Laser Power Dependent Surface-Enhanced Raman Spectroscopic

Laser Power Dependent Surface-Enhanced Raman Spectroscopic Study of 4-Mercaptopyridine on Uniform Gold Nanoparticle-Assembled Substrates...
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Laser Power Dependent Surface-Enhanced Raman Spectroscopic Study of 4‑Mercaptopyridine on Uniform Gold NanoparticleAssembled Substrates Xiao-Shan Zheng, Pei Hu, Jin-Hui Zhong, Cheng Zong, Xiang Wang, Bi-Ju Liu, and Bin Ren* State Key Laboratory of Physical Chemistry of Solid Surfaces, the MOE Key Laboratory of Spectrochemical Analysis & Instrumentation, and Department of Chemistry, College of Chemistry and Chemical Engineering and Collaborative Innovation Center of Chemistry of Energy Materials, Xiamen University, Xiamen 361005, China S Supporting Information *

ABSTRACT: A facile procedure was introduced to prepare uniform and highly active substrates assembled with gold nanoparticles for surface-enhanced Raman spectroscopy (SERS). The laser power and time dependent SERS study of 4-mercaptopyridine (4-MPy) adsorbed on this substrate was investigated. The relative intensity of the characteristic Raman peak pair (1575 and 1610 cm−1) was found to change with the laser power and the exposure time. The spectral changes may originate from change in the fraction of the double-end-bonded and the single-end-bonded states of 4-Mpy adsorbed on the gold nanoparticle-assembled substrate. In addition, this study offers a way to avoid the effect of laser power to achieve reliable spectral response to other factors.

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INTRODUCTION The adsorption of molecular monolayers of organic sulfur compounds on metal surfaces has aroused great interest due to the great importance in interfacial engineering for fundamental as well as practical research. Among the various molecules, 4mercaptopyridine (4-MPy) is of particular interest because 4MPy has bifunctional groups of sulfydryl (thiol) and pyridyl, which provide three possible ways for the interaction between 4-MPy and the metal surface, namely via the lone pair electrons of the S or N atom or via the π electrons. The surface modified with 4-MPy may display quite different chemical properties for future applications since different functional groups will be exposed in different binding modes. Therefore, investigation of the metal−adsorbate system of 4-MPy is of significance for revealing the structure (including the interaction mode and orientation) of the adsorbed species on the surface and lays the foundation for further applications in interfacial engineering and chemical sensing. Surface-enhanced Raman spectroscopy is a powerful technique for exploring metal−adsorbate interactions with high surface sensitivity and abundant molecular structural information. SERS studies of 4-MPy adsorbed on various metal surfaces have been reported in the past few years. These studies can be classified into the following three categories: (1) 4-MPy was as a probe molecule to demonstrate the uniformity and/or activity of a SERS-active substrate. The main reason is that it is an aromatic sulfhydryl compound with strong bonding interaction with most metals and presents strong and characteristic SERS signals.1−6 (2) Interactions of 4MPy with various metal surfaces, including Pt, Ag, Au, Cu, Pd, © 2014 American Chemical Society

semiconductors, and metal composites, were investigated to reveal the interaction mode and orientation.7−14 (3) Factors influencing the SERS behavior of 4-MPy were studied to understand the impact of the local environment on the SERS spectra of 4-MPy. Among the factors, potential and pH are the mostly investigated because both of them can influence the configuration of 4-MPy on the surface, which could result in changes in the SERS spectra and sensitively reflect the local environment.9,15,16 More recently, 4-MPy-functionalized nanoparticles were used as pH nanosensors to monitor the local pH environment of live cells.17,18 Although 4-MPy appears to be a well-studied system in SERS and has better stability than PATP and more effective pH responsive range than thiophenol,19−21 there are still some fundamental questions remained unanswered or even neglected. For example, how will the local environment influence the binding configuration of 4-MPy with the metal surface? Are there other factors besides the metal surface, potential, and pH which may also affect the SERS spectra? Why does the SERS feature obtained by different groups vary quite a lot? In this work, we developed a facile procedure to prepare uniform gold nanoparticle-assembled substrate, on which the laser power dependent SERS study of 4-MPy was investigated. Comparative SERS and tip-enhanced Raman spectroscopy (TERS) studies were conducted to understand the phenomReceived: September 30, 2013 Revised: January 21, 2014 Published: January 30, 2014 3750

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Figure 1. Schematic illustration of the procedure for preparing the gold nanoparticle-assembled substrate and the SERS sample.

Figure 2. Characterization of the gold nanoparticle-assembled substrate: (A) Dark-field image and (B, C, D) SEM images obtained with different magnification of 50 000, 25 000 and 10 000, respectively. (E) Raman spectrum of 4-MPy powder (blue, laser power: 2.5 mW, integration time: 10 s) and typical SER spectrum of 4-MPy adsorbed on a gold nanoparticle-assembled substrate (red, laser power: 0.05 mW, integration time: 10 s). (F) SERS mapping on the gold nanoparticle-assembled substrate (area 40 × 40 μm2, step size 2 μm) using the intensity of the 1095 cm−1 peak of 4-MPy. (G) The statistical histogram of all intensities in (F). The laser power was 0.2 mW, and the integration time was 1 s for each spectrum in (F).

under stirring and cooled to room temperature, resulting in nanospheres with a diameter of 30 nm and used as seeds. Then, monodispersed gold colloids were prepared by a seed-mediated growing method using hydroxylamine hydrochloride as the reductant.23 30 mL of 25 mM NH2OH·HCl aqueous solution and 30 mL of 2.5 mM HAuCl4 aqueous solution were added dropwise to 50 mL of the as-prepared colloid of 30 nm gold seeds successively for further growth to larger gold nanoparticles. The mixture was then remained stirring for 2 h at room temperature to react completely before used. The average size of the obtained spherical gold nanoparticles is approximately 60 nm in diameter to achieve a good balance between the SERS activity and stability of the sol. Preparation of Gold Nanoparticle-Assembled Substrate. The method for preparing the gold nanoparticleassembled substrate was slightly modified from the previously described method by our group and shown in Figure 1.24 The cover glass was cleaned ultrasonically in absolute ethanol and ultrapure water successively. The surface was then activated in a 3:7 (v/v) mixture solution of H2O2 (30%) and H2SO4 (98%) for 4 h. Subsequently, the cover glass was thoroughly rinsed with ultrapure water and dried in a stream of high-purity N2 gas. Afterward, the activated cover glass was immersed into the 0.2% (v/v) APTMS aqueous solution for 24 h to silanize the surface, followed by thorough rinsing with ultrapure water,

enon of the variation of the relative intensity in the SERS spectra. The result indicates the presence of the double-endbonded and single-end-bonded states of 4-MPy adsorbed on the gold surface. This study aims to emphasize the importance of controlling the laser power in the SERS measurement for a reliable spectral response to factors such as pH or potential.

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EXPERIMENTAL SECTION C h e m i c a l s a nd M a t e r i a ls . (3-Aminopropyl)trimethoxysilane (APTMS), 4-MPy, and 1-hexylthiol were purchased from Aldrich. Chloroauric acid (HAuCl4·4H2O), trisodium citrate dihydrate (C6H5Na3O7·2H2O), hydroxylamine hydrochloride (NH2OH·HCl), H2O2 (30%), H2SO4 (98%), and absolute ethanol were obtained from Sinopharm Chemical Reagent Co., Ltd. All the above chemicals were of reagent grade and used as received without further purification. Millipore ultrapure water (18 MΩ) was used throughout the experiments. Cover glass (22 × 22 mm) was purchased from VWR International, Inc. Synthesis of Gold Nanoparticles. The 30 nm gold nanosphere seeds were synthesized according to a modified Frens’ method.22 In brief, 100 mL of 0.01% (w/v) HAuCl4 aqueous solution was refluxed to boiling under vigorous stirring before 1.2 mL of 1% (w/v) sodium citrate aqueous solution was added quickly. The mixture was kept boiling for 30 min 3751

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Figure 3. Time evolution of the ratio of I1575/I1610 at different laser power of (A) 0.5, (B) 2.5, and (C) 5.0 mW. The corresponding time-series representative spectra at different exposure time at different laser power of (D) 0.5, (E) 2.5, and (F) 5.0 mW. The integration time for each spectrum was 50 ms. The red, green, and blue in (A), (B), and (C) represent the time-series measurements conducted at three different positions of the sample, which shows relatively good reproducibility. All the SER spectra were offset for clarity. The total number of spectra at each position was 100 with time lapse.

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drying in a stream of high-purity N2 gas, and baking at 110 °C for about 30 min. Finally, the cover glass was immersed in the above 60 nm gold colloidal suspension for 16 h for assembly of gold nanoparticles. The as-prepared substrate was kept in ultrapure water before used. For the SERS study, the substrate was first rinsed with absolute ethanol and then immersed in 1 mM 4-MPy ethanolic solution for 30 min. The 4-MPy adsorbed substrate was subsequently rinsed with absolute ethanol thoroughly and dried before the SERS measurement. Characterization. The morphology of the gold nanoparticle-assembled substrate was characterized using scanning electron microscopy (SEM) (Hitachi, S-4800) and dark-field microscopy (Leica, DMI 3000M). The normal Raman spectra and SERS spectra were collected on a modified Renishaw in Via Raman microscope equipped with a Leica DMI 3000M inverted microscope. A He−Ne 632.8 nm laser line was used for the measurement. The laser spot was approximately 2 μm in diameter on the sample by using a long working distance 50× objective (NA 0.55, BD). TERS measurement was performed in a home-built TERS setup, consisting of a home-built Raman optical fiber head, a nanoscope-E scanning tunneling microscope (Veeco), an optical fiber coupled Acton spectrometer, and a liquid nitrogen cooled, back-illuminated CCD detector.25 Gold tips with apex radius of ∼20 nm fabricated by the electrochemical etching were used for the TERS study.26 A 632.8 nm laser line was focused onto the tip−sample gap with a 2 μm spot through a 50× long working distance objective (NA = 0.45).

RESULTS AND DISCUSSION Preparation and Characterization of Gold Nanoparticle-Assembled Substrate. Gold nanoparticle-assembled substrate and SERS samples were prepared according to the procedure shown in Figure 1, and a detailed description is given in the Experimental Section. The morphology of the gold nanoparticle-assembled substrate was characterized by dark-field microscope and SEM. Figure 2A shows the reflective dark-field image over a large area (100 × 150 μm2) of the substrate. The high brightness indicates a high assembly density of the gold nanoparticle on the cover glass. The SEM images of the substrate under different magnification (Figure 2B−D) also indicate a high assembly density of the gold nanoparticle monolayer. Only a very small number of aggregates (bright white spots) can be found on the substrate in Figure 2D. SERS measurement was carried out to evaluate the SERS activity of the substrate. SER spectra of gold nanoparticleassembled substrate without 4-MPy did not show obvious signal that may interfere with that of the SERS of 4-MPy (see Figure S1-A). A comparison between the normal Raman spectrum of 4-MPy powder and the typical SER spectrum of 4MPy adsorbed on the gold nanoparticle-assembled substrate is shown in Figure 2E. As 4-MPy has C2v symmetry, its vibration modes can be classified into four symmetry types: a1, b2 modes (in-plane) and a2, b1 modes (out-of-plane). All of the modes are Raman-active although a1 and b2 modes tend to be stronger than the other two. Compared with the normal Raman spectrum, the SERS spectrum has broader peaks. Several typical 3752

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power (5.0 mW), the ratio increases markedly from about 0.8 to 1.4 within 10 s at the beginning and keeps constant at about 1.3 in the subsequent measurements. In addition, the intensities of the SER spectra under the three laser power decrease with the measuring time, and this trend is particularly obvious at a high laser power (see Figure 3F). Laser-induced desorption or decomposition of 4-MPy molecules under a high laser power may account for the overall decrease in intensity. Similar phenomena (both intensity ratio and absolute intensity change) were observed at different positions of the same sample (shown in different color in Figure 3) and samples of different batches (not shown). In the present experiment, no pH or potential control was applied to the surface. The change in the intensity ratio could not be understood by the effect of pH or potential. Instead, the above phenomena may reveal a laser power dependent change of the configuration of 4-MPy adsorbed on the gold nanoparticles, which, in turn, can lead to the spectral change of the characteristic Raman peak pair (1575 and 1610 cm−1). 4MPy has bifunctional groups, sulfydryl and pyridyl, and it may interact with the metal surface via the following three ways, namely the lone pair electrons of the S or N atom or the π electrons. Considering the molecular feature and inspired by the previous study of our group on the double-end-linked molecules at Au nanoparticles−Au film nanojunctions by SERS,28 we assume that the formation of double-end-bonded and single-end-bonded states of 4-Mpy on the gold nanoparticle-assembled substrate may lead to the change in the intensity ratio of the two peaks. To be explicit, the substrate has a high density of gold nanoparticles, and 4-MPy can be easily attached to the gap of nanoparticles to form the double-endbonded species. Therefore, there may be two configurations when 4-MPy is adsorbed on the surface: one is the single-endbonded state via the sulfydryl to form Au−S, and the other is the double-end-bonded state via both the sulfydryl and pyridyl to form a Au/4-MPy/Au nanojunction. Molecules with different adsorbed states show different SER spectra, which can be reflected by the spectral change of the characteristic Raman peak pair. To verify this assumption, we constructed two models to figure out the two interaction configurations: (1) a double-endbonded model using gap-mode SERS and (2) a single-endbonded model using TERS.28,29 It should be pointed out that in both configurations we used Au(111) single crystal as the substrates instead of the rough SERS substrates as have been used in the literature.30 Therefore, we are able to preserve a similar electromagnetic field distribution and a same surface selection rule for the Au NP/Au(111) and Au tip/Au(111) configurations. Furthermore, we can also avoid different adsorption behavior of molecules on the perfect single crystal surface and the rough SERS substrate with a lot of surface defects. Finally, the use of single crystal provides a flat and smooth substrate to avoid the double-end-bonded state before the coating of the gold nanoparticles. Figure 4A shows the scheme of the double-end-bonded sample of Au/4-MPy/Au nanojunction. A SAM of 4-MPy was formed by immersing the Au (111) single crystal into a 1 mM 4-MPy ethanolic solution for 2 h. The reason to use single crystal is to provide a flat and smooth substrate to avoid the double-end-bonded state before the coating of the gold nanoparticles. The Au (111) single crystal was treated according to the Clavilier method.31 After successive rinsing with ethanol and water to remove the physisorbed 4-MPy, the substrate was immersed in a sol

pyridyl-related Raman peaks such as 1002, 1060, 1095, 1212, 1575, and 1610 cm−1 were obtained with a high signal/noise ratio. Among them, the two characteristic Raman peaks (1575 and 1610 cm−1) are particularly important because 1575 cm−1 (8b2 ring stretching of pyridyl with deprotonated nitrogen) and 1610 cm−1 (8b2 ring stretching with protonated nitrogen) have been used as the marker bands for N-deprotonation and Nprotonation species, respectively. In this respect, 4-MPy has served as a pH-sensitive probe molecule in several previous SERS-based pH sensing studies.17,18,27 Although there are several peaks from 4-MPy other than 1575 and 1610 cm−1, we choose the pair of peaks at 1575 and 1610 cm−1 for our analysis because they are strong and located in a spectral region without too much interference. The most important is that they are sufficiently close so that we do not need to consider the temperature induced SPR position shift as a result of the possible change in the substrate nanostructure. A detailed analysis of these two bands will be discussed later on. Here, the strongest peak at 1095 cm−1 (ring breathing/C−S) was chosen to characterize the uniformity and SERS activity of the substrate. Figure 2F shows the SERS mapping of the substrate using the intensity of the 1095 cm−1 peak of 4-MPy. The mapping area is 40 × 40 μm2. The step size is 2 μm, equal to the laser spot on the sample. The brightest and darkest spots in the image represent the highest (9885 cps) and lowest (7310 cps) intensity in all the obtained spectra (see Figure S2), respectively. The difference is about 26%, very close to the requirement of SERS-active substrates for commercial use. Figure 2G shows the statistical histogram of all intensities from all the 441 spectra in Figure 2F. The result shows that the mean intensity is about 8510 cps, and the relative standard deviation (RSD) is only 5.35%. The excellent SERS enhancement and uniformity could be due to a compact structure with few aggregates or voids on the substrate surface. This facile method allows us to obtain a SERS-active substrate with a large area over a cover glass routinely, which is particularly important for the SERS analysis of live cells. Such kind of substrates also allows us to study dynamic changing systems in the following sections under well-controlled conditions. Laser Power Dependent SERS Spectra of 4-MPy Adsorbed on Gold Nanoparticle-Assembled Substrates. As aforementioned, the two Raman peaks appearing at 1575 and 1610 cm−1 are very unique in that they have been used as the marker bands for N-deprotonation and N-protonation in the literature. Because of this property, 4-MPy has been used as a probe molecule for SERS-based pH sensing by using the intensity ratio of these two peaks (I1575/I1610).17,18 However, in our study, we found that the ratio was influenced not only by pH but also by other experimental conditions. If the effect of these parameters can not be completely understood, it will hamper further application of this molecule for pH sensing. Parts A, B, and C of Figure 3 illustrate the time evolution of the ratio of I1575/I1610 at different laser powers of 0.5, 2.5, and 5.0 mW, respectively. The corresponding representative spectra at different exposure time for the three laser powers are shown in parts D, E, and F, respectively. It can be found the intensity ratio changes with the laser power as well as the exposure time. At a low power (0.5 mW), the ratio remains almost constant and the value is below 1. At a moderate power (2.5 mW), the ratio increases obviously with time during the whole period of measurement. The value changes from below 1 to above 1, whereas in the case of a high 3753

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Figure 4. (A) Schematic illustration of the double-end-bonded model based on SERS, and the sizes of molecule and the nanoparticles are not to scale. (B) Typical SERS spectra of the double-end-bonded 4MPy adsorbed on the gold surface: the black spectra were from five different positions on the same sample to show the reproducibility, and the red one is the average spectrum of these five spectra. The laser power was 0.3 mW, and the integration time for each spectrum was 10 s.

Figure 5. (A) Schematic illustration of the single-end-bonded configuration in TERS. (B) Typical TER spectra of the single-endbonded configuration; the black spectra were from three different positions on the same sample to show the reproducibility, and the green one is the average spectrum of the five spectra. The laser power was 0.7 mW, and the integration time for each spectrum was 60 s. The bias potential of the tip and the tunneling current were 200 mV and 1.0 nA, respectively.

containing 60 nm gold nanoparticles for 2 h to form a Au/4MPy/Au sandwiched structure. The double-end-bonded molecules stay right at the hot spot and will experience the highest enhancement, whereas some of the single-end-bonded molecules are in close vicinity of the double-end-bonded species and can still feel the enhanced electromagnetic field and contribute to the detected signal, as shown in Figure 4A. Therefore, in this configuration, we can observe the signal of the double-end-bonded species in the background of the singleend-bonded species.28 Figure 4B shows the SER spectra obtained at different positions of the above sample system. The average spectrum of the five points shows two characteristic Raman peaks at 1575 and 1610 cm−1, and the spectral feature is similar to that obtained from the gold nanoparticle-assembled substrate, especially that at a low laser power. To make a clear assignment of the two peaks at 1575 and 1610 cm−1, we performed TERS measurement of the single-end-bonded species. In TERS, the tip is controlled at a distance from the sample to avoid any physical or chemical interaction of the tip with the molecule, so that only single-end-bonded configuration can be formed and the formation of double-end-bonded species can be completely avoided (Figure 5A). To do this, we immersed the Au (111) single crystal into a 1 mM 4-MPy ethanolic solution for 10 min to obtain the single-end-bonded 4-MPy-coated substrate. The TERS measurement was performed using an electrochemically etched gold tip with a radius of ∼20 nm. Figure 5B shows the TERS spectra obtained from the singleend-bonded model. The average spectrum (in green) shows

that the peak at 1610 cm−1 is dominant, which is quite different from that of the double-end-bonded model. Similar structures of the single-end-bonded and double-endbonded model have been proposed by Cao et al.32 They gave schematic diagrams for single-end-bonded and double-endbonded species adsorbed on the gold surface, respectively. However, the SERS spectra of the single-end-bonded species also contain the contribution of double-end-bonded features. The reason may be that the surface used for single-end-bonded sample was gold electrode with a nanoscale roughness, on which double-end-bonded species will inevitably form on the surface. It can be seen from the model in Figure 4A that the signals obtained in this system are contributed by both the single-endbonded and the double-end-bonded 4-MPy, and the signals from Figure 5A are almost completely originated from the single-end-bonded 4-MPy. A difference spectrum, by subtracting the average spectrum of single-end-bonded 4-MPy from that of the double-end-bonded 4-MPy by a suitable scaling of the latter, may manifest the spectral feature of the double-endbonded species. As is shown in Figure 6, the peak at 1575 cm−1 can serve as the marker band for double-end-bonded 4-MPy adsorbed on the gold surface. Thus, we can conclude from the above discussion that 1575 cm−1 can be considered as the marker band for double-endbonded 4-MPy on the gold surface and 1610 cm−1 is the marker band for single-end-bonded species. It means that the ratio of these two peaks (I1575/I1610) can reflect the fraction of two types of species on the gold surface. With this understanding, we proposed a scheme (Figure 7) to explain 3754

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Figure 6. Difference spectrum (blue) by subtracting the average spectrum of the single-end-bonded 4-MPy (green) from that of the double-end-bonded 4-MPy (red).

the phenomenon of laser power dependent SERS spectral change observed in Figure 3. When 4-MPy is adsorbed on a gold surface with nanoscale roughness, some of the molecule will be in the single-end-bonded state while some in the doubled-end-bonded state. At a low laser power, the molecules remain stable in their original state on the gold surface (Figure 3A). With the increase in the laser power and due to the thermal movement, some single-end-bonded molecules will migrate on the surface and move to the nanojunction between the gold nanoparticles and be trapped in the double-endbonded state. As a result, the ratio of I1575/I1610 increases with the lapse of the exposure time (Figure 3B). At a high laser power, the surface movement of the single-end-bonded molecules is much faster, and they can be quickly trapped into the double-end-bonded state. However, since the surface sites (nanojunction) for the formation of double-end-bonded molecules are limited, there should be an equilibrium between the two states, and the change of the intensity ratio will be saturated with the further increase in the illumination time (Figure 3C). At a very high laser power, due to the SPR induced thermal effect, 4-MPy molecules will be desorbed from the gold surface and some gold nanoparticles will even be merged together, resulting in an overall decrease in SERS intensity (Figure 3F).33 The hypothesis of laser-induced thermal movement was verified by an in situ temperature dependent experiment. First, the 4-MPy adsorbed gold nanoparticle-assembled substrate was placed in a homemade Raman cell. Then, the temperature of the cell was elevated to 26 and 70 °C. To avoid the laserinduced thermal effect, a very low laser power (0.02 mW) was used for the measurement. Figure 8A illustrates the time evolution of the ratio of I1575/I1610 at 26 and 70 °C, respectively. The corresponding average spectra at different temperatures are

Figure 8. (A) Time evolution of the ratio of I1575/I1610 at 26 °C (square point) and 70 °C (circular point). (B) The corresponding average spectra at 26 °C (solid line) and 70 °C (dashed line). The laser power was 0.02 mW, and the integration time for each spectrum was 10 s. The red, green, and blue in (A) and (B) represent the measurements conducted at three different positions of the sample, which shows relatively good reproducibility. The SER spectra were offset for clarity.

shown in Figure 8B. It can be found that the intensity ratio remains almost constant with little fluctuation at a same temperature, whereas the ratio increases obviously when the temperature was changed from 26 to 70 °C. The result indicates that the mechanism of the thermal induced migration of molecules is reasonable. Since we have discovered and proved the laser power dependent SERS spectral change of 4-MPy, it is crucial to find ways to avoid the laser-induced effect, which will allow us to investigate systematically and reliably the influence of other factors, such as pH and potential. Obviously, one way is to choose a laser power as low as possible in the SERS measurement and meanwhile increase the integration time to obtain spectra with high signal/noise ratio. The molecules

Figure 7. Schematic illustration of the laser power dependent molecular reorientation of 4-MPy adsorbed on the gold nanoparticle-assembled substrate. 3755

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S1B). Parts B, C, and D of Figure 9 illustrate the time evolution of the ratio of I1575/I1610 at different laser powers of 0.5, 2.5, and 5.0 mW, respectively. The corresponding time-series spectra can be found in Figure S3. At a low laser power (0.5 mW), the intensity ratio remains at a low value of about 0.6. At a moderate power (2.5 mW), the intensity ratio changes from about 0.6 to 0.9 gradually. At a high power (5.0 mW), the intensity ratio increases slightly at the beginning and then stays at about 1.0 in the remaining time. Compared with the case without template molecules shown in Figure 3, similar trends are observed, but both the rate and the extent of spectral change have been decreased after the introduction of the template molecules. Interestingly, all the values of intensity ratio at the three laser power in the presence of template molecules shown in Figure 9 are lower than that in the absence of template molecules shown in Figure 3. In our previous discussion, we have concluded that 1575 cm−1 can be used as the marker band for double-end-bonded 4-MPy on the gold surface and 1610 cm−1 for single-end-bonded 4-MPy. Therefore, a low intensity ratio of I1575/I1610 indicates the predominant configuration in the single-end-bonded state. All the above discussions imply that the impact of laser power could be mitigated effectively when the template molecules are used as expected.

could remain stable even in a long-term measurement at a relatively low power as shown in Figure 3A. Another way is to confine the adsorbed state of the 4-MPy molecules. On the basis of this concept, we proposed a template moleculemediated model depicted in Figure 9A. The gold nanoparticle-

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CONCLUSION In summary, we have developed a facile procedure to prepare large-scale gold nanoparticle-assembled substrates with uniform and high SERS activity, on which the laser power and time dependent SERS study of adsorbed 4-MPy was performed systematically. We constructed the double-end-bonded 4-MPy model by dispersing gold nanoparticles over a 4-MPy adsorbed Au single crystal surface and the single-end-bonded 4-MPy model in TERS configuration. A comparison of the two models was made, and the results well explain the phenomenon. The 1575 and 1610 cm−1 peaks can be used as the marker bands for the double-end-bonded state and the single-end-bonded state of 4-MPy on the gold surface, respectively. The fraction of the two states can be reflected by the ratio of I1575/I1610. Ways to avoid the influence of laser power were also proposed, which is crucial for the reliable spectral response to other factors such as pH or potential. We anticipate that the large-area gold nanoparticle-assembled substrate with adsorbed 4-MPy can provide reliable pH response under the improved experimental conditions and has the potential to serve as a reliable pH sensitive substrate for extracellular pH analysis of live cells. The related work is now underway.

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Figure 9. (A) Schematic illustration of the template molecules mediated model and time evolution of the ratio of I1575/I1610 at different laser power of (B) 0.5, (C) 2.5, and (D) 5.0 mW. The integration time for each spectrum was 50 ms. Each color (red, green, or blue) of (B), (C), and (D) represents one different position of the same sample to show the reproducibility.

ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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assembled substrate was immersed into a 0.1 mM 1-hexylthiol solution in ethanol for 15 s to form a template molecule-coated surface prior to the 4-MPy adsorption. Then, the adsorbed state of 4-MPy molecules will be confined to the interspaces of the molecular template so that the movement of 4-MPy on the surface is hindered. SER spectra of template molecule-coated gold nanoparticle-assembled substrate without and with 4-MPy clearly indicated that both the APTMS and template molecule will not interfere with the SERS signal of 4-MPy (see Figure

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (B.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the MOST (973 Program nos. 2013 CB933703 and 2011YQ03012406), NSFC 3756

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The Journal of Physical Chemistry C

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(21021120456, 21227004, and 21321062), and Fundamental Research Funds for the Central Universities (2010121019).

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dx.doi.org/10.1021/jp409711r | J. Phys. Chem. C 2014, 118, 3750−3757