Gold nanoclusters performing as contrast agents for non-invasive imaging of tissue-like phantoms via two-photon excited fluorescence lifetime imaging

Dual-emissive solid-state histidine-stabilized gold nanoclusters for applications in white-light generation

The majority of present-day white-light emitting devices (WLEDs) are built upon the use of rare-earth elements, which have a short supply, are expensive and can become extremely toxic. Thus, in this work, we synthesized an eco-friendly, efficient and cheap white-light emitting material (WLEM) based on solid-state histidine-stabilized gold nanoclusters (His-AuNCs), obtained through the lyophilization of microwave-synthesized photoluminescent His-AuNCs. Their morphological and structural characterization was followed by thorough evaluation of their intrinsic solid-state photoluminescence properties via steady-state and time-resolved fluorescence spectroscopy and microscopy, at multiple excitation wavelengths. A white-light emission was observed under UV light excitation due to the two-band broad emission, with maxima at 475 and 520 nm, covering a large area of the visible spectrum. In order to evaluate the purity of the white-light emission we calculated the chromaticity coordinates, at different wavelengths, and displayed them on a CIE (Commision Internationale d'Eclairage) diagram. An excellent value of (0.36, 0.33) was found at 420 nm excitation, which falls within the range of pure white-light emission. Moreover, the His-AuNCs show great photo- and thermo-stability, thus proving their ability to perform as a reliable WLEM with potential use in the development of eco-friendly WLEDs.

Glutathione-capped gold nanoclusters as near-infrared-emitting efficient contrast agents for confocal fluorescence imaging of tissue-mimicking phantoms

An innovative research has been conducted focused on demonstrating the ability of novel dual-emissive glutathione-stabilized gold nanoclusters (GSH-AuNCs) to perform bright near-infrared (NIR)-emitting contrast agents inside tissue-mimicking agarose-phantoms via two complementary confocal fluorescence imaging techniques. First, using a new and fast microwave-assisted approach, we synthesized photostable dual-emitting GSH-AuNCs with an average size of 3.2 ± 0.4 nm and NIR emission quantum yield of 9.9%. Steady-state fluorescence measurements coupled with fluorescence lifetime imaging microscopy (FLIM) assays performed on lyophilized GSH-AuNCs revealed that the obtained GSH-AuNCs exhibit PL emissions at 610 nm (red PL) and, respectively, 800 nm (NIR PL) in both solution and powder solid-state. Time-resolved fluorescence measurements showed that the two PL components are characterized by average lifetimes of 407 ns (red PL) and 1821 ns (NIR PL), respectively. Additionally, due to a partial overlap between the red PL and the absorption of the NIR PL, an energy transfer between the two coexisting emissive centers was discovered and confirmed via steady-state and time-resolved fluorescence measurements. Furthermore, the FLIM analysis performed on powder GSH-AuNCs under 640 nm, an excitation more suitable for bioimaging applications, revealed a homogeneous and photostable NIR PL signal from GSH-AuNCs. Finally, the ability of GSH-AuNCs to operate as reliable NIR-emitting contrast agents inside tissue-mimicking agarose-phantoms was demonstrated here for the first time via complementary FLIM and re-scan confocal fluorescence imaging techniques. In consequence, GSH-AuNCs show great promise for future in vivo imaging applications via confocal fluorescence microscopy.