A fluorescent ratiometric nanosensor for detecting NO in aqueous media and imaging exogenous and endogenous NO in live cells

As an endogenously generated signal-inducing agent in the immune, cardiovascular and nervous systems, nitric oxide (NO) is involved in various biological processes and a large number of pathologies. Hence, sensitive and reliable detection of NO in biological samples and inside living cells has been of great interest. Herein, we reported a carbon-dot-based fluorescent ratiometric nanosensor for NO detection in water and imaging in living cells. Ratiometric sensing is realized through the fluorescence resonance energy transfer (FRET) process, and the carbon dots (CDs) serve as both the energy donor and the anchoring site for the NO probe moiety, which turns into the energy acceptor in the presence of NO. This nanosensor is highly selective for NO, and is operable in totally aqueous media with a very low detection limit of 3 nM. Due to the biocompatible nature and small size of the carbon dots, the nanosensor exhibits very low cytotoxicity and can easily permeate through the cell membrane. The nanosensor is capable of tracking the exogenous NO levels in several cell lines such as Hela, L929 and Raw 264.7 cells, and it can be also used to visualize the endogenously produced NO in the Raw 264.7 macrophage cell line.