Smartphone-assisted visual detection of arsenate based on Cu²⁺-functionalized Zr-MOF fluorescent nanozyme

A ratiometric fluorescence platform for sensing arsenate with high-performance has been developed with Cu²⁺-functionalized Zr metal organic framework (Cu@Zr-MOF) fluorescent nanozyme. The doping of Cu²⁺ provides the catalytic active site to decompose O₂ into •OH and ¹O₂, which oxidizes non-fluorescent substrate o-phenylenediamine (OPD) into product (oxOPD) with yellow fluorescence. When As(V) is present, the intrinsic blue fluorescence of Zr-MOF enhances remarkably by weakening the ligand-to-metal charge transfer (LMCT) accompanied by the fluorescence decrease of oxOPD owing to inner filter effect (IFE). Furthermore, the solutions present a distinguishable color tonality from yellow to blue, which is converted to digital value by smartphone, achieving visual detection of As(V) with a low LOD of 0.67 μM. The specific recognition of Zr-O clusters toward As(V) enables this sensor high selectivity. Compared to other MOF-based ratiometric fluorescence assays for As(V), this method is portable and cost-effective owing to the direct coordination of Cu@Zr-MOF nanozyme instead of fragile and high-cost natural enzyme with As(V). This work not only develops a portable method for sensing As(V) but also expands the potential application of functionalized MOF in on-site monitoring.