Theoretical and experimental studies on manipulation of fluorescence by gold nanoparticle: Application for molecular imaging.

摘要

Gold nanoparticles (GNPs) have shown beneficial properties for biomedical use, e.g., their non-toxic nature and surface properties for easy modification. Upon receiving light, they generate a strong surface plasmon field, which can alter the fluorescence of fluorophores. The level and type of the fluorescence alteration depend on the GNP size and shape, excitation (Ex)/emission (Em) wavelengths and quantum yield of the fluorophore, as well as the distance between the fluorophore and GNP.;Utilizing the quenching/enhancement ability of GNPs, a near-infrared (NIR) contrast agent that emits fluorescence at a higher level only at the particular cancer site was developed. Cypate, a safe NIR fluorophore, was selected as the fluorophore because NIR penetrates deeper into tissue and because Cypate is non-toxic. Cypate was conjugated to a GNP via two spacers. One is short for the quenching and with a substrate for a breast cancer-specific enzyme, urokinase-type plasminogen activator (uPA). The other is a long, biocompatible polymer chain for fluorescence enhancement. The fluorescence of the contrast agent was quenched by GNP by 93%. In the presence of uPA, the short spacer was cleaved and the remaining long spacer enhanced fluorescence 1.8 times.;The study results are beneficial for developing efficacious optical contrast agents. This novel contrast agent can detect and diagnose breast cancer with high specificity and sensitivity, as FRET or molecular beacon but with a higher sensitivity and without the restriction of using DNA/RNA segments.;In this dissertation, the effect of the properties listed above on the fluorescence output was theoretically analyzed for the fluorophores frequently used in biomedical studies. For fluorescence quenching, fluorophores with the Em wavelength near the GNP plasmon resonance peak (520 nm) are better suited. As the Em wavelength increases, a shorter distance is required for achieving the same level of quenching. A bigger GNP requires shorter distance for quenching. To obtain fluorescence enhancement, the Em wavelength of the fluorophore needs to be longer than the GNP plasmon resonance peak (e.g., > 650 nm). The fluorophore with lower intrinsic quantum yield tends to be enhanced more. The GNP needs to be sufficiently large (> 5 nm), and a bigger GNP provides a higher maximum enhancement.