Nitrogen-Doped Graphene Quantum Dots and Reduced Graphene Oxide Quantum Dots As Intracellular Temperature Sensors

Non-invasive temperature sensing is necessary for the analysis of biological processes occurring in the human body including cellular enzyme activity, protein expression, and ion regulation. In order to probe temperature-sensitive processes at the nanoscale we develop novel luminescence nanothermometers based on graphene quantum dots (GQDs) synthesized via top-down (RGQDs) and bottom-up (N-GQDs) approaches from reduced graphene oxide and glucosamine precursors. Because of their small 3-6 nm size, non-invasive optical sensitivity to temperature change, and high biocompatibility, GQDs enable biologically safe sub-cellular resolution sensing. Both GQD types exhibit temperature-sensitive, yet photostable fluorescence in the visible and near-infrared for RGQDs, utilized as a sensing mechanism in this work. Distinctive linear and reversible fluorescence quenching with temperature by up to 19.3 % is observed for the visible and near-infrared GQD emission in aqueous suspension, in a small temperature range from 25 to 49℃. An even more pronounced trend is observed with GQD nanothermometers internalized into the cytoplasm of HeLa cells as they are tested in vitro in the temperature range of 25℃ to 45℃ with over 40% quenching response. Our findings suggest that the temperature-dependent fluorescence quenching of bottom-up and top-down-synthesized GQDs studied in this work can serve as non-invasive reversible photostable deterministic mechanism for temperature sensing in microscopic sub-cellular biological environments.