Barrierless growth of precursor-free, ultrafast laser-fragmented noble metal nanoparticles by colloidal atom clusters – A kinetic in situ study

Monitoring Photoinduced Interparticle Chemical Communication In Situ

Monitoring interparticle chemical communication plays a critical role in the nanomaterial synthesis as this communication controls the final structure and stability of global nanoparticles (NPs). Yet most ensemble analytical techniques, which could only reveal average macroscopic information, are unable to elucidate NP-to-NP interactions. Herein, we employ stochastic collision electrochemistry to track the morphology transformation of Ag NPs in photochemical process at the single NP level. By further statistical analysis of time-resolved current transients, we quantitatively determine the dynamic chemical potential difference and interparticle communication between populations of large and small Ag NPs. The high sensitivity of stochastic collision electrochemistry enables the in situ investigation of chemical communication-dependent transformation kinetics of NPs in photochemical process, shedding light on designing nanomaterials.

Parts-Per-Million of Soluble Pd0 Catalyze the Semi-Hydrogenation Reaction of Alkynes to Alkenes

The synthesis of cis-alkenes is industrially carried out by selective semi-hydrogenation of alkynes with complex Pd catalysts, which include the Lindlar catalyst (PdPb on CaCO₃) and c-Pd/TiS (colloidal ligand-protected Pd nanoparticles), among others. Here, we show that Pd⁰ atoms are generated from primary Pd salts (PdCl₂, PdSO₄, Pd(OH)₂, PdO) with H₂ in alcohol solutions, independently of the alkyne, to catalyze the semi-hydrogenation reaction with extraordinarily high efficiency (up to 735 s^-1), yield (up to 99%), and selectivity (up to 99%). The easy-to-prepare Pd⁰ species hold other potential catalytic applications.

Impact of Sterilization on the Colloidal Stability of Ligand-Free Gold Nanoparticles for Biomedical Applications

Sterilization is a major prerequisite for the utilization of nanoparticle colloids in biomedicine, a process well examined for particles derived from chemical synthesis although highly underexplored for electrostatically stabilized ligand-free gold nanoparticles (AuNPs). Hence, in this work, we comprehensively examined and compared the physicochemical characteristics of laser-generated ligand-free colloidal AuNPs exposed to steam sterilization and sterile filtration as a function of particle size and mass concentration and obtained physicochemical insight into particle growth processes. These particles exhibit long-term colloidal stability (up to 3 months) derived from electrostatic stabilization without using any ligands or surfactants. We show that particle growth attributed to cluster-based ripening occurs in smaller AuNPs (∼5 nm) following autoclaving, while larger particles (∼10 and ∼30 nm) remain stable. Sterile filtration, as an alternative effective sterilizing approach, has no substantial impact on the colloidal stability of AuNPs, regardless of particle size, although a mass loss of 5-10% is observed. Finally, we evaluated the impact of the sterilization procedures on potential particle functionality in proton therapy, using the formation of reactive oxygen species (ROS) as a readout. In particular, 5 nm AuNPs exhibit a significant loss in activity upon autoclaving, probably dedicated to specific surface area reduction and surface restructuring during particle growth. The filtered analog enhanced the ROS release by up to a factor of ∼2.0, at 30 ppm gold concentration. Our findings highlight the need for carefully adapting the sterilization procedure of ligand-free NPs to the desired biomedical application with special emphasis on particle size and concentration.

Comparing Direct and Pulsed-Direct Current Electrophoretic Deposition on Neural Electrodes: Deposition Mechanism and Functional Influence

Electrophoretic deposition (EPD) of platinum nanoparticles (PtNPs) on platinum-iridium (Pt-Ir) neural electrode surfaces is a promising strategy to tune the impedance of electrodes implanted for deep brain stimulation in various neurological disorders such as advanced Parkinson's disease and dystonia. However, previous results are contradicting as impedance reduction was observed on flat samples while in three-dimensional (3D) structures, an increase in impedance was observed. Hence, defined correlations between coating properties and impedance are to date not fully understood. In this work, the influence of direct current (DC) and pulsed-DC electric fields on NP deposition is systematically compared and clear correlations between surface coating homogeneity and in vitro impedance are established. The ligand-free NPs were synthesized via pulsed laser processing in liquid, yielding monomodal particle size distributions, verified by analytical disk centrifugation (ADC). Deposits formed were quantified by UV-vis supernatant analysis and further characterized by scanning electron microscopy (SEM) with semiautomated interparticle distance analyses. Our findings reveal that pulsed-DC electric fields yield more ordered surface coatings with a lower abundance of particle assemblates, while DC fields produce coatings with more pronounced aggregation. Impedance measurements further highlight that impedance of the corresponding electrodes is significantly reduced in the case of more ordered coatings realized by pulsed-DC depositions. We attribute this phenomenon to the higher active surface area of the adsorbed NPs in homogeneous coatings and the reduced particle-electrode electrical contact in NP assemblates. These results provide insight for the efficient EPD of bare metal NPs on micron-sized surfaces for biomedical applications in neuroscience and correlate coating homogeneity with in vitro functionality.

On the Use of Laser Fragmentation for the Synthesis of Ligand-Free Ultra-Small Iron Nanoparticles in Various Liquid Environments

Traditionally, the synthesis of nanomaterials in the ultra-small size regime (1-3 nm diameter) has been linked with the employment of excessive amounts of hazardous chemicals, inevitably leading to significant environmentally detrimental effects. In the current work, we demonstrate the potential of laser fragmentation in liquids (LFL) to produce highly pure and stable iron ultra-small nanoparticles. This is carried out by reducing the size of carbonyl iron microparticles dispersed in various polar solvents (water, ethanol, ethylene glycol, polyethylene glycol 400) and liquid nitrogen. The explored method enables the fabrication of ligand-free iron oxide ultra-small nanoparticles with diameter in the 1-3 nm range, a tight size distribution, and excellent hydrodynamic stability (zeta potential > 50 mV). The generated particles can be found in different forms, including separated ultra-small NPs, ultra-small NPs forming agglomerates, and ultra-small NPs together with zero-valent iron, iron carbide, or iron oxide NPs embedded in matrices, depending on the employed solvent and their dipolar moment. The LFL technique, aside from avoiding chemical waste generation, does not require any additional chemical agent, other than the precursor microparticles immersed in the corresponding solvent. In contrast to their widely exploited chemically synthesized counterparts, the lack of additives and chemical residuals may be of fundamental interest in sectors requiring colloidal stability and the largest possible number of chemically active sites, making the presented pathway a promising alternative for the clean design of new-generation nanomaterials.

How the Physicochemical Properties of the Bulk Material Affect the Ablation Crater Profile, Mass Balance, and Bubble Dynamics During Single-Pulse, Nanosecond Laser Ablation in Water

Understanding the key steps that drive the laser-based synthesis of colloids is a prerequisite for learning how to optimize the ablation process in terms of nanoparticle output and functional design of the nanomaterials. Even though many studies focus on cavitation bubble formation using single-pulse ablation conditions, the ablation efficiency and nanoparticle properties are typically investigated under prolonged ablation conditions with repetition rate lasers. Linking single-pulse and multiple-pulse ablation is difficult due to limitations induced by gas formation cross-effects, which occur on longer timescales and depend on the target materials' oxidation-sensitivity. Therefore, this study investigates the ablation and cavitation bubble dynamics under nanosecond, single laser pulse conditions for six different bulk materials (Au, Ag, Cu, Fe, Ti, and Al). Also, the effective threshold fluences, ablation volumes, and penetration depths are quantified for these materials. The thermal and chemical properties of the corresponding bulk materials not only favor the formation of larger spot sizes but also lead to the highest molar ablation efficiencies for low melting materials such as aluminum. Furthermore, the concept of the cavitation bubble growth linked with the oxidation sensitivity of the ablated material is discussed. With this, evidence is provided that intensive chemical reactions occurring during the very early timescale of ablation are significantly enhanced by the bubble collapse.

Stability evolution of ultrafine Ag nanoparticles prepared by laser ablation in liquids

Understanding the stability evolution of the silver nanoparticles (Ag NPs) in colloid has great benefits for its controllable preparation, storage and application. Herein, uncapped Ag NPs with diameter of 1.66 ± 0.37 nm are obtained by laser ablation of Ag target in deionized water, corresponding surface plasma resonance (SPR) bands, ζ potential and particle size distribution are monitored to investigate uncapped Ag NPs' stability evolution. Due to negatively charged surface, uncapped Ag NPs show an excellent dispersion stability in 70 days without any external disturbance. But its dispersion stability and structure stability are destroyed easily by an oscillation treatment, resulting in a tardy growth and the formation of one-dimensional Ag nanochain. In addition, the chemical stability of uncapped Ag NPs is dramatically varied by a displacement reaction with an inserted copper wire. As comparison, two typical cationic and anionic surfactant molecules, N-hexadecyl trimethyl ammonium chloride (CTAC) and sodium dodecyl benzene sulfonate (SDBS) are severally used to prepare surface capped Ag NPs. With same treatment of Ag colloid, both two kinds of capped Ag NPs display better dispersion stability and structure stability than uncapped Ag NPs. Moreover, CTAC capped Ag NPs keep a better chemical stability than SDBS capped Ag NPs.

Increasing the Size-Selectivity in Laser-Based g/h Liquid Flow Synthesis of Pt and PtPd Nanoparticles for CO and NO Oxidation in Industrial Automotive Exhaust Gas Treatment Benchmarking

PtPd catalysts are state-of-the-art for automotive diesel exhaust gas treatment. Although wet-chemical preparation of PtPd nanoparticles below 3 nm and kg-scale synthesis of supported PtPd/Al₂O₃ are already established, the partial segregation of the bimetallic nanoparticles remains an issue that adversely affects catalytic performance. As a promising alternative, laser-based catalyst preparation allows the continuous synthesis of surfactant-free, solid-solution alloy nanoparticles at the g/h-scale. However, the required productivity of the catalytically relevant size fraction <10 nm has yet to be met. In this work, by optimization of ablation and fragmentation conditions, the continuous flow synthesis of nanoparticles with a productivity of the catalytically relevant size fraction <10 nm of >1 g/h is presented via an in-process size tuning strategy. After the laser-based preparation of hectoliters of colloid and more than 2 kg of PtPd/Al₂O₃ wash coat, the laser-generated catalysts were benchmarked against an industry-relevant reference catalyst. The conversion of CO by laser-generated catalysts was found to be equivalent to the reference, while improved activity during NO oxidation was achieved. Finally, the present study validates that laser-generated catalysts meet the size and productivity requirements for industrial standard operating procedures. Hence, laser-based catalyst synthesis appears to be a promising alternative to chemical-based preparation of alloy nanoparticles for developing industrial catalysts, such as those needed in the treatment of exhaust gases.

Room-Temperature Laser Synthesis in Liquid of Oxide, Metal-Oxide Core-Shells, and Doped Oxide Nanoparticles

Although oxide nanoparticles are ubiquitous in science and technology, a multitude of compositions, phases, structures, and doping levels exist, each one requiring a variety of conditions for their synthesis and modification. Besides, experimental procedures are frequently dominated by high temperatures or pressures and by chemical contaminants or waste. In recent years, laser synthesis of colloids emerged as a versatile approach to access a library of clean oxide nanoparticles relying on only four main strategies running at room temperature and ambient pressure: laser ablation in liquid, laser fragmentation in liquid, laser melting in liquid and laser defect-engineering in liquid. Here, established laser-based methodologies are reviewed through the presentation of a panorama of oxide nanoparticles which include pure oxidic phases, as well as unconventional structures like defective or doped oxides, non-equilibrium compounds, metal-oxide core-shells and other anisotropic morphologies. So far, these materials showed several useful properties that are discussed with special emphasis on catalytic, biomedical and optical application. Yet, given the endless number of mixed compounds accessible by the laser-assisted methodologies, there is still a lot of room to expand the library of nano-crystals and to refine the control over products as well as to improve the understanding of the whole process of nanoparticle formation. To that end, this review aims to identify the perspectives and unique opportunities of laser-based synthesis and processing of colloids for future studies of oxide nanomaterial-oriented sciences.

Manipulation of the Size and Phase Composition of Yttrium Iron Garnet Nanoparticles by Pulsed Laser Post-Processing in Liquid

Modification of the size and phase composition of magnetic oxide nanomaterials dispersed in liquids by laser synthesis and processing of colloids has high implications for applications in biomedicine, catalysis and for nanoparticle-polymer composites. Controlling these properties for ternary oxides, however, is challenging with typical additives like salts and ligands and can lead to unwanted byproducts and various phases. In our study, we demonstrate how additive-free pulsed laser post-processing (LPP) of colloidal yttrium iron oxide nanoparticles using high repetition rates and power at 355 nm laser wavelength can be used for phase transformation and phase purification of the garnet structure by variation of the laser fluence as well as the applied energy dose. Furthermore, LPP allows particle size modification between 5 nm (ps laser) and 20 nm (ns laser) and significant increase of the monodispersity. Resulting colloidal nanoparticles are investigated regarding their size, structure and temperature-dependent magnetic properties.

In situ structural kinetics of picosecond laser-induced heating and fragmentation of colloidal gold spheres

Fragmentation of colloidal 54 nm gold nanoparticles by picosecond laser pulses is recorded by time-resolved X-ray scattering, giving access to structural dynamics down to a 80 ps resolution. Lattice temperature and energy dissipation have been quantified to verify that the maximum applied fluence of 1800 J m^-2 heats up the particles close to boiling. Already within 30 ns, particles with significantly lower particle sizes of 2 to 3 nm are detected, which hints towards an ultrafast process either by a thermal phase explosion or Coulomb instability. An arrested growth is observed on a microsecond time scale resulting in a final particle size of 3-4 nm with high yield. In this context, the fragmentation in a NaCl/NaOH solution seems to limit growth by electrostatic stabilization of fragments, whereas it does not modify the initial product sizes. The laser-induced fragmentation process is identified as a single-step, instantaneous reaction.

Synergism between Specific Halide Anions and pH Effects during Nanosecond Laser Fragmentation of Ligand-Free Gold Nanoparticles

Gold nanoclusters (AuNCs) with diameters smaller than 3 nm are an emerging field of research because they possess interesting optical properties, such as photoluminescence. However, to date, it is still difficult to distinguish whether these properties originate from the cores of the nanoparticles or from the adsorbates on their surfaces. Hence, there is a high demand for ligand-free, ultra-small particles because they make it possible to study ligand and core effects separately. Pulsed laser fragmentation in liquids (LFL) is a convenient route for the synthesis of ligand-free AuNCs. The influence of physical parameters, such as melting and evaporation, on the LFL process is well understood both theoretically and experimentally. However, the impact of the chemical composition of the medium during LFL, which critically affects the particle formation process, has been less well examined. Therefore, in this work, we elucidate the extent to which the ionic strength, the pH value, and the nature of the halide anion that is present, that is, F^-, Cl^-, Br^-, or I^-, influence the particle size distribution of the LFL product and the mean yield of small particles (<3 nm) of the product. We showed that the yield of small particles can be enhanced by the synergism between pH and specific ion effects, which probably is attributable to the adsorption of specific anions. In addition, our findings indicated that anion-based stabilization depends critically on the type of anion. A direct Hofmeister effect was observed for anions in the neutral pH regime, whereas an indirect Hofmeister series was reported in alkaline solution, which probably was due to the more hydrophilic surfaces of the AuNCs that were formed.

Early appearance of crystalline nanoparticles in pulsed laser ablation in liquids dynamics

The size and crystallinity of gold and silver nanoparticles during the process of pulsed laser ablation in water (PLAL) is investigated with microsecond and sub-microsecond time resolution. While basic observations have already been established, such as detection of particles inside the cavitation bubble, trapping of ablated matter by the bubble or the action of size quenching on a sub-millisecond time scale, the structure formation mechanism is still a matter of debate. Quantifying the nanoparticle release and crystallinity close to the irradiated metal target by wide and small angle X-ray scattering reveals the presence of nanoparticles ahead of the developing vapour bubble and inside the bubble. While the (temporal) distribution is in agreement with a homogeneously particle-filled bubble, solid particles are detected at the advancing bubble front. Wide-angle X-ray scattering confirms the crystalline nature of these large particles. This reveals that for picosecond ablation the expulsion of condensed phases of material during the ablation process adds significantly to the bimodal size distribution, relating to recent models of film lift-off and liquid metal Rayleigh instabilities.

Time and Mechanism of Nanoparticle Functionalization by Macromolecular Ligands during Pulsed Laser Ablation in Liquids

Laser ablation of gold in liquids with nanosecond laser pulses in aqueous solutions of inorganic electrolytes and macromolecular ligands for gold nanoparticle size quenching is probed inside the laser-induced cavitation bubble by in situ X-ray multicontrast imaging with a Hartmann mask (XHI). It is found that (i) the in situ size quenching power of sodium chloride (NaCl) in comparison to the ablation in pure water can be observed by the scattering contrast from XHI already inside the cavitation bubble, while (ii) for polyvinylpyrrolidone (PVP) as a macromolecular model ligand an in situ size quenching cannot be observed. Complementary ex situ characterization confirms the overall size quenching ability of both additive types NaCl and PVP. The macromolecular ligand as well as its monomer N-vinylpyrrolidone (NVP) are mainly effective for growth quenching of larger nanoparticles on later time scales, leading to the conclusion of an alternative interaction mechanism with ablated nanoparticles compared to the electrolyte NaCl, probably outside of the cavitation bubble, in the surrounding liquid phase. While monomer and polymer have similar effects on the particle properties, with the polymer being slightly more efficient, only the polymer is effective against hydrodynamic aggregation.

Two Birds with One Stone: Spontaneous Size Separation and Growth Inhibition of Femtosecond Laser-Generated Surfactant-Free Metallic Nanoparticles via ex Situ SU-8 Functionalization

Laser ablation in liquids (LAL) offers a facile technique to develop a large variety of surfactant-free nanomaterials with high purity. However, due to the difficulty in the control of the particle synthesis process, the as-prepared nanomaterials always have a broad size distribution with a large polydispersity (σ). Surfactant-free properties can also cause problems with particle growth, which further increases the difficulty in size control of the colloids. Therefore, searching for strategies to simultaneously unify the sizes of colloids and inhibit particle growth has become significantly important for LAL-synthesized nanomaterials to be extensively used for biological, catalytic, and optical applications, in which fields particle size plays an important role. In this work, we present a facile way to simultaneously realize these two goals by ex situ SU-8 photoresist functionalization. Ag nanoparticles (NPs) synthesized by femtosecond laser ablation of silver in acetone at laser powers of 300 and 600 mW were used as starting materials. The synthesized Ag NPs have a broad size distribution between 1 and 200 nm with an average size of ca. 5.9 nm and σ of 127-207%. After ex situ SU-8 functionalization and 6 months storage, most particles larger than 10 nm become aggregates and precipitate, which makes the size distribution narrow with an average diameter of 4-5 nm and σ of 48-78%. The precipitation process is accompanied by the decrease in colloid mass from the initial ∼0.2 to 0.10-0.11 mg after ex situ SU-8 functionalization and 6 months colloid storage. Morphology analysis indicates that ex situ SU-8 functionalization inhibits the particle growth into polygonal nanocrystals. Radical polymerization of SU-8 on Ag NPs is considered to be the reason for both spontaneous size separation and growth inhibition phenomena. Benefiting from Ag NPs embedment and acetone dissolution, the glass-transition temperature of SU-8 photoresist increased from 314 to 331 °C according to thermogravimetric analysis. The universality of ex situ SU-8 functionalization-induced growth inhibition and size separation behaviors is further proved using the Au colloids generated by LAL in acetone. This work is expected to provide a new route for better size control of LAL-synthesized colloids via ex situ photoresist functionalization, although a half of colloidal mass is wasted due to radical polymerization-induced colloidal precipitation.

Spontaneous Shape Alteration and Size Separation of Surfactant-Free Silver Particles Synthesized by Laser Ablation in Acetone during Long-Period Storage

The technique of laser ablation in liquids (LAL) has already demonstrated its flexibility and capability for the synthesis of a large variety of surfactant-free nanomaterials with a high purity. However, high purity can cause trouble for nanomaterial synthesis, because active high-purity particles can spontaneously grow into different nanocrystals, which makes it difficult to accurately tailor the size and shape of the synthesized nanomaterials. Therefore, a series of questions arise with regards to whether particle growth occurs during colloid storage, how large the particle size increases to, and into which shape the particles evolve. To obtain answers to these questions, here, Ag particles that are synthesized by femtosecond (fs) laser ablation of Ag in acetone are used as precursors to witness the spontaneous growth behavior of the LAL-generated surfactant-free Ag dots (2–10 nm) into different polygonal particles (5–50 nm), and the spontaneous size separation phenomenon by the carbon-encapsulation induced precipitation of large particles, after six months of colloid storage. The colloids obtained by LAL at a higher power (600 mW) possess a greater ability and higher efficiency to yield colloids with sizes of <40 nm than the colloids obtained at lower power (300 mW), because of the generation of a larger amount of carbon ‘captors’ by the decomposition of acetone and the stronger particle fragmentation. Both the size increase and the shape alteration lead to a redshift of the surface plasmon resonance (SPR) band of the Ag colloid from 404 nm to 414 nm, after storage. The Fourier transform infrared spectroscopy (FTIR) analysis shows that the Ag particles are conjugated with COO– and OH– groups, both of which may lead to the growth of polygonal particles. The CO and CO₂ molecules are adsorbed on the particle surfaces to form Ag(CO)_x and Ag(CO₂)_x complexes. Complementary nanosecond LAL experiments confirmed that the particle growth was inherent to LAL in acetone, and independent of pulse duration, although some differences in the final particle sizes were observed. The nanosecond-LAL yields monomodal colloids, whereas the size-separated, initially bimodal colloids from the fs-LAL provide a higher fraction of very small particles that are <5 nm. The spontaneous growth of the LAL-generated metallic particles presented in this work should arouse the special attention of academia, especially regarding the detailed discussion on how long the colloids can be preserved for particle characterization and applications, without causing a mismatch between the colloid properties and their performance. The spontaneous size separation phenomenon may help researchers to realize a more reproducible synthesis for small metallic colloids, without concern for the generation of large particles.

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.

Optimizing in Vitro Impedance and Physico-Chemical Properties of Neural Electrodes by Electrophoretic Deposition of Pt Nanoparticles

Neural electrodes suffer from an undesired incline in impedance when in permanent contact with human tissue. Nanostructures, induced by electrophoretic deposition (EPD) of ligand-free laser-generated nanoparticles (NPs) on the electrodes are known to stabilize impedance in vivo. Hence, Pt surfaces were systematically EPD-coated with Pt NPs and evaluated for impedance as well as surface coverage, contact angle, electrochemically active surface area (ECSA) and surface oxidation. The aim was to establish a systematic correlation between EPD process parameters and physical surface properties. The findings clearly reveal a linear decrease in impedance with increasing surface coverage, which goes along with a proportional reduction of the contact angle and an increase in ECSA and surface oxidation. EPD process parameters, prone to yield surface coatings with low impedance, are long deposition times (40-60 min), while high colloid concentrations (>250 μg mL^-1 ) and electric field strengths (>25 V cm^-1 ) should be avoided due to detrimental NP assemblage effects.

Ultrasmall Yttrium Iron Garnet Nanoparticles with High Coercivity at Low Temperature Synthesized by Laser Ablation and Fragmentation of Pressed Powders

Pulsed laser ablation of pressed yttrium iron garnet powders in water is studied and compared to the ablation of a single-crystal target. We find that target porosity is a crucial factor, which has far-reaching implications on nanoparticle productivity. Although nanoparticle size distributions obtained by analytical disc centrifugation and transmission electron microscopy (TEM) are in agreement, X-ray diffraction and energy dispersive X-ray analysis show that only nanoparticles obtained from targets with densities close to that of a bulk target lead to comparable properties. Our findings also show why the gravimetrical measurement of nanoparticle productivity is often flawed and needs to be complemented by colloidal productivity measurements. The synthesized YIG nanoparticles are further reduced in size by laser fragmentation to obtain sizes smaller than 3 nm. Since the particle diameters are close to the YIG lattice constant, these ultrasmall nanoparticles reveal an immense change of the magnetic properties, exhibiting huge coercivity (0.11 T) and irreversibility fields (8 T) at low temperatures.

Laser Synthesis and Processing of Colloids: Fundamentals and Applications

Driven by functionality and purity demand for applications of inorganic nanoparticle colloids in optics, biology, and energy, their surface chemistry has become a topic of intensive research interest. Consequently, ligand-free colloids are ideal reference materials for evaluating the effects of surface adsorbates from the initial state for application-oriented nanointegration purposes. After two decades of development, laser synthesis and processing of colloids (LSPC) has emerged as a convenient and scalable technique for the synthesis of ligand-free nanomaterials in sealed environments. In addition to the high-purity surface of LSPC-generated nanoparticles, other strengths of LSPC include its high throughput, convenience for preparing alloys or series of doped nanomaterials, and its continuous operation mode, suitable for downstream processing. Unscreened surface charge of LSPC-synthesized colloids is the key to achieving colloidal stability and high affinity to biomolecules as well as support materials, thereby enabling the fabrication of bioconjugates and heterogeneous catalysts. Accurate size control of LSPC-synthesized materials ranging from quantum dots to submicrometer spheres and recent upscaling advancement toward the multiple-gram scale are helpful for extending the applicability of LSPC-synthesized nanomaterials to various fields. By discussing key reports on both the fundamentals and the applications related to laser ablation, fragmentation, and melting in liquids, this Article presents a timely and critical review of this emerging topic.

Continuous multigram nanoparticle synthesis by high-power, high-repetition-rate ultrafast laser ablation in liquids

Utilizing a novel laser system consisting of a 500 W, 10 MHz, 3 ps laser source which is fully synchronized with a polygon scanner reaching scanning speeds up to 500 m/s, we explore the possibilities to increase the productivity of nanoparticle synthesis by laser ablation in liquids. By exploiting the high scanning speed, laser-induced cavitation bubbles are spatially bypassed at high repetition rates and continuous multigram ablation rates up to 4 g/h are demonstrated for platinum, gold, silver, aluminum, copper, and titanium. Furthermore, the applicable, ablation-effective repetition rate is increased by two orders of magnitude. The ultrafast ablation mechanisms are investigated for different laser fluences, repetition rates, interpulse distances, and ablation times, while the resulting trends are successfully described by validating a model developed for ultrafast laser ablation in air to hold in liquids as well.