Isoquinoline based alkaloid chemosensor for detection of alkanes and subsequent fluorescence switching inside surfactant coated bio-mimicking nanocavity

Abstract

In this article, an attempt has been taken to endeavour the physiochemical modulation of an isoquinoline based alkaloid sanguinarine (SANG) in various nonpolar (alkane) medium and inside surfactant coated bio-mimicking nanocavity. Notably, in different alkane medium, SANG undergoes cationic iminium to neutral alkanolamine conversion. This conversion generally occurs as an outcome of nucleophilic attack by water (or hydroxyl group) at the electron deficient centre of iminium form of SANG in water at pKa >7.2. Hence, such conversion in nonpolar mediumis surprising, as alkane cannot act as a nucleophile. After investing much effort behind theoretical and experimental techniques, we have directly monitored that ‘water of crystallization’ of the alkaloid itself take part in the intranucleophilic attack at the electron deficient centre in alkane medium, which switches the fluorescence from orange (iminium) to bluish-violet (alkanolamine) even detectable by naked eye. Notably, we have employed such fluorescence switching of SANG into the detection of alkane mixed with the drinking water, and we have noticed that SANG alkaloid may be potentially useful for the detection of hazardous alkane, like, hexane, heptane, octane, decane etc. from drinking water. This kind of fluorescence switching of organic cationic fluorophore in alkane medium is rare, and such switching would be highly interesting for the application in petrochemistry, environmental and geological sciences for development of sensors to detect alkanes and other nonpolar solvents. Moreover, in presence of anionic (AOT) surfactant in nonpolar solvent (n-heptane and benzene), the newly created alkanolamine form is found to enter inside the reverse micellar (RMs) nanocavity. Intriguingly, subsequent addition of water results in the return of alkaloid into its initial cationic iminium form by second conversion inside the reverse micellar water nanopool, exhibiting an isoemissive point at ~530 nm.