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Abstract
The rapid and sensitive detection of plant-growth-regulator (PGR) residue is essential for ensuring food safety for consumers. However, there are many disadvantages in current approaches to detecting PGR residue. In this paper, we demonstrate a highly sensitive PGR detection method by using terahertz time-domain spectroscopy combined with metamaterials. We propose a double formant metamaterial resonator based on a split-ring structure with titanium-gold nanostructure. The metamaterial resonator is a split-ring structure composed of a titanium-gold nanostructure based on polyimide film as the substrate. Also, terahertz spectral response and electric field distribution of metamaterials under different analyte thickness and refractive index were investigated. The simulation results showed that the theoretical sensitivity of resonance peak 1 and peak 2 of the refractive index sensor based on our designed metamaterial resonator approaches 780 and 720 gigahertz per refractive index unit (GHz/RIU), respectively. In experiments, a rapid solution analysis platform based on the double formant metamaterial resonator was set up and PGR residues in aqueous solution were directly and rapidly detected through terahertz time-domain spectroscopy. The results showed that metamaterials can successfully detect butylhydrazine and N-N diglycine at a concentration as low as 0.05 mg/L. This study paves a new way for sensitive, rapid, low-cost detection of PGRs. It also means that the double formant metamaterial resonator has significant potential for other applications in terahertz sensing.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Plant growth regulators (PGRs) are natural hormones and synthetic hormone analogs that can be used to regulate the physiology and metabolism of crops. They have been widely used in fruit production to increase yield and improve quality [1–2]. At low concentrations, PGRs can affect cell division, cell expansion, and cell structure and function, and can also limit environmental stress [3]. Although PGRs are often applied to agricultural plants, their excessive application can cause fruits and vegetables to retain PGR residues, which are potentially toxic to humans [4]. To ensure food safety for consumers, PGR residues in food products need to be detected before the products are sold. Several advances have been made in the detection and identification of PGRs over recent decades using conventional analytical methods such as thin-layer chromatography, capillary electrophoresis, gas chromatography, high-performance liquid chromatography, enzyme-linked immunosorbent assays, and immune-affinity column assays [5–7]. However, these methods are time-consuming and require complex sample-pretreatment steps. It is therefore desirable to develop novel techniques for rapid and sensitive detection of PGR residues in routine assays.
Terahertz (THz) spectroscopy provides numerous unique advantages, including non-destruction, non-ionization, and molecular fingerprints for sensing applications [8–9]. However, because of mismatch between THz wavelengths and the sizes of the target molecules, the sensitivity for recognizing trace amounts of analytes is quite limited for free-space THz radiation. Therefore, terahertz time-domain spectroscopy (THz-TDS) combined with metamaterials has been developed to boost the interaction of matter and THz radiation by local electric-field enhancement, thus improving the sensitivity. Metamaterials are artificially defined periodic structures that simultaneously exhibit negative permittivity and permeability [10–11]. By appropriate design, they have been manipulated for various applications in the microwave and millimeter wave domain, especially in the terahertz band [12–13]. In recent years, metamaterials have been created to artificially and independently adjust their electrical and magnetic responses to external electric fields. Man-made materials with specialized properties can overcome some of the shortcomings of natural materials and have attracted the attention of researchers exploring terahertz sensors [14]. Metamaterials have an array of unusual properties, such as being thermal emitters, acting as imaging systems, having negative refraction, acting as perfect lens, and having a cloak of invisibility [15–19]. Because of requirements in practical applications, more and more attention has been paid to achieving both broadband and multi-band absorption.
In recent years, metamaterials based on different material systems have drawn increased attention. Qin et al. used metamaterials consisting of an array of circular ring apertures deposited in a metal film to detect trace amounts of TCH. The results showed that the metamaterial was about 105 times more effective in measuring TCH on a silicon substance. However, the detection range of the metamaterial was only 0.2-0.8 THz [20]. Liu et al. proposed an optically transparent metamaterial absorber with ultra-broadband absorption properties having a composite resonant structure with different dielectric layers, which could achieve 22.3-GHz-wide high-absorption [21]. Lee et al. found that single-stranded deoxyribonucleic acids (ss DNAs) at very low concentrations could be detected using graphene-combined nano-slot-based THz resonance. In addition, quantitative analysis of ss-DNA-molecule adsorption was carried out based on a change in conductivity, using a theoretical THz transmission model [22].
Research on metamaterials has followed the triangle “structure-component-function” relation. The response characteristics of different bands and different physical properties can be obtained through structural design and size adjustments. Split-ring resonators and complementary split ring resonators are the two most exploited topologies of metamaterials, and have been utilized for synthesis of various structures/components, such as radio-frequency antennas [23], sensors [24], filters [25], and absorbers [26–27]. Recently, Wu et al. presented a novel broadband microwave absorber, in which the upper metal layer consists of a novel double-split-ring resonator and a central octagonal ring pattern. The results show that absorption exceeds 90% between 5.7 and 13.1 GHz, and that the relative bandwidth of this structure reaches 78.1% [28]. Mehdi Aslinezhad theoretically and numerically investigated a perfect metamaterial absorber consisting of a semiconductor ring resonator, a semiconductor film, and a quartz dielectric that sandwiched between the ring and the semiconductor film. The results showed a high refractive index, high temperature sensitivity, and ultra-narrow bandwidth in the terahertz range [29]. Although this design can obtain high refractive index, high absorption, and small size, its absorption frequency band is narrow and it has only one absorption peak, which greatly limits its practical application. It remains challenging to achieve specific detection of highly absorptive liquid samples with high sensitivity in the THz range.
In this study, we demonstrate a highly sensitive method for detecting plant growth regulators using terahertz time-domain spectroscopy combined with metamaterials. We propose a double-formant metamaterial resonator with titanium-gold nanostructure, and we have investigated the THz spectral response and electric-field distribution of metamaterials under different analyte thickness and refractive indexes. Based on the sensitivity characteristics, a rapid-solution analysis platform based on the double formant metamaterial resonator was set up and the PGRs in aqueous solution were directly and rapidly detected through THz-TDS spectroscopy. Specifically, the results show that metamaterials can successfully detect butylhydrazine and N-N diglycine at a concentration as low as 0.05 mg/L. This study paves the way for practical sensitive detection of these compounds. Metamaterials combined with THz-TDS therefore offer a potential new approach for food quality and safety control.
2. Design and method
To achieve a high absorbing efficiency at multiple THz frequencies, a metamaterial-based split-ring terahertz resonator was designed. The metamaterial structure is shown in Fig. 1, with Fig. 1(a) showing a perspective view of the layout of one unit cell of the split-ring resonator. Basically, there are two circular rings structures with four gaps removed. Figures 1(b) and (c) show, respectively, the top and side views of the absorber. As can be seen, it consists of three layers: polyimide-titanium-gold. The detailed structural parameters are as follows: l = 130 μm, t = 14 μm, D1 = 120 μm, D2 = 100 μm, d1 = 52 μm, d2 = 40 μm, and g = 10 μm (see Fig. 1 for definitions of these dimensions). The thicknesses of the Au, Ti, and polyimide films are 0.2, 10, and 14 μm, respectively. The total size of the metamaterial used in the experiment is about 10 mm × 10 mm.
Fig. 1. (a)-(c) Schematic diagrams of a unit cell. (d) Partial image of the design metamaterial. (e) The principle of the double formant metamaterial resonator for PGR detection.
Numerical simulation of the metamaterial structure was carried out using CST Microwave Studio frequency domain solver with tetrahedral mesh [30]. The polyimide substrate was modeled as a lossless dielectric with dielectric permittivity ε = 3.5, the dielectric constant conductivity σ of Au was set to be 4.561 × 107 S/m, and the dielectric constant conductivity σ of Ti was set to be 2.38 × 106 S/m. The detailed processing steps and parameters are given in Appendix 1.
To demonstrate the potential of double formant metamaterial resonators to serve as sensors, the prepared plant-growth-regulator (PGR) liquid was dropped on the surface of the double formant metamaterial resonator. Butylhydrazine (>99%, BR) and N-N diglycine (>97%, BR) were purchased from Dalian Meilum Biotechnology Co., Ltd. and used without further purification. Liquid samples of 0, 0.05, 0.1, 1, 5, 100, 1000, and 2000 mg/L solution were prepared by dissolving 0.1g of PGRs in 50 ml pure water and then successively diluting. Subsequently, 50 μL of PGRs (butylhydrazine and N-N diglycine) were dropped on the surface of a metamaterial biosensor and dried before spectra collection (THz-TDS system, TAS7400SP, Advantest INC, Japan) with three duplicates. The effective frequency range that TAS7400SP can detect is 0.2–2.5 THz. The frequency resolution can be 7.9 GHz and 1.9 GHz. During our researches, we choose the frequency resolution 3.8 GHz. The principle of the double formant metamaterial resonator for PGRs detection is illustrated in Fig. 1(e). All measurements are performed in a nitrogen atmosphere at a temperature of 25 ± 1 °C and a relative humidity of less than 3%.
3. Results and discussion
In order to understand the underlying resonant mechanism of designed double formant metamaterial resonator structures, the transmission of metamaterial resonator was simulated and measured, respectively, seen from Fig. 2 (a). It is shown that the experimental and simulation results agree well except for slight difference in intensity, and the difference in intensity results from the system parameter error between simulation and experiment environment and the fabrication error in the experimental process (Appendix 2).
Fig. 2. (a) The transmission curves obtained by simulation (black line) and experiment (red line). (b)-(e) The corresponding electric field, magnetic field, and surface current distribution on top at the resonance frequencies of 1.25 and 2.01 THz.
To further analyze the physical mechanism of the double formant metamaterial resonator. We first investigated the electric energy density distribution in the plane of z=0μm and the surface current at two resonant frequencies. As shown in Fig. 2(a), the designed metamaterials structure shows multi-modal resonances in the THz frequency band of 0.3-2.1 THz, which including an asymmetric resonance dip at 1.25 THz, an asymmetric resonance dip at 2.01 THz, and a transmittance peak between the two resonance modes at 1.48 THz. The electric, magnetic field and the surface current are concentrated mainly at the inner circular ring and the outer circular ring, seen Fig. 2 (b)-(e). For 1.25 THz, the left and right symmetric subunits of the outer circular ring absorb much more energy than the other subunits (Fig. 2 (b)). Meanwhile, we observed Fano-like antiparallel currents at 1.25 THz (Fig. 2(c)). The results show that these subunits mainly contributes to the absorption of this frequency. Similarly, the up and down symmetric subunits of the outer circular ring absorb mainly contribute the absorptions of 2.01 THz. But the dipole-like parallel currents is mainly distributed in the whole outer circular ring. It is clear that the transmittance peak distributions of this absorber are distributed at a different part for different frequencies, which indicating that different bands and different physical properties can be obtained through the structural design and size adjustment.
In order to analyze the transmission of the double formant metamaterial resonator, we considered the influence of the analyte thickness and the refractive index on the sensitivity of the sensor. First, theoretical analysis of the designed metamaterial sensor was performed through simulations. An analyte layer was selected to add onto the metamaterial resonator structures. By changing the refractive index of the analyte with a thickness of 15 µm, the theoretical sensitivity, defined by the derivative of transparency frequency shift with respect to the refractive index, can be acquired. It can be concluded from Fig. 3(a) that when the refractive index varies from 1.0 to 1.9, resonant peak 1 redshifts from 1.20 to 1.01 THz and resonant peak 2 redshifts from 1.96 to 1.65 THz. Meanwhile, the corresponding frequency shift Δf, defined as the absolute value of the difference in peak frequencies on the curve of double formant metamaterial resonator with and without the analyte layer, was extracted. It is displayed in Fig. 3(c). A linear relationship of Δf and the refractive index n is seen in the graph. The sensitivity S of a sensor is usually defined as the shift in the resonance wavelength per unit change of the refractive index; it can be obtained by calculating the corresponding derivative of the Δf vs. n curve [31]. The theoretical sensitivities of the two resonance peaks of the refractive index sensor based on our designed metamaterials approaches 780 GHz/RIU and 720 GHz/RIU, respectively. These results are higher than the maximum theoretical sensitivity of 156.3 GHz/RIU by Zhang et al. [32]. Furthermore, the thickness of the analyte layer can also affect the frequency shift of metamaterial. From Fig. 3(b), it can be seen that when the thickness increases from 0 to 15 µm for a fixed refractive index of 1.9, the redshifts of resonant peaks 1 and 2 are improved from 54 to 270 GHz and from 72 to 384 GHz, respectively. In addition, it also shows that the effect of analyte-layer thickness on Δf appears to be nonlinear. The results show that with increasing analyte thickness, the redshift of the resonance is closer to x(0,0). This is more obvious in Fig. 3(d). We can see that with increasing analyte thickness, the sensitivity of the metasensor also increases. As we all know, Q (Quality factor) value and FOM (the figure of merit) are also used to quantity the sensitivity of the sensor, and FOM is related to the thickness of sensing medium layer [33–34]. Especially, Q = Fr/FWHM, FOM = S/FWHM, where Fr is the resonance frequency of peak and FWHM is the full width at half-maxima. High Q value and FOM make the resonance peak sharper, which makes it easier to observe the resonance peak shift. According to the formula, the max Q value of two peaks are 12.82 and 61.85 and the max FOM of two peaks are 8 and 22.15, which are higher than the results in Refs. [34–36]. The above results theoretically suggest that the terahertz spectroscopy technique combined with a metamaterial biosensor can quantitatively analyze trace substances. In addition, we find that the resonant peak 2 redshift is greater than the resonant peak 1 redshift. The main reason for this is that resonant peak 1 is caused by Fano-like antiparallel currents and resonant peak 2 is caused by dipole-like parallel currents. So the redshift of the resonance peak frequency is related not only to the thickness of the analyte but also to the structure of the metamaterial.
Next, we systematically changed structural parameters, including ring open or not open and varying the open ring width g of inside/outside ring, to study the influence of these parameters on the redshift of the resonance peak. The calculated results are shown in Fig. 4. As can be seen from Fig. 4(a), with an increase in the size of the split ring, the redshift of each resonance peak gradually increases. At the same time, the resonance redshift of the inner split ring is more evident than that of the outer split ring at resonant peak 1. At resonant peak 2, however, the transmittance of the metamaterial resonator with the inner split ring is relatively poor. These results show that the redshift of both resonance peaks are influenced by the split ring. It is further demonstrated that the sensitivity of metamaterials is related to their structural design. Figure 4(b) shows that as the width g of the open ring increases, the redshift of each resonance peak gradually moves to higher frequency. More important, as the width g increases, the redshift of each resonance peak has a slightly decrease trend. The results show that the redshift of the metamaterial sensor is related to its shape and size. It has previously been established that the response characteristics of different bands and different physical properties can be obtained through structural design, substrates and size adjustment [33,37,38].
Fig. 3. (a) Transmission vs. frequency for an analyte thickness of 15 μm and refractive indexes from 1.1 to 1.9. (b) Transmission vs. frequency for a fixed refractive index of 1.9 μm and analyte thicknesses from zero to 15 μm. (c) Frequency shift Δf as a function of refractive index of analyte as extracted from (a). (d) Frequency shift Δf as a function of analyte thickness as extracted from (b).
Fig. 4. The resonant peak of the proposed structure (a) for different split rings and (b) different open rings size.
Considering that sensitivity and linewidth change are desirable features of a biosensor platform, it is highly promising to consider a double formant metamaterial resonator for this application, given its advantages of high sensitivity, non-labelling, and low cost. To investigate the performance and sensitivity of double formant metamaterial resonators for biosensors, the PGRs butylhydrazine and N-N diglycine were selected as our analytes for tests. We dropped them on the surface of the double formant metamaterial resonator. Figures 5(a)-(d) indicate that, when the butylhydrazine concentration is higher, the resonance redshifts of the two peaks move, respectively, from 1.245 to 1.178 THz and from 1.904 to 1.747 THz. For resonance peak 1, the resonance frequency shift increases linearly with increase of the butylhydrazine concentration in the range 0 to 0.05 mg/L. When the concentration of butylhydrazine is in the range from 0.05 mg/L to 1000 mg/L, the resonance frequency shift changes very little. When the concentration of butylhydrazine is greater than 1000 mg/L, the resonance frequency shift is similar to that with a concentration of 0.05 mg/L. In our experiment, we calculate the sensitivity, defined as S = Δf /Δc [13], where Δc represents the change of PGR concentration, and we find that the maximum sensitivity approaches 109 GHz/mgL-1at 2000 mg/L, which means that butylhydrazine per milliliter would lead to a shift of 109 GHz in peak frequency. For resonance peak 2, the resonance frequency shift increases linearly with increase of the butylhydrazine concentration in the range from 0 to 0.05 mg/L. The redshift of resonance peak 2 is twice that of resonance peak 1—that is, a frequency shift Δf = 99.18 GHz. The main reason is that the transmittance of the two resonance peaks is different. When the concentration of butylhydrazine is in the range from 0.05 to 2000 mg/L, the resonance frequency of the peak 2 shift shows a trend of first slowly increasing, then decreasing, and then increasing rapidly. Additionally, the maximum sensitivity can approach 137 GHz/mg L-1 at 2000 mg/L. These results show only that the concentration of butylhydrazine determines the detection sensitivity of the metamaterial resonator, not that the detection sensitivity always increases as the concentration of the sample increases. Moreover, resonant peaks at 0.05 mg/L are 1.194 and 1.804 THz, which are evidently different from those at 0 mg/L. The resonant peaks frequency redshifts at 0.05 mg/L are 51.499 and 99.182 GHz. Therefore, the minimum detectable butylhydrazine concentration on the metamaterial is determined to be 0.05 mg/L, which is about 106 times enhancement compared to the solid sample of squash method for THz-TDS detection in our previous research [39].
Fig. 5. (a)-(c) The transmission of spectra of a double formant metamaterial resonator under different butylhydrazine concentrations from 0 to 2000 mg/L. (d) The resonant peaks shift Δf for different butylhydrazine concentrations. (e)-(g) The transmission of spectra of a double formant metamaterial resonator under different N-N diglycine concentrations from 0 to 2000 mg/L. (h) The resonant peaks shift Δf for different N-N diglycine concentrations.
As shown in Figs. 5(e)-(g), when the N-N diglycine concentration is higher, the resonance redshift of the two peaks move from 1.245 to 1.179 THz and from 1.904 to 1.747 THz, respectively. The overall trend of the results is that the resonance frequency shift increases with increasing N-N diglycine concentration, which is similar to Qin’s test results for carbendazim [40]. From Fig. 5(h), it is easy to see that, as the N-N diglycine concentration increases, the resonance frequency shift of resonance peak 1 shows a trend of first increasing rapidly, then slowly decreasing, and then increasing again. The maximum sensitivity approaches 67 GHz/mg L-1 at 2000 mg/L. For resonance peak 2, the resonance frequency shift first increases linearly and then decreases as the N-N diglycine concentration increases from 0 to 100 mg/L. At a concentration of 1 mg/L, the greatest resonance frequency shift is 156.43 GHz, so the maximum sensitivity can approach 156.43 GHz/mg L-1 at 1 mg/L. When the concentration of N-N diglycine is in the range from 100 to 2000 mg/L, the resonance frequency shift first increases and then decreases rapidly. In addition, resonant peaks at 0.05 mg/L are 1.186 and 1.835 THz, not the same as for 0 mg/L. That is, the resonant peaks redshifts at 0.05 mg/L are 59.128 and 68.665 GHz. Therefore, the detection limit of plant growth regulators achieved through the combination of metamaterials and terahertz is 0.05 mg/L, and the detection sensitivity is increased by 106 times compared with the tablet method [39].
In addition, we can see that the transmittance of the two PGR examples are different. The redshifts of the resonant peaks are different for different types of PGR at the same concentration, indicating that the metamaterial resonator can detect a variety of trace substances. We believe that the difference in response in our tests reflects a difference in the samples. This conclusion was also reached by Lin et al., who found that the concentration of the anti-CEA plays an important role in the response of the sensor [41].
4. Conclusion
We have demonstrated a highly sensitive method for detecting butylhydrazine and N-N diglycine by using terahertz time-domain spectroscopy (THz-TDS) combined with metamaterials. A double-formant metamaterial resonator based on a titanium-gold nanostructure was designed, analyzed, and fabricated. THz spectral response and electric field distribution of metamaterials under different analyte thicknesses and refractive indexes were investigated. The results of our simulations showed that the theoretical sensitivities of resonance peaks 1 and 2 of the refractive index sensor based on our designed metamaterial resonator approached 780 and 720 gigahertz per refractive index unit (GHz/RIU), respectively. Relative to previous metamaterial terahertz absorbers, our designed metamaterials have higher sensitivity and higher transparency. In experiments, a rapid-solution analysis platform based on the double formant metamaterial resonator was set up and showed its ability to directly and rapidly detect plant growth regulators (PGRs) in aqueous solutions through THz-TDS spectroscopy. Because of the sensitivity of the metamaterial resonator, it can successfully detect butylhydrazine or N-N diglycine at a concentration of 0.05 mg/L. The results demonstrate the feasibility of a sensor based on a sensitive metamaterial resonator to realize measurement of PGRs, and to achieve fast and low-cost detection of PGR residues.
Funding
National Natural Science Foundation of China (32071905, 61771224).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicy available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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