Controlled Molecular Assembly via Dynamic Confinement of Solvent

Abstract

Assembly from ultrasmall solution droplets follows a different dynamic from that of larger scales. Using an independently controlled microfluidic probe in an atomic force microscope, subfemtoliter aqueous droplets containing polymers 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 play significant roles in the final assembly of polymers due to the ultrafast evaporation rate and spatial confinement by the small droplets. These effects are used to control the final molecular assembly in terms of feature geometry and distribution and packing of individual molecules within the features. This work introduces new means of control over molecular assembly, bringing us closer to programmable synthesis for chemistry and materials science. The outcomes pave the way for three-dimensional (3D) nanoprinting in additive manufacturing.

Controlled Molecular Assembly of Tetrazine Derivatives on Surfaces

“…The concept of controlled assembly using small liquid droplets was first reported by our team using macromolecules (e.g., star polymers). 13 This result extends the concept to a much smaller molecule, PEG-bisTz, which has a hydrodynamic radius of 1.5 nm.…”

“…This setup enabled delivery of aqueous droplets as small as 0.4 attoliter (aL). 13 First, we demonstrated that the geometry and size of the individual and concrete assemblies can be controlled with PEG-bisTz molecules tightly packed within. Under a contact force of 40 nN, delivery pressure of 200 mbar, and contact time of 0.2 s, 250 aL droplets of PEG-bisTz solution were delivered on the AAPTMS SAM surfaces, as shown in Figure 2a.…”

“…31 SAMmodified glass slides were prepared as follows: slides were heated at 70−80°C in a sealed Teflon container (100 mL) containing 200 μL of AAPTMS for 2 h, then rinsed with ethanol and MQ water, and then dried again under nitrogen gas. 13 PEG-bisTz coated glass slides were prepared as follows: 20 μL of 2.5 × 10 −4 M PEG-bisTz aqueous solution was deposited by 100 μL Eppendorf pipette (Sigma-Aldrich, St. Louis, MO) on AAPTMS-modified glass surfaces and dried under ambient conditions for 30 min.…”

“…Typical probesurface contact during delivery is also shown in Figure 1, where the contact force was measured and controlled via similar means as conventional AFM with a deflection configuration. 13,32,35 The square pyramidal tip tilted 11°from the surface normal. The nanopipette probe had a 300 nm diameter pore located at the probe apex connected to a microchannel within the probe body.…”

“…Controlled assembly to produce zero-dimensional (0D) assemblies has been demonstrated by our team using macromolecules, including star polymers [molecular weight (M w ) of 383 kDa, hydrodynamic radius of 13.8 nm]. 13 The study indicated that factors governing the solute assembly in ultrasmall droplets significantly differs from that of larger droplets because ultrasmall droplets exhibit ultrafast evaporation; and therefore, the initial droplet geometry and concentration of the solute play dominant roles in dictating the final assembly of solute molecules. The applicability of this methodology to the assembly of low M w molecules, such as small linear polymers, has become our new task.…”