T. S. Ramulu

Nanowires array modified electrode for enhanced electrochemical detection of nucleic acid

The gold nanowires array electrode (AuNWsA) was synthesized by two step electrodeposition, which provided well oriented vertically aligned nanowires. The dimensions of the nanowires were determined by scanning electron micrograph and found to be around 1.5 μm in length with 200 nm diameter. Each nanowire was separated by a distance of 2-3 times the diameter of the nanowire itself. The electrochemical performance of the AuNWsA electrode was evaluated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Using these analytical tools, this AuNWsA electrode was shown to have a high effective surface area and excellent electron transfer surfaces compared with flat bare Au electrode. The AuNWsA electrode was then used as an electrochemical biosensor electrode by immobilizing probe DNA and analyzed by CV, EIS and Fourier transform infrared measurements. The results of this analysis suggested that the AuNWsA electrode provides good surfaces for the immobilization and hybridization of DNA. The selectivity of the probe DNA immobilized AuNWsA electrode was tested using non-complementary and one base pair mismatching DNA. The detection limit of the AuNWsA electrode was determined to be 6.78×10(-9) M, which is two times smaller than the bare Au electrode.

Planar Hall resistance ring sensor based on NiFe/Cu/IrMn trilayer structure

We have investigated the sensitivity of a planar Hall resistance sensor as a function of the ring radius in the trilayer structure Ta(3)/IrMn(10)/Cu(0.2)/NiFe(10)/Ta(3) (nm). The diagonal components of magnetoresistivity tensor in rectangular prism corresponding to anisotropic magnetoresistance are few ten times larger than that of off-diagonal component corresponding to planar Hall resistance. However, it is noteworthy that the resultant contribution is governed by the off-diagonal components due to the cancellation of diagonal components in the self-balanced bridge configuration. Both the experimental and theoretical results show that the sensitivity varies linearly with the ring radius. In multi-ring architecture, the circumference can be increased to a limit, which consequently enhances sensitivity. We found the sensitivity of the investigated 7-rings planar Hall to be more than 600 μV/Oe.

Planar Hall magnetoresistive aptasensor for thrombin detection.

The use of aptamer-based assays is an emerging and attractive approach in disease research and clinical diagnostics. A sensitive aptamer-based sandwich-type sensor is presented to detect human thrombin using a planar Hall magnetoresistive (PHR) sensor in cooperation with superparamagnetic labels. A PHR sensor has the great advantages of a high signal-to-noise ratio, a small offset voltage and linear response in the low-field region, allowing it to act as a high-resolution biosensor. In the system presented here, the sensor has an active area of 50 µm × 50 µm with a 10-nm gold layer deposited onto the sensor surface prior to the binding of thiolated DNA primary aptamer. A polydimethylsiloxane well of 600-µm radius and 1-mm height was prepared around the sensor surface to maintain the same specific area and volume for each sensor. The sensor response was traced in real time upon the addition of streptavidin-functionalized magnetic labels on the sensor. A linear response to the thrombin concentration in the range of 86 pM-8.6 µM and a lower detection limit down to 86 pM was achieved by the proposed present method with a sample volume consumption of 2 µl. The proposed aptasensor has a strong potential for application in clinical diagnosis.

Planar Hall Effect Ring Sensors for High Field-Sensitivity

Planar Hall effect sensor has been explored using multi-layer cross-shaped and bridge geometry. We present planar Hall effect in a ring-shaped geometry experimentally that shows progress of sensor sensitivity as well as output signals. Sensitivity improves about 170 times compare to cross-shaped geometry and about 1.4 times to bridge geometry in conventional measurement system. These values become 2.5 times larger at 20o measurement system. The presented ring geometry may take great potential in Planar Hall effect sensor applications.

NiCo sensing layer for enhanced signals in planar hall effect sensors

NiCo alloy materials have been investigaged as a potential sensing layer for planar Hall effect (PHE) sensors in magnetic multilayer structures. In this study, the magnetoresistive performance of the NiCo alloy is compared with that of the NiFe alloy. With an optimum thickness of 10 nm, the increment of the PHE voltage (Vmax) for the NiCo-based sensor was approximately 1.5 times larger than that of the NiFe-based sensor. The field sensitivity of both sensor types appeared to be nearly equal. However, the dynamic field range for the NiCo sensor was increased by approximately 40% compared with that of the NiFe sensor. The measuring configuration was optimized in order to obtain higher field sensitivity in the sensor. The field sensitivity was measured to be at a maximum at a 20° angle between the easy axis of the sensor and the applied external field, which was approximately three times higher than that in the perpendicular direction.

Electrochemical Synthesis and Characterization of NiFe/Au Multisegmented Nanowires

This paper represents the synthesis and characterization of NiFe single segment and NiFe/Au multisegmented nanowires. We fabricated these nanowires by electrodeposition technique using track etched polycarbonate membrane as a template with a nominal pore diameter of 50 nm. In recent years, much attention has been drawn to magnetic and nonmagnetic multisegmented nanowires because the magnetic properties of the nanowires can be useful for the manipulation and detection of biomolecules in a suspension by the application of relatively small magnetic fields where as the nonmagnetic segment has biocompatibility with high surface functionality. So in this work we studied the growth of NiFe/Au multisegmented nanowires. The length of the nanowires is found to be very uniform. The magnetic properties are measured by VSM. The measured coercivity and saturation magnetization of NiFe/Au multisegmented nanowires are lower than the NiFe nanowires. The structural morphology was observed by SEM. The EDS result shows the purity of both NiFe nanowires and NiFe/Au multisegmented nanowires.