Design and sonochemical synthesis of water-soluble fluorescent silver nanoclusters for Hg 2+ sensing Shyama Prasad Bayen a , Maloy Kr. Mondal a , Shagufta Naaz a , Sudip Kr. Mondal b , Pranesh Chowdhury a, * a Polymer & Nano Research Laboratory, Department of Chemistry, Visva-Bharati University, Santiniketan 731 235, India b Spectroscopy Laboratory, Department of Chemistry, Visva-Bharati University, Santiniketan 731 235, India A R T I C L E I N F O Article history: Received 1 September 2015 Received in revised form 21 December 2015 Accepted 11 January 2016 Available online 13 January 2016 Keywords: Sonochemical synthesis Fluorescent Ag nanoclusters Hg 2+ sensing A B S T R A C T Highly fluorescent, stable, water-soluble Ag nanoclusters (AgNCs) have been successfully prepared via a sonochemical approach using lipoic acid as a capping agent to sense the presence of mercury selectively in the environmental samples. The synthetic route follows the principles of green chemistry. Mass spectrometric analysis shows that nanoclusters are made of two to six silver atoms (Ag 2 –Ag 6 ) and its size remains within 2–3 nm (diameter). Such nanoclusters exhibit high fluorescence property as evident from photoluminescence spectroscopy. Based on fluorescence property, an analytical method has been developed to trace mercuric ions (Hg 2+ ) selectively, even at subnanomolar concentrations. ã 2016 Elsevier Ltd. All rights reserved. 1. Introduction In the past few years, novel metal nanoclusters are very attractive due to their unique behavior. The physical and chemical properties of metal nanoclusters differ significantly from both the discrete atomic state and bulk molecular state. Hence the clusters may be viewed as a transition regime [1]. Within the regime, there is a wide fluctuation of properties, which depend on the size, geometry, electronic state, and packing of the clusters [1]. The metal nanoclusters have marvelous optical, fluorescence, ferro- magnetism, chirality, quantum behavior, and electrical properties because of their tiny size and quantum confinement effect [2]. These nanoclusters find several applications in catalysis, sensing, probing (DNA-hybridization), biological imaging, and single- molecular spectroscopy [3,4]. There are several synthetic routes for the preparation of water- soluble fluorescent AgNCs such as radiolysis (harmful UV irradiation/gamma irradiation), chemical, and photochemical approaches in the presence of various toxic reducing and stabilizing agents [5]. However, there is very few literature related to the synthesis of fluorescent noble metal nanoclusters using a ‘green approach’ [6]. Ultrasound is considered a ‘green’ technology due to its high efficiency, low instrumental requirements, significantly reduced process time compared with other conven- tional techniques, and its low cost [7]. Ultrasound based sonochemical technique has important synthetic applications in the materials chemistry [8]. The reported fluorescence property of the water-soluble AgNCs (synthesized sonochemically) [8,9] is not impressive toward the detection of heavy toxic metals like mercury. Adhikari and coworkers [10] synthesized the fluorescent AgNCs using undesirable toxic sodium borohydride as a reducing agent. Hence there is an enough scope to improve the sonochem- ical technique. Quick, easy and accurate sensing of the trace level Hg 2+ in real environmental samples is very essential and challenging. Several analytical methods for the detection of Hg 2+ have been reported [11] but fluorescence probe is the most recent technique. Conventional methods such as AAS, CV, ICPMS, etc. have been used to measure Hg 2+ with high sensitivity and accuracy. However, these analytical methods demand bulky and expensive sophisti- cate instruments, and complicated sample treatment, which limit their applications for rapid and on-field analysis [12]. Colorimetric sensing [12] and fluorescence probe [13] of Hg 2+ have drawn attention recently due to their simplicity and high sensitivity. The fluorescence probe based on fluorescent AgNC which is stimulated by lipoic acid [10], oligonucleotide [14], DNA duplexes [15,16], polyinosinic acid (PI), polycytidylic acid (PC) [17], glutathione [18], 1,4-disubstituted azines [13] and denatured bovine serum albumin (dBSA) [19] have been reported recently. However, the bottle neck of the reported probes is its detection limit. It can detect Hg 2+ up to nano molar range. Accurate detection of subnanomolar level of Hg 2+ is highly essential for the environmental point of view. * Corresponding author. Tel.: +91 3463 264983; fax: +91 3463 261526. E-mail address: pranesh_02@yahoo.co.in (P. Chowdhury). http://dx.doi.org/10.1016/j.jece.2016.01.014 2213-3437/ ã 2016 Elsevier Ltd. All rights reserved. Journal of Environmental Chemical Engineering 4 (2016) 1110–1116 Contents lists available at ScienceDirect Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/je ce