A guaiazulene-based highly selective colorimetric sensor for cyanide ion detection in aqueous medium and food samples

A novel colorimetric sensor (4) for cyanide detection was designed based on guaiazulene which is a natural blue pigment. The sensor 4 was synthesized in excellent yield starting from guaiazulene in two simple reaction steps consisting of formylation of guaiazulene by the Vilsmeier-Haack reaction and subsequent Knoevenagel condensation reaction of 3-formylguaiazulene (2) with 1,3-indanedione (3). Structural characterization of sensor 4 was elucidated by means of various spectroscopic techniques such as ¹H-, ¹³C NMR, and HRMS. The nucleophilic addition of cyanide to the β-conjugated carbon of 1,3-indanedione scaffold breaks intramolecular charge transfer (ICT), subsequently causing a significant change in its photophysical properties resulting in a color change, that can be observed with naked eye. Cyanide sensory properties of 4 were studied in detail with UV/Vis spectroscopy in aqueous DMSO and revealed a limit of detection as low as 1.72 μM with a very high selectivity among the various competing anions. The detection of cyanine in tab water and a food sample was also demonstrated along with filter paper-based smart phone application.