Todd Klein

Magnetoresistive performance and comparison of supermagnetic nanoparticles on giant magnetoresistive sensor-based detection system

Giant magnetoresistive (GMR) biosensors have emerged as powerful tools for ultrasensitive, multiplexed, real-time electrical readout, and rapid biological/chemical detection while combining with magnetic particles. Finding appropriate magnetic nanoparticles (MNPs) and its influences on the detection signal is a vital aspect to the GMR bio-sensing technology. Here, we report a GMR sensor based detection system capable of stable and convenient connection, and real-time measurement. Five different types of MNPs with sizes ranging from 10 to 100u2005nm were investigated for GMR biosensing. The experiments were accomplished with the aid of DNA hybridization and detection architecture on GMR sensor surface. We found that different MNPs markedly affected the final detection signal, depending on their characteristics of magnetic moment, size, and surface-based binding ability, etc. This work may provide a useful guidance in selecting or preparing MNPs to enhance the sensitivity of GMR biosensors, and eventually lead to a versatile and portable device for molecular diagnostics.

Magnetic Detection of Mercuric Ion Using Giant Magnetoresistance-Based Biosensing System

We have demonstrated a novel sensing strategy employing a giant magnetoresistance (GMR) biosensor and DNA chemistry for the detection of mercuric ion (Hg(2+)). This assay takes advantages of high sensitivity and real-time signal readout of GMR biosensor and high selectivity of thymine-thymine (T-T) pair for Hg(2+). The assay has a detection limit of 10 nM in both buffer and natural water, which is the maximum mercury level in drinking water regulated by U.S. Environmental Protection Agency (EPA). The magnitude of the dynamic range for Hg(2+) detection is up to three orders (10 nM to 10 μM). Herein, GMR sensing technology is first introduced into a pollutant monitoring area. It can be foreseen that the GMR biosensor could become a robust contender in the areas of environmental monitoring and food safety testing.

Giant magnetoresistive-based biosensing probe station system for multiplex protein assays

In this study, a sensitive immune-biosensing system capable of multiplexed, real-time electrical readout was developed based on giant magnetoresistive (GMR) sensor array to detect a panel of protein biomarkers simultaneously. PAPP-A, PCSK9, and ST2 have been regarded as promising candidate biomarkers for cardiovascular diseases. Early detection of multiple biomarkers for a disease could enable accurate prediction of a disease risk. 64 nano-size GMR sensors were assembled onto one 16 mm × 16 mm chip with a reaction well, and they could work independently and be monitored simultaneously. A detect limit of 40 pg/mL for ST2 antigen had been achieved, and the dynamic ranges for the three proteins detection were up to four orders of magnitude. The GMR sensing platform was also selective enough to be directly used in serum samples. In addition, a lab-based probe station has been designed to implement quick lab-on-a-chip experiments instead of wire bonding. It has a potential application in clinical biomarkers identification and screening, and can be extended to fit other biosensing schemes.

Surface Modification for Protein and DNA Immobilization onto GMR Biosensor

Giant magnetoresistance (GMR) biosensor with 20 nm SiO 2 on surface was successfully modified by 3-aminopropyltriethoxy silane (APTES) and glutaraldehyde (Glu). The resultant functionalized surface with terminal aldehyde groups was able to efficiently capture Interleukin-6 (IL-6) antibody and amine modified DNA (deoxyribonucleic acid) oligonucleotide. The immobilized IL-6 antibody could bind to IL-6 antigen, and fluorescence sandwich assay was demonstrated. The immobilized DNA could also hybridize with complementary DNA oligonucleotide. Streptavidin labeled magnetic nanoparticles with a diameter of 30 nm were both successfully bound to IL-6 antibody and DNA immobilized GMR biosensors after their respective sandwich binding and complementary hybridization. This APTES-Glu modification method could be also applicable to other surface for protein and DNA microarrays.

Spin transfer torque programming dipole coupled nanomagnet arrays

We experimentally demonstrated spin transfer torque (STT) programming of dipole coupled nanomagnets using magnetic tunnel junctions. The STT write operations were performed in conjunction with a clock field used in magnetic quantum cellular automata (MQCA) operations. The spacing and number of nanomagnets in the transmission line strongly affected the STT programming of the individual pillars. These MQCA transmission lines ranged in length from 2 elements to 20 elements, while device sizes ranged between 50u2009nmu2009×u200980u2009nm and 70u2009nmu2009×u2009100u2009nm with spacing between 10u2009nm and 15u2009nm. With the application of the clock field, currents of 100-200u2009μA are sufficient to STT program the device. The demonstration of STT programming of individual nanomagnets in a dipole coupled array marks a significant step forward for applications such as MQCA logic device.

Evaluation of Hyperthermia of Magnetic Nanoparticles by Dehydrating DNA

A method based on the thermodynamic equilibrium reached between the hybridization and denaturation of double-stranded DNA (ds-DNA) is opened up to evaluate the hyperthermia performance of magnetic nanoparticles (MNPs). Two kinds of MNPs with different sizes and magnetic performance are chosen, and their temperature increments at the surface area under an alternating magnetic field (AMF) are calculated and compared through the concentration variation of ds-DNA modified on the surface. The temperature difference between the surface area of MNPs and bulk solution is also investigated, which can reach as high as 57.8°C when AMF applied for 300u2005s. This method provides a direct path way of comparison hyperthermia ability of MNPs, and serves as a good reference to choose MNPs and decides the therapy parameters based on the unique drug response of individual patient.

Measurement of Brownian and Néel Relaxation of Magnetic Nanoparticles by a Mixing-Frequency Method

A detection scheme for both Brownian and Néel relaxation of magnetic nanoparticles (MNPs) is demonstrated by a mixing-frequency method in this paper. MNPs are driven into the saturation region by a low-frequency sinusoidal magnetic field. A high-frequency sinusoidal magnetic field is then applied to generate mixing-frequency signals that are highly specific to the Brownian relaxation of MNPs. These highly sensitive mixing-frequency signals from MNPs are picked up by a pair of balanced built-in detection coils. The relationship between MNPs relaxation time and phase delays of the mixing-frequency signals behind the applied field is derived, and is experimentally verified. Magnetite MNPs with the core diameter of 35 nm are used for the measurement of Brownian relaxation, and Magnetite MNPs with the core diameter of 12 nm are used for the measurement of Néel relaxation. The results show that both Brownian and Néel relaxation depend on the magnetic offset field. This study provides an in-depth understanding of the relaxation mechanisms of MNPs.

Magnetic nanoparticles colourization by a mixing-frequency method

Brownian and Neel relaxation of magnetic nanoparticles (MNPs) can be characterized by a highly sensitive mixing-frequency method using a search-coil based detection system. The unique magnetic properties of MNPs have been used for biomarkers detection. In this paper, we present a theory and implement an experimental detection scheme using the mixing-frequency method to identify different MNPs simultaneously. A low-frequency sinusoidal magnetic field is applied to saturate the MNPs periodically. A high-frequency sinusoidal magnetic field is then applied to generate mixing-frequency signals that are highly specific to the magnetization of MNPs. The spectra of each MNP can be defined as the complex magnetization of the MNPs over the field frequency. The magnetic spectra of various MNPs and magnetic beads have been characterized and compared. The differences between the MNPs spectra enable us to identify the individual MNPs at the same time. A test has been done to verify the ratio of two different MNPs in mixed samples based on the proposed theory. The experimental results show that the mixing-frequency method is a promising method for MNPs colourization.

Integration of spintronic interface for nanomagnetic arrays

An experimental demonstration utilizing a spintronic input/output (I/O) interface for arrays of closely spaced nanomagnets is presented. The free layers of magnetic tunnel junctions (MTJs) form dipole coupled nanomagnet arrays which can be applied to different contexts including Magnetic Quantum Cellular Automata (MQCA) for logic applications and self-biased devices for field sensing applications. Dipole coupled nanomagnet arrays demonstrate adaptability to a variety of contexts due to the ability for tuning of magnetic response. Spintronics allows individual nanomagnets to be manipulated with spin transfer torque and monitored with magnetoresistance. This facilitates measurement of the magnetic coupling which is important for (yet to be demonstrated) data propagation reliability studies. In addition, the same magnetic coupling can be tuned to reduce coercivity for field sensing. Dipole coupled nanomagnet arrays have the potential to be thousands of times more energy efficient than CMOS technology for log...

Quantitative analysis of interaction between domain walls and magnetic nanoparticles

We have studied the potential energy and effective field induced by the presence of a single superparamagnetic particle above a magnetic domain wall in a 5 nm ferromagnetic film (Msu2009=u2009800 emu/cm3) with uniaxial crystalline anisotropy (Kuu2009<u2009107 erg/cm3). The wall width, wall type (head-to-head, Neel, and perpendicular Bloch), film dimensions, particle height, and external applied field are found to affect the performance of particle sensing systems. Results and optimization strategies derived from this model are presented. The calculated change in depinning field (ΔHdp) is compared against experimental data and micromagnetic simulation. This comparison provides justification for further development in terms of integration with micromagnetic simulations.