Probing the Long Range Distance Dependence of Noble Metal Nanoparticles
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Probing the Long Range Distance Dependence of Noble Metal Nanoparticles Amanda J. Haes and Richard P. Van Duyne Northwestern University, Department of Chemistry, 2145 Sheridan Road Evanston, Illinois 60208-3113, U.S.A. ABSTRACT The localized surface plasmon resonance (LSPR) of noble metal nanoparticles has recently been the subject of extensive studies. Previously, it has been demonstrated that Ag nanotriangles that have been synthesized using nanosphere lithography (NSL) behave as extremely sensitive and selective chemical and biological sensors. The present work reveals information regarding the long range distance dependence of the localized surface plasmon resonance (LSPR) of silver and gold nanoparticles. Multilayer adsorbates based on the interaction of HOOC(CH2)10SH and Cu2+ were assembled onto surface-confined nanoparticles. Measurement of the LSPR extinction peak shift versus number of layers and adsorbate thickness is non-linear and has a sensing range that is dependent on the composition, shape, in-plane width, and out-of-plane height of the nanoparticles. Theoretical modeling confirms and offers a mathematical interpretation of these results. These experiments indicate that the LSPR sensing capabilities of noble metal nanoparticles can be tuned to match the size of biological and chemical analytes by adjusting the aforementioned properties. The optimization of the LSPR nanosensor for a specific analyte will improve an already sensitive nanoparticle-based sensor. INTRODUCTION For ~20 years, surface plasmon resonance (SPR) sensors, that is, copper, gold, or silver planar films have been used as refractive index based sensing devices to detect analyte binding at or near a metal surface.1 This sensor exhibits an extremely large refractive index sensitivity (~2x106 nm/RIU) and modest decay length (200-300 nm),2 and this sensitivity is proportional to the square of the electric field that extends from the metal film. Recently, it was realized that this refractive index sensitivity also exists for noble metal nanoparticles. Although it has been demonstrated to operate successfully for nanoparticles, details regarding the aforementioned properties of planar SPR sensors still need a more complete explanation. Advancements in technology due to nanoscale phenomena of materials will be made and/or optimized when the chemical and physical properties of materials are more thoroughly understood. Prior to the realization of this technology, methods to synthesize isolated monodisperse nanoparticles in a controlled environment must be developed and their properties must be thoroughly characterized. The development of nanoparticle-based optical sensors is an extremely active area of nanoscience research. One such nanoparticle based optical sensor is known as the localized surface plasmon resonance (LSPR) nanosensor. The LSPR of noble metal nanoparticles arises when electromagnetic radiation induces a collective oscillation of the conduction electrons of the individual nanoparticles and has two primary consequences
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