Gold-Coated Silver Dendrites as SERS Substrates with an Improved

Article pubs.acs.org/Langmuir

Gold-Coated Silver Dendrites as SERS Substrates with an Improved Lifetime Albert Gutés, Roya Maboudian,* and Carlo Carraro Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720, United States ABSTRACT: Nanostructured silver is known to yield the highest signal-enhancement factors in surface-enhanced Raman spectroscopy, but its low chemical stability toward oxidation presents a challenge in the realization of Ag-based SERS substrates with long operating lifetimes. Here, a study of the long-term stability of silver dendrites as SERS substrates is reported. SERS spectra of 1,2-benzenedithiol monolayers on Ag dendrites, acquired over a period of time in excess of 1 year, shows appreciable degradation with time. However, no degradation is observed in the spectra of monolayers deposited on Ag dendrites that were coated with a monolayer-thin Au film deposited by an immersion plating process. X-ray photoelectron spectra confirm the oxidation of the uncoated Ag dendrites whereas no chemical changes are detected in the Au-coated ones. These results suggest that the galvanic displacement of Au on preformed Ag nanostructures provides a suitable route to producing SERS-active substrates with long operating and/or shelf lifetimes.

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INTRODUCTION Since the first reports1 of surface-enhanced Raman spectroscopy, interest has grown in developing simple, stable, efficient, and cost-effective substrates for single-molecule detection2−4 and biomedical analysis.5−8 Many metals have been studied as SERS substrates, with silver exhibiting the highest enhancement factor,9 but because of intrinsically low stability toward oxidation,10 Ag is often inadequate for applications requiring long shelf lives. Stability studies of SERS substrates are of crucial importance, especially in cases where the final application requires long-term substrate stability as, for example, when used as a biological in vivo tag.11 Several recent works have focused on this topic, reporting stabilities of days,12−14 weeks,15,16 or months.17−19 The different approaches for obtaining improved stability of the substrates include atomic-layer deposition of alumina on silver substrates,12,18 roughening of silver surfaces by the action of alumina nanoparticles while applying an oxidation potential to the silver surface,13,14 substrate entrapment in biopolymeric shells of chitosan,17 and embedding silver nanoparticles in ionexchanged glass.19 However, in the majority of previous works a direct comparison of the Raman spectra obtained before and after the aging of the substrates is not reported, and the stability study is based on indirect measurements, such as the lack of change in UV−vis optical properties.17 Facile and versatile methods of synthesizing SERS-active substrates from Ag nanostructures were reported previously,20−23 but their long-term viability was not characterized. Here, we present a study of the long-term stability of these substrates toward oxidation under ambient conditions. Furthermore, we propose a simple method for protecting these substrates with a monolayer-thin Au coating and show that the substrates protected in this manner exhibit superior © XXXX American Chemical Society

long-term stability, well in excess of 1 year, with respect to the uncoated Ag substrates.

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EXPERIMENTAL SECTION

Silver dendrites are synthesized as described in previous work.10 In short, aluminum foil is degreased in acetone and etched in concentrated HF to remove the native oxide prior to immersion in a 20 mM AgF solution for 24 h in the dark. Large amounts of Ag dendrites are formed on the Al foil. After being detached, rinsed, and dried, the Ag dendrite powder is compressed using a conventional IR pellet press to a thickness of about 1 mm. The dendrites are coated with gold by dipping the Ag dendrite pellets into a 1 mM KAuCl4 solution for 60 s. A thin Au film is formed by the spontaneous reduction of Au3+ ions on the substrate by galvanic displacement.24 Final rinsing in deionized water and drying are then performed. The effect of exposure to ambient air is monitored by X-ray photoelectron spectroscopy (XPS) analysis using an Omicron analyzer (EA 125) and an Omicron DAR400 source with Al Kα X-rays at an energy of 1486.6 eV. In particular, Ag 3d, Au 4f, and O 1s regions are recorded on freshly prepared Ag dendrites and Au-coated Ag dendrites and are recorded again after 15 months of storage in room air. Four sets of dendrites are incubated in a 100 μM solution of 1,2benzenedithiol in ethanol: (i) freshly prepared Ag dendrites, (ii) freshly prepared Ag dendrites coated with Au, (ii) 1-year-old Ag dendrites, and (iv) 1-year-old Ag dendrites coated with Au. After incubation for 24 h, ultrasonication in fresh ethanol is performed to remove any physisorbed molecules. A gentle flow of N2 is used to dry the dendrite pellets. Once they are dried, the Raman spectra are immediately recorded in the 950 to 1825 cm−1 region using a JYHoriba LabRAM spectrometer in backscattering configuration with an excitation line provided by a HeNe laser (632.8 nm wavelength, 7 Received: May 17, 2012 Revised: October 26, 2012

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mW power at the sample) through an Olympus BX41 confocal microscope (100× objective, numerical aperture = 0.8).

Table 1. XPS Ag Peak Areas in 15-Month-Aged Uncoated Ag Dendrites

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RESULTS AND DISCUSSION Figure 1a shows the scanning electron microscopy (SEM, Agilent U9320A 8500) image of the Ag dendrites. The

peak positions (eV) peak intensities (cts/s) Ag 3d5/2 (metal) Ag 3d3/2 (metal) Ag 3d5/2 (charged) Ag 3d3/2 (charged)

368 62 700 62 700

374.3 88 300

380.5 30 000

41 800 46 500 30 000

possibility of synthesizing Au dendrites is investigated without success because Au deposition is governed by much slower kinetics and thus a film is formed on the Al foil, in combination with Au nanoparticles and larger crystals. Figure 1b shows the nanostructure formed on top of the Al foil when the synthesis is pursued by the same wet chemical approach used for Ag dendrites. As can be observed in Figure 1a, many edges and sharp tips are formed, resulting in a high density of SERS hot spots per unit area. To monitor the evolution of the chemical state of the surface with time, XPS analysis is performed on both Au-coated and uncoated Ag specimens. The thickness of the Au film deposited on the dendrites can be estimated by the Au 4f to Ag 3d XPS

Figure 1. SEM image of (a) Ag dendrites obtained by the immersion of Al foil in a 20 mM AgF solution for 24 h in the dark. (b) Au structures obtained by the immersion of Al foil in a 20 mM KAuCl4 solution for 24 h in the dark.

Figure 2. X-ray photoelectron spectra of the Au-coated Ag dendrites: (a) Au 4f region on day 1 (black) and after 15 months (red, dashed), and (b) Ag 3d region on day 1 (black) and after 15 months (red, dashed). X-ray photoelectron spectra of the uncoated Ag dendrites: (c) Ag 3d spectrum on the aged Ag dendrites showing the doublet peak split. Dashed line (right y axis) corresponding to the fresh Ag dendrite spectrum and (d) O 1s of the aged dendrites showing a doublet peak. The O 1s signal for the regalvanized dendrites shows the typical single peak is shown with a dashed line. B

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absence of oxidation and thus the chemical inertness of the Aucoated Ag dendrites. The X-ray photoelectron spectra of uncoated Ag dendrites freshly prepared and after 15 months of storage are also recorded and are shown in Figure 2c for the Ag 3d region. The most striking difference is the appearance of a new peak at around 380.5 eV. The reversal of the intensity ratio of the Ag 3d doublet components (Ag 3d5/2 at 368 eV and Ag 3d3/2 at 374.3 eV, expected in an intensity ratio of 3:2) suggests that the aged spectrum results from the superposition of two Ag doublets, with a relative shift of about 6.3 eV. The analysis of the intensities of the three peaks in Figure 2c confirms that this is indeed the case (Table 1). We believe that the shift of the Ag doublet of about 6.3 eV can be attributed to charging, caused by large areas of the sample surface becoming insulating via Ag oxidation over time. The fact that this shift is approximately equal to the Ag doublet split is purely accidental, of course. This explanation is confirmed by the analysis of the O 1s photoemission line in the aged sample spectra (Figure 2d). This peak is also split with a large component at an unusually high binding energy of 539 eV. Because surface charge shifts all peaks equally, this component corresponds to oxygen with a binding energy of about 533 eV, which is attributable to oxygen in water,25 indicating that a large amount of atmospheric water adsorbs on hydrophilic and insulating regions of the aged dendrites. These regions likely consist of patches of silver oxides. For comparison, the O 1s spectrum of Au-coated Ag dendrites is also shown in Figure 2d, revealing only a small, and comparatively narrower, peak at about 533 eV, which is expected from the ubiquitous water monolayer coverage. The effectiveness of the dendrites as SERS substrates is also studied. First, a comparison between the silver dendrites and a 10 nm evaporated Ag film on a silicon substrate was performed. Both substrates were incubated in 1,2-benzendithiol for 24 h as explained in the Experimental Section. Immediately after rinsing to ensure only the presence of a monolayer, the Raman spectrum is collected for both substrates. Figure 3

Figure 3. Raman spectra of a 1,2-benzendithiol monolayer on freshly synthesized Ag dendrites (black), on a 10 nm Ag film on silicon (red, dashed), and on Au nanoparticles of different sizes: 10 nm (dashed green) and 200 nm (orange).

peak intensity ratio. In contrast to smooth films, where the film thickness can be extracted with high accuracy,24 for films coating a rough surface the peak ratio provides only an upper limit to the film thickness. With this proviso, the gold thickness is estimated to be ∼0.25 nm, indicative of a monolayer film. The dendrite pellet is kept in laboratory air and exposed to light for more than 15 months. Figure 2 shows (a) Au 4f and (b) Ag 3d regions obtained on Au-coated Ag dendrites on day 1 and after 15 months. No significant difference can be observed in the peak positions and (relative) intensities, revealing the

Figure 4. Raman spectra of a 1,2-benzendithiol monolayer on different dendrites. (a) Uncoated Ag dendrites freshly synthesized (black) and 15 months old (red, dashed). (b) Au-coated Ag dendrites freshly synthesized (black) and 15 months old (red, dashed). C

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(3) Qian, X.-M.; Nie, S. M. Single-molecule and single-nanoparticle SERS: from fundamental mechanisms to biomedical applications. Chem. Soc. Rev. 2008, 37, 912−920. (4) Kneipp, J.; Kneipp, H.; Kneipp, K. SERS - a single-molecule and nanoscale tool for bioanalytics. Chem. Soc. Rev. 2008, 37, 1052−1060. (5) Vo-Dinh, T.; Stokes, D. L.; Griffin, G. D.; Volkan, M.; Kim, U. J.; Simon, M. I. Surface-enhanced Raman scattering (SERS) method and instrumentation for genomics and biomedical analysis. J. Raman Spectrosc. 1999, 30, 785−793. (6) Schlucker, S. SERS microscopy: nanoparticle probes and biomedical applications. ChemPhysChem 2009, 10, 1344−1354. (7) Bonora, S.; Di Foggia, M.; Tugnoli, V.; Righi, V.; Benassi, E.; Maris, A. Raman and SERS study on cimetidine-metal complexes with biomedical interest. J. Raman Spectrosc. 2011, 42, 612−620. (8) Premasiri, W. R.; Moir, D. T.; Klempner, M. S.; Krieger, N.; Jones, G.; Ziegler, L. D. Characterization of the surface enhanced Raman scattering (SERS) of bacteria. J. Phys. Chem. B 2005, 109, 312− 320. (9) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Ultrasensitive chemical analysis by Raman spectroscopy. Chem. Rev. 1999, 99, 2957−2975. (10) Hillenkamp, M.; Di Domenicantonio, G.; Eugster, O.; Felix, C. Instability of Ag nanoparticles in SiO2 at ambient conditions. Nanotechnology 2007, 18, 015702. (11) Maiti, K. K.; Dinish, U. S.; Fu, C. Y.; Lee, J. J.; Soh, K. S.; Yun, S. W.; Bhuvaneswari, R.; Olivo, M.; Chang, Y. T. Development of biocompatible SERS nanotag with increased stability by chemisorption of reporter molecule for in vivo cancer detection. Biosens. Bioelectron. 2010, 26, 398−403. (12) John, J. F.; Mahurin, S.; Dai, S.; Sepaniak, M. J. Use of atomic layer deposition to improve the stability of silver substrates for in situ, high temperature SERS measurements. J. Raman Spectrosc. 2010, 41, 4. (13) Yang, K. H.; Liu, Y. C.; Hsu, T. C.; Juang, M. Y. Strategy to improve stability of surface-enhanced raman scattering-active Ag substrates. J. Mater. Chem. 2010, 20, 7530−7535. (14) Yang, K. H.; Liu, Y. C.; Yu, C. C. Enhancements in intensity and stability of surface-enhanced Raman scattering on optimally electrochemically roughened silver substrates. J. Mater. Chem. 2008, 18, 4849−4855. (15) Larmour, I. A.; Faulds, K.; Graham, D. SERS activity and stability of the most frequently used silver colloids. J. Raman Spectrosc. 2012, 43, 202−206. (16) Hu, J. W.; Lu, L. H.; He, W. M.; Pan, J. G.; Wang, W. Y.; Xiang, J. N. Ligand exchange based water-soluble, surface-enhanced Raman scattering-tagged gold nanorod probes with improved stability. Chem. Phys. Lett. 2011, 513, 241−245. (17) Potara, M.; Gabudean, A. M.; Astilean, S. Solution-phase, dual LSPR-SERS plasmonic sensors of high sensitivity and stability based on chitosan-coated anisotropic silver nanoparticles. J. Mater. Chem. 2011, 21, 3625−3633. (18) Zhang, X. Y.; Zhao, J.; Whitney, A. V.; Elam, J. W.; Van Duyne, R. P. Ultrastable substrates for surface-enhanced Raman spectroscopy: Al2O3 overlayers fabricated by atomic layer deposition yield improved anthrax biomarker detection. J. Am. Chem. Soc. 2006, 31, 10304− 10309. (19) Simo, A.; Joseph, V.; Fenger, R.; Kneipp, J.; Rademann, K. Long-term stable silver subsurface ion-exchanged glasses for SERS applications. ChemPhysChem 2011, 12, 1683. (20) Gutes, A.; Carraro, C.; Maboudian, R. Silver dendrites from galvanic displacement on commercial aluminum foil as an effective SERS substrate. J. Am. Chem. Soc. 2010, 132, 1476−1477. (21) Gutes, A.; Carraro, C.; Maboudian, R. Silver nanodesert rose as a substrate for surface-enhanced raman spectroscopy. ACS Appl. Mater. Interfaces 2009, 1, 2551−2555. (22) Wilson, E. K. Foil and tape serve Raman. Chem. Eng. News 2010, 88, 30−30. (23) Gutes, A.; Laboriante, I.; Carraro, C.; Maboudian, R. Silver nanostructures on silicon based on galvanic displacement process. J. Phys. Chem. C 2009, 113, 16939−16944.

presents the spectra clearly displaying the higher resolution and enhancement provided by the dendrites in comparison to a 10 nm Ag film and 10 and 200 nm Au nanoparticles. As can be observed, both the spectral resolution and Raman intensity are much higher in the case of the Au-coated dendrites when compared to that of the other SERS-active substrates. After the initial SERS test, freshly synthesized uncoated Ag dendrites, freshly synthesized Au-coated Ag dendrites, 15months-old uncoated Ag dendrites, and 15-months-old Aucoated Ag dendrites are also incubated in 1,2-benzenedithiol for 24 h as explained in the Experimental Section. Figure 4a presents the spectra obtained for the uncoated Ag dendrites. As can be observed, both fresh and aged substrates exhibit SERS behavior, but the fresh dendrites clearly present better-resolved spectral features. Figure 4b presents the spectra obtained for the Au-coated Ag dendrites. In this case, spectral resolution, when compared to that of freshly synthesized and 15-monthold dendrites, does not show significant degradation with aging. When comparing the uncoated and Au-coated dendrites after 15 months (red dashed lines in Figure 4a,b), a significant decrease is observed in the peak intensities at 1075 cm−1 and 1575 cm−1/1600 cm−1, as well as an increase in background on the uncoated dendrites. The increased background can be attributed to the fluorescence of the Ag2O layer, and the decreases in all of the peaks corresponding to the target molecule can be explained as the decrease in the coverage of thiol molecules that are not able to bind to Ag because of the formation of the oxide layer. It can therefore be concluded that the monolayer Au coating offers suitable long-term stability for SERS properties of the dendrites.

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CONCLUSIONS A simple process to prevent the oxidation of Ag SERS substrates by a Au monolayer coating through electroless deposition is presented. Long-term stability toward substrate oxidation is demonstrated by XPS analysis. Au-coated dendrites are shown to be better SERS enhancers than uncoated dendrites and also to present better spectral resolution. Aucoated Ag dendrite substrates are shown to be stable for SERS analysis, exhibits no degradation even after 15 months of storage in ambient air.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1 (510) 643-7957. Fax: +1 (510) 642-4778. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation under grant EEC-0832819 (Center of Integrated Nanomechanical Systems).

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REFERENCES

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(24) Gutes, A.; Carraro, C.; Maboudian, R. Ultrasmooth gold thin films by self-limiting galvanic displacement on silicon. Appl. Mater. Interfaces 2011, 3, 1581−1584. (25) Au, C.-T.; SinghBoparai, S.; Roberts, M. W.; Joyner, R. W. Chemisorption of oxygen at Ag(110) surfaces and its role in adsorbate activation. J. Chem. Soc., Faraday Trans. 1983, 79, 1779−1791.

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