Liquid-Phase Molecular-Ionic Layer Deposition of Surface-Supported Metal-Organic Frameworks of Cu_3(BTC)_2 HKUST-1 for Electrochemical Sensors

摘要

Metal-organic frameworks (MOFs) are traditionally developed as effective gas separation/storage materials, however, recently they are receiving increasing attention as multifunctional layers in electronic devices. MOFs can possess unique combinations of properties derived from complex interaction between inorganic and organic constituents that can provide better functionality in comparison to conventional materials. Recently, applications of electrochemically active MOFs for small molecules sensing, such as glucose, ethanol, NADH and H_2O_2 have been demonstrated. However, MOFs are still rarely used in electrochemical sensors, because fabrication of thin films of MOFs and their integration into electronic devices is challenging due to incompatibility of MOFs with conventional physical or chemical vacuum deposition methods, while conventional solvent-assisted methods, such as spin-coating and die-coating, or solvothermal methods are difficult to realize in controllable manner due to extremely low solubility of MOFs. From the other hand, the low solubility of MOFs becomes an advantage in the case of liquid-phase molecular ionic layer deposition (LP-MILD) method, which belongs to the family of layer-by-layer deposition methods similar to the atomic layer deposition (ALD). In the LP-MILD method the growing film is sequentially exposed to solutions of reactive precursors mediated by washing steps that remove excess of precursors and reaction byproducts, and effectively confine the reaction space to only one surface monolayer of the growing film. As a result, the film grows in steps of a half monolayer per process cycle, which allows easy control over film thickness and composition, including heteroepitaxy and/or doping. The LP-MILD method can be technically realized in different flavors, where sequential substrate exposures to precursor solutions and washing steps are facilitated by dip-coating, spray-coating or spin-coating.