Graphene Quantum Dots for Multifaceted Diagnostics

Abstract

Due to their unique physical and electronic properties as well as high biocompatibility and water solubility, functionalized graphene quantum dots (GQDs) have found utilization in a variety of biomedical applications. GQDs can serve as carriers for drugs and genes while tracing their delivery pathways in vitro and in vivo with their visible and near-infrared fluorescence. In addition to therapeutic applications, our recent work explores diagnostic modalities of these novel nanomaterials. GQDs synthesized bottom-up form a single glucosamine precursor, or top-down from reduced graphene oxide can serve as nanoscale temperature sensors for biological processes. They exhibit linear and reversible visible/NIR fluorescence quenching by up to 19.3 % in suspension with temperature increase form 25 to 49 ⁰C. A more pronounced trend is observed in HeLa cells with over 40% quenching in that temperature range. Our findings suggest that such deterministic response can serve as non-invasive reversible/photostable mechanism for temperature sensing in microscopic subcellular biological environments. Another diagnostic modality of GQDs involves their capability for magnetic resonance imaging. When doped with Mn2+ ions during their bottom-up synthesis, GQDs exhibit substantial r₂/r₁ relaxation parameter ratios of 11.19, showing potential as dual-mode T1 or T2 contrast agents. Their Gd3+-doped counterparts possess r₂/r₁ ratios of 1.148 with high r1 of 9.546 mM-1s-1 compared to commercial contrast agents, suggesting their utilization as T1 contrast agents. Given that doping did not affect their biocompatibility or fluorescence properties, such doped GQDs can serve as multimodal MR and fluorescence imaging agents. Metal-doped GQDs possess the capability ultrasonic contrast imaging that can be utilized for higher precision and deeper tissue diagnostic applications. They demonstrate high-contrast properties in ultrasound brightness mode by a factor of 10 surpassing undoped GQD structures. The successful imaging enhancement was observed in tissue phantom and animal tissue, suggesting that GQDs can be tracked by ultrasound imaging and used as contrast agents for the improvement of the deterministic power of this widely available imaging approach. GQD intrinsic fluorescence can itself serve as a non-invasive diagnostic sensing tool, as it can be affected by the binding of different drugs, genes and biomolecules to the GQDs. This optical sensing mechanism is used in our work for identifying trace quantities of cancer-generated genes for early personalized cancer detection. The sensor is prepared from GQDs complexed with the bait DNA sequence complementary to the target pancreatic cancer miRNA. It effectively discriminates between complementary single-stranded DNA sequences and random control genes with the sensitivity in the nanomolar range. Cancer gene detection is further verified by identifying both, the stem and the loop portions of the pancreatic cancer-generated pre-miR-132. Given the success of these different diagnostic applications, GQDs can be considered a highly perspective theragnostic tool for a variety of disease targets.