WO2015009239A1 - Sers-active device and method of manufacturing the device - Google Patents

Abstract

A surface enhanced Raman scattering (SERS)-active device is provided. The SERS-active device includes at least one SERS-active nanostructure attached to a substrate, wherein arrangement of the at least one SERS-active nanostructure on the substrate is adapted to provide a surface enhanced Raman signal having an intensity that is tunable depending on at least one of (i) orientation angle of the at least one SERS-active nanostructure, (ii) polarization of incident polarized light, and (iii) wavelength of the incident polarized light. An identification tag comprising the SERS-active device, method of identifying an object using the identification tag, and method of manufacturing the SERS-active device are also provided.

Description

SERS-ACTIVE DEVICE AND METHOD OF MANUFACTURING THE DEVICE

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of US provisional application No. 61/847,196 filed on 17 July 2013, the content of which is incorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD

[0002] The invention relates to a surface enhanced Raman scattering (SERS)-active device, and method of manufacturing the device. The invention also relates to an identification tag for identifying an object that contains the SERS-active device, and method of identifying the object using the identification tag.

BACKGROUND

[0003] Counterfeiting and forgery pose security threats to individuals, companies, and society, and may cause considerable financial damages. Many types of security labels have been developed and incorporated into currency notes, banknotes, identity cards, and legal documents to counter against global counterfeiting. Typically, these security labels possess unique physical features that are hard to copy, such as fine prints, security inks, watermarks, and holograms. They are generally made from stimuli-responsive molecules, polymer and/or photonic structures. A change in their optical or physical property may be induced by heat, light, and other external stimuli, which may then be directly visualized and validated using colorimetry and fluorometry.

[0004] With advances in technology, however, some of the security labels may be circumvented by counterfeiter, which renders them easy to copy. It is important to continually develop new security labels with improved levels of security for product authentication so as to counter against counterfeiters.

[0005] In view of the above, there remains a need for improved security labels that overcome or at least alleviate one or more of the above-mentioned problems. SUMMARY

[0006] In a first aspect, the invention refers to a surface enhanced Raman scattering (SERS)-active device. The device comprises at least one SERS-active nanostructure attached to a substrate, wherein arrangement of the at least one SERS-active nanostructure on the substrate is adapted to provide a surface enhanced Raman signal having an intensity that is tunable depending on at least one of (i) orientation angle of the at least one SERS-active nanostructure, (ii) polarization of incident polarized light, and (iii) wavelength of the incident polarized light.

[0007] In a second aspect, the invention refers to an identification tag comprising a SERS- active device according to the first aspect.

[0008] In a third aspect, the invention refers to a method of identifying an identification tag according to the second aspect. The method comprises

a) obtaining a surface enhanced Raman signal from the identification tag;

b) checking the obtained signal against a reference signature, and determining degree of overlap or similarity between the obtained signal and the reference signature.

[0009] In a fourth aspect, the invention refers to a method of manufacturing a SERS- active device. The method comprises forming at least one SERS-active nanostructure on a substrate, wherein arrangement of the at least one SERS-active nanostructure is adapted to provide a surface enhanced Raman signal having an intensity that is tunable depending on at least one of (i) orientation angle of the at least one SERS-active nanostructure, (ii) polarization of incident polarized light, and (iii) wavelength of the incident polarized light.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0011] FIG. 1(A) and (B) show tapping mode atomic force microscopy (AFM) topographic image of silver film and its corresponding cross-section profile. FIG. 1(C) and (D) show three-dimensional tapping mode AFM topographic image of silver nano wires and its corresponding cross-section profile. FIG. 1(E) shows a schematic representation of the Ag nanowire used in the simulation. [0012] FIG. 2(A) and (B) show simulated electric field distributions of silver (Ag)-coated nanowire when polarized at x-axis and y-axis, respectively. (C) and (D) are expanded views of the highlighted areas in (A) and (B), respectively. FIG. 2(E) and (F) depict simulated cross-sectional profiles of the electric field intensity enhancement of the two orthogonally polarized light.

[0013] FIG. 3 shows scanning electron microscopy (SEM) image of a (A) single polymer line fabricated by two-photon lithography; and (B) after thermal evaporation with 2-nm chromium (Cr) and 100-nm Ag. FIG. 3(C) and (D) show x-polarized and y-polarized 2D Raman imaging of the Ag nanowire coated with 4-methylbenzenethiol (4-MBT) molecules. The intensity was collected from 1079 cm^"1 band. FIG. 3(E) shows the SERS spectra of 4- MBT taken from different spots (location 1 - 4), as indicated in (C) and (D). The scale bar in FIG. 3(A) to (D) denotes a length of 1 μιη. FIG. 3(F) and (G) are cross-sectional Raman profiles of the Ag nanowires (collected from the center location) at x-polarization and y- polarization, respectively.

[0014] FIG. 4 shows (i) SEM images, (ii-iii) x-polarized and y-polarized 2D Raman imaging of different encrypted nanostructures formed by parallel and horizontal Ag nanowires, for (A) cross-line, (B) bowtie, (C) overlaid letter "A" and inversed "A", and (D) superimposed letters of "NTU" and "CBC". All scale bars denote a length of 10 μπι.

[0015] FIG. 5 shows (A) SEM images; (B, C) x-polarized and y-polarized 2D Raman imaging of Ag nanowires at orientation angles of Θ = 0°, 15°, 30°, 45°, 60°, 75°, and 90°. FIG. 5(D) and (E) depict x-polarized and y-polarized surface enhanced Raman spectroscopy (SERS) spectra collected from Ag nanowires with orientation angles ranging from Θ = 0° to 90°, respectively. FIG. 5(F) and (G) show plots of simulated and normalized experimental SERS intensity versus Ag nanowire orientation angle. The insets of (F, G) illustrate the definition of Θ between nanowire and the incident laser, respectively. All scale bars denote a length of 5 μιη.

[0016] FIG. 6 shows (i) schematics of Ag nanowires oriented at different angles to form concentric structures of (A) triangles, (B) squares, (C) pentagons, (D) hexagons, and (E) octagons. The respective orientation angle (Θ) of the Ag nanowires are indicated in the respective schemes; (ii) SEM images; (iii) and (iv) x-polarized and y-polarized 2D Raman imaging of concentric structures All scale bars denote a length of 10 μω.. FIG. 6(F) and (G) are plots of normalized SERS intensity versus Ag nanowires at different orientation angles at x-polarization and y-polarization, respectively.

[0017] FIG. 7 shows (i) SEM images; (ii) and (iii) x-polarized and y-polarized 2D Raman imaging, of alphabet A written entirely by (A) horizontal nanowires, (B) nanowires oriented at 45°, and (C) vertical nanowires.

[0018] FIG 8(A) shows a typical Raman spectrum of 2-naphthalenethiol. FIG. 8(B) and (C) shows (i) SEM images, (ii) dark-field microscope images, (iii and iv) x-polarized and y- polarized 2D SERS imaging of different molecularly encrypted nanostructures formed by horizontal and vertical Ag nanowires, for (B) superimposed letters of NTU and CBC; and (C) overlaid alphabets of upright and inverted "A"s. All scale bars are 10 mm.

[0019] FIG. 9(A) to (C) show SEM images, and (D) to (F) their corresponding x- polarized grayscale 2D Raman images of Escher lizard pattern with different nanowires orientation, respectively. Orientation of the nanowires used to draw the individual lizards turns 45° from (A) to (B), and from (B) to (C) to create different gray scale images. Scale bar in the figures denotes a length of 10 μιη. FIG. 9(G) is an optical image of corresponding grayscale of Escher lizard pattern (B). FIG. 9(H) shows average SERS spectra, and (I) intensity profile across the lizards with different gray scales in (D) to (F).

[0020] FIG. 10(A) is a SEM image of the line arrays with vertical and horizontal lines, (i) 2D SERS imaging; and (ii) SERS intensity profile along x-axis and y-axis of line arrays functionalized with 4-MBT molecules and second layer of Ag coating with the thickness of (B) 0 nm, (C) 10 nm, (D) 30 nm, (E) 50 nm, (F) 70 nm, (G) 100 nm, (H) 120 nm when incident laser is polarized along the x-axis. The 1078 cm^"1 peak was selected for imaging and measuring intensity profile. All the scale bars represent a length of 10 μιη.

[0021] FIG. 11 shows SERS spectra of 4-MBT from (A) Ag vertical lines and Ag/4- MBT/Ag sandwich structure, (B) Ag horizontal lines and Ag/4-MBT/Ag sandwich structure, with second layer of Ag of 10 nm, 30 nm, 50 nm, 70 nm, 100 nm and 120 nm. Variation of the average SERS intensity with different thickness of the second layer Ag coating from (C) vertical lines, and (D) horizontal lines.

[0022] FIG. 12 shows homogeneous bimolecular plasmonic anti-counterfeiting. FIG. 12(A) is a SEM image of a panda pattern formed by vertical lines and horizontal lines. FIG. 12(B) shows (i) scheme of the sandwich structure with a layout of 100 nm Ag/4-MBT/50 nm Ag sandwich structure, (ii) x-polarized 2D SERS image collected from 1078 cm^"1 band of 4- MBT embedded plasmonic panda pattern, (iii) x-polarized 2D SERS image collected from 1647 cm^"1 band, a character band of RhBITC from 4-MBT embedded panda plamonic pattern. FIG. 12(C) shows (i) scheme of the bimolecular sandwich structure, (ii) x-polarized 2D SERS image collected from 1078 cm^"1 band of 4-MBT, and (iii) x-polarized 2D SERS image collected from 1647 cm^"1 band. FIG. 12(D) shows (i) SERS spectra of 4-MBT from 100 nm Ag/4-MBT/50 nm Ag sandwich (red line), and (ii) composite SERS spectra of 4-MBT and RhBITC from 100 nm Ag/4-MBT/50 nm Ag/RhBITC/50 nm Ag sandwich structure (green line). The schemes of two types of structures are also shown in the inset. FIG. 12(E) shows SERS spectra of RhBITC from 100 nm Ag/RhBITC/50 nm Ag as a comparison. All scale bars represent a length of 10 μηι.

[0023] FIG. 13 depicts heterogeneous bimolecular plasmonic anti-counterfeiting. Scheme for (A) conventional direct laser writing (DLW), and (B) Dip-in laser lithografie (DiLL) technique used for heterogeneous bimolecular plasmonic anti-counterfeiting fabrication. Two examples, (C, D) merlion and (E, F) dove are used to show how heterogeneous bimolecular plasmonic anti-counterfeiting works. The first sandwich structure (merlion, dove with olive leaf) embedded with 4-MBT as shown in (C, E) is obtained from DLW technique and thermal evaporation, (i) SEM image, (ii) x-polarized 2D SERS image collected from 1078 cm^"1 band of 4-MBT embedded plasmonic pattern, (iii) x-polarized 2D SERS image collected from 1647 cm^"1 band, a character band of RhBITC embedded plamonic pattern, show that the physical information of first pattern was successfully encrypted into the x-polarized 2D SERS images. The second sandwich structure (merlion with gushing water stream, second dove) as shown in (B, F) was obtained from the DiLL technique and thermal evaporation, (i) SEM image, (ii) x-polarized 2D SERS image collected from 1078 cm^"1 band of 4-MBT, (iii) x- polarized 2D SERS image collected from 1647 cm^"1 band show that the physical information of first pattern may be revealed from 4-MBT, while both first and second pattern may be revealed from RhBITC. All scale bars represents a length of 10 μηι.

DETAILED DESCRIPTION

[0024] Raman spectroscopy is based on an inelastic light scattering by molecules (the Raman effect). In the Raman scattering process, a photon interacts momentarily with a molecule and is then scattered into surroundings in all directions. During the brief interaction with molecule, photon loses or gains energy which is then detected and analyzed. One important aspect of the Raman scattering is the correlation between the amount of the frequency shifts and the vibrational modes of the molecules. Here, vibrational modes refer to the "manner" in which the molecule vibrates. Since vibrational modes are sensitive to the chemical nature of the molecule, probing molecular vibrations may thus reveal information regarding its chemical geometry.

[0025] Surface-enhanced Raman spectroscopy refers to a form of Raman spectroscopy, which possesses high sensitivity brought about by intense enhancement of the local electromagnetic fields in the proximity of SERS-active metal, such as noble metal. This mechanism has been used herein to fabricate surface enhanced Raman scattering (SERS)- active devices. In particular, it has been demonstrated herein that the SERS-active device may form at least part of an identification tag for identifying an object, for example, or a data storage medium.

[0026] Advantageously, identification tags disclosed herein are capable of being equipped with two or more security levels. A first security level may be based on simple colored and/or holographic features that are easily verified by the public, while a second security level may include features that may only be authenticated by advanced and sophisticated analytical systems that are not available readily to the public. For example, the second security level may include use of SERS-active nanostructures that allow selective chemical Raman image read-outs to be carried out spatially and spectroscopically by controlling, for example, at least one of orientation angle of SERS-active nanostructures, polarization of incident polarized light, and wavelength of the incident polarized light.

[0027] It has been demonstrated herein that chemical SERS signals with tunable intensities may be obtained using complex 2D SERS plasmonic structures. The nanostructured security features disclosed herein provide molecular spectrum with fingerprint specificity. Advantageously, the method to manufacture the SERS-active device is highly versatile, flexible, and yet difficult to copy. In addition, Raman spectroscopy is a nondestructive technique that is operational at all ranges of light excitation, making it an ideal chemical imaging tool for various applications.

[0028] In a first aspect, the present invention refers to a surface enhanced Raman scattering (SERS)-active device.

[0029] As used herein, the term "SERS-active" refers to materials that enhance Raman scattering of a Raman-active molecule adsorbed thereon. In various embodiments, a SERS- active material enhances Raman scattering of a Raman-active molecule adsorbed thereon by a factor of 10⁴, 10⁶, 10¹⁰, or more. Accordingly, the term "non SERS-active" refers to materials that provide minimal or no enhancement of the Raman scattering.

[0030] The SERS-active device disclosed herein comprises at least one SERS-active nanostructure attached to a substrate.

[0031] The nanostructures may be formed entirely from a SERS-active material. Examples of a SERS-active material include, but are not limited to, noble metals such as silver, palladium, gold, platinum, iridium, osmium, rhodium, ruthenium; copper, aluminium, or alloys thereof. In various embodiments, the nanostructures may consist of a metal selected from the group consisting of a noble metal such as gold or silver, copper, aluminium, and alloys thereof. In some embodiments, the nanostructures may be formed from a non-SERS active material, such as plastic, ceramics, composites, glass or organic polymers, which has a layer of SERS-active material coated thereon. In specific embodiments, the nanostructures may comprise a polymer having a layer of silver coated thereon to render its plasmonic characteristic.

[0032] The terms "at least one" or "one or more" as used interchangeably herein in connection with nanostructures relates to 1, 2, 3 or more, for example at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 15, 20, 25 or a plurality of nanostructures. In this connection, the term "plurality" means more than two.

[0033] The term "nanostructure" refers to a material having at least one dimension that is in the nanometer range. At least one dimension of the nanostructure may be less than 1000 nm. In various embodiments, a nanostructure has a dimension typically ranging from 100 nm to 1000 nm. Examples of a nanostructure include, but are not limited to, nanotubes, nanoflowers, nanowires, nanofibers, nanoflakes, nanoparticles, nanodiscs, nanofilms, and combinations of the aforementioned.

[0034] In various embodiments, the at least one SERS-active nanostructure comprises or consists of one or more nanowires.

[0035] Each of the one or more nanowires may have a diameter in the range of about 300 nm to about 1000 nm. For example, each nanowire may have a diameter in the range of about 300 nm to about 750 nm, about 300 nm to about 500 nm, about 500 nm to about 1000 nm, about 750 nm to about 1000 nm, about 400 nm to about 800 nm, or about 500 nm to about 750 nm. [0036] Each of the one or more nanowires may have a length in the range of about 1 μιη to about 10 μηι. For example, each nanowire may have a length in the range of about 1 μιη to about 8 μηι, about 1 μηι to about 5 μηι, about 3 μηι to about 10 μιη, about 5 μηι to about 10 μηι, about 3 μπι to about 8 μπι, or about 4 μιη to about 6 μηι.

[0037] The at least one SERS -active nanostructure may comprise a first metal layer arranged on the substrate, and a first Raman-active marker compound layer arranged on the first metal layer.

[0038] As used herein, the term "Raman-active marker compound" refers to a compound which has a high Raman cross section. Examples of Raman-active marker compounds include, but are not limited to, 4-methylbenzenethiol, rhodamine B isothiocyanate, 2- napthalenethiol, methylene blue, coumarin, melamine, and combinations thereof.

[0039] In various embodiments, the at least one SERS-active nanostructure comprises a second metal layer arranged on the first Raman-active marker compound layer. In such a configuration, the SERS-active nanostructure may have a sandwich structure in which the first Raman-active marker compound layer is arranged between the first metal layer and the second metal layer.

[0040] In some embodiments, the at least one SERS-active nanostructure comprises a second Raman-active marker compound layer deposited on the second metal layer and a third metal layer deposited on the second Raman-active marker compound layer. In such a configuration, the SERS-active nanostructure may have a dual-sandwich structure in which the first and the second Raman-active marker compound layers are arranged between two metal layers.

[0041] The first metal layer, the second metal layer and the third metal layer may independently comprise or consist of a SERS-active material. Examples of SERS-active material have already been described above. In various embodiments, the first metal layer, the second metal layer and the third metal layer independently comprise or consist of silver.

[0042] Thickness of the first metal layer, the second metal layer, and the third metal layer may independently be in the range from about 30 nm to about 120 nm. For example, the first metal layer may have a thickness of about 100 nm, the second metal layer may have a thickness of about 50 nm, and the third metal layer may have a thickness of about 50 nm. In various embodiments, thickness of the first metal layer is about 100 nm. [0043] In various embodiments, the at least one SERS-active nanostructure comprises two metal layers and a Raman-active marker compound layer, wherein the Raman-active marker compound layer is arranged between the two metal layers to form a sandwich structure. In some embodiments, thickness of the second metal layer is about 80 nm to about 120 nm, such as about 80 nm to about 100 nm, about 90 nm to about 110 nm, about 90 nm, about 100 nm or about 110 nm.

[0044] It has been surprisingly found by the inventors that intensity of SERS signal increases with an increase in thickness of the second metal layer, with an optimal thickness of the second metal layer at 100 nm, whereby enhancements in intensity of more than 100-fold have been achieved compared to that generated using a layer thickness of 10 nm.

[0045] The first Raman-active marker compound and the second Raman-active marker compound may have a different signature vibrational mode. By the term "signature vibrational mode", it refers to one or more specific wavelengths at which a molecule or compound may be characterized by.

[0046] Advantageously, by having two Raman-active marker compounds in the SERS- active nanostructures, such as in the form of a sandwich or dual-sandwich structure, two probe molecules with non-overlapping SERS peaks may be used such that different peaks may be selected to construct individual images, which are not apparent based on appearance alone. In applications such as an identity tag for identifying an object for example, such a bimolecular anti-counterfeiting approach may complicate process of deciphering the number and type of probe molecules used in embedding the molecular information.

[0047] In various embodiments, the first Raman-active marker compound and the second Raman-active marker compound are independently selected from the group consisting of 4- methylbenzenethiol, rhodamine B isothiocyanate, and combinations thereof.

[0048] Arrangement of the at least one SERS-active nanostructure on the substrate is adapted to provide a surface enhanced Raman signal having an intensity that is tunable depending on at least one of (i) orientation angle of the at least one SERS-active nanostructure, (ii) polarization of incident polarized light, and (iii) wavelength of the incident polarized light.

[0049] In various embodiments, arrangement of the at least one SERS-active nanostructure on the substrate is adapted to provide a surface enhanced Raman signal having an intensity that is tunable depending on orientation angle of the at least one SERS-active nanostructure.

[0050] The term Orientation angle" as used herein refers to angle defined by the long-axis of an object with the y-axis, as measured from the positive y-axis in a clockwise direction to the long-axis of the object, and having a value in the range from 0 ° to 180 ° (180 ° non- inclusive). The term "long axis" and "longitudinal axis" are used interchangeably, and refers to an axis passing through a center of an object and which runs parallel to the length of the object.

[0051] For purposes of illustration only, when orientation angle of the SERS-active nanostructure on the substrate is 0 °, this means that the long axis of the SERS-active nanostructure is parallel to the y-axis. As another example, when orientation angle of the SERS-active nanostructure on the substrate is 90 °, the long axis of the SERS-active nanostructure is parallel to the x-axis. Exemplary orientation angles of SERS-active nanostructure on the substrate may be seen from FIG. 6(A) to (E).

[0052] In various embodiments, more than one SERS-active nanostructure is present. The plurality of SERS-active nanostructures may be arranged in a periodic array. The term "periodic array" as used herein refers to repetition at regular intervals of a structure within an area. For example, the SERS-active nanostructures may be uniformly aligned to one another, with each nanostructure being spaced apart from a neighboring nanostructure by a predetermined distance.

[0053] In various embodiments, the SERS-active nanostructures are arranged in a vertical array. By the term "vertical", it means that the SERS-active nanostructures are arranged such that long-axis of the SERS-active nanostructures is in a direction that is substantially parallel to the y-axis. In various embodiments, the SERS-active nanostructures are arranged in a horizontal array. By the term "horizontal", it means that the SERS-active nanostructures are arranged such that long-axis of the SERS-active nanostructures is in a direction that is substantially parallel to the x-axis. Accordingly, the terms "vertical" and "horizontal" are related in that lines drawn in the vertical and horizontal direction are perpendicular to each other, and intersect to define an angle of 90 °.

[0054] The SERS-active nanostructures may be arranged to form a concentric arrangement about a specific point. As such, each nanostructure comprised in the concentric arrangement may have a different orientation angle depending on how it is arranged on the substrate.

[0055] In various embodiments, the concentric arrangement is a regular shape such as, but not limited to, a triangle, a square, a pentagon, a hexagon, an octagon, a trapezium, a parallelogram, a rectangle, a circle, or an ellipse. In some embodiments, the concentric arrangement may be a combination of shapes, such as an alphabet, a number, or a graphic, an irregular shape, or combinations thereof.

[0056] In various embodiments, arrangement of the at least one SERS-active nanostructure on the substrate is adapted to provide a surface enhanced Raman signal having an intensity that is tunable depending on polarization of incident polarized light.

[0057] The term "polarized light" may alternatively be termed as "linearly polarized light", and refers to light having a single vibration direction. The polarized light may be polarized in a direction that is substantially perpendicular to or substantially parallel to a long axis of the at least one SERS-active nanostructure. In various embodiments, the polarized light may be polarized in a direction that lies between a direction that is substantially perpendicular to and a direction that is substantially parallel to a long axis of the at least one SERS-active nanostructure.

[0058] By manipulating the polarization angle of incident light relative to the long axis of the at least one SERS-active nanostructure, SERS response of the at least one SERS-active nanostructure may be selectively read-out.

[0059] For example, strong electromagnetic field and enhanced SERS intensity may be obtained when incident polarized light is transverse to the long axis of a SERS-active nanostructure. The intensity formed by the transverse incident polarized light may constitute a maximal value. Conversely, there may be minimal influence or effect on electromagnetic field and SERS intensity when incident polarized light is parallel to the long axis of a SERS- active nanostructure. The intensity formed by the parallel incident polarized light may constitute a minimal value.

[0060] In embodiments where the polarized light is polarized in a direction that lies between a direction that is substantially perpendicular to and a direction that is substantially parallel to a long axis of the at least one SERS-active nanostructure, a surface enhanced Raman signal having an intensity that is intermediate between the maximal (transverse) and minimal (parallel) enhancement may be obtained. Accordingly, intensity of the surface enhanced Raman signal may be tuned depending on the polarization of incident polarized light. Using polarized Raman chemical imaging, information that is not visible by mere visual observation may be encrypted into the SERS-active device.

[0061] Apart from or in addition to the afore-mentioned, arrangement of the at least one SERS-active nanostructure on the substrate may be adapted to provide a surface enhanced Raman signal having an intensity that is tunable depending on wavelength of the incident polarized light.

[0062] As mentioned above, one or more Raman-active marker compounds may be included in the at least one SERS-nanostructure. Each Raman-active marker compound may have a different signature vibrational mode. By controlling wavelength of the incident polarized light, the one or more Raman-active marker compounds may provide a surface Raman signal having an intensity that is dependent on their signature vibrational mode. For example, the incident polarized light may comprise or consist of a wavelength corresponding to a peak wavelength of the SERS-active nanostructure.

[0063] Advantageously, in applications such as identity tags for identifying an object, for example, this allows a further level of encryption to be present or stored within the at least one SERS-nanostructure, since only personnel who are aware of the type of Raman-active marker compounds present, for example, would possess the key in the form of the wavelength of the incident polarized light, to unlock such encrypted data for authentication purposes.

[0064] In various embodiments, the first Raman-active marker compound and the second Raman-active marker compound independently form a self-assembled monolayer on the first metal layer and the second metal layer, respectively.

[0065] Arrangement of the at least one SERS-active nanostructure on the substrate may be adapted to provide a surface enhanced Raman signal of a signature intensity based on at least one of (i) orientation angle of the at least one SERS-active nanostructure, (ii) polarization of incident polarized light, and (iii) wavelength of the incident polarized light.

[0066] By the term "signature intensity", it refers to a unique intensity value that arises due to the specific combination of orientation angle, polarization and wavelength of incident polarized light. One or more of orientation angle of the at least one SERS-active nanostructure, polarization of incident polarized light, and wavelength of the incident polarized light may be manipulated or controlled to provide a surface enhanced Raman signal having a signature intensity at a specific combination of the afore-mentioned parameters.

[0067] SERS signal is strongly dependent on incident field polarization and wavelength with respect to plasmonic SERS-active nanostructures. While such a trait is undesirable for general molecule sensing purpose, such polarization dependent plasmonic and SERS response holds great potential for encryption of molecular information with its enhanced directional optical properties. For example, Raman scattering of silver nanowires at its longitudinal modes is much weaker (and nearly negligible) compared to its transverse plasmon owing to the momentum mismatch between incident photon with the propagating plasmons.

[0068] The SERS-active device disclosed herein may be used in a myriad of applications, such as bio(chemical) sensing, imaging, anti-counterfeiting, and optical data storage. The widespread use of SERS arises from its high sensitivity, with single molecule detection capabilities already demonstrated. In various embodiments, the SERS-active device may form at least part of (i) a data storage medium, or (ii) an identification tag for identifying an object.

[0069] In some embodiments, the SERS-active device form at least part of a data storage medium. In application as a data storage medium, for example, the tunable gradient of SERS intensities obtained in the SERS-active nanostructures may be used as basic data storage element (or bit) for multiple-bit 2D plasmonic molecular information storage. By exploiting polarization-dependent SERS responses of a SERS molecular image, for example, multiple shades of SERS intensities may be obtained, where such properties may be exploited in manufacturing flexibility plasmonic nanostructures for multiple-bit data storage with enhanced spatial information.

[0070] The SERS-active device may form at least part of an identification tag for identifying an object. The invention refers accordingly in a second aspect to an identification tag comprising a SERS-active device according to the first aspect, and in a third aspect, to a method of identifying an object comprising an identification tag according to the second aspect.

[0071] Plasmonic Au and Ag nanostructures may hold great promises as the next generation security labels, as it enhances the Raman scattering signals by 10⁴ to 10¹⁰ orders of magnitude due to the coherent oscillation of conduction electrons (LSPR) on metal nanostructures with incident light that increases scattering and enhances the electromagnetic field strength on their surfaces. Such surface-enhanced Raman scattering provide an attractive molecular detection system yet complementary to current color-based or visual-based anticounterfeiting systems. In particular, SERS (or Raman spectroscopy) gives rise to highly specific molecular information, which may be incorporated into security label to further increase the security level in authentication. Advantageously, a vast library of molecules may be used as probes, each with its characteristic unique spectral fingerprint. This makes SERS- based anti-counterfeiting system more difficult to forge.

[0072] As used herein, the term "object" refers to an individual object or tag in which or on which identification information may be incorporated or arranged. The term "object" may also refer to an article that is to be identified or tagged with an identification tag.

[0073] The method includes obtaining a surface enhanced Raman signal from the identification tag; checking the obtained signal against a reference signature, and determining degree of overlap or similarity between the obtained signal and the reference signature.

[0074] In various embodiments, identity of the object is verified if values of the obtained signal differ from corresponding values of the reference signature by less than a predetermined threshold. The predetermined threshold may be set by the user depending on the type of Raman-active marker compounds used and arrangement of the at least one SERS- active nanostructure on the substrate, for example. Advantageously, due to narrow bandwidth of the SERS bands, a high spectral resolution may be obtained, giving rise to an ultrasensitive anti-counterfeiting technology that cannot be achieved with other colorimetry-based optical techniques.

[0075] Obtaining a surface enhanced Raman signal from the identification tag may comprise irradiating the at least one SERS-active nanostructure with a polarized light. In various embodiments, the polarized light comprises or consists of a wavelength corresponding to a peak wavelength of the SERS-active nanostructure.

[0076] In a fourth aspect, the invention refers to a method of manufacturing a SERS- active device. The method comprises forming at least one SERS-active nanostructure on a substrate, wherein arrangement of the at least one SERS-active nanostructure is adapted to provide a surface enhanced Raman signal having an intensity that is tunable depending on at least one of (i) orientation angle of the at least one SERS-active nanostructure, (ii) polarization of incident polarized light, and (iii) wavelength of the incident polarized light. [0077] Examples of SERS-active nanostructures, and their arrangement have already been discussed above.

[0078] In various embodiments, forming at least one SERS-active nanostructure on a substrate comprises fabricating at least one nanostructure on a polymeric template using direct laser writing, and alternately depositing one or more metal layers and one or more Raman-active marker compounds on the at least one nanostructure.

[0079] As mentioned above, the nanostructures may be formed from a non-SERS active material, such as plastic, ceramics, composites, glass or organic polymers, having a layer of SERS-active material coated thereon. By using direct laser writing to fabricate nanostructures on a polymeric template, nanostructures of a myriad of shapes and sizes may be fabricated easily.

[0080] The method may include alternately depositing one or more metal layers, and one or more Raman-active marker compounds on the at least one nanostructure. Examples of suitable metals and Raman-active marker compounds that may be used have already been described above.

[0081] In various embodiments, depositing one or more metal layers on the at least one nanostructure comprises depositing metal on the at least one nanostructure by thermal evaporation.

[0082] In various embodiments, depositing one or more Raman-active marker compound layers on the at least one nanostructure comprises incubating the substrate comprising at least one nanostructure in a liquid reagent comprising the respective Raman-active marker compounds. By incubating the substrate in the liquid reagent containing the Raman-active marker compound, the Raman-active marker compound may be deposited on the at least one nanostructure by self assembly.

[0083] Depositing one or more metal layers and one or more Raman-active marker compounds on the at least one nanostructure may include alternately depositing two or more metal layers and one or more Raman-active marker compound layers on the at least one nanostructure such that the one or more Raman-active marker compound layers are sandwiched between two metal layers. In embodiments where two Raman-active marker compound layers are present, for example, a dual-sandwich structure in which the first and the second Raman-active marker compound layers are arranged between two metal layers may be formed. [0084] Capability to embed information using more than one Raman-active marker compound further increases the complexity and/or density of information encoded within the SERS platform. In terms of anti-counterfeiting technology, an added layer of information stored increases the difficulty of decoding exponentially for the counterfeiters to overcome.

[0085] Hereinafter, the present invention will be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, lengths and sizes of layers and regions may be exaggerated for clarity.

[0086] As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0087] Embodiments of the invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing.

[0088] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0089] The invention illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[0090] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0091] Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group. EXPERIMENTAL SECTION

[0092] In embodiments disclosed herein, security labels using plasmonic nanowire structures and its polarization dependent localized surface plasmon resonance (LSPR)- enhanced Raman imaging are provided. Owing to the unique enhanced directional optical properties of Ag nano wires, chemical information encrypted under the nanostructures cannot be revealed directly. It is demonstrated herein that selective chemical Raman image may only be read-out spatially and spectroscopically by manipulating the orientation angles of Ag nanowires with respect to incident light. In addition, the Ag nanostructured security features disclosed herein also provide molecular spectrum with fingerprint specificity. Line-scanned confocal Raman scanning was used to achieve fast and efficient SERS chemical imaging mapping to demonstrate the anti-counterfeiting properties of plasmonic Ag nanowire platform.

[0093] Fabrication strategy in embodiments disclosed herein focuses on using two-photon lithography technique to construct tailored polymeric nanowires, followed by silver deposition to obtain plasmonic Ag nanowire structures. Chemical information is encrypted onto the plasmonic structures via Ag-thiol coordination functionalization of analyte molecules. The fabrication technique disclosed herein is highly versatile, flexible, and yet difficult to copy.

[0094] Selective SERS response of single nanowire and nanowire arrays with different orientation angles was demonstrated. This provides a dynamic SERS intensity profile under different polarization angles. Subsequently, more complex 2D SERS plasmonic structures, including concentric squares, triangles, pentagons, hexagons, octagons, and concentric ring, a curvature structure to obtain chemical SERS images with predictable SERS intensities were fabricated. Finally, a single plasmonic structure was designed to encrypt two different SERS images at different polarization angles, despite having the entire structure functionalized with a homogeneous layer of molecules over its surface.

[0095] A SERS platform based on arrays of plasmonic nanowires which are capable of generating highly reproducible SERS signals is disclosed herein. Plasmonic silver (Ag) nanowire arrays are first fabricated using a 2-photon lithographic approach and subsequently metalized via thermal evaporation. SERS enhancement factors from such plasmonic Ag nanowire arrays consistently reach 10⁶, and the anisotropic Ag nanowire morphology gives rise to a unique polarization-dependent SERS intensity. This polarization dependence was used to develop an anti-counterfeiting platform as well as a data storage device.

[0096] A bimolecular SERS-based anti-counterfeiting technology using a sandwich structure is also disclosed herein. This bimolecular system is also based on the polarization- dependent SERS signals of the Ag nanowire arrays. Using these Ag nanowire arrays, fabrication of both homogeneous and heterogeneous bimolecular anti-counterfeiting platforms was demonstrated. In the homogeneous platform, the same image may be produced at two distinct spectral positions corresponding to the signature vibrational modes of the two individual probe molecules. In the heterogeneous platform, images with additional features may be readout from one of the two distinct spectral positions but not from the other. Both of these techniques highlight the increased complexity of information readout, since the SERS spectra alone is a composite of the individual SERS spectra of both molecules used. Without prior knowledge to the number and types of molecules used, it will be challenging for the counterfeiters to decipher the probe molecules' identities. Furthermore, this anti- counterfeiting technology is extremely sensitive, being capable of resolving different images with a spectral resolution of around 2 nm. This is not possible using conventional colorimetry-based counterfeiting techniques.

[0097] Example 1: Materials

[0098] IP-L 780 photoresist (Nanoscribe Inc, Germany) that contains pentaerythritol triacrylate (greater than 95%) and 7-(diethlamino)-3-(2-thienylcarbonyl)-2H-l-benzopyran 2- one (less than 5%) was used as a negative photoresist for two photon lithography. 4- methylbenzenethiol (MBT, 98%), propylene glycol, monomethyl ether acetate, isopropyl alcohol, ethanol were purchased from Sigma- Aldrich chemical company. All chemicals were used without further purification, unless otherwise stated. Milli-Q water (greater than 18.0 ΜΩ .cm) was purified with a Sartorius Arium® 611 UV ultrapure water system.

[0099] Example 2: Fabrication of well-defined plasmonic structures

[00100] The fabrication method consists of two parts - (i) fabrication of a polymeric template, and (ii) deposition of Ag film.

[00101] To begin, polymeric nano and/or microstructures were fabricated using direct laser writing system (Nanoscribe Inc., Germany). In brief, a droplet of IP-L 780 monomer drop- casted on a glass substrate was polymerized by a computer-assisted femto second pulsed fiber laser with a center wavelength of 780 nm to form a polymer structure predefined by graphic program. The direct laser writing was performed using an inverted microscopy with an oil immersion lens (100 ^χ, NA1.4), and a computer-controlled piezoelectric stage. The average laser power was around 12 mW. A writing speed of 30 μιη/s was used. After writing, unexposed IP-L 780 was removed in propylene glycol monomethyl ether acetate for 30 min, and then washed with isopropyl alcohol for another 30 min. The polymeric structures were subsequently thermal evaporated with 2 nm chromium (Cr) and 150 nm silver (Ag) using thermal evaporation method.

[00102] Example 3 : Simulation [00103] Free standing polymer bases on a glass substrate are sputtered with about 150 nm Ag resulting in a nanowire like structure and also an aperture beneath. Since the distance between the nanowires, P, and the nanowire length, l_y, are large, compared to the illumination wavelength, the simulation of the electromagnetic fields is performed on an equivalent structure that is infinitely extended in the direction parallel to the nanowire axis and periodically extended in the direction perpendicular to it. Although the polymer bases are nearly rectangular in shape, the sputtering process results in the deposited silver layer on top of the polymer to assume a rounded shape as seen in the atomic force microscopy (AFM) measurements (FIG. 1C and D). A rounded shape is assumed for the top silver layer to match closely the experimentally observed morphology.

[00104] The structure is simulated with the frequency domain solver of CST microwave studio. Unit cell boundary conditions are assumed along the x and y directions and Floquet ports are used along the z direction. The periodicity along x is 2000 nm, the polymer layer is assumed to be 600 nm tall and 500 nm wide. The glass substrate and the polymer are both assumed to have a refractive index of n = 1.45 and fully dispersive permittivity data is used for Ag.

[00105] Example 4: High speed slit-scanning confocal Raman spectroscopy measurements

[00106] The plasmonic structures were incubated in 10 mM 4-MBT in ethanol solution overnight. After that, samples were removed and rinsed with copious of ethanol, and dried in nitrogen gas. Owing to a strong Ag-S coordination bond, 4-MBT is expected to form a self- assembled monolayer (SAM) on the Ag nanostructures. SERS spectra and SERS mapping were obtained with the sample mounted on the Ramantouch microspectrometer (Nanophoton Inc, Osaka, Japan). A 532 nm laser was used as an excitation laser. The excitation laser light was focused into a line on a sample through a cylindrical lens and an air objective lens (LU Plan Fluor 100 ^χ NA 0.9). The back-scattered Raman signal from the line illuminated site was collected with the same objective lens, and a one-dimensional Raman image (ID space and Raman spectra) was obtained with a two-dimensional image sensor (pixels 400 BR, -70 °C, 1340 x 400 pixels) at once. Two-dimensional (2D) Raman spectral images were obtained by scanning the line-shaped laser focus in a single direction.

[00107] At a single acquisition, line-shaped illumination is shone on the sample, where 400 Raman scattering points are then collected simultaneously in the x-direction. To a complete chemical imaging map, the laser confocal is shifted in the y-direction by scanning mirror, where another line scan is performed. The line illumination drastically reduces the acquisition time for x-y axis Raman mapping to less than half an hour for a 6400 μιη² area, as compared to the few hours required when using conventional Raman system. This makes the use of such ultrafast Raman system sufficiently competitive comparing to simple optical and/or colorimetric detection techniques, while offering much richer (molecular) information. Typical spectra of 4-MBT are shown in FIG. IE. The two characteristic peaks for 4-MBT at 1079 cm^"1 and 1594 cm^"1 can be clearly resolved. The peak at 1079 cm^-1 is due to a combination of the phenyl ring-breathing mode, CH in-plane bending, and CS stretching, while the peak at 1594 cm^-1 can be assigned to phenyl stretching vibrational mode.

[00108] The excitation laser power was 0.09 mW on the sample plane. The exposure time for each line and slit width of the spectrometer were 2 s and 50 μηι for 2D Raman imaging. The line scan mode with the resolution of y direction around 300 nm was used for x-y imaging. A half wave plate and a polarizer were used to change the polarization direction of laser from initial y direction to x direction.

[00109] Example 5; Characterization

[00110] Scanning electron microscopy (SEM) was performed using a JEOL-JSM-7600F with an accelerate voltage of 5 kV. 10-nm Pt was sputtered onto substrates to increase their conductivity for SEM imaging. The morphology and height profiles of the structures were measured using JPK Nanowizard 3 Bioscience atomic force microscopy (AFM) (Berlin, Germany) on a Nikon inverted microscope. The system was equipped with Ultra scanner head (max. scan size is 30 * 30 μπι² with z-range of 6.5 μιη) and 2-axes TAO stage with scan range of 100 ^χ 100 μη ². Silicon cantilevers from Nanosensors were used for AC mode (non- contact mode) operation. The typical free amplitude set-point of the cantilever was around 2 V, which is roughly around 40 nm to 50 nm. The slightly higher free amplitude used was to overcome the structure with feedback control maximized before the oscillation appear. Scan rate is varied from 0.3 Hz to 1 Hz.

[00111] Example 6; Results and Discussion

[00112] Plasmonic nano wires exhibit attractive security feature owing to its enhanced directional optical properties. Its one-dimensional morphology, i.e. nanometer-sized diameter and micrometer longitudinal length, has rendered anisotropic surface plasmonic and surface- enhanced Raman scattering responses with incident light at different polarizations. The polarization dependent SERS response is an excellent security feature of higher level security that has never been explored in anti-counterfeiting application.

[001 13] Structure of plasmonic nanowire disclosed herein is schematically illustrated in FIG. 2.

[001 14] Firstly, well defined polymeric nano wires are fabricated using two-photon lithography technique. The designed polymer lines have a diameter of 500 nm and length of 4 μηι (FIG. 3). The polymer structure is not suitable for SERS since it does not support surface plasmon responsible for the SERS effect.

[00115] In a second step, the polymeric nano wires are thermal evaporated with a 2-nm Cr film, followed by another 150-nm Ag film to render its plasmonic characteristic.

[001 16] As a start, simulation was used to verify the plasmonic responses of the nanowire at x- and y-polarizations, respectively; where x-polarization (0 = 0°, <→) and y-polarization (Θ = 90 ° , ) denote to the incident electric (E) field orientation is being perpendicular and parallel to the long axis of the nanowire, respectively (FIG. 2). A distinct polarization related difference in the response of the nanowire to a normally incident light is observed. When the E field is polarized in the x direction, a strong field enhancement is seen at the corners of the silver layers, at the top and the bottom layer (FIG. 2A and C). At the top layer, the field enhancement is related to the excitation of a localized surface plasmon resonance (LSPR). A time domain visualization of the fields shows that a surface wave propagates at the interface between the substrate and the deposited silver. In comparison, no localized field enhancement is seen for the electric field polarized along the y axis (FIG. 2B and D).

[001 17] A quantitative comparison of the localized field enhancement is provided by looking at the cross-sectional profile of electric field intensity enhancement (FIG. 2E and F). At x-polarization, the electric field distribution (FIG. 2E) indicates that sharp and intense electric field, with a about 12 x enhancement, at the edges of the Ag nanowire. In comparison, a modest about 2 x electric field enhancement is observed when scanned from y-polarization (FIG. 2F).

[001 18] Following this, slit-confocal SERS mapping on a single nanowire at x- and y-polarizations were performed to verify the simulation result. All of the plasmonic nano wires are vertically aligned during the Raman measurements, as illustrated as in FIG. 3 (A and B). 4-methylbezenethiol (4-MBT) was selected as the analyte molecules for the Raman measurement. It is an aromatic thiol that forms a self-assembled monolayer (SAM) on metal via metal-thiol coordination bond, allowing more accurate calculation of the number of molecules on the surface of plasmonic nanowires contributing to SERS response. For fast and efficient SERS chemical imaging, high speed slit confocal Raman imaging is performed, where the embedded chemical vibrational information may be rapidly read-out spatially and spectroscopically in a matter of less than 30 min (FIG. 2C to E). From here onwards, the integral Raman intensity at 1079 cm^"1 peak was used, corresponding to the combination of phenyl ring-breathing mode, CH in-plane bending, and CS stretching for the 2D SERS mapping, and refer to the selected peak as the Raman intensity, unless otherwise stated.

[001 19] The 2D Raman map of the single plasmonic nanowire at x-polarization (FIG. 3C) demonstrated that bright areas, which indicated strong Raman signals, were concentrated at the edges and tips of the Ag nanowire. However, at y-polarization (FIG. 3D), only a few pixels at the tips of Ag nanowire exhibited Raman intensities compared to that of the Ag nanowire. body. Quantitative Raman intensity comparison in FIG. 3E once again indicated that strongest Raman signals were observed at the edges (at x polarization) and tips (both x and y polarizations) of Ag nanowires.

[00120] The Raman cross-sectional profile at the x-polarization also demonstrated two strong SERS signals at the edges of the Ag nanowires (FIG. 3F-Raman), whereas negligible enhancement was observed when polarized at y-axis (FIG. 3G-Raman). The SERS enhancement factor of 10⁵ has been achieved at the edges, as obtained by the calculation as followed.

[00121] The experimental Raman enhancement factor (EF) for the Ag coated polymer

^ SERS

^ SERS

I ref

substrate is estimated using the equation: EF = ^ref , where ISERS s the intensity of the specific Raman band from the analyte adsorbed on a SERS active substrate, and NSERS is the number of molecules contributing to ISERS- Similarly, I_ief is the intensity of the same Raman band from the bulk analyte, normalized with the laser power and acquisition time, and N_ref is the number of molecules that yield I_ref.

[00122] Here, 1079 cm^"1 Raman band is selected for EF calculation. For a more comprehensive estimate of EF, average intensity from 10 sample points of the substrate were used. The average ISERS is around 337.8 counts per second (cps). The radius of the diffraction-limited laser beam is obtained by measuring the intensity profile of 100 nm fluorescence beads. The diameter of the laser beam is measured at about 520 nm, and the area is A = 7P<r² = 2.12 10⁵ nm2. Taking 4.5 x 10¹⁴ molecules/cm² for a monolayer of 4-MBT on silver (assume the binding of 4-MBT on silver is similar to that of gold), the number of molecules excited under the laser spot is approximated at 9.56 x lO⁵ molecules.

[00123] The average I_ref is measured at 0.57 cps using an 1 M 4-MBT ethanolic solution as a reference solution. For the determination of 7V_ref, the confocal volume is obtained by measuring the intensity profile of fluorescence beads immersed in ethanol solution, which simulates the condition of normal Raman signal (I_ref) measurement. The measured laser diameter and the focal depth (h) are 910 nm and 4320 nm, respectively. Effective excitation volume (V) i V = A*h = 2.81 10⁹ nm³. The larger confocal volume in ethanol solution than that in ideal dry condition is caused by the distortion of laser spot in solution. The number of 4-MBT molecules in ethanol (1 molxL^"1) excited by the laser beam is therefore N_ref= 1.69 x 10⁹ molecules. The average enhancement factor, based on mean intensity from 10 sample points, is estimated to be about 1.05 10⁶.

[00124] The result is in good agreement with simulated electric field cross-sectional profile in FIG. 2(E-F- Simulation) disclosed herein. The results validate the simulation results that the edges of Ag nanowire support localized surface plasmon resonance and localize intense electric field surrounding them. The tip of Ag nanowire with an isotropic hemispheric morphology (FIG. 3C to D) functioned as an antenna with identical localized surface plasmon resonance in all polarization directions, contributed to strong SERS hot spots in both x and y polarizations.

[00125] At y-polarization, the Raman signal at the body of Ag nanowire (FIG. 3F and E- 3) was significantly weaker than the tips, indicating minimal excitation of localized surface plasmon resonance. There was a negligible SERS background from the substrate (FIG. 3D and E-4), as already predicted in the simulation in FIG. 2. The result indicated that the Ag film of 3.5-nm root-mean-square roughness was not enough to produce significant electromagnetic field enhancement (FIG. 1A and B).

[00126] Generally, the simulated and experimental results demonstrated that Ag nanowire possessed advanced covert security feature, i.e. its SERS response can be selectively read-out by manipulating the polarization angle of incident light. The chemical information, in the form of Raman vibrational spectrum, is only turned "on" when the plasmonic nanowires were coupled with light at the transverse axis, resulting in strong electromagnetic■ field and enhanced SERS intensity at the edges of the nanostructures.

[00127] The ability to tune and prescribe the Raman intensity from "off to "on" is the foundation for the design of the "nanostructured SERS marker" disclosed herein. Nanostructures with customized polarization-dependent Raman scattering response were designed and fabricated. Despite being coated with a homogeneous monolayer of molecules over the entire surface, the encrypted chemical information may only be authenticated by polarized Raman chemical imaging, and not by simple visualization.

[00128] Herein, four examples of tailored made plasmonic nanostructures from simple shape orthogonal line pattern to more complicated information encryption were presented.

[00129] Example 6.1 : Horizontal and vertical Ag nanorod array in an alternating fashion

[00130] In the first example, horizontal and vertical Ag nanorod array in an alternating fashion (FIG. 4A) were designed and fabricated. Under the x-polarization, only the vertical lines are coupled to the incident light, hence yield higher SERS intensity. In y-polarization, the effect is reversed, where only horizontal lines exhibit strong SERS intensity (FIG. 4A).

[00131] Example 6.2: Structure made of circular and vertical Ag nanowires

[00132] The second example is a structure made of circular and vertical Ag nanowires. Without wishing to be bound by theory, the hypothesis is that circular nanowires do not have polarization-dependence behavior and always exhibit "bright" image at both polarizations. By adding non-polarization dependent circular nanowires in the design (FIG. 4B), possibility of the chemical information encrypted in structures disclosed herein had been broadened. At x-polarization, the vertical nanowires in FIG. 4B-ii appeared "bright', giving rise to unique double bowtie pattern. Contrary, the vertical nanowires were Raman inactive under y-polarized excitation; hence, single bowtie pattern was displayed (FIG. 4B-iii). It should be noted that, a careful look at the Raman imaging (FIG. 4B-H and iii) indicated that the circular lines actually exhibit dipole moment oriented along the respective laser polarization direction. The dipole of circular lines yielded very strong Raman intensity, giving rise to bright Raman imaging at both polarizations.

[00133] Example 6.3: Alphabet "A"

[00134] In addition to graphical patterns, capability disclosed herein was further extended by encrypting chemical information within alphabets. FIG. 4C shows a structure superimposed with two alphabets 'A's, with the upright 'A' consisted of horizontal lines only, and the inverted 'A' was written using vertical lines only. Using x-polarized SERS mapping, the vertically lined inverted 'A' may be clearly read-out, and the upright 'A' was invisible. Conversely, when polarized along y-axis, only the chemical imaging of upright 'A' was apparent. The inverted Ά', despite having the same molecular monolayer on its surface remained 'invisible' owing to the selective plasmonic coupling of Ag nanowire with the polarization angle of incident light .

[00135] Example 6.4: Alphabets

[00136] Based on the above mentioned design, it is possible to have more sophisticated encryption of chemical information in the plasmonic structure. A plasmonic structure that consists of alphabets 'CBC of vertical lines overlaid on alphabets 'NTU' of horizontal lines was constructed (FIG. 4D). 'CBC and 'NTU' are the acronyms of the division of Chemistry and Biological Chemistry, and Nanyang Technological University, respectively. From the SEM imaging, the alphabetical information embedded within the structure cannot be distinguished. Only via incident light polarized Raman imaging, the alphabetical 'CBC and 'NTU' may be clearly distinguished independently by x- and y-polarizations, respectively.

[00137] As disclosed herein, a few examples of Ag nanowire structures from simple cross line patterns to more sophisticated encryption of information by SERS imaging of MBT molecules have been demonstrated. Technique disclosed herein is simple but versatile, and may be easily applied to any encryption design (pattern, shape or information) and can be potentially serve as 3 -dimensional chemical information storage. With the combination of fabrication and plasmonic structures, SERS chemical imaging as anti-counterfeit has been demonstrated. It may be readily incorporated with current visual based system but is more complicated to forge.

[00138] Example 7: Line arrays with different orientation angles - a good platform for polarization SERS research

[00139] Having examined the. SERS response at x- and y-polarizations, SERS intensity with respect to the change in orientation angle of Ag nanowire in the range from Θ = 0° - 90° was examined.

2 2

[00140] The simulated SERS intensity was anticipated to follow the cos Θ and sin Θ relationship for x and y polarization, respectively, with respect to the orientation of the nanostructure (FIG. 5A). This is because the SERS intensity is generally attributable to

, where E(w) and E(w_s) are the enhanced local fields at the excitation and scattering frequencies, respectively and E₀ is the incident field. At incident polarization angle (Θ), the local electric field is E(w,6) = E(w).cos(9) for x polarization, and E(w,0)=E(w).sin(9) for y polarization, respectively. Since induced Raman scattering is isotropic, E(w_s) is polarization independent. For example, at polarization angle Θ = 0°, the SERS intensity is I_x,e = o° ⁼ Ιχ cos²(0°) = I_x, which is the maximum SERS intensity at x- polarization. Whereas at Θ = 30°, I_x, ₀ = 30° = Ιχ cos²(30°) = 0.74I_X. Consequently, a dynamic gradient of SERS intensities are expected at both polarization angles.

[00141] A series of Ag nanowire arrays with orientation angles ranging from Θ = 0°, 15°, 30°, 45°, 60°, 75°, and 90° was fabricated using two photon lithography (FIG. 5A). The 2D SERS mapping (FIG. 5B) and respective Raman spectra (FIG. 5D) of Ag nanowire arrays at x polarization at various orientation angles demonstrated that the Raman map of 4-MBT molecules gradually changed from the brightest (Θ = 0°) to very dim (0 = 90°), indicating the progressive reduction of SERS intensity reduced from its maximum to near zero from 0 = 0° to 90° (FIG. 5B). At x-polarization, the plot of normalized Raman intensity, obtained from the longitudinal body of nanowires at different orientation angles (excluding the Raman intensity from the respective tips), against the nanowires' orientation angle (Θ) demonstrated a good fit to cos Θ function (FIG. 5F). Similarly, at y-polarization, the trend of SERS intensity with respect to nanowire orientation was reversed (FIG. 5C and 5E), and a sin²0 function fitting (FIG. 5G) was found for the plot of normalized SERS intensity and nanowires' orientation angle.

[00142] Based on results obtained in FIG. 5(F, G), an intensity gradient of SERS response that may be tunable by the orientation of the Ag nanowire array was obtained. The variation in intensity also validated simulation studies disclosed in FIG. 2E and 2F. It highlighted that a dynamic range of SERS intensity may be realized by changing the orientation of Ag nanowires and the polarization of the incident laser. The ability to manipulate the SERS response of plasmonic nanowires disclosed herein through design may be crucial for anticounterfeiting. This provided additional design advantage that is not restricted to the "bright" or "dark" chemical images but instead towards a gray "chemical" scale.

[00143] Example 8; Concentric triangle, square, pentagon, hexagon, octagon [00144] The tunable Raman intensity study disclosed herein was extended to more complicated structures, such as concentric triangle, square, pentagon, hexagon, and octagon. The design strategy was firstly, to demonstrate a predictable change of Raman signal within a single concentric structure based upon that these structures may be decomposed into lines with different orientation angles with respect to incident laser. Secondly, to demonstrate the Ag nanowires as the basic building block that may be bottom up to very complicated structures, and yet the polarization dependent Raman signal rule still applied.

[00145] The SEM images of triangle, square, pentagon, hexagon, octagon structures after evaporation with Ag are shown in FIG. 6(i). The insets demonstrated the deconvoluted nanowire components at various orientation angles in the respective concentric structures. The width of nanowires was about 400 nm, and the nanowire periodicity was about 2 μπι for easy visualization without the plasmonic coupling between nanowires. These are structures may be easily fabricated using two-photon lithography but challenging using normal wet chemical synthesis and assembly techniques.

[00146] The SERS mapping of each structure at x and y polarizations are shown in FIG. 6(ii-iii), respectively. By tailoring the orientation of the nanowire array, a range of Raman chemical imaging with predictable intensity was designed, and the Raman maps were presented in dynamic Raman intensities according to ISERS oc cos²0 for x polarization, and ISERS oc sin²9 for y polarization. For instance, the concentric triangle nanowire structure (FIG. 6A) consisted of three nanowire components at orientation angles of 30°, 90°, and 150°, respectively. At x-polarized excitation, the Raman map (FIG. 6A-ii) indicated that only the nanowires at 30° and 150° exhibited detectable Raman intensity, whereas the nanowires oriented at 90° are "Raman invisible". Quantitative Raman intensity profile (FIG. 6F) validates that the Raman intensity of nanowires at 30° and 150° are identical, with their Raman intensities about 0.75 times of that the maximum normalized Raman intensity (I_max)- The result is in good agreement with the theoretical calculation of I = I_x.cos (30°) = I_x.cos²(150 ) = 0.75 I_x. At y-polarization, nanowires at 30° and 150° exhibit weak Raman intensity, with their Raman intensities, I being about 0.25I_y while nanowires reach I_max when oriented at 90° (FIG. 6G). The result matches well to I_y.sin²(30°) = I_y.sin²(150°) = I_y.0.25 and I_y.sin²(90°) = I_y = I_max.

[00147] Similarly, square structure can be decomposed into nanowires with orientation angles of 0° and 90° (FIG. 6B). Hence, only two types of Raman intensities are expected, i.e. x-polarized incident light, only the nanowires oriented at 0° show maximum Raman intensities (FIG. 6B- ii), whereas the one oriented at 90° exhibits undetectable Raman responses (FIG. 6B- iii), and vice versa. Further increasing the number of sides of a polygon increases the number of orientation in the nanowire structures, and a more dynamic Raman intensity range is projected. For a pentagonal nanowire structure (FIG. 6C), the nanowires are aligned at 18°, 54°, 90°, 126°, and 162°, respectively. In principle, the total orientation angles can be coupled into three pairs, i.e. 18° (18° and 162°), 54° (54° and 126°), and 90°. Hence, three Raman intensities are predicted. At x-polarization, three normalized Raman intensities may be obtained, i.e. about 0.91I_max, about 0.34I_max, 0 (FIG. 6C - ii and FIG. 6F). In contrary, at y-polarization, about 0.096I_max, about 0.65I_max, and I_max may be obtained at the 18° (18° and 162°), 54° (54° and 126°), and 90° oriented nanowires (FIG. 6C- iii and FIG. 6G). Hexagonal nanowire structure consists of two sets of symmetrical nanowires at 30° (30° and 150°), and 90°. Two Raman intensities at 0.75I_max, and 0 were obtained experimentally at x-polarization, which are also predicted theoretically (FIG. 6D-ii and FIG. 6F). At y- polarization, 0.25I_max, and I_raax were obtained at the 30° (150°), and 90° oriented nanowires (FIG. 6D-iii and FIG. 6G), respectively. The Ag nanowires in an octagon may be decomposed into 0°, 45°(45°, 135°), and 90° corresponding to Raman intensities of I_max, 0.5I_max, and 0 when incident light is polarized at x-axis (FIG. 6E-ii), and vice versa when change to y-polarization (FIG. 6E-iii).

[00148] A 2D dynamic chemical imaging based on Raman molecular information embedded on Ag nanowires with tunable orientation angle has been constructed. Intensity of the chemical (Raman) image was tunable according to the orientation angle of the Ag nanowire structure and the Raman incident laser polarization, as demonstrated in the FIG. 6F and 6G. Such dynamic Raman intensity range in Ag nanowire is different from the basic color change (from red to green, or vice versa) in nanorods upon change in polarization. A higher level of security is introduced with the intensity of the embedded chemical information can be manipulated by the orientation angle of the Ag nanowires and polarization angle of light. Nevertheless, the system disclosed herein may be complemented and extended to nanorod systems to become a security label that respond to color and chemical Raman intensity change upon polarization of incident light. This is a simple yet elegant technique that is yet to be explored. [00149] The combination of two photon lithography based nano fabrication tools for the fabrication of plasmonic nanowires and polarization-dependent tuning of Raman signal provided an opportunity for anti-counterfeiting application. The Ag nanowires may be an attractive security labels because both nanofabrication tool and detection technique (SERS) are highly sophisticated that rely on cutting-edge fabrication instrument and accurate design protocol to produce reproducible and reliable security labels and SERS signals. In addition, the molecular information encrypted within the Ag nanowires is highly specific, and correspond explicitly to specific structural orientation of the Ag nanowires and polarization angle of incident light. Such potential SERS anti-counterfeiting substrate cannot be easily re- produced with accuracy by counterfeiter(s).

[00150] In summary, SERS substrates of single line, line arrays, concentric structures by two-photon lithography followed by thermal evaporation technique were obtained, and their application in systematic polarization dependent SERS research by using 4-MBT as the probe molecule was demonstrated. The experimental results show that the dependence of SERS intensity and the relative angle between the transverse axis of lines and a given laser polarization can be fitted to cos²9 distribution when the incident laser is in x direction and can be fitted to sin Θ when the laser is polarized along y direction. Capability of the polarization dependent SERS mapping for future application as anti-counterfeit labels was demonstrated.

[00151] Example 9: Application of tunable gradient of SERS intensity

[00152] The tunable gradient of SERS intensities obtained in nanowire structures may be used as the basic data storage element (or bit) for multiple-bit 2D "plasmonic molecular information storage". The results shown in FIG. 5, FIG. 6 and FIG. 7 highlight that, despite having similar physical morphology and homogeneously coated with a monolayer of probe molecules, the Ag nanowires of different orientation exhibit different "bit" of SERS intensities that are dependent on their orientation angle and polarization of the incident light. Hence, a SERS molecular image with multiple shades of SERS intensities may be obtained simply by exploiting their polarization-dependent SERS responses. The directional light- matter interaction between plasmonic nanowire's structural orientations with respect to incident light was exploited to achieve a dynamic range of grayscale SERS intensities.

[00153] To demonstrate the flexibility plasmonic nano structures for multiple-bit data storage with enhanced spatial information, a tessellated pattern consisting of twelve Escher's reptiles using a combination of Ag nanowires oriented at Θ = 0°, 45°, and 90° was fabricated (FIG. 9). Both SEM image and darkfield optical micrograph (FIG. 9A, G) of the tessellated pattern yielded indistinguishable images with no physical feature and were not able to reveal the distinct molecular information stored within the structures. Using SERS imaging, a microstructure with rich molecular and spatial information was revealed. The tessellated microstructure may be decoded into three categories of reptiles carrying different bit of SERS intensities, i.e. the reptiles fabricated using vertical, 45°-oriented, horizontal nanowires exhibited I_raax, 0.5I_max, and 0 SERS signals, respectively (FIG. 9D). Furthermore, by simply switching the orientation of nanowires in the same tessellated microstructure (FIG. 9B, C), completely different SERS images carrying different bit of SERS intensities and spatial information may be obtained (FIG. 9E, F). This highlights the flexibility and reproducibility of our two-photon lithography tool to fabricate different micro/nanostructures over a large area for molecular information storage. In addition, the seemingly similar physical appearance of tessellated microstructure encoded with different molecular information can be potentially useful for anti-counterfeiting application.

[00154] Example 10: Materials

[00155] IP-L 780 photoresist with refractive index, n « 1.485 and IP -Dip resist with n « 1.52 (Nanoscribe Inc, Germany) were used as a negative photoresist for two photon lithography in direct laser writing (DLW) and dip-in laser lithography (DiLL) configuration, separately. 4-methylbenzenethiol (4-MBT, 98%), rhodamine B isothicocyanate (RhBITC), lH,lH,2H,2H-perfluorodecyltriethoxysilane ( > 98%), propylene glycol monomethyl ether acetate, isopropyl alcohol, ethanol were purchased from Sigma-Aldrich chemical company. All chemicals were used without further purification, unless otherwise stated. Milli-Q water (greater than 18.0 ΜΩ . cm) was purified with a Sartorius Arium® 611 UV ultrapure water system.

[00156] Example 11: Fabrication of plasmonic anti-counterfeitine structures

[00157] The fabrication method consists of two parts - the fabrication of a polymeric template, and the deposition of Ag film. To begin, polymeric nano and/or microstructures were fabricated using direct laser writing system (Nanoscribe Inc., Germany). In brief, a droplet of IP-L 780 or IP -Dip monomer drop-casted on a glass substrate was polymerized by a computer-assisted femtosecond pulsed fiber laser with a center wavelength of 780 nm to form a polymer structure pre-defined by graphic program. The DLW was performed using an inverted microscopy with an oil immersion lens (100 x, numerical aperture (NA) 1.4), and a computer-controlled piezoelectric stage. DiLL technique involved the use of an oil immersion lens (100 x, NA 1.3), instead. The average laser power was around 12 mW for DLW and 6 mW for DiLL. A writing speed of 30 μχη/s was used. After writing, unexposed IP-L 780 was removed in propylene glycol monomethyl ether acetate for 30 min, and then washed with isopropyl alcohol for another 30 min. Ag film was deposited on the substrates using a home- built thermal evaporator deposition system. The deposition rate Ag was 0.5 A/s, which was monitored in-situ by a quartz crystal microbalance. Ag target with 99.99 % purity was purchased from Advent Research Materials, UK.

[00158] Example 12: High speed slit-scanning confocal Raman spectroscopy measurements

[00159] The plasmonic structures were incubated in 100 mM 4-MBT in ethanol solution for 6 hours. After that, samples were removed and rinsed with copious amounts of ethanol, and dried in nitrogen gas. Owing to a strong Ag-S coordination bond, 4-MBT is expected to form a self-assembled monolayer (SAM) on the Ag nanostructures. Another layer of Ag film with various thicknesses was coated again to form a sandwich structure. SERS spectra and SERS mapping were obtained with the sample mounted on the Ramantouch microspectrometer (Nanophoton Inc, Osaka, Japan). A 532 nm laser was used as an excitation laser. The excitation laser light was focused into a line on a sample through a cylindrical lens and an air objective lens (LU Plan Fluor 100 ^χ NA 0.9). The back-scattered Raman signal from the line illuminated site was collected with the same objective lens, and a one-dimensional Raman image (ID space and Raman spectra) was obtained with a two- dimensional image sensor (Princeton Instrument, PIXIS 400 BR, -70 °C, 1340 400 pixels) at once. At a single acquisition, line-shaped illumination is shone on the sample, where 400 Raman scattering points are then collected simultaneously in the x-direction. Two- dimensional (2D) Raman spectral images were obtained by scanning the line-shaped laser focus in a single direction. The line illumination drastically reduces the acquisition time for x-y axis Raman mapping to less than half an hour for a 6400 μτη² area, as compared to the few hours required when using conventional Raman system. The excitation laser power was 0.09 mW on the sample plane. The exposure time for each line and slit width of the spectrometer were 5 s and 50 μιη for 2D Raman imaging respectively. The line scan mode with the resolution of y direction around 300 nm was used for x-y imaging. A half wave plate and a polarizer were used to change the polarization direction of laser from initial y direction to x direction. The SERS intensities are obtained from the longitudinal body of nanowires at different orientation angles (excluding the Raman intensity from the respective tips).

[00160] Example 13: Characterization

[00161] Scanning electron microscopy (SEM) was performed using a JEOL-JSM-7600F with an accelerate voltage of 5 kV. 10-nm Pt was sputtered onto substrates to increase their conductivity for SEM imaging.

[00162] Example 14: Results and Discussion

[00163] A sandwich structure is designed to realize the bimolecular SERS anti- counterfeiting technology. The nanowire structures are first fabricated using 2-photon lithography via a direct laser-writing process. These nanowires are then metalized via a thermal evaporation process which coats the nanowires with 100 nm-thick Ag. The probe molecules, 4-methylbenzenethiol (4-MBT), are then functionalized onto the metalized nanowires via a ligand exchange process. A second metallic layer is then thermally evaporated over the first Ag layer coated with 4-MBT to create the sandwich structure. Such a sandwich structure can lead to stronger SERS signals and at the same time enable a second probe molecule to be attached onto the second Ag layer. It is demonstrated herein that both of these advantages lead to a better security label with increased sensitivity and complexity.

[00164] In the first part of this study, influence of the thickness of the second Ag layer on the SERS intensity of 4-MBT was investigated. Arrays of vertical and horizontal lines were used for this investigation (FIG. 10A), with the thickness of the second Ag layer varied over the range of 10 ran, 30 nm, 50 nm, 70 nm, 100 nm, and 120 nm. The resulting sandwich structures were labeled as Ag/4-MBT/Ag. Use of both horizontal and vertical lines allowed testing if the polarization-dependence of the SERS signals on the nanowires will be affected by the sandwich structure. The x-y SERS images of the Ag/4-MBT/Ag sandwich structures created using the 1078 cm^"1 peak of 4-MBT at x-polarization showed that the sandwich structures do not disrupt the SERS polarization-dependence of the nanowire arrays (FIG. 10(B)-i to (H)-i). The vertical lines show strong SERS response due to the excitation of LSPR at x-polarization. On the other hand, SERS intensities were much weaker for the horizontal lines because of the momentum mismatch between incident photon with the propagating plasmons. In addition to the SERS images, the SERS intensity profiles along the vertical lines (along x axis) and horizontal lines (along y axis) are shown in FIG. 10(B)-ii to (H)-ii. Average SERS signals along vertical lines and horizontal lines increased with increasing thickness of second layer of Ag film. The much stronger SERS intensity along vertical lines was also clearly evident. In addition, SERS intensities remain consistent across the various nanowires in the same array.

[00165] The SERS spectra from both horizontal and vertical nanowire arrays with different second Ag layer thickness are shown in FIG. 11 A and B. It has been found herein that SERS intensities increase with increasing thickness of the second Ag layer up to 100 ran of second Ag layer coated for both the horizontal and vertical nanowire arrays (FIG. 11C and D). At 100 nm of second Ag layer coating, the SERS signals are enhanced 117-fold for the vertical nanowire array and 36-fold for the horizontal nanowire array. A decrease in SERS intensity is observed when the thickness of second Ag layer coating is increased to 120 nm. The sandwich structure leads to a significant enhancement of SERS signals as compared to a single Ag layer. This phenomenon arises from the creation of additional hotspots brought about by the deposition of the second Ag layer. The small size of 4-MBT (less than 1 nm from first Ag layer to -CH₃ group) implies that the second Ag layer is very closely spaced from the first Ag layer in the sandwich structure. Consequently, plasmon coupling between these two Ag layers leads to a strong enhancement of the local electromagnetic fields, giving rise to intense SERS signals. Furthermore, the thermal evaporation process creates nanoscale asperities on the nanowire surface and this serves to further enhance the SERS signals from the sandwich structures.

[00166] Next, a micro-panda structure to demonstrate a homogeneous bimolecular SERS anticounterfeiting capability of the sandwich structure was designed (FIG. 12A). This structure was composed of horizontal and vertical lines with the structure remaining covert under normal characterization techniques. The first Ag layer thickness was fixed at 100 nm, and the second Ag layer thickness was fixed at 50 nm thick. 4-MBT was chosen as the probe molecule for the first Ag layer and rhodamine B isothiocynanate (RhBITC) was used as the probe molecule for the second Ag layer. Similar to the Ag-S affinity, the isothiocyanate group (N=C=S) also exhibited affinity for the Ag surface and may be used to prepare a monolayer of RhBITC onto the second Ag layer with the formation of the Ag-S bond. 4- MBT was introduced to the first Ag layer after the first metallization step; a second metallization step then took place after this ligand exchange process, followed by the RhBITC functionalization. After the RhBITC functionalization, a further 50 nm Ag layer was thermally evaporated onto the nanowires to create a double sandwich structure. As with earlier measurements, the 1078 cm^"1 peak of 4-MBT was used to map the SERS image from the first Ag layer. With the addition of RhBITC on the second Ag layer, the 1647 cm^"1 peak from RhBITC may be used to map the SERS image from the second Ag layer since this peak was unique to RhBITC {vide infra).

[00167] In the presence of one Ag layer with just 4-MBT present as the single probe molecule, only a single SERS image of the panda showed up when the 1078 cm^"1 peak was selected (FIG. 12B). Selecting the peak at 1647 cm^"1 did not produce the panda pattern. In this image formed using the 1078 cm^"1 of 4-MBT, the features of a smiling panda can be distinguished, unlike the covert SEM pattern shown in FIG. 12 A. With the addition of a second Ag layer functionalized with RhBITC, two images may be produced by selecting the 1078 cm^"1 and 1647 cm^"1 peaks respectively (FIG. 12C). The composite SERS spectrum of the sandwich structure (FIG. 12D, green line) showed a series of distinct peaks arising from both 4-MBT and RhBITC. A comparison with the individual SERS spectra of 4-MBT (FIG. 12D, red line) and RhBITC (FIG. 12E) showed no overlap between the two probe molecules at 1078 cm^"1 and 1651 cm^"1. Consequently, this enabled the bimolecular counterfeiting technology to work seamlessly in the sandwich setup disclosed herein.

[00168] Two major advantages were demonstrated using this simple bimolecular counterfeiting technique in conjunction with a sandwich design.

[00169] Firstly, the composite SERS spectrum was much more complex than either of the individual SERS spectrum of 4-MBT and RhBITC. This increase in spectral complexity further enhances security labeling. Without the knowledge of molecule choice, it may be extremely challenging for counterfeiters to resolve the SERS spectra to deduce the number and type of probe molecules used.

[00170] The second advantage relates to the narrow bandwidth of the SERS bands. By itself, 4-MBT exhibits a SERS peak at about 1600 cm^"1. Yet, selecting at 1647 cm^"1 for the single sandwich structure did not produce the panda image. It was only when an actual peak from RhBITC is present at 1647 cm^"1 that allows the second panda image to be produced. This peak difference of 47 cm^"1 corresponds to a mere 2 nm difference in spectral positions of the two vibrational modes. As a result, such a high spectral resolution gives rise to an ultrasensitive anti-counterfeiting technology that cannot be achieved with other colorimetry- based optical techniques. [00171] In addition to the homogeneous bimolecular SERS anti-counterfeiting technology demonstrated, the anti-counterfeiting technology of the sandwich platform disclosed herein was further enhanced by creating a heterogeneous anti-counterfeiting technique. In the homogeneous platform, the number of patterns created with two probe molecules was limited to just one. In contrast, the heterogeneous platform allowed fabrication of additional security features that only show up in the full sandwich structure. This capability was demonstrated by the inventors using the merlion symbol and the dove with olive branch icon. Direct laser writing technique was used to create the base pattern with an array of horizontal and vertical nanowires (FIG. 13A). This laser writing process gave rise to the merlion structure (FIG. 13C-i) and a dove with an olive branch (FIG. 13E-i). The nanowire structures were then metallized with 100 nm of Ag, followed by the functionalization of 4-MBT. A second layer of 30 nm thick Ag was thermally evaporated onto the 4-MBT functionalized first Ag layer to create the first sandwich layer. This metallization process caused the glass coverslip to become opaque. In the second step, dip-in laser lithography (DiLL) technique was used to fabricate the second part of the security feature (FIG. 13B). DiLL is a patent-pending technique developed by the company Nanoscribe GmBH and it allows additional features to be fabricated on opaque substrates. The second half of the pattern includes the water stream gushing from the merlion's mouth (FIG. 13D-i) and a second dove (FIG. 13F-i). The fabricated structures were then metallized with a 30 nm thick Ag layer, functionalized with RhBITC, followed by another metallization process to complete the second sandwich structure.

[00172] SERS imaging was then conducted using the 1078 cm^"1 peak for 4-MBT and 1647 cm^"1 peak for RhBITC. Consistent with earlier observations, the SERS images at 1078 cm^"1 gave rise to distinct features of the merlion (FIG. 13C-U) and the dove with the olive branch (FIG. 13E-U) which were indistinguishable in their respective SEM images. No feature shows up at 1647 cm^"1 after the fabrication of the first sandwich structure (FIG. 13C-iii and E-iii). In addition, no additional feature from the second step of structure fabrication (the gushing water stream and second dove) were observed at 1078 cm^"1 even after the second sandwich structure was fabricated (FIG. 13D-ii and 13F-U). On the other hand, the merlion with the gushing water stream (FIG. 13D-iii) and two doves with an olive branch (FIG. 13F- iii) were observed at 1647 cm^"1 for the double sandwich structure. These data successfully demonstrated the added security of our heterogeneous bimolecular anti-counterfeiting SERS- based technology, with additional features showing up in a second spectral window.

[00173] In summary, a bimolecular SERS-based anti-counterfeiting technology using a sandwich structure was demonstrated. With use of Ag nanowire arrays, both homogeneous and heterogeneous sandwich structures with unique readouts that are much more secure compared to state of the art anti-counterfeiting technique were fabricated.

[00174] The sandwich structure enables the use of two probe molecules with non- overlapping SERS peaks so that different peaks may be selected to construct individual images in the homogeneous platform. Additional features may also be fabricated on the same platform to create a heterogeneous substrate with different peaks giving rise to different structures. Such a bimolecular anti-counterfeiting approach complicated the process of deciphering the number and type of probe molecules used to embed the molecular information. The complication arises from the composite SERS spectra of both molecules, which is much more sophisticated than the individual SERS spectrum of the respective molecules. In addition, the spectral sensitivity is ultrasensitive, with the capability to resolve peaks that are less than 2 nm apart from each other. Such high sensitivity has yet to be demonstrated using conventional colorimetry-based optical detection techniques.

[00175] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Claims

Surface enhanced Raman scattering (SERS)-active device comprising at least one SERS-active nanostracture attached to a substrate, wherein arrangement of the at least one SERS-active nanostracture on the substrate is adapted to provide a surface enhanced Raman signal having an intensity that is tunable depending on at least one of (i) orientation angle of the at least one SERS-active nanostracture, (ii) polarization of incident polarized light, and (iii) wavelength of the incident polarized light.

SERS-active device according to claim 1, wherein the at least one SERS-active nanostracture comprises or consists of one or more nanowires.

SERS-active device according to claim 1 or 2, wherein the at least one SERS-active nanostracture comprises a first metal layer arranged on the substrate and a first Raman-active marker compound layer arranged on the first metal layer.

SERS-active device according to claim 3, wherein the at least one SERS-active nanostracture comprises a second metal layer arranged on the first Raman-active marker compound layer.

SERS-active device according to claim 4, wherein the at least one SERS-active nanostracture comprises a second Raman-active marker compound layer deposited on the second metal layer and a third metal layer deposited on the second Raman-active marker compound layer.

SERS-active device according to any one of claims 3 to 5, wherein the first metal layer, the second metal layer, and the third metal layer independently comprise or consist of silver.

SERS-active device according to any one of claims 3 to 6, wherein thickness of the first metal layer, the second metal layer, and the third metal layer are independently in the range from about 30 nm to about 120 nm.

8. SERS-active device according to any one of claims 3 to 7, wherein thickness of the first metal layer is about 100 nm.

9. SERS-active device according to any one of claims 4 to 8, wherein thickness of the second metal layer is about 80 nm to about 120 nm.

10. SERS-active device according to any one of claims 3 to 9, wherein the first Raman- active marker compound and the second Raman-active marker compound have a different signature vibrational mode.

11. SERS-active device according to any one of claims 3 to 10, wherein the first Raman- active marker compound and the second Raman-active marker compound are independently selected from the group consisting of 4-methylbenzenethiol, rhodamine B isothiocyanate, 2-napthalenethiol, methylene blue, coumarin, melamine, and combinations thereof.

12. SERS-active device according to any one of claims 3 to 1 1, wherein the first Raman- active marker compound and the second Raman-active marker compound independently form a self-assembled monolayer on the first metal layer and the second metal layer, respectively.

13. SERS-active device according to any one of claims 1 to 12, wherein the incident polarized light comprises or consists of a wavelength corresponding to a peak wavelength of the SERS-active nanostructure.

14. SERS-active device according to any one of claims 1 to 13, wherein arrangement of the at least one SERS-active nanostructure on the substrate is adapted to provide a surface enhanced Raman signal of a signature intensity based on at least one of (i) orientation angle of the at least one SERS-active nanostructure, (ii) polarization of incident polarized light, and (iii) wavelength of the incident polarized light.

15. SERS-active device according to any one of claims 1 to 14, wherein the SERS-active device forms at least part of (i) a data storage medium, or (ii) an identification tag for identifying an object.

16. An identification tag comprising a SERS-active device according to any one of claims 1 to 15.

17. Method of identifying an object comprising an identification tag according to claim 16, the method comprising

a) obtaining a surface enhanced Raman signal from the identification tag;

b) checking the obtained signal against a reference signature, and determining degree of overlap or similarity between the obtained signal and the reference signature.

18. Method according to claim 17, wherein identity of the object is verified if values of the obtained signal differ from corresponding values of the reference signature by less than a predetermined threshold.

19. Method according to claim 17 or 18, wherein obtaining a surface enhanced Raman signal from the identification tag comprises irradiating the at least one SERS-active nanostructure with a polarized light.

20. Method according to claim 19, wherein the polarized light comprises or consists of a wavelength corresponding to a peak wavelength of the SERS-active nanostructure.

21. Method of manufacturing a SERS-active device, the method comprising forming at least one SERS-active nanostructure on a substrate, wherein arrangement of the at least one SERS-active nanostructure is adapted to provide a surface enhanced Raman signal having an intensity that is tunable depending on at least one of (i) orientation angle of the at least one SERS-active nanostructure, (ii) polarization of incident polarized light, and (iii) wavelength of the incident polarized light.

22. Method according to claim 21, wherein forming at least one SERS-active nanostructure on a substrate comprises

a) fabricating at least one nanostructure on a polymeric template using direct laser writing, and

b) alternately depositing one or more metal layers and one or more Raman-active marker compounds on the at least one nanostructure.

23. Method according to claim 22, wherein depositing one or more metal layers on the at least one nanostructure comprises depositing metal on the at least one nanostructure by thermal evaporation.

24. Method according to claim 22 or 23, wherein depositing one or more Raman-active marker compound layers on the at least one nanostructure comprises incubating the substrate comprising at least one nanostructure in a liquid reagent comprising the respective Raman-active marker compound.

25. Method according to any one of claims 22 to 24, wherein depositing one or more metal layers and one or more Raman-active marker compounds on the at least one nanostructure comprises alternately depositing two or more metal layers and one or more Raman-active marker compound layers on the at least one nanostructure such that the one or more Raman-active marker compound layers are sandwiched between two metal layers.