Sensors and Actuators B 231 (2016) 584–592 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Gold nanorods as peroxidase mimetics and its application for colorimetric biosensing of malathion Sharmila Biswas, Pranav Tripathi, Nitin Kumar, Seema Nara ∗ Department of Biotechnology, Motilal Nehru National Institute of Technology Allahabad, 211004 U.P., India a r t i c l e i n f o Article history: Received 26 October 2015 Received in revised form 11 March 2016 Accepted 16 March 2016 Available online 17 March 2016 Keywords: Gold nanorods Malathion Peroxidase Horseradish peroxidase Enzyme mimetics a b s t r a c t Nanozymes have drawn lot of attention as they overcome the limitations of natural enzymes and present a more robust alternative. Herein, the potential of Gold nanorods (GNRs) as nanozyme or enzyme mimetics is investigated. The detailed catalytic parameters of GNRs are studied and compared with other gold nanostructures (+ve and −ve nanoparticles) and horseradish peroxidase (HRP). Gold nanorods of 2.8 aspect ratio were synthesized and demonstrated to have 2.5 times higher peroxidase activity than HRP and +ve gold nanoparticles. GNRs are found to be robust enzyme mimetics displaying stability over a wide range of pH and temperature. Proof of concept is demonstrated for developing a simple, colorimetric inhibition assay for malathion using GNRs as nanozyme. The assay shows an inverse relationship between GNRs catalytic activity and malathion concentration. The sensitivity of the assay is 1.78 g/mL and the assay is specific with <0.01% cross-reaction with organosphosphates and metal salts. The study presents the intrinsic catalytic potential of GNRs for the first time and its subsequent use thereof for developing an assay for malathion. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Enzyme mimetics are those materials which possess simi- lar catalytic properties as enzymes but overcome their demerits [1]. They are also defined as “nanozyme” to describe a man- made nanomaterial that is capable of simulating catalytic function demonstrated by natural enzyme [2,3]. Recently, nanoparticles (NPs) have received increasing attention in enzyme mimetic research as they overcomes the problems of natural enzymes such as their low operational stability (denaturation and digestion), sen- sitivity of catalytic activity to environmental conditions, difficulties in recovery and recycling, high costs in preparation and purification [4,5] and also the demerits of artificial enzymes like unsatisfac- tory activity and selectivity, and complex synthesis [6]. Contrary to natural or artificial enzymes, NP enzyme mimetics provides high stability against harsh conditions, versatile chemistry, cost- effective synthesis and tunable catalytic activities [6,7]. These NP mimetics have been reported to possess peroxidases, oxidases or catalases like activities and have been excellently reviewed [3,7,8]. NP mimetics can be broadly classified into three major classes, carbon [9,10], metal [11–15] and metal oxides [16–19]. Out of ∗ Corresponding author. E-mail addresses: seemanara@mnnit.ac.in, seemanara@gmail.com (S. Nara). these, the noble gold nanoparticles have been exploited for devel- oping various detection methods due to their surface plasmon resonance (SPR), Raman, fluorescence and conductance properties. However, their catalytic property largely remains ignored or rela- tively unexplored ever since the first report in 2004 demonstrates their intrinsic glucose oxidase like activity [20]. Substrate-supported gold nanoparticles (GNPs) possess high catalytic activities toward the oxidation of CO or NO. Recently, col- loidal GNPs are found to exhibit glucose oxidase (GOx)-like activity [20–22]. This GOx like activity of GNPs has been exploited for nanoplasmonic detection of DNA hybridization by coupling it with HRP in an enzyme cascade reaction. The system is although highly sensitive but is limited by the polydispersity of the nanoparticles and the fact that citrate capped GNPs can not be directly (that is, without prior sample separation) applied in biological fluids owing to strong protein adsorption [23]. More recently, a nanocompos- ite of V 2 O 5 and GNPs having intrinsic peroxidase and GOx like activities respectively have been applied as a mimic in a dual enzyme cascade reaction for glucose detection and DNA hybridiza- tion opening up new avenues in biocatalysis [17]. In addition to oxidase like activity, peroxidase like activity of the different gold nanostructure has also been reported. BSA stabilized gold nanoclus- ters (AuNCs) have been reported to have intrinsic peroxidase like activity which is used to detect as low as 2.0 × 10 −8 M H 2 O 2 [24]. The concept is used to develop a sensitive method for xanthine http://dx.doi.org/10.1016/j.snb.2016.03.066 0925-4005/© 2016 Elsevier B.V. All rights reserved.