Review of Some Interesting Surface Plasmon Resonance-enhanced Properties of Noble Metal Nanoparticles and Their Applications to Biosystems Prashant K. Jain & Xiaohua Huang & Ivan H. El-Sayed & Mostafa A. El-Sayed Received: 12 February 2007 / Accepted: 29 May 2007 / Published online: 13 July 2007 # Springer Science + Business Media, LLC 2007 Abstract Noble metal, especially gold (Au) and silver (Ag) nanoparticles exhibit unique and tunable optical properties on account of their surface plasmon resonance (SPR). In this review, we discuss the SPR-enhanced optical properties of noble metal nanoparticles, with an emphasis on the recent advances in the utility of these plasmonic properties in molecular-specific imaging and sensing, photo-diagnostics, and selective photothermal therapy. The strongly enhanced SPR scattering from Au nanoparticles makes them useful as bright optical tags for molecular- specific biological imaging and detection using simple dark-field optical microscopy. On the other hand, the SPR absorption of the nanoparticles has allowed their use in the selective laser photothermal therapy of cancer. We also discuss the sensitivity of the nanoparticle SPR frequency to the local medium dielectric constant, which has been successfully exploited for the optical sensing of chemical and biological analytes. Plasmon coupling between metal nanoparticle pairs is also discussed, which forms the basis for nanoparticle assembly-based biodiagnostics and the plasmon ruler for dynamic measurement of nanoscale distances in biological systems. Keywords Surface plasmon resonance (SPR) . SPR sensing . Mie scattering . Metal nanocrystals for biodiagnostics . Photothermal therapy . Plasmon coupling Introduction The bright and fascinating colors of noble metal nano- particles have attracted considerable interest right since historical times as decorative pigments in stained glasses and artworks [1, 2]. In recent times, the tunable photo- physical attributes of metal nanocrystals [3–5], their efficient addressability via optical and spectroscopic tech- niques, and rapid advances in nanoparticle synthesis and fabrication [6] have brought these nanostructures to the forefront of nanotechnology research directed toward applications ranging from photonics [7, 8] to biomedicine [9–13]. As first pointed out by Gustav Mie in 1908 [14], the interaction of light with metal nanoparticles results in the collective oscillation of the metal-free electrons with respect to the nanoparticle lattice in resonance with the light field [3–5, 15–17]. This phenomenon is known as the surface plasmon resonance (SPR). For a spherical metal nanoparticle with a size much smaller than the wavelength of light, the surface plasmon oscillation is dominated by the dipolar mode with a polarizability α given by [4]: a ¼ 3" 0 V " À " m " þ 2" m   ; ð1Þ Plasmonics (2007) 2:107–118 DOI 10.1007/s11468-007-9031-1 P. K. Jain : X. Huang : M. A. El-Sayed (*) Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332-0400, USA e-mail: melsayed@gatech.edu I. H. El-Sayed Department of Otolaryngology—Head and Neck Surgery, Comprehensive Cancer Center, University of California at San Francisco, San Francisco, California 94143, USA M. A. El-Sayed University of California, Berkeley, California 94720, USA