Sensors and Actuators B 210 (2015) 519–532 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo ur nal home page: www.elsevier.com/locate/snb A highly selective fluorescent chemosensor for Hg 2+ based on a squaraine–bis(rhodamine-B) derivative: Part II Sehoon Lee 1 , Boddu Ananda Rao 1 , Young-A Son ∗ Department of Advanced Organic Materials Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, South Korea a r t i c l e i n f o Article history: Received 17 November 2014 Received in revised form 26 December 2014 Accepted 3 January 2015 Available online 12 January 2015 Keywords: Squaraine Rhodamine B Mercury sensing Cyanide anion Fluorescent chemosensor a b s t r a c t Herein, we report an effective strategy based on coordination-induced signaling by introducing a rho- damine B group linkage into squaraine–diamine dyads as a reversible switch. The optimized design, synthesis and application of a new optical squaraine–bis(rhodamine-B) chemosensor (SRB) as an ‘off–on’ fluorescent probe for the detection of Hg 2+ ions were investigated. SRB exhibited high selectivity toward Hg 2+ in the presence of various metal ions, such as Al 3+ , Ag + , Co 2+ , Cs + , Cu 2+ , Fe 3+ , K + , Li + , Mg 2+ , Na + , Ni 2+ , Pb 2+ , and Zn 2+ , and the resulting complex [SRB–Hg 2+ ] was investigated using UV–vis and fluorescence spectroscopy in acetonitrile (CH 3 CN). The ‘off–on’ fluorescence and color signal change of the probe are based on a Hg 2+ -triggered domino reaction that employs the open-ring form of rhodamine spirolactam to regain the conjugated system of the rhodamine skeleton. The mechanism for the opening of the rho- damine spirolactam ring induced by Hg 2+ binding and the 1:1 stoichiometric structure of SRB and Hg 2+ were confirmed using a Job’s plot estimation, optical titration and FT-IR. Subsequently, a SRB–Hg 2+ com- plex chemosensor was employed to detect CN - in the presence of different anions, such as Br - , CH 3 COO - or AcO - , Cl - , ClO 4 - , F - , HPO 4 - , HSO 4 - , I - , N 3 - , NO 3 - , PF 6 - and SCN - , in acetonitrile. In addition, this sensor exhibited highly selective and sensitive recognition of cyanide ions upon the addition of Hg 2+ with a color change back to colorless in the same solution. Finally, SRB was successfully applied with the PEGDMA polymer to sense Hg 2+ ions, which was analyzed using fluorescence confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) images. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Squaraines are a particularly promising class of organic dyes that exhibit unique photophysical properties, namely, a sharp and intense absorption band in the visible to near-infrared (NIR) region. In recent years, squaraine dyes have been extensively used in optical data storage, [1,2] solar cells [3–7], non-linear optics [8–10], photosensitizers for photodynamic therapy (PDT) [11,12] and chemosensors [13–18]. The electrophilicity of these dyes, which is due to their central electron-deficient four-membered ring, enables them to react with nucleophilic reagents, such as cyanide anions and thiols [19,20]. Although the use of squaraine dyes as chemosensors for the detection of Hg 2+ has been reported, the majority of previous studies involved the chelation of mer- cury ions [21–24]. Thus, there is still a great need to develop ∗ Corresponding author. Tel.: +82 42 821 6620; fax: +82 42 821 8870. E-mail address: yason@cnu.ac.kr (Y.-A. Son). 1 These authors contributed equally to this work. new approaches for analyte-specific chemosensors based on the squaraine chromophore. The rhodamine framework is an ideal template for construc- ting chelation-enhanced fluorescence (CHEF) OFF–ON fluorescent chemosensors for metal ions because of its structural properties. Without cations, these rhodamine-based chemosensors exist in a spirocyclic form that is colorless and non-fluorescent. The addi- tion of a specific metal ion leads to opening of the spirocycle via coordination or an irreversible chemical reaction [25–29], which results in the appearance of a pink color or orange fluorescence. In addition, rhodamine-based compounds are ideal candidates for fluorescent sensors because of their good photophysical properties, such as absorption and emission wavelengths extended to the visi- ble region, high fluorescence quantum yields, and large absorption coefficients [30]. Therefore, rhodamine-containing compounds are typically utilized for naked-eye chemosensors [31]. Rhodamine with acyclic xanthene has a long emission wavelength (>550 nm), which is superior for detection or imaging with a minimal influence of background fluorescence (<500 nm). Thus, rhodamine substrates are ideal choices for the molecular construction of “turn-on” Hg 2+ probes through the use of an appropriate design and synthesis. In http://dx.doi.org/10.1016/j.snb.2015.01.008 0925-4005/© 2015 Elsevier B.V. All rights reserved.