Differential Magnetic Catch and Release: Separation, Purification, and Characterization of Magnetic Nanoparticles and Particles Assemblies

Open Access
Beveridge, Jacob Stanley
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
April 24, 2012
Committee Members:
  • Mary Beth Williams, Dissertation Advisor
  • Raymond Edward Schaak, Committee Member
  • Benjamin James Lear, Committee Member
  • Siyang Zheng, Committee Member
  • Magnetic Nanoparticles
  • Differential Magnetic Catch and Release
  • Hybrid Nanocrystals
  • Nanoparticles
Magnetic nanoparticles uniquely combine superparamagnetic behavior with dimensions that are smaller than or the same size as molecular analytes. The integration of magnetic nanoparticles with analytical methods has opened new avenues for sensing, purification, and quantitative analysis. Applied magnetic fields can be used to control the motion and properties of magnetic nanoparticles; in analytical chemistry, use of magnetic fields provides methods for manipulating and analyzing species at the molecular level. The ability to use applied magnetic fields to control the motion and properties of magnetic nanoparticles is a tool for manipulating and analyzing species at the molecular level, and has led to applications including analyte handing, chemical sensors, and imaging techniques. This is clearly an area where significant growth and impact in separation science and analysis is expected in the future. In Chapter 1, we describe applications of magnetic nanoparticles to analyte handling, chemical sensors, and imaging techniques. Chapter 2 reports the purification and separation of magnetic nanoparticle mixtures using the technique developed in our lab called differential magnetic catch and release (DMCR). This method applies a variable magnetic flux orthogonal to the flow direction in an open tubular capillary to trap and controllably release magnetic nanoparticles. Magnetic moments of 8, 12, and 17 nm diameter CoFe¬2¬O¬4 nanoparticles are calculated using the applied magnetic flux density and experimentally determined force required to trap 50% of the particle sample. Balancing the relative strengths of the drag and magnetic forces enable separation and purification of magnetic CoFe2O4 nanoparticle samples with < 20 nm diameters. Samples were characterized by transmission electron microscopy to determine the average size and size dispersity of the sample population. DMCR is further demonstrated to be useful for separation of a magnetic nanoparticle mixture, resulting in samples with narrowed size distributions. Differential magnetic catch and release has been used as a method for the purification and separation of magnetic nanoparticles. In Chapter 3 the separation metrics are reported. DMCR separates nanoparticles in the mobile phase by magnetic trapping of magnetic nanoparticles against the wall of an open tubular capillary wrapped between two narrowly spaced electromagnetic poles. Using Au and CoFe2O4 nanoparticles as model systems, the loading capacity of the 250 m diameter capillary is determined to be ~130 g, and is scalable to higher quantities with larger bore capillary. Peak resolution in DMCR is externally controlled by selection of the release time (Rt) at which the magnetic flux density is removed, however longer capture times are shown to reduce the capture yield. In addition, the magnetic nanoparticle capture yields are observed to depend on the nanoparticle diameter, mobile phase viscosity and velocity, and applied magnetic flux. Using these optimized parameters, three samples of CoFe2O4 nanoparticles whose diameters are different by less than 10 nm are separated with excellent resolution and capture yield, demonstrating the capability of DMCR for separation and purification of magnetic nanoparticles. Individual hybrid nanocrystals possess multiple structural units with solid state interfaces, giving them a wide range of possible applications. Synthesis of truly monodisperse nanoparticles and hybrid nanocrystals is a formidable task, which has led us to apply our analytical technique, differential magnetic catch and release, to separate and purify magnetic nanoparticles. Using an open tubular capillary column and electromagnet, DMCR separates magnetic nanoparticles based on a balance of their magnetic moment and hydrodynamic size. Chapter 4 focuses on the purification of real world samples of hybrid nanocrystals including Au-Fe3O4 heterostructures and FePt-Fe3O4 dimers. Samples are characterized with transmission electron microscopy, UV-Vis, X-ray diffraction spectroscopy, selected area electron diffraction, electron dispersive X-ray spectroscopy, and superconducting quantum interference device magnetometry. After DMCR purification, the hybrid nanocrystals have distinct properties, suggesting that purification is vital for the proper characterization and utilization of these in new applications. As magnetic nanostructures become more complex, development of new separation/purification tools in parallel with optimizing hybrid nanocrystal syntheses is paramount for nanostructure construction and use. Probing the kinetics of nanoparticle nucleation, crystallization, and oxidation has historically been difficult to achieve. In Chapter 5 preliminary results using DMCR to monitor chemical reactions involving nanoparticles are discussed. Many characteristics of nanoparticles affect their magnetic properties, i.e. size, composition, crystallinity, phase purity etc. DMCR thus provides a method to assess these characteristics during a chemical reaction, and plot the kinetics of these transitions. Insights into nanoscale reaction kinetics will allow mechanistic insights for designing high quality nanocrystal syntheses that produce well defined pure products and hybrid materials. As a proof of concept, the crystallization of Fe3O4 is tracked using X-Ray diffraction analysis while the nanoparticle morphology is examined using transmission electron microscopy. In conjunction with these well-established techniques, DMCR is used to quantitatively assess the crystallization by magnetically tracking the magnetic properties as the reactions progresses, as well as imparting mechanistic insight into their architectural transformations. Chapter 6 presents unpublished data regarding using magnetic nanoparticles for magnetically manipulated/magnetic bursting of microcapsules, magnetic manipulation through microfluidic channels, non-spherical synthesis of magnetic nanoparticles, and DMCR of hollow Fe3O4 nanoparticles. Also discussed are ideas for the future improvement of DMCR to address a more inclusive size regime and composition of magnetic nanoparticles.