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Curr. Opt. Photon. 2023; 7(4): 463-470

Published online August 25, 2023 https://doi.org/10.3807/COPP.2023.7.4.463

Copyright © Optical Society of Korea.

Terahertz Characteristics of Hydroxygraphene Based on Microfluidic Technology

Boyan Zhang1,2,3,4, Siyu Qian1,2,3,4, Bo Peng1,2,3,4, Bo Su1,2,3,4 , Zhuang Peng4, Hailin Cui1,2,3,4, Shengbo Zhang1,2,3,4, Cunlin Zhang1,2,3,4

1Key Laboratory of Terahertz Optoelectronics, Ministry of Education, Beijing 100048, China
2Beijing Key Laboratory for Terahertz Spectroscopy and Imaging, Beijing 100048, China
3Beijing Advanced Innovation Centre for Imaging Theory and Technology, Beijing 100048, China
4Department of Physics, Capital Normal University, Beijing 100048, China

Corresponding author: *su-b@163.com, ORCID 0000-0003-1851-2621

Received: April 19, 2023; Revised: June 19, 2023; Accepted: June 22, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Hydroxygraphene as a kind of functionalized graphene has important applications in composite, photoelectric and biological materials. In the present study, THz and microfluidic technologies were implemented to study the THz transmission characteristics of hydroxygraphene with different concentrations and residence times in magnetic and electric fields. The results show that the THz transmission intensity decreases with the increase in sample concentration and duration of an applied electric field, while it increases by staying longer in the magnetic field. The phenomenon is analyzed and explained in terms of hydrogen bond, conductivity and scattering characteristics. The results establish a foundation for future research on the THz absorption characteristics of liquid graphene based on microfluidic technology in different external environments. It also provides technical support for the application and development of graphene in THz devices.

Keywords: Electric field, Hydroxygraphene, Magnetic field, Microfluidic chip, Terahertz

OCIS codes: (300.1030) Absorption; (300.6495) Spectroscopy, terahertz

The terahertz (THz) wave, also known as a far infrared ray, is a kind of high-frequency electromagnetic wave, with frequencies and wavelengths in the ranges of 0.1–10 THz and 30–3,000 μm [15], respectively. In recent years, the rapid development of ultrafast laser and semiconductor material technologies has provided a reliable and stable excitation light source for the generation of THz waves. Accordingly, THz waves have been widely used in the fields of safety detection, biomedicine, and spectroscopy. Moreover, due to the low photon energy of THz waves, it does not ionize samples, so it is a safe and reliable method to use for nondestructive testing of materials.

Graphene has recently become a popular material in both academic and industrial applications because of its special and excellent performance. Graphene has a honeycomb single-layer carbon structure composed of planar covalent bonds formed by carbon atoms. Hydroxygraphene as a kind of functionalized graphene has been widely used in many fields, such as photoelectric materials, biology, and sensor materials. The aim of functionalization is to improve dispersibility and optimize the physical or chemical properties of graphene. Therefore, functionalized graphene is one of the most important branches in graphene research with wide applications.

Because of the special structure of graphene, it is naturally characterized by THz waves. At the same time, THz waves can represent the characteristic spectra of plasma and biomacromolecules. This is due to the vibration of molecular groups and rotation of macromolecules, which characterizes the substance structure [6, 7]. In the THz band, graphene conductivity is dominated by the in-band transition of carriers, which affects the absorption of THz waves. So, the application of graphene can be studied by THz spectroscopy [8]. Various graphene materials can be produced with different thicknesses and substrates for THz devices, THz waveguides, and THz filters. Maeng et al. [9] studied the THz response of graphene to the applied gate voltage and found nonlinear behavior. Zhou et al. [10] used a THz time-domain spectroscopy (THz-TDS) system to study the THz spectral characteristics of graphene. Considering chemical vapor deposition with different stacked layers and growth temperatures, it was found that with an increase in frequency at the same growth temperature, graphene conductivity increased and THz transmission decreased. Moreover, in randomly stacked multilayer graphene, the effect of electron coupling between layers on conductivity could be neglected. The results indicated that the quality and doping of graphene considerably affect conductivity in the THz frequency band. However, regarding functionalized graphene, especially hydroxygraphene, there is still lack of research on THz spectroscopy.

Functionalized graphene destroys the stable structure of graphene and makes its surface active. It can be dispersed in solvents so that most of the active groups on the graphene surface can actively utilize its application performance. For the hydroxygraphene used in this study, hydroxyl functional groups with the structure shown in Fig. 1 are introduced. Since the solvent may considerably absorb THz waves, microfluidic technology is used for the present study. This technology can accurately control micro-scale fluids and involves advantages such as low consumption of liquid samples, fast detection speed and simple operation. Accordingly, microfluidic technology has been widely used in chemistry, physics, biological detection and other related fields. Wen et al. [11] adopted a new microfluidic chip with easy assembly/disassembly and reusability. They examined the THz transmission intensity of 17 electrolyte solutions in the range of 0.1–1 THz and compared them with deionized water [11], but their sealing performance was poor. Fan et al. [12] designed a THz sandwich microfluidic chip, which expanded the application range of microfluidic technology and was suitable for the detection of different solutions. They used Zeonor 1420R (Zeon Co., Seoul, Korea) material to construct the sandwich microfluidic chip. This material has no characteristic absorption peak in the THz band and is transparent to visible light. Furthermore, manufacturing time and chip cost are greatly reduced due to favorable characteristics of this material such as simple manufacturing, no leakage and reusability. Graphene materials have been widely used in photoelectric equipment, especially in electrical and magnetic modulations. In this study, the THz characteristics of hydroxygraphene are examined by using this microfluidic chip and THz spectroscopy. It is found that the THz transmission intensity decreases with the increase in concentration. Under the action of external electric and magnetic fields, the increase in standing time leads to opposite trends for THz transmission intensity, i.e. a descending trend in the magnetic field and an ascending trend in the electric field.

Figure 1.Structure diagram of hydroxygraphene.

2.1. Experimental Light Path

The research optical path is a self-built THz-TDS system, and the light source comes from a self-locking fiber femtosecond laser (center wavelength 1,550 nm, pulse width 75 fs, pulse repetition frequency 100 MHz, output power 130 mW; Peking University, Beijing, China). The output laser is divided into pump light and probe light after passing through a polarizing beam splitter prism, and the pump light is coupled into a fiber photoconductive antenna (bPCA-100-05-10-1550-c-f; Batop GmbH, Jena, Germany) after passing through the mechanical translation stage to generate THz waves. The detection light is directly coupled into a fiber photoconductive antenna (bPCA-180-05-10-1550-c-f; Batop GmbH) to detect THz waves. The fabricated microfluidic chip is placed in the middle of two antennas. The THz wave emitted by the THz generation antenna passes through the microfluidic chip filled with samples, and is received by the detection antenna and input into the locking amplifier for amplification. Finally, the computer is used for data acquisition and processing. The THz-TDS system is shown in Fig. 2.

Figure 2.THz time-domain spectroscopy system.

2.2. Fabrication of Microfluidic Chip

Because the water solution has high THz wave absorption characteristics, which affects the detection results, a microfluidic chip is designed in this study. The main material for making microfluidic chips is the 2 mm thick Zeonor 1420R, whose THz transmittance spectrum is shown in Fig. 3(a). The transmittance can reach more than 90%. In addition, this material is transparent to visible light and has no absorption peak in the THz band, so it is an ideal material for making THz microfluidic chips [13]. The manufacturing process is as follows:

Figure 3.Microfluidic chip description. (a) THz transmittance spectrum of Zeonor 1420R, (b) Schematic diagram of microfluidic chip.

Two pieces of Zeonor 1420R are used, one of which is a base plate and the other is a cover plate, and two round holes with a radius of 1 mm are carved on the cover plate as a liquid inlet and liquid outlet. A double-sided adhesive with a thickness of 50 μm is used as an intermediate layer between the substrate and the cover sheet. Areas cut out of the double-sided adhesive tape with a length and width of 2 cm and 1 cm, respectively, act as channels for liquid samples, and then it is bond with the substrate and cover. Finally, it is cleaned with an ultrasonic cleaning machine and alcohol. The preparation process of the microfluidic chip is shown in Fig. 3(b). The microfluidic chip can make less THz absorption when passing through liquid samples, consume fewer samples, and can be reused, which is environmentally friendly.

2.3. External Magnetic Field and Electric Field Device

As shown in Fig. 4, the magnetic field device includes two electromagnets, which are powered by a WYJ-9B transistor stabilized power supply. The output voltage of this power supply ranges from 0–30 V, and the magnetic field intensity of the electromagnets can be changed by adjusting the output voltage. The voltage used in the experiment is 20 V, and the magnetic field intensity of the two electromagnets at the chip position is about 70 mT.

Figure 4.Schematic diagram of the external magnetic field device.

In this study, an electric field device and a magnetic field device are designed independently. As shown in Fig. 5, it mainly consists of a high-voltage module, two electrode plates and a bracket. This device can change the output voltage between 0 and 15,000 V by adjusting the potentiometer. The distance between the electrodes is 3.5 cm, and the high-voltage power supply module is DW-P153-05C51. The voltage used in this study is 10,500 V, so the electric field intensity between the electrode plates is about 3,000 V/cm.

Figure 5.Schematic diagram of external electric field device.

3.1. THz Transmission Characteristics of Hydroxygraphene with Different Concentrations

Hydroxygraphene destroys the stable structure of graphene, so it can be dispersed in solvent. Dispersants of 0.5, 1, 3 and 5 mg/ml were obtained by dispersing pure hydroxygraphene into deionized water. Hydroxygraphenes with different concentrations were subsequently injected into the microfluidic chip, which was fixed in the optical path for THz transmission measurement. The THz time-domain spectra are shown in Fig. 6(a). It can be seen that the rise of the hydroxygraphene concentration weakens the THz time-domain spectra, which corresponds to a higher absorption level. The THz frequency-domain spectra are obtained by applying Fourier transform (logarithmic scale) to the THz time-domain spectra, as illustrated in Fig. 6(b). As can be seen, the THz transmission intensity in the frequency spectra decreases with the increase in hydroxygraphene concentration. In order to obtain the relationship between transmission coefficient and hydroxygraphene concentration at a certain frequency, the data was extracted based on linear fitting at 0.2 THz corresponding to the peak of the frequency-domain spectra. Figure 6(c) indicates that the growth of the concentration results in a reduction of the peak value of the spectrum. A high linear correlation (fitting degree of 0.95994) was obtained between the transmission coefficient and the concentration at 0.2 THz.

Figure 6.THz spectra for different concentrations: (a) THz time-domain spectra, (b) THz transmission frequency-domain spectra (logarithmic scale), and (c) linear fitting at 0.2 THz.

3.2. THz Transmission Characteristics under External Magnetic Field

Hydroxygraphene with a concentration of 5 mg/ml was injected into the microfluidic chip. It was then fixed on a plexiglass support and exposed to a self-made magnetic field with an intensity of 70 mT. THz transmission was measured every 5 minutes, i.e. at t = 0, 5, 10, 15, and 20 min. Figures 7(a) and 7(b) presents the THz time-domain spectra and transmission frequency-domain spectra, respectively. By increasing the residence time of hydroxygraphene in the magnetic field, both of these spectra weaken due to a higher absorption extent. In order to obtain the relationship between the transmission coefficient and time spent in the magnetic field of hydroxygraphene at a certain frequency, the data was extracted at 0.2 THz corresponding to the peak of the frequency-domain spectra, as illustrated in Fig. 7(c). As can be seen, the peak value of the spectra decreased with the increase in residence time in the magnetic field.

Figure 7.THz spectra for different residence times in the magnetic field: (a) THz time-domain spectra, (b) THz transmission frequency-domain spectra (logarithmic scale), and (c) THz transmission frequency spectra at 0.2 THz.

3.3. THz Transmission Characteristics of Hydroxygraphene under External Electric Field

Hydroxygraphene with a concentration of 5 mg/ml was injected into the microfluidic chip and then exposed to a self-made electric field with an intensity of 3,000 V/cm. THz transmission was measured every 5 minutes, i.e. at t = 0, 5, 10, 15, 20, and 25 min, and the THz time-domain and frequency-domain spectra are shown in Figs. 8(a) and 8(b), respectively. It can be seen that these spectra are relatively enhanced with the increase in residence time of hydroxygraphene in the electric field, which indicates a lower absorption level. The data at 0.2 THz (corresponding to the peak of the frequency-domain spectra) is employed to obtain the relationship between the transmission coefficient and time spent in the electric field of hydroxygraphene at a certain frequency, as illustrated in Fig. 8(c). It is found that the peak of the spectra increases as the residence time in the electric field increases.

Figure 8.THz spectra for different residence times in the electric field: (a) THz time-domain spectra, (b) THz transmission frequency-domain spectra (logarithmic scale), and (c) THz transmission frequency spectra at 0.2 THz.

The volume of deionized water decreases with the increase in hydroxygraphene concentration. According to the absorption coefficient of liquid water to THz [14], absorption should decrease while transmission should increase. However, experimental results indicate that the THz time-domain and frequency-domain spectra are relatively weakened, leading to a reduction of both transmission and absorption. Accordingly, absorption of THz by hydroxygraphene can be considered as the main reason for these trends. The presence of hydroxyl functional groups alters the stable structure of graphene. With the diffusion of hydroxygraphene in the solvent, hydroxyl functional groups easily combine with water molecules, and thus form more hydrogen bonds [15]. Figure 9 illustrates hydrogen bonds in hydroxygraphene. Intermolecular hydrogen bonds are very sensitive to THz waves and cause strong absorption of these waves. Moreover, because of the larger diameter of hydroxygraphene particles, an increase in concentration results in more polymerization and the formation of clusters, and consequently reduces the transmission of THz waves.

Figure 9.Structure of hydroxygraphene: (a) structural formula, (b) schematic diagram of hydrogen bond in hydroxygraphene.

The intensity of THz time-domain and frequency-domain spectra weakens when hydroxygraphene stays longer in the magnetic field. This also indicates a reduction of transmission. In the THz bandwidth, hydroxygraphene leads to an obvious magneto-optical effect. This is related to its in-band electrons, and produces strong cyclotron motion in a magnetic field [16]. Hydroxyl functional groups can make efficient and stable magnetic moments, and therefore they can form high-density magnetic clusters [17]. The larger the magnetic cluster, the greater the scattering. Rayleigh scattering is the main scattering mechanism that will reduce THz transmission [18]. Therefore, the intensity of the time-domain spectra and transmission frequency-domain spectra weakens with the increase in residence time in the magnetic field.

When hydroxygraphene stays longer in the electric field, the THz time-domain spectra and transmission frequency-domain spectra are more considerable, indicating a higher transmission level. In the following, theoretical calculations are carried out to further analyze the reasons for the higher spectral transmittance. Based on the Drude model, the carrier density N can be expressed as follows:

N=mε0ωp2e2

where m is the effective electron mass, ε0 is the vacuum permittivity and e is the electron charge. ωp represents the plasma frequency, which is expressed approximately as follows:

ωp=εi2(1εr)ω

where εr and εi are the real and imaginary parts of the dielectric constant, respectively, and ω is the THz frequency. It was found that the carrier density decreases with time, which also means a reduction of conductivity because of the proportionality between carrier density and conductivity. This trend is mainly caused by two reasons. First, when placed in an electric field, hydroxygraphene will form an electrophoresis phenomenon with the passage of time because of its large particle size, resulting in the particles moving to the two poles, thus reducing the cluster and conductivity [19]. Second, prolongation of residence time in an electric field causes more destruction of the structure of hydroxyl functional groups. This weakens the conjugated hydroxygraphene system and leads to a reduction in conductivity, which in turn decreases absorption and increases transmission.

In this study, microfluidic and THz technologies were combined, and different concentrations of hydroxygraphene were detected by a COC chip. The results indicated that the intensity of the THz time-domain and frequency-domain spectra decreases with the increase in hydroxygraphene concentration. It is believed that hydroxyl groups in hydroxygraphene can easily form hydrogen bonds with water molecules, which increases THz absorption. In addition, the THz transmission characteristics of hydroxygraphene after staying in a magnetic field with an intensity of 70 mT for different durations were studied. It was found that the intensity of the THz time-domain and frequency-domain spectra of hydroxygraphene decreased when it kept longer in the magnetic field. This is because the free electrons in the hydroxygraphene belt exhibit strong gyratory motion in the magnetic field and form high-density magnetic clusters. Accordingly, they reduce the transmission of THz waves and weaken the strength of the THz time-domain and frequency-domain spectra to a certain extent. The THz transmission characteristics of hydroxygraphene after staying in an electric field of 3,000 V/cm for different durations were also studied. The opposite phenomenon was observed in this case in comparison with that of the magnetic field. This is because of the occurrence of electrophoresis when the hydroxygraphene stays in an electric field. Such a condition leads to a decrease in its conductivity, and therefore a reduction of THz wave absorption. Since functional graphene is easily dispersed in solvent, it is a suitable choice for the study of THz spectral characteristics and development of graphene research scope. In a future study, it is intended to focus on the implementation of the above findings with optoelectronic devices for the design and development of THz metamaterials.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The data used to support the findings of this study are available from the corresponding author upon request.

National Key R&D Program of China (Grant No.2021 YFB3200100); National Natural Science Foundation of China (NSFC) (61575131); General Project of the Beijing Natural Science Foundation; Research on Microfluidic Biosensor Technology Based on Terahertz System-on-Chip (4232066).

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Article

Research Paper

Curr. Opt. Photon. 2023; 7(4): 463-470

Published online August 25, 2023 https://doi.org/10.3807/COPP.2023.7.4.463

Copyright © Optical Society of Korea.

Terahertz Characteristics of Hydroxygraphene Based on Microfluidic Technology

Boyan Zhang1,2,3,4, Siyu Qian1,2,3,4, Bo Peng1,2,3,4, Bo Su1,2,3,4 , Zhuang Peng4, Hailin Cui1,2,3,4, Shengbo Zhang1,2,3,4, Cunlin Zhang1,2,3,4

1Key Laboratory of Terahertz Optoelectronics, Ministry of Education, Beijing 100048, China
2Beijing Key Laboratory for Terahertz Spectroscopy and Imaging, Beijing 100048, China
3Beijing Advanced Innovation Centre for Imaging Theory and Technology, Beijing 100048, China
4Department of Physics, Capital Normal University, Beijing 100048, China

Correspondence to:*su-b@163.com, ORCID 0000-0003-1851-2621

Received: April 19, 2023; Revised: June 19, 2023; Accepted: June 22, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Hydroxygraphene as a kind of functionalized graphene has important applications in composite, photoelectric and biological materials. In the present study, THz and microfluidic technologies were implemented to study the THz transmission characteristics of hydroxygraphene with different concentrations and residence times in magnetic and electric fields. The results show that the THz transmission intensity decreases with the increase in sample concentration and duration of an applied electric field, while it increases by staying longer in the magnetic field. The phenomenon is analyzed and explained in terms of hydrogen bond, conductivity and scattering characteristics. The results establish a foundation for future research on the THz absorption characteristics of liquid graphene based on microfluidic technology in different external environments. It also provides technical support for the application and development of graphene in THz devices.

Keywords: Electric field, Hydroxygraphene, Magnetic field, Microfluidic chip, Terahertz

I. INTRODUCTION

The terahertz (THz) wave, also known as a far infrared ray, is a kind of high-frequency electromagnetic wave, with frequencies and wavelengths in the ranges of 0.1–10 THz and 30–3,000 μm [15], respectively. In recent years, the rapid development of ultrafast laser and semiconductor material technologies has provided a reliable and stable excitation light source for the generation of THz waves. Accordingly, THz waves have been widely used in the fields of safety detection, biomedicine, and spectroscopy. Moreover, due to the low photon energy of THz waves, it does not ionize samples, so it is a safe and reliable method to use for nondestructive testing of materials.

Graphene has recently become a popular material in both academic and industrial applications because of its special and excellent performance. Graphene has a honeycomb single-layer carbon structure composed of planar covalent bonds formed by carbon atoms. Hydroxygraphene as a kind of functionalized graphene has been widely used in many fields, such as photoelectric materials, biology, and sensor materials. The aim of functionalization is to improve dispersibility and optimize the physical or chemical properties of graphene. Therefore, functionalized graphene is one of the most important branches in graphene research with wide applications.

Because of the special structure of graphene, it is naturally characterized by THz waves. At the same time, THz waves can represent the characteristic spectra of plasma and biomacromolecules. This is due to the vibration of molecular groups and rotation of macromolecules, which characterizes the substance structure [6, 7]. In the THz band, graphene conductivity is dominated by the in-band transition of carriers, which affects the absorption of THz waves. So, the application of graphene can be studied by THz spectroscopy [8]. Various graphene materials can be produced with different thicknesses and substrates for THz devices, THz waveguides, and THz filters. Maeng et al. [9] studied the THz response of graphene to the applied gate voltage and found nonlinear behavior. Zhou et al. [10] used a THz time-domain spectroscopy (THz-TDS) system to study the THz spectral characteristics of graphene. Considering chemical vapor deposition with different stacked layers and growth temperatures, it was found that with an increase in frequency at the same growth temperature, graphene conductivity increased and THz transmission decreased. Moreover, in randomly stacked multilayer graphene, the effect of electron coupling between layers on conductivity could be neglected. The results indicated that the quality and doping of graphene considerably affect conductivity in the THz frequency band. However, regarding functionalized graphene, especially hydroxygraphene, there is still lack of research on THz spectroscopy.

Functionalized graphene destroys the stable structure of graphene and makes its surface active. It can be dispersed in solvents so that most of the active groups on the graphene surface can actively utilize its application performance. For the hydroxygraphene used in this study, hydroxyl functional groups with the structure shown in Fig. 1 are introduced. Since the solvent may considerably absorb THz waves, microfluidic technology is used for the present study. This technology can accurately control micro-scale fluids and involves advantages such as low consumption of liquid samples, fast detection speed and simple operation. Accordingly, microfluidic technology has been widely used in chemistry, physics, biological detection and other related fields. Wen et al. [11] adopted a new microfluidic chip with easy assembly/disassembly and reusability. They examined the THz transmission intensity of 17 electrolyte solutions in the range of 0.1–1 THz and compared them with deionized water [11], but their sealing performance was poor. Fan et al. [12] designed a THz sandwich microfluidic chip, which expanded the application range of microfluidic technology and was suitable for the detection of different solutions. They used Zeonor 1420R (Zeon Co., Seoul, Korea) material to construct the sandwich microfluidic chip. This material has no characteristic absorption peak in the THz band and is transparent to visible light. Furthermore, manufacturing time and chip cost are greatly reduced due to favorable characteristics of this material such as simple manufacturing, no leakage and reusability. Graphene materials have been widely used in photoelectric equipment, especially in electrical and magnetic modulations. In this study, the THz characteristics of hydroxygraphene are examined by using this microfluidic chip and THz spectroscopy. It is found that the THz transmission intensity decreases with the increase in concentration. Under the action of external electric and magnetic fields, the increase in standing time leads to opposite trends for THz transmission intensity, i.e. a descending trend in the magnetic field and an ascending trend in the electric field.

Figure 1. Structure diagram of hydroxygraphene.

II. Experimental Instruments and Devices

2.1. Experimental Light Path

The research optical path is a self-built THz-TDS system, and the light source comes from a self-locking fiber femtosecond laser (center wavelength 1,550 nm, pulse width 75 fs, pulse repetition frequency 100 MHz, output power 130 mW; Peking University, Beijing, China). The output laser is divided into pump light and probe light after passing through a polarizing beam splitter prism, and the pump light is coupled into a fiber photoconductive antenna (bPCA-100-05-10-1550-c-f; Batop GmbH, Jena, Germany) after passing through the mechanical translation stage to generate THz waves. The detection light is directly coupled into a fiber photoconductive antenna (bPCA-180-05-10-1550-c-f; Batop GmbH) to detect THz waves. The fabricated microfluidic chip is placed in the middle of two antennas. The THz wave emitted by the THz generation antenna passes through the microfluidic chip filled with samples, and is received by the detection antenna and input into the locking amplifier for amplification. Finally, the computer is used for data acquisition and processing. The THz-TDS system is shown in Fig. 2.

Figure 2. THz time-domain spectroscopy system.

2.2. Fabrication of Microfluidic Chip

Because the water solution has high THz wave absorption characteristics, which affects the detection results, a microfluidic chip is designed in this study. The main material for making microfluidic chips is the 2 mm thick Zeonor 1420R, whose THz transmittance spectrum is shown in Fig. 3(a). The transmittance can reach more than 90%. In addition, this material is transparent to visible light and has no absorption peak in the THz band, so it is an ideal material for making THz microfluidic chips [13]. The manufacturing process is as follows:

Figure 3. Microfluidic chip description. (a) THz transmittance spectrum of Zeonor 1420R, (b) Schematic diagram of microfluidic chip.

Two pieces of Zeonor 1420R are used, one of which is a base plate and the other is a cover plate, and two round holes with a radius of 1 mm are carved on the cover plate as a liquid inlet and liquid outlet. A double-sided adhesive with a thickness of 50 μm is used as an intermediate layer between the substrate and the cover sheet. Areas cut out of the double-sided adhesive tape with a length and width of 2 cm and 1 cm, respectively, act as channels for liquid samples, and then it is bond with the substrate and cover. Finally, it is cleaned with an ultrasonic cleaning machine and alcohol. The preparation process of the microfluidic chip is shown in Fig. 3(b). The microfluidic chip can make less THz absorption when passing through liquid samples, consume fewer samples, and can be reused, which is environmentally friendly.

2.3. External Magnetic Field and Electric Field Device

As shown in Fig. 4, the magnetic field device includes two electromagnets, which are powered by a WYJ-9B transistor stabilized power supply. The output voltage of this power supply ranges from 0–30 V, and the magnetic field intensity of the electromagnets can be changed by adjusting the output voltage. The voltage used in the experiment is 20 V, and the magnetic field intensity of the two electromagnets at the chip position is about 70 mT.

Figure 4. Schematic diagram of the external magnetic field device.

In this study, an electric field device and a magnetic field device are designed independently. As shown in Fig. 5, it mainly consists of a high-voltage module, two electrode plates and a bracket. This device can change the output voltage between 0 and 15,000 V by adjusting the potentiometer. The distance between the electrodes is 3.5 cm, and the high-voltage power supply module is DW-P153-05C51. The voltage used in this study is 10,500 V, so the electric field intensity between the electrode plates is about 3,000 V/cm.

Figure 5. Schematic diagram of external electric field device.

III. Experimental Results

3.1. THz Transmission Characteristics of Hydroxygraphene with Different Concentrations

Hydroxygraphene destroys the stable structure of graphene, so it can be dispersed in solvent. Dispersants of 0.5, 1, 3 and 5 mg/ml were obtained by dispersing pure hydroxygraphene into deionized water. Hydroxygraphenes with different concentrations were subsequently injected into the microfluidic chip, which was fixed in the optical path for THz transmission measurement. The THz time-domain spectra are shown in Fig. 6(a). It can be seen that the rise of the hydroxygraphene concentration weakens the THz time-domain spectra, which corresponds to a higher absorption level. The THz frequency-domain spectra are obtained by applying Fourier transform (logarithmic scale) to the THz time-domain spectra, as illustrated in Fig. 6(b). As can be seen, the THz transmission intensity in the frequency spectra decreases with the increase in hydroxygraphene concentration. In order to obtain the relationship between transmission coefficient and hydroxygraphene concentration at a certain frequency, the data was extracted based on linear fitting at 0.2 THz corresponding to the peak of the frequency-domain spectra. Figure 6(c) indicates that the growth of the concentration results in a reduction of the peak value of the spectrum. A high linear correlation (fitting degree of 0.95994) was obtained between the transmission coefficient and the concentration at 0.2 THz.

Figure 6. THz spectra for different concentrations: (a) THz time-domain spectra, (b) THz transmission frequency-domain spectra (logarithmic scale), and (c) linear fitting at 0.2 THz.

3.2. THz Transmission Characteristics under External Magnetic Field

Hydroxygraphene with a concentration of 5 mg/ml was injected into the microfluidic chip. It was then fixed on a plexiglass support and exposed to a self-made magnetic field with an intensity of 70 mT. THz transmission was measured every 5 minutes, i.e. at t = 0, 5, 10, 15, and 20 min. Figures 7(a) and 7(b) presents the THz time-domain spectra and transmission frequency-domain spectra, respectively. By increasing the residence time of hydroxygraphene in the magnetic field, both of these spectra weaken due to a higher absorption extent. In order to obtain the relationship between the transmission coefficient and time spent in the magnetic field of hydroxygraphene at a certain frequency, the data was extracted at 0.2 THz corresponding to the peak of the frequency-domain spectra, as illustrated in Fig. 7(c). As can be seen, the peak value of the spectra decreased with the increase in residence time in the magnetic field.

Figure 7. THz spectra for different residence times in the magnetic field: (a) THz time-domain spectra, (b) THz transmission frequency-domain spectra (logarithmic scale), and (c) THz transmission frequency spectra at 0.2 THz.

3.3. THz Transmission Characteristics of Hydroxygraphene under External Electric Field

Hydroxygraphene with a concentration of 5 mg/ml was injected into the microfluidic chip and then exposed to a self-made electric field with an intensity of 3,000 V/cm. THz transmission was measured every 5 minutes, i.e. at t = 0, 5, 10, 15, 20, and 25 min, and the THz time-domain and frequency-domain spectra are shown in Figs. 8(a) and 8(b), respectively. It can be seen that these spectra are relatively enhanced with the increase in residence time of hydroxygraphene in the electric field, which indicates a lower absorption level. The data at 0.2 THz (corresponding to the peak of the frequency-domain spectra) is employed to obtain the relationship between the transmission coefficient and time spent in the electric field of hydroxygraphene at a certain frequency, as illustrated in Fig. 8(c). It is found that the peak of the spectra increases as the residence time in the electric field increases.

Figure 8. THz spectra for different residence times in the electric field: (a) THz time-domain spectra, (b) THz transmission frequency-domain spectra (logarithmic scale), and (c) THz transmission frequency spectra at 0.2 THz.

IV. DISCUSSION

The volume of deionized water decreases with the increase in hydroxygraphene concentration. According to the absorption coefficient of liquid water to THz [14], absorption should decrease while transmission should increase. However, experimental results indicate that the THz time-domain and frequency-domain spectra are relatively weakened, leading to a reduction of both transmission and absorption. Accordingly, absorption of THz by hydroxygraphene can be considered as the main reason for these trends. The presence of hydroxyl functional groups alters the stable structure of graphene. With the diffusion of hydroxygraphene in the solvent, hydroxyl functional groups easily combine with water molecules, and thus form more hydrogen bonds [15]. Figure 9 illustrates hydrogen bonds in hydroxygraphene. Intermolecular hydrogen bonds are very sensitive to THz waves and cause strong absorption of these waves. Moreover, because of the larger diameter of hydroxygraphene particles, an increase in concentration results in more polymerization and the formation of clusters, and consequently reduces the transmission of THz waves.

Figure 9. Structure of hydroxygraphene: (a) structural formula, (b) schematic diagram of hydrogen bond in hydroxygraphene.

The intensity of THz time-domain and frequency-domain spectra weakens when hydroxygraphene stays longer in the magnetic field. This also indicates a reduction of transmission. In the THz bandwidth, hydroxygraphene leads to an obvious magneto-optical effect. This is related to its in-band electrons, and produces strong cyclotron motion in a magnetic field [16]. Hydroxyl functional groups can make efficient and stable magnetic moments, and therefore they can form high-density magnetic clusters [17]. The larger the magnetic cluster, the greater the scattering. Rayleigh scattering is the main scattering mechanism that will reduce THz transmission [18]. Therefore, the intensity of the time-domain spectra and transmission frequency-domain spectra weakens with the increase in residence time in the magnetic field.

When hydroxygraphene stays longer in the electric field, the THz time-domain spectra and transmission frequency-domain spectra are more considerable, indicating a higher transmission level. In the following, theoretical calculations are carried out to further analyze the reasons for the higher spectral transmittance. Based on the Drude model, the carrier density N can be expressed as follows:

N=mε0ωp2e2

where m is the effective electron mass, ε0 is the vacuum permittivity and e is the electron charge. ωp represents the plasma frequency, which is expressed approximately as follows:

ωp=εi2(1εr)ω

where εr and εi are the real and imaginary parts of the dielectric constant, respectively, and ω is the THz frequency. It was found that the carrier density decreases with time, which also means a reduction of conductivity because of the proportionality between carrier density and conductivity. This trend is mainly caused by two reasons. First, when placed in an electric field, hydroxygraphene will form an electrophoresis phenomenon with the passage of time because of its large particle size, resulting in the particles moving to the two poles, thus reducing the cluster and conductivity [19]. Second, prolongation of residence time in an electric field causes more destruction of the structure of hydroxyl functional groups. This weakens the conjugated hydroxygraphene system and leads to a reduction in conductivity, which in turn decreases absorption and increases transmission.

V. CONCLUSION

In this study, microfluidic and THz technologies were combined, and different concentrations of hydroxygraphene were detected by a COC chip. The results indicated that the intensity of the THz time-domain and frequency-domain spectra decreases with the increase in hydroxygraphene concentration. It is believed that hydroxyl groups in hydroxygraphene can easily form hydrogen bonds with water molecules, which increases THz absorption. In addition, the THz transmission characteristics of hydroxygraphene after staying in a magnetic field with an intensity of 70 mT for different durations were studied. It was found that the intensity of the THz time-domain and frequency-domain spectra of hydroxygraphene decreased when it kept longer in the magnetic field. This is because the free electrons in the hydroxygraphene belt exhibit strong gyratory motion in the magnetic field and form high-density magnetic clusters. Accordingly, they reduce the transmission of THz waves and weaken the strength of the THz time-domain and frequency-domain spectra to a certain extent. The THz transmission characteristics of hydroxygraphene after staying in an electric field of 3,000 V/cm for different durations were also studied. The opposite phenomenon was observed in this case in comparison with that of the magnetic field. This is because of the occurrence of electrophoresis when the hydroxygraphene stays in an electric field. Such a condition leads to a decrease in its conductivity, and therefore a reduction of THz wave absorption. Since functional graphene is easily dispersed in solvent, it is a suitable choice for the study of THz spectral characteristics and development of graphene research scope. In a future study, it is intended to focus on the implementation of the above findings with optoelectronic devices for the design and development of THz metamaterials.

DISCLOSURES

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

DATA AVAILABILITY

The data used to support the findings of this study are available from the corresponding author upon request.

ACKNOWLEDGMENTS

The authors would like to thank EditSprings (https://www.editsprings.cn) for providing English proofreading.

FUNDING

National Key R&D Program of China (Grant No.2021 YFB3200100); National Natural Science Foundation of China (NSFC) (61575131); General Project of the Beijing Natural Science Foundation; Research on Microfluidic Biosensor Technology Based on Terahertz System-on-Chip (4232066).

Fig 1.

Figure 1.Structure diagram of hydroxygraphene.
Current Optics and Photonics 2023; 7: 463-470https://doi.org/10.3807/COPP.2023.7.4.463

Fig 2.

Figure 2.THz time-domain spectroscopy system.
Current Optics and Photonics 2023; 7: 463-470https://doi.org/10.3807/COPP.2023.7.4.463

Fig 3.

Figure 3.Microfluidic chip description. (a) THz transmittance spectrum of Zeonor 1420R, (b) Schematic diagram of microfluidic chip.
Current Optics and Photonics 2023; 7: 463-470https://doi.org/10.3807/COPP.2023.7.4.463

Fig 4.

Figure 4.Schematic diagram of the external magnetic field device.
Current Optics and Photonics 2023; 7: 463-470https://doi.org/10.3807/COPP.2023.7.4.463

Fig 5.

Figure 5.Schematic diagram of external electric field device.
Current Optics and Photonics 2023; 7: 463-470https://doi.org/10.3807/COPP.2023.7.4.463

Fig 6.

Figure 6.THz spectra for different concentrations: (a) THz time-domain spectra, (b) THz transmission frequency-domain spectra (logarithmic scale), and (c) linear fitting at 0.2 THz.
Current Optics and Photonics 2023; 7: 463-470https://doi.org/10.3807/COPP.2023.7.4.463

Fig 7.

Figure 7.THz spectra for different residence times in the magnetic field: (a) THz time-domain spectra, (b) THz transmission frequency-domain spectra (logarithmic scale), and (c) THz transmission frequency spectra at 0.2 THz.
Current Optics and Photonics 2023; 7: 463-470https://doi.org/10.3807/COPP.2023.7.4.463

Fig 8.

Figure 8.THz spectra for different residence times in the electric field: (a) THz time-domain spectra, (b) THz transmission frequency-domain spectra (logarithmic scale), and (c) THz transmission frequency spectra at 0.2 THz.
Current Optics and Photonics 2023; 7: 463-470https://doi.org/10.3807/COPP.2023.7.4.463

Fig 9.

Figure 9.Structure of hydroxygraphene: (a) structural formula, (b) schematic diagram of hydrogen bond in hydroxygraphene.
Current Optics and Photonics 2023; 7: 463-470https://doi.org/10.3807/COPP.2023.7.4.463

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