Synthesis, Characterization, and Tunable Optical Properties of Hollow Gold Nanospheres ² Adam M. Schwartzberg, ‡,§ Tammy Y. Olson, ‡,§ Chad E. Talley, § and Jin Z. Zhang* ,‡ Department of Chemistry and Biochemistry, UniVersity of California, Santa Cruz, California 95064, and Department of Chemistry and Materials Science, Lawrence LiVermore National Laboratory, LiVermore, California 94550 ReceiVed: April 5, 2006; In Final Form: May 5, 2006 Nearly monodisperse hollow gold nanospheres (HGNs) with tunable interior and exterior diameters have been synthesized by sacrificial galvanic replacement of cobalt nanoparticles. It is possible to tune the peak of the surface plasmon band absorption between 550 and 820 nm by carefully controlling particle size and wall thickness. Cobalt particle size is tunable by simultaneously changing the concentration of sodium borohydride and sodium citrate, the reducing and capping agent, respectively. The thickness of the gold shell can be varied by carefully controlling the addition of gold salt. With successful demonstration of ensemble as well as single HGN surface-enhanced Raman scattering, these HGNs have shown great potential for chemical and biological sensing applications, especially those requiring nanostructures with near-IR absorption. Introduction Nanostructured materials of coinage metals such as gold and silver have unique optical properties due to strong surface plasmon absorption in the visible region of light and provide excellent substrates for surface plasmon resonance (SPR) spectroscopy 1-9 and surface-enhanced Raman scattering (SERS). 10-19 The surface plasmon is a collective oscillation of conduction-band electrons within the nanoparticle induced by the oscillating dipole of a resonant wavelength of light. 20 The electron oscillation induces a surface electromagnetic (EM) field, which is largely responsible for the SERS effect. 21 The wavelength at which a given nanoparticle is resonant depends on the size, shape, chemical nature of the metal, and the embedding enviroment. 22-24 In solid spherical particles, there is a single resonance at approximately 520 nm for gold and 400 nm for silver, varying slightly depending on size and embedding media. However, when one axis is extended, for example, a nanorod, the resonance will break into two absorption bands, one corre- sponding to the short axis, or transverse mode, and another to the long axis, or longitudinal mode. 25-28 The longitudinal mode has lower energy or redder absorption than the transverse mode. This is also true for aggregated systems in which there are multiple resonances within each given cluster of particles. 29-35 Therefore, controlling the size and shape of these metal nanostructures allows control of their optical properties that have potential applications in nanophotonics and sensing. Control of the structure and thereby optical properties of gold and silver nanomaterials is especially important for SERS applications because the SERS enhancement factor depends on the optical absorption of the substrate. 11,36-38 Only nanostruc- tures with absorption on-resonance with the incident light will contribute to the SERS signal, whereas those with absorption off-resonance with the incident light wavelength will contribute none or little to SERS. 39-42 In ensemble average experiments involving a large number of nanoparicles, optical and structural variations from particle to particle average out to yield consistent results from measurement to measurement. However, when examined at the single-nanoparticle or single-molecule level, the SERS spectrum and enhancement can vary significantly from one nanostructure to another because of their different structure and optical absorption. This is often complicated further by the necessity of aggregation of nanoparticles to achieve enhance- ments large enough for single-molecule observations. 39,40,42 Because aggregation is generally a random process, structural homogeneity is nearly impossible. It is essential to have high homogeneity or uniformity in the structure and optical absorp- tion of the different nanostructures to achieve good consistency on a single nanostructure level. The solution to this is to design homogeneous SERS substrates that can provide significant single-particle enhancement without aggregation. The first works to realize this goal were the so-called core/ shell systems. 43,44 There are two major advantages of the core/ shell system over standard solid gold or silver particles. First, because the synthesis of the core silica particle is well characterized, size tunable from 100 nm to more than a micrometer, and monodisperse, so is the resultant core/shell structure. 45 This leads to a nanostructure that is optically tunable from the visible to the IR and highly consistent on a particle- by-particle basis. 44 This is ideal for in situ biological studies because tissue has an absorption minimum in the IR. The second major advantage is their SERS response, even at the single- particle level. 46 Using a shell of gold or silver versus a solid particle allows the electromagnetic field to extend further from the surface, inducing greater enhancements than single, spherical particles. 43 There is also, most likely, some portion of the increased enhancement coming from surface roughness of the shells that is not present in the single particles. In these systems, the shell is most likely not single-crystalline and involves aggregates of nanoparticles. ² Part of the special issue “Charles B. Harris Festschrift”. * Corresponding author. E-mail: zhang@chemistry.ucsc.edu. Phone: (831) 459-3776. Fax: (831) 459-2935. ‡ University of California. § Lawrence Livermore National Laboratory. 19935 J. Phys. Chem. B 2006, 110, 19935-19944 10.1021/jp062136a CCC: $33.50 © 2006 American Chemical Society Published on Web 06/29/2006