RESEARCH ARTICLE Carbazole–azine based fluorescence ‘off–on’ sensor for selective detection of Cu 2+ and its live cell imaging Denzil Britto Christopher Leslee 1 | Sekar Karuppannan 1 | Karmegam Muthu Vengaian 1 | Sivaraman Gandhi 2 | Singaravadivel Subramanian 3 1 Department of Chemistry, Anna University – University College of Engineering, Dindigul, India 2 Institute for Stem Cell Biology and Regenerative Medicine, Bangalore, India 3 Department of Chemistry, SSM Institute of Engineering and Technology, Dindigul, India Correspondence Sekar Karuppannan, Department of Chemistry, Anna University – University College of Engineering, Dindigul – 624622, India. Email: karuppannansekar@gmail.com Funding Information DST‐SERB Fast Track, New Delhi, India, Grant/Award Number: SB/FT/CS‐062/2013 Abstract A new carbazole–azine based fluorescent sensor was synthesized and characterized. The selec- tivity of the sensor for Cu 2+ over other counter ions in a dimethyl sulfoxide/H 2 O mixture was shown through enhancement in fluorescence – an off to on transformation. The specificity of the probe towards Cu 2+ was evident in ultraviolet/visible, fluorescence, Fourier transform infra- red and mass studies. Application of the probe in the cell imaging and cytotoxicity of living cells is illustrated. KEYWORDS azine, carbazole, cell imaging, copper ion, fluorescence 1 | INTRODUCTION The design and synthesis of new chemosensors for monitoring ionic species, especially heavy and transition metal ions, are of great interest in chemical, biological and environmental analyses. [1–9] Copper is an essential trace element in biological systems, although under overdose conditions, it causes neurodegenerative diseases (e.g. Alzheimer's and Wilson's diseases). [10–16] The toxicity of copper ions towards humans is rather low compared with that of other heavy metals, but certain microorganisms are affected by low concentrations of copper. [17] The United States Environmental Protection Agency recommends an upper limit of 1.3 mg copper in humans. [18] Because of the toxicity of copper ions and their role as a critical catalytic co‐factor in a variety of metallo enzymes, such as superoxide dismutase, cytochrome c oxidase and tyrosinasemore, researchers have explored efficient recognition probes for Cu 2+ . [19–21] Although many analytical techniques have been developed, they are time consuming and expensive, and a simple, distinct reaction of toxic metal ions via colorimetric quantification is required. Thus, the development of fluorescence‐based sensors for metal ions has attracted much interest among the research commu- nity. [22–27] Cu 2+ is well known among transition metals as a fluorescent quencher due to its paramagnetic nature. It is therefore practically dif- ficult to develop fluorescent sensors for the detection of Cu 2+ . [28–32] In recent years, there have been several attempts to design chemosensors for paramagnetic metal ions. Recently, rhodamine B‐based fluorescent probes have been reported as ‘turn‐on’ fluores- cence, and colorimetric, sensors for Cu 2+ . [33–42] In these sensors, the mechanism involves opening of the spirolactam ring upon binding with a Cu 2+ ion, resulting in fluorescence enhancement. In addition, several other chemosensors have been reported with different signaling mech- anisms for the optical detection of paramagnetic ions based on photo‐ induced electron transfer, metal–ligand charge transfer, intramolecular charge transfer (ICT), excimer/exciplex formation, and excited‐state intermolecular proton transfer. [43–58] Carbazole and its derivatives possess highly conjugated systems, engage in active ICT and are used as electron‐donating chromo- phores. [59–63] Carbazole compounds are promising candidates for electroluminescent and photorefractive devices because of their elec- tron‐donating and hole‐transporting properties, and their remarkable application as fluorescent sensors. [64–76] In the bis‐azine type ligand, the electron density is located on the [C=N-N=C] moiety. Binding Abbreviations used: [CzA] 2 ‐Cu, Carbazole–azine–copper complex; CzA, Carbazole–azine based sensor (N,N′‐bis(9‐ethyl 3‐carbazolylmethylidene) azine); DFT, Density functional theory; DMSO, Dimethyl sulfoxide; ESI‐MS, Electrospray ionization mass spectrometry; FTIR, Fourier transform infrared spectroscopy.; HOMO, Higher occupied molecular orbital; ICT, Intramolecular charge transfer; LUMO, Lower unoccupied molecular orbital; MTT, 3‐(4,5‐ Dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide; NMR, Nuclear magnetic resonance spectroscopy; PBS, Phosphate‐buffered saline; TDDFT, Time‐dependent density functional theory; UV, Ultraviolet; Vis, Visible Received: 7 October 2016 Revised: 4 March 2017 Accepted: 6 March 2017 DOI: 10.1002/bio.3332 Luminescence. 2017;1–7. Copyright © 2017 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/bio 1