JP2020084205A - Control method for particle size of metal nanoparticle and method for production of metal nanoparticle - Google Patents

Abstract

To provide a metal nanoparticle production method capable of surely obtaining metal nanoparticles having desired properties.SOLUTION: A metal nanoparticle production method in this invention comprises: firstly introducing a solution 4 for synthesis of metal nanoparticles including superfine bubbles having the diameter of 1 μm or less produced under pressure dissolution in a container 7, and then generating cavitation by intermittently irradiating the solution 4 in the container 7 with the ultrasonic wave S1 of 100 kHz or more in a repeating cycle of 2 ms or more but 30 ms or less to reduce the metal ion in the solution 4. Thereby, the metal nanoparticle having an average particle size of 25 nm or less and the standard deviation of particle size of 6 nm or less is synthesized in the solution 4.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for controlling the particle size of metal nanoparticles and a method for producing metal nanoparticles.

It is known that noble metal particles typified by gold and the like show characteristic properties that are not shown in the bulk size when the size is 1 μm or less in nano size. Specific examples of such characteristic properties include, for example, catalytic properties for oxidation reaction and hydrogenation reaction, optical properties derived from surface plasmon resonance, and conductive properties. Therefore, it is considered that the metal nanoparticles can be applied in the fields of DNA sensing technology, light energy exchange efficiency improving technology and the like. In addition to these, metal nanoparticles have already been used in the medical field such as drug delivery systems and contrast agents for ultrasonic diagnostics.

Here, as a method for synthesizing the metal nanoparticles, a solution for synthesizing metal nanoparticles containing a metal ion adjusted to a predetermined concentration, temperature, etc. (for example, a gold ion aqueous solution) is prepared, and the metal ion in the solution is mixed. A method of reducing by a chemical method is well known in the art. In the chemical reduction method as described above, a reducing agent (for example, citric acid) by hydrogen radicals and a reducing auxiliary agent are added to cause a reduction reaction, and the synthesized metal nanoparticles are aggregated with each other. A protective agent such as a surfactant or an amphipathic polymer is added in order to prevent the above and disperse each other (see, for example, Patent Document 1). Further, Patent Document 2 discloses a technique of adding a reducing agent to cause a reduction reaction and irradiating an ultrasonic wave of 20 to 50 kHz to improve the dispersibility of metal nanoparticles. However, in the conventional techniques described in Patent Documents 1 and 2, impurities such as organic substances are inevitably contained in the solution, and there is a concern that the purity of the metal nanoparticles decreases and the environment and the like are adversely affected. Therefore, a clean synthesis method is considered desirable.

Under such circumstances, in recent years, a method of synthesizing metal nanoparticles by reducing metal ions using ultrasonic cavitation (ultrasonic reduction method) has been proposed (for example, Patent Document 3). reference). In this method, specifically, ultrasonic waves are applied to generate ultrasonic cavitation in the liquid. At this time, water is thermally decomposed by ultrasonic cavitation to generate hydrogen radicals, which is characterized by being utilized for the reduction reaction of metal ions. According to this method, in addition to the need for a reducing agent and a reduction auxiliary agent, since the bubble surface is charged, particles do not aggregate, and surfactants, amphipathic polymers, etc. No need for protective agents. Therefore, it is a clean synthesis method that can obtain highly pure metal nanoparticles as compared with the conventional method.

By the way, in the conventional technique described in Patent Document 3, the size of the metal nanoparticles is controlled to be small by changing the pH of the solution for synthesizing the metal nanoparticles. However, when the pH is changed, a chemical that becomes an impurity must be used, which may reduce the purity of the metal nanoparticles. In addition, in the above-mentioned ultrasonic reduction method, the inventors of the present invention include UFB (Ultrafine bubbles), which are ultrafine bubbles having a diameter of 1 μm or less, in a solution for synthesizing metal nanoparticles and change the number density of UFB. , A technique for controlling the size of the metal nanoparticles to be small has already been proposed (see Patent Document 4).

Japanese Patent No. 5293202 (paragraphs [0014], [0015], [0027], etc.) Japanese Patent No. 4958082 (claims 6, 9, 10 and paragraph [0019] etc.) JP 2009-57594 A (claim 2, paragraphs [0017], [0020], [0021], etc.) Japanese Patent Application No. 2018-037988 (Claims 1, 2, etc.)

However, when trying to synthesize the metal nanoparticles by the ultrasonic reduction method described in Patent Document 4, it was possible to obtain a relatively large number of particles having a particle size of 20 to 50 nm, but a smaller particle size (especially 20 nm or less). It's not easy to get a lot of things). In addition to this, conventionally, there was no effective method capable of controlling the particle size of the metal nanoparticles by a clean method, so that it was extremely difficult to obtain the metal nanoparticles of a desired size. ..

The present invention has been made in view of the above problems, and an object thereof is to provide a method for controlling the particle size of metal nanoparticles, which can control the size of the particle size of the metal nanoparticles to be surely reduced by a clean method. To provide. Further, a further object of the present invention is to provide a method for producing metal nanoparticles capable of surely obtaining metal nanoparticles having desired properties.

The present inventors have conducted intensive research to solve the above problems, ultrafine bubbles in a solution in which ultrasonic waves act contribute to promotion of radical reaction due to cavitation and improvement of dispersibility of metal nanoparticles. Focusing on the fact that the ultrasonic wave irradiation mode has a considerable influence on the particle size of the metal nanoparticles, it was newly found. Then, the present inventors finally conducted the following research based on the above findings and finally completed the following invention. The inventions for solving the above problems are listed below.

That is, the invention according to claim 1 is to reduce the metal ions in the solution by causing cavitation by irradiation of ultrasonic waves into a solution for synthesizing metal nanoparticles containing ultrafine bubbles having a diameter of 1 μm or less. Thus, when synthesizing the metal nanoparticles in the solution, by ultrasonically irradiating the solution with ultrasonic waves of 100 kHz or more at a repeating cycle of millisecond order and shortening the repeating cycle, the metal nanoparticles are obtained. The gist of the present invention is a method for controlling the particle size of metal nanoparticles, which is characterized by controlling the size of the metal nanoparticles to be small.

Therefore, according to the invention of claim 1, the size of the metal nanoparticles is reduced by intermittently irradiating the solution with ultrasonic waves of 100 kHz or more at a repeating cycle of millisecond order and shortening the repeating cycle. Can be controlled to be small. Therefore, the particle size of the metal nanoparticles can be controlled without using additives such as a reducing agent, a reduction auxiliary agent, a surfactant, and an amphipathic polymer. That is, the size of the metal nanoparticles can be reliably reduced by a clean method.

The gist of the invention according to claim 2 is the method for producing metal nanoparticles, which comprises the step of controlling the size of the particle diameter by the method according to claim 1.

Therefore, according to the second aspect of the invention, the metal nanoparticles having the desired properties can be reliably obtained through the step of controlling the size of the particle size.

In the invention according to claim 3, a step of introducing a solution for synthesizing metal nanoparticles containing ultrafine bubbles having a diameter of 1 μm or less, which is produced by a pressure dissolution method, into a container, and then the solution in the container is added. In contrast, by intermittently irradiating ultrasonic waves of 100 kHz or more with a repeating cycle of 2 ms or more and 30 ms or less to generate cavitation and reduce the metal ions in the solution, the average particle size is 25 nm or less and the particles are The gist is a method for producing metal nanoparticles, which comprises performing a step of synthesizing metal nanoparticles having a standard deviation of diameter of 6 nm or less in the solution.

According to a fourth aspect of the present invention, in the third aspect, the solution in the container is intermittently irradiated with ultrasonic waves at a repetition cycle of 4 ms or more and 20 ms or less, so that the metal nanoparticle having an average particle size of less than 20 nm is irradiated. The point is to synthesize particles.

Therefore, according to the invention described in claims 3 and 4, by intermittently irradiating the ultrasonic wave of a suitable frequency with the repetition cycle of irradiating the ultrasonic wave adjusted to an appropriate value, that is, It is possible to reliably obtain metal nanoparticles having an extremely small particle size.

As described in detail above, according to the invention of claim 1, there is provided a method for controlling the particle size of metal nanoparticles, which can control the size of the metal nanoparticles to be surely reduced by a clean method. be able to. According to the invention as set forth in claim 2, it is possible to provide a method for producing metal nanoparticles capable of surely obtaining metal nanoparticles having desired properties. According to the third aspect of the present invention, it is possible to provide a method for producing metal nanoparticles that can reliably obtain metal nanoparticles having an extremely small particle size.

The schematic block diagram which shows the gold nanoparticle manufacturing apparatus of this embodiment. The time chart which shows the pulse waveform of ultrasonic waves. The graph which shows the particle size distribution of the gold nanoparticle in each presence or absence of UFB. (A) is a graph showing voltage and current waveforms when the ultrasonic transducer is driven at a frequency of 43 kHz and an ON time of 10 ms, and (b) shows an ultrasonic transducer at a frequency of 43 kHz and an ON time of 2 ms. A graph showing voltage and current waveforms when driven, (c) is a graph showing voltage and current waveforms when the ultrasonic transducer is driven at a frequency of 495 kHz and an ON time of 10 ms, and (d) shows frequency. 6 is a graph showing voltage and current waveforms when an ultrasonic transducer is driven at 495 kHz and an ON time of 2 ms. (A) is an electron micrograph (magnification: 120,000 times) of gold nanoparticles obtained by irradiating ultrasonic waves (pulse wave) with ON time = 2 ms and OFF time = 2 ms, (b) shows ON time = Electron micrograph (magnification: 80,000 times) of gold nanoparticles obtained when irradiated with ultrasonic waves (pulse wave) at 600 ms, OFF time = 600 ms, (c) shows ultrasonic waves (continuous wave) at OFF time = 0 ms Electron micrograph (120,000 times magnification) of gold nanoparticles obtained when irradiated with. The graph which shows the particle size distribution of the gold nanoparticle at the time of irradiating each of a continuous wave and a pulse wave. The graph which shows the relationship between the ON time and OFF time of an ultrasonic wave, and the average particle diameter of a gold nanoparticle. The graph which shows the average particle diameter of a gold nanoparticle, and the relationship of ON time and OFF time of ultrasonic waves of 2 ms or more.

Hereinafter, an embodiment in which the present invention is embodied in a gold nanoparticle manufacturing apparatus will be described in detail with reference to the drawings.

As shown in FIG. 1, the gold nanoparticle manufacturing apparatus 10 irradiates the gold ion aqueous solution 4 (solution for synthesizing metal nanoparticles) 4 containing UFB, which is ultrafine bubbles having a diameter of 1 μm or less, with ultrasonic waves S1. Device. The gold nanoparticle manufacturing apparatus 10 includes an ultrasonic oscillator 1, a power amplifier 2, an ultrasonic vibrator 3, a pump 5, a constant temperature bath 6, and a processing bath 7 (vessel).

The treatment tank 7 of the present embodiment has an upper opening and a flat bottom (generally cylindrical shape), and the upper opening is the same as the inner tank 7a (glass container or the like) in which the gold ion aqueous solution 4 is stored. In addition, the processing tank has a flat bottom shape (a substantially rectangular tube shape) and an outer tank 7b in which an inner tank 7a is housed. An inlet for introducing constant temperature water into a space formed by the inner tank 7a and the outer tank 7b and an outlet for discharging constant temperature water from the space are provided at two locations on the side surface of the outer tank 7b. ing. The constant temperature water adjusted to a constant temperature is stored in the constant temperature tank 6, and the constant temperature water is constantly circulated and supplied to the processing tank 7 side by driving the pump 5. As a result, the temperature in the processing tank 7 can be kept constant (for example, 0° C. to 60° C.).

The ultrasonic vibrator 3 is a means for irradiating the gold ion aqueous solution 4 contained in the inner tank 7a of the processing tank 7 with the ultrasonic wave S1, and is fixed to the outer surface of the bottom of the outer tank 7b. Examples of the ultrasonic oscillator 3 in the present embodiment include a 130 kHz oscillator having a diameter of 45 mm, a 200 kHz oscillator having a diameter of 50 mm, a 300 kHz oscillator, a 500 kHz oscillator, a 1 MHz oscillator, and a 2 MHz oscillator. Etc. can be used (all are manufactured by Honda Electronics Co., Ltd.). Among these vibrators, a bolted Langevin type vibrator is suitable for low frequencies, and a vibrator composed of a ceramic element alone is suitable for high frequencies.

Further, in the gold nanoparticle manufacturing apparatus 10 shown in FIG. 1, the ultrasonic oscillator 1 and the power amplifier 2 constitute a driving device for driving the ultrasonic vibrator 3 at a predetermined frequency. As the ultrasonic oscillator 1, for example, a function generator can be used. This ultrasonic oscillator 1 is electrically connected to an ultrasonic oscillator 3 via a power amplifier 2. The ultrasonic oscillator 1 outputs a sine wave having a predetermined frequency as a continuous or intermittent oscillation signal. This oscillation signal is amplified by the power amplifier 2 and then supplied to the ultrasonic transducer 3 to drive the ultrasonic transducer 3. Although not shown, an impedance matching circuit may be provided between the power amplifier 2 and the ultrasonic transducer 3. Then, the ultrasonic transducer 3 generates an ultrasonic wave S1 (see FIG. 2) having a frequency corresponding to the oscillation frequency of the ultrasonic oscillator 1. As a result, the gold ion aqueous solution 4 in the inner bath 7a of the treatment bath 7 is indirectly irradiated with the ultrasonic wave S1 from the bottom side of the outer bath 7b of the treatment bath 7 toward the inner bath 7a.

Next, a method of manufacturing the gold nanoparticles 15 (metal nanoparticles) shown in FIG. 5 using the gold nanoparticle manufacturing apparatus 10 configured as described above will be described.

First, a solution preparation step for producing the gold ion aqueous solution 4 containing UFB is performed. The solution 4 can be prepared by, for example, preparing UFB water by a pressure dissolution method using ultrapure water, and dissolving tetrachloroauric acid tetrahydrate in the UFB water. .. The procedure may be such that an aqueous solution prepared by previously dissolving tetrachloroauric acid tetrahydrate in ultrapure water is prepared, and UFB water is prepared from this aqueous solution by a pressure dissolution method. The advantage of UFB water production by the pressure dissolution method is that high-density UFB water can be obtained at a relatively low cost with a relatively simple device.

Next, a solution introducing step of introducing the gold ion aqueous solution 4 containing UFB into the processing tank 7 is performed. In the gold nanoparticle synthesis step after the solution introduction step, ultrasonic cavitation is generated by the irradiation of ultrasonic waves S1 from the ultrasonic oscillator 3 into the solution 4 in the treatment tank 7, and the gold ions (metal The gold nanoparticles 15 are synthesized in the solution 4 by reducing (ions). More specifically, the gold ions in the solution 4 are reduced (nucleated) to Au by ultrasonic cavitation, and further grown from Au into gold nanoparticles 15 (particle growth). It is considered that as the ultrasonic cavitation increases, more gold ions are used for nucleation and less gold ions are required for particle growth, so that the particle size of the gold nanoparticles 15 becomes smaller.

Further, in the gold nanoparticle synthesis step, the solution 4 is intermittently irradiated with ultrasonic waves S1 of 495 kHz at a repetition cycle T0 of millisecond order (4 ms in the present embodiment) and the repetition cycle T0 is shortened. The particle size of the particles 15 is controlled to be small. At this time, the operator inputs the optimum ON time T1 (see FIG. 2) and OFF time T2 (see FIG. 2) of the ultrasonic wave S1 to the ultrasonic oscillator 1. Then, the ultrasonic wave S1 is emitted from the ultrasonic transducer 3 based on the input ON time T1 and OFF time T2. In this embodiment, the ON time T1 of the ultrasonic wave S1 is 2 ms and the OFF time T2 of the ultrasonic wave S1 is 2 ms. As a result, gold nanoparticles 15 (see FIG. 5A) having an average particle size of 13.1 nm and a standard deviation of particle sizes of 5.9 nm were synthesized in the solution 4, and the gold nanoparticles 15 were formed. The dispersed state (that is, gold nanocolloid) can be obtained in the solution 4.

Hereinafter, examples in which the above embodiment is more concretely introduced will be introduced.

[Example 1]: Experiment on effect of UFB on gold nanoparticle synthesis by ultrasonic waves In this example, an experiment was conducted by the following method.

1. Experimental Method As a sample used in the experiment, 0.1 mM gold ion aqueous solution 4 was prepared by dissolving tetrachloroauric acid tetrahydrate in a predetermined amount of water as a solvent. Here, UFB water was used as water, and distilled water (that is, noUFB water) was also used for comparison. UFB water was prepared from ultrapure water by a pressure dissolution method (ultrafineGaLF, IDEC). The number density was 5 billion/mL when UFB contained in UFB water was measured by the nanoparticle Brownian motion tracking method (NanoSight, Malvern).

Next, the prepared gold ion aqueous solution 4 was put into the processing tank 7 (inner tank 7a) of the gold nanoparticle manufacturing apparatus 10 shown in FIG. Then, with the frequency set to 500 kHz and the time average power applied to the vibrator set to 50 W, ultrasonic wave S1 was irradiated from the ultrasonic vibrator 3 for 10 minutes to synthesize gold nanoparticles 15 (gold nanocolloid). Furthermore, after the irradiation treatment with the ultrasonic wave S1, the synthesized gold nanocolloid was sampled, and the gold nanoparticles 15 were observed and evaluated by an electron microscope. The result is shown in FIG.

2. Experimental Results and Discussion The graph of FIG. 3 shows the particle size distribution of the gold nanoparticles 15 with and without UFB. According to this, in the absence of UFB, the gold nanoparticles 15 were present over 50 nm to 180 nm, the average particle size was 119 nm, and the standard deviation was 80 nm. On the other hand, when UFB was present, the gold nanoparticles 15 were present over 15 nm to 45 nm, the average particle size was 22 nm, and the standard deviation was 6 nm. That is, it was found that in the presence of UFB, both the average particle size and the standard deviation of the gold nanoparticles 15 are small. It was also found that the average particle size of the gold nanoparticles 15 decreases as the number density of UFB increases. It is considered that the decrease in the average particle size is because UFB in the solution 4 behaves as a nucleus of the gold nanoparticles 15 and increases the production amount of reducing radicals such as hydrogen radicals, hydrogen peroxide, and nitrous acid. Furthermore, it was also found that the presence of UFB improves the stability of the gold nanoparticles 15 in the gold nanocolloid. It is considered that this is because the gold nanoparticles 15 were adsorbed on the UFB surface.

[Conclusion]
From the above experimental results, by including UFB in the solution 4, the reaction rate of the ultrasonic chemical reaction in the synthetic reaction of the gold nanoparticles 15 increases, and the particle size of the gold nanoparticles 15 is stably reduced. It turned out to be controllable.

Example 2 Experiment on Influence of Frequency of Ultrasonic Wave on Driving State of Ultrasonic Transducer In this example, an experiment was conducted by the following method.

1. Experimental Method First, pure water was put into the processing tank 7 (outer tank 7b) of the gold nanoparticle manufacturing apparatus 10 shown in FIG. Then, with the frequency set to 43 kHz and 495 kHz, the ON time T1 and the OFF time T2 set to the same length, and T1 and T2 set to 10 ms and 2 ms, the voltage applied to the ultrasonic transducer 3 and the ultrasonic transducer 3 are set. The flowing current was measured with an oscilloscope and a current probe.

2. Experimental Results and Discussion The above results are shown in FIG. FIG. 4A shows voltage and current waveforms when the ultrasonic transducer 3 is driven at a frequency of 43 kHz and an ON time T1 of 10 ms. The current flowing through the ultrasonic oscillator 3 is proportional to the vibration speed of the surface of the ultrasonic oscillator 3. Therefore, it was found that when a voltage was applied to the ultrasonic vibrator 3, the ultrasonic vibrator 3 gradually started to vibrate and reached a constant vibration velocity 2 ms after the voltage was applied. Next, FIG. 4B shows voltage and current waveforms when the ultrasonic transducer 3 is driven at a frequency of 43 kHz and an ON time T1 of 2 ms. In this case, when the voltage is applied, the vibration speed of the ultrasonic vibrator 3 gradually increases, and the vibration speed of the ultrasonic vibrator 3 is 2 ms after the voltage is applied and immediately before the voltage is turned off. Became constant. Further, even if the voltage was turned off, the vibration speed of the ultrasonic transducer 3 did not immediately become zero but gradually decreased. Then, 2 ms after the voltage was turned off, the vibration of the ultrasonic transducer 3 stopped. Therefore, it was found that in the case of a low frequency such as 43 kHz, it is not possible to irradiate the pulsed ultrasonic waves having the short ON time T1 and OFF time T2 such as T1, T2=2 ms.

FIG. 4C shows voltage and current waveforms when the ultrasonic transducer 3 is driven at a frequency of 495 kHz and an ON time T1 of 10 ms. In this case, it was found that when a voltage was applied to the ultrasonic vibrator 3, the ultrasonic vibrator 3 immediately reached a constant vibration speed. Next, FIG. 4D shows voltage and current waveforms when the ultrasonic transducer 3 is driven at a frequency of 495 kHz and an ON time T1 of 2 ms. Even in this case, it was found that the ultrasonic transducer 3 reached a constant vibration speed immediately when a voltage was applied. Therefore, it has been found that a pulsed ultrasonic wave having a short ON time T1 and OFF time T2 such as T1 and T2=2 ms can be emitted at a high frequency of 100 kHz or higher such as 495 kHz.

[Example 3]: Experiment relating to effect of ultrasonic wave irradiation mode on gold nanoparticle synthesis In this example, an experiment was conducted by the following method.

1. Experimental Method As a sample used for the experiment, the same gold ion aqueous solution 4 as in Example 1 was prepared. Here, UFB water was used as the solvent of the solution 4. UFB water was prepared from ultrapure water by a pressure dissolution method. When UFB contained in UFB water was measured by the nanoparticle Brownian motion tracking method, the number density was 3 to 4×10 ⁹ cells/mL, and the average bubble diameter was about 140 nm.

Next, the prepared gold ion aqueous solution 4 was put into the processing tank 7 (inner tank 7a) of the gold nanoparticle manufacturing apparatus 10 shown in FIG. Then, while the solution 4 is maintained within 10° C.±2° C. by the constant temperature bath 6, the frequency is set to 495 kHz, and the time average power applied to the vibrator is set to 50 W, the ultrasonic wave S1 is transmitted from the ultrasonic vibrator 3 to the ultrasonic wave S1. It was irradiated for 4 minutes to synthesize gold nanoparticles 15 (gold nanocolloid). In this embodiment, the ON time T1 (see FIG. 2) is 2 ms, the OFF time T2 (see FIG. 2) is 2 ms, the applied power at the ON time T1 is set to 100 W, and the applied power at the OFF time T2 is set. Was set to 0 W, and a pulse wave (Pulse wave) was applied as the ultrasonic wave S1. At this time, the duty ratio is 50%, and the time average power applied to the ultrasonic transducer 3 is 50W. The duty ratio can be expressed by the following formula.
Formula Duty ratio [%]=100×ON time T1/(ON time T1+OFF time T2)
Further, in the present embodiment, a continuous wave with the OFF time T2 set to 0 ms is also emitted as the ultrasonic wave S1. Then, after the irradiation treatment with the ultrasonic wave S1, the synthesized gold nanocolloid was collected, and the gold nanoparticles 15 were observed and evaluated by an electron microscope. The results are shown in the photograph of FIG. 5 and the graph of FIG.

Further, with the duty ratio kept constant at 50% and the time-average power kept constant at 50 W, pulse waves (ultrasonic waves S1) were irradiated for 4 minutes in different irradiation modes, and gold nanoparticles 15 (gold nanocolloid) were irradiated. Synthesized. Specifically, with the ON time T1 and the OFF time T2 set to 0.2 ms, 0.5 ms, 1 ms, 2 ms, 5 ms, 10 ms, 15 ms, 20 ms, 200 ms, 400 ms, 600 ms, and 800 ms, respectively, the pulse wave is set. Irradiated. Then, after the irradiation treatment with the ultrasonic wave S1, the synthesized gold nanocolloid was collected, and the gold nanoparticles 15 were observed and evaluated by an electron microscope. The results are shown in the photograph of FIG. 5, the graph of FIG. 7 and Table 1.

2. Experimental Results and Discussion The photograph in FIG. 5 shows electron micrographs when a pulse wave and a continuous wave were irradiated respectively. Specifically, FIG. 5(a) shows an electron micrograph when irradiating a pulse wave with the ON time T1 and the OFF time T2 set to 2 ms, and the ON time T1 and the OFF time T2 are respectively shown. FIG. 5B shows an electron microscope photograph when a pulse wave is irradiated in the state of setting to 600 ms. Further, FIG. 5C shows an electron micrograph when a continuous wave (OFF time T2=0 ms) was irradiated. According to this, it was found that when the pulse wave is irradiated, the particle size of the gold nanoparticles 15 becomes smaller as the ON time T1 becomes shorter.

The graph of FIG. 6 shows a particle size distribution when a continuous wave and a pulse wave (ON time T1=2 ms, OFF time T2=2 ms) are respectively irradiated. According to this, when the continuous wave was irradiated, the average particle size of the gold nanoparticles 15 was 25.3 nm, and the standard deviation was 5.8 nm. On the other hand, when irradiated with a pulse wave, the gold nanoparticles 15 had an average particle size of 13.1 nm and a standard deviation of 5.9 nm. That is, it was found that the average particle size of the gold nanoparticles 15 was smaller when the pulse wave was irradiated than when the continuous wave was irradiated. The following are possible reasons. That is, when the time-average power is constant (50 W) in the pulse wave, the intensity of the ultrasonic wave S1 generated during the ON time T1 increases, so that the reduction rate increases and nucleation is promoted. .. Further, since the ultrasonic wave S1 is not generated during the OFF time T2, it is considered that the growth of the gold nanoparticles 15 is suppressed and the particle size of the metal nanoparticles 15 becomes small. Due to the interaction between these two times (ON time T1 and OFF time T2), the nucleation proceeds more favorably than the particle growth, so that the growth of the gold nanoparticles 15 is suppressed and the finer gold nanoparticles 15 are formed. Is considered to be generated.

The graph of FIG. 7 shows the relationship between the ON time T1 and the OFF time T2 of the ultrasonic wave S1 and the average particle size of the gold nanoparticles 15, and Table 1 shows the average when the ON time T1 and the OFF time T2 are changed. The particle size and standard deviation are shown. According to this, even when the pulse wave is irradiated, when the ON time T1 and the OFF time T2 are any of 20 ms, 200 ms, 400 ms, 600 ms, and 800 ms, the average particle size of the gold nanoparticles 15 is continuous. It was found to be larger than the average particle size (25.3 nm) when irradiating with waves. On the other hand, if the ON time T1 and the OFF time T2 are set to 0.5 ms, 1 ms, 2 ms, 5 ms, 19.9 ms, or 22.1 ms when the pulse wave is irradiated, the average particle diameter of the gold nanoparticles 15 is increased. However, it was found that the particle size was smaller than the average particle size when the continuous wave was irradiated. In particular, it was found that when the ON time T1 and the OFF time T2 were set to 2 ms, the average particle size became the smallest. In addition, when irradiating a pulse wave, when the ON time T1 and the OFF time T2 are set to 0.2 ms, ultrasonic cavitation does not occur, so that the gold nanoparticles 15 themselves cannot be synthesized (that is, the average particle size is 0 nm. I realized). It is considered preferable that the duty ratio is such that the ON time T1 and the OFF time T2 are substantially the same (about 50%). For example, the duty ratio is preferably 40% or more and 60% or less, and particularly preferably 45% or more and 55% or less.

[Conclusion]
According to the above series of experimental results, the ultrasonic wave S1 irradiated to the solution 4 is a pulse wave, for example, the repetition period T0 is set to 2 ms or more and 30 ms or less, and the ON time T1 and the OFF time T2 are set to 1 ms or more and 15 ms or more. It has been found that the average particle size of the gold nanoparticles 15 is smaller than the average particle size in the case of irradiating with continuous wave by the following. Therefore, it was found that the fine gold nanoparticles 15 can be controlled by the pulse wave.

Therefore, according to this embodiment, the following effects can be obtained.

(1) In the method for controlling the particle size of the metal nanoparticles 15 in the present embodiment, the ultrasonic wave S1 having a high frequency (100 kHz or more) is repeated at a millisecond-order (4 ms) repetition period T0 that is not generally used. By intermittently irradiating and making the repeating cycle T0 extremely short (to 2 ms), the size of the gold nanoparticles 15 can be controlled to be small. Therefore, the particle size of the gold nanoparticles 15 can be controlled without using additives such as a reducing agent, a reduction auxiliary agent, a surfactant, and an amphipathic polymer. That is, the size of the gold nanoparticles 15 can be reliably reduced by a clean method.

(2) In the present embodiment, the gold nanoparticles 15 are synthesized only by the irradiation of the ultrasonic wave S1 without adding the above-mentioned additive to the solution 4, so that the surface of the solution 4 is of high purity with nothing attached. Gold nanoparticles 15 can be synthesized.

In addition, you may change the said embodiment as follows.

In the above embodiment, the present invention is embodied in the method for manufacturing the gold nanoparticles 15, but may be embodied in a method for manufacturing nanoparticles of a noble metal other than gold (silver, platinum, palladium, etc.).

In the above-described embodiment, the gold ion aqueous solution 4 containing UFB was prepared by the pressure dissolution method, but a method other than the pressure dissolution method (for example, a swirling liquid flow method, a static mixer method, a micropore method, and a relatively low method). Such a solution may be prepared by a method utilizing high frequency ultrasonic radiation. In the above embodiment, the gas in the UFB is air, but a gas other than air (for example, oxygen, nitrogen, argon, etc.) may be used.

In the gold nanoparticle manufacturing apparatus 10 of the above-described embodiment, the operator inputs the optimum ON time T1 and OFF time T2 of the ultrasonic wave S1 to the ultrasonic oscillator 1 so that the ultrasonic wave from the ultrasonic wave oscillator 3 to the ultrasonic wave S1. Was being irradiated. However, the gold nanoparticle manufacturing apparatus 10 automatically calculates the optimum ON time T1 and OFF time T2 of the ultrasonic wave S1 when the operator inputs the target value of the particle diameter of the gold nanoparticle 15. A system for irradiating the ultrasonic wave S1 from the ultrasonic transducer 3 based on the calculated result may be provided. When the ON time T1=OFF time T2 and the particle size of the gold nanoparticles 15 is 10 ms or more, the calculation formula for calculating the ON time T1 (=OFF time T2) is as follows (see FIG. 8). ..
T1=1e (0.102×d-1.12)
Here, T1 (=T2) is ON time [ms] (=OFF time), and d is the particle size [nm] of the gold nanoparticles 15.

Next, in addition to the technical ideas described in the claims, the technical ideas grasped by the above-described embodiment will be listed below.

(1) The particle size of the metal nanoparticles according to claim 1, wherein the size of the metal nanoparticles is controlled by irradiating the solution with ultrasonic waves of 100 kHz or more and 3 MHz or less. Control method.

(2) The method for controlling the particle size of metal nanoparticles according to claim 1, wherein the metal nanoparticles are noble metal nanoparticles.

(3) The method for controlling the particle size of metal nanoparticles according to claim 1, wherein the metal nanoparticles are gold nanoparticles.

4... Gold ion aqueous solution as a solution (solution) for synthesizing metal nanoparticles 7... Processing tank 15 as a container... Gold nanoparticles S1 as metal nanoparticles... Ultrasound T0... Repeating cycle

Claims (4)

By irradiating ultrasonic waves into a solution for synthesizing metal nanoparticles containing ultrafine bubbles having a diameter of 1 μm or less, cavitation is generated to reduce metal ions in the solution to synthesize metal nanoparticles in the solution. When doing
It is possible to control the size of the metal nanoparticles to be small by intermittently irradiating the solution with ultrasonic waves of 100 kHz or more at a repeating cycle of millisecond order and shortening the repeating cycle. A characteristic method for controlling the particle size of metal nanoparticles. A method for producing metal nanoparticles, comprising the step of controlling the particle size according to the method of claim 1. After performing a step of introducing a solution for synthesizing metal nanoparticles containing ultrafine bubbles having a diameter of 1 μm or less, which is produced by a pressure dissolution method, into a container,
By intermittently irradiating the solution in the container with ultrasonic waves of 100 kHz or more at a repetition cycle of 2 ms or more and 30 ms or less, cavitation is generated to reduce the metal ions in the solution, and thus the average particle size is reduced. A method for producing metal nanoparticles, comprising a step of synthesizing metal nanoparticles having a particle size standard deviation of 25 nm or less and 6 nm or less in the solution. The metal nanoparticles having an average particle size of less than 20 nm are synthesized by intermittently irradiating the solution in the container with ultrasonic waves at a repeating cycle of 4 ms or more and 20 ms or less. Method for producing metal nanoparticles of the above.