Ultrasensitive and highly selective detection of formaldehyde via an adenine-based biological metal–organic framework

Abstract

Formaldehyde poses a carcinogenic risk to human beings, yet it would be difficult to accurately quantify by spectroscopic methods if other volatile organic compounds were present. Herein, we report a robust adenine-based BioMOF (JNU-100) constructed through a mixed-ligand strategy with BPDC-(NH₂)₂, an amine-functionalized dicarboxylate of intrinsically strong fluorescence, as the second ligand. JNU-100 exhibits an exceptional fluorescence “turn-on” exclusively for formaldehyde in aqueous solutions with a detection limit of 0.020 μM. In situ single-crystal X-ray diffraction studies reveal the hemiaminal formation on the Watson–Crick sites of adenines and the hemiaminal further stretching out to bridge over the neighboring BPDC-(NH₂)₂ through hydrogen bonding, which may suppress the nonradiative decay upon photoexcitation, resulting in restoration of the strong fluorescence of BPDC-(NH₂)₂. ¹³C solid-state NMR and isothermal titration calorimetry studies corroborate the hemiaminal formation on the Watson–Crick sites. Theoretical calculations confirm the fluorescence “turn-on” is originated from ligand-to-ligand charge transfer (LLCT) through the bridging hemiaminal. As a proof-of-concept, the rapid and visualizable response to formaldehyde is demonstrated by test strips. This work illustrates a successful design of MOF materials specifically for formaldehyde recognition without the interference of other volatile organic compounds by utilizing its reactivity on Watson–Crick sites and MOF compartmentalization.