Graphene Quantum Dots as Intracellular Imaging-Based Temperature Sensors

Abstract

Non-invasive temperature sensing is necessary to analyze biological processes occurring in the human body, including cellular enzyme activity, protein expression, and ion regulation. To probe temperature-sensitive processes at the nanoscale, novel luminescence nanothermometers are developed based on graphene quantum dots (GQDs) synthesized via top-down (RGQDs) and bottom-up (N-GQDs) approaches from reduced graphene oxide and glucosamine precursors, respectively. 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 by up to 19.3% is observed for the visible and near-infrared GQD emission in aqueous suspension from 25 °C to 49 °C. A more pronounced trend is observed with GQD nanothermometers internalized into the cytoplasm of HeLa cells as they are tested in vitro from 25 °C to 45 °C 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 mechanisms for temperature sensing in microscopic sub-cellular biological environments.

Graphene Quantum Dots – Hydrothermal Green Synthesis, Material Characterization and Prospects for Cervical Cancer Diagnosis Applications: A Review

“…[178] Chemical inertness, appreciable photostability, non-toxicity, hydrophilicity, and high solubility in an aqueous environment make GQDs lucrative for biosensing applications. [35,36] The higher surface area of GQDs maximizes their contact with analyte molecules and also results in a higher electrochemical surface area. This promotes interaction with electroactive species and facilitates direct electron transfer reactions from enzymes and proteins.…”

Fluorescence Microscopy—An Outline of Hardware, Biological Handling, and Fluorophore Considerations

“…For instance, pH sensing in HeLa cells using folic acid-encapsulated GQDs has been reported, where a pH increase from 5–8 results in an increase in emission intensity in the green detection window (500–570 nm, λ ex 488 nm), using confocal microscopy, while the emission intensity in the blue detection window (425–490 nm, λ ex 405 nm) remained relatively unchanged ( Figure 4 ) [ 220 ]. A more recent example has used nitrogen-doped GQDs to detect temperature in live HeLa cells by measuring the temperature-dependent fluorescence quenching as the cells are warmed from 25–45 °C [ 221 ].…”

Near infrared bioimaging and biosensing with semiconductor and rare-earth nanoparticles: recent developments in multifunctional nanomaterials

“…In this regard, the design of reversible Advantages associated with the lifetime-based imaging can also be translated into the field of PL thermometry, which, in terms of steady-state PL approach, has been a staple for over a Carbonaceous materials, such as graphene NPs, are a biocompatible, water soluble alternative to SNCs. Although their quantum confinement-defined intrinsic PL lies in the ultraviolet to visible range, recent examples have shown that this can be shifted to the NIR through the modification of the synthesis process, [109][110][111] or judicious doping, 112 both yielding materials with PL properties tuned to the NIR-I or NIR-II. This way, reduced graphene oxide-derived NPs have been added…”

“…to the existing library of NIR nanothermometers. 111 In addition, their strong absorption makes them ideal light-to-heat converters 110,112,113 for in vivo PTT and PL imaging when the absorption bands are tuned to the NIR. 110 Finally, the adequate dopant can also change graphene NPs sensitivity to the environment, which has been applied for pH sensing in the NIR.…”