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Abstract
The combined application of metasurface and terahertz (THz) time-domain spectroscopy techniques has received considerable attention in the fields of sensing and detection. However, to detect trace samples, the THz wave must still be enhanced locally using certain methods to improve the detection sensitivity. In this study, we proposed and experimentally demonstrated a fano resonance metasurface-based silver nanoparticles (FaMs-AgNPs) sensor. AgNPs can enhance the sensitivity of the sensor by generating charge accumulation and inducing localized electric field enhancement through the tip effect, thereby enhancing the interaction between the THz waves and analytes. We investigated the effects of four different contents of AgNPs, 10 µl, 20 µl, 30 µl and 40 µl, on the detection of acetamiprid. At 30 µl of AgNPs, the amplitude change of the FaMs-AgNPs sensor was more pronounced and the sensitivity was higher, which could detect acetamiprid solutions as low as 100 pg/ml. The FaMs-AgNPs sensor has the advantages of a simple structure, easy processing, and excellent sensing performance, and has a great potential application value in the field of THz trace detection and other fields.
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1. Introduction
With the widespread use of pesticides, concern regarding pesticide residues in agricultural products is increasing. Acetamiprid (ACE), a selective agonist of the nicotinic acetylcholine receptor, is a widely used neonicotinoid insecticide. However, according to certain studies, long-term accumulation and exposure to ACE may damage the male reproductive system, resulting in decreased testicular weights; significantly lower testosterone concentrations, sperm counts, and viability; and reduced sperm fertilization in rodents [1–4]. Near-infrared spectroscopy and hyperspectral imaging can be used to achieve ACE detection with high selectivity and sensitivity; however, the detection time is long and damaging to the test object [5]. Hence, a method that can rapidly detect ACE at low concentrations is urgently needed.
Terahertz (THz) waves have low energy and fingerprint properties and can therefore provide a fast, nondestructive detection method [6–10]. However, detection sensitivity is limited by the mismatch between the size of the target analyte and the THz wavelength. To improve detection sensitivity, the use of metasurfaces consisting of periodic manmade structures with unnatural optical properties has been proposed for analyte detection [11–13]. This can facilitate the interaction between the THz waves and the analyte by enhancing the local electric field [14–16]. Nevertheless, the combination of metasurfaces and THz time-domain spectroscopy still cannot meet the demand for trace detection in the field of biosensing, and certain methods of local enhancement of THz waves are required to improve the detection sensitivity [17–20]. Metal nanoparticles (AgNPs or AuNPs) have been vital nanomaterials in biology and medicine owing to their controllable geometries, optics, and special chemical properties [21–23]. Metal nanoparticles can generate charge accumulation, induce local electric field enhancement owing to the tip effect, and enhance the interaction between the THz wave and the analyte, thus improving the sensitivity of the THz metasurface sensor. Zhao et al. presented a sensor utilizing AuNPs to enhance THz glucose sensitivity and measured the physiological levels of glucose solutions ranging from 0 to 0.8 mg/mL, the absorption coefficients of each concentration of glucose could be clearly distinguished when compared with the THz time-domain spectra of pure aqueous solutions. Finally, the sensitivity enhancement was verified to be due to the pH change caused by glucose oxidation catalyzed by AuNPs [24]. Mu et al. [25] proposed a THz metasurface sensor integrated with AgNPs and applied it to enhance the sensitivity of substance detection. The integration of AgNPs of different diameters with a metasurface demonstrated that smaller diameter AgNPs enhanced the sensing capability more significantly and achieved chlorothalonil nanoscale detection. Because the AgNPs can enhance the trapping of biomarker molecules on the metasurface, this approach offers exciting new ideas for sensing in the THz band and allows the detection of trace amounts of biomolecules. However, few studies have focused on the effect of different contents of AgNPs on sensor performance, and their experimental demonstration in the THz portion of the spectrum remains unexplored.
In this paper, we proposed and experimentally demonstrated a fano resonance metasurface -based the silver nanoparticles (An-graphene-Ms) sensor that can detect picogram-level ACE. AgNPs can enhance the sensitivity of the sensor by generating charge accumulation and inducing localized electric field enhancement through the tip effect, thereby enhancing the interaction between the THz waves and analytes. We investigated the effects of four different contents of AgNPs, 10 μl, 20 μl, 30 μl and 40 μl, on the detection of ACE. At 30 μl of AgNPs, the amplitude change of the FaMs-AgNPs sensor was more pronounced and the sensitivity was higher, which could detect acetamiprid solutions as low as 100 pg/ml.
2. Design and analysis of Fano resonance metasurface
Figure 1(a) shows a schematic of a symmetrical split-ring (SSR) metasurface. From the bottom substrate to the top surface, it includes a 5-µm-thick polyimide (PI) film, a 0.2-µm-thick aluminum (Al) structure. In Fig. 1(a), x denotes the length of the gap translated to the right, and l1 and l2 denote the lengths of the metal rings to the left and right of the gap, respectively. A schematic of a THz wave passing through a metasurface is shown in Fig. 1(b). In this study, the proposed metasurfaces were quantified by simulations using the CST software. The THz wave was incident vertically on the upper surface of the metasurface, with an electric field along the y-direction (Ey) and a magnetic field along the x-direction (Hx).
Fig. 1. (a) Geometric parameters of the SSR structure: P = 100 µm, l = 48 µm, w = 6 µm, d = 8 µm, l1 = l2 = 112 µm, x = 0 µm. (b) Schematic of a THz wave passing through the metasurface. (c) Transmission spectra of the SSR under simulated conditions. (d) Surface currents and (e) electric field distributions in the x-y plane of the SSR.
The transmission spectrum of the SSR-containing metasurface was calculated in the frequency range of 0.7–2 THz. The transmission spectrum exhibited clear symmetrical characteristics, with a trough at 1.38 THz, as shown in Fig. 1(c). To better understand the resonance mechanism, the corresponding distributions of the surface current and electric field in the x-y plane at a frequency of 1.38 THz are illustrated in Fig. 1(d) and (e), respectively. At the resonance point, the direction of the surface current on both sides of the SSR pointed toward the + y axis, forming an electric dipole (ED) resonance. The oscillating charges gathered at the opening of the SSR, forming an ED moment pointing towards the + y axis (Fig. 1(e)).
The fano resonance was caused by electromagnetic near-field coupling between bright and dark modes, and this coupling could be controlled by symmetry breaking of the structure or the size of the gap between the structural subunits. We translated the position of the opening above the SSR to the right by x µm to achieve broken symmetry. Figure 2(a) shows the transmission spectrum obtained after shifting the gap to the right by x µm. The degree of asymmetry of the asymmetric split-ring (ASR) could be expressed by the dimensionless parameter, a, defined as a = (l1-l2)/(l1 + l2) × 100%. As shown by x = 4 µm in Fig. 2(a), the change of the split-ring from symmetry to asymmetry produced two extremely weak Fano resonances, at which point the degree of asymmetry of the ASR was 8%. The intensity and linewidth of the two fano resonances gradually increased as x increased, that is, the degree of asymmetry increased. The intensity and linewidth of the fano resonance were positively correlated with the asymmetry of the structure. To verify the reason for the formation of the fano resonances, we calculated the scattered power of the multipole based on the surface current strength of the structure and analyzed the contribution of the multipole. Figure 2(b) shows the magnitude of the far-field scattering power of the SSR. The ED rose sharply and always dominated, followed by the energy of the circumpolar moment toroidal dipole (TD). The energies of the magnetic quadrupole (MQ), electric quadrupole (EQ), and MD were negligible with respect to the ED. Figure 2(c) shows the magnitude of the far-field scattering power of the ASR, respectively. At point A, as the ED energy was suppressed by the MD and the MD energy increased sharply, the electromagnetic field was well confined by the MD within the metamaterial structure; thus, point A was dominated by the MD resonance. Nevertheless, the ED was dominant at points B and C, the EQ energy increased, and the MQ energy tapered. The ED resonance (bright mode) excited the MD resonance, that is, the dark mode, by means of near-field coupling. The dark mode, in turn, counteracted the bright mode, and the interference phenomenon between them led to the generation of the fano resonance..
Fig. 2. (a) Transmission spectra of the proposed metasurface at different values of x. (b) Calculated far-field scattering power of different multipoles of SSR. (c) Calculated far-field scattering power of different multipoles of ASR.
3. Fano resonance metasurface for pesticide detection
When x = 16 µm, the fano resonance generated by this metasurface was the strongest. Therefore, we chose the metasurface structure with x = 16 µm as the platform to detect ACE. Figure 3(a) shows the experimental fabrication process for the proposed metasurface. A 5-µm thick PI film (Fig. 3(a-II) was first spin-coated onto a cleaned 300-µm thick quartz substrate (1.5 cm × 1.5 cm, Fig. 3(a-I) and baked for 5 h. Next, two layers of photoresist were spin-coated on the PI film. The coated photoresist substrate was aligned with the mask plate on the photolithography machine and then exposed to UV light. Subsequently, the substrate was placed in a magnetron sputtering system to complete the metal vapor deposition, thus completing the drawing and preparation of 0.2-µm thick Al structural units (Fig. 3(a-III). Figure 3(b-I) shows an optical microscopy image of the sensor that generally meets the experimental requirements. Figures 3(b-II) and 3(c) show a schematic of the Al structural unit and a sensing schematic of the sensor, respectively. We observed the surface morphology of FaMs-AgNPs by scanning electron microscope (SEM), as shown in Fig. 3(d). The distribution of AgNPs solution on the proposed metasurface sensor is dense and non-uniform. The structural design of the proposed FaMs sensor was simple and easy to process, and had important applications in chemical biosensing and other fields.
Fig. 3. Fabrication and characterization of the designed metamaterial. (a) Manufacturing process. I: Quartz substrates. II: Spin-coated PI films on quartz substrates. III: Processing of metal structures on the PI film. (b) Enlarged diagram of the metamaterial structure under an optical microscope. (c) Sensing schematic of the sensor. (d) Silver nanoparticles observed under scanning electron microscopy.
The FaMs sensor (without AgNPs) and the FaMs-AgNP sensor were used as platforms for detecting different concentrations of ACE solutions and validated the sensing effect of different contents of AgNPs on the proposed sensors. To ensure the reliability of the experimental data as well as to prevent the THz waves from being absorbed by water vapor, the entire measurement environment was maintained dry at all times. The experimental data provided in this study were the average values of three tests. The concentrations of ACE detected in the experiment are presented in Table 1. Figure 4(a) depicts the sensing effect of the FaMs sensor without AgNPs for detecting different concentrations of ACE. The transmission spectra at all concentrations did not differ significantly from those without ACE. The results indicated that the FaMs without the AgNPs sensor did not yet have the ability to detect picogram-level concentrations of ACE. To show more clearly the ability of the proposed sensors to detect different concentrations of ACE, the amplitude variation, ΔT, of the sensor at different concentrations, C1–C9, was defined as ΔT = | (TCc – TC0) |%, where TCc (TC0) denotes the transmittance at the resonance point with (without) analyte. As shown in Fig. 4(b), the change in ΔT with ACE concentration was not significant, and realizing low concentration and high sensitivity detection of ACE was impossible.
Fig. 4. (a) Transmission spectra of the FaMs sensor for various concentrations of ACE from C1 to C9. (b) Amplitude variations of the FaMs sensor at resonant points and at different solution concentrations. (c) Transmission spectra of the FaMs-AgNPs sensor for various concentrations of ACE from C1 to C5. (d) Amplitude variations of the FaMs-AgNPs sensor at resonant points and at different solution concentrations.
Table 1. Concentrations of ACE Measured by the Experiment
To further improve the sensing effect of the FaMs sensor, we combined AgNPs with the FaMs sensor and investigated the effect of four different contents of AgNPs, 10 µl, 20 µl, 30 µl, and 40 µl, on the detection of ACE. Figures 4(c) and (d) show the transmittance curves and the variation of amplitude ΔT of the AgNPs-FaMs sensor in detecting different concentrations of ACE when the content of AgNPs is 10 µl, respectively. When the concentration of ACE increased from 100 pg/ml (C1) to 3.2 ng/ml (C4), ΔT gradually increased. When the concentration of ACE continued to increase to 4.2 ng/ml (C5), the ΔT of the AgNPs-FaMs sensor barely changed, that is, the effect of ACE on the sensor reached saturation. Because AgNPs could generate charge accumulation and induce local electric field enhancement through the tip effect, the ability of the FaMs-AgNPs sensor to detect ACE was significantly enhanced compared with that of the FaMs sensor.
To analyse the effect of AgNPs on the fano resonance metasurface, we randomly distributed a certain amount of AgNPs on the metasurface and calculated the electric field distribution on the metasurface. Figure 5(a)–(c) represent the electric field distributions at f1, f2 and f3 of the metasurface without AgNPs, respectively. Figure 5(d)–(f) represent the electric field distributions at f1, f2 and f3 of the metasurface with AgNPs, respectively. We can find that the electric field strength of the metasurface containing AgNPs is significantly higher than that of the metasurface without AgNPs. This is due to the fact that metal nanoparticles can generate charge accumulation, induce local electric field enhancement owing to the tip effect, and enhance the interaction between the THz wave and the analyte, thus improving the sensitivity of the THz metasurface sensor.
Fig. 5. (a)–(c). The electric field distributions at f1, f2 and f3 of the metasurface without AgNPs are shown, respectively. (d)–(f) The electric field distributions at f1, f2 and f3 of the metasurface with AgNPs are shown, respectively.
To verify whether the content of AgNPs had any effect on the sensing effect of the sensors, we verified the detection effect of the AgNPs-FaMs sensor on different concentrations of ACE when the content of AgNPs was 20 µl and 30 µl. Figures 6(a) and (b) show the transmittance curves and the variation of amplitude ΔT of the AgNPs-FaMs sensor in detecting different concentrations of ACE when the content of AgNPs is 20 µl, respectively. When the concentration of ACE increased from 100 pg/ml (C1) to 3.2 ng/ml (C4), ΔT gradually increased. Compared to the AgNPs at 10 µl, the sensor with 20 µl of AgNPs showed a more pronounced change in the ΔT of the sensor when detecting the same concentration of ACE. When the concentration of ACE continued to increase to 5.2 ng/ml (C6), the ΔT of the AgNPs-FaMs sensor barely changed, that is, the effect of ACE on the sensor was similarly saturated. When the content of AgNPs is 30 µl, the transmittance curves and the variation of amplitude ΔT of the AgNPs-FaMs sensor in detecting different concentrations of ACE are shown in Fig. 6(c) and (d), respectively. As the concentration of ACE increased, the ΔT of both f1 and f3 changed significantly; ΔTmax = 24.8% that was higher than the level of ΔT of the modulation depth of similar sensors. When the concentration of ACE was increased from 9.2 ng/ml (C8) to 10.2 ng/ml (C9), ΔT remained essentially unchanged. The effect of the ACE molecule on the AgNPs-FaMs sensor was again saturated, that is, the coupling effect between the analyte and the AgNPs-FaMs sensor was saturated. Similar analyte concentrations saturation has also appeared in other studies [10,26,27]. The ability of the 30 µl AgNPs-FaMs sensor to detect ACEs was significantly enhanced compared to the 10 µl and 20 µl AgNPs-FaMs sensors, respectively, with the detection range extended to 10.2 ng/ml and modulation depth ΔT reaching a maximum value. These results indicated that higher contents of AgNPs could generate more charge accumulation through the tip effect and induce the local electric field to reach its strongest value, thus enhancing the interaction between the THz wave and the analyte and improving the sensitivity of the sensor as much as possible. However, this does not mean that the content of this AgNPs continues to increase and the corresponding sensing effect will be better. Figures 6(e) and (f) depict the sensing effect of the FaMs sensor containing 40 μl of AgNPs solution for detecting different concentrations of ACE. In the absence of ACE, the transmittance of the sensor containing 40 μl of AgNPs solution is too small especially at the resonance point f1, and even if the concentration of ACE is increased, the change of the amplitude of the sensor is still insignificant as shown in Fig. 6(f). This is because the resonance unit of the metasurface in addition to the generation of fano resonance, due to the addition of AgNPs, the surface of the proposed sensor will also generate a localized surface plasma resonance [28–30], the content of AgNPs increased from 30 μl to 40 μl, the transmission loss of the proposed sensor continues to increase, the transmittance continues to decrease, thus, the sensing effect of sensor will be affected. Therefore, we must fully consider the effect of the content of AgNPs on the sensing of the sensor in practical applications.
Fig. 6. (a) Transmission spectra and (b) ΔT of the FaMs sensor-integrated 20 µl of AgNPs for various concentrations of ACE from C1 to C6. (c) Transmission spectra and (d) ΔT of the FaMs sensor-integrated 30 µl of AgNPs for various concentrations of ACE from C1 to C9. (e) Transmission spectra and (f) ΔT of the FaMs sensor-integrated 40 µl of AgNPs for various concentrations of ACE from C1 to C8.
4. Conclusion
In this study, we proposed a fano resonance metasurface integrated with a AgNPs sensor that could detect picogram-level ACE. AgNPs could enhance the sensitivity of the sensor by generating charge accumulation and inducing localized electric field enhancement through the tip effect, thereby enhancing the interaction between THz waves and analytes. We investigated the effects of four different contents of AgNPs, 10 µl, 20 µl, 30 µl and 40 µl, on the detection of acetamiprid. At 30 µl of AgNPs, the amplitude change of the FaMs-AgNPs sensor was more pronounced and the sensitivity was higher, which could detect acetamiprid solutions as low as 100 pg/ml. However, the proposed sensors could only quantitatively detect analytes and could not achieve analyte-specific detection. The biological samples detected in the experiments were single substances after processing. However, in practical applications, many types of biological substances exist, and continued research is required to improve the reliability of detection and specific identification. Despite this, the proposed sensors still have the advantages of a simple structure, easy processing, and excellent sensing performance, and have great potential for application in areas such as THz trace detection.
Funding
Medical Special Cultivation Project of Anhui University of Science and Technology (YZ2023H2B014); funding from the Qingchuang Science and Technology Plan of Shandong Universities (2019KJN001); National Key Research and Development Program of China (2017YFA0700202, 2017YFB1401203); Natural Science Foundation of Shandong Province (ZR2020FK008, ZR202102180769, ZR2021MF014, ZR2022QF054); Special Funding of the Taishan Scholar Project (tsqn201909150); National Natural Science Foundation of China (61675147, 61701434, 61735010, 62201496).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time, but may be obtained from the authors upon reasonable request.
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