H. C. Yang

Origin of field-dependent optical transmission of magnetic fluid films

The physical mechanism of the optical transmission of magnetic fluid films under perpendicular magnetic fields is investigated in this work. Under perpendicular magnetic fields, originally dispersed magnetic particles agglomerate to form magnetic columns. The liquid phase is transparent, whereas the columns are opaque. Hence, the liquid phase dominates the optical transmission of the magnetic fluid film. When the field strength is raised, more columns are formed, and the area of the liquid phase is reduced. This leads to the decrease in the optical transmission of the film under higher field strength. The variation in the concentration of the liquid phase under various field strengths also contributes to the transmission of the film. By taking account of the variations of the effective concentration and the area ratio of the liquid phase in the magnetic fluid film under magnetic fields, the resultant magnetic field dependence of the optical transmission was calculated and found to be consistent with the experimental results. This provides evidence for the origin of the field-dependent optical transmission of the magnetic fluid film under external fields.

Magnetic relaxation measurement in immunoassay using high-transition-temperature superconducting quantum interference device system

Due to their ultrahigh sensitivity to magnetic flux, superconducting quantum interference devices (SQUIDs) are able to detect biomagnetic signals. By labeling biotargets with magnetic nanoparticles, several groups have shown that SQUIDs are promising as quantitative probes of biotargets by measuring their magnetic properties. In this work, we describe the design and construction of a high-transition-temperature radio-frequency SQUID magnetometer system for measuring the magnetic relaxation of labeled avidin. We also describe the synthesis of magnetic nanoparticles coated with biotin for use in labeling the avidin. Furthermore, the specifications of the SQUID-based magnetically labeled immunoassay of avidin are explored.

Multi-Channel SQUID-Based Ultra-High-Sensitivity In-Vitro Detections for Bio-Markers of Alzheimer's Disease Via Immunomagnetic Reduction

Via immunomagnetic reduction assay, bio-molecules can be quantitatively detected with the aid of bio-functionalized magnetic nanoparticles, which are used as labeling markers for specific bio-molecules. To achieve an ultra-high sensitivity in the detection of bio-molecules, the superconducting quantum interference device (SQUID) looks very promising as a sensor for the magnetic signal that are related to the concentration of the detected bio-molecules. We had developed a single channel SQUID-based magnetosusceptometer, but for increasing the detection through-put, we have developed a multi-channel SQUID-based magnetosusceptometer. In this work, the design and the working principle of a 4-channel SQUID-based magnetosusceptometer are introduced. By utilizing scanning technology, 4 samples can be logged into a SQUID-based magnetosusceptometer simultaneously. Be noted that only one single SQUID magnetometer has been used in the magnetosusceptometer. The precision and the sensitivity of detecting bio-molecules by using a 4-channel SQUID-based magnetosusceptometer have been investigated. The detected bio-molecules are biomarkers for Alzheimers disease.

Universal Behavior of Biomolecule-Concentration-Dependent Reduction in AC Magnetic Susceptibility of Bioreagents

Magnetic nanoparticles biofunctionalized with antibodies are useful for specifically labeling target biomolecules. By measuring magnetic signals after the association between biofunctionalized magnetic nanoparticles and target biomolecules, the concentration of target biomolecules can be quantitatively detected. One of measuring methodologies is so-called immunomagnetic reduction (IMR), in which the reduction in ac magnetic susceptibility of magnetic reagent is a function of the concentration of target biomolecules. In this letter, the relationship between the magnetic reduction signal of reagent and the concentration of target biomolecules is explored. According to the experimental results on various kinds of target biomolecules, such as proteins and chemicals, the magnetic reduction signal as a function of concentration of various target biomolecules can be scaled to a universal curve. This universal curve is a logistic function. This implies that there exists a unique mechanism for the association between the target biomolecules and biofunctionalized magnetic nanoparticles in an IMR assay.

Pattern formation in microdrops of magnetic fluids

The pattern formation of magnetic fluid films containing magnetic microdrops under applied perpendicular magnetic fields was investigated. At a zero field, these drops of size several μm were initially near-circular. As the field strength increased, these drops changed to dumbbells or branched structure. When the field strength increased again, a labyrinthine structure appeared. At even higher field strength, the connectivity of the drops broke up; and finally an equilibrium hexagonal structure formed inside the film. A new phase transition exists between the labyrinthine structure and the hexagonal structure, which has not been seen before. Also, the reverse process of pattern formation is irreversible.

In Vivo and Real-Time Measurement of Magnetic Nanoparticles Distribution in Animals by Scanning SQUID Biosusceptometry for Biomedicine Study

Magnetic nanoparticles have been widely applied to biomagnetism, such as drug deliver, magnetic labeling, and contrast agent for in vivo image, etc. To localize the distribution of these magnetic particles in living organism is the first important issue to confirm the effects of magnetic nanoparticles and also evaluate the possible untoward effects. In this study, a scanning high Tc rf-SQUID superconducting quantum interference devices (SQUIDs) biosusceptometry, composed of static SQUID unit and scanning coil sets, is developed for biomedicine study with the advantages of easy operation and unshielded environment. The characteristics tests showed that the system had the low noise of 8 pT/Hz at 400 Hz and the high sensitivity with the minimum detectable magnetization around 4.5 × 10-3 EMU at distance of 13 mm. A magnetic nanoparticle detection test, performed by ex vivo scanning of the magnetic fluids filled capillary under swine skin for simulation of blood vessels in living bodies, confirmed that the system is feasible for dynamic tracking of magnetic nanoparticles. Based on this result, we performed further studies in rats to clarify the dynamic distribution of magnetic nanoparticle in living organism for the pharmacokinetics analysis like drug delivers, and propose the possible physiological metabolism of intravenous magnetic nanoparticles.

Spatially Resolved Imaging on Photocarrier Generations and Band Alignments at Perovskite/PbI2 Heterointerfaces of Perovskite Solar Cells by Light-Modulated Scanning Tunneling Microscopy

The presence of the PbI2 passivation layers at perovskite crystal grains has been found to considerably affect the charge carrier transport behaviors and device performance of perovskite solar cells. This work demonstrates the application of a novel light-modulated scanning tunneling microscopy (LM-STM) technique to reveal the interfacial electronic structures at the heterointerfaces between CH3NH3PbI3 perovskite crystals and PbI2 passivation layers of individual perovskite grains under light illumination. Most importantly, this technique enabled the first observation of spatially resolved mapping images of photoinduced interfacial band bending of valence bands and conduction bands and the photogenerated electron and hole carriers at the heterointerfaces of perovskite crystal grains. By systematically exploring the interfacial electronic structures of individual perovskite grains, enhanced charge separation and reduced back recombination were observed when an optimal design of interfacial PbI2 passivation layers consisting of a thickness less than 20 nm at perovskite crystal grains was applied.

Field-dependent phase diagram of the structural pattern in a ferrofluid film under perpendicular magnetic field

Abstract Influences of both the magnetic field strength H and the sweep rate d H /d t on the structural pattern evolution in a magnetic fluid film under perpendicular fields are systematically investigated. When the magnetic field is increased at a given rate, the structure evolves from a monodispersed state to a disordered one, then to the first-level ordered hexagonal structure, and finally to the second-level ordered structure through a transition state. With a higher d H /d t , the range of the applied magnetic field corresponding to the first-level ordered structure is widened and the field H s , at which the transition occurs becomes higher. An empirical power law H s ∝(d H /d t ) 0.4 was found. A phase diagram for the magnetic fluid film in the H –d H /d t plane is presented.

Evaluation of the flaw depth using high-Tc SQUID

Abstract Using high- T c SQUID, we investigated quantitative nondestructive evaluation for flaws in conducting samples. The spatial derivative of the defect field was found to provide valuable information about the position and the depth of the flaw. By analyzing the spatial derivative of the defect field, the quantitative flaw evaluation was demonstrated.

High quality step-edge substrates for high-Tc superconducting devices

Despite the significant progress in fabrication methods of step edge, the lack of reproducibility still hinders their use in more complicated systems. To pursue the high reproducibility and quality of step edge for high-Tc superconducting devices, we have developed the technique to fabricate high quality step-edge substrates with arbitrary step angles. We used two steps to improve the step ramp quality substantially. The surface microscopy of step substrates shows high uniformity with respect to any step angle. There are no needles, waves, trenches, cascades, or other flaws on these surfaces. Serial Josephson junctions and superconducting quantum interference device arrays were fabricated onto step-edge substrates. The step-edge devices exhibit excellent results.