Journal of Fluorescence, Vol. 14, No. 4, July 2004 (©2004) Using Solution-Phase Nanoparticles, Surface-Confined Nanoparticle Arrays and Single Nanoparticles as Biological Sensing Platforms Amanda J. Haes, 1 Douglas A. Stuart, 1 Shuming Nie, 2 and Richard P. Van Duyne 1,3 Received November 1, 2003; revised January 29, 2004; accepted January 29, 2004 The intense colors of noble metal nanoparticles have inspired artists and fascinated scientists for hundreds of years. In this review, we describe three sensing platforms based on the tunability of the localized surface plasmon resonance (LSPR) of gold and silver nanoparticles. Specifically, the color associated with solution-phase nanoparticles, surface-confined nanoparticle arrays, and single nanoparticles will be shown to be tunable and useful as platforms for biological sensing. KEY WORDS: Nanoparticles; localized surface plasmon resonance; biosensor; streptavidin; immunoassay. INTRODUCTION Recent years have seen an increase in the imple- mentation of fluorescence for biological assays, detec- tion, labeling, and sensing. While fluorescence based bi- ological assays have become standard in industry and academia, nanoparticle based techniques provide sev- eral alternatives to these conventional methods, includ- ing: nanoparticle barcode labels [1], resonant Rayleigh scattering [2,3], nanoparticle aggregation [4], local re- fractive index changes [5,6], and charge transfer inter- actions [5,7]. The intense scattering and absorption of light from noble metal nanoparticles is the source of the beautiful col- ors in stained glass windows and has attracted the interest of scientists for generations (Fig. 1). Although scientists have learned that the characteristic hues of these noble metal nanoparticle suspensions arise from their strong in- teraction with light, the advent of the field of nanoparticle 1 Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113. 2 Departments of Biomedical Engineering and Chemistry, Emory Uni- versity and Georgia Institute of Technology, Pierce Drive Suite 2001, Atlanta, Georgia. 3 To whom correspondence should be addressed. E-mail: vanduyne@ chem.northwestern.edu. optics allowed for a deep understanding of the relationship between material properties such as composition, size, shape, and local dielectric environment and the observed color of a metal suspension. An understanding of the op- tical properties of noble metal nanoparticles holds both fundamental and practical significance. Fundamentally, it is important to systematically explore the nanoscale struc- tural and local environmental characteristics that cause optical property variation as well as provide access to regimes of predictable behavior. Practically, the tunable optical properties of nanostructures can be applied as ma- terials for surface-enhanced spectroscopy [8–12], optical filters [13,14], plasmonic devices [15–17], and sensors [6,7,18–24]. Noble metal nanoparticles exhibit a strong UV- visible absorption band that is not present in the spectrum of the bulk metal. This absorption band results when the incident photon frequency is resonant with the collective excitation of the conduction electrons and is known as the localized surface plasmon resonance (LSPR). LSPR excitation results in wavelength selective absorption with extremely large molar extinction coefficients ∼3 × 10 11 M −1 cm −1 [25–27], resonant Rayleigh scattering [28,29] with an efficiency equivalent to that of 10 6 fluorophors [3,30,31], and the enhanced local electromagnetic fields near the surface of the nanoparticle which are responsible 355 1053-0509/04/0700-0355/0 C  2004 Plenum Publishing Corporation