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Abstract
To improve the sensitivity of surface-enhanced Raman spectroscopy (SERS) detection, we propose a three-dimensional (3D) SERS chip based on an inverted pyramid micro-reflector (IPMR) that converges Raman scattering light signals to improve the signal collection efficiency. The influence of the geometric parameters of the inverted pyramid structure on the Raman signal collection efficiency was analyzed by simulation for the determination of the optimal design parameters. The inverted pyramid through-hole structure was prepared on the silicon wafer through an anisotropic wet etching process, followed by the sputtering of a gold film to form the IPMR. The 3D SERS chip was constructed by bonding the IPMR and the active substrate that assembled with silver nanoparticles. Using Rhodamine 6G molecules, the Raman intensity measured with the 3D SERS chip was threefold greater than that of the silicon-based SERS substrate under the same test conditions. These experimental results show that the 3D SERS chip can significantly improve the SERS signal intensity. Its 3D structure is convenient for integration with microfluidic devices and has great potential in biochemical detection applications.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman spectroscopy (SERS) is a powerful tool for the detection and analysis of chemicals [1,2], biological molecules [3–6], and environmental pollutants [7–9]. SERS-active substrates are critical factors in SERS applications [10–12]. Much research has been devoted to the development of high-performance SERS substrates [13–15]. To date, various new high-sensitivity SERS substrates have been developed and used for biological and chemical component detection and analysis [16–18]. For example, Choi et al. prepared a hybrid SERS substrate by combining gold nanoislands with periodic MgF2 nanopillar arrays, which could significantly improve SERS intensity [19]; Gao et al. reported an “AuNPs@GO mesh@AgNPs” hybrid structure SERS substrate that could detect crystal violet at concentrations as low as 10−15 M [20].
In SERS applications, the collection efficiency of the Raman signals affects the detection sensitivity directly. According to the principle of Raman scattering, the Raman signal produced by sample molecules when irradiated with exciting light will be scattered randomly in all directions, of which only the signal located within the aperture angle of the objective lens will be detected. Signals beyond the aperture angle cannot be collected, thus reducing the effective Raman signal intensity and affecting the sensitivity of their detection.
It is well known that the numerical aperture (NA) of the objective lens determines its ability to collect light: the larger the NA, the greater the ability to collect light. Each objective lens has a specific aperture angle and the only way to increase the NA is to increase the refractive index of the medium. Based on this principle, there are water immersion objectives and oil immersion objectives. However, for SERS applications, this method of increasing the NA to enhance the signal intensity is often not feasible. The use of a high numerical aperture objective lens can also increase the intensity of the Raman signal. However, the working distance of such a lens is usually very short, limiting its ability to meet the requirements of some applications. In addition, to improve the intensity of the Raman signal, a commonly used method is to increase the power of the excitation light source. However, this may damage the sample due to local heating and, therefore, affect the experimental results [21].
Here, we describe a three-dimensional (3D) SERS chip based on an inverted pyramid micro-reflector (IPMR). The micro-reflector converges the Raman scattering light signals to improve the signal collection efficiency to obtain higher detection sensitivity. The SERS chip is essentially composed of a substrate and an inverted pyramid through-hole structure micro-reflector cover-plate. The Raman signal that would otherwise have been outside the aperture angle is collected by the objective lens due to the reflection and convergence of the substrate and the IPMR, thus improving the SERS detection sensitivity.
2. Structures
2.1 Optical simulations
The 3D SERS chip was modeled and simulated using the ray optics module of COMSOL Multiphysics 5.4 (COMSOL AB, Stockholm, Sweden). The geometric model includes an IPMR and a collection objective lens. The chip model and the optical path principle are shown in Fig. 1(a). The angle between the sidewall and the bottom surface of the IPMR is 54.74° (the angle between the (111) plane and the (100) plane of monocrystalline Si). It is assumed here that the Raman signal is uniformly scattered with the same power in all directions from the center of the substrate. After the Raman signal is reflected by the bottom and sidewalls of the inverted pyramid, only the signal that reaches the surface of the objective lens has a chance to be collected by the objective lens. The Raman signal collection intensity is determined by calculating the total optical power on the surface of the objective lens. The design parameters can be adjusted and optimized.
Fig. 1. (a) Schematic diagram of the optical path of the 3D SERS chip. (b) Electromagnetic field simulation model.
Moreover, the distributions of the electric field were obtained with finite element method by using the wave optics module of COMSOL Multiphysics 5.4. The SERS substrate used in our experiment was composed of approximately 4 layers of silver nanoparticles (AgNPs). For simplicity, the simulation model was set to place 4 layers of nanodimer on the substrate, and the electric field simulation model is shown in Fig. 1(b). The 785nm incident laser with X-polarization traveled along the -Z direction and the incident field intensity was set as E0 = 1 V/m. The diameter of the AgNPs was 50 nm, which is the average size of the AgNPs used in this experiment. The gap between the nanoparticle and the substrate and the gap between two adjacent AgNPs were set as 2 nm.
2.2 Preparation of the SERS chip
The 3D SERS chip is essentially composed of a substrate and an inverted pyramid through-hole structure cover-plate, as shown in Fig. 2. SERS-active nanoparticles are deposited on the substrate to provide Raman enhancement, and a high-reflective film is coated on the inner walls of the inverted pyramid structure to reflect and converge the Raman scattering signals. The detailed steps for the preparation of the SERS chip are as follows: (1) A layer of 300 nm SiO2 film is oxidized on the surface of (100) Si wafer, to be used as a mask. (2) Photolithography followed by removal of the exposed SiO2 to form square openings on the Si wafer. (3) Corrosion with tetramethyl ammonium hydroxide (TMAH) etchant solution (12.5%wt) under 90℃ to form an inverted pyramid through-hole structure. (4) Sputter an Au film on the surface of the inverted pyramid through-hole structure to form an IPMR. (5) Sputter an Au film on the surface of the second Si wafer. (6) Assemble nanoparticles on the Au film as the SERS-active substrate. (7) Bond the SERS substrate and the IPMR through an adhesive layer to obtain the 3D SERS chip.
3. Results and discussion
3.1 Simulation results and analysis
The Raman spectrometer used in this experiment was configured with an incident laser of 785 nm, a 40× objective lens with a numerical aperture of 0.5 and a working distance of 3 mm. These optical parameters were used in the COMSOL Multiphysics 5.4 software for the simulation. The simulated light trajectory is shown in Fig. 3(a), with the color legend in the figure representing the optical path length of the ray. From the simulation results, it can be seen that rays originally scattered in all directions are now reflected by the IPMR and are all scattered upwards, with most of the signals entering the scope of the objective lens. Figures 3(b) and 3(c) show the total light energy distribution diagram on the lens surface and the average light energy density distribution along the radial coordinate, respectively. It can be seen from Fig. 3(b) that the light energy is mainly concentrated in the central area, and a clear bright spot is formed.
Fig. 3. (a) The light trajectory. (b) Distribution of light energy on the surface of the lens. (c) Distribution of average light energy density along the radial direction. (d) Relation curve between the bottom side length of the inverted pyramid and the total surface light energy. (e) Relation curve between the depth of the IPMR and the total surface light energy. (f) Electric field distribution of AgNPs dimer on Si wafer. (g) Electric field distribution of AgNPs dimer on Au film.
Based on an IPMR with a depth of 300 µm, the reflection efficiencies of different bottom side lengths (10–350 µm) were simulated and analyzed. It can be seen from Fig. 3(d) that an increase in the bottom side length of the inverted pyramid results in a slow decrease of the collected signal power. When the bottom side length was in the range of 10–100 µm, the relative standard deviation (RSD) of the total surface optical power was less than 1%, which indicates that when the change in the bottom side length of the IPMR is in this range, the effect on the Raman intensity is very small.
We further explored the impact of changes in the IPMR depth on the Raman signal collection. Using an IPMR bottom side length of 50 µm as the model, we performed a simulation analysis for different depths (10–500 µm). Figure 3(e) shows that, as the depth of the IPMR increases, the total optical power on the surface increases at first, and then remains stable. When the depth was greater than 100 µm, the depth change had almost no effect on the Raman intensity. According to the simulation analysis, when the bottom side length and depth are 50 µm and 100 µm respectively, the reflection effect of the IPMR sidewalls produces an approximately twofold increase in the Raman intensity.
Figures 3(f) and 3(g) are the local electric field simulation results of multi-layer silver nanoparticle dimer placed on the Si wafer and Au film, respectively. The calculated local electric fields on Si wafer and Au film have reached the maximum of 59.3 V/m and 70.8 V/m. Thus, the corresponding calculated enhancement factor is ∼1.2×107 and 2.5×107, respectively. The simulation results show that SERS enhancement factor of the Au film-based substrate is about twice that of the Si-based substrate.
3.2 3D SERS chip structure characterization
An intrinsic Si wafer with a thickness of 100µm <100> crystal orientation was selected, based on the optimized design parameters acquired from the simulation, and an inverted pyramid through-hole structure was prepared by TMAH anisotropic wet etching [22]. For the control experiments, we designed an inverted pyramid structure with different gradient sizes. It is known that the corrosion rate of TMAH of the (111) crystal plane of Si is very low, while that on the (100) crystal plane is very fast, and that the angle between the two surfaces is 54.74°. A regular inverted pyramid structure is formed with prolonged corrosion times. Figure 4(a) shows a top-down scanning electron microscope (SEM) picture of the processed inverted pyramid through-hole structure, which is seen as a regular inverted pyramid structure. Figure 4(b) is a side-view SEM picture of the inverted pyramid, and the angle between the sidewall and the bottom surface is about 54.7°.
Fig. 4. (a) Top view of the inverted pyramid through-hole structure. (b) Side view SEM picture of the inverted pyramid. (c) AFM picture of the sidewall of IPMR. (d) AFM picture of the Au-plated substrate surface. (e) SEM picture of the AgNPs assembled on the Au film. (f) Photo of the 3D SERS chip.
Following this, a high-reflective film was sputtered on the surface of the inverted pyramid structure. In the visible to near-infrared band, the commonly used highly reflective materials include Au, Ag, Cu, and Al. Ag, Cu, and Al easily oxidize in the air and have poor stability. Au has good stability, so it was chosen as the reflective layer. An adhesion layer was first formed by sputtering 20 nm Cr, followed by the sputter of 120 nm Au film. The roughness of the Au film on the sidewall of the IPMR was characterized by an atomic force microscope (AFM) with the result shown in Fig. 4(c). The average surface roughness (Ra) was 10.83 nm on an area of 5×5 µm2. The principal reason for the greater roughness is that during the corrosion process, TMAH also slowly corrodes the (111) surface [23], resulting in a gradual increase in the surface roughness.
The Au film was sputtered on the surface of the polished Si wafer as a substrate. Figure 4(d) shows the AFM image of the substrate; its surface roughness is very minor, with a Ra of only 0.809 nm in an area of 5×5 µm2. AgNPs were assembled on the substrate as the SERS active substrate. Figure 4(e) shows the SEM picture of the AgNPs assembled on the Au film, which displays a good uniformity. Then, the IPMR was bonded with the SERS substrate to form the 3D SERS chip, as shown in Fig. 4(f).
3.3 SERS performance
The IPMR was bonded with a polished monocrystalline Si wafer and the Raman signal of the monocrystalline Si was tested directly in the IPMR to evaluate the Raman signal collection performance of the IPMR. The Raman test conditions were a laser power of 34.8 mW and an integration time of 3 s. Figure 5(a) shows the Raman spectra of Si collected in IPMRs with different bottom side lengths (10–100µm), and Fig. 5(b) shows the comparison of the peak values at 512 cm−1. The Raman intensity reached the maximum when the bottom side length of the IPMR was 50µm. Using the 512 cm−1 peak value for calculation, it was found that, compared with the Raman signal acquired directly on the Si wafer, the IPMR increased the Raman intensity by approximately 50%. The experimental results verified that while the IPMR was indeed conducive to the collection of Raman signals, it did not reach the levels predicted by the simulation.
Fig. 5. (a) The Raman spectra of Si obtained in IPMR with different bottom side lengths (10–100µm). (b) Comparison of peak values at 512cm−1. (c) The gap between the substrate and the IPMR after bonding. (d) Comparison of simulation results when the gap is 0 and 11.8µm. (e) SERS spectra of R6G obtained on Si wafer and Au film. (f) SERS spectra of R6G obtained after bonding IPMR on Si-based and Au film-based SERS substrates.
The main reasons for the difference between the experimental results and the simulation results are: (1) The calculations of the simulation were based on the total optical power on the surface of the lens; however, it is impossible for all Raman signals irradiated on the surface of the lens to be detected by a spectrometer in reality. (2) The IPMR is assumed to be an ideal specular reflector in the simulation. In fact, the IPMR has a certain degree of roughness (as shown in Fig. 4(c)). The rough surface will scatter the Raman signal, thus reducing the Raman intensity. (3) A gap is generated between the IPMR and the substrate after bonding (as shown in Fig. 5(b), the gap is about 11.8 µm measured under an optical microscope), which prevents part of the Raman signal being reflected into the objective lens. According to the simulation of the gap data measured in Fig. 5(c), the results show that the gap reduces the Raman intensity by about 45% (as shown in Fig. 5(d)), and a wider gap will cause greater Raman signal loss.
Four layers of AgNPs film were assembled on the Si wafer and Au-plated Si wafer as SERS-active substrates by self-assembly on water/oil interface [24]. Rhodamine 6G (R6G) was selected as the probe molecule for SERS testing. The SERS test conditions were a laser power of 1 mW and an integration time of 3 s. All SERS spectra were subtracted baseline by using the “Clean Peaks” function of OceanView software. Figure 5(e) shows the SERS spectra of R6G (10−6 M/L) collected on the Si and Au film substrates with each spectrum representing an average of spectra on 40 random points, and the raw SERS spectra are shown in Figure S1, Supplement 1.
The analytical enhancement factor (AEF) was calculated according to the formula [25]:
The calculation results of AEF for the Si wafer-based substrate and the Au film-based substrate were AEFSi wafer=1.13×107 and AEFAu film=2.33×107 respectively, and the details are shown in Figure S1, Supplement 1. The calculation results show that the experimental results are basically in agreement with the simulation. The AEF of the Au film-based substrate is about twice that of the Si-based substrate. The principal reasons for this result are as follows: (1) The Au film reflects the plasma electromagnetic field, which increases the electromagnetic field intensity at the hot spot [26]. (2) The excitation photons are also reflected by Au film, increasing the number of photons absorbed by the nanoparticles, thereby increasing the Raman intensity [27].
When the above-mentioned SERS substrates (Si wafer-based and Au film-based SERS substrates) were bonded with the IPMR and the R6G (10−6 M/L) SERS spectra were collected under the same test conditions, we obtained the results shown in Fig. 5(f). The raw SERS spectra are shown in Figure S2, Supplement 1. We calculated and compared the peaks at 612 cm−1, 1306 cm−1, 1360 cm−1, and 1506 cm−1 in the SERS spectra, and the results showed that the SERS intensity obtained in the IPMR is about 1.5 times of that on the flat substrates. According to the same method used above, the calculation result of the AEF of the 3D SERS chip (Au film-based SERS substrate + IPMR) was: AEF3D SERS=3.64×107. With the synergistic effects of the substrate and the IPMR, the SERS intensity measured with the 3D SERS chip (Au film-based SERS substrate + IPMR) was about three times that of the Si wafer-based SERS substrate.
4. Conclusions
In conclusion, we propose a 3D SERS chip based on an IPMR, which utilizes the Raman signal convergence effect of the micro-mirror to improve the signal collection efficiency. We used modeling and simulation to optimize the size of the IPMR and completed the production of the 3D SERS chip through anisotropic wet etching, sputtering of Au film, and bonding processes. We conclude that: the reflection convergence effect of the substrate and the IPMR can significantly improve the Raman signal collection efficiency; the Raman signal collection efficiency is greatest when the base length of the IPMR is 50µm, and its individual contribution to the Raman intensity is about 1.5 times; the coupling effect of the substrate has also an important influence on the Raman signal collection, and its individual contribution is about 2 times; under the same experimental conditions, due to the combined action of the substrate and the IPMR, the SERS intensity of R6G measured on the 3D SERS chip was about three times that of the silicon-based SERS substrate.
We also found that the roughness of the IPMR sidewall and the gap formed by the bonding process have a significant impact on the Raman signal collection efficiency. In follow-up research, we intend to make further improvements to the corrosion and bonding processes. The SERS chip has a simple structure and is convenient for mass manufacturing. Most importantly, its 3D structure is convenient to integrate with microfluidic devices and has great potential in the field of SERS biochemical detection.
Funding
National Key Research and Development Program of China (2018YFB2002302); National Natural Science Foundation of China (61971074); Fundamental Research Funds for the Central Universities (2019CDYGYB003); The research project of Sichuan University of Arts and Science (2019PT002Z).
Acknowledgments
We would like to thank Mr. Deng Chao (Electron Microscopy Center of Chongqing University), Mr. Zhou Kai, Mr. Gong Xiangnan (Analysis and Test Center of Chongqing University) for their help in SEM, AFM and Raman characterization, respectively.
Disclosures
The authors declare no conflicts of interest.
See Supplement 1 for supporting content.
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