Cysteamine modified Ruellia simplex carbon dots: tuning photoluminescence and unraveling the metal sensing mechanism

Carbon Dots (C-Dots), relatively the latest entrant into the vast area of nanochemistry have emerged as interesting fluorescent materials having diverse applications. However, these visible emitters may be tuned for specific applications by introducing functional groups on their surface. Herein, we present a state-of-the-art comparison of C-Dots synthesized using natural precursors alone and C-Dots synthesized with cysteamine. Interestingly, C-Dots produced in the latter case have sulfenic acid functional groups, which are known anti-cancer prodrugs and hence, may find applications as nano theranaustics. We used Ruellia simplex flowers as the precursor to synthesize C-Dots (RS-CD) via hydrothermal treatment followed by the synthesis of cysteamine-modified C-Dots (RS-Cys-CD). The studies on morphological and optical properties of RS-CD and RS-Cys-CD were conducted which concluded that the structural characteristics of these C-Dots can easily be tuned by surface modification. Further, we have utilized both the C-Dots as nanoprobes for metal ions detection. These fluorescent nanoprobes exhibited selectivity and sensitivity towards the Cr(VI) with the Limit of Detection (LOD) 1.28 and 1.58 µM for RS-CD and RS-Cys-CD respectively. The mechanism for the fluorescence quenching of C-Dots in the presence of  Cr(VI) is proposed to be the inner filter effect.