A multifunctional Si₃N₄ nanobrick metasurface for sensing†

Huimin Wang‡ , Lu Wang‡, Tao Wang*, Ming Shen, Xinzhao Yue, Enze Lv, Jinwei Zeng, Xuewen Shu* and Jian Wang*
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, 430074, China. E-mail: wangtao@hust.edu.cn; xshu@hust.edu.cn; jwang@hust.edu.cn

First published on 6th August 2024

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1. Introduction

Metasurfaces are artificial electromagnetic materials with a feature size smaller than the wavelength of the incident light.¹ In recent years, metasurfaces have quickly developed due to the flexibility of various artificially designed optical functions.^2–6 Besides, they have the advantages of being compatible with current CMOS (complementary metal–oxide semiconductor) nanofabrication technologies,⁷ easy to fabricate and integrate with flexible substrates⁸ and optofluidic platforms.^9,10 The manipulation of light waves has been extensively investigated in these metasurfaces in terms of rays, phases,¹¹ structural colors,^12–14 and holograms.^15,16 Except for exploring light-wave manipulation, there is growing interest in light–matter interactions in novel optical devices, which spans from fundamental investigations to practical applications.

Structural colors of metasurfaces are generated by the interaction of the incident light and nanostructure, and are more stable and safe in comparison with traditional dyes and pigments.^13,17 Colorimetric sensors based on structural colors can be achieved by changing the surrounding environment.^18–20 Especially in the visible region, dielectric metasurfaces are highly efficient and an excellent choice for colorimetric sensors due to their lossless properties. The all-dielectric metasurfaces support Mie resonances, which have the characteristic of the electromagnetic field enhancement. However, the enhanced electromagnetic fields are confined within the nanostructure and do not extend into the surrounding sensing regions, which limits the development of dielectric metasurface sensors. A metasurface composed of Si₃N₄ with a lower refractive index can result in more electromagnetic energy leaking into the surrounding environment compared with other dielectric materials,^21,22 such as Si^23–25 and Ge.^26,27 This characteristic can enhance the interactions between the electromagnetic energy and analytes, which is beneficial for sensing applications. However, the research of Si₃N₄ metasurfaces in the sensing field remains relatively scarce to date. Besides, various analytes have unique requirements for electromagnetic energy distributions based on their sizes.²⁸ It is crucial to investigate the interaction of different types of light and matter for the development of optical sensors.

In this study, we focused on two different applications to highlight the versatility of Si₃N₄ metasurfaces in the sensing field. (1) A size tuning of the nanobrick array provided different colors, which were sensitive to the refractive index changes of the surroundings. The colorimetric sensor obtained a sensitivity of about 104 nm per RIU at a resonance wavelength of 541 nm. (2) Antibody modified gold nanoparticles (AuNPs) were bound on the surface of the Si₃N₄ nanobrick array based on the specific recognition between the carcinoembryonic antigen (CEA) and antibodies. The interactions of the Mie resonance caused by the Si₃N₄ metasurface and localized surface plasmon resonance (LSPR) supported by the AuNPs resulted in blueshifts of the resonance wavelength. The potential of the Si₃N₄ metasurface biosensor for CEA detection was demonstrated.

2. Experimental

2.1. Materials and reagents

CEA, secondary antibody of CEA (Ab₂), and primary antibody of CEA (Ab₁) were from Beijing Key-Bio Biotech Co. Ltd. (Beijing, China). Bovine serum albumin (BSA) and phosphate-buffered saline (PBS) were purchased from Sigma-Aldrich (Shanghai, China). Silane-PEG–COOH (2000 Da) was purchased from Shanghai Ponsure Biotech, Inc. (Shanghai, China). N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China).

2.2. Fabrication of the Si₃N₄ metasurface

The Si₃N₄ metasurface was fabricated as previously described.¹² Briefly, a 350-nm-thick Si₃N₄ film was deposited on a quartz glass substrate using the plasma-enhanced chemical vapor deposition (PECVD) technology, followed by a 30-nm-thick Cr film deposition through the electron beam evaporation (EBE) technology as shown in Fig. 1. Nanobrick patterns were subsequently obtained using the electron beam lithography (EBL) technology with an AR-P 6200.09 photoresist. The inductively coupled plasma (ICP) technology was then used twice to etch Cr with Cl₂ gas, and Si₃N₄ with CF₄ and H₂ gases, successively. Finally, any remaining Cr was removed using a Cr etching solution.

2.3. Surface functionalization of the Si₃N₄ metasurface

Fig. 2 summarizes the procedure of a biomolecule-functionalized surface of the Si₃N₄ nanobrick array for CEA detection. Prior to the linker deposition, the Si₃N₄ metasurface was exposed to acetone and isopropanol, successively, followed by rinsing with deionized water and blow-drying under nitrogen. Initially, the Si₃N₄ metasurface was hydroxylated with an oxygen plasma (for 2 min). The Si₃N₄ metasurface was then immersed in silane-PEG–COOH toluene solution (1 μg mL⁻¹) for 16 h. Subsequently, the Si₃N₄ metasurface was incubated in a solution containing EDC (400 mM) and NHS (100 mM) in equal volumes for 30 min to activate the carboxylic group. The Si₃N₄ metasurface was immediately incubated in 10 μg mL⁻¹ Ab₁ solution for over 10 h at 4 °C. The Ab₁ modified Si₃N₄ metasurface was immersed in a BSA solution (30 μg mL⁻¹) for 30 min to prevent nonspecific binding. Then, the unbound and weakly attached Ab₁ and BSA were removed by rinsing with PBS. The Ab₁ modified Si₃N₄ metasurface was immersed in CEA solution for 45 min. Finally, the functionalized metasurface was incubated in the Ab₂ modified AuNP (AuNP–Ab₂) solution for 45 min after rinsing with PBS as previously described.²⁹ The surface functionalization of the Si₃N₄ metasurface was completed.

2.4. Optical measurement and numerical simulations

The schematic of the simplified experimental setup is shown in Fig. S1 (ESI†), which is composed of a metallographic microscope (MV5000) coupled to a charge coupled device (CCD) camera and a spectrometer (PG2000pro). The Si₃N₄ metasurface was placed in a container made of polydimethylsiloxane (PDMS). The colors of Si₃N₄ metasurfaces were observed using a 5× microscope objective lens. The colors and their corresponding reflection spectra were recorded using the CCD and spectrometer, respectively. The bulk sensitivity of the Si₃N₄ metasurface was determined in the air, water, 50% glycerol–water mixture, and glycerol solution, and the refractive indexes of the solutions at 28 °C were 1, 1.33, 1.39, and 1.47, respectively.

The stepwise molecule binding was investigated by monitoring the corresponding reflection spectra. The Si₃N₄ metasurface was rinsed with PBS before each analysis step. All reflection spectra were obtained at the normal incidence and were not background-subtracted. A 20× microscope objective lens was used for measurements of reflection spectra.

Numerical simulations were performed using the COMSOL Multiphysics software. The refractive indexes of the quartz glass and Si₃N₄ were 1.46 and 2, respectively, and the refractive index of gold was obtained from experimental data.³⁰ Floquet periodic boundary conditions were applied in horizontal directions, and a perfect match layer was used in the vertical direction. The incident light was the normal incident transverse magnetic wave.

3. Results and discussion

3.1. Colorimetric sensing

The scanning electron microscope (SEM) images in Fig. 3(a) show the top and tilt views of the metasurface, and the regularly distributed Si₃N₄ nanobricks have vertical sidewalls. One of the periodic nanobrick cells of the Si₃N₄ metasurface is shown in Fig. 3(b). H, L, and P represent the height, length, and period of each nanobrick, respectively. The H was fixed at 350 nm, and f = L/P represents the duty cycle of the Si₃N₄ metasurface. As shown in Fig. 3(c), the wavelength of the reflectance peak is 536 nm when the parameters of P = 360 nm, f = 0.6, and n = 2 (close to Si₃N₄) are used in the simulations. The coupling of individual meta-atoms to the surface lattice modes can produce far-field observable colors in Fig. 3(d). The reflectance peak resulted from the magnetic dipole resonance, which was typically characterized by the ring-shaped electric field distribution. The Si₃N₄ metasurface due to a moderate refractive index induces a stronger magnetic dipole resonance than TiO₂ (n = 2.5) at the same resonance wavelength in Fig. 3(c). The full width at half maximum (FWHM) of reflectance spectra decreased, and electric field enhancements were greater when the refractive indexes decreased from 2.5 to 2. The improvements were beneficial for enhancing the spectrum resolution and sensing performance. The ratio of sensitivity to FWHM, known as the figure of merit (FOM), was used to comprehensively evaluate the sensing performance of the sensors.³¹ The FOM increases as the refractive index of the nanobrick decreases in Fig. S2 (ESI†). Therefore, the Si₃N₄ metasurface with a lower refractive index had superior sensing performance.

The reflection spectra of Si₃N₄ metasurfaces were simulated, and the corresponding colors were calculated from the spectral data and color matching functions defined by the International Commission on Illumination (CIE). The chromaticity coordinates ((x = X/(X + Y + Z)), y = Y/(X + Y + Z)) in the CIE 1931 color space were calculated based on tristimulus values X, Y, and Z.^32,33 Further, X, Y, and Z could be calculated using the following equations:

	(1)

where S(λ) is the simulated reflection spectra, (λ), ȳ(λ), and (λ) are the standard observer functions defined by CIE, I(λ) is the relative spectral power distribution of the incident light, and is a normalizing constant. The results with different values of P and f are shown in Fig. S3 (ESI†), where a size tuning of the nanobrick array provides different colors. Green light, which was a very sensitive color to human eyes, was generated with f = 0.6.

The potential of the Si₃N₄ metasurface as a colorimetric sensor was explored. The experimentally fabricated Si₃N₄ metasurfaces were with f = 0.6 and P from 340 nm to 390 nm. Each of the fabricated Si₃N₄ metasurfaces had an area of 160 μm × 160 μm for the easy color observation. Fig. 4(a) shows the bright-field microscope images of the fabricated Si₃N₄ metasurfaces under different environmental conditions, where the color changes with the refractive indexes of the surroundings. For instance, the color of the Si₃N₄ metasurface with P = 360 nm changed from green to orange, and then to grayish purple and purple, which was a significant color change. The reflection spectra are measured and the main reflectance peaks are shown in Fig. 4(b), where redshifts are observed with the increases of the refractive indexes of the environments. The reflection spectra of the Si₃N₄ metasurface with P = 360 nm and f = 0.6 in different environments are shown in Fig. 4(c). Significant redshifts of the main reflectance peak were observed with the increase of the surrounding refractive index. The redshifts of reflectance peaks in the visible region resulted in the color change which can be observed by naked eyes, which showed the colorimetric properties of the proposed metasurface. The positions of reflectance peaks in air and glycerol solutions were at 541 nm and 590 nm, respectively, resulting in a sensitivity of about 104 nm per RIU. Furthermore, we compare sensitivities between the Si₃N₄ nanobrick metasurface and other different nanostructures in Table 1. In comparison with sensors based on Au nanospheres,^34,35 TiO₂ nanoblocks,³⁶ and gold grating covered with graphene,³⁷ the Si₃N₄ nanobrick metasurface had a higher sensitivity at a similar resonance wavelength, which showed superiority in the sensing application.

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3.2. Biosensing for CEA

CEA is a tumor biomarker used for diagnosing various cancers, such as breast carcinoma and lung cancer.^38,39 The potential of the Si₃N₄ metasurface as a biosensor for CEA detection was further explored. As shown in Table S1 (ESI†), the results of energy dispersive X-ray spectroscopy show that the contents of the C element significantly increase after the silane-PEG–COOH modification in comparison with the bare Si₃N₄ metasurface. Silane-PEG–COOH was successfully bound to the Si₃N₄ metasurface for the covalent immobilization of Ab₁. The reflection spectra are characterized in different linker depositions, which are shown in Fig. 5. Periods P of Si₃N₄ metasurfaces are 340 nm and 390 nm in Fig. 5(a) and (b), respectively, and their values of f are 0.8. Different structural parameters significantly affected the resonance wavelengths of the reflection spectra of Si₃N₄ metasurfaces. We treated the reflection spectra of Si₃N₄ metasurfaces modified by silane-PEG–COOH (metasurface/silane-PEG–COOH) as an initial reference. The resonance wavelengths of metasurface/silane-PEG–COOH are 549.52 nm and 583.66 nm in Fig. 5(a) and (b), respectively. We observed the redshifts of the resonance wavelengths after Ab₁ and 500 ng mL⁻¹ CEA biofunctionalization procedures. After silane-PEG–COOH, Ab₁, and CEA were modified on Si₃N₄ metasurfaces (metasurface/silane-PEG–COOH/Ab₁/CEA), the resonance wavelengths of Si₃N₄ metasurfaces changed to 550.9 nm and 584.59 nm in Fig. 5(a) and (b), respectively. The linker-molecular chains of the surface modification induced an increase of the surrounding refractive index, leading to dielectric environment-sensitive responses. After the AuNP–Ab₂ modification on the metasurface/silane-PEG–COOH/Ab₁/CEA (metasurface/silane-PEG–COOH/Ab₁/CEA/AuNP–Ab₂), the reflectance peaks have blueshifts and then 500 ng mL⁻¹ CEA can be detected in Fig. 5(a) and (b). The resonance wavelengths of metasurface/silane-PEG–COOH/Ab₁/CEA/AuNP–Ab₂ are 548.59 nm and 582.74 nm in Fig. 5(a) and (b), respectively. These results revealed that the proposed Si₃N₄ metasurface had the ability to detect CEA. The abnormal slight spectral buleshifts could not result in a readily recognizable color difference, so the proposed Si₃N₄ metasurface could not be used as a colorimetric biosensor based on current detection methods.

There were Ab₁, CEA, and Ab₂ between Si₃N₄ nanobricks and AuNPs, and their thicknesses could be treated as 15 nm.^29,40,41 The average diameter of AuNPs was 12 nm as described in our previous work.^29,42 Therefore, the overall sensing volume was in the range of approximately 27 nm above the surface of the Si₃N₄ nanobricks when biosensing for CEA was conducted. The model of Si₃N₄ nanobricks covered with a 15-nm-thick dielectric (metasurface/dielectric) was used to simulate Si₃N₄ nanobricks modified by Ab₁, CEA, and Ab₂. Furthermore, on the surface of metasurface/dielectric, 10 AuNPs (marked as metasurface/dielectric/AuNPs), and 20 AuNPs (marked as metasurface/dielectric/more AuNPs) were randomly placed to simulate the actual situation of AuNPs on the Si₃N₄ metasurface. As shown in Fig. S4 (ESI†), colors correspond to simulated reflection spectra for the Si₃N₄ metasurface, metasurface/dielectric/AuNPs, and metasurface/dielectric/more AuNPs at periods from 340 nm to 390 nm. The duty cycles (f) in Fig. S4(a) and (b) (ESI†) are 0.6 and 0.8, respectively. Colors only had a slight change when simulation models changed from the Si₃N₄ metasurface through metasurface/dielectric/AuNPs to metasurface/dielectric/more AuNPs, which was in agreement with the experimental results.

To illustrate the underlying physics, numerical simulations based on the finite element method were performed to model resonance responses between the Si₃N₄ metasurface and AuNPs. The scattering cross section could be decomposed into electric dipole (ED), magnetic dipole (MD), electric quadrupole (EQ), and magnetic quadrupole (MQ) components, as outlined in the multipole scattering theory.⁴³ When considering a harmonic excitation, exp(iωt), the total scattering cross-section could be represented as

	(2)

where c, ω, A, M, Q_αβ, and M_αβ represent the speed of light in a vacuum, angular frequency, ED moment, MD moment, EQ moment, and MQ moment, respectively. The reflection spectrum and multipole expansion for metasurface/dielectric are shown in Fig. 6(a) and (b), respectively. The resonance peak at 562 nm was primarily dominated by the MD and EQ components. The reflection spectrum and multipole expansion for AuNPs are shown in Fig. 6(c) and (d), respectively. AuNPs could support LSPR, which resulted from the ED component. The reflection spectrum and multipole expansion for metasurface/dielectric with top AuNPs (metasurface/dielectric/AuNPs) are shown in Fig. 6(e) and (f), respectively. The addition of AuNPs enhanced the MD resonance. The resonance peak at 559 nm of metasurface/dielectric/AuNPs had a blueshift of 3 nm in comparison with metasurface/dielectric, which was in excellent agreement with experiment results.

Reflection spectra of the Si₃N₄ metasurface and metasurface/dielectric with a top 12-nm-thick gold film (metasurface/dielectric/GF) were further simulated with f values ranging from 0.5 to 0.7 and P from 340 nm to 390 nm, and the corresponding colors are calculated in Fig. S5 to S7 (ESI†). The reflection spectra showed significant differences, leading to observable color changes between the Si₃N₄ metasurface and metasurface/dielectric/GF. This demonstrated the potential of the proposed Si₃N₄ metasurface for colorimetric biosensors by adding a gold film. The addition of a gold film onto the Si₃N₄ metasurface, which had been modified with biomolecules, is expected to be achieved by a wet transfer method.⁴⁴ Besides, the more alterations in the refractive index on the surface of Si₃N₄ nanobricks contribute to the development of colorimetric biosensors. For instance, optimizing biological modification techniques facilitates the capture of more CEA and AuNP–Ab₂^45,46 or refining the self-assembly method of AuNPs generates a dense layer of AuNPs on the surface of Si₃N₄ nanobricks modified by biomolecules.⁴⁷

4. Conclusion

In conclusion, we focused on two different applications of the Si₃N₄ nanobrick metasurface in the sensing field. On the one hand, a size tuning of the nanobrick array provided different colors, each of which was sensitive to the refractive index change of surroundings. On the other hand, the potential for CEA detection was demonstrated by forming the structure of metasurface/silane-PEG–COOH/Ab₁/CEA/AuNP–Ab₂. In the process, the interactions of the Mie resonance caused by the Si₃N₄ nanobrick array and LSPR supported by AuNPs resulted in blueshifts of the resonance wavelength. Furthermore, we discussed a situation where a significant color change could appear with a gold film on the surface of Si₃N₄ nanobricks, which provided a guide for colorimetric biosensors.

Author contributions

Huimin Wang: conceptualization, data curation, formal analysis, investigation, methodology, visualization, software, writing – original draft, and writing – review & editing. Lu Wang: conceptualization, investigation, methodology, visualization, writing – original draft, and writing – review & editing. Tao Wang: funding acquisition, project administration, supervision, and writing – review & editing. Ming Shen: data curation, resources, and writing – review & editing. Xinzhao Yue: methodology, and validation. Enze Lv: investigation and validation. Jinwei Zeng: investigation and validation. Xuewen Shu: funding acquisition, project administration, supervision, and writing – review & editing. Jian Wang: funding acquisition and project administration. All the authors have accepted responsibility for the entire content of the submitted manuscript and approved the submission.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2023YFE0105800), the National Natural Science Foundation of China (62275084 and 62275093), and the Key R and D Program of Hubei Province (2021BAA036). The authors thank Pan Li (engineer), Zhiwen li (engineer), Cheng Zeng (engineer), and Jun Su (engineer) in the Center of Optoelectronic Micro & Nano Fabrication and Characterizing Facility of WNLO for their support in PECVD, ICP, EBL, and SEM tests.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc01992j

‡ These authors contributed equally to this work.

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