  The self-assembly of DNA-coated colloids into highly-ordered structures
offers great promise for advanced optical materials. However, control of
disorder, defects, melting, and crystal growth is hindered by the lack of a
microscopic understanding of DNA-mediated colloidal interactions. Here we use
total internal reflection microscopy to measure in situ the interaction
potential between DNA-coated colloids with nanometer resolution and the
macroscopic melting behavior. The range and strength of the interaction are
measured and linked to key material design parameters, including DNA sequence,
polymer length, grafting density, and complementary fraction. We present a
first-principles model that quantitatively reproduces our experimental data
without fitting parameters over a wide range of DNA ligand designs. Our theory
identifies a subtle competition between DNA binding and steric repulsion and
accurately predicts adhesion and melting at a molecular level. Combining
experimental and theoretical results, our work provides a quantitative and
predictive approach for guiding material design with DNA-nanotechnology and can
be further extended to a diversity of colloidal and biological systems.
