Gold Nanoparticle-Based Terahertz Metamaterial Sensors

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Gold nanoparticle-based terahertz metamaterial sensors: mechanisms and applications Wendao Xu, Lijuan Xie, Jianfei Zhu, Xia Xu, Zunzhong Ye, Chen Wang, Yungui Ma, and Yibin Ying ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00463 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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Gold nanoparticle-based terahertz metamaterial sensors: mechanisms and applications Wendao Xua‡, Lijuan Xiea‡, Jianfei Zhub, Xia Xuc, Zunzhong Yea, Chen Wanga, Yungui Mab and Yibin Ying*a a

College of Biosystems Engineering and Food Science, Zhejiang University, 866 Yuhangtang

Rd., 310058 Hangzhou, PR China b

State Key Laboratory for Modern Optical Instrumentation, Center for Optical and

Electromagnetic Research, Department of Optical Engineering, Zhejiang University, 866 Yuhangtang Rd., 310058 Hangzhou, PR China c

Department of Food Science and Technology, Ocean College, Zhejiang University of

Technology, Hangzhou 310014, Zhejiang, PR China ‡

These authors contributed equally to this work.

*Corresponding Author: Yibin Ying E-mail address: [email protected] Phone: 0086-571-88982885 Fax: 0086-571-88982885

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ABSTRACT Terahertz (THz) waves, especially those assisted by THz metamaterials, have great potential for detecting trace amounts of sensing targets. Some nanomaterials, such as gold nanoparticles (AuNPs), also show promising characteristics; however, their application in this context is hindered by a lack of plasmonic activity in the THz region. This study is the first to introduce AuNPs into THz metamaterial applications as a new tool for improving the sensitivity of protein detection. We demonstrate the mechanism of THz metamaterial detection through sensing different targets by using metamaterials with distinct resonance peaks. Furthermore, we used an AuNP based THz metamaterial sensing method to detect avidin. The limit of detection of conjugated avidin-AuNPs reached 7.8 fmol, presenting greater than a 1,000-fold sensitivity improvement compared with that of avidin alone. Our present work illustrates the feasibility of AuNP-based protein sensing, which may lay a foundation for the development of numerous metallic nanoparticle-based THz metamaterial biosensors. Keywords: detection, metamaterials, gold nanoparticles, proteins, terahertz


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Metamaterials are artificially designed at the subwavelength scale and can exhibit electromagnetic responses that are not observed in naturally occurring materials.1-3 Another distinguishing property of metamaterials is their resonance frequency can be intended to fall anywhere in a large portion of the electromagnetic spectrum, ranging from radio frequencies4 to the terahertz (THz)5, 6 and near-infrared regions.7 THz metamaterials have attracted copious interest in THz technology and hold a potential to become a most up dated and novel tool,8 especially for the identification of biomolecules because of the collective vibrational modes occurring in the THz region.9, 10 Through ingenious structural designs, THz metamaterials can be used for various applications, such as THz modulation,11-13 polarization,14 generation,15 and absorption,16-18 as well as compressive imaging.19

The appearance of a spoof surface plasmon,20 showing an enhancement of

electromagnetic field near the metal, enables THz metamaterials to detect trace amount of a target.21-23

Our recent study demonstrated that metamaterials can exhibit greatly increased

sensitivity for sensing targets, e.g., antibiotics.24 Despite these promising results, the plasmonic activity of few promising nanomaterials, such as gold nanoparticles (AuNPs),25 does not fall in the THz region, which limits their application in this region. Furthermore, the mechanisms of THz metamaterial sensing haven’t yet been fully elucidated, and the feasibility of matching the resonance frequency of a desired target with that of a metamaterial by design to further enhance the selectivity and sensitivity of the sensor hasn’t yet to be determined.26-29

Thus, THz

metamaterial sensing technology remains under development. In this study, we firstly investigated the mechanisms of THz metamaterial sensing, focusing on the optical properties (e.g. refractive index and absorption coefficient) of the target and the detection sensitivity of the THz metamaterial. To determine the relation between the absorption


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coefficient of the target and the resonance peak of the metamaterial (ranging from 0.78 THz to 0.95 THz), fructose and L-histidine were selected as sensing targets. Since L-histidine shows a strong absorption peak at ~0.78 THz, the influence of the absorption peak of the target on the detection sensitivity could be inspected. To investigate the influence of refractive index of the target on the detection sensitivity, the sensing results of fructose and L-histidine were analyzed according to their refractive indices. In this case, by evaluating the sensitivities of different metamaterials for detecting fructose and L-histidine, the influence of the refractive index and the absorption coefficient on THz metamaterial sensing technology could be investigated further. This study illustrates that THz metamaterial is sensitive to the refractive index of sensing targets rather than their absorption peaks, which corresponds with the simulated data. The mechanism of detection by THz metamaterials can be extended to AuNP-based THz sensing.

By

introducing AuNPs, the refractive index of which far surpasses that of most dielectric materials, the limit of detection (LOD) for studies on conjugated avidin-AuNPs increased to 7.8 fmol based on three times the signal-to-noise ratio (SNR) criterion, presenting more than a 1,000-fold sensitivity improvement compared with that of avidin alone.

Our study demonstrates the

potential applications of AuNP-based THz sensors and may lead to the development of numerous nanoparticle-based biosensors operating in the THz region. RESULTS AND DISCUSSION Our study is based on the regular reflection detection mode of THz metamaterials, as shown in Figure 1. We used a simple metamaterial form consisting of square metal patch arrays for THz spoof surface plasmon excitation. This monolayer array of metal patches on an aluminum background spaced by thin dielectric films behaves as horizontal waveguides and strongly interacts with the incident wave at the excitation of the waveguide harmonic mode.16 Therefore,


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it leads to a strong wave attenuation at the resonances of these modes by transferring the electromagnetic energy into heat, showing a strong absorption peak, as shown in Figures 1b-1e. The localized enhancement of the electromagnetic field coupled with the metamaterial enables a new signal-enhancement technology by measuring the resonance frequency shift caused by the sensing targets. The reflection of the metamaterial was measured via commercial THz timedomain spectroscopy (THz-TDS); reflectance is defined as R = (Esample/Ereference)2, and Esample (Ereference) is the THz electric field intensity of the sample (reference). Figures 1b-1e reveal the simulated and experimental results of metamaterials with metal patches of different sizes d (Figure 1b, d = 105 µm; Figure 1c, d = 95 µm; Figure 1d, d = 90 µm; and Figure 1e, d = 85 µm). These figures demonstrate that the simulated results correspond well with the experimental results. The minor distinctions between the simulated and experimental data are mainly due to errors in fabricating the metamaterials. The numerical simulations of the metamaterials were performed by the finite-difference time-domain (FDTD) method. The resonance frequencies of the metamaterials increased with decreasing the metal patch size, illustrating the same trend observed in the simulated results (Figure 1b-1e). Compared with those of the metasurface, the magnetic and the electric resonance field enhancements of the impedance-matched absorption cavity enable stronger interactions with the dielectric analyte for the same pattern of metamaterial absorber, which may lead to better sensitivity.30 In this study, to investigate the detection mechanism of THz metamaterials and specifically probe deeply into the relation between the optical properties of a sample and the detection sensitivity of a metamaterial, samples with different refractive indices and absorption coefficients were prepared along with metamaterials with distinct resonance peaks.


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Figure 1. Schematic diagram of detection by terahertz metamaterials and terahertz spectra of metamaterials. a, The regular reflection detection mode of the terahertz metamaterials. b-e, Simulated and experimental data of terahertz metamaterials; the length of the metal patch is 105 µm (b), 95 µm (c), 90 µm (d), and 85 µm (e).

As the metamaterials in this study showed resonance peaks ranging from 0.78 THz to 0.95 THz, special attention was paid to this region. Fructose and L-histidine were selected for experiments performed to determine the relation between the absorption peak of the target and the resonance peak of the metamaterial.

First, a sample was formed into pellets with

polyethylene (PE) at a mixing ratio of 1:1. The refractive indices and absorption coefficients of fructose and L-histidine are shown in Figure S1. Figure S1b illustrates that fructose does not have any obvious absorption peaks from 0.78 THz to 0.95 THz, while L-histidine has a strong absorption peak at ~0.78 THz with an absorption coefficient higher than that of fructose. Notably, this peak of L-histidine at ~0.78 THz matches the resonance peak of the metamaterials shown in Figure 1b. However, the refractive index of fructose is higher than that of L-histidine between 0.78 THz and 0.95 THz, as shown in Figure S1c.

Therefore, by evaluating the


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sensitivities of different metamaterials for detecting fructose and L-histidine, the influences of the refractive index and absorption coefficient on THz metamaterial sensing applications can be investigated further. The results of sensing fructose and L-histidine with THz metamaterials are shown in Figure 2. Figure 2a illustrates a side view of sample detection using metamaterials. The reflectance was calculated, and the shifts in resonance frequency caused by the samples were obtained by analyzing the minimum reflectance with or without the sample. In this study, metamaterials with d = 105 µm (showing a resonance peak at 0.78 THz) and d = 95 µm (showing a resonance peak at 0.86 THz) were selected for the target detection analysis. The frequency shifts of the different metamaterials caused by the targets are shown in Figures 2b and 2c. The frequency shift slowly increased with increasing target concentrations, presenting the same trend as reported in the literature.31 For metamaterials with a resonance frequency of 0.78 THz, the frequency shift increased more promptly with fructose than it did with L-histidine, as shown in Figure 2b; indicating a better sensitivity for fructose. The solid lines in the figure are logarithm fitting curves of the frequency shift data, which fit well with the experimental data. The absorption of fructose is much smaller than that of L-histidine at 0.78 THz, while the refractive index of fructose is larger than that of Lhistidine, as shown in Figure S1. In this case, the refractive index may be the major factor accounting for the metamaterial sensing sensitivity. In the mid-infrared region, matching the resonance frequency of a metamaterial and the absorption peak of a target can lead to a split in the metamaterial resonance peak caused by Fano coupling.32 However, in the THz range, this phenomenon has not yet been reported, which may be because the absorption peaks of the targets in the THz range are too broad to generate Fano coupling with metamaterials. For metamaterials


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with a resonance frequency of 0.86 THz, the detection of fructose also has a better sensitivity, as shown in Figure 2c. The absorption coefficient of fructose is slightly greater than that of Lhistidine at 0.86 THz, and the refractive index of fructose is also greater than that of L-histidine, as shown in Figure S1. Therefore, the major factor of THz metamaterial sensing is likely the refractive index of the sensing target as opposed to the absorption coefficient.

Figure 2. Fructose and L-histidine sensing using metamaterials with different resonance peaks. a, The side view of detection by metamaterials. b, Detection of fructose and L-histidine by metamaterials with a resonance peak at 0.78 THz. c, Detection of fructose and L-histidine by metamaterials with a resonance peak at 0.86 THz. d-f, The electric field distribution (0.78 THz) of terahertz metamaterials with a dielectric film thickness of 0 µm (d), 1 µm (e), and 3 µm (f). The refractive index of the dielectric film was 1.8.


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To further understand the frequency shift curves shown in Figures 2b and 2c, the FDTD method was used to simulate the electric fields of metamaterials. The refractive index of the dielectric film was set to 1.8, and the film thicknesses were 0 µm, 1 µm and 3 µm, as shown in Figures 2d, 2e and 2f, respectively. The electric field intensity decreased in the space above the metamaterials at 0.78 THz, as shown in Figures 2d, 2e, and 2f.

Detecting a target on a

metamaterial surface region with a higher electric intensity could yield an improved sensitivity.22 Therefore, the interaction between a THz metamaterial and a target mainly depends on the intensity of the excited electric field caused by a spoof surface plasma. In this case, with a decrease in the electric field intensity in the space above the metamaterial, the sensitivity of the metamaterial decreased, resulting in a slower frequency shift, as shown in Figures 2b and 2c. Additionally, compared with Figure 2d, increased dielectric film thickness decreased the electric field intensity either in the space above the metamaterial or in the polyimide layer, as shown in Figures 2e and 2f. This is due to the dielectric film on the metamaterial surface, which reflects the THz wave; the attenuated THz wave then excites a spoof surface plasma with a weaker intensity. By comparing the results presented in Figures 2e and 2f, it is also obvious that with increasing dielectric film thickness, the intensity of the excited electric field decreases. Since the detection sensitivity of the metamaterials mainly depends on the electric field intensity and the decrease in the frequency shift speed caused by adding a sample can be explained by the following two interpretations: first, as a common property of surface waves, the intensity of the electric field decreases above the metamaterials; second, this result may be due to the reflection of the sensing target, which reduces the energy of the incident THz wave. Based on these two explanations, the frequency shift speed becomes slower during detection by THz metamaterials, which corresponds well with the trends shown in Figures 2b and 2c.


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Meanwhile, to further investigate the mechanism of detection by THz metamaterials, the FDTD method was utilized to simulate the frequency shifts caused by dielectric films with different refractive indices and extinction coefficients. In literature the relation between the absorption coefficient and the extinction coefficient has been already explained.33 With an absorption coefficient of 200 cm-1, the extinction coefficient is approximately 0.5 at 1 THz. Since the absorption coefficient of a dielectric sample is less than 200 cm-1 in most cases, at 1 THz (as in our case, the absorption coefficients of fructose and L-histidine are approximately 20 cm-1 at 1 THz; Figure S1), THz metamaterial sensing was simulated with a highest refractive index of 0.5.

Figure 3. Simulated results of detection by terahertz metamaterials with different refractive indices and extinction coefficients.

a, The simulated results of detection by terahertz

metamaterials with refractive indices ranging from 1.0 to 2.0. b, The simulated results of detection by terahertz metamaterials with extinction coefficients ranging from 0.0 to 0.5.

Figure 3 shows the simulated sensing results of metamaterials with dielectric films having different optical properties. Figure 3a demonstrates the influence of the refractive index on THz


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metamaterial sensing with the refractive index varying from 1.0 to 2.0. Larger frequency shifts in the resonance frequency occurred with increasing refractive index but without any obvious changes in the reflectance. However, as shown in Figure 3b, there were no obvious frequency shifts; only changes in the reflectance caused by increases in the extinction coefficient were observed. Therefore, the principal factor of THz metamaterial frequency shift is the refractive index of the sensing target as opposed to the extinction coefficient. This result coordinates well with the data illustrated in Figures 2b and 2c. Finally, selecting a sensing target with a high refractive index is of great significance for obtaining a remarkable frequency shift.

This

mechanism can be subtly extended to the signal-enhancing mechanism of AuNP-based detection by THz metamaterials. The study of introducing AuNPs into the THz region is also based on the regular reflection mode of THz metamaterials, as shown in Figure 4a. A simple form of metamaterials, consisting of square metal patch arrays, was used for THz spoof surface plasma excitation. An opaque aluminum film was pre-deposited as the background mirror on silicon (silicon not shown), and then the photoresist SU-8 was deposited on its surface. The SU-8 layer is used as a dielectric layer in the middle, and gold is used as a metallic layer on the top. Figure S2a presents scanning electron microscopy (SEM) images of the metamaterials. The curves from the simulation and the experiments from 0.2 to 1.6 THz are presented in Figure S2b.

A narrow peak at

approximately 0.8 THz is shown with resonance in the first-order parallel-plate waveguide mode. AuNPs show surface plasmon resonance in the visible range, while they perform poorly and cannot be utilized in THz sensing immediately. To confirm this point, an experiment was conducted by dropping a colloidal AuNP solution (20 µL, 20 nmol L-1) on a glass slide and allowing the solution to dry. A transmission electron microscopy (TEM) image of the AuNPs is


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exhibited in Figure S2c, revealing the diameter of the AuNPs was approximately 10 nm, consistent with the calculations based on reference 31 (see details in supporting information). The THz spectra were collected by THz-TDS using a transmission mode, and the transmittances of a glass slide with and without AuNPs are presented in Figure S2d. No significant differences in transmittance were observed between clean glass slides and glass slides with AuNPs, which limits applications based solely on AuNPs.

Figure 4. Schematic diagram and results of gold nanoparticle detection by terahertz metamaterials.

a, The regular reflective detection mode of terahertz metamaterials.

b,

Frequency shifts resulting from different amounts of gold nanoparticles varying from 1 fmol to 10 fmol. c, Reflective curves resulting from different amounts of gold nanoparticles. d, e, SEM images of 100 fmol gold nanoparticles on photoresist (d) and a gold patch (e).


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In our study, we demonstrate that AuNPs can be used as a sensitivity improvement tool in the THz region via the use of metamaterials. Various concentrations of AuNPs, ranging from 1 to 10 fmol, were dropped and dried on the surfaces of metamaterials in triplicate. The findings are presented in Figures 4b-4e. Figure 4b illustrates the relation between the resonance frequency shift and the AuNP concentration, which was linear with a determination coefficient of R2 = 0.9723. For 10 fmol AuNPs, the frequency shift reached up to 4.22 GHz. In our study, the LOD for AuNPs was calculated as 6.1 fmol based on the three times the SNR (see details in supporting information). Figure 4c illustrates the normalized reflectance waveforms of AuNPs from 0 to 10 fmol.

To emphasize the frequency shifts, all the lowest points of these waveforms were

normalized to the same value. Figure 4c demonstrates that the resonance peak shifts to lower frequencies with increasing AuNP concentrations. AuNPs will cause a change in the electric field on the surface of the metamaterials, which is equivalent to diminishing the gap between these adjacent metal patches and causes shift in the resonance frequency.

A detailed

interpretation of the frequency shift of the minimum resonance is available in reference 22. The intensity of the electric field decreases with increasing height from the metamaterial surface; therefore, the sensitivity decreases.34 However, in our study, we present good linear relation between the amount of AuNPs and the frequency shift. To interpret this contradiction, SEM images of 100 fmol AuNPs on SU-8 and gold patch arrays are displayed in Figures 4d and 4e (after depositing a 2 to 3 nm thick gold layer, the observed AuNP diameter increased). Figures 4d and 4e illustrate that the AuNPs were dispersed as a single layer on the metamaterial surface, which corresponds with the theoretical calculations (see supporting information). Since the thickness of the sensing target is constant as a single layer of AuNPs, the frequency shift can exhibit decent linearity.


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Detection of avidin and conjugated avidin-gold nanoparticles by terahertz

metamaterials. a, The principle of conjugating avidin and gold nanoparticles. b, Frequency shifts resulting from avidin and conjugated avidin-gold nanoparticles.

Generally, the sensitivity of the metamaterials has increased with the permittivity or the refractive index of the target, i.e., for the same metamaterials, the sensitivities are closely related to the dielectric constants or the refractive indices of the targets. Under most circumstances, the permittivity of the metal far outweighs that of the dielectric material.35 Hence, the detection of AuNPs reaches a lower LOD than that of most dielectric materials. Therefore, the detection sensitivity of THz metamaterials can be improved by adding only a few fmols of AuNPs. To confirm this point, avidin and conjugated avidin-AuNPs ranging from 2 to 10 fmol was detected; the results are revealed in Figure 5. Figure 5a illustrates the principle of conjugating avidin and AuNPs. In our study, an AuNP (with a diameter of ~10 nm) is negatively charged and coated with citrate, while avidin (with a diameter of ~7 nm) is positively charged with given pH (~7)


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which is lower than its isoelectric point (pH = 10) (see details in supporting information). Therefore, electrostatics can account for the conjugation of avidin and AuNPs. Taking the interspace hindrance and the electrostatic repulsion between adjacent avidin molecules into consideration, the proportion of avidin to AuNPs will be no larger than 15:1. Figure 5b presents the frequency shifts resulting from avidin and conjugated avidin-AuNPs. Avidin did not lead to an obvious frequency shift over concentrations ranging from 2 to 10 fmol, showing poor sensitivity, as demonstrated in Figure 5b. However, for conjugated avidin-AuNPs, the frequency shifts showed good linearity with the amount of AuNPs, yielding R2 = 0.9371. The LOD of conjugated avidin-AuNPs was calculated to be 7.8 fmol. For 150 pmol avidin, a frequency shift of 2.61 GHz occurred; for 10 fmol conjugated avidin-AuNPs, the frequency shift attained up to 3.12 GHz, showing more than a 1,000-fold improvement in sensitivity.

Therefore, the

introduction of AuNPs has the potential to provide a strong enhancement in the detection sensitivity of THz metamaterials.

CONCLUSIONS Our study focused on determining the mechanisms and potential applications of AuNP-based detection by THz metamaterials with improved sensitivity. Our results reveal that introducing high refractive index particles, such as AuNPs, improves the detection sensitivity. Notably, binding the target with high refractive index particles can boost the sensitivity of THz metamaterial sensing, which may lay a foundation for the development of numerous nanoparticle-based biosensors operating in the THz region. We expect that our work will provide new insights for metamaterial sensing, including the selection of targets and new signal-


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enhancing methods, bringing detection by THz metamaterials forward into a new developmental stage.

METHODS All the THz time-domain spectra (including the transmission and reflection spectra) were collected by a Z-3 THz time-domain spectrometer system (Zomega Corporation, East Greenbush, NY, USA) with a low-temperature GaAs photoconductive antenna as the THz emitter and a ZnTe electro-optical crystal as the THz detector. The spectral range was 0.1-3.5 THz with different detection modes, including transmission and reflection. The humidity was less than 1% after nitrogen purging, and the temperature was 23±1°C. All spectra were collected after the spectroscopy system had warmed up for half an hour. The SNR was greater than 60 dB for all sample collections. For the metamaterials, a 200-nm-thick, opaque aluminum film was pre-deposited as the background mirror on the bottom silicon layer. Polyimide with a measured permittivity ε = 2.980.165 i (at 1 THz) was deposited on the aluminum film with a thickness h = 4 µm. The polyimide layer was used as a dielectric layer in the middle, and Au was used as a metallic layer with the thickness of 200 nm on the top. The period of the THz metamaterials was p = 140 µm with different individual patch widths of d = 105 µm, 95 µm, 90 µm and 85 µm. For metamaterials used to sense avidin, SU-8 with a measured permittivity ε = 2.79 – 0.31 i (at 1 THz) was deposited in a 4-µm-thick layer on the aluminum film. The SU-8 layer was used as a dielectric layer in the middle, and Au was used as a metallic layer with a thickness of 200 nm on the top. The period of the THz metamaterials was p = 140 µm with a patch width d = 105 µm.


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Fructose and L-histidine were purchased from Sigma-Aldrich (Milwaukee, WI, USA). Fructose and L-histidine solutions with different concentrations were prepared for metamaterial sensing (the concentrations were 60 mg L-1, 80 mg L-1, 100 mg L-1, 500 mg L-1, 1000 mg L-1 and 2000 mg L-1).

Before adding the fructose or L-histidine sample, the metamaterials were

thoroughly washed by deionized water thrice. In the metamaterial sensing tests, 10 µL the desired fructose or L-histidine solution was dropped on the surface of the THz metamaterial with three duplicates. Then, the solution was dried at 40°C for 40 minutes before measurement. Each spectrum represents an average of four scans. The incident THz wave was controlled on the middle of the sensing target using an aligned laser diode. The preparation of AuNPs has been previously described.36 The determinations of AuNP size and concentration were based on UV-Vis spectra (see supporting information).37 The volumetric ratio of the avidin (1 mg mL-1, pH = 7) and AuNP solutions (20.3 nmol L-1, pH = 10) was 3:5 for preparing the conjugated avidin-AuNPs. The conjugated avidin-AuNPs were then throughly mixed and centrifuged to remove excess avidin. Deionized water was obtained from the Milli-Q SP Reagent Water System (18 MΩ, Millipore, Billerica, USA) and used in all tests. Other chemical reagents, including ethanol and sodium hydroxide, were commercially available, of high-purity grade, and used without further purification. Simulations were conducted using FDTD solutions (Lumerical Solutions, Vancouver, Canada).

ADDITIONAL INFORMATION Supporting Information


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This Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.XXXXXXX. The synthesis of gold nanoparticles; The procedure for producing and the experimental results of fructose and L-histidine pellets. (Figure S1); The properties of terahertz metamaterials and the results of AuNP detection. (Figure S2); UV-Vis spectrum of gold nanoparticles (Figure S3); Simulated results of the relation between sample refractive index and metamaterial frequency shift. (Figure S4).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] NOTES The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 31471410).

REFERENCES (1) Veselago, V. G. The Electrodynamics of Substances with Simultaneously Negative Values of ε and µ. Phys-Usp. 1968, 10, 509-514.


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For Table of Contents Use Only

Gold nanoparticle-based terahertz metamaterial sensors: mechanisms and applications Wendao Xua‡, Lijuan Xiea‡, Jianfei Zhub, Xia Xuc, Zunzhong Yea, Chen Wanga, Yungui Mab and Yibin Ying*a


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Figure 1. Schematic diagram of detection by terahertz metamaterials and terahertz spectra of metamaterials. a, The regular reflection detection mode of the terahertz metamaterials. b-e, Simulated and experimental data of terahertz metamaterials; the length of the metal patch is 105 µm (b), 95 µm (c), 90 µm (d), and 85 µm (e).

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Figure 2. Fructose and L-histidine sensing using metamaterials with different resonance peaks. a, The side view of detection by metamaterials. b, Detection of fructose and L-histidine by metamaterials with a resonance peak at 0.78 THz. c, Detection of fructose and L-histidine by metamaterials with a resonance peak at 0.86 THz. d-f, The electric field distribution (0.78 THz) of terahertz metamaterials with a dielectric film thickness of 0 µm (d), 1 µm (e), and 3 µm (f). The refractive index of the dielectric film was 1.8.

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Figure 3. Simulated results of detection by terahertz metamaterials with different refractive indices and extinction coefficients. a, The simulated results of detection by terahertz metamaterials with refractive indices ranging from 1.0 to 2.0. b, The simulated results of detection by terahertz metamaterials with extinction coefficients ranging from 0.0 to 0.5.

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Figure 4. Schematic diagram and results of gold nanoparticle detection by terahertz metamaterials. a, The regular reflective detection mode of terahertz metamaterials. b, Frequency shifts resulting from different amounts of gold nanoparticles varying from 1 fmol to 10 fmol. c, Reflective curves resulting from different amounts of gold nanoparticles. d, e, SEM images of 100 fmol gold nanoparticles on photoresist (d) and a gold patch (e).

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Figure 5. Detection of avidin and conjugated avidin-gold nanoparticles by terahertz metamaterials. a, The principle of conjugating avidin and gold nanoparticles. b, Frequency shifts resulting from avidin and conjugated avidin-gold nanoparticles.

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Gold Nanoparticle-Based Terahertz Metamaterial Sensors

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