The Synthesis of Fluorescent Gold Nanoparticles

Gold nanoparticles (GNPs) containing fluorescent labels are in high demand for the development of disease diagnostics, including cancer. A key challenge in this field is that GNPs are effective fluorescence quenchers, which dramatically reduces the detection sensitivity of flow cytometry and fluorescence bioimaging. This is particularly relevant for fluorescein, whose emission wavelength is lower than the plasmon resonance wavelength of GNPs. However, no studies are available addressing methods for overcoming its quenching upon adsorption onto GNPs. We propose an approach for covalently attaching fluorescein to GNPs via an optimized linker –PEG71–NH–(CH2)6–NH–, which allows the GNPs and fluorophore to be separated by the distance necessary to overcome the quenching effect. The method involves modifying GNP-S-PEG-COOH with hexamethylenediamine, followed by the addition of a fluorophore in the form of a succinimide ester. Fluorescent GNP-based conjugates with diameters of 12.7 and 6.1 nm (GNP-Flu-13 and GNP-Flu-6, respectively) capable of efficient cell penetration, were synthesized. All intermediate products and the target GNPs, Flu-13 and Flu-6, were characterized using dynamic light scattering, optical spectroscopy, transmission electron microscopy, and gel electrophoresis. The developed fluorescent GNP conjugates can be effectively used for in vivo tumor tissue bioimaging.