DW2B.3.pdf Biophotonics Congress: Optics in the Life Sciences Congress 2019 (BODA,BRAIN,NTM,OMA,OMP)© OSA 2019 Gold-Silver Alloy Nanodisk Array for Smartphone Colorimetric Biosensing Ibrahim Misbah 1 and Wei-Chuan Shih 1, 2, 3, 4. 1. Department of Electrical and Computer Engineering, University of Houston; 2. Department of Biomedical Engineering, University of Houston; 3. Program of Materials Science and Engineering, University of Houston; 4. Department of Chemistry, University of Houston; 4800 Calhoun Rd, Houston, Texas 77004, USA. Author e-mail address: wshih@uh.edu Abstract: Plasmonic hybridization in a gold-silver alloy nanodisks array results in a pair of high and low energy LSPR modes. This high energy mode is applied for colorimetric detection of sub-nM and sub-monolayer biotin-streptavidin surface binding using a smartphone camera. © 2019 The Author(s). OCIS codes: 240.6680, 250.5403. 1. Introduction Light-induced collective oscillation of free electrons in the conduction band of metallic nanostructures leads to remarkable optical properties known as localized surface plasmon resonance (LSPR) [1]. The LSPR wavelength and bandwidth typically depend on the shape, material, size and the dielectric medium surrounding it [2]. For example, nanoporous gold array has recently been shown to provide broad band field enhancement from visible to near-infrared for enhanced spectroscopy[3, 4] as well as photo-thermal conversion [5, 6] However, red and near- infrared LSPR peak shift are difficult to detect using RGB cameras. This has prompted us to consider strategies to blue shift LSPR peaks. For example, the interaction between nanoparticles in an ensemble of nanoparticles leads to the generation and modification of the resonances [7]. Plasmonic hybridization theory has been used to explain the origin of these resonances [8]. In accordance with this theory, LSPR splits into two separate high and low energy modes when nanoparticles are placed in between two contrasting medium (i.e. superstrate and substrate) [9]. More importantly, the higher energy mode has higher refractive index sensitivity compared to the lower energy mode because of their differences in the distribution of the E-field. In this article, we show that we can fully harvest the potential of this high energy mode from disk-shaped gold-silver alloy nanoparticles in a high-density, coupled plasmonic array. This plasmonic substrate is of broad and significant interest in developing novel biosensing schemes under limited resources. By utilizing the high energy mode, low-cost RGB cameras on smartphones can be employed for colorimetric readout. In this work, the high energy mode in alloy nanodisk array has been carefully engineered to have a peak around ~540 nm such that it can be used in a smartphone-based RGB colorimetry. 2. Fabrication Method and Properties of Alloy Nanodisks A planar array of gold-silver alloy nanodisks was fabricated on coverslips using Nanosphere lithography [10]. The fabrication steps involve deposition of 2 nm of Titanium as an adhesion layer and then 3 nm of the gold as a protective layer using electron-beam evaporation. Next, a gold-silver alloy film in the ratio of 30:70 respectively was sputtered using an alloy target. The combined deposited thickness was 80 nm. A monolayer of polystyrene beads of average diameter 460 nm was assembled over the alloy film. The samples were treated with oxygen plasma etching to shrink the size of the polystyrene beads into 350 nm. Fig. 1. (a) SEM image of the fabricated gold-silver alloy nanodisks in an array. (b) Measured and FDTD simulated spectra of alloy nanodisks in an array in air (black) and water (red) (c) E-field distribution of the high energy mode (P1) and low energy mode (P2).