Synergistic Effects between Gold Nanoparticles and Nanostructured

Sep 10, 2015 - Email a Colleague · Order Reprints · Rights & Permissions · Citation Alerts. Add to ACS ChemWorx. SciFinder Subscribers Sign in · Retri...
4 downloads 10 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCC

Synergistic Effects between Gold Nanoparticles and Nanostructured Platinum Film in Surface-Enhanced Raman Spectroscopy Da-Young Hong,† Seong Kyu Kim,† and Young-Uk Kwon*,†,‡ †

Department of Chemistry and ‡SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon, 440-746 Korea S Supporting Information *

ABSTRACT: We studied the effects of reflective supports on the surface-enhanced Raman scattering (SERS) by Au nanoparticles (NPs) placed on top of them. When Au NPs are placed on a flat Au or Pt thin film, the SERS intensity of 4-aminobenzenethiol is doubled from that of Au NPs on a Si wafer. This phenomenon can be explained, through finite difference time domain (FDTD) simulations, with a strongly enhanced electromagnetic (EM) field at the gap space between the Au NP and the metallic surface originating from the coupling of the localized surface plasmon of Au NPs with the reflected beam from the surface. When a nanostructured Pt thin film is used as the support, the SERS intensity is further enhanced. Interestingly, the SERS intensity in this case is higher than the sum of the intensities on Au NPs and the nanostructured Pt thin film, suggesting a synergistic effect between them. However, such enhancement cannot be explained by EM field strengths obtained by simulations. In this case, the explanation needs considerations on the distribution of analyte molecules on the available surfaces of SERS active regions.

1. INTRODUCTION

In the prevailing platform of SERS measurements, NPs (or NRs) of coinage metal elements are spread on a support such as a silicon wafer, and then, the analyte molecules are placed on it. The spread NPs are most likely to form a submonolayer with local aggregations. In such an arrangement, the absorption of incident photons for SERS excitation cannot be high, which may be one of the reasons of lowering the SERS efficiency. On the other hand, if the NPs are stacked to increase the absorption, the SERS efficiency per NP is bound to be decreased. These problems have been the subject of several precedent papers.22−24 Notably, Oh et al. showed that SERS signal increases proportionally when Au NPs of 21.5 nm in diameter are stacked until four layers and then is saturated.22 In this regard, we considered that a substrate with high reflectivity would be able to increase the amount of photon absorption per NP, providing an additional means to increase the SERS efficiency. Further, we considered that, if the substrate had a nanostructure so that it could induce localized field around it, there may be a coupling effect between the NPs and the substrate in addition to the effect of reflection. To test these ideas, in this study, we used Pt thin films with and without a nanostructure as the substrate and located Au NPs on the Pt thin films for SERS measurements. Although Pt is not the highest reflecting material in the UV−vis region,25 its mechanical and chemical stability permits stable nanostructures.26,27 Therefore, for the purpose of this present study, comparing the SERS efficiency per Au NP depending on the

Surface-enhanced Raman scattering (SERS), the phenomenon associated with the sharp increase of intensity of Raman spectrum when analyte molecules are adsorbed on roughened metal surface, has been a subject of intensive and extensive studies over the past three decades.1,2 One of the SERS mechanisms responsible for the electromagnetic (EM) field enhancement is localized surface plasmon resonance (LSPR), which is a result of the collective oscillation of conduction electrons in a metallic nanoparticle excited by the incident light.3−5 The local electromagnetic field experienced by analyte molecules on the metal surface is dramatically enhanced, producing strongly enhanced Raman intensity.6,7 Coinage metals such as Au and Ag are the prime materials used as SERS substrates because of their ability to produce LSPR by visible lights.8−10 One of the most active branches of research on SERS is the design of nanomaterials of Au or Ag in order to develop highly efficient SERS substrates.11,12 Nanomaterials with various shapes,13−15 sizes,15,16 and composition17,18 have been explored. In addition to these attributes, the gap space between nanoscopic entities (viz., nanoparticles (NPs) and nanorods (NRs)) is found to be very important for SERS because the gap is the locus of strong EM field enhancement through coupling of plasmons of involved NPs or NRs. Various efforts have been put forth to optimize this “hot spot” or “hot junction” effect.19−21 Hot spots can be created by aggregating NPs or NRs, and therefore, forming one or more close-packed layers of NPs or NRs has been demonstrated to be an efficient way to create a high density of hot spots with some control over the gap size.22 © XXXX American Chemical Society

Received: June 1, 2015 Revised: September 2, 2015

A

DOI: 10.1021/acs.jpcc.5b05204 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

with a MEEP program (version 0.20.3).34 The FDTD is a classical method for understanding light−matter interaction in a metallic nanostructure. This method, based on the time-domain solution of curl-Maxwell’s equation, can reflect the behavior of EM field as time passes.35,36 Through the employment of the dispersion equation of optical constants based on an extended Drude model,37 the dispersion function of metal in modeling can be accurately described.22 For modeling SERS substrates experimented in this work, a nanostructure composed of a Au NP and Pt NRs was modeled as a Au NP (30 nm) laid on a hexagonal array of cylinders with a diameter of 8 nm, a height of 60 nm, and 3 nm gap between the cylinders. The gap between Au NP and Pt NRs was set to 0 nm. We put the plasma frequency of metals in the Drude model, and the dielectric constants of Au and Pt were set as value of 5.967 and 5, respectively. The dielectric constant of medium (air) was also set as 1. The propagation direction of plane waves was chosen to be along the z-axis, parallel to the longitudinal axis of the Pt cylinder. To apply boundary conditions, the perfectly matched layers (PMLs) method34,38 was used. Because PMLs were placed at both ends for ∼200 nm thick, some artifacts produced by computational simulations in FDTD can be reduced.38 Under the same conditions of the experiment, continuous wave light source at a fixed wavelength of 532 nm was operated. More details of the calculation setup are found in ref 22.

properties of the substrate, Pt appears to be a suitable choice. To gain deeper understanding on the observed results, we also have conducted FDTD (finite difference time domain) calculations on several models to represent the experimental systems.

2. EXPERIMENTAL SECTION 2.1. Syntheses of Materials. Nanostructured Pt thin films were prepared by using a mesoporous silica thin film (MSTF) with regularly ordered 9 nm sized vertical pores. The MSTF was synthesized on a Pt-coated Si wafer by using the selfassembly between a silica precursor (tetraethoxyorthosilicate) and a nonionic block copolymer surfactant F-127 ((EO)106(PO)70(EO)106; EO = ethylene oxide, PO = propylene oxide), followed by calcination. Pt was deposited into the pore of the MSTF by electrochemical deposition by using the MSTF-coated Pt/Si wafer as the working electrode at −0.02 V (vs Ag/AgCl (3 M KCl)) in a 10 mM K2PtCl6/0.5 M H2SO4 solution. The amount of charge passed through during the deposition was controlled to be 30 mC/cm2. Afterward, the MSTF was removed to reveal the nanostructured Pt. A scheme of synthesis and a scanning electron microscopy (SEM, JEOL JSM-7100) image of a nanostructured Pt thin film are in Figure S1. More details of the synthesis of MSTF and the nanostructured Pt thin film can be found in our previous papers.28−30 Colloid solutions of Au NPs with different sizes were synthesized by a seed-mediated growth method.31−33 To prepare 13 nm Au NPs, 195 mL of triply deionized water and 5 mL of 20 mM HAuCl4 were mixed, stirred, and then heated until about 250 °C. After that, 10 mL of 38.8 mM sodium citrate was added to the boiling solution and was stirred for 20 min. The Au NPs so obtained were used as the seed to synthesize Au NPs with larger size. To synthesize 50 nm Au NPs colloid, 167 mL of triply deionized water and 3 mL of 13 nm seed solution were mixed with 4 mL solution of 20 mM HAuCl4, 400 μL of 10 mM AgNO3, and 30 mL of 5.3 mM ascorbic acid. The mixture was stirred for 1 h. Au NPs with different sizes were also synthesized in the range of 30−80 nm by using different amounts of reagents. Details of the Au NPs including the SEM images and particle size distribution are in Figure S2and Table 1S. 2.2. SERS Measurements of Au NPs/Metallic Substrate. Raman spectra were recorded on a micro-Raman (Renishaw) spectrometer. The excitation source was a diode pumped solid state (DPSS) laser operating at a wavelength of 532 nm, focused to a spot size of ∼5 μm2. A 50× objective lens was used. Raman spectra were acquired on different places of the substrates by illuminating the Pt film perpendicularly with an excitation laser beam. SERS substrates were prepared by placing Au NPs on different supports. As for the support, Si wafer, Au film (grown on a Si wafer), Pt film (grown on a Si wafer), and nanostructured Pt film mentioned earlier, all in the dimension of 0.5 × 0.5 cm2, were used. Twenty microliters of a 0.421 mM Au NPs solution was dropped on a support and was dried. Ten microliters of an ethanol solution of 4-aminobenzenethiol (4ABT) of a designed concentration (10−8∼10−3 M) was dropped on the substrate. For comparison, the Raman spectrum of an ethanol solution of 1 M 4-ABT without a substrate was also recorded. 2.3. Simulations of Electromagnetic Fields. The finite difference time domain (FDTD) calculations were carried out

3. RESULTS AND DISCUSSION In this study, we investigated the SERS effects of composite substrates. The composite substrates are formed by spreading Au NPs on various metallic thin films. We used Au and Pt thin films for this purpose. Especially, we used a nanostructured Pt thin film with the feature size less than 10 nm in addition to a sputter-grown flat Pt thin film (Figure S1). The composite substrates investigated in this study are summarized in Table 1. Table 1. Types of SERS Substrates and Their Names constituents

EFa (×106)

Au NPs spread on Si wafer

7.5

Au/ Au NPs spread on PtPt_film coated Si wafer Au/ Au NPs spread on AuAu_film coated Si wafer Pt NRs nanostructured Pt thin film

9.6

name Au/Si

Au/ Pt_NR a

Au NPs spread on nanostructured Pt thin film

remarks for inherent SERS of Au NPs

8.1 2.1

see text for the synthesis and characteristics

23

EF (enhancement factor) defined as in eq 1 of text.

To systematically investigate, we synthesized ∼50 nm sized Au NPs and used them for all of the composite substrates in Table 1. The reason for us to use this size of Au NPs is based on the literature data which indicate that ∼50 nm is the optimal size to produce the strongest SERS signal under the same concentration of Au NP solution.39 Our own experimental data on the effect of the size of Au NPs also conforms to the literature trend (Figure S3). However, determining the optimum size is debatable. For instance, Krug et al. reported the optimal size of Au NPs to 60 nm when a 647 nm laser source is used.40 The reason for the existence of the optimum B

DOI: 10.1021/acs.jpcc.5b05204 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Au/Au_film and Au/Pt_film show higher EFs than Au NPs alone (Au/Si). Second, the Au/Pt_NR substrate shows a higher EF than Au/Au_film or Au/Pt_film substrate. Third, the EF from the Au/Pt_NR substrate is larger than the sum of the EFs of the two components, Au NPs (Au/Si) and Pt NRs. In other words, the Au NPs and Pt NRs substrates show a synergistic effect when used in combination. These observations will be discussed separately below. 3.1. Au/Au_Film and Au/Pt_Film Substrates. In most of the literature studies on SERS, Si wafer is a standard material to disperse and support SERS-active NPs or nanomaterials. This is because Si has the plasmon frequency above 1107 nm,43 by which Si can be considered not to generate an EM field by the incident photons of Raman excitations and, thus, does not influence the EM field generated by the NPs on top of it. In other words, Si has been used because it is considered to be neutral to the SERS effect. Therefore, the comparison between the SERS on Au/Si and Au/Au_film or Au/Pt_film can show the composite effects in the latter two cases. As seen in Figure 1, the Au/Au_film and Au/Pt_film composite substrates show stronger SERS than Au NPs alone (Au/Si). We interpret the additional enhancement by the presence of the metallic supports as arising from the reflection of incident photons. From a geometrical point of view taking into account the size and quantity of Au NPs, the Au NPs form a submonolayer on the Si wafer or the Au or Pt film. Therefore, a large fraction of the incident photons is wasted in case of Au/ Si substrate. Even if the packing of Au NPs becomes denser to form a monolayer, because a single NP cannot absorb all of the incident photons on it, there must be some wasted photons. By using a reflective support underneath the Au NPs, some of the unabsorbed photons can be scattered back to the Au NPs, increasing the chance of photons to be absorbed and utilized in the excitation of plasmon resonance. To see this effect more clearly, we have performed FDTD simulations on many models for the Au/Pt_film case. Figure 2 shows the plots of cross-section-averaged EM field, ⟨E2⟩, along the perpendicular direction of the substrate in three different cases of a Au NP (30 nm in diameter), a Pt film (60 nm in thickness), and a composite of the Au NP on the Pt film. The beam propagates from the positive side to the negative side of the horizontal z-axis in the plots. In the case of the composite substrate (Figure 2c), the Pt wafer is located at z = −60−0 nm region and the Au NP is located at z = 0−30 nm region (center at z = +15 nm). These locations are used in the plots for the Au NP and Pt film cases (in Figure 2b and c, respectively) for easy comparison. The plots clearly show that the EM field around

size is because there are many different factors that can influence the SERS intensity, and the size of Au NPs affects these factors differently. For instance, as the particle size increases, the plasmon resonance intensity increases while the available surface area per Au atom decreases. In addition, photon absorption and reflection also vary differently depending on the particle size. The SERS of 4-ABT (10−5 M) on various substrates is shown in Figure 1. For direct comparison, the thin films in the

Figure 1. Raman spectra of 4-ABT on various substrates: (a) 1 M 4ABT and (b) Pt_NR, (c) Au/Si, (d) Au/Au_film, (e) Au/Pt_film, and (f) Au/Pt_NR substrates. For measurements in b−f, 10−5 M 4-ABT solution was used.

composite substrates were cut into the same size of 0.5 × 0.5 cm2. Assuming even coating of the analyte molecules, the areal density of the analyte molecules must be the same in all of the cases. The Raman spectra in all cases in this figure can be well explained with the peaks assigned to the δ(NH) (1656 cm−1), ν(CC) (1586 cm−1, 1455 cm−1), ν(CC) + δ(CH) (1455 cm−1, 1390 cm−1), and δ(CH) (1186 cm−1, 1145 cm−1) modes of a 4ABT molecule.41,42 The SERS efficiency of a substrate is often quantified by calculating the enhancement factor (EF),33 defined as in eq 1 EF = ISERS·C NR /INR ·CSERS

(1)

where ISERS/INR and CSERS/CNR are the intensities and concentrations of 4-ABT with/without a SERS substrate. The EFs of the substrates are about 106 in agreement with those in the literature. However, the EFs vary by a factor of up to 10 depending on the type of substrate (Table 1). Comparisons among different substrates for the EFs reveal some interesting observations: First, the composite substrates

Figure 2. Plot of cross-section-averaged ⟨E2⟩ along the direction of incident beam (z-axis): (a) a Pt film, (b) a Au NP, and (c) a Au NP on Pt film. In b and c, the dotted lines are for the case of two Au NPs with a 1 nm gap between them, which are scaled by 50% to compare the ⟨E2⟩ per Au NP. Au NP of 30 nm in diameter and Pt film of 60 nm in thickness were used in the models. The locations of the NP and film in a and b are the same as in c where the contact point between the NP and the film is placed at z = 0 nm level. C

DOI: 10.1021/acs.jpcc.5b05204 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 3. Plots of ⟨E2⟩ of a Au NP on Pt film at (a) the contact point (z = 0 nm) and (b) the center of the Au NP (z = +15 nm) and (c) of two Au NPs on Pt film at the center of the Au NPs (z = +15 nm). The color codes are in relative scale for better visibility. The maximum ⟨E2⟩ values are (a) 73.5, (b) 10.8, and (c) 232.6.

Figure 4. Plot of cross-section-averaged ⟨E2⟩ along the direction of incident beam (z-axis): (a) Pt NRs, (b) a Au NP, and (c) a Au NP on Pt NRs. A Au NP of 30 nm in diameter and an array of Pt NR layer of 60 nm in thickness were used in the models. The Pt NR layer was modeled as a hexagonal array of Pt NRs with 8 nm in diameter each and a separation from the nearest NRs by 3 nm. The locations of the NP and NRs in a and b are the same as in c where the contact point between the NP and the NRs is placed at z = 0 nm level.

the Au NP is significantly enhanced by the presence of the Pt film. On the other hand, there is no EM field developed inside the Pt film because Pt reflects light almost completely above 500 nm.25 More importantly, a spike is developed in the plot of the composite substrate (Figure 2c). The location of this spike corresponds to the gap space around the contact point between the Au NP and the Pt film. This gap space is accessible for the analyte molecules and thereby can be viewed as a hot spot. We also include plots for the case of two Au NPs without and with a Pt film (dotted lines in Figure 2b and c, respectively). The two Au NPs are placed to have a gap of 1 nm between them in order to simulate the situation of a hot spot. To directly compare with the single Au NP case, these plots are scaled down by 50%. Therefore, the difference between the dotted line and the solid line in these plots can be directly attributed to the interaction between the two Au NPs. For instance, the EM field strength of the hot spot generated by the interaction between two Au NPs can be appreciated by comparing the two lines in Figure 2b. The presence of a reflective support under Au NPs produces two effects: First, the EM field around each Au NP is strengthened. In case a hot spot is present by having two Au NPs in close vicinity, the EM field strength between them is further strengthened by the action of the reflecting plane. If the analyte molecules have strong affinity to Au so that all of them are on the surface of the Au NPs, this effect alone can increase the SERS intensity. Second, the hot spot generated around the contact point between the Au NP and the Pt film provides an additional mechanism to enhance SERS. This latter effect may be useful when the analyte does not have strong affinity to Au. In most of the cases of SERS

sampling, a solution of analyte is dropped on the substrate and is dried before the Raman measurement. During the drying, it is highly likely that capillary force exerted on the solution accumulates the analyte molecules in the gaps (hot spots), increasing their chances to show SERS effect. These two effects explain the higher EF of Au/Pt_film than that of Au/Si (Au NP alone). The 2D plots of the EM fields at different z-levels are shown in Figure 3. The plot at z = 0 nm level, the level of the contact point, shows that the hot spot spans rather a large area around the contact point. These plots allow comparison of ⟨E2⟩ at specific locations. For instance, the maximum ⟨E2⟩ around a single Au NP is 1.34 at z = +15 nm level. It increases to 95.9 between two Au NPs, which further increases to 232.6 by including a Pt film. The maximum ⟨E2⟩ value at z = 0 nm level is 73.50 and 133.9 for the cases of one Au NP and the two Au NPs, respectively. Because bulk Au is more efficient in reflecting visible light than Pt, the same simulation using a Au film in the place of the Pt film will produce even stronger EM field in the gap space. However, because the enhancement mechanism mainly relies on reflection from the support surface, the different reflection properties between Au and Pt cannot produce any strong effect. 3.2. Pt_NR and Au/Pt_NR Substrates. The synthesis of Pt NR and its SERS effect have been reported in our previous papers.28−30 In short, the morphology of Pt NR substrate can be described as constituted by arrays of Pt NRs of 9 nm in diameter. The length of a Pt NR can be tunable, and the Pt NR substrate in this study has ∼200 nm long Pt NRs. The NRs are separated from each other by an idealized gap of 3 nm. However, the mechanical stability is not high; tilting and D

DOI: 10.1021/acs.jpcc.5b05204 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 5. Plots of ⟨E2⟩ of a Au NP on Pt NRs at (a) the contact point (z = 0 nm) and (b) the center of the Au NP (z = +15 nm) and (c) inside the Pt NRs (z = −15 nm). The color codes are in relative scale for better visibility. The maximum ⟨E2⟩ values are (a) 23.0, (b) 8.5, and (c) 10.6.

size. Not only the EM field strength but also the distribution of the analyte molecules must be taken into consideration. In our case, simple geometric calculations on the molecular dimension and the size of the 50 nm Au NPs show that, in any one of the SERS spectra in Figure 1, the total surface area of the Au NPs is only 20% of that of the analyte molecules (assuming that the ring of the 4-ABT molecule is in contact with the Au NP surface). In other words, in the samples of the spectra in Figure 1, there is an excess of analyte molecules to the available surfaces of Au NPs. Therefore, it is highly likely that the excess analyte molecules do not have chances to contribute to the SERS. This will be clearly so in the case of Au/Si where Si is SERS inactive. Even in the cases of Au/Au_film and Au/ Pt_film, because the metallic films have smooth surfaces, the analyte molecules spilled onto them will not contribute to the SERS. On the contrary, Pt NRs substrate itself is SERS-active thanks to its nanoscopic morphology.29 Therefore, the excess analyte molecules can contribute to the SERS adding to the SERS produced by those in the hot spots. Even though the EM field enhancement in the Au/Pt_NR is weaker than in the Au/ Pt_film case, there is an additional mechanism that can more than offset the disadvantage of the insufficient reflection coming from the textured surface. To verify these ideas, we have compared SERS on Au/ Pt_film and Au/Pt_NR substrates by varying the analyte concentration in the range of 10−8∼10−3 M in Figure 6. The concentration of 10−5 M is the point when the surface areas of the Au NPs and the analyte molecules are close to each other

bending of the NR can take place, by which some NRs are in contact with one another (Figure S1). In fact, the orientation of the Pt NR can affect the SERS effect. In our previous paper, we showed that the Pt NR lying horizontally to the film surface is about 2 times more efficient than those standing vertically in producing SERS effect. Although a Pt NP has the plasmon absorption at ∼200 nm, deep in the UV, SERS effects from collections of Pt NPs have been reported in the literature.44,45 In the case of Pt NRs, the DDA simulation results in our previous studies29 show that the interaction between the Pt NRs shifts the absorption to the visible region, enabling the SERS effect. Remarkably, Pt NRs alone have an EF comparable to that of Au/Si (Figure 1). More remarkable is that the composite substrate Au/Pt_NR between Au NPs and Pt NRs shows the strongest SERS with an EF larger than the sum of EFs of the two constituents, suggestive of a synergistic effect. The FDTD simulation results on Au/Pt_NR are shown in Figures 4 and 5. The plots of ⟨E2⟩ of Pt NRs, a Au NP, and their composite are compared in Figure 4. The overall effect of the Pt NR support is the same as that of the Pt film. However, the magnitude of effect arising from the reflection of incident light is considerably lessened because Pt NRs have void spaces between the Pt NRs, which effectively reduces the reflectivity. Therefore, when compared with those on Au/Pt_film in Figure 2, the enhancement of the EM field both around the Au NP and at the gap space is weaker than in Au/Pt_film case. The same conclusion can be drawn from the 2D plots of ⟨E2⟩ at different levels of z (Figure 5). These results contradict the observations in Figure 1 where the SERS of Au/Pt_NR is at least 3 times more efficient than Au/Pt_film. Our explanation of these contradictions lies on the realization that the EF field strength of several spots cannot represent the whole situation. A Raman spectrum is bound to be an average of signals of many molecules under the influence of the incident beam. Some molecules may experience the highly enhanced EM field, while there will be others that do not. Even within a spot, the EM field is not uniform over an area but generally forms a gradient. Also, the analyte molecules are bound to be spread over the entire surface of the SERS substrate even though the interaction between the substrate surface and a certain type of molecule may make the distribution uneven. In short, in the conventional platform of SERS measurement, the SERS spectrum is the accumulation of all the analyte molecules under the incident beam of 5 μm2 in

Figure 6. Plots of the ratio of the intensity on Au/Pt_film to Au/ Pt_NR substrates as a function of analyte concentration. E

DOI: 10.1021/acs.jpcc.5b05204 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

■

(Figure 4Sand Table 2S). Assuming that the 4-ABT molecules have strong affinity to the Au surface, concentrations lower than 10−6 M will give conditions where most of the analyte molecules are accumulated to the Au NPs. In such a case, the EM field strength will dictate the SERS efficiency. On the basis of the simulation data in Figures 2 and 4, one can estimate that Au/Pt_film is about 2 times more efficient than Au/Pt_NR. On the contrary, when the analyte concentration becomes 10−5 M or higher, the spillover effect will take place. In this case, Au/ Pt_NR substrate is expected to give stronger SERS than Au/ Pt_film because in the former the spilled over analyte molecules can contribute to SERS while those in the latter are more or less wasted. The plots of EF on the two types of substrates show that 10−5 M concentration is the turning point around which the EFs of the substrates are inverted.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05204. SEM image and scheme of Pt NRs (Figure S1), size and concentration of Au NPs (Table S1), SEM images and size distribution of Au NPs (Figure S2), SERS EF of Au/ Pt_NR and Au/Si (Figure S3), and SERS spectra as a function of analyte molecules on different substrates (Figure S4 and Table S2) (PDF)

■

REFERENCES

(1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. Raman spectra of pyridine adsorbed at a silver electrode. Chem. Phys. Lett. 1974, 26, 163−166. (2) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. Surfaceenhanced Raman scattering. J. Phys.: Condens. Matter 1992, 4, 1143− 1212. (3) Ghosh, S. K.; Nath, S.; Kundu, S.; Esumi, K.; Pal, T. Solvent and ligand effects on the localized surface plasmon resonance (LSPR) of gold colloids. J. Phys. Chem. B 2004, 108, 13963−13971. (4) Gabudean, A.; Biro, D.; Astilean, S. Localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering (SERS) studies of 4-aminothiophenol adsorption on gold nanorods. J. Mol. Struct. 2011, 993, 420−424. (5) Nehl, C. L.; Hafner, J. H. Shape-dependent plasmon resonances of gold nanoparticles. J. Mater. Chem. 2008, 18, 2415−2419. (6) Campion, A.; Kambhampati, P. Surface-enhanced Raman scattering. Chem. Soc. Rev. 1998, 27, 241−250. (7) Jiang, J.; Bosnick, K.; Maillard, M.; Brus, L. Single molecule Raman spectroscopy at the junctions of large Ag nanocrystals. J. Phys. Chem. B 2003, 107, 9964−9972. (8) Chen, L. Y.; Yu, J. S.; Fujita, T.; Chen, M. W. Nanoporous copper with tunable nanoporosity for SERS applications. Adv. Funct. Mater. 2009, 19, 1221−1226. (9) Sharma, B.; Frontiera, R. R.; Henry, A.-I.; Ringe, E.; Van Duyne, R. P. SERS: materials, applications, and the future. Mater. Today 2012, 15, 16−25. (10) Joo, T. H.; Kim, K.; Kim, M. S. Surface-enhanced Raman scattering (SERS) of 1-propanethiol in silver sol. J. Phys. Chem. 1986, 90, 5816−5819. (11) Xu, B.-B.; Ma, Z.-C.; Wang, L.; Zhang, R.; Niu, L.-G.; Yang, Z.; Zhang, Y.-L.; Zheng, W.-H.; Zhao, B.; Xu, Y. Localized flexible integration of high-efficiency surface enhanced Raman scattering (SERS) monitors into microfluidic channels. Lab Chip 2011, 11, 3347−3351. (12) Saito, Y.; Wang, J.; Smith, D.; Batchelder, D. A simple chemical method for the preparation of silver surfaces for efficient SERS. Langmuir 2002, 18, 2959−2961. (13) Hu, J.; Wang, Z.; Li, J. Gold nanoparticles with special shapes: controlled synthesis, surface-enhanced Raman scattering, and the application in biodetection. Sensors 2007, 7, 3299−3311. (14) Wang, T.; Hu, X.; Dong, S. Surfactantless synthesis of multiple shapes of gold nanostructures and their shape-dependent SERS spectroscopy. J. Phys. Chem. B 2006, 110, 16930−16936. (15) Ray, P. C. Size and shape dependent second order nonlinear optical properties of nanomaterials and their application in biological and chemical sensing. Chem. Rev. 2010, 110, 5332−5365. (16) Emory, S. R.; Haskins, W. E.; Nie, S. Direct observation of sizedependent optical enhancement in single metal nanoparticles. J. Am. Chem. Soc. 1998, 120, 8009−8010. (17) Li, J.; Zhang, J. Z. Optical properties and applications of hybrid semiconductor nanomaterials. Coord. Chem. Rev. 2009, 253, 3015− 3041. (18) Li, W.; Zamani, R.; Rivera Gil, P.; Pelaz, B.; Ibáñez, M.; Cadavid, D.; Shavel, A.; Alvarez-Puebla, R. A.; Parak, W. J.; Arbiol, J. CuTe nanocrystals: Shape and size control, plasmonic properties, and use as SERS probes and photothermal agents. J. Am. Chem. Soc. 2013, 135, 7098−7101. (19) Stockman, M. I.; Faleev, S. V.; Bergman, D. J. Localization versus delocalization of surface plasmons in nanosystems: can one state have both characteristics? Phys. Rev. Lett. 2001, 87, 167401. (20) Tsai, D.; Kovacs, J.; Wang, Z.; Moskovits, M.; Shalaev, V. M.; Suh, J.; Botet, R. Photon scanning tunneling microscopy images of optical excitations of fractal metal colloid clusters. Phys. Rev. Lett. 1994, 72, 4149−4152. (21) Liver, N.; Nitzan, A.; Gersten, J. Local fields in cavity sites of rough dielectric surfaces. Chem. Phys. Lett. 1984, 111, 449−454.

4. CONCLUSION In this study, we investigated the combination effects of two different materials for producing SERS. The composite substrates are constituted of Au NPs spread on various metallic films. The reflection from the metallic films enhances the EM fields around the Au NPs on them increasing the SERS intensity. In addition, the space around the contact point between the Au NP and the film creates a large area of hot spot that can further fortify its ability to produce SERS. When a nanostructure is introduced to the metallic film, the reflection effect is reduced. Consequently, mechanisms that rely only on the EM field strength are weakened. However, on the other hand, the nanostructure of the reflector film itself can have its own capacity to produce SERS, which can add up to the SERS generated by the (fortified) EM fields around the Au NPs. The additional effect from the nanostructured reflector film may appear depending on the amount of analyte molecules relative to the hot spots (around Au NPs). When all of these factors are combined in an appropriate way, one can see synergistic effects between Au NPs and a nanostructured reflector.

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82 31 290 7070. Fax: +82 31 290 7075. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Agency for Defense Development through Chemical and Biological Defense Research Center. This work was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF-2009-0083540 and NRF20090081018). F

DOI: 10.1021/acs.jpcc.5b05204 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C (22) Oh, M. K.; Park, S.; Kim, S. K.; Lim, S.-H. Finite difference time domain calculation on layer-by-layer assemblies of close-packed gold nanoparticles. J. Comput. Theor. Nanosci. 2010, 7, 1085−1094. (23) Gunnarsson, L.; Bjerneld, E.; Xu, H.; Petronis, S.; Kasemo, B.; Käll, M. Interparticle coupling effects in nanofabricated substrates for surface-enhanced Raman scattering. Appl. Phys. Lett. 2001, 78, 802− 804. (24) King, M. D.; Khadka, S.; Craig, G. A.; Mason, M. D. Effect of local heating on the SERS efficiency of optically trapped prismatic nanoparticles. J. Phys. Chem. C 2008, 112, 11751−11757. (25) Bartlett, P.; Baumberg, J.; Coyle, S.; Abdelsalam, M. Optical properties of nanostructured metal films. Faraday Discuss. 2004, 125, 117−132. (26) Abdelsalam, M. E.; Mahajan, S.; Bartlett, P. N.; Baumberg, J. J.; Russell, A. E. SERS at structured palladium and platinum surfaces. J. Am. Chem. Soc. 2007, 129, 7399−7406. (27) Mrozek, M. F.; Xie, Y.; Weaver, M. J. Surface-enhanced Raman scattering on uniform platinum-group overlayers: Preparation by redox replacement of underpotential-deposited metals on gold. Anal. Chem. 2001, 73, 5953−5960. (28) Lee, K. R.; Kim, M. H.; Hong, J. E.; Kwon, Y. U. Templated syntheses of Pt thin films with feature sizes of 3, 6 and 9 nm and their effects on SERS. J. Raman Spectrosc. 2013, 44, 6−11. (29) Kim, M.-H.; Lee, E.-S.; Kim, S. K.; Kwon, Y.-U. Low-Cost Fabrication of Pt Thin Films with Controlled Nanostructures and Their Effects on SERS. Plasmonics 2011, 6, 715−723. (30) Lee, H. J.; Lee, U.; Park, J. Y.; Yoo, S. H.; Park, S.; Kwon, Y. U. Platinum films with controlled 3-dimensional nanoscopic morphologies and their effects on surface enhanced Raman scattering. Chem. Asian J. 2009, 4, 1284−1288. (31) Jana, N. R.; Gearheart, L.; Murphy, C. J. Seed-mediated growth approach for shape-controlled synthesis of spheroidal and rod-like gold nanoparticles using a surfactant template. Adv. Mater. 2001, 13, 1389− 1393. (32) Park, S.; Yang, P.; Corredor, P.; Weaver, M. J. Transition metalcoated nanoparticle films: vibrational characterization with surfaceenhanced Raman scattering. J. Am. Chem. Soc. 2002, 124, 2428−2429. (33) Park, Y.-K.; Yoo, S.-H.; Park, S. Assembly of highly ordered nanoparticle monolayers at a water/hexane interface. Langmuir 2007, 23, 10505−10510. (34) Oskooi, A. F.; Roundy, D.; Ibanescu, M.; Bermel, P.; Joannopoulos, J. D.; Johnson, S. G. MEEP: A flexible free-software package for electromagnetic simulations by the FDTD method. Comput. Phys. Commun. 2010, 181, 687−702. (35) Talley, C. E.; Jackson, J. B.; Oubre, C.; Grady, N. K.; Hollars, C. W.; Lane, S. M.; Huser, T. R.; Nordlander, P.; Halas, N. J. Surfaceenhanced Raman scattering from individual Au nanoparticles and nanoparticle dimer substrates. Nano Lett. 2005, 5, 1569−1574. (36) Taflove, A.; Hagness, S. C. Computational Electrodynamics; Artech House: Norwood, MA, 2005. (37) Vial, A.; Grimault, A.-S.; Macías, D.; Barchiesi, D.; de La Chapelle, M. L. Improved analytical fit of gold dispersion: Application to the modeling of extinction spectra with a finite-difference timedomain method. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 085416. (38) Marburg, S.; Nolte, B.; Bernhard, R.; Wang, S. Computational Acoustics of Noise Propagation in Fluids: Finite and Boundary Element Methods; Springer: Berlin, 2008. (39) Hong, S.; Li, X. Optimal size of gold nanoparticles for surfaceenhanced raman spectroscopy under different conditions. J. Nanomater. 2013, 2013, 790323. (40) Krug, J. T.; Wang, G. D.; Emory, S. R.; Nie, S. Efficient Raman enhancement and intermittent light emission observed in single gold nanocrystals. J. Am. Chem. Soc. 1999, 121, 9208−9214. (41) Abdelsalam, M. E. Surface enhanced raman scattering of aromatic thiols adsorbed on nanostructured gold surfaces. Cent. Eur. J. Chem. 2009, 7, 446−453. (42) Yoon, J. K.; Kim, K.; Shin, K. S. Raman scattering of 4aminobenzenethiol sandwiched between Au nanoparticles and a

macroscopically smooth Au substrate: Effect of size of Au nanoparticles. J. Phys. Chem. C 2009, 113, 1769−1774. (43) Alvarez-Puebla, R. A.; Bravo-Vasquez, J. P.; Cheben, P.; Xu, D.X.; Waldron, P.; Fenniri, H. SERS-active Ag/Au bimetallic nanoalloys on Si/SiOx. J. Colloid Interface Sci. 2009, 333, 237−241. (44) Li, J.-F.; Huang, Y.-F.; Duan, S.; Pang, R.; Wu, D.-Y.; Ren, B.; Xu, X.; Tian, Z.-Q. SERS and DFT study of water on metal cathodes of silver, gold and platinum nanoparticles. Phys. Chem. Chem. Phys. 2010, 12, 2493−2502. (45) Gómez, R.; Pérez, J. M.; Solla-Gullón, J.; Montiel, V.; Aldaz, A. In situ surface enhanced Raman spectroscopy on electrodes with platinum and palladium nanoparticle ensembles. J. Phys. Chem. B 2004, 108, 9943−9949.

G

DOI: 10.1021/acs.jpcc.5b05204 J. Phys. Chem. C XXXX, XXX, XXX−XXX