Soft colloids such as polymer brushes and polymer-coated particles have a wide range of applications and could be used as biomimetic lubricants, for example in artificial implants. “Even though the microscopic properties of these colloids are widely recognized to play a key role in tailoring their macroscopic properties, they so far are poorly addressed in a variety of physically distinct soft and penetrable colloidal systems,” said researcher E. Stiakakis. With colleagues from the Institute of Complex Systems at Forschungszentrum Jülich, he combined smart material preparation and confocal fluorescent microscopy to visualize the microscopic behavior of two-dimensional (2D) lattices of polymer-coated particles.
In the December 2014 issue of Physical Review Letters (DOI: 10.1103/PhysRevLett.113268303), the researchers report grafting fluorescent markers onto the DNA arms of charged DNA-coated particles. By modifying the position of the markers along the arm, the arm extension and the particle shape could be quantified. Furthermore, the researchers can image the way soft colloids deform when they are compressed by the action of a magnetic field – using superparamagnetic particles to control the density of the 2D arrays of particles.
It turns out that the DNA arms get compacted without much entanglement with the arms of the neighboring particles, under pressures approaching the MPa range. This confirms that the low friction between particles comes from the lack of arm entanglement, hence the well-known lubrication properties. The researchers finally tested the defect tolerance of the 2D arrays and found that these accommodate large and small particles alike through particle shrinking and anisotropic deformations of the corona.
E. Eiser, an expert in soft materials at the University of Cambridge working on self-assembly of DNA-functionalized colloids, said that this confirms work done in the 1990s with neutral polymer brushes, whose remarkable lubrication properties are due to the fact that the brushes are being compacted rather than interpenetrating each other. She underlines that one of the challenges facing the field is the highly interdisciplinary nature of the work, where knowledge of synthesis, biology, and physics is required if these new findings are to be applied to real-world problems.
The DNA brushes have wide functionalization possibilities and could be used in new electronic or photonic applications: “The DNA brushes can be end-functionalized with many different types of markers. This could be employed in multiplex detection applications in sensors. The mechanical strength of their corona could be used in materials science to design large-area photonic or plasmonic arrays,” the researchers report.
