Metal-organic framework membrane-based probe for on-site and sensitive detection of Cr(VI) in groundwater using a portable system

Groundwater sources are often contaminated by heavy metal ions such as hexavalent chromium (Cr(VI)) produced by various industrial processes, which can accumulate in the human tissues through drinking of water, resulting in serious health hazards. For this reason, on-site and sensitive detection of heavy metals in contaminated groundwater is essential for effective heavy metal pollution evaluation. Here, we develop a carboxymethyl cellulose (CMC)-metal–organic framework (MOF) fluorescence sensing membrane and couple with portable laser-induced fluorescence spectrometers (LIFs) for on-site detection of trace Cr(VI) in groundwater. The membrane was prepared using coordination assembly approach between flexible CMC and MOF and subsequent vacuum-assisted filtration. The CMC-MOF membrane exhibits very sensitive behaviour to Cr(VI) with a detection limit as low as 3.72 ppb, lower than the World Health Organization defined action level (50 ppb). Density functional theory calculations show that the formation of coordination bonds between CMC and MOF results in the excellent stability of the CMC-MOF membrane, and Cr(VI) simultaneously binds to two nitrogen atoms in MOFs, enabling CMC-MOF membrane to enrich and be on-site detection of trace Cr(VI) in contaminated groundwater by portable LIFs. This work provides an approach for on-site and highly sensitive assessment of heavy metal in contaminated groundwater.