Highly luminescent gold nanoparticles prepared via a facile photochemical method for bioimaging applications

Luminescent gold nanoparticles (L-AuNPs) with diameters exceeding 2 nm hold great promise for biomedical imaging due to their unique optical properties and excellent biocompatibility. However, they typically exhibit weak photoluminescence (PL) because of surface plasmon resonance (SPR) effects. Moreover, conventional synthesis of L-AuNPs, often through thermal or chemical reduction, tends to be complex and labor-intensive. It is crucial, therefore, to develop more straightforward synthesis methods that enhance PL emission efficiency. Herein, we introduce a facile photochemical method for synthesizing highly luminescent AuNPs coated with 2-n-hexylthio-1,3,4-thiadiazole-5-thiol (L-AuNP@HTT). These nanoparticles, with a diameter of 3.19 nm, exhibit outstanding optical properties, including a high quantum yield (φ ~ 12%), an extremely long luminescence lifetime (~ 1 µs), a symmetric PL spectrum, and a narrow full width at half maximum (FWHM ≤ 49 nm). They also feature an exceptionally large two-photon absorption cross-section (σ), reaching up to 8.0 × 10⁴ GM (1 GM = 10⁻⁵⁰ cm⁴ s photon⁻¹). Upon encapsulation in a polymer matrix (p-AuNPs), the TPA cross-sections were further enhanced to 1.1 × 10⁸ GM. These p-AuNPs demonstrated high photostability and efficient targeting to mitochondria, making them highly effective for mitochondrial-targeted two-photon excited luminescence (TPEL) imaging. Deep-tissue time-gated TPEL imaging and in vivo computed tomography (CT) imaging have also been achieved with p-AuNPs. This work establishes a straightforward synthesis route for highly luminescent gold nanoparticles larger than 2 nm, significantly broadening their potential in various bioimaging applications.