Ink and image reading method

Abstract

To provide an ink capable of having excellent invisibility in a visible light range and also producing an invisible image with excellent readability in a near-infrared range, as well as having excellent storage stability.SOLUTION: An ink comprises: water; a gold nanorod; and at least one surfactant selected from the group consisting of an acetylene glycol-based surfactant having an HLB value of 15 or less and a silicone-based surfactant having an HLB value of 15 or less, wherein when the aspect ratio distribution of the gold nanorod contained in the ink is measured, the mean value μ in the aspect ratio distribution is 6.0 to 13.0, and the standard deviation σ in the aspect ratio distribution is 0.5 to 4.5.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to ink and image reading methods. [Background technology]

[0002] In recent years, "invisible printing," which embeds invisible information into printed materials, has been attracting attention for the purpose of strengthening security, such as copyright protection and counterfeit prevention. Because invisible printing does not degrade the appearance even when it is overlaid on a visible image, it can be used in conjunction with the embedded information while maintaining the quality of normal printed materials, and is expected to be applied in a variety of fields, including security.

[0003] Patent Document 1 discloses that by incorporating specific gold nanorods into toner, it is possible to form an invisible image without impairing the quality of a visible image. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-219103 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-127085 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-118036 [Patent Document 4] Japanese Patent Application Laid-Open No. 2006-169544 [Non-patent literature]

[0005] [Non-Patent Document 1] Chemistry of Materials, 2003, No. 15, pp. 1957-1962 Summary of the Invention [Problem to be solved by the invention]

[0006] As a result of the inventors' investigation of the toner described in Patent Document 1, they recognized that further improvement is required in terms of achieving both image invisibility in the visible light region and image readability in the near-infrared region.

[0007] Furthermore, the inkjet recording method is a method that can easily record high-quality images, and is one of the image recording methods that has been increasingly developed in recent years. In the security field, invisible printing is also required for use in recording images on the spot, such as for checking visitors at events. Therefore, the present inventors have investigated invisible printing using an inkjet method that can easily record high-quality images.

[0008] As a result, the inventors' investigations revealed that, unlike the toner described in Patent Document 1 in which gold nanorods are dispersed in a solid such as a resin, in ink, gold nanorods must be dispersed in a liquid, which poses storage stability issues.

[0009] Patent Document 2 discloses a method for surface treatment of gold nanorods to obtain dried gold nanorods that are redispersible in water, rather than to improve the storage stability of gold nanorods in liquid.

[0010] However, when the gold nanorod surface treatment method described in Patent Document 2 is applied to gold nanorods used in ink, the gold nanorods must be re-dispersed before use, leaving room for improvement in terms of usability. Therefore, it was necessary to stabilize the dispersion of gold nanorods in the ink and improve the storage stability of the ink.

[0011] Therefore, one aspect of the present disclosure is to provide an ink that can provide an invisible image that can have excellent invisibility in the visible light region and excellent readability in the near-infrared region, and that also has excellent storage stability.

[0012] Another aspect of the present disclosure is directed to providing a method for reading an image formed using the ink of the present disclosure. [Means for solving the problem]

[0013] According to one embodiment of the present disclosure, there is provided an ink comprising water, gold nanorods, and at least one surfactant selected from the group consisting of an acetylene glycol-based surfactant having an HLB value of 15 or less and a silicone-based surfactant having an HLB value of 15 or less, When the aspect ratio distribution of the gold nanorods contained in the ink was measured, the average value μ in the aspect ratio distribution is 6.0 to 13.0, The standard deviation σ of the aspect ratio distribution is 0.5 to 4.5. the law of nature, The content of the gold nanorods in the ink is 0.005% by mass to 0.150% by mass based on the total mass of the ink. It is characterized by For forming an image that is invisible in the visible light region and readable in the near infrared region An ink is provided.

[0014] According to another aspect of the present disclosure, there is provided an image reading method comprising the step of reading an image formed using the ink of the present disclosure using a device equipped with a near-infrared sensor. [Effects of the Invention]

[0015] According to one aspect of the present disclosure, an ink can be provided that can obtain an invisible image that can have excellent invisibility in the visible light region and excellent readability in the near-infrared region, and that also has excellent storage stability.

[0016] According to another aspect of the present disclosure, there is provided an image reading method including a step of reading an image formed using the ink of the present disclosure using a device equipped with a near-infrared sensor. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram showing an image observation situation in image quality evaluation in the near-infrared region. DETAILED DESCRIPTION OF THE INVENTION

[0018] Unless otherwise specified, the expressions "xx or more and xx or less" and "xx to xx" representing a numerical range mean a numerical range including the lower and upper limits, which are the endpoints. When a numerical range is described in stages, the upper and lower limits of each numerical range can be combined in any way.

[0019] The ink according to the present disclosure is preferably an ink for forming an invisible image.

[0020] <Background to the invention> The use of gold nanorods described in Patent Document 1 certainly improves the readability of images in the near-infrared region, even in ink. However, the inventors have discovered that increasing the content of gold nanorods to improve image readability tends to reduce the invisibility of the image. The inventors speculate that this is because, in the optical absorption spectrum of an image recorded on a recording medium, the maximum absorptance in the near-infrared region is not sufficiently greater than the maximum absorptance in the visible light region. Therefore, the inventors speculate that increasing the content of gold nanorods in the ink significantly increases not only the maximum absorptance in the near-infrared region but also the maximum absorptance in the visible light region, thereby reducing the invisibility of the image.

[0021] Furthermore, as mentioned above, inks containing gold nanorods may have poor storage stability, which is presumably due to the poor storage stability of the gold nanorod dispersion itself.

[0022] Based on the above considerations, the present inventors conducted further studies and found that an ink containing water, gold nanorods, and at least one surfactant selected from the group consisting of acetylene glycol-based surfactants with an HLB value of 15 or less and silicone-based surfactants with an HLB value of 15 or less, and measuring the aspect ratio distribution of the gold nanorods contained in the ink, where the mean value μ in the aspect ratio distribution is 6.0 to 13.0 and the standard deviation σ in the aspect ratio distribution is 0.5 to 4.5, can form images with the above-mentioned properties while also exhibiting high storage stability.

[0023] By controlling the average and standard deviation of the aspect ratios of the gold nanorods contained in the ink within the above ranges, it was found that the peaks in the optical absorption spectrum of the image become sharper and the maximum absorptance in the near-infrared region becomes relatively large, making it easier to obtain images with excellent invisibility and readability even when the content of gold nanorods in the ink is reduced.

[0024] Furthermore, it was found that by including at least one surfactant selected from the group consisting of acetylene glycol-based surfactants with an HLB value of 15 or less and silicone-based surfactants with an HLB value of 15 or less, the dispersion of gold nanorods in the ink is stabilized, resulting in an ink with excellent storage stability.

[0025] Although the details of the mechanism by which the dispersion of gold nanorods in the ink is stabilized are unknown, the present inventors speculate as follows.

[0026] Gold nanorods are typically synthesized in water containing a cationic surfactant, such as the quaternary ammonium salt hexadecyltrimethylammonium bromide (CTAB). The cationic surfactant, such as CTAB, adsorbs to the surface of the gold nanorods, forming a double layer, which allows the nanorods to disperse in water. However, the adsorption of cationic surfactants, such as CTAB, is reversible, and molecular motion is thought to facilitate their detachment from the surface of the gold nanorods. On the other hand, if the solution used to synthesize gold nanorods contains at least one surfactant selected from the group consisting of acetylene glycol-based surfactants with an HLB value of 15 or less and silicone-based surfactants with an HLB value of 15 or less, these surfactants are further adsorbed to the outer layer of the double layer. This is thought to prevent the cationic surfactant, such as CTAB, from detaching from the surface of the gold nanorods, improving the dispersion stability of the gold nanorods. While there are many other types of surfactants, the inventors believe that the above surfactants specifically exert this effect.

[0027] <Gold nanorods> The ink contains gold nanorods. In this disclosure, gold nanorods refer to metal nanorods whose main component is gold. Furthermore, gold-based metal nanorods refer to metal nanorods whose gold content by mass is 80% or more.

[0028] Metal nanorods are made of precious metals such as gold and silver, and refer to minute metal materials that have a major axis and a minor axis in a TEM image. In other words, metal nanorods appear as roughly rectangular shapes in a TEM image. Generally, the minor axis of a gold nanorod is 1 nm to 60 nm long, and the major axis is 20 nm to 500 nm long. In the present disclosure, the average minor axis length of gold nanorods is preferably 3 nm to 20 nm, and more preferably 5 nm to 10 nm. Furthermore, the average major axis length of gold nanorods is preferably 20 nm to 200 nm, and more preferably 50 nm to 110 nm.

[0029] The value obtained by dividing the length of the long axis of a metal nanorod by the length of its short axis is called the aspect ratio. In this disclosure, a metal nanorod with an aspect ratio of 1.5 or greater is considered a gold nanorod. Gold nanorods exhibit two characteristic plasmon absorption bands (bands corresponding to the excitation of the surface plasmon band), one due to the long axis of the rod and the other due to the short axis of the rod. For example, in the case of gold nanorods, the absorption band due to the short axis is located around 530 nm, and the absorption band due to the long axis is located between 650 nm and 2000 nm.

[0030] The arrangement of gold and metal elements other than gold in gold nanorods can be either an alloy structure complexed at the atomic level, or a core-shell structure in which a gold nanorod is coated with a metal element other than gold. Gold nanorods can also be coated with an inert shell such as silica or polystyrene. Furthermore, depending on the purpose, such as dispersing gold nanorods in a medium, the surface of the gold nanorods can be modified with appropriate molecules such as surfactants.

[0031] <Aspect ratio distribution of gold nanorods> The aspect ratios of the gold nanorods contained in the ink have a distribution, and the aspect ratio distribution is expressed by the mean value and standard deviation of the distribution. The distribution may be a normal distribution, a skewed distribution, or a multi-modal distribution with multiple peaks.

[0032] The light absorption spectrum of gold nanorods changes in a complex manner as the aspect ratio distribution changes. As a result of extensive research, the inventors of the present invention have found that when the aspect ratio distribution of gold nanorods contained in ink is measured, The average value μ in the aspect ratio distribution is 6.0 to 13.0, Ink with a standard deviation σ of 0.5 to 4.5 in aspect ratio distribution It has been found that, when the above-mentioned compound is used, it is easy to obtain an invisible image that can have excellent invisibility in the visible light region and excellent readability in the near-infrared region.

[0033] When the average value μ is within the range of 6.0 to 13.0, an image having a peak in the wavelength range of 1000 nm to 1600 nm in the optical absorption spectrum is easily obtained. It has been found that when the average value is less than 6.0, the peak position of the optical absorption spectrum tends to be closer to the visible light region, resulting in insufficient image invisibility. This is thought to be due to the tendency to absorb light with wavelengths in the visible light region. Furthermore, when the average value is greater than 13.0, the peak position of the optical absorption spectrum may be too far toward the long wavelength side, which tends to reduce readability in devices that read wavelengths in the near-infrared region, such as InGaAs cameras.

[0034] For the above reasons, the average value μ in the aspect ratio distribution is 6.0 or more, preferably 7.0 or more, and more preferably 8.0 or more. Furthermore, the average value μ is 13.0 or less, preferably 12.0 or less, and more preferably 11.0 or less. That is, the preferred range of the average value μ is 7.0 to 12.0, and the more preferred range is 8.0 to 11.0.

[0035] Furthermore, the inventors have found that when the standard deviation σ of the aspect ratio distribution is 4.5 or less, the absorption wavelength of gold nanorods in the near-infrared region is less likely to vary, and the maximum absorptance in this region is relatively greater than the maximum absorptance in the visible light region.

[0036] It is easy to obtain an invisible image having excellent invisibility and readability of the invisible image. Therefore, the standard deviation σ is 4.5 or less, preferably 3.5 or less, and more preferably 2.5 or less. There is no particular lower limit, but the standard deviation σ is 0.5 or more.

[0037] That is, a preferred range of the standard deviation σ is 0.5 to 3.5, and a more preferred range is 0.5 to 2.5.

[0038] The above average value μ and standard deviation σ can be controlled by adjusting the reaction conditions and purification conditions, as will be described later.

[0039] <Gold nanorod content> The content of gold nanorods in the ink is preferably 0.005% by mass to 0.150% by mass based on the total mass of the ink. If it is 0.005% by mass or more, it is easy to obtain an invisible image with excellent readability in the near-infrared region. Therefore, it is preferably 0.005% by mass or more, and more preferably 0.010% by mass or more. Furthermore, if it is 0.150% by mass or less, coloring is difficult to see in the visible region, and stable ink ejection from an inkjet recording head is easy to achieve. Therefore, it is preferably 0.150% by mass or less, and more preferably 0.100% by mass or less.

[0040] <Gold nanoparticles> In this disclosure, gold nanoparticles are nanomaterials primarily composed of gold, and are considered to have an aspect ratio of less than 1.5 in TEM images. In other words, in this disclosure, those with an aspect ratio of 1.5 or greater are considered "gold nanorods," and those with an aspect ratio of less than 1.5 are considered "gold nanoparticles." The ink preferably contains gold nanoparticles at a number percentage of 30% or less of the gold nanorods. Gold nanoparticles may be generated during the preparation of gold nanorods. Gold nanoparticles exhibit absorption in the visible light range according to their particle size. For example, a particle size of 20 nm exhibits light absorption around 520 nm.

[0041] When the gold nanoparticle content is 30% or less by number relative to the number of gold nanorods, the light absorption rate in the visible light region corresponding to the absorption wavelength of gold nanoparticles is low, and an image with excellent invisibility is easily obtained. More preferably, the above proportion is 15% or less by number, even more preferably 10% or less by number, and particularly preferably 5% or less by number.

[0042] The proportion of gold nanoparticles can be controlled by the reaction conditions and purification conditions, as described below.

[0043] <Preparation of gold nanorods> Gold nanorods can be synthesized, for example, by the method proposed by B. Nikoobakft and M.A. El-Sayed (Non-Patent Document 1). Specifically, this method involves reducing chloroauric acid (HAuCl4) with ascorbic acid in an aqueous solution containing two surfactants (hexadecyltrimethylammonium bromide and benzyldimethylhexadecylammonium chloride).

[0044] Another method for synthesizing gold nanorod particles involves reducing gold ions in an aqueous solution containing an excess of the quaternary ammonium salt cetyltrimethylammonium bromide (CTAB), as described in Patent Documents 3 and 4. In this method, a solution containing seed particles is first prepared by adding an aqueous solution of CTAB to an aqueous solution of chloroauric acid tetrahydrate and then adding sodium borohydride. A solution containing a mixture of silver nitrate, chloroauric acid tetrahydrate, L-ascorbic acid, and CTAB is then added to the solution, and the mixture is left to stand for a certain period of time, or the solution is added in small amounts at a time. This facilitates anisotropic growth of the seed particles as nuclei, resulting in gold nanorods.

[0045] In addition, adding benzyldimethylhexadecylammonium chloride during seed particle growth can produce gold nanorods with a high aspect ratio. Alternatively, gold nanorods with a high aspect ratio can be obtained by first reducing the seed particles with sodium borohydride, a strong reducing agent, and then reducing them with triethylamine, a weak reducing agent.

[0046] Furthermore, gold nanorods can be purified as needed to adjust the aspect ratio distribution before use. Any commonly known purification method can be used, such as density gradient ultracentrifugation. Specifically, mixed solutions of sucrose and CTAB with different concentrations are first prepared and layered in a centrifuge tube in order of concentration gradient. A gold nanorod sample is then layered on top of this and centrifuged to separate the gold nanorods by density and size. By separating and purifying the gold nanorods in this manner, the standard deviation σ is reduced, resulting in gold nanorods with a narrower aspect ratio distribution.

[0047] <Surfactant> The ink contains at least one surfactant selected from the group consisting of acetylene glycol surfactants with an HLB value of 15 or less and silicone surfactants with an HLB value of 15 or less. Examples of acetylene glycol surfactants with an HLB value of 15 or less include nonionic surfactants with a central acetylene group and a symmetrical structure. Those with an added ethylene oxide group are preferred. Specific examples include Surfynol 104 (HLB value 4), Surfynol 440 (HLB value 8), and Surfynol 465 (HLB value 13) (all manufactured by Nissin Chemical Industry Co., Ltd.), and Acetylenol E40 (HLB value 10), E60 (HLB value 12), and E100 (HLB value 14) (all manufactured by Kawaken Fine Chemicals).

[0048] Examples of silicone surfactants with an HLB value of 15 or less include siloxane surfactants having one or more ethylene oxide groups and / or one or more propylene oxide groups on the side chain and / or both ends of the polydimethylsiloxane chain. Specific examples include BYK-347 (HLB value 9), BYK-348 (HLB value 11) (trade names, manufactured by BYK Japan), KF-351A (HLB value 12), KF-352A (HLB value 7), KF-353 (HLB value 10), KF-355A (HLB value 12), KF-615A (HLB value 10), KF-945 (HLB value 4), KF-640 (HLB value 14), KF-642 (HLB value 15), and the like. KF-6015 (HLB value 5), KF-6017 (HLB value 5), KF-6204 (HLB value 10) (all manufactured by Shin-Etsu Chemical Co., Ltd.). These may be used alone or in combination of two or more.

[0049] The content of the surfactant is preferably 0.01% by mass to 1% by mass, and more preferably 0.05% by mass to 0.5% by mass, based on the total mass of the ink. When the content is 0.01% by mass or more, the dot diameter tends to be sufficiently enlarged. When the content is 1% by mass or less, the storage stability of the ink is further improved.

[0050] <HLB value of surfactant> In the present disclosure, the HLB value means the HLB value determined by the Griffin method. When two or more of the above surfactants are used in combination, the weighted average value is used as the HLB value. The HLB value according to the Griffin method is calculated as follows: HLB value = 20 x surfactant Formula weight of ethylene oxide group of agent / surfactant The HLB value obtained by the Griffin method can be calculated from the formula: surfactant HLB is a physical property value that represents the degree of hydrophilicity or lipophilicity of a surfactant (compound), and takes a value between 0 and 20. The smaller the HLB value, the higher the lipophilicity, and the higher the HLB value, the higher the hydrophilicity. If the HLB value is greater than 15, the storage stability of the ink may decrease due to the presumed mechanism described above. Furthermore, it is preferable that the HLB value of the surfactant is 5 or greater. An HLB value of 5 or greater makes it easier to obtain sufficient dot spreading on the recording medium, further improving the uniformity of the printed matter. The HLB value determined by the Griffin method is a physical property value that applies to compounds that do not have ionic groups (non-ionic compounds).

[0051] <Aqueous medium> The ink is an aqueous ink containing water as an aqueous medium. Deionized water or ion-exchanged water is preferred. The water content (mass %) in the ink is preferably 50.00% to 99.00% by mass, and more preferably 70.00% to 99.00% by mass, based on the total mass of the ink. The ink may further contain a water-soluble organic solvent as an aqueous medium. The water-soluble organic solvent is not particularly limited as long as it is water-soluble, and alcohols, (poly)alkylene glycols, glycol ethers, nitrogen-containing polar solvents, sulfur-containing polar solvents, and the like can be used. The water-soluble organic solvent content (mass %) in the ink is preferably 1.00% to 30.00% by mass, and more preferably 3.00% to 20.00% by mass, based on the total mass of the ink. By keeping the water-soluble organic solvent content within the above range, surfactants such as CTAB on the gold nanorods are less likely to be detached, further improving the dispersion stability of the gold nanorods in the ink. Furthermore, reliability such as sticking resistance and ejection stability is further improved.

[0052] <Additives> The ink may contain various additives as needed, such as surfactants other than the above surfactants, pH adjusters, surface slip agents, rust inhibitors, preservatives, antifungal agents, antioxidants, antireducing agents, evaporation accelerators, and chelating agents. In addition, the ink according to the present disclosure preferably does not contain a coloring component that would make the printed image a visible image.

[0053] <Recording Media> Any known recording medium can be used, but it is preferable to use a recording medium that has ink permeability, such as a recording medium with an ink-receiving layer (glossy paper or PET film with a receiving layer).

[0054] <Light absorption rate in the visible light range> Using the ink according to the present disclosure, the ink application amount was 1.9 mg / cm 2In a spectroscopic analysis of the solid image formed as above, it is preferable that the maximum light absorptance in the wavelength range of 400 nm to 800 nm is 10% or less. This wavelength range corresponds to the visible light range, and if the light absorptance is 10% or less, the image becomes substantially difficult to recognize with the naked eye, and an image with excellent invisibility in the visible light range is obtained.

[0055] <Light absorption rate in the near-infrared region> In addition, the amount of ink applied was 1.9 mg / cm 2 year Te form In spectroscopic analysis of the solid image, the maximum light absorptance in the wavelength range of 900 nm to 1800 nm is preferably 5% or more. This wavelength range corresponds to the near-infrared region, and if the light absorptance is 5% or more, it is easy to clearly read the invisible image using a near-infrared camera such as an InGaAs camera. More preferably, it is 10% or more.

[0056] Furthermore, when actually using it as invisible ink, light absorption in the invisible region can be visualized by using a camera equipped with a sensor (such as an InGaAs sensor) that is sensitive to the 900 nm to 1800 nm wavelength range.

[0057] Visible light region and near The light absorption rate in the infrared region can be controlled by adjusting the aspect ratio distribution of gold nanorods.

[0058] <How to read an image> The method for reading an image formed using the ink according to the present disclosure is not particularly limited. Because images formed using the ink according to the present disclosure readily absorb wavelengths in the near-infrared region, an image reading method using a device equipped with a near-infrared sensor is preferred. More preferably, an device equipped with an InGaAs sensor is used, and even more preferably, an InGaAs camera is used. Furthermore, an image reading method using light with a wavelength of 900 nm to 2500 nm is preferred. More preferably, the wavelength is 900 nm to 1800 nm.

[0059] <Ink manufacturing method> The ink according to the present disclosure can be obtained by a typical ink manufacturing method, except that gold nanorods having the mean value μ and standard deviation σ of the aspect ratio distribution described above and a surfactant having the HLB value described above are used. Specifically, the ink can be manufactured through the following steps: (1) preparing the gold nanorods described above; and (2) mixing ink components containing the gold nanorods and the surfactant described above. Step (1) above can be performed according to the method for manufacturing gold nanorods described above.

[0060] Furthermore, if necessary, a purification step of purifying the gold nanorods may be provided between the above steps (1) and (2) in order to increase the content of gold nanorods in the gold nanorod dispersion.

[0061] <Various measurement methods, etc.> The various physical properties are measured as follows.

[0062] <Method for quantifying metal content in metal nanorod dispersion> The metal content in a metal nanorod dispersion can be quantified by ICP optical emission spectroscopy in accordance with JIS K 0116-2014. First, the metal nanorod dispersion is heated to 60°C on a hot plate to dryness. Aqua regia is added to the dispersion, and microwave acid decomposition is performed using an ETHOS PRO (Milestone General) or similar device. The metal content can be quantified by ICP optical emission spectroscopy of the resulting liquid using a CIROS CCD (SPECTRO).

[0063] <Method for measuring the aspect ratio distribution of gold nanorods contained in ink> To measure the aspect ratio distribution of the gold nanorods contained in the ink, first, the gold nanorods contained in the ink are separated and collected.

[0064] The procedure is to first sediment the gold nanorods in the ink by centrifugation and separate the supernatant. After separating the supernatant, new solvent is added to the dispersion containing the gold nanorods, and the mixture is centrifuged again to separate the supernatant. This process is repeated several times to wash the mixture, and finally the solvent is dried off, allowing the gold nanorods to be separated and recovered from the ink. The recovered gold nanorods are then redispersed in a solvent such as THF to serve as the measurement sample.

[0065] The measurement sample dispersed in a solvent is dropped onto a support film and dried, and then TEM observation is performed using a transmission electron microscope such as the Technai F30 (FEI). The lengths of the long and short axes of the observed gold nanorods are determined using image processing software such as Photoshop, which allows the average and standard deviation of the aspect ratio distribution, as well as the average length of the long axis of the gold nanorods, to be determined.

[0066] <Method for measuring the ratio of gold nanoparticles to gold nanorods contained in ink> The ratio of the number of gold nanoparticles to the number of gold nanorods contained in the ink can be measured by counting the number of gold nanoparticles and the number of gold nanorods using the TEM observation described above and calculating the ratio.

[0067] <Method for measuring the amount of gold nanorods contained in ink> The content (mass %) of gold nanorods in the ink is determined by fluorescent X-ray analysis in accordance with JIS K 0119-1969, specifically as follows.

[0068] The measurement equipment used was a wavelength dispersive X-ray fluorescence analyzer "Axios" (PANalytical) and the accompanying dedicated software "SuperQ ver.4.0F" (PANalytical) for setting measurement conditions and analyzing measurement data. Rh was used as the anode of the X-ray tube, the measurement atmosphere was vacuum, the measurement diameter (collimator mask diameter) was 27 mm, and the measurement time was 10 seconds. Light elements were detected using a proportional counter (PC), and heavy elements were detected using a scintillation counter (SC).

[0069] The measurement sample is prepared by applying 4 mL of ink to a glass substrate so as to have a diameter of 39 mm and allowing it to dry.

[0070] Measurements are performed under the above conditions, and elements are identified based on the peak positions of the obtained X-rays. The counting rate (unit: kcps), which is the number of X-ray photons per unit time, is then measured. A separately prepared calibration curve is then used to calculate the content of gold nanorods in the ink. When preparing the calibration curve, a sample of ink containing a predetermined amount of gold nanorods, prepared in the same manner as above, is used as the measurement sample.

[0071] <Ink cartridges> The ink of the present disclosure may be contained in an ink cartridge. The ink cartridge includes ink and an ink storage section that stores the ink. The ink stored in the ink storage section is the ink of the present disclosure described above. The ink storage section may be configured to hold all of the ink stored therein using an absorber. Alternatively, the ink storage section may not have an absorber and may store all of the ink in a liquid state. Furthermore, the ink cartridge may be configured to have an ink storage section and an inkjet recording head.

[0072] <Inkjet recording method> The ink of the present disclosure can be used in an inkjet recording method. The inkjet recording method is a method in which the ink of the present disclosure described above is ejected from an inkjet recording head to record an image on a recording medium. Methods for ejecting the ink include a method in which mechanical energy is applied to the ink and a method in which thermal energy is applied to the ink. In the present invention, it is particularly preferable to employ a method in which the ink is ejected by applying thermal energy to the ink. Other than using the ink of the present disclosure, the steps of the inkjet recording method may be known.

[0073] In addition, from the viewpoint of the drying property of the ink applied to the recording medium, the amount of ink applied to the recording medium is set to 3.0 mg / cm 2 2 Preferably, it is 2.5 mg / cm or less. 2 It is even more preferable that: [Example]

[0074] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited thereto. The measurement results in the examples were measured using the measurement methods described above.

[0075] [Preparation example of gold nanorod dispersion A] First, a seed particle solution was prepared by mixing 500 mL of a 0.0005 mol / L aqueous solution of chloroauric acid tetrahydrate (Kishida Chemical Co., Ltd.) and 500 mL of a 0.2 mol / L aqueous solution of cetyltrimethylammonium bromide (Kishida Chemical Co., Ltd.). 60 mL of 0.01 mol / L sodium borohydride (Tokyo Chemical Industry Co., Ltd.) was added to this solution to obtain a seed particle solution (Solution A).

[0076] Next, 10 g of cetyltrimethylammonium bromide was dissolved in 500 mL of a 0.15 mol / L aqueous solution of benzyldimethylhexadecylammonium chloride (Tokyo Chemical Industry Co., Ltd.). 20 mL of a 0.004 mol / L aqueous solution of silver nitrate was added to the aqueous solution containing these two surfactants. 500 mL of a 0.001 mol / L aqueous solution of chloroauric acid tetrahydrate was added to this solution, followed by 7 mL of a 0.078 mol / L aqueous solution of L-ascorbic acid (Kishida Chemical Co., Ltd.). This solution was designated Solution B.

[0077] Next, 10 g of cetyltrimethylammonium bromide was dissolved in 500 mL of a 0.15 mol / L aqueous solution of benzyldimethylhexadecylammonium chloride (Tokyo Chemical Industry Co., Ltd.). 20 mL of a 0.004 mol / L aqueous solution of silver nitrate was added to the aqueous solution containing these two surfactants. 500 mL of a 0.0005 mol / L aqueous solution of chloroauric acid tetrahydrate was added to this solution, followed by 3.6 mL of a 0.078 mol / L aqueous solution of L-ascorbic acid (Kishida Chemical Co., Ltd.). This solution was designated Solution C.

[0078] 1.2 mL of Solution A was added dropwise to Solution B, followed by 2.0 mL of Solution C at a rate of 1.0 mL / 20 min, allowing seed particles to grow anisotropically as nuclei. After 5 minutes of centrifugation at 10,000 × g, the gold nanorods were redispersed in water to a concentration of 0.35 mass % to obtain a gold nanorod dispersion (Dispersion A). The physical properties of the gold nanorods contained in Dispersion A are shown in Table 1.

[0079] [Preparation examples of gold nanorod dispersions B to J] Dispersions B to J were obtained by carrying out the same operations as in the preparation example of gold nanorod dispersion A, except that the amount of solution C and whether or not the following purification step was performed were changed as shown in Table 1.

[0080] (purification process) Six milliliters of the resulting gold nanorod dispersion was centrifuged at 10,000 x g for 5 minutes, and the resulting pellet was resuspended in 0.05 mL of 0.01 M CTAB aqueous solution to obtain a gold nanorod suspension. Additionally, solutions were prepared by adding sucrose to the 0.01 M CTAB solution at concentrations of 10, 15, 20, and 25% by mass. These sucrose-containing solutions were layered in 15 mL polyallomer tubes, 3 mL each, in descending order of sucrose concentration, and finally the gold nanorod suspension was layered on top. The solution was then centrifuged at 10,750 x g for 15 minutes at 25°C using an Avanti JXN-30 high-speed refrigerated centrifuge. After centrifugation, the solution was divided into 300 μL fractions, and TEM images of each fraction containing gold nanorods were observed. The fractions with the desired average aspect ratio μ and standard deviation σ calculated from the TEM images were centrifuged at 10,000 × g for 5 minutes, and the resulting precipitate, gold nanorods, were recovered and redispersed in water to a content of 0.35% by mass.

[0081] [Table 1]

[0082] The average major axis length in Table 1 refers to the average major axis length of the gold nanorods contained in the ink, and the ratio of the number of gold nanoparticles refers to the ratio of the number of gold nanoparticles to the number of gold nanorods contained in the ink.

[0083] [Ink 1 manufacturing example] The components shown below were mixed and thoroughly stirred, and then pressure filtered through a microfilter (manufactured by Fujifilm) with a pore size of 3.0 μm to obtain Ink 1. The remainder of the ion-exchanged water refers to the amount that makes the total amount of all components of the ink 100.0%. Gold nanorod dispersion A: 29% Surfactant (product name: Acetylenol AE60): 0.2% Ethylene glycol: 10% Ion-exchanged water: Remainder

[0084] [Ink 2-29 manufacturing example] Inks 2 to 29 were obtained in the same manner as in the production example of ink 1, except for the changes shown in Tables 2 to 4. Details of the surfactants listed in Tables 2 to 4 are given below.

[0085] <Surfactant> AE60: Product name "Acetylenol E60" (Kawaken Fine Chemicals) (HLB value 12) AE40: Product name "Acetylenol E40" (Kawaken Fine Chemicals) (HLB value 10) AE100: Product name "Acetylenol E100" (Kawaken Fine Chemicals) (HLB value 14) KF945: Product name "KF-945" (manufactured by Shin-Etsu Chemical Co., Ltd.) (HLB value 4) KF6015: Product name "KF-6015" (manufactured by Shin-Etsu Chemical Co., Ltd.) (HLB value 5) KF6204: Product name "KF-6204" (manufactured by Shin-Etsu Chemical Co., Ltd.) (HLB value 10) BYK348: Product name "BYK-348" (manufactured by BYK Japan) (HLB value 11) KF642: Product name "KF-642" (manufactured by Shin-Etsu Chemical Co., Ltd.) (HLB value 12) KF640: Product name "KF-640" (manufactured by Shin-Etsu Chemical Co., Ltd.) (HLB value 14) AE200: Product name "Acetylenol E200" (Kawaken Fine Chemicals) (HLB value 16) KF354L: Product name "KF-354L" (manufactured by Shin-Etsu Chemical Co., Ltd.) (HLB value 16) Emulgen 105: Product name "Emulgen 105" (Kao) (HLB value 10) Megafuck F-444: Product name "Megafuck F-444" ( DIC (HLB value 9)

[0086] AE60, AE40, AE100, and AE200 are acetylene glycol surfactants. KF945, KF6015, KF6204, BYK348, KF642, KF640, and KF354L are silicone surfactants. Emulgen 105 is a polyoxyalkylene alkyl ether surfactant. Megafac F-444 is a fluorine-based surfactant.

[0087] [Table 2]

[0088] [Table 3]

[0089] [Table 4]

[0090] Example 1 <Evaluation of image quality in the near-infrared region> The image recording device used was a material printer (product name: DMP-2850, manufactured by Fujifilm Corporation), which is an inkjet recording device. A PET film with an ink-receiving layer (product name: CG3110, manufactured by 3M Corporation) was used as the output medium, and images were output in an environment of 25°C and 60% relative humidity. The resolution was 1270 dpi, the ejection volume was 7.5 pL, and the applied amount of Ink 1 was 1.9 mg / cm. 2 A rectangular solid image of 2 cm x 3 cm was formed so as to obtain an evaluation image.

[0091] Next, a light source 202 and a camera 203 were set up as shown in FIG. 1, and an evaluation image 201 was observed. Specifically, the evaluation image was placed on a desk, and infrared light was irradiated from the light source 202 at a 15° angle from a distance of approximately 1 m relative to the evaluation image. Camera 203 was set up 15 cm directly above the evaluation image, and an image was taken. The light source 202 was a halogen lamp light source (product name: PCS-UHX-150, manufactured by Nippon PI Corporation) equipped with a visible light cut filter unit. The camera 203 was a near-infrared camera (product name: NVU3VD, manufactured by IR Spec, InGaAs camera) with a lens filter that cuts wavelength components of 800 nm or less. The spectral sensitivity wavelength range of the near-infrared camera was 970 nm to 1650 nm.

[0093] <Evaluation of invisibility in the visible light region> <Method for measuring light absorption in the wavelength range of 400 nm to 800 nm> Using the above image recording device and the above PET film, the amount of ink 1 applied was 1.9 mg / cm 2 A rectangular image of 2 cm x 3 cm was formed so as to obtain a sample image.

[0094] The sample image was subjected to spectroscopic analysis using a photometer in the wavelength range of 400 nm to 800 nm, and the maximum absorbance in the measurement was taken as the measured value (%) of the sample image. The photometer used was an ultraviolet-visible-near-infrared spectrophotometer (product name: UV-3600, manufactured by Shimadzu Corporation). Spectroscopic analysis of a PET film alone (a PET film without an image formed on it) was also performed as a blank.

[0095] The above measured value was taken as the visible light absorptance (%), and the invisibility of the ink was evaluated using the visible light absorptance (%). The results are shown in Table 5. When an image with a visible light absorptance of 13% was visually confirmed, it was at a level that could be used as an invisible image.

[0096] <Evaluation of readability in the near-infrared region> <Method for measuring light absorption in the wavelength range of 900 nm to 1800 nm> The sample images used in the above invisibility evaluation were subjected to spectroscopic analysis measurement in the wavelength range of 900 nm to 1800 nm using an ultraviolet-visible-near-infrared spectrophotometer (product name: UV-3600, manufactured by Shimadzu Corporation). The maximum absorption rate in the measurement was determined as the maximum absorption rate of the sample image. near The infrared absorption rate (%) was measured. Spectroscopic analysis of the PET film alone was also performed as a blank. The results are shown in Table 5. near When an image with an infrared absorption rate of 5% was checked using the above camera, it was only readable as an invisible image. near When an image with an infrared absorption rate of 2% was checked using the above camera, it was determined that the level was below the level at which it could be read as an invisible image.

[0097] <Evaluation of ink storage stability> 100 g of ink was placed in a 180 mL glass airtight container, which was then sealed and stored in an oven at 60° C. The ink was then visually observed after 3, 6, and 7 days of storage, and the storage stability of the ink was evaluated according to the following evaluation criteria. A: No change in the appearance of the ink was observed up to 7 days after storage. B: No change in the appearance of the ink was observed up to 6 days after storage, but after 7 days of storage, the color of the ink gradually became darker from the top to the bottom of the container. C: After three days of storage, the ink color gradually darkened from the top to the bottom of the container. D: Sediment had formed in the container after 3 days of storage.

[0098] <Example 2~ 10、13~ twenty three, Reference examples 1~2, and Comparative Examples 1 to 6> As shown in Table 5, the same evaluation as in Example 1 was carried out, except that ink 1 was changed to inks 2 to 29. The results are shown in Table 5.

[0099] [Table 5] [Explanation of symbols]

[0100] 201 evaluation images 202 Light source 203 Camera

Claims

1. An ink containing water, gold nanorods, and at least one surfactant selected from the group consisting of an acetylene glycol surfactant having an HLB value of 15 or less and a silicone surfactant having an HLB value of 15 or less, When the aspect ratio distribution of the gold nanorods contained in the ink was measured, the average value μ in the aspect ratio distribution is 6.0 to 13.0, the standard deviation σ in the aspect ratio distribution is 0.5 to 4.5; An ink for forming an image that is invisible in the visible light region and readable in the near-infrared region, characterized in that the content of gold nanorods contained in the ink is 0.005% by mass to 0.150% by mass based on the total mass of the ink.

2. 2. The ink according to claim 1, wherein the mean value μ is 7.0 to 12.0 and the standard deviation σ is 0.5 to 3.

5.

3. 3. The ink according to claim 1, wherein the average value μ is 8.0 to 11.0, and the standard deviation σ is 0.5 to 2.

5.

4. 4. The ink according to claim 1, wherein the gold nanorods contained in the ink have an average major axis length of 50 nm to 110 nm.

5. 5. The ink according to claim 1, wherein the content of gold nanoparticles having an aspect ratio of less than 1.5 in the ink is 30% or less by number relative to the number of gold nanorods.

6. 6. The ink according to claim 1, wherein the content of gold nanoparticles having an aspect ratio of less than 1.5 in the ink is 10% or less by number relative to the number of gold nanorods.

7. The ink according to any one of claims 1 to 6, wherein the gold nanorods are protected with CTAB.

8. 8. The ink according to claim 1, wherein the content of the surfactant in the ink is 0.01% by mass to 1% by mass based on the total mass of the ink.

9. 9. The ink according to claim 1, wherein the content of the surfactant in the ink is 0.05% by mass to 0.5% by mass based on the total mass of the ink.

10. 10. The ink according to claim 1, wherein the surfactant in the ink has an HLB of 5 or more.

11. Using the ink, the ink loading was 1.9 mg / cm ２ The ink according to any one of claims 1 to 10, wherein, in spectroscopic analysis of an image formed as above, the maximum light absorptance in the wavelength range of 400 nm to 800 nm is 10% or less.

12. Using the ink, the ink loading was 1.9 mg / cm ２ The ink according to any one of claims 1 to 11, wherein, in spectroscopic analysis of an image formed as above, the maximum light absorptance in the wavelength range of 900 nm to 1800 nm is 5% or more.

13. A method for reading an image, comprising:

13. A method for reading an image, comprising the step of reading, using a device equipped with a near-infrared sensor, an image that is invisible in the visible light region and readable in the near-infrared region, the image being formed using the ink according to any one of claims 1 to 12.

14. 14. The image reading method according to claim 13, wherein the device equipped with the near-infrared sensor is a device equipped with an InGaAs sensor.

15. 15. The image reading method according to claim 13, wherein the device equipped with the near-infrared sensor is an InGaAs camera.

Images