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Gold Films Deposited over Regular Arrays of Polystyrene Nanospheres as Highly Effective SERS Substrates from Visible to NIR L. Baia,*,† M. Baia,† J. Popp,‡,§ and S. Astilean† Faculty of Physics, Babes-Bolyai UniVersity, M. Kogalniceanu 1, 400084 Cluj-Napoca, Romania, Institute of Physical Chemistry, Friedrich-Schiller UniVersity, Helmholtzweg 4, D-07743 Jena, Germany, and Institute of Physical High Technologies e. V. Jena, Albert Einstein 9, D-07745 Jena, Germany ReceiVed: July 14, 2006; In Final Form: September 18, 2006
Gold nanostructured films of various thicknesses (15, 30, and 60 nm) are deposited over regular arrays of polystyrene nanospheres in an attempt to evaluate their potential as SERS-active substrates. Atomic force microscopy is used to topographically characterize the substrates as well as to ensure the thickness of the deposited gold films. The optical response of the prepared substrates recommends their use in SERS experiments with multiple laser lines from visible and NIR spectral domains. The assessment of the substrates’ SERS activity is performed by using the 532, 633, and 830 nm excitation lines and different average enhancement factor (EF) values are obtained depending on the film thickness and employed laser line. The 60 nm gold nanostructured film generates the greatest local electromagnetic field confinement under NIR excitation and consequently gives rise to maximum SERS enhancement. The large tunability of surface plasmon excitation combined with the advantage of relatively high exhibited average EF values obtained under NIR excitation recommends these substrates as outstanding candidates for upcoming investigations of biological relevant molecules.
1. Introduction The considerable interest in noble metal nanoarchitectures study comes from their potential in optical, chemical, and biomolecular sensing,1-5 optoelectronics,6,7 catalysis,7,8 and surface-enhanced Raman spectroscopy (SERS).8-10 The SERS technique is one of the most sensitive spectroscopic tools available for the detection of a wide range of adsorbate molecules down to the single-molecule detection limit.11-14 Two well-known mechanisms have been found to be responsible for the giant Raman enhancement.12,15,16 The first one (electromagnetic mechanism) arises from a huge enhancement of the local electromagnetic (EM) field close to surface roughness, due to the excitation of a localized surface plasmon, while the second one (chemical or charge-transfer mechanism) appears as a consequence of the molecular adsorption onto specific sites when resonant charge-transfer (CT) occurs. The recent advances in SERS are mainly guided toward understanding and developing SERS as an analytical tool. For example, one of the major problems in hospitals, pharmaceutical clean rooms, and food-processing technology is microbial contamination, which requires a rapid and sensitive identification of microorganisms. Previous studies have shown the great capabilities of micro-Raman spectroscopy for identifying single microorganisms.17-20 The use of 532 nm as the Raman excitation line for the investigation yields Raman spectra with a good S/N ratio. To further improve the detection of single bacteria, the acquisition time should be lowered and the quality of the bacterial spectra should be increased by quenching the fluorescence. These goals can be reached by SERS.21 However, * Corresponding author. Fax: +40 264 591906. E-mail: lucb@ phys.ubbcluj.ro. † Babes-Bolyai University. ‡ Friedrich-Schiller University. § Institute of Physical High Technologies e. V. Jena.
these attempts strongly depend on the fabrication of stable and highly reproducible SERS-active substrates. Typical SERS substrates such as roughened gold and silver electrodes or colloidal suspensions are disordered, and their structures are unstable and quickly lose their SERS activity.22,23 Therefore, controlled preparation of nanostructured substrates is essential for providing useful correlations between surface morphology and Raman signal enhancement. In this respect, a wide variety of metallic nanostructures have been fabricated and investigated as SERS substrates in past decades,22,24 one of the most important targets being the designing of active SERS substrates that can efficiently be used under both visible and near-infrared (NIR) excitation. Nanosphere lithography has been successfully used to fabricate arrays of nanometer-scale noble-metal particles with controllable size, shape, and interparticle spacing by using a close-packed monolayer of nanospheres as a metal deposition mask.25-29 In a recent study10 we showed that periodic corrugated gold films deposited on top of highly ordered polystyrene nanosphere arrays obtained by using this technique are effective SERS substrates for visible excitation. In the present work, we attempt to extend the SERS potential of similarly fabricated substrates, from visible to NIR excitation, by using nanospheres with larger diameter as a deposition mask. Encouraged by the substrates’ plasmonic response evidenced from reflectance measurements, our further interest was to optimize the SERS activity of gold films for different excitation wavelengths (532, 633, and 830 nm) depending on the film thickness by quantitative and comparative assessment of the exhibited Raman enhancement. 2. Experimental Section Substrates and Samples Preparation. The ordered metallic nanostructures employed as SERS substrates were prepared
10.1021/jp064458k CCC: $33.50 © 2006 American Chemical Society Published on Web 11/03/2006
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Figure 1. AFM image of a 10 × 10 µm2 region of the 60 nm gold nanostructured substrate together with its section analysis. The inset shows the truncated tetrahedra formed in the spaces between spheres (400 nm diameter) at an expanded scale.
according to the drop-coat method of nanosphere ordering as described in the literature.30 The solid substrates (silica of dimensions 25 mm × 25 mm × 1 mm) were cleaned by immersion in piranha solution (3:1 concentrated H2SO4:H2O2) for 2 h. The substrates were repeatedly rinsed with deionized water and then sonicated for 30 min in a 5:1:1 H2O:NH4OH: H2O2 solution. Following sonication the substrates were again repeatedly rinsed and stored in deionized water prior to nanosphere ordering. A suspension of monodisperse polystyrene nanospheres of 400 nm diameter was drop-coated onto the substrates where they, after water evaporation, self-assembled into hexagonally close-packed two-dimensional (2D) colloidal crystals that served as a deposition mask. The substrates were then mounted into the vacuum chamber of a vapor deposition system and gold films of 15, 30, and 60 nm thickness were thermally evaporated onto the substrates under a pressure of 5 × 10-6 Torr. For recording the SERS spectra the p-aminothiophenol (pATP) probe molecules were adsorbed on the nanostructured gold substrates by immersion into 10-3 M p-ATP MeOH solution for 24 h. Prior to recording of the SERS spectra, the substrates were repeatedly rinsed with methanol to ensure the p-ATP was adsorbed as a monomolecular layer. p-ATP and all materials involved in samples and substrates preparation were purchased from commercial sources as analytical pure reagents. Experimental Measurements. A Digital Instrument Nanoscope atomic force microscope (AFM) was used to examine the substrates morphology and to ensure the thickness of the deposited gold films. Reflectance spectra were recorded with a Cary 5000 UVVis-NIR spectrometer by using unpolarized light with a probe beam size of approximately 2 mm2. All Raman and SERS spectra were recorded in backscattering geometry with a Dilor Labram system equipped with a Leica PL FLUOTAR 100×/0.75 microscope objective and a 300 lines/ mm grating. The employed microscope objective ensures the excitation and collection of the Raman scattered light from a solid angle of approximately 50° off normal incidence. In the recording of the spectra three lasers with emission wavelengths of 532, 633, and 830 nm and powers incident on the sample of 20, 4.3, and 1.1 mW, respectively, were employed. The spectral resolution used for all measurements was smaller than 5 cm-1. The SERS spectra were collected from more than 10 different points of the sample.
Figure 2. Reflectance spectra of gold nanostructured substrates of different thicknesses as depicted recorded at 50° off normal incidence. The arrows indicate the employed excitation laser lines.
3. Results and Discussion The nanosphere assemblies covered with gold films of different thicknesses were firstly analyzed by AFM to probe the designed morphology. Figure 1 shows a 10 × 10 µm2 image of the 60 nm nanostructured gold film together with its section analysis. It is remarkable to note the existence of large domains of 2D colloidal crystals. The majority of the self-assembled nanospheres have a hexagonally close-packed symmetry. However, a few features like linear and local dislocations, which usually appear on such nanosphere masks, can be seen. Nevertheless, these characteristics should not considerably affect the reproducibility of the SERS spectra. One should also emphasize that the surface morphology of the fabricated metallic films is quite complex due to the superimposition of two gratings consisting of half-shells and truncated tetrahedra that are formed in the spaces between the spheres. The latter are highlighted in the inset from the upper left corner of Figure 1. To probe the plasmonic response of the investigated substrates, reflectance measurements have been performed and those recorded at 50° off normal incidence are presented in Figure 2. As one can see the reflectance spectra show strong reflectivity around 540 and 800 nm separated by pronounced reflectivity dips around 350, 440, 640, and 840 nm. The existence of two spatially separated metallic gratings, one arising from the
23984 J. Phys. Chem. B, Vol. 110, No. 47, 2006 obvious half-shells and the other one from less obvious truncated tetrahedra, which are geometrically different and give their own optical responses, as well as the scattering contribution of the polystyrene spheres substrate, make difficult the accurate analysis of the overall optical response of these nanostructured films. Similar reflectivity minima were observed around 525 and 650 nm in the spectra recorded on arrays of gold oblate spheroidal particles deposited onto a 20-nm thick gold film and were assigned to the mixed modes proceeding from the simultaneous excitation of localized and propagating surface plasmon modes and from the excitation of an ensemble of particles strongly coupled by propagating surface plasmon wave, respectively.31 Taking into account previously reported results,10 where optical properties of 54 nm gold films deposited on top of highly ordered polystyrene nanosphere (220 nm) arrays were investigated in relation to those of the flat metallic film, we assume that the optical characteristics at wavelength values lower than 700 nm are due to the simultaneous excitation of both localized plasmons, supported by quasi-isolated gold nanoprisms, and the propagating ones, from both metaldielectric interfaces. The change of the reflectivity dip positions with increasing film thickness can be explained by assuming that the surface waves on two metal-dielectric interfaces are resonant at different wavelength values and additionally they could interfere constructively or destructively. This behavior can consequently imply that certain thicknesses are efficient either in transmission or in reflectance at a specific wavelength. The calculation of plasmonic band structure is in progress. This would provide further insight concerning the coupling between the localized and propagating plasmons and would certainly contribute to a deep understanding of the optical properties of the fabricated gold nanostructured films. In the NIR spectral domain of the reflectance spectra (Figure 2) one observes a slight shift to lower wavelengths of the reflectance dip in the 830 nm wavelength range with the progressive increase of the film thickness. Keeping in view that strongly localized EM fields exist both at the top of the spheres and between them, we assume that the observed dip is most probably due to the EM coupling between the interconnected gold half-shells. The progressive increase of the reflectance dip as the gold film becomes thicker could lead to the EM field confinement either between calottes or at the top of them. If this is the case, the gold nanostructured substrate with the highest thickness should consequently provide the greatest SERS enhancement under NIR excitation. The above analysis suggests that by using a given laser line the SERS signal can be maximized for an optimum gold film thickness. Therefore, we effectively test the enhancement capabilities of the prepared substrates with laser lines covering a wide wavelengths domain from visible to NIR. The recorded SERS spectra of the adsorbed p-ATP molecules are presented in Figure 3 together with their corresponding Raman spectra. Significant differences can be observed between the SERS spectra recorded with a 830 nm laser line and those obtained with visible excitation wavelengths relative to the conventional Raman spectra. To prove the reproducibility of the SERS spectra for each substrate and laser line, the measurements were repeated many times from different points of the substrates and similar spectra, i.e., position and relative intensity of the bands, were obtained. It is also worth mentioning that the SERS measurements were repeated after a few weeks and identical spectra were obtained. This shows the remarkable stability of the prepared SERS substrates. The differences
Baia et al.
Figure 3. Normal Raman spectra of solid p-ATP (a) and the SERS spectra of p-ATP adsorbed on gold nanostructured films with thicknesses of 15 nm (b), 30 nm (c), and 60 nm (d) recorded with 532 (A), 633 (B), and 830 (C) nm excitation laser lines. The SERS spectra recorded with the 532 nm line were baseline-corrected.
observed between the SERS spectra recorded with various laser lines and the corresponding Raman spectra are comparable with those evidenced in a previous study where p-ATP molecules adsorbed on self-assembled gold colloidal nanoparticles and were explained from the perspective of different contributions of the enhancement mechanisms to the total enhancement when
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visible and NIR excitations were used.32 It was shown there that the enhancement of the p-ATP Raman band located at 1080 cm-1 and assigned to the C-S stretching vibration has a pure EM origin. Its further use for the assessment of the enhancement efficiency of the gold films is justified since such noble-metal ordered nanostructures are at least designed to control and optimize the EM contribution to the Raman enhancement as well as to tune the surface plasmon excitation band toward the NIR region, extending thus their applicability area to biological molecules investigations. It should be noted that the UV-Vis absorption spectrum of the probe molecule presents two bands around 256 and 297 nm33 and thus no resonant Raman contribution would be involved in the overall SERS enhancement when visible and NIR excitations are employed. The enhancement factor (EF) values were calculated for each laser line and substrate using the following equation,7,34
EF )
Isurf Nvol Ivol Nsurf
(1)
where Ivol, Isurf, Nvol, and Nsurf are the measured intensity of a selected band from the bulk p-ATP Raman spectrum, the measured intensity of the corresponding SERS band, the number of probe molecules located in the focus volume within the bulk sample, and the number of p-ATP molecules adsorbed on the gold surface, respectively. Nvol can be expressed as follows,34
F Nvol ) Alsh m
(2)
where Als, h, F, and m are the laser spot area, the focus length, the density of solid p-ATP (1.17 g/cm3), and its molecular weight, respectively. The parameters related to the focus dimensions, i.e., Als and h, for 532, 633, and 830 nm laser lines have been obtained by considering a focal “tube” with waist diameters of ∼0.9, 1, and 1.3 µm and depths of ∼16, 19, and 25 µm, respectively.35 Nsurf has been determined by using the following equation,
Nsurf )
Als Asc + NTT Ac Am
(3)
where Ac, Asc, Am, and NTT represent the area of a circle with a diameter equal with that of the polystyrene spheres, the area of a spherical calotte, the area occupied by a single p-ATP molecule (22 Å2),36 and the number of p-ATP molecules adsorbed on the truncated tetrahedra, respectively. It should be mentioned that the exposed area of the truncated tetrahedra was calculated based on its in-plane and out-of-plane dimensions.37 For the gold film with 60 nm it was considered, as previously reported,24 that only 7% from the entire exposed area of the truncated tetrahedra is SERS-active because of the huge EM fields that are located along the edges and corners.38 Substituting all the above values into eq 1, we calculated the EFs for each laser line and substrate. These values are displayed in Figure 4 as a function of the employed excitation wavelength. The Raman intensities used to evaluate the EF for a certain laser line have been normalized for laser power and acquisition time, under the same collection conditions. Keeping in view that a considerably intense EM field exists at the top of the sphere, but especially between two touching metal spheres, i.e., 1011 enhancement with respect to the localized surface plasmon excitation,39 the calottes geometrical area used in the assessment of the SERS enhancement is an overestimation of the active
Figure 4. Calculated average enhancement factors as a function of the excitation laser line.
area and thus the calculated EFs can be considered only average values. In addition, any disorder in the self-assembled monolayer would also yield a larger EF. Nevertheless, their evaluation is of great importance since it enable us to compare the enhancements provided by the substrates when different excitation lines are employed. The close inspection of Figure 4 reveals the existence of a more than 220 times gain of the average enhancement of the Raman signal of the p-ATP molecules adsorbed on the 60 nm nanostructured film when an 830 nm line was used for excitation (1.23 × 106) in comparison with that exhibited under 532 nm excitation (5.4 × 103). Moreover, for the 830 nm laser line one also notes the significant increase of the average EF for the 30 nm gold nanostructured film. The relatively reduced average EFs obtained for 532 nm excitation are not surprising at all as long as it is known that the interband transition of deep d-electrons to 6s-conduction band plays on the strength of local field at this wavelength.40 By comparing the average EF values obtained from different substrates excited with the same laser line, one can see the existence of a wavelength-dependent enhancement (see Figure 4). Thus, for the excitation with 532 nm the most enhanced SERS signal was obtained from the sample with 15 nm thickness, while for excitation with 830 nm the most enhanced SERS signal was achieved from the sample with 60 nm thickness. When the 633 nm line was used for excitation, the most enhanced SERS signal was recorded from the sample of 30 nm thickness, due probably to a stronger coupling between surface plasmons at this thickness. The major differences of the average EF values were obtained for the gold nanostructured films when 830 nm excitation line was employed. To get further insight into the exhibited enhancement behavior, the relative Raman enhancements were calculated by multiplying the SERS intensities ratio for the 1080 cm-1 band with the inverse ratio of the exposed areas of the metallic films. Thus, one obtains that for the 830 nm excitation line the metallic film with 60 nm thickness provides a stronger enhancement by 6.7 times compared to that of the film with 15 nm. Since the exposed areas ratio value calculated for the films with the smallest and the highest thickness is 0.8, one assumes the existence of a supplementary enhancement, which does not originate from the metallic geometrical area of the surface where the molecules are adsorbed. The reflectance spectra support this result; a sharpening of the reflectivity dip around 840 nm for the gold film of 60 nm thickness could be observed in Figure 2. Moreover, the differences obtained between the average EF ratio and the inverse ratio of the theoretically exposed areas for
23986 J. Phys. Chem. B, Vol. 110, No. 47, 2006 the above-mentioned nanostructured gold films prove again the existence of the propagating plasmons. Similarly, the ratio of the average EFs exhibited by the gold films with thickness of 30 and 15 nm when an 830 nm excitation line was employed was found to be 4.5, while the inverse areas ratio value was 0.92. Thus, one can conclude that the 60 nm gold nanostructured film with the smallest metallic area exhibits the greatest local EM field confinement under NIR excitation as was supposed from reflectance spectra analysis. Nevertheless, much effort is still certainly needed for complete understanding of the optical properties of these complex nanostructured films. Even so, the presented results demonstrate that such substrates could be outstanding candidates for forthcoming investigations of biological relevant molecules by using the SERS technique. Conclusion Gold nanostructured films of various thicknesses (15, 30, and 60 nm) were deposited over regular arrays of polystyrene nanospheres in an attempt to evaluate their potential as SERSactive substrates. While the morphology of the prepared substrates was examined by using AFM, their plasmonic response was achieved by recording reflectance spectra at 50° off normal incidence. The large tunability observed from the reflectance spectra was confirmed by the SERS spectra recorded with excitation laser lines from visible (532 and 633 nm) and NIR (830 nm) spectral range. Different average enhancement factors (EF) were obtained depending on the deposited film thickness and employed laser lines, and possible explanations for this behavior were given based on the reflectance spectra analysis. The large tunability of surface plasmon excitation combined with the advantage of relatively high exhibited average EF values recommends these substrates as outstanding candidates for upcoming investigations of various biological relevant molecules. In addition, the prepared nanostructured films are stable in air and water at ambient temperature and show no loss in SERS activity over a long period of time. Acknowledgment. We thank Dr. Lucian Prejbeanu from Spintec Laboratory in Grenoble for helping us with AFM measurements. M.B., L.B., and J.P. acknowledge financial support from the Deutsche Forschungsgemeinschaft (P0563/71). Part of this work was supported by the Ministry of Education and Research and National Research Council of Romania under Contract CNCSIS No. 341/2006 and CEEX No. 71/2006. References and Notes (1) Shafer-Peltier, K. E.; Haynes, C. L.; Glucksberg, M. R.; Van Duyne, R. P. J. Am. Chem. Soc. 2003, 125, 568. (2) Kim, Y.; Johnson, R. C.; Hupp, J. T. Nano Lett. 2001, 1, 165167. (3) Haes, A. J.; Stuart, D. A.; Nie, S.; Van Duyne, R. P. J. Fluoresc. 2004, 14, 355-367. (4) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2001, 123, 1471-1482.
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