WO2025058063A1 - Raman spectroscopic signal-enhanced nanoparticles, raman spectroscopic signal-enhanced nanoparticle dispersion, and method for producing raman spectroscopic signal-enhanced nanoparticles - Google Patents

Abstract

[Problem] To provide Raman spectroscopic signal-enhanced nanoparticles having high reproducibility for the Raman scattering enhancement effect, a Raman spectroscopic signal-enhanced nanoparticle dispersion, and a method for producing the Raman spectroscopic signal-enhanced nanoparticles. [Solution] The Raman spectroscopic signal-enhanced nanoparticles are composed of a copper-substituted Prussian blue analog prepared by a step in which a mixed solution obtained by mixing a hydrochloric acid solution of ferricyanide ions ([Fe(CN)₆]^3-) and a hydrochloric acid solution of divalent copper ions (Cu²⁺ ions, copper (II) ions) is heated at 80°C for 24 hours to obtain a hydrochloric acid dispersion of the copper-substituted Prussian blue analog, and a step for drying the hydrochloric acid dispersion of the copper-substituted Prussian blue analog to obtain a powder of the copper-substituted Prussian blue analog, and have a particle size of 10-100 nm. In a state where a measurement target for Raman spectroscopy is adsorbed on the surface of the nanoparticles, Raman scattered light from the measurement target in light emitted from a light source is enhanced.

Description

Raman spectroscopy signal enhancing nanoparticles, Raman spectroscopy signal enhancing nanoparticle dispersion and method for producing Raman spectroscopy signal enhancing nanoparticles

The present invention relates to Raman spectroscopic signal enhancing nanoparticles, a Raman spectroscopic signal enhancing nanoparticle dispersion, a dropping substrate for measuring Raman scattered light, and a method for manufacturing Raman spectroscopic signal enhancing nanoparticles.

Raman spectroscopy is used to obtain chemical and structural information about materials and to measure various physical properties such as stress, temperature, electrical properties, orientation, and crystallinity.

On the other hand, Raman spectroscopy has the problem of low sensitivity because the Raman scattering intensity is very weak, about one millionth that of Rayleigh scattering. For this reason, surface-enhanced Raman spectroscopy (SERS) is known, which enhances Raman scattered light and improves the sensitivity of Raman spectroscopy. For example, the following Patent Documents 1 and 2 disclose SERS that enhances Raman scattered light using metal nanostructures.

Patent No. 6196159 Patent No. 6429318

However, when using metals for SERS, there was a problem in that the reproducibility of the enhancement effect of Raman scattering light was low.

The object of the present invention is to provide Raman spectroscopic signal enhancing nanoparticles, Raman spectroscopic signal enhancing nanoparticle dispersions, and methods for producing Raman spectroscopic signal enhancing nanoparticles that have a highly reproducible effect of enhancing Raman scattered light.

In order to achieve the above object, the present invention includes the following embodiments.
[1] A Raman spectroscopy signal enhancing nanoparticle comprising a copper-substituted Prussian blue analogue and having a particle size of 10 to 100 nm, the nanoparticle having a surface on which a substance to be measured by Raman spectroscopy is adsorbed, and which enhances Raman scattered light from the substance to be measured in light irradiated from a light source.
[2] The Raman spectroscopic signal enhancing nanoparticle described in [1], wherein the copper-substituted Prussian blue analogue constituting the nanoparticle contains an atomic group that causes charge transfer to the substance to be measured adsorbed on the surface of the nanoparticle.
[3] The Raman spectroscopic signal enhancing nanoparticle according to [2], wherein the atomic group is an atomic group containing bonds between carbon and carbon, between carbon and nitrogen, and between carbon and oxygen.
[4] A Raman spectroscopic signal enhancing nanoparticle dispersion liquid, in which the Raman spectroscopic signal enhancing nanoparticles according to any one of [1] to [3] are dispersed in a dispersion medium.
[5] A method for producing nanoparticles for enhancing Raman spectroscopy signals, comprising the steps of: heating a mixture of a hydrochloric acid solution of ferricyanide ions ([Fe(CN) ₆ ] ³⁻ ) and a hydrochloric acid solution of divalent copper ions (Cu ²⁺ ions, copper (II) ions) at 80° C. for 24 hours to obtain a hydrochloric acid dispersion of a copper-substituted Prussian blue analogue; and drying the hydrochloric acid dispersion of the copper-substituted Prussian blue analogue to obtain a powder of the copper-substituted Prussian blue analogue.

The present invention provides Raman spectroscopic signal enhancing nanoparticles, Raman spectroscopic signal enhancing nanoparticle dispersions, and methods for producing Raman spectroscopic signal enhancing nanoparticles that have a highly reproducible effect of enhancing Raman scattered light.

FIG. 1 is a diagram showing an example of the operation of the Raman spectroscopy signal enhancing nanoparticles according to an embodiment. FIG. 1 shows the results of EDS line analysis of a powder of a copper-substituted Prussian blue analog as a Raman spectroscopic signal enhancing nanoparticle according to an embodiment. FIG. 1 is a diagram showing the measurement results of the binding energy of Raman spectroscopy signal enhancing nanoparticles according to an embodiment. FIG. 1 is a diagram showing the measurement results of the enhancement of Raman scattered light of Raman spectroscopy signal enhancing nanoparticles according to an embodiment. FIG. 13 is a diagram showing the results of Raman scattered light measurement at different concentrations of R6G, which is a substance to be measured.

Below, a form for implementing the present invention (hereinafter referred to as an embodiment) will be explained with reference to the drawings.

Figure 1 shows an example of the operation of the Raman spectroscopy signal enhancing nanoparticles according to the embodiment. In Figure 1, a substance (sample) 12 to be measured by Raman spectroscopy is adsorbed on the surface of the Raman spectroscopy signal enhancing nanoparticle 10. When excitation light 14 such as laser light is irradiated from a light source in this state, the Raman scattered light scattered from the substance to be measured 12 is enhanced by the Raman spectroscopy signal enhancing nanoparticle 10, generating enhanced Raman scattered light (SERS scattered light) 16.

The Raman spectroscopy signal enhancing nanoparticles 10 are composed of a copper-substituted Prussian blue analog. The crystal structure of Prussian blue is such that Fe ³⁺ ions form a face-centered cubic lattice, Fe ²⁺ ions are located at the midpoints of each side of the cube, and CN ^- ions are present between the Fe ³⁺ ions and the Fe ²⁺ ions, and the CN ^- ions are coordinated to the Fe ³⁺ ions with nitrogen atoms and to the Fe ²⁺ with carbon atoms. The copper-substituted Prussian blue analog is a Prussian blue in which a part of the Fe ³⁺ ions (iron (II) ions) is replaced with Cu ²⁺ ions (copper (II) ions). The chemical formula when the Fe ³⁺ ions (iron (II) ions) of Prussian blue are replaced with Cu ²⁺ ions (copper (II) ions) can be written as (K ₂ Cu [Fe (CN) ₆ ]), for example. The atomic ratio of copper ions (Cu ²⁺ ions) to iron ions (Fe ²⁺ ions + Fe ³⁺ ions) in copper-substituted Prussian blue analogues is approximately 1 to 1. Hereinafter, copper-substituted Prussian blue analogues may be referred to as Cu-PBAs, and Prussian blue analogues may be referred to as PBAs.

Furthermore, the particle size of the Raman spectroscopy signal enhancing nanoparticles 10 is preferably 10 to 100 nm. Raman spectroscopy signal enhancing nanoparticles 10 with such a particle size are advantageous for biological applications such as cell imaging.

The Raman spectroscopic signal enhancing nanoparticles 10 in this embodiment enhance Raman scattered light by a resonance effect (chemical enhancement) due to an electron (charge) transfer interaction between the nanoparticle surface and the measured substance 12 adsorbed thereon. In this case, as described below, it is preferable that the Raman spectroscopic signal enhancing nanoparticles 10 contain an atomic group that causes charge transfer between the measured substance 12, and it is considered that the atomic group that causes charge transfer between the Raman spectroscopic signal enhancing nanoparticles 10 and the measured substance 12 can be increased by replacing a part of the Fe ³⁺ ions in the crystal structure of Prussian blue with Cu ²⁺ ions. Examples of the atomic group that is increased include atomic groups containing bonds between carbon and carbon, carbon and nitrogen, and carbon and oxygen.

The method of using the Raman spectroscopy signal enhancing nanoparticles 10 in this embodiment is not particularly limited as long as the target substance 12 can be adsorbed to the surface, but it is preferable for convenience of handling to disperse the Raman spectroscopy signal enhancing nanoparticles 10 in an appropriate dispersion medium, since this makes it easy to drop them onto various substrates such as silicon substrates, glass substrates, or metal substrates. As the dispersion medium, water (demineralized water), an aqueous methanol solution, an aqueous ethanol solution, or the like is preferable.

For example, the substance to be measured 12 can be mixed with a dispersion liquid in which Raman spectroscopy signal enhancing nanoparticles 10 are dispersed in the above-mentioned dispersion medium, the mixed liquid is dropped onto a dropping substrate for Raman scattering light measurement, dried, and set in an appropriate Raman spectroscopy device to perform various measurements while enhancing the Raman scattering light. Examples of dropping substrates for Raman scattering light measurement include silicon substrates, carbon paper, nickel plates, copper plates, glass plates, etc.

The Raman spectroscopic signal enhancing nanoparticles 10 according to the embodiment can be produced by the steps of: heating a mixture of a hydrochloric acid solution of ferricyanide ions ([Fe(CN) ₆ ] ³⁻ ) and a hydrochloric acid solution of divalent copper ions (Cu ²⁺ ions, copper (II) ions) at 80° C. for 24 hours to obtain a hydrochloric acid dispersion of a copper-substituted Prussian blue analog; and drying the hydrochloric acid dispersion of the copper-substituted Prussian blue analog to obtain a powder of the copper-substituted Prussian blue analog (a powder of aggregated nanoparticles of the copper-substituted Prussian blue analog).

Below, specific examples of the present invention will be described. Note that the following examples are provided to facilitate understanding of the present invention, and the present invention is not limited to these examples.

<Production of Nanoparticles for Enhancing Raman Spectroscopy Signal>
A hydrochloric acid solution of ferricyanide ions ([Fe(CN) ₆ ] 3- ) was prepared by dissolving 3 g of potassium ferricyanide (potassium hexacyanoferrate (III) K ₃ [Fe(CN) ₆ ] manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) in 26 mL of hydrochloric acid (0.05 M manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., diluted with 1 M (mol/ ^L ) HCl aqueous solution). A hydrochloric acid solution of Cu ²⁺ ions was prepared by dissolving 1.5 g of copper (II) sulfate (CuSO ₄ manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) in 13 mL of hydrochloric acid (0.05 M).

Next, a hydrochloric acid solution of ferricyanide ions and a hydrochloric acid solution of ^Cu ions were mixed in a glass bottle of about 50 ml, and the mixed solution was heated at 80° C. for 24 hours to synthesize a copper-substituted Prussian blue analogue in which some of the iron ions (Fe ^ions ) that make up the crystal lattice of Prussian blue were replaced with copper ions (Cu ^ions ).

The copper-substituted Prussian blue analogue synthesized above was obtained in a dispersed state in hydrochloric acid, so the dispersion of the copper-substituted Prussian blue analogue (aqueous hydrochloric acid solution) was heated in an oven at 80°C for 24 hours and dried to obtain a powder of the copper-substituted Prussian blue analogue (particle size 40-60 nm) as the Raman spectroscopic signal enhancing nanoparticles of the embodiment. The particle size of the powder of the copper-substituted Prussian blue analogue was measured by observing the powder with an electron microscope (field emission transmission electron microscope JEM-2100F manufactured by JEOL Ltd.).

<Evaluation of Nanoparticles for Raman Spectroscopy Signal Enhancement>
The powder of the copper-substituted Prussian blue analogue as the Raman spectroscopic signal enhancing nanoparticles of the embodiment obtained as described above was subjected to line analysis using an energy dispersive X-ray analyzer (EDS) mounted on the electron microscope, and elements at each position of an arbitrary line segment in the image were measured. The results of the line analysis are shown in Figure 2. As can be seen from Figure 2, the ratio of the detected amount of elements (iron, copper, nitrogen, carbon) at each position was almost constant, and it can be seen that copper (II) ions were introduced into the crystal lattice of Prussian blue without any variation.

Figures 3(a), (b), and (c) show the results of measurements of the Raman spectroscopic signal-enhancing nanoparticles by X-ray photoelectron spectroscopy (XPS). For the measurements, a photoelectron spectroscopy (XPS) JPS-9010MC (using AlKα radiation) manufactured by JEOL Ltd. was used. The upper side of each figure shows copper-substituted Prussian blue analogues (Cu-PBAs), and the lower side shows Prussian blue analogues (PBAs).

FIG. 3(a) shows the bond energy of the 2p orbital of iron atoms, and Fe ³⁺ ions and Fe ²⁺ ions that constitute the crystal lattice of Prussian blue are observed. FIG. 3(b) shows the bond energy of the 1s orbital of carbon, and in PBAs and Cu-PBAs, bonds such as O-C=O, C=O, C-N, and C-C are confirmed as atomic groups. However, from the emitted photoelectron intensity on the vertical axis of FIG. 3(b), it can be seen that the number of atomic groups having these bonds (especially C=O and C-N) is increased compared to PBAs. Furthermore, FIG. 3(c) shows the bond energy of the 1s orbital of nitrogen, and in Cu-PBAs, bonds such as O-N, pyridine-type nitrogen (denoted as pyridinicN), and cyanide nitrogen (denoted as cyanideN) are confirmed as atomic groups, and these atomic groups are increased compared to PBAs.

The above-mentioned atomic groups, the presence of which was observed in Cu-PBAs, are likely to cause charge transfer in the substance to be measured by Raman spectroscopy that is adsorbed on the surface of the Raman spectroscopy signal enhancing nanoparticles, and therefore the ability to enhance Raman scattered light can be improved by the resonance effect (chemical enhancement) caused by the charge transfer interaction between the surface of the Raman spectroscopy signal enhancing nanoparticles and the substance to be measured.

4(a), (b), and (c) show the measurement results of the enhancement factor of Raman scattered light of the Raman spectroscopy signal enhancing nanoparticles according to the embodiment. FIG. 4(a) shows a Raman spectrum measured by placing a powder (sample) of rhodamine 6G (R6G obtained from Sigma-Aldrich Japan LLC) on a silicon substrate. FIG. 4(b) shows a Raman spectrum as a comparative example measured after dropping a dispersion (sample) in which PBAs was used as the Raman spectroscopy signal enhancing nanoparticles and dispersed in desalted water to a concentration of 2.5 mg/mL, and R6G was further dispersed to a concentration of 5×10 ⁻⁵ M onto a silicon substrate and dried in the air. 4(c) shows a Raman spectrum as an example in which Cu-PBAs was used as Raman spectroscopy signal enhancing nanoparticles, which were dispersed in demineralized water to a concentration of 2.5 mg/mL, and R6G was further dispersed to a concentration of 5×10 ⁻⁵ M to form a dispersion (sample), which was dropped onto a silicon substrate and dried in the air. The Raman spectrum was obtained by setting each of the above samples in a Raman spectrometer (confocal Raman microscope RM2000 manufactured by Renishaw) and measuring the enhanced Raman scattering light. The measurement was performed using a 532 nm laser beam (output 0.6 mW) with an integration time of 10 seconds.

The enhancement of Raman scattered light of the Raman spectroscopic signal enhancing nanoparticles was calculated by the following formula (1).
Enhancement factor (EF) = (Isers / Ir6g) / Mf ... (1)
Here, Isers is the peak intensity at a Raman shift of 613 cm ^-1 in the Raman spectrum of Figure 4(c) using Cu-PBAs, Ir6g is the peak intensity at a Raman shift of 613 cm ^-1 in the Raman spectrum of R6G alone shown in Figure 4(a), and Mf is the mass fraction of R6G in the measurement solution. In the above examples and comparative examples, the R6G concentration is 5 x ^10-5 M and Mf is 2.4 x ^10-3 %.

From the above, it was found that the enhancement of Raman scattered light of the Raman spectroscopy signal enhancing nanoparticles as an example using Cu-PBAs was about 10 ^7. On the other hand, the enhancement of Raman scattered light of the Raman spectroscopy signal enhancing nanoparticles as a comparative example using PBAs was calculated using the peak intensity at a Raman shift of 613 cm ^-1 in the Raman spectrum of FIG. 4(b) instead of FIG. 4(c). As a result, it was found that the enhancement of the comparative example was about 10 ^6. As a result, it was found that when Cu-PBAs is used, an enhancement 10 times that of PBAs can be obtained.

<Measurement of Raman scattering enhancement effect>
A measurement solution (aqueous solution) containing rhodamine 6G (R6G) as the substance to be measured at concentrations of 5 μM, 0.5 μM, and 5 nM, and a dispersion of copper-substituted Prussian blue analogue (Cu-PBAs) powder as the Raman spectroscopic signal enhancing nanoparticle according to the embodiment dispersed in desalted water at a concentration of 2.5 mg/mL were prepared, and the same volume of the dispersion was mixed with each solution (Cu-PBAs concentration was 1.25 mg/mL). This mixed solution was dropped onto a silicon wafer as a dropping substrate for Raman scattering light measurement, and then heated at 80 ° C. on a heating plate and dried for 12 hours, and then set in a Raman spectrometer (Renishaw RM2000 confocal Raman microscope) to measure the enhanced Raman scattering light. The measurement was performed using a 532 nm laser light (output 0.6 mW) with an integration time of 10 seconds.

As a comparative example, the concentrations of R6G in the solution were set to 5 x 10 μM, 5 μM, and 0.5 (5 x 0.1) μM, and Prussian blue analogs (PBAs) without copper substitution were used as Raman spectroscopic signal enhancing nanoparticles, and the enhanced Raman scattering light was measured in the same manner as above.

Figures 5(a) and (b) show the measurement results of Raman scattered light. Figure 5(a) shows the measurement results of the Example, and Figure 5(b) shows the measurement results of the Comparative Example. In Figure 5(a), Raman scattered light could be measured up to an R6G concentration of 5 nM. On the other hand, in Figure 5(b), Raman scattered light could only be measured up to an R6G concentration of 5 μM. Therefore, in the Example (where a copper-substituted Prussian blue analogue was used), the detection sensitivity could be increased by about 1000 times compared to the Comparative Example (where a Prussian blue analogue not substituted with copper was used).

10 Raman spectroscopy signal enhancing nanoparticles, 12 Measurement target substance, 14 Excitation light, 16 Enhanced Raman scattering light.

Claims (5)

Nanoparticles composed of a copper-substituted Prussian blue analogue and having a particle size of 10-100 nm,
A Raman spectroscopy signal enhancing nanoparticle that enhances Raman scattered light from the substance to be measured by Raman spectroscopy when the substance is adsorbed on the surface of the nanoparticle and that is irradiated from a light source. The Raman spectroscopy signal enhancing nanoparticle of claim 1, wherein the copper-substituted Prussian blue analogue constituting the nanoparticle contains an atomic group that causes charge transfer to the substance to be measured adsorbed on the surface of the nanoparticle. The Raman spectroscopy signal enhancing nanoparticle of claim 2, wherein the atomic group is an atomic group containing carbon-carbon, carbon-nitrogen, and carbon-oxygen bonds. A Raman spectroscopy signal enhancing nanoparticle dispersion in which the Raman spectroscopy signal enhancing nanoparticles according to any one of claims 1 to 3 are dispersed in a dispersion medium. A step of heating a mixture of a hydrochloric acid solution of ferricyanide ions ([Fe(CN) ₆ ] ³⁻ ) and a hydrochloric acid solution of divalent copper ions (Cu ²⁺ ions, copper (II) ions) at 80° C. for 24 hours to obtain a hydrochloric acid dispersion of a copper-substituted Prussian blue analogue;
drying the hydrochloric acid dispersion of the copper-substituted Prussian blue analog to obtain a powder of the copper-substituted Prussian blue analog;
23. A method for producing a Raman spectroscopy signal enhancing nanoparticle, comprising:

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