The Surface of Colloidal Metal–Organic Framework Nanoparticles Revealed by Vibrational Sum Frequency Scattering Spectroscopy

Abstract

Solvation shells strongly influence the interfacial chemistry of colloidal systems, from the activity of proteins to the colloidal stability and catalysis of nanoparticles. Despite their fundamental and practical importance, solvation shells have remained largely undetected by spectroscopy. Furthermore, their ability to assemble at complex but realistic interfaces with heterogeneous and rough surfaces remains an open question. Here, we apply vibrational sum frequency scattering spectroscopy (VSFSS), an interface-specific technique, to colloidal nanocrystals with porous metal–organic frameworks (MOFs) as a case study. Due to the porous nature of the solvent-particle boundary, MOF particles challenge conventional models of colloidal and interfacial chemistry. Their multiweek colloidal stability in the absence of conventional surface ligands suggests that stability may arise in part from solvation forces. Spectra of colloidally stable Zn(2-methylimidazolate)2 (ZIF-8) in polar solvents indicate the presence of ordered solvation shells, solvent–metal binding, and spontaneous ordering of organic bridging linkers within the MOF. These findings help explain the unexpected colloidal stability of MOF colloids, while providing a roadmap for applying VSFSS to wide-ranging colloidal nanocrystals in general.

Solvation of Nanoscale Materials

“…2: 𝑊(𝐷) = 𝑊 ! "# + 𝑊 $%$& + 𝑊 '()'*+& + 𝑊 $,*-'.+& (2) where the addition of the Wosmotic term accounts for the repulsive energy caused by neighboring nanoparticles with an increased overlap region (typically due to increased surface sterics), while the Wentropic term accounts for the attractive energy arising from an increase in solvent entropy upon particle coalescence. This entropy-driven aggregation is often termed the hydrophobic effect.…”

“…1 In fact, while traditional surfactants such as dodecanoic acid or cetyltrimethylammonium bromide can be included in a MOF synthesis to impart size or shape control, 131,132 these ligands do not remain with the MOF nanoparticles after washing, as we have observed by nuclear magnetic resonance spectroscopy, thermogravimentric analysis, and other analytical techniques. 1,2 Instead, while post-synthetic addition of ligand dyes 123 or DNA/protein coronas 133 leads to functionalized surfaces, few if any studies detail how they impact colloidal stability. Instead, we recently demonstrated that the solvent identity plays a greater role in stabilizing the surface as only solvents that can dissolve the organic linker can suspend the MOF nanoparticle.…”

“…136 Binding of solvent or polymers to open metal sites at the surface of MOF nanoparticles improve colloidal stability, while also reduce the apparent surface charge. 2,125 In other words, zeta potentials may not relate directly to MOF colloidal stability.…”

“…Our recent VSFSS study indicates that the ordering of solvation shells correlates with colloidal stability, with strong similarities to protein hydration shells and the Hofmeister series. 2 Binding of solvent through covalent or van der Waals forces with monomer units also improves the stability of bare inorganic, organic, or porous colloids. As with the dissolution of proteins, favorable enthalpic interactions between solvent and the components of a porous colloid improve stability, such that solvent that dissolves the constituent components also disperses the particle.…”

“…Take for example, a 5 nm metal-organic framework (MOF) nanoparticle of Zn(2-methylimidazolate)2 dispersed in N,N-dimethylformamide (DMF). 1,2 The particles remain stable in solution without the need for surface ligands to protect against particle aggregation and retain their sizes for weeks or months. In any case, attempts at surface functionalization would face the challenge that MOF particles are mostly empty space.…”