Surface-Enhanced Raman Spectroscopy of Double-Shell Hollow Nanoparticles: Electromagnetic and Chemical Enhancements Mahmoud A. Mahmoud* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States * S Supporting Information ABSTRACT: Enhancements of the Raman signal by the newly prepared gold-palladium and gold-platinum double- shell hollow nanoparticles were examined and compared with those using gold nanocages (AuNCs). The surface-enhanced Raman spectra (SERS) of thiophenol adsorbed on the surface of AuNCs assembled into a Langmuir-Blodgett monolayer were 10-fold stronger than AuNCs with an inner Pt or Pd shell. The chemical and electromagnetic enhancement mechanisms for these hollow nanoparticles were further proved by comparing the Raman enhancement of nitrothiophenol and nitrotoulene. Nitrothiophenol binds to the surface of the nanoparticles by covalent interaction, and Raman enhancement by both the two mechanisms is possible, while nitrotoulene does not form any chemical bond with the surface of the nanoparticles and hence no chemical enhancement is expected. Based on discrete dipole approximation (DDA) calculations and the experimental SERS results, AuNCs introduced a high electromagnetic enhancement, while the nanocages with inner Pt or Pd shell have a strong chemical enhancement. The optical measurements of the localized surface plasmon resonance (LSPR) of the nanocages with an outer Au shell and an inner Pt or Pd shell were found, experimentally and theoretically, to be broad compared with AuNCs. The possible reason could be due to the decrease of the coherence time of Au oscillated free electrons and fast damping of the plasmon energy. This agreed well with the fact that a Pt or Pd inner nanoshell decreases the electromagnetic field of the outer Au nanoshell while increasing the SERS chemical enhancement. ■ INTRODUCTION Plasmonic nanostructures attract much interest because of their unique optical, 1-3 photothermal, photoelectromagnetic, 4 and photoacoustic 5 properties which result from the large scattering and absorption cross sections and strong electromagnetic plasmon field. 1,2 The electromagnetic plasmon field of the plasmonic nanoparticles enhances different optical phenomena such as surface-enhanced IR (SEIR), 6 sum frequency generation (SESFG), 7 second harmonic generation (SESHG), 8 and surface-enhanced fluorescence 9 by factor 10 4 and enhanced Raman signal in surface-enhanced Raman spectroscopy (SERS) by a factor of 10 14 . Different shapes and sizes of plasmonic nanoparticles have been prepared with a localized surface plasmon resonance (LSPR) spectrum that covers visible and near-infrared regions. 10-17 Recently, hollow double-shell nanoparticles with different touching metal shells such as platinum, palladium, and gold were synthesized. However, their optical properties remain far from clear. 18,19 Irradiation of plasmonic nanoparticles with electromagnetic radiation of resonance frequency causes their free electrons to oscillate collectively along the nanoparticle. 1,2,18 The oscillation of the conduction band free electrons dephases in a short time scale 20 (less than ∼5 fs) by different processes such as scattering of the electrons into empty levels in the conduction band, electron-phonon coupling, electron-surface scattering, and radiation damping. 21,22 The extinction surface plasmon resonance spectrum of the plasmonic nanoparticles consists of absorption and scattering. The scattering spectrum arises when the plasmon oscillation decays radiatively with electromagnetic energy of the same frequency as the surface plasmon oscillation (elastic or Rayleigh scattering). The nonradiative decay occurs via excitation of electron-hole pairs either within the conduction band (intraband excitation) or between the d-band and the conduction band (interband excitations). 12,13,20 The non- radiative pathways of plasmon decay are responsible for the absorption of light by the nanoparticle. The enhancement of Raman signal by plasmonic nano- particles has been described by two acceptable mechanisms, namely chemical 23 and electromagnetic mechanisms. 24 The chemical mechanism of enhancement involves the charge transfer between the analyte, chemosorbed on the surface of the metallic nanoparticles, and the metallic nanoparticles. 25 The Fermi level of the metallic nanoparticles facilitate the HOMO- LUMO transitions in the chemisorbed molecule. 26 This is affecting the Raman polarizability of the adsorbed molecule. Received: March 7, 2013 Revised: April 3, 2013 Published: May 6, 2013 Article pubs.acs.org/Langmuir © 2013 American Chemical Society 6253 dx.doi.org/10.1021/la400845z | Langmuir 2013, 29, 6253-6261