Mingliang Zhang

Nanosized corners for trapping and detecting magnetic nanoparticles

We present a device concept based on controlled micromagnetic configurations in a corner-shaped permalloy nanostructure terminated with two circular disks, specifically designed for the capture and detection of a small number of magnetic beads in suspension. A transverse head-to-head domain wall (TDW) placed at the corner of the structure plays the role of an attracting pole for magnetic beads. The TDW is annihilated in the terminating disks by applying an appropriate magnetic field, whose value is affected by the presence of beads chemically bound to the surface. In the case where the beads are not chemically bound to the surface, the annihilation of the TDW causes their release into the suspension. The variation of the voltage drop across the corner, due to the anisotropic magnetoresistance (AMR) while sweeping the magnetic field, is used to detect the presence of a chemically bound bead. The device response has been characterized by using both synthetic antiferromagnetic nanoparticles (disks of 70 nm diameter and 20 nm height) and magnetic nanobeads, for different thicknesses of the protective capping layer. We demonstrate the detection down to a single nanoparticle, therefore the device holds the potential for the localization and detection of small numbers of molecules immobilized on the particles functionalized surface.

Sombrero-Shaped Plasmonic Nanoparticles with Molecular-Level Sensitivity and Multifunctionality

We demonstrate top-down synthesis of monodisperse plasmonic nanoparticles designed to contain internal Raman hot spots. Our Raman-active nanoparticles are fabricated using nanoimprint lithography and thin-film deposition and are composed of novel internal structures with sublithographic dimensions: a disk-shaped Ag core, a Petri-dish-shaped SiO2 base whose inner surface is coated with Ag film, and a sub-10 nm scale circular gap between the core and the base. Confocal Raman measurements and electromagnetic simulations show that Raman hot spots appear at the inside perimeter of individual nanoparticles and serve as the source of a 1000-fold improvement of minimum molecular detection level that enables detection of signals from a few molecules near hot spots. A multimodality version of these nanoparticles, which includes the functionality offered by magnetic multilayers, is also demonstrated. These results illustrate the potential of direct fabrication for creating exotic monodisperse nanoparticles, which combine engineered internal nanostructures and multilayer composite materials, for use in nanoparticle-based molecular imaging and detection.

High-Density 2D Homo- and Hetero- Plasmonic Dimers with Universal Sub-10-nm Gaps.

Fabrication of high-density plasmonic dimers on a large (wafer) scale is crucial for applications in surface-enhanced spectroscopy, bio- and molecular sensing, and optoelectronics. Here, we present an experimental approach based on nanoimprint lithography and shadow evaporation that allows for the fabrication of high-density, large-scale homo- (Au-Au and Ag-Ag) and hetero- (Au-Ag) dimer substrates with precise and consistent sub-10-nm gaps. We performed scanning electron, scanning transmission electron, and atomic force microscopy studies along with a complete electron energy-loss spectroscopy (EELS) characterization. We observed distinct plasmonic modes on these dimers, which are well interpreted by finite-difference time-domain (FDTD) and plasmon hybridization calculations.

Fabrication of planar, layered nanoparticles using tri-layer resist templates

A simple and universal pathway to produce free multilayer synthetic nanoparticles is developed based on lithography, vapor phase deposition and a tri-layer resist lift-off and release process. The fabrication method presented in this work is ideal for production of a broad range of nanoparticles, either free in solution or still attached to an intact release layer, with unique magnetic, optical, radioactive, electronic and catalytic properties. Multi-modal capabilities are implicit in the layered architecture. As an example, directly fabricated magnetic nanoparticles are evaluated to illustrate the structural integrity of thin internal multilayers and the nanoparticle stability in aggressive biological environments, which is highly desired for biomedical applications.

Functionalization of high-moment magnetic nanodisks for cell manipulation and separation

AbstractSynthetic antiferromagnetic (SAF) nanoparticles are layer-structured particles with high single-particle magnetic moments. In order to covalently bind these nanoparticles to cells, they were coated with a silica shell followed by conjugation with streptavidin. The silica coating generates both SAF@SiO2 core-shell nanoparticles and silica core-free nanoparticles. Using a simple magnetic separation, silica nanoparticles were removed and SAF@SiO2 nanoparticles were purified. After streptavidin conjugation, these particles were used to stain lung cancer cells, making them highly magnetically responsive. The stained cells can rotate in response to an external magnetic field and can be captured when a blood sample containing these cells flows through the sifter.

Raman-Active Two-Tiered Ag Nanoparticles with a Concentric Cavity

A two-tiered Ag nanoparticle containing a cavity at the center of each nanoparticle is generated by two simple steps of nano-imprinting and metal vacuum deposition. It enables sub-zeptomole detection of organic molecules and five orders of the dynamic sensing range.

Wafer-scale synthesis of monodisperse synthetic magnetic multilayer nanorods

A double exposure technique has been used to fabricate nanoimprint stamps for making monodisperse nanorods with controllable lengths. The nanorod length is defined by a normal photolithography projection process whereas the nanorod width is defined by an edge-lithography process using a soft polydimethylsiloxane (PDMS) contact mask. Taking advantage of edge-lithography, the nanorod width can be less than the diffraction limit of the exposure light. Using these nanorod stamps, synthetic magnetic multilayer (SMM) nanorods have been fabricated using nanoimprint lithography, resulting in a length variation of ∼3%. Nanorod magnetic properties have been characterized in both longitudinal and in-plane transverse directions of the nanorods. A theoretical model has been established to explain the magnetic responses and has revealed that both shape anisotropy and interlayer interactions are important in determining the properties of SMM nanorods.

Detection of a single synthetic antiferromagnetic nanoparticle with an AMR nanostructure: Comparison between simulations and experiments

The depinning field of a domain wall in a permalloy nanostructure can be used to detect the presence of a magnetic particle. In this device the displacement of the domain wall in a sweeping magnetic field produces a variation of the voltage drop across a corner due to the anisotropic magnetoresistance effect and hence an electrical signal. In this paper we use micromagnetic simulations to calculate the output signal of a particularly shaped device in the presence of a single synthetic antiferromagnetic nanoparticle. The calculated magnetoresistive signal is in good agreement with corresponding experimental data

Silane-based functionalization of synthetic antiferromagnetic nanoparticles for biomedical applications

Synthetic antiferromagnetic nanoparticles (SAFNPs) have been successfully coated with two different kinds of silanes, 3-aminopropyltrimethoxysilane and 2-[methoxy(polyethyleneoxy)propyl]trimethoxysilane. The morphology of SAF particles is characterized by scanning electron microscopy and magnetic properties by alternating gradient magnetometry. The attachment of silane molecules is verified by Fourier-transform infrared spectroscopy and colloidal stability is studied using dynamic light scattering. These two silanes change the surface chemical properties of SAFNPs dramatically in different ways, which in turn affects the stability of these particles.