Unveiling Fundamentals of Multi-Beam Pulsed Laser Ablation in Liquids toward Scaling up Nanoparticle Production

Pulsed laser ablation in liquids (PLAL) is a versatile technique to produce high-purity colloidal nanoparticles. Despite considerable recent progress in increasing the productivity of the technique, there is still significant demand for a practical, cost-effective method for upscaling PLAL synthesis. Here we employ and unveil the fundamentals of multi-beam (MB) PLAL. The MB-PLAL upscaling approach can bypass the cavitation bubble, the main limiting factor of PLAL efficiency, by splitting the laser beam into several beams using static diffractive optical elements (DOEs). A multimetallic high-entropy alloy CrFeCoNiMn was used as a model material and the productivity of its nanoparticles in the MB-PLAL setup was investigated and compared with that in the standard single-beam PLAL. We demonstrate that the proposed multi-beam method helps to bypass the cavitation bubble both temporally (lower pulse repetition rates can be used while keeping the optimum processing fluence) and spatially (lower beam scanning speeds are needed) and thus dramatically increases the nanoparticle yield. Time-resolved imaging of the cavitation bubble was performed to correlate the observed production efficiencies with the bubble bypassing. The results suggest that nanoparticle PLAL productivity at the level of g/h can be achieved by the proposed multi-beam strategy using compact kW-class lasers and simple inexpensive scanning systems.

Comparative Analysis of the Silver Nanoparticle's Yield for Pico-Femto-Nanosecond Laser Generation

Comparative analysis of different laser regimes of silver nanoparticle generation in water was performed for laser pulsewidth in the range of 300 fs-100 ns. Optical spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy and method of dynamic light scattering were used for nanoparticle characterization. Different laser regimes of generation were used with varying pulse duration, pulse energy and scanning velocity. The proposed universal quantitative criteria as productivity and ergonomicity of the obtained colloidal solutions of nanoparticles were investigated to compare different laser regimes of production. The efficiency per unit energy for picosecond generation of nanoparticles, free from the influence of nonlinear effects, turns out to be higher by 1-2 orders of magnitude than for nanosecond generation.

Photomechanical Laser Fragmentation of IrO2 Microparticles for the Synthesis of Active and Redox-Sensitive Colloidal Nanoclusters

Pulsed laser fragmentation of microparticles (MPs) in liquid is a synthesis method for producing high-purity nanoparticles (NPs) from virtually any material. Compared with laser ablation in liquids (LAL), the use of MPs enables a fully continuous, single-step synthesis of colloidal NPs. Although having been employed in several studies, neither the fragmentation mechanism nor the efficiency or scalability have been described. Starting from time-resolved investigations of the single-pulse fragmentation of single IrO₂ MPs in water, the contribution of stress-mediated processes to the fragmentation mechanism is highlighted. Single-pulse, multiparticle fragmentation is then performed in a continuously operated liquid jet. Here, 2 nm-sized nanoclusters (NCs) accompanied by larger fragments with sizes ranging between several ten nm and several µm are generated. For the nanosized product, an unprecedented efficiency of up to 18 µg J^-1 is reached, which exceeds comparable values reported for high-power LAL by one order of magnitude. The generated NCs exhibit high catalytic activity and stability in oxygen evolution reactions while simultaneously expressing a redox-sensitive fluorescence, thus rendering them promising candidates in electrocatalytic sensing. The provided insights will pave the way for laser fragmentation of MPs to become a versatile, scalable yet simple technique for nanomaterial design and development.

An empirical experimental investigation on the effect of an external electric field on the behaviour of laser-induced cavitation bubbles

This study is an attempt to empirically investigate the behaviour of laser-induced cavitation bubbles under the influence of an external electric field. As such two targets (copper and iridium) were subjected to a high-power Nd:YAG laser beam while being submerged in a liquid. Three different liquids were chosen for this purpose viz. acetone, ethanol, and distilled de-ionized water. The choice of the liquids was made with the underlying assumption that the conductivity of the liquids would play a significant role in responding to the applied external electric field and thus dictate the behaviour of the cavitation bubbles. A probe-beam method known as a beam deflection setup was employed for this experiment and the results were analyzed using the Rayleigh-Plesset model. The results revealed that the maximum radii of the cavitation bubbles increased in response to an increasing electric field. This effect was more pronounced in the presence of acetone medium and decreased successively while using ethanol and water media owing to their varying magnitudes of electrical conductivity. The bubble collapse speeds and their energies were also measured and similar trends were observed in both cases. The results from cavitation bubble dynamics were then applied to a Gilmore model and the sizes of the NPs synthesized using laser ablation with and without an external electric field were calculated using classical nucleation theory.

Direct 3D Printing of Binder-Free Bimetallic Nanomaterials as Integrated Electrodes for Glycerol Oxidation with High Selectivity for Valuable C3 Products

Net-zero carbon strategies and green synthesis methodologies are key to realizing the United Nations' sustainable development goals (SDGs) on a global scale. An electrocatalytic glycerol oxidation reaction (GOR) holds the promise of upcycling excess glycerol from biodiesel production directly into precious hydrocarbon commodities that are worth orders of magnitude more than the glycerol feedstock. Despite years of research on the GOR, the synthesis process of nanoscale electrocatalysts still involves (1) prohibitive heat input, (2) expensive vacuum chambers, and (3) emission of toxic liquid pollutants. In this paper, these knowledge gaps are closed via developing a laser-assisted nanomaterial preparation (LANP) process to fabricate bimetallic nanocatalysts (1) at room temperature, (2) under an ambient atmosphere, and (3) without liquid waste emission. Specifically, PdCu nanoparticles with adjustable Pd:Cu content supported on few-layer graphene can be prepared using this one-step LANP method with performance that can rival state-of-the-art GOR catalysts. Beyond exhibiting high GOR activity, the LANP-fabricated PdCu/C nanomaterials with an optimized Pd:Cu ratio further deliver an exclusive product selectivity of up to 99% for partially oxidized C₃ products with value over 280000-folds that of glycerol. Through DFT calculations and in situ XAS experiments, the synergy between Pd and Cu is found to be responsible for the stability under GOR conditions and preference for C₃ products of LANP PdCu. This dry LANP method is envisioned to afford sustainable production of multimetallic nanoparticles in a continuous fashion as efficient electrocatalysts for other redox reactions with intricate proton-coupled electron transfer steps that are central to the widespread deployment of renewable energy schemes and carbon-neutral technologies.

Single-Particle Hyperspectral Imaging Reveals Kinetics of Silver Ion Leaching from Alloy Nanoparticles

Gold-silver alloy nanoparticles are interesting for multiple applications, including heterogeneous catalysis, optical sensing, and antimicrobial properties. The inert element gold acts as a stabilizer for silver to prevent particle corrosion, or conversely, to control the release kinetics of antimicrobial silver ions for long-term efficiency at minimum cytotoxicity. However, little is known about the kinetics of silver ion leaching from bimetallic nanoparticles and how it is correlated with silver content, especially not on a single-particle level. To characterize the kinetics of silver ion release from gold-silver alloy nanoparticles, we employed a combination of electron microscopy and single-particle hyperspectral imaging with an acquisition speed fast enough to capture the irreversible silver ion leaching. Single-particle leaching profiles revealed a reduction in silver ion leaching rate due to the alloying with gold as well as two leaching stages, with a large heterogeneity in rate constants. We modeled the initial leaching stage as a shrinking-particle with a rate constant that exponentially depends on the silver content. The second, slower leaching stage is controlled by the electrochemical oxidation potential of the alloy being steadily increased by the change in relative gold content and diffusion of silver atoms through the lattice. Interestingly, individual nanoparticles with similar sizes and compositions exhibited completely different silver ion leaching yields. Most nanoparticles released silver completely, but 25% of them appeared to arrest leaching. Additionally, nanoparticles became slightly porous. Alloy nanoparticles, produced by scalable laser ablation in liquid, together with kinetic studies of silver ion leaching, provide an approach to design the durability or bioactivity of alloy nanoparticles.

Dynamics of laser-induced cavitation bubble during expansion over sharp-edge geometry submerged in liquid - an inside view by diffuse illumination

Laser ablation in liquids is growing in popularity for various applications including nanoparticle production, breakdown spectroscopy, and surface functionalization. When laser pulse ablates the solid target submerged in liquid, a cavitation bubble develops. In case of "finite" geometries of ablated solids, liquid dynamical phenomena can occur inside the bubble when the bubble overflows the surface edge. To observe this dynamics, we use diffuse illumination of a flashlamp in combination with a high-speed videography by exposure times down to 250  ns. The developed theoretical modelling and its comparison with the experimental observations clearly prove that this approach widens the observable area inside the bubble. We thereby use it to study the dynamics of laser-induced cavitation bubble during its expansion over a sharp-edge ("cliff-like" 90°) geometry submerged in water, ethanol, and polyethylene glycol 300. The samples are 17  mm wide stainless steel plates with thickness in the range of 0.025-2  mm. Bubbles are induced on the samples by 1064-nm laser pulses with pulse durations of 7-60  ns and pulse energies of 10-55  mJ. We observe formation of a fixed-type secondary cavity behind the edge where low-pressure area develops due to bubble-driven flow of the liquid. This occurs when the velocity of liquid overflow exceeds ~20  m s^-1. A re-entrant liquid injection with up to ~40  m s^-1 velocity may occur inside the bubble when the bubble overflows the edge of the sample. Formation and characteristics of the jet evidently depend on the relation between the breakdown-edge offset and the bubble energy, as well as the properties of the surrounding liquid. Higher viscosity of the liquid prevents the generation of the jet.

Dynamics of laser-induced cavitation bubble during expansion over sharp-edge geometry submerged in liquid - an inside view by diffuse illumination

Laser ablation in liquids is growing in popularity for various applications including nanoparticle production, breakdown spectroscopy, and surface functionalization. When laser pulse ablates the solid target submerged in liquid, a cavitation bubble develops. In case of "finite" geometries of ablated solids, liquid dynamical phenomena can occur inside the bubble when the bubble overflows the surface edge. To observe this dynamics, we use diffuse illumination of a flashlamp in combination with a high-speed videography by exposure times down to 250  ns. The developed theoretical modelling and its comparison with the experimental observations clearly prove that this approach widens the observable area inside the bubble. We thereby use it to study the dynamics of laser-induced cavitation bubble during its expansion over a sharp-edge ("cliff-like" 90°) geometry submerged in water, ethanol, and polyethylene glycol 300. The samples are 17  mm wide stainless steel plates with thickness in the range of 0.025-2  mm. Bubbles are induced on the samples by 1064-nm laser pulses with pulse durations of 7-60  ns and pulse energies of 10-55  mJ. We observe formation of a fixed-type secondary cavity behind the edge where low-pressure area develops due to bubble-driven flow of the liquid. This occurs when the velocity of liquid overflow exceeds ~20  m s^-1. A re-entrant liquid injection with up to ~40  m s^-1 velocity may occur inside the bubble when the bubble overflows the edge of the sample. Formation and characteristics of the jet evidently depend on the relation between the breakdown-edge offset and the bubble energy, as well as the properties of the surrounding liquid. Higher viscosity of the liquid prevents the generation of the jet.

Recycling silver nanoparticle debris from laser ablation of silver nanowire in liquid media toward minimum material waste

As silver nanowires (Ag NWs) are usually manufactured by chemical synthesis, a patterning process is needed to use them as functional devices. Pulsed laser ablation is a promising Ag NW patterning process because it is a simple and inexpensive procedure. However, this process has a disadvantage in that target materials are wasted owing to the subtractive nature of the process involving the removal of unnecessary materials, and large quantities of raw materials are required. In this study, we report a minimum-waste laser patterning process utilizing silver nanoparticle (Ag NP) debris obtained through laser ablation of Ag NWs in liquid media. Since the generated Ag NPs can be used for several applications, wastage of Ag NWs, which is inevitable in conventional laser patterning processes, is dramatically reduced. In addition, electrophoretic deposition of the recycled Ag NPs onto non-ablated Ag NWs allows easy fabrication of junction-enhanced Ag NWs from the deposited Ag NPs. The unique advantage of this method lies in using recycled Ag NPs as building materials, eliminating the additional cost of junction welding Ag NWs. These fabricated Ag NW substrates could be utilized as transparent heaters and stretchable TCEs, thereby validating the effectiveness of the proposed process.

Nanoparticles Engineering by Pulsed Laser Ablation in Liquids: Concepts and Applications

Laser synthesis emerges as a suitable technique to produce ligand-free nanoparticles, alloys and functionalized nanomaterials for catalysis, imaging, biomedicine, energy and environmental applications. In the last decade, laser ablation and nanoparticle generation in liquids has proven to be a unique and efficient technique to generate, excite, fragment and conjugate a large variety of nanostructures in a scalable and clean way. In this work, we give an overview on the fundamentals of pulsed laser synthesis of nanocolloids and new information about its scalability towards selected applications. Biomedicine, catalysis and sensing are the application areas mainly discussed in this review, highlighting advantages of laser-synthesized nanoparticles for these types of applications and, once partially resolved, the limitations to the technique for large-scale applications.

A simple strategy for enhanced production of nanoparticles by laser ablation in liquids

Upgrading the productivity of nanoparticles (NPs), generated by pulsed laser ablation in liquid (PLAL), still remains challenging. Here a novel variant of PLAL was developed, where a doubled frequency Nd:YAG laser beam (532 nm, ∼5 ns, 10 Hz) at different fluences and for different times was directed into a sealed vessel, toward the interface of the meniscus of ethanol with a tilted bulk metal target. Palladium, copper and silver NPs, synthesized in the performed proof of concept experiments, were mass quantified, by inductively coupled plasma optical emission spectrometry, and characterized by ultraviolet-visible extinction spectroscopy, transmission electron microscopy and x-ray diffraction. The NPs consist of crystalline metals of a few nm size and their ablation rates and agglomeration levels depend on the employed laser fluences. The ensuing laser power-specific productivity curves for each metal, peaked at specific laser fluences, were fitted to the results of a simple model accounting for plasma absorption and heat transfer. The resulting peaked yields and concentrations were more than an order of magnitude higher than those obtained for totally immersed targets. Besides, the measured productivities showed nearly linear dependencies during time intervals up to 30 min of ablation, but became saturated at 1 h, due to accumulation of a significant number of NPs along the laser beam path, reducing the laser intensity reaching the target. The suggested approach that led to enhanced productivities and to generation of high concentrations of NPs in a single vessel could inspire future studies that will contribute to further developments of efficient generation of NPs with controlled characteristics.

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.

Pilot-scale synthesis of metal nanoparticles by high-speed pulsed laser ablation in liquids

The synthesis of catalysis-relevant nanoparticles such as platinum and gold is demonstrated with productivities of 4 g h(-1) for pulsed laser ablation in liquids (PLAL). The major drawback of low productivity of PLAL is overcome by utilizing a novel ultrafast high-repetition rate laser system combined with a polygon scanner that reaches scanning speeds up to 500 m s(-1). This high scanning speed is exploited to spatially bypass the laser-induced cavitation bubbles at MHz-repetition rates resulting in an increase of the applicable, ablation-effective, repetition rate for PLAL by two orders of magnitude. The particle size, morphology and oxidation state of fully automated synthesized colloids are analyzed while the ablation mechanisms are studied for different laser fluences, repetition rates, interpulse distances, ablation times, volumetric flow rates and focus positions. It is found that at high scanning speeds and high repetition rate PLAL the ablation process is stable in crystallite size and decoupled from shielding and liquid effects that conventionally occur during low-speed PLAL.

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.

Highly-repeatable generation of very small nanoparticles by pulsed-laser ablation in liquids of a high-speed rotating target

Very small and pure Ag nanoparticles are generated by pulsed-laser ablation in water of a silver target rotated at a high-speed.

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

Productivity of laser ablation in liquids we found that the productivity is enhanced for a wire tip laser ablation due to a springboard-like effect of the flexible target releasing the cavitation bubble.

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.

Out-of-Plane Plasmonic Antennas for Raman Analysis in Living Cells

Out-of-plane plasmonic nanoantennas protruding from the substrate are exploited to perform very sensitive surface enhanced Raman scattering analysis of living cells. Cells cultured on three-dimensional surfaces exhibit tight adhesion with nanoantenna tips where the plasmonic hot-spot resides. This fact provides observable cell adhesion sites combined with high plasmonic enhancement, resulting in an ideal system for Raman investigation of cell membranes.

Biosynthesis of Silver Nanoparticles and Its Applications

Silver nanoparticles possess unique properties which find myriad applications such as antimicrobial, anticancer, larvicidal, catalytic, and wound healing activities. Biogenic syntheses of silver nanoparticles using plants and their pharmacological and other potential applications are gaining momentum owing to its assured rewards. This critical review is aimed at providing an insight into the phytomediated synthesis of silver nanoparticles, its significant applications in various fields, and characterization techniques involved.

Hollow plasmonic antennas for broadband SERS spectroscopy

The chemical environment of cells is an extremely complex and multifaceted system that includes many types of proteins, lipids, nucleic acids and various other components. With the final aim of studying these components in detail, we have developed multiband plasmonic antennas, which are suitable for highly sensitive surface enhanced Raman spectroscopy (SERS) and are activated by a wide range of excitation wavelengths. The three-dimensional hollow nanoantennas were produced on an optical resist by a secondary electron lithography approach, generated by fast ion-beam milling on the polymer and then covered with silver in order to obtain plasmonic functionalities. The optical properties of these structures have been studied through finite element analysis simulations that demonstrated the presence of broadband absorption and multiband enhancement due to the unusual geometry of the antennas. The enhancement was confirmed by SERS measurements, which showed a large enhancement of the vibrational features both in the case of resonant excitation and out-of-resonance excitation. Such characteristics indicate that these structures are potential candidates for plasmonic enhancers in multifunctional opto-electronic biosensors.