Amino-Incorporated Tricarboxylate Metal-Organic Framework for the Sensitive Fluorescence Detection of Heavy Metal Ions with Insights into the Origin of Photoluminescence Response.

Metal-organic frameworks (MOFs) are powerful chemosensors when designed to undergo a detectable change in optical properties upon interacting with target analytes. This work contributes to the overall understanding of metal-ion interactions with MOFs to elicit changes in fluorescence emission, a necessary step en route to developing more sensitive and selective systems for metal-ion sensing. Toward this goal, the photophysical properties of an amino-containing MOF, Cu3(NH2BTC)2, were investigated. The MOF was highly sensitive for the detection of Fe2+ and Fe3+ exhibiting the most intense fluorescence quenching with the lowest detectable change in signal occurring at 1.55 ppm (27.8 μM) at room temperature in minutes. Other metal ions, including Pb2+, Cu2+, Mn2+, Ni2+, and Co2+, were also detected at 5.7, 12, 3.0, 1.6, and 0.2 ppm, respectively, demonstrating the range of sensing capabilities. Additional experiments were performed to elucidate the pathway of metal-ion detection, including the investigation of photoluminescent changes upon the introduction of acids (HCl, ZrCl4, and AlCl3) and several anions (CO32-, OAc-, and Cr2O72-). To determine the influence of the amino functional group on interactions with the analytes, isoreticular Cu3BTC2 and postsynthetically modified Cu3(NH2BTC)2 (to hinder access to the free-amine moiety) were also investigated, revealing that the free amine is essential for the detection of the anions and is likely involved in the detection of several divalent metal ions. On the other hand, divalent metal ion Fe2+ likely induces an emission intensity change by interaction with the carboxylate of the MOF ligand or the open Cu2+ sites. Taken together, this report demonstrates that Cu3(NH2BTC)2 has unique photoluminescent properties and likely elicits detection of ions (ppm, μM) via multiple pathways. As such, future development of MOF-based sensors should include MOFs with free accessible functional handles (such as an amine) to be strategically modified to selectively detect specific metal ions.