Loading fluorescent dyes into MOF pores for the optical sensing and adsorption of heavy metal ions: synthesis, characterization, and performance.

The detection and removal of Cu(II) cations are an important topic in environmental protection and human health. Optical sensing platforms based on luminescent probes seem attractive due to their optical signal outputs which are free of electromagnetic interference, low cost, fast response, and high sensitivity. In this work, two azobenzene-based sensing probes (P1 and P2) were synthesized, and their sensing performance was firstly evaluated by their spectroscopic response towards various ions. P1 showed good sensing selectivity towards Cu by forming an adduct with stoichiometric ratio of 1:1, as confirmed by Job's plot, NMR, XPS, and EPR (electron paramagnetic resonance) comparison. P2 showed sensing behavior not only towards Cu but also towards Fe, Fe, and Hg due to the increased bonding affinity of -OH group. These two azobenzene-based probes were then covalently loaded into a MOF (metal-organic framework) matrix of bio-MOF-1 to endow it with optical sensing ability, as well as to improve the adsorption stability/capacity for metal cations. P@BMOF (n = 1, 2) samples were characterized by SEM, XRD, N adsorption/desorption, IR, and elemental analysis. Their optical sensing and adsorption performance for metal cations were evaluated as well. Linear working equations were obtained with quenching constants and LOD values of 0.0884 μM and 0.22 μM for P1@BMOF, and 0.0998 μM and 0.20 μM for P2@BMOF. The adsorption levels for Cu were determined as 0.97 mmol/g for P1@BMOF and 1.03 mmol/g for P2@BMOF.