FRET-based ‘ratiometric’ molecular switch for multiple ions with efficacy towards real-time sampling and logic gate applications

Abstract

Fluorescent nanoaggregates comprised of rhodamine based amphiphilic probe has been designed, which can simultaneously detect Al 3+ , Cr 3+ and F¯ ions in the aqueous medium (pH 7.4). Mechanistic investigation indicates that both the carbonyl oxygen of the spirolactam ring and the nitrogen center of indole participate in the coordination of metal ions. Consequently, metal ion-binding induces the opening of the spirolactam ring and allows the Förster energy transfer from indole to xanthene moiety. As a result, the colorless probe solution turns pink in presence of metal ions, with ratiometric fluorescence change from blue to yellow. Further, the addition of fluoride leads to dissociation of preformed Al 3+ -complex, resulting in the fading of pink color and reverting the fluorescence signal to the original. This not only serves as a tool to discriminate between Al 3+ and Cr 3+ ion but also acts as a strategy for detecting fluoride in the aqueous medium. Thus, using a single FRET probe, ‘ratiometric’, naked-eye sensing of multiple ions was achieved. Also, considering the differential response of probe molecule towards Cr(III) and Cr(VI), a colorimetric assay for biogenic thiols was developed. Simultaneously, a combinatorial molecular logic gate was designed based on the multiple ion sensing ability of the probe molecule. Further, the system showed real-time detection of Al 3+ and Cr 3+ in natural water samples, estimation of Al 3+ in antacid tablets and fluoride in mouthwash, etc. Lastly, low-cost, reusable paper strips were developed, which offers an alternative strategy for on-site detection of metal ions without engaging any sophisticated instruments or trained personnel. • Ratiometric sensing of Al 3+ , Cr 3+ and F − ions at nanomolar levels in water. • Assay for biogenic thiols based on differential response for Cr(III) and Cr(VI). • Design combinatorial logic gate based on multiple ion sensing. • Analysis of real-life samples: fluoride in mouthwash, Aluminium in antacid tablets. • Low-cost, reusable paper strips for onsite detection of ions.