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Fluorescence Enhancement by Au Nanostructures: Nanoshells and Nanorods
Rizia Bardhan,†,§ Nathaniel K. Grady,‡,§ Joseph R. Cole,‡,§ Amit Joshi, and Naomi J. Halas†,‡,§,*

Department of Chemistry and ‡Department of Electrical and Computer Engineering, §Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, and Department of Radiology, Baylor College of Medicine,Houston, Texas 77005

ABSTRACT Metallic nanoparticles influence the quantum yield and lifetime of adjacent fluorophores in a manner dependent on the properties of the nanostructure. Here we directly compare the fluorescence enhancement of the near-infrared fluorophore IR800 by Au nanoshells (NSs) and Au nanorods (NRs), where human serum albumin (HSA) serves as a spacer layer between the nanoparticle andthe fluorophore. Our measurements reveal that the quantum yield of IR800 is enhanced from 7% as an isolated fluorophore to 86% in a NSs HSA IR800 complex and 74% in a NRs HSA IR800 complex. This dramatic increase in fluorescence shows tremendous potential for contrast enhancement in fluorescence-based bioimaging.
KEYWORDS: fluorescence enhancement · IR800 · gold nanoshells · gold nanorods · frequencydomain lifetime decay

*Address correspondence to halas@rice.edu. Received for review January 2, 2009 and accepted February 11, 2009. Published online February 20, 2009. 10.1021/nn900001q CCC: $40.75
© 2009 American Chemical Society

luorescence imaging has seen widespread use in clinical diagnosis and monitoring processes in biological systems.1 The development of contrast agents, such asfluorescent probes with engineered biomarker functionalities, has become integral to the advancement of new bioimaging technologies.2 Fluorescent molecules emitting at wavelengths in the physiologically relevant “water window” (700 900 nm) are of particular interest due to the large penetration depth of nearinfrared (NIR) light in most biological media and offer the potential for imaging at significantdepths in living tissues.3 However, achieving bright fluorescent emission with photostable and biocompatible near-IR fluorophores has proven to be extremely difficult. It has long been known that, in the proximity of a metallic surface, fluorescence emission of molecules can be enhanced; this is also the case for metallic nanostructures and nanoparticles adjacent to a fluorophore.4 7 The presence of anearby metallic nanoparticle can not only enhance the quantum yield but also stabilize adjacent fluorophores against photobleaching, further enhancing their practical use in bioimaging applications.8 In new and emerging light-assisted therapeutic appli-


cations such as photothermal cancer therapy, the addition of bright near-IR fluorescence to a therapeutic nanostructure complex couldprovide additional diagnostic imaging capabilities for this treatment strategy that could facilitate clinical use. Understanding precisely how metallic nanostructures enhance molecular fluorescence is of general fundamental interest and may ultimately provide practical routes to enhancing light emission from a variety of materials systems and devices far beyond the specific application of bioimaging.Metal nanostructures exhibit remarkable optical properties due to excitation of their surface plasmons by incident light, which results in a significant enhancement of the electromagnetic field at the nanoparticle surface. This enhanced near field can be used to design highly sensitive chemical and biosensors with specific plasmon resonances tailored by the nanoparticle geometry.9,10 Metallicnanoparticles have been shown to enhance the fluorescence emission and decrease the molecular excited-state lifetimes of vicinal fluorophores. The fluorescence enhancement is attributable to a combination of processes including enhanced absorption by the molecule, modification of the radiative decay rate of the molecule, and enhanced coupling efficiency of the fluorescent emission to the far field.8,11 The...
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