Rectangular Silver Nanorods: Controlled Preparation, Liquid-Liquid Interface Assembly, and Application in Surface-Enhanced Raman Scattering Shaojun Guo, Shaojun Dong, and Erkang Wang*
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 372–377
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, China ReceiVed June 5, 2008; ReVised Manuscript ReceiVed October 5, 2008
ABSTRACT: In this paper, we for the first time report a polyol method for large-scale synthesis of rectangular silver nanorods in the presence of directing agent and seeds. This method has some clear advantages including simplicity, high quality, and ease of scaleup. Silver nanowires or silver nanorods with a submicrometer diameter could also be facilely prepared when the reaction parameters are slightly changed. Furthermore, a liquid-liquid assembly strategy has been employed to construct uniform rectangular silver nanorod arrays on a solid substrate which could be used as surface-enhanced Raman scattering (SERS) substrates with high SERS activity, stability, and reproducibility. It is found that the SERS spectra obtained from the probe molecules with the different concentrations show different SERS intensities. As the concentration of 4-aminothiophenol (4-ATP) or rhodamine 6G (R6G) increases, the SERS intensities progressively increase. The enhancement factor for 4-ATP and R6G should be as large as 5.06 × 104 or much larger than the value of 5.06 × 108, respectively. Most importantly, the SERS spectra of R6G on the assembling film are well reproducible at different sites on a substrate, with a standard deviation of 95%), * To whom correspondence should be addressed. E-mail:
[email protected].
particularly no papers reporting the large-scale preparation of rectangular silver nanorods with a high aspect ratio. Herein, we first report a polyol method for controlled synthesis of rectangular silver nanorods in the presence of a directing agent and seeds. This method has some clear advantages including simplicity, high quality, and ease of scaleup. On the other hand, silver nanostructures with diverse morphologies as excellent surface-enhanced Raman scattering (SERS) substrates have gained great interest because metallic silver exhibits the best SERS effects compared with other metals such as gold and copper under certain conditions.28,29 Although silver nanoparticles with a size of 20-100 nm exhibited high SERS activity, it is necessary to further explore new SERS substrates based on novel silver nanostructures such as rectangular silver nanorods in the scientific research. Furthermore, recent calculations and experimental data have shown that large enhancement on the order of 1014-1015 for certain probe molecule can be obtained at the junctions of two aggregated nanoparticles due to the coupling of the localized surface plasmon (LSP) of the metal nanoparticles produced by the hot spots.30 Thus, there has been a considerable research effort in SERS focusing on the fabrication of metallic nanostructures that generate such hot junctions.29,31,32 However, how to achieve a large, reliable, stable, uniform, and reproducible SERS signal spanning a wide dynamical range is still a tremendous challenge. Van Duyne and co-workers have used nanosphere lithography,33 while Liu and Lee exploited soft lithography,34 to fabricate Ag nanoparticle arrays (hot spots) with high SERS activity and improved uniformity. More recently, Wang et al. presented SERS measurements of molecules adsorbed on arrays of Ag nanoparticles grown in porous anodic aluminum oxide (AAO) nanochannels with a precisely controlled variation of interparticle gaps between 5 and 25 nm.29 However, the soft lithography technique and preparation of the template (AAO) are very tedious. Therefore, it is very necessary to develop a very simple, rapid, and reproducible method for constructing nanostructure
10.1021/cg800583h CCC: $40.75 2009 American Chemical Society Published on Web 11/26/2008
Rectangular Silver Nanorods
arrays with relatively uniform hot spots. Controlled selfassembly of nanostructured materials provides a feasible project. Recently, it has been demonstrated that hydrophilic nanoparticles can be trapped at liquid-liquid interfaces and assembled into two-dimensional arrays, which is induced by the destabilization of nanoparticles.35-38 Herein, we expand this strategy to rectangular silver nanorod systems and first construct uniform rectangular silver nanorod arrays on a solid substrate via a very simple and rapid method which exhibit good, stable, and reproducible SERS activity. This liquid-liquid assembly method can probably be extended to prepare other silver or gold nanostructure arrays (hot spots) with high, stable, and reproducible SERS activity. Our results will also open new possibilities for applying SERS to detect different analytical species due to the good reproducibility of the present SERS substrate. Experimental Section Chemicals. EG, Na2S, PVP · K30 (molecular weight 30000-40000), and AgNO3 were purchased from the Beijing Chemical Factory (Beijing, China) and used as received without further purification. 4-Aminothiophenol (4-ATP) and rhodamine 6G (R6G) were obtained from Sigma-Aldrich Chemical Co. and used as received. Water used throughout all experiments was purified with a Millipore system. Apparatus. An XL30 ESEM scanning electron microscope was used to determine the morphology and composition of the products. Transmission electron microscopy (TEM) measurements were made on a Hitachi H-8100 electron microscope with an accelerating voltage of 200 kV. HRTEM images were measured with a JEOL 3010 highresolution transmission electron microscope operating at 300 kV. The sample for TEM characterization was prepared by placing a drop of prepared solution on a carbon-coated copper grid and drying it at room temperature. X-ray diffraction (XRD) analysis was carried out on a D/Max 2500 V/PC X-ray diffractometer using Cu (40 kV, 30 mA) radiation. The UV-vis spectrum was acquired using a Cary 500 UV-vis-NIR spectrometer (Varian). SERS spectra were measured with a Renishaw 2000 model confocal microscopy Raman spectrometer with a CCD detector and a holographic notch filter (Renishaw Ltd., Gloucestershire, U.K.) at ambient conditions. Radiation of 514.5 nm from an air-cooled argon ion laser was used for the SERS excitation. The laser beam was focused to a spot with a diameter of approximately 1 mm using a 50× microscope objective. The data acquisition time was 10 s for one accumulation. The Raman band of a silicon wafer at 520 cm-1 was used to calibrate the spectrometer. Synthesis of Rectangular Silver Nanorods. In a typical synthesis, 36 mL of ethylene glycol (EG; Beijing Chemical Factory) was heated at ∼157 °C under stirring with a Teflon-coated magnetic stirring bar for 1 h in a conical flask. Shortly after injection of 30 µL of Na2S solution (48 mM), 9 mL of PVP (177 mM) solution was quickly added to the above solution. Then 3 mL of AgNO3 solution (284 mM) was dropwise injected. When AgNO3 was reduced completely (the introduction of a strong reducing reagent such as NaBH4 into the solution after centrifugation cannot result in further change of the solution color, suggesting that AgNO3 has been totally reduced during the thermal process), the solution color changed to gray-white and rectangular silver nanorods (sample 1; the concentration of Ag nanorods is about 17.75 mM, calculated by using the Ag atom) with a high yield (>95%) were obtained finally. Self-Assembly of Rectangular Silver Nanorods. An 8 mL sample of the rectangular silver nanorod aqueous solution was transferred to the surface of a plate at room temperature, and 6 mL of toluene was added to the top of a colloid solution surface to form an immiscible water/toluene interface. Then 2 mL of ethanol was added dropwise to the surface of the water/toluene layers in about 5 min, leading to rectangular silver nanorod arrays at the interfaces at ambient conditions. SERS Measurements. After the silver nanorod assembling film was transferred to glass slides, the glass slides were immersed in 4-ATP (10-4, 10-5, and 10-6 M) and R6G (10-6, 10-7, and 10-8 M) solutions with different concentrations for about 6 h. After being thoroughly rinsed with water and dried by nitrogen, they were subjected to Raman characterization. In Raman experiments, more than six SERS-active substrates were prepared, and 10 different points on each substrate were
Crystal Growth & Design, Vol. 9, No. 1, 2009 373 selected to detect the R6G and 4-ATP probes, to verify the stability and reproducibility of these SERS-active substrates.
Results and Discussion The morphology of the resulting product was investigated by scanning electron microscopy (SEM) and TEM. Parts A-C of Figure 1 show the typical SEM images of the as-prepared silver nanostructures coated on the ITO substrate at different magnifications. As shown in Figure 1B, the substrate is covered with a great deal of silver nanorods. From the magnified image (Figure 1C), it is interestingly observed that these silver nanorods with an average diameter of about 200 nm own a cross section of rectangular shape. The corresponding size distribution of Ag nanorods is shown in Figure S1, Supporting Information. TEM was used to further characterize the morphology and crystal structure of the samples. Figure 1D provides a representative TEM overview image of the silver nanorods. It still can be seen that the silver nanorods were shaped with a regular rectangular cross section. Figure 1F presents the selected-area electron diffraction (SAED) pattern of an individual silver nanorod (Figure 1E), which was obtained by focusing the electron beam on a nanorod lying flat on the TEM grid. It is found that the SAED of the silver nanorod is consistent with cubic silver with strong diffraction patterns due to (111) and (100) planes. The HRTEM image (Figure 1G) of a part of one silver nanorod (Figure 1F) shows many lattice planes with perfect crystallinity. The measured interplanar spacing for all the lattice fringes is 0.205 nm, which corresponds to the (200) lattice plane of facecentered-cubic (fcc) silver, revealing that the growth plane of the nanorod is one of the (200) planes. XRD gives further support to the phase structure of the silver nanorods. The five diffraction peaks shown in Figure 1I correspond to the (111), (200), (220), (311), and (222) diffraction peaks of metallic silver, which indicates that the products are composed of pure crystalline silver. It is worth noting that the ratio between the intensities of the (200) and (111) diffraction peaks was higher than the conventional value (0.63 versus 0.4), indicating that our nanorods were abundant in (200) facets. The silver nanorods were additionally characterized by energy-dispersive X-ray spectroscopy (EDS; Figure 1H) and X-ray photoelectron spectroscopy (XPS; data not shown), which confirmed that the nanorods are pure silver. It is commonly accepted that the shape of fcc nanocrystal is mainly determined by the ratio (R) of the growth rates along the [100] versus the [111] direction.39 Tetrahedrons and icosahedrons bound by the most (111) planes will be formed when R is large (1.73), and perfect cubes bound by the less stable (100) planes will result if R is reduced (0.58). In our experiments, special interaction between PVP and a certain face, (100), of the silver nanocrystal is believed to play a key role here. We have investigated the effect of the reaction time on the morphologies of the resulting nanocrystals. It is found that when the reaction time is short (30 s), some small silver nanoparticles will be produced (data not shown). Continuously increasing the reaction time to ∼8 min, it is very interesting that silver nanocubes with a diameter of 50-70 nm were facilely obtained (Figure 1J). This further indicates that PVP is easily attached to the surface of silver nanocrystals and increases the growth rates of a certain crystal face, (100).39 Xia et al.39b developed a polyol strategy for rapid synthesis of silver nanocubes by mediating polyol reduction with a trace amount of Na2S. They thought that Na2S could interact quite strongly with silver for the creation of Ag2S nanoparticles when silver existed at concentrations above the micromolar level with trace sulfides
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Figure 1. SEM images (A-C), TEM images (D, E), SAED (F), HRTEM (G), EDX (H), and XRD (I) patterns of the rectangular silver nanorods, and TEM image (J) of the silver nanocubes at a reaction time of ∼8 min.
in aqueous systems. Then Ag2S nanoparticles could catalyze the reduction of Ag+, and at this enhanced rate, the formation of silver nanocubes would be dominated by the fast kinetic growth of single-crystal seeds instead of the formation of twinned seeds. In the present synthesis system, Na2S could probably be reacted with AgNO3 to form Ag2S nanoparticles in a similar way and probably play an important role in rapidly forming silver nanocubes in a short time (about 8 min).39b It should be noted that, in this case (about 8 min), a great deal of AgNO3 has still not been reduced. When the reaction time is prolonged, a great number of rectangular silver nanorods will be obtained finally. Therefore, the silver nanocubes produced in the reaction process (∼8 min) probably acted as seeds for 1D growth of rectangular silver nanorods, while PVP acted as a directing agent via its strong attraction to the (100) planes of silver nanocrystals (proved by data such as HRTEM, SAED, and XRD). However, the exact formation mechanism of rectangular silver nanorods in this synthesis system is not very clear at the present time and needs further investigation. The morphology and dimensions of the product were found to strongly depend on reaction parameters such as the concentration of Na2S and the molar ratio between the repeating unit of PVP and AgNO3. For example, when 40 µL of Na2S solution (48 mM) was injected under conditions identical to thosed used for preparing sample 1, large-scale silver nanowires were facilely synthesized (sample 2). Figure 2A-C shows the typical TEM images of the obtained silver nanowires. From the magnified images (Figure 2C), it can be seen that silver nanowires own an average diameter of 70 nm. When synthesis is performed without addition of Na2S, uniform silver nanowires and some cubic silver nanoparticles dominate in the product (Figure 2D-F). If the molar ratio between the repeating unit of PVP and AgNO3 was decreased from 1.87 to 0.93, silver nanorods
with a bigger diameter (∼400 nm) and cubic silver nanoparticles became the major product (Figure 2G,H). When the molar ratio between the repeating unit of PVP and AgNO3 was increased from 1.87 to 8.4, small silver nanoparticles with different sizes could be obtained (Figure 2I). It would be interesting to explore whether the as-prepared rectangular silver nanorods could be used for fabricating intense and stable SERS substrates. Yang et al.40 reported that the assembling monolayers of aligned silver nanowires could be facilely obtained via the Langmuir-Blodgett (LB) technique and also serve as excellent SERS substrates. However, the above LB technique for reproducible SERS substrates usually needs special equipment and a complex operating process. Also the surface of the nanowires (hydrophilic) must be hydrophobic for the LB experiment and therefore results in a tedious process for obtaining good SERS substrates. Herein, a simple and rapid liquid-liquid interface assembly strategy has been employed to obtain well-defined rectangular silver nanorod arrays on a solid surface, which further could be explored as good and stable SERS substrates for molecular sensing. Figure 3A shows a typical optical image of the as-prepared assembling film. It is found that a uniform film has been obtained over a large area. Note that the area of the assembling film can be controlled simply by adjusting the concentration of rectangular silver nanorods in solution and the interfacial area of the two phases. SEM was used to obtain more in-depth information regarding the film profile of the transferred films (Figure 1A). The SEM image of the assembling film shows a homogeneous morphology over a large area. Significantly, these rectangular silver nanorod monolayers can be readily used as good and stable SERS substrates for molecular sensing with high sensitivity and specificity. 4-ATP was first chosen as the probe molecule due to its distinct spectral feature and easy binding to the silver
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Figure 2. TEM images (A-C) of the silver nanowires obtained by increasing Na2S (48 mM) to 40 µL, TEM images (D-F) of the silver nanowires obtained without the addition of Na2S, TEM images (G, H) of the silver nanorods at a molar ratio of the repeating unit of PVP to AgNO3 of 0.93, and TEM image (I) of the silver nanoparticles at a molar ratio of the repeating unit of PVP to AgNO3 of 8.4.
Figure 3. (A) Photograph of the self-assembled rectangular silver nanorod array film. (B) SERS spectra of 4-ATP on rectangular silver nanorod arrays with different molecular concentrations: 10-4 M (curve c), 10-5 M (curve b), and 10-6 M (curve a). The laser power was 12.5 mW. (C) SERS spectra of R6G on rectangular silver nanorod arrays with different molecular concentrations: 10-6 M (curve c), 10-7 M (curve b), and 10-8 M (curve a). The laser power was 0.5 mW. (D) SERS spectra of R6G (10-8 M) at different points (curves a-c) on the as-prepared substrate. The laser power was 0.5 mW.
nanorod surface through the Ag-S bond. Figure 3B shows the SERS spectra of 4-ATP with different concentrations on an assembling film of rectangular silver nanorods for visible (514 nm, 12.5 mW) excitation. Relative to the Raman spectrum (Figure S2, Supporting Information) obtained for the solid
4-ATP, the differences in the SERS spectrum (line c, Figure 3B) for the assembling film of rectangular silver nanorods were frequency shifts and changes in the relative intensities of some bands. Two sets of bands were observed on the SERS spectra of 4-ATP on the assembling film; one set is located at 1078
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and 1179 cm-1, which are assigned to the a1 vibration modes, and the other set is located at 1141, 1390, 1435, and 1577 cm-1, which are assigned to the b2 vibration modes.41 As reported by Osawa et al.,42 the enhancement of b2 modes is attributed to the chemical mechanism, most likely from the charge transfer (CT) of the adsorbate to the metal. Because rectangular silver nanorods are dominated by a (100) facet, which has a higher surface energy, the chemical enhancement effects are probably from the preferential binding of 4-ATP on the (100) facet of silver nanorods.43 To determine the enhancement effect (EF) of 4-ATP on the assembling film quantitatively, the EF values of 4-ATP in the assembling film are calculated with the following expression (the detailed calculation can be seen in the Supporting Information):
EF ) (ISERS/Nads)/(Ibulk/Nbulk) Then the EF at the assembling film for the band located at 1078 cm-1 can be calculated to be as large as 5.06 × 104 at 514 nm excitation. This value is comparable to that obtained from silver nanoparticle (∼22 nm)44 or LB silver nanowire (∼50 nm) monolayers.40 The relatively lower value than that of silver nanoparticles or nanowires was probably caused by the larger dimension (100-200 nm) of the rectangular silver nanorods because an earlier report shows that spheres of 20-100 nm or anisotropic particles of 10-20 nm width were better SERS substrates. Therefore, further experiments are needed to synthesize rectangular silver nanorods with a width or edge of small size (under way). In addition, relatively high SERS activities of the assembling film were probably caused by the fact that rectangular silver nanorods have more well-defined edges and corners and generally sharper surface features than spherical nanostructures, thus leading to intense local electromagnetic field enhancement.45 Solutions with different concentrations (10-4, 10-5, and 10-6 M) of 4-ATP are used to study the SERS dynamic range. It is found that the SERS spectra obtained from solutions with different concentrations show different SERS intensities. As the concentration of 4-ATP increases, the SERS intensities progressively increase. SERS activity of the assembling film has also been successfully demonstrated for dye molecules. Figure 3C shows the SERS spectra of R6G for a range of R6G concentrations (10-6, 10-7, and10-8 M), which is usually used to detect SERS activity on the silver surface on the assembling silver nanorod film. Apparently, vibrations at 1186, 1309, 1362, 1507, and 1647 cm-1, which are assigned to C-H in-plane bending, C-O-C stretching, and C-C stretching of the aromatic ring, are enhanced greatly.46 As the concentration of R6G increases, the SERS intensities progressively increase. The EF for R6G at the assembling film is also calculated. In this work, the ratio of the Raman intensities of the R6G- and thiol-related C-C stretching bands at R6G saturation coverage was about 104, as reported by Yang et al.40 Using this ratio and the EF for 4-ATP, the EF for R6G should be about 5.06 × 108 for the rectangular silver nanorod assembling film at 514 nm excitation. However, it should be noted that the SERS signal of R6G greatly exceeded the measurement range of the apparatus when it was measured in the condition of high laser power (the same as that for 4-ATP, 12.5 mW). Thus, we had to change the laser power to 0.5 mW. Therefore, in fact, the EF for R6G should be much larger than the value of 5.06 × 108. In addition, the SERS spectra of R6G on the assembling film are well reproducible at different sites on a substrate (Figure 3D), with a standard deviation of
