Laser-ablative aqueous synthesis and characterization of elemental boron nanoparticles for biomedical applications

Boron-based nano-formulations look very promising for biomedical applications, including photo- and boron neutron capture therapies, but the fabrication of non-toxic water-dispersible boron nanoparticles (NPs), which contain the highest boron atom concentration, is difficult using currently available chemical and plasma synthesis methods. Here, we demonstrate purely aqueous synthesis of clean boron NPs by methods of femtosecond laser ablation from a solid boron target in water, thus free of any toxic organic solvents, and characterize their properties. We show that despite highly oxidizing water ambience, the laser-ablative synthesis process follows an unusual scenario leading to the formation of boron NPs together with boric acid (H₃BO₃) as an oxidation by-product coating the nanoparticles, which acts to stabilize the elemental boron NPs dispersion. We then demonstrate the purification of boron NPs from residual boric acid in deionized water, followed by their coating with polyethylene glycol to improve colloidal stability and biocompatibility. It was found that the formed NPs have a spherical shape with averaged size of about 37 nm, and are composed of elemental boron in mostly amorphous phase with the presence of certain crystalline fraction. The synthesized NPs demonstrate low toxicity and exhibit strong absorption in the NIR window of relative tissue transparency, promising their use in photoacoustic imaging and phototherapy, in addition to their promise for neutron capture therapy. This combined potential ability of generating imaging and therapy functionalities makes laser-synthesized B NPs a very promising multifunctional agent for biomedical applications.

Pulsed Laser in Liquids Made Nanomaterials for Catalysis

Catalysis is essential to modern life and has a huge economic impact. The development of new catalysts critically depends on synthetic methods that enable the preparation of tailored nanomaterials. Pulsed laser in liquids synthesis can produce uniform, multicomponent, nonequilibrium nanomaterials with independently and precisely controlled properties, such as size, composition, morphology, defect density, and atomistic structure within the nanoparticle and at its surface. We cover the fundamentals, unique advantages, challenges, and experimental solutions of this powerful technique and review the state-of-the-art of laser-made electrocatalysts for water oxidation, oxygen reduction, hydrogen evolution, nitrogen reduction, carbon dioxide reduction, and organic oxidations, followed by laser-made nanomaterials for light-driven catalytic processes and heterogeneous catalysis of thermochemical processes. We also highlight laser-synthesized nanomaterials for which proposed catalytic applications exist. This review provides a practical guide to how the catalysis community can capitalize on pulsed laser in liquids synthesis to advance catalyst development, by leveraging the synergies of two fields of intensive research.

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.

Laser wavelength modulated pulsed laser ablation for selective and efficient production of graphene quantum dots

Graphene quantum dots (GQDs) and graphene oxide quantum dots (GOQDs) can be selectively produced by wavelength-modulated pulsed laser ablation in liquid (PLAL) method, which can used in different applications such as optoelectronic and biomedical applications, respectively.

Spheroidization of Nickel Powder and Coating with Carbon Layer through Laser Heating

We developed a simple and efficient process, laser heating of nickel powder in ethanol, to produce carbon-encapsulated nickel microspheres. Long-pulse-width laser heated nickel powder suspended in pure ethanol into liquid droplets. In turn, the latter droplets became sphere-like, pyrolyzed surrounding ethanol and dissolved the produced carbon atoms. Because of their lower solubility in solid nickel, excess carbon atoms were then expelled from the metal core after solidification, thus forming graphite-like shells on the laser-modified Ni spheres. Hence, after pyrolysis the transformation of carbon was found to follow the dissolution-precipitation mechanism. The produced carbon-encapsulated nickel microspheres exhibited higher oxidation resistance compared with the initial nickel powder, while keeping their magnetic properties essentially unchanged.

Ultrathin Oxide Layer-Wrapped Noble Metal Nanoparticles via Colloidal Electrostatic Self-Assembly for Efficient and Reusable Surface Enhanced Raman Scattering Substrates

Controllable and flexible fabrication of ultrathin and uniform oxide layer-wrapped noble metal nanoparticles (NPs) has been expected. Here a new strategy is presented for them based on colloidal electrostatic attraction and self-assembly on the metal NPs via one-step laser ablation of noble metal targets in the hydrolysis-induced hydroxide sol solutions at room temperature. The Au NPs, with several tens of nanometers in size, are taken as core part and TiO₂ as shell-layer to demonstrate the validity of the presented strategy. It has been shown that the TiO₂ shell-wrapped Au NPs are obtained after laser ablation of Au target in the hydrolysis-induced Ti(OH)₄ sol solution. The Au NPs are about 35 nm in mean size, and the TiO₂ shell layers are amorphous in structure and about 2.5 nm in thickness. The shell thickness is nearly independent of the Au NPs' size. Further experiments have shown that the thickness and crystallinity of the shell-layer can be tuned and controlled via changing the temperature or pH value of the Ti(OH)₄ sol solution or prolonging the laser ablation duration. The formation of the TiO₂ shell-wrapped Au NPs is attributed to attachment and self-assembly of Ti(OH)₄ colloids on the laser-induced Au NPs due to the electrostatic attraction between them. Importantly, the presented strategy is universal and suitable for fabrication of many other ultrathin oxide-wrapped noble metal NPs. A series of oxide shell-wrapped noble metal NPs have been successfully fabricated, such as Au@oxides (Fe₂O₃, Al₂O₃, CuO, and ZnO) as well as Pt@TiO₂ and Pd@TiO₂, etc. Further, compared with the pure gold NPs-built film, the TiO₂-wrapped Au NPs-built film has exhibited much stronger surface enhanced Raman scattering (SERS) performance to the anions NO₃^-, which weakly interact with noble metals, and the good reusability for the SERS-based detection of 4-nitrophenol, which could be photodegraded by xenon lamp irradiation. This work provides a flexible and universal route to the ultrathin and uniform oxide layer-wrapped noble metal NPs.

Ultrathin tin oxide layer-wrapped gold nanoparticles induced by laser ablation in solutions and their enhanced performances

A simple and flexible method of preparing an ultrathin semiconducting oxide layer-wrapped gold nanoparticles (NPs) is presented. The method is a single-step procedure based on laser ablation in a precursor solution. The spherical Au NPs (<20nm in mean size) wrapped with a SnO₂ layer of approximately 2nm in thickness are formed after the laser ablation of a gold target in SnCl₄ solutions with concentrations of 0.01-0.1M. The thickness of such SnO₂ shell is nearly independent of Au particle sizes. Results reveal that the formation of Au@SnO₂ NPs involves a two-step process: the laser ablation-induced formation of Au NPs and subsequent Coulomb effect-based colloidal attachment and self-assembly on the Au NPs. Au@SnO₂ NPs-built film exhibits significantly stronger surface-enhanced Raman scattering effect to organic phosphor molecules (phenylphosphonic acid) and much better gas sensing performance to H₂S at room temperature compared with the bare Au NPs and pure SnO₂ NPs films, respectively. This work presents a simple route to fabricating noble-metal NPs wrapped with symmetrical and ultrathin semiconducting oxide shells.

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.

Facile fabrication of bioactive ultra-small protein–hydroxyapatite nanoconjugates via liquid-phase laser ablation and their enhanced osteogenic differentiation activity

Ultra-small protein–hydroxyapatite nanoconjugates promote the osteogenic differentiation of mesenchymal stem cells.

Extensive Characterization of Oxide-Coated Colloidal Gold Nanoparticles Synthesized by Laser Ablation in Liquid

Colloidal gold nanoparticles are a widespread nanomaterial with many potential applications, but their aggregation in suspension is a critical issue which is usually prevented by organic surfactants. This solution has some drawbacks, such as material contamination and modifications of its functional properties. The gold nanoparticles presented in this work have been synthesized by ultra-fast laser ablation in liquid, which addresses the above issues by overcoating the metal nanoparticles with an oxide layer. The main focus of the work is in the characterization of the oxidized gold nanoparticles, which were made first in solution by means of dynamic light scattering and optical spectroscopy, and then in dried form by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and finally by surface potential measurements with atomic force microscopy. The light scattering assessed the nanoscale size of the formed particles and provided insight in their stability. The nanoparticles’ size was confirmed by direct imaging in transmission electron microscopy, and their crystalline nature was disclosed by X-ray diffraction. The X-ray photoelectron spectroscopy showed measurements compatible with the presence of surface oxide, which was confirmed by the surface potential measurements, which are the novel point of the present work. In conclusion, the method of laser ablation in liquid for the synthesis of gold nanoparticles has been presented, and the advantage of this physical approach, consisting of coating the nanoparticles in situ with gold oxide which provides the required morphological and chemical stability without organic surfactants, has been confirmed by using scanning Kelvin probe microscopy for the first time.

Ultrasmall, Ligand-Free Ag Nanoparticles with High Antibacterial Activity Prepared by Pulsed Laser Ablation in Liquid

Since ancient times, silver and its compounds have been known to have a broad spectrum of antimicrobial activities for bacteria, fungi, and viruses. Due to the increasing bacterial resistance to classic antibiotics, the investigations of Ag NPs have increased. Herein, we present the preparation of ligand-free Ag NPs with 3 and 20 nm sizes by applying picosecond laser ablation in liquid at 355 and 1065 nm. Our laser processing system allows a high control on particle sizes. The produced nanoparticles were characterized by means of transmission electron microscopy, UV-Vis spectroscopy, and X-ray diffraction. The size effect on the antibacterial activity of Ag NPs was tested againstE. coliandS. aureus. The growth curves of bacteria were monitored at 0–5 mg/L of Ag NPs by a multimode microplate reader. The size effects as well as the concentration of Ag NPs on their antibacterial activity are discussed.