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Abstract
A silver nanoparticles (AgNPs)-integrated terahertz (THz) metasurface sensor was presented and applied to enhance the sensitivity of substance detection. Simulations and experiments were conducted to study the influence of AgNPs on the sensor performance. The enhancement of the local electric field excited by AgNPs can substantially strengthen the interaction between the THz waves and analytes, thereby increasing detection sensitivity. The experimental results indicate that the detection limit of the AgNPs-integrated metasurface sensor is improved by two orders of magnitude compared to that of the bare metasurface sensor without AgNPs. This study provides a convenient method to enhance the sensitivity of THz metasurface sensors.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Terahertz (THz) radiation lies between the microwave and infrared waves in the electromagnetic spectrum. It exhibits many excellent properties, such as fingerprint spectroscopy, nonionizing radiation damage, good penetration, and high transmittance [1–3]. In addition, since the vibrational/rotational energy levels of most bio-macromolecules and polar substances lie in the THz region [4], THz spectroscopy has been demonstrated as a powerful tool for substance detection [5,6]. Conventionally, natural materials exhibit a weak response to THz irradiation [7], which restricts the application of THz in the detection field. Therefore, metasurfaces composed of artificial structure arrays have been proposed to enhance the interaction between THz waves and substances [8]. The electromagnetic responses of the metasurface, such as resonant intensity and frequency, are highly sensitive to the surrounding dielectric environment, which renders THz metasurfaces suitable for substance detection. A large number of studies have revealed that designing a suitable metasurface structure can increase its detection sensitivity by tens of thousands of times. Xie et al. [9] used a metasurface sensor composed of periodic arrays of ring square apertures over a Si substrate to detect kanamycin sulfate molecules. And its detection limit was as low as 100 pg/L. In contrast, the lowest detectable concentration of Si without any metallic structure is approximately 1 g/L. Therefore, the sensitivity was increased by 1010 times. Zhang et al. [10] designed a THz metasurface sensor consisting of a four-division ring resonator to detect chlorothalonil. Its sensitivity improved by 106 times compared to that of the detection strategy without a metasurface. Qin et al. [11] proposed a metasurface sensor composed of metal ohm-ring arrays to detect carbendazim. Its sensitivity was approximately 104 times greater than the detection strategy without a metasurface.
Currently, medical treatment, food safety, and other fields have put forward much higher sensor sensitivity requirements. In the medical field, scholars have added antibodies and targeted biomarkers to enhance sensor sensitivity for specific proteins [12,13]. But in the field of food safety, this approach does not work. Optimizing the unit structure of the metasurface is one method for fulfilling them [5,14]. However, it incurs a large production cost. Adding functional materials, such as graphene, on the top of the metasurface is an effective alternative method. Yao et al. [15] proposed a novel THz biosensor consisting of a metasurface and graphene. They found that graphene increased the sensor sensitivity by 80 times compared to that of the detection strategy without graphene. This is because the Fermi level of graphene is close to the Dirac point, which allows the sensor to become ultrasensitive to external stimuli. However, conventionally, graphene transfer technology is a complicated process that requires a professional operation. Adding nanoparticles to the metasurface may be another convenient and suitable method for increasing sensor sensitivity. Nanoparticles have a wide range of applications in many fields due to their physical, chemical, and optical properties associated with the size effect [16–19]. In the field of condensed matter physics, to generate strong local fields that enhance the interaction between light and matter, many scholars have used the tip effect between silver nanoparticles (AgNPs) [20,21]. This phenomenon has been applied successfully to enhance Raman detection sensitivity [22–24]. However, metal nanoparticles have rarely been studied in the THz field.
In this study, we propose a type of AgNPs-integrated THz metasurface sensor that can detect nanogram-level chlorothalonil, which is a type of pesticide. In addition, we studied the factors affecting the enhancement effect of AgNPs on the sensor performance.
2. Research methods
Our previous study suggests spiral metal structures work as an ideal platform for pesticide detection [10], thus here we select this structure to construct an AgNPs-integrated sensor to investigate the enhancement effect of AgNPs on the sensitivity of metasurface sensor. The schematic of the proposed THz metasurface is displayed in Fig. 1(a). It consists of a periodic array of spiral metal structures deposited on a polyimide substrate. And the AgNPs are randomly distributed on the periodic structure. The metal layer is gold and has a thickness of 200 nm. The polyimide substrate layer has a thickness of 10 µm. As shown in Fig. 1(a), the period P of periodic structure is 80 µm, the side length T of one metal unit is 70 µm, the line width b of the gold layer is 5 µm, and the gap size a is 5 µm. We used the finite-difference time-domain algorithm (FDTD, from Lumerical Solutions, Vancouver, Canada) to theoretically study the sensor sensitivity and the electric field intensity on the metasurface. In simulation, we set the polyimide index to be 1.78. Terahertz waves were set as plane waves, and incident along the negative direction of the z-axis. The boundary condition along x(y) direction was periodic, and that along z direction was set to be perfectly matched layers (PML). To achieve the accurate results, the minimum mesh step is set to be 12.5 nm. Gold and silver were set to be perfect conductors. We generate two groups of random numbers through the uniformly distributed random numbers (rand) [25] through Matlab tool (Mathworks, Natick, United States), and then form the coordinate points through these two groups of random numbers. Then the coordinate points were input into FDTD simulation tool to model AgNPs.
Fig. 1. Schematic of (a) the proposed THz metasurface sensor and (b) the unit structure and distribution of AgNPs. The inset of Fig. 1(b) preparation process of AgNPs-integrated sensor.
In the preparation of liquid samples, by dissolving the appropriate amount of chlorothalonil powder into acetone, chlorothalonil solutions with different concentrations (0 µg/mL, 0.01 µg/mL, 0.05 µg/mL, 0.1 µg/mL, 0. µg/mL, 1 µg/mL and 2 µg/mL) were prepared. During the sensing procedure, firstly we measured the response of the bare sensor to different concentrations of chlorothalonil solutions. Then we rinsed the bare sensor sequentially with acetone and absolute ethanol. And then AgNPs solution were dripped onto the surface of the bare sensor. Then let the sensor stands for 30 minutes to prepare the AgNPs-integrated sensor (as shown in Fig. 1(b)). Finally, the response of the AgNPs-integrated sensor to different concentrations of chlorothalonil solution was measured. After each concentration measurement, we immersed the sensor in the acetone for 30 min to wash off the residual chlorothalonil, rinsed it in anhydrous ethanol for 5 min to wash away the acetone, and finally it was dried in a nitrogen-filled environment for the next test.
To investigate the effect of AgNPs on the limit of detection (LOD) of the sensor, we successively measured the LOD of the bare sensor and integrated sensor using a THz time-domain spectroscopy system. To guarantee the experimental environment and prevent the THz wave from being absorbed by the water vapor, dry nitrogen was introduced continuously into the instrument chamber until the air humidity dropped below 5%. For system stability, the temperature was maintained at 23 ± 0.1 °C. The frequency resolution of the system was set to 1.9 GHz in the measurement, and the average of 1024 measurement results for each spectrum was taken as the final result. The metasurface sensors were rinsed successively with acetone and anhydrous ethanol after each measurement to avoid cross-contamination. Various concentrations of chlorothalonil solutions were considered as the research target. The concentration unit of the chlorothalonil solutions and AgNPs were respectively taken as µg/mL and ng/mm2.
3. Results and discussion
3.1 Effecting mechanism of AgNPs on sensitivity
First, we verified whether AgNPs can enhance the sensitivity of THz metasurface sensors and explained their operating mechanism through experiments and simulations. The theoretical sensitivity of THz sensors is defined by the derivative of the transmission frequency shift with the refractive index change [10]. We simulated the responses of the bare sensor and AgNPs-integrated metasurface sensor against a refractive index change from 0.2 THz to 2 THz, as displayed in Fig. 2. The ambient medium was the air with a refractive index of 1.0. The refractive index of the analyte layer varied from 1.0 to 2.0 in increments of 0.2. Figures 2(a) and 2(b) respectively display the obtained frequency shift of the bare sensor and the AgNPs-integrated metasurface sensor. It should be noted that for both sensors, the resonance peaks at 1.0 THz red-shifted with increasing refractive index. As the refractive index changed from 1.0 to 2.0, the resonance peak position of the bare sensor changed from 1.076 THz to 0.974 THz, and the variation was approximately 102 GHz. For the same refractive index change, the resonance peak position of the AgNPs-integrated metasurface sensor changed from 1.074 THz to 0.94 THz, and the variation was approximately 134 GHz. Figure 2(c) displays the comparison results. To obtain the respective sensitivities of the two sensors, we performed linear fitting of the two sets of data. The sensitivity of the bare sensor and the AgNPs-integrated sensor was obtained as approximately 106 GHz/RIU and 125.42 GHz/RIU, respectively. The sensitivity of the AgNPs-integrated sensor increased by 19.42 GHz/RIU, which indicates that AgNPs had a significant impact on the sensitivity of the THz metasurface sensor.
Fig. 2. Resonance peak shifts of (a) the bare sensor and (b) the AgNPs-integrated sensor. (c) Comparison of the sensitivity of the two sensors. (d) Schematic of the electric field enhancement of the bare sensor. (e) Schematic of the electric field enhancement of the AgNPs-integrated sensor.
To analyze the mechanism of the enhancement effect of the AgNPs, we simulated the electric field at the resonance peak of the bare sensor and the AgNPs-integrated sensor, as displayed in Figs. 2(d) and 2(e), respectively. When irradiated by THz waves, AgNPs generate charge accumulation and induce local electric field enhancement due to the tip effect. The maximum electric field intensity of the AgNPs-integrated sensor (3.4 × 107 V/m) was nearly five times stronger than the bare sensor (7.0 × 106 V/m). According to the theory of matter polarization through electromagnetic waves [26], the charges in atoms can move under an electric field E. As a result, the positive and negative charges separate, which causes an induced dipole to polarize. Therefore, the AgNPs induced an enhanced local electric field and strengthened the interaction between the THz waves and analytes, thereby improving sensor sensitivity.
We also conducted experiments to validate the above theoretical results. Figures 3(a)–(c) respectively display the THz spectra of the three sensors (bare, 50 nm AgNPs-integrated, and 100 nm AgNPs-integrated) to detect different concentrations of chlorothalonil. For the 50 nm and 100 nm AgNPs-integrated sensors, the mass of AgNPs per unit area was identically set at approximately 31.25 ng/mm2. Figure 3(d) displays the resonance peaks of the three sensors after they were exposed to different concentrations of chlorothalonil. The bare sensor did not respond to the increase in the concentration of the chlorothalonil solutions from 0 µg/mL to 0.5 µg/mL. It began to respond when the concentration increased to 1.0 µg/mL, and the resonance peak shift was approximately 7.63 GHz. Therefore, the LOD of the bare sensor is approximately 1.0 µg/mL. Next, we analyzed the LOD of the metasurface sensors with integrated AgNPs of 50 nm and 100 nm, respectively. The 50 nm AgNPs-integrated sensor began to respond when the concentration of the chlorothalonil solution reached 0.01 µg/mL, whereas the 100 nm AgNPs-integrated sensor began to respond when the concentration of chlorothalonil solution reached 0.05 µg/mL. Therefore, the respective LODs of the 50 nm and 100 nm AgNPs-integrated sensors are approximately 0.01 µg/mL and 0.05 µg/mL, respectively. Consequently, as per the experimental results, AgNPs are highly effective for enhancing the detection sensitivity of the metasurface sensor.
Fig. 3. (a)-(c) Respective THz spectra of the three sensors (bare, 50 nm AgNPs-integrated, and 100 nm AgNPs-integrated) to detect different concentrations of chlorothalonil. (d) Positions of the resonance peaks in Figs. 3(a)-(c). The inset shows an enlarged view of the concentration of 0-0.1 µg/mL. (e) Frequency shifts of a resonance peak in Figs. 3(a)-(c).
3.2 Factors influencing the sensitivity of the AgNPs-integrated sensor
As shown in Figs. 3(d) and 3(e), LOD is affected by the diameter and concentration of AgNPs. Therefore, we discuss the factors affecting the enhancement effect of AgNPs.
To explain the influence of AgNP size on the sensitivity enhancing effect, a simulation was conducted to analyze the surface electric field intensity of the AgNPs-integrated sensor with different sizes of AgNPs. We selected 2 µm × 2 µm areas on the metasurfaces (as seen in the magnified parts of Figs. 4(a)–4(c)) and observed their respective electric field intensity. Similar to the experiments, the mass of the AgNPs per unit area was identical in the simulation, which was achieved by controlling the number of AgNPs using the formula $m = N\rho V$. Herein, $m$ represents the mass of AgNPs, $N$ represents the number of added AgNPs, $\rho $ represents the density of silver, and $V$ represents the volume of one single AgNP. The simulated results are displayed in Figs. 4(a)–4(c). We can find that the 50 nm-diameter AgNPs cause the localized electric field enhancement and enhance the interaction between THz wave and analyte, thereby leading to higher sensor sensitivity. This is because under the condition that the added AgNPs have an identical mass per unit area, decreasing the particle size results in smaller spacing between particles. The detailed reasoning steps regarding this explanation are given in Appendix 1. For the 50 nm AgNPs-integrated sensor, the average distance between nanoparticles was smaller. As a result, the sensor exhibited the most obvious enhancement effect on the surface electric field and obtained higher sensitivity, thereby verifying the experimental results displayed in Fig. 3.
Fig. 4. Electric field enhancement of the (a) bare sensor, (b) 50 nm AgNPs-integrated sensor, and (c) 100 nm AgNPs-integrated sensor.
Figure 5 displays the effect of adding different concentrations of AgNPs on the sensitivity enhancement. As shown in the figure, as the mass of the 50 nm AgNPs per unit area increases from 3.12 ng/mm2 to 31.25 ng/mm2, the LOD of the AgNPs-integrated sensor decreases from 0.5 µg/mL to 0.01 µg/mL. Therefore, under the condition that the added AgNPs have identical sizes, as the mass of the AgNPs per unit area increases, the electric field intensity increases, thereby enhancing sensor sensitivity.
Fig. 5. (a)-(c) Respective THz spectra of three sensors to detect different concentrations of chlorothalonil. (d) LOD of the three sensors.
4. Conclusion
In this study, we improved the theoretical sensitivity of the metasurface sensor via the coupling of AgNPs. The experimental results showed that the sensor sensitivity was enhanced by 19.42 GHz/RIU, and the sensor LOD was enhanced to 10 ng/mL. The coupling of AgNPs enhanced the electric field intensity on the sensor surface, which strengthened the interaction between the THz waves and analytes and improved sensor performance. Furthermore, decreasing the size of AgNPs and increasing their concentration increased the sensitivity of the sensor. Compared to the conventional bare metasurface sensor without AgNPs, the proposed sensor integrated with AgNPs offers superior high-sensitivity detection and can be used for various applications in the fields of environmental monitoring, agricultural production, and food safety.
Appendix 1. Reasoning steps
The inset in Fig. 1(b) displays that the AgNPs are tiled on the surface without any overlap. Therefore, during our experiments, the AgNPs were distributed in the same plane. As a result, each nanoparticle occupied a space of $S/N$, where $S$ represents the area and $N$ represents the number of particles.
The average distance between neighboring nanoparticles can be represented by:
where $D$ is the diameter of the AgNPs. According to the formula $m = N\rho V$, for the same mass per unit area, the number density of the 50 nm AgNPs was eight times that of the 100 nm AgNPs. Therefore, we represented the particle numbers of the 50 nm AgNPs and 100 nm AgNPs as N and $N/8$, respectively. For simplicity, we selected an area of 1 µm2 to calculate the interval $d$. The calculated results were obtained as: where d50 nm and d100 nm represent the interval between the 50 nm AgNPs and 100 nm AgNPs, respectively. According to the results, d100 nm is always larger than d50 nm when N is between 0 and 400 (400 is a critical value where the 50 nm AgNPs can cover the exact surface area of 1 µm2).Funding
Six Talent Peaks Project in Jiangsu Province (GDZB-020).
Disclosures
No conflict of interest exists in the submission of this manuscript, and the manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was the original research that has not been published previously and is not under consideration for publication elsewhere, in whole or in part.
Data availability
The simulation code/data are available from the corresponding author upon reasonable request.
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