Optochemically Responsive 2D Nanosheets of a 3D Metal–Organic Framework Material

Abstract

chemical tunability, structural flexibility, and ease of exfoliation. [6] There is recent intensifying interest in an emergent class of 2D nanosheets constructed by downsizing 3D metal-organic frameworks (MOFs), [3a,7] which are inorganic-organic (hybrid) structures possessing an enormous physicochemical [8] and structural versatility. [9] Furthermore, the nanoscale porosity of MOFs could function as a vessel to "host" a variety of functional "guest" molecules, [10] imparting a unique combination of properties through intimate host-guest interactions. [11] Yet, preparation of a functionalized MOF as 2D nanosheets exemplifying a tunable host-guest sensing response is uncommon in literature, unlike its traditional counterparts. [12] In this work, we present a simple supramolecular self-assembly strategy to accomplish concomitant 2D nanosheet synthesis and functionalization of a porous MOF system, and demonstrate its efficacy for application as a tunable optochemical sensor. We leverage our recently elucidated supramolecular "high-concentration reactions" (HCR) approach, [13] to realize one-pot synthesis of functionalized MOF nanosheets at ambient conditions; the basic concept is illustrated in Scheme 1. Here we describe a representative study employing 1,4-benzenedicarboxylic acid (BDC) as the organic linker, because of its strong propensity to construct an extended chemical network upon coordination with metal centers; for instance, here we utilized divalent Zn 2+ . Triethylamine base (NEt 3 ) featuring a flexible tripodal geometry was used to trigger fast activation (deprotonation) of the BDC linkers. [14] It is striking to see that, a white gel-like fibrous soft matter was immediately obtained at room temperature (Figure 1a), arising from the HCR between Zn 2+ and BDC 2− , augmented by the NEt 3 + cations. We observed a discernible twostage material transformation via optical microscopy ( Figure 1b): initially witnessing development of highly oriented fibers, prior to formation of a visually shiny phase prevalent on the fiber surfaces. The gel fiber diameter was found to be ≈1-10s µm by scanning electron microscopy (SEM), see Figure 1c. Intriguingly, SEM revealed those supramolecular fibers are, in fact, constituting densely packed crystalline nanosheets (Figure 1d), thus confirming the (shiny) faceted appearance detected under optical microscopy ( Figure 1b). To establish the detailed 2D morphologies, we examined nanosheets harvested from the supramolecular gels using transmission electron micro scopy (TEM) and SEM (Figure 1e,f), as well as by atomic force microscopy (AFM) with which we have estimated the nominal thickness of 2D nanosheets are contemporary materials with exceptional physical and functional properties, derived from a broad class of low-dimensional solids containing atomically thin structures, [1] exfoliated 2D frameworks, [2] and molecular membranes.[3] Considerable efforts are being devoted to 2D graphene-related materials [4] to yield improved mechanical, electronic, and optical modulat...

2D and 3D Metal–Organic Framework at the Oil/Water Interface: A Case Study of Copper Benzenedicarboxylate

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