Talanta 80 (2009) 532–538 Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta An efficient thiourea-based colorimetric chemosensor for naked-eye recognition of fluoride and acetate anions: UV–vis and 1 HNMR studies Arvind Misra ∗ , Mohammad Shahid, Pratibha Dwivedi Nucleic Acids Research Laboratory, Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, UP, India article info Article history: Received 27 April 2009 Received in revised form 3 July 2009 Accepted 6 July 2009 Available online 14 July 2009 Keywords: Anion recognition Colorimetric sensor Thiourea abstract An efficient colorimetric chemosensor with a thiourea binding site and 2-amino-6-nitrobenothiazole as a signaling unit has been synthesized by dithiocarbamate approach. The chemosensor has been utilized for selective recognition of fluoride and acetate anions in dry DMSO solution by UV–vis and 1 H NMR titration experiments. The chemosensor has shown naked-eye sensitivity for both the anions in solution. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The selective recognition and sensing of anions via artifi- cial organic chemosensor molecule/probe, containing a suitable receptor site, have attracted considerable attention for chemists in past decades [1]. Anions especially, fluoride and carboxylate play significant roles in chemical, environmental and biochem- ical processes [2–8], hence their recognition is important. The ionic recognition can be confirmed by different analytical meth- ods like electrochemical methods – by variation in the redox potential, by change in chemical shifts in 1 H NMR signal or by change in fluorescence intensity signal – utilizing fluorescence spectroscopy [9]. The non-covalent ionic recognition generally, governs by the energy of the receptor–anion interaction, geom- etry, basicity of the employed anion and nature of the solvent systems. In fact, the strongest hydrogen-bond interactions are established with anions that contain the most electronegative atoms, such as, fluoride and carboxylate, and that display the most pronounced basic strength in a particular solvent. Recently, major impetus is given on strategies to develop colorimetric sensors for ionic species by utilizing binding site-signaling unit approach, in which an appropriate chromophore is attached to a specific anion receptor [10–18], to make sensitive recognition behaviors simple and convenient through naked-eye. The designing of such kind of chemosensor usually involves the covalent linking of a chromogenic fragment to a neutral receptor that should be capa- ∗ Corresponding author. Tel.: +91 542 2307321x104/6702503; fax: +91 542 2368127/2368175. E-mail address: arvindmisra2003@yahoo.com (A. Misra). ble in establishing selective interaction with the envisaged anion either by directional hydrogen bonding or by donation of one or more hydrogen of N–H fragment to the anions–deprotonation. In this sense several synthetic receptors (also referred as host) have been designed, to include, urea [19–23], thiourea [24–28], amides [29,30], macrocyclic ammonium/guanidinium [31], phenylhydra- zone [32,33], indole [34], pyrrole [35–37] and phenol [38] units for interaction study with negatively charge ionic species (also referred as guest). The phenylhydrazone and indole type of receptors is neutral and the synthetic chemistry involved to synthesize these kinds of molecule is also simpler. Moreover, such kind of receptor molecule could recognize anions through color change, which is visible to naked-eye in aqueous medium. In continuation of anion recognition by neutral receptors, urea and thiourea subunits, owing to their ability to act as hydrogen- bond donors have also been extensively utilized to produce chemosensors [39–41]. In fact, urea is capable for donating two hydrogen bonds to get an orientation complementary to Y-shaped oxoanions, parallel to carboxylates [42]. Further, by incorporating N–H unit into the chromogenic subunit, it is reasonable to have modification in the dipole and associated charge transfer transi- tion in the influence of anions. Consequently, the ultimate negative charge being developed on the receptor will modulate UV–vis spec- trum and thus, a prominent color change of medium that could be visible through naked-eye. To make chemosensor and recognition phenomenon more sensitive, presence of electron-withdrawing substituent on the chromophoric unit is useful. The presence of electron withdrawing group, like –NO 2 , –CF 3 , persuades for more acidity to the anion binding subunit—receptor site or more polariza- tion in the N–H bond to facilitate more promising hydrogen-bond donating or interaction tendency, to support further an “incipient” 0039-9140/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2009.07.020