Michael Fedoruk

Controlled Nanometric Phase Transitions of Phospholipid Membranes by Plasmonic Heating of Single Gold Nanoparticles

The development of remotely controlled nanoscopic sources of heat is essential for investigating and manipulating temperature sensitive processes at the nanoscale. Here, we use single gold nanoparticles to rapidly deposit controlled amounts of heat in nanoscopic regions of defined size. This allows us to induce and control nanoscale reversible gel-fluid phase transitions in phospholipid membranes. We exploit the optical control over the phase transition to determine the velocity of the fluid phase front into the gel phase membrane and to guide the nanoparticles to specific locations. These results illustrate how single gold nanoparticles enable local thermodynamic investigation and manipulation on nanoscale (bio-) systems.

DNA-assembled nanoparticle rings exhibit electric and magnetic resonances at visible frequencies.

Metallic nanostructures can be used to manipulate light on the subwavelength scale to create tailored optical material properties. Next to electric responses, artificial optical magnetism is of particular interest but difficult to achieve at visible wavelengths. DNA-self-assembly has proved to serve as a viable method to template plasmonic materials with nanometer precision and to produce large quantities of metallic objects with high yields. We present here the fabrication of self-assembled ring-shaped plasmonic metamolecules that are composed of four to eight single metal nanoparticles with full stoichiometric and geometric control. Scattering spectra of single rings as well as absorption spectra of solutions containing the metamolecules are used to examine the unique plasmonic features, which are compared to computational simulations. We demonstrate that the electric and magnetic plasmon resonance modes strongly correlate with the exact shape of the structures. In particular, our computations reveal the magnetic plasmons only for particle rings of broken symmetries, which is consistent with our experimental data. We stress the feasibility of DNA self-assembly as a method to create bulk plasmonic materials and metamolecules that may be applied as building blocks in plasmonic devices.

Single-Step Injection of Gold Nanoparticles through Phospholipid Membranes

We propose and demonstrate a new method of an all-optical, contactless, one-step injection of single gold nanoparticles through phospholipid membranes. The method is based on the combination of strong optical forces acting on and simultaneous optical heating of a gold nanoparticle exposed to laser light tuned to the plasmon resonance of the nanoparticle. A focused laser beam captures single nanoparticles from the colloidal suspension, guides them toward a phospholipid vesicle and propels them through the gel-phase membrane, resulting in the nanoparticle internalization into the vesicle. Efficient resonant optical heating of the gold nanoparticle causes a pore to form in the gel-phase membrane, a few-hundred nanometers in size, which remains open for several minutes.

Nanolithography by Plasmonic Heating and Optical Manipulation of Gold Nanoparticles

Noble-metal particles feature intriguing optical properties, which can be utilized to manipulate them by means of light. Light absorbed by gold nanoparticles, for example, is very efficiently converted into heat, and single particles can thus be used as a fine tool to apply heat to a nanoscopic area. At the same time, gold nanoparticles are subject to optical forces when they are irradiated with a focused laser beam, which renders it possible to print, manipulate, and optically trap them in two and three dimensions. Here, we demonstrate how these properties can be used to control the polymerization reaction and thermal curing of polydimethylsiloxane (PDMS) at the nanoscale and how these findings can be applied to synthesize polymer nanostructures such as particles and nanowires with subdiffraction limited resolution.

Two-Color Laser Printing of Individual Gold Nanorods

We report on the deposition of individual gold nanorods from an optical trap using two different laser wavelengths. Laser light, not being resonant to the plasmon resonances of the nanorods, is used for stable trapping and in situ alignment of individual nanorods. Laser light, being resonant to the transversal mode of the nanorods, is used for depositing nanorods at desired locations. The power and polarization dependence of the process is investigated and discussed in terms of force balances between gradient and scattering forces, plasmonic heating, and rotational diffusion of the nanorods. This two-color approach enables faster printing than its one-color equivalent and provides control over the angular orientation (±16°) and location of the deposited nanorods at the single-nanorod level.

Triggering the volume phase transition of core–shell Au nanorod–microgel nanocomposites with light

We have coated gold nanorods (NRs) with thermoresponsive microgel shells based on poly(N-isopropylacrylamide) (pNIPAM). We demonstrate by simultaneous laser-heating and optical extinction measurements that the Au NR cores can be simultaneously used as fast optothermal manipulators (switchers) and sensitive optical reporters of the microgel state in a fully externally controlled and reversible manner. We support our results with optical modeling based on the boundary element method and 3D numerical analysis on the temperature distribution. Briefly, we show that due to the sharp increase in refractive index resulting from the optothermally triggered microgel collapse, the longitudinal plasmon band of the coated Au NRs is significantly red-shifted. The optothermal control over the pNIPAM shell, and thereby over the optical response of the nanocomposite, is fully reversible and can be simply controlled by switching on and off a NIR heating laser. In contrast to bulk solution heating, we demonstrate that light-triggering does not compromise colloidal stability, which is of primary importance for the ultimate utilization of these types of nanocomposites as remotely controlled optomechanical actuators, for applications spanning from drug delivery to photonic crystals and nanoscale motion.

Subdiffraction-Limited Milling by an Optically Driven Single Gold Nanoparticle

We propose and demonstrate a hybrid lithographic technique capable of nanopatterning surfaces by optothermal decomposition of a polymeric film induced by a single metal nanoparticle. A tightly focused laser beam exerting a strong optical force onto the nanoparticle is used to move it inside the polymer film. Due to efficient plasmonic absorption of the laser light, the nanoparticle is heated up to temperatures of several hundred degrees, causing melting or even thermal decomposition of the polymer film. By this method, grooves less than 100 nm wide and tens of micrometers long can be directly milled in a polymer layer.

Plasmonic coupling between metallic nanocavities

We demonstrate strong coupling of nanocavities in metal films, sparked by propagating surface plasmons. Unlike the coupling of metallic nanoparticles which decays over distances of tens of nanometers, the metallic nanocavities display long range coupling at distances of hundreds of nanometers for the properly selected metal/wavelength combinations. Such strong coupling drastically changes the symmetry of the charge distribution around the nanocavities as is evidenced by the nonlinear optical response of the medium. We show that when strongly coupled, equilateral triangular nanocavities lose their individual symmetry to adopt the lower symmetry of the coupled system and respond like a single dipolar entity. A quantitative model is suggested for the transition from individual to strongly coupled nanocavities.

Analyzing the Movement of the Nauplius 'Artemia salina' by Optical Tracking of Plasmonic Nanoparticles

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A single gold nanoparticle held by an optical tweezer is used as a sensor to quantify the rhythmic motion of a Nauplius larva (Artemia salina) in a water sample. This is achieved by monitoring the time dependent displacement of the trapped nanoparticle as a consequence of the Nauplius activity. A Fourier analysis of the nanoparticles position then yields a frequency spectrum that is characteristic to the motion of the observed species. This experiment demonstrates the capability of this method to measure and characterize the activity of small aquatic larvae without the requirement to observe them directly and to gain information about the position of the larvae with respect to the trapped particle. Overall, this approach could give an insight on the vitality of certain species found in an aquatic ecosystem and could expand the range of conventional methods for analyzing water samples.