Superhydrophobic SERS Substrates Based on Silver-Coated

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Superhydrophobic SERS Substrates Based on Silver-Coated Reduced Graphene Oxide Gratings Prepared by Two-Beam Laser Interference Zhao-Xu Yan,‡ Yong-Lai Zhang,*,† Wei Wang,† Xiu-Yan Fu,† Hao-Bo Jiang,† Yu-Qing Liu,† Prabhat Verma,‡ Satoshi Kawata,*,‡ and Hong-Bo Sun*,† †

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China ‡ Department of Applied Physics, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan S Supporting Information *

ABSTRACT: Reported here is the fabrication of reduced graphene oxide (RGO) grating structures by two-beam laser interference (TBLI) for the development of highly efficient SERS substrates via simple physical vapor deposition (PVD) coating of silver. TBLI has been utilized to make hierarchical RGO grating structures with microscale gratings and nanoscale folders through a laser treatment induced ablation and photoreduction process. The hierarchical structures contribute to the formation of plasmonic structures after silver coating, giving rise to the formation of plenty of SERS “hot spots”, while the RGO substrate would provide chemical enhancement of Raman signal through interaction with analytes molecules. The significantly increased roughness with respect to the hierarchical structures in combination with the removal of hydrophilic oxygen-containing groups endow the resultant substrates with unique superhydrophobicity, which leads to the enrichment of analytes and further lowers the detection limit. The synergistic effects make the silver coated RGO gratings a highly efficient SERS substrate; in the detection of Rhodamine B, our SERS substrates showed high SERS enhancement and good reproducibility, a detection limit of 10−10 M has been achieved. KEYWORDS: SERS, superhydrophobic, silver nanoparticles, RGO gratings, two-beam laser interference

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INTRODUCTION Surface enhanced Raman spectroscopy (SERS) has attracted more and more research interest1−5 because it not only can provide the fingerprints information on target molecules but also shows very high sensitivities, which reveals great potential for practical analysis and detection, such as chemicals identification and analytes concentration judgment.6−9 To realize effective SERS measurements, basically, one needs to devote more effort to rational design of effective plasmonic structures using proper materials as SERS active substrates, which can enhance the Raman signal by means of localized surface plasmon resonance (LSPR).10 In this regard, the fabrication of highly efficient SERS substrates is generally considered as the key issue for practical SERS detection. With the help of classical micronanofabrication technologies, represented by “top-down” and “bottom-up” approaches, many different kinds of metallic micronanostructures were designed and fabricated for SERS measurements, such as closepacked metallic nanoparticles, nanorods arrays, nanogaps and multibranched metallic nanocrystals etc.11−17 However, these methods were either proved to be very complicated, or depending on expensive instruments (e.g., electron beam lithography), which limits the broad application of SERS technique. To realize a facile preparation of SERS substrates, © XXXX American Chemical Society

rough metal surface have also been prepared by coating silver or gold nanoparticles on rough templates, such as DVD disk, paper, and even rose petal.18−20 However, such templates usually possess strong photoluminescence (PL). Furthermore, both the Au and Ag films also have a strong photoluminescence (PL) background under laser excitation, which sets huge obstacle in signal collection, not to mention the detailed molecular vibrational information on fluorescence molecules. From this point of view, 2-dimensional (2D) graphene and its derivations (e.g., graphene oxide, GO, and reduced graphene oxide, RGO) seem to be a kind of favorable template for SERS, because the exceptional electronic properties may suitably address the above-mentioned concerns by effective quenching the fluorescence signal from probe molecules.21 Additionally, graphene derivations such as GO and RGO showed great potential in SERS investigation due to the additional SERS chemical enhancing ability,22 high adsorption to target molecules,23 very good biocompatibility24 and antioxidation of silver nanoparticels.25 Thus, rational combination of GO/ RGO and noble metal NPs could dramatically improve the Received: May 14, 2015 Accepted: November 23, 2015

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DOI: 10.1021/acsami.5b09128 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

of RhB (4 μL) was dropped onto the Ag-RGO gratings substrate. Due to the superhydrophobicity, it keeps a sphere shape, the RhB droplet was evaporated naturally for further Raman tests. Characterization. The reflectance spectra of the substrate were measured by UV−vis spectrometer (spectrophotometer (UV-2550, SHIMADZU Co., Inc., Japan). The static water contact angle (CA) was measured by using the Contact Angle System OCA 20 (DataPhysics Instruments GmbH, Germany) at room temperature. The CA was measured by the sessile-drop method with a water droplet of 4 μL. Scanning electron microscopy (SEM) images were taken by a field-emission scanning electron microscope (JSM-7500F, JEOL, Japan). The morphology of the substrate was measured by atomic force microscopy (AFM, iCON, Veeko). Raman spectra were recorded on a LabRAM HR Evolution from Horiba scientific equipped with a He−Ne laser at 633 nm as excitation source, the laser power was about 30 μW on the samples, and the average spot size was 1 μm in diameter.

conventional SERS performance in various aspects. However, currently, it is still challenging to integrate hierarchical micronanostructures with RGO film, which more or less limits the development of RGO-based SERS substrates. On the other hand, in addition to the attempts of enhancing the E-field, to get higher Raman enhancement, other strategies have also been implemented, among which enriching analytes into confined area by controlling the surface wettability is an effective one, especially for the detection of analytes with low concentration.20,26 As typical examples, Gentile et al.,26 first realized the Raman detection of Rhodamine 6G with ultralow concentration (10−18 M), by using superhydrophobic substrates comprising a regular lattice of silicon micropillar arrays covered with silver nanograin aggregates. After that, Li et al.27 directly confirmed the contribution of superhydrophobicity to SERS detection by comparing the SERS signal on silver nanoparticles coated zinc oxide nanorods (Ag@ZnO) arrays before and after stearic acid (hydrophobic material) modification, and the superhydrophobic one showed improved detection limit due to the condensation effect. Generally, in the case of superhydrophobic SERS substrates, the contact area between analyte solution and SERS substrate could be significantly reduced, which gives rise to enrichment of analytes and further lowers the detection limit. In this study, we reported the fabrication of superhydrophobic SERS substrate by coating silver on hierarchically structured RGO gratings, which was prepared by two-beam laser interference (TBLI) induced ablation and photoreduction of graphene oxide (GO) films. The hierarchical RGO grating structures contribute to both the formation of plasmonic structures after silver coating, and the unique superhydrophobicity. The synergistic effects of enhanced E-field originated from the plasmonic structures and the enrichment effect due to the superhydrophobicity make the silver coated RGO (AgRGO) gratings an efficient SERS substrate. It shows high SERS enhancement, low detection limit (10−10 M) and high reproducibility in the test of rhodamine B (RhB). As a rapid, mask-free, chemical-free, and flexible laser processing technology,28−30 TBLI holds great promise for making RGO micronanostructures toward the development of SERS substrates.

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RESULTS AND DISCUSSION In this work, TBLI was used to tailor the surface structures of the GO film. Figure 1 shows the schematic illustration of our

Figure 1. Schematic illustration of TBLI fabrication of RGO gratings and subsequent silver coating toward the development of SERS substrates. (a) TBLI treatment of GO film; (b) light intensity distribution of two interfered laser beams; (c) RGO gratings; and (d) Ag-RGO gratings as SERS substrates.

EXPERIMENTAL SECTION

experimental procedure. Two laser beams with a diameter of ∼9 mm was guided to interfere at the surface of the GO film, forming a periodically distributed light field. To give an intuitionistic illustration, we calculated theoretical simulation of the laser intensity distributions on the interference area using Matlab, as shown in Figure 1b. When the GO film was exposed to the interfered laser beams, the GO surface could be treated in a similar pattern, in which the region exposed to high laser intensity could be partially ablated and reduced to RGO,31−33 whereas the region under low intensity survived. In this way, the RGO gratings could be readily fabricated without any masks or chemicals. Here, it is necessary to point out that by tuning the optical path, such as the angle between two laser beams, the period of the RGO gratings could be adjusted freely within a certain range; this has been confirmed in our previous works,34 in which the period of ∼2 μm was found to be suitable for achieving superhydrophobicity. So, RGO gratings with other periods have not been adopted for the fabrication of SERS substrates in this work. After TBLI treatment, gratings structures have been successfully created. As shown in Figure 2a, gratings with a

Fabrication of SERS Substrates Based on Structured GO Films. GO aqueous was prepared from purified natural graphite (Aldrich, < 150 um) using Hummers’ method. The GO films were prepared by spin coating GO aqueous solution on a piece of cover glass substrate at 1000 rpm, and then dried in air at room temperature. Then, the GO films were used for fabrication of RGO gratings by TBLI system. Typically, a frequency-tripled, Q-switched, single-mode Nd:YAG laser (Spectra-Physics), with an emission wavelength of 355 nm (frequency of 10 Hz and pulse duration of 10 ns) and a beam diameter of 9 mm was split into two beams, which had the same optical path length to the sample and interfered on the surface of GO film. RGO gratings could be fabricated by exposing the GO films to the laser interference region for several second. To fabricate the SERS substrates based on the RGO gratings, silver was coated using thermal evaporation system DM-300B. The PVD was carried out in a high vacuum (less than 5 × 10−4 Pa), with a low deposition rate at 0.03 nm/s, and the final Ag thickness was established to be ∼18 nm by AFM (Figure S1). SERS Detection of RhB. RhB was chosen as Raman probe material to evaluate the efficiency of our SERS substrates. RhB aqueous solution with different concentrations ranging from 10−6 to 10−10 M was prepared using distilled water. In a typical test, a droplet B


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Figure 2. (a) Optical image of RGO gratings fabricated by TBLI and (inset) photograph of a water droplet on its surface; the CA is ∼150°. (b) Diffraction pattern of the RGO gratings. (c) SEM image of the RGO gratings fabricated by TBLI. (d) Magnified SEM image of the red square marked in panel c.

Figure 3. (a) SEM images of Ag-RGO gratings. (b) Magnified SEM image at the square area marked in panel a. (c and d) Magnified SEM images of the area marked by red and green squares, respectively, in panel b.

period of ∼2 μm could be clearly observed by optical microscope. Moreover, the periodic structure could be confirmed by diffraction tests. Figure 2b shows the diffraction pattern, which suggests that the periodic gratings were very uniform. Notably, the uniform structure is a very important character for the SERS substrates because it would guarantee a high reproducibility in real SERS detection. To investigate the detailed surface structures of the RGO gratings, we characterized our sample by SEM. Figure 2c,d shows the SEM image of the RGO gratings and the magnified SEM image of the area marked by red square in Figure 2c, which shows the details of the RGO gratings structure. We noticed that, in addition to microscale gratings, there exist special nanofolder structures at the edge part of each grating. The formation of nanoscale layered structure could be attributed to the laser treatment induced reduction and the emission of carbon species (e.g., CO and CO2). As we know that both the microscale grating and the nanoscale nanofolders of the resultant RGO surfaces would lead to a rough surface, in combination with the laser treatment induced drastic removal of hydrophilic oxygen groups, the resultant RGO grating surface shows unique superhydrophobicity. The inset of Figure 2a is the photograph of a water droplet on the surface; a water contact angle (CA) of ∼150° is obtained. Using the RGO gratings as templates, we could fabricate the superhydrophobic SERS substrates by simply coating a thin layer of silver nanoparticles using a physical vapor deposition (PVD) technique. Here, the RGO gratings structure played an important role for the formation of a large amount of dispersed Ag NPs; and the oxygen functionalities on RGO would contribute to better control over the nucleation and growth of silver nanostructures.35 As shown in Figure 3, the hierarchical structures have not changed after decoration with silver. The microscale gratings as well as nanoscale layers could be clearly identified from the SEM image. To get further insight into the morphology of silver layer, we further magnified the SEM image. As shown in Figure 3c,d, we observed both the flat region (RGO strips) and the structured region marked by red and green squares, respectively. On the flat region, Ag nanoislands were uniformly deposited on the RGO surface (RGO composition was confirmed by Raman, Figure S2) and connected together, whereas on the structured region, the nanoscale layers could separate the Ag nanoparticles by providing large substrate contact region. Silver nanoparticles with smaller particle size of 10−30 nm closely packed over the entire nanofolder structures homogeneously (Figure 3d). The

space between each other was measured to be 1−2 nm, the nanoscale gaps between silver nanoparticles could be considered as plasmonic structures, which contribute to the enhancement of electromagnetic field. The dimensions and density of the silver nanoparticles are similar to that deposited on other substrates, for instance, on rose petal.20 As compared with the Ag nanoislands in the flat RGO region, structured RGO substrates would generate plenty of SERS “hot spot” after silver coating and contribute to the SERS detection performance accordingly. To check the plasmonic property of the Ag-RGO gratings, reflectance spectrum was provided (Figure 4). It shows strong

Figure 4. Reflectance spectrum of Ag-RGO grating structures.

SPPs absorbance band centered at 600 nm, which fits well with the 633 nm laser used in this research. For comparison, Aucoated RGO grating structures have also been prepared; they showed SPPs absorbance band in the infrared region (Figure S4), which is not suitable for our 633 nm laser. As reported elsewhere, the presence of graphene or its derivations would affect the metal LSPR,36,37 for instance, obvious blue shift has been observed in the case of graphene/Au nanostructures due to charge transfer between graphene and Au NPs.38 However, in our work, the SPPs of Ag-RGO gratings show obvious red shift as compared with bare Ag NPs on flat substrate (Figure S3). The red shift would be attributed to the very narrow gaps (∼1 nm) formed among Ag NPs on the RGO layers and the high refractive index (n = 4.39) of the RGO substrate.39 In addition to the electromagnetic field enhancement that originated from the plasmonic structures, the presence of RGO could also give rise to Raman enhancement by means of chemical interaction with the detected molecules, which is called chemical enhancement.40,41 The unique dewetting C


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ACS Applied Materials & Interfaces property would contribute to the enrichment effects, so-called condensation effect, which can also further lower the SERS detection limit. The superhydrophobicity was characterized by the contact angle of the Ag-RGO gratings, which was checked by dropping a drop of RhB solution on the Ag-RGO gratings SERS substrate. As shown in Figure 5a, the CA was measured

Figure 5. (a−d) The optical images during the evaporation of the RhB droplet. In image a, the CA was measured to be ∼152°. The images were recorded after evaporation for different times.

to be ∼152°. As compared with RGO gratings (CA = 150°) and pure Ag NPs coated cover glass substrate (CA = 107°, Figure S5), the slightly increased CA of Ag NPs coated RGO gratings (CA = 152°) could be attributed to the increased surface roughness after silver coating.42 Importantly, the sphere shaped RhB droplet has a significantly decreased contact area with the substrate (Ag-RGO gratings). In our experiment, the RhB drop was evaporated naturally in ambient condition. We recorded the photographs of the droplets during the evaporation process, as shown in Figure 5b−d, the droplet became smaller and smaller, the contact area almost keeps a consistent value. In other words, all of the RhB molecules in the 4 μL droplet were deposited within the contact area, about 0.9 mm2. For comparison, we also measured a hydrophilic glass surface that has a low CA of ∼15°, the contact area between the glass surface and a 4 μL droplet is measured to be 12.5 mm2. From this point of view, the superhydrophobic Ag-RGO gratings substrate would lower the SERS detection limit to more than 1 order of magnitude. To evaluate the SERS performance of our Ag-RGO gratings substrate, we used RhB as a probe molecule for Raman detection. In our experiments, to avoid the photo bleaching of RhB, we used 633 nm laser with relatively low power (30 μw) and short exposure time (several seconds) for the Raman measurement. We have checked the SERS signals of RhB on both flat region (Figure 3c) and structured region (Figure 3d), as shown in Figure S6. Generally, the SERS signal intensity on the structure region is about 5 times higher than that on the flat region, so all of the SERS spectra were collected on the structure region. Because the unique grating structure shows obvious anisotropic properties, we first checked the SERS performance with different laser polarization directions. In this test, the concentration of RhB was 10−6 M, the directions of the laser polarization are illustrated in the inset of Figure 6a with respect to the gratings direction, and we compared the SERS signal of linear polarized laser that parallel and perpendicular to the gratings, as shown in Figure 6a. It was obvious that when the laser polarization direction was parallel to the gratings, the Raman signal was almost twice stronger than that along the perpendicular direction. By carefully observing the AgNPs decorated on the RGO layers, it could be explained as that when the laser polarization is parallel to the gratings, most of the inner layer Ag NPs could interact with each other and generate strong localized SPPs; while on the other hand, when the laser polarization is perpendicular to the gratings, the SPPs were mainly generated from the interlayer interactions among AgNPs, which were separated by the RGO layers, and thus

Figure 6. (a) Comparison of SERS measured with two different laser polarization directions with respect to the gratings, red and blue curves show the SERS spectra measured with the polarization parallel and perpendicular to the gratings, respectively. (Inset) SEM image of the Ag-RGO gratings; red and blue marks show the laser polarization directions. (b) SERS spectra of RhB solution with different concentrations, from top to bottom the concentration decreased from 10−6 to 10−10 M.

provided relative weaker E-field enhancement. In this regard, the SERS measurements in the following experiments were all implemented with the laser polarization parallel to the gratings’ direction. To measure the limit concentration that could be detected by our SERS substrate, we first prepared RhB solutions with concentration ranging from 10−6 to 10−10 M. Figure 6b shows the SERS signals of different concentrations of RhB. For all the samples the characteristic Raman peaks (at 611, 1360, 1507, and 1651 cm−1) could be clearly observed, even for the low concentration sample (10−10 M), and the intensity of each peak increased with the solution concentration. Actually, in the case of RhB solutions with concentration lower than 10−10 M, the SERS signal could also be collected because the signal-to-noise ratio still looked good at this concentration. However, the noise became obvious, and the SERS signal became unreproducible every times. In this case, we estimated 10−10 M to be the limit of detection (LOD) of our Ag-RGO gratings SERS substrate. As compared with other works reported elsewhere, the LOD of 10−10 M is not very low. For instance, Galopin et al. reported a detection limit of 10−14 M and an enhancement factor of 2.3 × 108 using Ag nanoparticles deposited silicon nanowires prepared using an in situ electroless metal deposition technique;43 whereas Huang et al. reported the enhancement factor of 106 and low relative standard deviation (RSD) of 7% based on silver coated silicon nanowire prepared by sphere lithography.44 However, we feel that it is not necessary to pursuit low LOD, because signal fluctuation would become obvious when the concentration was further decreased. We calculated the enhancement factor (EF) of our SERS substrates, D


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spectra also showed very high uniformity and low fluctuation, the case was the same in the detection of RhB solution at a concentration of 10−6 M, indicating the high reproducibility of our Ag-RGO gratings substrate. As direct proof of the remarkable reproducibility, Figure 7c,d shows the Raman signal fluctuation at band 1360 cm−1 of RhB with 10−10 and 10−6 M, respectively. It is much clearer to judge the signal stability, and we calculated the relative standard deviation (RSD) as 10.4% for the 10−10 M and 8.0% for the 10−6 M sample, respectively. The low RSD values indicate the excellent SERS signal reproducibility of the Ag-RGO gratings substrate. Here, the little differences between the RSD values of two samples were due to the low signal-to-noise (S/N) ratio of the low concentration (10−10 M) sample, where the noise signal float could somehow affect the signal fluctuation. The highly reproducible SERS signals could be attributed to the uniformity of the grating structures. It is necessary to point out that we measured all the SERS signals on the structured region of the gratings. As complemental supports, SERS signals of RhB (10−6 M) on 4 different pieces of substrates were also collected and compared, in which high reproducibility (RSD = 8.3%) has been achieved (Figure S7). Additionally, our SERS substrates also show high temporal stability, stable SERS performance has been observed even the SERS substrate has been placed in atmosphere for long time (Figure S8). That is because the RGO layers would prevent Ag NPs from oxidation (Figure S9) and increase the substrate temporal stability.25,45 And similar results were also reported by other researchers.42 The excellent SERS performance could be attributed to the combined structures of both RGO gratings and the welldispersed Ag NPs. To prove this hypothesis, we measured the Raman performance of RhB (10−10 M) on the bare RGO gratings and Ag coated flat GO film, respectively (Figure S10). As compared with Ag-RGO gratings substrate, both RGO gratings without Ag and Ag coated flat GO film show very weak Raman enhancement under the same excitation conditions, which confirms the unique advantages of the combined structures.

which is calculated to be 2 × 107 (for details of the calculation, see Supporting Information). It is worthy pointing out that the stability and signal reproducibility of a SERS substrate are more important for practical detection. Generally, Raman spectra of GO or RGO would display two broad picks at 1354 and 1599 cm−1, corresponding to D and G band, respectively. The G band peak is attributed to an E2g mode of graphite associated with the vibration of sp2 bonded carbon atoms. The D band peak is related to the vibrations of carbon atoms with dangling bonds in plane terminations of disordered graphite. In our experiment, to avoid the influence of RGO signals, we used a 633 nm laser with very low power (30 μw) and very short exposure time (several seconds) for the Raman measurement. In this case, the SERS signals of G-band and D-band were much weaker than that of RhB molecules. For SERS substrates, the signal reproducibility is a very important factor that affects their performance and reliability. In this section, we evaluated the reproducibility of the SERS signals of our Ag-RGO gratings SERS substrate in the detection of RhB solution with concentrations of 10−10 and 10−6 M, respectively. For each sample, we chose seven different positions randomly on the Ag-RGO gratings. As shown in Figure 7a,b, even at a concentration of 10−10 M, the Raman

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CONCLUSIONS In this report, we have successfully developed a novel SERS substrate by combining TBLI fabrication of RGO gratings with PVD coating of silver. As a result, RGO gratings-based SERS substrates with hierarchical micronanostructures including microscale gratings, nanoscale RGO nanofolers as well as Ag nanparticles have been achieved. The hierarchical structures not only contribute to the enhancement of local electromagnetic field, giving rise to high SERS enhancement, but also lead to superhydrophobicity induced enrichment effect, which further lowers the detection limit of target analytes. By using the AgRGO gratings as SERS substrate, the SERS detection limit as low as 10−10 M has been achieved, and the SERS signals on different positions of the substrate showed high reproducibility. TBLI fabrication of RGO gratings may hold great promise for the development of highly efficient SERS substrates.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09128. Thickness of silver layer on GO substrate; Raman spectra of the structured region and flat region on the RGO/GO

Figure 7. Raman spectra of RhB with a concentration of (a) 10−10 M and (b) 10−6 M at seven different positions. The peak intensity of the Raman mode at 1360 cm−1 on seven measured sites of (c) 10−10 M and (d) 10−6 M samples. E


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substrate; reflectance spectra comparison of Ag-RGO gratings and pure AgNPs coated on cover glass; reflectance spectrum of Au-RGO gratings; contact angle of water droplet on AgNPs coated cover glass; Raman spectra of 10−6 and 10−10 M RhB measured on both flat and structured regions of Ag-RGO substrate; Raman spectra of RhB (10−6 M) at four different pieces of substrates; Raman signal of RhB (10−6 M) at same spot measured at different time; XPS Ag 3d peaks obtained from as-prepared Ag-RGO structures and AgRGO structures placed in atmosphere for 3 weeks; comparison of Raman spectra of 10−10M RhB measured on the substrate with Ag-RGO gratings, Ag-GO film and pure RGO gratings; enhancement factor (EF) calculation process; and comparison of RhB Raman signal with and without SERS substrate. (PDF)

AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected]. * E-mail: [email protected]. * E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the National Basic Research Program of China and National Natural Science Foundation of China under grants #2011CB013000, #61522503, #2014CB921302, #61376123, and #61435005 for support.

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ABBREVIATIONS TBLI, two-beam laser interference NPs, nanoparticles SERS, surface enhanced Raman spectroscopy RhB, Rhodamine B SPPs, surface plasmon polaritons CA, contact angle GO, graphene oxide PVD, physical vapor deposition

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Superhydrophobic SERS Substrates Based on Silver-Coated

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