JP6624515B2 - Method and apparatus for detecting volatile organic compounds - Google Patents

Description

  The present invention relates to a method and an apparatus for detecting a volatile organic compound.

Among volatile organic compounds that account for about 80% of harmful air pollutants, aromatic gases such as benzene may cause carcinogenicity and serious effects on the cerebral nervous system even at low ppb levels. . Some aromatic gases represented by nitrobenzene and the like have explosive properties. From the above background, the importance of developing sensors for real-time detection of volatile organic compounds in the air on-site has been increasing for the purpose of measures against air pollution, biological effects assessment, and security improvement in important facilities such as airports.
A typical example of a conventional gas sensor is a quartz oscillator gas sensor. This is a sensor that detects the presence of gas molecules by utilizing the property that the resonance frequency fluctuates (decreases) according to the mass of the substance when it adheres to the surface of the crystal resonator (Non-Patent Document 1). When this method is used, the following problems occur. Since the surface of the crystal unit does not exhibit excellent gas adsorption characteristics, when used as a gas sensor, it is necessary to manufacture a sensor by forming a detection film, such as a surface coating with an organic thin film material, on which the gas to be detected is easily adsorbed. is there. The formation of the sensing film is complicated, such as material selection, fabrication method, film thickness conditions, etc., which itself becomes the subject of research, and the characteristics of each fabricated sensor greatly depend on the performance of the sensing film. There are issues with stability and reproducibility.

On the other hand, there is graphene as a material exhibiting excellent adsorption characteristics for aromatic gas. Graphene is a carbon allotrope having a two-dimensional planar structure composed only of carbon sp2 bonds (Non-Patent Document 2). A typical example of a graphene manufacturing method is a chemical vapor deposition (CVD) method. It is possible to grow one layer of graphene over a large area on a copper foil surface (Non-Patent Document 3), and recently it is also possible to grow single-crystal graphene on a wafer scale (Non-Patent Document 4).
Graphene exhibits a strong adsorptive interaction with an aromatic molecule by a π-π interaction (Non-Patent Document 5), and sensor applications utilizing this interaction are also being studied. In addition, it has advantages in practical and application aspects, such as excellent thermal and chemical stability. Since it has a two-dimensional crystal structure, there is an advantage that the characteristics as a gas sensor material are less likely to fluctuate than an organic thin film material whose structure changes flexibly.

  Since graphene has high charge mobility based on a unique electronic structure derived from its two-dimensional structure, a transistor-type sensor using the charge mobility of graphene has been reported (Non-Patent Document 6). A field-effect transistor can be used as a technique for evaluating the charge mobility of a material, but can be operated as a gas sensor. By forming a channel made of, for example, graphene on the surface of the gate insulator between the source electrode and the drain electrode, and detecting a change in the value of current flowing between the source and the drain caused by adsorption of gas molecules to the channel surface, The presence of a molecule can be detected (Non-Patent Document 7).

Kazutoshi Noda, Hidenobu Aizawa, IEICE Transactions E, 135, 292, (2015). A. K. Geim, K.S.Noselov, Nature Materials, 6, 183 (2007). A. Reina et al., "Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition", Nano Letters, 9, 30 (2009). J. H. Lee et al., "Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium", SCIENCE, 344, 286 (2014). A. Rochefort and J. D. Wuest, “Interaction of Substituted Aromatic Compounds with Graphene” Langmuir, 25, 210 (2009). Y. Ohno et al., “Label-Free Biosensors Based on Aptamer-Modified Graphene Field-Effect Transistors” J. Am. Chem. Soc., 132, 18012 (2010). Yoshihiro Iwasa, Daishi Takenobu, Applied Physics 77, 432 (2008). Y. Honsho et al, “Evaluation of Intrinsic Charge Carrier Transport at Insulator-Semiconductor Interfaces Probed by a Non-Contact Microwaved Based Technique”, Scientific Reports, 3, 3182, (2013).

  However, the gas sensor using the field effect transistor has the following problems. Due to the structure of the device, it is not possible to avoid the occurrence of resistance at the contact portion between the channel and the source / drain. Therefore, the characteristics of the material used for the channel cannot be sufficiently exhibited, and there are problems in stability and reproducibility. In addition, when forming a detection film that easily adsorbs the gas to be detected, or when modifying the surface of a recognition molecule that adsorbs gas molecules, the characteristics of each manufactured sensor are detected as in the case of a quartz crystal gas sensor. Problems such as a large dependence on the performance of the membrane and the modifying molecule occur.

  The present invention has been made in order to solve the above problems, and a volatile organic compound can be analyzed more stably and reproducibly without using a detection film. The aim is to provide a method.

One embodiment of the present invention detects a change in charge mobility of the graphene due to adsorption of a volatile organic compound to graphene based on a time change in microwave dielectric loss,
A volatile organic compound detection method characterized by detecting a volatile organic compound adsorbed on the graphene based on the detected change in the charge mobility.

  One embodiment of the present invention is the above-mentioned method for detecting a volatile organic compound, wherein the amount of the volatile organic compound adsorbed on the graphene is quantified based on a change amount of the charge mobility of the graphene. .

  One embodiment of the present invention is the method for detecting a volatile organic compound, wherein the graphene is single-layer graphene.

  One embodiment of the present invention is the method for detecting a volatile organic compound, wherein the graphene is a multilayer graphene in which graphene is stacked in two or more layers.

  One embodiment of the present invention is a detection device used for the method for detecting a volatile organic compound, including a microwave source and a microwave cavity resonator, wherein the microwave cavity resonator is a sensor device having graphene. It is characterized by having an installation part which can be installed.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the volatile organic compound detection method which can analyze a volatile organic compound stably with good reproducibility.

FIG. 2 is a schematic diagram illustrating an example of a sensor device according to an embodiment of the present invention and a method for manufacturing the same. It is a schematic diagram which shows an example of the detection apparatus which concerns on one Embodiment of this invention. FIG. 3 is a schematic diagram illustrating an example of a microwave cavity resonator portion in the detection device according to the embodiment of the present invention. 4 is a graph showing the results of detecting nitrobenzene obtained in the examples.

Hereinafter, an embodiment of the method for detecting a volatile organic compound according to the present invention will be described with reference to the drawings.
In this embodiment, a volatile organic compound is detected using a sensor device having graphene. First, the sensor device according to the present embodiment will be described.

  FIG. 1 is a schematic view illustrating an example of a sensor device according to an embodiment of the present invention and a method for manufacturing the sensor device. The sensor device 1 has a structure in which a lower electrode 20, an insulating layer 30, a graphene 40, and an upper electrode 50 are stacked on a surface of a substrate 10 in this order. The graphene 40 is exposed on the surface of the sensor device 1 to adsorb the volatile organic compound.

  As a material for the substrate 10, an insulator material is suitable, and examples thereof include quartz, sapphire, other inorganic solid materials, polyimide, Teflon (registered trademark), and other organic polymer materials. Here, an example in which quartz is used as the material of the substrate 10 is shown.

  First, the lower electrode 20 is formed on the surface of the substrate 10. Examples of the lower electrode 20 include a metal electrode. Examples of the material of the metal electrode include metals such as gold, platinum, copper, nickel, and titanium, and may be formed by an existing evaporation method, for example, resistance heating evaporation, sputtering evaporation, electron beam heating evaporation, or the like. Here, an example in which titanium (Ti) 5 nm and gold (Au) 30 nm are stacked is shown.

Next, an insulating layer 30 covering the lower electrode is formed. The material of the insulating layer 30 may be quartz, another inorganic insulator material, or an organic insulator such as a polymer. Here, an example in which silicon dioxide (SiO ₂ ) is stacked to a thickness of 200 nm as the insulating layer 31 is shown. The insulating layer 30 may be formed using a material obtained by combining these materials in order to enhance the insulating property, or may be formed by using a multilayer in which these materials are stacked. Optimum polymer materials include amorphous fluororesin (CYTOP, manufactured by Asahi Glass Co., Ltd.), polyvinylidene chloride film (PVDC film, Saran resin, manufactured by Asahi Kasei) and the like. Here, an example is shown in which an insulating layer 32 having an amorphous fluororesin is stacked on the insulating layer 31 as the insulating layer 30. The insulating layer 32 is obtained by annealing Cytop at 180 ° C. for 1 hour.

Next, graphene 40 is laminated on the surface of the insulating layer 30. A polymer thin film 41 (for example, polymethyl methacrylate (PMMA)) is formed by spin coating on the surface of single-layer graphene grown on a copper foil by a chemical vapor deposition method (CVD method), thereby producing a graphene film-carrying film. Subsequently, the lower copper foil is etched with an acid (for example, a solution in which FeCl ₃ is dissolved at a concentration of 0.05 g / mL), and the graphene supported on the polymer is formed in a state of floating on water. This carrier film is scooped on the lower electrode 20 of the substrate 10 on which the insulating layer shown in FIG. 1 has been formed. After drying, the polymer thin film 41 is treated with an organic solvent (for example, acetone) overnight to dissolve and remove the polymer thin film 41.

Subsequently, an upper electrode 50 is formed on a part of the surface of the obtained graphene 40. The arrangement and shape of the upper electrode 50 may be made so as to expose the graphene area where the adsorption site for gas molecules necessary for gas detection is obtained. For example, a shape that covers a part of the entire surface of the graphene, a shape obtained by hollowing the inside such as a ring or a square, or a comb shape may be used. The configuration of the upper electrode 50 may be the same as or different from that of the lower electrode 20. For example, the material of the upper electrode 50 may be gold, platinum, copper, nickel, titanium or another metal as in the case of the lower electrode 20, and may be any of the existing evaporation methods, for example, resistance heating evaporation, sputter evaporation, electron beam heating evaporation, Others may be used. Here, an example in which 30 nm of gold is laminated is shown.
Thus, the sensor device 1 according to the embodiment can be manufactured.

  Next, detection of a volatile organic compound using the sensor device 1 by the detection device of the embodiment will be described. In addition, the detection method of the volatile organic compound of the present invention is not limited to the method implemented by the detection device of the following embodiment.

The sensor device 1 is placed on the detection device 100 shown in FIG. 2 and measures the charge mobility.
FIG. 2 is a schematic diagram illustrating an example of the detection device according to the embodiment of the present invention.
The detection device 100 includes a microwave cavity resonator 110, a microwave source 120, a gate voltage control unit 130, an amplifier 140, and an oscilloscope 150. The microwave source 120, the microwave cavity resonator 110, and the amplifier 140 are connected by a waveguide 160 that transmits microwaves. As shown in FIG. 2, the waveguide 160 includes a mixer 161, a circulator 162, an isolator 163, a directional coupler 164, a phase shifter 165, and an attenuator 166. The microwave cavity resonator 110 has an installation section 112 on which the sensor device 1 can be installed. The detection device may have an introduction path for introducing the volatile organic compound to be detected into the microwave cavity resonator. As the microwave source, a known microwave generator can be used.

Hereinafter, an example of a procedure of the detection method according to the embodiment will be described. FIG. 3 is a cross-sectional view schematically illustrating an example of the microwave cavity resonator 110 in the detection device 100. As shown in FIG. 3, at least a part of the graphene of the sensor device 1 is placed in the microwave cavity resonator 110 so that the charge mobility can be measured. Here, a gas A containing a volatile organic compound to be detected is contained in the microwave cavity resonator 110. That is, the gas A and the sensor device 1 are accommodated in the microwave cavity resonator 110. The microwave generated from the microwave source 120 is irradiated on the graphene of the sensor device 1 installed in the microwave cavity resonator, and the dielectric loss of the microwave under the gas A accommodation condition is measured using the oscilloscope 150. . Next, the gas A is discharged from the inside of the microwave cavity resonator 110, and the gas B containing no volatile organic compound to be detected is introduced instead. That is, the gas B and the sensor device 1 are accommodated in the microwave cavity resonator 110. The microwave is applied to the graphene of the sensor device 1 installed in the microwave cavity resonator, and the dielectric loss of the microwave under the gas B accommodation condition is measured. Then, by comparing the dielectric loss of the microwave under the condition of containing gas A and the dielectric loss of the microwave under the condition of containing gas B, it is possible to detect a time change of the dielectric loss of the microwave.
Here, the “time change” of the dielectric loss according to the present embodiment is a value of the dielectric loss related to the same sensor device, and the volatilization of the detection target to the same sensor device in the detection method according to the embodiment. This is because the state of adsorption of the volatile organic compound changes with time.
The gas B is a gas that does not contain a volatile organic compound to be detected. However, the gas B contains a volatile organic compound at a concentration different from that of the gas A within a range in which the time change of dielectric loss of microwaves can be detected. It may be a gas.
Next, a change in the charge mobility of the graphene is detected based on a time change in the detected dielectric loss of the microwave. It is known that the microwave absorption of a material in a microwave cavity is proportional to the electrical conductivity of the material. That is, detection of the change over time in the dielectric loss of microwaves is detection of a change in the charge mobility of the graphene. The change in the charge mobility of graphene is due to the change in the adsorption of the volatile organic compound to graphene, and the presence of the volatile organic compound can be detected from the change over time in the value of the microwave dielectric loss. By utilizing this phenomenon, it is possible to detect the charge mobility of the material without contacting and destructing the graphene from changes in microwave absorption when charge carriers are injected into the material (graphene). The graphene 40 is exposed on the surface of the sensor device 1, and the charge mobility of the graphene of the sensor device 1 in the microwave cavity resonator 110 can be measured without using a detection film.

Also, by utilizing this phenomenon, it is possible to absolutely quantify the charge mobility of a material (non-contact and non-destructive) from the change in microwave absorption when charge carriers are injected into the material (graphene). is there. That is, the amount of the volatile organic compound adsorbed on the graphene can be quantified based on the change amount of the charge mobility of the graphene. For example, from the amount of microwave dielectric loss under the gas A storage condition and the amount of microwave dielectric loss under the gas B storage condition, the amount of change in microwave dielectric loss is obtained, and from the amount of change in microwave dielectric loss. Then, the change amount of the charge mobility of the graphene is obtained. Since the amount of change in the charge mobility of graphene reflects the amount of change in the amount of the volatile organic compound adsorbed on the graphene, the amount of the volatile organic compound can be quantified from the value of the microwave dielectric loss. In order to quantify the volatile organic compound, for example, the amount of change in the absolute value of the signal with respect to the volatile organic compound having a known concentration may be measured in advance and used as a calibration curve.
According to the volatile organic compound detection method of the present embodiment, by comparing the states before and after the charge carriers are injected into the graphene, there is an advantage that the method is not affected by the charge injection process by the electrodes.

In the method for detecting a volatile organic compound according to the present embodiment, the detection target substance may be any compound that can be adsorbed on graphene and causes a change in charge mobility by adsorption. Here, that the detection target adsorbs to the graphene means that the detection target and the graphene are in contact with or close to each other to such an extent that the charge mobility of the graphene is changed. The detection target and the graphene may be in direct contact.
The volatile organic compound to be detected is preferably a gas, and is preferably an aromatic gas. A volatile organic compound is an organic compound that can be volatilized in air at normal temperature and normal pressure. Normal temperature refers to a range of 20 ° C. ± 15 ° C., and normal pressure refers to one atmosphere. The aromatic gas is a gas in which an aromatic compound is vaporized. Examples of the aromatic compound include a compound having a benzene ring.

  The concentration of the volatile organic compound in the gas of the detection target substance in the gas may be 0.01 to 2000 ppm, 0.1 to 2000 ppm, 1 to 500 ppm, or 10 to 10 ppm by volume. It may be 300 ppm.

  In the method for detecting a volatile organic compound according to the embodiment, the graphene may be single-layer graphene. Since single-layer graphene has high charge mobility, highly sensitive detection is possible. The graphene may be a multilayer graphene in which two or more single-layer graphenes are stacked. Since the mobility of multilayer graphene is different depending on the orientation of superposition, changing the gas detection sensitivity by using the orientation of a multilayer graphene film in which two or more single-layer graphenes having different orientations are stacked (controlling) Is possible.

  According to the detection device of the embodiment, the method for detecting a volatile organic compound of the embodiment can be performed.

  According to the method for detecting a volatile organic compound of the embodiment, by using graphene exhibiting excellent gas adsorption characteristics for a volatile organic compound or the like to be detected, an aromatic gas can be formed without forming a detection film. A method for analyzing volatile organic compounds excellent in the adsorption of water is realized. Since a volatile organic compound can be detected without using a detection film, the volatile organic compound can be detected stably with good reproducibility. In addition, graphene can be analyzed for volatile organic compounds with good stability and reproducibility by utilizing the characteristics of graphene, which has excellent thermal and chemical stability and exhibits uniform properties. Further, in the present embodiment, the charge mobility is changed due to molecular adsorption to graphene. However, since graphene has high mobility, not only the function of adsorbing gas as a detection film but also the charge mobility At the same time.

  According to the method for detecting a volatile organic compound of the present embodiment, the change in charge mobility of graphene due to adsorption of gas molecules is measured in a non-contact and non-destructive manner with graphene, thereby eliminating the influence of contact resistance. In addition, the present invention can be implemented with higher stability and reproducibility without impairing the charge mobility of graphene. The method of evaluating the charge mobility used in the present embodiment is a method called “time-resolved microwave conductivity measurement method”, which evaluates the charge mobility of a material using the time change of the dielectric loss of microwaves (non- Patent Document 8). According to this method, a positive charge or a negative charge is selectively generated in a material to be measured (in this embodiment, graphene), and the charge mobility can be absolutely determined in a non-contact and non-destructive manner. From the change in the charge mobility, the number of molecules or the concentration of the adsorbed volatile organic compound can be determined.

  As described above, the embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to the embodiments, and includes a design and the like within a range not departing from the gist of the present invention.

  Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples.

The sensor device 1 manufactured as exemplified in the above embodiment was inserted into a microwave cavity resonator having a capacity of about 10 cm ³ . An aromatic gas introduction line and a reference gas introduction line were connected to the microwave cavity resonator. Nitrogen gas containing nitrobenzene at a saturated vapor pressure (20 Pa, 200 ppm) is supplied from the aromatic gas introduction line, and nitrogen gas is supplied from the reference gas introduction line at a flow rate of 10 to 20 L / min every 2 minutes. Switched and introduced.
After adjusting the frequency of the incident microwave to the resonance frequency, a voltage of 30 V, 100 Hz rectangular wave was applied to the sample, and the change in the reflected microwave intensity was observed with an oscilloscope. In the device used in this measurement, the exposed area of the graphene is 0.14 cm ² and the injected charge carriers are 1011.

  FIG. 4 shows the result of plotting the product (Nμ) of the number N of injected charge carriers and the mobility μ thereof with respect to the time after gas introduction. The relationship between the charge carrier and the aromatic gas can be approximated by a linear relationship C = k · Nμ (C represents gas concentration and k represents a constant). Here, FIG. 4 (upper) shows the result for hole injection, and FIG. 4 (lower) shows the result for electron injection. The shaded area in the figure corresponds to the nitrobenzene introduction time. In both the hole injection and the electron injection, the absolute value of the signal (a positive signal for holes and a negative signal for electrons) increased when nitrobenzene gas was introduced. To determine nitrobenzene, the amount of change in the absolute value of the signal with respect to a known concentration of aromatic gas may be measured in advance and used as a calibration curve.

From the above, the effect of contact resistance can be measured by measuring the change in charge mobility of graphene due to the adsorption of aromatic gas molecules based on the time change of microwave dielectric loss without contacting and destructing graphene. , And was shown to be feasible without impairing the properties of graphene.
This example shows that aromatic gas can be detected without forming a detection film on graphene.

DESCRIPTION OF SYMBOLS 1 ... Sensor device, 10 ... Substrate, 20 ... Lower electrode, 30 ... Insulating layer, 31 ... Insulating layer, 32 ... Insulating layer, 40 ... Graphene, 41 ... Polymer thin film, 50 ... Upper electrode, 100 ... Detector, 110 ... Microwave cavity resonator, 112 installation section, 120 microwave source, 130 gate voltage control section, 140 amplifier, 150 oscilloscope, 160 waveguide, mixer 161 circulator 162, isolator 163, directionality Coupler 164, phase shifter 165, attenuator 166

Claims (5)

A change in the charge mobility of the graphene due to the adsorption of the volatile organic compound to the graphene is detected based on a time change of the dielectric loss of the microwave,
A method for detecting a volatile organic compound, comprising: detecting a volatile organic compound adsorbed on the graphene based on the detected change in the charge mobility.   The method for detecting a volatile organic compound according to claim 1, wherein the amount of the volatile organic compound adsorbed on the graphene is quantified based on the amount of change in the charge mobility of the graphene.   The method for detecting a volatile organic compound according to claim 1, wherein the graphene is a single-layer graphene. The graphene is characterized in that graphene is multilayer graphene stacked into two or more layers, the detection method of the volatile organic compound according to claim 1 or 2.   A detection device used for the method for detecting a volatile organic compound according to any one of claims 1 to 4, comprising a microwave source and a microwave cavity resonator, wherein the microwave cavity resonator is: A detection device having an installation portion on which a sensor device having graphene can be installed.

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