Ink and image reading methods

Abstract

An ink comprising water, gold nanorods, and at least one surfactant selected from the group consisting of acetylenic diol surfactants with an HLB value of less than 15 and organosilicon surfactants with an HLB value of less than 15, characterized in that, when the aspect ratio distribution of the gold nanorods contained in the ink is measured, the average value μ of the aspect ratio distribution is from 6.0 to 13.0, and the standard deviation σ of the aspect ratio distribution is from 0.5 to 4.5.

Description

Technical Field

[0001] This disclosure relates to methods for reading ink and images. Background Technology

[0002] In recent years, "invisible printing," which embeds invisible information into printed materials to enhance security, such as for copyright protection and anti-counterfeiting, has attracted attention. The expectation is that invisible printing can be applied to various fields, including security, because even when superimposed on a visible image, it is unlikely to degrade the appearance of the visible image, allowing the use of embedded information while maintaining normal print quality.

[0003] Patent document 1 discloses that a toner containing specific gold nanorods can form an invisible image without degrading the quality of a visible image.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2007-219103

[0007] Patent Document 2: Japanese Patent Application Publication No. 2009-127085

[0008] Patent Document 3: Japanese Patent Application Publication No. 2006-118036

[0009] Patent Document 4: Japanese Patent Application Publication No. 2006-169544

[0010] Non-patent literature

[0011] Non-patent literature 1: Chemistry of Materials, 2003, Vol.15, pp.1957-1962 Summary of the Invention

[0012] The problem the invention aims to solve

[0013] The inventors examined the toner disclosed in Patent Document 1 and recognized that the toner should be further improved in terms of compatibility between image invisibility in the visible light region and image readability in the near-infrared region.

[0014] Inkjet printing, which can easily record high-quality images, is one of the image recording methods that has seen increasing development in recent years. Invisible printing in the security field also requires on-site use, for example, for entry checks at events. Therefore, the inventors have researched inkjet invisible printing that can easily record high-quality images.

[0015] As a result, the inventors discovered that the ink has a problem with storage stability because the ink requires gold nanorods to be dispersed in a liquid, which is different from the toner disclosed in Patent Document 1, in which gold nanorods are dispersed in a solid.

[0016] Patent document 2 discloses a surface treatment method for gold nanorods to obtain dried gold nanorods that can be redispersed in water, rather than to improve the storage stability of gold nanorods in liquids.

[0017] However, when the surface treatment method for gold nanorods disclosed in Patent Document 2 is applied to gold nanorods used in ink, the gold nanorods must be redispersed before use, leaving room for improvement in usability. Therefore, the dispersion of gold nanorods in ink should be stabilized to improve the storage stability of the ink.

[0018] Therefore, one aspect of this disclosure aims to provide an ink that can form invisible images with excellent invisibility in the visible light region and excellent readability in the near-infrared region, and has excellent storage stability.

[0019] Another aspect of this disclosure aims to provide a method for reading images formed by the ink disclosed herein.

[0020] Solution for solving the problem

[0021] One aspect of this disclosure provides an ink comprising water, gold nanorods, and at least one surfactant selected from the group consisting of acetylenic diol surfactants with an HLB value of less than 15 and organosilicon surfactants with an HLB value of less than 15.

[0022] When measuring the aspect ratio distribution of gold nanorods,

[0023] The average μ of this aspect ratio distribution ranges from 6.0 to 13.0; and

[0024] The standard deviation σ of this aspect ratio distribution is 0.5 to 4.5.

[0025] Furthermore, another aspect of this disclosure provides an image reading method, which includes reading an image formed with the ink disclosed herein using a device including a near-infrared sensor.

[0026] The effects of the invention

[0027] One aspect of this disclosure provides an ink that can form invisible images with excellent invisibility in the visible light region and excellent readability in the near-infrared region, and has excellent storage stability.

[0028] Furthermore, another aspect of this disclosure may provide an image reading method comprising reading an image formed with the ink disclosed herein using a device including a near-infrared sensor. Attached Figure Description

[0029] [ Figure 1 [Illustration] is a schematic diagram showing the image observation when evaluating image quality in the near-infrared region. Detailed Implementation

[0030] Unless otherwise stated, expressions indicating numerical ranges, such as "above ○○ and below ××" and "○○ to ××", refer to ranges that include both the lower and upper limits as endpoints. When some numerical ranges are presented in stages, the lower and upper limits of each range can be combined as needed.

[0031] The ink according to this disclosure is preferably an ink used for forming invisible images.

[0032] <Background of the Invention>

[0033] Using the gold nanorods disclosed in Patent Document 1, even when used in ink, can certainly enhance the readability of images in the near-infrared region. However, the inventors have found that when the gold nanorod content is increased to enhance image readability, the invisibility of the image tends to decrease. The inventors speculate that this is because the optical absorption spectrum of the image recorded on the recording medium exhibits a less than large maximum absorption rate in the near-infrared region compared to the visible light region. The inventors speculate that thus increasing the gold nanorod content in the ink not only increases the maximum absorption rate in the near-infrared region but also correspondingly increases the maximum absorption rate in the visible light region, thereby reducing the invisibility of the image.

[0034] Furthermore, some inks containing gold nanorods exhibit insufficient storage stability. This may be due to the inherently low storage stability of the gold nanorod dispersion itself.

[0035] The inventors examined the above considerations and determined that the ink can simultaneously achieve high storage stability and the formation of images with the aforementioned characteristics. The ink comprises water, gold nanorods, and at least one surfactant selected from the group consisting of acetylenic diol surfactants with an HLB value of 15 or less and organosilicon surfactants with an HLB value of 15 or less, and when the aspect ratio distribution of the gold nanorods is measured, the average value μ of the aspect ratio distribution is 6.0 to 13.0, and the standard deviation σ is 0.5 to 4.5.

[0036] It was found that controlling the average and standard deviation of the aspect ratio of gold nanorods in ink within the aforementioned range resulted in narrower peaks in the optical absorption spectrum of the image and a relatively increased maximum absorption rate in the near-infrared region. Therefore, even with a reduced content of gold nanorods in the ink, it is still possible to produce images with excellent invisibility and readability.

[0037] Furthermore, it was found that when the ink contains at least one surfactant selected from the group consisting of acetylenic diol surfactants with an HLB value of less than 15 and organosilicon surfactants with an HLB value of less than 15, the gold nanorods are stably dispersed in the ink, thereby providing ink with excellent storage stability.

[0038] The detailed mechanism by which gold nanorods are stably dispersed in ink is unknown, but the inventors speculate as follows.

[0039] Typically, gold nanorods are synthesized in water containing a cationic surfactant such as hexadecyltrimethylammonium bromide (CTAB) as a quaternary ammonium salt. In this process, the cationic surfactant, such as CTAB, adsorbs onto the surface of the gold nanorods to form a bilayer, thereby dispersing the gold nanorods in the water. However, the adsorption of cationic surfactants such as CTAB is reversible, and molecular motion readily causes the surfactant to detach from the surface of the gold nanorods. When the liquid used to synthesize the gold nanorods contains at least one surfactant selected from the group consisting of acetylenic diol surfactants with an HLB value of 15 or less and organosilicon surfactants with an HLB value of 15 or less, the surfactant is further adsorbed onto the outer side of the bilayer. It is highly likely that the detachment of cationic surfactants such as CTAB from the surface of the gold nanorods is thus suppressed, thereby improving the dispersion stability of the gold nanorods. The inventors believe that surfactants possessing the above-described properties particularly function in this manner among various surfactants.

[0040] <Gold Nanorods>

[0041] The ink contains gold nanorods. In this disclosure, gold nanorods refer to metallic nanorods primarily composed of gold. The metallic nanorods primarily composed of gold contain gold in a proportion of 80% or more relative to the mass of the metallic nanorods.

[0042] Metal nanorods are fine metallic materials made of precious metals such as gold or silver, exhibiting both a long axis and a short axis in TEM images. In other words, the metal nanorods are observed to be substantially rectangular in TEM images. Gold nanorods typically have a short axis with a length of 1 nm to 60 nm and a long axis with a length of 20 nm to 500 nm. In this disclosure, the average length of the short axis of the gold nanorods is preferably 3 nm to 20 nm, more preferably 5 nm to 10 nm. Furthermore, the average length of the long axis of the gold nanorods is preferably 20 nm to 200 nm, more preferably 50 nm to 110 nm.

[0043] The value obtained by dividing the length of the major axis of a metal nanorod by the length of its minor axis is called the aspect ratio. In this disclosure, metal nanorods with an aspect ratio of 1.5 or higher are used as gold nanorods. The gold nanorods exhibit two characteristic plasmon absorption bands (corresponding to the excitation of surface plasmon bands) originating from the major and minor axes of the rod, respectively. For example, the gold nanorods exhibit an absorption band originating from the minor axis near 530 nm and an absorption band originating from the major axis in the range of 650 nm to 2000 nm.

[0044] Regarding the configuration of gold and other metallic elements within a single gold nanorod, the gold nanorod can be in the form of an alloy of atomic combinations, or in a core-shell form where elemental gold nanorods are coated with other metallic elements. Furthermore, the gold nanorods can be coated with, for example, an inert shell, such as silica or polystyrene. Additionally, depending on the purpose, such as dispersing the gold nanorods in a medium, the surface of the gold nanorods can be appropriately molecularly modified with, for example, molecules of surfactants.

[0045] Aspect Ratio Distribution of Gold Nanorods

[0046] The aspect ratio of the gold nanorods in the ink exhibits a distribution, which is represented by the mean and standard deviation of the aspect ratio. The distribution can be a normal distribution, a skewed distribution, or a multimodal distribution with multiple peaks.

[0047] The optical absorption spectra of gold nanorods vary complexly with changes in their aspect ratio distribution. Through their in-depth research, the inventors have determined that inks containing gold nanorods exhibit varying absorption spectra when the aspect ratio distribution of the gold nanorods is measured.

[0048] The mean μ of this aspect ratio distribution ranges from 6.0 to 13.0, and the standard deviation σ of the aspect ratio distribution ranges from 0.5 to 4.5.

[0049] The ink containing gold nanorods is likely to form invisible images that exhibit excellent invisibility in the visible light region and excellent readability in the near-infrared region.

[0050] When the average μ is in the range of 6.0 to 13.0, it is easy to obtain images with peaks in the optical absorption spectrum at wavelengths in the range of 1000 nm to 1600 nm. When the average μ is less than 6.0, the peak positions of the optical absorption spectrum tend to be close to the visible light region; therefore, some images may be insufficient in terms of visibility. This may be because such images readily absorb light at wavelengths in the visible light region. It was also found that when the average μ is greater than 13.0, in some cases, the peaks of the optical absorption spectrum are located at excessively long wavelengths, which is likely to reduce the readability of devices used to read wavelengths in the near-infrared region (e.g., InGaAs cameras).

[0051] Therefore, the average value μ of 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. For example, the preferred range for the average value μ can be 7.0 to 12.0, and more preferably 8.0 to 11.0.

[0052] Furthermore, the inventors have discovered that when the standard deviation σ of the aspect ratio distribution is below 4.5, the absorption wavelength of the gold nanorods is unlikely to change in the near-infrared region, and the maximum absorption rate in this region is relatively greater than the maximum absorption rate in the visible light region.

[0053] Therefore, invisible images with excellent invisibility and readability can be easily obtained. Thus, the standard deviation is 4.5 or less, preferably 3.5 or less, and more preferably 2.5 or less. The lower limit is not limited, but the standard deviation σ is 0.5 or more.

[0054] The preferred range for the standard deviation σ is, for example, 0.5 to 3.5, more preferably 0.5 to 2.5.

[0055] The mean value μ and the standard deviation σ can be controlled by adjusting the reaction conditions and purification conditions.

[0056] <Gold Nanorod Content>

[0057] The gold nanorod content in the ink is preferably between 0.005% by mass and 0.150% by mass relative to the total mass of the ink. When it is 0.005% by mass or more, invisible images with excellent readability in the near-infrared region can be easily obtained. From this perspective, 0.005% by mass or more is preferred, and 0.010% by mass or more is more preferred. Furthermore, when it is below 0.150% by mass, the coloring is difficult to see in the visible region, and the ink is likely to continuously eject from the inkjet recording head. From this perspective, 0.150% by mass or less is preferred, and 0.100% by mass or less is more preferred.

[0058] <Gold nanoparticles>

[0059] In this disclosure, gold nanoparticles, which are nanomaterials primarily composed of gold, have an aspect ratio of less than 1.5 in TEM images. Therefore, in this disclosure, nanomaterials with an aspect ratio of 1.5 or higher are designated as "gold nanorods," while gold nanomaterials with an aspect ratio of less than 1.5 are designated as "gold nanoparticles." Preferably, the content of gold nanoparticles in the ink is less than 30% relative to the number of gold nanorods. Gold nanoparticles can be produced during the production process of gold nanorods. Gold nanoparticles exhibit absorption in the visible light region according to their particle size. For example, gold nanoparticles with a particle size of 20 nm absorb light around 520 nm.

[0060] When the content of gold nanoparticles relative to the number of gold nanorods is less than 30%, the light absorption rate of the gold nanoparticles in the visible light region corresponding to their absorption wavelength is small, thus it is likely to produce images with excellent invisibility. More preferably, this percentage is less than 15%, even more preferably less than 10%, and particularly less than 5%.

[0061] The percentage of gold nanoparticles can be controlled by the reaction and purification conditions, as will be described later.

[0062] <Preparation of Gold Nanorods>

[0063] Gold nanorods can be synthesized, for example, by a method proposed by B. Nikoobakft and MAEl-Sayed (non-patent literature 1). More specifically, chloroauric acid (HAuCl4) is reduced with ascorbic acid in an aqueous solution containing two surfactants (hexadecyltrimethylammonium bromide and benzyldimethylhexadecylammonium chloride).

[0064] Alternatively, as disclosed in Patent Documents 3 and 4, gold nanorod particles can be synthesized by reducing gold ions in an aqueous solution containing an excess of the quaternary ammonium salt hexadecyltrimethylammonium bromide (CTAB). In this method, an aqueous solution of CTAB is first added to an aqueous solution of chloroauric acid tetrahydrate, and sodium borohydride is added to prepare a solution containing seed particles. A mixed solution of silver nitrate, chloroauric acid tetrahydrate, L-ascorbic acid, and CTAB is added to this solution, and the resulting solution is then maintained for a certain period of time, or a small amount of the mixed solution is added. Therefore, the seed particles, serving as the core, readily grow anisotropically to obtain gold nanorods.

[0065] During seed grain growth, benzyl dimethyl hexadecyl ammonium chloride can be added to produce gold nanorods with a high aspect ratio. Gold nanorods with a high aspect ratio can also be produced by a method involving reduction in a first stage with the strong reducing agent sodium borohydride, followed by reduction with the weak reducing agent triethylamine.

[0066] Furthermore, gold nanorods can be purified to adjust their aspect ratio distribution as needed before use. For purification, any known technique can be used; for example, density gradient ultracentrifugation can be applied. More specifically, first, mixed solutions containing sucrose and CTAB at different concentrations are introduced into centrifuge tubes in a stratified manner according to the concentration gradient. Then, the gold nanorod sample is layered on top of each layer and subsequently separated by centrifugation based on density and size. This separation and purification reduces the standard deviation σ, resulting in gold nanorods with a narrower aspect ratio distribution.

[0067] <surfactants>

[0068] The ink contains at least one surfactant selected from the group consisting of acetylenol surfactants with an HLB value of 15 or less and organosilicon surfactants with an HLB value of 15 or less. Acetylene diol surfactants with an HLB value of 15 or less include nonionic surfactants with a symmetrical structure centered on an acetylene group. Preferably, an ethylene oxide group is added. 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); and the Acetylenol series E40 (HLB value: 10), E60 (HLB value: 12), and E100 (HLB value: 14) (all manufactured by Kawaken Fine Chemicals).

[0069] 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 at the side chains and / or ends of a polydimethylsiloxane chain. Specific examples include BYK-347 (HLB value: 9) and BYK-348 (HLB value: 11) (both are trade names, manufactured by BYK). (manufactured by JapanKK); and 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: 12), KF-643 (HLB value: 14), KF-6020 (HLB value: 4), X-22-4515 (HLB value: 5), KF-6011 (HLB value: 12), KF-6012 (HLB value: 7), KF-6015 (HLB value: 5), KF-6017 (HLB value: 5) and KF-6204 (HLB value: 10) (all manufactured by Shin-Etsu). (Chemical production). These can be used alone or in combination.

[0070] The amount of surfactant relative to the total mass of the ink is preferably 0.01% to 1% by mass, more preferably 0.05% to 0.5% by mass. When the amount of surfactant is 0.01% by mass or more, the dots are likely to diffuse sufficiently in terms of diameter. Furthermore, when the amount of surfactant is 1% by mass or less, the storage stability of the ink is further improved.

[0071] <HLB value of surfactant>

[0072] In this disclosure, the HLB value refers to the value obtained by the Griffin method. When two or more of the surfactants specified above are used in combination, a weighted average of their HLB values ​​is applied. The HLB value obtained by the Griffin method can be calculated using the equation "HLB value = 20 × (formula weight of the ethylene oxide group of the surfactant) / (molecular weight of the surfactant)". The HLB value obtained by the Griffin method is a physical property value representing the hydrophilicity or lipophilicity of the surfactant (compound), falling within the range of 0 to 20. The smaller the HLB value, the higher the lipophilicity; the larger the HLB value, the higher the hydrophilicity. When the HLB value is greater than 15, the storage stability of the ink decreases based on the mechanism described above. The HLB value of the surfactant is preferably 5 or higher. When the HLB value is 5 or higher, the dots are more likely to spread sufficiently on the recording medium, further improving the uniformity of the printed matter. The HLB value obtained by the Griffin method is a physical property value of a compound without ionic groups (nonionic compound).

[0073] <Aqueous Media>

[0074] The ink is an ink containing water as an aqueous medium (aqueous ink). The water is preferably deionized water or ion-exchanged water. The water content (mass%) in the ink is preferably from 50.00% to 99.00% by mass, more preferably from 70.00% to 99.00% by mass, relative to the total mass of the ink. Furthermore, the ink may also contain a water-soluble organic solvent as an aqueous medium. Any water-soluble organic solvent can be used, such as alcohols, (poly)alkylene glycols, glycol ethers, nitrogen-containing polar solvents, and sulfur-containing polar solvents, provided that it is soluble in water. The water-soluble organic solvent content (mass%) in the ink is preferably from 1.00% to 30.00% by mass, more preferably from 3.00% to 20.00% by mass, relative to the total mass of the ink. When the water-soluble organic solvent content is within this range, surfactants such as CTAB on the gold nanorods are less likely to detach, further stabilizing the dispersion of the gold nanorods in the ink. Furthermore, reliability, such as resistance to adhesion and spray stability, is further enhanced.

[0075] <Additives>

[0076] The ink may optionally contain various additives, such as surfactants other than those described above, pH adjusters, surface lubricants, rust inhibitors, preservatives, antifungal agents, antioxidants, reduction inhibitors, evaporation promoters, and chelating agents. Preferably, the ink according to this disclosure does not contain coloring components that make the printed image visible.

[0077] <Recording Medium>

[0078] Any known recording medium can be used, but preferably, an ink-permeable recording medium, such as a recording medium with an ink-receiving layer (glossy paper or a PET film with a receiving layer), is used.

[0079] <Light absorption rate in the visible light region>

[0080] Using ink according to this disclosure at 1.9 mg / cm³ 2 In the spectral analysis of the solid image formed by the applied amount, the maximum light absorption rate in the wavelength range of 400 nm to 800 nm is preferably less than 10%. This wavelength range corresponds to the visible region. When the light absorption rate in this region is less than 10%, the image is practically undetectable to the naked eye; therefore, an image with excellent invisibility in the visible light region can be obtained.

[0081] <Light absorption rate in the near-infrared region>

[0082] In addition, when using ink at 1.9 mg / cm³ 2 In the spectral analysis of the solid image formed by the applied amount, the maximum light absorption rate in the wavelength range of 900 nm to 1800 nm is preferably 5% or more. This wavelength range corresponds to the near-infrared region. When the light absorption rate in this region is 5% or more, the invisible image can be easily read by a near-infrared camera (e.g., an InGaAs camera). More preferably, the light absorption rate in this region is 10% or more.

[0083] When the ink is used in practice as an invisible ink, the light absorption in the invisible region can be visualized using a camera or other device that includes a sensor sensitive to 900 nm to 1800 nm (e.g., an InGaAs sensor).

[0084] The light absorption rate in the visible and near-infrared regions can be controlled by adjusting the aspect ratio distribution of gold nanorods.

[0085] <Image Reading Methods>

[0086] Images formed with the ink according to this disclosure can be read without limitation by any method. Images formed with the ink according to this disclosure readily absorb wavelengths in the near-infrared region; therefore, it is preferable to use a device including a near-infrared sensor for reading. More preferably, a device including an InGaAs sensor is used, and even more preferably, an InGaAs camera is used. Furthermore, the image reading method preferably uses light with a wavelength of 900 nm to 2500 nm, more preferably 900 nm to 1800 nm.

[0087] <Ink Production Methods>

[0088] The ink according to this disclosure can be produced by a conventional ink production method, provided that the method uses gold nanorods exhibiting an aspect ratio distribution having the aforementioned average value μ and standard deviation σ, and a surfactant having the aforementioned specified HLB value. More specifically, the ink can be produced by the following steps: (1) preparing the aforementioned specified gold nanorods; and (2) mixing the ink components comprising the gold nanorods and the aforementioned specified surfactant. Step (1) can be performed according to the aforementioned method for producing gold nanorods.

[0089] If necessary, a purification step for purifying gold nanorods can be performed between steps (1) and (2) to increase the gold nanorod content in the gold nanorod dispersion.

[0090] <Performance measurement, etc.>

[0091] The physical properties are measured as follows.

[0092] Quantitative Determination of Metal Content in Metal Nanorod Dispersions

[0093] The metal content in a metal nanorod dispersion can be quantitatively determined by ICP emission spectroscopy according to JIS K 0116-2014. First, the metal nanorod dispersion is heated to 60°C on a hot plate to dry and solidify. Then, aqua regia is added to the resulting dispersion, and microwave acid digestion is performed using an ETHOS PRO (manufactured by Milestone General KK). The liquid is then analyzed by ICP emission spectroscopy using a CIROS CCD (manufactured by SPECTRO). Therefore, the metal content can be quantitatively determined.

[0094] Measurement of aspect ratio distribution of gold nanorods in ink

[0095] To measure the aspect ratio distribution of gold nanorods in ink, the gold nanorods are first separated from the ink and collected.

[0096] In this process, the gold nanorods in the ink are first precipitated by centrifugation, and then the supernatant is removed. Fresh solvent is added to the dispersion containing the gold nanorods from which the supernatant has been removed, and centrifugation is performed again, followed by removal of the supernatant. This process is repeated several times for washing, and finally the solvent is removed, thereby separating and collecting the gold nanorods from the ink. The collected gold nanorods are then redispersed in a solvent such as THF and used as a measurement sample.

[0097] The measurement sample, dispersed in a solvent, was dropped onto a support film, dried, and observed using a transmission electron microscope, such as a Technai F30 (manufactured by FEI). The long and short axes of the observed gold nanorods were determined using image processing software such as Photoshop, thereby obtaining the mean and standard deviation of the aspect ratio distribution of the gold nanorods and the average length of the long axis.

[0098] <Measurement of the percentage of gold nanoparticles relative to gold nanorods in ink>

[0099] The percentage of gold nanoparticles relative to gold nanorods in ink can be measured by counting the number of gold nanoparticles and gold nanorods observed in the above TEM observations and calculating their ratio.

[0100] Measurement of Gold Nanorod Content in Ink

[0101] The gold nanorod content (mass%) in the ink was measured by X-ray fluorescence analysis. This measurement was performed according to JIS K0119-1969, as detailed below.

[0102] The measurements were performed using an Axios wavelength dispersive X-ray fluorescence analyzer (manufactured by PANalytical) and its dedicated software, Super Q ver. 4.0F (manufactured by PANalytical), for setting measurement conditions and analyzing measurement data. The X-ray tube anode was Rh; the measurement atmosphere was vacuum; the measurement diameter (collimator mask diameter) was 27 mm; and the measurement time was 10 s. Light elements were detected using a proportional counter (PC), and heavy elements were detected using a scintillation counter (SC).

[0103] A sample prepared by applying 4 mL of ink to a glass substrate with a diameter of 39 mm and drying the ink was used as a measurement sample.

[0104] The samples were measured under the conditions described above. Elements were identified based on the obtained X-ray peak positions, and the count rate (in kcps), which is the number of X-ray photons per unit time, was measured. The gold nanorod content in the ink was then calculated using a separately prepared calibration curve. For the preparation of the calibration curve, samples prepared in the same manner as described above using inks each containing a predetermined amount of gold nanorods were used as the measurement samples.

[0105] <Ink cartridge>

[0106] The ink disclosed herein can be contained in an ink cartridge. The ink cartridge includes ink and an ink container for containing the ink. The ink in the ink container is the ink described above. The ink container can be in the form of an absorber that holds the entire amount of ink to be contained. Alternatively, the ink container can be such that it can hold the entire amount of ink in liquid form without an absorber. The ink cartridge can have a structure including an ink container and an inkjet recording head.

[0107] <Inkjet Recording Methods>

[0108] The ink disclosed herein can be used in inkjet recording methods. In inkjet recording methods, the ink of the present disclosure is ejected from an inkjet recording head to record an image on a recording medium. For inkjet printing, mechanical energy or thermal energy can be applied to the ink. In this invention, the method of applying thermal energy to the ink is particularly preferred for inkjet printing. Except for using the ink of the present disclosure, the steps of the inkjet recording method can be the same as those of known inkjet recording methods.

[0109] From the perspective of the ink applied to the recording medium after drying, preferably, the amount of ink applied to the recording medium is 3.0 mg / cm³. 2 Below, 2.5 mg / cm² is preferred. 2 the following.

[0110] Example

[0111] This disclosure will be described in further detail with reference to the embodiments, but this disclosure is not limited to the following embodiments. The measurement results in the embodiments are obtained by the measurement methods described above.

[0112] [Example of preparation of gold nanorod dispersion A]

[0113] First, a solution of seed particles is prepared. Specifically, 500 mL of a 0.0005 mol / L aqueous solution of chloroauric acid tetrahydrate (manufactured by Kishida Chemical) and 500 mL of a 0.2 mol / L aqueous solution of hexadecyltrimethylammonium bromide (manufactured by Kishida Chemical) are mixed. 60 mL of 0.01 mol / L sodium borohydride (manufactured by Tokyo Chemical Industry) is then added to this aqueous solution to obtain the seed particle solution (solution A).

[0114] Subsequently, 10 g of hexadecyltrimethylammonium bromide was dissolved in 500 mL of 0.15 mol / L benzyldimethylhexadecylammonium chloride aqueous solution (manufactured by Tokyo Chemical Industry). To this solution containing the two surfactants, 20 mL of 0.004 mol / L silver nitrate aqueous solution was added. To the resulting aqueous solution, 500 mL of 0.001 mol / L chloroauric acid tetrahydrate aqueous solution was added, followed by 7 mL of 0.078 mol / L L-ascorbic acid aqueous solution (manufactured by Kishida Chemical). The resulting solution was designated as solution B.

[0115] Subsequently, 10 g of hexadecyltrimethylammonium bromide was dissolved in 500 mL of 0.15 mol / L benzyldimethylhexadecylammonium chloride aqueous solution (manufactured by Tokyo Chemical Industry). To this solution containing the two surfactants, 20 mL of 0.004 mol / L silver nitrate aqueous solution was added. To the resulting aqueous solution, 500 mL of 0.0005 mol / L chloroauric acid tetrahydrate aqueous solution was added, followed by 3.6 mL of 0.078 mol / L L-ascorbic acid aqueous solution (manufactured by Kishida Chemical). The resulting solution was designated as solution C.

[0116] To solution B, 1.2 mL of solution A was added, followed by 2.0 mL of solution C at a rate of 1.0 mL / 20 min, thereby anisotropically growing seed particles as nuclei. After centrifugation at 10,000 × g for 5 min, the gold nanorods were redispersed in water to a concentration of 0.35% by mass, thus obtaining a gold nanorod dispersion (dispersion A). Table 1 shows the physical properties of the gold nanorods in dispersion A.

[0117] [Examples of preparation of gold nanorod dispersions B to J]

[0118] Except for changing the amount of solution C and whether or not the following purification process is applied, as shown in Table 1, dispersions B to J are prepared by the same operation as in the preparation example of gold nanorod dispersion A.

[0119] <Purification Steps>

[0120] The obtained 6 mL gold nanorod dispersion was centrifuged at 10000 × g for 5 minutes, and the resulting pellets were resuspended in 0.05 mL of 0.01 M CTAB aqueous solution to obtain a gold nanorod suspension. Furthermore, sucrose was added to the 0.01 M CTAB solution to prepare solutions containing 10%, 15%, 20%, and 25% sucrose by mass, respectively. These sucrose-containing solutions (3 mL each) were layered in descending order of sucrose concentration in a 15 mL polyhexamethylene transistor to allow the gold nanorod suspension to finally separate into layers. The resulting samples were then centrifuged at 25 °C and 10750 × g for 15 minutes using an Avanti JXN-30 high-speed refrigerated centrifuge. After centrifugation, the samples were aliquoted into 300 μL fractions, and TEM images of the fractions containing gold nanorods were observed. The TEM images were exported, and the portions with the desired average value μ and standard deviation σ were centrifuged at 10000×g for 5 minutes, and the resulting deposits, i.e., gold nanorods, were collected. The collected gold nanorods were then redispersed in water to a concentration of 0.35% by mass.

[0121] [Table 1]

[0122] Table 1

[0123]

[0124] In Table 1, "Average length of the major axis" refers to the average length of the major axis of the gold nanorods in the ink. Furthermore, "Percentage of gold nanoparticles" refers to the percentage of the number of gold nanoparticles in the ink relative to the number of gold nanorods.

[0125] [Example of ink production 1]

[0126] Mix the following ingredients and stir thoroughly. Filter the resulting mixture under pressure through a 3.0 μm microfilter (manufactured by Fujifilm) to obtain ink 1. The "balance" of ion-exchanged water is the amount that makes the total amount of all components in the ink reach 100.0%.

[0127] • Gold nanorod dispersion A: 29%

[0128] Surfactant (trade name: Acetylenol AE60): 0.2%

[0129] Ethylene glycol: 10%

[0130] • Ion-exchanged water: Balance

[0131] [Production examples from ink 2 to 29]

[0132] Apart from the changes shown in Tables 2 to 4, Ink 2 to 29 are produced using the same procedures as in the production example of Ink 1. The surfactants shown in Tables 2 to 4 are detailed below:

[0133] <surfactants>

[0134] • AE60: Trade name "Acetylenol E60" (manufactured by Kawaken Fine Chemical) (HLB value: 12)

[0135] • AE40: Trade name: "Acetylenol E40" (manufactured by Kawaken Fine Chemical) (HLB value: 10)

[0136] • AE100: Trade name: "Acetylenol E100" (manufactured by Kawaken Fine Chemical) (HLB value: 14)

[0137] • KF945: Trade name "KF-945" (manufactured by Shin-Etsu Chemical) (HLB value: 4)

[0138] • KF6015: Trade name "KF-6015" (manufactured by Shin-Etsu Chemical) (HLB value: 5)

[0139] • KF6204: Trade name "KF-6204" (manufactured by Shin-Etsu Chemical) (HLB value: 10)

[0140] • BYK348: Product name "BYK-348" (manufactured by BYK Japan KK) (HLB value: 11)

[0141] • KF642: Trade name "KF-642" (manufactured by Shin-Etsu Chemical) (HLB value: 12)

[0142] • KF640: Trade name "KF-640" (manufactured by Shin-Etsu Chemical) (HLB value: 14)

[0143] • AE200: Trade name: "Acetylenol E200" (manufactured by Kawaken Fine Chemical) (HLB value: 16)

[0144] • KF354L: Trade name "KF-354L" (manufactured by Shin-Etsu Chemical) (HLB value: 16)

[0145] • Emulgen 105: Trade name "Emulgen 105" (manufactured by Kao) (HLB value: 10)

[0146] • Megafac F-444: Trade name "Megafac F-444" (manufactured by DIC Corporation) (HLB value: 9)

[0147] AE60, AE40, AE100, and AE200 are all alkynyl diol surfactants. Additionally, KF945, KF6015, KF6204, BYK348, KF642, KF640, and KF354L are all silicone surfactants. Emulgen 105 is a polyoxyalkylene alkyl ether surfactant. Megafac F-444 is a fluorinated surfactant.

[0148] [Table 2] Table 2

[0149]

[0150] [Table 3] Table 3

[0151]

[0152] [Table 4] Table 4

[0153]

[0154] <Example 1>

[0155] <Evaluation of image quality in the near-infrared region>

[0156] The material printer (trade name: DMP-2850, manufactured by Fujifilm) is used as an image recording device. It outputs images to an output medium—a PET film with an ink-receiving layer (trade name: CG3110, manufactured by 3M)—at an environment of 25°C and 60% relative humidity. The output volume is 7.5 pL and the ink concentration is 1.9 mg / cm³. 2 The amount of ink 1 applied forms a 2cm×3cm rectangular solid image with a resolution of 1270dpi, thereby obtaining an evaluation image.

[0157] Then, as Figure 1The light source 202 and camera 203 are set up as shown, and an evaluation image 201 is observed. More specifically, the evaluation image is placed on a table and illuminated by infrared light from the light source 202 at an angle of 15° from a distance of approximately 1 meter. Furthermore, the evaluation image is captured by the camera 203, which is placed 15 cm directly above the evaluation image. A halogen lamp light source (trade name: PCS-UHX-150, manufactured by Nippon P·I) with a visible light cutoff filter unit is used as the light source 202. Furthermore, a near-infrared camera (trade name: NVU3VD, InGaAs camera, manufactured by IRspec) with a filter in its lens section having a cutoff wavelength component of 800 nm or less is used as the camera 203. The near-infrared camera has spectral sensitivity in the wavelength range of 970 nm to 1650 nm.

[0158] <Evaluation of Invisibility in the Visible Light Region>

[0159] <Method for measuring light absorbance in the wavelength range of 400 nm to 800 nm>

[0160] Using the aforementioned image recording equipment and PET film, at a concentration of 1.9 mg / cm³... 2 The amount of ink 1 applied is used to form a rectangular image of 2cm × 3cm to obtain a sample image.

[0161] Sample images were measured using a photometer at wavelengths ranging from 400 nm to 800 nm, and the maximum absorbance was recorded as the measured value (%) of the sample image. A UV-Vis-NIR spectrophotometer (trade name: UV-3600, manufactured by Shimadzu Corporation) was used as the photometer. Individual PET films (PET films without an image) were also used as blanks for spectral analysis.

[0162] The measured value was taken as the visible light absorption rate (%), which was used to evaluate the invisibility of the ink. The results are shown in Table 5. When the visible light absorption rate was 13% upon visual observation, the image could be used as an invisible image.

[0163] <Evaluation of readability in the near-infrared region>

[0164] <Methods for measuring light absorption in the wavelength range of 900nm to 1800nm>

[0165] Sample images used for invisibility evaluation were subjected to spectral analysis measurements in the wavelength range of 900 nm to 1800 nm using a UV-Vis-NIR spectrophotometer (trade name: UV-3600, manufactured by Shimadzu Corporation). The maximum absorbance measured was taken as the near-infrared absorbance (%) of the sample. PET films were also used separately as blanks for spectral analysis. The results are shown in Table 5. When an image with a near-infrared absorbance of 5% was examined with the aforementioned camera, the image was deemed readable as an invisible image. Conversely, when an image with a near-infrared absorbance of 2% was examined with the same camera, the image was determined to be below the readable level for invisibility.

[0166] Evaluation of the storage stability of ink

[0167] 100g of ink was placed into a 180mL sealed glass container, sealed, and stored in an oven at 60°C. The ink was then visually inspected after 3, 6, and 7 days of storage to evaluate its storage stability according to the following criteria:

[0168] A: The ink's appearance remained unchanged even after 7 days of storage.

[0169] B: The ink did not change in appearance until 6 days after storage, but after 7 days of storage, the ink color gradually darkened from the top to the bottom of the container.

[0170] C: After 3 days of storage, the ink color gradually darkens from the top to the bottom of the container.

[0171] D: After 3 days of storage, precipitate was observed in the container.

[0172] <Examples 2 to 23 and Comparative Examples 1 to 6>

[0173] Except for inks 2 to 29 replacing ink 1, the evaluation was performed in the same manner as in Example 1, as shown in Table 5. The results are shown in Table 5.

[0174] [Table 5] Table 5

[0175] Visible light absorption rate Near-infrared absorption rate Storage stability Example 1 Ink 1 9％ 19％ A Example 2 Ink 2 6％ 23％ A Example 3 Ink 3 5％ 11％ A Example 4 Ink 4 8％ 19％ A Example 5 Ink 5 3％ 17％ A Example 6 Ink 6 4％ 23％ A Example 7 Ink 7 4％ 20％ A Example 8 ink 8 3％ 19％ A Example 9 Ink 9 2％ 9％ A Example 10 Ink 10 1％ 5％ A Example 11 Ink 11 12％ 81％ A Example 12 Ink 12 13％ 98％ A Example 13 Ink 13 4％ 23％ A Example 14 Ink 14 4％ 23％ C Example 15 Ink 15 4％ 23％ A Example 16 Ink 16 4％ 23％ A Example 17 Ink 17 4％ 23％ A Example 18 Ink 18 4％ 23％ A Example 19 Ink 19 4％ 23％ A Example 20 ink 20 4％ 23％ B Example 21 Ink 21 4％ 23％ B Example 22 Ink 22 4％ 23％ A Example 23 Ink 23 4％ 23％ A Comparative Example 1 Mexico 24 1％ 1％ D Comparative Example 2 Mexico 25 0％ 2％ D Comparative Example 3 Ink 26 4％ 23％ D Comparative Example 4 Ink 27 4％ 23％ D Comparative Example 5 Ink 28 4％ 23％ D Comparative Example 6 Mexico 29 4％ 23％ D

[0176] This invention is not limited to the above-described embodiments, and various modifications and alterations can be made without departing from the spirit and scope of the invention. Therefore, the claims are appended to disclose the scope of the invention.

[0177] This application claims the benefit of Japanese Patent Application No. 2021-175756, filed on October 27, 2021, which is incorporated herein by reference in its entirety.

Claims

1. An ink comprising: water; gold nanorods; and at least one surfactant selected from the group consisting of acetylenic diol surfactants with an HLB value of less than 15 and organosilicon surfactants with an HLB value of less than 15. When measuring the aspect ratio distribution of the gold nanorods contained in the ink, The average μ of the aspect ratio distribution is between 6.0 and 13.0; and The standard deviation σ of the aspect ratio distribution is between 0.5 and 4.

5. The gold nanorods are protected by CTAB.

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

5.

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

5.

4. The ink according to claim 1 or 2, wherein the content of gold nanorods in the ink is from 0.005% by mass to 0.150% by mass relative to the total mass of the ink.

5. The ink according to claim 1 or 2, wherein the gold nanorods contained in the ink have an average length of 50 nm to 110 nm along their long axis.

6. The ink according to claim 1 or 2, wherein the amount of gold nanoparticles with an aspect ratio of less than 1.5 in the ink is less than 30% relative to the number of gold nanorods.

7. The ink according to claim 1 or 2, wherein the amount of gold nanoparticles with an aspect ratio of less than 1.5 in the ink is less than 10% relative to the number of gold nanorods.

8. The ink according to claim 1 or 2, wherein the amount of the surfactant in the ink is from 0.01% to 1% by mass relative to the total mass of the ink.

9. The ink according to claim 1 or 2, wherein the amount of the surfactant in the ink is from 0.05% by mass to 0.5% by mass relative to the total mass of the ink.

10. The ink according to claim 1 or 2, wherein the surfactant in the ink has an HLB value of 5 or higher.

11. The ink according to claim 1 or 2, wherein when the ink is used at 1.9 mg / cm³ 2 When the image formed by the applied amount is subjected to spectral analysis, the image exhibits a maximum light absorption rate of less than 10% in the wavelength range of 400 nm to 800 nm.

12. The ink according to claim 1 or 2, wherein when the ink is used at 1.9 mg / cm³ 2 When the image formed by the applied amount is subjected to spectral analysis, the image exhibits a maximum light absorption rate of more than 5% in the wavelength range of 900nm to 1800nm.

13. A method for reading an image, comprising: An image formed with ink is read using a device including a near-infrared sensor, wherein the ink is the ink according to any one of claims 1 to 12.

Images