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 (CO₃^2-, OAc^-, and Cr₂O₇^2-). 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.