We report the formation of colloidal macromolecules consisting of chains of micron-sized paramagnetic particles assembled using a magnetic field and linked with DNA. The interparticle spacing and chain flexibility was controlled by varying the magnetic field strength and the linker spring constant. Variations in the DNA lengths allowed for the generation of chains with an improved range of flexibility, as compared to previous studies. These chains adopted the rigid-rod, semi-flexible, and flexible conformations that are characteristic of linear polymer systems. These assembly techniques were investigated to determine the effects of the nanoscale DNA linker properties on the properties of the microscale colloidal chains. With stiff DNA linkers (564 base pairs) the chains were only stable at moderate to high field strengths and produced rigid chains. For flexible DNA linkers (8000 base pairs), high magnetic field strengths caused the linkers to be excluded from the gap between the particles, leading to a transition from very flexible chains at low field strengths to semi-flexible chains at high field strengths. In the intermediate range of linker sizes, the chains exhibited predictable behavior, demonstrating increased flexibility with longer DNA linker length or smaller linking field strengths. This study provides insight into the process of directed assembly using magnetic fields and DNA by precisely tuning the components to generate colloidal analogues of linear macromolecular chains.

The anisotropy of dipolar interactions can sometimes be a hindrance when assembling colloids, as it limits the diversity of structures that can be manufactured. Here we demonstrate that a mixture of paramagnetic and diamagnetic colloids in a ferrofluid can be used to create a variety of fractal aggregates in the presence of a field. These aggregates exhibit growth both parallel and perpendicular to the field, a distinct departure from the linear chains that are typical of dipolar assembly. The fractal dimension of these aggregates displays a parabolic character as the ferrofluid concentration is increased and varies between 0.94 AE 0.03 and 1.54 AE 0.03-a wider range than that which is seen when colloids are assembled using shortrange forces. This behavior is explained by examining how the ferrofluid concentration affects the relative strength of the dipolar interactions between each type of particle. These dipolar fractal aggregates may find use in the study of gelation via long-range forces or the preparation of gels that can be activated using an external field.

Self-assembly is a spontaneous process in which small objects could combine to form larger and more complex constructs. Here we present experiments on how to use two sequence-designed single stranded nucleotideoligos (50 bases) to form fairly long (>500 base pairs) double stranded DNA concatemers by hybridization due to the recognition of complementary bases on the two strands. To optimize the construction of long DNA concatemers, the incubation conditions have been varied, such as oligo concentration, salt concentration, hybridization time and presence of congesting agents as poly vinyl alcohol. Formed concatemers were characterized by gel-electrophoresis and Atomic Force Microscopy (AFM) to investigate the size and shape distribution.