An analytical model for the prediction of the dielectric properties of gold–silver alloys is developed. This multi‐parametric model is a modification of the usual Drude–Lorentz model that takes into account the band structure of the metals. It is fitted by a genetic algorithm to the dielectric function of thin alloy films of different gold–silver ratio obtained by ellipsometry. The model is validated for arbitrary alloy compositions by comparing the experimental extinction spectra of alloy nanoparticles with the spectra predicted by Mie theory.
Brillouin measurements revealed notable differences between healthy and keratoconic corneas. Importantly, Brillouin imaging showed that the mechanical loss is primarily concentrated within the area of the keratoconic cone. Outside the cone, the Brillouin shift was comparable with that of healthy corneas. The results demonstrate the potential of Brillouin microscopy for diagnosis and treatment monitoring of keratoconus.
A femtosecond laser-assisted method has been developed to produce stable, size-tunable (3 to ∼80 nm) and low dispersed gold nanoparticles. The method implies the formation of initial nanosized seeds (<10 nm) by laser ablation from a gold target in aqueous solutions of biopolymers. Those seeds are then irradiated by a femtosecond laser-induced white-light supercontinuum to initiate a controlled growth of larger nanoparticles. The use of biopolymers (chitosan, R,ω-dithiol poly(N-isopropylacrylamide), poly(ethylene glycol), and dextran) of different concentrations enables in situ surface functionalization and size control during the process. We discuss mechanisms of nanoparticle growth and demonstrate an example of their application in biosensing, using a model of nanoparticle aggregation via specific binding of the gold-dextran bioconjugate to concanavalin A.
A femtosecond laser irradiation approach has been developed for the production of homogeneous AuAg nanoalloys of various compositions. The mean size was controlled by the dextran-nanoparticle affinity and resulted in the production of 5-7 nm nanoalloys for all nanoparticle compositions. Strong improvement of the oxidation resistance resulted from the increase of the atomic gold fraction in the nanoparticles. At gold fractions above 0.4, most of the nanoparticle oxidation was quenched, inhibiting the release of toxic silver ions in solution. The oxidation of the produced nanoparticles was mainly attributed to the generation of free radicals (O • , H • , • OH) and of molecular reactive oxygen species (O 2 , H 2 , H 2 O 2 ) formed by the decomposition of the water molecules through femtosecond laser-induced optical breakdown. For some biological applications, like surface-enhanced Raman spectroscopy (SERS), it is anticipated that AuAg nanoalloys would be the best compromise in terms of chemical stability and plasmonic response, as they possess much better resistance to oxidation in comparison to pure silver and a much stronger and narrower plasmon peak in comparison to pure gold.
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