Optical Injection of Gold Nanoparticles into Living Cells

The controlled injection of nanoscopic objects into living cells with light offers promising prospects for the development of novel molecular delivery strategies or intracellular biosensor applications. Here, we show that single gold nanoparticles from solution can be patterned on the surface of living cells with a continuous wave laser beam. In a second step, we demonstrate how the same particles can then be injected into the cells through a combination of plasmonic heating and optical force. We find that short exposure times are sufficient to perforate the cell membrane and inject the particles into cells with a survival rate of >70%

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

Optical Force Stamping Lithography

Here we introduce a new paradigm of far-field optical lithography, <i>optical force stamping lithography</i>. The approach employs optical forces exerted by a spatially modulated light field on colloidal nanoparticles to rapidly stamp large arbitrary patterns comprised of single nanoparticles onto a substrate with a single-nanoparticle positioning accuracy well beyond the diffraction limit. Because the process is all-optical, the stamping pattern can be changed almost instantly and there is no constraint on the type of nanoparticle or substrates used

Electron Transfer Rate vs Recombination Losses in Photocatalytic H₂ Generation on Pt-Decorated CdS Nanorods

Cadmium chalcogenide nanocrystals combined with co-catalyst nanoparticles hold promise for efficient solar to hydrogen conversion. Despite the progress, achieving high efficiency is hampered by high charge recombination rates and sample degradation. Here, we vary the decoration of platinum nanoparticles on CdS nanorods to demonstrate the important role of pathways for the photoelectrons to the co-catalyst. Contrary to expectations, the shortening of the path, by increasing the number of co-catalyst particles, increases the transfer rate but decreases the photocatalytic performance. This is because subsequent electron transfer to the acceptor is much slower; therefore, the recombination rate with the nearby holes increases. We show that with tip-decorated nanorods, the quantum yield of H₂ production can reach and sustain nearly 90%, provided an efficient mechanism of mediated hole extraction is employed. The approach demonstrates that highly efficient photocatalysts may be prepared with only a minimal amount of co-catalyst and thereby suggests future pathways for solar to H₂ conversion with semiconductor nanocrystals

Stretching and Heating Single DNA Molecules with Optically Trapped Gold–Silica Janus Particles

Self-propelled micro- and nanoscale motors are capable of autonomous motion typically by inducing local concentration gradients or thermal gradients in their surrounding medium. This is a result of the heterogeneous surface of the self-propelled structures that consist of materials with different chemical or physical properties. Here we present a self-thermophoretically driven Au–silica Janus particle that can simultaneously stretch and partially melt a single double-stranded DNA molecule. We show that the effective force acting on the DNA molecule is in the ∼pN range, well suited to probe the entropic stretching regime of DNA, and we demonstrate that the local temperature enhancement around the gold side of the particle produces partial DNA dehybridization

Migration of Constituent Protons in Hybrid Organic–Inorganic Perovskite Triggers Intrinsic Doping

The crucial separation of photocarriers in solar cells can be efficiently driven by contacting semiconductor phases with differing doping levels. Here we show that intrinsic doping surges in methylammonium lead iodide (MAPbI₃) crystals as a response to environmental basicity. MAPbI₃ crystals were passivated with polybases to induce the deprotonation of its methylammonium ions (MA⁺). Stable crystals showed marked increases in photoluminescence and radiative decay, attributed to the presence of unbalanced charges acting as doped carriers. This emulates in a controlled manner the proton-withdrawing conditions of polycrystalline films, where excess basic precursors are found between grains. Raman spectroscopy showed the collective alignment of MA⁺ cations within the intrinsically doped lattices, thus revealing the buildup of electric fields. On this basis, we propose a mechanism for the formation of doping-gradients toward grain boundaries, potentially explaining the extended photocarrier lifetimes and diffusion lengths observed in perovskite solar cells

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

Enhancing Single-Nanoparticle Surface-Chemistry by Plasmonic Overheating in an Optical Trap

Surface-chemistry of individual, optically trapped plasmonic nanoparticles is modified and accelerated by plasmonic overheating. Depending on the optical trapping power, gold nanorods can exhibit red shifts of their plasmon resonance (i.e., increasing aspect ratio) under oxidative conditions. In contrast, in bulk exclusively blue shifts (decreasing aspect ratios) are observed. Supported by calculations, we explain this finding by local temperatures in the trap exceeding the boiling point of the solvent that cannot be achieved in bulk

Quantum-Dot-Sensitized Solar Cells with Water-Soluble and Air-Stable PbS Quantum Dots

The sensitization of dispersed P25 TiO₂ nanoparticles (NPs) and macroporous TiO₂ films with water-soluble and air-stable PbS quantum dots (QDs) capped with l-glutathione (GSH) ligands was investigated. Optimum sensitization was achieved by careful adjustment of the surface charges of TiO₂ and PbS QDs by controlling the pH of the QD solution. Efficient electron transfer from photoexcited PbS QDs via the GSH ligands into the conduction band of TiO₂ was demonstrated by photoluminescence (PL) spectroscopy of PbS-sensitized P25 nanoparticles. The PbS QD-sensitized porous TiO₂ electrodes were used to prepare quantum-dot-sensitized solar cells (QDSSCs) utilizing a Cu_<i>x</i>S_<i>y</i> counter electrode and aqueous polysulfide electrolyte. Cells with up to 64% injection efficiency, 1.1% AM 1.5 conversion efficiency, and short circuit current density of 7.4 mA cm^–2 were obtained. The physical parameters of the cells were investigated using impedance spectroscopy

Tuning DNA Binding Kinetics in an Optical Trap by Plasmonic Nanoparticle Heating

We report on the tuning of specific binding of DNA attached to gold nanoparticles at the individual particle pair (dimer) level in an optical trap by means of plasmonic heating. DNA hybridization events are detected optically by the change in the plasmon resonance frequency due to plasmonic coupling of the nanoparticles. We find that at larger trapping powers (i.e., larger temperatures and stiffer traps) the hybridization rates decrease by more than an order of magnitude. This result is explained by higher temperatures preventing the formation of dimers with lower binding energies. Our results demonstrate that plasmonic heating can be used to fine tune the kinetics of biomolecular binding events