Short Communication Chromogenic sensing of biological thiols using squarylium dye In-Sub Shin a , Seon-Yeong Gwon a , Sung-Hoon Kim a,b,⇑ a Department of Advanced Organic Materials Science and Engineering, Kyungpook National University, Daegu 702 701, Republic of Korea b School of Chemistry Science & Technology, Zhanjiang Normal University, Zhanjiang 524048, PR China highlights  A new biothiol probe SQ was synthesized.  The probe displayed a color change from Cys, Hcy and GSH.  SQ was highly selective for Cys detection.  SQ could serve as a ‘‘naked-eye’’ probe for Cys with a minimum concentration of approximately 4 lM. graphical abstract N N HO HO OH HO O OH SR N N HO HO OH OH O O 2+ RSH SQ article info Article history: Received 17 July 2013 Received in revised form 7 October 2013 Accepted 17 October 2013 Available online 26 October 2013 Keywords: Chemosensor Cysteine Homocysteine Glutathione Biothiols Squarylium dye abstract A new highly selective probe for biothiols, squarylium dye (SQ), was designed and synthesized. The probe displayed a color change from blue to colorless upon reaction with biothilos such as cysteine (Cys), homo- cysteine (Hcy) and glutathione (GSH). The competition experiments revealed that no obvious interfer- ence was observed by performing the titration with the mixtures of Cys and other amino acids. The results indicated that SQ was highly selective for Cys detection. Moreover, SQ could also serve as a ‘‘naked-eye’’ probe for Cys with a minimum detectable concentration of approximately 4 lM. Ó 2013 Elsevier B.V. All rights reserved. Introduction Naturally occurring thiols exhibit a variety of structures and important physiological properties. In which, Cys, Hcy, GSH have the similar structures (Fig. 1). Biological thiols such as Cys, Hcy and GSH play crucial roles in the cellular antioxidant defense system [1]. Their abnormal levels have been directly linked to some diseases and cancers. Deficiency of Cys would lead to many diseases, such as hematopoiesis de- crease, leucocyte loss [2], and psoriasis, while Hcy is a risk factor for cardiovascular [3] and Alzheimer’s disease [4]. GSH is a major component of the cellular antioxidant system, and it plays an important role in the detoxification of xenobiotic compounds and in the antioxidation of reactive oxygen species and free radicals [5]. The determination of biothiols in body fluids is very important from the biological and pharmacological stand point. Some Cys and Hcy analyses have been developed in conjunction with HPLC [6], capillary electrophoresis [6], immunoassay [6], colorimetric and fluorescence detection [7–11], etc. Recently, colorimetric sensors are popular due to their capabil- ity to detect analyte by the naked eye without resorting to any expensive instruments. Therefore, the design of ratiometric and colorimetric biothiol probes has been the focus of numerous re- search efforts because of their remarkable importance in the qual- itative and quantitative detection. Squarylium dyes and derivatives are 1,3-disubstituted compounds synthesized from squaric acid and two equivalents of various types of electron donating carbocy- cles or heterocycles such as azulene [12], pyrroles [13], or hetero- cyclic methylene bases [14]. Squarylium dyes are expected to have 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.10.066 ⇑ Corresponding author at: Department of Advanced Organic Materials Science and Engineering, Kyungpook National University, Daegu 702 701, Republic of Korea. Tel.: +82 53 950 5641; fax: +82 53 950 6617. E-mail address: shokim@knu.ac.kr (S.-H. Kim). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 642–645 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa