Security feature

Abstract

The invention relates to a visual security feature consisting of a substrate (2) and elongate metal nanoparticles (3) arranged in a non-random manner in the substrate (2). When illuminated with a linearly polarized visible light (4), the feature will produce a colour according to the angle between the electric field vector of the linearly polarized light (4) and the longitudinal axis of the elongate metal nanoparticles (3). As the angle changes when the object or the light emitter (5) is rotated, the colour of the substrate (2) changes. The nanoparticles (3) can incorporate fluorescent materials, so that, when excited by linearly polarized light (4), the device will produce fluorescence that varies in intensity according to the angle between the electric field vector of the exciting light and the longitudinal axis of the elongate metal nanoparticles (3).

Description

Description Safety feature Technology sector

[0001] The invention belongs to the field of security printing and relates to a security element that produces a color-changing effect and, in one embodiment, a variable fluorescence effect. Background of the invention

[0002] Some security features produce optical effects whereby a portion of an object, for example an identity document, a passport, a banknote or other valuable document, changes its color when the angle of the incident light varies as a result of the object being rotated or tilted, or viewed by reflection or transmission or in response to certain stimuli, such as heat, pressure, different lights or others.

[0003] Optical effects can be achieved by various conventional means, such as optically variable inks containing optically variable pigments, diffractive gratings, interference-coated particles, thermochroic pigments, photochromic pigments, luminescent pigments, infrared-absorbing pigments, ultraviolet-absorbing pigments, or dichroic effects.

[0004] This patent addresses, in part, dichroic effects as a safety feature. In this context, dichroism should be understood as the property of some materials whereby incident electromagnetic radiation is partially absorbed, partially reflected, and partially transmitted at different wavelengths. Consequently, if the electromagnetic radiation is visible light, the reflected and transmitted light will be perceived as different colors. If the electromagnetic radiation is ultraviolet or infrared, the dichroic effect will not be visible to the naked eye but will require a specific detector.

[0005] Several ways of achieving dichroic effects are known, for example, document WO 2019068655A1 (Giesecke+Devrient Currency Technology GmbH et al), April 11, 2019 (11.04.2019), Two-Sided Transparent Window Element with Dichroic Dyes, discloses a method for producing a foil security element to secure a valuable document by means of a liquid crystalline mixture containing dichroic dyes that provide latent images to display different images of respectively opposite sides in a cured state.

[0006] US 2005 / 0012998A1 (Transitions Optical Inc), January 20, 2005 (20.01.2005), Polarizing Photochromic Devices and Methods for Manufacturing Them, discloses the formation of a coating on a substrate; and the adaptation of the coating to change from a first state to a second linear polarization state in response to actinic radiation and to return to the first state in response to thermal energy.

[0007] EP 3706086 A1 (Alise Devices SL), September 9, 2020 (09.09.2020), Method for manufacturing custom document optical security elements and element obtained, describes a security element using confinement surfaces where at least one of the confinement surfaces contains an electrode pattern and an alignment layer on its inner face, disposing of a liquid crystal doped with at least one dichroic dye between the confinement surfaces and applying an electric field to the electrodes to orient the liquid.

[0008] Document EP 1680282 B1 (Giesecke+Devrient GmbH), July 19, 2006 (19.07.2006), Security element with a liquid crystalline material, discloses a security element for securing valuables, the security element having at least one liquid crystalline material that causes linear polarization of light and / or has a dichroic effect with a refractive index dependent on the direction, the liquid crystalline material being formed by a lyotropic liquid crystal.

[0009] Dichroic compounds not intended for security printing are known, for example, Kool et al, Gold and silver dichroic nanocomposite in the quest for 3D printing the Lycurgus cup, Beilstein J. Nanotechnol. 2020, 11, 16-23 (doi.org / 10.3762 / bjnano.11.2) describes the synthesis of gold and silver nanoparticles with dichroic properties and their incorporation into a printable polymer solution to obtain dichroic objects.

[0010] Dichroic inks are known, such as the one described in document KR 102057801 B1 Anti-counterfeiting security printing ink composition, dichroic mixture and dichroic ink comprising a carbazole-based blue fluorescent compound, December 19, 2019 (12.19.2019), which discloses a security printing ink composition comprising a carbazole-based blue fluorescent compound, a dichroic mixture and a dichroic ink.

[0011] WO 2019 / 020682 A1 (Basf Se), "Process for the preparation of metallic nanoparticle coatings and their use in decorative or security elements," dated January 31, 2019 (31 / 01 / 2019), describes a process for preparing thin coatings containing silver nanoparticles, which are produced directly onto a substrate as part of a coating or printing process. The coatings can exhibit different colors in transmittance and reflectance, so that when applied to a security element, such as a hologram, the resulting products can display different colors in reflectance and transmission.

[0012] Document WO2021214244 A1 (Sicpa Holding SA), Process for the production of dichroic security elements for the protection of valuable documents, October 28, 2021 (10 / 18 / 2021), describes a security ink that exhibits a blue color in transmitted light and a metallic yellow color in incident light. The manufacturing process comprises the following steps: a) printing a specific UV-Vis curable ink onto a transparent or partially transparent region of the substrate of a valuable document; b) heating the ink layer obtained in step a) to a temperature of approximately 55 °C to approximately 100 °C for at least one second, so that the ink layer exhibits a metallic yellow color when viewed in incident light; and c) UV-Vis curing the ink layer obtained in step b) to form the security element.

[0013] WO 2020 / 083794 A1 (Basf Se), Compositions comprising silver nanoplatelets, April 30, 2020 (30.04.2020) describes a composition comprising silver nanoplatelets having an average diameter in the range of 20 to 70 nm and an average thickness in the range of 5 to 30 nm, wherein the average aspect ratio of the silver nanoplatelets is greater than 1.5.

[0014] The dichroic effect described in the aforementioned documents, due to the use of metallic nanoparticles, is visible when observing the document by both reflection and transmission; that is, the color observed by reflection differs from that observed by transmission. The polarization of the light is irrelevant; that is, the effect is the same whether natural (unpolarized) or linearly polarized light is used. The nanoparticles are deposited randomly on the substrate. Furthermore, none of these documents describes fluorescent effects that vary depending on the polarization of the excitation light used.

[0015] Carlos Campos-Cuerva et al., in "Screen-printed nanoparticles as anti-counterfeiting labels", 2016 Nanotechnology 27 095702 (doi 10.1088 / 0957-4484 / 27 / 9 / 095702), have described the use of nanorods as security markers. However, this paper does not describe the multiplasmonic effect of the nanorods, the use of linearly polarized light to check the optical characteristics, the non-random arrangement of the nanorods, or the use of fluorescent materials.

[0016] Nanoplasmonic nanoparticles have been used to identify and authenticate individual objects, as described in document W02008010822A2 (Ultradots, Inc.), "Authentication and Identification of Objects Using Nanoparticles," January 24, 2008 (01 / 24 / 2008), which describes apparatus, systems, and methods for authenticating and identifying objects using nanoparticles. In one embodiment, a computer-readable storage medium includes executable code to: obtain an index based on an authentication image of a mark; select a reference image of the mark based on the index; compare the authentication image with the reference image to determine whether the authentication image matches the reference image; and generate an indication of authenticity based on whether the authentication image matches the reference image.

[0017] The present invention provides a dichroic security device that, when viewed under natural light, will display one color when viewed by transmission and another when viewed by reflection. This is the dichroic effect of known security devices. Furthermore, the security device provides an additional change in appearance when viewed by transmission and reflection, this time when illuminated with linearly polarized light. Rotating the object under linearly polarized light, or rotating the light emitter, will change the color. Additionally, in one example, the security device provides a fluorescence-based optical effect, where the fluorescence emission changes when excited with linearly polarized light and when the excited object or light emitter is rotated. Explanation of the invention

[0018] The object of the invention is a visual safety element, as defined in claim 1. Preferred embodiments are defined in the dependent claims. The visual safety element is applied to an object (1) and comprises a substrate (2) located in a region of the object (1) and elongated metallic nanoparticles (3) non-randomly arranged in at least a portion of the substrate (2). Under linearly polarized light (4), the visual safety element produces an optical effect that depends on the angle between the electric field vector of the linearly polarized light (4) and the longitudinal axis of the elongated metallic nanoparticles (3). At any given angle, at least a portion of the substrate (2) can be predefined where the elongated metallic nanoparticles (3) reflect and / or transmit light at the same wavelength.

[0019] According to one example, linearly polarized light is visible light and the optical effect produced by the visual safety characteristic under linearly polarized visible light (4) is a color change effect.

[0020] According to another example of the invention, the substrate (2) is transparent or semi-transparent, and the color-changing effect is produced by reflected and transmitted light. In one example, the substrate (2) is semi-transparent and tinted. In another example, the substrate (2) is opaque, and the color-changing effect is produced solely by reflected light. In yet another example, the substrate (2) is opaque and painted, and elongated metallic nanoparticles (3) are arranged on a portion of the substrate (2).

[0021] In one embodiment, the elongated metallic nanoparticles (3) incorporate at least one fluorescent material. Under linearly polarized excitation light (4), the visual safety device produces a fluorescent color whose intensity depends on the angle between the electric field vector of the linearly polarized excitation light (4) and the longitudinal axis of the elongated metallic nanoparticles (3). At any given angle, at least a portion of the substrate (2) can be predefined where the elongated metallic nanoparticles (3) emit fluorescent light of the same intensity.

[0022] In one example, the substrate (2) is painted with fluorescent ink of a different color than the fluorescent color emitted by the elongated metallic nanoparticles (3) arranged on the substrate (2). In another example, the substrate (2) is painted with fluorescent ink and the elongated metallic nanoparticles (3) are arranged on a portion of the substrate (2).

[0023] In one example, the substrate is divided into at least two zones that produce a different color under linearly polarized visible light (4) and a fluorescent color of different intensity under linearly polarized excitation light (4).

[0024] In one example, elongated metallic nanoparticles (3) incorporate at least one magnetic material. In another example, elongated metallic nanoparticles (3) incorporating at least one fluorescent material also incorporate at least one magnetic material.

[0025]

[0025] As defined in other independent claims, the invention also relates to an object (1) comprising the security feature, to the use of elongated metallic nanoparticles (3) to construct the security feature, and to a printing ink containing elongated metallic nanoparticles (3) configured according to the invention. Brief description of the drawings

[0026] Fig. 1: Visual security feature incorporated into a banknote, shown in two different positions (a, b). The banknote has a rectangular substrate with elongated metallic nanoparticles arranged in parallel.

[0027] Fig. 2: Example of an acceptable parallel arrangement of elongated nanoparticles.

[0028] Fig. 3: Absorption diagram of nanoparticles with different aspect ratios and a medium with a dielectric constant of four.

[0029] Fig. 4: Examples of different nanoparticle arrangements on a substrate.

[0030] Fig. 5: Nanoparticles arranged radially on a substrate.

[0031] Fig. 6: Magnetic field lines drawn around a magnet.

[0032] Fig. 7: Diagram representing the absorbance peaks of the silver nanorods.

[0033] Fig. 8: Diagram representing the UV-vis spectrum of a gold nanorod solution.

[0034] Fig. 9: Silica-coated gold nanobar with embedded organic fluorophores.

[0035] Fig. 10: Diagram representing the fluorescence intensity of organic fluorophores embedded in silica around gold nanorods.

[0036] Fig. 11: Fe3O3-SiO2 gold nanorods. Preferred embodiment of the invention

[0037] The security element that is the subject of this patent applies to an object (1). The object (1) can be a valuable object, a security object, a consumer product, a security document, a valuable document, a certificate (e.g., birth, residence, marital status, academic achievements), a certified copy of a document, a card (e.g., an identity document, a smart card, an access card, a credit card), or a passport.

[0038] The visual safety element comprises a substrate (2) and elongated metallic nanoparticles (3) non-randomly arranged on the substrate. The substrate (2) is understood to be the element configured to host the elongated metallic nanoparticles (3). In some examples, the elongated metallic nanoparticles (3) are embedded in the substrate (2); for example, the substrate (2) would be a security wire and the elongated metallic nanoparticles (3) would be embedded in it. The substrate (2) can also be a polymer matrix in which the elongated metallic nanoparticles (3) are embedded. In some examples, the elongated metallic nanoparticles (3) are deposited onto the substrate, for example, by printing it with an ink containing them. In one example, the substrate (2) is transparent. In another example, it is semi-transparent. In another example, the substrate (2) is opaque.

[0039] To illustrate a known dichroic effect, the object (1) can be a banknote and the substrate (2) a rectangular transparent film applied to a region of the banknote. Metallic nanoparticles, either amorphous or elongated, have been randomly distributed on the substrate (2), for example, by printing it with an ink containing them. A user holding the banknote in front of them shines a light onto the front of the substrate (2), that is, the side facing the user. This light can be unpolarized, for example, from a smartphone flashlight, or linearly polarized, for example, from a flashlight with a polarizing filter.

[0040] The user will perceive the light reflected by the metallic nanoparticles. Due to the dichroic effect caused by the surface plasmonic resonance phenomenon in the metallic nanoparticles, the wavelengths of the reflected light (understood as a set of wavelengths of varying intensity) will determine the color perceived by the user. If the user moves the light emitter so that it points directly at the back of the substrate (2), they will perceive the light transmitted by the nanoparticles and passing through the transparent substrate (2). The wavelengths of the transmitted light will be different (understood as a set of wavelengths of varying intensity), so the user will perceive a different color. This color change between the reflected and transmitted light perceived is a known dichroic optical effect associated with metallic nanoparticles.

[0041] The present invention utilizes dichroism as a safety feature, going beyond the known optical effect. In its basic configuration, new optical effects are obtained by using elongated metallic nanoparticles (3) arranged non-randomly on a substrate (2) and illuminating the safety feature with linearly polarized light (4). The new optical effect consists of the fact that, when the object (1) or the linearly polarized light (4) is rotated, the color observed by the user changes.

[0042] To explain this new optical effect, as illustrated in Fig. 1, the object (1) is a banknote (1) with a substrate (2) consisting of a rectangular transparent film. The elongated metallic nanoparticles (3) have been non-randomly distributed on the substrate (2); for example, they have been arranged parallel to each other, so that their longitudinal axis is parallel to the shorter side of the rectangle. It should be noted that a precisely parallel arrangement of the nanoparticles (3) is not feasible, since all or some of them will deviate from a straight line. Fig. 2 is an example of an acceptable parallel arrangement of elongated nanoparticles (3). As can be seen, there are deviations that, in the context of the nanoparticles (3), can be defined as small and acceptable, in the approximate range of -10° to 10°, or -20° to 20°, or a maximum of -45° to 45° with respect to a straight line.This circumstance must be considered when interpreting the terms "parallel" and "perpendicular" referring to the arrangement of the nanoparticles (3) in this patent.

[0043] In Fig. 1(a), a user holds the banknote (1) in front of them in a first position, with the rectangular substrate (2) horizontal. The user directs a linearly polarized light (4)—for example, using a flashlight with a polarizing filter or a flashlight without a filter and placing a linear polarizer between the flashlight and the banknote—towards the front of the substrate (2). The linearly polarized light (4) strikes the substrate (2) such that the electric field vector of the light is parallel to the longitudinal axis of the elongated metal particles (although, as explained above, the electric field vector will not be exactly parallel to all the elongated metal particles). The user rotates the banknote (1) to a second position, where the rectangular substrate (2) is vertical, as illustrated in Fig. 1(b).Therefore, the electric field vector of the light is perpendicular (according to the meaning that should be given to this term) to the longitudinal axis of the elongated metallic particles, or in other words, parallel to the transverse axis. In both positions, the user will perceive the light reflected by the elongated metallic nanoparticles (3).

[0044] Due to surface plasmonic resonance in the elongated metallic nanoparticles (3), the light reflected by these nanoparticles in the first position will have longer wavelengths, which could be red, for example, than the light reflected in the second position, which could be blue. Consequently, the reflected light perceived by the user will change color from the first to the second position. Therefore, tilting the banknote (1) produces a color-changing effect. The same effect would be obtained if, instead of tilting the banknote, the user tilted the light emitter (5).

[0045] For any given angle between the electric field vector of linearly polarized light (4) and the longitudinal axis of the elongated metal nanoparticles (3), at least a portion of the substrate (2) can be predefined where the elongated metal nanoparticles (3) reflect or transmit light at the same wavelength. In this example, such a portion would be the entire substrate (2), since the nanoparticles (3) are arranged along the entire substrate and are all in the same position. It should be understood that small differences in wavelength would exist due to the impossibility of obtaining a precisely parallel arrangement of the nanoparticles (3), as explained above. Such a portion can be predefined in the sense that its existence and location are known beforehand, since they are derived from the arrangement of the nanoparticles (3), without having to determine its location by illuminating the substrate (2) and verifying the optical response.

[0046] In the previous example, red and blue represent the two extremes of the rotation, that is, the electric vector of linearly polarized light (4) that goes from parallel to perpendicular to the longitudinal axis of the elongated metallic nanoparticles (3). However, the color change occurs across the entire range of intermediate angles, so that, as the rotation takes place, the color continues to change from the initial red to blue after a 90° rotation, with all intermediate combinations. And it will continue to change if the rotational motion continues, until it reaches red again upon completing a 180° rotation.

[0047] In this example, the substrate (2) on which the elongated metallic nanoparticles (3) are arranged is transparent or semi-transparent. Therefore, the optical color-change effect would occur not only with respect to the light reflected by the substrate (2) but also with respect to the light transmitted through it. To obtain a color-change effect with transmitted light, the user would direct the light toward the reverse of the banknote (1), instead of the obverse, and, as described, rotate the document or the light emitter (5). By illuminating the reverse, the color change would occur in the transmitted light perceived by the user.

[0048] In one example, the substrate (2) is opaque. In this case, the color change effect would only occur when the front is illuminated, since, being opaque, it would not transmit light when the back is illuminated. The elongated metallic nanoparticles (3) are arranged on the substrate (2), which therefore acts as a background for the nanoparticles (3). Some of the light received by the nanoparticles (3) will be transmitted to the background substrate (2). If the substrate (2) is opaque and black, that portion of the light will be absorbed. Therefore, the color perceived by the user will only be determined by the light reflected by the nanoparticles (3) arranged on the substrate (2).

[0049] In one example, the background substrate (2) is opaque and painted a color, or it is semi-transparent and tinted a color. In these cases, two lights would be reflected: the light from the background substrate (2) and the light from the nanoparticles (3). Consequently, the color perceived by the user would be a combination of both. For example, if the substrate (2) is opaque and painted yellow, and referring to the same example illustrated in Fig. 1, in the first position, the color perceived by the user would be a combination of the yellow reflected by the background substrate (2) and the red reflected by the nanoparticles (3), that is, orange. And in the second position, the perceived color would be green, as a combination of the yellow reflected by the background and the blue reflected by the nanoparticles (3).

[0050] The combination of colors reflected by the background substrate (2) and the nanoparticles (3) can form the basis of a variation of the described color-changing effect. Assuming, as in the previous example, that in the first position of the object (1) the nanoparticles (3) produce a red color, the background substrate (2) can be opaque and painted the same color, or semi-transparent and tinted the same color. The nanoparticles (3) are applied, for example, by printing, only to a portion of the rectangular substrate (2). This portion has a specific shape, for example, triangular. Therefore, there would be a triangle of nanoparticles (3) inscribed on an opaque or semi-transparent rectangular substrate (2). In the first position, the rectangle would reflect red light, since both the background and the triangular portion of the nanoparticles (3) would reflect that color.When the banknote (1) or the light emitter (5) is rotated, the nanoparticles (3) begin to change color towards blue, causing the triangular shape to appear against the red rectangular background. If the rotation continues, the triangle turns red and disappears from the background. The elongated metallic nanoparticles (3), which form a triangle, reflect and / or transmit light at the same wavelength. Therefore, a portion of the substrate (2) can be predefined where the nanoparticles (3) reflect or transmit light at the same wavelength. As explained previously, this portion can be predefined because its existence and location are known beforehand.

[0051] The safety feature claimed in this patent is constructed using nanoparticles (3). A nanoparticle is defined as a particle whose dimension is less than or substantially less than 400 nm. This dimension was selected because 400 nm is the wavelength of blue light, and the safety feature relates to the interaction of the nanoparticles (3) with electromagnetic waves, including visible light.

[0052] The nanoparticles (3) of the invention are metallic. In one example, the metal is selected from a group consisting of Au, Ag, Al, Cu, Co, Fe, Ga, Mg, Ni, Pb, Pd, Pt, Rh, and Zn, alloys containing any of these metals, and combinations thereof.

[0053] The nanoparticles (3) of the invention are elongated, i.e., nanoparticles (3) whose dimensions are clearly discernible due to their aspect ratio being greater than one. Therefore, one of their dimensions is larger than the others; that is, they have a discernible longitudinal dimension, unlike amorphous nanoparticles (3), whose dimensions are indistinguishable. The elongated nanoparticles (3) were selected because of their anisotropic plasmonic properties.

[0054] Surface plasmonic resonance refers to the behavior of surface electrons of light-excited nanoparticles in metallic nanoparticles (3). In the previous examples, producing the color-changing effect when the banknote (1) or the light emitter (5) is rotated requires an angle to form between the electric vector of the light and an axis of the nanoparticle (3). This can only occur if two conditions are met: that the light is linearly polarized (4) and that the nanoparticles (3) have a longitudinal axis and, therefore, a transverse axis.

[0055] When the electric vector of linearly polarized light (4) is parallel to the longitudinal axis of the nanoparticle, electrons moving along this axis will travel a greater distance than if they were moving along any other axis of the nanoparticle (3). The greater the distance, the longer the wavelength of the light reflected by the nanoparticles; therefore, the reflected light will typically have a wavelength similar to that of red light (it being understood that an elongated nanoparticle (3) could be long enough to reflect light in the infrared spectrum). This is called the longitudinal plasmon of the nanoparticle (3).When the electric vector of linearly polarized light (4) is parallel to the transverse axis of the nanoparticle (3), electrons moving along this axis will travel a shorter distance, i.e., across the width of the nanoparticle (3), than if they were moving along any other axis of the nanoparticle (3). The shorter the distance, the shorter the wavelength of the light reflected from the nanoparticle (3); therefore, the reflected light will typically have approximately the wavelength corresponding to blue light (it being understood that an elongated nanoparticle (3) could be short enough to reflect light in the ultraviolet spectrum). This is called the transverse plasmon of the nanoparticle.A difference in reflected light, or in transmitted light, depending on the angle between the electric field vector of linearly polarized light (4) and an axis of the nanoparticles (3), means that the elongated metallic nanoparticles (3) exhibit an anisotropic two-surface plasmonic resonance.

[0056] According to the described color-changing effect, a substrate (2) will display a range of colors when the object (1) or the light emitter (5) is rotated. One of these colors will have the longest wavelength relative to the others. Consequently, another color in the range will have the shortest wavelength. As mentioned previously, the longest wavelength in the substrate (2) typically produces a red color, and the shortest, a blue color, with all combinations in between. However, the range of colors could vary depending on the dimensions of the elongated nanoparticles (3), but always maintaining the principle that the color of the longitudinal plasmon will correspond to a longer wavelength than that of the transverse plasmon. Therefore, it should be clarified that, in this patent, "red" refers to the color corresponding to the longest wavelength in the range of colors produced by the substrate (2).Likewise, "blue" is understood to be the color that corresponds to the shortest wavelength in the range of colors produced by the substrate (2). And "purple" refers to the color produced by the combination in equal quantities of the two extremes.

[0057] Unlike elongated nanoparticles (3), the amorphous nanoparticles, nanospheres, or nanoplatelets used to construct security devices have indistinguishable dimensions, as they lack discernible width, length, or aspect ratio. Their plasmonic effect is always the same, i.e., isotropic, since there is no longitudinal or transverse axis along which electrons can travel. Therefore, even when illuminated with linearly polarized light (4), there would be no angle between the electric vector of the light and the longitudinal or transverse axis of the nanoparticles. Under these conditions, the light absorbed, reflected, or transmitted by these nanoparticles will always remain at the same wavelengths.Therefore, in a security device constructed with such nanoparticles, a color shift associated with dichroism would occur, in which the reflected and transmitted light perceived by the user would differ in color, but these colors would not change. In particular, no color shift effect would occur when rotating the document or the light emitter (5), as this would not involve any variation in the angle between the electric vector of linearly polarized light (4) and the axis of the nanoparticles. For this reason, known security devices made with amorphous nanoparticles, nanospheres, or nanoplatelets do not use linearly polarized light (4).

[0058] The invention can be implemented with elongated metallic nanoparticles (3) such as nanorods (also known as nanocylinders), nanowires, nanofibers, dimers, nanocrescents, nanorice, nanocarrots, nanobipyramids, nanodumbbells, nanoneedles or combinations thereof.

[0059] In one example, the elongated metallic nanoparticles (3) are selected from a group consisting of nanorods, nanowires, and combinations thereof. Nanorods are rods, pillars, or columns with a lateral dimension on the order of tens of nanometers, and their aspect ratio is typically on the order of 10 or less. Nanowires are structures with a thickness or diameter limited to tens of nanometers or less, and an unrestricted length, with an aspect ratio greater than 10. While nanorods and nanowires usually have a circular cross-section, for the purposes of this patent they may also have a polygonal cross-section.

[0060] The wavelength of longitudinal and transverse plasmons depends on the material used, the dielectric constant of the medium in which the nanoparticles are located (3), and their dimensions, particularly their aspect ratio (i.e., the ratio between their length and thickness). This allows, to some extent, the selection of the color produced in the longitudinal and transverse plasmons.

[0061] In one embodiment, the selected elongated metallic nanoparticles (3) (whether of the same type or combinations thereof) have substantially the same length and thickness. "Substantially" means that, in the context of the nanoparticles (3), small variations in the dimensions of some nanoparticles (3) with respect to others are permissible.

[0062] In one embodiment, the elongated metallic nanoparticles (3) have virtually identical thickness (the dimension that determines the transverse plasmon) but differ in length. Figure 3 (taken from Goldys and Krystyna Drozdowicz-Tomsia, “Gold and silver nanowires for fluorescence enhancement,” doi10.5772 / 16330) shows the absorption pattern of the nanoparticles (3), specifically nanorods, with different lengths for a given thickness (i.e., different aspect ratios) and a medium with a dielectric constant of four. For a given thickness, the longitudinal plasmon can be adjusted at will by controlling the length of the nanorod (3), while keeping the transverse plasmon virtually unchanged. The nanoparticle dimensions could also induce a plasmon in the ultraviolet or infrared regions of the electromagnetic spectrum, imperceptible to the naked eye.As explained, the longitudinal plasmon tends towards frequencies close to red and the transverse plasmon, towards blue.

[0063] In one embodiment, the elongated nanoparticles (3) differ in both length and thickness. In this embodiment, the perceived color will be a combination of the colors corresponding to the plasmons of the elongated metallic nanoparticles (3) of different sizes.

[0064] Another condition for obtaining the optical effects of this invention is that the elongated metallic nanoparticles (3) are arranged non-randomly on the substrate (2).

[0065] In the known safety features, the amorphous or spheroid nanoparticles are randomly distributed on the substrate (2). Since these nanoparticles lack the elongated configuration necessary to obtain the desired color-changing effect when rotating the object (1) or the linearly polarized light emitter (5) (4), it is not necessary to arrange them in any specific order, as this would not affect the optical effect.

[0066] The nanoparticles (3) proposed in this invention have the required configuration (elongated and metallic) to produce the desired optical effect, since the electric vector of linearly polarized light (4) can form angles with respect to the longitudinal axis of the elongated nanoparticles (3). However, if they were randomly distributed on the substrate (2), the electric vector of linearly polarized light (4) would form very diverse angles with the nanoparticles (3), so the nanoparticles (3) on the substrate (2) would emit light at different wavelengths, thus losing the overall color-changing effect when the object (1) or the light emitter (5) is rotated. The same would occur if the elongated metallic nanoparticles (3) were arranged in a geometric pattern where no single plasmon predominated, for example, perpendicular to each other.

[0067] Different arrangements of the elongated metal nanoparticles (3) are possible according to the invention, for example as shown in Fig. 4. Taking a rectangular substrate (2) as a reference, the elongated metal nanoparticles (3) are arranged parallel to the short side of the rectangle (Fig. 4(a)), parallel to its long side (Fig. 4(b)), and diagonally across the substrate (2) (Fig. 4(c)). In these examples, for any given position of the substrate (2) and the linearly polarized light emitter (5) (4), the electric vector of the light will be at the same angle with respect to all the nanoparticles (3) on the substrate (2). It should be understood that the angle is the same, although there are acceptable variations due to the fact that the particles cannot be arranged in a precisely parallel position, as explained above.Assuming that the electric vector of linearly polarized light (4) is parallel to the short side of the rectangle, the reflected light with the longest wavelength will be in Fig. 4(a) and the shortest in Fig. 4(b), while in Fig. 4(c) the wavelength would be intermediate and the perceived color a combination of the other two. When the substrate (2) is illuminated with linearly polarized visible light (4), the nanoparticles (3) reflect and transmit light at the same wavelength, so the color is the same throughout the substrate (2) for any of the arrangements shown in Fig. 4. For example, in Fig. 4(a), the color would be red.When the document or light emitter (5) is rotated, the color change will be uniform throughout the substrate (2), gradually leaving behind the red until reaching a purple tone around 45° which gradually evolves towards a blue color that would be reached at an approximate degree of 90° between the longitudinal axis of the nanoparticles (3) and the vector of linearly polarized light (4).

[0068] In another example, with reference to Fig. 5, the elongated metallic nanoparticles (3) can be arranged radially. When the substrate (2) is illuminated with linearly polarized visible light (4), the color displayed by the substrate (2) will not be uniform, but will be divided into zones of different colors. This is because the electric vector of the linearly polarized light (4) forms different angles with the radially arranged nanoparticles (3). In Fig. 5, two zones have been marked. In zone A, substantially all the nanoparticles (3) (except those that converge with zone B) are arranged in a vertical line. Therefore, the angle formed by the electric vector of the linearly polarized light (4) and virtually all the nanoparticles (3) in zone A will be the same.Consequently, substantially all the nanoparticles (3) in zone A will reflect and transmit light at the same wavelength, so the light reflected or transmitted by zone A of the substrate (2) will be uniform, except in the confluence zones (in the example, assuming that the electric vector of the linearly polarized light (4) is parallel to the longitudinal axis of the particles in zone A, this color may be red). In zone B, substantially all the nanoparticles (3) (i.e., except those in the confluence zones with portion A) are arranged in a horizontal line.As in zone A, the angle formed by the electric vector of linearly polarized light (4) and substantially all the nanoparticles (3) in zone B will be the same, and the nanoparticles (3) will reflect and transmit light at the same wavelength (in the example, assuming the same position of the electric vector of linearly polarized light (4)—parallel to the longitudinal axis of the particles in zone A and therefore perpendicular to the longitudinal axis of the particles in zone B—the color reflected or transmitted by zone B will be blue). At the confluence of zones A and B, the color will be purple, a combination of red and blue. For nanoparticles (3) located outside zones A and B, their reflected or transmitted color will be determined by the different angles formed with the electric vector of linearly polarized light (4).This non-random arrangement of the nanoparticles (3) on the substrate (2) will cause portions of different colors to be observed simultaneously on the substrate (2). In the example, zone A would contain two portions (i.e., the entire area of ​​zone A except for the area where it meets zone B), and the same for zone B. The existence and location of these portions can be predefined. The colors observed simultaneously on the substrate (2) will change when the object (1) or the light emitter (5) is rotated.

[0069] The example in Fig. 5 defines the minimum condition that any arrangement of elongated metallic nanoparticles (3) must meet to be considered a non-random arrangement according to the invention: that the nanoparticles (3) within at least a predefined portion of the substrate (2) form the same angle with the electric vector of linearly polarized visible light (4), or with the linearly polarized exciting light (4) (in the embodiment where the nanoparticles (3) exhibit fluorescent properties).

[0070] Figure 6 depicts magnetic field lines drawn around a magnet. Nanoparticles (3) are arranged along these lines by printing them onto a substrate (2) using a printing machine equipped with a magnet. The nanoparticles (3) are made of a magnetic material. As in the previous example, when the substrate (2) is illuminated with linearly polarized visible light (4), the resulting color will not be uniform but will be divided into zones of different colors. In this example, four zones A and four zones B have been marked. In all of them, the minimum condition for a non-random arrangement is met: the nanoparticles (3) will form the same angle with the electric vector of the linearly polarized light (4). The predominant plasmon will be longitudinal in zones A and transverse in zones B.For example, in reflected light, this will result in the colors red and blue, respectively, while combinations of these colors will be displayed on the rest of the substrate (2) outside of zones A and B, all depending on the position of the nanoparticles (3) with respect to the electric vector of linearly polarized light (4) incident vertically on the substrate (2). In the predefined portions of zones A and B, the elongated metallic nanoparticles (3) reflect and transmit light at the same wavelength. The color-changing effect will originate from these initial colors when the light emitter (5) on the substrate (2) is rotated.

[0071] In one embodiment, the elongated metallic nanoparticles (3) incorporate at least one fluorescent material. Security features with fluorescent properties are known in the art, for example, banknotes that incorporate invisible images or markings or that display certain colors under visible light. The area where the feature is located is illuminated with an excitation light, usually ultraviolet. After excitation, the fluorophores return to their ground state, emitting fluorescence photons, which makes invisible features visible or changes the color or appearance of a visible feature.

[0072] According to one embodiment of the invention, the incorporation of fluorescent materials into the elongated metallic nanoparticles (3) produces a new fluorescence effect in addition to the color change effects that have been described.

[0073] To explain the novel fluorescent effect produced by the security element in this embodiment, the example of a banknote (1) with a rectangular transparent film substrate (2), as shown in Fig. 1, will be used. In this embodiment, the reference number (4) represents linearly polarized excitation light, rather than linearly polarized visible light. Accordingly, the reference number (5) represents a linearly polarized excitation light emitter. The elongated metallic nanoparticles (3) containing fluorescent materials are distributed parallel to each other, such that their longitudinal axis is parallel to the shorter side of the rectangle.

[0074] A user holds the banknote (1) in front of them in a first position, with the rectangular substrate (2) horizontal. The user directs a linearly polarized excitation light (4) towards the front of the substrate (2). It is important to note that linearly polarized light (4) is required, but this time not necessarily visible light, but rather excitation light. By "excitation light" is meant any light capable of exciting fluorophores. This is usually an ultraviolet light emitter, for example, a specific lamp. It can also be an infrared light emitter (this type of fluorescent excitation is called "upconversion") or light at other suitable wavelengths.

[0075] Linearly polarized excitation light (4) strikes the substrate (2), such that its electric field vector is parallel to the longitudinal axis of the elongated fluorescent metal particles (3). The user rotates the banknote (1) to a second position, where the rectangular substrate (2) is vertical. Therefore, the electric field vector of the light (4) is perpendicular to the longitudinal axis of the elongated fluorescent metal nanoparticles (3), that is, parallel to the transverse axis. In both positions, the user observes the fluorescent color emitted by the nanoparticles (3).

[0076] Due to metal-enhanced fluorescence (MEF), the luminescent response of fluorophores is intensified by their proximity to metal nanoparticles, whether amorphous, spheroidal, or elongated. If the metal nanoparticles are elongated and excited with linearly polarized excitation light (4), the plasmonic properties of the elongated metal nanoparticles (3) will influence their fluorescent emission, as it will be enhanced by the longitudinal plasmonic resonance of the elongated metal nanoparticles (3). Because of these plasmonic properties, in the first position of the banknote (1), the nanoparticles (3) will be excited longitudinally, where the fluorescence intensity is greater than in the second position, where they are excited transversely.Consequently, the fluorescent light perceived by the user will change in intensity from the highest in the first position to the lowest in the second, with intermediate intensities in the intermediate positions, as shown in Fig. 10. In this example, for any given angle between the longitudinal axis of the elongated metallic nanoparticles (3) and the electric field vector of the linearly polarized excitation light (4), the nanoparticles (3) in the substrate (2) emit fluorescent light of the same intensity. Thus, rotating the banknote (1) produces an optical effect whereby the fluorescent color of the emitted light changes intensity. The same effect would be obtained if, instead of rotating the banknote (1), the user rotated the excitation light emitter (5).

[0077] When the arrangement of the elongated metal nanoparticles (3) corresponds to that in Fig. 4, since all the nanoparticles (3) have the same orientation, the electric vector of the linearly polarized light (4) will form the same angle with all the nanoparticles (3). Consequently, the fluorescent color will show the same intensity across the entire substrate (2), and the intensity will change uniformly when the substrate (2) or the excitation light emitter (5) is rotated.

[0078] When the arrangement of the elongated metallic nanoparticles (3) with fluorescent properties corresponds to Fig. 5 (radial) or Fig. 6 (under a magnetic field), due to the different angles formed by the electric vector of the linearly polarized light (4) with the nanoparticles (3), the fluorescence intensity of a given color will not be uniform along the substrate (2). Instead, for any given position of the substrate (2) or the exciting light (4), there will be zones of varying intensity, and these zones will change as the substrate (2) or the exciting light emitter (5) is rotated. Within these zones, predefined portions (for example, two respective portions in zones A and B, excluding their confluence area) will be defined where the fluorescence emission from the nanoparticles (3) will have the same intensity.

[0079] Therefore, the use of elongated metallic nanoparticles (3) with fluorescent materials and a transparent or semi-transparent substrate (2) will produce three optical effects: two color-changing effects in the reflected and transmitted light, respectively, and a variable fluorescence effect. If the substrate (2) is opaque, the color-changing effect, as explained, will only occur in the reflected light. As for the variable fluorescence, the effect always consists of a change in the intensity of the emitted fluorescent light, regardless of whether the substrate (2) is transparent, semi-transparent, or opaque. To change from the color-changing effect to the variable fluorescence effect, it is sufficient to change the light-emitting device (5) from a linearly polarized visible light emitter to a linearly polarized excitation light emitter.

[0080] In one embodiment, the substrate (2) on which the elongated metallic fluorescent nanoparticles (3) are arranged can be painted with conventional fluorescent ink. If the fluorescent color emitted by the background ink differs from the color emitted by the elongated metallic nanoparticles (3), a combination of both will be displayed in response to an excitation light (4). By rotating the document (1) or the excitation light (4), the background fluorescence will remain constant, but the intensity of the fluorescent color emitted by the nanoparticles (3) will vary, causing a change in the color of the combined emitted fluorescence. Therefore, in this embodiment, a fluorescent color-changing effect is obtained, in addition to the variable intensity of the fluorescence.

[0081] In one embodiment, the nanoparticles (3) are arranged on a portion of the substrate (2), which is, for example, triangular in shape. This portion will produce a variable fluorescence effect, in addition to the constant fluorescence of the rest of the substrate (2). Depending on the rotational position of the object (1) and the chosen colors, the triangular shape could, in some positions, blend into the background, or in others, appear with greater or lesser intensity and with the same or a different color than the background. Again, with this embodiment, a fluorescent color-changing effect can be achieved.

[0082] After describing the safety feature in their embodiments with elongated metallic nanoparticles (3) with and without fluorescent materials, the description will focus on different examples of the production of elongated metallic nanoparticles (3) and substrates (2) according to the claims. The production of elongated metallic nanoparticles (3) is known in the art. Pietrobon et al., “Synthesis of size-controlled faceted pentagonal silver nanorods with tunable plasmonic properties and self-assembly of the nanorods”, ACS Nano 2009, 3, 1, 21-26 (doi: 10.1021 / nn800591), synthesized highly homogeneous, water-dispersible silver nanorods with custom dimensions, thickness, and length by thermal regeneration of decayed particles. The length varies from 50 nm to 2 nm, depending on the amount of reagent, and the width is determined by the size of the initial particle, in this case 49.5 ± 2.5 nm. As can be seen in Fig.7. These nanorods exhibit two absorbance peaks depending on their aspect ratio. When the aspect ratio is one, that is, when it is no longer an elongated nanoparticle, a single plasmon is observed.

[0083] Xie, Qin et al., in “Preparation of optically anisotropic nanocomposites with oriented gold nanorods embedded in polyvinyl alcohol”, Journal of Nanoscience and Nanotechnology, Volume 10, Number 3, March 2010, pp. 1829-1833(5) (doi 10.1166 / jnn.2010.2101) describes a process for producing custom-sized gold nanorods. A seed solution can be prepared by mixing 10 mL of an aqueous solution containing HAuCl₂ (2.5 x 10⁻⁴ M) and CTAB (0.1 M) with 0.6 mL of a freshly prepared and cooled NaBH₄ (10 mM) solution. A certain amount of these seeds was added to 40 mL of a growth solution composed of HAuCl₂ (5 x 10⁻⁴ M) and AgNO₃ (8 x 10⁻⁵ M) in 0.1 M CTAB mixed with a certain amount of fresh 0.0788 M ascorbic acid. The gold solutions were centrifuged and redispersed in 2 mL of ultrapure water. The resulting gold nanorods are relatively homogeneous in size and shape. Their sizes were approximately 15 nm x 40 nm with less than 15% dispersion, and the aspect ratio was approximately 2.7.Figure 8 shows the UV-vis spectrum of the gold nanowall solution with the typical peaks corresponding to transverse and longitudinal plasmons.

[0084] Regarding the embodiment where elongated metallic nanoparticles (3) incorporate fluorescent materials, in one example, this incorporation is achieved by coating the nanoparticles (3) with a silica layer containing organic fluorophores. The silica layers protect the organic fluorophores from two problems that affect their performance: photobleaching and photodegradation. Amorphous silica is particularly suitable for these purposes due to its optical transparency, chemical inertness, and photochemical stability. It is also ideal for both emitter accommodation and spacing control. Furthermore, the nanoparticles (3) can be coated with successive silica layers, for example, one with fluorophores and another with magnetic particles.

[0085] Along these lines, as depicted in Fig. 9, Tian Ming et al., “Strong Polarization Dependence of Plasmon-Enhanced Fluorescence on Single Gold Nanorods”, Nano Lett. 2009, 9, 11, 3896-3903 (doi 10.1021 / nl902095q), describe a gold nanorod (6) embedded in a silica layer (7). Organic fluorophores (8), specifically oxazine 725, are embedded in the silica layer (7). The average length, diameter, and aspect ratio of the bare nanorods (6) are 89 ± 7 nm, 42 ± 3 nm, and 2.1 ± 0.2 nm, respectively. The longitudinal plasmon wavelength of the assembly is 646 nm when the nanorods (6) are dispersed in aqueous solutions. The thickness of the silica layer (7) is uniform around the nanoshells (6), with an average value of 21 + / -2 nm.

[0086] The fluorescence effect produced by organic fluorophores embedded in a mesostructured silica layer around individual gold nanoshells is enhanced by the plasmonic resonance of the nanoshells. As exemplified in Fig. 10, the fluorescence intensity is maximum when the electric field of the linearly polarized light (4) used to excite the fluorophore is parallel to the major axis of the nanoshells.

[0087] In one example, quantum dots (QDs) are the fluorescent material incorporated into elongated metallic nanoparticles (3). This material has shown a generally narrower and more symmetrical emission fluorescence than that of other fluorophores, such as organic molecules. Furthermore, QDs can be excited by a wide range of light wavelengths, including near-ultraviolet (365 nm), which is commonly used to verify fluorescence in safety devices, allowing for a clear separation of the absorption and emission bands. Moreover, by varying their size (typically in the 2–6 nm diameter range), fluorescence across the entire visible spectrum can be achieved. They also exhibit greater physical and chemical resistance than organic fluorophores.

[0088] The incorporation of quantum dots (QDs) into elongated metal nanoparticles (3) is similar to the incorporation of other fluorescent materials. For example, Lu-Lu Wang et al., in "Exciton-plasmon-photon conversion in silver nanowires: polarization-dependent," Appl. Phys. Lett. 99, 061103 (2011) (doi.org / 10.1063 / 1.3625949), documented a method for coating silver nanowires with a silica layer of controllable thickness. In particular, a silica coating thickness of 15 nm was selected to achieve a high fluorescence enhancement of cadmium selenide quantum dots. This coating thickness ensures that the fluorophores are close enough to the metal to achieve increased fluorescence, but not so close as to be quenched. The greatest enhancement of plasmon-induced fluorescence typically occurs at distances between 5 and 30 nm.For shorter distances, the extinction of non-radioactive fluorescence predominates, while for longer distances, the fluorophore barely senses the presence of the metallic nanoparticle.

[0089] The wavelength of the excitation light can be greater than the wavelength of the fluorescence produced (upconversion). Jijun et al., “Plasmonic enhancement and polarization dependence of nonlinear upconversion emissions from single gold core-shell-satellite hybrid nanostructures @SiO2@CaF2:Yb3+,Er3+”, Light Science & Applications (2017) 6, e16217 (doi 10.1038 / lsa.2016.217), have produced gold nanoshells coated with a silica layer incorporating upconversion materials, specifically CaF2:Yb3+,Er3+, which, under illumination from a 980 nm infrared continuous-wave diode laser, produced polarization-dependent enhanced green and red fluorescence.

[0090] The silica layer containing QD or other fluorophores can be deposited directly onto the nanoparticle, but in this case, some of the fluorescent materials might be turned off.

[0091] In one embodiment, the elongated metallic nanoparticles (3) incorporate at least one magnetic material. This magnetic material allows the non-random arrangement of the nanoparticles (3) under the action of a magnet during the printing process, as explained in Figure 6.

[0092] In one example, elongated metallic nanoparticles (3) are coated with a silica layer containing magnetic materials. As previously described, this silica layer can be added to another silica layer containing fluorophores. In one example, the magnetic materials embedded in the silica layer consist of synthesized magnetic nanoparticles, preferably Fe3O4. Chapman et al., “Heteroaggregation approach for depositing magnetite nanoparticles on silica-coated gold nanofoils”, Chem. Mater. 2017, 29, 24, 10362-10368 (doi: 10.1021 / acs.chemmater.7b03481) have produced Fe3O4-SiO2 gold nanoshells by a simple heteroaggregation method to deposit magnetite (Fe3O4) nanoparticle coatings on the surface of silica-coated gold nanoshells (SiO2-GNR).This results in Fe3O4-S¡O2 gold nanowires that maintain the longitudinal surface plasmon resonance (LSPR) of the gold nanowires and are magnetically sensitive. Fe3O4-SiO2 gold nanowires are of special interest for biomedical applications. These nanowires are illustrated in Fig. 1 11.

[0093] Another example of nanoparticles (3) with magnetic properties would be nanoparticles (3) with a magnetic core, generally Fe3O4, although other magnetic materials are also possible, coated with a layer of silver or gold. For example, Rincón-Iglesias et al., “Fe3O4@Au core-shell loaded gels for anisotropic and tunable magnetothermal and photothermal energy,” ACS Appl. Mater. Interfaces 2022, 14, 5, 7130-7140 (doi / 10.1021 / acsami.1c20990), reported on the synthesis of monodisperse Fe3O4@Au core-shell nanoparticles. The size-tunable Fe3O4 nanoparticles were synthesized using a solvothermal method and subsequently coated with gold. In one example, a second coating with fluorophores is added to the nanoparticles thus formed, which also gives them fluorescent properties.

[0094] It is preferable that the magnetic materials used to magnetize nanoparticles (3) possess super paramagnetic properties, or have low remanence, to avoid agglomeration of the nanoparticles in the absence of an external magnetic field.

[0095] In one embodiment, the elongated metallic nanoparticles (3) possess magnetic and fluorescent properties. Ning Sui et al., “Magnetic and optical properties of Ag@SiO2-FITC-Fe3O4 hybrid nanoparticles,” Materials Science and Engineering, Volume 182, March 2014, pages 92–95, doi.org / 10.1016 / j.mseb.2013.11.029, described a method for obtaining silver nanoparticles coated with a five-nm silica layer. A dye, fluorescein isothiocyanate (FITC), is further encapsulated during the growth of a second silica layer onto the Ag@SiO2 nanoparticles. The proximity of the silver nanoparticles led to a maximum fourfold improvement in FITC fluorescence when the thickness of the first silica layer was fixed at five nm. After the aminofunctionalization of the Ag@S¡O2-FITC nanoparticles, iron oxide nanoparticles were attached to their surface.The metal- and magnet-enhanced fluorescence properties appeared simultaneously when the Ag@SiO2-FITC-Fe3O4 hybrid nanoparticles were dispersed in a solution.

[0096] Elongated metallic nanoparticles (3) with fluorescent or magnetic properties, or both, can be formed from multiple segments, each of which provides one or more of these properties. Multisegment nanoparticles have been developed for biomedical applications. For example, U.S. Patent 20020187504 A1 (Johns Hopkins University), "Multifunctional Magnetic Nanowires," dated December 12, 2002 (12 / 12 / 2002), describes multifunctional, multisegment nanowires for probing and manipulating molecules at the cellular and subcellular levels. These nanowires are composed of segments formed from different materials selected from the group consisting essentially of platinum, iron, cobalt, nickel, gold, silver, copper, iron oxide, copper oxide, zinc oxide, and their alloys, wherein at least one of these segments is formed from a fluorescent or photoluminescent material.Furthermore, at least one segment of the nanowire may have an associated photoluminescent tag. These nanowires exhibit plasmonic and fluorescent functionalities.

[0097] Alternatively, multisegmented elongated metallic nanoparticles can have only two segments, a plasmonic segment and a magnetic segment, where one or both segments can have an added fluorophore, for example, by coating. Two-component nanoparticles consisting of gold and nickel segments have been produced, for example, by Carlos M. Hanganter, “Magnetic alignment of nanowires”, Chem. Mater. 2005, 17, 6, 1320-1324 (doi.org / 10.1021 / cm047955r). Thanks to the nickel segment, the nanorods can be magnetically oriented. Subsequently, either of the two segments can be functionalized. For example, Jin et al., “Development of a two-component nanorod complex for dual fluorescence imaging and RNAi delivery”, J. Microbiol. Biotechnol. 2014; 24(9): 1291-1299 (doi.org / 10.4014 / jmb.1406.06055) attached fluorescent tags to gold-nickel composite nanorods.

[0098] As mentioned previously, elongated metallic nanoparticles (3) can be embedded in the substrate (2). In particular, objects (1), such as identity documents, credit cards, or passport data sheets, consist of a polymer sheet or a layering of several polymeric materials. Similarly, banknotes can be printed on a polymeric substrate (2), or paper banknotes can contain a polymeric element (generally referred to as a "window"). In these cases, the elongated metallic nanoparticles (3) can be embedded in the polymeric element (polymer matrix).

[0099] A method for embedding elongated nanoparticles (3) in a polymer matrix, achieving parallel alignment, has been described by Marco Bernabé et al., “Polymeric nanocomposites containing anisotropic metallic nanostructures as indicators of internal strain,” Materials 2010, 3(2), 1461–1477 (doi.org / 10.3390 / ma3021461). It involves adding silver nanoparticles (narods) to an aqueous solution containing polyvinyl alcohol (PVA). A homogeneous film is then obtained by solution casting after solvent evaporation. The silver-based nanocomposite film (0.5 wt% Ag) was uniaxially stretched at 110 °C to induce anisotropic properties. Typically, stretch ratios greater than 1.5 produce a color change. With stretch ratios of 4 to 6, a strong color change is induced in the film when viewed under polarized light (4).The alignment of the nanorods is governed by the elongation of the OVA polymer molecules. Since PVA is not an elastomer, the plasmonic dichroic effect is permanent.

[0100] The same document describes a process in which 200 mg of PVA are dissolved in 15 ml of water with vigorous stirring at room temperature, and subsequently an aqueous suspension of silver nanorods or nanowires is added at a concentration that allows the preparation of composite films with an absorbance of less than 1 (i.e., 0.8 wt%). After evaporation of the water (two days in a fume hood at room temperature), a solid film can be obtained. The Ag / PVA nanocomposite films are then stretched successively by uniaxial tensile testing.

[0101] The film's color changes from bluish-gray to reddish-brown when illuminated with linearly polarized light, rotating the direction of the electric vector from parallel to perpendicular with respect to the drawing axis. Silver nanorods have aspect ratios (i.e., their length-to-diameter ratio) of approximately 5–10, with an average length of approximately 200 nm, while silver nanowires have aspect ratios greater than 90 and reach lengths of approximately 15–20 microns.

[0102] When, as in the previous example, the nanoparticles are embedded in a polymer matrix, and assuming they do not exhibit fluorescent properties, there is an additional possibility of obtaining such properties by embedding a conventional fluorescent material in the polymer matrix containing the elongated metallic nanoparticles (3). Part of the fluorescent material will be sufficiently close to the nanoparticles (3) so that when the excitation light is parallel to them, the fluorescence is amplified, while when it is perpendicular, it is not amplified, resulting in a different fluorescence intensity in the visual safety device.

[0103] A non-random parallel arrangement of elongated metallic nanoparticles (3) can be obtained through shear or flow alignment forces generated during a printing process. When an elongated object (1) is subjected to shear forces in a fluid, for example, elongated nanoparticles suspended in ink and printed using various printing techniques, especially screen printing, they tend to align in parallel. Patent EP1769484A1 (Landqart AG), "Safety Element and Method for its Production," dated April 4, 2007 (04 / 04 / 2007), describes a method for aligning, at least partially, anisotropic shape particles using shear forces during printing. This occurs, for example, through shear forces generated during processing. In interaction with the anisotropic shape of the particles (e.g.(rod-shaped particles), shear forces result in a collective alignment of the particles. Once printing is complete, the ink dries, usually through UV polymerization, keeping the particles aligned.

[0104] Elongated metallic nanoparticles (3) can be aligned in parallel using a squeegee, similar to a screen printing process. Andrii B. Golovin et al., “Aligned layers of silver nanofibers”, Materials 2012, 5(2), 239-247 (doi 10.3390 / ma5020239), described a method for aligning silver nanofibers. This alignment can be enhanced by adding alignment liquid crystals to the particles. In particular, the dye IR806 can be added to a dispersion of silver nanofibers, which is then sheared by mechanical shearing with a metal squeegee. As a result, uniaxial layers of silver nanoparticles (3) can be produced from a colloidal dispersion. In this example, the silver nanofibers exhibit an absorption peak corresponding to wavelengths of 3.5 microns, i.e., in the infrared. This effect is not visible to the naked eye, but it can be detected through forensic analysis to validate the authenticity of the document.

[0105] Extrusion, stretching, flow, or shearing techniques are particularly suitable for the parallel arrangement of nanoparticles with high aspect ratios, such as nanowires, but are less effective for the parallel alignment of shorter, but still elongated, nanoparticles, such as nanowalls. To overcome this limitation, magnetic alignment forces can be used to align any type of elongated nanoparticle. According to one embodiment of the invention, the elongated metallic nanoparticles (3) must contain magnetic materials. These elongated metallic nanoparticles (3) would be printed onto the substrate (2).

[0106] Known security printing devices include one or more magnets that induce the arrangement of optically variable magnetized pigments (MOVPs) magnetized parallel to the magnetic field lines. By modifying the number, shape, and position of the magnets using known methods, different MOVP arrangement patterns can be obtained. The ink is then dried, usually by UV polymerization, fixing the MOVPs to the substrate (2) according to the desired pattern.

[0107] The same printing devices and techniques can be used with an ink containing elongated, magnetized metallic nanoparticles (3) instead of MOVP. When subjected to a magnetic field, the longitudinal axes of the nanoparticles (3) will be parallel to that field. Therefore, it is possible to actively induce a non-random arrangement of elongated metallic nanoparticles (3) incorporating magnetic materials. For example, a set of two nearby magnets with their opposite poles facing each other a short distance apart—a typical example being two neodymium magnets—will create a set of substantially parallel magnetic field lines between them.

[0108] All the preceding examples of non-random arrangement of elongated metallic nanoparticles (3), whether by extrusion, stretching, flow, or shearing techniques, or by magnetic printing, result in a parallel arrangement. However, other arrangements are possible according to the invention, for example, a curved arrangement. This can be obtained by the aforementioned magnetic printing techniques, using two magnets in the printing machine, as shown in Figure 6. In elongated metallic nanoparticles (3) with magnetic properties arranged according to this magnetic field, at least two portions could be established where the nanoparticles (3) would be, respectively, parallel and perpendicular to the electric field vector of the linearly polarized visible light (4) or of the linearly polarized excitation light (4).

Claims

Claims 1. A visual safety element comprising a substrate (2) and elongated metallic nanoparticles (3) arranged non-randomly on the substrate (2), characterized in that, under linearly polarized light (4), the visual safety element produces an optical effect that depends on the angle between the electric field vector of the linearly polarized light (4) and the longitudinal axis of the elongated metallic nanoparticles (3); and in that, at any given angle, at least a portion on the substrate (2) can be predefined where the elongated metallic nanoparticles (3) reflect and / or transmit light at the same wavelength.

2. The visual safety element of claim 1, characterized in that linearly polarized light is visible light and in that the optical effect produced by the visual safety device under linearly polarized visible light (4) is a color change effect.

3. The visual safety element of claim 2, characterized in that the substrate (2) is transparent or semi-transparent and the color change effect is produced by reflected light and transmitted light.

4. The visual safety element of claim 3, wherein the substrate (2) is semi-transparent and is dyed a color.

5. The visual safety element of claim 2, characterized in that the substrate (2) is opaque and the color change effect is produced solely by reflected light.

6. The visual safety element of claim 5, wherein the opaque substrate (2) is painted a color and the elongated metallic nanoparticles (3) are arranged in a portion of the substrate (2).

7. A visual safety element according to claim 1, characterized in that the elongated metallic nanoparticles (3) incorporate at least one fluorescent material and in that, under linearly polarized excitation light (4), the visual safety element produces a fluorescent color whose intensity depends on the angle between the electric field vector of the linearly polarized excitation light (4) and the longitudinal axis of the elongated metallic nanoparticles (3); and in that, at any given angle, at least a portion on the substrate (2) can be predefined in which the elongated metallic nanoparticles (3) emit a fluorescent light of the same intensity.

8. A visual safety element according to claim 7, wherein the substrate (2) is painted with fluorescent ink of a color different from the fluorescent color emitted by the elongated metallic nanoparticles (3) arranged on the substrate (2).

9. Visual safety element according to claim 7, wherein the substrate (2) is painted with fluorescent ink and the elongated metallic nanoparticles (3) are arranged in a portion of the substrate (2).

10. Visual safety element of claims 2 and 7, characterized in that the substrate is divided into at least two zones that produce a different color under linearly polarized visible light (4) and a fluorescent color of different intensity under linearly polarized excitation light (4).

11. Visual safety element according to claim 1, characterized in that the elongated metallic nanoparticles (3) incorporate at least one magnetic material.

12. Visual safety element according to claim 7, characterized in that the elongated metallic nanoparticles (3) incorporating at least one fluorescent material also incorporate at least one magnetic material.

13. An object (1) comprising the safety element of any of the preceding claims.

14. Use of elongated metallic nanoparticles (3) to construct the safety element of any of the preceding claims.

15. A printing ink containing elongated metallic nanoparticles (3) configured according to any of claims 1 to 12.

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