Quantitative Mixing of Fluids Using Dip-Pen Nanolithography for Combinatorial Materials Science

Dip-pen nanolithography (DPN) represents a versatile approach for depositing nanoscale quantities of fluids onto surfaces. Here, we show that overwriting─or patterning fluids onto previously deposited features─results in patterns with predictable size and composition that are useful for combinatorial materials experiments. We combine fluorescence microscopy and inertial sensing to show that multiple fluid reservoirs can be mixed together to realize combinatorial libraries of nanoscale features with known mass and composition. As an example of the utility of this approach from a materials discovery perspective, we employ this process to study the mechanics and swelling behavior of polyethylene glycol hydrogels with different compositions. Given that this approach allows one to prepare compositional gradients using less than a microgram of material and functionality to evaluate these using the versatility of the atomic force microscope, it has tremendous potential for the discovery and optimization of performance materials for catalysis, mechanics, photonics, and electronics.