Identification of the optimal spectral region for plasmonic and nanoplasmonic sensing

This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in ACS Nano, copyright © American Chemical Society after peer review. To access the final edited and published work see http://pubs.acs.org/articlesonrequest/AOR-jfCkKFwsJwtEYkHTFzG7We present a theoretical and experimental study involving the sensing characteristics of wavelength interrogated plasmonic sensors based on Surface Plasmon Polaritons (SPP) in planar gold films and on Localized Surface Plasmon Resonances (LSPR) of single gold nanorods. The tunability of both sensing platforms allowed us to analyze their bulk and surface sensing characteristics as a function of the plasmon resonance position. We demonstrate that a general figure of merit (FOM), which is equivalent in wavelength and energy scales, can be employed to mutually compare both sensing schemes. Most interestingly, this FOM has revealed a spectral region for which the surface sensitivity performance of both sensor types is optimized, which we attribute to the intrinsic dielectric properties of plasmonic materials. Additionally, in good agreement with theoretical predictions, we experimentally demonstrate that, although the SPP sensor offers a much better bulk sensitivity, the LSPR sensor shows an approximately 15% better performance for surface sensitivity measurements when its FOM is optimized. However, optimization of the substrate refractive index and the accessibility of the relevant molecules to the nanoparticles can lead to a total 3-fold improvement of the FOM in LSPR sensors.This research was carried out with the support of the Commission for Universities and Investigation belonging to the Department of Innovation, Universities and Companies of the Generalitat de Catalunya, and the Social European Fund. Financial support of the M. Botn Foundation and the Nanomag CIBER-BBN Project is also greatly acknowledged. L.M.L.-M. acknowledges financial support from Ministerio de Ciencia e Innovacin (MAT2007-62696 and PCI2005-A7-0075).Peer reviewe