Article pubs.acs.org/Langmuir
Ag-Nanoparticle-Decorated Ge Nanocap Arrays Protruding from Porous Anodic Aluminum Oxide as Sensitive and Reproducible Surface-Enhanced Raman Scattering Substrates Jing Liu,† Guowen Meng,*,†,‡ Xiangdong Li,† and Zhulin Huang† †
Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, China ‡ University of Science and Technology of China, Hefei 230026, China S Supporting Information *
ABSTRACT: We report on the fabrication of Ag nanoparticle (Ag NP) decorated germanium (Ge) nanocap (Ag-NPs@Genanocap) arrays protruding from highly ordered porous anodic aluminum oxide (AAO) template as highly sensitive and uniform surface-enhanced Raman scattering (SERS) substrates. The hybrid SERS substrates are fabricated via a combinatorial process of AAO template-assisted growth of Ge nanotubes with each tube having a hemispherical nanocap on the AAO pore bottom, wet chemical etching of the remaining aluminum and the AAO barrier layer to expose the Ge nanocaps, and sputtering Ag NPs on the Ge nanocap arrays. Because sufficient SERS “hot spots” are created from the electromagnetic coupling among the Ag NPs on the Ge nanocap and the highly ordered Ge nanocap arrays also have semiconducting chemical supporting enhancement, the hybrid SERS substrates have high SERS sensitivity and good signal reproducibility. Using the hybrid SERS substrates, Rhodamine 6G with a concentration down to 10−11 M is identified, and one congener of highly toxic polychlorinated biphenyls with a concentration as low as 10−6 M is also recognized, showing great potential for SERS-based rapid detection of organic pollutants in the environment.
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hybrid substrates with higher SERS activity.19−24 For example, with the help of chemical modification, hybrid SERS substrates of Au nanoparticle (NP)-decorated ZnO particles,11 Au/AgNP-decorated ZnO multipods, and Ag-coated Si nanowires (NWs) were obtained.25,26 Nevertheless, the coupling agents for linking the noble metal NPs onto the semiconductor surface bring in extra background signals in the SERS measurements, which would interfere with experimental results, especially if the target analytes have Raman fingerprint peaks similar to those of the coupling agents. To avoid the interference of chemical coupling agents to the SERS signals of the analytes, in situ hydrolysis of Sn2+-sensitized titanium glycolate microspheres in the presence of Ag+ ions was used to create a porous TiO2−Ag core−shell hybrid SERS substrate.27 However, the random distribution of TiO2−Ag core−shell nanostructures results in nonuniform hot spots, which makes it difficult to achieve reproducible signals in SERS map measurements, restricting practical applications. Semiconducting germanium (Ge) has chemical supporting SERS effect together with good biocompatibility and structural
INTRODUCTION Surface-enhanced Raman scattering (SERS) spectroscopy has promising potentials in rapid trace-level analysis in biological and environmental analysis because of its high sensitivity,1,2 rapid response, and fingerprint effect.3−6 From the viewpoint of practical applications, it is essential to develop effective SERS substrates with not only strong enhancement factors but also reproducible SERS signals.7 Generally, the SERS effect originates from both electromagnetic and chemical enhancement.8 The former is responsible for the large enhancement, which relies on the excitation of localized surface plasmon resonance (LSPR)-induced SERS “hot spots”, usually in the nanometer-sized gaps between the neighboring noble metallic nanostructures.9,10 Under the laser excitation, a strong surface plasmon resonance (SPR) can be generated in the hot spots, resulting in an extraordinary electromagnetic enhancement. In this way, Raman signals of analytes anchored to the hot spots can be amplified enormously. The latter originates from the charge transfer between the analyte and the conducting sublayer, usually a semiconductor.11,12 Thus, hybrid substrates consisting of semiconductors and noble metals are beneficial to high SERS sensitivity for rapid trace analysis. To date, various semiconductors (Si,13 ZnO,14,15 ZnS,16 TiO2,17 and Cu2O18) and noble metals (Au and Ag) have been chosen to build © 2014 American Chemical Society
Received: May 14, 2014 Revised: October 30, 2014 Published: October 31, 2014 13964
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Figure 1. Schematic for the fabrication of Ag-NPs@Ge-nanocap arrays protruding from AAO template.
stability;28−32 therefore, Ge−noble metal hybrid SERS substrates have been built. For example, Ag-NP-decorated Ge nanowires (Ge NWs) were achieved by reducing Ag+ on the hydrogen-terminated surface of Ge NWs,33 and Au/Ag-NPdecorated Ge disks were also fabricated via galvanic displacement.34 However, both of the supporting Ge NWs in the former and Ge disks in the latter were randomly distributed; therefore, the previously reported Ge−noble metal hybrid SERS substrates could not provide reproducible signals in SERS map measurements, limiting their practical applications in SERS-based analysis. Recently, we developed a simple synthetic approach to large-scale highly ordered and uniformly distributed vertically aligned Ge nanotubes (Ge NTs) via predecorating metal catalyst on the inner pore walls of nanoporous anodic aluminum oxide (AAO) template and subsequent metal cluster catalyst-induced chemical vapor deposition (CVD) of Ge NTs.35 If these highly ordered Ge nanostructure arrays can be used as chemical-supporting framework for the building of Ge−noble metal hybrid SERS substrates, both high SERS activity and good SERS signal reproducibility could be achieved. Herein, we show a fabrication approach to large-area arrays of highly ordered germanium nanocaps (Ge nanocaps) protruding from the planar surface of AAO template with each Ge nanocap decorated with tens of uniformly distributed Ag NPs (denoted as Ag-NPs@Ge-nanocap arrays), as highly sensitive SERS substrates with excellent signal reproducibility, as shown schematically in Figure 1. First, nickel nitrate (Ni(NO3)2) was decorated on the inner wall of the AAO pores as catalyst precursors, where they are reduced into nickel clusters that provide nucleation sites for the subsequent Ge NT growth. Then, both the pore bottom and the inner pore walls of the AAO were deposited with a thin film of Ge in a CVD process,35 forming highly ordered arrays of Ge NTs with each tube having a closed cap at the pore bottom of the AAO template. Next, the remaining aluminum (Al) and the bottom AAO barrier layer were selectively removed in aqueous tin tetrachloride (SnCl4) solution and sodium hydroxide (NaOH) solution sequentially to expose the Ge nanocaps out of the AAO template. Finally, Ag NPs were ion-sputtered onto the Ge nanocap arrays and the top surface of the networked anodized alumina between the Ge nanocaps. The Ag-NPs@Ge-nanocap arrays have demonstrated remarkable SERS activity and good Raman signal reproducibility for both laboratory probing of molecules of Rhodamine 6G (R6G) and an environmental pollutant 3,3′,4,4′-polychloronated (PCB-77, one congener of polychlorinated biphenyls (PCBs), a notorious class of
persistent organic pollutants defined in the Stockholm convention),36 showing promising potential in SERS-based rapid trace-level detection of toxic, persistent organic pollutants in the environment.
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EXPERIMENTAL SECTION
Preparation of Highly Ordered Nanoporous AAO Template. The highly ordered nanoporous AAO template with barrier layer and remaining Al was achieved by constant potential anodization of pure Al foil in 0.3 M oxalic acid solution under 40 VDC at 5 °C for 6 h.37 CVD Growth of Ge Nanotubes in the Porous AAO Template. To decorate metal catalyst precursor on the inner pore walls of AAO template, the AAO template was immersed in the mixture of 0.5 M Ni(NO3)2 and 5% phosphate (H3PO4) at 40 oC for 20 min and rinsed with 0.5 M Ni(NO3)2 to remove the residual H3PO4. After being dried, the above-treated AAO template with Ni catalyst precursors dispersed on the pore walls was loaded in a ceramic boat and into the center zone of a horizontal quartz tube furnace. The quartz tube was pumped and purged by high-purity argon (Ar) (99.999%) for two cycles. Then the furnace was heated to the growth temperature under 60 standard-state cubic centimeter per minute (sccm) flow of Ar at a heating rate of 10 oC/min. The growth was performed under a 60 sccm flow of Ar in tandem with a 30 sccm flow of germane (GeH4) (5% GeH4 in hydrogen) at 330 oC for 27 min. Exposing the Ge Nanocap Arrays from the AAO Template. The AAO template embedded with Ge NTs was immersed in saturated SnCl4 aqueous solution to selectively etch the remaining Al and then in 0.1 M NaOH at 40 oC for 2 min to remove the barrier layer to expose the Ge nanocaps out of the planar surface of the AAO template. Sputtering Ag NPs on the Ge Nanocaps. Ag NPs were assembled on the Ge nanocap arrays via top-view ion-sputtering Ag at a current density of 10 mA. Ag sputtering was performed for different durations to tailor the sizes and the gaps of the Ag NPs for the SERS activity improvement. SERS Measurements. For SERS measurements, the as-fabricated Ag-NPs@Ge-nanocap arrays were immersed in R6G aqueous solutions with different concentrations and dried in air before Raman spectral examination. For the detection of PCB-77, PCB-77 was first dissolved in acetone to form PCB-77 solutions with different concentrations (from 10−3 to 10−7 M). The Raman spectra were recorded using a confocal microprobe Raman spectrometer (Renishaw Invia Reflex) with a 532 nm argon ion laser line, where the effective power of the laser source was 0.03 mW. The laser spot focused on the sample surface was about 5 μm in diameter. The integration durations were maintained for 5 and 25 s for R6G and PCB-77, respectively.
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RESULTS AND DISCUSSION Structural Characterization. After CVD growth of Ge NTs embedded in the AAO template and removal of the remaining Al and the bottom AAO barrier layer, large-area
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can be tailored by simply tuning the Ag-sputtering duration. For short Ag sputtering of 4 min, small Ag NPs were sparsely dispersed on the Ge nanocaps, where the typical gap between the neighboring Ag NPs is about 20 nm (Figure 3a). With the increase of the Ag-sputtering duration, the Ag NPs grow and the gaps between the neighboring Ag NPs decrease correspondingly, being beneficial to the plasmonic coupling between the adjacent Ag NPs. With the elongation of Ag sputtering to 8 min, the Ag NPs grow to about 12 nm while still maintaining the isolated state from each other, and the gaps between the neighboring Ag NPs decrease to less than 15 nm (Figure 3b). When the Ag-sputtering duration increases to 10 min, some Ag NPs tend to aggregate and the number of nanogaps starts to decrease, as shown in Figure 3c. After the Ag-sputtering duration was elongated to about 12 min, a continuous film of Ag NPs was formed on the Ge nanocaps and the planar surface of the AAO template (Figure 3d), and many of the nanogaps among the Ag NPs disappeared. The UV−vis absorption spectra of Ag-NPs@Ge-nanocap arrays are shown in Figure S2 of Supporting Information, indicating that the absorption band for silver particles is located around 330 nm. SERS Map Measurements. To obtain an optimal SERS effect, the SERS activities of the Ag-NPs@Ge-nanocap arrays with different Ag-sputtering durations were tested by using 10−7 M R6G as probe molecules, as shown in Figure 4a. It can be
arrays of highly ordered and hexagonally arranged Ge nanocaps protruding from the planar surface of the AAO template are achieved, as shown in Figure 2. Scanning electron microscopy
Figure 2. Characterization of the Ge nanocaps protruding from the AAO template: (a) SEM oblique view; (b) SEM cross-sectional view of the Ge nanocaps and the Ge NTs embedded in AAO; (c) TEM image of a small section of the Ge nanotube arrays; and (d) EDX spectrum.
(SEM) observations (Figure 2a,b) clearly show that the ordered Ge nanocaps are about 50 nm high with tube diameters of 70 nm. The closed ends were further confirmed by the transmission electron microscopy (TEM) observation of nanotube arrays after being separated from the AAO template (Figure 2c). The energy dispersive X-ray (EDX) spectrum (Figure 2d) taken from the Ge nanocaps protruding from AAO template reveals Ge, Al, and O elements. Obviously, Ge element comes from the Ge nanocaps, whereas Al and O originate from the AAO template itself. After Ag sputtering onto the arrays of Ge nanocaps protruding from AAO template, the Ge nanocaps were covered with discontinuous Ag NPs, as shown in Figure 3 and Figure S1 of Supporting Information. It is demonstrated that the size and morphology of the Ag NPs
Figure 4. SERS sensitivity to R6G. (a) SERS spectra of 10−7 M R6G adsorbed on the Ag-NPs@Ge-nanocap arrays with different Agsputtering durations. (b) SERS spectra of R6G with different concentrations adsorbed on Ag-NPs@Ge-nanocap arrays with Ag sputtering for 8 min. The inset is the molecular structure of R6G.
observed that the SERS map peak intensities of R6G increase with the Ag-sputtering duration for the first 8 min and then decrease with longer Ag-sputtering duration. With further elongation of the Ag-sputtering duration, the Ag NPs aggregate on the Ge nanocaps and in between, resulting in rapid decrease of the hot spots between the neighboring Ag NPs;
Figure 3. Typical SEM top-view observations on the Ag-NPs@Genanocap arrays with different Ag-sputtering durations of (a) 4 min, (b) 8 min, (c) 10 min, and (d) 12 min. 13966
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Figure 5. Map of electron-transfer process between (a) Ge nanocaps and R6G and (b) Ag-NPs@Ge-nanocaps and R6G.
nanomembranes as SERS substrate but not by using Ag-NPs@ AAO as SERS substrate. As previously mentioned, the chemical enhancement of semiconducting Ge nanocaps originates from the charge transfer between analytes and themselves. A map of electron-transfer process between Ge and R6G is shown in Figure 5a.13 Because the work function of Ge is higher than that of Ag, the Fermi level of Ge rises when Ag contacts Ge. Thus, electrons are liable to transfer from Ag to Ge. Therefore, the sputtered Ag promotes the electron transfer (Figure 5b),38 resulting in enhanced SERS effect. In addition to the high SERS sensitivity, the SERS signal homogeneity of the Ag-NPs@Ge-nanocap arrays was also investigated by examining ten random spots across the substrate under identical experimental conditions, such as the laser power and the integration duration. The results reveal good reproducibility as the deviation of the band intensity of R6G (614 cm−1 shift) is less than 10% compared to that of the average relative peak intensity (see Supporting Information, part S6 and Figure S9). Analytical application of the Ag-NPs@Ge-nanocap arrays was tested toward the rapid detection of 3,3′,4,4′-tetrachlorobiphenyl (PCB-77), a congener of PCBs that are detrimental to human health.36 Traditional detection methods for PCBs, such as phosphorimetry,39 chromatographic analysis,40 and gas chromatography-isotope dilution time-of-flight mass spectrometry,41 are normally expensive, time-consuming, and complicated. Figure 6a shows the SERS spectra of PCB-77 in acetone with different concentrations (10−3, 10−4, 10−5, and 10−6 M) dispersed on the optimal Ag-NPs@Ge-nanocap arrays. The five fingerprint characteristic peaks at 677 cm−1 (C−Cl stretching), 1033 cm−1 (ring breathing), 1246 cm−1 (C−H wagging), 1300 cm−1 (biphenyl C−C bridge stretching), and 1600 cm−1 (ring stretching) in the SERS spectra (Figure 6)42 correspond well with those in the normal Raman spectrum of PCB-77. It can be seen that even for the concentration of PCB-77 down to 10−5 M, the crucial characteristic bands are still distinguishable. In comparison with Figure 4b and Figure 6a, the SERS sensitivity of the optimal Ag-NPs@Ge-nanocap arrays to PCBs is weaker than that to R6G, which can be attributed to the fact that PCB molecules do not have good affinity for Ag and cannot be readily anchored onto the bare surface of the Ag NPs.43 To decrease the low detection limitation, we tried to modify the optimal Ag-NPs@Ge-nanocap arrays with sulfhydryl-β-cyclodextrin (HS-β-CD), which can trap PCB-77 molecules into the cavities of β-CD to form the inclusion complex with β-CD.44 By modifying HS-β-CD, the Ag-NPs@Ge-nanocap arrays can efficiently capture more PCB-77 molecules on the hot spots between the adjacent Ag NPs. Figure 6b shows the SERS
consequently, the SERS peak intensity decreases. These results indicate that the coupling of the localized surface plasmons between the neighboring Ag NPs can be tailored by simply tuning the Ag-sputtering duration. Under the current experimental conditions, the Ag-NPs@Ge-nanocap arrays with Ag sputtering of 8 min is the optimal SERS substrate with the highest SERS activity. To show the SERS sensitivity of the optimal Ag-NPs@Ge-nanocap arrays, the SERS spectra of R6G with different concentrations (10−8, 10−9, 10−10, and 10−11 M) adsorbed on the optimal Ag-NPs@Ge-nanocaps are shown in Figure 4b. The spectral features of R6G can be identified clearly even at a concentration as low as 10 −11 M, demonstrating the high sensitivity of the large-area arrays of uniformly distributed Ag-NPs@Ge-nanocaps for SERS detection. The high SERS sensitivity originates from the following reasons. First, the sputtered Ag NPs on the large surface area of Ge nanocap arrays protruding from the planar surface of AAO template provide sufficient high density of SERS hot spots between the neighboring Ag NPs. Second, the roughed surface of large-area Ag-NPs@Ge-nanocap arrays are conducive for adsorbing sufficient analyte molecules. Third, the chemical supporting enhancement effect of the semiconducting Ge nanocaps may further improve the SERS sensitivity. To further test the chemical enhancement of the semiconducting Ge nanocaps, we compared the SERS performance of the Ge nanocap arrays (Figure S3a of Supporting Information), Ge film (Figure S3b), and Al2O3 nanocap arrays (Figure S3c) by using 10−3 M R6G aqueous solution. The SERS spectra taken from Ge nanocap arrays (curve I in Figure S3d) and Al2O3 nanocap arrays (curve II in Figure S3d) with identical morphology and size demonstrate that the Ge nanocap arrays show higher SERS signal intensity. Further SERS measurement indicates that the Ge nanocap arrays with a large surface area are liable to adsorb more analyte molecules to increase SERS signal than that of the Ge film (curves I and II in Figure S3d). Then the average enhancement factor was calculated by comparing the normalized SERS band intensity and the normal Raman band intensities of the 4-aminothiophenol (PATP) molecules. The typical average enhancement factor for the optimal Ag-NPs@Ge-nanocap arrays is estimated to be about 3.15 × 106 (part S4, Supporting Information). To further demonstrate that the semiconducting Ge supporter is remarkably important, we also compared the SERS performance between the Ag-NPs@Al2O3-nanostructures and the Ag-NPs@Ge-nanostructures (part S5, Supporting Information). The results show that R6G with a concentration down to 10−10 M can be identified by using the Ag-NPs@Ge13967
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Ag-NPs@Ge nanocap arrays for detecting PATP, SERS performance between Ag-NPs@Al2O3 nanostructures and AgNPs@Ge-nanostructures, and SERS spectra of R6G obtained from 10 random points on as-prepared SERS substrate. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Fax: (86) 0551-65591434. Tel.: (86) 0551-65592749. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program of China (Grant 2013CB934304), the CAS/SAFEA International Partnership Program for Creative Research Teams for financial support, and the NSFC (Grants 11274312, 51202254, and 51201159).
■ Figure 6. SERS sensitivity to PCBs. (a) SERS spectra of PCB-77 with different concentrations collected on the as-prepared substrates with Ag sputtering for 8 min. The inset is the molecular structure of PCB77. (b) SERS spectra of 10−6 and 10−7 M PCB-77 from HS-β-CD modified Ag-NPs@Ge-nanocap arrays. The inset is the schematic for PCB-77 molecule being captured by HS-β-CD.
spectra of 10−6 and 10−7 M PCB-77 collected from the HS-βCD-modified optimal Ag-NPs@Ge-nanocap arrays. From curve I in Figure 6b and curve IV in Figure 6a, it can be seen that the peak intensities of 10−6 M PCB-77 increase distinctly, and 10−7 M PCB-77 can be detected after modification of HS-β-CD. Therefore, with modification of HS-β-CD molecules, the AgNPs@Ge-nanocap arrays demonstrate improved ability to capture the target PCB molecules for SERS detection.
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CONCLUSION In summary, large-area hexagonally ordered Ag-NPs@Genanocap arrays have been achieved by ion-sputtering Ag NPs on the CVD-grown Ge nanocaps protruding from the planar surface of an AAO template. The large surface area of the Ge nanocaps is essential for loading a large amount of Ag NPs for high-density SERS hot spots between the Ag NPs, and thus leads to high SERS activity. Moreover, the chemical supporting enhancement effect of semiconducting Ge nanocaps also contributes to the SERS sensitivity. Furthermore, the uniformity of the Ge nanocap frameworks embedded in AAO template guarantees the homogeneous distribution of the large quantities of Ag NPs and thus reproducible SERS signals. The Ag-NPs@Ge-nanocap arrays show promising potential in SERS-based rapid detection of environmental organic pollutants, such as PCBs.
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ASSOCIATED CONTENT
S Supporting Information *
Images of Ag-NPs@Ge-nanocaps, UV−vis spectra of the AgNPs@Ge-nanocap arrays, description of chemical enhancement of semiconductor Ge, calculation of the enhancement factor of 13968
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(32) Alvarez, S. D.; Derfus, A. M.; Schwartz, M. P.; Bhatia, S. N.; Sailor, M. J. The compatibility of hepatocytes with chemically modified porous silicon with reference to in vitro biosensors. Biomaterials 2009, 30, 26−34. (33) Peng, M.; Gao, J.; Zhang, P.; Li, Y.; Sun, X.; Lee, S.-T. Reductive self-assembling of Ag nanoparticles on germanium nanowires and their application in ultrasensitive surface-enhanced Raman spectroscopy. Chem. Mater. 2011, 23, 3296−3301. (34) Brejna, P. R.; Griffiths, P. R.; Yang, J. Nanostructural silver and gold substrates for surface-enhanced Raman spectroscopy measurements prepared by galvanic displacement on germanium disks. Appl. Spectrosc. 2009, 63, 396−400. (35) Li, X.; Meng, G.; Xu, Q.; Kong, M.; Zhu, X.; Chu, Z.; Li, A. P. Controlled synthesis of germanium nanowires and nanotubes with variable morphologies and sizes. Nano Lett. 2011, 11, 1704−1709. (36) Ross, G. The public health implications of polychlorinated biphenyls (PCBs) in the environment. Ecotoxicol. Environ. Saf. 2004, 59, 275−291. (37) Huang, Z.; Meng, G.; Huang, Q.; Yang, Y.; Zhu, C.; Tang, C. Improved SERS performance from Au nanopillar arrays by abridging the pillar tip spacing by Ag sputtering. Adv. Mater. (Weinheim, Ger.) 2010, 22, 4136−4139. (38) Wang, X.; Kong, X.; Yu, Y.; Zhang, H. Synthesis and Characterization of Water-Soluble and Bifunctional ZnO-Au Nanocomposites. J. Phys. Chem. C 2007, 111, 3836−3841. (39) Arruda, A. F.; Goicoechea, H. C.; Santos, M.; Campiglia, A. D.; Olivieri, A. C. Solid-liquid extraction room temperature phosphorimetry and pattern recognition for screening polycyclic aromatic hydrocarbon and polychlorinated biphenyls in water samples. Environ. Sci. Technol. 2003, 37, 1385−1391. (40) Schurig, V.; Reich, S. Determination of the rotational barriers of atropisomeric polychlorinated biphenyls (PCBs) by a novel stoppedflow multidimensional gas chromatographic technique. Chirality 1998, 10, 316−320. (41) Focant, J. F.; Sjödin, A.; Turner, W. E.; Donald, G.; Patterson, J. Measurement of selected polybrominated diphenyl ethers, polybrominated and polychlorinated biphenyls, and organochlorine pesticides in human serum and milk using comprehensive two-dimensional gas chromatography isotope dilution time-of-flight mass spectrometry. Anal. Chem. 2004, 76, 6313−6320. (42) Zhu, C.; Meng, G.; Huang, Q.; Zhang, Z.; Xu, Q.; Liu, G.; Huang, Z.; Chu, Z. Ag nanosheet-assembled micro-hemispheres as effective SERS substrates. Chem. Commun. (Cambridge, U.K.) 2011, 47, 2709−2711. (43) Huang, Z.; Meng, G.; Huang, Q.; Chen, B.; Zhu, C.; Zhang, Z. Large-area Ag nanorod array substrates for SERS: AAO templateassisted fabrication, functionalization, and application in detection PCBs. J. Raman Spectrosc. 2013, 44, 240−246. (44) Liu, P.; Zhang, D.; Zhan, J. Investigation on the inclusions of PCB52 with cyclodextrins by performing DFT calculations and molecular dynamics simulations. J. Phys. Chem. A 2010, 114, 13122− 13128.
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Ag-Nanoparticle-Decorated Ge Nanocap Arrays Protruding from Porous Anodic Aluminum Oxide as Sensitive and Reproducible Surface-Enhanced Raman Scattering Substrates Jing Liu,† Guowen Meng,*,†,‡ Xiangdong Li,† and Zhulin Huang† †
Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, China ‡ University of Science and Technology of China, Hefei 230026, China S Supporting Information *
ABSTRACT: We report on the fabrication of Ag nanoparticle (Ag NP) decorated germanium (Ge) nanocap (Ag-NPs@Genanocap) arrays protruding from highly ordered porous anodic aluminum oxide (AAO) template as highly sensitive and uniform surface-enhanced Raman scattering (SERS) substrates. The hybrid SERS substrates are fabricated via a combinatorial process of AAO template-assisted growth of Ge nanotubes with each tube having a hemispherical nanocap on the AAO pore bottom, wet chemical etching of the remaining aluminum and the AAO barrier layer to expose the Ge nanocaps, and sputtering Ag NPs on the Ge nanocap arrays. Because sufficient SERS “hot spots” are created from the electromagnetic coupling among the Ag NPs on the Ge nanocap and the highly ordered Ge nanocap arrays also have semiconducting chemical supporting enhancement, the hybrid SERS substrates have high SERS sensitivity and good signal reproducibility. Using the hybrid SERS substrates, Rhodamine 6G with a concentration down to 10−11 M is identified, and one congener of highly toxic polychlorinated biphenyls with a concentration as low as 10−6 M is also recognized, showing great potential for SERS-based rapid detection of organic pollutants in the environment.
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hybrid substrates with higher SERS activity.19−24 For example, with the help of chemical modification, hybrid SERS substrates of Au nanoparticle (NP)-decorated ZnO particles,11 Au/AgNP-decorated ZnO multipods, and Ag-coated Si nanowires (NWs) were obtained.25,26 Nevertheless, the coupling agents for linking the noble metal NPs onto the semiconductor surface bring in extra background signals in the SERS measurements, which would interfere with experimental results, especially if the target analytes have Raman fingerprint peaks similar to those of the coupling agents. To avoid the interference of chemical coupling agents to the SERS signals of the analytes, in situ hydrolysis of Sn2+-sensitized titanium glycolate microspheres in the presence of Ag+ ions was used to create a porous TiO2−Ag core−shell hybrid SERS substrate.27 However, the random distribution of TiO2−Ag core−shell nanostructures results in nonuniform hot spots, which makes it difficult to achieve reproducible signals in SERS map measurements, restricting practical applications. Semiconducting germanium (Ge) has chemical supporting SERS effect together with good biocompatibility and structural
INTRODUCTION Surface-enhanced Raman scattering (SERS) spectroscopy has promising potentials in rapid trace-level analysis in biological and environmental analysis because of its high sensitivity,1,2 rapid response, and fingerprint effect.3−6 From the viewpoint of practical applications, it is essential to develop effective SERS substrates with not only strong enhancement factors but also reproducible SERS signals.7 Generally, the SERS effect originates from both electromagnetic and chemical enhancement.8 The former is responsible for the large enhancement, which relies on the excitation of localized surface plasmon resonance (LSPR)-induced SERS “hot spots”, usually in the nanometer-sized gaps between the neighboring noble metallic nanostructures.9,10 Under the laser excitation, a strong surface plasmon resonance (SPR) can be generated in the hot spots, resulting in an extraordinary electromagnetic enhancement. In this way, Raman signals of analytes anchored to the hot spots can be amplified enormously. The latter originates from the charge transfer between the analyte and the conducting sublayer, usually a semiconductor.11,12 Thus, hybrid substrates consisting of semiconductors and noble metals are beneficial to high SERS sensitivity for rapid trace analysis. To date, various semiconductors (Si,13 ZnO,14,15 ZnS,16 TiO2,17 and Cu2O18) and noble metals (Au and Ag) have been chosen to build © 2014 American Chemical Society
Received: May 14, 2014 Revised: October 30, 2014 Published: October 31, 2014 13964
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Figure 1. Schematic for the fabrication of Ag-NPs@Ge-nanocap arrays protruding from AAO template.
stability;28−32 therefore, Ge−noble metal hybrid SERS substrates have been built. For example, Ag-NP-decorated Ge nanowires (Ge NWs) were achieved by reducing Ag+ on the hydrogen-terminated surface of Ge NWs,33 and Au/Ag-NPdecorated Ge disks were also fabricated via galvanic displacement.34 However, both of the supporting Ge NWs in the former and Ge disks in the latter were randomly distributed; therefore, the previously reported Ge−noble metal hybrid SERS substrates could not provide reproducible signals in SERS map measurements, limiting their practical applications in SERS-based analysis. Recently, we developed a simple synthetic approach to large-scale highly ordered and uniformly distributed vertically aligned Ge nanotubes (Ge NTs) via predecorating metal catalyst on the inner pore walls of nanoporous anodic aluminum oxide (AAO) template and subsequent metal cluster catalyst-induced chemical vapor deposition (CVD) of Ge NTs.35 If these highly ordered Ge nanostructure arrays can be used as chemical-supporting framework for the building of Ge−noble metal hybrid SERS substrates, both high SERS activity and good SERS signal reproducibility could be achieved. Herein, we show a fabrication approach to large-area arrays of highly ordered germanium nanocaps (Ge nanocaps) protruding from the planar surface of AAO template with each Ge nanocap decorated with tens of uniformly distributed Ag NPs (denoted as Ag-NPs@Ge-nanocap arrays), as highly sensitive SERS substrates with excellent signal reproducibility, as shown schematically in Figure 1. First, nickel nitrate (Ni(NO3)2) was decorated on the inner wall of the AAO pores as catalyst precursors, where they are reduced into nickel clusters that provide nucleation sites for the subsequent Ge NT growth. Then, both the pore bottom and the inner pore walls of the AAO were deposited with a thin film of Ge in a CVD process,35 forming highly ordered arrays of Ge NTs with each tube having a closed cap at the pore bottom of the AAO template. Next, the remaining aluminum (Al) and the bottom AAO barrier layer were selectively removed in aqueous tin tetrachloride (SnCl4) solution and sodium hydroxide (NaOH) solution sequentially to expose the Ge nanocaps out of the AAO template. Finally, Ag NPs were ion-sputtered onto the Ge nanocap arrays and the top surface of the networked anodized alumina between the Ge nanocaps. The Ag-NPs@Ge-nanocap arrays have demonstrated remarkable SERS activity and good Raman signal reproducibility for both laboratory probing of molecules of Rhodamine 6G (R6G) and an environmental pollutant 3,3′,4,4′-polychloronated (PCB-77, one congener of polychlorinated biphenyls (PCBs), a notorious class of
persistent organic pollutants defined in the Stockholm convention),36 showing promising potential in SERS-based rapid trace-level detection of toxic, persistent organic pollutants in the environment.
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EXPERIMENTAL SECTION
Preparation of Highly Ordered Nanoporous AAO Template. The highly ordered nanoporous AAO template with barrier layer and remaining Al was achieved by constant potential anodization of pure Al foil in 0.3 M oxalic acid solution under 40 VDC at 5 °C for 6 h.37 CVD Growth of Ge Nanotubes in the Porous AAO Template. To decorate metal catalyst precursor on the inner pore walls of AAO template, the AAO template was immersed in the mixture of 0.5 M Ni(NO3)2 and 5% phosphate (H3PO4) at 40 oC for 20 min and rinsed with 0.5 M Ni(NO3)2 to remove the residual H3PO4. After being dried, the above-treated AAO template with Ni catalyst precursors dispersed on the pore walls was loaded in a ceramic boat and into the center zone of a horizontal quartz tube furnace. The quartz tube was pumped and purged by high-purity argon (Ar) (99.999%) for two cycles. Then the furnace was heated to the growth temperature under 60 standard-state cubic centimeter per minute (sccm) flow of Ar at a heating rate of 10 oC/min. The growth was performed under a 60 sccm flow of Ar in tandem with a 30 sccm flow of germane (GeH4) (5% GeH4 in hydrogen) at 330 oC for 27 min. Exposing the Ge Nanocap Arrays from the AAO Template. The AAO template embedded with Ge NTs was immersed in saturated SnCl4 aqueous solution to selectively etch the remaining Al and then in 0.1 M NaOH at 40 oC for 2 min to remove the barrier layer to expose the Ge nanocaps out of the planar surface of the AAO template. Sputtering Ag NPs on the Ge Nanocaps. Ag NPs were assembled on the Ge nanocap arrays via top-view ion-sputtering Ag at a current density of 10 mA. Ag sputtering was performed for different durations to tailor the sizes and the gaps of the Ag NPs for the SERS activity improvement. SERS Measurements. For SERS measurements, the as-fabricated Ag-NPs@Ge-nanocap arrays were immersed in R6G aqueous solutions with different concentrations and dried in air before Raman spectral examination. For the detection of PCB-77, PCB-77 was first dissolved in acetone to form PCB-77 solutions with different concentrations (from 10−3 to 10−7 M). The Raman spectra were recorded using a confocal microprobe Raman spectrometer (Renishaw Invia Reflex) with a 532 nm argon ion laser line, where the effective power of the laser source was 0.03 mW. The laser spot focused on the sample surface was about 5 μm in diameter. The integration durations were maintained for 5 and 25 s for R6G and PCB-77, respectively.
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RESULTS AND DISCUSSION Structural Characterization. After CVD growth of Ge NTs embedded in the AAO template and removal of the remaining Al and the bottom AAO barrier layer, large-area
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can be tailored by simply tuning the Ag-sputtering duration. For short Ag sputtering of 4 min, small Ag NPs were sparsely dispersed on the Ge nanocaps, where the typical gap between the neighboring Ag NPs is about 20 nm (Figure 3a). With the increase of the Ag-sputtering duration, the Ag NPs grow and the gaps between the neighboring Ag NPs decrease correspondingly, being beneficial to the plasmonic coupling between the adjacent Ag NPs. With the elongation of Ag sputtering to 8 min, the Ag NPs grow to about 12 nm while still maintaining the isolated state from each other, and the gaps between the neighboring Ag NPs decrease to less than 15 nm (Figure 3b). When the Ag-sputtering duration increases to 10 min, some Ag NPs tend to aggregate and the number of nanogaps starts to decrease, as shown in Figure 3c. After the Ag-sputtering duration was elongated to about 12 min, a continuous film of Ag NPs was formed on the Ge nanocaps and the planar surface of the AAO template (Figure 3d), and many of the nanogaps among the Ag NPs disappeared. The UV−vis absorption spectra of Ag-NPs@Ge-nanocap arrays are shown in Figure S2 of Supporting Information, indicating that the absorption band for silver particles is located around 330 nm. SERS Map Measurements. To obtain an optimal SERS effect, the SERS activities of the Ag-NPs@Ge-nanocap arrays with different Ag-sputtering durations were tested by using 10−7 M R6G as probe molecules, as shown in Figure 4a. It can be
arrays of highly ordered and hexagonally arranged Ge nanocaps protruding from the planar surface of the AAO template are achieved, as shown in Figure 2. Scanning electron microscopy
Figure 2. Characterization of the Ge nanocaps protruding from the AAO template: (a) SEM oblique view; (b) SEM cross-sectional view of the Ge nanocaps and the Ge NTs embedded in AAO; (c) TEM image of a small section of the Ge nanotube arrays; and (d) EDX spectrum.
(SEM) observations (Figure 2a,b) clearly show that the ordered Ge nanocaps are about 50 nm high with tube diameters of 70 nm. The closed ends were further confirmed by the transmission electron microscopy (TEM) observation of nanotube arrays after being separated from the AAO template (Figure 2c). The energy dispersive X-ray (EDX) spectrum (Figure 2d) taken from the Ge nanocaps protruding from AAO template reveals Ge, Al, and O elements. Obviously, Ge element comes from the Ge nanocaps, whereas Al and O originate from the AAO template itself. After Ag sputtering onto the arrays of Ge nanocaps protruding from AAO template, the Ge nanocaps were covered with discontinuous Ag NPs, as shown in Figure 3 and Figure S1 of Supporting Information. It is demonstrated that the size and morphology of the Ag NPs
Figure 4. SERS sensitivity to R6G. (a) SERS spectra of 10−7 M R6G adsorbed on the Ag-NPs@Ge-nanocap arrays with different Agsputtering durations. (b) SERS spectra of R6G with different concentrations adsorbed on Ag-NPs@Ge-nanocap arrays with Ag sputtering for 8 min. The inset is the molecular structure of R6G.
observed that the SERS map peak intensities of R6G increase with the Ag-sputtering duration for the first 8 min and then decrease with longer Ag-sputtering duration. With further elongation of the Ag-sputtering duration, the Ag NPs aggregate on the Ge nanocaps and in between, resulting in rapid decrease of the hot spots between the neighboring Ag NPs;
Figure 3. Typical SEM top-view observations on the Ag-NPs@Genanocap arrays with different Ag-sputtering durations of (a) 4 min, (b) 8 min, (c) 10 min, and (d) 12 min. 13966
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Figure 5. Map of electron-transfer process between (a) Ge nanocaps and R6G and (b) Ag-NPs@Ge-nanocaps and R6G.
nanomembranes as SERS substrate but not by using Ag-NPs@ AAO as SERS substrate. As previously mentioned, the chemical enhancement of semiconducting Ge nanocaps originates from the charge transfer between analytes and themselves. A map of electron-transfer process between Ge and R6G is shown in Figure 5a.13 Because the work function of Ge is higher than that of Ag, the Fermi level of Ge rises when Ag contacts Ge. Thus, electrons are liable to transfer from Ag to Ge. Therefore, the sputtered Ag promotes the electron transfer (Figure 5b),38 resulting in enhanced SERS effect. In addition to the high SERS sensitivity, the SERS signal homogeneity of the Ag-NPs@Ge-nanocap arrays was also investigated by examining ten random spots across the substrate under identical experimental conditions, such as the laser power and the integration duration. The results reveal good reproducibility as the deviation of the band intensity of R6G (614 cm−1 shift) is less than 10% compared to that of the average relative peak intensity (see Supporting Information, part S6 and Figure S9). Analytical application of the Ag-NPs@Ge-nanocap arrays was tested toward the rapid detection of 3,3′,4,4′-tetrachlorobiphenyl (PCB-77), a congener of PCBs that are detrimental to human health.36 Traditional detection methods for PCBs, such as phosphorimetry,39 chromatographic analysis,40 and gas chromatography-isotope dilution time-of-flight mass spectrometry,41 are normally expensive, time-consuming, and complicated. Figure 6a shows the SERS spectra of PCB-77 in acetone with different concentrations (10−3, 10−4, 10−5, and 10−6 M) dispersed on the optimal Ag-NPs@Ge-nanocap arrays. The five fingerprint characteristic peaks at 677 cm−1 (C−Cl stretching), 1033 cm−1 (ring breathing), 1246 cm−1 (C−H wagging), 1300 cm−1 (biphenyl C−C bridge stretching), and 1600 cm−1 (ring stretching) in the SERS spectra (Figure 6)42 correspond well with those in the normal Raman spectrum of PCB-77. It can be seen that even for the concentration of PCB-77 down to 10−5 M, the crucial characteristic bands are still distinguishable. In comparison with Figure 4b and Figure 6a, the SERS sensitivity of the optimal Ag-NPs@Ge-nanocap arrays to PCBs is weaker than that to R6G, which can be attributed to the fact that PCB molecules do not have good affinity for Ag and cannot be readily anchored onto the bare surface of the Ag NPs.43 To decrease the low detection limitation, we tried to modify the optimal Ag-NPs@Ge-nanocap arrays with sulfhydryl-β-cyclodextrin (HS-β-CD), which can trap PCB-77 molecules into the cavities of β-CD to form the inclusion complex with β-CD.44 By modifying HS-β-CD, the Ag-NPs@Ge-nanocap arrays can efficiently capture more PCB-77 molecules on the hot spots between the adjacent Ag NPs. Figure 6b shows the SERS
consequently, the SERS peak intensity decreases. These results indicate that the coupling of the localized surface plasmons between the neighboring Ag NPs can be tailored by simply tuning the Ag-sputtering duration. Under the current experimental conditions, the Ag-NPs@Ge-nanocap arrays with Ag sputtering of 8 min is the optimal SERS substrate with the highest SERS activity. To show the SERS sensitivity of the optimal Ag-NPs@Ge-nanocap arrays, the SERS spectra of R6G with different concentrations (10−8, 10−9, 10−10, and 10−11 M) adsorbed on the optimal Ag-NPs@Ge-nanocaps are shown in Figure 4b. The spectral features of R6G can be identified clearly even at a concentration as low as 10 −11 M, demonstrating the high sensitivity of the large-area arrays of uniformly distributed Ag-NPs@Ge-nanocaps for SERS detection. The high SERS sensitivity originates from the following reasons. First, the sputtered Ag NPs on the large surface area of Ge nanocap arrays protruding from the planar surface of AAO template provide sufficient high density of SERS hot spots between the neighboring Ag NPs. Second, the roughed surface of large-area Ag-NPs@Ge-nanocap arrays are conducive for adsorbing sufficient analyte molecules. Third, the chemical supporting enhancement effect of the semiconducting Ge nanocaps may further improve the SERS sensitivity. To further test the chemical enhancement of the semiconducting Ge nanocaps, we compared the SERS performance of the Ge nanocap arrays (Figure S3a of Supporting Information), Ge film (Figure S3b), and Al2O3 nanocap arrays (Figure S3c) by using 10−3 M R6G aqueous solution. The SERS spectra taken from Ge nanocap arrays (curve I in Figure S3d) and Al2O3 nanocap arrays (curve II in Figure S3d) with identical morphology and size demonstrate that the Ge nanocap arrays show higher SERS signal intensity. Further SERS measurement indicates that the Ge nanocap arrays with a large surface area are liable to adsorb more analyte molecules to increase SERS signal than that of the Ge film (curves I and II in Figure S3d). Then the average enhancement factor was calculated by comparing the normalized SERS band intensity and the normal Raman band intensities of the 4-aminothiophenol (PATP) molecules. The typical average enhancement factor for the optimal Ag-NPs@Ge-nanocap arrays is estimated to be about 3.15 × 106 (part S4, Supporting Information). To further demonstrate that the semiconducting Ge supporter is remarkably important, we also compared the SERS performance between the Ag-NPs@Al2O3-nanostructures and the Ag-NPs@Ge-nanostructures (part S5, Supporting Information). The results show that R6G with a concentration down to 10−10 M can be identified by using the Ag-NPs@Ge13967
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Ag-NPs@Ge nanocap arrays for detecting PATP, SERS performance between Ag-NPs@Al2O3 nanostructures and AgNPs@Ge-nanostructures, and SERS spectra of R6G obtained from 10 random points on as-prepared SERS substrate. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Fax: (86) 0551-65591434. Tel.: (86) 0551-65592749. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program of China (Grant 2013CB934304), the CAS/SAFEA International Partnership Program for Creative Research Teams for financial support, and the NSFC (Grants 11274312, 51202254, and 51201159).
■ Figure 6. SERS sensitivity to PCBs. (a) SERS spectra of PCB-77 with different concentrations collected on the as-prepared substrates with Ag sputtering for 8 min. The inset is the molecular structure of PCB77. (b) SERS spectra of 10−6 and 10−7 M PCB-77 from HS-β-CD modified Ag-NPs@Ge-nanocap arrays. The inset is the schematic for PCB-77 molecule being captured by HS-β-CD.
spectra of 10−6 and 10−7 M PCB-77 collected from the HS-βCD-modified optimal Ag-NPs@Ge-nanocap arrays. From curve I in Figure 6b and curve IV in Figure 6a, it can be seen that the peak intensities of 10−6 M PCB-77 increase distinctly, and 10−7 M PCB-77 can be detected after modification of HS-β-CD. Therefore, with modification of HS-β-CD molecules, the AgNPs@Ge-nanocap arrays demonstrate improved ability to capture the target PCB molecules for SERS detection.
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CONCLUSION In summary, large-area hexagonally ordered Ag-NPs@Genanocap arrays have been achieved by ion-sputtering Ag NPs on the CVD-grown Ge nanocaps protruding from the planar surface of an AAO template. The large surface area of the Ge nanocaps is essential for loading a large amount of Ag NPs for high-density SERS hot spots between the Ag NPs, and thus leads to high SERS activity. Moreover, the chemical supporting enhancement effect of semiconducting Ge nanocaps also contributes to the SERS sensitivity. Furthermore, the uniformity of the Ge nanocap frameworks embedded in AAO template guarantees the homogeneous distribution of the large quantities of Ag NPs and thus reproducible SERS signals. The Ag-NPs@Ge-nanocap arrays show promising potential in SERS-based rapid detection of environmental organic pollutants, such as PCBs.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
Images of Ag-NPs@Ge-nanocaps, UV−vis spectra of the AgNPs@Ge-nanocap arrays, description of chemical enhancement of semiconductor Ge, calculation of the enhancement factor of 13968
dx.doi.org/10.1021/la5033338 | Langmuir 2014, 30, 13964−13969
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