treatment is preferably lower than the atmospheric pressure. More preferably the pressure is between 2 and 50 torr, and the most preferably the pressure is between 2 and 20 torr. However, depending on characteristic of a precursor, the atmospheric pressure can be also used.
As a precursor for producing noble metal nanowires of the present invention, oxides of noble metal, noble metals, or noble metal halides can be used. The said oxide of the noble metal is selected from silver oxide, gold oxide or palladium oxide. The said noble metal is selected from silver, gold or palladium. The said noble metal halide is preferably selected from noble metal fluoride, noble metal chloride, noble metal bromide, or noble metal iodide. More preferably, it is selected from noble metal chloride, noble metal bromide or noble metal iodide. Most preferably, it is noble metal chloride. The said noble metal halide is preferably selected from gold halide, silver halide or palladium halide. In addition, the said gold halide is preferably selected from gold fluoride, gold chloride, gold bromide or gold iodide. The said silver halide is preferably selected from silver fluoride, silver chloride, silver bromide or silver iodide. The said palladium halide is preferably selected from palladium fluoride, palladium chloride, palladium bromide or palladium iodide. Furthermore, the said noble metal halide includes a hydrate of noble metal halide.
For the said oxides of noble metal, gold oxide, silver oxide, palladium oxide, platinum oxide, iridium oxide, osmium oxide, rhodium oxide or ruthenium oxide can be used. By using the said oxides of noble metal, single-crystal nanowires made of gold, silver, palladium, platinum, iridium, osmium, rhodium, or ruthenium can be produced.
The said oxides of noble metal including gold oxide, silver oxide, palladium oxide, platinum oxide, iridium oxide, osmium oxide, rhodium oxide or ruthenium oxide can be an oxide having a stoichemical ratio that is thermodynamically stable at the room temperature and the atmospheric pressure. In addition, it can be an oxide of noble metal which does not have the said stable stoichemical ratio due to the presence of a point defect that is caused by noble metal or oxygen.
The above-described precursor is preferably an oxide of noble metal or a noble metal. More preferably, it is an oxide of noble metal.
Especially for Ag and Au nanowires, silver, silver oxide or silver halide is used as a precursor to produce Ag single-crystal nanowire. In this case, the temperature of the front end of a reacting furnace is preferably about from 250 to 650° C. higher than of the rear end. Preferably, the said precursor (oxide of noble metal) is maintained at the temperature of between 850 and 1050° C. and a single-crystal substrate is maintained at the temperature of between 400 and 600° C. For Au nanowires, gold, gold oxide or gold halide is used as a precursor to produce Au single-crystal nanowire. In this case, the temperature of the front end of a reacting furnace is preferably about from 0 to 300° C. higher than of the rear end. Preferably, the said precursor is maintained at the temperature of between 1000 and 1200° C. and the said single-crystal substrate is maintained at the temperature of between 900 and 1000° C.
Among the noble metal nanowires that are added to the SERS spectral sensor of the present invention, Ag nanowire and Au nanowire were produced in the following Example 1 and Example 2 according to the above-described preparation method.
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Ag single-crystal nanowire was produced in a reacting furnace using a vapor phase transfer method. The reacting furnace has a separate front end and a rear end, and is independently equipped with a heating element and a temperature controlling device. The tube inside in the reacting furnace is based on a quartz material that is 60 cm long and has a diameter of 1 inch.
At the center of the front end of the furnace, a boat-shaped vessel which is made of highly pure alumina and contains 0.5 g of Ag2O (Sigma-Aldrich, 226831) as a precursor was placed. At the center of the rear end of the furnace, a silicon plate was placed. Argon gas was injected to the front end of the furnace and escaped through the rear end of the reacting furnace. At the rear end a vacuum pump was also attached. By using the vacuum pump, the pressure inside said quartz tube was kept at 15 torr, and using a MFC (Mass Flow Controller), a stream of 500 sccm Ar gas was flowed.
For said silicon substrate, a silicon wafer having (100) crystal plane on which a oxide layer has been formed was used.
Ag single-crystal nanowire was produced by heat treatment for 30 min while maintaining the temperatures of the front end (i.e., the alumina boat containing the precursor) and the rear end of the reacting furnace (i.e., the silicon wafer) at 950° C. and 500° C., respectively.
Au single-crystal nanowire was synthesized in a reacting furnace using a vapor phase transfer method. Except precursor, temperature for heat treatment and single-crystal substrate material, Au nanowire was synthesized using the same condition and the devices as described in Example 1.
For a precursor, 0.05 g Au2O3 (Sigma-Aldrich, 334057) was used. A sapphire substrate of (0001) plane was used as a single-crystal substrate.
Au single-crystal nanowire was produced by heat treatment for 30 min while maintaining the temperatures of the front end (i.e., the alumina boat containing the precursor) and the rear end of the reacting furnace (i.e., the sapphire substrare) at 1100° C. and 900° C., respectively.
For the noble metal single-crystal nanowires that compose of the SERS spectral sensors as prepared in the said Example 1 and Example 2, quality, shape and purity, and etc. of the nanowire were determined.
Close determination of the shape and the structure of the Au using TEM suggest that the Au nanowire has a smooth surface, as it is shown in
Noble metal nanowire which is prepared by the method described above and composes of the SERS spectral sensor of the present invention has a uniform size regardless of base materials, is a single crystal with high quality, and a highly pure nanowire free of any impurities. In addition, a great amount of the nanowires can be formed on a substrate and each nanowire can be individually separated without entanglement. Especially, the Ag or Au nanowires that are applied to the SERS sensor of the present invention have high qualities, high purities and favorable shapes.
Noble metal nanowires obtained by the method described above have a short axis of which diameter is within the range of between 50 and 200 nm and a long axis of which length is at least 1 μm, and they are individually separated. Noble metal nanowires having the said dimension can be observed by an optical microscope and with the aid of general apparatuses their individual position on a substrate or relative position to each other can be adjusted. The said dimension of the nanowire is within the range that a specific structure consisting of at least one nanowire can be optionally formed.
Therefore, by using noble metal nanowires which have no entanglement, are individually separated to single-crystal substrates, and have a diameter of its short axis within the range of between 50 and 200 nm and the length of the long axis at least 1 μm, the position of the said noble metal nanowire on the said substrate can be decided by physically and individually controlling a single noble metal nanowire. In addition, the relative position between the noble metal nanowires can be also physically controlled. Especially regarding spectral sensor Structure B, various structures can be defined including a structure wherein the long axes of two noble metal nanowires are crossed over, a structure wherein the long axes of two noble metal nanowires are crossed at a right angle to each other, and a structure wherein two noble metal nanowires are in contact with each other in a direction of their long axes, etc.
As it is explained above, the direction and the position of the noble metal nanowire singularly present in spectral sensor Structure A can be physically and individually controlled. Regarding spectral sensor Structure B, two noble metal nanowires can be individually controlled so that they are made to be in contact with each other. For spectral sensor Structure B, the position and the direction of two noble metal nanowires can be also physically and individually controlled. In addition, having the said single noble metal nanowire of spectral sensor Structure A or two noble metal nanowires of spectral sensor Structure B that are physically contacting each other as one unit, many units can be present and the direction and the position of an individual unit can be also controlled.
As a result, the noble metal nanowire of the spectral sensor of the present invention becomes to have a well-defined structure as well as a well-controlled hot spot (for spectral sensor Structure A, a contact point between a noble metal thin film and a single noble metal nanowire serves as a hot spot and for spectral sensor Structure B, a contact region of noble metal nanowires that are in physical contact with each other serves as a hot spot).
Difference between spectral sensor Structure A and spectral sensor Structure B is determined by the type of the contact points (or contact lines) which create a local electrical field serving as a hot spot. As it is shown in
Therefore, the said spectral sensor Structure B is not limited to the structure wherein two nanowires are in a simple physical contact with each other but also includes a spectral sensor structure wherein many nanowires are individually and physically controlled to have controlled contact points.
In addition, regarding spectral sensor Structure A of the present invention, the structure of nanowire on a noble metal thin film can be a cluster in which many noble metal nanowires are individually controlled and determined. For spectral sensor Structure A of the present invention, the contact points along a single nanowire on a noble metal thin film can form a line. Also, by adjusting the roughness of the noble metal thin film, number of said contact points can be controlled. Roughness of the noble metal thin film can be adjusted by a physical, chemical or thermal method, or a combination thereof. As a physical method, a fine particle having a certain size can be used for forming a physical scratch evenly on said noble metal thin layer, or considering that the noble metal is highly ductile but weak in strength a highly solid material having fine pattern formed on its surface can be brought in contact with said noble metal thin film and then pressurized to modify the surface roughness of the thin film. As a chemical method, an etching can be carried out by using a solution which can selectively etch grain boundary of the noble metal thin film which is made of polycrystalline material to modify the surface roughness of the thin film. As a thermal method, a mean particle size of polycrystalline material which constitutes the noble metal thin layer can be adjusted or a thermal grooving can be formed in grain boundary to modify the surface roughness of the thin film. In addition, base on a heat-treatment with an addition of chemical or physical elements, the surface roughness can be modified with recrystallization of polycrystalline material which constitutes the noble metal thin film it is known that, especially based on recrystallization of the surface of the noble metal using a chemical surface treatment using piranha solution or aqua regia, a mean particle size can be reduced and more even surface can be obtained.
The noble metal wires for the said spectral sensor Structure A or spectral sensor Structure B can be any of noble metal nanowires from which SERS enhancement is observed. Preferably, Ag nanowire or Au nanowire are used. In this case, since the noble metal thin film is provided in order to form a contact point with noble metal nanowire in the said spectral sensor Structure A, the thickness of the film is not specifically limited. Therefore, it also can be a thick film as well as a thin film. The noble metal thin film can be any one which can form a local electric field at a contact point with the noble metal nanowire that is present on top of the film, consequently forming a hot spot. Preferably, Ag film or Au film is used. More preferably, it is a thick or thin film made of a material which is the same as the noble metal nanowire that is present on top of the film (e.g., Ag nanowire-Ag thin film or Au nanowire-Au thin film).
Noble metal nanowire applied to the spectral sensor of the present invention does not involve a linking compound such as dithiol to link the nanowire to a substrate or to a noble metal thin film. Instead, the present invention is characterized in that thanks to its great mass and van der Waals bonding force the nanowire becomes strongly fixed to the substrate or to the noble metal thin film.
Spectral sensors having the above-described physical and structural properties were prepared in the following Examples 3 to 7. Following examples are provided as an example to fully deliver the spirit of the present invention to a skilled person in the art.
Thus, the present invention is not limited to the following examples and it can be carried out according to other possible embodiments.
To top of Si single-crystal substrate (1 cm×1 cm) a solution of Ag nanowire which has been prepared in above Example 1 and diluted with ethanol (ethanol 2 ml, Ag nanowire 0.001 g) was added dropwise, in order to place Ag nanowire on top of the Si substrate.
A highly concentrated nanowire solution which has been prepared by dispersing Ag nanowire of Example 1 in ethanol (ethanol 2 ml, Ag nanowire 0.001 g) was dispersed onto a glass substrate (2.5 cm×2.5 cm) to observe a nanowire structure having nanowires that are crossed at a right angle.
A highly concentrated nanowire solution which has been prepared by dispersing Ag nanowire of Example 1 in ethanol (ethanol 2 ml, Ag nanowire 0.001 g) was dispersed onto a glass substrate (2.5 cm×2.5 cm) to observe a nanowire structure having parallel nanowires.
On top of a Si single-crystal substrate having (100) surface Ag thin film was formed using E-beam evaporation apparatus (Korea vacuum, KVE T-0500200) under the condition of UHV (ultra high vacuum) with deposit speed of 0.2 nm/s (thickness of the film; 300 nm).
Ag nanowire solution which has been prepared by diluting Ag nanowire of Example 1 in ethanol (ethanol 2 ml, Ag nanowire 0.001 g) was sprinkled on top of said substrate (1 cm×1 cm) having a Ag thin film, in order to place Ag nanowire on top of Ag thin film.
On top of a Si single-crystal substrate having (111) or (100) surface Au thin film was formed using E-beam evaporation apparatus (Korea vacuum, KVE T-0500200) under the condition of UHV (ultra high vacuum) with deposit speed of 0.2 nm/s (thickness of the film; 300 nm).
Au nanowire solution which is prepared by diluting Au nanowire of Example 2 in ethanol (ethanol 2 ml, Au nanowire 0.001 g) was sprinkled on top of said substrate (1 cm×1 cm) having a Au thin film, in order to place Au nanowire on top of Au thin film.
Structure of the above-described Example 3 corresponds to the most basic structure of a spectral sensor, comprising a single nanowire formed on top of a SERS inert substrate. According to the structures given in the above-described Examples 3 to 7, contact points with a single nanowire or between nanowires were made by controlling the concentration of a noble metal nanowire which has been dispersed in ethanol. Such method exemplifies the simplest way for mass producing spectral sensors. It is evident that the spectral sensors of the present invention can be produced by individually controlling nanowires using typical apparatuses, considering that noble metal nanowire that is applied to the spectral sensor of the present invention is an individually separated nanowire having a short axis of which diameter is from 50 to 200 nm and a long axis of which length is at least 1 μm. Furthermore, thanks to the said advantages of the noble metal nanowire that is applied to the spectral sensor of the present invention, a specific nanowire among many noble metal nanowires constituting the spectral sensor and a specific part of any specific nanowire can be selected and determined using a simple optical microscope during the measurement based on the spectral sensor of the present invention.
By using the spectral sensor of the present invention, an operation condition of a spectral sensor for improving sensitivity, level of qualitative/quantitative analysis, reproducibility and reliability of data measurement is provided. Further, use of the spectral sensor of the present invention for chemical and biological sensing is provided.
The spectral sensor of the present invention can be used in conjunction with laser beam and Raman spectrometer. Preferably, the said lasers are argon ion laser having a wavelength of 514.5 nm, helium-neon laser having a wavelength of 633 nm, or diode laser having a wavelength of 785 nm. The said Raman spectrometer is preferably a confocal Raman spectrometer. As it is shown in
In order to control and optimize Raman enhancement by the spectral sensor of the present invention, it is preferred that polarized laser beam is irradiated to a single noble metal nanowire so that Raman spectrum is observed from a single noble metal nanowire. Because the spectral sensor of the present invention has a well-defined structure and the contact point (i.e., hot spot) also has a controlled structure, in order to achieve a quantitative, reproducible and reliable analysis and an analysis of a sample in ultra low amount, it is preferred that focal position of laser is controlled so that laser beam can be irradiated to a single noble metal nanowire and the focal position of laser beam can be focused to the noble metal nanowire that is being irradiated. In addition, when there are contact points made by nanowires, it is preferred that laser beam is irradiated to the said points and the focal position of laser beam is focused to the said points.
After sprinkling 10−2M ethanol solution of Brilliant Cresyl Blue (BCB) to the spectral sensor of single Ag nanowire which has been prepared in Example 3 above, the sensor was dried. Polarization of the laser beam was changed to measure a change in Raman spectrum enhancement of BCB by the difference between the direction of the long axis of Ag nanowire and the polarization direction of laser beam (θ). As it has been described before, focal position of laser is varied so that laser beam can be irradiated to a single noble metal nanowire and the focal position of laser beam can be focused to the noble metal nanowire that is being irradiated.
Results shown in
Thus, in order to control and optimize the Raman enhancement of a spectral sensor, it is preferable that polarized laser beam is irradiated to the noble metal nanowire that is applied to the spectral sensor of the present invention to obtain a Raman spectrum. It is also preferred that the angle (θ) between the polarization direction of laser beam and the direction of the long axis of noble metal nanowire is between 30° and 150° or between 210° and 330°. More preferably, it is between 60° and 120° or between 240° and 300°.
After sprinkling 10−4M ethanol solution of BCB to the Ag-spectral sensor which has been prepared in Example 4 above (i.e., the nanowire structure having nanowires crossed at a right angle), the sensor was dried and then laser beam was irradiated thereto. Green dots at the center of Ag nanowire in
After sprinkling10−4M ethanol solution of BCB to the Ag-parallel spectral sensor which has been prepared in Example 5 above, the sensor was dried and then laser beam was irradiated thereto.
a) is an AFM image of the spectral sensors in which two Ag nanowires are in contact with each other in a direction of their long axis, and overlapped to each other. As it is shown in the Raman spectrum of BCB molecule in
Results shown in
As it is described above, for spectral sensor Structure B in which a contact point is formed by a physical contact between two noble metal nanowires, it is preferred that laser beam is irradiated to said contact point and said point is a focus of laser beam to generate Raman spectrum at said contact point. When polarized laser beam is irradiated, the angle between the polarization direction of laser beam and the direction of the long axes of two nanowires should be optimized depending on a contact structure of the two noble metal nanowire that are in physical contact with each other. When a contact point is formed by two noble metal nanowires that are in contact with each other in direction of the long axis, thesaid direction of the long axis of the two noble metal nanowires is almost parallel to each other.
Thus, it is also preferred that the angle (θ) between the polarization direction of laser beam and the direction of the long axis of two noble metal nanowires is between 30° and 150° or between 210° and 330°. More preferably, it is between 60° and 120° or between 240° and 300°.
When a contact point is formed by crossing over of the long axis of two noble metal nanowires, the angle (θ) between the polarization direction of laser beam and the direction of the long axis of single nanowire that is selected from the two noble metal nanowires is between 30° and 150° or between 210° and 330°. More preferably, it is between 60° and 120° or between 240° and 300°. Therefore, when a contact point is formed by perpendicular crossing over the long axis of two noble metal nanowires, the angle (θ) between the polarization direction of laser beam and the direction of the long axis of single nanowire that is selected from the two noble metal nanowires is between 60° and 120° or between 240° and 300° for each noble metal nanowire, respectively. Thus, on the basis of the long axis of one specific noble metal nanowire selected from the two noble metal nanowires that are crossed at a right angle to each other, said angle is preferably between 330° and 30°, between 60° and 120°, between 150° and 210°, or between 240° and 300°.
As it has been described above regarding the problems associated with prior art, a great difficulty remains for developing a chemical, biological or medical sensor using SERS sensor due to a difficulty for synthesizing noble metal nanowires having high quality, high purity and excellent shape and for establishing a sensor having a well-defined structure and a controlled hot spot. The present invention solved such problems. By placing a chemical or biological substance on top surfaces of a noble metal nanowire, noble metal thin film or noble metal nanowire and noble metal thin film that are applied to the spectral sensor of the present invention, it becomes possible to obtain a reproducible and reliable result for an analyte in ultra low amount. As a result, the sensor of the present invention can be used as a chemical, biological or medical sensor having maximized sensitivity, selectivity and precision for quantitative analysis.
The said biological or chemical substance as an analyte can be present in a state of being adsorbed or chemically bonded to the noble metal nanowire that is applied to the spectral sensor of the present invention. For an actual application, an analyte sample or a solution comprising diluted analyte sample can be sprayed over the spectral sensor. Said analyte sample can be any of chemical or biological substances present in an analyte that is added to the spectral sensor. The said biological substances include body fluid, cell extract and tissue homogenate, etc.
Furthermore, by placing a complex comprising functional groups which can form a spontaneous bonding with a chemical or biological substance on top surfaces of a noble metal nanowire, noble metal thin film or noble metal nanowire and noble metal thin film that are applied to the spectral sensor of the present invention, a sensor which can be used as a chemical, biological or medical sensor can be prepared and used. Among the spectral sensors of the present invention, with respect to the spectral sensor having a noble metal thin film, said analyte or said complex can be present not only on top surface of the nanowire but also on top surface of the noble metal thin film. In addition, said analyte or said complex can be present only on top surface of the noble metal thin film and be measured.
For introducing specific functional groups on surface of nanomaterials, a method based on well known self-assembly phenomena can be preferably used. Thus, it is preferred that the above-described complex is self-assembled and formed a monolayer on the surface of the noble metal nanowire applied to the spectral sensor of the present invention. More preferably, said complex comprises sulfur so that self-assembly based on bonding between said sulfur and the noble metal particles present on the surface of the noble metal nanowire is induced to yield a monolayer. In most cases, such self-assembly occurs spontaneously by a chemical bonding between sulfur and a metal. In particular, since a self-assembly of alkane thiol on surface of gold has been reported, its use is broadened not only to surface of metals including Ag, Pd, Pt and Cu, etc. but also to metal oxides including SiO2, etc. The favorable aspect of such chemical reaction is that, as it is shown in
To Ag-thin film spectral sensor that has been prepared in Example 6, self-assembled monolayer (SAM) of para-mercaptoaniline (pMA) was attached and laser beam was irradiated thereto. As it is shown in the result of
To Au-thin film spectral sensor that has been prepared in Example 7, adenine, which is one of the four fundamental bases of DNA, was attached and laser beam was irradiated thereto. Among the four bases, adenine is known for its ability for forming a strong bond with gold. Meanwhile, gold can form a strong bond with thiol so that it can be easily linked to biomolecules, and being free of toxicity it is widely used for the development of biosensors. In addition to silver, gold can show a strong enhancement of SERS.
As it has been described in detail above, spectral sensor of the present invention has a well-defined structure and a controlled hot spot. Thus, based on highly sensitive SERS phenomena which can provide information on molecular structure of a sample and has a high selectivity and can be used for a measurement to the level of a single molecule, the spectral sensor of the present invention can be employed for a quantitative or qualitative analysis of chemical or biological samples and for obtaining a reproducible and reliable data. Further, the data obtained from a measurement can be calibrated to determine an absolute concentration, thus providing another advantage.
Spectral sensor for SERS (surface-enhanced Raman scattering) of the present invention is advantageous in that it has a geometric structure consisting of a single noble metal single-crystal nanowire and several single nanolines, and it can be used for obtaining surface-enhanced Raman scattering having high sensitivity, high selectivity, and strong Raman intensity based on experiments made on surface-enhanced Raman scattering, depending on the polarization direction of laser beam. Further, by using a noble metal single-crystal nanowire which has a high quality, a high purity and an excellent shape, position of individual nanowires and the geometric structure made by several nanowires can be controlled, and intensity of surface-enhanced Raman scattering can be improved by adjusting hot spot between noble metal film layer and noble metal nanowire and polarization direction of laser beam. In addition, the spectral sensor for SERS of the present invention can be used as a sensor for detecting chemicals by using surface of the noble metal nanowire, and by introducing specific functional groups on the surface of the noble metal nanowire to detect biological substances, a nano-bio hybrid structure can be formed and used for obtaining highly sensitive Raman spectrum of the biological substances. Consequently, it can be advantageously used as a biological sensor or a medical sensor for early diagnosis of disease.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2007-0065063 | Jun 2007 | KR | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/KR2007/005563 | Nov 2007 | US |
| Child | 12654665 | US |
