NANO LETTERS
Aggregation of Gold Nanoframes Reduces, Rather Than Enhances, SERS Efficiency Due to the Trade-Off of the Inter- and Intraparticle Plasmonic Fields
2009 Vol. 9, No. 8 3025-3031
M. A. Mahmoud and M. A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 Received May 11, 2009; Revised Manuscript Received June 17, 2009
ABSTRACT It is usually observed and understood that aggregation of silver and gold solid nanoparticles gives rise to enhanced SERS spectra due to the increased plasmon field between the particles. In the present work, we observed that the increase of aggregation of Langmuir-Blodgett assembled 80 nm gold nanoframe particles reduces the efficiency of the surface-enhanced Raman spectra of adsorbed thiophenol molecules. Using discrete dipole approximation simulation of the plasmonic fields of a pair of nanoframes as a function of their interparticle separation, it is found that at large separation the fields inside the cavities are stronger than those outside. As the interpair separation decreases, the gain in the interparticle field does not make up for the loss in the field within the cavities, supporting the observation of the decrease in the SERS intensity with aggregation.
Introduction. Since the discovery of surface-enhanced Raman spectroscopy (SERS) on roughed surfaces, this technique has been used widely in studying molecules, proteins, and membranes.1 It is now realized that the “roughed silver surfaces” that were used in the early studies of SERS were actually composed of assembled silver nanoparticles.2 There are many theoretical studies3 and models4 developed to describe surface Raman enhancement. Two mechanisms proposed by Moskovits have been used to describe the origin of SERS: electromagnetic5 and chemical6 mechanisms. In the electromagnetic mechanism, the plasmonic field is responsible for the Raman enhancement. More recently, when single molecule Raman spectroscopy was used to study SERS on a very few number of nanoparticles, enhancement factors of the order of 1014-1015 were reported.7 Various sizes and shapes of nanoparticles have been used to enhance SERS, with a general preference toward gold spheres8 and rods.9 However, silver nanoparticles and aggregates10 give better signal enhancement due to the stronger plasmon field that results from the overlapping fields of the individual particles and the long dephasing time (and thus the sharp plasmon band) of the surface plasmon absorption of the Ag nanoparticle. This results from the fact that the silver surface plasmon resonance is at a wavelength away from the interband absorption.11,12 The silver cubes give * Corresponding author,
[email protected]. 10.1021/nl901501x CCC: $40.75 Published on Web 07/08/2009
2009 American Chemical Society
strong SERS for 1,4-benzenedithiol,13,14 while Pd triangular nanoplates, hexagonal nanoplates, and cuboctahedra were used to enhance the SERS of 4-mercaptopyridine.15 For these experiments, the films of gold nanoparticles used in the SERS experiments were assembled via a template using colloidal crystals.16 Many researchers have reported that SERS enhancement requires aggregation of the nanoparticles,17 while others showed that there could be active hot spots resulting in single particle SERS enhancement.18 The hot spots were later shown by Brus et al.19 to likely occur at interstitial spots between two nanoparticles where the fields are most intense. This explained the importance of aggregation to SERS. The assembly of plasmonic nanoparticles (gold and silver) leads to enhancement of the plasmonic field between the nanoparticles.20 This leads to red shifts in the wavelength of the surface plasmon resonance extinction, an increase in the intensity of the surface enhanced plasmon resonance extinction, and an increase in the intensity of the scattered light. The Langmuir-Blodgett (LB) technique employed in this study is one of a number of methods used for the assembly of plasmonic solid silver nanocubes.21-23 It is clear that as the LB surface pressure used for assembly of these nanoparticles increases, the SERS intensities also increase due to the increase in aggregation. In the present study, we examine the effect of this type of assembly on the wavelength of the surface plasmon absorption of gold nanoframes as well
as the intensity of the surface-enhanced Raman spectra of thiophenol adsorbed on these nanoparticles. These nanoframes are empty cubes with no faces; only the frames are present. The thiophenol molecules are expected to be adsorbed on the inside and outside of the surfaces of the frame. Upon aggregation, the surface fields on each side are expected to change with different dependence on the interparticle separation. The SERS intensity of the adsorbed molecules can thereby increase or decrease upon aggregation depending on the changes in the inside and outside surface Plasmon fields. In this Letter, we assembled gold nanoframes (AuNFs) into LB monolayers on the surface of a quartz substrate at different interstitial particle distances. The SERS of thiophenol adsorbed on the inner and outer surfaces of the AuNFs were measured as a function of the number of nanoparticles present in the cross sectional area of the exciting laser light used for different LB monolayer particle densities. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to image the topography and count the number of particles under study. We found that aggregation of the nanoframes decreases, rather than increases, the SERS intensity. The discrete dipole approximation (DDA) was utilized to estimate the electromagnetic field enhancement that is responsible for the SERS as a function of the interparticle separation of a pairs of AuNFs used. The theoretical DDA simulations suggest that as the interparticle distance between the pair decreases, the electromagnetic plasmonic surface field enhancement of the internal surfaces of the frame decreases more than the increase in the field enhancement on the outside surfaces of the frame. These calculations support the experimental observation that aggregation decreases, not increases, the SERS intensity when nanoframe nanoparticles with dimensions of 80 nm edge length and 16 nm axis diameter are used. Experimental Section. Gold nanoframes were prepared from silver nanocubes (AgNCs) by galvanic replacement.14,21 AgNCs were prepared by heating 30 mL of ethylene glycol (EG) at 150 °C for 1 h, followed by the addition of a solution of poly(vinylpyrolidone) (PVP) with molecular weight of ∼55000 g (0.2 g dissolved in 10 mL of EG). A 0.4 mL sodium sulfide (3 mM) solution in EG was then added to the flask followed by 3.2 mL of 282 mM silver nitrate, dissolved in EG. The silver ion was completely reduced after 15 min, producing AgNCs. To purify the AgNCs, 10 mL of the AgNC solution was diluted with water and acetone and centrifuged (13000 rpm) for 5 min. The particle precipitate was then redispersed in 50 mL of deionized water. The solution of purified AgNCs was boiled for 2 min and a 10 mg/L hydrogen tetrachloroaurate solution was injected slowly. The mixture was continuously refluxed until its color stabilized. The wall thickness of AuNFs can be decreased and the plasmon peak position red-shifted by increasing the amount of gold salt added.14,21 The optical spectrum was measured after each addition until the plasmon peak shifted to approximately 1030 nm. Vigorous stirring was maintained throughout the synthesis. The particles were removed from this medium by centrifugation, and the pure particles were redispersed in chloroform for monolayer preparation. 3026
The Langmuir-Blodgett monolayer was prepared by a Nima 611D trough filled with ultrapure water. The surface pressure was measured with a Wilhelmy plate attached to a D1L-75 model pressure sensor. Nanoparticles in chloroform (1 mL) were sprayed on the water surface, and after 20 min, the surface pressure-surface area curve (isotherm) was plotted by decreasing the area at which the particles were dispersed mechanically and measuring the surface pressure at each surface area. The LB film was transferred to the quartz and silicon substrates (cleaned with piranha solution 30% H2O2 + 70% H2SO4 volume ratio) by the vertical dipping method, with the pressure kept constant during the deposition. Thiophenol (Sigma-Aldrich) was placed in a small receptacle affixed in the center of a Petri dish, and the quartz substrates covered with AuNF monolayer were supported around the periphery of the dish. This was then tightly covered. After 24 h, the thiophenol vapor was adsorbed on the surface of AuNFs. A Holoprobe Raman microscope (Kaiser Optical Systems) with 785 nm laser excitation was used for surface Raman measurements. For the UV-vis absorption measurements, a Carry UV-vis-NIR (Cary 500, Version 8.01) was used. A Zeiss Ultra60 was used for SEM image collection, and the Picoscan 5 Molecular Imaging instrument was used for AFM. Imaging of the Langmuir-Blodgett Monolayer Film. The two-dimensional (2D) assembly has many possible applications in both optics and in biology.24,25 The LB 2D ordered monolayer has advantages over the random self-assembled monolayer in many ways, as the optical and other physical properties depend greatly on the distance between the nanoparticles and the arrangement of the particles in space.21 The LB technique facilitates particle ordering and allows for the control of the interparticle distance.26 Here, several techniques were employed to image the topography of the monolayer (SEM, STEM, and AFM). Figure 1 shows the SEM of the AuNFs assembled on the surface of quartz substrate at different surface pressures (22, 16, and 0 mN/ m). For the sample collected at a surface pressure of 22 mN/m there is a large number of aggregates, and most of the particles stick together due to large surface particle density. The large number of aggregates observed in SEM agrees well with the optical measurement in Figure 3A, which shows a broad band that appears at long wavelengths. Upon increase of the surface pressure, the density of both the single particles and aggregates increases, but the density of the aggregates increases faster due to the shift of the equilibrium in favor of the aggregate. SEM produces good 2D images of the surface topography. However, AFM gives more details about particle arrangement of the monolayer by producing 3D images. Figure 2 shows the AFM images of AuNFs assembled at surface pressures of 2 and 22 mN/m. A large degree of aggregating is observed in the image of the monolayer assembled at 22 mN/m, in contrast to the monolayer assembled at 2 mN/m, which displays much less aggregation. The blur in the AFM images implies particle movement on the surface of the substrate. This could be due to the lack of capping material, which is Nano Lett., Vol. 9, No. 8, 2009
Figure 1. SEM images of gold nanoframes adsorbed on the surface of quartz substrate assembled at surface pressures of 0, 16, and 22 mN/m (A-C), respectively. (The scale bar is 500 nm.)
Figure 2. AFM of AuNF LB monolayer adsorbed on the surface of a quartz substrate, assembled at surface pressures of 22 mN/m (left) and 2 mN/m (right).
generally responsible for the adsorption of the particle to substrate surface. The Spectroscopy. Figure 3A shows the observed extinction spectra of the assembled nanoframes at different surface pressures. The sharp peaks on the short wavelength side correspond to the surface plasmon resonance (SPR) of the AuNF. At longer wavelength are the broad bands whose relative intensity to that of the SPR increases as the surface pressure increases. This suggests that these bands are the aggregation extinction of AuNF nanoparticles. It is interesting that the aggregation band is observed even at 0 mN/m pressure, where the number of dimers is very small. This is undoubtedly due to the enhanced surface field of the AuNF of the aggregates. The SERS of the AuNFs at different assembled pressures are shown in Figure 3B. As for the extinction spectra, the intensity increases as the surface pressure increases. This is due to the increase in the total number of particles present in the laser beam. In order to examine whether, in addition to this effect, aggregation gives rise to an increase in the Raman cross section, a plot of the Raman intensity versus the total number of particles in the laser beam is plotted in Figure 4B. The average interparticle separation was calculated from the number of particles in the area exposed to the Raman laser spot and the edge length of the particle (80 nm). The concave shape of the plot shows that as the fraction of AuNFs increases with increasing surface pressure, the slope decreases. The optical spectra show that increasing the surface pressure increases the aggregation peak intensity. These two results taken together suggest that aggregation of the AuNFs decreases the observed SERS intensity. Nano Lett., Vol. 9, No. 8, 2009
For comparison, we present the SERS results of adsorbed PVP on solid silver nanocubes in Figure 4A, for which the excitation was carried out within the aggregate band. It is clear that as the number of particles increases (and the percent of aggregates increases), the SERS intensity continues to increase nonlinearly. This suggests that the increase in the Raman intensity is not simply due to an increase in the percentage of the aggregates in the laser beam but rather to an increase in the SERS intensity in the aggregate. The relationship between the concentration and the SERS intensity is nonlinear for both the solid Ag nanocubes and the AuNFs. For the solid Ag nanocubes, the curve is concave; i.e., the tangential slopes increase with the percentage of the aggregates. The shape of the curve representing the AuNFs is convex, i.e., the tangential slopes decrease as the percentage of the aggregates increases. This strongly suggests that aggregation of the AuNF particles decreases the SERS intensity. This could be the result of one of two possibilities. It could be due to an actual decrease in the plasmonic field of the aggregated nanoparticles compared to that of the sum of the fields of the same number of the individual particles. However, it could also be due to the red shift of the surface resonance of the aggregates away from the enhancement wavelength region (the region between the wavelength of the laser used and that of the Raman band of the largest stock shifted vibration examined, 1570 cm-1). An examination of the optical spectra in Figure 3A shows that there is an apparent red shift of the broad aggregation band as the surface pressure of the LB increases. In order to examine whether or not this shift in the aggregation band is the cause of the decrease in the slope observed in Figure 4B as the particle density increases, a deconvolution of the optical spectrum observed at different surface pressures into two bands is carried out, a narrow short wavelength monomer band and a broad long wavelength aggregate band (Figure 5). The number of both the free and aggregated AuNF particles in the laser beam increases with increasing the surface pressure, as seen in Figure 5. The question that needs to be answered is what happens to the intensity ratio of these two bands in the plasmon enhancement region as the surface pressure increases. This is shown in Figure 5. The ratio (R) of the aggregation spectral area to that of the spectrum corresponding to the free Au nanoframe in the area in the enhancement region is also given in Figure 5 for each pressure used. From Figure 5 it is clear that the ratio increases slowly first, then increases suddenly, levels off, and then 3027
Figure 3. (A) The surface plasmon resonance extinction spectrum of gold nanoframe assembled on a surface of quartz substrate at surface pressures of 22, 20, 16, 8, 2, and 0 mN/m of a Langmuir Blodget. (B) The Raman spectra of thiophenol in the liquid form (bottom spectrum) and adsorbed on the surface of LB gold nanoframe monolayer film assembled at surface pressures of 22, 20, 16, 8, 2, and 0 mN/m.
Figure 4. (A) The relationships between the Raman band intensity (at maximum) of PVP and the number of silver nanocubes. (B) The relationships between the logarithm of Raman band intensity of thiophenol (at maximum) and the number of AuNF particles (bottom axis) and the average interparticle separation (top axis).
increases again. This trend follows the phase changes from gas phase to the liquid and then to the solid phase in the surface pressure-surface area isotherm. Thus, upon aggregation, the red shift of the band maximum of the aggregate band does not cause a decrease in the relative intensity in the enhancement region. This is due to the fact that the band becomes broader as the surface pressure increases. The observed decrease in the slope must then be due to a decrease in the effective Raman surface field enhancement by the AuNF upon aggregation. There is no doubt that upon aggregation the outside field between neighboring nanoframes increases. Thus the surface field enhancement of the 3028
thiophenol molecules adsorbed on the inside surfaces must then decrease to a greater extent than the increase in the enhancement of the molecules adsorbed on the outside surfaces of the frames. DDA calculations below support this experimental conclusion. DDA Simulation of the Surface Field Enhancements for Gold Nanoframe Pairs at Different Separations. It is wellknown that the surface plasmon field that results from interaction between a pair of solid nanoparticles is much stronger than the surface field on a single particle.27-30 According to the electromagnetic theory of SERS, the plasmon field is responsible for the observed enhancement Nano Lett., Vol. 9, No. 8, 2009
Figure 5. The deconvolution of the extinction spectra of the assembled gold nanoframes at different surface pressures by the LB technique (A-F assembled at 22, 20, 16, 8, 2, and 0 mN/m, respectively) into monomeric (black narrow short wavelength spectra) and aggregation (red broad spectra) nanoparticles. The ratio (R) of the aggregated integrated spectral area to that of the monomeric area in the plasmonic Raman enhanced region (between the red and blue vertical lines) is given for each surface pressure.
of the Raman signal.31-33 It follows from this that nanoparticle aggregation is responsible for Raman enhancement on these types of particles.28,34,35 However, as observed in the previous section, the SERS band intensity of thiophenol adsorbed on the AuNF monolayer decreases as the number of surface pressure increases. The experimental results suggest that aggregation of the AuNFs reduces the observed SERS intensity. DDA calculation of the field of a pair of nanoframes as a function of the interparticle distance has been carried out. Nano Lett., Vol. 9, No. 8, 2009
The SPR Spectrum of Dimer Pairs of Nanoframes as a Function of Their Separation. The AuNF used is an empty structure, a frame with dimensions of 16 × 16 × 80 nm. DDA calculation gives the maximum plasmon peak position of the single particle at 860 nm. This peak position is redshifted for the dimer as the gap separation decreases. Figure 6 shows the maximum plasmon peak position is at 1014, 970, 920, 891, 878, 871, 865,and 862 nm for dimer gap separations of 8, 16, 40, 80, 120, 160, 200, and 240 nm, respectively. The calculated wavelengths can be off by as 3029
Figure 6. Simulated DDA calculation of the surface plasmon resonance of a pair of gold nanoframes at different distances showing that most of the curves are within the surface plasmon wavelength enhancement region (that the laser wavelength (780 nm) and that of the vibronic Raman band of largest frequency (∼900 nm). A correction of 50 nm is added, which is the error in these calculations. This shows that most of the dimer bands will have their SPR band within the Raman enhancement region.
many as 50 nm due to the fact that the calculation is very sensitive to the nanoframe thickness which is very hard to fix carefully in the experiment. Thus by applying a correction of 50 nm to each of the peaks above changes these numbers
to 964, 920, 870, 841, 828, 821, 815, 812, and 810 nm, respectively. This would make the wavelength of the SPR 920 and 964 nm for the pair at the smallest separation distance. The enhancement region in our experiment for the Raman bands is between 785 and 893 nm. Thus, according to these calculations, all pairs separated by all the distances used here, except the last one, can enhance the entire vibronic bands. Experimentally, it is concluded that all the aggregates are found to have more optical extinction intensity in the enhancement region than do the monomers. Thus it was concluded that aggregation reduces (not increases) the efficiency of the plasmon field Raman enhancement of molecules adsorbed on these surfaces. Below is the DDA simulation of the surface field of the monomer compared to those on the frames of dimers. DDA Simulation of the Surface field Enhancement of Gold Nanoframe Monomer Ws Dimer. For the DDA calculation for the surface field enhancement, we assume that the light is propagating along the z-axis with the polarization direction along the x-axis. Figure 7 shows that across any empty cube along the x-axis, the AuNF has four intense electric field enhancement regions: two inside and two outside the cavity. It is found that for the single nanoparticle, the field inside the particle is twice as strong as the field outside (Figure 7A). Also, the value of the field enhancement inside and outside is equal on each of the empty frame faces. The field intensity around and inside each empty cube changes in the AuNF dimer depending on the interparticle separation. As the gap increases, the field enhancement
Figure 7. DDA calculations for the electromagnetic field enhancement (contours of E) of light field polarized along the x-axis and propagating along the z-axis for a single gold nanoframe (A), and for a pair of gold nanoframes at different separations: 8 nm (B), 120 nm (C), and 240 nm (D). 3030
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increases inside the frame of each cube at the expense of the field outside. Moreover, the field inside the frame but close to the gap region between the particles is different in value from the field far away from the gap region. For the dimer at the smallest gap separation, the field outside close to the gap region was found to be higher than that for the far away one. In all cases, the enhancement outside the frame is less than the enhancement in the gap region. The field enhancement inside the frame and opposite to the gap region increases as the separation gap increases. The Raman enhancement factor is proportional to the square of the value of the field enhancement factor.42 According to our calculations; the single particles and the dimers with large separations are more efficient in enhancing the Raman spectra of the same number of observed molecules than dimers with small gap separation. Due to this fact, most of the Raman spectrum intensity observed is probably of molecules adsorbed on the inside of the nanoframe cavity. It is also clear that nanoframe aggregation leads to a decrease, rather than the expected increase, in the SERS intensity. If hot spots are responsible for SERS enhancement and these hot spots occur in the region of the highest field, it is possible that the hot spots could be observed in the interior of the nanoframes. Experimental studies of the hot spots in single nanoframe might be most interesting. Acknowledgment. The authors thank DMR-NSF for support under Grant No 0527297. The authors also thank K. B. T. Draine and P. J. Flatau for the use of their DDA code, DDSCAT 6.1, Steven Hayden for proof reading of the manuscript, and Erik Dreaden for helpful input. The DDA simulations were completed using the computing facilities at the Center for Computational Molecular Science and Technology (CCMST), Georgia Tech. References (1) Qian, X. M.; Nie, S. M. Chem. Soc. ReV. 2008, 37, 912–920. (2) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163–166. (3) Schatz, G. C. Acc. Chem. Res. 1984, 17, 370–376. (4) Wang, D. S.; Kerker, M. Phys. ReV. B 1981, 24, 1777–1790. (5) Moskovits, M. J. Chem. Phys. 1978, 69, 4159–4161. (6) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783–826. (7) Doering, W. E.; Nie, S. M. J. Phys. Chem. B 2002, 106, 311–317.
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