Magnetic Control and Optical Imaging of Nanoparticles for Biosensing

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

0853963 S. Majetich Intellectual Merit. The proposed research program will develop two types of plasmonic magnetic particles, and investigate the ability to control the motion of single particles magnetically while imaging them optically. Though these particles are smaller than the optical diffraction limit, their positions can be tracked by dark field optical microscopy through the surface plasmon resonance of the gold coating, or by fluorescence microscopy when fluorophores are bound to the particle surface. The rotation of rod-shaped particles will be monitored from the polarization of the longitudinal surface plasmon mode. Magnetically guided translation of nanoparticles is challenging because both viscous drag forces and random Brownian diffusion forces become significant for small sizes. Peclet number analysis suggests that it should be easier to control the motion of nanorods than nanospheres of the same volume, but current models of nanorod diffusion do not agree quantitatively. Another complicating factor is aggregate formation in aqueous dispersions. While electron microscopy shows highly monodisperse particle cores, the dispersions in water or biological media may contain aggregates with a broad size distribution. For single particle-based sensing as well as magnetic hyperthermia applications, the magnetic response will depend on the size of the aggregate, and optimal control requires uniform samples. We will coat 35-50 nm nominally spherical magnetite particles with gold, and then with polymers to form stable dispersions in phosphate buffered saline (PBS) solution. Our nanorod samples will be based on uniform but non-magnetic hematite nanorods ~300 nm long and 20 nm in diameter, that are reactively coated with thin layers of magnetite prior to the gold and polymer coating stages. The size of the agglomerates in aqueous and PBS dispersions will be determined from dynamic light scattering (DLS). The objective of the synthetic portion of the proposed research will be to prepare highly uniform, minimally agglomerated particles that show rapid magnetic response and have a strong surface Plasmon resonance. We will investigate the translational and rotational dynamics as a function of the applied field and field gradient first macroscopically by the AC magnetic susceptibility, and for the nanorods, optical modulation of the plasmon spectra. We will also construct an optical microscope cell with magnetic control for single nanoparticle and nanorod translation and rotation studies using dark field optical microscopy and fluorescence microscopy. The results will be compared with the predictions of ellipsoidal and cylindrical models of nanorod dynamics. In the final phase of the proposed work, these results will be used to explore new biosensing approaches where magnetic fields are used to move the particles and modulate their optical response. We will investigate magnetically guided translation within living cells to probe local viscosity and pH, and magnetically induced rotation to monitor selective binding events through the sensitivity of fluorescence intensity to the surface plasmons of the nanorods. The Broader Impact aims will be linked with the research aims of the proposed program. A graduate student will be co-advised by the PIs. There will also be several undergraduate research projects involving magnetic, optical, and light scattering measurements on the particle dispersions. We have already recruited an African-American undergraduate to work on this project. A hands-on experimental module on ferrofluids and magnetic forces that will be developed for use in the Engineering Your Future program at Carnegie Mellon that targets middle school and high school girls, and also provides the opportunity to disseminate information to science teachers in the Pittsburgh Public Schools. This connection will be used to recruit a middle school teacher to work with us in creating age-appropriate classroom materials, demonstrations, and activities based on colloidal stabilization forces in both ferrofluids and plasmonic sols.