Metallic Cu nanoparticles dispersed into porous glass: A simple green chemistry approach to prepare SERS substrates
Introduction
Nanoparticles have attracted widespread interest by scientists because of excellent optical properties, catalyst properties and sensing properties [1], [2], [3]. Metal nanoparticles have high surface area, which is an important characteristic to be applied as catalyst as well as sensing devices [2], [3]. Copper nanoparticles have potential application in the fields of chemistry and electronic devices because of high electron conductivity and so they have attracted wide attention [4], [5]. Besides those applications, copper nanoparticles can also be used in plasmonic devices due to localized surface plasmon (LSP) in the visible range, that is appropriate to be applied as a Surface-Enhanced Raman Scattering (SERS) substrate [5], [6].
Since the SERS phenomenon was discovered by Fleischmann et al. in 1974 [7] it has become an attractive subject in surface-sensitivity research and in nanoscience [8], [9]. The contribution of this surface phenomenon to the amplification of optical fields was quickly confirmed after its discovery [7], and since then, the role of nanostructures in this phenomenon has been fully recognized [10]. Unfortunately, strong enhancements with practical applications can be observed only on coinage metals, i.e., Ag, Au, and Cu [11]. Substrates of these metals can support SERS over the spectral range from the visible to the near infrared, which is the range of most laser sources commonly used in the Raman spectrometers [11]. Extensive investigations have demonstrated that SERS enhancements of nanostructured Ag and Au strongly rely on their size and morphology as well as assembly, and many nanostructured Ag and Au have been developed as SERS sensitive substrates [12], [13]. However, amongst the three coinage metals, the SERS effect of nanostructured Cu is much less investigated [6], [14]. This is probably due to its weak SERS effects and poor chemical stability, which can be overcome by developing new Cu nanostructures and new synthesis approach [5].
Recently, the synthesis of metallic copper nanoparticles on porous materials have been reported to be applied mainly as catalysts [15], [16]. In the present work, a simple chemistry approach to prepare metallic Cu nanoparticles within the PVG porous structure by reduction of Cu2+ ions to Cu0 is reported. The used methodology allows controlling of the metallic Cu nanoparticles size because the reduction step occurs inside PVG porous structure. The control of the nanoparticles size has been reported to be possible for semiconductor oxide nanoparticles [17], [18], suggesting that the methodology can be used for other materials. PVG was chosen rather than other mesoporous materials for preparation of metallic Cu nanoparticles for SERS application because aqueous and other solvents were reported to diffuse in its porous structure [17], [18]. This fact is attributed to the extensive surface area (250 m2 g−1) and three-dimensional pore structure available in the PVG mesoporous framework.
Section snippets
Experimental
The chemicals were used as obtained. CuCl2 (Aldrich, 99%) was dissolved in distilled water to prepare the solution of concentration 0.25 mol L−1. Sodium borohydride (Acros Organics, 99%) was dissolved in distilled water to prepare a solution of concentration 1.5 mol L−1.
Preparation of Cu nanoparticles was made onto Porous Vycor® Glass (PVG, Corning Glass code 7030) discs having a radius of 0.6 cm and a thickness of 0.1 cm. PVG discs were soaked into HCl solution (1.0 mol L−1) for 2 h, washed with
Results and discussion
Synthesis of metallic Cu nanoparticles within the PVG porous structure was achieved using the impregnation–reduction procedure. This procedure initially involves ion-exchange between Cu2+ ions, from CuCl2 solution, and hydrogen in the silanol groups (Si-OH) in the PVG structure. In the reduction step an excess of sodium borohydride was used to ensure complete Cu2+→Cu0 reduction. Fig. 1 shows a representative drawing of the PVG/Cu discs and the UV–vis absorption spectrum of the PVG/Cu
Conclusions
Direct reduction, by sodium borohydride, of Cu2+ ions impregnated inside PVG porous structure has been demonstrated to be a simple route to synthesize copper nanoparticles. This green chemistry approach has several advantages as it is a simple procedure and free-carbon procedure with low-cost substrate preparation. The PVG/Cu porous structure allowed 4-ABT molecules to diffuse and attach on Cu nanoparticles, and so preconcentrate before the analysis. The PVG/Cu samples are shown to be an active
Acknowledgments
EBS thanks FAPESP for a post-doc fellowship. The authors would like to thank the FAPESP, CAPES and CNPq for financial supports. Contributions from Multiuser Laboratory of Advanced Optical Spectroscopy (LMEOA/IQ/UNICAMP) for Raman analysis and Brazilian Nanotechnology National Laboratory (LNNano, Campinas—SP, Brazil) for HRTEM analysis are also gratefully acknowledged. This is a contribution of the National Institute of Science and Technology in Complex Functional Materials (CNPq-MCT/FAPESP).
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