Residual ligands from colloidal synthesis of nanoparticles influence adsorption of nanoparticles to supports and may complicate fabrication of nanoparticle-decorated microparticles. In this work, we studied the adsorption of completely ligand-free metal nanoparticles and controlled ligand-functionalized nanoparticles to chemically inert microparticle supports. Adsorption of ligand-free silver nanoparticles to barium sulfate microparticle supports is a quantitative, nonreversible process following Freundlich adsorption isotherm. However, adsorption efficiency is very sensitive to ligand concentration applied during laser-based synthesis of silver nanoparticles: exceeding a specific threshold concentration of 50 μmol/L citrate equal to a nanoparticle ligand surface coverage of about 50%, results in an almost complete prevention of nanoparticle adsorption because of electrosteric repulsion by ligand shell. Laser-based synthesis of nanoparticle-decorated microparticles is demonstrated with a variety of metal nanoparticles (Ag, Au, Pt, Fe) and supporting microparticles (calcium phosphate, titanium dioxide, barium sulfate) with application potential in heterogeneous catalysis or biomedicine where ligand control offers extra value, like enhanced catalytic activity or biocompatibility.

We applied a high-power (25 W) picosecond-pulsed laser system in combination with fast scanner optics for pulsed laser ablation in liquids in order to generate zinc/zinc oxide nanoparticles in tetrahydrofuran with optimized efficiency. Systematic variation of repetition rate and interpulse distance of subsequent laser pulses strongly affects the ablation efficiency. Shielding of subsequent laser pulses by induced cavitation bubbles could be minimized by these parameters. The analysis of experimental data results in a time constant of 55 μs concerning the cavitation bubble decay and a nonspherical shape with a lateral elongation of 120 μm after 100 μs. Regarding these parameters allows temporal and spatial bypassing of the cavitation bubble to enhance ablation efficiency and nanoparticle productivity. Furthermore, there is a nonlinear dependency of ablation efficiency on interpulse distance even if an effect coupled by cavitation bubbles can be excluded. We interpret this as a competition between two ablation mechanisms including thermal vaporization and phase explosion. For that purpose, we assume a transient preheating of the target by previous pulse, which leads to less efficient heat conduction that favors phase explosion instead of thermal vaporization. Calculations of 1D-heat conduction and analysis of generated nanoparticles support that interpretation. We were able to model the shape of the cavitation bubble and the experimental data by an adopted fit function.

The influence of fundamental and second harmonic wavelength on ablation efficiency and nanoparticle properties is studied during picosecond laser ablation of silver, zinc, and magnesium in polymer-doped tetrahydrofuran. Laser ablation in stationary liquid involves simultaneously the fabrication of nanoparticles by ablation of the target material and fragmentation of dispersed nanoparticles by post irradiation. The ratio in which the laser pulse energy contributes to these processes depends on laser wavelength and colloidal properties. For plasmon absorbers (silver), using the second harmonic wavelength leads to a decrease of the nanoparticle productivity over process time along with exponential decrease in particle diameter, while using the fundamental wavelength results in a constant ablation rate and linear decrease in particle diameter. For colloids made of materials without plasmon absorption (zinc, magnesium), laser scattering is the colloidal property that limits nanoparticle productivity by Mie-scattering of dispersed nanoparticle clusters.A. Schwenke · P. Wagener · S. Barcikowski ( ) Laser Zentrum Hannover e.V.,

The crucial step in the production of solid nanocomposites is the uniform embedding of nanoparticles into the polymer matrix, since the colloidal properties or specific physical properties are very sensitive to particle dispersion within the nanocomposite. Therefore, we studied a laser-based generation method of a nanocomposite which enables us to control the agglomeration of nanoparticles and to increase the single particle dispersion within polyurethane. For this purpose, we ablated targets of silver and copper inside a polymer-doped solution of tetrahydrofuran by a picosecond laser (using a pulse energy of 125 μJ at 33.3 kHz repetition rate) and hardened the resulting colloids into solid polymers. Electron microscopy of these nanocomposites revealed that primary particle size, agglomerate size and particle dispersion strongly depend on concentration of the polyurethane added before laser ablation. 0.3 wt% polyurethane is the optimal polymer concentration to produce nanocomposites with improved particle dispersion and adequate productivity. Lower polyurethane concentration results in agglomeration whereas higher concentration reduces the production rate significantly. The following evaporation step did not change the distribution of the nanocomposite inside the polyurethane matrix. Hence, the in situ coating of nanoparticles with polyurethane during laser ablation enables simple integration into the structural analogue polymer matrix without additives. Furthermore, it was possible to injection mold these in situ-stabilized nanocomposites without affecting particle dispersion. This clarifies that sufficient in situ stabilization during laser ablation in polymer solution is able to prevent agglomeration even in a hot polymer melt.

Surface pre-endothelialization is a promising approach to improve the hemocompatibility of implants, medical devices, and artificial organs. To promote the adhesive property of thermoplastic polyurethane (TPU) for endothelial cells (ECs), up to 1 wt % of gold (Au) or platinum (Pt) nanoparticles, fabricated by pulsed laser ablation in polymer solution, were embedded into the polymer matrix. The analysis of these nanocomposites showed a homogenous dispersion of the nanoparticles, with average diameters of 7 nm for Au or 9 nm for Pt. A dose-dependent effect was found when ECs were seeded onto nanocomposites comprising different nanoparticle concentrations, resulting in a fivefold improvement of proliferation at 0.1 wt % nanoparticle load. This effect was associated with a nanoparticle concentration-dependent hydrophilicity and negative charge of the nanocomposite. In dynamic flow tests, nanocomposites containing 0.1 wt % Au or Pt nanoparticles allowed for the generation of a confluent and resistant EC layer. Real-time polymerase chain reaction quantification of specific markers for EC activation indicated that ECs cultivated on nanocomposites remain in an inactivated, nonthrombogenic and noninflammatory state; however, maintain the ability to trigger an inflammatory response upon stimulation. These findings were confirmed by a platelet and leukocyte adhesion assay. The results of this study suggest the possible applicability of TPU nanocomposites, containing 0.1 wt % Au or Pt nanoparticles, for the generation of pre-endothelialized surfaces of medical devices.

The near- and off-resonant optical limiting properties of gold, silver and gold–silver alloy nanoparticles in methyl 2-methylprop-2-enoate for nanosecond laser pulses are presented. The nanoparticles are generated by picosecond pulsed laser ablation in liquid having hydrodynamic diameters from 26 to 30 nm. We use a Q-switched Nd:YAG laser working at a wavelength of 1064 or 532 nm, with a pulse width of 3 ns to characterize their behaviour by laser energy and fluence dependent transmittance measurements. To elucidate the contribution of nonlinear scattering to the optical limiting properties the scattered light energy at an angle of 90° is measured. The experimental results show that these nanoparticles have a strong nonlinear attenuation which can be attributed to intraband, interband and free carrier absorption and a thermal-induced scattering only at high input energies. Our results indicate in addition that the surface plasmon resonance does not contribute to the nonlinear processes at high input energies.

Medical devices made of polymers are often protected against infection-relevant biofilm formation by embedding nanoparticles as a source of bioactive metal ion release. Safe application of such nanocomposites requires finding the optimal ion dose and identifying the cross-effects caused by material mixtures. This study investigated the safety and antimicrobial efficacy of thermoplastic polyurethane (TPU), which is widely used for medical devices, e.g., catheters containing zinc, silver, copper and magnesium nanoparticles, respectively, and combinations thereof. Nanoparticles were generated by using pulsed laser ablation in polymer solution. We found that the composites embedded with nanosilver were noncytotoxic to cells but toxic to bacteria, with an optimal effect at 0.5 wt%. In contrast, zinc, copper, and magnesium nanoparticle composites did not inhibit bacteria growth. Interestingly, by combining the antibacterial metals (Ag, Cu) with nanoparticles made of elements required in biological systems (Zn, Mg), we observed an altered ion release and corresponding changes to their antibacterial efficacy and biocompatibility. The combination of silver with magnesium in the nanocomposites did increase both the amount and rate of silver ion release, and resulted in an increased antimicrobial effect of this Ag-Mg-TPU composite material. The therapeutic window of silver could not be changed quantitatively by the Ag-Mg combination, but less wt% silver was required for achieving antimicrobial efficacy because of faster ion release in the clinically relevant, critical initial phase of immersion. According to our observations, the mechanism of Mg increasing the mass-specific bio-effectivity of silver is possibly nonelectrochemical but volumetric. A fine-tuning of the Mg to Ag ratio and the overall load would be required to test whether a larger therapeutic window compared with Ag composites can be gained by the mixed Mg-Ag nanocomposites. Overall, the addition of Mg to Ag reduces the lag phase of bioactivity by increasing the Ag ion release in the critical first days after application of the medical device.

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