Multifunctional fluorescent “Off-On-Off” nanosensor for Au^sup 3+^ and S^sup 2−^ employing N-S co-doped carbon–dots

Out of the plethora of sensors being developed using carbon dots, the use of green alternatives with the potential of multi-functionality is still in its infancy. In view of this, the present work shows the use of Rosa indica derived heteroatom i.e. nitrogen and sulfur co-doped rose carbon dot (N-S@RCD) for dual fluorescence based sensing of multi-analytes. The N-S@RCD shows interesting fluorescence turn “off-on-off” response towards S2− and Au3+ with limit of detection (LOD) 92.4 nM and 63.1 nM, respectively. To the best of our knowledge, this is first report on direct “Turn-on” of c-dots with S2− ions without any intermediate c-dot–quencher complex. Further, the bioimaging and flow cytometry studies revealed the potential of live cell imaging, intracellular sensing and cytocompatibility of N-S@RCD. The hemocompatibility of N-S@RCD was also investigated for potential in vivo applications. Moreover, the “off-on-off” fluorescence behaviour was used to construct molecular logic gates which simulate single input logic “YES” and multi-input “INHIBIT” logic system.