Inorganic nanoparticle core-polymer shell structural coloring material, particle film, and method for producing nanoparticle film

Abstract

Provided are nanoparticles each having a polymer shell attached thereto and a nanoparticle film, in which the direct contact between nanoparticle cores is prevented so as to significantly suppress the formation of dimers or clusters so that brighter color development in a target wavelength band can be achieved. An inorganic nanoparticle core-polymer shell structural coloring material is used, in which a polymer shell having a thickness of 20-200 nm is formed around an inorganic nanocrystal having a particle diameter of 40-300 nm and a refractive index of 3 or more. The polymer shell formed around the inorganic nanocrystal comprises a hydroxyl group on the surface of the inorganic nanocrystal and a surface-modified functional group. For example, a hydrophilic OH group (hydroxyl group) on the silicon surface is substituted with a hydrophobic group, and N-isopropylacrylamide (NIPAM) is modified on the silicon surface to coat the silicon surface.

Description

Production method of inorganic nanoparticle core-polymer shell structure colorant, particle film, and nanoparticle film

[0001] The present invention relates to nanoparticles made of a high refractive index material such as silicon and nanoparticle films.

[0002] Nanoparticles (100 - 200 nm) made of a high refractive index material such as silicon are expected to be used as structural color pigments and paints because they emit color due to Mie resonance. The present inventors have proposed a dispersion solution of nanoparticles that does not change color or fade, has high monochromaticity, and can achieve high resolution, and a method for producing the same (see Patent Document 1). This is based on the fact that silicon etc. exhibits large light scattering in the visible wavelength range and can be used as a color-emitting nanostructure without using a periodic array structure. By forming a dispersion solution and controlling the average particle diameter, high scattered light is generated in the visible to near-infrared region by a single particle. In addition, the present inventors have also proposed nanoparticle films and dispersion solutions that can achieve high hiding power with a small number of coating times and can adjust the hue (see Patent Document 2). This is by using nanoparticles (100 - 220 nm) such as silicon, and it has been confirmed that a single-layer or two-layer or three-layer particle film has high hiding power. By using nanoparticles with different particle size distributions, high hiding power and hue adjustment are simultaneously realized. However, there is a problem that when particles contact each other in a nanoparticle film (coating film) and form dimers or clusters, the color changes. Therefore, a technique for preventing particles from contacting each other in the coating film is desired.

[0003] On the other hand, Patent Document 3 describes nanosemiconductor particles in which the core part is silicon particles and the shell part around it is mainly silicon oxide. In the nanosemiconductor particles of Patent Document 3, when the sum of the core part particle diameter and the shell part thickness exceeds 100 nm, it becomes a bulk property and the quantum size effect cannot be obtained, so the particle diameter of the shelled particles is set to 100 nm or less. Also, it is described that the thickness of the shell part needs to be 0.2 nm or more in order to isolate the cores from each other and avoid aggregation. However, there is no description regarding suppressing the formation of dimers or clusters by forming a polymer shell on the surface of the nanoparticles.

[0004] Furthermore, Patent Document 4 describes particles having a silicon compound shell, the surface of which is modified with a silane coupling agent. The silica-coated nanoparticles described in Patent Document 4 have high solvent dispersibility and are intended for immobilization on substrates, etc., and are not intended for color development.

[0005] Japanese Patent Publication No. 2021-025023, Japanese Patent Publication No. 2024-027742, International Publication Pamphlet WO2007 / 086321, Japanese Patent Publication No. 2010-001555

[0006] As described above, in nanoparticle films (coating films), particles come into contact with each other, forming dimers and clusters during drying, which changes the particle size and consequently alters the color development due to Mie resonance. In view of this situation, the present invention aims to provide polymer shell-equipped nanoparticles and nanoparticle films that significantly suppress the formation of dimers and clusters by preventing direct contact between nanoparticle cores, thereby enabling brighter color development in the target wavelength range.

[0007] To solve the above problems, the polymer shell-equipped nanoparticles of the present invention are inorganic nanoparticle core-polymer shell structural colorants in which a polymer shell with a thickness of 20 to 200 nm is formed around inorganic nanocrystals with a particle size of 40 to 300 nm and a refractive index of 3 or higher. A polymer shell of a predetermined thickness is formed on the surface of nanoparticles made of a high refractive index material such as silicon that develops color by Mie resonance, so that the nanoparticle core does not come into direct contact with the nanoparticle film (coating film), thereby suppressing the formation of dimers and clusters and obtaining bright coloration. With the above configuration, improvements in brightness and saturation can be expected in coating films of structural color particles, thus expanding the applications as paints. Furthermore, polymer shell-equipped nanoparticles may be able to promote chemical bonding with binders, enabling the formation of more stable coating films.

[0008] Here, Mie resonance refers to the phenomenon where, when light of wavelength λ (nm) is incident on an inorganic nanoparticle (refractive index n), the effective wavelength within the nanoparticle becomes λ / n (nm). When the effective wavelength of light λ / n (nm) equals the diameter of the nanoparticle, a standing wave is formed, and the lowest-order Mie resonance occurs. Furthermore, the particle size of the nanoparticle refers to the average particle size, and the distribution of a certain type of particle size is within the range obtained by dividing the standard deviation by the average particle size (for example, 4-50%).

[0009] In the polymer shell-equipped nanoparticles of the present invention, the particle size is preferably 100 to 300 nm, and more preferably 100 to 220 nm. By selecting a particle size of 100 to 220 nm, when the color development by Mie resonance is in the visible light range, the particle size is narrowed down to the range of 100 to 220 nm.

[0010] Inorganic nanocrystals with a refractive index of 3 or higher include, for example, silicon (Si), GaAs, GaP, and InP. The refractive index of silicon is 4.32, the refractive index of the inorganic compound GaAs is 4.27, the refractive index of GaP is 3.6, and the refractive index of InP is 3.0. Note that the refractive index depends on wavelength, but it refers to the value at a wavelength of 500 nm. Inorganic nanocrystals are preferably silicon or GaAs with a refractive index of 4 or higher, and more preferably silicon, which is the second most abundant element on Earth after oxygen, found in soil, rocks, natural water, trees, and plants, and is widely used in semiconductors.

[0011] In inorganic nanoparticle core-polymer shell structure colorants, the polymer shell formed around the inorganic nanocrystals is preferably one of polystyrene, polyacrylic, polyethylene glycol, or N-isopropylacrylamide (NIPAM). The polymer material may have both hydrophobic and hydrophilic groups, or be either hydrophobic or hydrophilic.

[0012] Preferably, the polymer shell formed around the inorganic nanocrystal is formed by bonding to or adsorbing to hydroxyl groups or surface-modified functional groups on the surface of the inorganic nanocrystal. For example, hydrophilic OH groups (hydroxyl groups) on the silicon surface are replaced with hydrophobic groups, and the silicon surface is modified and coated with N-isopropylacrylamide (NIPAM).

[0013] Inorganic nanoparticle core-polymer shell structure colorant may have an inorganic nanoparticle core with an inorganic nanocrystal particle size of 100-200 nm, and may exhibit a structural color in the visible light range. Here, a particle size of 100-110 nm results in a pinkish structural color, 115-125 nm results in a blueish structural color, 130-140 nm results in a greenish structural color, 145-160 nm results in a yellowish structural color, and 165-200 nm results in a skin-tone structural color.

[0014] Furthermore, the inorganic nanoparticle core-polymer shell structure colorant may also exhibit a structural coloration under white light by having a mixture of particles in the inorganic nanocrystal with particle size distribution peaks of approximately 100 nm, 130 nm, and 200 nm, respectively. White coloration is produced by mixing three types of particles: blue (particle size distribution with a peak of approximately 100 nm), green (particle size distribution with a peak of approximately 130 nm), and red (particle size distribution with a peak of approximately 200 nm). Note that the particle size distribution here refers to a distribution with a width of approximately ±10%.

[0015] Furthermore, the inorganic nanoparticle core-polymer shell structure colorant may have inorganic nanocrystals with a particle size of 200-300 nm, and may emit structural colors in the near-infrared region. The particle size is standardized to emit structural colors in the near-infrared light region. Alternatively, the inorganic nanoparticle core-polymer shell structure colorant may have inorganic nanocrystals with a particle size of 40-99 nm, and may emit structural colors in the ultraviolet light region. The particle size is standardized to emit structural colors in the ultraviolet light region.

[0016] The inorganic nanocrystals are preferably selected from Si, GaAs, GaP, and InP. They are specific inorganic materials with a refractive index of 3 or higher.

[0017] The nanoparticle film of the present invention is a single-layer film, multilayer film, or laminated film formed from the above-described inorganic nanoparticle core-polymer shell structure colorant, wherein the surface filling rate of nanoparticles in the single layer is 15 to 74%. Cosmetic products and highly opaque nanoparticle films can be manufactured using this nanoparticle film. Cosmetic products include, for example, foundation, lipstick, and eyeshadow.

[0018] The present invention relates to a method for producing a nanoparticle film, comprising the steps of: 1) preparing inorganic nanoparticles with a refractive index of 3 or higher to a particle size of 40 to 300 nm; 2) coating the inorganic nanoparticles with a polymer shell having a thickness of 20 to 200 nm; and 3) floating the polymer-coated inorganic nanoparticles on a liquid surface and transferring them to a substrate. Furthermore, it is preferable that the above method for producing a nanoparticle film further comprises the step of washing the particle film on the substrate with an organic solvent or performing oxygen plasma treatment to remove only the polymer shell.

[0019] The polymer shelled nanoparticles and nanoparticle films of the present invention have the effect of significantly suppressing the formation of dimers and clusters by preventing direct contact between the nanoparticle cores, thereby achieving brighter color development in the target wavelength range.

[0020] Schematic diagram of Si nanoparticle dispersion Schematic diagram of polymer shell formation using functional groups on the surface of Si nanoparticles Flowchart for fabrication of Si nanoparticle film Optical microscope image after particle size separation of Si nanoparticle film Magnified view of optical microscope image after particle size separation of Si nanoparticle film Explanatory diagram of polymer shell formation on the surface of Si nanoparticles (Example 1) Schematic diagram of the method for fabricating Si nanoparticle film SEM image of Si nanoparticle film (after oxygen plasma treatment) Dark-field image of Si nanoparticle film Quenching spectrum of Si nanoparticle dispersion Reflection spectrum of Si nanoparticle film (after oxygen plasma treatment) Color space obtained from the reflection spectrum of Si nanoparticle film (difference with or without surface shell)

[0021] Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings. It should be noted that the scope of the present invention is not limited to the following embodiments or illustrated examples, and numerous modifications and variations are possible.

[0022] Figure 1 is a schematic diagram of the dispersion of Si nanoparticles, and the current challenges will be explained using Figure 1. As shown in Figure 1, isolated Si nanoparticles 2, which are inorganic nanocrystals, emit color through Mie resonance. However, when they become dimers or clusters (multimers), the coloration changes, causing a decrease in saturation and brightness. Therefore, by forming a polymer shell of a predetermined thickness (20 to 200 nm) around the Si nanoparticles 2, the nanoparticles are prevented from touching each other, resulting in an inorganic nanoparticle core-polymer shell structure colorant. In this specification, nanoparticles refer to particles of nanometer size, and Si nanoparticles are spherical particles made of crystalline Si with a particle diameter of nanometer size, and Si is a nanoparticle with a positive real part of its dielectric constant, exhibiting Mie resonance.

[0023] Figure 2 shows a schematic diagram of forming a polymer shell using functional groups on the surface of Si nanoparticles. First, Si nanoparticles are dispersed in a solvent. A solution containing the functional groups to be modified is added to the surface of the Si nanoparticles to modify the surface of the Si nanoparticles (see Figure 2(1)). Polymers are bonded to or adsorbed onto the surface-modified functional groups to form a polymer shell (see Figure 2(2)). The functional groups modified on the surface of the Si nanoparticles are tightly crosslinked, while the polymers bonded to or adsorbed onto these functional groups are relatively loosely crosslinked.

[0024] Figure 3 shows the flow diagram for the fabrication of the Si nanoparticle film. First, Si nanoparticles are prepared to a particle size of 40 to 300 nm (step S01). For example, commercially available silicon monoxide powder is used as a raw material, and the silicon monoxide powder is heated from a temperature condition lower than the melting point of elemental silicon (1414°C) to a temperature condition higher than the melting point (1350 to 1700°C), for example at 1450°C, with nitrogen (N ２The annealing treatment is performed for 30 minutes in a controlled atmosphere. By controlling the annealing temperature, the particle size of crystalline Si nanoparticles can be controlled. The powder obtained after annealing is silicon dioxide particles containing Si nanoparticles. The annealed powder is etched using hydrofluoric acid (HF). By etching with hydrofluoric acid, only the silicon dioxide portion is etched, and the Si nanoparticles contained within the particles are extracted. After etching, methanol is used as a polar solvent to replace the alcohol and etching solution. Then, the mixture is stirred with an ultrasonic homogenizer to disperse the Si nanoparticles in the alcohol, and the particle size of the Si nanoparticles is standardized using density gradient centrifugation (2000-3000 G; 60 minutes) on the dispersion. Through the above process, a dispersion of Si nanoparticles with controlled particle size is prepared. In addition, density gradient centrifugation and particle size sorting by adding a poor solvent can be used as methods to standardize the particle size of nanoparticles in the dispersion. When using density gradient centrifugation, it is best to centrifuge the centrifuge tube containing the dispersion on a horizontal plane to achieve high particle size resolution. By positioning the material horizontally and centrifuging it laterally, the direction of the centrifugal force and the longitudinal direction toward the bottom of the centrifuge tube become the same, increasing the sedimentation path length and thus improving the resolution of density gradient centrifugation.

[0025] Next, a polymer shell with a thickness of 20 to 200 nm is coated onto the Si nanoparticles (step S02). Then, the Si nanoparticles coated with polymer shells are floated on the surface of a liquid and transferred to a substrate (step S03). For example, the film is formed by self-assembling the Si nanoparticles coated with polymer shells onto a glass substrate. Alternatively, the film may be formed by spin-coating or drop-coating a dispersion of Si nanoparticles coated with polymer shells onto a substrate. After that, only the polymer shell is removed from the particle film on the substrate (step S04).

[0026] This section describes a comparison of the particle size distribution of Si nanoparticles and Si nanoparticles coated with polymer shells. The particle size in a dispersed state in solution was measured using DLS (dynamic light scattering). The average particle sizes of two types of Si nanoparticles (NIPAM: 50 mM and 25 mM) were 175 nm, 275 nm, and 399 nm, respectively, indicating an increase in particle size due to the polymer shell coating. From this, the shell thickness can be calculated as 224 ÷ 2 = 112 nm for NIPAM 50 mM and 100 ÷ 2 = 50 nm for NIPAM 25 mM.

[0027] Figure 4 shows an optical microscope image of the fabricated Si nanoparticle film after particle size separation, and Figure 5 shows a magnified view thereof. In Figures 4 and 5, (1) shows the case without a polymer shell, and (2) shows the case with a polymer shell. As shown in Figure 5(1), in the case without a polymer shell, as indicated by the arrows in the image, it can be seen that there are many clusters, whereas as shown in Figure 5(2), in the case with a polymer shell, individual particles can be clearly seen.

[0028] An example of an embodiment for forming a polymer shell on the surface of Si nanoparticles is described. Figure 6 shows an explanatory diagram of forming a polymer shell on the surface of Si nanoparticles, and Figure 7 shows a schematic diagram of the method for fabricating the Si nanoparticle film. First, the dispersion of Si nanoparticles in alcohol is centrifuged to remove the alcohol, and dried overnight at 70°C. Then, the Si nanoparticles are dispersed in DMF (N,N-dimethylformamide) (concentration 0.04 wt%). This is then subjected to sonication, and MPS ((trimethoxysilyl)propyl methacrylate) is added (surface area of ​​Si nanoparticles 1 nm). ２ The mixture is reacted with over 100 molecules per unit, followed by ultrasonic treatment at 50°C for 2 hours. After that, it is washed with alcohol 4 to 5 times.

[0029] Next, 25 mM or 50 mM NIPAM, BIS as a crosslinking agent, and ammonium persulfate (APS) as a polymerization accelerator are added to a nitrogen-purged glass container. While heating to 80°C and stirring, a dispersion of MPS-modified Si nanoparticles is added little by little using a syringe to form a polymer (poly(N-isopropylacrylamide)) shell on the surface of the Si nanoparticles. In the laboratory, as shown in Figure 7, Si nanoparticle films were fabricated by dispersing Si nanoparticles coated with polymer shells in alcohol and gently dropping the dispersion onto the surface of water to float the particles on the water surface and transfer them to the substrate.

[0030] Figure 8 shows an SEM image of the Si nanoparticle film after oxygen plasma treatment. The Si nanoparticles have an average particle size (D ａｖｅ The wavelength is 188.31 nm. As shown in Figure 8, the crystalline Si nanoparticles constituting the fabricated Si nanoparticle film have a circularity close to that of a perfect sphere.

[0031] Figure 9 shows a dark-field image of a Si nanoparticle film. Figures 9(1) to (8) show the average particle size (D) of the Si nanoparticles. ａｖｅ The values ​​for ) and standard deviation ÷ average particle size (CV) are dark-field images of the Si nanoparticle films of the samples (F1 to F8) shown in Table 1 below. The average particle size was controlled to approximately 110 to 200 nm ± 10% to confirm the differences in hue.

[0032]

[0033] Figure 10 shows the quenching spectrum of a Si nanoparticle dispersion. It can be seen that the spectral peak shifts to longer wavelengths as the average particle size increases.

[0034] Figure 11 shows the total reflectance spectrum of a Si nanoparticle film (after oxygen plasma treatment). From Figure 11, it can be seen that, reflecting the quenching spectrum of the dispersion in Figure 10, the reflectance spectrum changes in the range of approximately 450 to 800 nm as the nanoparticle size increases. Total reflectance is expressed as the ratio of the sum of specularly reflected light and diffusely reflected light to the incident light.

[0035] Figure 12 shows the color space obtained from the reflection spectra of Si nanoparticle films with and without surface shells. Figure 12 shows the CIE 1931 color space. The CIE 1931 color space quantitatively represents the relationship between visible light and color in human color vision. From Figure 12, it can be seen that the color gamut changes over a wider range when polymer shells are formed on the surface of Si nanoparticles than when polymer shells are not formed on the surface of Si nanoparticles. From this, it can be seen that the color gamut is expanded by suppressing cluster formation due to the polymer shells.

[0036] This invention is useful for cosmetics, particularly foundations that are effective in small amounts and highly opaque inks.

[0037] 1. Structural colorant 2. Si nanoparticles 3. Polymer shell 4. Air 5. Water 6. Glass substrate 7. Beaker 8. Dropper 9. Glass rod

Claims

1. An inorganic nanoparticle core-polymer shell structure colorant characterized by the formation of a polymer shell with a thickness of 20 to 200 nm around inorganic nanocrystals having a particle size of 40 to 300 nm and a refractive index of 3 or higher.

2. The inorganic nanoparticle core-polymer shell structure colorant according to claim 1, wherein the polymer shell is one of polystyrene, polyacrylic, polyethylene glycol, or N-isopropylacrylamide (NIPAM).

3. The inorganic nanoparticle core-polymer shell structure colorant according to claim 1, wherein the polymer shell is formed by bonding to or adsorbing to hydroxyl groups or surface-modified functional groups on the surface of the inorganic nanocrystal.

4. The inorganic nanoparticle core-polymer shell structural colorant according to claim 1, wherein the particle size of the inorganic nanocrystals is 100 to 200 nm and produces a structural color in the visible light range.

5. The inorganic nanoparticle core-polymer shell structural colorant according to claim 1, wherein the inorganic nanocrystal contains particles with particle size distribution peaks of approximately 100 nm, approximately 130 nm, and approximately 200 nm, respectively, and exhibits a structural color under white light.

6. The inorganic nanoparticle core-polymer shell structural colorant according to claim 1, wherein the particle size of the inorganic nanocrystals is 200 to 300 nm and produces a structural color in the near-infrared region.

7. The inorganic nanoparticle core-polymer shell structural colorant according to claim 1, wherein the particle size of the inorganic nanocrystals is 40 to 99 nm and produces a structural color in the ultraviolet light region.

8. The inorganic nanoparticle core-polymer shell structure colorant according to claim 1, characterized in that the inorganic nanocrystals are selected from Si, GaAs, GaP, and InP.

9. A particle film made of an inorganic nanoparticle core-polymer shell structure colorant, characterized in that it is a single-layer film, a multilayer film, or a laminated film formed of an inorganic nanoparticle core-polymer shell structure colorant according to any one of claims 1 to 8, wherein the surface packing rate of nanoparticles in the single layer is 15 to 74%.

10. Cosmetic product using a particle film made of an inorganic nanoparticle core-polymer shell structure colorant according to claim 9.

11. A highly opaque nanoparticle film having a particle film composed of an inorganic nanoparticle core-polymer shell structure colorant according to claim 9.

12. A method for producing a particle film comprising an inorganic nanoparticle core-polymer shell structure colorant according to any one of claims 1 to 8, comprising the steps of: preparing inorganic nanoparticles having a refractive index of 3 or more to a particle size of 40 to 300 nm; coating the inorganic nanoparticles with a polymer shell having a thickness of 20 to 200 nm; and floating the inorganic nanoparticles coated with the polymer shell on a liquid surface and transferring them to a substrate.

13. A method for producing a nanoparticle film according to claim 12, further comprising the step of washing the particle film on the substrate with an organic solvent or performing oxygen plasma treatment to remove only the polymer shell.

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