Facile Photochemical Synthesis and Characterization of Highly Luminescent Gold Nanoparticles as Versatile Probes for Bioimaging Applications

Abstract Luminescent gold nanoparticles (L-AuNPs) with sizes larger than 2 nm have attracted significant interest as a novel type of nanomaterial, particularly in the field of biomedical imaging, owing to their distinctive optical properties and excellent biocompatibility. Nevertheless, the prevalent method for synthesizing these L-AuNPs is thermal reduction, and there remains room for enhancing their photoluminescence (PL) emission efficiency. Herein, we report a facile photochemical route for the preparation of highly L-AuNPs (L-AuNP@HTT) coated with 2-n-hexylthio-1,3,4-thiadiazole-5-thiol (HTT). These L-AuNP@HTT possess a diameter of 3.19 nm and exhibit exceptional optical properties, including a high quantum yield (φ ~12%), an extremely long luminescence lifetime (~1 μs), a symmetric PL profile, as well as a narrow full width at half maximum (FWHM ≤ 49 nm). Furthermore, they display an unusually large two-photon absorption (TPA) cross-sections (σ) of up to 8.0 × 104 GM (1 GM = 10-50 cm4 s photon-1). After encapsulation in a polymer matrix (namely p-AuNPs), the TPA cross-sections could be further enhanced to 1.1 × 108 GM per particle, and these prepared p-AuNPs show high photostability and efficient mitochondrial targeting. With these p-AuNPs, high-quality mitochondrial-targeted two-photon excited luminescence (TPEL) imaging, deep tissue time-gated TPEL imaging, and in vivo computed tomography (CT) imaging have been successfully achieved. These results pave the way for the facile synthesis of highly luminescent gold nanoparticles for a wide range of bioimaging applications.