Abstract
We report a magnetofluidic device with integrated strong ferromagnetically-coupled and hysteresis-free spin valve sensors for dynamic monitoring of ferrofluid droplets in microfluidics. The strong ferromagnetic coupling between the free layer and the pinned layer of spin valve sensors is achieved by reducing the spacer thickness, while the hysteresis of the free layer is eliminated by the interplay between shape anisotropy and the strength of coupling. The increased ferromagnetic coupling field up to the remarkable 70 Oe, which is five-times larger than conventional solutions, brings key advantages for dynamic sensing, e.g., a larger biasing field giving rise to larger detection signals, facilitating the operation of devices without saturation of the sensors. Studies on the fundamental effects of an external magnetic field on the evolution of the shape of droplets, as enabled by the non-visual monitoring capability of the device, provides crucial information for future development of a magnetofluidic device for multiplexed assays.
Highlights
Modern medical research calls for the development of new technologies or platforms
The optimized spin valve sensors integrated in a microfluidic device allow the detection of ferrofluid droplets with about six-times enhanced signal responses compared to the commonly-used giant magnetoresistive (GMR) multilayers with the same measurement configuration [24]
To optimize the coupling strength aiming at the increase of the biasing point of the sensor as needed for dynamic detection, we prepared a series of spin valve sensors with different thicknesses of the Cu spacer in the range from 1.8 nm to 3.0 nm (Figure 1a)
Summary
Modern medical research calls for the development of new technologies or platforms. Droplet microfluidics is a prominent example of such new platforms and has picked up momentum in recent years [1]. The free layer and the pinned layer are usually weakly coupled, so as to bring the sensing point to a near zero magnetic field [29] In this respect, in order to detect superparamagnetic nanoparticles or beads, which are frequently used as the carrier materials for labeling or manipulation of species in fluidics, an external out-of-plane magnetic field is applied to induce net magnetic moments. We report a magnetofluidic platform for in-flow detection of ferrofluid droplets in microfluidics For this purpose, we optimize the performance of the integrated spin valve sensor device to be hysteresis free and to possess a biasing point shifted to a remarkable 70 Oe, which is at least five-times larger than applied previously [23]. The optimized spin valve sensors integrated in a microfluidic device allow the detection of ferrofluid droplets with about six-times enhanced signal responses compared to the commonly-used GMR multilayers with the same measurement configuration [24]. The fundamental understanding provides crucial information for the future design of micro-magnetofluidic devices for multiplexed magnetic barcoding assays
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