In Situ Investigations of Laser-Generated Ligand-Free Platinum Nanoparticles by X-ray Absorption Spectroscopy: How Does the Immediate Environment Influence the Particle Surface?

Pulsed laser ablation in liquid (PLAL) has proven its usefulness as a nanoparticle (NP) synthesis method alternative to traditional chemical reduction methods, where the absence of any molecular ligands or residual reactants makes laser-generated nanoparticles ideal reference materials for charge-transfer experiments. We synthesized additive-free platinum nanoparticles by PLAL and in-situ characterized their interaction with H2O, sodium phosphate buffer, and sodium citrate as well as a TiO2 support by X-ray absorption fine structure (XAFS), i.e., X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Differences in the white-line intensity among the colloidal particles in the three liquids indicate that the respective NP-solvent interaction varies in strength. The ions added ex situ diffuse through the particles' electric double layer and interact electrostatically with the Stern plane. Consequently, these ions weaken the interaction of the functional OH groups that are bound to the partially oxidized platinum surfaces and cause their partial reduction. Comparing XAFS spectra of laser-generated Pt NPs in citrate with wet-chemically synthesized ones (both ligand-covered) indicates different types of Pt-O bonds: a Pt(IV)O2 type in the case of wet-chemical NPs and a Pt(II)O type in the case of laser-generated NPs. A comparison of unsupported laser-generated platinum NPs in H2O with TiO2-supported ones shows no white-line intensity differences and also an identical number of Pt-O bonds in both cases. This suggests that in the deposition process at least part of the double-layer coating stays intact and that the ligand-free Pt particle properties are preserved in the TiO2-supported Pt particles, relevant for heterogeneous catalysis.

Target geometry and rigidity determines laser-induced cavitation bubble transport and nanoparticle productivity - a high-speed videography study.

Laser-induced cavitation has mostly been studied in bulk liquid or at a two-dimensional wall, although target shapes for the particle synthesis may strongly affect bubble dynamics and interfere with particle productivity. We investigated the dynamics of the cavitation bubble induced by pulsed-laser ablation in liquid for different target geometries with high-speed laser microsecond videography and focus on the collapse behaviour. This method enables us observations in a high time resolution (intervals of 1 µs) and single-pulse experiments. Further, we analyzed the nanoparticle productivity, the sizes of the synthesized nanoparticles and the evolution of the bubble volume for each different target shape and geometry. For the ablation of metal (Ag, Cu, Ni) wire tips a springboard-like behaviour after the first collapse is observed which can be correlated with vertical projectile motion. Its turbulent friction in the liquid causes a very efficient transport and movement of the bubble and ablated material into the bulk liquid and prevents particle redeposition. This effect is influenced by the degree of freedom of the wire as well as the material properties and dimensions, especially the Young's modulus. The most efficient and largest bubble movement away from the wire was observed for a thin (500 µm) silver wire with velocities up to 19.8 m s(-1) and for materials with a small Young's modulus and flexural rigidity. We suggest that these observations may contribute to upscaling strategies and increase of particle yield towards large synthesis of colloids based on targets that may continuously be fed.

Adsorption of colloidal platinum nanoparticles to supports: charge transfer and effects of electrostatic and steric interactions.

Adsorption of colloidal nanoparticles to surfaces and supports is a convenient approach to heterogeneous catalysts, polymer additives, or wastewater treatment. We investigated the adsorption efficiency of laser-generated and initially ligand-free platinum nanoparticles to TiO2 supports as a function of pH, ionic strength, and ligand surface coverage. The nanoparticle adsorption is dominantly controlled by electrostatic interactions: if the pH of the suspension is between the isoelectric point of the nanoparticles and the support, nanoparticles are adsorbed and transfer a net charge to the support. This charge-driven adsorption is not affected by steric repulsion due to various ligands attached to the nanoparticle surface. In addition to electrostatic interactions, colloidal stability given by moderate ionic strengths and pH values above the isoelectric point of nanoparticles are prerequisites for colloidal deposition.

Dynamics of silver nanoparticle formation and agglomeration inside the cavitation bubble after pulsed laser ablation in liquid.

The formation of nanoparticles within the laser-induced cavitation bubble is studied in situ using small angle X-ray scattering with high spatiotemporal resolution. Directly after laser ablation, two different particle fractions consisting of compact primary particles of 8-10 nm size and agglomerates of 40-60 nm size are formed. The abundance of these species is strongly influenced by the dynamics of the oscillating cavitation bubble. Primary particle mass is most abundant during maximal expansion of the first bubble and reappears a little weaker in the rebound. In contrast to this, the mass abundance of agglomerates is relatively low in the first bubble but strongly increases during first bubble collapse and following rebound. Although most of the ablated material is trapped inside the bubble and follows its oscillation, a minor fraction of both species could be detected outside the cavitation bubble even before its final collapse.

Pulsed laser ablation of a continuously-fed wire in liquid flow for high-yield production of silver nanoparticles.

Using wires of defined diameters instead of a planar target for pulsed laser ablation in liquid results in significant increase of ablation efficiency and nanoparticle productivity up to a factor of 15. We identified several competitive phenomena based on thermal conductivity, reflectivity and cavitation bubble shape that affect the ablation efficiency when the geometry of the target is changed. On the basis of the obtained results, this work represents an intriguing starting point for further developments related to the up-scaling of pulsed laser ablation in liquid environments at the industrial level.

How citrate ligands affect nanoparticle adsorption to microparticle supports.

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.

Stoichiometry of alloy nanoparticles from laser ablation of PtIr in acetone and their electrophoretic deposition on PtIr electrodes.

Charged Pt-Ir alloy nanoparticles are generated through femtosecond laser ablation of a Pt9Ir target in acetone without using chemical precursors or stabilizing agents. Preservation of the target's stoichiometry in the colloidal nanoparticles is confirmed by transmission electron microscopy (TEM)-energy-dispersive x-ray spectroscopy (EDX), high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM)-EDX elemental maps, high resolution TEM and selected area electron diffraction (SAED) measurements. Results are discussed with reference to thermophysical properties and the phase diagram. The nanoparticles show a lognormal size distribution with a mean Feret particle size of 26 nm. The zeta potential of -45 mV indicates high stability of the colloid with a hydrodynamic diameter of 63 nm. The charge of the particles enables electrophoretic deposition of nanoparticles, creating nanoscale roughness on three-dimensional PtIr neural electrodes within a minute. In contrast to coating with Pt or Ir oxides, this method allows modification of the surface roughness without changing the chemical composition of PtIr.

Impact of in situ polymer coating on particle dispersion into solid laser-generated nanocomposites.

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.

Magnetic alloy nanoparticles from laser ablation in cyclopentanone and their embedding into a photoresist.

The generation of nonoxidized magnetic alloy nanoparticles is still a challenge using conventional chemical reduction methods. However, because these nanoparticles are currently attracting much attention, alternative methods are required. In this context, the applicability of femtosecond laser ablation, which has evolved as a powerful tool for the generation of colloidal metal nanoparticles, has been investigated using the example of Ni(48)Fe(52) and Sm(2)Co(17) ablation in cyclopentanone. Besides stability and size measurements, the focus has been placed on the analysis of the elemental composition of nanoparticles, which proved the preservation of the stoichiometry of the target in Ni-Fe nanoparticles but not in Sm-Co. It is assumed that this is due to a greater difference in the heat of evaporation of the bulk alloy components in Sm-Co than in Ni-Fe. Hence, the successful generation of magnetic alloy nanoparticles is possible for alloys composed of elements with similar heats of evaporation. This one-step approach allows the fabrication of nanomagnetic polymer composites (e.g., with application prospects in microtechnology such as microactuators).

Laser synthesis, structure and chemical properties of colloidal nickel-molybdenum nanoparticles for the substitution of noble metals in heterogeneous catalysis.

Platinum and iridium are rare and expensive noble metals that are used as catalysts for different sectors including in heterogeneous chemical automotive emission catalysis and electrochemical energy conversion. Nickel and its alloys are promising materials to substitute noble metals. Nickel based materials are cost-effective with good availability and show comparable catalytic performances. The nickel-molybdenum system is a very interesting alternative to platinum in water electrolysis. We produced ligand-free nickel-molybdenum nanoparticles by laser ablation in water and acetone. Our results show that segregated particles were formed in water due to the oxidation of the metals. X-ray diffraction shows a significant change in the lattice parameter due to a diffusion of molybdenum atoms into the nickel lattice with increasing activity in the electrochemical oxygen evolution reaction. Even though the solubility of molecular oxygen in acetone is higher than in water, there were no oxides and a more homogeneous metal distribution in the particles in acetone as seen by TEM-EDX. This showed that dissolved molecular oxygen does not control oxide formation. Overall, the laser ablation of pressed micro particulate mixtures in liquids offers a combinational synthesis approach that allows the screening of alloy nanoparticles for catalytic testing and can convert micro-mixtures into nano-alloys.

Pulsed laser ablation in liquids: Impact of the bubble dynamics on particle formation.

Pulsed laser ablation in liquids (PLAL) is a multiscale process, involving multiple mutually interacting phenomena. In order to synthesize nanoparticles with well-defined properties it is important to understand the dynamics of the underlying structure evolution. We use visible-light stroboscopic imaging and X-ray radiography to investigate the dynamics occurring during PLAL of silver and gold on a macroscopic scale, whilst X-ray small angle scattering is utilized to deepen the understanding on particle genesis. By comparing our results with earlier reports we can elucidate the role of the cavitation bubble. We find that symmetry breaking at the liquid-solid interface is a critical factor for bubble motion and that the bubble motion acts on the particle distribution as confinement and retraction force to create secondary agglomerates.

Solvent-surface interactions control the phase structure in laser-generated iron-gold core-shell nanoparticles.

This work highlights a strategy for the one-step synthesis of FeAu nanoparticles by the pulsed laser ablation of alloy targets in the presence of different solvents. This method allows particle generation without the use of additional chemicals; hence, solvent-metal interactions could be studied without cross effects from organic surface ligands. A detailed analysis of generated particles via transmission electron microscopy in combination with EDX elemental mapping could conclusively verify that the nature of the used solvent governs the internal phase structure of the formed nanoparticles. In the presence of acetone or methyl methacrylate, a gold shell covering a non-oxidized iron core was formed, whereas in aqueous media, an Au core with an Fe3O4 shell was generated. This core-shell morphology was the predominant species found in >90% of the examined nanoparticles. These findings indicate that fundamental chemical interactions between the nanoparticle surface and the solvent significantly contribute to phase segregation and elemental distribution in FeAu nanoparticles. A consecutive analysis of resulting Fe@Au core-shell nanoparticles revealed outstanding oxidation resistance and fair magnetic and optical properties. In particular, the combination of these features with high stability magnetism and plasmonics may create new opportunities for this hybrid material in imaging applications.

A hierarchical view on material formation during pulsed-laser synthesis of nanoparticles in liquid.

Pulsed-laser assisted nanoparticle synthesis in liquids (PLAL) is a versatile tool for nanoparticle synthesis. However, fundamental aspects of structure formation during PLAL are presently poorly understood. We analyse the spatio-temporal kinetics during PLAL by means of fast X-ray radiography (XR) and scanning small-angle X-ray scattering (SAXS), which permits us to probe the process on length scales from nanometers to millimeters with microsecond temporal resolution. We find that the global structural evolution, such as the dynamics of the vapor bubble can be correlated to the locus and evolution of silver nanoparticles. The bubble plays an important role in particle formation, as it confines the primary particles and redeposits them to the substrate. Agglomeration takes place for the confined particles in the second bubble. Additionally, upon the collapse of the second bubble a jet of confined material is ejected perpendicularly to the surface. We hypothesize that these kinetics influence the final particle size distribution and determine the quality of the resulting colloids, such as polydispersity and modality through the interplay between particle cloud compression and particle release into the liquid.

Dose-dependent surface endothelialization and biocompatibility of polyurethane noble metal nanocomposites.

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.

Bioconjugated silicon quantum dots from one-step green synthesis.

Biofunctionalized silicon quantum dots were prepared through a one step strategy avoiding the use of chemical precursors. UV-Vis spectroscopy, Raman spectroscopy and HAADF-STEM prove oligonucleotide conjugation to the surface of silicon nanoparticle with an average size of 4 nm. The nanoparticle size results from the size-quenching effect during in situ conjugation. Photoemissive properties, conjugation efficiency and stability of these pure colloids were studied and demonstrate the bio-application potential, e.g. for nucleic acid vector delivery with semiconducting, biocompatible nanoparticles.

Metal ion release kinetics from nanoparticle silicone composites.

Metal ion release kinetics from silver and copper nanoparticle silicone composites generated by laser ablation in liquids are investigated. The metal ion transport mechanism is studied by using different model equations and their fit to experimental data. Results indicate that during the first 30 days of immersion, Fickian diffusion is the dominant transport mechanism. After this time period, the oxidation and dissolution of nanoparticles from the bulk determine the ion release. This second mechanism is very slow since the dissolution of the nanoparticle is found to be anisotropic. Silver ion release profile is best described by pseudo-first order exponential equation. Copper ion release profile is best described by a second order exponential equation. For practical purposes, the in vitro release characteristics of the bioactive metal ions are evaluated as a function of nanoparticle loading density, the chemistry and the texture of the silicone. Based on the proposed two-step release model, a prediction of the release characteristics over a time course of 84 days is possible and a long-term ion release could be demonstrated.

Ultrafast intramolecular electronic energy transfer in rigidly linked aminopyrenyl-aminobenzanthronyl dyads--a femtosecond study.

Ultrafast electronic excitation transfer (EET) followed by structural and vibrational relaxation (VER) of the acceptor have been characterised using transient absorption and transient lens techniques.

Photoinduced isomerization kinetics of diiodomethane in supercritical fluid solution: local density effects.

The density dependence of diiodomethane photoinduced isomerization in supercritical (sc) CO2, CHF3, and C2H6 was investigated by transient absorption spectroscopy, covering a fluid density range from 0.7 to 2.5 (in reduced units). The solvent-caged photoproduct iso-diiodomethane is formed even at the lowest density, and its yield increases about 4-fold over the whole range. At the same time, isomer formation rate constants increase by roughly an order of magnitude and show little variation between CO2, C2H6, and CHF3. Furthermore, the formation rate constant decreases significantly with increasing excitation energy. We propose an isomer formation mechanism involving a rapidly established preequilibrium between a solvent-caged iodine atom-methyliodide radical pair and a loosely bound iodine-methyliodide radical complex, from which the reaction subsequently proceeds to the isomer. The latter step seems to be controlled by collisional stabilization of the initially hot radical moiety, as the formation rate constant increases linearly with sc solvent viscosity. The model predicts a quadratic dependence of relative isomer yield on fluid density. A corresponding correlation is found with the local fluid density, calculated via solute-solvent radial distribution functions obtained from molecular dynamics (MD) simulations.