Maik Eichelbaum

Photoluminescence of atomic gold and silver particles in soda-lime silicate glasses

We report the chemistry and photophysics of atomic gold and silver particles in inorganic glasses. By synchrotron irradiation of gold-doped soda-lime silicate glasses we could create and identify unambiguously the gold dimer as a stable and bright luminescing particle embedded in the glassy matrix. The gold dimer spectra coincide perfectly with rare gas matrix spectra of Au(2). The glass matrix is, however, stable for years, and is hence perfectly suited for various applications. If the irradiated gold-doped sample is annealed at 550 degrees C a bright green luminescence can be recognized. Intense 337 nm excitation induces a decrease of the green luminescence and the reappearance of the 753 nm Au(2) emission, indicating a strong interrelationship between both luminescence centers. Time-dependent density functional theory (TD-DFT) calculations indicate that the green luminescence can be assigned to noble metal dimers bound to silanolate centers. These complexes are recognized as the first stages in the further cluster growth process, which has been investigated with small-angle x-ray scattering (SAXS). In silver-doped glasses, Ag(0) atoms can be identified with electron paramagnetic resonance (EPR) spectroscopy after synchrotron activation. Annealing at 300 degrees C decreases the concentration of Ag(1), but induces an intense white light emission with 337 nm excitation. The white luminescence can be decomposed into bands that are attributed to small silver clusters such as Ag(2), Ag(3) and Ag(4), and an additional band matching the green emission of gold-doped glasses.

Three-photon-induced luminescence of gold nanoparticles embedded in and located on the surface of glassy nanolayers

We report on the multiphoton-induced luminescence of gold nanoparticles embedded in thin glassy silicate–titanate films. The glassy layers doped with gold(III) chloride are synthesized by a sol–gel coating process. Gold nanoparticles are generated by subsequent annealing of the thin films at 300 ◦ C. Intensive near-infrared femtosecond laser irradiation also initiates the formation of gold particles, providing the possibility of spatially resolved photoactivation of the film. The reduction of gold ions to gold nanoparticles is monitored by Au L3-edge x-ray absorption near edge spectroscopy (XANES), UV–vis absorption spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The particle sizes and shapes can be tuned by changing the metal concentration in the matrix. We demonstrate that the particles exhibit an efficient, long time stable, white luminescence during near-infrared Ti:sapphire femtosecond laser excitation. The laser power-emission intensity law indicates that the luminescence is induced by the absorption of three laser photons. Cross-sectional TEM images show that gold nanoparticles are both embedded in the glassy matrix and located on the film surface. Hence, the particles should be accessible for viable applications, for example as sensor materials, and could therefore become a powerful alternative to organic and semiconducting fluorophores in biological imaging. (Some figures in this article are in colour only in the electronic version)

SERS and multiphoton-induced luminescence of gold micro- and nanostructures fabricated by NIR femtosecond-laser irradiation.

possible applications have not been reported so far. The described multiphoton-induced luminescence is indeed very promising due to the excitation of the light emission by nearinfrared (NIR) photons. The low tissue autofluorescence and the deep penetration depth of light waves into the tissues in the NIR spectral range make the particles excellent candidates for contrast reagents in the field of biological imaging. A second major advantage of using gold is that it is non-toxic and superior for bio-labeling as it can be easily functionalized with diverse compounds by the reaction with thiolates and other organic molecules. In addition, gold nanoparticles have attracted much attention in ex-vivo and in-vivo biological imaging due to the surface-enhanced Raman scattering (SERS) effect of adsorbed molecules. [6, 7] For the utilization of gold nanoparticles as luminescence and SERS sensor materials and especially for trace analysis, a simple, fast and reproducible technique for the production of SERS-active and luminescent substrates is necessary. [8, 9] Furthermore, the mean particle size, the size distribution and the spatial arrangement of nanoparticles must fulfill certain criteria for optimum SERS enhancement. [10] As an additional requirement for analytical applications the particles should be very stable towards any kinds of solvents. An ideal SERS-active substrate should be usable more than once since the analyte molecules can be washed away after the detection without damaging the substrate. Herein, we introduce an efficient and facile chemical sol–gel derived technique to generate gold nanoparticles in glassy silicate-titanium nanolayers by titaniumdoped sapphire (Ti:Sa) femtosecond(fs)-laser activation. Our new laser scanning microscope (LSM) and surface-electron microscopy (SEM) data support our previous findings and clearly demonstrate that well-defined micro- and nano-patterns of gold nanoparticles can be fabricated over extended areas on stable films by employing a confocal LSM, thus showing the ease and feasibility of this method for “real world” applications. This procedure is fast and straightforward. The partially embedded or tethered gold nanoparticles are stable and show a multiphoton NIR excited photoluminescence (PL). The PL is helpful for directly visualizing (or imaging) the gold nanostructures. It might become very useful for detecting reporter molecules in combination with SERS. Additionally, we show that the luminescent gold nanoparticle films are indeed applicable as very powerful and stable SERS-active substrates.

Gold-ruby glass in a new light: On the microstructuring of optical glasses with synchrotron radiation

We describe the microstructuring of gold-ruby glasses with synchrotron radiation. Plasmonic or luminescent microstructures with a lateral width of minimum 5 μm can be written directly into the glasses by implementing X-ray lithography. The technique involves two steps: First, gold containing glass samples are irradiated with synchrotron X-rays through a microstructured mask. And second, subsequent annealing at minimum 500°C induces the growth of gold nanoparticles. The patterned sites are ruby coloured due to the gold surface plasmon resonance of gold nanoparticles. Furthermore we investigated the photoluminescence of the microstructured glass. After synchrotron irradiation a red photoluminescence is observed under UV light excitation. Subsequent annealing for a few minutes at 300°C induces the quenching of the red luminescence. If the irradiated sample is annealed for 5 minutes at a higher temperature of 500°C a bright green light emission is detected. The green photoluminescence decreases after further annealing and finally vanishes. We assume that the origin of the luminescence are silicate hole centres. The technique of generating gold particles with synchrotron X-ray lithography has potential to produce micro-optical devices like optical storage units, photonic crystals, gratings or sensors.