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Abstract
Toroidal dipole resonance can significantly reduce radiation loss of materials, potentially improving sensor sensitivity. Generally, toroidal dipole response is suppressed by electric and magnetic dipoles in natural materials, making it difficult to observe experimentally. However, as 2D metamaterials, metasurfaces can weaken the electric and magnetic dipole, enhancing toroidal dipole response. Here, we propose a new graphene-integrated toroidal resonance metasurface as an ultra-sensitive chemical sensor, capable of qualitative detection of chlorothalonil in the terahertz region, down to a detection limit of 100 pg/mL. Our results demonstrate graphene-integrated toroidal resonance metasurfaces as a promising basis for ultra-sensitive, qualitative detection in chemical and biological sensing.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Terahertz (THz) waves are located in the transition region between photonics and electronics, and have many excellent properties, such as high penetration, low energy, and a characteristic fingerprint [1–3]. Correspondingly, terahertz spectroscopy also has many distinctive advantages when used as a sensing technique, such as providing non-destructive and label-free detection [4,5]. In addition, such a technique can reveal rich information on intramolecular and intermolecular vibrations [6]. However, in some cases, since the wavelength of the THz wave does not match the size of the analyte molecule, the interaction between the THz wave and analyte is weak [7], making it difficult to achieve a picogram-level detection. Thus, a novel method to enhance the sensitivity of THz sensors would greatly benefit the optics community.
The toroidal dipole is the third family of electromagnetic multipoles, first proposed by Zel'dovich (1958) to solve the parity breaking of weak interactions in atomic and nuclear physics [8]. By a circular head-to-tail arrangement of magnetic dipoles, all compressed into a single point, called a toroidal dipole [9]. The toroidal dipole resonance can significantly reduce radiation loss of resonators, leading to a high Q-factor [10–12]. Thus, it could provide an excellent basis for enhancing the sensitivity of a sensor [13]. However, the excitation of a toroidal dipole is often accompanied by electric and magnetic multipoles. Because the toroidal dipoles response is much weaker than that of electric and magnetic dipoles, the toroidal dipole response is obscured in natural materials [14,15], making it difficult to experimentally observe. To observe the response of toroidal dipoles, the excitation of electric and magnetic dipoles must be suppressed, but this is difficult to achieve in natural media [16,17]. This can be accomplished by carefully designing the artificial subwavelength unit cell of metasurfaces, effectively suppressing electric and the magnetic dipoles and quadrupoles [9]. As a result, enhancement of the toroidal dipole response occurs in the metasurface [18], appearing as a subwavelength-scale, artificially designed medium with unusual optical properties [19,20]. Metasurfaces have recently been used as high-sensitivity biochemical sensors [21–23], but further enhancement in sensitivity has reached a problem on picogram-level detection.
Here, we present graphene-integrated toroidal dipole resonance metasurfaces (Gr-TDRMFs) used for the ultra-sensitive qualitative detection of chlorothalonil in the terahertz region, down to a detection limit of 100 pg/mL. This was achieved by jointly exploiting the unique electromagnetic properties of toroidal dipole resonance, together with the fact that the initial Fermi level (EF) of triple-layered chemical vapor deposited (CVD) graphene deviates slightly from the Dirac point. This method has strong potential for use in biological and chemical sensing.
2. Experimental details
Figure 1(a) shows an illustration of the Gr-TDRMF sample, composed of complementary structures of multi-split elliptical rings. From top surface to bottom substrate, the Gr-TDRMFs include three layers of graphene, a 2 µm-thick polyimide (PI) film, 0.2 µm-thick gold structure, a 10 µm-thick PI film, and a quartz glass (JGS1). In the experiment, we use a Gr-TDRMF sample as a sensor, qualitatively detecting the concentration of the chlorothalonil by measuring changes in the amplitude of the THz reflectance spectrum. As shown in Fig. 1(b), the toroidal dipolar response is from a meta-atom. Our metasurface design is based on one I-shaped resonant cavity and two crescent cavities, surrounded by a gold film. By controlling the distance between the cavities and the polarization of the terahertz waves, it is possible to excite oscillating currents in opposite directions. These currents are induced on either side of the resonant cavities with x-axis symmetry, suppressing electric dipoles and enhancing the toroidal dipole response [9]. This enhancement of the toroidal dipole can be demonstrated by the current density and magnetic field induced in the Gr-TDRMFs, as displayed in Fig. 1(c)and Fig. 1(d).
Fig. 1. Illustration of Gr-TDRMFs. a) Illustration of a Gr-TDRMF used to detect the chlorothalonil molecule. b) Relationship between toroidal dipole and circulating magnetic field generated by a current-carrying loop. c) Surface currents at 1.27 THz. d) H-field at 1.27 THz.
3. Results and discussion
Figure 2(a) shows a microscope image of toroidal dipole resonance metasurfaces (TDRMFs). The structure is fabricated using conventional photolithography, and the inset shows parameters of a meta-atom. After sample preparation, the graphene was carefully transferred onto the top surface of the metasurface, as shown in Fig. 2(b), with the inset showing the Raman spectra of the three-layer graphene on the silicon oxide substrate. We can determine the quality of graphene using spectroscopic characterization. The inset shows a weak peak, D, at ∼1355 cm−1, and two more peaks, G at 1580 cm−1 and 2D at 2688 cm−1. The 2D peak has a full width at half-maximum of ∼58 cm−1, and an intensity lower than that of the G peak. Based on these values, we consider the graphene to be of high quality [24,25]. Figure 2(c) and 2(d) show illustrations of TDRMFs and the Gr-TDRMFs, respectively. Figure 2(e) and 2(f) show experimental and simulated reflectance spectra, respectively, with good agreement in the two cases. However, there is a difference in the amplitude of the reflectance spectra between TDRMFs and the Gr-TDRMFs. There are two reasons for this. Firstly, the toroidal dipolar response has a high Q-factor, and is very sensitive to changes in the dielectric environment. The second reason is because the inevitable influence of impurities, defects, and disorders make CVD graphene effectively p-doped, with the EF in the valence band, close to the Dirac point [26]. In other words, the conductivity of CVD graphene is not zero, which can cause significant changes in the dielectric environment of TDRMFs, leading to inhibition of the toroidal dipolar response. This causes a decrease in amplitude of the reflectance spectra in Gr-TDRMFs compared with the TDRMFs.
Fig. 2. a) Microscope image of an experimental TDRMFs, with parameters of a unit cell inset, the width of the elliptical ring is 3 microns. b) Microscope image of an experimental Gr-TDRMFs, with a Raman spectrum of the triple-layered graphene on the silicon oxide substrate inset. c) Illustration of a simulated TDRMFs. d) Illustration of a simulated Gr-TDRMFs. e) Reflectance spectra of the TDRMFs, f) Reflectance spectra of the Gr-TDRMFs. g) Reflectance spectra of the TDRMFs as the loss tangent of an analyte changes from 0 to 0.05. h) Reflectance spectra of the Gr-TDRMFs with the Fermi level of graphene changing from 0 to 0.12 eV.
To elucidate the sensing mechanism based on the amplitude change of the reflection spectrum, we add a 500 nm layer of analyte to the metasurface and study the change in amplitude by changing the loss tangent of the analyte. Considering that, in our study, the analyte concentrations detected are in the order of picograms, the value of loss tangent was taken very small. As shown in Fig. 2 g, the change in amplitude is almost indistinguishable as loss tangent changes from 0 to 0.05, meaning that picogram-scale detection can be challenging on metasurfaces. In Gr-TDRMFs, we evaluate the sensing performance by observing the regularity of amplitude variation caused by the conductivity of graphene. The change in the dielectric environment is mainly due to the conductivity of the graphene. Figure 2 h shows variation in the reflectance spectrum of the bare Gr-TDRMFs as the graphene Fermi level changes from 0 to 0.12 eV. The amplitude of the reflection spectrum gradually drops with increasing Fermi levels. This suggests that Gr-TDRMFs may have higher sensitivity in picogram-scale detection. We apply the TDRMFs as a platform for the qualitative detection of chlorothalonil. The pesticide solutions with six concentrations of 100 pg/mL, 200 pg/mL, 300 pg/mL, 1 ng/mL, 12 ng/mL and 22 ng/mL were formulated by dissolving chlorothalonil into double distilled water. Then, the pesticide solution of 25 µL was dropped on the top surface of TDRMFs and dried in the air. In the measurement, we used the commercial THz TDS system (TAS7400TS, Advantest Cop.) with a high resolution (1.9 GHz) and fast (16 ms) THz scan to collect experimental data. The experimental environment is under a humidity below 10% and a temperature of 297 ± 1 K. Figure 3(a) shows the experimental reflectance spectra at concentrations from 100 to 300 pg/mL. Compared with the bare TDRMFs, the change in reflectance spectra under all concentrations is not obvious. The results demonstrate that picogram-scale detection is challenging with TDRMFs. How- ever as the chlorothalonil concentrations increase to 1 ng/mL, the change in reflectance spectra significantly reduces, as shown in Fig. 3(b). As the concentration continues to increase, further change in the reflectance spectrum is lesspronounced. To clarify the sensing performance clearly, defined as ΔR = (RCc–RBare)/RBare×100%, where RCc(RBare) is the reflectivity at the peak with (without) chlorothalonil. From Fig. 3(a), ΔR for the concentration of 100 pg/mL is 1%, and ΔR for concentrations of 200 and 300 pg/mL are 1.1% and 0.98%, respectively. These values further demonstrate that metasurfaces are challenging to probe on the picogram scale. However, ΔR for the concentration at 1 ng/mL achieves 5%, resulting in an obvious sensing effect. With an increase in the concentration, the values of ΔR become even greater. The limit of detection is close to 1 ng/mL. The sensitivity of this biosensor shows great improvement compared with sensors of previous works (see Fig. 3(d)) [27–30]. In summary, we can find the TDRMFs have great potential in terms of sensitivity enhancement.
Fig. 3. Sensing performance of the toroidal dipolar response metasurface. a)–b) Experimental reflectance spectra. c) The ΔR calculated from a)-b). d) Comparison with previous studies.
To achieve picogram-scale detection, the Gr-TDRMFs were used as a basis for the qualitative detection of chlorothalonil. For ease of comparison, the concentration used with the Gr-TDRMFs and TDRMFs is the same. In detection, graphene is in contact with chlorothalonil molecular. The change in the dielectric environment is mainly due to the doping effect of chlorothalonil on the graphene, observed by reflectance spectra. Figure 4(a) shows experimental reflectance spectra with chlorothalonil concentrations from 100 to 300 pg/mL. Compared with the bare TDRMFs, the reflectance spectra are enhanced significantly when the concentration of chlorothalonil is 100 pg/mL, facilitating qualitative sensing of chlorothalonil. When the concentration of chlorothalonil is increased to 200 and 300 pg/mL, the reflectance is reduced, almost coinciding with that of bare Gr-TDRMFs. However, as shown in Fig.4a, as concentration is increased from 1 to 22 ng/mL, the reflectance spectra are significantly increased. As shown in Fig. 4(c), ΔR is 15% at 100 pg/mL, 0.8% at 200 pg/mL, and 2% at 300 pg/mL. We find that Gr-TDRMFs have the potential to achieve picogram-scale detection.
Fig. 4. Sensing performance of the toroidal dipolar response metasurface. a)–b) Experimental reflectance spectra. c) The ΔR calculated from a)-b). d) Comparison with previous studies.
In addition, with the increase of concentration from 1 to 22 ng/mL, ΔR has a greater value, for example, ΔR = 45% at 12 ng/mL. These results demonstrate that the Gr-TDRMFs have greatly improved sensing performance compared with previous studies [7,26,31,32] (see Fig. 4(d)). The limit of detection of Gr-TDRMFs is close to 100 pg/mL. In addition, there are two abnormal sensing phenomena. First, the amplitude of reflectance spectra in sensing is nonlinear, causing the reflectance spectra at 100 pg/mL to be significantly higher than those of the bare Gr-TDRMFs. As concentrations continue to increase to 300 pg/mL, the amplitude decreases. Second, ΔR is greatly increased at nanogram concentrations. These anomalous sensing phenomena are due to changes in the Fermi level of CVD graphene. Figure 5(a)–5(c) show the illustrations of sensing metasurfaces with concentrations from 0 to 200 pg/mL. As mentioned previously, the EF of p-doped CVD graphene is in the valence band and quite close to the Dirac point, causing the conductivity of CVD graphene to be non-zero, which leads to significant changes in the dielectric environment of TDRMFs. This results in inhibition of the toroidal dipolar response. Therefore, the reflectance spectra of Gr-TDRMFs have decreased amplitude compared with the TDRMFs, as shown in Fig. 5(d). When the 100 pg/mL chlorothalonil and the graphene covalently bond, the EF shifts to the Dirac point, as shown in Fig. 5(e). As a result, the conductivity decreases. This results in the observed marked enhancement of the toroidal dipolar response, and the increased amplitude of the reflectance spectra. However, when the concentration is 200 pg/mL, the EF shifts to the conduction band (see Fig. 5(f)), enhancing the conductivity and suppressing the toroidal dipolar response. Therefore, the reflectance spectra of the 200 pg/mL are lower than those of the 100 pg/mL and almost coincide with bare. The internal mechanism causing this is that the Fermi level of graphene moves to the Dirac point due to the chlorothalonil, causing a significant change in the dielectric environment.
Fig. 5. The mechanisms of sensing performance based on the Gr-TDRMFs. a)–c) Illustrations of of sensing metasurfaces with different concentrations of adsorbate. d)–f) The Fermi levels of graphene at different concentrations. g)–i) Current distributions. g)–l) Magnetic field distributions.
To give an in-depth understanding of the electromagnetic behavior, Fig. 5(g)–5 l shows the current and magnetic field distributions at their respective resonant frequencies and illustrates the influence of toroidal dipoles by the conductivity of graphene. Where the toroidal dipole is weak, Gr-TDRMFs have no chlorothalonil owing to the suppressed current and magnetic field. When the concentration is 100 pg/mL, the conductivity decreases, toroidal dipole increases in strength, and the current and magnetic field are enhanced. At a concentration of 200 pg/mL, the conductivity increases and the current and magnetic field are intensely suppressed, resulting in a weak toroidal dipole. For nanogram-scale detection (see Fig. 4(b)), the likely reason for the reduced electrical conductivity of graphene is the effect of the massive accumulation of chlorothalonil on the graphene surface [33].
4. Conclusion
We demonstrate that novel graphene-integrated toroidal dipole resonance metasurfaces (Gr-TDRMFs) can be employed as sensors for the ultra-sensitive and qualitative detection of chlorothalonil, using changes in amplitude of reflectance spectra. The Gr-TDRMF sensor is capable of picogram-level detection, with a level of sensitivity higher than that reported by previously published studies. This is accomplished via the unique electromagnetic properties of toroidal dipole resonances and the EF of the triple-layered CVD graphene, which deviates slightly from the Dirac point. This sensor has a detection limit of 100 pg/mL, achieved by observing the changing of reflectance spectra. These results show that Gr-TDRMFs are a promising basis for ultra-sensitive and qualitative detection in chemical and biological sensing.
Funding
National Natural Science Foundation of China (61675147, 61701434, 61735010); The University Synergy Innovation Program of Anhui Province (GXXT-2021-026); The High-level Talents Research Start-up Funding Project of West Anhui University (WGKQ2022007); Special Funding of the Taishan Scholar Project (tsqn201909150); Natural Science Foundation of Shandong Province (ZR2020FK008); Natural ScienceFoundation of Shandong Province (ZR202102180769); Natural Science Foundation of Jiangsu Province (SBK2022042453).
Acknowledgments
We thank Mark Hammonds PhD, from Liwen Bianji (Edanz) (www.liwenbianji.cn) for editing the English text of a draft of this manuscript.
Disclosures
The authors declare no conflicts of interest.
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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