Thermal dynamics of pulsed-laser excited gold nanorods in suspension

Physical Regimes and Mechanisms of Picosecond Laser Fragmentation of Gold Nanoparticles in Water from X-ray Probing and Atomistic Simulations

Laser fragmentation in liquids has emerged as a promising green chemistry technique for changing the size, shape, structure, and phase composition of colloidal nanoparticles, thus tuning their properties to the needs of practical applications. The advancement of this technique requires a solid understanding of the mechanisms of laser-nanoparticle interactions that lead to the fragmentation. While theoretical studies have made impressive practical and mechanistic predictions, their experimental validation is required. Hence, using the picosecond laser fragmentation of Au nanoparticles in water as a model system, the transient melting and fragmentation processes are investigated with a combination of time-resolved X-ray probing and atomistic simulations. The direct comparison of the diffraction profiles predicted in the simulations and measured in experiments has revealed a sequence of several nonequilibrium processes triggered by the laser irradiation. At low laser fluences, in the regime of nanoparticle melting and resolidification, the results provide evidence of a transient superheating of crystalline nanoparticles above the melting temperature. At fluences about three times the melting threshold, the fragmentation starts with evaporation of Au atoms and their condensation into small satellite nanoparticles. As fluence increases above five times the melting threshold, a transition to a rapid (explosive) phase decomposition of superheated nanoparticles into small liquid droplets and vapor phase atoms is observed. The transition to the phase explosion fragmentation regime is signified by prominent changes in the small-angle X-ray scattering profiles measured in experiments and calculated in simulations. The good match between the experimental and computational diffraction profiles gives credence to the physical picture of the cascade of thermal fragmentation regimes revealed in the simulations and demonstrates the high promise of the joint tightly integrated computational and experimental efforts.

From Multi- to Single-Hollow Trimetallic Nanocrystals by Ultrafast Heating

Metal nanocrystals (NCs) display unique physicochemical features that are highly dependent on nanoparticle dimensions, anisotropy, structure, and composition. The development of synthesis methodologies that allow us to tune such parameters finely emerges as crucial for the application of metal NCs in catalysis, optical materials, or biomedicine. Here, we describe a synthetic methodology to fabricate hollow multimetallic heterostructures using a combination of seed-mediated growth routes and femtosecond-pulsed laser irradiation. The envisaged methodology relies on the coreduction of Ag and Pd ions on gold nanorods (Au NRs) to form Au@PdAg core-shell nanostructures containing small cavities at the Au-PdAg interface. The excitation of Au@PdAg NRs with low fluence femtosecond pulses was employed to induce the coalescence and growth of large cavities, forming multihollow anisotropic Au@PdAg nanostructures. Moreover, single-hollow alloy AuPdAg could be achieved in high yield by increasing the irradiation energy. Advanced electron microscopy techniques, energy-dispersive X-ray spectroscopy (EDX) tomography, X-ray absorption near-edge structure (XANES) spectroscopy, and finite differences in the time domain (FDTD) simulations allowed us to characterize the morphology, structure, and elemental distribution of the irradiated NCs in detail. The ability of the reported synthesis route to fabricate multimetallic NCs with unprecedented hollow nanostructures offers attractive prospects for the fabrication of tailored high-entropy alloy nanoparticles.

One-Shot Laser-Pulse Modification of Bare and Silica-Coated Gold Nanoparticles of Various Morphologies

Gold nanoparticles are widely used in laser biomedical applications due to their favorable properties, mainly localized plasmon resonance. However, laser radiation can cause a change in the shape and size of plasmonic nanoparticles, thus resulting in an unwanted reduction of their photothermal and photodynamic efficiency due to a drastic alteration of optical properties. Most previously reported experiments were carried out with bulk colloids where different particles were irradiated by different numbers of laser pulses, thus making it difficult to accurately evaluate the laser power photomodification (PM) threshold. Here, we examine the one-shot nanosecond laser-pulse PM of bare and silica-coated gold nanoparticles moving in a capillary flow. Four types of gold nanoparticles, including nanostars, nanoantennas, nanorods, and SiO₂@Au nanoshells, were fabricated for PM experiments. To evaluate the changes in the particle morphology under laser irradiation, we combine measurements of extinction spectra with electron microscopy. A quantitative spectral approach is developed to characterize the laser power PM threshold in terms of normalized extinction parameters. The experimentally determined PM threshold increases in series were as follows: nanorods, nanoantennas, nanoshells, and nanostars. An important observation is that even a thin silica shell significantly increases the photostability of gold nanorods. The developed methods and reported findings can be useful for the optimal design of plasmonic particles and laser irradiation parameters in various biomedical applications of functionalized hybrid nanostructures.

Curvature and temperature-dependent thermal interface conductance between nanoscale-gold and water

<p>Plasmonic gold nanoparticles (AuNPs) can convert laser irradiation into thermal energy for a variety of applications. Although heat transfer through the AuNP-water interface is considered an essential part of the plasmonic heating process, there is a lack of mechanistic understanding of how interface curvature and the heating itself impact interfacial heat transfer. Here, we report atomistic molecular dynamics simulations that investigate heat transfer through nanoscale gold-water interfaces. We simulated four nanoscale gold structures under various applied heat flux to evaluate how gold-water interface curvature and temperature affect the interfacial heat transfer. We also considered a case in which we artificially reduced wetting at the gold surfaces by tuning the gold-water interactions to determine if such a perturbation alters the curvature and temperature dependence of the gold-water interfacial heat transfer. We first confirmed that interfacial heat transfer is particularly important for small particles (diameter {less than or equal to} 10 nm). We found that the thermal interface conductance increases linearly with interface curvature regardless of the gold wettability, while it increases non-linearly with the applied heat flux under normal wetting and remains constant under reduced wetting. Our analysis suggests the curvature dependence of the interface conductance coincides with changes in interfacial water adsorption, while the temperature dependence may arise from temperature-induced shifts in the distribution of water vibrational states. Our study advances the current understanding of interface thermal conductance for a broad range of applications.

Nanoparticle Fragmentation Below the Melting Point Under Single Picosecond Laser Pulse Stimulation

Understanding the laser-nanomaterials interaction including nanomaterial fragmentation has important implications in nanoparticle manufacturing, energy, and biomedical sciences. So far, three mechanisms of laser-induced fragmentation have been recognized including non-thermal processes and thermomechanical force under femtosecond pulses, and the phase transitions under nanosecond pulses. Here we show that single picosecond (ps) laser pulse stimulation leads to anomalous fragmentation of gold nanoparticles that deviates from these three mechanisms. The ps laser fragmentation was weakly dependent on particle size, and it resulted in a bimodal size distribution. Importantly, ps laser stimulation fragmented particles below the whole particle melting point and below the threshold for non-thermal mechanism. We propose a framework based on near-field enhancement and nanoparticle surface melting to account for the ps laser-induced fragmentation observed here. This study reveals a new form of surface ablation that occurs under picosecond laser stimulation at low fluence.

Femtosecond nearly resonant self-focusing in gold nanorod colloids

We evaluate the threshold power for self-focusing in gold nanorod colloids of varying concentration by a power limiting method in the femtosecond filamentation regime. The pulses are tuned near the longitudinal plasmon peak of the nanorods, leading to saturation of linear absorption and reshaping of the particles. We evaluated the last two effects by optical transmission measurements and spectroscopic analysis and estimated that considerable particle deformation does not occur before the collapse of the beam. We performed numerical simulations based on the experimental results, and evaluated only a subtle, monotonically increasing enhancement of the nonlinear refractive index of the host material (water) as the nanoparticles concentration increases. The role of higher-order contributions is discussed. Our work provides an alternative characterization approach of ultrafast nonlinearities in absorbing media. It further emphasizes that self-focusing of intense femtosecond pulses in gold nanocomposites is hampered by the ultrafast modulation of the susceptibility of the metal.

Quantitative Ultrafast Electron-Temperature Dynamics in Photo-Excited Au Nanoparticles

The femtosecond evolution of the electronic temperature of laser-excited gold nanoparticles is measured, by means of ultrafast time-resolved photoemission spectroscopy induced by extreme-ultraviolet radiation pulses. The temperature of the electron gas is deduced by recording and fitting high-resolution photo emission spectra around the Fermi edge of gold nanoparticles providing a direct, unambiguous picture of the ultrafast electron-gas dynamics. These results will be instrumental to the refinement of existing models of femtosecond processes in laterally-confined and bulk condensed-matter systems, and for understanding more deeply the role of hot electrons in technological applications.

Plasmon-Enhanced Photothermal and Optomechanical Deformations of a Gold Nanoparticle

Plasmon-enhanced photothermal and optomechanical effects on deforming and reshaping a gold nanoparticle (NP) are studied theoretically. A previous paper (Wang and Ding, ACS Nano 13, 32-37, 2019) has shown that a spherical gold nanoparticle (NP) irradiated by a tightly focused laser beam can be deformed into an elongated nanorod (NR) and even chopped in half (a dimer). The mechanism is supposed to be caused by photothermal heating for softening NP associated with optical traction for follow-up deformation. In this paper, our study focuses on deformation induced by Maxwell's stress provided by a linearly polarized Gaussian beam upon the surface of a thermal-softened NP/NR. We use an elastic model to numerically calculate deformation according to optical traction and a viscoelastic model to theoretically estimate the following creep (elongation) as temperature nears the melting point. Our results indicate that a stretching traction at the two ends of the NP/NR causes elongation and a pinching traction at the middle causes a dent. Hence, a bigger NP can be elongated and then cut into two pieces (a dimer) at the dent due to the optomechanical effect. As the continuous heating process induces premelting of NPs, a quasi-liquid layer is formed first and then an outer liquid layer is induced due to reduction of surface energy, which was predicted by previous works of molecular dynamics simulation. Subsequently, we use the Young-Laplace model to investigate the surface tension effect on the following deformation. This study may provide an insight into utilizing the photothermal effect associated with optomechanical manipulation to tailor gold nanostructures.

Cancer cell-specific protein delivery by optoporation with laser-irradiated gold nanorods

The delivery of macromolecules into living cells is challenging since in most cases molecules are endocytosed and remain in the endo-lysosomal pathway where they are degraded before reaching their target. Here, a method is presented to selectively improve cell membrane permeability by nanosecond laser irradiation of gold nanorods (GNRs) with visible or near-infrared irradiation in order to deliver proteins across the plasma membrane, avoiding the endo lysosomal pathway. GNRs were labeled with the anti-EGFR (epidermal growth factor receptor) antibody Erbitux to target human ovarian carcinoma cells OVCAR-3. Irradiation with nanosecond laser pulses at wavelengths of 532 nm or 730 nm is used for transient permeabilization of the cell membranes. As a result of the irradiation, the uptake of an anti-Ki-67 antibody was observed in about 50 % of the cells. The results of fluorescence lifetime imaging show that the GNR detached from the membrane after irradiation.

Impact of Kapitza resistance on the stability and efficiency of photoacoustic conversion from gold nanorods

Plasmonic particles have been proposed for a broad variety of optical and hybrid applications, including the photothermal ablation and photoacoustic imaging of cancer, or their integration in photonic sensors. Here, we address the effect of thermal resistance at the gold-water interface, or Kapitza resistance, on the performance of photoacoustic conversion of gold nanorods. Our findings point to possible strategies for the optimization of plasmonic particles as contrast agents for imaging, or even as transducers for biosensing. We perform numerical simulations that project a simultaneous increase of efficiency and stability of photoacoustic conversion with a decrease of Kapitza resistance. We suggest an effective approach to modulate Kapitza resistance by including underresolved features as roughness or the presence of adsorbates. Inspired by this idea, we synthesize a rough variant of gold nanorods by the deposition and galvanic replacement of a silver shell, where roughness provides higher photoacoustic signals by about 70% and damage thresholds by 120%. In addition, we coat our particles with a protein corona and find a decrease of photoacoustic signals with shell thickness, which may inspire new solutions for biosensors based on a mechanism of photoacoustic transduction. Both our findings are consistent with an effective modulation of Kapitza resistance, which decreases upon roughening, due to an underlying increase of specific surface area, and increases upon coating with a protein shell that may act as a thermal insulation. We discuss possible directions to gain more advantage of our concept for topical applications at the crossroads of plasmonics, biomedical optics and biosensing.

Light Tailoring of Internal Atomic Structure of Gold Nanorods

Laser processing of gold nanorods (Au NRs) relying on light-matter interaction provides great opportunities in various potential applications. Unveiling the light-induced structure change is a crucial goal in order to control the shape and related properties for practical application. However, the internal atomic structure control of metallic NRs has long been a challenge. Here, the concept of internal atomic structure tailored with light is demonstrated and Au NRs with various internal atomic structures including point defects, twin structures, and polycrystalline nanospheres are fabricated. Experimental characterization and theoretical simulation show that light-induced localized energy deposition and dynamic stresses distribution give rise to atomic structure change. Au NRs with internal defects show enhanced potential to improve activity. The concept of light tailoring of internal atomic structure represents a promising strategy for the rational design of metallic NRs to boost wide applications.

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.

Mechanisms and advanced photothermal modelling of laser-induced shape transformations of colloidal gold nanorods by nanosecond laser pulses

We propose an advanced photothermal model based on a modified Takami model (MTM) to explain the mechanisms of shape changes of colloidal gold nanorods (NRs) induced by nanosecond laser pulses. This model takes into account the orientation of NRs, the radiative and convective losses, and the phase transitions of NRs. It was applied to the determination of the evolution of temperature and the shape and size transformations of NRs during the laser exposure. A series of measurements arising from the interaction between Au NRs and nanosecond laser pulses were analyzed by TEM measurements and the MTM model. We have demonstrated that the fragmentation and reshaping processes govern the nanoparticle (NP) shape. At high laser fluence, the complete fragmentation leads to a population of nearly spherical NPs, while at a moderate laser fluence, the partial fragmentation and reshaping processes generate a bimodal distribution. At low laser fluence, uncommon φ-shape NPs were produced as a result of the competition of cooling and reshaping processes. We also demonstrated that it is possible by the MTM model to determine the laser fluence required to suppress some specific NR shapes and to predict the NP size and shape distributions obtained after the laser exposure.

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.

Femtosecond Pulse Laser Near-Field Ablation of Ag Nanorods

Ag nanorods (Ag NRs) with a mean aspect ratio of 3.9 were prepared through a wet-chemical method, and the absorption spectra for various aspect ratios were obtained. The morphology transformation of Ag NRs irradiated with a femtosecond pulse laser was investigated through transmission electron microscopy (TEM). The near-field ablation was dependent on the laser polarization and wavelength. Laser-induced high electric field intensity was observed at the ends, middle, and junctions of the Ag NRs under various ablation conditions. Through simulation, the evolution mechanism was analyzed in detail. The effect of laser polarization angle on plasmonic junction welding was also investigated. By controlling the electronic field distribution, several nanostructures were obtained: bone-shaped NRs, T-shaped NRs, dimers, trimers, curved NRs, and nanodots. This study suggests a potentially useful approach for the reshaping, cutting, and welding of nanostructures.

Nucleation and growth of gold nanoparticles initiated by nanosecond and femtosecond laser irradiation of aqueous [AuCl4]. Phys Chem Chem Phys 2018;20:28465-28475. [PMID: 30411753 PMCID: PMC6310131 DOI: 10.1039/c8cp05774e]

Irradiation of aqueous [AuCl4]- with 532 nm nanosecond (ns) laser pulses produces monodisperse (PDI = 0.04) 5 nm Au nanoparticles (AuNPs) without any additives or capping agents via a plasmon-enhanced photothermal autocatalytic mechanism. Compared with 800 nm femtosecond (fs) laser pulses, the AuNP growth kinetics under ns laser irradiation follow the same autocatalytic rate law, but with a significantly lower sensitivity to laser pulse energy. The results are explained using a simple model for simulating heat transfer in liquid water and at the interface with AuNPs. While the extent of water superheating with the ns laser is smaller compared to the fs laser, its significantly longer duration can provide sufficient energy to dissociate a small fraction of the [AuCl4]- present, resulting in the formation of AuNPs by coalescence of the resulting Au atoms. Irradiation of initially formed AuNPs at 532 nm results in plasmon-enhanced superheating of water, which greatly accelerates the rate of thermal dissociation of [AuCl4]- and accounts for the observed autocatalytic kinetics. The plasmon-enhanced heating under ns laser irradiation fragments the AuNPs and results in nearly uniform 5 nm particles, while the lack of particles' heating under fs laser irradiation results in the growth of the particles as large as 40 nm.

From plasmon-induced luminescence enhancement in gold nanorods to plasmon-induced luminescence turn-off: a way to control reshaping

Photo-induced nanorod reshaping due to high-energy density illumination at their longitudinal plasmon resonance, as followed by two-photon luminescence measurements.