KING ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX www.acsnano.org A C XXXX American Chemical Society Fano Resonant Aluminum Nanoclusters for Plasmonic Colorimetric Sensing Nicholas S. King, †, ) Lifei Liu, †, ) Xiao Yang, †, ) Benjamin Cerjan, †, ) Henry O. Everitt, ‡,^ Peter Nordlander, †,‡, ) and Naomi J. Halas * ,†,‡,§, ) † Department of Physics and Astronomy, ‡ Department of Electrical and Computer Engineering, § Department of Chemistry, and ) Laboratory for Nanophotonics, Rice University, 6100 Main Street, Houston, Texas 77005 United States and ^ Army Aviation and Missile RD&E Center, Redstone Arsenal, Alabama 35898 United States A luminum is an excellent material for plasmonics, with an oxidation stabi- lity superior to that of silver and a functional range of plasmon resonant tun- ability across the ultraviolet and visible regime. 1À19 Furthermore, it is an abundant, sustainable material with the potential to transition laboratory-based research into practical applications and commercial pro- ducts. Much of the current and growing interest in aluminum plasmonics is due to the wavelength range of the plasmon resonances obtainable with simple geome- tries. Its use as a potentially superior chro- matic material relative to organic media for color displays has captured great inter- est. 6,7,10,14,17,20À22 A range of nanostruc- tures, such as disks, 7 holes, 22 rods, 6 and crosses, 21 have been fabricated and studied, showing localized plasmon tuning for optical filters and other colorimetric applications. An important application in plasmonics is localized surface plasmon resonance (LSPR) sensing. 23,24 This process involves monitoring the shift of the plasmon resonance of a finite nanostructure as its dielectric environment is changed. The screening charges induced in the surrounding dielectric red shift the plasmon resonance by an amount that de- pends on the permittivity of the dielectric: a larger permittivity results in a larger red shift. Although, in principle, the red shift of the LSPR provides a direct report of the dielectric permittivity surrounding the nanostructure, the permittivities of typical analytes are too similar to induce distin- guishable red shifts, so this approach does not provide analyte selectivity. To overcome this problem in realistic applications, the plasmonic nanostructure can be functiona- lized with molecules that can only bind specific analytes: an LSPR shift will only be observed if these analytes are present and bind to the receptors. For gold nanostruc- tures, the highest LSPR sensitivity is for plasmon resonances in the near-infrared, requiring substantial instrumentation for monitoring of the plasmon resonance, such as spectrophotometers. This relatively bulky experimental setup requires stable ambient * Address correspondence to halas@rice.edu, nordland@rice.edu. Received for review August 5, 2015 and accepted October 1, 2015. Published online 10.1021/acsnano.5b04864 ABSTRACT Aluminum is an abundant and high-quality material for plasmonics with potential for large-area, low-cost photonic technologies. Here we examine aluminum nanoclusters with plasmonic Fano resonances that can be tuned from the near-UV into the visible region of the spectrum. These nanoclusters can be designed with specific chromaticities in the blue- green region of the spectrum and exhibit a remarkable spectral sensitivity to changes in the local dielectric environment. We show that such structures can be used quite generally for colorimetric localized surface plasmon resonance (LSPR) sensing, where the presence of analytes is detected by directly observable color changes rather than through photodetectors and spectral analyzers. To quantify our results and provide a metric for optimization of such structures for colorimetric LSPR sensing, we introduce a figure of merit based on the color perception ability of the human eye. KEYWORDS: plasmon . aluminum . ultraviolet . Fano resonance . chromaticity . figure of merit ARTICLE