Cyanine-Conjugated Gold Nanospheres for Near-Infrared Fluorescence-Based Biomedical Imaging

Near-infrared (NIR) fluorescence imaging offers improved spatial precision by reducing light scattering and absorption in tissue. Despite this key advantage, the NIR region is limited by the availability of fluorophores, most of which exhibit a relatively low quantum yield (QY). In this study, gold nanospheres (AuNSs) with absorption peaks in the visible range (400–700 nm) were used to enhance the fluorescence intensity of the cyanine NIR fluorophore IRdye 800 in the first NIR window (NIR-I, 700–900 nm) of the electromagnetic spectrum. AuNSs with diameters ranging from 5 to 25 nm were chosen to investigate the impact of a nanoparticle size on fluorescence enhancement, functionalized with polyethylene glycol (PEG) of varying molecular weights (1, 2, 5, and 7.5 kDa) to optimize the distance between the fluorophore and the nanoparticle surface. Theoretical analyses using finite-difference time-domain (FDTD) simulations and experimental comparisons with nonmetallic nanoparticles were performed to identify the factors contributing to the enhancement of fluorescence. PEGylated AuNSs conjugated with IRdye 800 (AuNDs) exhibited decreased photoisomerization, resulting in an increased fluorescence intensity and altered fluorescence lifetimes (FLTs). The observed enhancement in the fluorescence intensity of the AuNDs was attributed to three primary mechanisms: metal-enhanced fluorescence, altered radiative decay rates, and steric stabilization. Among these three mechanisms, two are attributed to the tail-end absorption spectral overlap of the AuNSs with the IRdye 800. This study demonstrates the potential of the AuNDs to enhance NIR-I fluorescence imaging and enable dual-mode imaging through computed tomography (CT) and fluorescence lifetime imaging (FLI), offering opportunities for both structural and functional biomedical imaging.