Sabina Rodríguez-Hermida,1 Min Ying Tsang,2 Claudia Vignatti,1 Kyriakos C. Stylianou,1 Vincent Guillerm,1 Javier Pérez-Carvajal,1 Francesc Teixidor,2 Clara Viñas,2 Duane Choquesillo-Lazarte,3 Cristóbal Verdugo-Escamilla,3 Inmaculada Peral,4 Jordi Juanhuix,5 Albert Verdaguer,1 Inhar Imaz,1 Daniel Maspoch1,6 and José Giner Planas2
Material science and engineering is playing a key role in the necessary move towards a new energy paradigm. Smart solid surfaces that exhibit switchable wettability – mainly between high or superhydrophobicity, and superhydrophilicity – are actively being sought due to their diverse potential applications such as self-cleaning materials, microfluidics, tunable optical lenses, drug delivery vectors, and sensors. We now report for the first time that metal-organic frameworks (MOFs) (also known as porous coordination polymers [PCPs]) can be a novel class of responsive materials that exhibit switching of their crystal surface, between hydrophobic to superhydrophilic. MOFs are porous materials that are composed of two major components: a metal ion or cluster of metal ions and an organic molecules called linkers. One of the major problems of these materials is their instability in water, something that hampers their application. In order to solve this problem, we have designed a new hydrophobic linker, based on an icosahedral carborane cluster (oCB-L, CB:C2B10H10). The new MOF was assembled from ZnII ions, 1,4-benzenedicarboxylate and the hydrophobic carborane-based linker (oCB-L), which connects the constituent Zn4(bdc)2 2D layers to yield the 3D network of the final MOF, oCB-MOF-1 (Figure). This MOF is non-porous to N2 at 77 K and 1 bar, but it is porous to CO2 (69.4 cm3g-1 at 0.9 bar; BET surface area: 296 m2g-1) at 195 K and 0.8 bar. oCB-MOF-1 is stable in water for over 15h and its channels are highly hydrophobic as it barely adsorbs water-vapors (0.05 g H2O/g activated oCB-MOF-1 at 95% relative humidity). Moreover, as the internal surface, the external surface of the MOF is also highly hydrophobic. We found that the carborane moieties confer enhanced hydrophobicity to this MOF as shown by its high contact angle (Ɵc = 140°; Figure). A switching behavior between hydrophobic and hydrophilic is achieved by alternatively exposing the MOF to NaOH/DMF solution and to slightly acidic aqueous solution (Figure). Such switching phenomena can be explained by the selective removal of hydrophobic carborane layers (under base treatment) or Zn4(bdc)2 layers (in aqueous solution). Given the versatile compositions of MOFs, and the fact that they can be fabricated by design, we hope that our work here facilitates development of MOFs with reversible wettability properties triggered by stimuli such as light, temperature or pH. Such materials would ultimately prove utile for obtaining smart porous surfaces (e.g. membranes and coatings) that exhibit switchable wettability.
1Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC & BIST, Spain.
2Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain.
3Laboratorio de Estudios Cristalográficos, IACT, CSIC-Universidad de Granada, Spain.
4Campus Limpertsberg, Université du Luxembourg, Luxembourg.
5Alba Synchrotron Light Facility, Spain.
6Institució Catalana de Recerca i Estudis Avançats (ICREA), Spain.