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 email@example.com. 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...