Gold Nanotips Research Articles

Photomagnetically Powered Spiky Nanomachines with Thermal Control of Viscosity for Enhanced Cancer Mechanotherapy.

Nanomachines with active propulsion have emerged as an intelligent platform for targeted cancer therapy. Achieving an efficient locomotion performance using an external energy conversion is a key requirement in the design of nanomachines. In this study, inspired by diverse spiky structures in nature, a photomagnetically powered nanomachine (PMN) with a spiky surface and thermally dependent viscosity tunability is proposed to facilitate mechanical motion in lysosomes for cancer mechanotherapy. The hybrid nanomachine is integrated with magnetic nanoparticles as the core and covered with gold nanotips. Physical simulations and experimental results prove that the spiky structure endows nanomachines with an obvious photomagnetic coupling effect in the NIR-II region through the alignment and orienting movement of plasmons on the gold tips. Using a coupling-enhanced magnetic field, PMNs are efficiently assembled into chain-like structures to further elevate energy conversion efficiency. Notably, PMNs with the thermal control of viscosity are efficiently propelled under simultaneously applied dual external energy sources in cell lysosomes. Enhanced mechanical destruction of cancer cells via PMNs is confirmed both in vitro and in vivo under photomagnetic treatment. This study provides a new direction for designing integrated nanomachines with active adaptability to physiological environments for cancer treatment.

Parabola-Like Gold Nanobowtie on Sapphire Substrate as Nano-Cavity

Plasmonic metallic nanostructures have attracted much interest for their ability to manipulate light on a subwavelength scale and for their related applications in various fields. In this work, a parabola-like gold nanobowtie (PGNB) on a sapphire substrate was designed as a nano-cavity for confining light waves in a nanoscale gap region. The near-field optical properties of the innovative PGNB structure were studied comprehensively, taking advantage of the time-resolved field calculation based on a finite-difference time-domain algorithm (FDTD). The calculation result showed that the resonance wavelength of the nano-cavity was quite sensitive to the geometry of the PGNB. The values that related to the scattering and absorption properties of the PGNB, such as the scattering cross section, absorption cross section, extinction cross section, scattering ratio, and also the absorption ratio, were strongly dependent on the geometrical parameters which affected the surface area of the nanobowtie. Increased sharpness of the gold tips on the parabola-like nano-wings benefited the concentration of high-density charges with opposite electric properties in the narrow gold tips with limited volume, thus, resulting in a highly enhanced electric field in the nano-cavity under illumination of the light wave. Reduction of the gap size between the two gold nano-tips, namely, the size of the nano-cavity, decreased the distance that the electric potential produced by the highly concentrated charges on the surface of each gold nano-tip had to jump across, therefore, causing a significantly enhanced field in the nano-cavity. Further, alignment of the linearly polarized electric field of the incident light wave with the symmetric axis of the PGNB efficiently enabled the free electrons in the PGNB to concentrate on the surface of the sharp gold tips with a high density, thus, strongly improving the field across the nano-cavity. The research provides a new insight for future design, nanofabrication, and characterization of PGNBs for applications in devices that relate to enhancing photons emission, improving efficiency for energy harvesting, and improving sensitivity for infrared detection.

SERS substrates based on self-organized dimple nanostructures on polyethylene naphthalate films produced via oxygen ion beam sputtering

Surface-enhanced Raman spectroscopy (SERS) utilizes metal nanostructures to enhance the intensity of Raman signals. Although many methods have been developed for fabricating SERS nanostructures, most involve multiple steps. Herein, we employed oxygen ion beam sputtering (IBS), a one-step technique suitable for processing flexible substrates in roll-to-roll processes for mass production, Specifically, one-step oxygen IBS was used to fabricate self-organized dimple nanostructures, whose area and roughness could be controlled using the ion irradiation energy density, on the surfaces of polyethylene naphthalate films. Gold nano-tips for SERS were subsequently obtained by evaporating gold onto the dimple nanostructures. Finite-difference time-domain (FDTD) simulations revealed that nano-tip structures with spacings of less than 10 nm increased the localized E-field enhancement, which improved the SERS signal. Fabrication at a low energy density (5.8 J/cm2) produced more nano-tips with spacings of less than 10 nm, corresponding to a density of 61.4 nano-tips/μm2. SERS analysis conducted with methylene blue at 638 nm and 785 nm demonstrated that the Raman signal intensity was stronger for SERS substrates fabricated with low energy density (5.8 J/cm2) than for substrates fabricated with high energy density (17.3 J/cm2), because of the high density of nano-tips on the former substrate.

Fabrication of highly uniform nanoprobe via the automated process for tip-enhanced Raman spectroscopy

Abstract In a tip-enhanced Raman spectroscopy (TERS) system, using a sharp nanotip that comprises a noble metal is critical to attaining high spatial resolution and highly enhanced Raman scattering. A strongly acidic solution is typically used to fabricate gold nanotips in a quick and reliable manner. However, using an acidic solution could corrode the etching system, thereby posing hazardous problems. Therefore, both the corrosion of the etching system and human error induced by the conventional method considerably decrease the quality and reproducibility of the tip. In this study, we significantly increased the reproducibility of tip fabrication by automating the electrochemical etching system. In addition, we optimized the etching conditions for an etchant that comprised a KCl solution to which ethanol was added to overcome the limitations of the acidic etchant. The automated etching system significantly increases the yield rate of tip-fabrication reproducibility from 65 to 95%. The standard deviation of the radius of curvature decreased to 7.3 nm with an average radius of curvature of 30 nm. Accordingly, the automated electrochemical etching system might improve the efficiency of TERS.

Effects of electrolyte concentration on the morphology control of gold nanotips in electrochemical etching

Gold nanotips fabrication with electrochemical etching has advantages of low cost, short processing time and easy operation, while the morphology control and related electrochemical etching mechanism remain to be explored. Through analyzing the cyclic voltammetry curves during the etching processes, it was found that HCl solution is more stable in fabricating nanotips as compared to KCl solution. The effects of electrolyte concentration on the etching process were also investigated. Based on the constant meniscus descending rate and radial etching rate, a model to explain the unique tip morphology formed at different electrolyte concentrations was established, which indicated that the cone angles of the tips gradually decreased with the increase of the HCl concentration, while the curvature radius was not sensitive to the concentration. This investigation demonstrated the controllability of tip cone angle during the etching process, which would be of great significance to low cost preparation and application of gold nanotips on the high-resolution detection of biological and medical molecules.

Controlled fabrication of gold nanotip arrays by nanomolding-necking technology

The fabrication of nanotips has been driven by the increasing industrial demands in developing high-performance multifunctional nanodevices. In this work, we proposed a controlled, rapid as well as low cost nanomolding-necking technology to fabricate gold nanotips arrays. The geometries of gold nanotips having cone angle range of ∼28–77° and curvature radii of <5 nm can be prepared by tailoring the diameters of raw nanorods in nanomolding process or modulating the necking temperature. Molecular dynamics simulation reveals that the formation of the nanotip geometry is determined by the interplay between dislocation-based and diffusion-based deformation mechanisms, intrinsically arising from the nonlinear dependence of atom diffusion on temperature and sample size. The good controllability, mass production and low cost of the developed nanomolding-necking technology make it highly promising in developing nanodevices for a wide range of applications, such as probing, sensing, antireflection coating and nanoindentation.

An ultrafast nanotip electron gun triggered by grating-coupled surface plasmons

We demonstrate multiphoton photoelectron emission from gold nanotips induced by nanofocusing surface plasmons, resonantly excited on the tip shaft by a grating coupler. The tip is integrated into an electron gun assembly, which facilitates control over the spatial emission sites and allows us to disentangle direct grating emission from plasmon-triggered apex emission. The nanoscale source size of this electron gun concept enables highly coherent electron pulses with applications in ultrafast electron imaging and diffraction.

Real-space imaging of nanotip plasmons using electron energy loss spectroscopy

We report the spatial mapping of surface plasmons on conical gold nanotips by electron energy loss spectroscopy. We observe standing-wave patterns resulting from reflections of the fundamental surface-plasmon mode at the nanotip apex. The experimental results are in very good agreement with numerical calculations using the discontinuous Galerkin time-domain method and analytical computations based on a cylindrical mode expansion.

Enhanced Mechanical Stability of Gold Nanotips through Carbon Nanocone Encapsulation.

Gold is a noble metal that, in comparison with silver and copper, has the advantage of corrosion resistance. Despite its high conductivity, chemical stability and biocompatibility, gold exhibits high plasticity, which limits its applications in some nanodevices. Here, we report an experimental and theoretical study on how to attain enhanced mechanical stability of gold nanotips. The gold tips were fabricated by chemical etching and further encapsulated with carbon nanocones via nanomanipulation. Atomic force microscopy experiments were carried out to test their mechanical stability. Molecular dynamics simulations show that the encapsulated nanocone changes the strain release mechanisms at the nanoscale by blocking gold atomic sliding, redistributing the strain along the whole nanostructure. The carbon nanocones are conducting and can induce magnetism, thus opening new avenues on the exploitation of transport, mechanical and magnetic properties of gold covered by sp2 carbon at the nanoscale.

Large optical field enhancement for nanotips with large opening angles

We theoretically investigate the dependence of the enhancement of optical near-fields at nanometric tips on the shape, size, and material of the tip. We confirm the strong dependence of the field enhancement factor on the radius of curvature. In addition, we find a surprisingly strong increase of field enhancement with increasing opening angle of the nanotips. For gold and tungsten nanotips in the experimentally relevant parameter range (radius of curvature at laser wavelength), we obtain field enhancement factors of up to for Au and for W for large opening angles. We confirm this strong dependence on the opening angle for many other materials featuring a wide variety in their dielectric response. For dielectrics, the opening angle dependence is traced back to the electrostatic force of the induced surface charge at the tip shank. For metals, the plasmonic response strongly increases the field enhancement and shifts the maximum field enhancement to smaller opening angles.

Probing of Optical Near-Fields by Electron Rescattering on the 1 nm Scale

We present a new method of measuring optical near-fields within ~1 nm of a metal surface based on rescattering of photoemitted electrons. With this method, we precisely measure the field enhancement factor for tungsten and gold nanotips as a function of tip radius. The agreement with Maxwell simulations is very good. Further simulations yield a field enhancement map for all materials, which shows that optical near-fields at nanotips are governed by a geometric effect under most conditions, while plasmon resonances play only a minor role. Last, we consider the implications of our results on quantum mechanical effects near the surface of nanostructures and discuss features of quantum plasmonics.

A gold-nanotip optical fiber for plasmon-enhanced near-field detection

A wet-chemical etching and mechanical cleaving technique is used to fabricate gold nanotips attached to tapered optical fibers. Localized surface plasmon resonances (tunable from 500 to 850 nm by varying the tip dimensions) are excited at the tip, and the signal is transmitted via the fiber to an optical analyzer, making the device a plasmon-enhanced near-field probe. A simple cavity model is used to explain the resonances observed in numerical simulations.

Interaction of an ultrashort optical pulse with a metallic nanotip: A Green dyadic approach

We consider the interaction of an ultrashort optical pulse with a metallic nanotip. In the framework of the Green dyadic method, we calculate the time-dependent electric field in the vicinity of the metallic nano-object. The electric field enhancement at the tip apex, its temporal evolution, and its dependence upon the polarization of the incident electric field are addressed. By comparing the case of tungsten and gold nanotips, we show that significantly larger electric field enhancements can be obtained with the latter and point out different temporal responses. Our description provides the time-dependent spatial distribution of the electric field around metallic nano-objects of arbitrary geometries which is the first step to describe light-matter interaction in recently developed laser-driven femtosecond electron nanosources or recent results in ultrafast nanoplasmonics.

Large-Scale Atomistic Simulations of Environmental Effects on the Formation and Properties of Molecular Junctions

Using an updated simulation tool, we examine molecular junctions composed of benzene-1,4-dithiolate bonded between gold nanotips, focusing on the importance of environmental factors and interelectrode distance on the formation and structure of bridged molecules. We investigate the complex relationship between monolayer density and tip separation, finding that the formation of multimolecule junctions is favored at low monolayer density, while single-molecule junctions are favored at high density. We demonstrate that tip geometry and monolayer interactions, two factors that are often neglected in simulation, affect the bonding geometry and tilt angle of bridged molecules. We further show that the structures of bridged molecules at 298 and 77 K are similar.

Scanning Electrochemical Microscopy with Gold Nanotips: The Effect of Electrode Material on Electron Transfer Rates

The disk-type, polished Au nanoelectrodes were prepared, characterized, and used as tips in the scanning electrochemical microscope (SECM). Kinetic parameters were measured for several rapid heterogeneous electron transfer (ET) reactions and compared to those obtained previously at Pt nanoelectrodes. The kinetics of most investigated ET processes were found to be independent of the electrode material. In contrast to the previously reported results, the rate constant of hexaammineruthenium(III) reduction was somewhat higher at Pt than at Au electrodes in different electrolyte solutions. This finding suggests that this reaction exhibits some degree of nonadiabaticity.

CO oxidation on electrically charged gold nanotips

We report a study of the oxidation of CO on a gold nanotip in the presence of high electrostatic fields. With the binding energies obtained using density functional theory as a function of the electric field, a simple field-dependent kinetic model based on the Langmuir-Hinshelwood mechanism is set up. We show that the dissociative adsorption of oxygen on gold happens only below a negative critical value of the electric field while the binding of CO on gold is enhanced for positive values. We explain the propagation of a wave observed in field ion microscopy experiments and predict that the oxidation of CO occurs on negatively charged gold clusters.

Field-induced CO adsorption and formation of carbonyl waves on gold nanotips

We report a study of the adsorption and reaction of CO on a gold nanotip in high electrostatic fields. Field ion microscopy is used to investigate the emergence of a Au-carbonyl wave that is made visible with oxygen as the imaging gas. We set up a simple kinetic model that reproduces the adsorption wave and confirms that the presence of oxygen merely serves as an imaging gas and does not lead to field-induced oxidation of CO.