Superparamagnetic nanoparticles are widely used in biology and medicine for applications which include biomolecule purifications and cell separations, magnetic resonance imaging (MRI) contrast agents, bio-magnetic sensing, magnetic hyperthermia and drug delivery. These nanoparticles are usually synthesized by chemical routes, which are powerful but the size of nanoparticles are typically below 20 nm due to the superparamagnetic size limit. Beyond this size, it is difficult to attain monodispersity and the onset of ferromagnetism results in coercivity, remanent magnetization and consequently magnetically induced agglomeration. Magnetic nanoparticles with higher moments are often desired to produce large signals or to avoid restrictive requirements for high magnetic field gradients in separations. One conventional solution is to incorporate numerous magnetic nanoparticles into larger composites using matrices comprised of dextran or silica. However, there are still limitations associated with controlling the monodispersity, magnetic response and variations in the number and size of the embedded nanoparticles.
In this dissertation, I will present the physical fabrication of sub-100 nm monodisperse disk-shape synthetic nanoparticles with high magnetization ferromagnetic multilayers (e.g. Co-Fe alloy) using nanoimprint lithography (NIL) and high vacuum deposition, followed by release and stabilization of nanoparticles in solution. Antiferromagnetic interlayer interactions are exploited to achieve zero remanence and thus these nanoparticles are termed synthetic antiferromagnetic (SAF) nanoparticles, which posses magnetic moments well above those typical of superparamagnetic nanoparticles.
Unlike the chemical synthesis of magnetic nanoparticles, physical fabrication enables accurate control of particle shape, size and composition, and thus synthetic nanoparticles possess a lot of interesting properties which are not readily accessible to conventional superparamagnetic nanoparticles. For example, I demonstrate SAF nanoparticles with adjustable saturation fields, which are desired for multiplex magnetic labeling in biodetection or multiplex cell sorting. Their high magnetic moments afford great ease for magnetic manipulation in solutions with only modest field gradients, which is highly desired for magnetic sorting. Metallic synthetic nanoparticles strongly scatter light and can be individually tracked in solution under optical microscopy.
To further evaluate their application potential for biomedicine, we performed bio-magnetic detection with streptavidin functionalized SAF nanoparticles. A low concentration of analyte DNA molecules at 10 pM was clearly detectable. MRI measurements of nanoparticle enhanced proton transverse relaxation revealed that SAF nanoparticles are promising as contrast enhancement agents. In addition, hysteresis measurements indicate that magnetic nanoparticles with vortex domain structure (a second type of synthetic nanoparticles) could be efficient heating source for magnetic hyperthermia.
Last but not least, large scale fabrication of SAF nanoparticles with reasonable cost and high throughput is achieved using self-assembled stamps and a polymer sacrificial layer with the assistance of batch-process thermal evaporation. This fabrication technique is ideal for producing multi-modal nanoparticles by exploiting layers with unique magnetic, optical, radioactive, or electronic properties.