Substrate Effect on the Plasmonic Sensing Ability of Hollow Nanoparticles of Different Shapes Mahmoud A. Mahmoud and Mostafa A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States * S Supporting Information ABSTRACT: Gold hollow nanospheres (AuHSs) and gold hollow nanocubes (AuHCs) were synthesized by the galvanic replacement technique using silver nano templates. Colloidal AuHSs are found to have a higher sensitivity factor than that of AuHCs. This value decreases for both shapes when the nanoparticles are assembled on a quartz substrate by using the Langmuir-Blodgett technique. AuHSs are observed to have the larger effect. It is observed that as the separation gap between AuHCs nanoparticles decreases, their localized surface plasmon resonance band red shifts more than AuHSs. This is accounted for by the discrete dipole approximation (DDA) calculations. The coupling between the plasmon fields of the AuHCs pair is stronger than that between AuHSs pair. Using the DDA calculation, this is found to be due to geometric factors, as well as to the difference in the plasmonic field intensity. The calculation also showed that the plasmon field distributions of both AuHCs and AuHSs were distorted by the quartz substrate in a different manner. It is also observed that the surface-enhanced Raman spectrum of thiophenol is stronger when measured on AuHCs than on AuHSs. This is due to the difference in the plasmon field distribution as well as the fact that the AuHCs have a higher scattering/absorption yield ratio. ■ INTRODUCTION Plasmonic metallic nanoparticles are characterized by the presence of localized surface plasmon resonance (LSPR) and the associated electromagnetic plasmon fields induced when the free electrons oscillate collectively with resonant electro- magnetic radiation. 1,2 This plasmon field is generated on the surface of the nanoparticles, which enhances both absorption and scattering. Plasmonic nanoparticles have broad applications utilizing both the LSPR spectrum and the plasmon field. 3 The LSPR and the plasmon field intensity depend on a variety of factors. 4,5 (1) For nanoparticles of similar shapes, a red shift of the LSPR peak is observed as the nanoparticle size is increased. 6 (2) In nanoparticles dimers, the plasmon field coupling between a pair of nanoparticles placed at a separation distance less than twice their size shifts the LSPR peak to a lower energy. 4,7 (3) As the symmetry of the nanoparticles decreases, the LSPR peak red-shifts and a new peak is generated due to the variation of the electron restoring force energy and the polarizability. 3 (4) The LSPR peak red-shifts as the dielectric function of the medium increases. 8 (5) The presence of a substrate distorts the distribution of the plasmon field around the nanoparticles, due to the change of the dielectric function at one side of the nanoparticle, and this either red- or blue-shifts the LSPR peak. 9-11 Plasmonic nanoparticles of various shapes and sizes have been prepared for different optical or biological applications. The design of the synthesis of the plasmonic nanoparticles is mainly focused on tailoring the LSPR peak position and maximizing the plasmon field intensity. Plasmonic nano- particles of various shapes such as spheres, 6 cubes, 12 rods, 13 stars, 14 triangles, 15 shells, 16 hollow nanospheres, 17 and frames 18 with LSPR spectrum covering the visible and near-infrared regions have been prepared. Sun and Xia 12 have prepared gold nanocages by galvanic replacement method in which the LSPR peak of these hollow shaped nanoparticles red-shifts as the wall thickness decreases. 19 Two kinds of plasmon fields (inside and outside the hollow nanoparticle) were observed by surface- enhanced Raman spectroscopy (SERS 20 ) of thiophenol adsorbed on gold nanoframes and confirmed by discrete dipole approximation (DDA 21 ) calculation. 18,22 Plasmonic nanoparticles have been used in sensing biological systems, 23,24 detecting pollutant gases by SERS, 19 improving optical extinction, 25 and fluorescence techniques. 26 Most of these applications require the nanoparticles to be assembled on the surface of substrate. Different techniques have been used to prepare the plasmonic nanoparticles on the surface of substrate such as electron beam lithography, 27 soft lithography, DC sputtering, nanosphere lithography 2 and helium ion lithog- raphy. Although these methods succeeded in preparing different shapes of nanoparticles, the colloidal chemical method remains the most efficient method to control the shape of the nanoparticles. Currently, the difficulties of assembling the colloidal plasmonic nanoparticles on the surface of a substrate limit their applications but the Langmuir-Blodgett technique can overcome this problem. Different shapes of gold and silver Special Issue: Paul F. Barbara Memorial Issue Received: August 29, 2012 Revised: October 7, 2012 Published: October 17, 2012 Article pubs.acs.org/JPCB © 2012 American Chemical Society 4468 dx.doi.org/10.1021/jp3085793 | J. Phys. Chem. B 2013, 117, 4468-4477