Fluorescence lifetime correlation spectroscopy (FLCS) is presented as a single-step label-free detection method for probing the amine silanization-driven spontaneous 3D self-assembly of freestanding gold nanoparticles (GNPs) in solution. Unlike the conventional methods of studying self-assembly, for example, UV-vis spectroscopy and electron microscopy, FLCS utilizes the intrinsic gold fluorescence. The significance of this approach is to amalgamate the measurement of optical and hydrodynamic size properties simultaneously to achieve a more coherent description of the self-assembly pathway. GNP self-assembly has two-stage kinetics. Electrostatic interaction drives the initial amine silanization, and this is followed by siloxane bond formation between hydrolyzed ethoxy groups of GNP-attached APTES, resulting in the formation of micrometer-sized superstructures. The self-assembly has resulted in a 5-fold increase in the fluorescence lifetime (FL), and the FLCS study has shown an 8- to 10-fold increase in the diffusion coefficient using the pure diffusion model. This result is consistent with the transmission electron microscopy (TEM) observation, which shows a few hundred fold increase in the diameter due to assembly formation by the GNPs.
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