Miniaturized Layer-by-Layer Deposition of Metal–Organic Framework Coatings through Digital Microfluidics

Digital microfluidics (DMF) was demonstrated as a unique tool to scale down and automate the layer-by-layer deposition (LBL) process for depositing patterned metal-organic frameworks (MOF) coatings by making use of the coordinated movement of individual droplets containing the MOF building blocks. HKUST-1, a MOF material consisting of Cu(II) ions and trimesate (1,3,5-benzenetricarboxylate; BTC) linkers, was chosen. In each step, reactant droplets were kept in contact with the substrate to be coated for a few minutes before several washing droplets were used to remove leftover reactants. As could be expected from previous LBL studies, highly uniform dense HKUST-1 coatings are formed that cover the whole surface of the functionalized gold patches. After removing parts of the coating, it can easily be seen that the coating has a uniform thickness of approximately 550 nm after 40 cycles. A similar enhanced increase in layer thickness has been observed in the preparation of HKUST-1 LBL films by spraying the different solutions onto the substrate.