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A Janus particle is a nanoparticle with a surface that has two or more different physical properties. This allows multiple types of chemistry to occur on the same particle. The Janus particles produced by this new microfluidic technique have both fluorescent and magnetic properties, which makes them a powerful tool for bioassays. Not only can the particles be collected by magnetic separation, but the degree of conjugation can be monitored by fluorescence detection.

Quantum dots (QDs)

Quantum dots (QDs) are an exciting new development. They are nanocrystals that are quantum-confined by their size. This means that certain sizes of quantum dots are only capable of emitting one wavelength of light. Therefore, quantum dots can be synthesized to fluoresce sharply at a single color regardless of how much light they absorb. Combining the properties of quantum dots with the properties of magnetic nanoparticles creates a powerful bioassay tool.

Synthesis of QD-doped magnetic nanoparticles (MNPs) has been attempted before by using layer-by-layer assembly or encapsulation techniques. These techniques have not been very successful. One problem is that the QDs can leak out in harsh chemical environments. Another problem is that encapsulation techniques are time-consuming and result in particles with a very large size distribution. A large size distribution is undesirable because it introduces variability into the magnetic separation process and can lead to irreversible bead aggregation. A narrow size distribution and uniform particle size is extremely desirable. A new microfluidic technique for the generation of QD magnetic nanoparticles with a narrow size distribution has recently been developed.

All microfluidic techniques allow for controlled preparation of particles by combining a microfluidic emulsification with a solidification mechanism. Many different types of bead coatings are compatible with this technique. However, the solidification mechanisms used in the past relied on UV exposure or heating treatment.

Alginate hydrogel chemistry

This new technique does not use excessive heat or any UV treatment, but instead relied on alginate hydrogel chemistryBriefly, Fe3O4 nanoparticles and CdSe/ZnS QDs are dispersed in separate alginate solutions. The two solutions flow through separate channels and then join together as a co-flow in a focusing channel. The solutions don’t mix, they simple flow together. A third continuous flow orthogonal to the co-flow injects a solution of CaCl2, oleic acid, and a small amount of isobutanol. When the third channel hits the co-flow the interaction creates small janus droplets by a shear-rupturing mechanism. The size of the particles is quite uniform, and can be controlled by adjusting the continuous flow rate. As the droplets continue to flow down the channel the Ca2+ diffuses into the droplets and ionically crosslinks the sodium alginate, which causes them to solidify. Alginate hydrogel is a biocompatible material, and the surface contains carboxyl groups that will conjugate to biomolecules to allow surface functionalization of the microbeads.

After the QD-MNP Janus particles were synthesized they were tested in a DNA assay. The particles were functionalized with a complementary coding sequence and incubated with the DNA. The DNA had a green fluorescent molecule. The Janus particle had a red fluorescent quantum dot. After incubation the DNA-particle conjugates were collected by magnetic separation. The isolate was viewed under a fluorescent microscope to examine binding efficiency. It was found that the DNA bound properly because the green fluorescence co-localized with the red fluorescence. The new synthesis technique is easy and functional, and could become a very useful bioassay tool.

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Lluis M. Martínez | SEPMAG Chief Scientific Officer

Founder of SEPMAG, Lluis holds a PhD in Magnetic Materials by the UAB. He has conducted research at German and Spanish academic institutions. Having worked in companies in Ireland, USA and Spain, he has more than 20 years of experience applying magnetic materials and sensors to industrial products and processes. He has filed several international patents on the field and co-authored more than 20 scientific papers, most of them on the subject of magnetic particle movement.

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