Versatile Solution Phase Triangula Silver Nanoplates for Highly Sensit Plasmon Resonance Sensing Denise E. Charles, †, * Damian Aherne, ‡ Matthew Gara, ‡ Deirdre M. Ledwith, § YuriiK. Gun’ko, ‡ John M.Kelly, ‡ Werner J.Blau, † and Margaret E. Brennan-Fournet §, * † School of Physics, ‡ School of Chemistry, Trinity College Dublin, Ireland and § School of Physics, § National University Ireland, Galway T he extraordinary optical properties of noble metal nanoparticles have led to significant interest into their potential application as subwavelength op- tical elements in a diverse range of tech- nologies across all scientific fields. These op- tical properties are governed by their unique localized surface plasmon reso- nance (LSPR), that is, the collective oscilla- tion of the nanostructure’s conduction band electrons in resonance with the inci- dent electromagnetic field. 1 The spectrum of the LSPR oscillation is strongly reliant upon the nanostructure’s size, 2 shape, 3 di- electric constant, 4⫺7 and the dielectric con- stant of the surrounding environment. 8⫺10 The recognition of LSPR sensitivity to changes in these parameters has resulted in intense development of noble metal nanostructures for applications including molecular rulers, 11 bioimaging agents, 12 glu- cose concentration markers, 13 and chemi- cal and biological sensing. 14⫺16 The utiliza- tion of local medium refractive index induced LSPR sensitivity via specific bind- ing of analyte molecules to capture ligand- functionalized nanostructures, in particular, opens a route to ultrasensitive biosensors. Nonspherical nanostructures (e.g., nano- prisms, nanorods, or nanoshells) have been postulated to exhibit increased LSPR sensi- tivities via the support of large surface charge polarizability and increased local field enhancement, which they have been shown to display due to their sharp geometries. 17,18 High LSPR sensitivities for a variety of substrate bound, shaped, single nanostructures have been reported in the literature to date, including those for single silver nanoprisms, 19 silver nanocubes, 20 gold nanostars, 21 and gold nanoshells. 22 Sensitivity values as large as 0.79 eV · RIU ⫺1 for single silver nanocubes 20 and 1.41 eV · RIU ⫺1 for dielectric substrate coupled single gold nanostars 21 have been recorded. Significantly increased LSPR sensitivities have been measured for more complex coupled plasmonic nanostructures such as 801 nm · RIU ⫺1 for hematite core/Au shell nanorice at a LSPR peak wavelength ␭ max of 1160 nm 23 and 880 nm · RIU ⫺1 for gold nanorings at a ␭ max of 1545 nm. 24 As seen from these reported sensitivities, an incon- sistency exists in the units used to express the linear refractive index sensitivity of the varying nanostructures, with some authors quoting eV · RIU ⫺1 and others nm · RIU ⫺1 . Due to this inconsistency, Sherry et al. 20 de- fined a figure of merit (FOM) as a way of de- fining the overall sensitive response of a plasmonic nanostructure, where the FOM can be expressed as the ratio *Address correspondence to charled@tcd.ie, margaret.brennanfournet@nuigalway.ie. Received for review June 17, 2009 and accepted December 15, 2009. Published online December 23, 2009. 10.1021/nn9016235 © 2010 American Chemical Society ABSTRACT Solution phase triangular silver nanoplates (TSNP) with versat visibleⴚNIR wavelengths are presented as highly sensitive localized sur range of 20 TSNP solutions with edge lengths ranging from 11 to 200 nm studied comprehensively using AFM, TEM, and UVⴚvisⴚNIR spectroscopy plasmon resonance (LSPR) peak’s sensitivity to refractive index change concentration method whereby the surrounding refractive index can so other parameter. The dependence of the TSNP localized surface plasmo max and its bulk refractive index sensitivity on the nanoplate’s structure is d to increase linearly with ␭ max up to 800 nm, with the values lying within the upper li for optimal sensitivity, notwithstanding any diminution due to ensembl sensitivity is apparent at wavelengths within the NIR region with values ⴚ1 at ␭ max 1093 nm. Theoretical studies performed using a simple aspect ratio depende dipole approximation methods confirm the dependence of the LSPR bulk aspect ratio measured experimentally. These studies highlight the impo such high sensitivities and promote these TSNP sols for sensing applica biological samples. KEYWORDS: localized surface plasmon resonance (LSPR) · sensitivity · silver nanostructures · refractive index · sensor A R T I C L E www.acsnano.org VOL.4 ▪ NO. 1 ▪ 55–64 ▪ 2010 55