Probing the stability of magnetically assembled DNA-linked colloidal chains

Nanoparticle self-assembly: from interactions in suspension to polymer nanocomposites

Recent experimental results using in particular small-angle scattering to characterize the self-assembly of mainly hard spherical nanoparticles into higher ordered structures ranging from fractal aggregates to ordered assemblies are reviewed. The crucial control of interparticle interactions is discussed, from chemical surface-modification, or the action of additives like depletion agents, to the generation of directional patches and the use of external fields. It is shown how the properties of interparticle interactions have been used to allow inducing and possibly controlling aggregation, opening the road to the generation of colloidal molecules or potentially metamaterials. In the last part, studies of the microstructure of polymer nanocomposites as an application of volume-spanning and stress-carrying aggregates are discussed.

Formation of printable granular and colloidal chains through capillary effects and dielectrophoresis

One-dimensional conductive particle assembly holds promise for a variety of practical applications, in particular for a new generation of electronic devices. However, synthesis of such chains with programmable shapes outside a liquid environment has proven difficult. Here we report a route to simply ‘pull' flexible granular and colloidal chains out of a dispersion by combining field-directed assembly and capillary effects. These chains are automatically stabilized by liquid bridges formed between adjacent particles, without the need for continuous energy input or special particle functionalization. They can further be deposited onto any surface and form desired conductive patterns, potentially applicable to the manufacturing of simple electronic circuits. Various aspects of our route, including the role of particle size and the voltages needed, are studied in detail. Looking towards practical applications, we also present the possibility of two-dimensional writing, rapid solidification of chains and methods to scale up chain production.

Conductive colloidal chains are promising for electronics but difficult to synthesize outside of a liquid environment. Here, the authors use field-directed assembly and capillary effects to pull conductive particle chains out of a suspension, which remain held together by flexible liquid bridges even after the external field is turned off.

Multidirectional colloidal assembly in concurrent electric and magnetic fields

Dipolar interactions between nano- and micron sized colloids lead to their assembly into domains with well-defined local order. The particles with a single dipole induced by an external field assemble into linear chains and clusters. However, to achieve the formation of multidirectionally organized nano- or microassemblies with tunable physical characteristics, more sophisticated interaction tools are needed. Here we demonstrate that such complex interactions can be introduced in the form of two independent, non-interacting dipoles (double-dipoles) within a microparticle. We show how this can be achieved by the simultaneous application of alternating current (AC)-electric field and uniform magnetic field to dispersions of superparamagnetic microspheres. Depending on their timing and intensity, concurrent electric and magnetic fields lead to the formation of bidirectional particle chains, colloidal networks, and discrete crystals. We investigate the mechanistic details of the assembly process, and identify and classify the non-equilibrium states formed. The morphologies of different experimental states are in excellent correlation with our theoretical predictions based on Brownian dynamics simulations combined with a structural analysis based on local energy parameters. This novel methodology of introducing and interpreting double-dipolar particle interactions may assist in the assembly of colloidal coatings, dynamically reconfigurable particle networks, and bidirectional active structures.

Paper-based rapid detection of pork and chicken using LAMP–magnetic bead aggregates

Magnetic beads (MBs) have been widely used for DNA quantification.

Micro-mutual-dipolar model for rapid calculation of forces between paramagnetic colloids

Typically, the force between paramagnetic particles in a uniform magnetic field is calculated using either dipole-based models or the Maxwell stress tensor combined with Laplace's equation for magnetostatics. Dipole-based models are fast but involve many assumptions, leading to inaccuracies in determining forces for clusters of particles. The Maxwell stress tensor yields an exact force calculation, but solving Laplace's equation is very time consuming. Here, we present a more elaborate dipole-based model: the micro-mutual-dipolar model. Our model has a time complexity that is similar to that of other dipole-based models but is much more accurate especially when used to calculate the force of small aggregates. Using this model, we calculate the force between two paramagnetic spheres in a uniform magnetic field and a circular rotational magnetic field and compare our results with those of other models. The forces for three-particle and ten-particle systems dispersed in two-dimensional (2D) space are examined using the same model. We also apply this model to calculate the force between two paramagnetic disks dispersed in 2D space. The micro-mutual-dipolar model is demonstrated to be useful for force calculations in dynamic simulations of small clusters of particles for which both accuracy and efficiency are desirable.

Directing assembly of DNA-coated colloids with magnetic fields to generate rigid, semiflexible, and flexible chains

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 were 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, semiflexible, 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 semiflexible 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.

Numerical calculation of interaction forces between paramagnetic colloids in two-dimensional systems

Typically the force between paramagnetic particles in a uniform magnetic field is described using the dipolar model, which is inaccurate when particles are in close proximity to each other. Instead, the exact force between paramagnetic particles can be determined by solving a three-dimensional Laplace's equation for magnetostatics under specified boundary conditions and calculating the Maxwell stress tensor. The analytical solution to this multi-boundary-condition Laplace's equation can be obtained by using a solid harmonics expansion in conjunction with the Hobson formula. However, for a multibody system, finite truncation of the Hobson formula does not lead to convergence of the expansion at all points, which makes the approximation physically unrealistic. Here we present a numerical method for solving this Laplace's equation for magnetostatics. This method uses a smoothed representation to replace all the boundary conditions. A two-step propagation is used to dramatically accelerate the calculation without losing accuracy. Using this method, we calculate the force between two paramagnetic particles in a uniform and a rotational external field and compare our results with other models. Furthermore, the many-body effects for three-particle, ten-particle, and 24-particle systems are examined using the same method. We also calculate the interaction between particles with different magnetic susceptibilities and particle diameters. The Laplace's equation solver method described in this article that is used to determine the force between paramagnetic particles is shown to be very useful for dynamic simulations for both two-particle systems and a large cluster of particles.

New detection modality for label-free quantification of DNA in biological samples via superparamagnetic bead aggregation

Combining DNA and superparamagnetic beads in a rotating magnetic field produces multiparticle aggregates that are visually striking, enabling label-free optical detection and quantification of DNA at levels in the picogram per microliter range. DNA in biological samples can be quantified directly by simple analysis of optical images of microfluidic wells placed on a magnetic stirrer without prior DNA purification. Aggregation results from DNA/bead interactions driven either by the presence of a chaotrope (a nonspecific trigger for aggregation) or by hybridization with oligonucleotides on functionalized beads (sequence-specific). This paper demonstrates quantification of DNA with sensitivity comparable to that of the best currently available fluorometric assays. The robustness and sensitivity of the method enable a wide range of applications, illustrated here by counting eukaryotic cells. Using widely available and inexpensive benchtop hardware, the approach provides a highly accessible low-tech microscale alternative to more expensive DNA detection and cell counting techniques.

DNA bimodified gold nanoparticles

We report a general approach to bimodify gold nanoparticles (GNPs) with two different DNA strands via DNA template reaction. Two thioctic acid modified DNA strands, one at 5' end and one at 3' end, were attached to GNPs through bivalent thiol-gold bond. By sequence design, assemblies of 5 nm GNPs chains, 10 nm GNPs chains and alternative arrangement of 5 and 10 nm GNPs could be achieved. Gel electrophoresis, transmission electron microscope (TEM), UV-vis spectra were used to characterize the assemblies. It is believed that this new kind of bimodified GNPs with two different DNA strands at different ends would enrich the toolbox of DNA-GNP conjugates and provide diverse selectivity for further assembly.

Axial thermal rotation of slender rods

Axial rotational diffusion of rodlike polymers is important in processes such as microtubule filament sliding and flagella beating. By imaging the motion of small kinks along the backbone of chains of DNA-linked colloids, we produce a direct and systematic measurement of axial rotational diffusivity of rods both in bulk solution and near a wall. The measured diffusivities decrease linearly with the chain length, irrespective of the distance from a wall, in agreement with slender-body hydrodynamics theory. Moreover, the presence of small kinks does not affect the chain's axial diffusivity. Our system and measurements provide insights into fundamental axial diffusion processes of slender objects, which encompass a wide range of entities including biological filaments and linear polymer chains.