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Abstract
Terahertz (THz) metamaterials are widely used in biosensor devices due to their unique superiority, and the demand for new high sensitivity biosensors based on THz metamaterials is increasing. This paper presents a polarization-insensitive terahertz metamaterial sensor used for BSA detection. Simulation reveals that the peak of transmission spectrum shifts obviously when the sensor is covered with analytes of different refractive index and thickness. After the sensor is covered with 10 μm thick non-destructive analytes, its sensitivity is as high as 135 GHz/RIU. Experiments show that the lowest detectable concentration of BSA solutions by this sensor is 0.1 mg/mL, the peak red shift of the transmission spectrum reaches 137 GHz when the concentration is 17.6 mg/mL, and the frequency shift percentage is 16.4%. This study provides a highly sensitive solution for biosensor detection in the pharmaceutical and food fields.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Biological detection has always been a matter of human health. Efficient and fast biosensor detection technology is of urgent need. In daily life, albumin is an important indicator of cow health, milk and meat quality [1], so its measurement is necessary in medicine and food industries. BSA is the predominant protein in bovine serum and has a similar structure to human serum albumin (HSA), furthermore, it is widely available [1–3]. Hence, BSA is often used as an alternative protein to HSA model. BSA is very suitable for protein research on account of its cheap price, good water solubility, easy purification, good biodegradability, etc. [1,4]. Meanwhile, according to the previous work [5], there is no obvious transmission peak for BSA in the range of 0.05-1.2 THz near 30℃. At present, the commonly used methods for detecting protein include Biuret method, Lowry method and Bromocresol Green method [1,6–9]. Whereas, these methods have some limitations such as complex process, irreversible damage to proteins and long measurement time, and new type non-destructive and efficient biosensors for BSA are expected to be developed.
Ranging from 0.1-10 THz [10], terahertz wave has been used in a variety of fields such as broadband communications, radar, astronomy, nondestructive testing, medical imaging and etc. [11–15]. The terahertz wave has the following advantages. (1) It has low photon energy, does not cause ionizing damage to biological samples and also can excite the collective oscillation pattern of biomolecules. (2) It has wide bandwidth. (3) Its short pulses and high temporal resolution make it very suitable for biosensing and detection. Metamaterials are artificial materials which were first predicted theoretically by Veselago [16]. Its periodic arrangement of subwavelength structure enables it to have characteristics such as imaging beyond the diffraction limit, negative refraction, cloaking, abnormal transmission, abnormal Cherenkov radiation [17–21], etc. More importantly, metamaterials are sensitive to local enhancement of the electromagnetic field and changes in the dielectric properties of the surrounding environment, and various terahertz response functional devices such as absorbers, modulators, filters and sensors [22–24] can be realized by selecting various structures and substrate materials [25–27]. Furthermore, biosensing with terahertz metamaterials does not require sample pretreatment and damage the sample, has accurate results and short detection time. In addition, it can perform trace detection simply and efficiently [28].
Nowadays, a wide variety of terahertz metamaterial sensors have been developed. Reinhard et al. completed the sensing detection of glucose with terahertz metamaterials on paper substrates [29]. Singh et al. demonstrated high-Q ultrasensitive sensing via strong light-matter interaction at the ultra-sharp quadrupole and Fano resonances, which can be used in chemical and bio molecular detection [30]. Qin et al. completed the detection of tetracycline hydrochloride (TCH) using a THz high-sensitivity metamaterial sensor [31]. Park et al. completed the detection of 60 nm-PRD1 and 30 nm-MS2 nano viruses by using the designed THz SRR sensors with various capacitive gap widths [32]. Srivastava et al. experimentally demonstrated a ultrathin, low index, and flexible design of the metasensor, which enables sensing of the analyte from the top and bottom surfaces of the metamaterial [33]. Yang et al. proposed a THz metamaterial biosensor and tested genomic DNA of transgenic tomatoes [34]. Srivastava et al. demonstrated a quasibound state in the continuum (BIC) resonance for sensing of a nanometer scale thin analyte deposited on a flexible metasurface [35]. Gupta et al. designed and fabricated a terahertz metamaterial cavity featuring large electromagnetic energy confined in low mode volume that gives rise to large Q/Veff, opens an enabling platform for efficient probing of light–matter interaction [36]. Huo et al. completed the sensing study of imidacloprid pesticide with the designed transmissive THz metamaterial [37].
For BSA sensing, Li et al. devised a THz metamaterial sensor with four resonators at the same frequency to complete the sensing of BSA. The experimental results show that the frequency shift of peak in spectrum is only 50 GHz when the BSA concentration reaches 765 μmol/ L [38]. The sensing effect of this sensor is good, but the sensing sensitivity has a large room for improvement.
So, we designed and fabricated a THz metamaterial for BSA high-sensitivity detection and obtained the minimum sensing concentration and sensing sensitivity for BSA in experiment. Compared with prior work [38], the sensing sensitivity of BSA has been improved greatly.
2. Design and simulation results
2.1 Simulation settings and results
We use FITD numerical simulation software CST Microwave Studio 2016 for simulation. Figure 1(a) shows the unit cell of our biosensor device which is composed of two layers. The top layer of yellow ring with cross notch is aluminum(Al), and its conductivity is σ = 3.56 × 107 S/m; the bottom layer of blue substrate is quartz with a relative dielectric constant εr = 4.41 and a loss tangent tanδ = 0.0004. The optimized metal parameters are as follows, outer radius R1 = 37 μm, inner radius R2 = 29 μm, ring and cross width w = 8 μm, width of center notch b = 15 μm, thickness of substrate h = 1000 μm, period L = 80 μm. In simulation settings, there are periodic boundary conditions in the x and y directions, open boundary conditions in the z direction. The THz wave is incident along the positive direction of the z-axis.
The simulation result is shown in Fig. 1(b). It can be seen that in the range of 0.1-1.6 THz, only 0.835 THz has a unique resonance peak with a depth of -36 dB. And it has a quality factor (Q) of 10.62. Figure 1(c) shows the current distribution of the structure at peak resonance. As can be seen from the figure, the surface current oscillates strongly on the inner and outer rings of the ring structure, inducing a typical LC resonance. The structure can be divided into four parts, and each part can be equivalent to the circuit model in Fig. 1(c). The decrease in the transmission spectrum is the result of the superposition of all LC resonances. The resonant frequency is calculated as: [39]
where L is the resonator inductance determined by geometric parameters of the metamaterial and C is the overall capacitance. The overall capacitance consists of the following components: [40]
Fig. 1. (a)3D diagram of the designed structure. (b)Simulated transmission spectrum of the metamaterial sensor. (c)Simulated surface current distribution at resonance frequency and equivalent circuit diagram. (d)Simulated transmission spectrum under different polarization angles.
Ca is the substrate capacitance, Cb is the capacitance between the substrate and the metal in metamaterial, Cc is the capacitance of metal in metamaterial itself, Cd is the capacitance between the biological sample and the metamaterial, Ce is the capacitance of sample itself. When the biological sample is dropped on the metamaterial, the overall capacitance C will change due to the change of the surrounding dielectric constant, resulting in the change of resonance frequency. In addition, we changed the polarization angle of the incident wave in the simulation, the result is shown in Fig. 1(d). As can be seen, when the polarization angle is changed by 90°, the transmission spectrum does not change significantly, indicating that this structure is insensitive to polarization.
The principles about how the symmetrical structure affects the polarization insensitivity are shown below [41]. How light transmits through a metamaterial can be described by the Jones matrix, which we call it T matrix in the next manuscript. The new T matrix ${\widehat T_{new}}$ of the rotated sample by an angle $\varphi$ can be written by applying the following matrix operation:
The T matrix superscript f designates propagation in the forward direction. Our structure is C4- symmetric with respect to the z axis, so we have:
It shows that A = D, C = -B. That is to say, as the device rotates, the T matrix doesn’t change. Hence the structure is insensitive to linearly polarized light of any state. Also, the metamaterial is mirror-symmetric with respect to the xz and yz plane, so it has no polarization conversion function. In practical applications, there is no need to ensure the structure is placed in the same position when testing samples of different concentrations, so this structure is convenient for rapid detection.
2.2 Simulation of analytes with different thicknesses and dielectric constants
We select the analyte with certain dielectric constant and thickness to explore the response of sensor to the refractive index and thickness of the analyte deposited on sensor surface. An important parameter to measure the resonance characteristics of the sensor is the sensitivity S, which is the degree of response caused by changing the unit analyte. After the structural parameters and materials of the metamaterial are determined, only the change of refractive index of the surface analyte affects the sensitivity, which is expressed as follows:
Δf is the frequency shift of the resonance peak, Δn is the change of refractive index, its unit is THz/RIU (reflective index unit). Due to the fact that the refractive index range of most biomolecules is 1.4 to 2.0 [42], we select non-destructive analytes with refractive indices of 1.0, 1.2, 1.4, 1.6, 1.8 and 2.0 to cover the metamaterial surface and fix the analyte thickness to 10 μm. The simulation results are shown in Fig. 2 (a). As can be seen, the resonance peak shift to lower frequency with the increase of refractive index. While the refractive index is 2.0, the redshift is 135 GHz, so the sensor sensitivity can be calculated to be 135 GHz/RIU. We proceed to simulate the situation with thicknesses of 5 μm and 15 μm and plot the refractive index against the frequency shift at the same time, and the linear fitting results are shown in Fig. 2(b). It can be observed that there is a linear relationship between them, the R2 are 0.9989, 0.9998 and 0.9998 at the thickness of 5 μm, 10 μm and 15 μm respectively, and the sensitivity increases with the increase of the thickness. These results reveal that this sensor has an efficient linear response to the analytes with refractive index in the range of 1.0-2.0, and the frequency shift of the resonance peak varies with thickness. In order to further explore the sensing effect of the sensor on the thickness, the refractive index of the analytes is fixed at 1.4 and the thickness is changed from 1 μm to 40 μm for simulation. The results are shown in Fig. 2(c) and (d). It can be seen that the resonance peak has a redshift with the increase of the thickness, and the redshift is obvious when the thickness is small, such as 10 μm, Δf is 54 GHz, when thickness continues to increase, the change of redshift slows down, the frequency shift at 40 μm is only 2.85 GHz more than that at 20 μm, indicating that there is an optimal sensing thickness for this sensor, and it is in the range of 0-10 μm, in this range, the frequency shift varies significantly. We regard that the farther away from the metasurface, the weaker the local electric field, so the weakening of the interaction between electric field and surface analyte reduces the sensitivity.Fig. 2. (a)The transmission spectra of changing refractive index with fixed thickness of 10 μm. (b)The frequency shift of changing the refractive index when the thickness is 5 μm, 10 μm and 15 μm. (c)The transmission spectra of changing the thickness with fixed refractive index of the analyte of 1.4. (d)The frequency shift of changing the thickness of analyte.
3. Experimental results and discussion
3.1 Metamaterial fabrication
We first apply photoresist on the quartz substrate, dry it and use fabricated mask to do transmissive UV exposure, and then sequentially go through development, development inspection, etching, and striping to finish the fabrication, as shown in Fig. 4(b), the yellow part is aluminum (Al) and the black part is silicon dioxide (SiO2).
3.2 System of THz-TDS
As shown in Fig. 3, THz-TDS is installed in a dry air purification chamber, the excitation source is a Mai Tai Ti:sapphire femtosecond laser (800 nm, 70 fs). The laser pulse is divided into pump light and probe light by the beam splitter, which are used for terahertz wave generation and detection, respectively. The photoconductive antenna PCA generates THz wave with power of 200 mW, and passes through the delay line, then the THz wave is collimated by a gold-plated paraboloidal mirror and passes through the metamaterial covered with BSA solution. Next, the probe light is time-coherent with the THz wave on the ZnTe crystal, the THz pulse makes the crystal produce electro-optical effect, so the polarization state of the passed femtosecond light is modulated, and the energy difference between two polarized lights after polarization splitting is not 0. By changing delay line, the electric field signal of the THz pulse can be scanned in time, finally the obtained time domain signal is Fourier transformed to transmitted spectrum signal in frequency domain. The terahertz spot diameter where the sample placed is ф4 mm. We performed an 80 ps time scan to achieve a frequency resolution of 12.5 GHz. Figure 4(a) is a schematic diagram of the sensor transmission.
Fig. 4. (a)Schematic diagram of a terahertz wave transmission metamaterial biosensor (The light blue part is BSA solution). (b)Microscopic image of fabricated metamaterial. (c) Microscopic image of dried metamaterial after dripping BSA solution. (d) Measured and simulated transmission spectra.
3.3 Metamaterial sensing experiment of BSA solution
THz-TDS is used to complete the transmission measurement of the prepared metamaterial, and the experimental results are in good agreement with the simulation, as shown in Fig. 4(d), the resonance peaks are all located at 0.835 THz, and the peak depth is below -30 dB.
In the experiment, BSA was purchased from Shanghai Yuanye Bio-Technology Co., Ltd. It contains no fatty acids and has a purity of 98%. We use deionized water as the solvent. We prepare BSA solutions at concentrations of 0.1 mg/mL, 0.5 mg/mL, 2 mg/mL, 5 mg/mL and 10 mg/mL under laboratory aseptic conditions, then mix the solution evenly and keep it placed static. Shake well before using, then use a quantitative adjustable pipet to get 70 μL solution and drop it in the metamaterial to keep the transmission perforation of THz-TDS fully covered, the THz transmission spot diameter is ф4 mm. To avoid water absorption, the metamaterial with solution is dried in a 30℃ oven for 30 minutes before THz-TDS detection. Transmission spectrum of blank quartz substrate was used as contrast.
Figure 5(a) tells there is almost no change for the blank quartz substrate before and after adding 0.1 mg/mL of BSA solution, indicating that the substrate is unable to sense the BSA, however, metamaterial sensor can achieve effective discrimination of BSA. The results of multiple concentration groups are displayed in Fig. 5(b). The resonance frequency shift and the variation of peak amplitude of BSA at different concentrations are also given in Table 1, Fig. 5(c) and (d). The results show that the detectable minimum BSA concentration by this metamaterial is 0.1 mg/mL, the resonance peak gradually shift to lower frequency with the increase of the solution concentration. When the concentration change is less than 2.6 mg/mL, the frequency shift changes significantly. However, when the concentration exceeds 7.6 mg/mL, the frequency shift changes slowly. The frequency shift reaches 137 GHz when BSA is 17.6 mg/mL. As indicated in Tab.1, the frequency shift percentage changes rapidly at low concentrations and slows down at higher concentrations, ending up at 16.4% at 17.6 mg/mL. In addition, the intensity changes at different concentrations is distinct, the initial rate of change is greater than the follow-up. This may be caused by the difference in the resonance of terahertz waves at different concentrations. The experimental results demonstrate that this metamaterial biosensor can realize the efficient sensing of BSA solution.
Table 1. Frequency shift of BSA solution at different concentrations and its percentage
Fig. 5. (a)Transmission spectra of quartz substrate and metamaterial before and after dropping solution of 0.1 mg/mL. (b)Transmission spectra of BSA at different concentrations. (c)The frequency shift varies with the concentration of BSA solution. (d)The transmission intensity varies with the concentration of BSA solution.
4. Summary
In this paper, a polarization insensitive metamaterial biosensor for BSA detection is designed and fabricated, both simulations and experiments confirm it can realize high sensitive sensing of BSA solutions with a concentration as low as 0.1 mg/mL. Its quality factor (Q) can reach 10.62. The resonance peak frequency shift is as high as 137 GHz at the highest measured concentration of 17.6 mg/mL. In conclusion, this new biosensor realizes the rapid and efficient detection of BSA solution, which provides a novel method for BSA sensing and a valuable reference for the design of other metamaterial biosensors in food and medical fields.
Funding
National Natural Science Foundation of China (11574159); Open Fund of the State Key Laboratory of High Field Laser Physics, China; the Special Research Foundation of the Central University of Nankai University (63191108); Tianjin Municipal Fund for Distinguished Young Scholars (20JCJQJC00190); the Key Fund of Shenzhen Natural Science Foundation (JCYJ20200109150212515).
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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