Contents lists available at ScienceDirect LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt Rapid analysis of herbicide diquat in apple juice with surface enhanced Raman spectroscopy: Effects of particle size and the ratio of gold to silver with gold and gold-silver core-shell bimetallic nanoparticles as substrates Nianwei Xu a,b , Keqiang Lai b , Yuxia Fan b , Barbara A. Rasco c , Yiqun Huang a,* a School of Chemistry and Food Engineering, Changsha University of Science and Technology, 960, 2nd Section, Wanjiali South Rd, Changsha, Hunan, 410114, PR China b College of Food Science and Technology, Shanghai Ocean University, No. 999 Hucheng Huan Road, LinGang New City, Shanghai, 201306, PR China c School of Food Science, Washington State University, Pullman, WA, 99164-6376, USA ARTICLE INFO Keywords: Surface enhanced Raman scattering Nanoparticles Pesticide Fruit juice Rapid analysis ABSTRACT Twelve different Au nanoparticles (NPs; diameter, 19–87 nm) and Au–Ag core-shell NPs (diameters of core and particles: 19 nm, 35–91 nm; 43 nm, 66–127 nm) varying in size and the ratio of gold to silver were employed as surface enhanced Raman scattering (SERS) substrates for rapid analysis of herbicide diquat cation in apple juice. The minimum detectible concentrations for diquat cation standards with three selected optimal SERS substrates were 0.005 mg/L (43 nm Au NPs), 0.05 mg/L (91 nm Au–Ag NPs, 19 nm core), and 0.025 mg/L (78 nm Au–Ag NPs, 43 nm core), respectively. However, only the 78 nm Au–Ag NPs led to acceptable SERS enhancement effect for diquat cation in apple juice with minimum sample preparation, allowing its detection at as low as 0.025 mg/ L. In addition to providing a rapid analysis approach for diquat in fruit juices, this study indicates that the influences of particle size and composition of NPs on SERS enhancement effects for an analyte in standard solutions could be quite different from that in a real food sample, while the optimal ratio of gold to silver of the bimetallic NPs depended on the Au seeds and sample matrix. 1. Introduction Diquat dibromide (1,1′-ethylene-2,2′-dipyridylium dibromide) is widely used in agricultural practices as a non-selective contact herbi- cide, algicide and defoliant to control weeds in crop and non-crop areas (EPA, 1995). Diquat dibromide could be accumulated into human body through direct skin contact and ingestion, and a large amount of bioaccumulation can cause toxicity to heart, liver, lung and other or- gans as well as fetal malformation (Jones & Vale, 2000). The estab- lished maximum residue limits (MRLs) or tolerance for diquat di- bromide (calculated as the diquat cation) in fruits range from 0.01 ppm to 0.05 ppm according to the standards of the European Commission (EU Pesticide database, 2018), the USA (EPA, 1995), and the Codex Alimentarius (FAO Pesticides Database, 2018). Typically, the MRLs for diquat cation in citrus fruits (such as oranges, lemons and grapefruits) and pome fruits (such as apples and pears) are set as 0.02 ppm, but for other fruits, such as banana (0.02–0.05 ppm) and grape (0.01–0.05 ppm), the MRLs or tolerance may slightly vary among these three standards. Among a number of alternatives to replace commonly used chromatographic methods (Nagatomi, Yoshioka, Yanagisawa, Uyama, & Mochizuki, 2013; Pizzutti et al., 2016) for rapid analysis of residual pesticides in foods, surface enhanced Raman spectroscopy or surface enhanced Raman scattering (SERS, used interchangeable in this article) has caught an increasing attention in the past decade. Unlike conven- tional Raman spectroscopy applicable only for bulk composition ana- lysis, SERS could be employed for trace chemical analysis through utilizing a special surface enhancement phenomenon of Raman scat- tering of an analyte on the surface of some nanostructured metals or metallic nanoparticles (NPs) commonly made of Au and/or Ag. As compared to Au NPs, Ag NPs may lead to higher enhancement effects, but it is difficult to synthesize Ag NPs with uniform size and to keep them stable. Some studies indicated that gold coated by silver (Au–Ag) core-shell bimetallic NPs have the advantages of both Au and Ag NPs with tunable localized surface plasmon resonance, controllable particle size and good stability that allows the bimetallic NPs to be applied in a wide range of chemical analyses (Pei et al., 2015; Samal et al., 2013). SERS has been applied to analyze residual pesticides like phosmet, carbaryl, thiabendazole and flusilazole in fruits and fruit juices (Fan, Lai, Rasco, & Huang, 2014, 2015; Alsammarraie et al., 2018; Chen, https://doi.org/10.1016/j.lwt.2019.108547 Received 30 May 2019; Received in revised form 28 July 2019; Accepted 25 August 2019 * Corresponding author. E-mail addresses: yiqunh@csust.edu.cn, yiqunh@hotmail.com (Y. Huang). LWT - Food Science and Technology 116 (2019) 108547 Available online 27 August 2019 0023-6438/ © 2019 Elsevier Ltd. All rights reserved. T