Article pubs.acs.org/JPCC
Tailoring the SERS Enhancement Mechanisms of Silver Nanowire Langmuir−Blodgett Films via Galvanic Replacement Reaction Nathan L. Netzer,† Zuki Tanaka,‡ Bin Chen,‡ and Chaoyang Jiang*,† †
Department of Chemistry, University of South Dakota, 414 East Clark Street, Vermillion, South Dakota 57069, United States Advanced Studies Laboratories, MS 245-3 NASA Ames Research Center, Moffett Field, California 94035, United States
‡
ABSTRACT: We report a new facile route for preparing surface-enhanced Raman scattering (SERS) substrates with tailored enhancement mechanisms. Silver nanowires were assembled using the Langmuir−Blodgett (LB) technique and further processed via galvanic replacement reactions (GRRs). The GRRs provided an efficient method to decrease the spectral noise caused by the capping agent polyvinylpyrrolidone. A decrease in noise along with the addition of gold nanostructures to the system revealed Raman signals from nonfluorescent molecules associated with a charge-transfer mechanism. The GRR LB substrates exhibited ultrasensitive SERS ability with a detection limit as low as 8 nM using 4aminothiophenol, partially due to the strong chemical binding between the SERS substrates and probe molecules. Furthermore, the GRRs provide a facile route in tailoring SERS substrates to target molecules in a controlled manner.
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INTRODUCTION Over the past few decades, surface-enhanced Raman scattering (SERS) has generated increased interest due to its ability in detecting extremely low concentrations of analytes.1−5 The SERS technique has been utilized in a variety of fields ranging from space exploration6 and chemical warfare agent detection7 to biomedical sensing8 and conservation science.9 By providing fingerprint information at a molecular level, SERS has proven to be extremely useful for improving safety by early detection of cancerous or hazmat materials.10−12 Current SERS research focuses on, but is not limited to, novel nanostructures and substrates,13,14 biological sensitivity,11,15,16 and organic material and mineralogy detection.17−19 The SERS research field is an innovative discipline that moves forward at a very fast pace and is frequently in search of new materials, shapes, and assembly processes that will generate the best SERS enhancements. In its infancy, SERS was commonly thought to be a coincidence of disparate effects produced by two mechanisms: the electromagnetic or physical mechanism (EM) and the chemical mechanism (CM).5,20 In 2008, Lombardi and Birke presented a unified expression for the resonances that generate the overall enhancement displayed in SERS.21 In their work, they described that the SERS enhancement is caused by three resonance processes including surface-plasmon resonances, charge-transfer resonances, and molecular resonances. Those three processes can be unified in a single expression with each resonance denominator contributing in a multiplicative fashion coupled together by terms in the numerator. This indicates that SERS is not simply a coincidence of several unrelated effects but can be considered as a single effect that invokes up to three © 2013 American Chemical Society
resonances. Those resonances are intimately tied together and cannot easily be considered separately. The surface plasmon resonance (SPR) in metallic nanostructures, previously described as the EM, is believed to be a major contributor to the SERS process.20 Such enhancement is brought forth by an amplification of the electromagnetic field, which increases the overall intensity of the electromagnetic wave near the nanomaterial’s surface as a result of the SPR.22−24 The structure and morphology of metallic nanomaterials play important roles in the resonance condition of the surface plasmon. At locations of nanostructure gaps, fissures, and tips, SPRs can generate hot spots with superior electromagnetic fields, thus resulting in significant enhancement in Raman scatterings. A SERS enhancement factor (EF) for such hot spots can be on the order of 1011, as reported in 2000 by Xu and co-workers.25 Such a large SERS EF is best suited for testing physisorbed species since the enhanced electromagnetic field surrounds the nanostructures and is extended out for a few nanometers. For chemisorbed species, additional SERS enhancement can be achieved via charge-transfer resonances between the probe molecules and the SERS substrates, which was previously termed as the CM.26 Extensive calculations previously determined that the maximum SERS enhancement caused by the CM is around 103 in some early reports. However, more recent computational and experimental studies have proposed that the enhancement of the CM can be as large Received: June 11, 2013 Revised: July 11, 2013 Published: July 11, 2013 16187
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as 5−7 orders of magnitude.26−28 Computational approaches have not only made significant contributions to enhancement factor calculations but have also contributed largely in predicting and understanding the localized electromagnetic fields, assemblies, morphologies, and size of nanostructures that play pivotal roles in generating large SERS signals.25,29 It should be noted that in both physisorbed and chemisorbed species there will also be the third type of enhancement, molecular enhancement, which can also play a critical role in the overall Raman scattering in SERS studies. Many research groups have successfully demonstrated the ability to manufacture highly sensitive SERS substrates allowing trace chemical detections and even single molecule detections.30,31 For example, Tao and co-workers used Langmuir− Blodgett (LB) films of silver nanowires (Ag NWs) to detect fluorescent Rhodamine 6G (R6G) molecules at a concentration of 10−10 M.30 In their work, the ligand 1-hexadecanethiol was used to facilitate the LB assembly of Ag NWs. However, the 1hexadecanethiol also exhibited SERS signals with an EF of 105, which could cause unwanted spectral lines (noise) when trying to detect trace amounts of nonfluorescent molecules. The same issue exists in a variety of SERS substrates containing assembled nanostructures. However, the relative intensities of the spectral lines are strictly governed by the selection rules of an intensity borrowing mechanism, which are induced by vibronic couplings. As discussed by Lombardi and Birke, it is important to note that the combination of the transition moments, Herzberg−Teller coupling, and the direction of the plasmoninduced electric field place restrictions on the nature of the spectrum and the symmetry of the normal modes.21 The appearance of nontotally symmetric lines in the spectrum displays a clear indication of the charge-transfer contribution; whereas totally symmetric lines can arise from charge-transfer contributions as well as pure surface plasmons. How to reduce the spectroscopic noise, utilize these restrictions, and prepare a clean SERS substrate is a bottleneck in the development of novel SERS detection. To the best of our knowledge, there is little if any literature that focuses on the tailoring of the SERS substrates to a specific resonance process or enhancement mechanism while decreasing the spectral noise caused by surrounding molecules. In this article, we utilize a simple room temperature reaction to convert a SERS substrate that employs the EM or surfaceplasmon resonances to obtain the majority of the enhancement into one that predominantly utilizes the CM or charge-transfer resonances, while decreasing spectral noise from the organic capping molecules. It is believed that in both cases the molecular resonances will be a factor in the overall SERS enhancement but is not instrumental to the desired tailoring ability of the SERS substrates. Here, we would like to show the possibility to tailor various substrates and obtain optimum enhancements by using the galvanic replacement reactions (GRRs) and monitoring the molecular vibrations of the capping molecules (PVP) and probe molecules (R6G and 4aminothiophenol, 4-ATP). We demonstrate the ability to identify both SERS enhancement mechanisms (surfaceplasmon resonances and charge-transfer resonances) and increase the signal-to-noise ratio from the resulting SERS substrates in a more confident manner. Our designed SERS substrates exhibited remarkable reproducibility in detecting nonfluorescent 4-ATP molecules at a concentration as low as 8 nM, which is several orders of magnitude better in the limit of
detection (LOD) as compared to other commonly reported SERS substrates.
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EXPERIMENTAL METHODS All the chemicals used for experimentation were commercially available and used as received without any further purification. Ethylene glycol, silver nitrate, polyvinylpyrrolidone (PVP, FW = 1,300,000), sodium chloride, chloroform, chloroauric acid, and Rhodamine 6G were all purchased from Acros. 4Aminothiophenol was purchased from Sigma-Aldrich. Deionized (DI) water (18.2 MΩ·cm) was obtained from a Barnstead diamond nanopure system. Acetone and methanol were purchased from Fisher Scientific. All glassware was washed using a royal solution (3:1, HCl:HNO3) and then rinsed thoroughly with DI water. Solvothermal Synthesis of Ag NWs. A solvothermal method was used to prepare silver nanowires according to a known procedure.32 Briefly, 4.6 mg of sodium chloride and 1.776 g of PVP were added to 60 mL ethylene glycol (solution A), which was then kept under vigorous stirring at a slightly raised temperature (45 °C). Meanwhile, solution B was prepared, which contained 0.68 g of silver nitrate in 40 mL ethylene glycol. Once both solutions appeared homogeneous; solution A was added dropwise to solution B under vigorous stirring. The final solution turned from transparent to an opaque one. The solution was allowed to mix for an additional five minutes and then transferred to a 125 mL Teflon-lined autoclave, which was then kept at 160 °C for six hours. The autoclave was allowed to naturally cool to room temperature and a dark gray suspension was obtained. The resulting product was washed at-least four times using a precipitation and redispersion process. The purified Ag NWs were stored in methanol until further usage. Langmuir−Blodgett Assembly of Ag NWs. A solvent exchange method was used to disperse Ag NWs in chloroform. In brief, the as synthesized Ag NWs were first precipitated by high speed centrifugations. The precipitation was then redissolved in chloroform with the aid of sonication. After the solvent exchange, the Ag NWs were ready to be used in the LB experiments. Glass and silicon substrates were cleaned using a piranha solution (a mixture of 3:1 concentrated sulfuric acid and 30% hydrogen peroxide). These substrates were rendered hydrophilic after cleaning and can be used as solid substrates for the deposition of the Ag NW monolayer in the LB experiment. All the LB experiments were performed using a KSV 2000 system equipped with a Teflon trough that has a total surface area of 870 cm2 (L580 × W150 × D9 mm3), a dipping well (L37 × W117 × D90 mm3), total subphase volume of 1172 mL, and a platinum Wilhelmy plate. In a typical experiment, 3.5 mL of Ag NW solution was spread dropwise onto the water subphase. The system was then left alone for 30 min to allow full evaporation of the solvent and equilibrium of the surface pressure. To obtain the surface pressure−area (π−A) isotherm, Teflon coated barriers are used to compress the Ag NWs at the air−water interface with a speed of 2 mm/min. Such compression will proceed until the collapse point is reached. To prepare the Ag NW LB films on solid substrates, the Ag NW monolayers were transferred at the desired surface pressure (45 mN/m) using a robotic dipper. Galvanic Replacement Reaction of Ag NW LB Films. GRRs of Ag NWs were simply conducted at room temperature by dropping a solution of chloroauric acid onto the LB films. 16188
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Figure 1. (a−e) SEM micrographs and (f) Raman spectra of the Ag NW LB films as the GRR proceeds with time. All the image scale bars are 500 nm.
Briefly, 50 μL of chloroauric acid (0.5 mM) was dropped onto the Ag NW LB films via a pipet. The solution was then allowed to react with the silver nanowires on the substrates for a desired amount of time. After that, the substrates were rinsed using copious amounts of water to ensure the termination of the GRR process. Characterization and SERS Measurements. The scanning electron microscopy (SEM) measurements on the morphology of Ag NWs were carried out using a Hitachi S4800 field emission scanning electron microscope with a secondary electron detector. Confocal Raman images and SERS spectra were obtained using an Aramis confocal Microscope (Horiba Jobin Yvon) equipped with a 50× objective (NA = 0.75), a diode-pump solid state laser operating at a wavelength of 532 nm (0.40 mW), and a CCD detector outfitted with electrical cooling. The samples were raster scanned with an integration time of 1 s at each location, and the intensity ratios of Raman peaks at 1394 and 1437 cm−1 were utilized to form the Raman ratio maps. We also utilized a modified microRaman Imaging Microscope (Renishaw, Inc.) that is equipped with an excitation of 632.8 nm (0.15 mW on the sample) from a helium−neon laser in the Raman mappings.
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fabrication, the Ag NW LB films were ready for modification using a room temperature galvanic replacement reaction. The pristine LB films were composed of well-defined smooth Ag NWs as shown in Figure 1a. Then the Ag NW LB films were modified by the galvanic replacement reaction, which resulted in significant changes to the morphology of the silver nanowires. SEM micrographs shown in Figure 1 clearly illustrates that with prolonged reaction time, the surface roughness of silver nanowires increases. As discussed in the literature, the galvanic replacement reaction is an efficient way in producing alloys that consist of Au−Ag bimetallic nanostructures.33,34 The replacement reaction or corrosion process of Ag NWs can be explained by the stripping of electrons from the elemental silver via oxidation. After the release of electrons has occurred, the AuCl4− ions capture the migrating negatively charged electrons to generate gold atoms via a reduction reaction. With the GRR process, the smooth surface of the Ag NWs became rough due to the addition of nanostructures (Figure 1b). The etched locations and growth of nanostructures are more pronounced with longer GRR time. As shown in Figure 1c,d, the Ag NWs not only have increased roughness on their surface but also have some etched locations that can be clearly observed. These etched locations are the outcome of chloroauric acid converting surface elemental silver into to silver ions.35 Such changes in morphology can be further observed with even longer etching time and can play important roles in impacting the SERS performance. Typical Raman spectra for the as-prepared Ag NW LB films and GRR modified LB films are shown in Figure 1f. The Raman signals obtained from both types of LB films can be attributed to the capping agent, PVP. The corresponding vibrational modes of PVP have been previously studied, and the spectral features we observed are consistent with the literature data.36 The decreased intensity in the PVP SERS spectra was observed for the GRR modified LB films. Such decrease can be attributed to the nanostructure changes due to the galvanic reaction, during which the chloroauric acid will etch elemental silver on
RESULTS AND DISCUSSION
Our method provides a simple and novel approach in fabricating SERS-active substrates from well-assembled arrays of silver nanowires. In sample preparation, Ag NWs were first uniformly packed using the LB assembly, a same approach reported by Tao and co-workers.30 The SEM micrograph (Figure 1a) displays the typical morphological characteristics for LB films obtained at high surface pressure (45 mN/m), with a well-defined longitudinal alignment for Ag NWs in the LB films. The Ag NW π−A isotherm displayed a typical three phase (gas, liquid, and quasi-solid state) fatty-acid type curve with a collapse pressure around 50 mN/m. After the LB 16189
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Figure 2. SERS spectra obtained from Ag NW LB films and GRR LB films of probe molecules (a) fluorescent, R6G, and (b) nonfluorescent, 4-ATP.
Figure 3. SERS spectra of (a) as-fabricated LB films before and after the addition of 4-ATP, (b) Ag NW LB film as the GRR proceeds, and (c) 1437:1078 cm−1 and 1437:1394 cm−1 ratio trends as GRR time increases, respectively.
and that the etching process somehow actually causes a reduction of the pre-existing hot spots due to the structural damage of the silver nanowires. The SERS experiments of nonfluorescent probe molecule, 4ATP, have demonstrated interesting spectroscopic features when the Ag NW LB films were treated with the GRR process. Figure 2b shows the typical SERS spectra of 4-ATP acquired from LB films before (8 × 10−6 M of 4-ATP) and after (8 × 10−9 M of 4-ATP) GRR process, respectively. While signature peaks from 4-ATP could be observed in the spectra taken from both substrates, spectral interference within the Ag NW LB substrate from PVP caused spectral noise to be large enough to generate uncertainty in the molecular identification at trace concentrations. However, 4-ATP is a unique Raman probe in evaluating the SERS substrates, due to its unique Raman asymmetric mode at 1437 cm−1 (marked by green arrow in Figure 2b). This nontotally symmetric b2 mode can indicate the dominance of the CM by contributions from charge-transfer resonance or dimerization.36−38 That peak was clearly observed only after the GRR processing of the Ag NW LB films, indicating the existence of charge-transfer resonance (a CM mechanism) on 4-ATP molecules brought forth by the GRR process of silver nanowire LB films. Le Ru and co-workers displayed a similar evaluation of the charge-transfer resonances for the crystal violet molecule, which the nontotally symmetric e modes were responsible for a majority of the most enhanced spectral lines.39 Spectral interference is a very critical and practical issue in SERS application for ultrasensitive molecular detection. Our results clearly show that the measurements of nonfluorescent
nanowire surface and form gold (and gold/silver alloy) nanostructures along the nanowires.35 With such etching process, some PVP can be delaminated from the nanowires and be removed during the washing step. Chen et al. reported similar work with the preferential etching of truncated Ag nanocubes to form Au−Ag nanocages.34 It is evident from Figure 1f that increased GRR time causes a decrease in the signal intensities of the PVP molecules. Such decrease might also be related to the loss of Raman hot spots and will be discussed below. Nevertheless, reductions in SERS signals of residual polymers are quite favorable in considering the SERS detection for trace amounts of probe molecules. The Ag NW LB films, both before and after GRR process, exhibit strong SERS ability in detecting the fluorescent R6G molecule at a concentration as low as 10 ppb. As shown in Figure 2a, typical R6G SERS peaks can be clearly observed on both types of LB films at this low concentration, as the results of the existence of the Raman hot spots from the LB assembly of silver nanowires.30 It should be noted that the SERS intensity of R6G from the GRR LB films is lower than that of Ag NW LB films before the GRRs. This was initially hard to understand since we did observe a dramatic enhancement of SERS activities on individual silver nanowires with GRR process.35 Overall, we believe that the observation should be evaluated by the effectiveness of the individual hot spots and their percentages in the entire substrates. For the case of individual Ag NWs, the GRR does increase the density of Raman hot spots. However, for Ag NW LB films, there were already an adequate number of Raman hot spots throughout the films. We think that GRR will not necessary add additional hot spots to the Ag NW LB films 16190
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Figure 4. Optical images, Raman intensity ratio map (1437:1394 cm−1), and statistical data for the mapped area for (a−c) Ag NW LB film with 10−6 M 4-ATP and (d−f) GRR LB film with 8 × 10−9 M 4-ATP, respectively. All the images are 30 × 30 μm.
factors: (i) decrease in spectral noise caused by the capping agent PVP and (ii) chemical enhancement of 4-ATP molecules to gold and silver nanostructures formed during GRR treatment. Such chemical enhancement can be caused by either a photoinduced charge transfer between 4-ATP molecules and substrate or the dimerization of 4-ATP molecules on the metal surface. In the following sections, we will focus on specific Raman peaks at 1078, 1394, and 1437 cm−1, which are marked by red, blue, and green arrows in Figure 3, respectively. Indicated by the red arrow in Figure 3a, the 4-ATP Raman signal at 1078 cm−1 can be confidently identified in both types of the SERS substrates. This signal (1078 cm−1) has previously been assigned to an a1 in-plane vibrational mode, which is known to dominate the normal Raman spectra, as well as become highly enhanced from the EM mechanism.37 Therefore, by comparing the peak ratio intensities of 1437 and 1078 cm−1 (Figure 3c top), a specific correlation between the EM and CM mechanisms (or say the SPR resonance and charge-transfer resonance) is possible, thus a better understanding of the enhancement mechanisms can be achieved as the GRR time proceeds. It is clearly noticed that the 1437:1394 cm−1 peak ratio is nearly 2 orders of magnitude larger, as well as the 1437:1078 cm−1 peak ratio is about 1 order of magnitude larger for the GRR LB films when compared to the LB films before GRR process. The statistical data in Figure 3c gives an asymptotic type fit to both ratios of 1437:1078 and 1437:1394 cm−1. The peak at 1437 cm−1, indicating the onset of a greater contribution from chemical enhancement mechanism (i.e.,
molecule 4-ATP on the Ag NW LB substrates resulted in large spectral noise (Figure 2b, red line), making it extremely difficult to confidently identify the probe molecules. Spectral determination of 4-ATP, as well as other organic molecules and biomolecules, is thus challenging because many of the characteristic Raman signals from the probe molecule overlap with Raman signals from the residual PVP, the capping agent. The vibrational modes at 1004, ∼1390, and ∼1576 cm−1 are significant Raman signals for both PVP polymers and 4-ATP molecules since their chemical structures include carbon rings as well as groups containing nitrogen and oxygen atoms. A detailed investigation on the SERS spectra of Ag NW LB films, before and after the GRR process, with and without probe molecules, can provide a much clearer understanding on the peak assignments of the Raman scattering in the spectra. Because of a large background noise generated by the PVP capping agent of Ag NWs, only two Raman signals of the probe molecule 4-ATP (1078 and 1183 cm−1) can be confidently identified in the Ag NW LB + probe spectrum in Figure 3a, whereas after the GRR process, the GRR LB film exhibits a much less number of noticeable PVP Raman peaks. There are only four peaks at 1007, 1134, 1240, and 1394 cm−1 in the spectra, which causes a dramatic increase in the signal-to-noise ratio when distinguishing the probe molecules’ signature peaks. Signature 4-ATP peaks of 1078, 1145, 1437, and 1579 cm−1 are expressed in the spectra obtained from the GRR LB film and can be confidently identified in the GRR + probe spectrum (Figure 3a). The increase in the characteristic 4-ATP peaks displayed in the GRR LB films can be possibly attributed to two 16191
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displays a Gaussian type fit, centered at a ratio of ∼1.1 with 97% of the total GRR LB area having intensity ratios above 0.8. The different SERS characteristics on Ag NW LB films before and after the GRR process could be the result of nanoscale structural variations caused by the GRR process. In the as-prepared LB films, the Ag NWs are coated with the PVP capping agent, and there are very few bare sites for the direct attachment of 4-ATP molecules. Therefore, the two appearances of SERS peaks at 1078 and 1183 cm−1 can be due to the physical enhancement and the SPR resonance. A similar explanation can be used for the absence of the 1437 cm−1 peak since that is a characteristic signal from a chemical enhancement. With GRR process, the amount of PVP is significantly reduced, and meanwhile, gold and gold/silver alloy nanostructures are formed on the surface of silver nanowires. Such nanostructures can dramatically increase the number of binding sites for 4-ATP to chemically attach to the SERS substrates, thus increasing the contribution of the chemical enhancement and charge-transfer resonance in the overall SERS performance. Our results on SERS spectra with GRR processed Ag NW LB films allow us to get a better insight into the characteristics of substrates. The Raman mapping reveals the specific vibrational mode at 1437 cm−1 after the GRR process of LB films. Furthermore, we have shown that unmodified Ag NW LB films exhibit large amounts of fluorescence background noise. Such spectral noise must be lowered for the trace detections of nonfluorescent molecules. Our proposition is that Ag NW LB films display a majority of EM type enhancement mechanism and that the GRR LB films exhibit much more CM-type mechanism in overall SERS activities. There are two simple yet straightforward facts supporting such a mechanistic proposal. First, PVP must still be coating the Ag NWs during the LB deposition to allow them buoyancy and the ability to “float like logs on a river” at the air/water interface. This residual capping agent can cause hindrance for probe molecules to chemically attach to the substrate, thus resulting in an extremely low 1437:1394 cm−1 ratio in SERS mapping of the Ag NW LB films. Second, the gold and gold/silver alloy nanostructures, as well as the etched locations along the Ag NWs, facilitate plenty of surface active sites for the 4-ATP molecules to bind, therefore increasing chemical binding and peak ratio of 1437:1394 cm−1. We believe that the most intriguing and promising aspect of these findings is the ability to use the simple GRR method to tune the substrates between various enhancement and resonance conditions. As far as we know, this is one of few studies that concentrate on understanding and tuning the SERS mechanisms, which can significantly advance our knowledge and allow the fabrication of novel SERS substrates for detections of various chemicals and biomolecules.
charge-transfer resonance), contributes to the SERS signals with time until the GRR reaches around 10 min on such type of SERS substrates. After 10 min, little chemical enhancement can be further gained for the overall SERS improvement. A more detailed study on the change of spectra along the GRR time was conducted and a better understanding can thus be achieved on the SERS peak assignment and enhancement mechanism. Figure 3b shows a series of SERS spectra of 4-ATP on Ag NW LB films with various time of GRRs. While the overall spectral intensities were generally decreasing with the longer GRR treatment, the relative intensities of several peaks can provide more detailed information. The ratio shown in Figure 3c bottom correlates the 1437 cm−1 peak in which its onset has previously been described as a chemical attachment to the substrate,37 with the 1394 cm−1 peak, which is chemically specific to both PVP and 4-ATP. As we know, the SERS peak at 1437 cm−1 has extensively been studied36,40 and can be due to either a b2 vibrational mode expressed by a photoinduced charge-transfer resonance between the 4-ATP molecule and substrate or the dimerization between 4-ATP molecules that generates an ag vibrational mode of the p,p'-dimercaptoazobenzene molecule. Furthermore, the increase of the b2 mode at 1437 cm−1 is a clear sign that the Fermi level of the Ag (4.3 eV) NW LB films was raised after the addition of gold (5.0 eV) to the system, which brought the overall Fermi level of the alloy closer to resonances with the charge-transfer transition, as previously identified by Osawa et al.37 In either case, the 4-ATP molecule needs to be chemically bound to the substrate to exhibit enhancement by these mechanisms. It is noteworthy that the GRR process can also cause a redshift in surface-plasmon resonance of the LB films, placing the SPR closer to the resonance of the excitation sources (532 and 633 nm), which would indicate surface-plasmon resonances and EM. However, it is quite unlikely to be counted as the main contribution of enhancement because the experimental results indicate that the b2 mode has the largest enhancement and the a1 mode is rather small. According to models provided by Moskovits and Suh, the a2, b1, and b2 modes are enhanced in the order b1 = b2 > a2 (standing orientation) and a2 = b1 > b2 (flat orientation).37,41 The significant and selective enhancement of the b2 mode at 1437 cm−1 thus cannot be explained solely by the EM mechanism (SPR resonance). By comparing spectral intensity ratios between 1437 and 1394 cm−1 in the SERS maps of Ag NW LB films, it is possible to qualitatively assess the amount of EM or CM enhancements throughout the SERS substrates. The intensity ratio is an excellent indicator of the enhancement mechanism because of the unique origination of the Raman peak at 1437 cm−1. We acquired the Raman maps to investigate SERS performance of the overall LB films. The results were converted to intensity ratio maps, representing relative peak intensities between 1437 and 1394 cm−1. Shown in Figure 4b,e are two typical ratio maps obtained from Ag NW LB films before and after the GRR process. By examining Figure 4b, we noticed there are several locations where the ratios are about 1. This can be explained by possible 4-ATP binding to some surface of silver nanowires where the PVP capping agents were absent. Statistics graphed in Figure 4c show a Gaussian type fit centered at a ratio of ∼0.6, with 86% of the overall Ag NW LB area in Figure 4b exhibiting an intensity ratio of 1437 to 1394 cm−1 smaller than 0.8. However, the ratio map for the SERS substrate after GRR gave a significantly different result. As shown in Figure 4f, the statistical data for the intensity ratio map of Figure 4e also
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CONCLUSIONS In conclusion, we have demonstrated a facile room-temperature approach to tailor the SERS enhancements in the LB films of silver nanowires in order to obtain the optimum enhancement factors and excellent SERS performance. The galvanic replacement reaction was successfully utilized in fabricating SERS substrates that possessed a higher sensitivity to nonfluorescent probe molecules as well as a greater selectivity to molecules with a great binding affinity to metallic substrates. Meanwhile, such reaction also increases the signal-to-noise ratio for SERS spectra via reducing the amount of PVP capping agents on the silver nanowires. Our mapping results clearly demonstrated that the Ag NW LB films were initially 16192
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dominated by an EM enhancement and consequently with increased CM enhancement after the GRR process. Overall, using the GRRs to tailor the SERS substrates for a specific resonance process allows the researchers to select a desired enhancement mechanism to detect specific molecules of interest in a controlled manner.
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AUTHOR INFORMATION
Corresponding Author
*(C.J.) E-mail:
[email protected]. Fax: +1 605 677 6397. Tel: +1 605 677 6250. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the NSF (EPS-0903804 and DGE-0903685), NASA (Cooperative Agreement Number: NNX10AN34A), and by the State of South Dakota. Purchase of the LB trough was made possible through the US Department of Energy, contract number DE-EE0000270. We thank Ms. Tam Ho for sample preparation and valuable discussions. We also appreciate the anonymous referees for their valuable comments and suggestions.
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