Exploring the Chemical Enhancement of Surface-Enhanced Raman

Dec 18, 2012 - School of Chemistry and Chemical Engineering, Nantong University, Nantong 226007, P. R. China. § Department of Chemistry, The City ...
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Exploring the Chemical Enhancement of Surface-Enhanced Raman Scattering with a Designed Silver/Silica Cavity Substrate Shu Tian,†,‡ Qun Zhou,*,† Chuanhong Li,† Zhuomin Gu,† John R. Lombardi,§ and Junwei Zheng*,† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China School of Chemistry and Chemical Engineering, Nantong University, Nantong 226007, P. R. China § Department of Chemistry, The City College of New York, New York, New York 10031, United States ‡

S Supporting Information *

ABSTRACT: Silver nanoparticles were assembled onto the bottom of closed-packed silica cavity using polystyrene (PS) spheres as template. Charge transfer between the adsorbed 4aminothiophenol (PATP) and the silver nanoparticles was studied using surface-enhanced Raman spectroscopy with 514, 633, 785, and 1064 nm excitation, and compared to that of the immobilized silver nanoparticles without the modification of silica cavity. Using the concept of degree of charge transfer, we directly observed the additional chemical enhancement without a deliberate distinction between electromagnetic (EM) enhancement and chemical enhancement. It was demonstrated that the negative charges of the silica could induce the formation of the dipole in the nanoparticles, thus enlarging the electron density at the sites where probe molecules adsorbed, and leading to higher charge transfer from the metal to the adsorbed PATP molecules. We also proposed another model to further elucidate the relationship between the electron density and the charge transfer. The result showed that the reduction of the electron density of silver nanoparticles will cause the redistribution of the dipole, thereby reducing the charge transfer degree. section;23−25 this only happens when the photon energy of the excitation line is strong enough (usually in the UV region). The other is an electronic resonance caused by photo-driven charge transfer between the affinity level of the adsorbed molecule and the Fermi level of the metal. This leads to an increase in the polarizability of the molecule and results in a Raman enhancement.8,21,26−28 In this mechanism, CT is considered to be a resonance Raman-like process, which can be tuned into and out of resonance by changing either the Fermi level (Ef) of the metal or the photon energy of the excitation line (hν). Campion and co-workers7 employed smooth metal surfaces as a substrate to minimize the EM enhancement and attributed all improvement to a chemical mechanism, which resulted in an enhancement factor of ∼30. Shegai and co-workers28 created hot spots by depositing Tollen’s silver island films on an ITO electrode; the singlemolecule SERS of nonresonant 4-mercaptopyridine were collected from individual electromagnetic hot spots. They estimated 3 orders of magnitude for the chemical enhancement using R6G as reference. Park and co-workers fabricated a sandwich structure of Au thin film/p-aminothiophenol (PATP)/Au nanoparticle;29 they demonstrated that only a small fraction of the molecules at the junction were CT-active, and the CT-enhancement factor could be 101 to 103. Zhao et

1. INTRODUCTION Since its emergence in the 1970s,1−3 surface-enhanced Raman scattering (SERS) has always been a hot topic due to its nondestructive nature, ultrahigh sensitivity, and the richness of molecular information offered by it.4−6 It is generally accepted that both long-range electromagnetic (EM) enhancement and short-range chemical enhancement contribute simultaneously to the total enhancement.7,8 The dominant EM enhancement is associated with giant electromagnetic field resulting from the localized surface plasmon resonance (LSPR) of collective oscillation of free electrons and the incident electromagnetic radiation. Nie and Kneipp independently reported single molecule detection in 1997,9,10 for which an enhancement of 1014 was observed. Experimental evidence further verified that coupling of the localized surface plasmon of nanosized neighboring particles can produce a series of “hot spots”; these “hot spots” usually localized in the interstices or sharp clefts in the dimers and the detected molecules were adsorbed between closely spaced nanoparticles.11−13 Through theoretical calculations, Xu et al. also demonstrated that vast enhancement from 10 to 11 orders of magnitude can be obtained at the junction of two touching silver particles.14 The contribution of chemical enhancement to SERS has also been examined by several researchers.15−22 Generally speaking, there are two possible effects involved in this mechanism. One is the ground-state charge transfer (CT) between the molecule and the metal which can change the polarizability of the molecule, therefore changing the Raman scattering cross © 2012 American Chemical Society

Received: September 17, 2012 Revised: December 18, 2012 Published: December 18, 2012 556

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al.30 performed a theoretical calculation for the SERS of pyrazine molecules located at the junction of two Ag20 clusters. Since nanoclusters with such a small size cannot support collective plasma oscillations, the junction between Ag20 clusters does not provide an electromagnetic “hot spot”. A calculated enhancement factor of approximately 10 5 is suggested to stem mainly from the chemical enhancement. From the examples given above, the estimates of the CTenhancement factors range from a factor of 10−103, even as large as 107−108.31 The magnitude of the chemical enhancement varies from publication to publication and varies with the experimental conditions; so, for a correct evaluation of a CT mechanism, there are two main problems that must be solved. The first question is how to quantify CT-enhancement factors. Because chemical enhancement is normally inextricably linked with EM enhancement, it becomes exceedingly difficult to estimate the magnitude of each mechanism. Second, what can be the main influences on the CT-enhancement? The spectroelectrochemical method has always been regarded as one of the most reliable methods to study CT enhancement. This is done experimentally by examining the dependence of SERS spectra on electrochemical potentials and photon energy of excitation lines. Osawa et al.15 systematically measured the SERS of PATP adsorbed on an electrochemically roughened silver electrode; by changing the applied potential, they found that the b2 bands of PATP (the enhancement of the b2 modes only arises from the Herzberg−Teller effect based on the CT mechanism) show a resonance-shaped intensity profile as a function of applied potential, and demonstrated a resonance Raman-like process associated with the photon-induced charge transfer from the metal to an affinity level of the adsorbed molecule. However, sometimes one may encounter difficulties when using this method to prove the presence of CT in many systems.28 The reasons could be as follows: First, the SERS intensity is influenced not only by the CT process, but also by the EM enhancement, which may also be influenced by the applied electrode potential.32 Second, the coverage of molecules on the metal surface usually decreases at very negative potentials or due to the electrochemical reaction that may occur.33,34 Moreover, electrochemical reactions may occur in an electrolyte solution during the electrochemical process. A hydrogen evolution reaction will occur at very negative potentials, while an oxygen evolution reaction will occur at very positive potentials. The most appropriate potential range should be somewhere in between. However, the highest intensity for some molecules, such as mercaptopyridine, pyridine, and pyrazine, appears at a very negative potential or even in the potential region of hydrogen evolution with visible laser excitations.28,35 So, the third reason is that the narrow electrochemical window impedes the observation of the highest intensity. In our previous studies,32,36 we managed to separate the EM and chemical enhancement using NIR excitation (1064 nm) to measure the SERS enhancement behavior of PATP residing in a layer-by-layer construction. Because the 1064 nm excitation is far away from the SPR of the nanoparticles, the contribution from the chemical enhancement can be amplified relative to EM enhancement. We demonstrated that the SERS enhancement of the interconnecting molecules between the particles is dependent on the direction of the molecular dipole and the charge density of the metal surface. Very recently, Ikeda and coworker37 reported an atomically smooth single crystalline gold substrates-molecule-gold nanoparticle (plane−sphere) type for

SERS investigation; the degree of each contribution to SERS signals was examined on this well-defined substrate. They also drew the same conclusion that CT-activity of “SERS-hotspots” is closely related to the density and directions of interfacial dipoles formed at metal−molecular junctions. In this paper, we report a direct examination of the chemical enhancement by using a well-designed silver/silica cavity array as SERS substrate, which can provide exceptionally strong enhancement for PATP molecules adsorbed on it. We find the Raman intensities of b2 modes of PATP molecules on silver/ silica cavity array, as compared to the intensities observed from monolayer silver nanoparticles (AgNPs), were selectively enhanced. UV−vis spectra of these two different substrates indicate that the particle’s electromagnetic properties do not change after the modification of silica cavity. Thus, we believe that it is the chemical enhancement that accounts for this extra improvement; we also demonstrate that CT-activity is closely related to the density and directions of interfacial dipoles formed at metal−molecular junctions, which we called charge density dependent CT behavior. We investigated this behavior by using 514, 632.8, 785, and 1064 nm laser light as incident radiation, and the computational value of the extent of charge transfer PCT is used as quantitative index. There are several novel and important features to the work reported here. First, we directly observed the chemical enhancement effect without separating the EM and chemical enhancement artificially. Second, we demonstrated that optimization of the environmental condition can result in an alteration of the local Fermi level of the metal nanoparticles, thus leading to better energy matching between the energy levels of the interconnecting molecules and the Fermi level of the metal. As a result, a much higher CT-enhancement can be obtained. Third, this decorated structure of the silica voids may have many potential applications, such as a reproducible Raman substrate and a device available for enrichment of analyte in bulk solution, which is important for detecting objects with ultralow concentration.

2. EXPERIMENTAL SECTION 2.1. Materials. LUDOX AS-40 colloidal silica (40% water solution), PATP, and sidium dodecyl sulfate (SDS) were obtained from Sigma-Aldrich; polyvinylpyridine (PVP) was purchased from Acros Organics. Silver nitrate (AgNO3) and silver oxide (Ag2O) was purchased from Shanghai Chemical Reagent Company. Ultrapure water used in washings and all solution preparation was produced using a Milli-Q system with resistivity greater than 18.2 MΩ cm−1. The other chemicals were of analytical reagent grade and all the reagents were used as received. 2.2. Experimental Procedures. Preparation of Substrates. Silver colloids were prepared according to the literature protocols.38 The average diameter of the particles was ca. 80 nm, estimated from the TEM image. Polystyrene (PS) spheres of 1.2 μm diameter were synthesized by dispersion polymerization of styrene in ethanol−water mixtures according to the literature.39 Monolayer immobilized AgNPs were used as a comparison SERS substrate and for further modification. For preparation, a cleaned glass slide was first coated with PVP by immersing the glass slides into a 2% PVP ethanol solution for 5 h. The positively charged slide was exposed to the silver colloid suspension for 12 h; a layer of the AgNPs was assembled onto 557

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on a Shimadzu UV-3600 spectrometer. The SERS spectrograms were measured on a LabRam Aramis Raman microscope (Jobin Yvon technology) with the excitation wavelengths of 514 (0.12 mW on the sample), 633 (0.40 mW on the sample), and 785 nm (1.00 mW on the sample). The system employed a chargecoupled device detector with a scanning time of 2 s and 50× (NA 0.5) microscope objective to focus the laser beam onto a spot of ∼2 μm2. The resulting scanning resolution at each wavelength was ∼4 cm−1. For the laser line 1064 nm, a Nicolet 960 FT-Raman spectrometer equipped with a liquid-nitrogencooled Ge detector and a Nd/VO4 laser as an excitation source, and the laser power used was about 300 mW at the samples. At each recording, 128 runs were performed giving a resolution of 4 cm−1. For each substrate, we randomly measured 20 spots; the spectra and the extent of charge transfer calculations (PCT value) were based on the mean value. A baseline correction was conducted for PCT value calculations at each spectrum.

the surface of the slides. After adsorption, the glass slide was rinsed with water, blown dry, and stored with nitrogen. Gold or silver thin films used in this work were prepared by electromagnetic sputtering of a 10-nm-thick chromium layer, followed by a 200-nm-thick gold or silver layer onto 1-mmthick glass slides. These gold or silver coated slides (1 cm × 1 cm) were thoroughly cleaned and stored with nitrogen. Assembly of the Colloidal Templates. The monolayer of PS particles was generated by self-assembly at air/water interface in a 5-cm-diameter Petri dish. In brief, a water/ethanol dispersion containing monodisperse PS spheres (5 wt %) was injected slowly onto the surface of water by a microsyringe, and the PS spheres were spread freely over the top surface of water until it covered nearly the whole surface area. A few drops of 2 wt % SDS solution were then added onto the water surface to lower the surface tension and closely pack the PS spheres. The whole system was kept in an environment free from outside disturbances in order to sediment the suspended PS particles in the water for 24 h. A close-packed monolayer of PS spheres could be prepared in this way. The PS microsphere template was fabricated as follows: a piece of monolayer AgNP modified glass slide or gold/silver thin film was placed into the solution at a 30−40° tilt angle, and the particles were transferred onto the surface of the glass by careful water drainage at a controlled rate; the glass slide was then dried at 90 °C under the protection of nitrogen for 1 h to remove the remaining solvent in the monolayerthis was done to generate the highly ordered template structure of the closepacked PS particles. They were then preserved under a nitrogen atmosphere. Fabrication of Silica and Silver Cavity. The fabrication of silica cavities was carried out by spin-coating the diluted LudoxAS 40 colloidal silica sol (sol:water = 1:30) through the PS template glass slide, and then dehydrated at room temperature under the protection of nitrogen for 24 h. Following spincoating, the PS spheres were removed by dissolving in toluene to leave an ordered array of interconnected sphere segment voids with AgNPs on the bottom. The two-dimensional silver cavity array was fabricated by electrochemical deposition. Electrochemical deposition was performed in a thermostatted cell at room temperature, using a conventional three-electrode configuration controlled by a CHI 660D electrochemical station (Shanghai ChenHua Instruments Co Ltd., China). The template-coated gold or silver substrate was the working electrode, and a platinum and a Hg/Hg2SO4 electrode were used as the auxiliary and reference electrodes, respectively. Silver was deposited from a cyanide-free plating bath (0.1 M AgNO3+0.1 M EDTA+0.05 M NH4NO3, and several milliliters of concentrated ammonia were added to ensure the pH value 9−10) using pulse plating.40 The templated silver films were produced using multicurrent pulse plating with the first pulse to a current density of 30 mA cm−2 for 100 ms (this is important to ensure good adhesion) followed by a train of pulses of 5 mA cm−2 for 60 ms separated by a rest time of 1 s (zero current). The adsorption of the PATP molecules on the assembled AgNPs was carried out by immersion of the different substrates into 0.1 mM PATP ethanol solution for 2 h. The slides were then thoroughly rinsed with ethanol to remove the physically adsorbed PATP molecules. 2.3. Characterization. The surface morphologies of the samples were measured on a Hitachi S4700 FE-SEM microscope. The electronic absorption spectra were measured

3. RESULTS AND DISCUSSION The fabrication of the silver/silica cavity array is schematically illustrated in Figure S1a. The protonated pyridine groups of PVP coated on a glass slide provided the active sites for the adsorption of negatively charged AgNPs by electrostatic interaction. A layer of AgNPs was assembled on the surface as shown in SEM in Figure S1a. The drying process of PS template will lead to deformation of the PS spheres, and the deformed spheres will cover several nanoparticles. The interstice of PS spheres was filled with diluted Ludox-AS 40 colloidal silica sol through spin-coating; after the template was removed by dissolution of the PS template with toluene, ordered silica voids opened at the top and bottom can be obtained and the covered AgNPs will be exposed. The fabrication of the silver cavity on gold or silver thin film is schematically illustrated in Figure S1b. Ordered arrays of close-packed PS spheres, supported onto sputtered gold or silver thin film, were used as templates. Despite the high quality of the deposits obtained from the alkaline cyanide solutions, however, this type of bath plating is highly toxic, so we chose a cyanide free plating bath. The galvanostatic multistep method is one of the methods used to fabricate relatively uniform metal particles that strongly adhere to the conducting substrate. The first pulse is used to initiate the formation of nuclei, the second pulse is used to control the growth of the nuclei formed during the previous pulse, and the introduction of the rest time ensures that the silver ion complex in the solution has enough time to spread to the deposition interface. When the electrochemical deposition was complete, usually 30 min later, the samples were rinsed with water to remove the residual electrolyte and then soaked in toluene to dissolve the PS template. SEM images of silver/silica cavities and silver cavities are shown in Figure S2a and b, respectively. It can be clearly seen that there are ∼10 AgNPs in each silica cavity, and no apparent change regarding the distribution of AgNPs is observed after the modification of silica cavities and the adsorption of PATP molecules. As for the silver cavities, in this work, we used the second pulse 1400 times to generate about 500 nm thickness of Ag deposition, which is almost half the height of the template, so a complete hemisphere cavity was formed. It is well-known that the molecular dipolar momentum and surface properties of the AgNPs have large effects on the enhancement of the Raman scattering of the interconnecting molecules. We have previously demonstrated that the Fermi 558

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level of a metal can be changed through altering the electrode potential and coadsorption of chloride ions, making the b2 modes selectively enhanced.32,41 In this case, we assumed that the negative charges of the silica surface could alter the charge distribution in the immobilized nanoparticles, and the direction of dipole in the nanoparticles, in turn, altered the extent of charge transfer between the adsorbed PATP molecules and the silver. To verify our assumption, we chose two different silver substrates for SERS detection. For the first substrate, AgNPs were electrostatically attracted on positively charged glass substrate; for the second substrate, AgNPs were placed inside the silica cavities. We measured the SERS spectra of attached PATP on these two kinds of substrates with near-infrared (NIR) excitation; see Figure 1. This excitation wavelength was chosen because it is far away from the surface plasmon resonance of the AgNPs.

Figure 2. Extinction spectra of (a) the AgNPs monolayer and (b) the AgNPs/silica cavity assembly.

should make a contribution to the overall enhancement, the spectral difference in the SPR bands should not render such a large difference in FT-SERS spectra. Additionally, according to the surface-selection role of the EM model proposed by Moskovits,42,43 only polarizability components in a direction perpendicular to the surface will be enhanced. Since the out-ofplane modes b1 and a2 are very weakly observed in both of the spectra, this indicates that the PATP molecule adopts a standing-up orientation. If reorientation of the PATP molecule occurred, the PATP molecule should then adopt a tilted or even lying-down orientation, and the vibrational direction of inplane modes of a1 and b2 should be more parallel to the surface as compared to that of the standing-up orientation, the intensity of b2 vibration should not be maximized but be minimized. For this reason, it is unlikely that the enlargement of the b2 modes observed in Figure 1 is due to the reorientation of PATP molecule, which means the selective enhancement of the b2 modes cannot be explained by the EM mechanism. On the basis of the parameters described above, the only possible explanation of the selective enhancement of the b2 modes is the CT mechanism. It is generally agreed that the enhancement of the b2 modes of the adsorbed PATP molecules is associated with a charge transfer from metal to the adsorbed molecules, and dependent on the degree of matching between the energy levels of the adsorbed molecules and the metal.15,21,37,44 According to the CT model for SERS proposed by Lombardi et al.,8,27 Raman polarizability tensors can be depicted as three terms, A, B, and C. Term A represents the Franck−Condon contribution, and only totally symmetric modes are enhanced by this mechanism. Terms B and C belong to the Herzberg−Teller contribution and stand for the enhancement via molecule-to-metal CT and metal-to-molecule CT, respectively. Different from term A, both totally and nontotally symmetric modes can be enhanced by terms B and C. For the current case of PATP adsorbed on silver, the nontotally symmetric b2 modes are merely enhanced through the C term, whereas both the A and C terms determine enhancement of the totally symmetric a1 modes. So, the different behaviors of the a1 and b2 modes of the PATP molecules as demonstrated in Figure 1 can be attributed to the differences in the CT resonance condition of the A and C terms. The CT resonance is something similar to the resonance Raman process, and the resonance can be reached by changing either the Fermi level of the metal (Ef) or the energy of incident light (hν). The Ef of Ag is 4.3 eV below the vacuum level and the lowest unfilled orbital (LUMO) of PATP is 3.03 eV;45 thus,

Figure 1. FT-SERS spectra of PATP molecules adsorbed on (a) immobilized AgNPs and (b) AgNP/silica cavities. The excitation wavelength was 1064 nm.

For the PATP molecules adsorbed on the AgNPs monolayer, the predominant bands in the FT-SERS spectrum (Figure 1a) are located at 1003, 1077, 1178, and 1587 cm−1, which belong to a1 modes (totally symmetric, 18a, 7a, 8a, and 9a, respectively) of the PATP molecule.15 Upon assembly of the silica cavities, the other set at 1140, 1390, and 1435 cm−1, which are assigned to the b2 modes (asymmetric, 9b, 3b, and 19b, respectively) of the PATP molecules can also be observed in Figure 1b. In particular, the intensities of b2 modes relative to that of the a1 modes gain additional enhancement. For example, we normalized peak intensities of the 7a and 9b bands with the highest intensity value of 1587 cm−1; the intensity of the 9b band of spectrum a was ∼10 times greater than that of spectrum b, while the intensity of the 7a band was almost the same. How could this selective enhancement of b2 vibrational modes happen? Normally, EM enhancement contributes most of the total enhancement, so we first tried to solve this problem with EM enhancement mechanism. Because EM enhancement is always associated with the SPR band, we examined the extinction spectra of these two different substrates, as depicted in Figure 2. By comparison of these two spectra, the SPR band of AgNPs immobilized on PVP-coated glass substrate locates at 429 cm−1 (Figure 2a), a red shift of 6 nm of the SPR band can be seen after the assembly of the silica cavities (Figure 2b), and the broad SPR bands of the AgNPs on both substrates can reach the NIR region. In our present study, SERS spectra were measured with 1064 nm excitation, which is far from the SPR bands of both substrates, so although the EM mechanism 559

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therefore, the direction of the dipole was reversed. Osawa et al.15 studied the SERS of PATP molecules adsorbed on an electrochemically roughened Ag electrode. They drew an intensity−potential curve and pointed out that, as for the metal-to-molecule CT, the curve shifted to more positive potentials as the excitation energy increased. Therefore, to detain an identical intensity, the more negative the potential applied, the lower the photon energy needed. Note that making the applied potential more negative is similar to increasing the electron density of Ag electrode; although the CT in the present study is excited by the light irradiation that differs from the potential in the electrochemical systems, the CT mechanism in the two cases should be identical. In this case, the electron density of the place where the PATP molecules adsorbed changed from relatively electron deficient to excess, and the positive pole of interface dipole was also reversed. This will reduce the energy difference of LUMO level of the PATP molecules and Fermi levels of the silver particles, and be favorable to the overall CT process between the metal nanoparticles and the PATP molecules. As mentioned above, it is the combination of changes in the static polarizability of the molecule and the photoexcited charge transfer between metal and molecule that lead to the overall chemical enhancement. In our present case, the direction of transference of electrons is from metal nanoparticles to molecules, the dipolar direction of the PATP molecule may go against this CT process, due to the existence of electrondonating amine and the electron-withdrawing thiolate. We compared the expressions to the computational results reported by Zhao et al.24 They demonstrated that changes in the static polarizability of the molecule contributed about 101 to the enhancement of the Raman signal, whereas CT resonances enhanced the Raman intensity by 103, which can be attributed to borrowing of intensity from the strong plasmon absorption transition of the metallic nanoparticles.28 Correlating these numbers to our experimental results suggests that CT resonances must be the main factor in the case of chemical

the energy difference between the LUMO levels of PATP and the Fermi level of AgNPs is calculated to be 1.27 eV. In this case, the incident energy was fixed, and the energy of a photon at 1064 nm (1.16 eV) was too low to induce sufficient charge transfer to provide measurable enhancement of the b2 modes. However, when the AgNPs were surrounded by the silica cavities, a remarkable enhancement of the b2 modes occurred. Therefore, the modification of the silica cavities was considered to be the main factor responsible for the change of Ef and the CT enhancement. As graphically illustrated in Figure 3, the

Figure 3. Schematic illustration of the charge distribution in AgNPs (a) immobilized on a PVP-derivatized glass slide and (b) dwelling on the bottom of the silica cavity.

negatively charged AgNPs were assembled on the surface of the positively charged PVP-coated glass surface via the electrostatic interaction. Due to the electrostatic attraction of the positive charges of the PVP layer, the electron density may move toward the bottom of the AgNPs, generating a dipole in the particles (Figure 3a). The top part of the particles, where the PATP molecules were adsorbed, became relatively electron deficient. The modification of silica cavities, on the other hand, the negatively charged surface caused by residual hydroxyl group, resulted in redistribution of the charges in the nanoparticles. As shown in Figure 3b, the electrons moved to the top of the particles where the PATP molecules attached;

Figure 4. SERS spectra of PATP on the AgNPs monolayer (lower) and in the AgNPs/silica cavity assembly (upper) by using 514, 633, 785, and 1064 nm laser excitation. 560

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with increasing excitation energy, and that the value of PCT also shows the same tendency for modification of structured silica cavities. We noticed that, no matter what the excitation source is, modification of silica cavities also increases the PCT. The interpretation could exist in twofold: EM and CT. First, the broad SPR band of the AgNPs can also reach the region of longer wavelengths, and the EM mechanism can make a contribution to the overall enhancement. At the same time, UV−vis absorbance at 514.5, 632.8, and 785 nm shows a gradual decrease, meaning the intensities of EM field they formed will decrease in that order and result in the decrease of the overall intensity. Second, as mentioned above, the energy difference is 1.27 eV, the introduction of a photon at 514.5 nm (2.40 eV), 632.8 nm (1.96 eV), and 785 nm (1.58 eV) will be sufficient to transfer from Ag surface to an excited unfilled level of PATP, so it is not surprising to see such an enormous enhancement of b2 bands as compared to that of 1064 nm. Meanwhile, due to the excitation with different laser energy, the electron could be transferred to a different state (for example, LUMO+N excited state) to obtain a better match between the excitation energy and the energy gap,46 resulting in a different maximum intensity of b2 modes. Additionally, even though the incident energy is sufficient for electron transfer, the effects of the negatively charged surface of the silica cavities will always exist. Consequently, this will influence the redistribution of the charges in the nanoparticles and result in an additional enhancement of b2 modes. As described above, the b2 bands are enhanced via both EM and CT enhancement mechanisms, while the a1 bands are enhanced merely via the EM enhancement mechanism. The regular change of the PCT further demonstrates that the negatively charged silica could reorientate the dipole of the nanoparticles, increase the electron density of the site where PATP adsorbed, and generate an additional contribution to CT enhancement. If this is indeed the case, then the chemical enhancement should also be reduced if the electron density of silver nanoparticles is decreased. To test this hypothesis, we managed to prepare two different kinds of 2D silver cavity arrays; the only difference between them is that the arrays were built on a gold thin film or a silver film, as described in the Experimental Section. In our present case, the Femi level of gold is about 5.2 eV, which is a little higher than that of silver (4.3 eV). The contact of these two kinds of metal causes electron transfer spontaneously from the surface of silver to gold, making the electron density of the sites where the PATP molecules adsorbed much lower. If our assumption is correct, the charge transfer process should be hindered under this condition. We measured the SERS spectra of PATP molecules adsorbed on these two different substrates; corresponding spectra are shown in Figure 6. Upon the substrate of Ag cavities assembled on silver thin film, the band positions in the spectra (top spectra of panels a−c) are in good agreement with those results obtained from roughened silver substrates, demonstrating that these silver cavity arrays are active substrates for the SERS detection of molecule species. As can be seen from Figure 6, the absolute intensity of both a1 and b2 modes at different laser power show the same variation tendency as that of Figure 5; that is, the peak intensity increases with increasing laser power, and all of the b2 modes get additional enhancement as compared to those of a1 modes. We also plotted PCT as a function of excitation energy, using the ratios of intensities of 9b mode to 7a mode. As shown in Figure 6d, the extent of the charge transfer shows an increase with the increase of laser energy, which is very close to the

enhancement of b2 modes of PATP; consequently, the adverse factor resulting from the opposite molecular dipole can be compensated. To gain a deeper insight into the contribution of silica cavities to the CT enhancement, we measured the SERS spectra of PATP molecules on immobilized AgNPs with and without the modification of silica cavities by using 514, 632, 785, and 1064 nm laser excitation; the corresponding spectra are shown in Figure 4. In each panel, we normalized two spectra with respect to the peak intensity values of the 7a band. As can be seen in Figure 4, all of the spectra are dominated by five bands identical to the spectrum measured under 1064 nm excitation; three of them at about 1140, 1390, and 1430 cm−1 can be attributed to the b2 modes of PATP, while the remaining two at 1078 cm−1 and 1578 cm−1 can be assigned to the a1 modes. Moreover, the intensities of both a1 and b2 modes show a growing tendency in the order 514.5 > 632.8 > 785 nm (note that laser power of 514 nm laser excitation is only one-third of the 633 nm laser excitation and one-eighth of the 785 nm laser excitation). It is interesting to point out that, at a fixed excitation, the intensities of b2 modes obtain additional enhancement as compared to that of a1 modes, due to the modification of silica cavities. As described above, we attribute this selective enhancement to the contribution of CT resonance. In order to obtain a quantitative measurement of the relative contribution of the charge-transfer to the overall SERS intensity, we introduced the degree of charge-transfer PCT as a quantification index; Lombardi et al.20,27 defined PCT as

PCT =

R 1+R

where R is the ratio of intensity of a nontotally symmetric b2 band to that of a totally symmetric a1 band. Use of the ratio has the advantage of making our measurement of charge transfer independent of other effects on the total enhancement factor. It can be seen from the equation that, when PCT is zero, there are no charge-transfer contributions, while as PCT approaches 1, the charge-transfer contributions will tend to dominate the spectrum. In this work, all of the values used for calculating have been corrected for background; the curves in Figure 5 present the calculation results of PCT as a function of excitation energy, using the ratios of intensities of the nontotally symmetric 9b mode to the totally symmetric 7a mode, which show the variation of the extent of charge transfer of b2 modes in Figure 4. It is seen more clearly that PCT tends to increase

Figure 5. Degree of charge transfer (PCT) as a function of excitation energy. 561

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Figure 6. SERS spectra of PATP on Ag cavities by using (a) 514 nm, (b) 633 nm, and (c) 785 nm laser excitation: top spectra represent Ag cavities fabricated on silver thin film, bottom spectra on gold thin film. (d) Degree of charge transfer (PCT) as a function of excitation energy.

corresponding PCT value of PATP adsorbed on AgNPs monolayer. However, when we built Ag cavity arrays on gold thin films, an obvious decrease in the intensity of the b2 modes can be observed (bottom spectra of panels a−c in Figure 6), and the extent of this decrease became greater with the reduction of the laser energy. Therefore, the experimental results were fully consistent with what we had originally envisaged. As described above, this difference can be well explained by the difference of the work function between Au and Ag, and we attributed this decrease to the possibility that the Au thin film could lower the electron density of Ag cavities. Now, we can summariaze the relationship between charge density and the extent of charge transfer in a sentence, that is, the greater the surplus of electrons on the metal, the higher the extent of charge transfer from metal to the molecule.

promise for applications in theoretical study and analytical SERS. First, the cavity structure can be conveniently changed by the size and arrangement of the template PS particles; this assembly of AgNPs with modification of the silica cavity array may be used as a reproducible SERS substrate. Second, the cavities can also be functionalized as the host for the positively charged analyte, for example, methyl viologen cation, a kind of high-persistence pesticide. It is expected that the negatively charged silica cavity can enrich this kind of cationic phytocide and, as a result, lower the limit of detection.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic illustration of the procedures for fabricating AgNPs/ silica cavities and silver cavities; FE-SEM images of AgNPs/ silica cavities and silver hemisphere cavities. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSIONS In the present study, we have demonstrated the formation of a silver/silica cavity array using PS spheres as template. The array exhibits SERS enhancement for the adsorbed PATP molecules under the 514, 633, 785, and 1064 nm excitation. Furthermore, the b2 mode of the adsorbed PATP, which is a characteristic of chemical enhancement, is greatly enhanced by CT from the silver to PATP molecules. Through the introduction of the concept of degrees of charge transfer, we directly observed the additional chemical enhancement without a deliberate distinction between EM enhancement and chemical enhancement. We have demonstrated that the negatively charged silica cavity can alter the dipolar orientation of the AgNPs inside it, enlarging the electron density at the sites where probe molecules adsorbed. The interface dipole with its positive pole toward the metal decreases the Fermi energy of the metal, making it favorable for the charge transfer from metal to molecule, and in turn, a greater CT enhancement can be achieved. Given the relative ease of preparation and their reproducible nature, we believe that these surfaces have great

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Q.Z), [email protected] (J.Z). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Nature Science Foundation of China (Nos. 20975073, 20873089, 21173122) and Nature Science Foundation of Jiangsu Province (No. BK2010034) are gratefully acknowledged. This work was also supported by Scientific and Technological Innovation Projects of Nantong City (HS2012006).

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