Magnetic nanoparticles (MNPs), including magnetite, are attractive materials for use in a variety of sensing applications. This presentation describes the synthesis of cubic and spherical MNPs having varying dimensions, and compares their fundamental magnetic properties as a function of their size and shape, with the ultimate goal of creating MNP-based sensors with the ability to detect single molecules and/or single binding events/forces. We synthesized Fe3O4 nanospheres and nanocubes having tunable diameters and body diagonals via solvothermal and thermal decomposition synthesis methods, respectively. We tuned the dimensions of the MNPs to obtain samples with equivalent volumes and body diagonals/diameters. The diameters of the polycrystalline spherical MNPs ranged from 100 to 700 nm, and the body diagonals of the monocrystalline cubic MNPs ranged from 100 to 225 nm. We used a vibrating sample magnetometer (VSM) to show that the saturation magnetization of the cubic MNPs was 1.4 to 3 times that observed for the spherical MNPs. For applications relying on both enhanced contact areas and magnetic properties, monocrystalline Fe3O4 nanocubes offered a distinct advantage over polycrystalline Fe3O4 nanospheres with the same-volume or same-body diagonal/diameter, thereby highlighting the importance of shape and crystallinity. More importantly for sensing, high contact areas in the case of nanocubes led to stronger binding on the surface and significantly amplified magnetic signals. We validated this difference using force-induced remnant magnetization spectroscopy (FIRMS), which found for samples subjected to the same force and same initial concentration, that greater numbers of nanocubes remained bound to the sensor surface. In addition, we coated three sizes of spherical Fe3O4 MNPs (average diameter 120, 440, and 700 nm), with silica and subsequently with streptavidin for use in studies of protein-protein interactions. We demonstrated specific protein detection with the Fe3O4 MNPs using the FIRMS technique and evaluation of IgG protein interactions using the related technique known as exchange-induced remnant magnetization (EXIRM). For both spherical and cubic Fe3O4 MNPs, the larger MNPs showed greater binding. Since greater binding translates to stronger signal and enhanced sensitivity, these studies argue for the use of large sizes of MNPs and specifically cubic MNPs rather than spherical MNPs for sensing applications.

Magnetic Fe3O4 nanocubes and nanospheres: Synthesis, properties, and sensing capabilities

254th National Meeting (2017)

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