New Means to Control Molecular Assembly

While self-assembly of molecules is relatively well-known and frequently utilized in chemical synthesis and materials science, controlled assembly of molecules represents a new concept and approach. The present work demonstrates the concept of controlled molecular assembly using a nonspherical biomolecule, heparosan tetrasaccharide (MW = 1.099 kD). The key to controlled assembly is the fact that ultrasmall solution droplets exhibit different evaporation dynamics from those of larger ones. Using an independently controlled microfluidic probe in an atomic force microscope, sub-femtoliter aqueous droplets containing designed molecules produce well-defined features with dimensions as small as tens of nanometers. The initial shape of the droplet and the concentration of solute within the droplet dictate the final assembly of molecules because of the ultrafast evaporation rate and dynamic spatial confinement of the droplets. The level of control demonstrated in this work brings us closer to programmable synthesis for chemistry and materials science which can be used to develop vehicles for drug delivery and three-dimensional nanoprinting in additive manufacturing.