Metallic Nanowire Arrays and Methods for Making and Using Same

Abstract

Freestanding metallic nanowires attached to a metallic substrate are disclosed. A method of creating the nanowire structure using an anodized layer is presented. In one embodiment an optical SERS sensor is formed. The sensor head has at least one array of nanowires chemically functionalized to recognize molecules of interest. A method of forming a SERS sensor and using the sensor to analyze a sample is presented.

Description

This non-provisional patent application claims priority from non-provisional patent application Ser. No. 11/206,632, filed on Aug. 18, 2005, entitled Metallic Nanowire Arrays and Methods for Making and Using Same which claims priority from provisional application Ser. No. 60/603,203, filed Aug. 20, 2004, entitled Nanowire Optical Sensor System And Methods Of Use Thereof, both of which are incorporated herein by reference in their entirety.

This invention was supported in part by U.S. Government contract number Phase I SBIR Navy Contract N65540-03-0055 and NSF Phase I DMI-0339668 and portions of this invention are subject to a paid-up license to the U.S. Government.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to arrays of nanowires attached to a substrate that thereby form an array. Throughout this disclosure the term “nanowires” refers to high aspect ratio, solid wire structures (made from, for example, a metal or a semiconductor) having a length in the range from about 5 nm to about 5 microns with appropriate, application specific diameters. It is preferred that nanowires are made of a metal, such as for example, silver, nickel, iron, gold, cadmium or copper. In an additional embodiment, semiconductors for use as nanowires are silicon, Germanium and GaAs. Nanowires in an array can all be of the same length, or vary in length. Arrays of nanowires are a useful structure. In one embodiment, the nanowire array structure is used as an SERS sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic of a planar array of free-standing nanowires attached to the surface of a substrate.

FIG. 2 Schematic showing the template process steps for growing free-standing nanowires on a substrate.

FIG. 3 Schematic diagram of SERS application of nanowire arrays, where the nanowire array is grown on an optical fiber.

DETAILED DESCRIPTION OF THE INVENTION

The self-organizational properties of certain materials (e.g., anodic aluminum oxide (AAO)) are useful for nanowire production in the present invention. When aluminum is anodically oxidized in an acidic electrolyte, a uniform and oriented porous-structured layer, i.e., nanobores, is formed with nearly parallel pores organized in a hexagonal geometry. Thus, in an aspect of the present invention, nanowires may be formed on a substrate in part by coating a surface with a desired thickness of aluminum (for example, 1-3 microns) and then anodizing the aluminum metal forming a porous AAO template. Aluminum may be sputter coated onto surface of substrates. Prior to aluminum coating, a thin (e.g., about 10-100 nm) strike layer of a desired metal, such as gold, may be deposited onto surface of an optical fiber substrate for later use as the electrode for electrochemically forming nanowires, i.e., for the electrodeposition of gold to form gold nanowires. In this way, freestanding metal nanowires can be attached to a metallic substrate layer where the metals are substantially the same. Optically transparent conductors may also be used as strike layer electrode, e.g., indium-tin-oxide (ITO). Other such optically transparent materials that are useful in the present invention are ZnO:M and V₂O₅.

The versatility in processing allows nanowire arrays to be formed with uniform wire density using a variety of substrates, including flat solid substrates or, the outer surface of an optical fiber. Substrates may include any of the optical materials well known in the art for use in optical systems, such as glass, sapphire, etc., or other conductive substrates. Substrates may also include conductors and metals. In addition, one can control the length and diameter of nanowires in an array, which facilitates tuning the array and optical sensor application to a particular excitation wavelength. Additionally, the density of nanowires may be chosen to provide an array with a number of nanowires per unit area adapted to maximize an output signal. For example, the lengths may range from 5 nm to 200 microns, preferably up to 100 microns long, and wire densities can be up to 10¹¹ cm⁻² or even up to 10¹² cm⁻².

The AAO can be produced with the desired nanobore diameter, spacing and depth to control the dimensions of nanowires formed on AAO template. The depth of nanobores is controlled by the duration of the anodization process. The separation of nanobores is controlled by the anodization voltage and nanobore diameter is controlled by the duration of a post anodization chemical etch process, as will be disclosed in further detail herein below. The array of nanobores is formed by self-assembly. For example, arrays of gold (Au) nanowires may be produced using porous AAO with nanobore diameters of about 20-80 nm, nanobore depths of about 1-5 μm, and a center-to-center spacing of about 20-250 nm. Spectral analysis performed on the nanowire arrays can be used to determine the proper dimensions that will optimize these parameters for plasmon resonance.

A SEM micrograph of a porous anodic aluminum oxide template will reveal the relationship between nanobore diameter and anodization voltage. FIG. 2 shows a cross-section of AAO template as produced on aluminum metal. As the aluminum layer is anodized, AAO physically forms in a uniform porous formation to create template (FIG. 2)(5). The structure can be used directly to template nanowires. A barrier layer can be removed to form nanowires in contact with aluminum metal, or the oxidation can be carried out until all the Aluminum metal is consumed and nanobores completely penetrate the AAO structure.

In a representative anodization process, the aluminum layer, FIG. 2A, is anodized in a solution of 0.3-wt % oxalic acid at 2° C. The anodization is carried out until all the aluminum metal is consumed and nanobores in alumina template penetrate through to strike layer, FIG. 2B. The typical anodization rate is about 1 microns/hour. Anodization is performed at about 20-1000 V DC depending upon the desired nanobore diameter and spacing. A brief post anodization etch in phosphoric acid will remove any residual AAO from the bottom of nanobore, exposing strike layer. For example, following anodization, the AAO nanobores can be widened and strike layer cleared in a 0.5 wt % solution of phosphoric acid. It is possible to accurately control the diameter and center-to-center spacing of nanobores by adjusting the anodization voltage, and the electrolyte composition and concentration. For example, by optimizing voltage, electrolyte composition and concentration, and nanobore widening, the diameter and pitch may be controlled over a range of about 10-250 nm. Nanobore depth is often a linear function of the anodization time and can be extended up to several hundred microns.

Metal nanowire arrays, which in one embodiment are made from Gold (Au), may also be formed using the basic AAO templating technique shown schematically in FIG. 2 except the anodization is carried out until nanobores completely penetrate aluminum metal. Briefly, the first step is to create porous AAO template structure, FIG. 2B. The next step fills nanobores in template with metal by e.g., electrodeposition techniques, FIG. 2C. The anodized layer is partially or totally removed by chemical etching using phosphoric acid, leaving an ordered array of nanowire tips protruding from an AAO matrix of nanobores, FIGS. 2D and 2E. Nanowires may be formed using Ag, Ni, Fe, Au, and Cu. In one embodiment this produces the SERS active surface. The AAO can be further removed while still maintaining ordered, well-aligned nanowires FIG. 2D (2). Further removal of the AAO exposes ever-increasing lengths of nanowires. If the AAO structural support is completely removed, nanowires may collapse. However, if the nanowires bond to the substrate, they can remain free-standing. FIG. 2E shows the free-standing nanowires (2) attached to the substrate (3). The nanowire array structure is shown in FIG. 1, with the free-standing nanowires (1) attached at one end to the substrate (2). In one embodiment, the length and diameter of nanowires may be such that it most effectively leads to Raman enhancement. Preferably, arrays of gold nanowires are produced about the circumference of optical fiber. Gold nanowires may be from about 20 nm to about 80 nm in diameter, with a center-to-center spacing of about 20-250 nm, and an exposed length of about 1-5 microns.

In an embodiment of the invention, gold nanowires may be prepared inside porous AAO templates by standard AC electrolysis conducted at, for example, about 100-1000 Hz at a level of about 0.1-30 V_ac using a function generator. Gold nanowires may also be prepared inside template using a DC electrodeposition technique, for example, using 1.2 Vdc applied to the substrate as the cathode in the electrodeposition bath. A typical electrodeposition bath may have a solution of potassium gold cyanide and citric acid that is pH controlled using potassium hydroxide and phosphoric acid. Alternatively, a gold sulfide plating solution can be used. After plating, the AAO matrix can be partially etched back in phosphoric acid to expose a desired length of each nanowire, shown in FIG. 2D. In a preferred embodiment, nanowires will have direct optical contact with optical fiber, see FIG. 3 (4). Optical fibers that are coated with nanowire arrays may be imaged using SEM at various stages of production process to determine their dimensions and structural integrity and to document the work. By way of further example, Ag, Ni, Fe, Au, and Cu nanowire arrays may be patterned on a variety of substrate materials using the AAO templating technique. Nanowire arrays using porous AAO as template have been successfully engineered using electrodeposition of cadmium, iron, gold, silver, copper, nickel, and other metals from aqueous solution. Nanowire arrays can also be built using electronics fabrication methods such as photolithography and electrodeposition.

In another specific aspect, the present invention provides an SERS optical sensor with nanowires grown on a suitable substrate within a removable template such as a removable self-assembled template. For example, an alumina template can be used in which a series of blind holes or nanobores have been formed, e.g., by etching (FIG. 2). Nanowires are grown within nanobores in alumina template formed according to a method of the invention using electrochemical processes so that nanowires are individually attached to substrate 20 after complete removal of alumina template (FIG. 2). A conductive layer and a glass substrate base may be disposed below template (FIG. 2.)(4).

EXAMPLES

The following examples further illustrate the present invention, but of course should not be construed as in any way limiting its scope. The examples below are carried out using standard techniques, that are well known and routine to those of skill in the art, except where otherwise described in detail. The examples are illustrative, but do not limit the invention.

Example 1

Engineering of Gold Nanowires on Glass Substrates

An AAO template and gold nanowires of 90 nm diameters, 2 μm long were produced as follows:

The base material used is a multilayer structure comprising a standard 75 mm by 25 mm glass microscope slide, a layer of indium tin oxide (10 to 20 Ohms per square), and a layer of aluminum 1 to 1.5 microns thick. The gold nanowire arrays were produced on these substrates using the following procedure:

1. Prepare anodization bath using 0.1 to 0.3 weight percent oxalic acid solution. The concentration of the solution used to anodize the aluminum layer is dependent on voltage being used to anodize. Higher voltages require lower concentrations. Cool the bath to 5° C. by placing in an ice bath.

2. Clean aluminum substrate with acetone and alcohol followed by a DI water rinse for 1 minute.

3. Blow dry with air.

4. Mask off interface line on the sample with acrylic paint to protect areas not to be anodized.

5. Set up the data acquisition system to acquire data during processing.

6. Set power supplies to the required voltage.

7. Place the sample in the anodization bath so that the masked area falls slightly below the liquid-air interface of the bath.

8. Connect electrical connections. Positive lead attached to aluminum layer to be anodized, negative lead to a stainless steel mesh counter electrode.

9. As the initial oxide layer forms on the surface of the aluminum the measured current level drops rapidly to a “resting” level where anodization takes place.

10. As the anodization front reaches the back of the aluminum layer the current level will decrease slightly, and then start to increase. The increase in the current is associated with the thinning of the barrier layer at the bottom of the channels as it is consumed by the anodization process.

11. When the current level reaches twice the “resting” current, the power is removed from the cell to stop the anodization process. The sample appears uniformly translucent at the end of the process.

12. The sample is removed from the anodization bath and rinsed with DI water for 1 minute.

13. The sample is placed in a beaker filled with DI water to keep the anodized channels filled with water. This is important to achieve uniform widening of the channels as well as uniform filling during the plating process.

After the porous matrix has been formed in the aluminum layer the channels are widened using a pore widening etch. The etch serves two purposes: first, to widen the channels to the desired diameter and second, to remove the remainder of the barrier at the bottom of the channels to provide direct electrical contact to the ITO layer for electrodeposition. A specific process used to widen the pores created from the above anodization procedure is:

1. Prepare a 5 vol % phosphoric solution for pore widening.

2. Heat the solution to 37° C.

3. Place sample in the phosphoric solution so that the edge of the masked area lies below the liquid-air interface.

4. Allow the sample to remain in the bath for 7-10 minutes using mild agitation.

5. Remove sample and rinse in DI water for 1 minute.

6. Return sample to DI water bath to keep pores wet before plating step.

Electrodeposition of metals into the channels is accomplished using a standard electroplating bath. All metals, semiconductors and insulators that can be deposited using an electrodeposition process may be deposited into the channels. The specific materials of interest are the transition metals most notably copper, silver and gold. Below we enumerate a specific process used to deposit gold into the porous matrix. These can also be useful for one embodiment of the invention in the form of an SERS sensor

1. Pre-Heat TSG-250 gold sulfite plating solution to 55-60° C.

2. Turn on the power supply and set up plating parameters:

a. Frequency 20 kHz; b. Amplitude 400 mV; c. Offset 800 mV

3. Place the sample in the plating bath so that a portion of the masked area lies beneath the liquid interface.

4. Connect electrical leads. Positive to a gold anode and negative to the ITO conductive layer on the sample.

5. Plate the sample for 2 minutes 30 seconds with mild agitation.

6. Remove sample from plating bath.

7. Rinse residual plating solution from the sample for 1 minute followed by air blow dry.

Once the channels have been filled with metal, the remaining alumina matrix is removed using the following process:

1. Heat a 5 vol % phosphoric solution to 37° C.

2. Place the plated sample in the solution so that the masked area is inserted just beneath the solution-air interface.

3. Etch the sample for 45 to 60 minutes.

4. Remove the sample from the phosphoric solution and rinse in DI water for 1 minute.

5. Allow to air dry to prevent damage to the wire array.

Example 2

SERS Application

One embodiment of the present invention discloses a surface enhanced Raman scattering analysis system and methods for detection of target molecules in a test sample using the analysis system and a sensor for the same. The system has a SERS sensor containing an optical substrate and an array of often functionalized high aspect ratio nanowires disposed on the optical substrate. The nanowires are preferably cylindrical and/or solid structures, and are often formed from a metal, such as, silver, nickel, iron, gold, cadmium, copper, or the like, or a semiconductor, such as silicon, Germanium, GaAs or the like. Typically, the nanowires have a length ranging from about 5 nm to about 5 microns. An illumination source (e.g., diode laser excitation source or optical fiber laser excitation source) and optical data collection portion may also be included as part of an analysis system. The nanowires are often chemically functionalized so as to detect molecular species or biological agents in a test sample.

Raman spectroscopy (relying upon Raman effect) provides definitive information about the molecular structure of a material by investigating its vibrational spectrum. Different molecular species exhibit different Raman spectra. In fact, isomers of the same molecular species can be distinguished by this technique.

A method for detection of molecular species or biological agents in a test sample is also provided that uses the analysis system of the present invention having a SERS sensor formed in accordance with one aspect of the invention. The method includes contacting the nanowires on the sensor with a sample to be analyzed, illuminating an optical substrate, and collecting optical data from the system following the illumination.

Nanowires in a SERS optical sensor often comprise dimensions that coincide with a fundamental resonance of the exciting optical wave or a harmonic (integer multiple) of the fundamental resonance mode and/or can create a resonant cavity for the exciting optical wave or for surface plasmons in the nanowire. SERS optical sensor often includes nanowires with a geometry adapted for plasmon field enhancement and large reduction of plasmon damping. A typical SERS optical sensor formed in accordance with one embodiment of the present invention comprises an array of nanowires formed about a surface of an optical fiber serving as a substrate (FIG. 3). Arrays of nanowires are often produced using a templating technique (FIG. 2) that is dependable and relatively easy to incorporate into a manufacturing environment.

Surface Enhanced Raman Scattering (SERS) techniques may employ small structured materials. Noble metal nanoparticles exhibit very strong optical absorption in the ultra-violet through the visible range of the spectrum, which is not observed in their bulk counterparts. The absorption leads to tremendous electric field enhancement at the particle surface and in the regions between neighboring nanostructures. This field enhancement affect is utilized in SERS, by employing nanostructured materials to boost the Raman signal intensity. SERS techniques have often led to increases in the effective Raman cross-section by factors of 10¹⁴-10¹⁵, allowing the Raman spectra of single molecules to be probed in relatively short times (e.g., tens of seconds).

Referring to FIG. 3, a SERS optical sensor system is provided that includes an optical sensor comprising an array of nanowires for detecting, discriminating, and quantifying molecular species using spectroscopy methods. SERS optical sensor system often uses an illumination or excitation source, SERS optical sensor and an optical data collection and analysis portion. A diode laser or optic fiber laser may often be used as excitation source. SERS optical sensor may be integrated into a system including an optical detector. The preferred detector for a SERS optical sensor system is photodiode array fabricated from InGaAs technology. These InGaAs diode arrays will operate in the near infrared allowing the use of high power diode lasers. The advantage of near infrared Raman is that luminescence that interferes with the Raman spectrum can be avoided since the near IR photon energy is too low to generate these excitations. A portable version of SERS optical sensor system with handheld data collection and/or analysis units can be used for testing various samples.

Nanowires manufactured according to the methods of the present invention are preferably chemically functionalized to recognize or to trap selective chemical species in a test sample. Functionalization involves depositing one or more suitable active chemicals on the surfaces of the nanowires. The chemicals may include, but are not limited to thiocyanate, thiol alkanea, peptides, proteins, antibodies (monoclonal and polyclonal), DNA, RNA, PNA, histidine, streptavidin, biotin and inorganic elements, ions or compounds and other suitable chemicals capable of reacting or binding with counterparts in the test sample. In one embodiment, nanowires are coated on their respective outer surface by submersing the array in a solution of the functionalizing material. One or more monolayers may be added to surface of each nanowire in this way. Additional layers may include antibodies and antigens. In one embodiment of the invention, SERS optical sensor has an array of nanowires oriented normal, i.e., at or about 90°, with respect to the longitudinal axis of an optical fiber (FIG. 3). The array of nanowires can be used as a test platform for single or multiple chemical or biological species, for drug testing, or for environmental testing.

The nanowire arrays may be functionalized in any number of solutions to form molecular bridges to other target molecules. Here we detail a process for functionalizing the surface of gold nanowires with potassium thiocyanate (KSCN) as a bridging molecule for hemoglobin. In solution KSCN dissociates to become K⁺ and SCN⁻.

1. An array of gold nanowires was immersed in a 1 molar aqueous solution of potassium thiocyanate.

2. The nanowire array is allowed to sit in solution for 30 minutes to allow the KSCN molecules to fill all the locations on the surface of the gold nanowires. The sulfur atom in the SCN⁻ radical preferentially binds to the gold nanowire.

3. The nanowire array is removed from the KSCN solution and rinsed in DI water for 5 minutes.

Infrared data of the nanowire array after this treatment show that the SCN radicals are still present on the surface after rinsing, strongly suggesting the presence of the SCN attached to the gold nanowire.

In part, functional groups disposed on nanowires depend on the sample to be screened or the type of assays. Screening can involve detection of biochemical substances, such as proteins, metabolites, nucleic acids or biological agents such as fungi, bacteria and viruses. For example, nanowires can be functionalized for applications in genomics, proteomics and SNP assays, medical diagnostics, drug discovery screening, and detection of biological and chemical warfare agents.

Specifically, a nanowire may be functionalized by attaching a bridging molecule and/or a reactive molecule to nanowire using solution chemistry. The nanowire array is submersed in a solution containing a concentration of the bridge molecules large enough to coat surfaces of nanowires with a monolayer of the bridge molecule. In the specific case of gold nanowires, the bridging molecule has a thiol (sulfur containing) group that preferentially binds to the gold surface. The opposite end of the bridging molecule contains a chemical group that preferentially attaches to additional bridge molecules or the sensor target.

A specific embodiment uses mercaptoundecanoic acid which contains both a thiol group and a carboxylic acid group. The thiol group binds to gold nanowire and the carboxylic acid group can bind to antibodies or antigens specific to the molecule the sensor is targeting. Other molecules can also be used. For example, potassium thiocyanate can be used as a bridge for hemoglobin detection. The functionalization process is simply finding the molecules that will create the chemical bridge and bind to both nanowire and the target/additional bridge molecules. A specific advantage of nanowire geometry is that the bridging molecules will populate entire surface of nanowire providing many more sites for target binding to occur. Binding along the entire length of nanowire enables a greater volume of target molecules to be sampled, increasing detection efficiency.

The first step in functionalizing nanowire arrays may be the formation of a self-assembled monolayer (SAM). As an exemplary embodiment of the invention, gold nanowire arrays may be chemically functionalized to bind with hemoglobin. To functionalize the gold nanowire arrays to preferentially bind hemoglobin, a SAM is deposited onto surface of each gold nanowire by reacting a sulfur group, for example, in thiocyanate or thiol alkane with the gold. The sulfur group of the thiocyanate ion (SCN⁻) will bind to surface of each gold nanowire. The iron in the hemo-group of the hemoglobin will bind to the cyanate portion of the ion. For example, a 0.1 M aqueous solution of thiocyanate with a pH of 7 may be used at room temperature.

If needed, a different SAM using a thiol alkane (carbon chain with a sulfur group attached to the end) with a cyano-functional group on the terminal end (HS—CH₂—CH₂—CH₂— . . . CH₂—CH₂—CN) may also be deposited in place of, or in addition to using thiocyanate. The cyano group will be facing away from surface of nanowires so that the length of the alkane chain can be controlled to accurately penetrate the protein. Different lengths of the alkane chain functional groups may be prepared to determine the optimum length that will reach into the hemo crevice to bind to the iron. For example, gold nanowire arrays may be submerged in an ethanol solution of 1 mM thiol alkane (or aqueous solution of thiocyanate) for several minutes at room temperature. Nanowires are then removed from solution, rinsed with ethanol and dried under a stream of nitrogen. This should achieve 90 to 95% coverage of nanowires with the SAM and the attached functional groups.

Characterization of the resulting thiol alkane (or thiocyanate) layer can be done using, for example, infrared reflection-absorption spectroscopy and cyclic voltammetry. The infrared spectrum should show absorption peaks due to the thiol alkane (or thiocyanate) that are not present on untreated gold nanowires. Cyclic voltammetry in a buffer solution employing modified gold nanowires as the working electrode often display oxidation and reduction peaks due to the presence of thiol alkane (or thiocyanate) on gold nanowires. These measurements are performed on nanowire arrays with and without the functional groups attached. After preparation of the SAM covered, functionalized gold nanowires, which will serve as the platform for the attachment of hemoglobin, they may be stored under purified nitrogen.

After the functionalization has been performed, the ability of the cyanate group to bind to hemoglobin may be determined. For example, this can be accomplished by exposing (e.g., by immersing) a functionalized nanowire array to an aqueous solution of a commercial sample of hemoglobin dissolved in a phosphate buffer of pH 7.4 and then measuring the infrared spectrum at several positions on the surface of the array. Variations in the intensity of the infrared absorption from one position to another is an indication of how uniformly the hemoglobin has bound to the surface of nanowires. The presence of hemoglobin attached to the nanowires can be further verified using cyclic voltammetry to estimate the percent coverage of the hemoglobin on surfaces of the nanowires. Imaging of the array with the attached hemoglobin may be also be done using SEM.

In one embodiment, SERS optical sensor has an array of nanowires on a flat solid substrate. Incident light from excitation source interacts with, and excites nanowire which in turn causes molecule (located on the nanowire surface due to that surface's prior functionalization) to vibrate and thereby give off Raman shifted emissions that are collected by optical elements for further processing.

A large array of nanowires may be used to probe a sample of solution deposited onto the array. The nanowire array can act as a fluid wick spreading the solution throughout the array for maximum exposure of the solution to surfaces. Different areas of the array may be functionalized with different surface treatments to attach one or more specific molecules. In this way, the nanowire array may be used to analyze the concentration of one or more species in a solution. Such an optical sensor is often useful for drug testing where only small quantities of drugs are available for test.

In another embodiment, as shown schematically, an SERS optical sensor has an array of nanowires on a flat transparent substrate, e.g., glass. Excitation light is directed through substrate to excite nanowires, with emissions from molecules on the nanowire side of substrate being collected by optical elements for further processing. Such a sensor may be well suited for air and water monitoring systems for buildings or large public areas. Numerous plates of nanowires may be assembled into a parallel array with a single excitation source being used from outside array of plates. Since the plates are transparent to excitation source, the single excitation source can be used to probe many plates within an air or water stream without encumbering the stream. In the case of air sampling, a fluid, such as water, may be used to permeate the nanowire array to improve collection of airborne chemicals and/or biological agents.

In yet another embodiment, an SERS sensor has an array of nanowires on a flat transparent substrate. Excitation light is guided by transparent substrate and escapes from the light guide and excites nanowires which in turn causes molecule (located on the nanowire surface due to that surface's prior functionalization) to vibrate and thereby give off Raman shifted emissions that are then collected by external optical elements. This embodiment may be well suited for a portable system where excitation source power is limited and must be preserved. This arrangement may also be well suited for reducing the effects of sensor fouling.

In a further embodiment, a SERS sensor has an array of nanowires on a flat transparent substrate. Excitation light is guided by transparent substrate, and escapes from this light guide to excite nanowires which in turn causes molecule (located on nanowire surface due to that surface's prior functionalization) to vibrate and thereby give off Raman shifted emissions. Raman shifted light emissions from nanowires, re-entering the light guide, are then collected. This arrangement may be well suited for portable systems operating in a dirty environment where fouling may inhibit both the excitation source and the generated Raman signal.

In another embodiment, an SERS sensor has an array of nanowires on a shaped transparent substrate, preferably an optical fiber. Excitation light escaping from this light guide excite nanowire which in turn causes molecule to vibrate and thereby give off Raman shifted emissions which are collected by external optical elements. This arrangement forms a low cost disposable sensor that may be bundled to form a chemical/biological sensor suite in portable systems. A sensor may comprise one or more such devices bundled together. Custom sensor heads can be constructed to provide custom sensitivity for one or more agents of interest. A reflective enclosure can be used to collect all of the light emitted from the different species, i.e., chemical sensitivities, in fibers in the array.

In yet another embodiment, a SERS optical sensor has an array of nanowires on a shaped transparent substrate, preferably an optical fiber. Excitation light is guided by the transparent substrate and excitation light escaping from the light guide excites nanowires. Emissions from molecules, re-entering the light guide, are collected. This design is least affected by fouling where opaque and nonactive species settle on the nanowire array. This is the preferred embodiment when multiple excitation sources are employed to insure ultra high reliability for detecting a particular species in a diverse background. Multiple fiber sensors are bundled, each with an excitation source of unique wavelength.

In another embodiment, a SERS sensor has an array of nanowires 6 on a shaped transparent substrate, preferably a hollow tube. Nanowires are on an inner surface with an outer surface of tube covered by a reflecting layer. Excitation light is guided by the walls of tube. Excitation light escaping from the light guide excites nanowires, and emissions from molecules are collected by external optical elements positioned at one end of tube.

In a further embodiment, a SERS sensor has an array of nanowires on a shaped transparent substrate, preferably a hollow tube. Nanowires are on inner surface with outer surface of tube covered by reflecting layer. Excitation light is guided by the walls of tube and excitation light escaping from the light guide excites nanowires. Raman shifted light emissions from molecules re-entering the light guide are collected.

In another embodiment, an SERS sensor has an array of nanowires on a shaped transparent substrate, preferably a hollow tube. Nanowires are on inner surface with outer surface of tube covered by reflecting layer. Excitation light may be introduced into the center of tube so as to interact with, and excite nanowire which in turn causes molecule (located on surface due to that surface's prior functionalization) to vibrate and thereby give off Raman shifted emissions that enter the light guide to be collected. This design allows for the maximum excitation power level to be used and has the most efficient light collection. Primary use of this system may be for detecting trace amounts of agents in relatively clean gas/fluids.

Example 3

Raman Measurements to Determine the Sensitivity and Accuracy of Hemoglobin Detection

Raman spectra are acquired using the 514.5 nm line of an argon ion laser. The light is directed against the surface of the wire array in the backscattering geometry, but may be directed through the back side of the glass substrate. Data are acquired for two to ten minutes by a single grating monochrometer fitted with a linear CCD array with a resolution of 1 cm⁻¹. Referring again to FIG. 3, a Surface-Enhanced Raman Scattering (SERS) analysis system formed according to an aspect of the present invention operates as follows. A low-power diode laser that is integrated into a fiber laser cavity creates a narrow bandwidth optical excitation to probe molecular species located adjacent to SERS optical sensor. Nanowires act as a type of optical antenna, transferring the light energy from optical fiber 4 to molecules 8 located or trapped on surface of one or more nanowires in the array. This light then excites Raman emission from molecule. The Raman spectrum of the molecules provides a unique “fingerprint” for identification, as no two molecular species have the same Raman spectrum. Nanowires formed to the appropriate geometry and dimensions enable the enhanced optical signature of molecular species 8 to be detected using SERS. Raman shifted light that is collected back into optical fiber, while probe light is filtered out. For example, excitation source (e.g., diode laser) laser is removed from the signal path using a notch filter or other similar optical device placed in the optical signal capture path. The separate, isolated optical path within optical fiber reduces sensor-fouling errors. The emitted Raman spectra are then detected using photodiode spectrometer and the spectrum is analyzed in a manner similar to pattern recognition systems. Analyzer compares the detected spectrum to a database of known spectra, and quantifies the concentration of detected species. For example, such a system with nanowires functionalized to bind hemoglobin can be used for detecting, discriminating, and quantifying the molecule in a drop of blood sample used for analysis. A nanowire sensor head formed according to the invention can be integrated with the other optical and electronic components to form a compact and portable hemoglobin monitoring system.

SERS optical sensor system may be based upon arrays of metallic nanowires on a selected substrate (e.g., an optical fiber or other substrate material) and then chemically functionalizing nanowires to bind specific analytes. It will be understood that the present invention may be readily extended to detect and identify other important bio-molecules, nucleic acids, or protein sequences. SERS optical sensor system offers dramatically increased speed and sensitivity for detecting a large range of biologically important molecules, as well as, the potential for in vivo analysis.

Nanowire nanosensors such as optical sensor of the present invention can detect single molecules or chemicals at picomolar concentrations. One of the most practical physical properties of metallic nanowires for use in sensor technology is plasmon resonance. Plasmons are collective electron oscillations that are triggered when electromagnetic radiation is incident upon a metal structure. The resonance is due to a correlation between the wavelength of the incident radiation and the dimensions of the metallic nanowires. The absorption of light often peaks at the plasmon resonance frequency. Without intending to be bound by any particular theory, it is believed that the enhancement is due to the wire geometry acting like a perturbation on a spherical nanoparticle to form a plasmon resonator with modes that lie predominantly along the length of the wire. The nanowire geometry can be assumed to be similar to a prolate ellipsoid for the purposes of surface plasmon modes. In the present invention, nanowires have a size as to match the surface plasmaon modes to the laser frequency being used. Nanowires act as resonant cavities that enhance the strength of the electric field on surface of each nanowire. The size of the enhancement is often directly proportional to the quality factor for the mode. A cylindrical geometry of nanowire provides a distinct advantage to nanospheres in that nanowires 6 can be polarized in a specific mode along their length. Nanowires polarized in this way can interact with neighboring nanowires to increase the surface enhancement factor. Often there appear to be a linear relationship between the radius of a metallic cylindrical nanowires and the incident wavelength that governs the resonance effect. As classically derived, the resonant radius is a fraction of the wavelength, and for high-aspect cylinders, the resonance frequency is seemingly independent of the length. In these terms, high aspect ratio nanowires are usually height to diameter of about 5:1 or greater.

While some other factors, such as polarization, may be considered, this example demonstrates a key consideration in engineering nanowires to directly couple light energy from optical fiber to target molecule. According to an aspect of the present invention, nanowires may be tuned (formed) to the proper diameter that corresponds to the excitation wavelength resonance used in the Raman analysis. Nanowires are often grown to a specific length, as defined by the depth of in template, i.e., the thickness of the anodized layer. The voltage used in the creation of template controls the separation between the nanobores and hence the nanowires. The duration of a bore widening etch controls the diameter of nanobores and hence nanowire diameter. The particle size dependence of the optical enhancement from nanoparticles in SERS has been experimentally observed to have a linear relationship with the excitation wavelength.

Nanowire geometry (high-aspect ratio, metallic, cylindrical structure) is desirable. For example, plasmon field enhancement may result due to the large reduction of plasmon damping found in these structures. Further, SERS is more pronounced in composite groups of nanostructures like arrays of nanowires. Importantly, organized particle arrays, unlike random structures, can be tuned to a common plasmon resonance frequency due to coupling between the ordered structures in the array. Plasmon field enhancement is typically large at the ends of nanowires and in the regions between nanowires due to strong electromagnetic coupling where the materials are touching or in close proximity.

Nanowires are often formed in the range from about 5 nm to about 200 nm in diameter and, preferably in the range from about 20 nm to 80 nm in diameter. Nanowires are often spaced between about 5 nm and about 50 μm with a center-to-center spacing of about 20-250 nm. Nanowires formed on substrate may have an exposed length of about 1-5 microns to 10 microns. Nanowire densities may range from about 10⁹ to 10¹²/cm². With such densities and wire lengths of up to about 10 microns. Nanowire array substrates can have a surface area enhancement factor of 10,000 for SERS, for example.

It is to be understood that the present invention is by no means limited only to the particular constructions herein disclosed and shown in the drawings, but also comprises any modifications or equivalents within the scope of the claims. Although the present invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only, and is not to be taken by way of limitation. It is appreciated that various features of the invention which are, for clarity, described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment may also be provided separately or in any suitable combination. It is appreciated that the particular embodiment described in the Appendices is intended only to provide an extremely detailed disclosure of the present invention and is not intended to be limiting. The spirit and scope of the present invention are to be limited only by the terms of the appended claims.

Claims

1. A nanowire array comprised of a plurality of substantially free-standing nanowires, each nanowire with a first end and second end;

a substrate with a surface where the substantially free-standing nanowires are attached to the surface at the first end and the free-standing nanowires are not supported by a layer of material that is substantially distinct from the substrate material.

2. The nanowire array of claim 1 where the nanowires and the substrate are substantially the same material.

3. The nanowire array of claim 1 where the nanowires are made from one of silver, nickel, iron, gold, cadmium, silicon, Germanium or GaAs.

4. The nanowire array of claim 1 where the nanowires are made of copper.

5. A nanowire array comprised of a plurality of free-standing nanowires each attached to a substrate, where said nanowires are between about 1 micron and 10 microns in length and where the array exhibits nanowire densities between about 109 and 1012 nanowires per cm2.

6. The nanowire array of claim 5 where the nanowires are made of the substantially the same material as the substrate.

7. The nanowire array of claim 5 where the nanowires are made of one of silver, nickel, iron, gold, cadmium, silicon, germanium or GaAs.

8. The nanowire array of claim 5 where the nanowires are made of copper.

9. The nanowire array of claim 2 where the nanowires are made of copper.