Letter pubs.acs.org/JPCL
Ag@SiO2 Core−Shell Nanostructures: Distance-Dependent Plasmon Coupling and SERS Investigation M. Shanthil,† Reshmi Thomas,‡ R. S. Swathi,*,‡ and K. George Thomas*,†,‡ †
Photosciences and Photonics Group, National Institute for Interdisciplinary Science and Technology (NIIST), CSIR, Thiruvananthapuram, 695 019, India ‡ School of Chemistry, Indian Institute of Science Education and Research-Thiruvananthapuram (IISER-TVM), CET Campus, Thiruvananthapuram, 695 016, India S Supporting Information *
ABSTRACT: Enhancement of Raman signals of pyrene due to the enhanced electric fields on the surface of silver nanoparticles has been investigated by controlling the thickness of the silica shell. Dimeric nanostructures having well-defined gaps between two silver nanoparticles were prepared, and the gap size (d) was varied from 1.5 to 40 nm. The molecules trapped at the dimeric junctions showed higher Raman signal enhancements when the gap was less than 15 nm due to the presence of amplified electric field, in agreement with our theoretical studies. The experimental Raman enhancement factors at the hot spots follow a 1/dn dependence, with n = 1.5, in agreement with the recent theoretical studies by Schatz and co-workers. Experimental results presented here on the distance dependence of surface enhanced Raman spectroscopy (SERS) enhancement at the hot spots can provide insight on the design of newer plasmonic nanostructures with optimal nanogaps. SECTION: Plasmonics, Optical Materials, and Hard Matter influence of intense electric fields at the hot spots on Raman signal enhancements. Herein we report the Raman signal enhancement of an ammonium salt of pyrene (Py-A, Figure 1) when bound onto the surface of isolated silver nanoparticles having silica shells of varying thickness, “t” (Ag@SiO2 NPs; t = 0, 3, 6, 10, 15, and 25 nm). We have also prepared dimeric nanostructures of Ag@SiO2 NPs of varying shell thickness, and the signal enhancement of Py-A at the junctions was investigated. Further, we have used the finite difference time domain method (FDTD)18,19 to analyze the near-field and the far-field optical properties of the monomeric and the dimeric Ag@SiO2 nanostructures. In the present study, silver nanoparticles having an average diameter of 60 ± 5 nm were synthesized by the sodium citrate reduction method,20 and their surface was modified using polyvinyl pyrrolidone (mol. wt. 10 000), which acts as a silane coupling agent as well as a stabilizer. Two factors contribute to the Raman signal enhancement of a molecule near metal nanostructures: metal−molecule charge transfer and enhanced electromagnetic fields prevailing on the metal surface. The main purpose of overcoating with silica is to (i) eliminate the contribution of the former process and (ii) tune the electromagnetic field experienced by a probe, when bound on the surface, by varying the thickness of the silica shell.21,22 Uniform silica coating over silver nanoparticles was achieved by the Stöber condensation reaction and the shell thickness was varied from 3 to 25 nm by controlling the reaction time (see Supporting
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lasmonic nanostructures are proposed as promising platforms for the fast, sensitive and specific analysis of various molecules of importance in the biomedical field, the environment, and safety by using various spectroscopic techniques such as Raman scattering and fluorescence spectroscopy.1−4 The rationale behind the design of plasmonic nanostructures is based on the fact that noble metal nanoparticles on interaction with visible light generate intense electric fields on their surface. When molecules are placed on the surfaces of these metal nanostructures, an enhancement in the electronic and vibrational spectroscopic signals is observed.3−5 Compared to fluorescence, Raman scattering provides spectral fingerprints of molecules, which allows the identification of analytes. However, Raman spectroscopy is not widely used due to the low scattering cross sections of ∼10−25 to 10−31 cm2 per molecule. The interaction of incident light with plasmonic nanostructures provides newer opportunities in signal enhancement and hence can help in efficient detection of various analytes.6 Recently, it has been demonstrated that assembled nanostructures possess strong surface plasmon coupling, creating amplified electromagnetic fields (hot spots) at the nanogaps.7,8 Raman signals of analyte molecules can be significantly enhanced by placing them at the nanogaps of these plasmonic nanostructures. Several approaches have been reported for the design of such plasmonic nanostructures having nanogaps: various strategies include (i) lithographic methods,9,10 (ii) chemical functionalization methods,11,12 (iii) biomolecular and supramolecular organization,13−15 and (iv) salt-induced aggregation.16,17 One of the major challenges is to design rigid plasmonic nanostructures with tunable nanogaps for investigating the © 2012 American Chemical Society
Received: April 3, 2012 Accepted: May 11, 2012 Published: May 11, 2012 1459
dx.doi.org/10.1021/jz3004014 | J. Phys. Chem. Lett. 2012, 3, 1459−1464
The Journal of Physical Chemistry Letters
Letter
spectroscopic investigations on Py-A at Ag@SiO2 NPs. The stability of core−shell nanoparticles in aqueous medium was investigated by following the zeta potential (ζ) and the UV− visible extinction spectra as a function of time. The ζ values of Ag@SiO2 NPs remain constant, −33 ± 2 mV in H2O, and no spectral change was observed for several hours (trace a, Figure 2A), indicating high stability. The ζ studies indicate that Ag@
Figure 1. (A−E) HRTEM images of Ag@SiO2 NPs of varying t (dropcasted onto a carbon coated Cu grid). (A′−E′) Results of the FDTD simulations showing the intensity of the relative electric field (I) around the corresponding Ag@SiO2 NPs. Dashed circles represent the periphery of the silica shell surface. The scale bars are kept uniform for better comparison across various shell thickness. Refer to Figure S7 for the actual values of the electric field. (F) Extinction spectra of Ag@ SiO2 NPs: t = 0 nm (blue solid trace) and t = 25 nm (dash-dotted black trace). Extinction spectra of Ag@SiO2 NPs: t = 3 nm (red dotted trace) recorded immediately after the addition of Py-A. (G) Raman spectra recorded immediately after the (
