Anion Sensing Using Quinolinium Based Boronic Acid Probes

In this paper we review a series of water-soluble quinolinium boronic acid fluorescent probes, sensitive to halide and cyanide anions. We have intelligently combined the well-known halide, in particular chloride, bromide and iodide, sensing capability of the quinolinum nucleus with the fluoride or cyanide ion binding boronic acid moiety, to afford a range of water-soluble probes sensitive to fluoride, chloride, bromide, iodide and cyanide anions. These new probes offer the most attractive capability of sensing all important halides at physiological concentrations, coupled with a cyanide response at the lethal threshold level of about ≈ 20-30 μM. Some of the probes such as the BAQBAs show spectral shifts and intensity changes in the presence of fluoride and cyanide in a wavelength-ratiometric and colorimetric manner, enabling the detection of these anion concentrations at visible wavelengths. Although the sensing mechanism is different for fluoride and cyanide as compared to the other halides (chloride, bromide and iodide), the results reveal that these probes are in fact potential candidates towards the sensing of the all the halides and cyanide. Also we have constructed the corresponding control compounds that have no boronic acid moiety and are therefore insensitive to fluoride and cyanide but are mildly sensitive to chloride, bromide and iodide, similar to that of the boronic acid probes.