WO2023058539A1

WO2023058539A1 - Volatile fatty acid detection sensor, volatile fatty acid detection device, and method for manufacturing volatile fatty acid detection sensor

Info

Publication number: WO2023058539A1
Application number: PCT/JP2022/036344
Authority: WO
Inventors: 泉一ノ瀬; 仁川喜多
Original assignee: 国立研究開発法人物質・材料研究機構
Priority date: 2021-10-05
Filing date: 2022-09-29
Publication date: 2023-04-13
Also published as: JPWO2023058539A1

Abstract

The present invention provides a small sensor for detecting volatile fatty acids with high sensitivity. A volatile fatty acid detection sensor according to one embodiment of the present invention includes a quartz vibrator, a first electrode formed on a first main surface of the quartz resonator, and a second electrode formed on a second main surface of the quartz vibrator. A substance, which adsorbs a different amount of water from a gas phase according to the amount of volatile organic acid in the gas phase, is implanted in at least one surface of the first electrode and the second electrode, and volatile fatty acids are detected by changes in the frequency of the quartz vibrator.

Description

Volatile fatty acid detection sensor, volatile fatty acid detection device, and method for manufacturing volatile fatty acid detection sensor

The present invention relates to a volatile fatty acid detection sensor, a volatile fatty acid detection device, and a method for manufacturing a volatile fatty acid detection sensor.

Volatile fatty acids (VFA) such as propionic acid, butyric acid and acetic acid are substances that greatly affect the productivity and quality improvement of dairy farming such as cattle. In addition, when cows digest their food, they emit a large amount of methane gas, which contributes to global warming, but this amount can be greatly reduced by controlling VFA in the cow rumen (rumen). I have a report.
Against this background, there is a demand for a method and apparatus for detecting and quantitatively measuring VFA, particularly VFA volatilized from the liquid portion of bovine rumen.

Conventionally, an attempt has been made to detect and measure VFA by adsorbing VFA on an adsorbent and monitoring the change in the weight of the adsorbent and the change in infrared absorption spectrum due to the adsorption of VFA.
However, this method has the problem that the sensitivity is low and the apparatus is relatively large. As for the sensitivity, a liquid concentration of 25 mM (millimol) in the liquid part of the bovine rumen is required.

Japanese Unexamined Patent Application Publication No. 2012-13620

SUMMARY OF THE INVENTION An object of the present invention is to provide a small sensor, an apparatus, and a manufacturing method thereof that detect VFA with high sensitivity.
Another object of the present invention is to provide a compact device capable of detecting VFA in the liquid part of bovine rumen with high sensitivity.

The configuration of the present invention is shown below.
(Configuration 1)
a crystal oscillator, a first electrode formed on a first main surface of the crystal oscillator, and a second electrode formed on a second main surface of the crystal oscillator,
At least one surface of the first electrode and the second electrode has a film imprinted with a substance that changes the amount of water adsorbed from the gas phase depending on the amount of volatile organic acid in the gas phase. and a volatile fatty acid detection sensor, wherein the volatile fatty acid is detected by a change in the frequency of the crystal oscillator.
(Configuration 2)
A crystal oscillator micro, comprising a crystal oscillator, a first electrode formed on a first main surface of the crystal oscillator, and a second electrode formed on a second main surface of the crystal oscillator. consists of balance (QCM),
A volatile fatty acid detection sensor, wherein a titania porous film containing an alkoxide group is formed on the surface of at least one or more of the first electrode and the second electrode.
(Composition 3)
The volatile fatty acid detection sensor according to Structure 2, wherein the titania porous body is coated with an acid.
(Composition 4)
Having at least two QCMs,
The volatile fatty acid of configuration 3, wherein the acid deposited on one or more porous titania bodies of said QCM is different from the acid deposited on at least one porous titania body of the remainder of said QCM. detection sensor.
(Composition 5)
The volatile fatty acid detection sensor according to configuration 3 or 4, wherein the acid is selected from the group consisting of propionic acid, butyric acid and acetic acid.
(Composition 6)
Having a capsule that is permeable to and permeable to gases of volatile fatty acids and that is water-repellent is disposed on at least a portion of the surface that is in contact with the outside world;
A volatile fatty acid detection device, wherein the volatile fatty acid detection sensor according to any one of configurations 1 to 5 is arranged inside the capsule.
(Composition 7)
The volatile fatty acid detection device according to configuration 6, wherein a frequency measuring device for measuring the frequency of the crystal oscillator is arranged inside the capsule.
(Composition 8)
The volatile fatty acid detection device according to configuration 7, wherein a transmitter for wirelessly transmitting frequency information obtained by the frequency measuring device to the outside of the capsule is arranged inside the capsule.
(Composition 9)
A method for producing a volatile fatty acid detection sensor according to configuration 2,
Forming the porous titania film by imprinting the surface of at least one or more of the first electrode and the second electrode using at least Ti(O-nBu) ₄ and acid, volatile A method for manufacturing a fatty acid detection sensor.
(Configuration 10)
A crystal oscillator micro, comprising a crystal oscillator, a first electrode formed on a first main surface of the crystal oscillator, and a second electrode formed on a second main surface of the crystal oscillator. preparing a balance (QCM);
modifying the surface of at least one or more of the first electrode and the second electrode with mercaptoethanol;
immersing at least the modified surface in a Ti(O-nBu) ₄ solution consisting of Ti(O-nBu) ₄ and acid, water, toluene and ethanol;
After the immersion in the Ti(O-nBu) ₄ solution, the immersed surface is sequentially immersed in toluene, water-saturated toluene, and toluene;
drying the sequentially dipped surface;
A method for producing a volatile fatty acid detection sensor, comprising: treating the dried surface with a solution containing ammonia or an amine compound.
(Composition 11)
11. The method of manufacturing a volatile fatty acid detection sensor according to configuration 10, wherein said acid is selected from the group consisting of propionic acid, butyric acid and acetic acid.

ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the small sensor, apparatus, and its manufacturing method which detect VFA with high sensitivity.
Further, according to the present invention, it is possible to provide a small-sized device capable of detecting VFA in the liquid part of bovine rumen with high sensitivity.

FIG. 2 is a schematic configuration diagram for explaining the configuration of the sensor of the present invention; (a) Configuration example in which an imprint film is arranged on the surface of one electrode, (b) Configuration example in which an imprint film is arranged on the surfaces of both electrodes. FIG. 4 is a flow chart diagram showing the manufacturing process of the sensor of the present invention. 1 is a schematic configuration diagram for explaining the configuration of an apparatus according to the present invention; FIG. FIG. 2 is a characteristic diagram showing the relationship between the number of imprinting processes and the resonance frequency of a crystal oscillator in fabricating the sensor of Example 1; 4 is an SEM observation image of the imprinted film (water-adsorbing material) of the sensor of Example 1. FIG. (a) Cross-sectional image of the imprinted film on the silver electrode (magnification: 100,000 times), (b) Cross-sectional image of the imprinted film on the silver electrode (magnification: 60,000 times). FIG. 6 is a measurement diagram obtained by obtaining the film thickness of an imprint film based on the SEM observation image of FIG. 5 ; (a) when observed at a magnification of 100,000; (b) when observed at a magnification of 60,000. FIG. 3 is an explanatory diagram illustrating the procedure of an experiment for evaluating the VFA detection sensitivity of the sensor of Example 1; FIG. 4 is a characteristic diagram showing changes in vibration frequency (resonance frequency) with respect to elapsed time when the sensor of Example 1 is moved from the atmosphere to a box containing pure water. FIG. 4 is a characteristic diagram showing the amount of change in frequency when detecting propionic acid, butyric acid, and acetic acid using a sensor having, as an imprinted film, a titania porous film coated with different acids. (a) when the adherent is propionic acid, (b) when the adherent is butyric acid, and (c) when the adherent is acetic acid.

(Embodiment 1)
Embodiment 1 describes a VFA detection sensor of the present invention.

<Structure>
A VFA detection sensor 101 of the present invention, as shown in FIG. 1(a), has a crystal oscillator 11, a first electrode 12, a second electrode 13, and an imprint film 14 made of a water-adsorbing substance. The imprint film (water-adsorbing material) 14 is arranged on the surface of the electrode (that is, in contact with the electrode), but may be arranged on either one of the first electrode 12 and the second electrode 13, It may be arranged in both. A sensor 101 shown in FIG. 1A is a configuration example for the former case, and a sensor 102 shown in FIG. 1B is a configuration example for the latter case. The sensor 102 in which the imprinted film 14 is arranged on both electrodes can secure a large area where a substance can be adsorbed on the imprinted film 14, and thus has a feature of easily increasing detection sensitivity. The sensor 101 in which the membrane 14 is arranged on one electrode is characterized by high efficiency and low cost when only the electrode on which the imprinted membrane 14 is arranged contacts the gas phase during use. Moreover, in the

sensors

101 and 102, the imprint film 14 may be formed on the entire surface of the electrode (that is, on the entire portion that can contact the outside air), or may be formed on a part of the surface of the electrode (that is, on the portion that can contact the outside air). part).
The water adsorption substance 14 constituting the imprint film is a substance whose amount of water adsorbed from the gas phase changes depending on the amount of VFA in the gas phase, and the use of this substance is a feature of the present invention. .
On the other hand, a sensor with a normal QCM structure uses not the water-adsorbing substance 14 but a substance that adsorbs the target substance or its thin film (sensitive film). Here, QCM is a device that detects the adhesion amount of a specific substance using the principle that when some substance adheres to the vibrating body surface of a crystal unit, the resonance frequency changes according to the weight of the adhered substance. , for example, is disclosed in Patent Document 1.

When the inventor tries to detect VFA with high sensitivity by QCM using a conventional VFA adsorbent, it becomes necessary to monitor a very small amount of weight change. We have found that it becomes difficult to avoid complications.

Therefore, instead of using a substance that directly adsorbs VFA, the inventor attempted to use a substance that changes the amount of water adsorbed depending on the presence of VFA in the gas phase as an adsorption aid. Since there is a large amount of water in the atmosphere, by utilizing the change in the properties of this adsorption aid, it is possible, in principle, to prevent or suppress the attraction of a large amount of water with a small amount of VFA. , the amount of change in weight can be increased, and high detection sensitivity can be obtained.

Specifically, as a mechanism for drawing water in the gas phase into the water-adsorbing material 14, it is possible to use the polarity of the surface in contact with the outside air, that is, the degree of hydrophilicity of the surface in contact with the outside air. can. In addition, if a substance whose surface becomes hydrophobic or changes from hydrophobic to hydrophilic due to the presence of VFA in the gas phase is used as the water-adsorbing substance 14, water adsorption in an environment where there is no VFA in the gas phase can be achieved. The adsorption amount of water by the substance 14 and the adsorption amount of water by the water adsorption substance 14 when the polarity of the surface is changed by adsorbing a small amount of VFA in the gas phase change greatly.
Here, it is preferable that the water-adsorbing material 14 is a porous body in order to increase the amount of adsorbed water and improve the detection sensitivity. If the water-adsorbing substance 14 is a porous body, the surface area per volume becomes large, and a large amount of water can be adsorbed.

The inventors found that a porous titania material containing alkoxide groups adsorbs water well, and that there is a good correlation between the amount of VFA in the gas phase and the change in the amount of water adsorbed. Furthermore, when the titania porous body is coated with acid, the correlation between the amount of VFA in the gas phase and the change in the amount of water adsorption is enhanced (water adsorption to the titania porous body is further inhibited. ) and that the sensitivity to the type of VFA varies depending on the type of acid adhered to the titania porous material (acid that is the adherent). Here, propionic acid, butyric acid, acetic acid and the like can be mentioned as the acid of the adherend.
A specific example of this titania porous body is Ti(O-nBu) ₄ (titanium (IV) n-butoxide). In addition, as a method of arranging this titania porous body in the form of a film on the surface of an electrode, Ti(O-nBu) ₄ , an acid such as propionic acid, a small amount of water and an organic solvent are coated on a metal electrode such as gold or silver. A method of imprinting (coating) using a solution containing (such as a mixed solvent of toluene and ethanol) can be mentioned. Details will be described in the manufacturing method section. The porous titania film (organic titania film) thus formed has molecular-level porosity, which is different from a film (inorganic titania film) formed of titania (TiO ₂ ) containing no alkoxide group. and suitable for use as the imprint film 14 . This molecular level porosity is described in the Examples section.

Here, the imprint film 14 has different sensitivities to the types of VFAs to be detected, depending on the type of acid coated on Ti(O-nBu) ₄ . Therefore, VFA can be separated by appropriately selecting the acid to be added when the imprint film 14 is produced. This is explained with reference to examples in the examples.
In addition, using this property, a sensor having at least two or more QCMs is constructed, and the acid coated on one or more titania porous bodies of the QCMs is applied to at least one titania porous body of the rest of the QCMs. VFA type identification and VFA quantitative assessment can be performed using a single sensor.

Another example of the water-adsorbing substance 14 is a swelling rubber whose portion exposed to the outside air is hydrophobic. Here, the portion exposed to the outside air means the gas adsorption surface, and includes not only the surface forming the so-called outer shape but also the surface of the pores of the porous body through which the outside air enters. In this context, the term "swellable rubber" means rubber that has the property of swelling, and includes both those that are in a swollen state and those that are not in a swollen state when the

sensors

101 and 102 are used. shall be included.
Examples of the swellable rubber include acrylonitrile-butadiene rubber (NBR), which is in a swollen state when the

sensors

101 and 102 are used, and in which dodecane is adhered to portions exposed to the outside air. In this NBR, the surfaces of the gaps (voids) formed by the swelling are usually hydrophobic, but when they come into contact with VFA in the gas phase, they become hydrophilic, drawing in environmental water and increasing their weight. NBR having such properties is suitable for use as the water adsorption material 14 .

The thickness of the imprint film 14 is not particularly limited, but a thin film is preferable in terms of detection speed, and for example, 10 nm or more and 1 μm or less can be mentioned. The size of the imprint film 14 is also not particularly limited, but the larger the size, the larger the area where the substance can be adsorbed, so it is preferable from the viewpoint of detection sensitivity. Considering the balance with miniaturization of the sensor, the size (width) of the first electrode 12 and the second electrode 13 is preferable.

When the water-adsorbing substance 14 is porous, the density (ratio of the space to the external volume) is preferably as high as possible on the premise that the strength is maintained so as not to cause defects such as chipping. 5 g/cm ³ or more and 2.0 g/cm ³ or less can be mentioned.

The crystal oscillator 11 is not particularly limited, but an AT-cut crystal oscillator can be preferably used. Here, AT-cut means that the cut-out angle is in a specific direction with respect to the quartz crystal axis. It is characterized by excellent stability.
The first electrode 12 and the second electrode 13 are not particularly limited as long as they can apply an alternating voltage to the crystal oscillator 11. For example, gold (Au), silver (Ag), and platinum (Pt). , copper (Cu), tungsten (W), aluminum (Al), titanium (Ti), chromium (Cr), nickel (Ni), their alloys, their metallic compounds such as Al—Si, doped silicon (Si ) and doped polysilicon (PolySi). Here, the electrode does not have to be made of a single material, and the electrode has a base of Ni or Cr to increase adhesion to the crystal, and an outermost surface of Au or Ag to prevent oxidation. desirable.
The first electrode 12 and the second electrode 13 preferably have a high resonance Q value of the crystal oscillator 11, can follow the movement of the crystal oscillator 11, and have nanogram (ng) sensitivity. The thicknesses of the first electrode 12 and the second electrode 13 are not particularly limited, but are preferably thin films in order to improve the Q value, for example, 10 nm to 150 nm.

By applying an alternating voltage to the first electrode 12 and the second electrode 13 of the

sensors

101 and 102 and measuring the change in the frequency of the crystal oscillator 11, specifically the change in the resonance frequency, It is also possible to detect the VFA with high sensitivity and determine its amount.
As explained above,

sensors

101 and 102 provide sensors with high VFA detection sensitivity while being compact.

<Manufacturing method>
A method of manufacturing the sensor 101 will now be described with reference to FIG.
First, a crystal oscillator 11, a first electrode 12 formed on a first main surface of the crystal oscillator 11, and a second electrode 13 formed on a second main surface of the crystal oscillator 11. A crystal oscillator microbalance (QCM) having is prepared (step S11). Here, the crystal resonator 11, the first electrode 12, and the second electrode 13 may be those used in a normal QCM.

Next, at least one or more surfaces of the first electrode 12 and the second electrode 13 are modified with mercaptoethanol (step S12). Specifically, for example, the surface of the electrode to be modified may be immersed in a mercaptoethanol solution in which mercaptoethanol is dissolved at a predetermined concentration using ethanol as a solvent. The temperature for modification may be room temperature (20° C. or more and 25° C. or less), and the modification time is preferably 1 hour or more and 12 hours or less. Sufficient modification of the target surface can be achieved within this time range. If it is shorter than this time, the modification will be insufficient, and if it is longer, time will be wasted, leading to a decrease in production efficiency. This modification is necessary when the outermost surface of the first electrode 12 and the second electrode 13 is Au, and can be omitted when the outermost surface is Ag.
Thereafter, at least the modified surface is immersed in a Ti(O-nBu) ₄ solution composed of Ti(O-nBu) ₄ and an acid, a small amount of water, and an organic solvent such as toluene or ethanol (step S13). The temperature for immersion may be room temperature. The immersion time is preferably 1 minute or more and 5 minutes or less. Sufficient Ti(O-nBu) ₄ deposition can be performed on the target surface within this time range. If the time is shorter than this, deposition of Ti(O-nBu) ₄ will be insufficient, and if it is longer, time will be wasted, leading to a decrease in production efficiency.
After immersion in the Ti(O-nBu) ₄ solution, the immersed surface was sequentially immersed in toluene (step S14), immersed in water-saturated toluene (step S15), and immersed in toluene (step S16). conduct. Here, the temperature for immersion may be room temperature (20° C. or more and 25° C. or less), and each immersion time is preferably 30 seconds or more and 2 minutes or less. Imprinting with few defects and unevenness can be performed within this time range.
After that, drying is performed (step S17), and one course of imprint processing is completed. Here, examples of the drying method include nitrogen gas drying, dry air drying, vacuum drying, and heat drying at a temperature of 100° C. or less in a dry air environment.

Next, the number of cools n is compared with a predetermined number of times X (step S18), and if n is less than X (n<X), it is determined that the n+1th process (step S19) is necessary. Then, the processes from step S13 to step S17 are performed again. When n is X or more (n≧X), treatment with a solution containing an amine compound such as TMED (tetramethylethylenediamine) diluent (immersion in the solution) (step S20) to remove carboxylic acid, A series of imprint processes are completed (step S21), and the sensor 101 is manufactured. A solution containing ammonia (dilute ammonia solution) may be used to remove the carboxylic acid instead of the amine compound such as TMED.
The sensor 101 obtained in this manner is compact and has high VFA detection sensitivity.
Note that when the first electrode 12 and the second electrode 13 are electrodes having the same outermost surface and similar imprint films are formed on the surfaces of the

electrodes

12 and 13, the sensor 102 is manufactured.

(Embodiment 2)
In Embodiment 2, a VFA detection apparatus suitable for detecting and quantifying VFA in a solution environment such as a liquid portion of bovine rumen in which VFA is dissolved will be described.

As shown in FIG. 3, the VFA detection device 103 of the present invention includes a sensor 102 (see FIG. 1(b)) in which imprint films 14 are formed on the surfaces of both electrodes described in Embodiment 1 inside a capsule 56. )), receives the output from sensor 102 via signal line 57, measures the frequency of the crystal oscillator, processes the data, and transmits the data via signal line 58 to transmitter 54, which transmits the data wirelessly. A sending frequency measuring device 53 is arranged. In addition, on the surface of the capsule 56, more specifically, on at least part of the surface of the capsule 56 in contact with the outside world (environment outside the capsule), a filter 55 that is permeable to the VFA gas but blocks the liquid is arranged. there is
The material of the capsule 56 is not particularly limited, but it is preferable to use a material that does not corrode and break in the digestive tract such as inside the rumen of a cow, and examples thereof include polyethylene terephthalate, vinyl chloride, and high-strength glass. In addition, the interior of capsule 56 is desirably coated with a hydrophobic coating to prevent water vapor from condensing. By using a capsule as the housing of the VFA detection device, the cow can be swallowed as a capsule tablet.
Here, as the material of the filter 55, a porous material having super water repellency and permeable gas or a super water repellent non-woven fabric made of fibers can be used. Here, in order to maintain the strength of the nonwoven fabric and improve its durability, it is preferable to cover the nonwoven fabric with a net-like metal. More specifically, the filter 55 includes, for example, a gas-permeable ultrafiltration membrane in which both sides of a porous PTFE (polytetrafluoroethylene) ultrafiltration membrane are sandwiched between superhydrophobic PET (polyethylene terephthalate) nonwoven fabrics. A water-repellent multilayer film or the like can be preferably used.

According to the configuration of the device 103, the sensor 102, the frequency measuring device 53, and the wireless transmitter 54 housed therein can be miniaturized. Gas can be drawn into the interior of capsule 56 through filter 55 for desired detection and measurement. In addition, since the result of measurement by the frequency measuring device 53, which is transmitted from the transmitting device 54, can be externally received wirelessly or the like, the device 103 can monitor the state of the liquid portion of the bovine rumen in real time. A useful device. In the apparatus 103 shown in FIG. 3, the sensor 102 having the imprinted film 14 provided on the surface of both electrodes is used as the VFA detection sensor. The sensor 101 (FIG. 1A) having the imprinted film 14 disposed on the sensor 101 may be arranged so that the side on which the imprinted film 14 is disposed faces the direction in which the filter 55 is disposed.

(Example 1)
In Example 1, the sensor 102 (see FIG. 1B) described in Embodiment 1 was manufactured and its VFA detection characteristics were examined.
However, it should be noted that the present invention is of course not limited to such a particular form, and the scope of the present invention is defined by the appended claims.

<Production of sensor>
First, a crystal resonator 11 having electrodes (first electrode 12 and second electrode 13) on both main surfaces was prepared (step S11 in FIG. 2). Here, as the crystal oscillator 11, a 9 MHz AT-cut QCM (manufactured by Piezo Parts Co., Ltd.) was used, and its size was 8.7 mm in diameter and 0.17 mm in thickness. Both the first electrode 12 and the second electrode 13 are formed by vapor deposition and have a thickness of 0.1 μm. Gold (Au) or silver (Ag) was used as the electrode material. The procedure for fabricating the sensor when the electrode material is gold will be described below. When the electrode material was silver, the sensor was fabricated under the same conditions as for gold electrodes, except that the modification of the electrode surface with mercaptoethanol (step S12) described in the next paragraph was omitted.

The first electrode 12 and the second electrode 13 of the crystal oscillator 11 were immersed in the mercaptoethanol solution for 12 hours at room temperature (step S12). Here, the mercaptoethanol solution is obtained by dissolving 10 mM mercaptoethanol using ethanol as a solvent.
After that, it was immersed and washed with ethanol three times, washed with water for 1 minute, and then dried with nitrogen gas.

On the other hand, a Ti(O-nBu) ₄ solution for forming an organic titania film as the imprint film 14 was prepared.
This solution was prepared by first preparing solution #1 shown below, adding a small amount of water thereto to prepare solution #2, and further diluting it 20 times with toluene.
Solution #1 was a solution of 3.4 g of Ti(O-nBu) ₄ having a molecular weight (Mw) of 340 dissolved in 100 mL of a toluene-ethanol solvent of 2 parts toluene and 1 part ethanol (thus Ti(O-nBu ) ₄ is 100 mM) and 0.25 g of an acid (propionic acid with Mw of 74, butyric acid with Mw of 88, or acetic acid with Mw of 60) dissolved in 100 mL of toluene-ethanol solvent in a ratio of 2 parts toluene to 1 part ethanol. The resulting solutions (thus 33.8 mM for propionic acid, 28.4 mM for butyric acid, and 41.7 mM for acetic acid) were made by mixing for 2 hours.
Solution #2 was prepared by adding several drops of water to Solution #1 while rotating the stirrer at high speed, adding a total of 0.495 g of water.
A 20-fold dilution of toluene was prepared by mixing 200 μL of

solution #

2 and 4 mL of toluene.

Next, the QCM whose electrode surface was modified with mercaptoethanol was immersed in the prepared Ti(O-nBu) ₄ solution (20-fold diluted solution containing Ti(O-nBu) ₄ ) for 3 minutes at room temperature (step S13 ). Furthermore, this QCM is immersed in toluene for several seconds after replacing the solution with toluene (step S14), replacing the solution with water-saturated toluene and immersed in water-saturated toluene for 3 minutes (step S15), and the solution is replaced with toluene again. and immersed in toluene for several seconds (step S16). After that, it was dried using nitrogen gas (step S17). In this example, the number of imprint processes from step S13 to step S17 was set to 16 times (X=16) in advance, and this imprint process was repeated 16 times until n=16.

After that, the QCM was immersed in an aqueous TMED solution prepared by dissolving 50 μL of TMED in 5 mL of water at room temperature for 30 minutes (step S20), immersed in an aqueous 0.0001 M HCl solution for several minutes, washed with water by immersion, washed with methanol, and A sensor 102 in which an organic titania film was imprinted on the surfaces of the first electrode 12 and the second electrode 13 was produced through a drying process using nitrogen gas (step S21).
FIG. 4 shows the relationship between the number of times of imprint processing and the change in resonance frequency of the crystal oscillator. Here, the results are shown when gold is used as the electrode material and propionic acid is used as the acid in preparing the above solution #1. The resonance frequency increases almost monotonously except for the abnormal point at which the number of treatments is 5, and it can be seen that the formation of the organic titania film monotonically progresses with the number of imprint treatments. The increase in resonance frequency after 16 treatments is 2277 Hz, which corresponds to the thickness of the organic titania film of 60 nm.

<Evaluation of sensor characteristics>
FIG. 5 shows an SEM (Scanning Electron Microscope) photograph of the vicinity of the electrodes of the fabricated sensor 102 . Here, an observed image is shown when silver is used as the electrode material and propionic acid is used as the acid in preparing the above solution #1. 5A and 5B are cross-sectional images of the imprint film 14 on the silver electrode observed at magnifications of 100,000 and 60,000, respectively. It can be seen that the imprint film 14 made of the water-adsorbing substance has a rough surface, and the formed organic titania film is porous.

6(a) and 6(b) show the film thickness of the imprint film 14 obtained based on the SEM observation images of FIGS. 5(a) and 5(b). The film thickness of the formed organic titania film was about 70 nm, although there was some variation depending on the measurement location.
On the other hand, the resonance frequency of the (initial) crystal oscillator (when the electrode material is silver) before forming the imprint film 14 is 9005394 Hz, and after forming the imprint film 14 it is 9002654 Hz. rice field. In addition, after leaving the fabricated sensor in the atmosphere for a certain period of time (in an environment of about 20° C. to 25° C.) to desorb propionic acid excessively adhered to the imprint film 14, the frequency was 9002763 Hz, The decrease in resonance frequency (frequency) from the initial value was 2631 Hz.
The film thickness d (angstroms), the frequency F and the density ρ (g/cm ³ ) have the following relationship.
2d=-ΔF/1.82ρ (1)
The density of the imprint film 14 is estimated to be 1.04 g/cm ³ from equation (1).
Using this sensor, as shown in FIG. 7, which will be described later, the decrease in vibration frequency due to water absorption was measured by moving alternately between the atmosphere and a box containing pure water. The average frequency is 386 Hz, and the porosity obtained from this by 386×1.04/2631 is 0.153. Therefore, it is estimated that 15.3% of the imprinted film (water-adsorbing substance) 14 produced in the sensor of Example 1 is void.

The atomic composition ratio of the imprint film 14 was examined using XPS (X-ray Photoelectron Spectroscopy). The XPS used here is SigmaProbe (manufactured by Thermo Fisher Scientific). Here, the case of a sensor fabricated using silver as the electrode material and acetic acid as the acid in the preparation of the above solution #1 will be described.
As a result, when the atomic composition of Ti is 1.00, the composition ratio of carbon bonded to one oxygen of the butoxy group (CH ₂ in CH ₂ —O) is 0.98, and the carbon bonded to oxygen is The composition ratio of CH ₂ and CH ₃ of butoxy groups excluding butoxy groups is 3.08, the composition ratio of methyl group carbon (CH ₃ ) in acetic acid is 0.46, and the composition of carboxyl group carbon (OC=O) in acetic acid A ratio of 0.46 was obtained.
As a result, one of the four propoxides of Ti(O-nBu) ₄ used to fabricate the imprinted film 14 remained in the imprinted film 14, and 0.5 molecule of acetic acid used to fabricate the imprinted film 14 remained in the imprinted film 14. The extent means remaining within the imprint film 14 .
Therefore, the obtained imprint film 14 is an organic titania film containing alkoxide groups, and is considered to have molecular-level porosity, which is difficult with inorganic TiO ₂ films.

As shown in FIG. 7, the VFA detection sensitivity of the sensor 102 is measured by moving the sensor 102 into a box containing pure water, a box containing an aqueous solution of VFA, and the air, and measuring changes in vibration frequency (resonance frequency). It was evaluated by Here, the capacity of the box is 200 mL, the amount of pure water stored therein is 100 mL, and the amount of VFA aqueous solution is about 100 mL.

FIG. 8 shows an example in which the sensor 102 waiting in the atmosphere was moved to a box (closed space) containing pure water and the change in vibration frequency was measured. A frequency change (decrease) of about 500 Hz was observed due to the adsorption of water by the imprint film. Also, it can be seen that this change has responsiveness that settles down in about 40 seconds.

25 mM of propionic acid, butyric acid, and acetic acid were used as the acids to be added during the production of the imprinted film. and acetic acid are shown in FIG.
Here, propionic acid is 27 mM, butyric acid is 22 mM, and acetic acid is 35 mM in each VFA aqueous solution, and the frequency change (R ₁ (Hz)) and the frequency change (R ₂ (Hz)) when moving from the air to a box containing each VFA aqueous solution (environment with VFA), and the difference between these frequency changes (R ₁ -R ₂ ) was calculated to determine the amount of change in frequency (Hz). When the value of this variation is +, it means that adsorption of VFA to the imprint film inhibits co-adsorption of water thereto.

From the results of FIG. 9( a ), in the case of the organic titania film coated with propionic acid, in the environment with any VFA, a difference in frequency change from the environment with pure water was observed. adsorption inhibited the co-adsorption of water, and the inhibition of the co-adsorption was particularly large for butyric acid. In addition, since the value of the amount of change in frequency differs depending on the type of VFA, it can be seen that 27 mM propionic acid, 22 mM butyric acid, and 35 mM acetic acid can be detected.
From the results of FIG. 9(b), in the case of the organic titania film coated with butyric acid, the co-adsorption of water to the imprint film is inhibited by the adsorption of VFA to the imprint film in the presence of VFA. Especially in the presence of acetic acid, the value of the frequency change is large, indicating that the co-adsorption of water is reduced in the presence of acetic acid, that is, the detection sensitivity for acetic acid is high.
From the results of FIG. 9(c), in the case of the organic titania film coated with acetic acid, the environment with acetic acid has a larger frequency change than the environment with pure water, and the imprint film specifically (As a result, the value of the amount of change in frequency is negative). In addition, in an environment with propionic acid, the co-adsorption amount of water is remarkably reduced, indicating that propionic acid can be detected with high sensitivity.
From the above, the sensor 102 of the present embodiment can detect VFA with high sensitivity, and the sensitivity to the type of VFA varies depending on the type of acid added when producing the imprint film, and VFA can be separated. shown.

Here, the concentration of the aqueous solution of VFA used in the test, such as an aqueous solution of propionic acid, was used for evaluation _. It can be converted using Raoult's law. The relationship between the VFA aqueous solution concentration C _L (unit: mM), the relative concentration C _V (unit: %), and the partial pressure P (unit: Pa) at 25° C. is represented by the following formula (2) using propionic acid as an example: , (3). Here, "relative" represents a ratio to water (water vapor).
P=5.6×10 ⁻² C _L (2)
C _V =1.8×10 ⁻⁵ C _L (3)
In other words, VFA contained in the aqueous solution at a concentration in the low range used in the test in this example exists in the gas phase in a constant closed space at a relative concentration and partial pressure that correlates with the concentration in the aqueous solution. Therefore, it can be said that the sensor manufactured in this example has achieved high-sensitivity detection of VFA.

According to the present invention, a small sensor, device, and manufacturing method for detecting VFA with high sensitivity are provided. The device can also be encapsulated in the bovine ruminal fluid, allowing on-site, real-time measurements.

As described in the background art section, VFA is a substance that greatly affects the productivity and quality improvement of dairy farming such as cattle, and is also greatly related to the emission of methane gas through the digestion of food by cattle. It is a substance. Therefore, if VFA is measured with high accuracy and the results are fed back, the productivity and quality of dairy farming can be greatly improved, and it is believed that this will greatly contribute to the prevention of global warming.
In addition, the sensor and device of the present invention that detect VFA with high sensitivity lead to quantitative evaluation of the activity of anaerobic bacteria. ), environmental management of paddy fields and lakes, and various other cases where VFA is generated by microbial decomposition.

11: crystal oscillator 12: first electrode 13: second electrode 14: imprint film (water adsorption substance, organic titania film)
53: Frequency measuring device 54: Wireless transmitter 55: Filter (super water-repellent nonwoven fabric)
56: Capsule 57: Signal line 58: Signal line 101: VFA detection sensor 102: VFA detection sensor 103: VFA detection device

Claims

a crystal oscillator, a first electrode formed on a first main surface of the crystal oscillator, and a second electrode formed on a second main surface of the crystal oscillator,
At least one surface of the first electrode and the second electrode has a film imprinted with a substance that changes the amount of water adsorbed from the gas phase depending on the amount of volatile organic acid in the gas phase. and a volatile fatty acid detection sensor, wherein the volatile fatty acid is detected by a change in the frequency of the crystal oscillator.
A crystal oscillator micro, comprising a crystal oscillator, a first electrode formed on a first main surface of the crystal oscillator, and a second electrode formed on a second main surface of the crystal oscillator. consists of balance (QCM),
A volatile fatty acid detection sensor, wherein a titania porous film containing an alkoxide group is formed on the surface of at least one or more of the first electrode and the second electrode.
The volatile fatty acid detection sensor according to claim 2, wherein the titania porous body is coated with acid.
Having at least two QCMs,
4. The volatile of claim 3, wherein the acid deposited on one or more porous titania bodies of said QCM is different from the acid deposited on at least one porous titania body of the remainder of said QCM. Fatty acid detection sensor.
The volatile fatty acid detection sensor according to claim 3 or 4, wherein said acid is selected from the group consisting of propionic acid, butyric acid and acetic acid.
a volatile fatty acid gas-permeable and water-repellent filter having a capsule disposed on at least a portion of the surface in contact with the environment;
A volatile fatty acid detection device, wherein the volatile fatty acid detection sensor according to any one of claims 1 to 5 is arranged inside the capsule.
The volatile fatty acid detection device according to claim 6, wherein a frequency measuring device for measuring the frequency of the crystal oscillator is arranged inside the capsule.
The volatile fatty acid detection device according to claim 7, wherein a transmitter for wirelessly transmitting information on the frequency obtained by the frequency measuring device to the outside of the capsule is arranged inside the capsule.
A method for manufacturing a volatile fatty acid detection sensor according to claim 2,
Forming the porous titania film by imprinting the surface of at least one or more of the first electrode and the second electrode using at least Ti(O-nBu) 4 and acid, volatile A method for manufacturing a fatty acid detection sensor.
A crystal oscillator micro, comprising a crystal oscillator, a first electrode formed on a first main surface of the crystal oscillator, and a second electrode formed on a second main surface of the crystal oscillator. preparing a balance (QCM);
modifying the surface of at least one or more of the first electrode and the second electrode with mercaptoethanol;
immersing at least the modified surface in a Ti(O-nBu) 4 solution consisting of Ti(O-nBu) 4 and acid, water, toluene and ethanol;
After the immersion in the Ti(O-nBu) 4 solution, the immersed surface is sequentially immersed in toluene, water-saturated toluene, and toluene;
drying the sequentially dipped surface;
A method for producing a volatile fatty acid detection sensor, comprising: treating the dried surface with a solution containing ammonia or an amine compound.
The method for producing a volatile fatty acid detection sensor according to claim 10, wherein said acid is selected from the group consisting of propionic acid, butyric acid and acetic acid.