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

Abstract

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.

Fig. 1. (a) Excitation spectra (λ_em at 475 and 520 nm) and PL spectrum (λ_ex at 400 nm excitation) of His-AuNCs in the solid state. (b) Low resolution TEM image of His-AuNCs. Inset – HRTEM image.

Fig. 2. The ¹H-NMR superimposed spectra of His (red) and His-AuNCs (green), including the detailed view of the aliphatic region. The ¹H-NMR of both His and His-AuNCs were recorded at 600 MHz in D₂O.

Fig. 3. XPS survey spectra of His and His-AuNC samples.

Fig. 4. The FT-IR spectra of His (black) and His-AuNCs (red). The inset displays the extended spectra in the 700–900 cm⁻¹ region.

Fig. 5. (a) FLIM images of powder His-AuNCs deposited on a glass substrate, obtained at different excitation wavelengths (375, 405, 485 and 520 nm). (b) Corresponding optical images of the powder areas investigated in the FLIM images presented in (a).

Fig. 6. (a) PL spectrum of powder His-AuNCs under excitation at 365 nm. Inset – photographic image of His-AuNCs in the solid state under ambient and UV light exposure (365 nm) proving white-light generation. (b) The CIE color coordinates of His-AuNCs in the solid state under different excitation wavelengths in the 340–420 nm range.

Fig. 7. (a) PL intensity under continuous excitation at 365 nm. (b) Thermogravimetric analysis (TGA) curve. (c) PL spectra of powder His-AuNCs at 365 nm excitation, after thermal treatment at various temperatures in the 25–230 °C range.