Flow-Induced Transport via Optical Heating of a Single Gold Nanoparticle

Optothermal trapping has gained increasing popularity in manipulation such as selecting, guiding, and positioning submicron objects because of a few mW laser power much lower than that required for optical trapping. The optotothermal trapping uses thermal gradient-induced phoretic motions, but the underlying physics of driving force has not been fully understood. In this study, we performed optotothermal trapping of 500-nm-diameter colloidal silica via a continuous laser illumination of a single gold nanoparticle from the bottom in a closed chamber. Under illumination, the tracer particles were attracted to the gold nanoparticle and trapped. Notably, the direction of migrating particles was always to hot gold nanoparticle regardless of the configuration of gold nanoparticle placed at two opposite sides of the chamber, on the bottom surface of an upper substrate (ceiling) or on the top surface of a lower substrate (floor). The previous interpretation based on thermal convective flow from the bottom to the top and circulating inside the chamber was only applicable to floor configuration and failed to explain our observation for ceiling. Instead, temperature-induced Marangoni effect at the water/superheated water interface is likely to play a role. This study promoted a better understanding of the driving mechanism in optothermal trapping. Moreover, as an application of the single-particle platform, we showed the photothermal phase separation-induced microdroplet formation of thermoresponsive polymers and the coating of non-thermoresponsive polymers on nanoparticles

Liquid-Liquid Interface Can Promote Micro-Scale Thermal Marangoni Convection in Liquid Binary Mixtures

Liquid-liquid phase separation, a physical transition in which a homogeneous solution spontaneously demixes into two coexisting liquid phases, has been a key topic in the thermodynamics of two-component systems and may find applications in separation, drug delivery, and protein crystallization. Here we applied a microscale temperature gradient using optothermal heating of a gold nanoparticle to overcome the experimental difficulties inherent in homogeneous heating: we aimed at highlighting precise structural development by avoiding randomly nucleating/growing microdomains. In response to laser illumination, a single gold nanoparticle immersed in a binary mixture of aqueous 2,6-dimethylpiridine (lutidine) and N-isopropylpropionamide (NiPPA) was clearly sensitive to the phase transition of the surrounding liquid as demonstrated by light scattering signals, spectral red-shifts and bright-spot images. The local phase separation encapsulating the gold nanoparticle resulted in immediate formation and growth of an organic-rich droplet which was confirmed by Raman spectroscopy. Remarkably, the droplet was stable under a non-equilibrium steady-state heating condition because of strong thermal confinement. Microdroplet growth was ascribed to thermocapillary flow induced by a newly formed liquid-liquid interface around the hot gold nanoparticle. Based upon a tracer experiment and numerical simulation, it is deduced that the transport of solute to the high temperature area is driven by this thermocapillary flow. This study enhances our understanding of phase separation in binary mixtures induced by microscale temperature confinement

Plasmonic Nanofabrication through Optical Heating

A temperature gradient can induce solutes to migrate from a hot to a cold region, and vice versa, in solution. This process, termed thermophoresis, has been applied to manipulate, transport, and separate various macromolecules and colloids by exploiting a microscale temperature gradient. Here we describe using a single gold nanoparticle (AuNP) as an efficient nanoscale heating source to promote thermophoresis. Moreover, on introducing a substrate with high thermal conductivity such as sapphire, a strong local temperature gradient can be shaped in the medium near the AuNP under continuous wave laser illumination. We observed molecules such as polyethylene glycol and sodium dodecyl sulfate being transported toward the AuNP and attaching to its surface, forming a gold core–organic shell structure within several tens of seconds of illumination. Spectroscopically, because of the gradual increasing encapsulation, progressive red shifts with enhanced scattering intensities were seen for the localized surface plasmon resonance bands of the AuNP with increasing cycles of illumination. Post-mortem scanning electron microscopy provided direct evidence of shell formation. Our technique is relevant to nanofabrication based upon optical heating at the nanometer scale

Localized Phase Separation of Thermoresponsive Polymers Induced by Plasmonic Heating

Optical excitation-induced heating of a single gold nanoparticle potentially offers a high-temperature field confined to the immediate neighborhood of the particle. In this study, we applied darkfield microscopy imaging and Rayleigh scattering spectroscopy to pursue phase separation of aqueous thermoresponsive poly(N-isopropylacrylamide) and poly(vinyl methyl ether) adjacent to a gold nanoparticle that was heated by continuous wave laser illumination. Gold nanoparticles were supported on transparent substrates of glass or sapphire. From the imaging study, we observed that a 1−10-μm microdroplet covering the nanoparticle formed and grew in time scales of seconds to a few tens of seconds. The growth was triggered by the illumination and the droplet collapsed when the laser was blocked. At the same time, we observed scattering spectral changes characterized by a progressive redshift in the localized surface plasmon resonance (LSPR) band and an increasing scattering intensity in wavelengths region shorter than the LSPR band with increasing laser intensity. The scattering spectral changes were interpreted by the encapsulation of the nanoparticle by a polymer-rich droplet with increasing sizes. The present study revealed that thermoresponsive polymers were attracted to a hot gold nanoparticle and formed a microdroplet under illumination with a wavelength near the LSPR. Our findings demonstrate the potential of plasmonic heating to manipulate polymer migration and accumulation, which may find applications in protein crystallization

Laser-driven phase transitions in aqueous colloidal gold nanoparticles under high pressure : picosecond pump–probe study

Pump-probe transient extinction spectroscopy was applied to analyze 355-nm picosecond laser heating-induced phenomena of 60-nm-diameter aqueous gold nanoparticles (AuNPs) under a high pressure of 60 MPa. Kinetic spectroscopy revealed that a supercritical layer surrounding the AuNP nucleated with a lifetime of approximately 1 ns in its dynamic expansion and decay behavior for a fluence of 19.6 mJ cm−2. Moreover, we observed in the post-mortem transmission electron micrographs a number of fragments, a small percentage of size-reduced cores, and erupted particles, among the intact particles after 60 shots, suggesting that evaporation occurs under laser illumination. The particle temperature calculation indicated that evaporation begins with a liquid droplet AuNP surrounded by a supercritical layer at temperatures below the boiling point of gold. By applying high pressure, we are able to obtain a clear picture of the evaporation event, which was not possible at ambient pressure because the forming bubble caused particle temperatures to rise uncontrollably. In this study, we shed light on the critical role of the supercritical layer formed around the AuNP under high pressure during laser-induced evaporation.Pulsed-laser illumination of aqueous gold colloids under high pressure enables the evaporation from liquid droplets surrounded by supercritical water layer

Second-order Phase Transition Behavior behind Polymer Glass Transition

Glass transition has similarity to the second-order phase transition in temperature dependent changes in entropy, non-Arrhenius viscosity, and heat capacity of glass forming materials. However, it has primarily been considered to be not phase transition. Recent single-molecule spectroscopy developments prompted re-investigating glass transition at the nanometer scale probing resolution, showing that glass transition includes phenomena similar to the second-order phase transition. They are characterized by microscopic collective polymer motion and discontinuous changes in temperature dependent relaxation times, the latter of which resembles the critical slowing down of second-order phase transitions, within a temperature window above the polymer calorimetric glass transition temperature. Simultaneous collective motion and critical slowing down occurrences disclose that the second-order phase transition hides behind polymer glass transition

Polymer Relaxation Time Enhancement at Temperatures above Glass Transition Temperatures Predicted by Idealized Mode-Coupling Theory

The mode-coupling theory of glass transition predicts the relaxation time divergence of glass-forming materials at the crossover temperature, which is approximately 1.2 times the calorimetric glass transition temperature. However, this divergence has not been experimentally observed. This is known as the most serious drawback of the mode-coupling theory. The use of viscosity-sensitive single molecule fluorescence probes enables the detection of the poly(vinyl acetate) and poly(ethyl methacrylate) relaxation time enhancement around the crossover temperature, thereby supporting the prediction by the mode-coupling theory

Plasmonic Nanofabrication through Optical Heating

A temperature gradient can induce solutes to migrate from a hot to a cold region, and vice versa, in solution. This process, termed thermophoresis, has been applied to manipulate, transport, and separate various macromolecules and colloids by exploiting a microscale temperature gradient. Here we describe using a single gold nanoparticle (AuNP) as an efficient nanoscale heating source to promote thermophoresis. Moreover, on introducing a substrate with high thermal conductivity such as sapphire, a strong local temperature gradient can be shaped in the medium near the AuNP under continuous wave laser illumination. We observed molecules such as polyethylene glycol and sodium dodecyl sulfate being transported toward the AuNP and attaching to its surface, forming a gold core–organic shell structure within several tens of seconds of illumination. Spectroscopically, because of the gradual increasing encapsulation, progressive red shifts with enhanced scattering intensities were seen for the localized surface plasmon resonance bands of the AuNP with increasing cycles of illumination. Post-mortem scanning electron microscopy provided direct evidence of shell formation. Our technique is relevant to nanofabrication based upon optical heating at the nanometer scale

Localized Phase Separation of Thermoresponsive Polymers Induced by Plasmonic Heating

Optical excitation-induced heating of a single gold nanoparticle potentially offers a high-temperature field confined to the immediate neighborhood of the particle. In this study, we applied darkfield microscopy imaging and Rayleigh scattering spectroscopy to pursue phase separation of aqueous thermoresponsive poly­(<i>N</i>-isopropylacrylamide) and poly­(vinyl methyl ether) adjacent to a gold nanoparticle that was heated by continuous wave laser illumination. Gold nanoparticles were supported on transparent substrates of glass or sapphire. From the imaging study, we observed that a 1–10 μm microdroplet covering the nanoparticle formed and grew in time scales of seconds to a few tens of seconds. The growth was triggered by the illumination, and the droplet collapsed when the laser was blocked. At the same time, we observed scattering spectral changes characterized by a progressive redshift in the localized surface plasmon resonance (LSPR) band and an increasing scattering intensity in the region of wavelengths shorter than the LSPR band with increasing laser intensity. The scattering spectral changes were interpreted by the encapsulation of the nanoparticle by a polymer-rich droplet with increasing sizes. The present study revealed that thermoresponsive polymers were attracted to a hot gold nanoparticle and formed a microdroplet under illumination with a wavelength near the LSPR. Our findings demonstrate the potential of plasmonic heating to manipulate polymer migration and accumulation, which may find applications in protein crystallization

Size-Dependent Optical Properties of Grana Inside Chloroplast of Plant Cells

Well-packed thylakoids known as grana are one of the major functional sites for photosynthesis in algae and plants. Their highly ordered structures can be considered as a few hundred nanometer-sized particles having distinct scattering cross sections from other various macromolecular organizations inside plant cells. With this background we show that elastic light scattering imaging and microspectroscopy is an important tool for investigating structure and organization of grana inside a single chloroplast in plant cells. We have demonstrated this noninvasive method to identify the distribution of grana in intact fresh leaf of robust and rapidly growing <i>Egaria densa</i>, which is also known as <i>Anachris</i> and among the most popular aquarium plants. The scattering efficiency spectra of their individual grana fairly resemble cooperative absorption spectra of porphyrins and carotenoids. We found that the electronic structure of the stacked thylakoids shows granum size-dependence, indicating that size of grana is one of the critical parameters in the regulation of the photochemical functions in the thylakoid