12878
J. Phys. Chem. C 2010, 114, 12878–12884
Surface-Enhanced Raman and Fluorescence Spectroscopy of Dye Molecules Deposited on Nanostructured Gold Surfaces A. Merlen* UniVersite´ du Sud Toulon Var, IM2NP, Baˆtiment R, BP 20132, F83957 La Garde Cedex, France, and CNRS, IM2NP (UMR 6242), France
F. Lagugne´-Labarthet and E. Harte´ UniVersity of Western Ontario, 1151 Richmond Street, London, On, N6A5B7, Canada ReceiVed: February 22, 2010; ReVised Manuscript ReceiVed: June 15, 2010
Nanostructured gold surfaces were prepared over silicon and glass surfaces using evaporation and sputtering techniques, respectively. The surface enhanced Raman scattering (SERS) activity of these surfaces was investigated for dye molecules such as Methylene Blue and Rhodamine 6G using excitation wavelengths of 532 and 632.8 nm. Our results clearly emphasize that the SERS signal strongly depends on the nature of the surface and is further amplified under electronic resonance conditions. In addition, time-dependent spectral collection of the SERS and the surface-enhanced fluorescence (SEF) of Rhodamine 6G signals were obtained and showed similar behavior that could be fitted using a multiexponential model. This suggests a similar photobleaching mechanism for both SERS and SEF signals. We suggest that the study of molecules photostability under enhanced electromagnetic field should be extremely interesting for the further development of optical near-field spectroscopy. I. Introduction The optical properties of metallic nanoparticles such as gold or silver have led to the development of very sensitive spectroscopic analytical techniques. The localized near-field electromagnetic enhancement reported in the vicinity of such metallic particles can be applied to improve the sensivity of different optical techniques such as infrared spectroscopy through surface-enhanced infrared absorption (SEIRA),1 surfaceenhanced fluorescence (SEF),2 and surface-enhanced Raman spectroscopy.3-6 In the peculiar case of fluorescence, the interaction between a metallic nanoparticle and the analyte can either lead to a quenching of the fluorescence,7 commonly observed when the distance between the molecule and the surface is less than a few nanometers, or, in the ideal conditions, to an enhancement of the fluorescence that can also be triggered by the metallic nanoparticles.8 This can arise from two different origins: (i) an increase of the excitation magnitude due to the enhanced local field from the metallic nanoparticles or (ii) an increase of the quantum yield (correlated to an increase of the radiative decay rate). Both of them should occur for a slightly higher molecule-metallic nanoparticles distance. However, the relative contributions of those effects are still unclear and a fluorescence enhancement has been observed for molecules in direct contact with silver nanoparticles.8 On the other hand, the very intense Raman enhancement observed in SERS is due to an electromagnetic enhancement from the metallic nanoparticles and a charge transfer mechanism between the metallic nanoparticles and the surrounding molecules.9,10 Due to its extreme sensitivity, and with enhancements on the order of 108-1012, this technique is commonly used for the study of molecular traces in biology,11 forensic,12 * To whom correspondence should be addressed. E-mail: merlen@ univ-tln.fr.
archeology,13 art authentication,14 and many surface science problems in physics and chemistry. It is established that the electromagnetic enhancement in both SERS and SEF is due to surface plasmons properties localized at the surface of such metallic nanoparticles. As a consequence, any of those techniques requires metallic nanostructured substrates that can be produced by a broad variety of techniques to fabricate and tune such plasmonic platforms using chemical or physical methods.15,16 The main advantage of the physical protocol is that no functionalization of the gold particle is necessary, unlike in liquid-phase synthesis, where surfactants are usually used to prevent the formation of aggregates. This point is essential since such molecules can prevent the adsorption of other compounds, decreasing the SERS activity of the substrate. Among all physical techniques, evaporation17 or sputtering18 of a rough metallic thin film are efficient and costeffective preparation methods to fabricate these surfaces. For any protocol, the control of the surface plasmon band frequency is essential, since it plays a key role in the enhancement mechanism as outline above. For SERS measurements, it often appears that the highest enhancement is observed when the excitation wavelength matches the absorption band frequency of the substrate. More precisely, a recent study19 suggests that the highest enhancement is obtained when
λsp ≈
λexc + λRS 2
(1)
where λsp is the position of the surface plasmon band, λexc the excitation wavelength, and λRS the wavelength of the Stokesshifted Raman modes. Roughly, the highest enhancement is obtained when the absorption band is midway between the exciting wavelength and the Raman line. However, in a recent
10.1021/jp101576h 2010 American Chemical Society Published on Web 07/12/2010
Spectroscopy of Dye Deposited on Nanostructured Au work, Le Ru et al. have pointed out20 that the correlation between the macroscopic optical properties of the SERS substrate and the reported enhancement is not trivial. The enhancement is a pure local effect and the connection between surface plasmon band and highest enhancement depends strictly on the spatial localization of the collective resonances. This explains why in some reported SERS measurements a strong signal is observed with an excitation wavelength far from the surface plasmon band.21 The relation between the intensity of the observed SERS signal and the excitation wavelength also depends on the intrinsic optical properties of the probed molecules. Like for any Raman measurement, electronic resonant effects play a key role: when the excitation wavelength matches an absorption band of the molecule, electronic resonance occurs and the measured intensity is higher by several orders of magnitude. It has been shown that such a feature also occurs in SERS measurements performed in electronic resonance (SERRS).22 SERRS can be considered a highly sensitive spectroscopic technique. With dye molecules, whose absorption bands are generally located in the visible range, resonant effects can play a key role in the final observed SERS signal. Thus, many effects coming from the SERS substrate and the probed molecule can have a strong influence on the measured signal. In this paper, we report SERS measurements performed on dye molecules (Methylene Blue and Rhodamine 6G) using two different SERS substrates: one was obtained by the evaporation of a large gold quantity on a silicon surface, while other surfaces were prepared by sputtering very low quantity of gold on a glass substrate. The optical properties of those two substrates are totally different: the sputtered sample is partially transparent with a slight blue color, whereas the other one is totally opaque with a macroscopic appearance similar to bulk gold, suggesting a difference of thickness of the two films. Depending on the excitation wavelength, the absorption of the dye molecules, and the nature of the SERS substrate, the intensity of the SERS signal is strongly modified. Such features are discussed in term of resonant and local enhancement contributions. In the case of Rhodamine 6G, a clear fluorescence signal is superimposed on the SERS signal and depends on the nature of the substrate as well as the excitation wavelength. Few studies have been performed comparing the SERS and SEF signal. In this work, we show strong similarity between those two signals. More specifically, we report for both of them a progressive time decrease of their intensity for Rhodamine 6G. Those similar behaviors suggest that the observed enhanced Raman and fluorescence signals could have a common origin. II. Experimental Section Sputtered gold substrates were prepared following a protocol described elsewhere.18 Briefly, gold nanoparticles were deposited directly on previously washed glass substrates by sputtering under argon plasma. The deposition time was 20 s with a current of 40 mA. The presence of gold nanoparticles on such substrate is confirmed by a broad surface plasmon band absorption centered around 580 nm, as can be seen in Figure 1 and is clearly observed in the AFM measurement. Their SERS activity was compared with gold evaporated on silicon substrates. Before deposition, the silicon substrate was washed in a piranha solution during 15 min. A 6 nm layer of chromium was then evaporated and followed by a gold layer. During evaporation, the vacuum was fixed at 2 × 10-6 Torr. The gold thickness is around 190 nm, which is larger than the penetration depth of visible light (
