Island-like Nanoporous Gold:Smaller Island Generates Stronger

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Island-like Nanoporous Gold: Smaller Island Generates Stronger Surface-Enhanced Raman Scattering Jinglin Huang, Zhibing He, Xiaoshan He, Yansong Liu, Tao Wang, Guo Chen, Cuilan Tang, Ru Jia, Lei Liu, Ling Zhang, Jian Wang, Xing Ai, Shubing Sun, XiaoLiang Xu, and Kai Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08013 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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ACS Applied Materials & Interfaces

Island-like Nanoporous Gold:Smaller Island Generates Stronger Surface-Enhanced Raman Scattering Jinglin Huang,†Zhibing He,†Xiaoshan He,† Yansong Liu,†Tao Wang,†Guo Chen,†Cuilan Tang,†,‡RuJia,§ Lei Liu,†,‡ Ling Zhang,†Jian Wang,† Xing Ai,†Shubing Sun,†XiaoliangXu,ǁand Kai ⊥

Du†, ,* †

Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, PR

China ‡

School of Material Science and Engineering, Southwest University of Science and Technology,

Mianyang 621010, PR China §

Analytic and Testing Center, Southwest University of Science and Technology, Mianyang 621010,

PR China ‖

School of Physical Sciences, University of Science and Technology of China, Hefei 230026, PR

China ⊥

Collaborative Innovation Center of IFSA (CICIFSA), Shanghai Jiao Tong University, Shanghai

200240, China

*Corresponding author: E-mail: [email protected] 1

ACS Paragon Plus Environment


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ABSTRACT The surface-enhanced Raman scattering properties of nanoporous gold prepared by the dealloying technique have been investigated for many years.The relatively low enhancement factor or the poor uniformity of existing conventional or advanced nanoporous gold structures are still the main factors that limit their wide application as Raman enhancement substrates. Here, we report island-like nanoporous gold (INPG) fabricated by simply controlling the composition of the dealloying precursor.This nanostructure can generate ~10 times higher enhancement factor (above 107) with ~4 times lower gold consumption than conventional nanoporous gold. The dimensions of the gold islands can be controlled by the composition of the precursor. The enhancement factor can therefore be controlled by thegold island dimensions, which suggestsan effective approach to fabricate better Raman enhancement substrates. Furthermore, INPG exhibits excellent Raman enhancement uniformity and reproducibility with the relative standard deviation of only 2.5% and 6.5%, which originates from the extremely homogeneous structure of INPG at both the microscale and macroscale. The excellent surface-enhanced Raman scattering properties make INPG a potential surface-enhanced Raman scattering substrate. KEYWORDS Island-like; nanoporous gold; dealloying; surface-enhanced Raman scattering; electromagnetic enhancement; radiation damping

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1. INTRODUCTION The surface-enhanced Raman scattering (SERS) effect has attracted interest since its first discovery in the 1970s because of its diverseapplications to molecular detection, reaction dynamics, biosensors, explosives detection, and art conservation.1–5 The most challenging issue in SERS substrate studies is how to achieve the best performance, including high Raman enhancement, stability, uniformity and reproducibility, with low cost.6–9 A variety of SERS substrates have been reported,10–13 such as nonuniform substrates prepared by electrochemical oxidation and reduction cycles, nanoparticle sols with large size distributions prepared by wet chemical synthesis, nanoparticles with controlled shape and size prepared by chemical synthesis methods and surface nanostructures with defined shape, size, and interparticle spacing prepared by template, self-assembly, or lithography methods. It has recently been reported that nanoporous gold (NPG) fabricated by dealloying or self-organization methods is a potential SERS substrate owing to its excellent SERS properties.14–17 In particular, for dealloyedNPG, the SERS properties mainly originate from the unique bicontinuous ligament and pore nanostructures.18–21For conventional NPG, although the uniformity and reproducibility are excellent, the relatively low enhancement factor (EF) limits its applications. Therefore, much effort has been devoted to the design and fabrication of more advanced NPG structures.22–24 Various advanced NPG structures have been reported during the studies of NPG for SERS, for example, patterned NPG prepared by a direct imprinting process,22 wrinkled NPG prepared by thermal contraction of the NPG/PS composites,23 and NPG disks prepared by sputter etching.24These advanced structures can produce abundant electromagnetic “hot-spots” and therefore large Raman enhancement. Unfortunately, such high enhancement is 3



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generally difficult to reproduce over large areas, which significantly limits the practical applications of these NPG structures. Therefore, it is essential to explore more effective methods to fabricate more advanced NPG structures that simultaneously have a high EF, high Raman uniformity,high reproducibility, and low cost. In this study, we report an island-like nanoporous gold (INPG) prepared by dealloying Au–Ag alloy precursors with Au content as low as ~9 at.%. It is demonstrated that the fabricated INPG can produce ~10 times higher SERS EF with ~4 times lower gold consumption than conventional NPGs, as well as excellent Raman enhancement uniformity and reproducibility. To our knowledge, this is the best result using little gold to achieve excellent SERS properties during the research of NPG.

2. EXPERIMENTAL SECTION 2.1 Chemicals and Materials The Au (99.99% purity), Ag (99.9% purity) and Cr targets (99.99% purity) were purchased from the General Research Institute for Nonferrous Metals (Beijing, China). Ar (99.999% purity) and N2 (99.999% purity) were purchased from Chengdu Messer Gas Co., Ltd. (Chengdu, China). Acetone, ethanol, nitric acid (HNO3, 70%, AR), perchloric acid (HClO4, 70%, AR), sulfuric acid (H2SO4, 98%, AR), copper sulfate pentahydrate (CuSO4·5H2O, AR), crystal violet (CV, AR), rhodamine 6G (R6G, AR), and glucose (AR) were purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Silicon wafer was purchased from Suzhou Crystal Silicon 4


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Electronic & Technology Co., Ltd. (Suzhou, China). Deionized water (DI, 18 MΩ) was prepared by purification with a Milli-Q gradient quality system (Millipore, Bedford, MA, USA). 2.2 Fabrication of INPG To obtain the INPG, the composition of the Au–Ag alloy precursor should be carefully controlled. Four different Au–Ag alloy films (Au9Ag91, Au13Ag87, Au18Ag82, and Au33Ag77) were deposited on Si (111) substrates as precursors by the direct current magnetron sputtering technique. The Au and Ag targets were co-sputtered to form the alloys. First, the Si substrates were successively cleanedwith acetone, ethanol, and DI. The Au–Ag alloy films were sputtered in pure Ar at a working pressure of 0.5 Pa (3.8 mTorr) and room temperature. The base pressure was lower than 1.2 × 10–4 Pa (9 × 10–7Torr). To avoid collapse of the films during dealloying, it is important to first deposit a 10 nm Cr layer and a 10nm Au layer as an adhesion layer to increase the adhesion strength between the Au–Ag alloy and the Si substrate. The composition of the Au–Ag alloy was altered by changing the power of the targets based on the deposition rate of the Au and Ag elements. The power of the Ag target was set constant to 50 W, while the Au target power was set to 14, 19, 26, or 59 W. The thickness of the deposited Au–Ag alloy layer was approximately 100 nm. The Au–Ag alloy precursors were dealloyed by 50% HNO3 for 1.5 h at room temperature followed by DI rinsing and nitrogen drying to form INPG films.

2.3 Characterizations The microstructures of the INPG films were determined by field emission scanning electron microscopy (FE-SEM, ZEISS Merlin VP Compact) at an accelerating voltage of 15 kV. The compositions of the Au–Ag alloy precursors and INPG films were determined by energy 5



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dispersive X-ray spectroscopy (EDS) combined with SEM. The net surface areasof the INPG and NPG samples were measured by two methods: the capacitance ratio method using 1 M HClO4 as the electrolyte25 and Cu underpotential deposition (UPD) using 10-3 M CuSO4 and 0.5 M H2SO4as the electrolyte,26,27 respectively. The INPG or NPG sample, a Ag/AgCl electrode, and a Pt wire electrode were used as the working electrode, reference electrode, and counter electrode, respectively. The contact area between the working electrode and the electrolyte was set to 1 cm2. Cyclic voltammogramswere measured using an electrochemical workstation (Autolab PGSTAT 302N).

2.4 SERSmeasurements Standard surface-enhanced Raman spectroscopy test was performed using a micro-Raman spectrometer (RenishawInVia) with an excitation laser wavelength of 514 nm. The laser power was set to 0.06 mW and the focused spot size was set to 1 µm. The integration time was 50 s. To investigate the electromagnetic (EM) effect more precise, CV was used as a probe molecule to avoid the resonance Raman scattering (RRS) effect. R6G was used to perform the limit of detection (LOD) test with the assistance of RRS. The detection of glucose was performed with the laser power set to 0.6 mW and the integration time to 50 s. To perform the SERS measurements, the samples were immersed in corresponding solutions for 2 h, thoroughly rinsed with DI, and then dried under blowing N2.

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3. RESULTS AND DISCUSSION This simple method has long been ignored by researchers because it is believed that the bicontinuous structure of NPG would collapse during dealloying when the Au content is relatively low (in general, lower than 20 at.%) and the SERS properties of the collapsed structures would be poor. In this study, we successfully fabricated three types of INPG films by dealloying different Au–Ag alloy precursors (see Section 2.2). A NPG film was also fabricated for comparison. For simplicity, the prepared INPG samples are named based on the Au content in the precursor: INPG9, INPG13, andINPG18 for the 9, 13, and 18 at.% Au precursors, respectively.

3.1 Microstructure analysis of INPG After dealloying for 1.5 h, the residual Ag content in the INPG samples is less than 3 at.% (Table S1), while it is ~16 at.% in the NPG sample. The microstructures of the fabricated INPG films are shown in Figure 1a-c. After dealloying, the integrated Au–Ag alloy films decompose to form detached gold clusters (red arrows in Figure 1a–c). These irregular gold clusters randomly distribute on the Si substrate like islands scatteredin the sea. The gold islands are separated by continuous large gaps (yellow arrows in Figure 1a–c), while their inner structure is separated by some small gaps (blue arrows in Figure 1a–c). Interestingly, by comparing with the conventional NPG structure (Figure 1d), the inner structure of the gold islands is similar to the structure of NPG. Their microstructures can be considered to be composed of the same basic structural unit. An example is shown in Figure 2a. This gold island is composed of three structural units labeled 1, 2 7



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and 3. For NPG, this structural unit is called a gold ligament.28, 29 Therefore, for consistency, we also call it a gold ligament. The gold islands are composed of finite ligaments, while the bicontinuous structure of NPG is composed of infinite ligaments. Overall, the gold nanostructure fabricated in this work is a typical hierarchical nanostructure with two distinct structural features:an exterior island structure and an inner nanoporous structure. Therefore, we call this new type of gold nanostructure INPG. It should be noted that when the Au content is too low, the nanoporous structure of the INPG would degenerate to gold nanoparticles (GPs), as shown in Figure S1.

Figure 1. Microstructures of the INPG and NPG films. (a–d) Low magnification SEM images of the INPG9, INPG13, INPG18, and NPG films,respectively. The insets in (a–d) show the corresponding high magnification SEM images. The dealloying process for each sample was performed at room temperature for 1.5 h.

For analysis ofthe INPG structure, it is necessary to introduce parameters to describe the two structural features. Referring to the studies of NPG,30,31 we definethe ligament diameter (D) to represent the structural unit of the INPG, as shown in Figure 2a. It is not easy to quantify the features of the gold islands because of their irregular shapes. Here, we only discuss the variation 8


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of their geometrical dimensions. The mean dimensions of the gold islands were measured. For an easier discussion, the equivalent radius of a disk (Req) that has the same mean dimension as the measured islands is introduced to describe the geometrical dimensions of the gold islands, as shown in Figure 2b. With these two characteristic parameters, it is possible to describe the differences of the INPGsamples. Figure 2c shows the variations of Req and D with the Au content in the precursor.The variation of D is negligible, which indicates that the composition of the precursor has little influence on the structural unit of the INPG. Reqlinearly increases with increasing Au content, which shows that the island dimensions can be accurately controlled by the precursor composition. Overall, the composition of the precursor mainly determines the exterior gold island structure of INPG.

Figure 2. Structure dissection of the INPGfilms. (a) Schematic diagram of a gold island modeled from the SEM

image of INPG9 to describe its inner structure. (b) Schematic diagram of conversion of a gold island from the

SEM image of INPG13 to an equivalent disk for description of its geometrical dimensions. (c) Variations of the

two characteristic parameters (Req and D) with the Au content in the precursor.

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3.2 Formation mechanism of INPG To comprehensively investigate this new gold nanostructure, it is necessary to discuss its formation mechanism. Figure 3 shows the microstructure evolutionof dealloying of Au18Ag82 for variousdealloying times (0.25–60 min). The corresponding SEM images of the other two INPG samples are shown in Figure S2. Interestingly, gold islands appear after only 0.25 min dealloying (Figure 3a). However, the inner structure of the islands (the typical ligament structure) is not clear until 15 min dealloying (Figure 3c). These results indicate that formation of the gold islands occurs prior to formation of the internal nanoporous structures.Erlebacheret al.28 suggested that dealloying of the Au–Ag alloy involves two main processes: dissolution of less noble Ag and diffusion of noble Au. These two processes combine during the dealloying process. Dealloying starts with dissolution of Ag atoms.When the Ag atoms on the first layer are dissolved, the Au atoms on this layer would diffuse and agglomerate. As a result, two distinct regions form on the surface: pure gold clusters and patches of un-dealloyed alloy exposed to electrolyte. Diffusion of Au only occurs at the alloy-electrolyte interface. Therefore, the motion of the alloy-electrolyte interface has a significant effect on the final microstructures of the dealloyed gold. During formation of the INPG structures, we suggest that the grain boundaries of the Au–Ag alloy films act as rapid passages for motion of the alloy-electrolyte interface.Morrishet al.32 reported a similar phenomenon in their study of formation of nanoporous Au by dealloying the Au–Cu alloy. It is well known that volume shrinkage occurs during the dealloying process.33This could be another 10


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reason for formation of the island structures. We believe that volume shrinkage could expand the gaps between the islands, resulting in isolated gold islands. As well as the above two reasons, the interactions of Au/Cr/Si at their interfaces should not be ignored.34 We believe that mutual diffusion of these three atoms could result in a firm adhesion layer, which ensures that the isolated gold islands do not fall off the Si substrate. Based on the above three reasons, gold islands can form. The inner structure of the INPG would slowly form with further dissolution of Ag atoms. In addition, it is not surprising that the ligament size D increases with increasing dealloying time (Figure 3f, the values at 90 min were measured from Figure 1c) because of the diffusion dynamics of dealloying.35–37From the discussion of the composition effect, the inner nanoporous structure is mainly determined by the dealloying process.

Figure 3. SEM micrographs and characteristic size analysis of INPG18. (a–e) Low magnification SEM images of

INPG18 at dealloying times of (a) 0.25 min, (b) 1.5 min, (c) 15 min, (d) 35 min, and (e) 60 min. The insets show

the corresponding high magnification SEM images of samples. (f) Measured results of the ligament size Dversus

thedealloying time.

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3.3 Surface area of INPG Generally, the specific surface area of nanoporous materials has a significant effect on their properties, especially for catalysis, energy storage, sensors, and SERS.38–41 Consequently, the surface areas of the INPG samples were investigated. Here, we only discuss the net surface area of INPGbecause it is difficult to precisely measure the mass of thin films. However, this is sufficient to compare the INPGsamples. The net surface areas of the INPG samples were measured by the electrochemical capacitance ratio method.42The external surface area of each sample was set to 1 cm2. The cyclic voltammograms in the double-layer region at 40 mV/s are shown in Figure S3. Figure 4a shows the corresponding averages of the capacitive currents when = 0.3 V. The variation of the net surface area of the INPG samples with the Au content in the precursor is shown in Figure 4b. Considering possible contamination of Cr oxides during the surface area measurement, the reliability of the method was demonstrated by the Cu UPD test. The results are shown in Figure S4. The potential in Figure S4(a) is the relative value versus the Ag/AgCl electrode. The Cu UPD charge density of the polycrystalline Au film was estimated to be ~0.406 mC/cm2 after subtraction of the background. The surface areas of the INPG and NPG samples were estimated according to their Cu UPD charge densities, as shown in Figure S4(b). The results are only slightly smaller than the results in Figure 4. It is well known that there is a diffusion limited process during UPD measurements of nanoporous materials, which could lead to 12


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underestimated surface areas.26,27 Therefore, the surface areas measured by the two methods should be consistent. The net surface areas of all of samples are larger than 1 cm2, which indicates that the INPG samples have abundant internal structures. The net surface area increases with increasing Au content in the precursor. The net surface area of NPG is ~2.5 times larger than INPG9. It should be point out that the net surface area can be further improved by shortening the dealloying time (see Figure S5) because the ligaments tend to coarsen with increasing dealloying time.

Figure 4. Surface area characterization of the INPG and NPG films. (a) Mean current magnitude at = 0.3 V

of cyclic voltammograms measurement versus the scan rate. (b) Estimated net surface area versus the Au content

in the precursor.

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3.4 SERS analysis of INPG Raman spectra of the INPG samples were recorded to investigate their SERS properties. CV was chosen as the probe molecule with an excitation wavelength of 514 nm to avoid the effect of RRS. Figure 5a shows the Raman spectra of the INPG samples using 10-6 MCV as the probe molecule. The spectrum of the NPG sample is included for comparison. The INPG films have much stronger Raman signals than the NPG film. The Raman intensity of INPG9 is more than 4 times stronger than that of NPG, which is a considerable enhancement of the Raman intensity. The number of effective probe molecules is known to significantly affect the Raman intensity. Therefore, the EF is a more objective parameter to evaluate the SERS properties. The relative enhancement factor (EFR) of the INPG samples with respect to NPG was estimated by the following formula (the details are described in our previous work):43

EF = ( ⁄ )⁄( ⁄ )

(1)

where and are the intensity of the SERS signal and the number of probe molecules contributing to the Raman signal of INPG, respectively. and are the corresponding parameters of the NPG film. The variation of the EFR as a function of Req is shown in Figure 5b. For the INPG films, there is an approximately linear decrease of the Raman enhancement withincreasing Req, which indicates that smaller islands can produce stronger Raman signals. It is 14


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significant to note that INPG9 can produce ~10 times stronger Raman enhancement with ~4 times lower gold consumption compared with the NPG film, which indicates a ~40 times higherinput-output ratio of SERS for INPG9 than conventional NPG.It is well known that the EF of NPG films is above 106 magnitudes23, so the EF of INPG9 is above 107.

Figure 5. SERS spectra of 10-6 M CV molecules on the INPG and NPG films. (a) Comparison of the SERS spectra

of the INPG and NPG films. (b) Variation of the estimated relative enhancement factor (EFR) with the equivalent radius Req of the gold islands. The excitation wavelength was 514 nm. The intensities of the peak at 1621 cm-1 were chosen for the estimate of EFR.

The analytical performance including the sensitivity, uniformity and reproducibility of the 15



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best SERS substrate (INPG9) were investigated by the detection of both CV and R6G molecules. To obtain a strong signal, the dealloying time was set at 15 min. Figure 6a shows the SERS spectra of a series of concentrations of CV (1 × 10-5–1 × 10-8 M) loaded on the substrate. The intensities of the SERS signals increase with increasing CV concentration. The LOD, i.e. the lowest concentration at which the most intense signal (1621 cm-1) remain noticeable, is as low as 1 × 10-8 M. There is a good linear relationship for the log-log plot of the Raman intensity of the peak centered at ~1621 cm-1 versus the concentration of CV (Figure 6b), which demonstrates that INPG is a potential candidate for rapid and quantitative detection of organic molecules at trace concentrations. As shown in Figure 6c and d, the Raman results for R6G are similar to the results for CV. With the assistance of RRS, the LOD of R6G is as low as 1 × 10-10 M, which makes it possible for the detection of molecules at ultralow concentrations.

Figure 6. SERS response of CV and R6G at different concentrations on the INPG9 substrate. (a) and (b) SERS 16


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spectra of series of concentrations of CV and R6G, respectively. (b) and (d) Log-log plots of the intensity of the SERS signal at 1621 cm-1 versus the CV concentration and the intensity of the SERS signal at 1362 cm-1 versus the

R6G concentration, respectively.The excitation wavelength was 514 nm.

The uniformity and reproducibility of SERS substrates significantly affect their practical applications. Therefore, these two properties of the INPG9 substrate were investigated. The uniformity of INPG9 was investigated by point-by-point Raman mapping over a large area (10 µm × 10 µm) with a step size of 2 µm and 10-9 M R6G as the probe molecule, as shown in Figure 7a. The brightness of the map is proportional to the signal integral intensity of the peak at 1362 cm-1. The brightness is similar for the whole area, which indicates that the substrate has good uniformity over its surface. To confirm this conclusion, the SERS spectra of 25 randomly selected positions were recorded under the same experimental settings (Figure 7b). The relative standard deviation (RSD) values of the Raman intensities were calculated to less than 2.5%, which indicates excellent uniformity over the entire area of the sample.The reproducibility of INPG9 was evaluated by acquiring SERS spectra of R6G (1 × 10-7 M) foreight replicate samples, as shown in Figure S6. The results show similar and consistent SERS signals. The corresponding RSD is estimated to be less than 6.5%, which indicates excellent reproducibility of this SERS substrate. The SEM images in Figure 1 showthe microscopic uniformity of the INPG films. The results of INPG9 fabricated on a 3 inch wafer before and after dealloying (Figure S7) show the macroscopic large area uniformity of this structure. These results suggest that INPG can be used for applications where reproducibility over a large area is desired, such as large illumination area electrodes for plasmonic fuel cells, and remote/roadside detection of chemical warfare agents or illicit drugs.44–47 17



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Figure 7. Uniformity measurementsof the INPG9 sample. (a) Raman mapping of INPG9 (2 µm step size, 5 × 5=25

points), where the brightness of the image is proportional to the signal integral intensity at 1362 cm-1. The scale bar is 2 µm. (b) SERS spectra and corresponding RSD values of 10-9 M R6G at 25 different points on the INPG9

surface. The excitation wavelength was 514 nm.

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3.5 Enhancement mechanism of INPG To theoretically elucidate the effect of the INPG structure on SERS enhancement (here, we only focus on the EM mechanisms), the local electric field contours of INPG were calculated using the three-dimensional finite-difference time-domain (FDTD) method, as shown in Figure 8. The INPG structural models were directly constructed from the SEM images. The structures were placed on an infinite Si substrate and illuminated from the top by a linearly polarized plane wave with a wavelength of 514 nm. The simulation mesh was set to a 1.0 nm cubic grid with perfectly matched layers (PML) as the boundary condition. The contour maps visualize the enhanced electric fields of | ⁄ | , where E0 and E are the amplitudes of the incident and enhanced electric fields, respectively. The contour maps of the INPG films and NPG film are shown in Figure 8a–d. The calculated electric fields are mainly distributed in the gaps among adjacent ligaments. For the INPG films, there are also abundant “hot spots” on the edges of the gold islands. For quantitative comparison of the simulated results, statistical analysis of log (| ⁄ |" ), which reflects the EM effect of the SERS substrate, of each pixel was performed.48,49 The entire area of each field contour in Figure 8a–d was analyzed (600×600 pixel). The number of hot spots (counts) is plotted against log (| ⁄ |" )in Figure 8e. There are many more counts for the INPG films than for the NPG film, which indicates that the INPG films have stronger EM enhancement than the NPG film. These simulation results are in reasonable agreement with the experimental results.

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Figure 8. Three-dimensional FDTD simulations for evaluating the EM effect of the INPG samples. (a–d) Squared

magnitudes of the local electric field amplitudes of (a) INPG9, (b) INPG13, (c) INPG18, and (d) NPG. (e) Statistical results of the log (| ⁄ |" ) values for quantitative comparison of the simulated results. The incident light with a wavelength of 514 nm enters in the z-direction and is polarized in the x-direction, as indicated by the

white and yellow arrows, respectively. The scale bars are 100 nm.

We suggest that the large Raman enhancement of INPG should originate from the following three aspects: the residual silver, the nanoporous morphology, and the island structure. First, according to the EDS results, the residual silver in INPG cannot be ignored. Thus, it is better to consider the fabricated INPG as a hybrid material. The residual silver should contribute to the 20


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Raman enhancement of INPG because of its stronger SERS enhancement than gold.50The NPG sample has more residual silver but much lower Raman intensity than INPG, which indicates that the residual silver is not the major factor for the stronger Raman signals of INPG. Second, the nanoporous morphologies of INPG would also contribute to the Raman enhancement. This was demonstrated by comparing the Raman spectra of NPG and GPs, as shown in Figure S8. The Raman intensity of NPG is much stronger than that of the GPs, which indicates that the nanoporous gold structures have better SERS properties than dispersed GPs. The weaker Raman enhancement of the GPsis probably caused by the lack of coupling effects induced by nanoporous structures. Finally, we believe that formation of the island structure of INPG is the main reason for the enhancement of the Raman signals. Morphological studies of NPG have revealed that NPG has typical bicontinuous ligament and pore structures over the whole volume.51,52Formation of gold islands could divide the continuous ligament structures of NPG, which can be regarded as a complete structure with large dimensions, into fragmented structures. This process could effectively decrease the structural dimensions of NPG. Usually, smaller dimensions have a smaller radiation damping rate, which can produce larger EM enhancement.53,54Our experimental results show that decreasing the gold island dimensions could increase the EF. A theoretical simulation also shows that EM enhancement will increase with decreasing gold island dimensions (Figure 9). Therefore, we believe that formation of gold islands in the NPG structures could effectively decrease the radiation damping rate, and consequently increase EM enhancement. Our results suggest that it is beneficial to appropriately reduce the dimensions of the structural unit during the design and fabrication of SERS substrates. 21



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Figure 9. Three-dimensional FDTD simulations for comparison of the effect of the island dimensions on EM

enhancements. (a–d) Simulation models constructed by masking the SEM images of the NPG film to ensure the same inner structures. (e–h) Squared magnitudes of the local electric field amplitudes of the corresponding models. The incident light with a wavelength of 514 nm enters in the z-direction and is polarized in the x-direction, as

indicated by the white and yellow arrows, respectively. All of the scale bars are 100 nm.

3.6 Application of INPG to detection of glucose The excellent SERS properties of INPG encouraged us to further explore its practical applications. Careful regulation of glucose levels is important for diabetes to maintain their health. Most diabetics measure glucose levels several times per day with the electrochemically based finger-stick method, which brings pain and discomfort for diabetics. It is necessary to develop new 22


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techniques for the measurement of glucose levels without collecting blood. SERS is one of the potential techniques that can meet this requirement.55,56To characterize INPG for glucose sensing, three standard glucose aqueous solutions with concentrations from 1 × 10-3 to 1 × 10-5 M were introduced to INPG9 for SERS measurement. The results are shown in Figure 10. Unfortunately, the Raman signal is not quite obvious. While the two broad Raman bands near 1200 cm-1 and 1400 cm-1 should belong to the characteristic peaks of glucose(1073 cm-1, 1126 cm-1, 1365 cm-1, and 1462 cm-1).55 Generally, glucose molecule has terrible adsorbed ability with metals, therefore the weak Raman signals of glucose is probably caused by the terrible adsorption of glucose molecule with INPG.55,56 The exploration of additional surface modifies of INPG to improve its adsorption for glucose molecule will be the topic of our further work.

Figure 10. SERS spectra of a series of glucose standard solutions at various concentrations. The excitation

wavelength was 514 nm. The laser power and the integration time was set to 0.6 mW and 50 s, respectively. The

inset is the molecular structure of glucose.

23



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4. CONCLUSIONS Our study demonstrates that INPG can be simply fabricated by reducing the gold content in the dealloying precursor. The gold content in the precursor determines the final microstructure of the dealloying gold, that is, more gold results in continuous NPG while less gold results in INPG. The dimensions of the gold islands can be easily controlled by the gold content in the precursor. This INPG structure shows excellent SERS performance, including high EF, high Raman uniformity, and high reproducibility. The integrated actions of the nanoporous morphology and island structure of INPG mainly lead to the strong SERS. Formation of gold islands can result in higher electromagnetic enhancement than conventional nanoporous gold. Smaller gold islands can produce stronger SERS enhancement. The high uniformity and reproducibility of this advanced gold nanostructure are ascribed to the uniformly distributed gold islands on the substrate. This can lead to formation of uniform high density hot spotsthroughout the large surface area. Therefore, the INPG structure is considered to be a high-performance, low-cost, and easy to fabricate SERS substrate.

█ASSOCIATED

CONTENT

Supporting Information Ag content of the prepared samples before and after dealloying. Microstructure of the gold nanostructure by dealloyingthe Au3.9Ag96.1 alloy precursor for 1.5 h. SEM micrographs of INPG9 and INPG13 after various dealloying times. Cyclic voltammograms of the current and electrode potential of INPG and NPG. Surface area characterization of INPG and NPG films dealloyingfor 24


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15 min. Evaluation of the reproducibility of INPG9. Photograph of INPG9 on 3 inch wafers before and after dealloying. Comparison of the SERS spectra of the NPG and GP substrates.

█ACKNOWLEDGEMENTS

This research was supported by the National Natural Science Foundation of China (Grant No. 51401194).

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Island-like Nanoporous Gold:Smaller Island Generates Stronger

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