Photoluminescent lanthanide-based sensor distinguishes solvents

With the help of a little light, a metal-organic framework (MOF) containing photoluminescent lanthanides can uniquely identify and measure the concentrations of multiple solvents, a research team reports (Chem 2017, DOI: j.chempr.2017.02.010).

Applied to a dipstick, the material could enable rapid identification of environmental contaminants on-site rather than require responders to send samples to a lab. The approach could also solve the tricky analytical problem of determining how much H₂O contaminates D₂O, an issue for isotopic labeling in biomolecular experiments and calibration for spectroscopic methods such as nuclear magnetic resonance.

Led by University of Texas, Austin, chemistry professor Simon M. Humphrey, the team investigated a MOF composed of tris(p-carboxylato)triphenylphosphine and europium(III), gadolinium(III), and terbium(III). Each lanthanide has a unique emission spectrum: Eu^III emits red, Gd^III emits ultraviolet, and Tb^III emits green light.

But when solvent molecules bind within the MOF, the molecules’ vibrational frequencies quench the lanthanide emissions in a way that’s unique to each solvent-lanthanide combination. The researchers exploited those properties to identify solvents by creating several MOFs with varying ratios of the lanthanides. They exposed each of four MOF varieties to a solvent, excited them at 365 nm, and compared the resulting emission intensity at three wavelengths. The approach allowed the scientists to identify a characteristic fingerprint for each solvent, thereby distinguishing among H₂O, D₂O, methanol, ethanol, toluene, benzene, and 12 other solvents.

“We were lucky that we stumbled across the right combination of lanthanides to do this,” Humphrey says. He and colleagues created dipstick-like sensors by using spray glue to deposit the MOFs onto glass slides and immersing them in solvents. The researchers could reuse the dipsticks multiple times by heating them to drive off the solvent.

Preliminary data suggest the MOFs also work to selectively detect halogens and heavy metals, Humphrey says.

Distinguishing between H₂O and D₂O is particularly difficult because the two molecules are so similar. “Using a lanthanide MOF as a sensor to detect a trace amount of H₂O in D₂O is really fantastic work that not only greatly deepens the research of luminescent lanthanide MOFs but also proposes a new application,” comments Peng Cheng, a chemistry professor at Nankai University.

CORRECTION: This story was updated on April 19, 2017, to correct the spelling of Peng Cheng’s family name.