Structure Enhancement Factor Relationships in Single Gold Nanoantennas by Surface-Enhanced Raman Excitation Spectroscopy Samuel L. Kleinman, ‡ Bhavya Sharma, ‡ Martin G. Blaber, ‡ Anne-Isabelle Henry, ‡ Nicholas Valley, ‡ R. Griffith Freeman, § Michael J. Natan, § George C. Schatz, ‡ and Richard P. Van Duyne* ,‡ ‡ Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States § Cabot Security Materials Incorporated, 325 East Middlefield Road, Mountain View, California 94043, United States * S Supporting Information ABSTRACT: Determining the existence of any direct spectral relationship between the far-field scattering properties and the near-field Raman-enhancing properties of surface-enhanced Raman spectroscopy (SERS) substrates has been a challenging task with only a few significant results to date. Here, we prove that hot spot dominated systems show little dependence on the far-field scattering properties because of differences between near- and far-field localized surface plasmon resonance (LSPR) effects as well as excitation of new plasmon modes via a localized emitter. We directly probe the relationship between the near- and far-field light interactions using a correlated LSPR-transmission electron microscopy (TEM) surface-enhanced Raman excitation spectroscopy (SERES) technique. Fourteen individual SERS nanoantennas, Au nanoparticle aggregates ranging from dimers to undecamers, coated in a reporter molecule and encased in a protective silica shell, were excited using eight laser wavelengths. We observed no correlation between the spectral position of the LSPR maxima and the maximum enhancement factor (EF). The single nanoantenna data reveal EFs ranging from (2.5 ± 0.6) × 10 4 to (4.5 ± 0.6) × 10 8 with maximum enhancement for excitation wavelengths of 785 nm and lower energy. The magnitude of maximum EF was not correlated to the number of cores in the nanoantenna or the spectral position of the LSPR, suggesting a separation between near-field SERS enhancement and far-field Rayleigh scattering. Computational electrodynamics confirms the decoupling of maximum SERS enhancement from the peak of the scattering spectrum. It also points to the importance of a localized emitter for radiating Raman photons to the far-field which, in nonsymmetric systems, allows for the excitation of radiative plasmon modes that are difficult to excite with plane waves. Once these effects are considered, we are able to fully explain the hot spot dominated SERS response of the nanoantennas. 1. INTRODUCTION Surface-enhanced Raman spectroscopy (SERS) is a valuable analytical tool for the ultrasensitive identification and quantifi- cation of a host of molecules ranging from chemical warfare agents to biomolecules and artist dyestuffs. 1,2 At the outset, SERS was a niche technique with relatively few applications, namely, monitoring spectroelectrochemical and redox-related reac- tions 3-5 and elucidating fundamental aspects of surface- mediated spectroscopies. 6,7 Attention persisted in the field through the 1980s and into the next decade, and the seminal claims of observation of SERS signal from a single molecule in 1997 8,9 added interest to the field. The implementation of modern instrumentation for preparing and characterizing substrates and for measuring and interpreting spectra has driven activity to the present day. Presently, the incorporation of nanoparticles with a strong SERS response is attractive in a wide variety of applications ranging from in vivo measurements of cellular processes to pharmaceutical tracking. 10 One very promising and stable material for this application is aggregated nanoparticles encapsulated with silica, commonly known as SERS nano- antennas. The nanoantennas used in this work consist of 90 nm Au particles functionalized with a reporter molecule then lightly aggregated and subsequently coated with a protective SiO 2 shell. This coating makes them extremely stable, lasting in aqueous solution for years after creation. Additionally, glass particles are largely inert in biological systems, and if needed, the silica can be modified via silane chemical linker protocols. 11,12 The nano- antennas are composed of various numbers of Au cores, ranging from monomers to aggregates exceeding 10 constituents. Nanoantennas are not adversely affected by centrifugation methods; furthermore, centrifugal processing has been imple- mented to improve the average SERS signal per nanoantenna by removing the less-active monomer components from the resulting synthetic mixture. 13 The reporter molecules are added after nanoparticle synthesis, and the variety of distinct reporter options is extremely diverse. In this case we examine SERS nanoantennas labeled with trans-1,2-bis(4-pyridyl)- ethylene (BPE), a model SERS probe. Nanoantennas are easily integrated into a variety of materials and can provide a stable signal in many conditions. The question remains: What are the Received: September 19, 2012 Published: December 6, 2012 Article pubs.acs.org/JACS © 2012 American Chemical Society 301 dx.doi.org/10.1021/ja309300d | J. Am. Chem. Soc. 2013, 135, 301-308