† Corresponding author. E-mail:
Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0207104), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09040101), the National Natural Science Foundation of China (Grant No. Y6061111JJ), the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2015030), and the Key Technology Talent Program of Chinese Academy of Sciences (Grant Nos. Y8482911ZX and Y7602921ZX).
The hazard of Hg ion pollution triggers the motivation to explore a fast, sensitive, and reliable detection method. Here, we design and fabricate novel 36-nm-thick Ag–Au composite layers alternately deposited on three-dimensional (3D) periodic SiO2 nanogrids as surface-enhanced Raman scattering (SERS) probes. The SERS effects of the probes depend mainly on the positions and intensities of their localized surface plasmon resonance (LSPR) peaks, which is confirmed by the absorption spectra from finite-difference time-domain (FDTD) calculations. By optimizing the structure and material to maximize the intrinsic electric field enhancement based on the design method of 3D periodic SERS probes proposed, high performance of the Ag–Au/SiO2 nanogrid probes is achieved with the stability further enhanced by annealing. The optimized probes show the outstanding stability with only 4.0% SERS intensity change during 10-day storage, the excellent detection uniformity of 5.78% (RSD), the detection limit of 5.0×10−12 M (1 ppt), and superior selectivity for Hg ions. The present study renders it possible to realize the rapid and reliable detection of trace heavy metal ions by developing high-performance 3D periodic structure SERS probes by designing novel 3D structure and optimizing plasmonic material.
It is well known that mercury is a highly toxic pollutant that can do harms to human health even at a very low concentration due to the bio-concentration.[1,2] At present, several methods, such as atomic absorption spectrometry,[3] atomic fluorescence spectrometry,[4] X-ray fluorescence spectrometry,[5] and inductively coupled plasma mass spectroscopy[6] have been proposed for the detection of mercury. However, these methodologies are time-consuming and tedious, and thus it is quite necessary to develop a fast, sensitive, and reliable detection method of Hg ion residues. Surface-enhanced Raman scattering (SERS) is a very powerful technique for molecule detection which has extensive applications in chemical, biological and environmental fields with a low limit of detection.[7–11]
The detection limit of SERS probes does mean something especially for trace or single molecule detection, which is primarily dependent on the maximum intrinsic electromagnetic field (EMF) produced on plasmonic substrates.[12] So far, the main methods to improve the detection limit is by reducing the gaps at hot spots and/or utilizing coupling effects between surface plasmon polaritons (SPPs) and localized surface plasmon resonance (LSPR) in some specific nanostructures.[13–16] In addition, bimetallic composite nanostructures like Ag–Au nanoparticles have presented higher SERS activity than the nanoparticles made from either pure silver or gold.[17–20] Meanwhile, bimetallic Ag–Au materials have the potential to combine the advantages of both metals, i.e., higher SERS activity of Ag,[21,22] better biocompatibility, and chemical stability of Au.[23,24] Improving the stability of the SERS probes is beneficial to the reproducibility of signals, which is significant for the practical application.
Herein, we developed novel three-dimensional (3D) periodic Ag–Au/SiO2 hybrid SERS probes by electron beam evaporating alternately Ag and Au with different thickness ratios on 3D periodic hexagonal SiO2 nanogrids combining annealing treatment for exploration of SERS activity and stability. Based on the general design principles for the 3D periodic nanostructures SERS probes we proposed,[25] we designed the optimized sizes for Ag–Au SERS probes. The optimized SERS probes show the strongest intrinsic electric field enhancement, which is demonstrated by SERS experiments. The SERS activity of Ag–Au/SiO2 nanogrid probes with a certain thickness ratio of Ag and Au is stronger than that of pure Ag and Au structures with the same total thickness under the excitation of a 514 nm laser, which can be explained by the relative positions and intensities of LSPR absorption peaks of these probe structures. The optimized probes show the superior sensitivity, stability, uniformity, and selectivity which were successfully applied to the detection of trace Hg ions in water.
Three-dimensional (3D) Ag–Au/SiO2 periodic nanogrids with various thickness ratios of Ag and Au were designed and fabricated. Hydrogen silsesquioxane (HSQ XR-1541–006, Dow Corning, USA) was firstly spin-coated on silicon (100) substrates with the thickness of 198 nm. Patterning was realized by using electron beam lithography (EBL, Vistec EBPG 5000 plus ES, Raith Company, Germany) with an accelerating voltage of 100 kV and a beam current of 2 nA, followed by the development. The typical width of SiO2 sidewalls for all nanogrids was controlled to be around 11 nm (Fig.
The optical constants of Ag–Au composite layers were determined using a spectroscopic ellipsometer (SE 850 DUV, Sentech Company, Germany). Finite- difference time-domain (FDTD) method was used to calculate the absorption spectra and spatial distributions of the electromagnetic fields. For simplicity, we used the rough Ag–Au/SiO2 models with periodically arranged semiellipsoid-like Ag–Au composite nanoparticles with a smooth Ag–Au composite film of 2 nm thick on the sidewalls of SiO2 nanogrids to model the real Ag–Au/SiO2 nanogrids in which the sizes and center distance of particles were semi-principal axes, a = b = 20.0 nm, c = 10.0 nm, and d = 40.0 nm for the calculations of absorption spectra.[25] The width and height of SiO2 grids were 11 and 198 nm, respectively. The structures were illuminated with a plane wave with x-direction linear polarization incident along the –z axis. Periodic boundary conditions for the xz and yz planes were applied to simulate an infinite array of periodic nanogrids or nanowalls. Perfectly matched layer (PML) boundary conditions were used in the z-direction. The mesh size used in the simulation region was 2 nm for the calculations of absorption spectra and 0.5 nm for the calculations of the electromagnetic fields.
The samples were first immersed into 4,4’-Bipyridine (Bpy) absolute ethanol solution with a concentration of 10−5 M for 4 h, and then dried naturally in air as SERS probes for Hg ions detection. 35 μL of Hg ion solutions with different concentrations of 5.0 × 10−12 (1 ppt), 5.0 × 10−11 (10 ppt), 5.0 × 10−9, 5.0 × 10−7, and 5.0 × 10−5 M were dropped onto the SERS probes, respectively, then kept for 10 min, and finally dried in air. Likewise, 35 μL deionized water was prepared with the same procedure as SERS probes for the blank control group. The SERS measurements were performed using a 514 nm laser with a power of 0.5 mW and the x-polarization on a Raman microscope (Renishaw in-Via, Renishaw company, UK) equipped with a 20× objective (NA = 0.4) and an integration time of 10 s. For each sample, measurements on at least five different positions were taken.
We designed and fabricated alternate Ag and Au layers with various thicknesses deposited on hexagonal 3D periodic SiO2 nanogrids with a total thickness of 36 nm to maximize the SPP effects based on the proposed design method for exploration of SERS activity and stability.[25] Here, 2-nm-thick Ag and 2-nm-thick Au deposited alternately on hexagonal SiO2 nanogrids with a grid length of 188 nm and a height of 198 nm for 9 cycles is labeled by (2 nm Ag–2 nm Au)-9c/198 nm SiO2_h188. The scanning electron microscopy (SEM) images of these samples are shown in Fig.
In order to obtain optical properties of the Ag–Au composite layers above, their equivalent refractive indexes n and extinction coefficients k within the wavelength range from 360 nm to 930 nm were derived by fitting the spectroscopic ellipsometric curves using effective medium approximation method (EMA),[28] as shown in Figs.
To evaluate the performance, the Raman intensity of the peak at 1610 cm−1 of Bpy molecules is recorded to investigate SERS activity here, and its dependences on atomic ratio of Ag for 36 nm Ag–Au/198 nm SiO2 nanogrids_h188 are presented in Fig.
To get plasmon resonance modes for the hybrid structures with different Ag–Au components, we performed finite-difference time-domain (FDTD) simulations to derive the corresponding absorption spectra by assuming the same sidewall roughness and using the measured equivalent optical constants. The results are shown in Fig.
For periodic and rough Ag–Au/SiO2 hybrid nanogrids, SPP waves and their interference effects, LSPR effects, standing wave effect of incident light and coupling effects between LSPR and SPP are involved.[25] For 514 nm light, a height with 36% of the incident light wavelength, 185 nm, is adequate for the strongest intrinsic EMF enhancement due to the standing wave effect (the height here is 198 nm which can meet the condition).[25] The interference effects of SPP waves due to Fabry Perot (FP)-like resonance caused by multiple reflections in periodic structures,[29,30] can excite LSPR as a secondary source (incident light as primary sources), like Au/SiO2 nanogrid structures.[25] According to the calculation formula of SPP wavelength and the equivalent optical constants of the Ag–Au composite layers (Figs.
The general design principles for 3D structures also hold for Ag–Au/SiO2 nanogrids structures here.[25] It is known that the SPP interference effects depend mainly on the geometries of structure especially the sidewall spacing (materials independent) and the reflectance of SPP wave reflected by the sidewall (materials dependent).[25] Therefore, we need to know the sidewall spacing dependence of LSPR and the reflectance for Ag–Au/SiO2 nanogrids. Taking (5 nm Ag–1 nm Au)-6c/198 nm SiO2 as an example, sidewall spacing Lw dependences of the maximum and averaged |E/E0|4 from LSPR on the rough nanowalls at TE mode calculated by FDTD solutions are given in Fig.
SEM image of the optimized hexagonal 36 nm Ag–Au/198 nm SiO2 nanogrids_h157 samples with 28 nm sidewall width and 244 nm sidewall spacing (half of 488 nm SPP wavelength) is compared with those h87, h101, h105, h188, h194, h227, h274, and h282, as shown in Fig.
In order to study the detection performances of the Ag–Au and Ag/SiO2 nanogrid SERS probes for Hg ions, SERS measurements were conducted on the SERS probes onto which Hg ion solutions with different concentrations were dropped. Meanwhile, deionized water instead of Hg ion solutions was prepared as the blank control experiments with the same procedure. The intensity changes of Ag–Au and Ag/SiO2 nanogrid probes with the concentration of Hg ions and their SEM images after SERS measurements are shown in Fig.
We found that the change of signal intensity for (10 nm Ag–2 nm Au)-3c/SiO2 nanogrids probe during the storage of 10 days is smaller than that of (5 nm Ag–1 nm Au)-6c/SiO2 nanogrids probe, which indicates the better stability with 2 nm Au surface layer. Based on this, we fabricated (7 nm Ag–2 nm Au)-4c and (16 nm Ag–2 nm Au)-2c/198 nm SiO2 nanogrids_h157 probes with 2 nm thick Au outermost layer to study the stability of the unannealed and annealed probes after 10 days of storage. We annealed the Au, Ag, and Ag–Au/198 nm SiO2 nanogrids structures, and their SEM images are shown in Fig.
Experimental SERS intensities of unannealed and annealed hexagonal (2 nm Ag–2 nm Au)-9c, (7 nm Ag–2 nm Au)-4c, and (16 nm Ag–2 nm Au)-2c/198 nm SiO2 nanogrids_h157 probes are shown in Fig.
Here, the annealed hexagonal (7 nm Ag–2 nm Au)-4c/198 nm SiO2 nanogrids_h157 SERS probes with 10-day and without storage (0 day) were used to detect Hg ions, as shown in Figs.
In summary, we designed and fabricated novel 3D periodic Ag–Au/SiO2 nanogrids as SERS probes for trace Hg ions detection. High performances were realized by structural optimization in terms of maximization of electric field enhancement at hot spots responsible for SERS effects, and material optimization, i.e., deposition of alternate Au and Ag layers on SiO2 nanogrids with different thickness ratios and a total thickness of 36 nm. Annealing is demonstrated to favor stabilizing SERS probes. The SERS effects of the probes are related to not only the positions and intensities of their LSPR peaks but the Ag and Au proportions at the surface, which was basically confirmed by the absorption spectra derived from FDTD calculations. The design method of 3D periodic SERS probes proposed based on plasmonic gold proves to hold for not only single constituent plasmonic materials but also plasmonic composite materials such as Ag–Au composite layers here.[25] By structural optimization the annealed (7 nm Ag–2 nm Au)-4c/198 nm SiO2 nanogrids_h157 probes show the best stability with only 4.0% SERS intensity change during 10-day storage, good detection uniformity (RSD 5.78%), the detection limit of 5.0 × 10−12 M (1 ppt), and superior selectivity of detection for Hg ions. Therefore, the combination of structural design and material optimization makes it possible to develop novel 3D periodic structures as high-performance SERS probes which can be applied to rapid and reliable detection of trace heavy ion such as trace Hg ions in water.
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