Rsc_cc_c3cc42711k 3..5

Metal–organic frameworks (MOFs) are a class of sophisticated crystalline solids with well-defined coordination geometry and porosity. Rich selections of building blocks, long-range ordering in packing, and superior surface areas have promoted MOFs’ applications in gas adsorption, storage, and separation. While most studies utilized inner pores by regulating their interactions with small molecules, MOFs are rarely reported to bind large molecules, for instance, polymers or proteins. Two major reasons are the limited pore sizes and chemical instability of the frameworks. Though expanded pores have been reported for hosting large molecules such as proteins, most of the MOFs have pore size spanning from several angstroms to a couple of nanometers. The size exclusion prevents large molecules from entering the inner pores, and hence they stick to the outer surfaces only. Chemical instability makes MOFs vulnerable when in contact with liquid media such as water and organic solvent, through ligand exchange processes. A combination of both factors thereforemakes a controlled binding of large molecules challenging. We recently found that defects like cracks or edges in MOFs are effective traps for retaining large molecules like proteins (Fig. 1A). A simple mechanical crushing reveals more active sites and greatly enhances protein adsorption onto particular crystal planes (Fig. 1B–E; ESI†). To push this discovery further, we report sequential binding of large molecules on this same type of MOFs via salt etching, as illustrated in the Table-of