ARTICLE pubs.acs.org/JPCC
A Simple Route for the Synthesis of Morphology-Controlled and SERS-Active Ag Dendrites with Near-Infrared Absorption Wen Ren, Shaojun Guo, Shaojun Dong, and Erkang Wang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Graduate School of the Chinese Academy of Sciences, Changchun 130022, Jilin, People's Republic of China
bS Supporting Information ABSTRACT: Herein we developed a simple and low-cost route for morphology-controllable synthesis of Ag dendrites based on a facile wet chemical route. The morphology of Ag dendrites was tunable by changing the concentration of capping reagent PVP. It was demonstrated that for higher concentrations of PVP, smaller Ag dendrites with shorter and smoother branches were obtained, while the ratio of the length of the branches to the body diameter of the Ag dendrites (L/D) became lower. It was also shown that the concentration of added AgNO3 was very important for the formation of Ag dendrites. The presented method is amenable for extension to large-scale synthesis of Ag dendrites. The prepared Ag dendrites exhibited strong near-infrared absorption proved by UVvisible spectra. Using p-ATP as a probe molecule, the SERS activity of prepared Ag dendrites was estimated. Based on the apparent enhancement factor and the measured diameter and L/D of the morphology-controlled Ag dendrites, the relation between the morphology and the SERS activity of Ag dendrites was investigated. It was shown that the smaller Ag dendrites with a lower L/D exhibited larger SERS activity.
’ INTRODUCTION Because of the exceptional advantages over spherical nanoparticles supplied by the morphology, enabling functional manipulation of different properties,1,2 anisotropic nanostructures have drawn significant attention. Especially, silver as a metal with unique optical properties3 and strong surface-enhanced Raman scattering (SERS) activity4 have inspired considerable research. Ag dendrites have been intensively investigated because of their fractal-like structures and large surface area, both leading to applications in catalysis,5 chemical/biochemical sensors,6 and SERS.79 A broad absorption in near-infrared region of Ag dendrites is also observed due to the presence of nanometer sized branches with variable length and diameter.10 Numerous works about the synthesis of Ag dendrites have been reported, based on methods of photochemistry,11 sonochemistry,12 and electrochemistry.13,14 Among the above strategies, electrochemistry as a facile route is quite conventional for the fabrication of Ag dendrites. It was easy to modify the surface of substrates with massive Ag dendrites, though there are some disadvantages to the electrochemical methods: (i) most of the obtained Ag dendrites are quite large because of the long trunks and complex branches; (ii) although some attempts have been performed,7 products usually stick on the surface of the electrode which complicates further applications. Wet-chemical methods have been proved as facile and classic routes for the synthesis of Ag nanostructures with well-controlled shape and size, such as spheres,15,16 cubes,17 wires,18 rice,19 etc.20 For wet-chemical synthesis, morphological control was easily achieved by changing the amount of reactants r 2011 American Chemical Society
and capping reagents which could tune the driving forces for reaction of precursor ions, nucleation, and growth modes. Obtained nanostructures are dissolved in solvent, usually in water, and beneficial for further application. It is very attractive to develop a simple, low-cost, and morphology-controllable synthesis of Ag dendrites based on wet chemical methods. However, until now, only a few of papers about wet-chemical routes of synthesis of Ag dendrites have been published.2123 The synthesis of shape- and size-controlled Ag dendrites based on wet chemical methods is still difficult. Since the discovery in 1974,24 SERS has become an attractive analytical tool for biomedical detections25,26 and chemical sensing.27,28 SERS substrates have been extensively studied to obtain reproducible and economical substrates with strong SERS activity for further applications. To date, various SERS substrates of several materials in different shapes and structures have been investigated.2931 Compared with other materials, silver has been demonstrated as the most suitable material for SERS studies. Nie’s group and Kneipp’s group have reported a great enhancement of Ag substrates and thereby single molecule detection by SERS.4,32 On the basis of the high activity of silver, SERS is employed as an ultrasensitive method for protein recognition,26,33 DNA detection34, etc.3537 In general, there are two mechanisms to describe the overall SERS effect: the Received: November 3, 2010 Revised: April 18, 2011 Published: May 05, 2011 10315
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The Journal of Physical Chemistry C electromagnetic effect and the chemical effect. The great enhancement of SERS substrates mainly results from the electromagnetic effect. It has been demonstrated that great SERS enhancement from a large electromagnetic field would be caused by hot spots which reside in the nanoscale junctions or interstices due to the aggregation of nanostructures.38 Meanwhile, due to the surface plasmon polarization (SPP) of the high curvature surface of the nanostructures such as tips and sharp edges, strong SERS enhancement is established according to the experimental and simulation results.39,40 For Ag dendrites, the hot spots, which form in the nanoscale junctions and interstices from the aggregation of Ag dendrites and therefore the overlap of branches, are supposedly a reason of the SERS enhancement. It is distinctive that the branches and tips with strong SPP could also supply high SERS activity. On account of the factors discussed above, Ag dendrites are demonstrated to be used as SERS substrates, though it is attractive to further understand the relationship of SERS activity to the morphology of Ag dendrites. In this paper, a simple and low-cost method for the synthesis of Ag dendrites was reported, and the morphology was controlled by adjusting the concentration of capping agent PVP. It was demonstrated that smaller Ag dendrites were prepared with shorter and smoother branches in higher concentrations of PVP, while the ratio of the length of the branches to the body diameter of the Ag dendrites (L/D) became lower. It was also found that the concentration of AgNO3 was important to the morphology of Ag nanostructures. UVvisible spectra showed that obtained Ag dendrites exhibited strong near-infrared absorption. Furthermore, these shape-controlled Ag dendrites were suited to elucidate the relationship of the SERS activity to the morphology of the Ag dendrites. Using p-ATP as probe molecules, the SERS enhancements of the Ag dendrites with different diameters and L/D were evaluated. According to the apparent enhancement factor (AEF) of the morphology-controlled Ag dendrites, SERS activity due to SPP of high curvature surfaces and hot spots from the aggregation of Ag dendrites was estimated. Based on the data of diameter and L/D as morphologic characters and the results of AEF, the relationship between the morphology and the SERS activity was investigated.
’ EXPERIMENTAL SECTION Chemicals and Reagents. AgNO3 and poly(vinylpyrrolidone) (PVP 3 k30) were purchased from Shanghai Chemical Reagent Co. (Shanghai, China). Hydroxylamine and p-aminothiophenol (p-ATP) were obtained from Sigma-Aldrich. All the reagents were used as received. Water used throughout all these experiments was purified with a Millipore system. Synthesis of Ag Dendrites. All glassware used in the following procedures were cleaned in a bath of freshly prepared 3:1 HCl:HNO3 (aqua regia) and rinsed thoroughly in milli-Q grade water prior to use. In a typical synthesis of Ag nanostructures, at room temperature, 1.0 mL of 0.5 M AgNO3 aqua solution was rapidly added to 18 mL of water. Then 0.5 mL of 1.0 M PVP was added to the solution, followed by addition of 1.0 mL of 2.0 M hydroxylamine under strong stirring. A quick color change from colorless to charcoal gray was observed upon the addition of hydroxylamine in several seconds. The shape and size of the Ag nanostructures were controlled by changing the amount of AgNO3 or PVP. After the mixture was stirred for 6 h, the product was washed and centrifuged at 3000 rpm for 5 min several times, and the precipitate was dissolved in 10 mL of water.
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Figure 1. (A) HRTEM image of Ag dendrites synthesized in 12 mmol/ L PVP, (B) lattice fringe spacing of a branch, and selected area electron diffraction patterns obtained (C) from a branch and (D) from the body.
Characterization. An XL30 ESEM scanning electron microscope was used to characterize the morphology of the products. Transmission electron microscopy (TEM) was performed with a Hitachi H-800 EM with an accelerating voltage of 200 kV. Highresolution transmission electron microscopy (HRTEM) images were obtained with a TECNAI G2 high-resolution transmission electron microscope operating at 200 kV. The sample for TEM and HRTEM measurement was prepared by dropping the product solution on a carbon-coated copper grid and drying at room temperature. XRD analysis was performed on a D/Max 2500 V/PC X-ray diffractometer using Cu (40 kV, 30 mA) radiation. The samples for XRD characterization were made by depositing the product on glass plates. UVvis spectra were recorded with a Cary 50 UVvisible spectrophotometer. Surface-enhanced Raman scattering (SERS) spectra were obtained with a Renishaw 2000, equipped with an Arþ ion laser giving the excitation line of 514.5 nm with a laser spot diameter of 1.6 μm, and air-cooled charge-coupled device (CCD) as the detector (Renishaw Co., United Kingdom). The Raman band of a silicon wafer at 520 cm1 was used to calibrate the spectrometer.
’ RESULTS AND DISCUSSION The morphology of typical Ag dendrites obtained in our work was characterized by TEM. A representative Ag dendrite is shown in Figure 1A, and multiple branches were clearly seen in the TEM image from a dendrite body. Some tubers were also observed on the branches, which formed a rough surface, and are beneficial for the applications in catalysis and SERS. Unlike some reported Ag dendrites synthesized by electrochemical methods,41 the angles between branches were not certain, and the branch lengths of one Ag dendrite were not uniform. Measured at one of the branches, the lattice fringe spacing shown in Figure 1B was about 0.23 nm, corresponding to the (111) plane spacing of face-centered cubic (fcc) Ag (d111 = 0.2359 nm). 10316
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Table 1. Diameters and L/D of Ag Dendrites Synthesized in Different Concentrations of PVP in the Presence of 24 mmol/ L AgNO3 PVP concentration (mmol/L)
diameter (nm)
L/D
12
808.5 ( 154.9
1.32 ( 0.15
24
501.1 ( 82.4
0.874 ( 0.078
48
500.8 ( 96.2
0.726 ( 0.038
Figure 2. Morphologies of Ag nanoparticles synthesized in (A) 12 mmol/L, (B) 24 mmol/L, (C) 48 mmol/L, and (D) 110 mmol/L PVP in the presence of AgNO3 at C = 24 mmol/L. Scale bars correspond to 2 μm for main panels and 200 nm for insets.
To further characterize the structure of the branches, the diffraction pattern of the branch shown in Figure 1C proved that the obtained branches of Ag dendrites were a single crystal. Besides, the diffraction pattern at the center of the Ag dendrite showed a single crystal pattern in Figure 1D. It was evident from the diffraction patterns of the branch and body that Ag dendrites were obviously single crystals, and the crystal structures of the branches and the body were the same. It has been reported that PVP is a very important organic capping reagent for the shape-selective synthesis of Ag nanostructures, because PVP could selectively adhere to certain crystallographic planes.42,43 For Ag dendrite synthesis, PVP was reported to assist the formation of branches on the Ag nanostructures, while more PVP may cause more branches and tips.14 To examine the role of PVP in the formation of Ag dendrites in our work, a series of experiments were carried out in which the concentrations of PVP were changed while other parameters remained. The morphologic results are shown in Figure 2. At a low PVP concentration of 12 mmol/L, large Ag dendrites were obtained (Figure 2A), while it was observed that the length of the branches was long and there were tips and tubers on the branches. When the PVP concentration increased, the size of Ag dendrites decreased, while the branches became shorter with less tips and tubers on the branches. Although the shape of the Ag dendrites was not quite uniform, the ratio of the branch length to the body diameter of the Ag dendrites (L/D) in Figure 2C was lower compared to that of the products shown in Figure 2B. If the concentration of PVP was 110 mmol/L, the obtained Ag nanostructures were amorphous and of uncertain size with a smooth surface. We also tried a PVP concentration lower than 12 mmol/L (results not shown), but the products quickly dispersed and became aggregated in several seconds. It was obvious that in this reaction PVP was used to stabilize the Ag dendrites. Besides, PVP as a capping agent played a very important role in the shape and size control. According to the SEM and TEM images shown in Figure 2, the low concentration of PVP led to the formation of bigger Ag dendrites with more complex branches, while the L/D of the obtained Ag dendrites increased. Meanwhile, too much PVP caused the formation of amorphous Ag nanostructures. To further study the morphology of the Ag dendrites synthesized in different PVP concentrations, the diameter and the L/D
Figure 3. Morphologies of Ag nanoparticles synthesized with (A) 1.1 mmol/L, (B) 55 mmol/L, (C) 110 mmol/L, and (D) 440 mmol/L AgNO3 in the presence of PVP at C = 48 mmol/L. Scale bars correspond to 2 μm for main panels and 200 nm for insets.
of the Ag dendrites was measured, and the results are shown in Table 1 (the branch length was measured from the center of the Ag dendrites to the top of the branches). The diameter of the Ag dendrites synthesized in 12 mmol/L PVP was 808.5 ( 154.9 nm, which was largest compared with the products synthesized in 24 mmol/L and 48 mmol/L PVP, while its L/D was 1.32 ( 0.15, larger than that of the products obtained in higher PVP concentration. The diameter of the Ag dendrites synthesized in 24 mmol/L and 48 mmol/L PVP both decreased to about 500 nm; at the same time, the L/D reduced from 0.874 ( 0.078 to 0.726 ( 0.038. In summary, the concentration of PVP played a very important role in the morphology of the Ag dendrites: the increase of the PVP caused the decrease of the L/D, while the diameter reduced to a minimum of about 500 nm. It was reported that the morphology of Ag crystals depended on the distance of the formation conditions from thermodynamic equilibrium.44 As an important factor to the nucleation and growth of Ag nanostructures, the concentration of initial AgNO3 also played a key role for the formation of Ag dendrites. Serial concentrations were examined, and the obtained products were characterized by TEM and SEM. As shown in Figure 3A, the products were amorphous particles in a AgNO3 concentration of 1.1 mmol/L. The size and shape of these particles were not uniform, while there were some dots on the surface of the particles. When more AgNO3 was added, such as 55 mmol/L or 110 mmol/L, Ag dendrites were obtained. However, if the concentration of AgNO3 was 440 mmol/L or higher, the products became shapeless particles with a smooth surface compared with the particles synthesized in low AgNO3 concentrations. As shown in Figure 3D, there were some particles which were similar in structure to Ag dendrites with wider branches, suggesting that 10317
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Figure 4. (A) UVvisible spectra of Ag dendrites obtained in the presence of PVP (a) at C = 12 mmol/L, (b) 24 mmol/L, (c) 48 mmol/L, and (d) 110 mmol/L in 24 mmol/L AgNO3. (B) XRD pattern of Ag dendrites.
these particles may result from Ag dendrites. It was assumed that too much AgNO3 added to the solution induced further growth of Ag dendrites in the Ostwald ripening process. UVvisible spectra of nanostructures strongly depended on the morphology and therefore surface plasmon resonance (SPR) property of nanostructures.45 Because of the anisotropic structure, Ag dendrites have quite special SPR and thereby unique UVvisible spectra compared to those of isotropic nanoparticles.10 Figure 4A shows the UVvisible spectra of the obtained Ag nanostructures, and it was observed that Ag dendrites with different morphology and amorphous particles had broad absorptions ranging from about 410 nm to near-infrared for the dispersion of Ag nanostructures in water. Although the shape and the size of the Ag dendrites were different, UVvisible spectra shown in Figure 4A(ac) were similar. According to the morphologic characterization of the Ag nanostructures, the branches had different length and diameter, while the shape of the branches varied. The broad absorptions in UVvisible spectra of the Ag dendrites were attributed to the SPR properties depending on the morphology of the branches with different shapes.46 For the amorphous Ag nanostructures, the UVvisible spectrum (Figure 4A(d)) was like the spectra of Ag dendrites because of the SPR of various structures. An unclear peak was observed at about 410 nm which was attributed to the characteristic absorption for silver nanoparticles,47 which should result from the small structures in amorphous Ag nanoparticles. XRD characterization of Ag dendrites was also carried out, and a typical result is shown in Figure 4B. The four diffraction peaks
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Figure 5. (A) SERS spectra of p-ATP in solution of Ag dendrites synthesized in (a) 12 mmol/L, (b) 24 mmol/L, (c) 48 mmol/L, and (d) 110 mmol/L PVP in the presence of AgNO3 at C = 24 mmol/L and (B) the corresponding AEF of the Ag dendrites calculated based on the peak at 1435 cm1 in the SERS spectra of p-ATP.
were indexed as (111), (200), (220), and (311) planes of fcc Ag (R = 0.409 nm, JCPDS No. 4-783). The absence of other peaks indicated high purity silver. It has been reported that Ag dendrites are used as SERS substrates, and it is supposed that their SERS activity is attributed to two factors: SPP due to high curvature surface of Ag nanostructures and hot spots resulting from aggregation of Ag dendrites, both of which were relative to the morphology of the nanostructures. Ag dendrites prepared in this work were very fit for the investigation of the contributions of SPP of a high curvature surface and hot spots from aggregation of Ag dendrites to the SERS activity. p-ATP were chosen as probe molecules for examination of the SERS properties of Ag dendrites, which has been used to estimate the SERS activity of Ag nanostructures.4851 Figure 5A gave the SERS spectra for 104 M p-ATP in a solution of Ag dendrites of different shape and size. The SERS spectra were dominated by two sets of bands: 1580, 1193, and 1080 cm1 were one set, which belong to a1 vibration modes of p-ATP molecules; the other set was 1435, 1391, 1145 cm1, which were assigned to the b2 vibration modes.52 To estimate the SERS activity, the solutions of the Ag dendrites were prepared with the same concentration of Ag. Although each group of the Ag nanostructures showed SERS enhancement, it was clear that their SERS activities were quite different. Although the enhancement factor (EF) is usually used to estimate the SERS activity of a substrate, it is difficult to calculate reliable EF for the comparison of the SERS activity of Ag dendrites with different morphology, 10318
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The Journal of Physical Chemistry C because it is difficult to obtain accurate surface area and volume of the Ag dendrites. Instead, the apparent enhancement factor (AEF) was selected to represent the SERS activity of the Ag dendrites, and the Ag nanoparticle (Ag NP) colloid (for more detail, see Supporting Information) was chosen to calculate the AEF of Ag dendrites. The AEF of Ag dendrites was defined using the following expression: AEF = ISERS/I0 C0/CSERS, where ISERS is the intensity of the peak at 1435 cm1 in the SERS spectra of p-ATP on Ag dendrites, I0 is the intensity of the same peak in the SERS spectra on Ag NPs, and CSERS and C0 refer to the concentrations of p-ATP in Ag dendrites and Ag nanoparticle colloids, respectively.53,54 The AEF of Ag dendrites with error bar are shown in Figure 5B, and the SERS activities changed dramatically for Ag dendrites with different morphology. The AEF variation could be explained by morphology and aggregation of the Ag nanostructure. As presented previously, the shape and the size of the Ag dendrites (shown as groups a, b, c in Figure 5) were changing because of the synthesis condition. The results in Table 1 show that increasing the concentration of PVP caused smaller Ag dendrites with a lower L/D. It has been demonstrated that structures with a high curvature surface such as tips and sharp edges are beneficial for SERS.39,40 Meanwhile, hot spots residing in the junctions and interstices that result from the aggregation of nanostructures were established to supply strong SERS activity due to the large electromagnetic field.38 The SERS activity variation of Ag dendrites was determined for both of the above reasons. For Ag dendrites of group a (synthesized in 12 mmol/L with a diameter of 808.5 ( 154.9 nm and an L/D of 1.32 ( 0.15), long branches, the tips, and tubers on the branches which had high curvature surface were supposed to give SERS enhancement due to the SPP, while such structures disturbed the aggregation of Ag dendrites and influenced the formation of hot spots. When the size of the Ag dendrites became smaller with shorter and smoother branches, the contribution to SERS enhancement from the SPP of the high curvature surface decreased; meanwhile, more hot spots were formed because small Ag dendrites with shorter branches aggregated more easily compared with the bigger ones with longer branches. Therefore, the AEF of groups b (synthesized in 24 mmol/L with diameter of 501.1 ( 82.4 nm and L/D of 0.874 ( 0.078) and c (synthesized in 48 mmol/L with diameter of 500.8 ( 96.2 nm and L/D of 0.726 ( 0.038) were larger than that of group a, indicating a stronger SERS activity. The AEF of group c was strongest, although the diameters of Ag dendrites in groups b and c are comparable. It was supposed that the increasing hot spots due to the smaller L/D of the Ag dendrites in group c supplied strong SERS enhancement. For the Ag nanoparticles of group d (synthesized in 110 mmol/L), there was a weak SPP due to the smooth surface without branches, and a few hot spots were formed because of the amorphous structures with big size; therefore, the weakest SERS spectra were obtained compared with the spectra of Ag dendrites. According to the diameter and L/D in Table 1 and the AEF in Figure 5B, Ag dendrites with smaller size and lower L/D could exhibit stronger SERS activity.
’ CONCLUSION In summary, we have developed a simple and low-cost method for morphology-controllable synthesis of SERS-active Ag dendrites with near-infrared absorption. By changing the concentration of capping reagent PVP, the shape and size of the Ag dendrites were controlled. It has been demonstrated that in
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higher concentrations of PVP, smaller Ag dendrites were synthesized with shorter and smoother branches, while the L/D of Ag dendrites was lower. It was also found that the concentration of added AgNO3 was very important for the formation of the Ag dendrite morphology. The obtained Ag dendrites exhibited strong near-infrared absorption according to the UVvisible spectra. The overall method was amenable for extension to largescale synthesis of Ag dendrites. The SERS activity of prepared Ag dendrites was investigated using p-ATP as probe molecules. On the basis of these morphology-tunable Ag dendrites, SERS activity due to SPP of the high curvature surface and hot spots from the aggregation of Ag dendrites was estimated. The morphology of the Ag dendrites was proved to be important for the SERS activity. It was demonstrated that smaller Ag dendrites with a lower L/D exhibited stronger SERS activity.
’ ASSOCIATED CONTENT
bS
Supporting Information. Synthesis of Ag NPs; TEM image of the Ag NPs; SERS spectra of 104 mol/L p-ATP on the substrates of (a) Ag NPs and (b) Ag dendrites synthesized in 48 mmol/L PVP corresponding to the products shown in Figure 2C; SERS spectra of 105 mol/L crystal violet on the substrates of (a) Ag NPs and (b) Ag dendrites synthesized in 48 mmol/L PVP corresponding to the products shown in Figure 2C; SERS spectra of 104 mol/L p-ATP recorded at 10 points randomly selected on the substrate of the Ag dendrites synthesized in 48 mmol/L PVP corresponding to the products shown in Figure 2C. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: þ86-431-85689711.
’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 20735003 and 20820102037) and 973 Project (Nos. 2009CB930100 and 2010CB933600). ’ REFERENCES (1) Eustis, S.; El-Sayed, M. A. Chem. Soc. Rev. 2006, 35, 209. (2) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (3) Gould, I. R.; Lenhard, J. R.; Muenter, A. A.; Godleski, S. A.; Farid, S. J. Am. Chem. Soc. 2000, 122, 11934. (4) Nie, S. M.; Emery, S. R. Science 1997, 275, 1102. (5) Huang, J.; Vongehr, S.; Tang, S.; Lu, H.; Shen, J.; Meng, X. Langmuir 2009, 25, 11890. (6) Wen, X.; Xie, Y.-T.; Mak, W. C.; Cheung, K. Y.; Li, X.-Y.; Renneberg, R.; Yang, S. Langmuir 2006, 22, 4836. (7) Gutes, A.; Carraro, C.; Maboudian, R. J. Am. Chem. Soc. 2010, 132, 1476. (8) Xu, P.; Jeon, S. H.; Mack, N. H.; Doorn, S. K.; Williams, D. J.; Han, X. J.; Wang, H. L. Nanoscale 2010, 2, 1436. (9) Song, W.; Cheng, Y. C.; Jia, H. Y.; Xu, W. Q.; Zhao, B. J. Colloid Interface Sci. 2006, 298, 765. (10) Jiang, L. P.; Wang, A. N.; Zhao, Y.; Zhang, J. R.; Zhu, J. J. Inorg. Chem. Commun. 2004, 7, 506. (11) Zou, K.; Zhang, X. H.; Duan, X. F.; Meng, X. M.; Wu, S. K. J. Cryst. Growth 2004, 273, 285. 10319
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