Continuous, One-pot Synthesis and Post-Synthetic Modification of NanoMOFs Using Droplet Nanoreactors

Abstract

Metal-organic frameworks (MOFs); also known as porous coordination polymers (PCP) are a class of porous crystalline materials constructed by connecting metal clusters via organic linkers. The possibility of functionalization leads to virtually infinite MOF designs using generic modular methods. Functionalized MOFs can exhibit interesting physical and chemical properties including accelerated adsorption kinetics and catalysis. Although there are discrete methods to synthesize well-defined nanoscale MOFs, rapid and flexible methods are not available for continuous, one-pot synthesis and post-synthetic modification (functionalization) of MOFs. Here, we show a continuous, scalable nanodroplet-based microfluidic route that not only facilitates the synthesis of MOFs at a nanoscale, but also offers flexibility for direct functionalization with desired functional groups (e.g., -COCH₃, fluorescein isothiocyanate; FITC). In addition, the presented route of continuous manufacturing of functionalized nanosized MOFs takes significantly less time compared to state-of-the-art batch methods currently available (1 hr vs. several days). We envisage our approach to be a breakthrough method for synthesizing complex functionalized nanomaterials (metal, metal oxides, quantum dots and MOFs) that are not accessible by direct batch processing and expand the range of a new class of functionalized MOF-based functional nanomaterials.

Figure 1. MOF synthesis and post-synthetic modifications via different routes.

Direct comparison of our microfluidic nanodroplet route with conventional direct and post-synthetic modification of UIO-66-NH₂.

Figure 2. Microfluidic chip for nanoMOF synthesis and post-synthetic modification.

(a) Droplet based nanoreactors formation using PDMS based microfluidic chip; (b) Automated experimental setup to drive microfluidic chip for synthesis and functionalization of nano-sized MOFs and (c) Design of microfluidic chip that shows ports and important parts of the chip, flow-focusing junction and winding channel mixer. The chip consists of four ports, feeding oil (O) port, metal salt (MS), organic ligand (OL) and modulator (M) ports and one exit port to transport mixed droplet nanoreactors to a residence tube for subsequent MOF synthesis and PSM inside these nanoreactors.

Figure 3. Characterization of nanoMOFs.

(a) Comparison of PXRD patterns of UIO-66-NH₂ and its amino analog synthesized through microfluidic device and batch method; (b) Nitrogen adsorption isotherms of UIO-66-NH₂ MOF synthesized using microfluidic system; (c) SEM images of (i) UIO-66-NH₂ (ii) UIO-66-NH-COCH₃ and (iii) UIO-66-NH-FITC MOFs synthesized from the microfluidic system.

Figure 4. Amino functionalized nanoMOFs.

The FT-IR spectrum showing three prominent parts– skeletal peaks related to MOF, CO peaks, NH and NH₂ peaks of UIO-66- and UIO-66-NHCOCH₃.

Figure 5. FITC Functionalized nanoMOFs.

(a) Schematic illustration of FITC functionalization onto UIO-66-NH₂. (b) Fluorescent emission spectra of FITC, FITC tagged to UIO-66-NH₂ and FITC tagged to linker amino BDC.