ARTICLE pubs.acs.org/JPCC
Au/SBA-15-Based Robust and Convenient-to-Use Nanopowder Material for Surface-Enhanced Raman Spectroscopy Rafael Silva,† Ankush V. Biradar,† Laura Fabris,*,§ and Tewodros Asefa*,† †
Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, United States § Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, New Jersey 08854, United States
bS Supporting Information ABSTRACT:
Powdered nanocomposite materials composed of SBA-15 mesoporous silica and different types of Au nanoparticles within the mesoporous channel pores (Au/SBA-15) are shown to serve as robust and efficient substrates for surface-enhanced Raman spectroscopy (SERS). Using the Au/SBA-15 samples as SERS substrates and 4-mercaptopyridine (4-Mpy) as SERS reporter molecule, SERS enhancement factors as high as ∼105 are obtained. The degree of SERS enhancement is found to depend on the size of the Au nanoparticles as well as the synthetic procedures employed to synthesize the Au/SBA-15 materials. The high SERS enhancement given by the Au/SBA-15 as compared to any other powered SERS substrates appears to be the result of the formation of SERS “hot spots” due to the side-by-side alignment of Au nanoparticles within the cylindrical channel pores of the SBA-15 mesoporous silica host. Because of their powdered forms, longevity, simple sample preparation procedures, easily tunable surface, and high SERS enhancement factors, Au/SBA-15 can also be expected to serve as simple and convenient-to-use substrates for SERSbased analysis of various analytes, besides 4-Mpy. Furthermore, this work demonstrates the synthesis of supported “naked” Au nanoparticles and their use as SERS substrate directly and without any further chemical modification.
1. INTRODUCTION Surface-enhanced Raman spectroscopy (SERS) using colloidal solutions of noble metal nanoparticles is a potentially invaluable, appealing, and cost-effective tool for qualitative and quantitative analysis of a wide variety of chemical and biological samples. SERS is appealing for chemical analysis also because of its high sensitivity.1,2 On the other hand, however, the irreproducible results that SERS sometimes gives, mainly due to the inherent instability of colloidal nanoparticles in solution, still remain a challenging problem.3,4 Thus, nanostructured solid substrates have been increasingly considered and investigated as alternative candidate materials for reproducible SERS-based analyses.5,6 For instance, nanopatterned solid surfaces prepared using lithographic techniques, particle deposition, or electrochemical texturing have been shown to provide reproducible SERS enhancement factors (EFs) as high as 108.7,8 However, with such r 2011 American Chemical Society
SERS substrates, SERS enhancement is verified only at the socalled “hot spot” regions of the materials, where high electromagnetic field is generated by the nanostructures, while little or no significant signal enhancement is exhibited in neighboring regions. In addition, even though high enhancement factors are obtained with these substrates, the inherently smaller amount of molecules that can interact with two-dimensional surfaces, and the costly fabrication methods involved to make such materials limit their range of application. These existing inherent problems associated with colloidal nanoparticles as well as nanopatterned solid surfaces have essentially prevented SERS from being applied as a broad-spectrum analytical technique.9 To overcome Received: July 17, 2011 Revised: October 5, 2011 Published: October 10, 2011 22810
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Scheme 1. Schematic Representation of the Synthesis of Au Nanoparticles within the Pores of SBA-15 (Au/SBA-15) by Sequential Redox and Galvanic Reactions through an Intermediate Product, Ag/SBA-15
these problems, metal nanostructures adsorbed to or grown on silica networks or in dendrimers have been considered and recently utilized successfully as SERS substrates.1012 Mesoporous metal oxides that were first reported in the early 1990s have been widely considered for a broad range of applications and as “hard-templates” for synthesizing, stabilizing, and assembling various nanoparticles and nanowires.1315 Their large surface areas, pore volumes, and well-ordered nanoscale pore sizes and pore structures have also been widely exploited for the synthesis of various nanocomposite materials that are potentially useful in areas ranging from catalysis to solar cells and plasmonics.1618 When used as host material for the synthesis of nanoparticles, mesoporous metal oxides can also provide additional advantages, including high pore volume for the formation of a high density of stable nanoparticles. Furthermore, noble metal nanoparticles (e.g., Au) can be synthesized in “naked”, and yet stable, form within mesoporous materials. Such supported “naked” metal nanoparticles can, in principle, be more suitable as SERS substrates as they potentially enable the identification of a broad range of molecules, including those not capable of displacing strongly bound ligands on metallic surfaces.19 The term “naked” is used here to indicate that the Au nanoparticles are coated only loosely by residual ions in solution, or by agents with no strong binding groups such as alkanethiols or alkylamines. Thus, incoming probe molecules (analytes) do not need to displace typical strongly bound capping agents used with Au, such as alkanethiols and alkylamines. In addition, “naked” metal nanoparticles can potentially result in high SERS enhancements because of the direct interaction between the analytes and the metal nanoparticle surface. Despite all of these potential advantages, however, the possible use of mesoporous silica-supported metal nanoparticles as SERS substrates has not been explored previously. Herein, we report on the synthesis of nanocomposite materials consisting of SBA-15 mesoporous silica and metallic nanoparticles within the channel pores (Au/SBA-15) and their use as SERS substrates in a powdered form with high enhancement factors (EFs). The materials' SERS EFs are found to depend on the size of their Au nanoparticles as well as on the synthetic procedures employed to make them. Scheme 1 shows the schematic representation of the synthetic method used to obtain ultrasmall Au nanoparticles within the ∼8 nm diameter cylindrical channel-like pores, also presenting typical channel lengths of ∼1 μm, of the highly ordered SBA-15 mesoporous silica.20,21
The stable Au nanoparticles inside SBA-15 were prepared using the mesoporous channel pores of SBA-15 as nanoreactor to control the growth of the Au nanoparticles.20 SBA-15 was chosen rather than other mesoporous materials for the deposition of Au nanoparticles for SERS application because aqueous analytes were reported to diffuse in its mesopores more easily, as shown by Huang et al.22 This was attributed to the extensive intrawall pores and three-dimensional pore structures available in the mesoporous framework of SBA-15 material.
2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Absolute ethanol (99.99%) was purchased from Fisher Scientific. Cetyltrimethylammonium bromide (CTAB), AgNO3, SnCl4, hexamethyldisilazane (HMDS), tetraethyl orthosilicate (TEOS), 4-mercaptopyridine (4-Mpy), and trifluoroacetic acid were obtained from Sigma-Aldrich. A solution containing Au + (Na 3 Au(SO 3 )2 , Oromerse Part B) was received from Technic Inc., RI. A block copolymer, poly(ethylene glycol)-block-poly-(propylene glycol)-block-poly(ethylene glycol) (Pluronic 123, average molecular mass ≈ 5800), was obtained from BASF. 2.2. Synthesis of Au/SBA-15 Nanocomposites for SERS. The synthesis of Au nanoparticles within SBA-15 was achieved using an electroless deposition method23 as follows. First, assynthesized SBA-15 was prepared as per literature report.24 The as-synthesized SBA-15 material was treated with a solution of HMDS/toluene (5 mL/60 mL) under nitrogen atmosphere for 12 h. This allowed the external surface silanols of as-synthesized SBA-15 to be capped with methyl (Me) groups. The Pluronic 123 templates were then removed from this sample via solvent extraction by stirring 0.5 g of Me-capped mesostructured SBA-15 in 100 mL of diethyl ether:ethanol (1:1) for 5 h. This yielded mesoporous SBA-15 having Me groups on its external surface, or a sample labeled as Me-SBA-15. After filtration and air drying, 0.3 g of Me-SBA-15 was stirred in an aqueous SnCl2 solution (30 mL, 0.5 mM/1.5 mM trifluoroacetic acid) for 30 min. This process allowed Sn2+ ions to complex onto the pore walls of the mesoporous silica.25 The solution was filtered, and the precipitate was washed copiously with water:ethanol (1:1 ratio) and airdried. The resulting Sn2+-immobilized Me-SBA-15 material was stirred in aqueous ammonical AgNO3 solution (20 mL, 1.0 M) for 20 min. This produced a light yellowish sample, containing Ag 22811
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nanoclusters on the pore walls of the material (Ag/SBA-15). The resulting Ag nanoclusters of Ag/SBA-15 were then used as nucleation and growth sites for Au nanoclusters. This was performed in the presence or absence of CTAB surface capping agents for the Au nanoparticles. In the first case, the resulting Ag/ SBA-15 precipitate from the previous step was dispersed in 10 mL of H2O containing 400 mg of CTAB, and to which 3 mL of gold plating solution (Na3Au(SO3)2, Oromerse Part B, Technic Inc.) was added. The solution was sonicated for 3 min and filtered. The recovered solid material was copiously washed with water and then ethanol. After air drying, a solid sample containing CTAB-capped Au nanoparticles, which was designated as Au/ SBA-15-A, was obtained. A second material was prepared in the same way, but in the absence of CTAB to result in “naked” Au nanoparticles. The resulting sample was labeled as Au/SBA-15-B. 2.3. Characterizations. The UVvis absorption spectra of the Au/SBA-15 samples were measured with a Lambda 950 spectrophotometer (PerkinElmer). Transmission electron microscope (TEM) images of the samples were obtained with a Topcon 002B TEM that was working at an acceleration voltage of 200 kV. Powder X-ray diffraction (XRD) patterns were recorded with a Siemens, Daco-Mp X-ray diffractometer. 2.4. Surface-Enhanced Raman Scattering (SERS) Measurements. Raman and SERS spectra were obtained at room temperature with a micro-Raman instrument (Renishaw 1000, Gloucestershire, U.K.) equipped with a He/Ne laser (632.8 nm) and a CCD detector. The laser power at the sample position was typically 2.0 mW. An integration time of 1 s was used for both Raman and SERS spectra. All of the Raman spectra presented in this work consisted of an average of five spectra, which were collected at five different positions over the samples. 2.5. Evaluation of SERS Activity. To evaluate the SERS activities of the two Au/SBA-15 materials, their enhancement factors (EFs) have to be obtained first. This was done by using the following equation (eq 1):26 EF ¼ ðISERS =IRaman ÞðNbulk =Nads Þ
ð1Þ
where ISERS and IRaman are the intensity of the vibrational modes in the SERS and normal Raman spectra, respectively. Nbulk is the number of 4-Mpy molecules drop casted on the Me-SBA-15, and Nads is the number of 4-Mpy molecules placed on the Au/SBA15 materials. Thus, the determination of the EF requires that the spectra of the 4-Mpy molecules adsorbed in Me-SBA-15 and those adsorbed in Au/SBA-15 be measured under identical conditions. Raman spectra of a 1 105 mol/L solution of 4-Mpy dried over Au/SBA-15 and a 1 102 mol/L solution of 4-Mpy dried over Me-SBA-15 were measured to obtain information on the band intensities of molecules adsorbed in Au/SBA-15 and Me-SBA-15 directly under the same experimental conditions. For both experiments, the laser focal volume was kept constant (1 μm in diameter). To determine EF values, we assumed that all of the 4-Mpy molecules used in the SERS experiment were adsorbed on the gold surface. Thus, the EF values presented in this Article were underestimated values, and the real EF values could actually be some orders of magnitude higher.
3. RESULTS AND DISCUSSION To prepare Au/SBA-15 materials, first SBA-15 mesostructured silica was synthesized by standard supramolecular self-assembly of TEOS with Pluronic P-123 polymer micellar templates.22 The external surface of SBA-15 type mesostructured silica was then
capped with methyl (Me) groups using HMDS. The purpose of placing Me capping groups on the external surface of SBA-15 particles was to inhibit the uncontrolled growth of the Au nanoparticles on the outer surface of SBA-15, or to produce monodisperse Au nanoparticles only within the pores of SBA-15.17 The Pluronic 123 templates were then extracted by stirring Me-capped SBA-15 mesostructured silica in a solution of ethanol and diethyl ether. This resulted in mesoporous SBA-15 having Me groups on its external surface (or Me-SBA-15). Nitrogen gas sorption measurement of the resulting Me-SBA-15 sample gave a BrunauerEmmettTeller (BET) surface area of 428 m2 g1 and monodisperse mesopores with an average BarrettJoynerHalenda (BJH) pore diameter of 5.9 nm (Supporting Information, Figures S1 and S2). Synthesis of Au nanoparticles within the pores of Me-SBA-15 was then achieved using an electroless deposition method,25 which involved sequential redox and galvanic reactions on the channel walls of the mesoporous SBA-15. Typically, Me-SBA-15 was treated with an aqueous Sn2+ solution. This resulted in immobilization of Sn2+ ions onto silanol (SiOH) groups present in the internal channel walls of SBA-15 via electrostatic interaction. Upon addition of aqueous Ag+ solution, ultrasmall (
